Enzymatic reaction modulated gold nanorod end-to-end self-assembly for ultrahigh sensitively colorimetric sensing of cholinesterase and organophosphate pesticides in human blood

Valence States Modulation Strategy for Picomole Level Assay of Hg2+ in Drinking and Environmental Water by Directional Self-Assembly of Gold Nanorods

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.

Colorimetric biosensor for the assay of paraoxon in environmental water samples based on the iodine-starch color reaction

In this work, a new colorimetric biosensor for the assay of paraoxon was developed via the conventional iodine-starch color reaction and multi-enzyme cascade catalytic reactions. In the presence of acetylcholine chloride, acetylcholinesterase (AChE) and choline oxidase (ChO) catalyzed the formation of H₂O₂, which then activated horseradish peroxidase (HRP) to catalyze the oxidation of KI to produce an iodine-starch color reaction. Upon exposure to paraoxon, the catalytic activity of AChE was inhibited and less H₂O₂ generated, resulting in a decrease in the production of I₂ and a drop in the intensity of solution color. This colorimetric biosensor showed high sensitivity for the assay of paraoxon with a limit of detection 4.7 ppb and was applied for the assay of paraoxon in spiked real samples. By employing the conventional iodine-starch color reaction, this biosensor has the potential of on-site assay of OPs residues in environmental samples.

Oxidase-mimicking activity of ultrathin MnO2 nanosheets in colorimetric assay of acetylcholinesterase activity

In the present study, a novel colorimetric sensing platform was constructed for quantitative detection of acetylcholinesterase (AChE) activity and its inhibitor. Manganese dioxide (MnO₂) nanosheets as an oxidase-mimicking nanomaterial could directly oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into oxTMB without the need for horseradish peroxidase and H₂O₂. When AChE was introduced, acetylthiocholine could be catalytically hydrolyzed to produce thiocholine, which easily triggers the decomposition of MnO₂ nanosheets, causing the decrease of solution absorbance. Owing to the inhibition effect of organophosphorus pesticides, the enzymatic activity was suppressed, preventing the decomposition of MnO₂ and resulting in the increase of absorbance. Under optimal conditions, the colorimetric platform shows sensitive responses to AChE and paraoxon in the range of 0.1-15 mU mL^-1 and 0.001-0.1 μg mL^-1, respectively. The detection limits of AChE and paraoxon reached 35 μU mL^-1 and 1.0 ng mL^-1, respectively. Furthermore, the MnO₂-TMB platform has been used to fabricate test strips for rapid and convenient visual detection of AChE and its inhibitor with highly promising performance.

Exonuclease I-aided homogeneous electrochemical strategy for organophosphorus pesticide detection based on enzyme inhibition integrated with a DNA conformational switch

A homogeneous electrochemical strategy for the detection of organophosphorus pesticides based on the inhibition of acetylcholinesterase activity and configuration change of hairpin DNA probes was developed.

Design of an ultra-sensitive gold nanorod colorimetric sensor and its application based on formaldehyde reducing Ag+

HCHO could reduce Ag⁺ to Ag on the surface of AuNRs to form Au core–Ag shell nanorods (Au@Ag↓NRs) in AuNRs–Ag⁺–HCHO system, which caused LPAB of AuNRs to redshift. Thus, a responsive AuNRs colorimetric sensor for the detection HCHO has been developed.