Dual-mode sensing by cobalt oxyhydroxide nanosheets for antibiotics

Here, we present a cobalt oxyhydroxide (CoOOH) nanosheet-driven dual-mode sensor for fluoroquinolone antibiotic detection using fluorescence and colorimetric strategies. By using the intrinsic oxidizing ability of CoOOH nanosheets, 3,3',5,5'-tetramethylbenzidine (TMB) was directly oxidized, resulting in its blue product (oxTMB), enabling sensitive and selective detection. A fluorescence quenching effect was observed owing to the inner filter effects (IFEs), showing a noticeable change in the blue coloration through interaction with fluoroquinolones (ofloxacin, norfloxacin, and ciprofloxacin). The system exhibited excellent sensitivity with linear responses in the range of 0-250 nM with the lowest detection limits of 2.23 nM, 19.31 nM, and 29.03 nM for ofloxacin, norfloxacin, and ciprofloxacin, respectively, outperforming many existing sensors. These findings provide a novel scheme for the fluorescence and colorimetric analysis of antibiotics in water. Furthermore, a cost-effective, mobile-device-assisted paper-based sensing system was developed for the quick and visual field-level detection of fluoroquinolones. These sensors provide a practical and user-friendly tool for rapid antibiotic screening, which can be completed within minutes. The proposed system offers a promising strategy for environmental and pharmaceutical monitoring, using CoOOH nanosheets as versatile components for sensor development.