Co-reporter:Yilei Wang, Suresh Kumar Raman Pillai, Jianfei Che, and Mary B. Chan-Park
ACS Applied Materials & Interfaces March 15, 2017 Volume 9(Issue 10) pp:8960-8960
Publication Date(Web):February 21, 2017
DOI:10.1021/acsami.6b13197
To improve the interlaminar shear strength (ILSS) of carbon fiber reinforced epoxy composite, networks of multiwalled carbon nanotubes (MWNTs) were grown on micron-sized carbon fibers and single-walled carbon nanotubes (SWNTs) were dispersed into the epoxy matrix so that these two types of carbon nanotubes entangle at the carbon fiber (CF)/epoxy matrix interface. The MWNTs on the CF fiber (CF-MWNTs) were grown by chemical vapor deposition (CVD), while the single-walled carbon nanotubes (SWNTs) were finely dispersed in the epoxy matrix precursor with the aid of a dispersing agent polyimide-graft-bisphenol A diglyceryl acrylate (PI-BDA) copolymer. Using vacuum assisted resin transfer molding, the SWNT-laden epoxy matrix precursor was forced into intimate contact with the “hairy” surface of the CF-MWNT fiber. The tube density and the average tube length of the MWNT layer on CF was controlled by the CVD growth time. The ILSS of the CF-MWNT/epoxy resin composite was examined using the short beam shear test. With addition of MWNTs onto the CF surface as well as SWNTs into the epoxy matrix, the ILSS of CF/epoxy resin composite was 47.59 ± 2.26 MPa, which represented a ∼103% increase compared with the composite made with pristine CF and pristine epoxy matrix (without any SWNT filler). FESEM established that the enhanced composite did not fail at the CF/epoxy matrix interface.Keywords: carbon fiber; carbon nanotube; composite; epoxy matrix; ILSS;
Co-reporter:Chengyin Liu;Bo Liu;Mary B. Chan-Park
Polymer Chemistry (2010-Present) 2017 vol. 8(Issue 4) pp:674-681
Publication Date(Web):2017/01/24
DOI:10.1039/C6PY01933A
Polyimides have attractive properties such as a strong interaction with 1D/2D carbon nanomaterials but their solubilities in common organic solvents are limited. We report a new synthesis route for triblock copolymers of polycaprolactone-polyimide-polycaprolactone (PCL-PI-PCL) via polycondensation followed by ring-opening polymerization. The prepared OH-PI-OH homopolymer precursors were reacted with two equivalents of stannous(II) octoate (Sn(Oct)2) to afford α,ω-dihydroxyl-terminated polyimide macroinitiators which can polymerize with ε-caprolactone to obtain the final triblock copolymers (PCL-PI-PCL). Four different molecular weights of PCL-PI-PCL triblock copolymers with different lengths of PCL and PI blocks were synthesized to assess the best composition for carbon nanotube dispersion in a low boiling organic solvent (tetrahydrofuran, THF). The polyimide block interacts strongly with single walled carbon nanotubes (SWNTs) through charge transfer, as shown by Raman spectroscopy, while the polycaprolactone block has a good solubility in THF. An optimised triblock copolymer disperses the carbon nanotubes in THF well even after standing for 1 h, while the PI homopolymer-dispersed SWNTs settled completely under the same conditions. We applied the new PCL-PI-PCL in SWNT-reinforced epoxy composites with the use of THF as the casting solvent. The optimised triblock copolymer-dispersed SWNTs (2 wt%) increased the tensile strength, modulus, and elongation at maximum stress by 74%, 35%, and 62% respectively compared to the neat resin blend. The new synthesis route of the triblock copolymer is amenable to the synthesis of diverse PI-based triblock copolymers with various desired functionalities for myriad applications, such as for carbon nanotube-reinforced epoxy-based composites, water-based antibacterial dispersions, etc.
Co-reporter:Dicky Pranantyo;Li Qun Xu;Zheng Hou;En-Tang Kang;Mary B. Chan-Park
Polymer Chemistry (2010-Present) 2017 vol. 8(Issue 21) pp:3364-3373
Publication Date(Web):2017/05/30
DOI:10.1039/C7PY00441A
A series of four-arm star copolymers, incorporating glycopolymer and antimicrobial polypeptide domains, was developed in the design of forthcoming anti-infective agents. Mannose, glucose, and galactose-based glycopolymers with a variety of well-defined chain lengths were prepared via atom transfer radical polymerization, whereas linear α-polylysine was prepared via ring-opening polymerization of N-carboxyanhydride monomers. Copper-catalyzed azide–alkyne cycloaddition was employed for ‘click’ conjugation of the glycopolymer arms and the polypeptide chains. The glycopolymer–polypeptide conjugates were non-hemolytic and exhibited higher cytocompatibility than the linear α-polylysine. The conjugates with shorter chains of mannose-based glycopolymer arms showed an enhanced bactericidal efficacy against Gram-negative and Gram-positive bacteria, with a therapeutic selectivity half of that of the linear α-polylysine. The pendant mannose moieties of the conjugates increased microbial targeting due to their specific affinity for bacterial surfaces, and binding competition with free mannopyranoside was demonstrated. Therefore, the molecular combination of glycopolymers and polypeptides without loss of their respective activities provides an interesting concept in the design of antimicrobial agents to combat infectious disease.
Co-reporter:Yuji Pu, Zheng Hou, Mya Mya Khin, Rubi Zamudio-Vázquez, Kar Lai Poon, Hongwei DuanMary B. Chan-Park
Biomacromolecules 2017 Volume 18(Issue 1) pp:
Publication Date(Web):December 6, 2016
DOI:10.1021/acs.biomac.6b01279
Cationic polymethacrylates are interesting candidates for bacterial disinfectants since they can be made in large-scale by various well-established polymerization techniques such as atom transfer radical polymerization (ATRP). However, they are usually toxic or ineffective in serum and various strategies to improve their biocompatibility or nonfouling property have often resulted in compromised bactericidal activity. Also, star-shaped polymers are less explored than linear polymers for application as antibacterial compounds. In this paper, star polymers with poly[2-(dimethylamino)ethyl methacrylate] (PDMA) as the arms and polyhedral oligomeric silsesquioxane (POSS) as the core (POSS-g-PDMA) were successfully synthesized by ATRP. The minimum inhibition concentrations (MICs) of the synthesized POSS-g-PDMA are in the range of 10–20 μg/mL. POSS-g-PDMA was further modified by various hydrophilization strategies in attempting to reduce hemolysis. With quaternization of POSS-g-PDMA, the antibacterial activities of the obtained quaternary polymers are almost unchanged and the copolymers become relatively nonhemolytic. We also copolymerized sulfobetaine (SB) with POSS-g-PDMA to obtain random and block PDMA-co-PSB arm structures, where the PDMA and poly(sulfobetaine) were the cationic and zwitterionic blocks, respectively. The random cationic–zwitterionic POSS-g-(PDMA-r-PSB) copolymers showed poor antibacterial activity, while the block POSS-g-(PDMA-b-PSB) copolymers retained the antibacterial and hemolytic activity of the pristine POSS-g-PDMA. Further, the block copolymers of POSS-g-(PDMA-b-PSB) showed enhanced antifouling property and serum stability as seen by their nanoparticle size stability in the presence of serum and reduced red blood cell aggregation; the POSS-g-(PDMA-b-PSB) also somewhat retained its MIC in blood unlike the quaternized or random zwitterionic copolymers. The antibacterial kinetics study showed that Escherichia coli can be killed within 30 min by both random and block copolymers of POSS-g-(PDMA-co-PSB). Finally, our POSS star polymers showed low toxicity to zebrafish embryo and could be potentially used in aquaculture antibacterial applications.
Co-reporter:Yilei Wang, Ya Xuan Thong, Jing Wang, and Mary B. Chan-Park
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 35) pp:23338
Publication Date(Web):August 9, 2016
DOI:10.1021/acsami.6b04732
We recently reported a noncovalent chemistry bead-based method that can sort single-walled carbon nanotubes (SWNTs) to ultrahigh purity. The method is based on the higher affinity of functionalized agarose beads for M(etallic) carbon nanotubes in acidic pH versus S(emiconducting) SWNTs based on the selective charge reversal of metallic carbon nanotubes. However, fundamental understanding of the relative selective affinity of various functional groups for certain electronic type nanotubes remains somewhat lacking. We show that the chemical force microscopy (CFM) technique can measure the subtle differences between various common functional groups (such as −NH2, −OH, −SO3H/–SO3–Na+, −NO2, etc.) and the different electronic types of SWNTs. We show that the amine-functionalized alkane has significantly higher interaction forces with S SWNTs. On the other hand, SO3–Na+- and NO2-functionalized naphthalene show significantly higher interaction forces with M SWNTs compared with S SWNTs; the −SO3H substitution on an alkane, however, shows no significant selectivity for any single electronic type of SWNTs. We discovered two novel molecules (sodium 4-amino-1-naphthalenesulfonate and 1-amino-4-nitronaphthalene) that are able to have significantly higher interaction force with M SWNTs and provide complete electronic type discrimination over the entire nanotube diameter range. We also show that the CFM platform can be applied to distinguish between M or S tubes from an as-grown SWNTs mixture in air. The platform can also be applied for studying the effect of solvent (water) on the selectivity. It is anticipated that our new CFM method using functionalized tips will be able to accelerate the development of noncovalent separation strategies for improved nanotube electronic type separation.Keywords: carbon nanotubes; chemical-force microscopy; noncovalent interactions; PeakForce QNM; selectivity
Co-reporter:Ahmed G. El-Deen, Remko M. Boom, Hak Yong Kim, Hongwei Duan, Mary B. Chan-Park, and Jae-Hwan Choi
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 38) pp:25313
Publication Date(Web):September 2, 2016
DOI:10.1021/acsami.6b08658
Nanoporous graphene based materials are a promising nanostructured carbon for energy storage and electrosorption applications. We present a novel and facile strategy for fabrication of asymmetrically functionalized microporous activated graphene electrodes for high performance capacitive desalination and disinfection of brackish water. Briefly, thiocarbohydrazide coated silica nanoparticles intercalated graphene sheets are used as a sacrificial material for creating mesoporous graphene followed by alkaline activation process. This fabrication procedure meets the ideal desalination pore diameter with ultrahigh specific surface area ∼ 2680 m2 g–1 of activated 3D graphene based micropores. The obtained activated graphene electrode is modified by carboxymethyl cellulose as negative charge (COO–2) and disinfectant quaternary ammonium cellulose with positively charged polyatomic ions of the structure (NR4+). Our novel asymmetric coated microporous activated 3D graphene employs nontoxic water-soluble binder which increases the surface wettability and decreases the interfacial resistance and moreover improves the electrode flexibility compared with organic binders. The desalination performance of the fabricated electrodes was evaluated by carrying out single pass mode experiment under various cell potentials with symmetric and asymmetric cells. The asymmetric charge coated microporous activated graphene exhibits exceptional electrosorption capacity of 18.43 mg g–1 at a flow rate of 20 mL min–1 upon applied cell potential of 1.4 V with initial NaCl concentration of 300 mg L–1, high charge efficiency, excellent recyclability, and, moreover, good antibacterial behavior. The present strategy provides a new avenue for producing ultrapure water via green capacitive deionization technology.Keywords: asymmetric capacitive deionization; nanohybrid electrode; nanoporous graphene; water desalination; water disinfection
Co-reporter:Lu Pu, Jinbao Xu, Yimin Sun, Zheng Fang, Mary B. Chan-Park and Hongwei Duan
Biomaterials Science 2016 vol. 4(Issue 5) pp:871-879
Publication Date(Web):24 Feb 2016
DOI:10.1039/C5BM00545K
We report a new class of antimicrobial nanomaterials with biodegradable cationic polycarbonates grafted on superparamagnetic nanoparticles. Our results have shown that end-functionalized cationic polycarbonates, synthesized by organocatalytic ring opening polymerization, can be grafted onto superparamagnetic MnFe2O4 nanoparticles via ligand exchange. In comparison with the individual building blocks, the core–shell hybrid nanoparticles led to improved antimicrobial activities in two ways: first, the cationic polycarbonates in a brush form afforded a greater charge density than that of free polymer chains, resulting in stronger interactions with bacterial surfaces. Second, the structural integration of the “soft” polycarbonate shell and the “hard” superparamagnetic core in the hybrid nanoparticles brings about a synergistic action of membrane disruption by the cationic shell and magnetic hyperthermia by the nanoparticle core. The combination of two physical killing mechanisms holds great promise in fighting against a broad spectrum of bacterial pathogens.
Co-reporter:Edgar H. H. Wong, Mya Mya Khin, Vikashini Ravikumar, Zhangyong Si, Scott A. Rice, and Mary B. Chan-Park
Biomacromolecules 2016 Volume 17(Issue 3) pp:
Publication Date(Web):February 9, 2016
DOI:10.1021/acs.biomac.5b01766
The development of novel reagents and antibiotics for combating multidrug resistance bacteria has received significant attention in recent years. In this study, new antimicrobial star polymers (14–26 nm in diameter) that consist of mixtures of polylysine and glycopolymer arms were developed and were shown to possess antimicrobial efficacy toward Gram positive bacteria including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE) (with MIC values as low as 16 μg mL–1) while being non-hemolytic (HC50 > 10 000 μg mL–1) and exhibit excellent mammalian cell biocompatibility. Structure function analysis indicated that the antimicrobial activity and mammalian cell biocompatibility of the star nanoparticles could be optimized by modifying the molar ratio of polylysine to glycopolymers arms. The technology described herein thus represents an innovative approach that could be used to fight deadly infectious diseases.
Co-reporter:Dicky Pranantyo, Li Qun Xu, and En-Tang Kang, Mya Khin Mya and Mary B. Chan-Park
Biomacromolecules 2016 Volume 17(Issue 12) pp:
Publication Date(Web):November 9, 2016
DOI:10.1021/acs.biomac.6b01452
Enhancing the bactericidal activity and moderating the toxicity are two important challenges in the design of upcoming antimicrobial compounds. Herein, antimicrobial macromolecules were developed by conjugating CysHHC10 peptide and polyphosphoester for the modulation of microbiocidal activity and biocompatibility. The conjugation was carried out via thiol-yne “click” chemistry between the cysteine terminal of the peptide and the pendant propargyl moieties of the polyphosphoester. The bactericidal efficacy of the polyphosphoester–peptide conjugates were investigated by microbial growth inhibition toward the Gram-positive and Gram-negative bacteria. On the basis of peptide mass fraction, the polyphosphoester–peptide conjugates exhibited lower values of minimum inhibitory concentration than that of the free peptide. The polyphosphoester–peptide conjugates also exhibited ultralow hemolytic characteristic at a concentration of 4000 μg/mL, indicating significant improvement of erythrocytes compatibility as compared to the free peptide that readily caused lysis of 50% of red blood cells at 1000 μg/mL. Cytotoxicity of the polyphosphoester–peptide conjugates toward 3T3 fibroblast cells was also reduced in comparison to that of the free peptide. Conjugation of the polyphosphoester thus improves the bactericidal efficacy and biocompatibility of the antimicrobial peptide.
Co-reporter:Jing Wang, Tuan Dat Nguyen, Qing Cao, Yilei Wang, Marcus Y.C. Tan, and Mary B. Chan-Park
ACS Nano 2016 Volume 10(Issue 3) pp:3222
Publication Date(Web):February 22, 2016
DOI:10.1021/acsnano.5b05795
Semiconducting (semi-) single-walled carbon nanotubes (SWNTs) must be purified of their metallic (met-) counterparts for most applications including nanoelectronics, solar cells, chemical sensors, and artificial skins. Previous bulk sorting techniques are based on subtle contrasts between properties of different nanotube/dispersing agent complexes. We report here a method which directly exploits the nanotube band structure differences. For the heterogeneous redox reaction of SWNTs with oxygen/water couple, the aqueous pH can be tuned so that the redox kinetics is determined by the availability of nanotube electrons only at/near the Fermi level, as predicted quantitatively by the Marcus–Gerischer (MG) theory. Consequently, met-SWNTs oxidize much faster than semi-SWNTs and only met-SWNTs selectively reverse the sign of their measured surface zeta potential from negative to positive at the optimized acidic pH when suspended with nonionic surfactants. By passing the redox-reacted nanotubes through anionic hydrogel beads, we isolate semi-SWNTs to record high electrically verified purity above 99.94% ± 0.04%. This facile charge sign reversal (CSR)-based sorting technique is robust and can sort SWNTs with a broad diameter range.Keywords: carbon nanotubes; charge sign reversal; gel chromatography; purification; sorting
Co-reporter:Bernice H. L. Oh, Alexander Bismarck and Mary B. Chan-Park
Journal of Materials Chemistry A 2015 vol. 3(Issue 20) pp:4118-4122
Publication Date(Web):13 Mar 2015
DOI:10.1039/C5TB00303B
High internal phase emulsions (HIPEs) are indisputably a core technology for various industries involving pharmaceuticals, food, cosmetics, and biologics but they usually require surfactants/co-surfactants to form, which is often undesired. More specifically, micro-HIPEs are thermodynamically stable, optically clear emulsions with droplet sizes in the range of around 1–100 nm that form spontaneously with little energy input but are rare. Mini-/macro-HIPEs have larger droplet sizes in the range of 50–500 nm and >500 nm, respectively, and typically require high energy input for emulsification. We have synthesized a series of chitosan-graft-oligoN-isopropylacrylamide-graft-oligolysine (CSNLYS) copolymers that act as both emulsifiers for HIPEs without needing extraneous surfactants as well as the matrix material of the resulting porous solid polyHIPE. By merely adjusting the length of the oligolysine graft from relatively long to medium to short, we can form either a micro-, mini- or macro-HIPE, respectively. These emulsions can then be solidified into porous polymers, polyHIPEs, simply by increasing the temperature by exploiting the copolymer thermo-responsiveness and then removing the solvents. These porous polyHIPE, particularly the ones from micro-HIPEs, have surface areas as high as 988 m2 g−1 and pore sizes below 200 nm.
Co-reporter:Bo Duan, Jiajing Zhou, Zheng Fang, Chenxu Wang, Xiujuan Wang, Harold F. Hemond, Mary B. Chan-Park and Hongwei Duan
Nanoscale 2015 vol. 7(Issue 29) pp:12606-12613
Publication Date(Web):15 Jun 2015
DOI:10.1039/C5NR02164B
We have developed a new type of surface enhanced Raman scattering (SERS) substrate with thiolated graphene oxide (tGO) nanosheets sandwiched between two layers of closely packed plasmonic nanoparticles. The trilayered substrate is built up through alternative loading of interfacially assembled plasmonic nanoparticle arrays and tGO nanosheets, followed by coating the nanoparticle surfaces with poly(ethylene glycol) (PEG). Here tGO plays multifunctional roles as a 2D scaffold to immobilized interfacially assembled plasmonic nanoparticles, a nanospacer to create SERS-active nanogaps between two layers of nanoparticle arrays, and a molecule harvester to enrich molecules of interest via π–π interaction. In particular, the molecule harvesting capability of the tGO nanospacer and the stealth properties of PEG coating on the plasmonic nanoparticles collectively lead to preferential positioning of selective targets such as aromatic molecules and single-stranded DNA at the SERS-active nanogap hotspots. We have demonstrated that an SERS assay based on the PEGylated trilayered substrate, in combination with magnetic separation, allows for sensitive, multiplexed “signal-off” detection of DNA sequences of bacterial pathogens.
Co-reporter:Daniel E. Heath, Abdul Rahim Mohamed Sharif, Chee Ping Ng, Mary G. Rhoads, Linda G. Griffith, Paula T. Hammond and Mary B. Chan-Park
Lab on a Chip 2015 vol. 15(Issue 9) pp:2073-2089
Publication Date(Web):19 Mar 2015
DOI:10.1039/C4LC01416B
Polydimethylsiloxane stamp materials used during soft lithography undermine the non-fouling behaviour of bio-inert PEG-based hydrogels, resulting in increased protein adsorption and cell adhesion and migration on the gel. This previously unreported phenomenon undermines the function of lab-on-a-chip devices that require the device to be bio-inert, and slows the implementation of promising micromolding and imprinting methods for 3D culture and commercial cell culture systems. We illustrate that the degree of cell adhesion and protein adsorption to the gels correlates with the amount of residual stamp material remaining at the hydrogel interface after fabrication. After identifying this previously unreported phenomenon, we screened multiple polymer cleaning/fabrication techniques in order to maintain/restore the non-fouling properties of the gels including PDMS curing and extraction, use of other common soft lithography stamp materials, post-fabrication cleaning of the hydrogels, and changing the composition of the hydrogel. The optimal solution was determined to be incorporation of reactive sites into the hydrogel during micromolding followed by grafting of PEG macromers to these sites post-fabrication. This treatment resulted in micromolded hydrogels with robust cell resistant properties. Broadly, this work identifies and solves a previously unreported problem in hydrogel micromolding, and specifically reports the development of a cell culture platform that when combined with video microscopy enables high-resolution in situ study of single cell behaviour during in vitro culture.
Co-reporter:Bernice H. L. Oh;Alexer Bismarck;Mary B. Chan-Park
Macromolecular Rapid Communications 2015 Volume 36( Issue 4) pp:364-372
Publication Date(Web):
DOI:10.1002/marc.201400524
Co-reporter:Jien Yang, Yilei Wang, Jing Wang and Mary B. Chan-Park
RSC Advances 2015 vol. 5(Issue 15) pp:11124-11127
Publication Date(Web):06 Jan 2015
DOI:10.1039/C4RA13801E
An effective and facile method has been developed to reduce graphene oxide (GO) in alkyl amine solution using alkali metal as reducing agent. The alkali metal donates electrons to the GO and the oxygen-containing functional groups were eliminated to produce high-quality reduced graphene oxide (RGO).
Co-reporter:Yilei Wang, Ahmed G. El-Deen, Peng Li, Bernice H.L. Oh, Zanru Guo, Mya Mya Khin, Yogesh S. Vikhe, Jing Wang, Rebecca G. Hu, Remko M. Boom, Kimberly A. Kline, David L. Becker, Hongwei Duan, and Mary B. Chan-Park
ACS Nano 2015 Volume 9(Issue 10) pp:10142
Publication Date(Web):September 21, 2015
DOI:10.1021/acsnano.5b03763
Water disinfection materials should ideally be broad-spectrum-active, nonleachable, and noncontaminating to the liquid needing sterilization. Herein, we demonstrate a high-performance capacitive deionization disinfection (CDID) electrode made by coating an activated carbon (AC) electrode with cationic nanohybrids of graphene oxide-graft-quaternized chitosan (GO-QC). Our GO-QC/AC CDID electrode can achieve at least 99.9999% killing (i.e., 6 log reduction) of Escherichia coli in water flowing continuously through the CDID cell. Without the GO-QC coating, the AC electrode alone cannot kill the bacteria and adsorbs a much smaller fraction (<82.8 ± 1.8%) of E. coli from the same biocontaminated water. Our CDID process consists of alternating cycles of water disinfection followed by electrode regeneration, each a few minutes duration, so that this water disinfection process can be continuous and it only needs a small electrode voltage (2 V). With a typical brackish water biocontamination (with 104 CFU mL–1 bacteria), the GO-QC/AC electrodes can kill 99.99% of the E. coli in water for 5 h. The disinfecting GO-QC is securely attached on the AC electrode surface, so that it is noncontaminating to water, unlike many other chemicals used today. The GO-QC nanohybrids have excellent intrinsic antimicrobial properties in suspension form. Further, the GO component contributes toward the needed surface conductivity of the CDID electrode. This CDID process offers an economical method toward ultrafast, contaminant-free, and continuous killing of bacteria in biocontaminated water. The proposed strategy introduces a green in situ disinfectant approach for water purification.Keywords: antimicrobial; capacitive deionization; cationic; contact-active; graphene oxide; quaternary chitosan; water disinfection;
Co-reporter:Yao Li;Cheng-Kang Mai;Hung Phan;Xiaofeng Liu;Thuc-Quyen Nguyen;Guillermo C. Bazan;Mary B. Chan-Park
Advanced Materials 2014 Volume 26( Issue 27) pp:4697-4703
Publication Date(Web):
DOI:10.1002/adma.201400612
Co-reporter:Jing Wang, Jintao Zhang, Ashok Kumar Sundramoorthy, Peng Chen and Mary B. Chan-Park
Nanoscale 2014 vol. 6(Issue 9) pp:4560-4565
Publication Date(Web):25 Feb 2014
DOI:10.1039/C3NR06386K
In a simple, cost-effective, and solution-based process, a thin-film of single-walled carbon nanotubes is hybridized on a PET film which has been patterned with solution self-assembled Ag nanoparticles. Such a flexible and transparent electrode exhibits a sheet resistance down to ∼5.8 Ω sq−1 at ∼83.7% optical transmittance. The hybrid films are stable under ambient conditions and offer excellent bendability.
Co-reporter:Jintao Zhang, Jing Wang, Jien Yang, Yilei Wang, and Mary B. Chan-Park
ACS Sustainable Chemistry & Engineering 2014 Volume 2(Issue 10) pp:2291
Publication Date(Web):August 29, 2014
DOI:10.1021/sc500247h
Bicontinuous macroporous graphene foam composed of few-layered graphene sheets provides a highly conductive platform on which to deposit mesoporous polyaniline via incorporation of electrodeposition and inkjet techniques. The experimental results exhibit that the coating polyaniline thin layer on the surface of three-dimensional graphene foam via electrodeposition is of importance for changing the hydrophobic surface to a hydrophilic one and for the subsequent filling of the mesoporous polyaniline network into the macroporous graphene foam. The porous polyaniline network with high pseudocapacitance is highly efficient for adjusting the pore structure and capacitive properties of graphene foam. When used as electrode materials for supercapacitors, the resulted graphene foam–polyaniline network with high porosity renders a large areal capacitance of over 1700 mF cm–2, which is over two times the enhancement in comparison with the pure graphene foam and polyaniline thin layer coated one. The ultrahigh areal capacitance benefits from the synergistic effect of the good conductive graphene backbone and high pseudocapacitive polyaniline.Keywords: Areal capacitance; Graphene; Polyaniline; Porous structure; Supercapacitor;
Co-reporter:Shahrzad Rayatpisheh, Daniel E. Heath, Amir Shakouri, Pim-On Rujitanaroj, Sing Yian Chew, Mary B. Chan-Park
Biomaterials 2014 35(9) pp: 2713-2719
Publication Date(Web):
DOI:10.1016/j.biomaterials.2013.12.035
Co-reporter:Bernice H. L. Oh, Alexander Bismarck, and Mary B. Chan-Park
Biomacromolecules 2014 Volume 15(Issue 5) pp:
Publication Date(Web):March 24, 2014
DOI:10.1021/bm500172u
High internal phase emulsion (HIPE)-templating is an attractive method of producing high porosity polymer foams with tailored pore structure, pore size and porosity. However, this method typically requires the use of large amounts of surfactants to stabilize the immiscible liquid phases, and polymerizable monomers/cross-linker in the continuous minority phase to solidify the HIPE, which may not be desirable in many applications. We show that polyHIPEs with a porosity of 73% can be formed solely using a copolymer of chitosan-graft-PNIPAM-graft-oligoproline (CSN-PRO), which acts simultaneously as emulsifier and thermoresponsive gelator, and forms upon removal of the liquid templating phases, the bulk structure of the resulting polyHIPE. With only a small amount of surfactant (1%v/v in the aqueous phase), and varying the polymer concentration and internal phase volume ratio, different polyHIPEs with porosities of up to 99%, surface areas in excess of 300 m2/g and controlled pore interconnectivity can be formed. The poly(CSN-PRO)HIPEs are also shown to be thermoresponsive and remained intact when immersed into water above 34 °C but dissolve below their LCST, which is useful for applications such as drug delivery and tissue engineering scaffolds.
Co-reporter:Ashok Kumar Sundramoorthy ; Sara Mesgari ; Jing Wang ; Raj Kumar ; Mahasin Alam Sk. ; Siew Hooi Yeap ; Qing Zhang ; Siu Kwan Sze ; Kok Hwa Lim ;Mary B. Chan-Park
Journal of the American Chemical Society 2013 Volume 135(Issue 15) pp:5569-5581
Publication Date(Web):March 22, 2013
DOI:10.1021/ja312282g
Semiconducting single-walled carbon nanotubes (s-SWNTs) have emerged as a promising class of electronic materials, but the metallic (m)-SWNTs present in all as-synthesized nanotube samples must be removed for many applications. A high selectivity and high yield separation method has remained elusive. A separation process based on selective chemistry appears to be an attractive route since it is usually relatively simple, but more effective chemicals are needed. Here we demonstrate the first example of a new class of dual selective compounds based on polycyclic aromatic azo compounds, specifically Direct Blue 71 (I), for high-purity separation of s-SWNTs at high yield. Highly enriched (∼93% purity) s-SWNTs are produced through the simple process of standing arc-discharge SWNTs with I followed by centrifugation. The s-SWNTs total yield is up to 41%, the highest yet reported for a solution-based separation technique that demonstrates applicability in actual transistors. 91% of transistor devices fabricated with these s-SWNTs exhibited on/off ratios of 103 to 105 with the best devices showing mobility as high as 21.8 cm2/V s with on/off ratio of 104. Raman and X-ray photoelectron spectroscopic shifts and ultraviolet–visible–near-infrared (UV–vis–NIR) show that I preferentially complexes with s-SWNTs and preferentially suspends them. Preferential reaction of naphthyl radicals (generated from I with ultrasonication) with m-SWNTs is confirmed by changes in the D-band in the Raman spectroscopy, matrix-assisted desorption–ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and molecular simulation results. The high selectivity of I stems from its unique dual action as both a selective dispersion agent and the generator of radicals which preferentially attack unwanted metallic species.
Co-reporter:Jiangbo Li, Yinxi Huang, Peng Chen, and Mary B. Chan-Park
Chemistry of Materials 2013 Volume 25(Issue 22) pp:4464
Publication Date(Web):October 24, 2013
DOI:10.1021/cm401040d
Single-walled carbon nanotubes (SWNTs) are regarded to be potential building blocks for future electronics, because of their exceptional electrical, physical, and mechanical properties. A major obstacle to their practical use for high-performance nanoelectronics is the presence of both metallic SWNTs (M-SWNT) and semiconducting SWNTs (S-SWNT) in as-grown SWNT samples. Most metallicity-based SWNT sorting techniques involve suspension of the nanotubes in solutions, which generally also result in nanotube defects. As-grown SWNTs typically have far superior conductivity or carrier mobility than solution-suspended SWNTs; however, thus far, there is no simple or reproducible method to remove the electronic inhomogeneity in these as-grown nanotubes. We present a simple in situ method using an organic electron-acceptor compound, Acid Yellow (AY), to convert SWNTs from metallic to semiconducting to improve device field-effect behavior. By simply immersing the as-synthesized SWNTs (still attached to a wafer) into an AY solution, the originally metallic nanotubes behave similar to semiconducting ones. Using Raman spectroscopy, atomic force microscopy and single nanotube transistor device measurements, we show that the charge-transfer interaction between SWNTs and the organic electron-acceptor compound AY is diameter-dependent; in the large-diameter regime (1.60–2.60 nm), modulated metallic SWNTs exhibit semiconducting behavior, as evidenced by a pronounced field effect in the current–voltage (I–V) characteristics and up to 3 orders of magnitude increase in the on/off ratio of single M-SWNT field-effect transistors. This method presents a simple viable route toward the fabrication of switchable transistors with as-synthesized SWNTs.Keywords: Breit−Wigner−Fano (BWF) peak; charge-transfer; single-walled carbon nanotubes; π−π stacking interaction;
Co-reporter:Jintao Zhang, Peng Chen, Bernice H. L. Oh and Mary B. Chan-Park
Nanoscale 2013 vol. 5(Issue 20) pp:9860-9866
Publication Date(Web):02 Aug 2013
DOI:10.1039/C3NR02381H
Exploiting flexible and binder-free electrode materials is of importance for the fast development of smart supercapacitor devices. A simple and effective strategy is demonstrated to fabricate a flexible composite membrane of reduced graphene oxide and polypyrrole nanowire (RGO–PPy) via in situ reduction of graphene oxide and self-assembly. More importantly, the shape and thickness of the membrane can be reasonably controlled by varying either the concentration of GO and PPy nanowires or the filtration volume. By direct coupling of two membrane electrodes, symmetric supercapacitors can be fabricated without the use of a binder and conductive additive. The supercapacitor is able to offer large areal capacitance (175 mF cm−2) and excellent cycling stability (∼93% capacitance retention after 5000 charge–discharge cycles), thanks to the synergic integration between RGO sheets and PPy nanowires and the unique self-assembled porous structure.
Co-reporter:Jianfei Che, Peng Chen and Mary B. Chan-Park
Journal of Materials Chemistry A 2013 vol. 1(Issue 12) pp:4057-4066
Publication Date(Web):17 Jan 2013
DOI:10.1039/C3TA01421E
Buckypaper is an attractive candidate material for free-standing electrodes in supercapacitors due to its high electrochemical performance, light weight, and thin dimensions. At present, however, free-standing buckypapers exhibit severe limitations in terms of product quality, especially mechanical properties, which hinder their commercial applications. We here report a new method of co-packaging buckypaper with conducting polymer and thermosetting resin to fabricate cellular SWNT buckypaper materials with excellent mechanical properties, high electrical conductivity, and enhanced electrochemical properties. This new fabrication method involves wrapping of the as-prepared buckypaper with a uniform coaxial coating of polypyrrole (PPy) on the individual SWNT or SWNT bundle surfaces via a pulsed electrochemical deposition method, followed by further packaging with cyanate ester resin via a full dip infiltration. The resulting material exhibits a significant improvement in mechanical properties (improvement over unmodified buckypaper of about 400% in tensile modulus and strength) and enhanced electrochemical performance (320 F g−1 at a current density of 1 A g−1) without sacrificing electrical and thermal properties. This material is a promising candidate for use as a free-standing electrode material in small-size, light-weight, and high-temperature supercapacitors.
Co-reporter:Sara Mesgari, Yin Fun Poon, Yilei Wang, Ya Xuan Thong, Jing Wang and Mary B. Chan-Park
Journal of Materials Chemistry A 2013 vol. 1(Issue 41) pp:6813-6823
Publication Date(Web):22 Aug 2013
DOI:10.1039/C3TC31242A
Semiconducting single-walled carbon nanotubes (s-SWNTs) are attractive candidates for next-generation printable semiconductors. However, all current synthesis methods produce s-SWNTs which are co-mingled with metallic (m-) SWNTs. Agarose gel electrophoresis has been reported to be an effective technique for the separation of s-SWNTs from m-SWNTs but removal of the agarose gel after separation has proved to be non-trivial. To remove agarose and the organic dispersing agent, specifically chondroitin sulfate in this work, from sorted s-SWNTs obtained by agarose gel electrophoresis, we employ the multi-step process involving a chlorosulfonic acid (HSO3Cl) wash, a base wash and thermal annealing. Herein, we report the detailed analysis of the effects of the various steps for gel removal from SWNTs by Fourier transform infrared (FTIR) spectroscopy, thermal gravimetric analysis (TGA), FTIR-TGA, scanning electron microscopy, atomic force microscopy and Raman spectroscopy. The polymer-contaminated s-SWNTs were dissolved in HSO3Cl, then selectively precipitated in a large excess of water, then washed with a base (NaOH) and finally thermally annealed. A detailed analysis confirmed that the final annealed samples contained almost no residual polymers. Field effect transistors were also fabricated from the annealed s-SWNTs and they showed good performance metrics with on/off ratio and mobility in the ∼102 to 106 and ∼2.5–9.5 cm2 V−1 s−1 ranges, respectively. Our method of gel electrophoresis and chlorosulfonic acid treatment produces clean and defect-free tubes which may be used for electronic applications.
Co-reporter:Chuncai Zhou;Daniel E. Heath;Abdul Rahim Mohamed Sharif;Shahrzad Rayatpisheh;Bernice H. L. Oh;Xu Rong;Roger Beuerman;Mary B. Chan-Park
Macromolecular Bioscience 2013 Volume 13( Issue 11) pp:1485-1491
Publication Date(Web):
DOI:10.1002/mabi.201300191
Transparent, high water content (>65%), and cytocompatible hydrogels, which also possess super high refractive indices (RI > 1.5), are needed for ophthalmological applications. Most hydrogels can achieve either high RI or high water content but not both in the same system because water is a low RI material. Here, high water content/high RI hydrogels fabricated through elevated-temperature UV polymerization of an aqueous solution of acrylamide (AM) and methacrylamide (MAM) with tri(ethylene glycol) dimethacrylate (TEDA) crosslinker are reported. By varying the AM:MAM ratios (2:8 to 8:2) and crosslinker density (5 to 11 mol %), it is discovered that high water content (66%) AM:MAM copolymer hydrogels exhibiting anomalously high refractive indices (1.53); they are also colorless, transparent (99.4%), and cytocompatible with human keratinocytes.
Co-reporter:Gen Qiang Zhang, Hao Bin Wu, Harry E. Hoster, Mary B. Chan-Park and Xiong Wen (David) Lou
Energy & Environmental Science 2012 vol. 5(Issue 11) pp:9453-9456
Publication Date(Web):25 Jul 2012
DOI:10.1039/C2EE22572G
In this work, we have successfully grown single-crystalline nanoneedle arrays of NiCo2O4 on conductive substrates such as Ni foam and Ti foil through a simple solution method together with a post-annealing treatment. Remarkably, the NiCo2O4–Ni foam binder-free electrode exhibits greatly improved electrochemical performance with very high capacitance and excellent cycling stability.
Co-reporter:Peng Li;Chuncai Zhou;Shahrzad Rayatpisheh;Kai Ye;Yin Fun Poon;Paula T. Hammond;Hongwei Duan;Mary B. Chan-Park
Advanced Materials 2012 Volume 24( Issue 30) pp:4130-4137
Publication Date(Web):
DOI:10.1002/adma.201104186
Co-reporter:Jing Wang, Xiaochen Dong, Rong Xu, Shuzhou Li, Peng Chen and Mary B. Chan-Park
Nanoscale 2012 vol. 4(Issue 10) pp:3055-3059
Publication Date(Web):27 Mar 2012
DOI:10.1039/C2NR30465A
The morphologies/dimensions of Au nanostructures can be tailored by merely controlling the reduction degree of graphene oxide surface. Au nanoparticles, long Au nanowires, and semicircular-shaped Au nanoplates are in situ synthesized on slightly, moderately, and highly reduced graphene oxide films respectively, without the need of any templating agent.
Co-reporter:Liang Yu Yan, Hailan Chen, Peng Li, Dong-Hwan Kim, and Mary. B. Chan-Park
ACS Applied Materials & Interfaces 2012 Volume 4(Issue 9) pp:4610
Publication Date(Web):August 21, 2012
DOI:10.1021/am300985p
Here we demonstrate a polysaccharide hydrogel reinforced with finely dispersed single-walled carbon nanotubes (SWNTs) using biocompatible dispersants O-carboxymethylchitosan (OC) and chondroitin sulfate A (CS-A) as a structural support. Both of the dispersants can disperse SWNTs in aqueous solutions and hydrogel matrix as individual tubes or small bundles. Additionally, we have found that compressive modulus and strain of the hydrogels reinforced with SWNTs were enhanced as much as two times by the addition of a few weight percent of SWNTs. Moreover, the SWNT-incorporated hydrogels exhibited lower impedance and higher charge capacity than the alginate/dispersant hydrogel without SWNTs. The OC and the CS-A demonstrated much higher reinforcing enhancement than a commercially available dispersant, sodium dodecyl sulfate. Combined with the experimental data on the mechanical and electrical properties, the biocompatibility of OC and CS-A can provide the possibility of biomedical application of the SWNT-reinforced hydrogels.Keywords: alginate; biocompatibility; chondroitin sulfate A; O-carboxymethylchitosan; reinforced hydrogel; SWNT;
Co-reporter:Wei Yuan and Mary B. Chan-Park
ACS Applied Materials & Interfaces 2012 Volume 4(Issue 4) pp:2065
Publication Date(Web):March 20, 2012
DOI:10.1021/am300038d
Although carbon nanotubes have impressive tensile properties, exploiting these properties in composites, especially those made by the common solution casting technique, seems to be elusive thus far. The reasons could be partly due to the poor nanotube dispersion and the weak nanotube/matrix interface. To solve this dual pronged problem, we combine noncovalent and covalent functionalizations of nanotubes in a single system by the design and application of a novel dispersant, hydroxyl polyimide-graft-bisphenol A diglyceryl acrylate (PIOH-BDA), and use them with epoxidized single-walled carbon nanotubes (O-SWNTs). Our novel PIOH-BDA dispersant functionalizes the nanotubes noncovalently to achieve good dispersion of the nanotubes because of the strong π–π interaction due to main chain and steric hindrance of the BDA side chain. PIOH-BDA also functionalizes O-SWNTs covalently because it reacts with epoxide groups on the nanotubes, as well as the cyanate ester (CE) matrix used. The resulting solution-cast CE composites show 57%, 71%, and 124% increases in Young’s modulus, tensile strength, and toughness over neat CE. These values are higher than those of composites reinforced with pristine SWNTs, epoxidized SWNTs, and pristine SWNTs dispersed with PIOH-BDA. The modulus and strength increase per unit nanotube weight fraction, i.e., dE/dWNT and dσ/dWNT, are 175 GPa and 7220 MPa, respectively, which are significantly higher than those of other nanotube/thermosetting composites (22–70 GPa and 140–3540 MPa, respectively). Our study indicates that covalent cum noncovalent functionalization of nanotubes is an effective tool for improving both the nanotube dispersion and nanotube/matrix interfacial interaction, resulting in significantly improved mechanical reinforcement of the solution-cast composites.Keywords: covalent functionalization; cyanate ester; mechanical properties; noncovalent functionalization; single-walled carbon nanotubes;
Co-reporter:Suresh Kumar Raman Pillai and Mary B. Chan-Park
ACS Applied Materials & Interfaces 2012 Volume 4(Issue 12) pp:7047
Publication Date(Web):November 29, 2012
DOI:10.1021/am302431e
The large-scale application of semiconducting single-walled carbon nanotubes (s-SWCNTs) for printed electronics requires scalable, repeateable, as well as noncontaminating assembly techniques. Previously explored nanotube deposition methods include serial methods such as inkjet printing and parallel methods such as spin-coating with photolithography. The serial methods are usually slow, whereas the photolithography-related parallel methods result in contamination of the nanotubes. In this paper, we report a reliable clean parallel method for fabrication of arrays of carbon nanotube-based field effect transistors (CNTFETs) involving shadow mask patterning of a passivating layer of Hafnium oxide (HfO2) over the nanotube (CNT) active channel regions and plasma etching of the unprotected nanotubes. Pure (99%) semiconducting SWCNTs are first sprayed over the entire surface of a wafer substrate followed by a two-step shadow masking procedure to first deposit metal electrodes and then a HfO2 isolation/passivation layer over the device channel region. The exposed SWCNT network outside the HfO2 protected area is removed with oxygen plasma etching. The HfO2 thus serves as both the device isolation mask during the plasma etching and as a protective passivating layer in subsequent use. The fabricated devices on SiO2/Si substrate exhibit good device performance metrics, with on/off ratio ranging from 1 × 101 to 3 × 105 and mobilities of 4 to 23 cm2/(V s). The HfO2/Si devices show excellent performance with on/off ratios of 1 × 102 to 2 × 104 and mobilities of 8 to 56 cm2/(V s). The optimum devices (on HfO2/Si) have an on/off ratio of 1 × 104 and mobility as high as 46 cm2/(V s). This HfO2-based patterning method enables large scale fabrication of CNTFETs with no resist residue or other contamination on the device channel. Further, shadow masking circumvents the need for expensive and area-limited lithography patterning process. The device channel is also protected from external environment by the HfO2 film and the passivated device shows similar (or slightly improved) performance after 300 days of exposure to ambient conditions.Keywords: carbon nanotube; electrical characteristics; hafnium oxide; passivation; shadow mask patterning; thin film transistor;
Co-reporter:Yunxiao Liu, Shahrzad Rayatpisheh, Sing Yian Chew, and Mary B Chan-Park
ACS Applied Materials & Interfaces 2012 Volume 4(Issue 3) pp:1378
Publication Date(Web):February 1, 2012
DOI:10.1021/am201648f
For the development of vascular tissue engineering, the impact of endothelial cells (ECs) on smooth muscle cell (SMC) spreading, proliferation, and differentiation is explored in the current study using a coculture model. In this coculture model, SMCs were encapsulated in a biomimetic hydrogel based on methacrylated dextran-graft-lysine (Dex-MA-LA) and methacrylamide-modified gelatin (Gel-MA), and exposed to a monolayer of ECs. With EC coculture, SMC proliferation in 3D hydrogel was promoted at initial period, and the formation of denser cellular networks was enhanced. ECs dynamically modulated SMC phenotype by promoting a more contractile SMC phenotype initially (on day 2), indicated by the upregulated expression of contractile genes α-actin, calponin, smooth muscle-myosin heavy chain (SM-MHC), and smoothelin; however, the onset of maximum expressions was delayed by ECs. Full differentiation of SMCs was not obtained even with EC coculture. Higher level of platelet-derived growth factor (PDGF)-BB and latent transforming growth factor (TGF)-β1 were detected in medium of coculture. These biochemical cues together with the physical cue of tensional force within cellular networks may be responsible for the dynamic modulation of SMC phenotype in coculture. Synthesis of elastin was promoted by ECs at transcriptional level. The formation of denser cellular networks and synthesis of elastin suggest that coculture with ECs is a potential method to construct functional vessel media layer in vitro.Keywords: 3D coculture; endothelial cell; growth factors; hydrogels; phenotype; smooth muscle cell;
Co-reporter:Shahrzad Rayatpisheh;Peng Li ;Mary B. Chan-Park
Macromolecular Bioscience 2012 Volume 12( Issue 7) pp:937-945
Publication Date(Web):
DOI:10.1002/mabi.201100477
Co-reporter:Jiangbo Li, Xuena Luan, Yinxi Huang, Simon Dunham, Peng Chen, John A. Rogers and Mary B. Chan-Park
RSC Advances 2012 vol. 2(Issue 4) pp:1275-1281
Publication Date(Web):22 Dec 2011
DOI:10.1039/C1RA00817J
The key hurdle to the practical use of as-grown single-walled carbon nanotubes (SWNTs) for high performance nanoelectronics is the unavoidable presence of metallic nanotubes with current synthesis methods. We present a simple approach to convert metallic SWNTs to semiconducting ones in situ to improve device field effect behaviour, using an aromatic compound (2-ethylanthraquinone - EAQ) which is a mild radical initiator. We show that the reaction between EAQ-generated radicals and SWNTs is diameter and metallicity dependent; in the medium-diameter regime (0.8 to 1.0 nm), the EAQ-generated radicals preferentially attack the metallic SWNTs over semiconducting ones and convert them to semiconducting, as evidenced by several orders (10 and 104) increase in the on/off ratio.
Co-reporter:Sara Mesgari, Yin Fun Poon, Liang Yu Yan, Yuan Chen, Leslie S. Loo, Ya Xuan Thong, and Mary B. Chan-Park
The Journal of Physical Chemistry C 2012 Volume 116(Issue 18) pp:10266-10273
Publication Date(Web):April 24, 2012
DOI:10.1021/jp211562p
Pure semiconducting single-walled carbon nanotubes (SWNTs) are appealing for many electronic circuits and devices, but the presence of parasitic metallic SWNTs in all as-synthesized nanotube samples makes this application elusive. Agarose gel electrophoresis (AGE) can be used to separate metallic from semiconducting SWNTs when applied in conjunction with the use of an appropriate surfactant or dispersant. To date, only sodium dodecyl sulfate (SDS) has been reported to permit considerable separation with AGE. In this study, we report on the considerably better separation achieved using chondroitin sulfate (CS-A) as a dispersant in AGE compared with SDS-assisted AGE. The CS-A assisted AGE technique may be used to produce in a single pass semiconducting SWNTs with purity of 95%, compared with 85% purity achieved with SDS-assisted AGE for the same arc discharge nanotubes. Further, the yield of CS-A assisted AGE is about 25%, which is in the order of 5 to 10 times the yields of other reported highly selective techniques. Semiconducting SWNTs produced via CS-A/AGE were used to fabricate field effect transistors (FET) with mobilities of ∼2 to 8 cm2/(V s) and on/off ratios from 102 to 105, which are significantly higher than the mobility of 0.7 cm2/(V s), and on/off ratio of 104 reported for FETs made with semiconducting SWNTs produced by SDS-assisted AGE. The excellent yield-cum-purity single-pass separation is achievable with this unique chemically selective CS-A dispersant with AGE because of its ability to wrap the nanotubes well, high degree of sulfation making the nanotube/CS-A hybrid highly charged and amine functionality resulting in preselectivity of metallic nanotubes, causing the latter to migrate much more effectively under a uniform electric field.
Co-reporter:Wei Zhi Wang, Alam Sk Mahasin, Ping Qi Gao, Kok Hwa Lim, and Mary B. Chan-Park
The Journal of Physical Chemistry C 2012 Volume 116(Issue 43) pp:23027-23035
Publication Date(Web):October 2, 2012
DOI:10.1021/jp305724w
Brominated single-walled carbon nanotubes, with bromine covalently attached to the nanotube surface, have been synthesized by a mild reaction using n-bromosuccinimide (NBS). The latter preferentially attacks metallic single walled carbon nanotubes (SWNTs) over semiconducting ones, and the attached Br leads to a significant density differential between reacted and pristine nanotubes. The differential reactivity between semiconducting and metallic SWNTs enhances the density contrast between them, which may be more effectively spatially separated via density gradient ultracentrifugation than unchemically modified SWNTs. The results of optical absorbance, photoluminescence emission, and resonant Raman scattering show that bromination-assisted density gradient ultracentrifugation (hereafter labeled as Br-DGU) preferentially separated semiconducting nanotubes within a certain diameter range (0.829–0.966 nm, specifically (7,6), (8,4), (9,4), and (10,3)). We have applied the semiconducting species enriched SWNTs to prepare solution-processed FET devices with random nanotube network active channels. The devices exhibit stable p-type semiconductor behavior in air with very promising characteristics. The on–off current ratio reaches up to 1730 within a narrow gate voltage (−2 to 2 V) and an estimated hole mobility of 13 cm2 V–1 s–1.
Co-reporter:Guan Hong Guai;Yao Li;Dr. Chee Mang Ng; Chang Ming Li; Mary B. Chan-Park
ChemPhysChem 2012 Volume 13( Issue 10) pp:2566-2572
Publication Date(Web):
DOI:10.1002/cphc.201200156
Abstract
Different types of single-walled carbon nanotubes (SWCNTs), pristine (p-), metallic (m-) and semiconducting (s-) are incorporated into TiO2 photoanodes to improve the dye-sensitized solar-cell (DSSC) performance and their effects on the device performance are further investigated. Although all three types of SWCNTs are found to have comparable structural morphologies and a reduced charge transport resistivity for the photoanodes, only the semiconducting one was able to suppress charge-recombination events, resulting in even greater improvement of DSSC performances. This is very likely to be ascribed to the higher energy barrier of s-SWCNTs compared to both m- and p-SWCNTs to block the back flowing of dye-injected electrons for I3− reduction in the charge recombination process.
Co-reporter:Wei Zhi Wang;Wei Feng Li;Xiao Yong Pan;Chang Ming Li;Lain-Jong Li;Yu Guang Mu;John A Rogers;Mary B. Chan-Park
Advanced Functional Materials 2011 Volume 21( Issue 9) pp:1643-1651
Publication Date(Web):
DOI:10.1002/adfm.201002278
Abstract
Polymers which enrich semiconducting single-walled carbon nanotubes (SWNTs) and are also removable after enrichment are highly desirable for achieving high-performance field-effect transistors (FETs). We have designed and synthesized a new class of alternating copolymers containing main-chain fluorene and hydrofluoric acid (HF) degradable disilane for sorting and preferentially suspending semiconducting nanotube species. The results of optical absorbance, photoluminescence emission, and resonant Raman scattering show that poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-1,1,2,2-tetramethyl-disilane] preferentially suspends semiconducting nanotubes with larger chiral angle (25°–28°) and larger diameter (1.03 nm–1.17 nm) (specifically (8,7), (9,7) and (9,8) species) present in HiPCO nanotube samples. Computer simulation shows that P1 preferentially interacts with (8,7) (semiconducting) over (7,7) (metallic) species, confirming that P1 selects larger diameter, larger chiral angle semiconducting tubes. P1 wrapped on the surface of SWNTs is easily washed off through degradation of the disilane bond of the alternating polymer main chain in HF, yielding “clean” purified SWNTs. We have applied the semiconducting species enriched SWNTs to prepare solution-processed FET devices with random nanotube network active channels. The devices exhibit stable p-type semiconductor behavior in air with very promising characteristics. The on/off current ratio reaches up to 15 000, with on-current level of around 10 μA and estimated hole mobility of 5.2 cm2 V−1 s−1.
Co-reporter:Wei Yuan, Jianfei Che, and Mary B. Chan-Park
Chemistry of Materials 2011 Volume 23(Issue 18) pp:4149
Publication Date(Web):August 29, 2011
DOI:10.1021/cm200909x
Multiwalled carbon nanotubes (MWNTs) have high intrinsic conductivities and are thought to be ideal fillers for electrically conductive composites. However, so far, simple solution casting or blending of nanotubes has not been highly successful in producing highly conductive composites, because of the poor dispersion and low loading of the nanotubes. We show that, by using a novel poly(amic acid) (PAA) containing a rigid backbone with hydroxyl pendant groups, as both the nanotube dispersant and the matrix precursor, we can increase the nanotube content in the solution-cast polyimide (PI)-based composite to as high as 30 wt % and achieve ultrahigh composite electrical conductivity as well as high mechanical properties. The electrical conductivity of the MWNT/PI composites reaches a value of 38.8 S cm–1 at a nanotube loading of 30 wt % and the MWNT concentration for achieving the percolation threshold of conductivity of the composites is 0.48 wt %. These are, respectively, the highest and among the lowest reported values for any conventional solution-processed nanotube composites. The 30 wt % MWNTs composite has a higher Young’s modulus (9.43 ± 0.14 GPa) and tensile strength (179.2 ± 9.7 MPa) than other nanotube-reinforced polyimide composites. The high conductivity, as well as tensile properties, of the composite films is attributed to the good nanotube dispersion and strong nanotube–polymer interfacial adhesion achieved through use of a single polymer to perform the dual functions of nanotube dispersant and matrix precursor. The excellent properties, combined with the facile conventional solution-casting technique, make this MWNT/PI composite film a promising material for many potential applications. We have also demonstrated that uniform MWNT(30 wt %)/PI composite coatings can be deposited onto glass and aluminum substrates.Keywords: electrical conductivity; mechanical properties; multiwalled carbon nanotubes; noncovalent functionalization; polyimide;
Co-reporter:Wei Yuan, Weifeng Li, Yuguang Mu, and Mary B. Chan-Park
ACS Applied Materials & Interfaces 2011 Volume 3(Issue 5) pp:1702
Publication Date(Web):April 28, 2011
DOI:10.1021/am2002229
Three kinds of polymer, polyimide without side-chain (PI), polyimide-graft-glyceryl 4-nonylphenyl ether (PI-GNE), and polyimide-graft-bisphenol A diglyceryl acrylate (PI-BDA), have been synthesized and used to disperse single-walled carbon nanotubes (SWNTs) and to improve the interfacial bonding between SWNTs and cyanate ester (CE) matrix. Visual observation, UV–vis–near–IR (UV–vis–NIR) spectra, and atomic force microscopy (AFM) images show that both PI-GNE and PI-BDA are highly effective at dispersing and debundling SWNTs in DMF, whereas PI is less effective. Interaction between SWNTs and PI, PI-GNE or PI-BDA was confirmed by computer simulation and Raman spectra. A series of CE-based composite films reinforced with different loadings of SWNTs, SWNTs/PI, SWNTs/PI-GNE and SWNTs/PI-BDA were prepared by solution casting. It was found that, because of the unique side-chain structure of PI-BDA, SWNTs/PI-BDA disperse better in CE matrix than do SWNTs/PI-GNE, SWNTs/PI, and SWNTs. As a result, SWNTs/PI-BDA/CE composites have the greatest improvement in mechanical properties of the materials tested. These results imply that the choice of side-chain on a dispersant is very important to the dispersion of SWNTs in matrix and the filler/matrix interfacial adhesion, which are two key requirements for achieving effective reinforcement.Keywords: cyanate ester; mechanical properties; noncovalent; polyimide; side-chain; single-walled carbon nanotubes
Co-reporter:Shahrzad Rayatpisheh;Yin Fun Poon;Ye Cao;Jie Feng;Vincent Chan ;Mary B. Chan-Park
Journal of Biomedical Materials Research Part A 2011 Volume 98A( Issue 2) pp:235-244
Publication Date(Web):
DOI:10.1002/jbm.a.33085
Abstract
Tissue engineering of the small diameter blood vessel medial layer has been challenging. Recreation of the circumferentially aligned multilayer smooth muscle tissue has been one of the major technical difficulties. Some research has utilized cyclic stress to align smooth muscle cells (SMCs) but due to the long time conditioning needed, it was not possible to use primary human cells because of expeditious senescence occured. We demonstrate rapid buildup of a homogeneous relatively thick (30–40 μm) aligned smooth muscle tissue via layer by layer (LBL) technique within microchannels and a soft cell-adhesive hydrogel. Using a microchannelled scaffold with gapped microwalls, two layers of primary human SMCs separated by an interlayer hydrogel were cultured to confluence within the microchannels. The SMCs aligned along the microchannels because of the physically constraining microwalls. A novel double layered gel consisting of a mixture of pristine and oxidized alginate hydrogel coated with collagen was designed to place between each layer of cells, leading to a thicker tissue in a shorter time. The SMCs penetrated the soft thin interlayer hydrogel within 6 days of seeding of the 2nd cell layer so that the entire construct became more or less homogeneously populated by the SMCs. The unique LBL technique applied within the micropatterned scaffold using a soft cell-adhesive gel interlayer allows rapid growth and confluence of SMCs on 2D surface but at the same time aligns the cells and builds up multiple layers into a 3D tissue. This pseudo-3D buildup method avoids the typical steric resistance of hydrogel embedding. © 2011 Wiley Periodicals, Inc. J Biomed Mater Res Part A:, 2011.
Co-reporter:Chuncai Zhou, Peng Li, Xiaobao Qi, Abdul Rahim Mohamed Sharif, Yin Fun Poon, Ye Cao, Matthew W. Chang, Susanna Su Jan Leong, Mary B. Chan-Park
Biomaterials 2011 32(11) pp: 2704-2712
Publication Date(Web):
DOI:10.1016/j.biomaterials.2010.12.040
Co-reporter:Junluo Feng ; Sk. Mahasin Alam ; Liang Yu Yan ; Chang Ming Li ; Zaher Judeh ; Yuan Chen ; Lain-Jong Li ; Kok Hwa Lim ;Mary B. Chan-Park
The Journal of Physical Chemistry C 2011 Volume 115(Issue 13) pp:5199-5206
Publication Date(Web):March 4, 2011
DOI:10.1021/jp107865m
Noncovalent polymer sorting of single-walled carbon nanotubes (SWNTs) was carried out using polyvinylpyrrolidone (PVP) in dimethylformamide (DMF) solvent. With long-term (i.e., 14-day) standing of SWNTs/PVP/DMF in ambient condition, the semiconducting SWNTs remain suspended in solvent while metallic SWNTs precipitated out. The preferential semiconducting nanotube suspension was confirmed by optical absorption spectroscopy, Raman spectroscopy, and field effect transistor measurements. Field-effect transistors made from the enriched semiconducting SWNTs show enhanced switching performance with typical on/off ratios of at least 1000. We propose that noncovalent charge transfer occurs between PVP and SWNTs, and metallic nanotubes with mobile electrons at/near the Fermi level are more susceptible to environmental temperature fluctuations than semiconducting nanotubes, so that with the small temperature increase experienced during 14 days of standing under normal laboratory condition, polymer unwrapping from metallic nanotubes occurs, leading to their selective precipitation.
Co-reporter:Xiaoyong Pan ;Mary B. Chan-Park
Journal of Polymer Science Part B: Polymer Physics 2011 Volume 49( Issue 13) pp:949-960
Publication Date(Web):
DOI:10.1002/polb.22265
Abstract
We previously showed that in N,N-dimethylformamide (DMF), poly(9-anthracenylmethyl methacrylate) (PAMMA) and poly(2-naphthylmethacrylate) selectively disperse semiconducting and metallic single-walled carbon nanotubes (SWNTs), respectively. We have also proposed a new noncovalent polymer interaction based on photon induced dipole–dipole interaction to account for the metallicity-based selectivity. In this article, we investigate two other polymethacrylates, that is, poly(benzyl methacrylate) (PBMA) and poly(methylmethacrylate)-co-(9-anthracenylmethyl acrylate) (PMMA-c-PAMA) in the light of our previously proposed photon-induced dipole–dipole interaction. We find that PBMA and PMM-c-PAMMA in DMF show no metallicity selectivity. The different selective behavior of the four polymers in DMF manifests the decisive influence of the side aromatic group in determining their metallicity selectivity. The nonpreferential energy transfer from PMMA-c-PAMA to SWNTs and the nonoverlap of PBMA fluorescence (in the ultraviolet range) with nanotube absorption account for their nonselectivity of specific nanotube species. Further, the parallel relationship between the diameters of extracted tube species and the affinity between polymers and solvents suggests the leading role of the polymeric conformation on the diameter selectivity. A sufficient (i.e., 2 weeks) standing time of the SWNTs solution after sonication, during which the polymers presumably optimize their conformation to the SWNTs, was found to be essential to the enrichment. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011
Co-reporter:Wei Yuan, Junluo Feng, Zaher Judeh, Jie Dai, and Mary B. Chan-Park
Chemistry of Materials 2010 Volume 22(Issue 24) pp:6542
Publication Date(Web):December 2, 2010
DOI:10.1021/cm101785t
Dispersion of single-walled carbon nanotubes (SWNTs) into individuals or small bundles and strong nanotube/matrix interfacial strength remain as challenges in exploiting the excellent mechanical properties of SWNTs in structural composites. Noncovalent functionalization of SWNTs is an attractive option for compatibilization of nanotubes with the matrix as it does not destroy the nanotube graphene structure. We have designed and successfully synthesized a new reactive comb-like polymer, polyimide-graft-bisphenol A diglyceryl acrylate (PI-BDA), which is shown to be highly effective in dispersing SWNTs into individual nanotubes or small bundles, as evidenced by transmission electron microscopy (TEM) and UV−vis−NIR absorption spectra. SWNTs dispersed with PI-BDA were added to a thermosetting resin blend of cyanate ester and epoxy (CE-EP) (70/30, w/w) and composite fibers with SWNT loading ranging from 0 to 1.5 wt % were successfully fabricated. The strong π−π interaction between SWNTs and PI-BDA backbone was evidenced by Raman spectra, and the covalent reaction between PI-BDA side-chain and matrix was confirmed by FT-IR and 1H NMR. Our designed dispersant interacts noncovalently with nanotubes but reacts with the thermosetting matrix. With addition of PI-BDA dispersant, only 1 wt % SWNTs optimally increases the tensile modulus, strength, and toughness by 80% (to 4.70 ± 0.24 GPa), 70% (to 142.3 ± 6.9 MPa), and 58% (4.1 ± 0.4 MJ m−3) compared to the neat resin blend (with E = 2.61 ± 0.14 GPa, σ = 83.7 ± 3.3 MPa, and T = 2.6 ± 0.2 MJ m−3). Improvement in thermal properties was also observed.
Co-reporter:Yin Fun Poon, Ye Cao, Yunxiao Liu, Vincent Chan and Mary B. Chan-Park
ACS Applied Materials & Interfaces 2010 Volume 2(Issue 7) pp:2012
Publication Date(Web):June 22, 2010
DOI:10.1021/am1002876
Ultraviolet (UV) photo-cross-linkable hydrogels have been commonly used for three-dimensional (3D) encapsulation of cells. Previous UV cross-linkable hydrogels have employed one-shot hardening of mixtures of hydrogels and cells. Here we propose an alternative method of making hydrogel-encapsulated cell constructs through layer by layer (LBL) buildup of alternating layers of cells and hydrogel. The LBL method potentially permits better spatial control of different cell types and control of cell orientation. Each hydrogel layer must be hardened before deposition of the next layer of cells. A UV-curable gel precursor that can also be gelled at physiological temperature is desirable to avoid repeated UV exposure of cells after deposition of each successive hydrogel layer. We designed, synthesized, and applied such a precursor, dual-curable−both thermoresponsive and UV-curable−chitosan-graft-polyethylene glycol-graft-methacrylate (CEGx-MA) copolymer (x is the PEG molecular weight in Daltons). We found that CEG350-MA copolymer solutions (5 wt % polymer) formed physical gels at ∼37 °C and could be further photopolymerized to form thermally stable dual-cured hydrogels. This material was applied to the creation of a two-layer LBL smooth muscle cell (SMC)/hydrogel construct using temperature elevation to ∼37 °C to gel each hydrogel layer. The physically gelled two-layered hydrogel/cell construct was finally exposed to a single UV shot to improve its mechanical properties and render it thermally stable. CEG350-MA solution and gel are nontoxic to SMCs. Cells remained mostly viable when they were encapsulated inside both physically gelled and dual-cured CEG350-MA and suffered little damage from the single brief UV exposure. The combination of LBL tissue engineering with a dual curable hydrogel precursor such as CEG350-MA permits the buildup of viable thick and complex tissues in a stable, biocompatible, and biodegradable matrix.Keywords: chitosan; hydrogels; photopolymerization; polyethylene glycol; smooth muscle cells
Co-reporter:Xiaowei Pei, Ye Hai Yan, Liangyu Yan, Pan Yang, Junling Wang, Rong Xu, Mary B. Chan-Park
Carbon 2010 Volume 48(Issue 9) pp:2501-2505
Publication Date(Web):August 2010
DOI:10.1016/j.carbon.2010.03.023
A magnetically responsive nanomaterial was prepared by covalent attachment of a magnetic ionic liquid (MIL), specifically 1-butyl-3-methylimidazolium [FeCl4], to single-walled carbon nanotubes (SWCNTs). Our magnetic -SWCNTs, based on molecular FeCl4- magnetic ion bound to cationic imidazolium which was covalently conjugated to the nanotubes, were found to have a strong magnetic response and be easily dispersed in an organic solvent, N-methyl pyrrolidone. MIL-based magnetic SWCNTs is superior to magnetic particle-based nanotubes since particles attached typically are much bigger than the nanoscale diameter of SWCNTs and are prone to aggregation, detachment and non-homogeneity.
Co-reporter:Ye Cao, Yin Fun Poon, Jie Feng, Shahrzad Rayatpisheh, Vincent Chan, Mary B. Chan-Park
Biomaterials 2010 31(24) pp: 6228-6238
Publication Date(Web):
DOI:10.1016/j.biomaterials.2010.04.059
Co-reporter:Chuncai Zhou, Xiaobao Qi, Peng Li, Wei Ning Chen, Lamrani Mouad, Matthew W. Chang, Susanna Su Jan Leong and Mary B. Chan-Park
Biomacromolecules 2010 Volume 11(Issue 1) pp:
Publication Date(Web):December 3, 2009
DOI:10.1021/bm900896h
Antimicrobial peptides (AMPs), particularly those effective against methicillin-resistant Staphylococcus aureus (S. aureus) and antibiotic-resistant Pseudomonas aeruginosa (P. aeruginosa), are important alternatives to antibiotics. Typical peptide synthesis methods involving solid-phase sequential synthesis are slow and costly, which are obstacles to their more widespread application. In this paper, we synthesize peptides via ring-opening polymerization of α-amino acid N-carboxyanhydrides (NCA) using a transition metal initiator. This method offers high potential for inexpensive synthesis of substantial quantities of AMPs. Lysine (K) was chosen as the hydrophilic amino acid and alanine (A), phenylalanine (F), and leucine (L) as the hydrophobic amino acids. We synthesized five series of AMPs (i.e., P(KA), P(KL), P(KF), P(KAL), and P(KFL)), varied the hydrophobic amino acid content from 0 to 100%, and determined minimal inhibitory concentrations (MICs) against clinically important Gram-negative and Gram-positive bacteria and fungi (i.e., Escherichia coli (E. coli), P. aeruginosa, Serratia marcescens (S. marcescens), and Candida albicans (C. albicans). We found that P(K10F7.5L7.5) and P(K10F15) show the broadest activity against all five pathogens and have the lowest MICs against these pathogens. For P(K10F7.5L7.5), the MICs against E. coli, P. aeruginosa, S. marcescens, S. aureus, and C. albicans are 31 μg/mL, 31 μg/mL, 250 μg/mL, 31 μg/mL, and 62.5 μg/mL, while for P(K10F15) the respective MICs are 31 μg/mL, 31 μg/mL, 250 μg/mL, 31 μg/mL, and 125 μg/mL. These are lower than the MICs of many naturally occurring AMPs. The membrane depolarization and SEM assays confirm that the mechanism of microbe killing by P(K10F7.5L7.5) copeptide includes membrane disruption, which is likely to inhibit rapid induction of AMP-resistance in pathogens.
Co-reporter:Zhi Dai ; Liangyu Yan ; Sk. Mahasin Alam ; Junluo Feng ; Pyria Rose Divina Mariathomas ; Yuan Chen ; Chang Ming Li ; Qing Zhang ; Lain-Jong Li ; Kok Hwa Lim ;Mary B. Chan-Park
The Journal of Physical Chemistry C 2010 Volume 114(Issue 49) pp:21035-21041
Publication Date(Web):November 19, 2010
DOI:10.1021/jp106398k
Small-diameter metallic single-wall carbon nanotubes (SWNT) were separated from larger diameter semiconducting SWNT when CoMoCAT samples (supplied with abundance of (6,5) species) were left standing at ambient laboratory condition for a few days in dimethylformamide solution containing ethylanthraquinone (EAQ). SWNT enriched in larger diameter semiconducting species by this method were used to fabricate SWNT network-based thin film transistors (TFTs). The resulting devices had mobilities of 0.2 cm2/(V s) and on/off ratios of about 104. The on/off ratios were greatly improved, by approximately 2 orders of magnitude, over those of TFTs made with pristine SWNT. The enrichment in semiconducting tubes was corroborated by UV−vis−NIR absorption, photoluminescence excitation (PLE), and resonance Raman scattering spectroscopy. Density functional simulations show that hydroxyl radicals preferentially attack small-diameter metallic and then small-diameter semiconducting nanotubes over larger diameter semiconducting tubes. We propose that EAQ forms radicals in ambient lighted conditions, resulting in this diameter- and metallicity-selective reaction which increases the density of the target species, promoting their sedimentation under centrifugation. This simple method of obtaining semiconducting-enriched SWNT samples should be widely applicable for printed electronic devices.
Co-reporter:Yunxiao Liu, Mary B. Chan-Park
Biomaterials 2010 31(6) pp: 1158-1170
Publication Date(Web):
DOI:10.1016/j.biomaterials.2009.10.040
Co-reporter:Wen Xiu Zhou;Ye Hai Yan;Xiaoyong Pan;Rong Xu;Mary B. Chan-Park
Journal of Polymer Science Part B: Polymer Physics 2010 Volume 48( Issue 4) pp:442-450
Publication Date(Web):
DOI:10.1002/polb.21905
Abstract
Pristine-, poly(octafluorotoluene)- (POFT), and polyacetylene (PAc)-coated Si wafers were used as substrates for the study of segregation of silicone diacrylate (SA) from a formulation containing other oligomeric and monomeric acrylates. POFT and PAc were microwave plasma polymerized on the Si wafers. Three SAs with molecular weights ranging from 700 to 6000 Da were synthesized and characterized. Formulations with 2 wt % SA were ultraviolet cured on the silicon wafers. The surface composition of formulation-substrate side of the cured film was analyzed with X-ray photoelectron spectroscopy, and the depth profile was analyzed with time of flight-secondary ion mass spectrometry. The analysis results indicated that SA aggregated on all three types of Si surfaces. However, SA segregation is highest on the low surface energy POFT-coated Si substrate, and low-molecular-weight SA is favorable to the segregation. For high-molecular-weight SA, the different Si substrates do not affect the degree of aggregation at the formulation-substrate interface. The observed SA aggregation trend can be predicted by the Gibbs-adsorption equation correlated to the resin surface tension and contact angle on substrates. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 442–450, 2010
Co-reporter:Jianfei Che, Wei Yuan, Guohua Jiang, Jie Dai, Su Yin Lim and Mary B. Chan-Park
Chemistry of Materials 2009 Volume 21(Issue 8) pp:1471
Publication Date(Web):March 20, 2009
DOI:10.1021/cm8023549
Single-walled carbon nanotubes functionalized with generation (n) 0−2 dendritic poly(amidoamine) (denoted as SWNTs-Gn-NH2, n = 0, 1, or 2) were used as filler in thermosetting epoxy to prepare anisotropic microscale diameter fibers by reactive spinning. Dendritic poly(amidoamine) (PAMAM) was “grafted from” acid-modified and toluene 2,4-diisocyanate-activated SWNTs by repeating amidation of terminal ester groups via ethylenediamine (EDA) and Michael addition of methyl acrylate (MA) to amino groups. Fourier transform infrared (FTIR) and hydrogen nuclear magnetic resonance (1H NMR) spectroscopy and thermogravimetric analysis (TGA) confirmed the successful grafting and generation buildup. Optical and scanning electron microscope (SEM) observations revealed that PAMAM-functionalized SWNTs dispersed in epoxy much more uniformly than pristine nanotubes. The dispersibility improved with increasing generation number. Microsized fibers made of epoxy reinforced with aligned SWNTs-G2-NH2 by reactive spinning show high tensile strength and Young’s modulus per unit weight fraction (dσ/dWNT and dE/dWNT). The respective measures are 7022 MPa and 118.0 GPa, which is a high reinforcement efficacy in comparison to other fillers. The nanotube alignment and grafting of dendritic PAMAM play a crucial role in the enhancement of the tensile strength of these reinforced composite fibers.
Co-reporter:Qin Jia Cai, Ye Gan, Mary B Chan-Park, Hong Bin Yang, Zhi Song Lu, Chang Ming Li, Jun Guo and Zhi Li Dong
Chemistry of Materials 2009 Volume 21(Issue 14) pp:3153
Publication Date(Web):June 23, 2009
DOI:10.1021/cm900532q
A series of solution-processable oleic-acid capped barium titanate and strontium titanate nanoparticles was synthesized and spin-coated to form homogeneous high-k dielectric films for organic thin-film transistors (TFTs). The dielectric constant k of the nanoparticle films was tunable in the range from 4.1 to 9.3 by altering the molar ratio of oleic-acid surfactant to synthesis precursor. Low-voltage modulated high-performance organic TFTs were fabricated using nanoparticle films as the dielectric components. Flexible bottom-gate pentacene TFTs exhibited outstanding device performance with field-effect mobility, μ, in the range of 2.0−3.5 cm2 V−1 s−1 and on/off ratios of about 1 × 104 at low gate voltage. Top-gate poly(3,3′′′-didodecylquaterthiophene) TFTs also showed high device performance with μ of 0.05−0.1 cm2 V−1 s−1 and on/off ratios of 1 × 103 to 1 × 104. The low-voltage performance of the TFTs could be attributed to a low density of trapped states at the interfaces between the organic semiconductors and the nanoparticle dielectric films. This research provides a series of promising dielectric materials for fabrication of superior organic TFTs through a solution process and fundamentally suggests that low trapped state density at the semiconductor/dielectrics interface may be an important factor to achieve low-voltage modulation in organic TFTs.
Co-reporter:Ye Gan, Qin Jia Cai, Chang Ming Li, Hong Bin Yang, Zhi Song Lu, Cheng Gong and Mary B. Chan-Park
ACS Applied Materials & Interfaces 2009 Volume 1(Issue 10) pp:2230
Publication Date(Web):September 16, 2009
DOI:10.1021/am9003914
Oleic acid capped barium strontium titanate (OA-BST) nanoparticles were synthesized for solution-prepared dielectrics in organic thin-film transistors (OTFTs). The as-synthesized nanoparticles were well-dispersed in organic solvents to deposit very homogeneous dielectric films by direct spin coating. Bottom-gate pentacene TFTs fabricated using these nanoparticle dielectric films showed high mobilities of 1−2 cm2 V−1 s−1 with on/off ratios of 103 under a low driven voltage of −2.5 V. Top-gate poly(3,3′′′-didodecylquaterthiophene) (PQT-12) TFTs with nanoparticle dielectrics also exhibited a low-voltage operation (−5 V) performance with mobilities of 0.01−0.1 cm2 V−1 s−1 and on/off ratios of 103−104. Detailed studies on the gate voltage-dependent mobility of the devices showed that only a low gate electric field needed to achieve the saturated mobility for the OA-BST-based pentacene OTFTs could be attributed to the low trapped-state densities (<3.9 × 1011 cm−2) at the dielectric/semiconductor interfaces for these devices.Keywords: dielectrics; organic thin-film transistors; pentacene; solution-processed; trapped-state density
Co-reporter:Mary B. Chan-Park;Jin Ye Shen;Ye Cao;Yun Xiong;Yunxiao Liu;Shahrzad Rayatpisheh;Gavin Chun-Wei Kang;Howard P. Greisler
Journal of Biomedical Materials Research Part A 2009 Volume 88A( Issue 4) pp:1104-1121
Publication Date(Web):
DOI:10.1002/jbm.a.32318
Abstract
Small-diameter blood vessel substitutes are urgently needed for patients requiring replacements of their coronary and below-the-knee vessels and for better arteriovenous dialysis shunts. Circulatory diseases, especially those arising from atherosclerosis, are the predominant cause of mortality and morbidity in the developed world. Current therapies include the use of autologous vessels or synthetic materials as vessel replacements. The limited availability of healthy vessels for use as bypass grafts and the failure of purely synthetic materials in small-diameter sites necessitate the development of a biological substitute. Tissue engineering is such an approach and has achieved promising results, but reconstruction of a functional vascular tunica media, with circumferentially oriented contractile smooth muscle cells (SMCs) and extracellular matrix, appropriate mechanical properties, and vasoactivity has yet to be demonstrated. This review focuses on strategies to effect the switch of SMC phenotype from synthetic to contractile, which is regarded as crucial for the engineering of a functional vascular media. The synthetic SMC phenotype is desired initially for cell proliferation and tissue remodeling, but the contractile phenotype is then necessary for sufficient vasoactivity and inhibition of neointima formation. The factors governing the switch to a more contractile phenotype with in vitro culture are reviewed. © 2008 Wiley Periodicals, Inc. J Biomed Mater Res, 2009
Co-reporter:Yin Fun Poon, Ye Cao, Yabin Zhu, Zaher M. A. Judeh and Mary B. Chan-Park
Biomacromolecules 2009 Volume 10(Issue 8) pp:
Publication Date(Web):July 15, 2009
DOI:10.1021/bm801367n
Poly(malic acid) is water-soluble, functionalizable, and biodegradable, making it attractive as a precursor of hydrogels for biomedical applications. However, homopoly(malic acid), with pK1/2 of 4.3, is too acidic for biocompatibility. To overcome the acidity, we have synthesized β-malic acid-containing poly(ethylene glycol) dimethacrylate (PEGMAc) with pKa of 5.02. Solutions of methacrylated O-carboxymethylchitosan (OCMCS), PEGMAc, and poly(ethylene glycol) diacrylate (PEGDA; 7:7:86 and 6:20:74 (w/w/w)) in water (80%) have near neutral pHs (6.8−6.9). These solutions form firm hydrogels when photopolymerized. These are referred to as O7-PEGMAc7-B86 and O6-PEGMAc20-B74 (where the numerals refer to the weight content of each component, O is OCMCS and B is PEGDA added to make blend). The carboxyl groups in PEGMAc permit the surface grafting of hydrogels with Arg-Gly-Asp (RGD). The cytocompatibilities of smooth muscle cells (SMCs) on RGD-grafted hydrogels were studied. From the tetrazolium salt reduction assay, O6-PEGMAc20-B74 was found to have significantly better 10th day cytocompatibility compared to hydrogels containing lower or no PEGMAc. These gels degrade upon hydrolysis releasing malic acid, PEG and OCMCS. The increased cell compatibility of O6-PEGMAc20-B74 is possibly due to increased surface RGD content and near neutral pH even during biodegradation. Our novel PEGMAc-modified blends are a promising functionalizable biodegradable hydrogel precursor providing improved cell proliferation.
Co-reporter:Jianfei Che;Mary B. Chan-Park
Advanced Functional Materials 2008 Volume 18( Issue 6) pp:888-897
Publication Date(Web):
DOI:10.1002/adfm.200700919
Abstract
We report a new approach of reactive spinning to fabricate thermosetting cyanate ester micro-scale diameter fibers with aligned single walled carbon nanotubes (SWNTs). The composite fibers were produced by first dispersing the SWNTs (1 wt %) in cyanate ester (CE) via solvent blending, followed by pre-polymerization, spinning and then multiple-stage curing. The pre-polymerization, spinning and post-spinning cure temperatures were carefully controlled to achieve good spun crosslinked fibers. Both pristine and amino-functionalized SWNTs were used for the reinforced fiber spinning. Amino-functionalized SWNTs (f-SWNTs) were prepared by reacting acid-treated SWNTs with toluene 2,4-diisocyanate and then ethylenediamine (EDA). FTIR, optical microscopy and scanning electron microscopy (SEM) showed that the amino-functionalized SWNTs were covalently and uniformly dispersed into the cyanate ester matrix and aligned along the fiber axis. The alignment was further confirmed using polarized Raman spectroscopy. The composite fibers with aligned amino-functionalized SWNTs possess improved tensile properties with respect to neat CE fibers, showing 85, 140, and 420% increase in tensile strength, elongation and stress-strain curve area (i.e., toughness), respectively. NH2-functionalization of SWNTs improves their dispersibility, alignment and interfacial strength and hence tensile properties of composite spun fibers. Fiber spinning to align SWNTs using thermosetting resin is novel. Others have reported fiber spinning to align SWNTs in thermoplastics. However, thermosetting CE resins offer the advantages of low and controllable viscosity during spinning and reactivity with amino functional groups to enable f-SWNT/CE covalent bonding.
Co-reporter:Qin Jia Cai, Mary B. Chan-Park, Qin Zhou, Zhi Song Lu, Chang Ming Li, Beng S. Ong
Organic Electronics 2008 Volume 9(Issue 6) pp:936-943
Publication Date(Web):December 2008
DOI:10.1016/j.orgel.2008.06.014
Device performance of bottom-contact poly(3,3′′′-didodecylquaterthiophene) (PQT-12) thin-film transistors (TFTs) was significantly improved via surface-modification of Au source–drain (S–D) electrodes with 1-decanethiol and 1H,1H,2H,2H-perfluorodecanethiol self-assembled monolayers (SAMs). By improving the PQT-12 morphology and modulating the Schottky barrier at electrode/PQT-12 contacts, the thiol SAMs chemisorbed onto Au surfaces can improve the charge carrier injection at electrode/PQT-12 contacts and result in dramatic enhancements in device mobilities. Device mobilities up to 0.09 and 0.19 cm2 V−1 s−1 were obtained in high performance bottom-contact PQT-12 TFTs with 1-decanethiol and 1H,1H,2H,2H-perfluorodecanethiol SAMs surface-modified Au S–D electrodes, compared with 0.015 cm2 V−1 s−1 in PQT-12 TFTs with bare Au electrodes. This work may provide a simple path to the fabrication of high performance, low-cost, and solution-processable bottom-contact OTFTs using fine lithography technology.
Co-reporter:Qin Jia Cai, Mary B. Chan-Park, Jun Zhang, Ye Gan, Chang Ming Li, Tu Pei Chen, Beng S. Ong
Organic Electronics 2008 Volume 9(Issue 1) pp:14-20
Publication Date(Web):February 2008
DOI:10.1016/j.orgel.2007.07.002
A dramatic, ∼20-fold, reduction in the contact resistance of the bottom-contact poly(3,3‴-didodecylquaterthiophene) (PQT-12) thin-film transistors was achieved through a simple treatment of gold (Au) source and drain electrodes. The Au electrode treatment involved simply immersing the Au electrodes into Piranha solution prior to the deposition of the organic semiconductor. This treatment led to significant improvement of device performance. Channel length scaling analysis indicates that the contact resistance is reduced by about one order of magnitude, resulting in enhancement of estimated field-effect mobility by about a factor of five. Transport characteristic analysis suggests that the improved efficiency of charge carrier injection is probably due to increased dopant density of PQT-12 at the electrode/PQT-12 interface.
Co-reporter:Jun Zhang, Mary B. Chan-Park, Chang Ming Li
Sensors and Actuators B: Chemical 2008 Volume 131(Issue 2) pp:609-620
Publication Date(Web):14 May 2008
DOI:10.1016/j.snb.2007.12.048
SU-8 is an octafunctional epoxy-based negative resist supplied with a reactive diluent, gamma-butyrolactone (GBL). This paper characterizes the network properties and acid degradability of cured SU-8 resists with varying GBL monomer content and ultraviolet (UV) irradiation time. The SU-8/GBL network structure was characterized by Fourier transform infrared (FTIR) spectroscopy, 13C nuclear magnetic resonance (NMR) spectroscopy and gas chromatography–mass spectrometry (GC/MS). GBL was found to copolymerize with epoxy to bridge two neighbouring epoxy groups and does not homopolymerize. The maximum GBL:SU-8 molar ratio whereby all GBL fully reacted with available epoxy functionalities in the network was found to be 8:1. Excess GBL beyond the maximum GBL:SU-8 ratio remains in the network as a plasticizer. GBL content and UV irradiation time affect glass transition temperature (Tg), epoxy conversion and molecular weight between cross-links (Mc) which were measured by dynamic mechanical analysis (DMA) and FTIR. The mechanism of cross-linked network acid degradation was found to be surface erosion. Lower epoxy conversion, higher Mc and higher GBL content resulted in a higher dissolution rate, which can be exploited in applications requiring SU-8 removal. A patterned SU-8 grating with relatively high-GBL content (10%) was successfully used as a template for Cu electroforming.
Co-reporter:Mary B. Chan-Park, Chun Yang, Xun Guo, LQ Chen, Soon Fatt Yoon and Jung-Hoon Chun
Langmuir 2008 Volume 24(Issue 10) pp:5492-5499
Publication Date(Web):April 29, 2008
DOI:10.1021/la703608p
This paper describes a novel method of fabricating three-dimensional (3-D) curved microstructures with continuous relief through controlled argon gas expansion into a photocurable resin. A microstructured stamp is placed on top of a nonwetting photopolymerizable liquid resin. The setup is heated, and the argon gas in the blind holes of the stamp expands. The expanded gas displaces the resin at the mouth of the microcavities to form 3-D curved indentations in the liquid resin which is subsequently rapidly solidified by photopolymerization. By changing the duration of the preheating, different curvatures can be produced. Arrays of homogeneous 3-D curved microstructures having different cross-sectional geometries and heights were fabricated using various shapes of the blind holes and preheating times, respectively. As a demonstration of applications, high-quality and uniform polydimethylsiloxane microlens arrays were produced. In addition, thorough investigation was carried out to study the factors influencing the fabricated 3-D curved microstructures. Curved microstructures with diameters as small as 2 µm were demonstrated. A simple model was developed, and such a model allows for predicting the curvatures of indentations with different preheating times. It has been found that the predicted curvatures are in good agreement with experimental data.
Co-reporter:Liang Yu Yan ; Yin Fun Poon ; M. B. Chan-Park ; Yuan Chen ;Qing Zhang
The Journal of Physical Chemistry C 2008 Volume 112(Issue 20) pp:7579-7587
Publication Date(Web):April 19, 2008
DOI:10.1021/jp711039s
Chitosan and its various neutral pH water-soluble derivatives were investigated for dispersing single-walled carbon nanotubes (SWNTs). Chitosan (CS) can produce good dispersion of SWNTs, but only in acidic pH condition. Our two novel derivatives, O-carboxymethylchitosan (OC) and OC modified by poly(ethylene glycol) at the −COOH position (OPEG), were able to produce highly effective debundling and dispersion of SWNTs in neutral pH aqueous solution. Atomic force microscopy (AFM), transmission electron microscopy (TEM), photoluminescence, UV−vis−NIR spetroscopy, and Raman spectroscopy confirmed that SWNTs are present as individual nanotubes in the dispersions. The solubilities of individually dispersed SWNTs in neutral water are 0.021 and 0.032 g/L for OC and OPEG, respectively, which are comparable to 0.038 g/L for SWNTs using CS in acetic acid. Further, OC and OPEG aqueous solutions (1 wt %) do not significantly lower the surface tensions (65−67 mN/m). From the Fourier transform infrared spectroscopic results, we conclude that the free electron pair in the pendant amine groups of OC and OPEG plays a vital role in finely dispersing the SWNTs; the −NH2 contributes to the adsorption of these two chitosan derivatives on the nanotubes. Quaternary ammonium chitosan (QC), with alkyl substitution at the protonated amine, was found to be unable to disperse SWNTs; possibly cation−π interaction with nanotubes is diminished due to steric hindrance.
Co-reporter:Wei Yuan, Guohua Jiang, Jianfei Che, Xiaobao Qi, Rong Xu, Matthew W. Chang, Yuan Chen, Su Yin Lim, Jie Dai and Mary B. Chan-Park
The Journal of Physical Chemistry C 2008 Volume 112(Issue 48) pp:18754-18759
Publication Date(Web):2017-2-22
DOI:10.1021/jp807133j
A nanohybrid comprising silver nanoparticles within third-generation dendritic poly(amidoamine) (PAMAM) grafted onto multiwalled carbon nanotubes (MWNTs) was applied as an antimicrobial agent in solution. The high abundance of amine groups on the dendrimer-modified MWNTs (d-MWNTs) provided sites for reduction and precipitation of silver nanoparticles from silver acetate aqueous solution, resulting in carbon nanotubes/Ag nanohybrids (d-MWNTs/Ag). The content of PAMAM grafted on d-MWNTs determined by using a thermal gravimetric analyzer (TGA) was about 45%. The silver nanoparticles produced were determined to be face-centered cubic silver nanocrystals by X-ray powder diffraction (XRD). The nanohybrids were investigated with scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, and X-ray energy dispersive spectroscopy (EDS). The antimicrobial properties of acid-treated MWNTs (MWNTs-COOH), d-MWNTs, and d-MWNTs/Ag were investigated against Staphylococcus aureus (S. aureus), Escherichia coli (E. coli), and Pseudomonas aeruginosa (P. aeruginosa). Against E. coli and P. aeruginosa which are Gram-negative, d-MWNTs and d-MWNTs/Ag which are equally effective were found to have a stronger antimicrobial effect than MWNTs-COOH. Against S. aureus (Gram-positive), d-MWNTs/Ag showed a stronger antimicrobial effect than d-MWNTs (92.3% kill versus 71.6% kill), while MWNTs-COOH only killed 15.4% of this bacteria. Possible mechanisms are proposed to explain the higher antimicrobial activity by d-MWNTs/Ag nanohybrids. These findings suggest that PAMAM/Ag grafted onto insoluble MWNTs may be used as effective antimicrobial materials.
Co-reporter:Qin Jia Cai, Mary B. Chan-Park, Zhi Song Lu, Chang Ming Li and Beng S. Ong
Langmuir 2008 Volume 24(Issue 20) pp:11889-11894
Publication Date(Web):September 6, 2008
DOI:10.1021/la8009942
A series of alkanethiol monolayers (CH3(CH2)n−1SH, n = 4, 6, 8, 10, 12, 14, 16) were used to modify gold source-drain electrode surfaces for bottom-contact poly(3,3′′′-didodecylquaterthiophene) (PQT-12) thin-film transistors (TFTs). The device mobilities of TFTs were significantly increased from ∼0.015 cm2 V−1 s−1 for bare electrode TFTs to a maximum of ∼0.1 cm2 V−1 s−1 for the n = 8 monolayer devices. The mobilities of devices with alkanethiol-modified Au electrodes varied parabolically with alkyl length with values of 0.06, 0.1, and 0.04 cm2 V−1 s−1 at n = 4, 8, and 16, respectively. Atomic force microscopy investigations reveal that alkanethiol electrode surface modifications promote polycrystalline PQT-12 morphologies at electrode/PQT-12 contacts, which probably increase the density of states of the PQT-12 semiconductor at the interfaces. The contact resistance of TFTs is strongly modulated by the surface modification and strongly varies with the alkanethiol chain length. The surface modifications of electrodes appear to significantly improve the charge injection, with consequent substantial improvement in device performance.
Co-reporter:Y. F. Poon;Y. B. Zhu;J. Y. Shen;M. B. Chan-Park;S. C. Ng
Advanced Functional Materials 2007 Volume 17(Issue 13) pp:
Publication Date(Web):20 JUL 2007
DOI:10.1002/adfm.200600420
Ionic hydrogels are attractive for the protection, delivery and controlled release of charged biomacromolecules such as proteins, growth factors or DNA. We have prepared and characterized a series of photocrosslinked anionic hydrogels based on water soluble methacrylated (MA) O-carboxymethylchitosan (OCMCS) and polyethylene glycol (PEG) diacrylate. OCMCS samples with varying degree of substitution of carboxymethyl group ranging from 0.69 to 1.86 were prepared by reacting native chitosan with different amounts of monochloroacetic acid. The OCMCS products demonstrated differences in solubility, zeta potential (–52.7 to –12.8 mV) and thermal decomposition temperature (260 to 283 °C). The OCMCS products were then reacted with glycidyl methacrylate to make ultra-violet (UV) crosslinkable OCMCS-MAs which were blended with PEG diacrylate, a photoinitiator and water and successfully photocrosslinked to create OCMCS-MA/PEG hydrogels. Water uptake of the hydrogels varied between 226 % to 358 % and the porous structures of the prepared OCMCS-MA/PEG hydrogels could be modulated by the degree of methacrylation. All the OCMCS-MA/PEG hydrogel substrates similarly supported attachment and proliferation of Smooth Muscle Cells (SMCs). The in vitro biodegradation of these hydrogels, in the presence of SMCs, could be controlled by the degree of methacrylation; weight loss after 9-week was (15±1) % and (19±2) % using OCMCS 4-MA (12.7 % MA) and OCMCS 1-MA (4.6 % MA), respectively. In addition, the hydrogel based on the most anionic OCMCS 1 showed higher adsorption of basic TGF-β1 than similarly modified -agarose, -dextran, and -chondroitin sulfate hydrogels.
Co-reporter:Yabin Zhu, Mary B. Chan-Park
Analytical Biochemistry 2007 Volume 363(Issue 1) pp:119-127
Publication Date(Web):1 April 2007
DOI:10.1016/j.ab.2007.01.007
An improved technique for quantification of collagen immobilized on polymeric substrates is needed as tissue engineering evolves. Current immobilized protein quantification methods are indirect, time-consuming, and/or inaccurate. In this study, Sirius red colorimetric microassay was shown to be feasible for quantifying the density of collagen immobilized on aminolyzed poly(l-lactic acid) (PLLA) surfaces using the specific bonding of Sirius dye to collagen. It offers a number of advantages over traditional methods, including direct staining, high sensitivity, and high stability of the dye. The detection limit is approximately 0.1 μg/cm2, and the dynamic range is greater than 50. Sirius red dye has not been used previously for quantification of protein immobilized on polymers. The collagen densities achieved with each of the two crosslinking reagents investigated, namely glutaraldehyde (GA) and genipin, were compared. The latter is an alternative crosslinker derived from a traditional Chinese medicine. The collagen densities immobilized by the two reagents were measured to be similar. This was confirmed by the similar behaviors of esophageal primary smooth muscle cells (ESMCs) on these two modified PLLA membranes; collagen grafted with either coupler was found to greatly promote, to a similar extent, cell attachment and both short-term (4 days) and long-term (12 days) proliferation compared with unmodified PLLA. Smooth muscle cells on both modified membranes were stained to display contractile α-actin protein filaments.
Co-reporter:W.X. Zhou, Mary B. Chan-Park
Applied Surface Science 2006 Volume 253(Issue 4) pp:1921-1928
Publication Date(Web):15 December 2006
DOI:10.1016/j.apsusc.2006.03.038
Abstract
When silicone diacrylate was added in small amount (<5 wt.%) to ultraviolet (UV) curable formulations containing other oligomeric diacrylates, there was segregation of the silicone additive at the solid substrate–formulation interface. The amount was quantified by X-ray photoelectron spectroscopy measurement of the UV cured film surface composition. The effect of silicone diacrylate concentration, resin formulation and substrate polarity on silicone surface excess was systematically studied. Young's–Gibbs adsorption theory was applied to the prediction of the silicone surface excess at the solid substrate interface for these oligomeric mixtures. Further, we proposed a simplified Young's–Gibbs adsorption theory equation to predict the variation of surface excess from only formulation surface tension and substrate critical surface tension. The selective segregation is beneficial to demolding in UV embossing since only small amount of release added can result in large decrease of the mold–resin interfacial energy difference leading to easy demolding and high replication fidelity.
Co-reporter:Yabin Zhu, Kerm Sin Chian, Mary B. Chan-Park, Priyadarshini S. Mhaisalkar, Buddy D. Ratner
Biomaterials 2006 Volume 27(Issue 1) pp:68-78
Publication Date(Web):January 2006
DOI:10.1016/j.biomaterials.2005.05.069
A biodegradable and flexible poly(l-lactide-co-caprolactone) (PLLC) copolymer was synthesized and surface modification has been performed aiming at application as a scaffold in esophageal tissue engineering. The PLLC membrane surface was aminolyzed by 1,6-hexanediamine to introduce free amino groups. Using these amino groups as bridges, fibronectin and collagen were subsequently bonded with glutaraldehyde as a coupling agent. The presence of free amino groups on the aminolyzed PLLC surface was quantified using fluorescamine analysis method, which revealed that the surface NH2 density increased and eventually saturated with increasing 1,6-hexanediamine concentration or reaction time. X-ray photoelectron spectroscopy (XPS) confirmed the presence of both proteins separately on the modified PLLC surface. Water contact angle measurements evaluate the wettability of modified and unmodified PLLC surfaces. Protein-bonded surface presented more hydrophilic and homogeneous, yet PLLC can also adsorb some protein molecules. In vitro long-term (12 d) culture of porcine esophageal cells proved that fibronectin- and collagen-modified PLLC surface (denoted PLLC–Fn and PLLC–Col, respectively) can more effectively support the growth of smooth muscle cells and epithelial cells; both modified and unmodified PLLC support fibroblasts growth. Mitochondrial activity assay and cell morphology observation indicate that the PLLC–Fn surface is more favorable to epithelium regeneration than PLLC–Col. These culture results provide much valuable information for our subsequent research on the construction of artificial scaffolds with esophageal function. Fibronectin-integrated PLLC will be a good candidate scaffold to support the growth of all types of esophageal cells.
Co-reporter:Ai Ping Zhu, Ning Fang, Mary B. Chan-Park, Vincent Chan
Biomaterials 2006 Volume 27(Issue 12) pp:2566-2576
Publication Date(Web):April 2006
DOI:10.1016/j.biomaterials.2005.11.039
A simple and effective method of biomacromolecule immobilization on biomaterial surface for direct tuning of biophysical parameters such as the initial cell deformation rate, degree of cell spreading and adhesion kinetics is important for tissue engineering. The photochemical immobilization of azide-chitosan (Az-CS) on poly (lactide-co-glycolide) acid (PLGA) is applied here. Chitosan immobilization on PLGA through the photoactive azide group further facilitates subsequent grafting of other biocompatible biomacromolecules like gelatin (Gel) through the active amine groups on CS. This study quantitatively compares the 3T3 fibroblast adhesion dynamics on three PLGA surfaces (Gel-CS-PLGA, CS-PLGA and unmodified PLGA surfaces) using Confocal-Reflectance Interference Contrast Microscopy (C-RICM) together with phase contrast imaging. CS-PLGA and Gel-CS-PLGA surfaces developed were confirmed by X-ray photoelectron spectroscopy, atomic force microscopy and water contact angle and cell adhesion contact dynamics measurements. The cell adhesion was strongest on the Gel-CS-PLGA surface and lowest on unmodified PLGA. The steady state adhesion energy attained by the cells on gelatin modified PLGA surface is determined as 4.0×10−8 J/m2, which is about 400 times higher than that on PLGA surface (1.1×10−10 J/m2). Significantly increased cell adhesion with Gel-CS-PLGA is postulated to result in increased cell spreading. Our integrated biophysical method can quantify the transient contact dynamics and is sufficiently accurate to discriminate even between Gel and CS modified surfaces.
Co-reporter:Ai Ping Zhu;Mary B. Chan-Park;Jian Xia Gao
Journal of Biomedical Materials Research Part B: Applied Biomaterials 2006 Volume 76B(Issue 1) pp:76-84
Publication Date(Web):30 AUG 2005
DOI:10.1002/jbm.b.30348
Foldable hydrogel films with micropatterns measuring 480 μm by 45 μm by 54 μm by 2 cm (width of microchannel by width of microwall by height of wall by length of pattern) were made by UV embossing of a block copolymer of polycaprolactone (PCL) and poly(ethylene glycol) (PEG), specifically PCL-b-PEG-b-PCL-diacrylate (DA), with a polydimethylsiloxane mold. The mold was treated with Ar/CF4 plasma to simultaneously promote microchannel filling and demolding, and the glass substrate was modified with 3-(trimethoxysilyl) propyl acrylate to promote hydrogel adhesion to avoid delamination of the gel during demolding. The micropatterned hydrogel film was detached from the glass substrate by freeze-drying. As the films were demolded, the microstructured pattern was well replicated in the hydrogel. The gel pattern dimensions shrank with freeze-drying and increased with water swelling, but under both conditions, the gel micropattern morphology was perfectly preserved. PCL-b-PEG-b-PCL-DA hydrogel was found to have good biocompatibility compared with PEGDA hydrogel. A micropattern with a smaller microchannel width of 50 μm was also made. Micropatterned foldable and biocompatible hydrogel films have potential applications in the construction of tissue-engineering scaffolds. © 2005 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2006
Co-reporter:Jin-Ye Shen;Zhi-Qin Feng;Vincent Chan;Zhi-Wei Feng
Journal of Biomedical Materials Research Part B: Applied Biomaterials 2006 Volume 77B(Issue 2) pp:423-430
Publication Date(Web):16 NOV 2005
DOI:10.1002/jbm.b.30449
This article shows that ultra violet (UV) micro-embossing can be successfully used for fabricating biocompatible micropatterned films with microchannels separated by high aspect ratio microwalls. Eight series of micropatterns were investigated; the width of the microwall was either 10 or 25 μm and that of the microchannel either 40, 80, 120, or 160 μm. The material investigated was principally polyurethane diacrylate. The UV-embossed micropattern was extracted with methanol, converting the micropatterns from cytotoxic to biocompatible. The typical UV embossing method was modified by using a marginally adhesive polyester substrate, which facilitates demolding but is removable before methanol extraction to avoid fragmentation of the embossed micropatterns. The effect of the micropatterns on A7r5 smooth muscle cells and C2C12 skeletal muscle cells was investigated. The dimensions of both channel and wall have significant effects on the elongation of both muscle cells. In the narrower 40-μm channel, the C2C12 cells merged together to form myofibers. These results indicate that UV-embossed micropatterns may present a useful scaffold for in vitro cell shape and orientation control needed in vascular and muscle tissue engineering. © 2005 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2006
Co-reporter:W. X. Zhou and Mary. B. Chan-Park
Lab on a Chip 2005 vol. 5(Issue 5) pp:512-518
Publication Date(Web):06 Apr 2005
DOI:10.1039/B419330J
Large area molding of long and deep microchannels separated by high aspect ratio microwalls is important for high sensitivity and high throughput microfluidic devices. Ultraviolet (UV) casting is a feasible, economical and convenient method of replication of such microstructures in plastics. It is shown that a wide variety of polyacrylates with diverse properties such as those made from epoxy (EP), polyurethane (UR), polyester (ES), poly (ethylene glycol)
(EG) and poly(propylene glycol)
(PG) can be used for the high aspect ratio (7–9) UV casting of such linear microstructures over a 100 mm diameter, enlarging the range of applications of the replicated microstructures. Some challenges arise. With the EG formulation, wavy microstructures were observed; this can be overcome by stress relaxation. With non-polar PG formulation, poor adhesion between the polyester substrate and resin can lead to delamination of the casting from the substrate during demolding; this can be overcome by pre-coating a partially cured same resin on the polyester substrate. An optimum UV irradiation time was important for cure at the deepest end of the microstructure without excessive crosslinking leading to much increased demolding forces. The viscosity and wetting capability of the formulations were found to affect replication fidelity.
Co-reporter:Wee Koon Neo;Mary B. Chan-Park;Wee Koon Neo;Mary B. Chan-Park
Macromolecular Rapid Communications 2005 Volume 26(Issue 12) pp:1008-1013
Publication Date(Web):9 JUN 2005
DOI:10.1002/marc.200500134
Summary: A novel experimental set-up has been devised to measure simultaneously, in real time, the conversion and shrinkage of multi-acrylates during photopolymerization. The data show that the current practice of assigning the excess volume entirely as excess free volume is inappropriate as this leads to an increasing fractional free volume with conversion. We propose to partition the excess volume into free and occupied volume components. The new model produces satisfactory results.
Co-reporter:Y.H. Yan, M.B. Chan-Park, W.C. Ching, C.Y. Yue
Applied Surface Science 2005 Volume 249(1–4) pp:332-339
Publication Date(Web):15 August 2005
DOI:10.1016/j.apsusc.2004.12.013
Abstract
The durability of two silicone films, which were formed from silicone-based release agents with different molecular weights (MW), as anti-adhesive coatings for molds used for UV embossing was investigated. Flat electroless nickel-plated polyester stamp and UV-curable polyethylene glycol diacrylate (PEGDA) were the mold and molding material, respectively. The surface chemical composition of the silicone films and the PEGDA moldings was analyzed by X-ray photoelectron spectroscopy (XPS) as a function of molding times using the same nickelized polyester stamp. Silicone transfer was found to occur from the silicone films to the moldings with increased number of moldings. The silicone film formed from the release agent with the higher MW had the poorer durability. Two mechanisms are proposed for the material transfer.
Co-reporter:Ai Ping Zhu, Ning Fang, Mary B. Chan-Park, Vincent Chan
Biomaterials 2005 Volume 26(Issue 34) pp:6873-6879
Publication Date(Web):December 2005
DOI:10.1016/j.biomaterials.2005.05.021
O-carboxylmethylchitosan (OCMCS), a chitosan derivative, has emerged as a strong polymeric biomembrane perturbant. In this study, the interaction between OCMCS and dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was examined with cross-polarization microscopy, differential scanning calorimetry (DSC) and the surface pressure–area isotherms techniques. Cross-polarized light images showed that OCMCS induced the fusion of small DPPC multilamellar vesicles (MLV) to form large lamellar structures. From DSC measurement, the highest degree of fusion was found at the optimum OCMCS concentrations between 0.0625 and 0.2 mg/ml which are orders of magnitude lower than those required for similar reductions with unmodified chitosan as perturbant. At these concentrations, the association of DPPC and OCMCS enhances the fusion of DPPC vesicles. Surface pressure–area isotherms of DPPC monolayer in the presence of OCMCS imply interactions between OCMCS and DPPC in neutral condition. In comparison with chitosan, OCMCS proved to be a more effective membrane perturbant not only in neutral but also in acidic and basic conditions. The physical driving forces for OCMCS-induced perturbation of DPPC bilayer in neutral conditions are mainly hydrogen bonding and hydrophobic interactions. In acidic or basic conditions, the physical driving forces are dominated by the electrostatic interactions. The strong OCMCS–DPPC interaction will potentially increase the effectiveness of OCMCS for gene or drug delivery.
Co-reporter:Yabin Zhu;Mary B. Chan-Park;Kerm Sin Chian
Journal of Biomedical Materials Research Part B: Applied Biomaterials 2005 Volume 75B(Issue 1) pp:193-199
Publication Date(Web):15 JUL 2005
DOI:10.1002/jbm.b.30305
Synthetic polyester and the extracellular matrix component collagen are among the most widely used materials in tissue engineering. However, the integration of collagen into polyester scaffolds without loss of its biological function is a problem that has not been fully solved. This article investigates the covalent immobilization of collagen onto poly(DL-lactide-co-glycolide) (PLGA) membrane surfaces via a bridge of 1,8-diaminooctane and with glutaraldehyde as crosslinking agent. X-ray photoelectron spectroscopy (XPS) and fluorescence measurements confirmed the presence of bonded collagen. The effect of collagen grafting on cell behavior was investigated by comparing collagen-PLGA with unmodified PLGA sample and tissue culture polystyrene (TCPS) plates by using porcine esophageal smooth muscle cells (ESMC). DNA analysis showed that collagen-modified PLGA improved the overall proliferation of the ESMCs compared with unmodified PLGA and TCPS plates. Cells seeded on collagen-modified PLGA also showed a more extended morphology. Thus, we believe that collagen-modified PLGA shows good potential to be used as a scaffold material for tissue engineering of the esophagus. © 2005 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2005
Co-reporter:Jianxia Gao, Mary B. Chan-Park
Surface and Coatings Technology 2005 Volume 194(2–3) pp:244-250
Publication Date(Web):1 May 2005
DOI:10.1016/j.surfcoat.2004.08.189
The adhesion of Deoxyribose Nuclei Acid (DNA) on an electrode surface such as silicon is a key issue in the sensitivity of DNA chips, in order to maximize the DNA concentration on the electrodes of DNA chips and increase the detected signal, the chip should have good adhesion of DNA to its electrodes. In our study, several pieces of silicon wafer were treated by argon and oxygen plasma, respectively. We analyzed the DNA content on the different silicon surfaces using the X-ray photoelectron spectroscopy (XPS). The results indicate that the content of DNA molecules adhered on the silicon surface treated with an oxygen plasma is higher than that on the surface treated with an Ar plasma. This can be interpreted in terms of the interaction of parasitic charges and van der Waals forces on silicon oxide surfaces.
Co-reporter:Jun Zhang, Mary B. Chan-Park and Samuel R. Conner
Lab on a Chip 2004 vol. 4(Issue 6) pp:646-653
Publication Date(Web):01 Nov 2004
DOI:10.1039/B403304C
We are interested in using SU-8 dense gratings with very high aspect ratio microchannels as the master mold for fabrication of child molds needed for replication. For such applications, the sidewall taper angle and mask replication fidelity of SU-8 are very important. Increasing the exposure time was experimentally observed to decrease the width of the microchannel and the sidewall angle of SU-8 bars. A new diffraction–refraction–reflection model was also developed. The calculated microchannel width and sidewall angle at high exposure dose agreed well with the experimentally observed values indicating that reflection at the silicon substrate was significant. The larger than calculated actual microchannel width for low exposure dose was shown to be due to leaching of unreacted SU-8 in the developer. Dense gratings of high aspect ratio SU-8 bars separated by high aspect ratio (19.1) microchannels were also demonstrated.
Co-reporter:Mary B. Chan-Park;Ai P. Zhu;Jin Y. Shen;Ai L. Fan
Macromolecular Bioscience 2004 Volume 4(Issue 7) pp:665-673
Publication Date(Web):28 JUN 2004
DOI:10.1002/mabi.200300139
Summary: For use in micro-patterned scaffolds in tissue engineering, novel diacrylated triblock macromers (PLA-b-PCL-b-PLA, PGA-b-PCL-b-PGA and PCL-b-PEO-b-PCL) were synthesized and characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC). All diacrylated polymers were designed as triblock copolymers and involved biodegradable blocks of relatively non-polar ε-caprolactone (CL) and polar monomers such as glycolide (GA), lactide (LA) or ethylene oxide (EO). All triblock polymers were prepared in molecular weights of a few kilo daltons via the anionic ring-opening polymerization (ROP) of the corresponding lactide, glycolide or caprolactone using stannous octoate [Sn(Oct)2] as catalyst. The polymers had low polydispersity indices, ranging from 1.23 to 1.56. Biodegradable polymeric networks were prepared with conversions of 72–84% via photopolymerization of the triblock diacrylated polymers with 2,2-dimethoxy-2-phenylacetophenone (DMPA) as photoinitiator. PLA-b-PCL-b-PLA copolymers crumbled easily and were not suitable for micro-patterning. PGA-b-PCL-b-PGA copolymers had higher water contact angles than PCL-b-PEO-b-PCL and were also cytocompatible with Fibroblasts 3T3.
Co-reporter:Mary B. Chan-Park;Jin Y. Shen;Ai P. Zhu;Ai L. Fan
Macromolecular Bioscience 2004 Volume 4(Issue 7) pp:
Publication Date(Web):6 JUL 2004
DOI:10.1002/mabi.200490015
Co-reporter:Mary B. Chan-Park, Jun Zhang, Yehai Yan, C.Y. Yue
Sensors and Actuators B: Chemical 2004 Volume 101(1–2) pp:175-182
Publication Date(Web):15 June 2004
DOI:10.1016/j.snb.2004.02.049
Patterned SU-8 can be used as a master mold for soft lithography. Fabrication of SU-8 molds with high aspect ratio microchannels for this purpose is non-trivial due to the contrary imperatives of mold hardness and faithful replication of the photomask pattern. Increased UV exposure time, which improves resist hardness, is found to result in overexposure of the shadowed resist and unresolved pattern structure in the wafer center. We have found that reduction of UV exposure dose from the resist manufacturer’s recommendation of 500–600 mJ/cm2 to 350 mJ/cm2 permits the successful fabrication of dense SU-8 gratings with relatively wide (80 μm) SU-8 bars separated by narrow (10 μm) microchannels with aspect ratio of 10 over the entire 100 mm-diameter wafer. The underexposed SU-8 was rather soft but could be sufficiently hardened to attain a useable hardness (Vickers hardness (VH) number of 25) by hard baking at a relatively low temperature (95 °C). A hard baking time of 20 min resulted in saturation of the hardness; further increase of hard baking time resulted in no significant increase in hardness. The hard-baked SU-8 gratings were successfully used for replication of soft silicone rubber. Without hard-baking, the silicone rubber broke cohesively within the SU-8 gratings during demolding. A method of fabricating 100 mm-diameter SU-8 mold consisting of 80 μm wide SU-8 bars separated by 10 μm narrow channels with aspect ratio of 10 for soft lithography has been demonstrated.
Co-reporter:D. E. Katsoulis;M. B. Chan-Park;R. H. Baney
Polymer Composites 2003 Volume 24(Issue 1) pp:13-23
Publication Date(Web):15 APR 2004
DOI:10.1002/pc.10001
Silicone resins have been used as binders for ceramic frit coatings and can withstand temperatures of 650°C to 1260°C. Conceptually, silicone resins can potentially be used as matrices for high temperature fiber-reinforced composites. The mechanical and thermal properties of a commercially available silicone resin, Dow Corning® 6-2230, were characterized. Neat 6-2230 resin was found to have inferior room temperature mechanical properties such as flexural, tensile and fracture properties when compared to epoxy. The room temperature flexural properties and short beam shear strength of the silicone/glass composites were also found to be lower than those of epoxy/glass composite with similar glass content. However, the silicone resin had better elevated temperature properties. At an elevated temperature of 316°C, the retentions of flexural modulus and strength were 80% and 40% respectively of room temperature values; these were superior to those of phenolic/glass. Unlike the carbon-based resins, the drop in flexural properties of the silicon/glass laminates with temperature leveled off with increase in temperature beyond 250°C. The resin weight loss at 316°C in 100 cm3/min of flowing air was small compared to other carbon-based resins such as PMR-15 and LaRC TPI. Only Avimid-N appeared comparable to Dow Corning® 6-2230.
Co-reporter:M.B Chan-Park, S.S Tan
International Journal of Adhesion and Adhesives 2002 Volume 22(Issue 6) pp:471-475
Publication Date(Web):2002
DOI:10.1016/S0143-7496(02)00057-X
The surface of poly[N,N′-(oxydiphenylene)pyromellitimide] film, Kapton® HN, was modified to improve its adhesion to copper metal. The polyimide surface was argon plasma activated and then exposed to air. A nitrogen-containing monomer, 4-vinyl pyridine, was then polymerized at elevated temperature under constant pressure between the argon plasma activated polyimide film and copper foil without any added photoinitiator. Optimization of the argon pretreatment time, curing temperature and curing duration resulted in almost doubling of the single lap shear strength. It is postulated that failure occurred mainly between the polyimide and the poly(4-vinyl pyridine).
Co-reporter:Jianfei Che, Peng Chen and Mary B. Chan-Park
Journal of Materials Chemistry A 2013 - vol. 1(Issue 12) pp:NaN4066-4066
Publication Date(Web):2013/01/17
DOI:10.1039/C3TA01421E
Buckypaper is an attractive candidate material for free-standing electrodes in supercapacitors due to its high electrochemical performance, light weight, and thin dimensions. At present, however, free-standing buckypapers exhibit severe limitations in terms of product quality, especially mechanical properties, which hinder their commercial applications. We here report a new method of co-packaging buckypaper with conducting polymer and thermosetting resin to fabricate cellular SWNT buckypaper materials with excellent mechanical properties, high electrical conductivity, and enhanced electrochemical properties. This new fabrication method involves wrapping of the as-prepared buckypaper with a uniform coaxial coating of polypyrrole (PPy) on the individual SWNT or SWNT bundle surfaces via a pulsed electrochemical deposition method, followed by further packaging with cyanate ester resin via a full dip infiltration. The resulting material exhibits a significant improvement in mechanical properties (improvement over unmodified buckypaper of about 400% in tensile modulus and strength) and enhanced electrochemical performance (320 F g−1 at a current density of 1 A g−1) without sacrificing electrical and thermal properties. This material is a promising candidate for use as a free-standing electrode material in small-size, light-weight, and high-temperature supercapacitors.
Co-reporter:Sara Mesgari, Yin Fun Poon, Yilei Wang, Ya Xuan Thong, Jing Wang and Mary B. Chan-Park
Journal of Materials Chemistry A 2013 - vol. 1(Issue 41) pp:NaN6823-6823
Publication Date(Web):2013/08/22
DOI:10.1039/C3TC31242A
Semiconducting single-walled carbon nanotubes (s-SWNTs) are attractive candidates for next-generation printable semiconductors. However, all current synthesis methods produce s-SWNTs which are co-mingled with metallic (m-) SWNTs. Agarose gel electrophoresis has been reported to be an effective technique for the separation of s-SWNTs from m-SWNTs but removal of the agarose gel after separation has proved to be non-trivial. To remove agarose and the organic dispersing agent, specifically chondroitin sulfate in this work, from sorted s-SWNTs obtained by agarose gel electrophoresis, we employ the multi-step process involving a chlorosulfonic acid (HSO3Cl) wash, a base wash and thermal annealing. Herein, we report the detailed analysis of the effects of the various steps for gel removal from SWNTs by Fourier transform infrared (FTIR) spectroscopy, thermal gravimetric analysis (TGA), FTIR-TGA, scanning electron microscopy, atomic force microscopy and Raman spectroscopy. The polymer-contaminated s-SWNTs were dissolved in HSO3Cl, then selectively precipitated in a large excess of water, then washed with a base (NaOH) and finally thermally annealed. A detailed analysis confirmed that the final annealed samples contained almost no residual polymers. Field effect transistors were also fabricated from the annealed s-SWNTs and they showed good performance metrics with on/off ratio and mobility in the ∼102 to 106 and ∼2.5–9.5 cm2 V−1 s−1 ranges, respectively. Our method of gel electrophoresis and chlorosulfonic acid treatment produces clean and defect-free tubes which may be used for electronic applications.
Co-reporter:Lu Pu, Jinbao Xu, Yimin Sun, Zheng Fang, Mary B. Chan-Park and Hongwei Duan
Biomaterials Science (2013-Present) 2016 - vol. 4(Issue 5) pp:NaN879-879
Publication Date(Web):2016/02/24
DOI:10.1039/C5BM00545K
We report a new class of antimicrobial nanomaterials with biodegradable cationic polycarbonates grafted on superparamagnetic nanoparticles. Our results have shown that end-functionalized cationic polycarbonates, synthesized by organocatalytic ring opening polymerization, can be grafted onto superparamagnetic MnFe2O4 nanoparticles via ligand exchange. In comparison with the individual building blocks, the core–shell hybrid nanoparticles led to improved antimicrobial activities in two ways: first, the cationic polycarbonates in a brush form afforded a greater charge density than that of free polymer chains, resulting in stronger interactions with bacterial surfaces. Second, the structural integration of the “soft” polycarbonate shell and the “hard” superparamagnetic core in the hybrid nanoparticles brings about a synergistic action of membrane disruption by the cationic shell and magnetic hyperthermia by the nanoparticle core. The combination of two physical killing mechanisms holds great promise in fighting against a broad spectrum of bacterial pathogens.
Co-reporter:Bernice H. L. Oh, Alexander Bismarck and Mary B. Chan-Park
Journal of Materials Chemistry A 2015 - vol. 3(Issue 20) pp:NaN4122-4122
Publication Date(Web):2015/03/13
DOI:10.1039/C5TB00303B
High internal phase emulsions (HIPEs) are indisputably a core technology for various industries involving pharmaceuticals, food, cosmetics, and biologics but they usually require surfactants/co-surfactants to form, which is often undesired. More specifically, micro-HIPEs are thermodynamically stable, optically clear emulsions with droplet sizes in the range of around 1–100 nm that form spontaneously with little energy input but are rare. Mini-/macro-HIPEs have larger droplet sizes in the range of 50–500 nm and >500 nm, respectively, and typically require high energy input for emulsification. We have synthesized a series of chitosan-graft-oligoN-isopropylacrylamide-graft-oligolysine (CSNLYS) copolymers that act as both emulsifiers for HIPEs without needing extraneous surfactants as well as the matrix material of the resulting porous solid polyHIPE. By merely adjusting the length of the oligolysine graft from relatively long to medium to short, we can form either a micro-, mini- or macro-HIPE, respectively. These emulsions can then be solidified into porous polymers, polyHIPEs, simply by increasing the temperature by exploiting the copolymer thermo-responsiveness and then removing the solvents. These porous polyHIPE, particularly the ones from micro-HIPEs, have surface areas as high as 988 m2 g−1 and pore sizes below 200 nm.
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