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Mercury poses a significant threat to human health and the environment, and the United States Environmental Protection Agency has set a drinking water threshold of mercury (10 nM). Therefore, developing an environmentally friendly, on-site mercury ion (Hg2+) detection is highly desirable. Based on the synergetic catalysis of gold nanoparticles (AuNPs) and mercury, a simple and ultrasensitive colorimetric method for the determination of Hg2+ was established. The innovation of this work is to propose a strategy based on the self-assembly of two-dimensional (2D) AuNPs for colorimetric detection of Hg2+. The AuNPs are self-assembled into a densely arranged 2D AuNP layer at the liquid/liquid interface between dimethyl carbonate and water, which converts from the disordered distribution of AuNPs in 3D space to an orderly 2D AuNP layer distribution. This avoids the shortcomings of poor repeatability and weak detection signals caused by the uneven sampling before and greatly plays the catalytic role of each AuNP. In this work, gold amalgam catalyzes colorless 3,3′,5,5′-tetramethylbenzidine (TMB) and hydrogen peroxide (H2O2) to yield blue-green oxidized TMB (oxTMB), realizing the colorimetric quantitative detection of Hg2+ in water systems. In this process, the formation of gold amalgam greatly improves the catalytic ability of AuNPs, and the detection limit of mercury ions is as low as 0.021 nM. The practical application of this sensor in the determination of Hg2+ in tap water samples has also been successfully verified, providing an effective method for environmental monitoring. [ABSTRACT FROM AUTHOR]

This report describes dual-channel colorimetric array sensors for rapid distinction of metal ions. This dual-channel sensing array is built based upon gold nanoparticles (AuNPs)-catalyzed two colorimetric reactions [3,3′,5,5′-tetramethylbenzidine (TMB) and H2O2 as channel 1 and HAuCl4 and NH2OH as channel 2]. Different metal ions show diverse interactions toward AuNPs, producing differential variations of AuNPs' catalytic ability and yielding distinct colorimetric response modes (channel 1). Different metal ions show different affinities toward the DNA absorbed on the AuNP surface, and the decrease of the amount of DNA entangled on the AuNPs changes the catalytic ability of the AuNPs to varying degrees, generating varying visual response modes (channel 2). These signal modes as "fingerprints" are typical for each metal ion and are analyzed by linear discriminant analysis (LDA). The results display clear discrimination of each analyte on a LDA plot, and 10 kinds of metal ions are well identified at a low concentration (20 nM). [ABSTRACT FROM AUTHOR]

As a low-cost support, MXenes have attracted more and more attention in developing nanoenzymes with enhanced peroxidase- or oxidase-like activity. In this paper, we report a simple way to synthesize a N-doped Ti3C2Tx MXene, which provides a support platform for developing MXene-based nanozymes with an attractive structure and performance. By anchoring copper nanoparticles on N-doped Ti3C2Tx Cu nanoparticles, Cu nanoparticles (NPs)/N-doped Ti3C2Tx with satisfactory structural stability was prepared, displaying a synergistic effect and enhanced peroxidase-like activity. The superior peroxidase-like activity was confirmed by the oxidation kinetics of 3,3′,5,5′-tetramethylbenzidine (TMB) and density functional theory calculations. The characteristics of peroxidase-like nanozymes have promoted the development of colorimetric biosensors to evaluate the performance of Cu NPs/N-doped Ti3C2Tx. As expected, the nanozyme exhibits gratifying sensitivity, high selectivity, a low detection limit of 5 μM, and practicability in the detection of serum samples. Remarkably, the Cu NPs/N-doped Ti3C2Tx has remarkable stability and maintains its peroxidase-like activity after 1 month of storage. This work has laid a solid foundation for guiding the design of metal NPs/nonmetal-doped MXenes with outstanding peroxidase-like activity and expanding their application in biosensors. [ABSTRACT FROM AUTHOR]

A series of single-strand oligonucleotides functionalized catalytically active gold nanoparticle (AuNPs) as nonspecific receptors have been designed to build a protein sensing array. We take advantage of the correlation between the catalytic activity and the exposed surface area of AuNPs, i.e., DNA-proteins interactions mask the surface area of AuNPs, leading to poor catalytic performance of AuNPs. As the number of DNA-bound proteins increases, the surfaces of AuNPs become more masked; thus, the time of 4- nitrophenol/NaBH4 reaction for color change (yellow ? colorless) of the solution increases. Taking advantage of three nonspecific SH-labeled DNA sequences (A15, C15, and T15) as array sensing elements and the color-change time (CCT) of the solution as signal readout, colorimetric response patterns can be obtained on the array and identified via linear discriminant analysis (LDA). Eleven proteins have been completely distinguished with 100% accuracy with the naked eye at the 30 nM level. Remarkably, two similar proteins (bovine serum albumin and human serum albumin), two different proteins (bovine serum albumin and concanavalin) at the same concentration, and the mixtures of the two proteins with different molar ratios have been discriminated with 100%. The practicability of this sensor array is further validated by high accuracy (100%) identification of 11 proteins in human serum samples. [ABSTRACT FROM AUTHOR]

Nanozymes with high activity and stability have emerged as a potential alternative to natural enzymes in the past years, but the relationship between the electronic metal-support interactions (EMSI) and catalytic performance in nanozymes still remains unclear. Herein, a copper nanoparticle nanozyme supported on N-doped Ti 3 C 2 T x (Cu NPs@N-Ti 3 C 2 T x ) is successfully synthesized and the modulation of EMSI is achieved by introducing N species. The stronger EMSI between Cu NPs and Ti 3 C 2 T x , involving electronic transfer and an interface effect, is revealed by X-ray photoelectron spectroscopy, soft X-ray absorption spectroscopy, and hard X-ray absorption fine spectroscopy at the atomic level. Consequently, Cu NPs@N-Ti 3 C 2 T x nanozyme exhibits remarkable peroxidase-like activity, surpassing its counterpart (Cu NPs, Ti 3 C 2 T x , Cu NPs-Ti 3 C 2 T x ), suggesting that EMSI significantly enhances catalytic performance. Benefiting from the excellent performance, the colorimetric platform based on Cu NPs@N-Ti 3 C 2 T x nanozyme for detecting astaxanthin is constructed and shows a wide linear detection range of 0.01-50 μM and a limit of detection of 0.015 μM in the sunscreens. Density functional theory is further conducted to reveal that the excellent performance is ascribed to the stronger EMSI. This work opens an avenue for studying the influence of EMSI toward catalytic performance of nanozyme.

Great enthusiasm in single-atom catalysts for various catalytic reactions continues to heat up. However, the poor activity of the existing single/dual-metal-atom catalysts does not meet the actual requirement. In this scenario, the precise design of triple-metal-atom catalysts is vital but still challenging. Here, a triple-atom site catalyst of FeCoZn catalyst coordinated with S and N, which is doped in the carbon matrix (named FeCoZn-TAC/SNC), is designed. The FeCoZn catalyst can mimic the activity of oxidase by activating O 2 into • O 2 - radicals by virtue of its atomically dispersed metal active sites. Employing this characteristic, triple-atom catalysts can become a great driving force for the development of novel biosensors featuring adequate sensitivity. First, the property of FeCoZn catalyst as an oxidase-like nanozyme was explored. The obtained FeCoZn-TAC/SNC shows remarkably enhanced catalytic performance than that of FeCoZn-TAC/NC and single/dual-atom site catalysts (FeZn, CoZn, FeCo-DAC/NC and Fe, Zn, Co-SAC/NC) because of trimetallic sites, demonstrating the synergistic effect. Further, the utility of the oxidase-like FeCoZn-TAC/SNC in biosensor field is evaluated by the colorimetric sensing of ascorbic acid. The nanozyme sensor shows a wide concentration range from 0.01 to 90 μM and an excellent detection limit of 6.24 nM. The applicability of the nanozyme sensor in biologically relevant detection was further proved in serum. The implementation of TAC in colorimetric detection holds vast promise for further development of biomedical research and clinical diagnosis.

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 (CsPbBr 3 NCs and CsPbBr 3 /SiO 2 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.

Many enrichment techniques for phosphopeptides usually rely on the interaction of phosphate groups with metal ions or metal oxides. Based on this, we innovatively designed and fabricated an electrochemical sensor based on TiO 2 nanoparticles (NPs), which can sensitively and rapidly detect phosphopeptides in protein samples pretreated with AuNPs. When the phosphopeptide solution was pretreated with AuNPs, AuNPs can be linked to the polypeptide chain via the amino group at the tail of the polypeptide chain. When TiO 2 NPs are specifically bound to the phosphate group on the peptide, the modified AuNPs can improve the electron conduction ability of the electrode to detect the phosphopeptides. The designed electrochemical sensor had the advantages of high sensitivity, selectivity, and repeatability, and it showed a wide linear concentration range (1 pg/L to 1 mg/L) and a lower limit of detection (0.24 pg/L) for phosphopeptides. In order to improve the detection capability of the electrochemical sensor, we also synthesized TiO 2 and graphene oxide (GO) composite materials. The influence of the morphology and crystal form of TiO 2 NPs on phosphopeptide detection was studied by changing the feeding ratio and heat treatment temperature. We found that the uniformly dispersed anatase crystal TiO 2 and GO composite-modified electrode showed a lower detection limit (0.37 ag/L). This sensing strategy is expected to provide a novel solution for the direct detection of phosphate groups in polypeptides in complex environments.

The efficient discrimination of oxidizing anions is of considerable importance in environmental monitoring. Here, for the first time, we have developed a simple and fast colorimetric sensor array for detection and identification of oxidizing anions, which takes advantage of the etching of the Ag shell of two core-shell Au@Ag nanoparticles (Au@Ag nanospheres (Au@Ag NPs) and Au@Ag nanocubes (Au@Ag NCs)) by oxidizing anions. The differential etching ability of various oxidizing anions to the Ag shell of the two Au@Ag nanoparticles resulted in different absorbance and color change of the nanoparticles. Thus, employing Au@Ag NPs and Au@Ag NCs as the array's receptors and the indicators, six oxidizing anions (i.e., BrO 3 - , Cr 2 O 7 2- , ClO 4 - , IO 3 - , IO 4 - , and MnO 4 - ) down to 10 nM could be identified from each other by their own colorimetric response patterns. Moreover, the complex mixtures of oxidizing anions could be well discriminated. Most importantly, the sensor array was successfully applied to the discrimination of oxidizing anions in river water and tap water samples.

Flavonoids are closely related to human health, and the distinguishiment of flavonoids is an important but difficult issue. We herein unveil a novel colorimetric sensor array for the rapid identification of 7 flavonoids (e.g., gallocatechin (GC), morin hydrate (MH), puerarin (Pu), epigallocatechin gallate (EGCG), catechin (C), rac Naringenin (rN), and Flavone (Fla)) for the first time. The colorimetric performances of gold nanoparticles (AuNPs) are characteristically correlated with thiocholine, which is issued from the enzymatic hydrolysis of acetylcholine (AcCh). Therefore, as a proof-of-concept design, three sensors (Cu 2+ /acetylcholinesterase (AcChE)/AcCh/AuNPs, Zn 2+ /AcChE/AcCh/AuNPs, and Mn 2+ /AcChE/AcCh/AuNPs) were constructed to form our sensor array. The distinct affinities between flavonoids and metal ions would cause varying degrees of effective reactivation of AcChE, leading to unique colorimetric response patterns upon being challenged with the seven flavonoids for their pattern recognition, enabling an excellent identification of the seven flavonoids at a concentration of 20 nM and different concentrations of individual flavonoids, as well as mixtures of them.

Robust but ultrasensitive aptasensors with an ability to detect lower concentrations of heavy metal ions enable the detection of serious environmental and health issues. We herein develop a label-free aptasensor for ultrasensitive detection of the silver ion (Ag + ) utilizing gold nanoparticle (AuNP) intensity measurement methodology by dark-field microscopy, which is based on target Ag + and exonuclease III (Exo III)-dependent DNA cleavage recycling amplification. In the presence of target Ag + , thymine (T) bases at two termini of hairpin DNA bind with Ag + through C-Ag + -C coordination to form a DNA duplex, Exo III can recognize the blunt 3' end of the DNA duplex and digest it from the 3' end to the 5' direction. The released target Ag + then binds with another hairpin DNA via C-Ag + -C pairs. After many cycles of the digestion of the DNA duplex by Exo III, numerous remaining single-stranded DNA (ssDNA) are generated. These ssDNA are absorbed on the surface of AuNPs, enhancing the repulsion force between AuNPs, which further promotes the dispersion of AuNPs, leading to a significantly decreased intensity of yellow and red dots (aggregated AuNPs) under dark-field microscopy observation, in contrast to that of the blank solution (without target Ag + ). On this basis, the detection limits of 41 and 39 fM were achieved for Ag + in Tris-HCl buffer and river water, respectively.

Herein, we present a novel gold nanoparticle (AuNP) enumeration-based colorimetric aptamer biosensor for ultrasensitive detection of nucleic acid. This AuNP enumeration-based colorimetric method takes advantages of the distinctive and strong localized surface plasmon resonance light scattering with the dark-field microscope. In our model system, first, cost-effective DNA1 instead of expensive 2-thioethyl ether acetic acid was capped on the surface of AuNPs to form a dense DNA1 layer. Then, two DNA strands (DNA2 and DNA3) in two different solutions were separately asymmetrically functionalized on the AuNPs capped dense DNA1 layer. The subsequent binding of the target DNA could trigger the formation of perfect complementary DNA with a Y shape and adjust the distance between nanoparticles to form AuNP dimers, accompanied by a color change from green to yellow as observed, and thereby modulated the performance of the sensor, which resulted in the ultrahigh sensitivity. With this design, a 43 aM limit of detection was obtained, which exhibited an increase of at least 5-9 orders of magnitude in sensitivity over other colorimetric sensors fabricated using conventional strategies.

We presented a simple, cost-effective, and ultrasensitive colorimetric approach for visually detecting thrombin by the catalytic amplification of gold nanoparticles (AuNPs) and aptamer-thrombin recognition. Thrombin can be quantified in the presence of catalytic AuNP surface by using color-change time of 4-nitrophenol. Without thrombin, yellow 4-nitrophenol can freely access the surface of AuNP and becomes colorless 4-aminophenol. With the addition of thrombin, aptamer-thrombin with large size interaction masks the partial surfaces of AuNPs, and increases the reduction time of 4-nitrophenol to 4-aminophenol. The maximum number of bound thrombin fully mask the catalytic AuNP surface, and thus 4-nitrophenol cannot approach to AuNP surface, the color of the solution remains yellow. The limit of detection (LOD) of 0.1 nM can be achieved with naked eyes. Of note, the method was further applied for the detection of thrombin in human serum samples, showing the results in agreement with those values obtained in an immobilization buffer by the colorimetric method.

Mercury is a major threat to the environment and to human health. It is highly desirable to develop a user-friendly kit for on-site mercury detection. Such a method must be able to detect mercury below the threshold levels (10 nM) for drinking water defined by the U.S. Environmental Protection Agency. Herein, we for the first time reported catalytically active gold amalgam-based reaction between 4-nitrophenol and NaBH4 with colorimetric sensing function. We take advantage of the correlation between the catalytic properties and the surface area of gold amalgam, which is proportional to the amount of the gold nanoparticle (AuNP)-bound Hg(2+). As the concentration of Hg(2+) increases until the saturation of Hg onto the AuNPs, the catalytic performance of the gold amalgam is much stronger due to the formation of gold amalgam and the increase of the nanoparticle surface area, leading to the decrease of the reduction time of 4-nitrophenol for the color change. This sensing system exhibits excellent selectivity and ultrahigh sensitivity up to the 1.45 nM detection limit. The practical use of this system for Hg(2+) determination in tap water samples is also demonstrated successfully.

2-Azidoacrylates react with benzynes in the presence of PPh(3) and CsF to afford substituted indoles in good yields. The reaction involves the formation of iminophosphorane and benzyne and a subsequent double cyclization/hydrolysis/air-oxidation cascade. This methodology was utilized to synthesize 10H-indolo[1,2-a]indol-10-ones.