The visual detection of amoxicillin using a dual-mode probe based on CQD-doped MnO ₂ nanospheres

Amoxicillin (AMO), a broad-spectrum penicillin antibiotic, has been widely employed for the treatment of bacterial infections. However, the unreasonable use of amoxicillin may cause a serious threat to human health, and its visual determination remains a challenge. Herein, a MnO₂@CQD probe was used for the dual-mode detection of AMO, which was synthesized by doping CQDs into MnO₂ nanospheres. In the colorimetric mode, MnO₂@CQDs oxidized colorless 3, 3', 5, 5'-tetramethylbenzidine (TMB) into yellow oxidized TMB (oxTMB). In the presence of AMO, AMO could deoxidize oxTMB to cause multiple color changes from yellow to colorless. In the fluorescence mode, MnO₂@CQDs initially exhibited quenched fluorescence due to the presence of MnO₂. Upon the addition of AMO, MnO₂ was reduced to Mn²⁺ ions, and then CQDs were released and their fluorescence was recovered at an emission wavelength of 450 nm. The colorimetric detection range was wide from 0.005 to 0.09 μM, and the detection limit was 1.6 nM. With increasing concentration of AMO, the color changed from yellow to colourless, which allowed naked eye observation. The fluorescence method was rapid, requiring only 1 min, and exhibited a linear detection range from 0.003 to 0.045 μM, with a detection limit as low as 1 nM. In addition, a paper-based sensing platform was constructed, where fluorescence intensity transitioned from dark to blue with increasing AMO levels. The RGB analysis demonstrated that AMO concentration could be discriminated effectively using a smartphone-assisted detection system. The MnO₂@CQD sensor showed excellent selectivity and specificity for AMO colorimetric and fluorescence detection. Moreover, the probe exhibited good practicability in tap water, milk and serum, with results comparable to those obtained by high-performance liquid chromatography. Overall, the MnO₂@CQD dual-mode probe provides an efficient, convenient and rapid strategy for the visual and quantitative detection of AMO in complex environments.