•Carboxyfluorescein and aminofluorescein possess intrinsic peroxidase-like activity.•The carboxyl groups may act as substrate-binding sites, and the existence of amine groups decrease the catalytic activity.•The quantum theoretical calculation data support the experimental results.Recently, an important and intriguing property of fluorescein to function as an artificial peroxidase was found. Although, as a new class of small molecule enzyme mimics, the catalytic activities should be highly associated with the composition and structure, little is known about the mechanism underlying the functional groups mediated enzyme-like activities. Herein, we report for the first time that fluorescein derivatives, i.e., carboxyfluorescein (CF) and aminofluorescein (AF) possess peroxidase-like activity, and investigate the impact of different functional groups on the catalytic performance. By systematically evaluating absorption intensity, electron spin resonance (ESR) signals, enzyme kinetic parameters and activation energies, we obtain evidence that the carboxyl group can enhance catalytic activity and may serve as substrate-binding sites, but the amino group can even inhibit the catalytic reaction. Correspondingly, the theoretical study further identified these experimental results in molecular-level. These observations pave the way for identifying highly effective functional groups that promote the overall performance of fluorescein-based artificial peroxidase. Meanwhile, this work should facilitate the design and construction of tailor-made small molecule peroxidase mimics.Download full-size image

•H+-triggered bubble-generating nanosystem (BGNSs) was used to kill cancer cells.•Folic acid was modified onto BGNS surfaces.•Polydopamine was used as a universal coating for anchor folic acid.•Modification with folic acid reduced the toxicity of BGNS against normal cells.A novel targeting drug delivery system for cancer therapy based on H+-triggered bubble-generating nanosystems (BGNSs) was engineered. First, hollow mesoporous silica nanoparticles (HMSNs) were used to load doxorubicin (DOX). Then, the obtained drug-loaded HMSNs were treated with NaHCO3 to prepare the BGNSs. The BGNSs were coated with polydopamine (pDA), and finally, folic acids (FA) were anchored on the nanosystems to obtain the desired nanoscale drug delivery system (BGNSs@pDA-FA). BGNSs@pDA-FA was effectively internalized by cancer cells through folate receptor-mediated endocytosis and generated CO2 bubbles under the acidic environment of the lysosomes, thus enhancing lysosomal membrane permeability (LMP) to release caspase-3 into the cytoplasm, resulting in cancer cell death via an apoptosis-like pathway. Notably, we demonstrated that the BGNSs@pDA-FA exhibited a significant simultaneous synergetic cytotoxicity against MCF-7 cells and remarkably overcame the multidrug resistance (MDR) of MCF-7/ADR cells. Moreover, compared to free DOX and a nanosystem without FA modification (BGNSs), the BGNSs@pDA-FA induced relatively minor side effects in the MCF-10A cells. Therefore, the results showed that BGNSs@pDA-FA, as a targeted drug delivery system, have a good probability of overcoming the MDR of tumor cells with minor side effects on normal cells.Download high-res image (94KB)Download full-size image

Prion proteins (PrP) were successfully used to separate CdTe quantum dots (QDs) of different size based on the size-dependent aggregation. Even though the size disparity was only 0.8–2 nm, red-, yellow- and green-emitting QDs could be separated from one another by addition of different concentrations of PrP. This novel separation method allows controllable, rapid and simple separation of QDs without requiring large amounts of expensive reagents and complicated technology, which can be developed into a versatile method for separating diversiform nanoparticles.

•Reduced state carbon dots possess intrinsic peroxidase-like activity.•Reduced state carbon dots can catalytically oxidize TMB by H2O2 to produce a color reaction.•Reduced state carbon dots are applied to glucose detection.It was found that reduced state carbon dots (r-CDs) possessed intrinsic peroxidase-like activity, and could catalytically oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) by H2O2 to produce a color reaction. The effects of temperature, pH, incubating time and the concentration of H2O2 and TMB on catalytic activity of r-CDs were investigated. Finally we calculated the kinetic constant was ca. 0.00729 min−1 and applied r-CDs to glucose sensing by coupling glucose oxidase. As low as 2 µM H2O2 could be detected with a linear range from 0.010 to 0.40 mM via this method. This study offered a simple, sensitive, and high selectivity method for glucose determination even in serum.

•We propose a sandwich immunoassy method for the detection of Aβ1–42.•Aβ1–42 was a promising biomarker for the early diagnosis of Alzheimer’s disease.•We use QDs as a nanonabels due to their excellent fluorescent characteristics.•Magnetic beads as the substrate captured antibody enable easy separation.•This immunoassay was applied to the detection of Aβ1–42 in a cerebrospinal fluid sample.Alzheimer’s disease (AD) is the primary cause of dementia over the age of 60, affecting more than 35 million people worldwide. Methods for early diagnosis of AD are critical for the development of effective treatments to combat this debilitating disease. It was confirmed that amyloid-beta peptide 1–42 (Aβ1–42) is the biomarker of its early diagnosis. In this work, we present a new sandwich immunoassay method for the detection of Aβ1–42 based on quantum dot (QDs) nanolabels and magnetic separation. In the presence of Aβ1–42, QDs linked to magnetic beads (MB) via the formation of immune-sandwich complex and can be removed by a magnetic field. And as a result, fluorescence intensity from QDs in the supernatant decreased. Under the optimized conditions, there is a linear relationship between the fluorescence intensity of supernatant solution and the concentration of Aβ1–42 from 0.50 to 8.0 nM with a limit detection of 0.2 nM (3σ). This immunoassay was applied to detect Aβ1–42 in human cerebrospinal fluid (CSF) successfully.

In order to evaluate the effect of quantum dot (QD) labeling on virus activity, cell viability and cytokine secretion were measured after host cells were infected by QD-labeled H9N2 and unlabeled H9N2. Our results suggested that in the process of QD labeling, biotin modification (<1 mg mL−1) had no obvious effect on virus activity. However, follow-up QD labeling enhanced the virus toxicity toward host cells and caused a high level of cytokine secretion. And our results indicate that QD-labeling is not suitable for long time tracing of viruses.

•Carbon dots (CDs) and reduced state carbon dots (r-CDs) can react with potassium permanganate (KMnO4) in a strong acid to generate chemiluminescence (CL).•With different surface groups, the CL intensity of r-CDs-KMnO4 system is different from that of CDs-KMnO4 system.•The CL mechanisms of the two systems were investigated.Potassium permanganate (KMnO4) can react with two different carbon nanoparticles, i.e., carbon dots (CDs) and reduced state carbon dots (r-CDs), in a strong acid medium to generate chemiluminescence (CL). Furthermore, the different CL intensities and CL behaviors due to the different surface groups on these two kinds of carbon nanoparticles were confirmed. CL spectra, fluorescence spectra, UV–vis absorption spectra, and electron paramagnanetic resonance spectra were applied to investigate the CL mechanism. The main reaction pathways were proposed as follows: for the CL reaction between CDs and KMnO4, the excited states of CDs (CDs⁎) and Mn(II) (Mn(II)⁎) emerged as KMnO4 could inject holes into CDs, then, the CDs⁎ and Mn(II)⁎ acted as luminophors to yield CL; in the r-CDs-KMnO4 system, r-CDs were oxidized by KMnO4 directly, and CDs⁎ and Mn(II)⁎ were produced, at the same time, CL occurred. What is more interesting is that the CL intensity of the r-CD system is stronger than that of the CD system, which confirms that functional groups have strong effect on the CL behavior. It inspired us that new carbon nanoparticles with excellent luminous performance can be designed by tuning their surface groups.

Prions are a special class of pathogens that cause a number of fatal neurodegenerative diseases in mammals. This paper presents a very simple and convenient biosensor for detecting prion protein, in which a prion protein aptamer was used as the molecular recognition group and gold nanoparticles were used as the signal report group. Binding of the target molecular prion protein resulted in the enhancement of resonance light scattering (RLS) by the gold nanoparticles. A linear relationship was then identified between the enhanced RLS intensity and the concentration of prion protein in the range 0.2 to 50 nmol L−1, with a limit of detection of 0.07 nmol L−1 (3σ). The biosensor has very good selectivity for prion protein without interference by coexisting proteins, amino acids or metal ions. This “aptasensor” offers a rapid, selective and sensitive route for prion protein detection and has good potential for use in practical applications.

•A simple and facile strategy for the synthesis of fluorescent Cu NCs was developed.•The obtained Cu NCs were water soluble, stable and highly quantum efficient.•The Cu NCs were used as the favorable probe for Fe3+ ion over other common cations.•The Cu NCs show low cytotoxicity and are suitable for cell imaging.•The obtained Cu NCs can be used for imaging ferric ions in living cells.A simple, one-step facile route for preparation of water soluble and fluorescent Cu nanoclusters (NCs) stabilized by tannic acid (TA) is described. The as-prepared TA capped Cu NCs (TA-Cu NCs) are characterized by UV–vis spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, luminescence, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The TA-Cu NCs show luminescence properties having excitation and emission maxima at 360 nm and 430 nm, respectively, with a quantum yield of about 14%. The TA-Cu NCs are very stable even in 0.3 M NaCl, and their luminescent properties show pH independent. The fluorescence (FL) of Cu NCs is strongly quenched by Fe3+ through an electron transfer mechanism, but not by other metal ions. Furthermore, the FL of the TA-Cu NCs shows no changes with the addition of Fe2+ or H2O2 individually. On this basis, a facile chemosensor was developed for rapid, reliable, sensitive, and selective sensing of Fe3+ ions with detection limit as low as 10 nM and a dynamic range from 10 nM to 10 μM. The proposed sensor was successfully used for the determination of iron contents in serum samples. Importantly, the Cu NCs-based FL probe showed long-term stability, good biocompatibility and very low cytotoxicity. It was successfully used for imaging ferric ions in living cells, suggesting the potential application of Cu NCs fluorescent probe in clinical analysis and cell imaging.

Carbon dots (CDs) are luminescent nanomaterials with unique properties that show great potential in many applications. Herein, hollow CDs (HCDs) are prepared from bovine serum albumin by solvothermal reaction. The obtained HCDs are ca. 6.8 nm in diameter and have a quantum yield of 7%. Their bright photoluminescence means they can be used for cellular imaging. Structure and composition analyses indicate that the HCDs possess a hollow structure with a pore size of ca. 2 nm. The HCDs are used as a delivery system for doxorubicin (DOX). The DOX-HCD drug delivery system exhibits pH-controlled release, and is rapidly taken up by cells. Because of their specific nanostructure and photoluminescence properties, the multifunctional HCDs prepared here show potential for application in both cell imaging and cancer therapy.

The reduced state carbon dot (r-CD) is a new kind of carbon dot (CD) prepared by using sodium borohydride. Herein, we find that the r-CDs can directly reduce chloroauric acid to form gold nanoparticles (AuNPs) without adding other reducers or stabilizers. At the same time, we deduced that the hydroxyl groups (–OH) on the surfaces of r-CDs can act as both reducer and stabilizer for the synthesis of AuNPs. The AuNPs possess intrinsic catalytic activity, but compared with the AuNPs synthesized by citrate reduction, the AuNPs we prepared exhibit less excellent catalytic activity in the reduction of 4-nitrophenol (4-NP) with sodium borohydride. Conversely, for catalyzing oxidation of the substrate 3,3′,5,5′-tetramethylbenzidine (TMB) by H2O2, the AuNPs we prepared show much higher catalytic activity than the AuNPs synthesized by citrate reduction. This work may open up a new route for the applications of CDs.

•Inhibition of peroxidase mimetic activity of BSA-Au clusters can sensitively and selectively detect Hg2+.•Detection limit of Hg2+ can be as low as 3 nM.•The detection procedure is capable of estimating mercury contents in cosmetics by naked-eyed screening.It was found that Hg2+ can inhibit the peroxidase mimetic activity of bovine serum albumin (BSA) protected Au clusters (BSA-Au) due to the specific interaction between Hg2+ and Au+ existed onto the surface of BSA-Au clusters. By coupling with 3, 3′, 5, 5′-tetramethylbenzidine (TMB)–H2O2 chromogenic reaction, a novel method for Hg2+ detection was developed based on the inhibiting effect of Hg2+ on BSA-Au clusters peroxidase-like activity. This method exhibited high selectivity and sensitivity. As low as 3 nM (0.6 ppb, 3σ) Hg2+ could be detected with a linear range from 10 nM (2 ppb) to 10 µM (2 ppm) and this method was successfully applied for the determination of total mercury content in skin lightening products.

Because amyloidogenic proteins, such as prion protein, β-amyloid peptide and α-synuclein, are associated with a variety of diseases, methods for their detection are important. Recombinant prion protein (rPrP) can selectively induce aggregation of dihydrolipoic acid capped gold nanoparticles (DHLA-AuNPs), which reduces the absorbance of the DHLA-AuNPs and changes their color from red to blue. These changes were used for label-free qualitative and quantitative detection of amyloidogenic protein. The addition of NaCl improved the detection sensitivity considerably, and the detection limit was as low as 33 pmol/L.Highlights► rPrP selectively induce dihydrolipoic acid capped gold nanoparticles aggregrate. ► NaCl can influence the interaction between prion protein and AuNPs. ► The detection limit of amyloidogenic protein can be as low as 33 pM.

Recently, the intrinsic enzyme-like activity of nanoparticles (NPs) has become a growing area of interest. Compared with natural enzymes, these enzyme-like NPs are stable against denaturing, low in cost, and highly resistant to high concentrations of substrate. These advantages make them promising in various applications.In this review, we focus on recent advances in NPs as enzyme mimetics and their analytical and environmental applications. We pay special attention to the enzyme-like activity of magnetic NPs, cerium-oxide NPs, noble-metal NPs, carbon and other nanomaterials.Highlights► Analytical and environmental applications of nanoparticles (NPs) as enzyme mimetics. ► Review covers magnetic nanoparticles (NPs), cerium-oxide NPs and noble-metal NPs. ► Screening tools include colorimetric, electrochemical and chemiluminescence methods. ► Discussion of the future outlook of nanoparticles (NPs) as enzyme mimetics.

Blue luminescent reduced state carbon dots were prepared by reducing carbon dots with NaBH4. The quantum yield of the reduced state carbon dots increased from 2% to 24% and the maximum emission wavelength shifted from 520 to 450 nm. This offers a simple pathway to enhance the luminescence of carbon dots.

Carbon nanodots (C-Dots) were found to possess intrinsic peroxidase-like activity, and could catalytically oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) by H2O2 to produce a colour reaction. This offers a simple, sensitive and selective colorimetric method for glucose determination in serum.

Graphite oxide (GO), candle soot, conductive carbon black and lampblack were used to prepare fluorescent carbon dots (CDots) by heating under reflux in nitric acid. The CDots prepared from GO exhibited the highest quantum yield and narrowest emission of those produced. Microwave-assisted techniques were also used to synthesize CDots. Compared with conventional heating under reflux, microwave-assisted heating under reflux and a microwave–hydrothermal method both shortened the reaction time. CDots prepared using microwave-assisted techniques exhibited increased absorption, higher quantum yield and longer fluorescence lifetime than those prepared by conventional heating under reflux.

CdTe quantum dots (QDs) were used as a highly selective probe for the detection of prion protein. Orange-emitting precipitates appeared within 30 s of the addition of recombination prion protein (rPrP) to a solution of green-emitting CdTe QDs. This allowed colorimetric qualitative and semi-quantitative detection of rPrP. The decrease in fluorescence intensity of the supernatant could be used for quantitative detection of rPrP. The fluorescence intensity of the supernatant was inversely proportional to the rPrP concentration from 8 to 200 nmol L−1 (R2 = 0.9897). Transmission electron microscopy results showed that fibrils existed in the precipitates and these were partly transformed to amyloid plaques after the addition of rPrP.

Herein, we prepared water-soluble fluorescent carbon dots with diameter about 1.5 nm from cheap commercial lampblack. These fluorescent carbon nanoparticles are stable toward photobleaching and stable in water for more than half a year without fluorescence decrease. In order to improve its fluorescence properties, we passivated these nanoparticles with bisamino-terminated polyethylene glycol (PEG1500N). Therefore, both fluorescence quantum yield and lifetime increased after this progress. In addition, the passivated carbon dots were more inert to solvent than the bare one and showed different responses to pH change.

Herein, we reported the quenching effect of Ni2+ on bovine serum albumin protected fluorescent gold nanoparticles (BSA-GNPs). The quenching mechanism was discussed and a static quenching mechanism was proposed. The number of binding sites (n), apparent stability constants (K) and corresponding thermodynamic parameters of BSA-GNPs-Ni2+ complex were measured at different temperatures. Under optimum conditions, the fluorescence intensity of BSA-GNPs is linearly proportional to nickel concentration from 6.0 × 10−8 mol/L to 8.0 × 10−6 mol/L with a detection limit of 1.0 × 10−8 mol/L. The result indicated that BSA-GNP was a potential Ni2+ probe.

Carbon nanodots (C-Dots) were found to possess intrinsic peroxidase-like activity, and could catalytically oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) by H2O2 to produce a colour reaction. This offers a simple, sensitive and selective colorimetric method for glucose determination in serum.

Blue luminescent reduced state carbon dots were prepared by reducing carbon dots with NaBH4. The quantum yield of the reduced state carbon dots increased from 2% to 24% and the maximum emission wavelength shifted from 520 to 450 nm. This offers a simple pathway to enhance the luminescence of carbon dots.

Prions are a special class of pathogens that cause a number of fatal neurodegenerative diseases in mammals. This paper presents a very simple and convenient biosensor for detecting prion protein, in which a prion protein aptamer was used as the molecular recognition group and gold nanoparticles were used as the signal report group. Binding of the target molecular prion protein resulted in the enhancement of resonance light scattering (RLS) by the gold nanoparticles. A linear relationship was then identified between the enhanced RLS intensity and the concentration of prion protein in the range 0.2 to 50 nmol L−1, with a limit of detection of 0.07 nmol L−1 (3σ). The biosensor has very good selectivity for prion protein without interference by coexisting proteins, amino acids or metal ions. This “aptasensor” offers a rapid, selective and sensitive route for prion protein detection and has good potential for use in practical applications.

We constructed a H+-triggered bubble-generating nanosystem (BGNS), which generated CO2 bubbles in the acidic environment of lysosomes after being internalized by cancer cells. The quickly generated bubbles caused enhanced lysosomal membrane permeabilization. As expected, H+-triggered BGNS possessed remarkable cytotoxicity against MCF-7 breast cancer cells, and successfully overcame the multidrug resistance of MCF-7/ADR cells.