Hypochlorite (ClO–) is one of the most important reactive oxygen species (ROS), which plays an important role in sustaining human innate immunity during microbial invasion. Moreover, ClO– is a powerful oxidizer for water treatment. The safety of drinking water is closely related to its content. Herein, m-phenylenediamine (mPD) is used as a precursor to prepare carbon dots (named m-CDs) with highly fluorescent quantum yield (31.58% in water), and our investigation shows that the strong fluorescent emission of m-CDs can be effectively quenched by ClO–. Based on these findings, we developed a novel fluorescent nanoprobe (m-CDs) for highly selective detection of ClO–. The linear range was from 0.05 to 7 μM (R2 = 0.998), and the limit of detection (S/N = 3) was as low as 0.012 μM. Moreover, a portable agarose hydrogel solid matrix-based ratiometric fluorescent nanoprobe (m-CDs@[Ru(bpy)3]2+) sensor was subsequently developed for visual on-site detection of ClO– with the naked eyes under a UV lamp, suggesting its potential in practical application with low cost and excellent performance in water quality monitoring. Additionally, intracellular detection of exogenous ClO– was demonstrated via ratiometric imaging microscopy.Keywords: agarose hydrogel solid matrix; carbon dots (CDs); hypochlorite; m-CDs@[Ru(bpy)3]2+; ratiometric fluorescent nanoprobe; ratiometric imaging microscopy;

•We report the first multicolor biosensor for the evaluation of fish freshness with the naked eye.•Hypoxanthine has been chosen as the index of fish freshness, the concentration of which is directly determined to the color of the sensing system.•The color of the sensing system has a good relationship with hypoxanthine concentration in the concentration range of 0.05 mM–0.63 mM.•The proposed multicolor Hx sensor has successfully applied to estimate the freshness of the fish samples.Fish product is one of the main seafood with the disadvantage of easy to perish. Various indexes and diverse methods have been developed to evaluate the fish freshness, but most of them are complex and cannot be applied for on-field detection. In this study, we reported a simple, visual and economical multicolor biosensor based on the etching of gold nanorods (GNRs) to evaluate the fish freshness with the naked eye. Hypoxanthine (Hx) is chosen as the index of fish freshness and it react with the dissolved oxygen to produce H2O2 at the presence of xanthine oxidase (XOD). Then GNRs are etched quickly by the H2O2 in the presence of Fe2+. Correspondingly, the GNRs surface plasmon resonance (SPR) is regulated and results in a distinctly color change of the system, which can be easily distinguished with the naked eye. Therefore, the concentration of Hx in the fish samples can be semi-quantitatively analyzed with the naked eye. The proposed multicolor Hx sensor has been successfully applied to the detection of Hx in fish samples.

Gold nanoparticles (AuNPs) have been frequently utilized for the construction of diverse colorimetric biosensors. Normally, AuNPs with sharp edges could have better sensitivity. However, the poor monodipersity of AuNPs with sharp edges seriously confines their utility for colorimetric biosensing. Herein, we demonstrate the utility of highly uniform gold nanobipyramids (Au NBPs) for ultrasensitive colorimetric detection of H5N1 virus. The proposed method is based on the fact that alkaline phosphatase (ALP) could catalyze the decomposition of 4-aminophenyl phosphate (4-APP) to generate 4-aminophenol (4-AP), which would then reduce silver nitrate to metal silver and then deposited on Au NBPs. The metal silver shell coated on the Au NBPs changed the refractive index of gold and thus resulted in a blue shift of longitudinal localized surface plasmon resonance (LSPR) and accompanied a vivid color change. This method exhibited a higher sensitivity than that of other Au NPs such as gold nanorods due to the high-index-faceted on the tips of the Au NBPs. This method was used to detect the activity of ALP. It exhibited a linear range of 0.1–5 mU/mL with a limit of detection (LOD) of 0.086 mU/mL. Finally, the proposed method was used in immunoassay to detect H5N1 virus. The results showed that the corresponding linear range for the detection of H5N1 virus antigen was 0.001–2.5 ng/mL, and the LOD was determined to be 1 pg/mL, which is more sensitive than those in most of the colorimetric biosensors reported previously.

●We disclose for the first time that TMB2+ can react with AuNRs.●Instrument-free multicolor change by a simple one step mixed.●One kind of color refers to a different concentration of target.●Multicolor display can transform from the typical HRP H2O2/TMB strategy.●It can well accommodate conventional immunoassay platforms.3, 3′, 5, 5′-tetramethylbenzidine (TMB) has been widely used as a chromogenic substrate for colorimetric immunoassays. Normally, the colorless TMB is oxidized into yellow TMB2+ (in acidic solution) to indicate the presence of the target molecules. However, this kind of monochromic intensity changes seriously confine the accuracy of visual inspection. Herein, we demonstrate for the first time that TMB2+ can quantitatively and efficiently etch gold nanorods (AuNRs). The addition of AuNRs into a solution containing different amount of TMB2+ generates vivid color responses as colorful as a rainbow, and the etching process can be finished within 90 s. As a result, the exact concentration of TMB2+ can be easily distinguished with the naked eyes by the corresponding solution color. Based on this finding, we incorporate AuNRs into the well-developed, commercially available horseradish peroxidase (HRP)-TMB immunoassay system, so that it can be utilized for semiquantitative detection of a broad range of disease biomarkers with the naked eyes (termed ‘NEQ-IA’). Carcinoembryonic antigen (CEA) and Prostate specific antigen (PSA) had been chosen as example targets to test the feasibility of the proposed biosensor. The results showed good accordance with the conventional methods. Because no sophisticated apparatus but human eyes are used as the readout, the proposed NEQ-IA could be a good supplementary to current state-of-the-art immunoassay methods for those applications that require the use of portable and affordable devices, for example, for the detection of disease biomarkers at home and in the field.

The efficient extraction of targets from complex surfaces is vital for technological applications ranging from environmental pollutant monitoring to analysis of explosive traces and pesticide residues. In our present study, we proposed a proof-of-concept surface enhance Raman scattering (SERS) active substrate serving directly to the rapid extraction and detection of target molecules. The novel substrate was constructed by decorating the commercial tape with colloidal gold nanoparticles (Au NPs), which simultaneously provides SERS activity and “sticky” of adhesive. The utility of SERS tape was demonstrated by directly extracting pesticide residues in fruits and vegetables via a simple and viable “paste and peel off” approach. The obtained strong and easily distinguishable SERS signals allow us to detect various pesticide residues such as parathion-methyl, thiram, and chlorpyrifos in the real samples with complex surfaces including green vegetable, cucumber, orange, and apple.

Herein, we report for the first time a colorful chromogenic substrate, which displays vivid color responses in the presence of different concentration of analytes. Our investigation reveals that the selective shortening of gold nanorods (AuNRs) could generate a series of distinct colors that covers nearly the whole visible range from 400 to 760 nm. These vivid colors can be easily distinguished by the naked eye; as a result, the accuracy of visual inspection could be greatly improved. Next, we demonstrate the utility of AuNRs as multicolor chromogenic substrate to develop a number of colorimetric immunoassay methods, e.g., multicolor enzyme-linked immunosorbent assay (ELISA), multicolor competitive ELISA, and multicolor magnetic immunoassay (MIA). These methods allow us to visually quantify the concentration of a broad range of target molecules with the naked eye, and the obtained results are highly consistent with those state-of-the-art techniques that are tested by the sophisticated apparatus. These multicolor portable and cost-effective immunoassay approaches could be potentially useful for a number of applications, for example, in-home personal healthcare, on-site environmental monitoring, and food inspection in the field.

Seed-mediated synthesis of gold nanorods (AuNRs) has been widely used for diverse applications in the past decade. In this work, this synthetic process is demonstrated for multicolor biosensing for the first time. Our investigation reveals that ascorbic acid acts as a key factor to mediate the growth of AuNRs. This phenomenon is incorporated into the alkaline phosphatase (ALP)-enzyme-linked immunosorbent assay (ELISA) system based on the fact that ALP can catalyze the conversion of ascorbic acid–phosphate into ascorbic acid with high efficiency. This allows us to develop a multicolor ELISA approach for sensitive detection of disease biomarkers with the naked eye. We show the proof-of-concept multicolor ELISA for the detection of prostate-specific antigen (PSA) in human serum. The results show that different colors are presented in response to different concentrations of PSA, and a detection limit of 3 × 10−15 g mL−1 in human serum was achieved. The proposed multicolor ELISA could be a good supplement to conventional ELISA for POC diagnostics.

We report a very easy and effective approach for synthesizing unique palladium-on-gold supra-nanostructure (Au@Pd-SprNS)-decorated graphene oxide (GO) nanosheets. The SprNSs comprising Au nanorods as core and a unique close-packed assembly of tiny anisotropic Pd nanoparticles (NPs) as shell were homogeneously distributed on the GO surface via electrostatic self-assembly. Compared with the traditional one-pot method for synthesis of metal NPs on GO sheets, the size and shape of core–shell Au@Pd SprNSs can be finely controlled and uniformly distributed on the GO carrier. Interestingly, this Au@Pd-SprNSs/GO nanocomposite displayed high electrocatalytic activities toward the oxidation of methanol, ethanol, and formic acid, which can be attributed to the abundance of intrinsic active sites including high density of atomic steps, ledges and kinks, Au–Pd heterojunctions and cooperative action of the two metals of the SprNSs. Additionally, uniform dispersion of the SprNSs over the GO nanosheets prevent agglomeration between the SprNSs, which is of great significance to enhance the long-term stability of catalyst. This work will introduce a highly efficient Pd-based nanoelectrocatalyst to be used in fuel cell application.

Carcinoembryonic antigen (CEA) is recognized as a disease biomarker to reflect the existence of various cancers and tumors in the human body. Sensitive detection of CEA in body fluid is valuable for clinical diagnosis and treatment assessment of cancers. Herein, we present a new approach for ultrasensitive determination of CEA in human serum based on localized surface plasmon resonance (LSPR) enhanced electrochemiluminescence (ECL) of Ru(bpy)32+. In this surface-enhanced ECL (SEECL) sensing scheme, Ru(bpy)32+-doped SiO2 nanoparticles (Ru@SiO2) act as ECL luminophores, and AuNPs are used as LSPR source to enhance the ECL signal. Two different kinds of aptamers specific to CEA are modified on the surface of Ru@SiO2 and AuNPs, respectively. In the presence of CEA, a multilayer of Ru@SiO2–AuNPs nanoarchitectures would be formed. Our investigation reveals that the ECL signal of Ru@SiO2 can be effectively enhanced by AuNPs. One layer of Ru@SiO2–AuNPs nanoarchitectures would generate about 3-fold ECL enhancement compared with the ECL of the nanoarchitectures without the presence of AuNPs. As much as 30-fold ECL enhancement could be obtained by a multilayer of Ru@SiO2–AuNPs nanoarchitectures. Under the optimal conditions, a detection limit of 1.52 × 10–6 ng/mL of CEA in human serum was achieved. To the best of our knowledge, CEA assays with such a low LOD have never been reported for an ECL sensor.

Colorimetric sensors based on assembly or disassembly of isotropic gold nanoparticle (AuNP) aggregates have been frequently reported. Herein we demonstrate the first colorimetric aptasensor for sensitive detection of ochratoxin A, one of the most abundant food-contaminating mycotoxins, based on disassembly of aggregates of oriented AuNP dimers.

•Recent progress on different strategies for plasmonic nanosensors is highlighted.•An applications-oriented perspective in plasmonic nanosensors is provided.•The advantages and disadvantages of current detection platforms are discussed.Based on the localized surface plasmon resonance (LSPR) of metallic nanoparticles, plasmonic nanosensors have emerged as a powerful tool for biosensing applications. Many detection schemes have been developed and the field is rapidly growing to incorporate new methodologies and applications. Amidst all the ongoing research efforts, one common factor remains a key driving force: continued improvement of high-sensitivity detection. Although there are many excellent reviews available that describe the general progress of LSPR-based plasmonic biosensors, there has been limited attention to strategies for improving the sensitivity of plasmonic nanosensors. Recognizing the importance of this subject, this review highlights recent progress on different strategies used for improving the sensitivity of plasmonic nanosensors. These strategies are classified into the following three categories based on their different sensing mechanisms: (1) sensing based on target-induced local refractive index changes, (2) colorimetric sensing based on LSPR coupling, and (3) amplification of detection sensitivity based on nanoparticle growth. The basic principles and cutting-edge examples are provided for each kind of strategy, collectively forming a unifying framework to view the latest attempts to improve the sensitivity of nanoplasmonic sensors. Future trends for the fabrication of improved plasmonic nanosensors are also discussed.Figure optionsDownload full-size imageDownload high-quality image (135 K)Download as PowerPoint slide

•We demonstrate the first biosensor based on disassembly of the orient-aggregated AuNP dimers.•The sensitivity and stability of the proposed sensor are much better than those non-oriented sensors based on the formation of large AuNP aggregates.•The target molecules can be expended to arbitrary molecules that can specifically bind to aptamers.•The color change is completed within 5 min.Recently we demonstrated oriented formation of gold nanoparticle (AuNP) dimers for ultrasensitive sensing oligonucleotides (J. Am. Chem. Soc. 2013, 135, 12338). Herein, we investigate the reverse process of this sensing mechanism using target analytes to disassemble the orient-aggregated AuNP dimers. This enables us to expand the analytes from oligonucleotides to other molecules, e.g. highly sensitive and selective determination of microcystin-LR (MC-LR) is selected for a demonstration in this work. Aptamers specific to the target molecules are used as linkers to prepare the AuNP dimers. In the presence of the target molecule, the aptamer changes its structure to bind the target molecule. Thus the pre-formed AuNP dimers are disassembled. As a result, the solution color is changed from blue to red. This sensing design retains the advantages of the previously developed sensors based on target molecules guided formation of AuNP dimers, e.g. the overwhelming sensitivity and stability comparing with those non-oriented sensors based on the formation of large aggregates, with the additional advantages as follows: 1) the target molecules are expanded from oligonucleotides to arbitrary molecules that can specifically bind to aptamers; 2) the color change is completed within 5 min, while the previous sensor based on the formation of AuNP dimers cost ~1 hour to obtain stable responses.

•Partially functionalized AuNPs were synthesized under the assistance of CTAB.•A one-step synthesis of AuNP dimers with high yield was demonstrated.•The method could be generalized to other surfactants and nanoparticles.Herein, we report a versatile solid phase synthesis approach for partial functionalization of gold nanoparticles (AuNPs) via the assistance of surfactant. Hexadecyl trimethyl ammonium bromide (CTAB) is used as the bifunctional ligand to link the solid substrate and the AuNPs. This brings at least two advantages comparing to other bifunctional ligands: first, the thickness of the CTAB bilayer is flexible. During the “catch” process, the bilayer bound on different sites of the AuNPs could shrink or extend to a total thickness difference of ∼2.5 nm, which protects a spherical cap of the AuNPs with the height of ∼2.5 nm from chemical modification; second, after chemical modification, the “release” of the AuNPs from the substrate is quite simple. Not any second ligand, but only sonication is enough to release the AuNPs. As a result, the proposed approach produces partially functionalized AuNPs with a large surface area (80–95%) covered with unreactive ligands, while keeps the other small spatially limited region of the surface unmodified. This small unmodified surface can be directly used for further chemistry. Finally, the partially modified AuNPs is demonstrated for a one-step synthesis of AuNP dimers.Graphical abstract

•The first LSPR sensor based on weak enantioselective receptors was reported.•The results showed that 500 times enantioexcess can be accurately distinguished.•It could effectively expend the applicable receptors for enantioselective sensors.Chiral recognition based on enantioselective sensors is superior to conventional chromatographic enantioseparation techniques in terms of simplicity and rapidity. Normally, highly specific enantioselective receptors are used for the fabrication of enantioselective sensors. However, to date there only limited number of highly specific chiral selectors are reported, which greatly confines the development of enantioselective sensors. Herein, we demonstrate the feasibility of using relatively weak chiral selectors to construct an enantioselective biosensor for accurate chiral discrimination of enantiomers. The detection of racemic mixture of (R)- and (S)-1,2,3,4-Tetrahydro-1-naphthylamine (TNA) was demonstrated as an example. The sensor was made up of a dual-channel microfluidic chip. One channel of the chip was modified with human serum albumin (HSA), which was reported to be a weak chiral selector for TNA; while the other channel was modified with a monoclonal anti-TNA antibody, which was a non-enantioselective TNA receptor. A portable localized surface plasmon resonance (LSPR) detection system was integrated with the microfluidic chip to accomplish the signal collection. Our investigation revealed that the combination of LSPR responses obtained from the two channels can be used for quantitative discrimination of the (R)- and (S)-TNA. The limit of detection was found to be 150 nM for (R)-TNA and 100 nM for (S)-TNA. The feasibility of use relatively weak chiral selectors could potentially promote the development of various enantioselective sensors.

The impact of chiral compounds on pharmacological and biological processes is well known. With the increasing need for enantiomerically pure compounds, effective strategies for enantioseparation and chiral discrimination are in great demand. Herein we report a simple but efficient approach for the enantioselective determination of chiral compounds based on a localized surface plasmon resonance (LSPR) biosensor integrated with a microfluidic chip. A glass microfluidic chip with an effective volume of ∼0.75 μL was fabricated for this application. Gold nanorods (AuNRs) with an aspect ratio of ∼2.6 were self-assembled onto the surface of the inner wall of the chip to serve as LSPR transducers, which would translate the analyte binding events into quantitative concentration information. Human α-thrombin was immobilized onto the AuNR surface for enantioselective sensing of the enantiomers of melagatran. The proposed sensor was found to be highly selective for RS-melagatran, while the binding of its enantiomer, SR-melagatran, to the sensor was inactive. Under optimal conditions, the limit of detection of this sensor for RS-melagatran was found to be 0.9 nM, whereas the presence of 10000-fold amounts of SR-melagatran did not interfere with the detection. To the best of our knowledge, this is the first demonstration of an LSPR-based enantioselective biosensor.