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In this work, we report a simple, ultrasensitive, and feasible colorimetric assay for metal ion (K + , used as a model) via inherent peroxidase-like enzymatic amplification strategy of gold nanoparticles (AuNPs). It is shown that peroxidase-like activity of AuNPs can be improved dramatically by its surface activation with target-specific aptamer molecules. Whereas when the target exists, the aptamers leave the surface of AuNPs in a target concentration-dependent manner, resulting in a decrease of the nanoenzymatic catalytic ability of AuNPs. Thus, K + can be quantified in the presence of AuNPs by using a colorimetric sensing probe (3,3′,5,5′-tetramethylbenzidine). The color change of the solution is relevant to the dose of the target, and this can be achieved with the naked eyes and monitored by UV–vis spectrometry. A linear dependence between the absorbance and target K + concentration is obtained under optimal conditions in the range from 0. 1 nM to 1 μM with a detection limit (LOD) of 0.06 nM estimated at the 3S blank level . The sensitivity displays to be 2–9 orders of magnitude better than those of other K + detection methods. This sensing strategy may in principle be universally applicable for the detection of a range of environmental or biomedical molecules of interest. [ABSTRACT FROM AUTHOR]

In this study, a simple and highly sensitive colorimetric assay of H 2 O 2 using triangular Ag nanoplates has been carried out. In the presence of H 2 O 2 , the Ag nanoplates are etched from triangle to round, which leads to the vanishment of the surface plasmon resonance (SPR) absorption at 400 nm and significant decrease the absorbance of the band at 670 nm. The color change of the reaction system from the initial blue to mauve can be observed. This proposed method can determine H 2 O 2 with a dynamic range of 50 nM to 1 μM and a low detection limit of 1.36 nM. The high sensitivity of the triangular nanoplates can be ascribed to highly reactive edges/tips, and strongly tip sharpness and aspect ratio dependent SPR absorption of the triangular nanostructure. This sensing system is further extended to the detection of glucose by H 2 O 2 generated from the enzymatic oxidation of glucose, and thus holds great promises in clinical applications. [ABSTRACT FROM AUTHOR]

We provide a highly sensitive and selective colorimetric assay to detect mercury ions (Hg 2+ ) in aqueous environment using thiocyanuric acid (TCA) molecule-functionalized gold nanoparticles (AuNPs). This method is based on the thiophilicity of Hg 2+ and AuNPs as well as the unique optical properties of TCA-functionalized AuNPs. In the presence of TCA, AuNPs aggregate due to the strong attraction between thiol groups of TCA and surface-bounded AuCl 4 - /AuCl 2 - ions, which induces the visible color change from red to blue. With the addition of Hg 2+ , Hg 2+ is more apt to interact with thiols than AuNPs. Thus, Hg 2+ can remove the AuNPs of the TCA-functionalized AuNPs and trigger AuNP aggregation redisperse again. This assay can selectively detect Hg 2+ with the detection limits as low as 0.5 nM in aqueous solution. [ABSTRACT FROM AUTHOR]

Abstract We develop a glucose oxidase (GOx)-mediated strategy for detecting glucose based on oxidized 3,3′,5,5′-tetramethylbenzidine (oxTMB), which is generated from Cu2+-catalyzed 3,3′,5,5′-tetramethylbenzidine (TMB)-H 2 O 2 reaction, as colorimetric readout. The sensing system involves two processes: generation of H 2 O 2 from GOx-catalyzed oxidation of glucose, and H 2 O 2 -induced the oxidization of TMB via the catalysis of Cu2+. The H 2 O 2 formed by GOx-catalyzed oxidation of glucose oxidizes colorless TMB to blue oxTMB, thus enhancing the absorbance intensity at 670 nm. Therefore, we draw a conclusion that the enhancement in colorimetric signal relies directly on H 2 O 2 concentration, which, in turn, depends on glucose concentration. This color change can be used not only for visual detection of glucose by naked eyes but for reliable glucose quantification in the range from 1 to 100 nM with a detection limit of 0.21 nM. The method possesses the following advantages: simple design, low experimental cost, and no any additional experimental equipment for heating, illuminating, or bubbling. Graphical Abstract Unlabelled Image Highlights • A GOx-mediated strategy for detecting glucose based on oxTMB, which is generated from Cu2+-catalyzed TMB-H 2 O 2 reaction. • The enhancement in colorimetric signal relies directly on H 2 O 2 concentration. • The method exhibits an ultrasensitive detection of glucose with the detection limit down to 0.21 nM. • The method is universally applicable for detection of other H 2 O 2 -mediated substrates and enzymes. [ABSTRACT FROM AUTHOR]

A growing body of evidence indicates that lysozyme plays a significant role as an indicator for many diseases and a drug for treatment of infections, ulcers and to study the spatial conformation, enzyme kinetics, and molecular immunology. Therefore, highly sensitive determination of lysozyme is necessary and vital in a wide variety of fields. In this work, we put forward a simple but effective strategy for colorimetric visualization of lysozyme based on iodide-responsive Cu@Au nanoparticles (Cu@Au NPs) as well as the iodide-catalyzed H 2 O 2 -TMB (3,3,5,5-tetramethylbenzidine) reaction system. Colorimetric detection is applied because of its simplicity, fast response for analysis, high detection limit, low costs and practicality. In our strategy, iodide is applied for the reason that it can induce an obvious color change of the Cu@Au nanoparticles solution from gray to red, along with the change of morphologies of the Cu@Au nanoparticles from irregular to spherical. Consequently, this phenomenon results in colorimetric signal variation of the iodide-catalytic H 2 O 2 -TMB system. What's more, by quite simple biomolecule modification on the Cu@Au nanoparticles surface, an all-purpose colorimetric platform is established for the accurate detection of lysozyme, which could lead to the change of Cu@Au NP concentration through molecular recognition. The results show that modified Cu-Au NPs successfully achieved a simple, selective, visualized, and ultrasensitive detection of lysozyme with a linear range from 10 −7 to 10 −3 M and a detection limit of 60 nM. [ABSTRACT FROM AUTHOR]

We present a simple and efficient colorimetric assay strategy for ultrasensitive visual detection of human α-thrombin, which is essentially based on the formation of the DNA1-thrombin-DNA2 sandwich complex-bridged gold nanoparticle (Au NP) oligomers. Unlike the traditional colorimetric sensing strategies which induced the nanoparticle aggregates with uncontrolled aggregate size. In this work, the DNA1with rich G bases was firstly conjugated on the surfaces of Au NPs fixed on the hexadecyl trimethylammonium bromide (CTAB)-coated glass slide, and thrombin was captured by the DNA1. Then, the other DNA2 with rich G bases interacted with the former DNA1-thrombin complex and formed a DNA1-thrombin-DNA2 sandwich complex. The subsequently added Au NPs can be bound to the Au NP-DNA1-thrombin-DNA2 via Au-S bond to trigger the formation of Au NP oligomers, an apparent color change of the single Au NPs from green to yellow and red was observed under dark field microscopy. By measuring the intensity change of the yellow and red Au NPs, the concentration of target thrombin could be accurately quantified. As a proof of concept experiment, the formation of Au NP oligomers resulted in significantly improved sensitivity (10 fM of limit of detection and 20 fM of limit of quantity) and wider linear dynamic range of thrombin detection (20 fM-20 nM), the relative standard deviation (RSD) was less than 5.73% (n = 5). In addition, in order to validate the potential application in clinical diagnosis, the content of thrombin in a human serum samples was also quantified. [ABSTRACT FROM AUTHOR]

We present here a simple and sensitive sensing strategy for thrombin in buffer solution and human serum samples. The key features of this assay lie in target binding-induced DNA hybridization and the formation of gold nanoparticle (AuNP) aggregation. In the presence of thrombin, the binding of the two DNA strands to the same target triggers the hybridization between the complementary sequences of the two DNA strands. The hybridization is able to pull a few AuNPs together to form oligomers. A linear dependence between the absorbance and target thrombin concentration is obtained under optimal conditions in the range from 10 nM–5 μM with a detection limit (LOD) of 7.5 nM estimated at the 3S blank level. Moreover, this method is successfully applied to complex serum samples without complicated sample pretreatment and sophisticated instruments, a dynamic range (10 nM–5 μM) and a detection limit as low as 9.1 nM is achieved. [ABSTRACT FROM AUTHOR]

A facile colorimetric sensor array for detection of proteins was demonstrated using DNA as nonspecific receptors. We found that different proteins could trigger the DNA-protected gold nanoparticles (AuNPs) to show different aggregation behaviors in the presence of salt with high concentrations along with various color changes. As a proof-of-concept application, a multi-protein discrimination array was fabricated with two ssDNA strands (15A and 30A bases) as a receptor array. The combinatorial colorimetric response of this sensor array can be analyzed by linear discriminant analysis (LDA) and hierarchical cluster analysis (HCA). 12 proteins have been well distinguished with the naked eye at the 50 nM level. The proteins in human serum have also been discriminated. Furthermore, the accuracies of discrimination of the similar mixtures of proteins and unknown samples were all 100%. [ABSTRACT FROM AUTHOR]

Mercury ion (Hg 2+ ) has been recognized as one of the most hazardous metal pollutants that can cause deleterious effects on human health and the environment even at low concentrations. Therefore, monitoring of Hg 2+ levels in water is very important in terms of waste management, environmental analysis, toxicology, water safety, and water quality. Here, we provide a highly sensitive and selective colorimetric assay to detect Hg 2+ in aqueous solution using iodide responsive core-shell Cu@Au nanoparticles (Cu@Au NPs). Iodide can induce an appreciable color change of the Cu@Au NP solution from gray to red by transforming the interconnected, irregularly shaped nanoparticles to the single, separated, and nearly spherical ones. In the presence of Hg 2+ , the formation of Cu@Au-Hg nanoalloys can protect the Cu@Au NPs from absorbing more iodide in the solution, the solution color turns pale red. When Hg absorbed on the surface of Cu@Au NPs reaches saturation, Hg prevents iodide from reacting with Cu@Au NPs, thus the solution color remains gray. This method exhibits a wide response range from 0.1 nM to 70 nM, and the detection limit (LOD) of 0.037 nM is obtained. Besides, our Cu@Au NPs-based colorimetric sensor exhibits excellent selectivity for Hg 2+ . [ABSTRACT FROM AUTHOR]

We have developed aptamer-functionalized gold nanoparticles (AuNPs) for the rapid, selective, and sensitive detection of Ag + in aqueous solution. Two DNA strands are designed to functionalized on the surface of AuNPs through Au S bond. In the presence of Ag + , aptamer1 hybridizes with the partially complementary aptamer2 with the aid of C Ag + C bindings, which could lead to different degrees of AuNP aggregation. This process is accompanied by a color change from red to bluish-purple as observed by naked eyes. This colorimetric sensor is shown to detect the presence of as low as a 0.236 nM concentration of Ag + . Importantly, it could be applied to complex samples without complicated sample pretreatment and sophisticated instruments. [ABSTRACT FROM AUTHOR]

In this work, we report an ultrasensitive colorimetric sensing strategy for Cu 2+ based on the Cu 2+ catalyzed TMB-H 2 O 2 reaction, and the signal amplification induced by aptamer-K + -Cu 2+ complex. It is demonstrated that Cu 2+ exhibits an intrinsic peroxidase-like catalytic activity. Thus, we utilize the catalytic activity of Cu 2+ to TMB-H 2 O 2 reaction for direct colorimetric visualization of Cu 2+ for the first time. The limit of detection (LOD) is 2.6 μM. To further improve the sensitivity of Cu 2+ detection, we utilized G-quadruplex-Cu 2+ metalloenzyme complex formed by K + , G-rich DNA, and Cu 2+ to amplify colorimetric signal of TMB-H 2 O 2 reaction. The LOD is as low as 0.076 μM, which is 34-fold lower than that of only Cu 2+ -catalyzed TMB-H 2 O 2 system (2.6 μM). The LOD shows excellent potential for monitoring ultralow levels of Cu 2+ in water samples. [ABSTRACT FROM AUTHOR]

Organophosphorus pesticides (OPs) are widely used in pest control. The residual OPs can cause pollution to the environment and harm to the human body. Although OPs can inhibit the activity of acetylcholinesterase (AChE) and lead to neurological diseases, it also provides enlightenment for the development of a simple and sensitive method for the detection of OPs. Herein, we report a colorimetric sensor array using nanozymes (Fe-N-C nanozymes and Cu-N-C nanozymes) combined with natural enzymes to discriminate eight OPs (Iprobenfos (Ipr), Diazinon (Dia), Crotoxyphos (Cro), Bromophos ethyl (Bro), Dipterex (Dip), Chlorpyrifos methyl (Chl), Ethion (Eth), and Omethoate (Ome)) through a cascade reaction system. Surprisingly, eight OPs were successfully identified in tap water at 1 ng mL−1. • We report a colorimetric sensor array using two nanozymes as sensing elements to discriminate organophosphorus pesticides. • Cu-N-C nanozymes show better peroxidase activity than Fe-N-C nanozymes. • Eight OPs were successfully identified in tap water at 1 ng mL−1. • This method opens a paradigm for the development of colorimetric sensor arrays based on nanozymes. [ABSTRACT FROM AUTHOR]

Flavonoids are closely related to human health. The identification and determination of flavonoids is an important and difficult issue. In view of this, this work puts forward a colorimetric sensing array for flavonoid discrimination based on gold nanoparticle (AuNP) aggregation. Two substances (acetylthiocholine iodide (Atc) and S-propionylthiocholine iodide (Ptc)) are hydrolyzed to choline (Chi) under the action of acetylcholinesterase (AcChE). Choline can agglomerate AuNPs by forming Au S bond with its own sulfhydryl functional group. The presence of flavonoids inhibits the AcChE activity, resulting in the formation of different amounts of Chi and different degrees of AuNP aggregation. Controlled by Atc and Ptc as array's receptors, the five flavonoids, including proanthocyanidin (Pro), naringenin (Nar), quercetin (que), flavone (Fla), and curcumin (Cur), were differentiated through their unique colorimetric "fingerprint" pattern-based recognition with the aid of linear discriminant analysis (LDA). The assay is validated under phosphate buffered saline (PBS) buffer conditions as well as in human serum. In addition, identification for diverse concentrations of single flavonoid and different binary and ternary mixtures of flavonoids were also demonstrated. • The identification and determination of flavonoids is an important and difficult issue. • A colorimetric sensor array for flavonoid discrimination based on gold nanoparticle aggregation is developed. • The presence of flavonoids inhibits the AcChE activity, resulting in different degrees of AuNP aggregation. • Identification for diverse concentrations of single flavonoid and flavonoid mixtures were demonstrated. [ABSTRACT FROM AUTHOR]