A dual-channel sensor array for discrimination of biothiols based on manganese dioxide nanosheets.

A dual-signal sensor array for highly sensitive identification of biothiols is reported based on different optical responses of MnO2/curcumin (CUR) system to different biothiols. The addition of MnO2 nanosheets (MnO2 NSs) quenches the fluorescence of CUR, and the color of the mixture changes from yellow to brown. In the presence of reductive biothiols, MnO2 NSs are etched and lose their fluorescence quenching ability, resulting in an increase in the fluorescence intensity of CUR at 540 nm and a decrease in the absorbance at 430 nm. The sensor array generates specific response modes based on the varying reduction abilities of different biothiols, which can be distinguished by linear discriminant analysis (LDA). The sensor array successfully distinguished five biothiols (glutathione (GSH), dithiothreitol (DTT), cysteine (Cys), mercaptoethanol (ME), and homocysteine (Hcy)) across a wide concentration range (1 µM-100 µM) and biothiol mixtures with varing molar ratios.

Fabrication of glass-based analytical devices by immobilizing nanomaterials on glass substrate with a fluorescent glue for the highly sensitive determination of mercury ions.

A facile method is reported to develop glass-based analytical devices (GADs) based on immobilizing nanomaterials on a glass substrate with fluorescent glue. The fluorescent glue was first prepared by coupling bovine serum albumin (BSA)-protected Au nanoclusters (NCs) and sugars (i.e., ascorbic acid, AA). The glue was then used to immobilize carbon dots (C-dots) on glass substrates to fabricate the portable GADs. The liquid glue-C-dots mixture and probable GADs were developed for Hg2+ detection. Under 365-nm excitation wavelength, the emission at 652 nm from the glue is gradually quenched with increasing concentrations of Hg2+. This quenching is explained in terms of the Stern-Volmer equation and is ascribed to static quenching. The fluorescent color of the glue and GADs gradually changes from pink to blue, with increasing concentrations of Hg2+. The limits of detection (LODs) for Hg2+ determination by bare eyes are 1 nM both for the glue and GADs, suggesting an uncompromised sensing capability even after immobilization. The detection sensitivity of GADs shows a significant improvement compared with the same material-based papers (5 µM). A linear relationship is observed between the total Euclidean distances (EDs) and Hg2+ concentration in the range 0-100 nM, providing the potential for Hg2+ quantification using GADs. The LOD is estimated to be 0.84 nM. To show a potentially practical application, the GADs were used to detect Hg2+ in certified reference material and lake water.

CsPbBr3 and CsPbBr3/SiO2 Nanocrystals as a Fluorescence Sensing Platform for High-Throughput Identification of Multiple Thiophene Sulfides.

Air pollution is a serious problem. Refractory thiophene sulfides, which cause air pollution, bring great challenges to their rapid and accurate identification. In this work, we propose a fluorescent sensor array based on two perovskite nanocrystals (CsPbBr3 NCs and CsPbBr3/SiO2 NCs) to distinguish different thiophene sulfides. The hydrogen bonding force between the thiophenics of thiophene sulfides and the amino groups of the perovskite NCs results in the weakening of the fluorescence signals of the perovskite NCs. The diverse interactions between thiophene sulfides and two perovskite NCs provide rich information, which can be obtained on the sensor array and identified by linear discriminant analysis. Five thiophene sulfides (i.e., benzothiophene, dibenzothiophene, 2-methylbenzothiophene, 3-methylthiophene, and thiophene) were discriminated by the sensor array at concentrations of 10-50 ppm. The effectiveness of the sensor array was further verified in the discrimination of blinded samples, in which all 10 samples were correctly identified. In addition, it is gratifying that even binary mixtures of thiophene sulfides could be distinguished by the proposed sensor array.

A rapid reduction of Au(I→0) strategy for the colorimetric detection and discrimination of proteins.

A gold nanoparticle (AuNP)-based sensing strategy based on rapid reduction of Au(I→0) is proposed. As a proof-of-concept study, the proposed sensing principle is designed for simultaneous and colorimetric detection and discrimination of multiple proteins. In the presence of H2O2, the target proteins could reduce Au(I) (i.e. HAuCl2) to AuNPs with different sizes, shapes and dispersion/aggregation states, thus resulting in rapidly colorimetric identification of different proteins. The optical response (i.e. color) of AuNPs is found to be characteristic of a given protein. The color response patterns are characteristic for each protein and can be quantitatively differentiated by statistical techniques. The sensor array is capable of discriminating proteins at concentrations as low as 0.1 µg/mL with high accuracy. A linear relationship was observed between the total Euclidean distances and protein concentration, providing the potential for protein quantification using this sensor array. The limit of detection (LOD) for catalase (Cat) is 0.08 µg/mL. The good linear range (from 0 to 8 µg/mL) has been used for the quantitative assay of Cat. To show a potentially practical application, this method was used to detect and discriminate proteins in human urine and tear samples. Graphical abstract We report a facile gold nanoparticle (AuNP)-based sensing strategy, that is, "a rapid reduction of Au(I) to Au(0) nanoparticles with different sizes and shapes by analytes that having certain reducing capabilities, resulting in different colours." The proposed sensing principle is designed for simultaneous, colorimetric detection and discrimination of multiple proteins.

Sub-10-nm multicolored gold nanoparticles for colorimetric determination of antibiotics via formation of interlocking rings.

A general approach is presented for synthesis of multicolored gold nanoparticles (GNPs) by Au(I)-mediated generation of interlocking rings in proteins and antibiotics. The Au(I) ions are shuttled from proteins to antibiotics, and this causes the formation of interlocking rings. The multicolored GNPs of different sizes were synthesized in the rings by using the rapid nucleation method. To take the unique colors of GNPs, a functional array was designed for the colorimetric determination and discrimination of antibiotics, specifically of amoxicillin, chlortetracycline, erythromycin, spiramycin, neomycin, thiamphenicol, gentamycin and lincomycin. The method is based on the "three color" (RGB) principle. The color response patterns are characteristic for each antibiotic and can be quantitatively differentiated by statistical techniques. The limits of detection (LOD, at S/N = 3) for spiramycin (Sp) have been calculated to be 0.18 µM and 0.10 µM in water and milk, respectively. The good linear range (from 0.3 to 3.5 µM) has been used for the quantitative assay of Sp in a certified reference material. Graphical abstractSchematic presentation of gold nanoparticles (GNPs) synthesis via formation of interlocking rings in protein and antibiotics. The Au(I) ions mediate protein and antibiotics to be interlocking rings, which are quickly fixed via microwave reaction. The GNPs are synthesized and assembled in the rings.

ORF1a of highly pathogenic PRRS attenuated vaccine virus plays a key role in neutralizing antibody induction in piglets and virus neutralization in vitro.

BACKGROUND: Currently, porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most economically important viral pathogens in swine in most countries, especially China. Two PRRSV attenuated live vaccine strains (HuN4-F112 and CH-1R) are currently widely used in China. Our previous study showed that HuN4-F112, but not CH-1R, induced high anti-nucleocapsid (N) antibody and neutralizing antibody (NA) titers. Additionally, sera from HuN4-F112 inoculated pigs induced low cross neutralization of CH-1R. METHODS: In the present study, 6 chimeric viruses through exchanging 5' untranslated region (UTR) + open reading frame (ORF)1a, ORF1b, and ORF2-7 + 3'UTR between HuN4-F112 and CH-1R were constructed and rescued based on the infectious clones of rHuN4-F112 and rCH-1R. The characteristics of these viruses were investigated in vitro and vivo. RESULTS: All the three fragments, 5'UTR + ORF1a, ORF1b, and ORF2-7 + 3'UTR, could affect the replication efficiencies of rHuN4-F112 and rCH-1R in vitro. Additionally, both 5'UTR + ORF1a and ORF2-7 + 3'UTR affected the anti-N antibody and NA responses targeting rHuN4-F112 and rCH-1R in piglets. CONCLUSIONS: The 5'UTR + ORF1a region of HuN4-F112 played a key role in inducing NAs in piglets. Furthermore, we confirmed for the first time that ORF1a contains a neutralization region. This study provides important information that can be used for further study of the generation of anti-PRRSV NAs.

Synthesis of Gold Nanoparticles from Au(I) Ions That Shuttle To Solidify: Application on the Sensor Array Design.

Metal-mediated interlocking rings won the 2016 Nobel Prize in Chemistry. The metal-directed interlocking rings in macromolecular systems (e.g., proteins) may be similar to the form of Maxwell's electromagnetic waves; the metal ions may shuttle among the rings in the special environment. To verify this hypothesis, we designed a general approach to synthesize the multicolored gold nanoparticles (GNPs) mediated by Au(I)-directed interlocking rings in proteins. The Au(I) ions shuttled among these interlocking rings in the strong alkaline solution. Through the rapid nucleation method, the multicolored GNPs of different morphology and sizes were synthesized in the multiple honeycombed templates. On the basis of the "three-color" principle of Thomas Young, we extracted the red, green, and blue (RGB) alterations of GNPs to fabricate a visual sensor array for protein discrimination. The fingerprints (ΔRGB) were obtained from the target proteins and fed into computer programs. The proposed sensing platform was also applied to detect lysozyme in human tears with satisfactory results. Importantly, we forecasted that lysozyme could be the effective drug for curing dacryocystitis and nasolacrimal duct obstruction diseases.

Platinum single atom on CsPbBr3 nanocrystals as electrocatalyst boosts electrochemical sensing of ascorbic acid.

Monitoring ascorbic acid (AA) levels in human body can provide valuable clues for disease diagnosis. Anchoring noble metal single atoms on perovskite substrate is a promising strategy to design electrocatalysts with outstanding electrocatalytic performance. Herein, we design an electrochemical method for detecting AA by utilizing Pt single atoms-doped CsPbBr3 nanocrystals (Pt SA/CsPbBr3 NCs) fixed on a glassy carbon electrode as an electrochemical catalyst. The uncharged 3,5,3',5'-tetramethylbenzidine (TMB) undergoes oxidation to form the positively charged oxidized TMB (oxTMB) owing to the exceptional electrochemical catalytic performance of Pt SA/CsPbBr3 NCs. Subsequently, the target AA reduces oxTMB to TMB, which is then electrocatalytically oxidized to oxTMB, producing significant oxidation current. In this way, such characteristic provides a sensitive electrochemical strategy for AA detection, achieving a concentration range of 50-fold with the detection limit of 0.0369 µM. The developed electrochemical method also successfully generates accurate detection response of AA in complex sample media (urine). Overall, this approach is expected to offer a novel way for early disease diagnosis.

Tailoring the peroxidase-like properties of Mo atom nanoclusters/N-MXene nanozymes for sensitive colorimetric detection of glutathione.

Although nanozyme engineering has made tremendous progress, there is a huge gap between them and natural enzymes due to the enormous challenge of precisely adjusting the geometric and electronic structure of active sites. Considering that intentionally adjusting the metal-carrier interactions may bring the promising catalytic activity, in this work, a novel Mo atom nanocluster is successfully synthesized using nitrogen-doped Mxene (MoACs/N-MXene) nanozymes as carriers. The constructed MoACs/N-MXene displays excellent peroxidase-like catalytic activity and kinetics, outweighing its N-MXene and Mo nanoparticles (NPs)-MXene references and natural horse radish peroxidase. This work not only reports a successful example of MoACs/N-MXene nanozyme as a guide for achieving peroxidase-mimic performance of nanozymes for colorimetric glutathione sensing at 0.29 µM, but also expands the application prospects of two-dimensional MXene nanosheets by reasonably introducing metal atomic clusters and nonmetal atom doping and exploring related nanozyme properties.

Colorimetric response of dithizone product and hexadecyl trimethyl ammonium bromide modified gold nanoparticle dispersion to 10 types of heavy metal ions: understanding the involved molecules from experiment to simulation.

A new kind of analytical reagent, hexadecyl trimethyl ammonium bromide (CTAB), and dithizone product-modified gold nanoparticle dispersion, is developed for colorimetric response to 10 types of heavy metal ions (M(n+)), including Cr(VI), Cr(3+), Mn(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), Hg(2+), and Pb(2+). The color change of the modified gold nanoparticle dispersion is instantaneous and distinct for Mn(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), Hg(2+), and Pb(2+). The color change results from the multiple reasons, such as electronic transitions, cation-π interactions, formation of coordination bonds, and M(n+)-induced aggregation of gold nanoparticles (AuNPs). The different combining capacity of heavy metal ions to modifiers results in the different broadening and red-shifting of the plasmon peak of modified AuNPs. In addition, Cr(VI), Cu(2+), Co(2+), Ni(2+), and Mn(2+) cause the new UV-vis absorption peaks in the region of 360-460 nm. The interactions between the modifiers and AuNPs, and between the modifiers and M(n+), are investigated by using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The results confirm that AuNPs are modified by CTAB and dithizone products through electrostatic interactions and Au-S bonds, respectively, and the M(n+)-N bonds form between M(n+) and dithizone products. Furthermore, the experimental and density functional theory calculated IR spectra prove that dithizone reacts with NaOH to produce C6H5O(-) and [SCH2N4](2-). The validation of this method is carried out by analysis of heavy metal ions in tap water.

Cobalt-Nickel Layered Double Hydroxides on Electrospun MXene for Superior Asymmetric Supercapacitor Electrodes.

Flexible electrodes for energy storage and conversion require a micro-nanomorphology and stable structure. Herein, MXene fibers (MX-CNF) are fabricated by electrospinning, and Co-MOF nanoarrays are prepared on the fibers to form Co-MOF@MX-CNF. Hydrolysis and etching of Co-MOF@MX-CNF in the Ni2+ solution produce cobalt-nickel layered double hydroxide (CoNi-LDH). The CoNi-LDH nanoarrays on the MX-CNF substrate have a large specific surface area and abundant electrochemical active sites, thus ensuring effective exposure of the CoNi-LDH active materials to the electrolyte and efficient pseudocapacitive energy storage and fast reversible redox kinetics for enhanced charging-discharging characteristics. The CoNi-LDH@MX-CNF electrode exhibits a discharge capacity of 996 F g-1 at a current density of 1 A g-1 as well as 78.62% capacitance retention after 3,000 cycles at 10 A g-1. The asymmetric supercapacitor (ASC) comprising the CoNi-LDH@MX-CNF positive electrode and negative activated carbon electrode shows an energy density of 48.4 Wh kg-1 at a power density of 499 W kg-1 and a capacity retention of 78.9% after 3,000 cycles at a current density of 10 A g-1. Density-functional theory calculations reveal the charge density difference and partial density of states of CoNi-LDH@MX-CNF confirming the large potential of the CoNi-LDH@MX-CNF electrode in energy storage applications.

Rupture mechanism of aromatic systems from graphite probed with molecular dynamics simulations.

Intermolecular interactions involving aromatic rings are of pivotal importance in many areas of chemistry, biology and materials science. Mimicking recent atomic force microscopy (AFM) experiments that measured the adhesion forces of single pi-pi complexes, here interactions between pyrene/coronene and graphite have been probed by force-probe molecular dynamics (FPMD) simulations. The pyrene or coronene molecule was connected to a virtual spring through a flexible poly(ethylene glycol) (PEG) linker and was pulled away from graphite in water under constant velocity. Pyrene and coronene showed similar unbinding pathways featuring four states, with a transition and an intermediate state connecting the bound and unbound states in terms of distance and interplanar angles. Transient conformations with tilted orientations (approximately 40 degrees) and with one side of the aromatic structure still in contact with the graphite surface (approximately 70 degrees) were identified as the transition and intermediate states, respectively, similar to previously observed perpendicularly stacked benzene dimers. The distance to transition state x(tr) was determined to be 0.23 +/- 0.03 nm both for pyrene/graphite and coronene/graphite. The complexes share similar unbinding pathways, but coronene binds to graphite more strongly than to pyrene.

Simple, rapid, sensitive, selective and label-free lincomycin detection by using HAuCl4 and NaOH.

A simple, rapid, sensitive, selective and label-free method is presented for the colorimetric determination of lincomycin (Lin) by using HAuCl4 and NaOH. Upon the addition of Lin, the mixture of HAuCl4 and NaOH shows a color change from colorless to blue (or dark blue). The limit of colorimetric detection is as low as 1 µM, observed both in Milli-Q water and real samples. The selectivity of Lin detection is excellent compared with 9 other common antibiotics. On the basis of the "three-color" principle of Thomas Young, we extracted the red, green and blue (RGB) alterations of the sensor in the absence and presence of different concentrations of Lin. The color changes are quantitatively illustrated by the total Euclidean distances (EDs = [ΔR2 + ΔG2 + ΔB2]1/2). The good linear relationship between the EDs and Lin concentration is used for the quantitative assay of Lin. The developed method demonstrates great potential for the detection of Lin in environmental water and milk.

A fluorescent carbon-dots-based mitochondria-targetable nanoprobe for peroxynitrite sensing in living cells.

Mitochondria, the power generators in cell, are a primary organelle of oxygen consumption and a main source of reactive oxygen/nitrogen species (ROS/RNS). Peroxynitrite (ONOO-), known as a kind of RNS, has been considered to be a significant factor in many cell-related biological processes, and there is great desire to develop fluorescent probes that can sensitively and selectively detect peroxynitrite in living cells. Herein, we developed a fluorescent carbon-dots (C-dots) based mitochondria-targetable nanoprobe with high sensitivity and selectivity for peroxynitrite sensing in living cells. The C-dots with its surface rich in amino groups was synthesized using o-phenylenediamine as carbon precursor, and it could be covalently conjugated with a mitochondria-targeting moiety, i.e. triphenylphosphonium (TPP). In the presence of peroxynitrite, the fluorescence of the constructed nanoprobe (C-dots-TPP) was efficiently quenched via a mechanism of photoinduced electron transfer (PET). The nanoprobe exhibited relatively high sensitivity (limit of detection: 13.5nM) and selectivity towards peroxynitrite in aqueous buffer. The performance of the nanoprobe for fluorescence imaging of peroxynitrite in mitochondria was investigated. The results demonstrated that the nanoprobe showed fine mitochondria-targeting ability and imaging contrast towards peroxynitrite in living cells. We anticipate that the proposed nanoprobe will provide a facile tool to explore the role played by peroxynitrite in cytobiology.

Thermal treatment of hair for the synthesis of sustainable carbon quantum dots and the applications for sensing Hg2.

A facile, simple and low-cost approach for synthesizing highly fluorescent carbon quantum dots (CQDs) from thermal treatment of sustainable hair has been developed. The resultant CQDs exhibited strong blue emission with a quantum yield of 10.75%, excellent photostability and high stability in high salt conditions. As the fluorescence of CQDs can be efficiently quenched by Hg2+, the CQDs can be constructed as a nanosensor for Hg2+ with good sensitivity and selectivity. And as low as 10 nM Hg2+ can be successfully detected.

Single-indicator-based Multidimensional Sensing: Detection and Identification of Heavy Metal Ions and Understanding the Foundations from Experiment to Simulation.

Multidimensional sensing offers advantages in accuracy, diversity and capability for the simultaneous detection and discrimination of multiple analytes, however, the previous reports usually require complicated synthesis/fabrication process and/or need a variety of techniques (or instruments) to acquire signals. Therefore, to take full advantages of this concept, simple designs are highly desirable. Herein, a novel concept is conceived to construct multidimensional sensing platforms based on a single indicator that has capability of showing diverse color/fluorescence responses with the addition of different analytes. Through extracting hidden information from these responses, such as red, green and blue (RGB) alterations, a triple-channel-based multidimensional sensing platform could consequently be fabricated, and the RGB alterations are further applicable to standard statistical methods. As a proof-of-concept study, a triple-channel sensing platform is fabricated solely using dithizone with assistance of cetyltrimethylammonium bromide (CTAB) for hyperchromicity and sensitization, which demonstrates superior capabilities in detection and identification of ten common heavy metal ions at their standard concentrations of wastewater-discharge of China. Moreover, this sensing platform exhibits promising applications in semi-quantitative and even quantitative analysis individuals of these heavy metal ions with high sensitivity as well. Finally, density functional theory calculations are performed to reveal the foundations for this analysis.

Protein-directed synthesis of highly monodispersed, spherical gold nanoparticles and their applications in multidimensional sensing.

An in-situ reduction method has been reported to prepare gold nanoparticles (GNPs) of 40-110 nm by using the green reducing agents of proteins, which are activated by H2O2 and the superoxide anion (). The protein of collagen turns HAuCl4 to the aqueous Au(I) ainions, which are further reduced by other proteins to be highly monodispersed and spherical GNPs of different sizes. The GNPs reduced by different proteins are found to be with the exposed {100} facets, the distinctive UV-vis absorption spectra and various colors (See Fig. 1). By means of extracting the color responses, such as red, green and blue (RGB) alterations, an in-situ reduction method-based multidimensional sensing platform is fabricated in the process of GNPs synthesis. Without further modification of GNPs, nine common proteins are found to be well detected and discriminated at different concentrations. Moreover, this sensing platform also demonstrates great potentials in qualitative and semiquantitative analysis on the individuals of these proteins with high sensitivity. Furthermore, the validation of this multidimensional sensing platform has been carried out by analysis on the spiked proteins in human urine and the target proteins in complex matrix (e.g. lysozyme in human tear).

Gold-nanoparticle-based colorimetric array for detection of dopamine in urine and serum.

A highly selective method is presented for the colorimetric determination of dopamine (DA) using gold nanoparticles (AuNPs). DA caps on the surface of AuNPs (DA-AuNPs) induces the aggregation of AuNPs in alkaline solution. The DA-AuNPs are modified by the hydrolysate of thioglycolic acid (TGA(2)(-)) through Au-S bonds. The aggregation of AuNPs is accelerated by TGA(2-), due to the strong hydrogen-bonds (NH⋯OC and OH⋯OC) formed between TGA(2)(-) and DA. Upon the addition of DA, the solution shows a color change from red to purple (or yellow), which is also monitored to detect DA in human urine and fetal bovine serum samples. Here, the limits of colorimetric detection are as low as 10(-7)M observed in Milli-Q water, urine and serum. Based on UV-vis absorption spectra, the limits of detection have been calculated to be 3.3×10(-8)M, 1.0×10(-7)M and 9.4×10(-8)M in Milli-Q water, urine, and serum, respectively. All the limits of detection are lower than the lowest abnormal concentrations of DA in urine (5.7×10(-7)M) and blood (1.6×10(-5)M). The good linear ranges from 0 to 10(-6)M are used for the quantitative assay of DA in urine and serum samples. The applicability of our detection system is also verified by analysis of DA in urine and serum samples. The developed approach is without using complex financial instruments.

The colorimetric detection of Pb2+ by using sodium thiosulfate and hexadecyl trimethyl ammonium bromide modified gold nanoparticles.

A simple, rapid colorimetric detection method for Pb(2+) in aqueous solution has been developed by using sodium thiosulfate (Na2S2O3) and hexadecyl trimethyl ammonium bromide (CTAB) modified gold nanoparticles (Au NPs). Na2S2O3 was added into the Au NP solution and thiosulfate ions (S2O3(2-)) were adsorbed on the surface of the Au NPs due to electrostatic interactions. Au atoms on the surface of the Au NPs were then oxidized to Au(i) by the O2 that existed in the solution in presence of thiosulfate. The addition of Pb(2+) (the final concentration was lower than 10 µM), accelerated the leaching of the Au NPs, and Pb-Au alloys also formed on the surface of the Au NPs. There was an obvious decrease in the surface plasmon resonance (SPR) absorption of the Au NPs. The lowest concentration for Pb(2+) that could be detected by the naked eye was 0.1 µM and using UV-vis spectroscopy was 40 nM. This is lower than the lead toxic level defined by the US Environmental Protection Agency (US EPA), which is 75 nM. In this method, CTAB, as a stabilizing agent for Au NPs, can accelerate the adsorption of S2O3(2-) on the surface of the Au NPs, which shortened the detection time to within 30 min. Moreover, this detection method is simple, cheap and environmentally friendly.

A rapid and sensitive colorimetric assay method for Co2+ based on the modified Au nanoparticles (NPs): understanding the involved interactions from experiments and simulations.

We previously reported a colorimetric assay method for Co(2+) based on the thioglycolic acid (TGA) functionalized hexadecyl trimethyl ammonium bromide (CTAB) modified Au NPs. However, the detection limit of 3×10(-7) M was still higher than that of the sanitary standard for drinking water (6.8×10(-8) M). In addition, the interactions between the modifier and Au NPs, and between the modifier-Au NPs and Co(2+) remain to be clarified and confirmed. Thus, in the present study, the modified Au NPs solution was dialyzed and its detection limit was optimized to be 5×10(-10) M. The interactions between the modifier and Au NPs, and between the modifier-Au NPs and Co(2+) were investigated in both experimental characterizations and theoretical calculations, consistently confirming that the Au NPs were modified by the negatively charged anions of [SCH(2)CO(2)](2-) through Au-S bonds and Co(2+) was recognized by the modifier-Au NPs through Co-O chelate bonds. The results of X-ray photoelectron spectroscopy (XPS) suggest that there were no chemical bonds formed between CTAB and Co(2+). Moreover, the colorimetric assay of Co(2+) using the modified Au NPs has been proved to be a rapid, very sensitive and highly selective method. The validation of the method was carried out by analysis of a certified reference material, GSBZ 50030-94.