In order to explore novel colorimetric biosensors with high sensibility and selectivity, two new Keggin polyoxometalates (POMs)-based Cu-trz (1,2,4-triazole) metal–organic frameworks (MOFs) with suitable specific surface areas and multiple active sites were favorably fabricated; then single-walled carbon nanotubes (SWNTs) were merged with new POMOFs to construct POMOF/SWNT nanocomposites. Herein, POMOF/SWNT nanocomposites as peroxidase mimics were explored for the first time, and the peroxidase-mimicking activity of the prepared POMOF/SWNT nanocomposites is heavily dependent on the mass ratio of POMOFs and SWNTs, in which the maximum activity is achieved at the mass ratio of 2.5:1 (named PMNT-2). More importantly, PMNT-2 exhibits the lowest limit of detection (0.103 μM) among all reported materials to date and the assumable selectivity toward l-cysteine (l-Cys) detection. With these findings, a convenient, sensitive, and effective “on–off switch” colorimetric platform for l-Cys detection has been successfully developed, providing a promising prospect in the biosensors and clinical diagnosis fields.

A novel colorimetric aptamer sensor (aptasensor) for the detection of streptomycin (STR) is developed based on the peroxidase-like activity of gold nanoparticles (AuNPs) and their interaction with STRaptamer complex. It is known that AuNPs can easily oxidize substrates in the presence of hydrogen peroxide in a manner identical to peroxidase and 2,2 0 -azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) is a commonly used peroxidase substrate with green oxidation product and a characteristic absorption peak at 733 nm. In the absence of STR, STR1 aptamer is adsorbed on the surface of AuNPs, and restrains its catalytic activity due to shield effect of ssDNA sequence against substrates. STR first binds with STR1 aptamer to form an STR-aptamer complex; thus, aptamer sequences do not get absorbed on the surface of the nanoparticles and their peroxidase activity is further greatly enhanced by the STR-aptamer complex, which enables AuNPs to catalyse the oxidation of the substrates. To achieve a higher level of sensitivity, several key parameters of the proposed aptasensor such as pH, concentrations of STR1 aptamer and AuNPs, and incubation and reaction temperature have been investigated. Under optimal conditions, the proposed aptasensor can detect STR in a linear range from 0.1 mM to 0.5 mM with a limit of detection (LOD) as low as 86 nM and exhibits good selectivity. Moreover, further studies also validate the applicability of the proposed aptasensor in milk samples, revealing that it may have enormous potential utility for practical STR detection in food products in the future.

A novel capacitive biosensor based on electropolymerized molecularly imprinted polymer (MIP) for glutathione detection is reported. The capacitive sensor was prepared by electropolymerizing o-phenylenediamine (o-PD) on a self-assembled 2-mercaptoethane sulfonate (MES) monolayer modified gold electrode in the presence of template glutathione. 1-Dodecanethiol was used to block the defect of polymer film to improve its dielectric performance. Hydrolization procedure was selected to remove the template thoroughly for better sensitivity. The dielectric property of the sensitive layer was characterized by differential pulse voltammetry (DPV). The linear response range of the sensor for glutathione was between 0.025 and 0.30 mmol L À1 with a detection limit of 1.25 Â 10 À3 mmol L À1. Satisfactory results were obtained in the direct detection of real samples. The selectivity was evaluated by capacitance selective coefficient of glutathione and other compounds. The kinetic aspects of the recognition process were investigated by capacitive transduction. A two-step kinetic model was derived to describe the interaction between analyte and imprint sites. Fitted results were well in agreement with the corresponding experimental results.

Although the colorimetric methods can easily meet the demands of point-of-care and ease-of-use for antibiotic detection, they still face many challenges in the accuracy and stability of assay. Herein, a facile and stable colorimetric aptasensor is first developed for kanamycin residue detection based on the aptamer-enhanced peroxidase-mimicking activity of layered WS2 nanosheets. The investigation confirmed that aptamer sequences can improve the affinity of nanosheets to the chromogenic substrate 3,3′’,5,5′’-tetramethylbenzidine, resulting in a significant increase of the peroxidase-mimicking activity. Under the optimal conditions, the limit of detection of the proposed colorimetric aptasensor for kanamycin was determined to be as low as 0.6 μM, and such an aptasensor displays excellent selectivity against other competitive antibiotics. Moreover, further studies have verified the applicability of the established colorimetric aptasensor in several actual samples, indicating that the aptasensor may have bright application prospects for kanamycin detection in livestock husbandry and agriculture samples.

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