In this study, we present a valence states modulation strategy for picomole level assay of Hg2+ using directional self-assembly of gold nanorods (AuNRs) as signal readout. Hg2+ ions are first controllably reduced to Hg+ ions by appropriate ascorbic acid, and the reduced Hg+ ions react with the tips of the preadded AuNRs and form gold amalgam. Such Hg+ decorated AuNRs then end-to-end self-assemble into one-dimensional architectures by the bridging effects of lysine based on the high affinity of NH2–Hg+ interactions. Correspondingly, the AuNRs’ longitudinal surface plasmon resonance is gradually reduced and a new broad band appears at 900–1100 nm region simultaneously. The resulting distinctly ratiometric signal output is not only favorable for Hg2+ ions detection but competent for their quantification. Under optimal conditions, the linear range is 22.8 pM to 11.4 nM, and the detection limit is as low as 8.7 pM. Various transition/heavy metal ions, such as Pb2+, Ti2+, Co2+, Fe3+, Mn2+, Ba2+, Fe2+, Ni2+, Al3+, Cu2+, Ag+, and Au3+, do not interfere with the assay. Because of ultrahigh sensitivity and excellent selectivity, the proposed system can be employed for assaying ultratrace of Hg2+ containing in drinking and commonly environmental water samples, which is difficult to be achieved by conventional colorimetric systems. These results indicate that the present platform possesses specific advantages and potential applications in the assay of ultratrace amounts of Hg2+ ions.Keywords: gold nanorods; Hg2+ ions; ratiometric sensing; self-assembly; valence states modulation;

We herein report aqueous fabrication of well-defined Au@Cu2–xE (E = S, Se) core@shell dual plasmonic supraparticles (SPs) for multimodal imaging and tumor therapy at the in vivo level. By means of a modified self-limiting self-assembly based strategy, monodisperse core@shell dual plasmonic SPs, including spherical Au@Cu2–xS SPs, Au@Cu2–xSe SPs, and rod-like Au@Cu2–xS SPs, are reliably and eco-friendly fabricated in aqueous solution. Due to plasmonic coupling from the core and shell materials, the as-prepared hybrid products possess an extremely large extinction coefficient (9.32 L g–1 cm–1 for spherical Au@Cu2–xS SPs) at 808 nm, which endows their excellent photothermal effect. Furthermore, the hybrid core@shell SPs possess the properties of good biocompatibility, low nonspecific interactions, and high photothermal stability. So, they show favorable performances for photoacoustic imaging and X-ray computed tomography imaging as well as photothermal therapy of tumors, indicating their application potentials in biological field.Keywords: core@shell; dual plasmonic; multimodal imaging; self-assembly; supraparticles; tumor therapy;

•Ratiometric sensing of metabolites is achieved using doped QDs as sole luminophore.•A triple-layer “filter screen” is designed for overcoming non-specific interactions.•The metabolites in serum can be directly assayed without any sample pre-treatment.We herein present an effective and versatile platform for ratiometric sensing of metabolites using intrinsically dual-emitting ZnS:Mn2+ quantum dots (QDs) as sole reporter. To avoid notoriously non-specific interactions, a special triple-layer “filter screen” around the inorganic QD core is rationally constructed, which is made of oleic acid, cetyltrimethyl ammonium bromide and bio-enzymes. In the presence of the analytes, the in-situ enzymatic H2O2 molecules diffuse and pass through the “filter screen” along the molecule interspace, which then reacts with the inorganic core and leads to more dramatically quenching of the Mn2+ emission. The ratiometric signal readout is so distinct that can be observed by naked eyes (from orange to violet). In contrast, various coexisting bio-molecules, due to larger size, are well prevented from penetrating the filter screen by steric hindrance effect. So, various potential interfering substances do not disturb the assay. Under optimal conditions, five kinds of the corresponding substrates, namely glucose, cholesterol, lactate, xanthine and uric acid are well quantified by the emission intensity ratio of I470/I615, and the linear ranges are 0.1–200 µM, 0.1–200 µM, 1–200 µM, 1–200 µM and 1–200 µM, respectively. The detection limits can even reach quasi-picomole levels. Because of favorable analytical performances (excellent selectivity, appropriate sensitivity and broad linear range), the proposed system can direct assay the analytes in blood without any sample pre-treatment.Download high-res image (271KB)Download full-size image

We herein present a three-in-one nanoplatform for sensing, self-assembly, and cascade catalysis, enabled by cyclodextrin modified gold nanoparticles (CD@AuNPs). Monodisperse AuNPs 15–20 nm in diameter are fabricated in an eco-friendly way by the proposed one-step colloidal synthesis method using CD as both reducing agents and stabilizers. First, the as-prepared AuNPs are employed as not only scaffolds but energy acceptors for turn-on fluorescent sensing based on guest replacement reaction. Then, the macrocyclic supramolecule functionalized AuNPs can be controllably assembled and form well-defined one- and two-dimensional architectures using tetrakis(4-carboxyphenyl)porphyrin as mediator. Finally, in addition to conventional host–guest interaction based properties, the CD@AuNPs possess unpredictable catalytic activity and exhibit mimicking properties of both glucose oxidase and horseradish peroxidase simultaneously. Especially, the cascade reaction (glucose is first catalytically oxidized and generates gluconic acid and H2O2; then the enzymatic H2O2 and preadded TMB (3,3′,5,5′-tetramethylbenzidine) are further catalyzed into H2O and oxTMB, respectively) is well-achieved using the AuNPs as the sole catalyst. By employing a joint experimental–theoretical study, we reveal that the unique catalytic properties of the CD@AuNPs probably derive from the special topological structures of CD molecules and the resulting electron transfer effect from the AuNP surface to the appended CD molecules.

Correction for ‘Beyond “turn-on” readout: from zero background to signal amplification by combination of magnetic separation and plasmon enhanced fluorescence’ by Suqin Gong and Yunsheng Xia, Chem. Commun., 2016, 52, 9660–9663.

By magnetic separation and subsequent plasmon enhanced fluorescence, an assay platform with a signal output from completely “zero” background to fluorescence amplification is achieved, using quantum dots as reporters. So, it well breaks through the conventional “turn-on” strategy in both lower and upper limits. The sensitivity for hyaluronidase sensing is enhanced 104–106 times as compared with previous fluorescence methods.

In this study, we have reported a new colorimetric platform for sensitive and selective sensing of Hg2+ using near-infrared (NIR) plasmonic Cu2-xSe nanoparticles (NPs) as reporters. Because of ultrahigh affinity between Hg2+ and Se2–, the added Hg2+ can react with Cu2-xSe NPs and exchange their Cu+/Cu2+, yielding a HgSe layer around the host NPs. Accordingly, the absorption profiles of the Cu2-xSe NPs are modulated substantially: The absorbance at 400–600 nm is increased, and the NIR localized surface plasmon resonance dramatically decreases with a more than 150 nm bathochromic shift. Thus, the system possesses triple signal responses, namely, ratiometry, wavelength, and intensity, to the analytes simultaneously. Such uniquely multiple signal output not only provides more choices for the quantification, but also enhances the reliability in the analyte detection. By rationally choosing poly(allylamine hydrochloride) as the NP template, Hg2+ ions can be determined as ranging from 0–800 nM. The detection limit is as low as 2.7 nM, which is nearly 4 times lower than the limit value (10 nM) defined by the U.S. Environmental Protection Agency for drinking water. Other heavy/transition metal ions, such as Cu2+, Ag+, Pb2+, Cd2+, Ni2+, Co2+, Mn2+, Zn2+, Cr3+, Fe2+, and FeF63–, do not interfere with the sensing. Especially, Hg2+ contents can be quantified, even if their concentrations are as low 10 nM in tap water and common environmental water samples. Due to favorable analytical performance, the proposed Cu2-xSe NPs based system has potential applications in monitoring trace Hg2+ in various real samples, even in drinking water.Keywords: colorimetric chemosenosor; Cu2-xSe nanoparticles; Hg2+ assay; localized surface plasmon resonance

We present herein the first reported self-assembly modulation of gold nanorods (AuNRs) by enzymatic reaction, which is further employed for colorimetric assays of cholinesterase (ChE) and organophosphate pesticides (OPs) in human blood. ChE catalyzes its substrate (acetylthiocholine) and produces thiocholine and acetate acid. The resulting thiols then react with the tips of the AuNRs by S–Au conjunction and prevent subsequent cysteine-induced AuNR end-to-end (EE) self-assembly. Correspondingly, the AuNR surface plasmon resonance is regulated, which results in a distinctly ratiometric signal output. Under optimal conditions, the linear range is 0.042 to 8.4 μU/mL, and the detection limit is as low as 0.018 μU/mL. As ChE is incubated with OPs, the enzymatic activity is inhibited. So, the cysteine-induced assembly is observed again. On the basis of this principle, OPs can be well determined ranging from 0.12 to 40 pM with a 0.039 pM detection limit. To our knowledge, the present quasi pU/mL level sensitivity for ChE and the quasi femtomolar level sensitivity for OPs are at least 500 and 7000 times lower than those of previous colorimetric methods, respectively. The ultrahigh sensitivity results from (1) the rational choice of anisotropic AuNRs as building blocks and reporters and (2) the specific structure of the enzymatic thiocholine. Because of ultrahigh sensitivity, serum samples are allowed to be extremely diluted in the assay. Accordingly, various nonspecific interactions, even from glutathione/cysteine, are well avoided. So, both ChE and OPs in human blood can be directly assayed without any prepurification, indicating the simplicity and practical promise of the proposed method.

In this paper, we have presented a novel strategy to fabricate fluorescent boronic acid modified carbon dots (C-dots) for nonenzymatic blood glucose sensing applications. The functionalized C-dots are obtained by one-step hydrothermal carbonization, using phenylboronic acid as the sole precursor. Compared with conventional two-step fabrication of nanoparticle-based sensors, the present “synthesis-modification integration” strategy is simpler and more efficient. The added glucose selectively leads to the assembly and fluorescence quenching of the C-dots. Such fluorescence responses can be used for well quantifying glucose in the range of 9–900 μM, which is 10–250 times more sensitive than that of previous boronic acid based fluorescent nanosensing systems. Due to “inert” surface, the C-dots can well resist the interferences from various biomolecules and exhibit excellent selectivity. The proposed sensing system has been successfully used for the assay of glucose in human serum. Due to simplicity and effectivity, it exhibits great promise as a practical platform for blood glucose sensing.

In this study, we have presented a novel plasmon enhanced fluorescence (PEF) system for label-free sensing of small molecules in bulk solution. The amine-terminated gold nanodendrite (AuND) and carboxyl-terminated QDs directly assemble each other by amine–carboxyl attraction. Without any spacer layers, PEF can be increased by 4 times during the formation of the compact hybrid (AuND-QDs) assembly. Both experiment and finite-difference time domain calculation results indicate that the distinct solution-PEF effect is ascribed to two reasons: (1) The used AuNDs simultaneously possess four features in morphology and topology, well-defined superstructure, sharp tips and edges, moderately elongated subunits, and smaller size. (2) The hybrid (AuND-QDs) assembly has a very compact structure. So, the fluorescence is well enhanced by the effective increase of excitation and radiative decay rates with the decrease of scattering effect. The (AuND-QDs) assembly is then employed for sensing of trinitrotoluene (TNT), one of the highly explosive and environmentally detrimental substances, in bulk solution. The sensing principle is that the analytes can react with primary amines on the AuND surface and form Meisenheimer complexes, which break the preformed assemblies and result in the fluorescence recovery of the QDs. The linear range is 0–8.8 nM with 0.05 nM detection limit. The present quasi-picomole level sensitivity is one of the best results for fluorescent TNT sensing. The developed method is successfully applied to TNT sensing in real environmental samples, indicating the practical potential.

Intrinsic dual-emitting doped ZnS:Mn2+ quantum dots are promising as sole fluorophore for ratiometric sensing. The ratiometric signals are reliably output by three kinds of modulation modes, namely, electron transfer, energy transfer, and chemical reaction, respectively. Compared with a conventional QD-based pair-fluorophore system, such a comprehensively ratiometric signal readout from a single fluorophore not only means a fundamental breakthrough but will substantially simplify the design and greatly promote the application of ratiometric sensing.

A high concentration nitric acid oxidation strategy has been presented for one-step fabrication of strongly red-emitting fluorescent graphene quantum dots (GQDs) using activated carbon as a carbon source. In optimal conditions, the emission quantum yield at 600 nm wavelength is as high as 18%. The concentration of the used nitric acid is critical to the optical properties of the GQDs: concentrated nitric acid (14.6 M) can more sufficiently oxidize GQDs' surface and more efficiently dope the N element, resulting in a longer emission band and higher emission efficiency. Preliminary cell image study indicates the obtained GQDs possess a high signal to background ratio, good stability and low cytotoxicity, which endow their promise as a new type of near-infrared fluorophores for biological applications.

We present a previously unexplored mechanism for a quantum dot (QD) fluorescence switch, and introduce its application to turn-on sensing of anions. A hybrid composite is formed from polyacrylic acid (PAA) and the surface of CdTe QDs through strong coordination interactions between the carboxy groups of PAA and the Cd atoms on the surface of the QDs. Such interactions cause almost complete quenching of the fluorescence of the QDs via the pull effect of PAA. Carbonate, silicate or phosphate anions are then added in the pH 8.2 solution of the QD-PAA composites. These anions are partially hydrolyzed and protonated, and then react with the coordinating oxygen atoms due to the formation of hydrogen bonds. This leads to a decrease in the pull effect of PAA, and eventually gives rise to a strong recovery of fluorescence. The detection limits are 0.29, 0.02, and 0.76 mM, respectively, for carbonate, silicate and phosphate. The surface modulation method presented here provides a novel strategy for the design of QD-based chemosensors for carbonate, silicate or phosphate which otherwise are difficult to detect.

•A “facet dependent binding and etching” strategy for blood UA sensing is proposed.•The detection limit is as low as 10 nM.•This system can conveniently discriminate hyperuricemia by naked eyes.•This strategy needs not any surface modification and sample pre-treatments.By combination of experiments and density functional theory calculations, we present a simple but effective “facet dependent binding and etching” strategy for non-enzymatic and non-aggregated colorimetric sensing of blood uric acid (UA), using unmodified Ag nanoprisms as the signal readout. In the absence of UA, the triangular Ag nanoprisms are etched alongside (1 1 0) facets by H2O2 and form round nanodiscs, and a more than 160 nm surface plasmon resonance (SPR) blue shift is observed. Because of special affinity between UA and side facets of the Ag nanoprisms, pre-added UA can well protect the Ag nanoprisms from etching. Such protection effect can be used for well quantifying UA in the range of 10–3000 nM, based on the inverse proportion of the SPR blue shift with the added analyte. Due to very thin plate morphology (5 nm) and facet dependent binding/etching effects of the Ag nanoprisms, the sensing system has ultrahigh sensitivity. The detection limit is only 10 nM, which is about 2 to 4 orders of magnitude lower than that of previous colorimetric sensing systems. In addition to accurate quantitation, the proposed strategy can conveniently discriminate the patient of hyperuricemia from normal person by naked eyes. So, the present simple, low-cost and visualized UA chemosensor has great potential in the applications for point-of-care diagnostics.

In this study, we design a homogeneous system consisting of Ag nanoprisms and glucose oxidase (GOx) for simple, sensitive, and low-cost colorimetric sensing of glucose in serum. The unmodified Ag nanoprisms and GOx are first mixed with each other. Glucose is then added in the homogeneous mixture. Finally, the nanoplates are etched from triangle to round by H2O2 produced by the enzymatic oxidation, which leads to a more than 120 nm blue shift of the surface plasmon resonance (SPR) absorption band of the Ag nanoplates. This large wavelength shift can be used not only for visual detection (from blue to mauve) of glucose by naked eyes but for reliable and convenient glucose quantification in the range from 2.0 × 10–7 to 1.0 × 10–4 M. The detection limit is as low as 2.0 × 10–7 M, because the used Ag nanoprisms possess (1) highly reactive edges/tips and (2) strongly tip sharpness and aspect ratio dependent SPR absorption. Owing to ultrahigh sensitivity, only 10–20 μL of serum is enough for a one-time determination. The proposed glucose sensor has great potential in the applications of point-of-care diagnostics, especially for third-world countries where high-tech diagnostics aids are inaccessible to the bulk of the population.

The design of fluorescent probes for turn-on sensing of anions has been especially significant because it can effectively enhance sensing sensitivity by decreasing the background interference. In the present work, we have systematically studied the potential applications of fluorescent quantum dots (QDs) in turn-on anion sensing. The fluorescence of QDs are firstly quenched by three different mechanisms, i.e. fluorescence resonance energy transfer, electron transfer and surface states modulated fluorescence. The fluorescence of the pre-quenched QDs can then be recovered by various anions due to the modulating effects of added anions on the interaction between QDs and QDs, the interaction between QDs and quenchers, and the surface chemistry of the quenched QDs, respectively. The results described here indicate that turn-on sensing of various anions by QDs-based systems can be achieved by rationally choosing fluorescence modulating strategies, demonstrating the versatility of QDs in the corresponding applications.

Monodisperse inorganic supraparticles (SPs) are an emerging and hot research topic in the chemistry, physics and materials science communities in the past several years. Monodisperse inorganic SPs exhibit unique physiochemical properties due to their well-defined shape and distinctive topological structure. This review summarizes recent progress in the study of formation mechanism, properties and applications of inorganic monodisperse SPs. The future developments in this research area are also discussed.

Fluorescence sensing of enantiomers is a much needed yet very challenging task due to nearly identical chemical and physical properties of the chiral isomers also known as chiral equivalence. In this study, we propose a novel strategy for fluorescence sensing of enantiomers using chiral nanoparticles and their ability to form dynamic assemblies. Fluorescence resonance energy transfer (FRET) in nanoscale assemblies consisting of either l-cysteine- or d-cysteine-modified quantum dots (QDs) and gold nanorods (GNRs) was found to be strongly dependent on traces of cysteine. This occurs due to high sensitivity of dynamic assemblies to the weak internanoparticle interactions that can exponentially increase energy transfer efficiencies from QDs to GNRs. Comprehensive analysis of the fluorescence responses in the two types of chiral nanoscale assemblies enables accurate determination of both concentration and enantiomeric composition of the analyte, i.e., cysteine. The described method can quantify the composition of a chiral sample, even the content of one enantiomer is as low as 10% in the mixture. Exceptional selectivity in respect to d/l-cysteine in comparison to analogous small molecules was observed. Versatility of nanoparticle–nanorod assemblies and tunability of intermolecular interactions in them open the road to adaptation of this sensing platform to other chiral analytes.

In this study, a novel Au nanocluster (NC)-based fluorescent sensor has been designed for near-infrared (NIR) and turn-on sensing of glutathione (GSH) in both living cells and human blood samples. The large Stokes-shifted (140 nm) fluorescent Au NCs with NIR emission and long-wavelength excitation have been rapidly synthesized for 2 h by means of a microwave-assisted method in aqueous solution. The addition of HgII leads to an almost complete emission quenching (98%) of Au NCs because of the interaction of HgII and AuI on the surface of Au NCs. After introducing GSH to the Au NC–HgII system, a more than 20 times fluorescent enhancement is obtained because of the preferable affinity of GSH with HgII. Under optimum conditions, the fluorescence recovery is linearly proportional to the concentration of GSH between 0.04 and 16.0 μM and the detection limit is as low as 7.0 nM. This Au NC-based sensor with high sensitivity and low spectral interference has been proven to facilitate biosensing applications.

In this study, we design a FRET system consisting of gold nanorod (AuNR) and quantum dots (QDs) for turn-on fluorescent sensing of 2,4,6-trinitrotoluene (TNT) in near-infrared region. The amine-terminated AuNR and carboxyl-terminated QDs first form a compact hybrid assembly through amine−carboxyl attractive interaction, which leads to a high-efficiency (>92%) FRET from QDs to AuNRs and an almost complete emission quenching. Next, added TNT molecules break the preformed assembly because they can replace the QDs around AuNRs, based on the specific reaction of forming Meisenheimer complexes between TNT and primary amines. Thus, the FRET is switched off, and a more than 10 times fluorescent enhancement is obtained. The fluorescence turn-on is immediate, and the limit of detection for TNT is as low as 0.1 nM. Importantly, TNT can be well distinguished from its analogues due to their electron deficiency difference. The developed method is successfully applied to TNT sensing in real environmental samples.

Monodisperse inorganic supraparticles (SPs) are an emerging and hot research topic in the chemistry, physics and materials science communities in the past several years. Monodisperse inorganic SPs exhibit unique physiochemical properties due to their well-defined shape and distinctive topological structure. This review summarizes recent progress in the study of formation mechanism, properties and applications of inorganic monodisperse SPs. The future developments in this research area are also discussed.

By magnetic separation and subsequent plasmon enhanced fluorescence, an assay platform with a signal output from completely “zero” background to fluorescence amplification is achieved, using quantum dots as reporters. So, it well breaks through the conventional “turn-on” strategy in both lower and upper limits. The sensitivity for hyaluronidase sensing is enhanced 104–106 times as compared with previous fluorescence methods.

Correction for ‘Beyond “turn-on” readout: from zero background to signal amplification by combination of magnetic separation and plasmon enhanced fluorescence’ by Suqin Gong and Yunsheng Xia, Chem. Commun., 2016, 52, 9660–9663.