Co(OH)2/MXene-Ti3C2 nanocomposites with triple-enzyme mimic activities as hydrogel sensing platform for on-site detection of hypoxanthine.

Novel Co(OH)2/MXene-Ti3C2 nanocomposites with oxidase (OXD)-mimic, peroxidase (POD)-mimic, and catalase (CAT)-mimic activities were prepared by a simple two-step method. The Co(OH)2/MXene-Ti3C2 nanocomposites with triple-enzyme mimic activities were embedded into sodium alginate (SA) gels for the first time to fabricate a target-responsive hydrogel-assisted assay. The catalytic mechanism and steady-state kinetics of Co(OH)2/MXene-Ti3C2 nanocomposites were investigated. Subsequently, hypoxanthine (Hx) was catalyzed by xanthine oxidase (XOD) to form H2O2, which reacts with 3,3',5,5'-tetramethyl-benzidine (TMB) in the presence of Co(OH)2/MXene-Ti3C2 nanocomposites to form a blue oxide (ox-TMB) in the hydrogel. The visible color change of the hydrogel with the increase of Hx concentration can be recognized through a smartphone App to transfer the red (R), green (G), and blue (B) values for the quantitative determination of  Hx, with a detection range from 5 to 250 µM, and detection limit of 0.2 µM. The method was applied to the determination of Hx content in different aquatic products. The spiked recoveries of the aquatic products were from 94.1 to 106.4%, and the relative standard deviations (RSD) were less than 5.4%. Our results show that the Co(OH)2/MXene-Ti3C2 nanocomposites hydrogel-assisted colorimetric biosensor is cost-effective, sensitive, and selective and has excellent application prospects for in-the-field determination of Hx.

Cu nanoclusters decorated Ti3C2 nanosheets composite with tetraenzyme mimic activities and the application for smartphone-assisted detection of hypoxanthine.

MXene-based nanozymes have increased research enthusiasm in the field of food safety and environment monitoring. Herein, the Cu NCs/Ti3C2 NSs nanocomposites were prepared by modifying copper nanoclusters (Cu NCs) on the surface of Ti3C2 nanosheets (NSs) with a simple two-step method. The Cu NCs/Ti3C2 NSs nanocomposites had outstanding tetraenzyme mimic activities, i.e. peroxidase (POD)-mimics, catalase (CAT)-mimics, ascorbic acid oxidase (AAO)-mimics and superoxide dismutase (SOD)-mimics. Modification of Cu NCs on Ti3C2 NSs can enhance tetraenzyme mimic activities because of the synergistic catalytic effect between Cu NCs and Ti3C2 NSs. The catalytic mechanism and steady-state kinetics of Cu NCs/Ti3C2 NSs were also investigated. Based on the POD-mimic activity of Cu NCs/Ti3C2 NSs, a simple and rapid colorimetric method was established for the on-site detection of hypoxanthine (Hx), with the linear range of 5-200 µM and limit of detection (LOD) was 0.25 µM. The visible color change with the increase of Hx concentration can be recognized by a smartphone APP to transfer the red (R), green (G) and blue (B) value for the quantitative analysis of Hx, with the linear range of 10-200 µM, which provided a convenient method for the real-time detection of Hx. This work not only provides a significant route to fabricate nanocomposite with outstanding tetraenzyme mimic activities but also offers a low-cost and rapid method for monitoring the freshness of aquatic products.

Smartphone-assisted miniature device based on nitrogen and sulfur co-doped carbon dots for point-of-care testing of tetracycline.

A miniature device was design for the point-of-care testing (POCT) of tetracycline (TC) including a ratio fluorescence test strip, a sample slot, a UV lamp and a smartphone. The nitrogen and sulfur co-doped carbon dots (N, S-CDs) and Eu3+ were dropped onto the filter paper to construct the ratio fluorescence test strips for the specific detection of TC. Under the excitation at 390 nm, the fluorescence emission of N, S-CDs at 530 nm decreases through inner filter effect (IEF) after addition of Eu3+. When the further addition of TC, the emission of N, S-CDs at 530 nm kept unchanged while the emission of Eu3+ at 616 nm was obviously enhanced for the antenna effect (AE) between Eu3+ and TC. The ratio changes of the two-fluorescence emission realized the quantitative detection of TC. In addition, the test strips with different concentrations of TC showed different fluorescence color from green to red under a 365 nm UV lamp. The miniature device was designed as a fluorescence photo reader with the merits of the powerful functions of smartphones and the portability of test strips. The smartphone camera takes a fluorescent color image of the test strips and the photos are recognized by a color recognizer on the smartphone to obtain RGB (red-greenblue) values which reflect the concentrations of the analytes. Therefore, we established a fast, sensitive and efficient POCT of TC. In particular, the proposed nanomaterial-based POCT platform will open a new route towards the development of ratio fluorescence probe for TC analysis for environment samples.