Smartphone‐Assisted Visual Detection of Doxycycline Using S‐doped Fluorescent Carbon Dots in Environmental and Pharmaceutical Samples

Visual and easy detection of doxycycline (DOX) in various matrices, using a smartphone‐based readout, is crucial for monitoring antibiotic contamination, ensuring environmental safety, and preventing antibiotic resistance. In this study, a simple and efficient fluorescent ON–OFF sensing platform for DOX detection was developed using S‐doped fluorescent carbon dots (S‐CDs) synthesized via a cost‐effective route. The sensing mechanism exploits the quenching effect of DOX on the fluorescence of S‐CDs, enabling both conventional instrument‐based spectrofluorometric and instrument‐free smartphone‐assisted visual detection systems. The emission quenching of S‐CDs by DOX was attributed to competitive absorption resulting from the spectral overlap between the absorption and excitation spectra of S‐CDs and DOX. The latter approach combines high sensitivity and user‐friendly operation, making it suitable for on‐site applications. The sensor exhibited excellent performance, with a limit of detection of 0.33 µM and a linear range from 1 to 60 µM. Moreover, the method demonstrated high recovery rates for spiked samples, with values ranging from 94.4% to 103.3%, thereby confirming its reliability for real‐world applications. The developed system is a promising candidate for routine monitoring of DOX in diverse matrices, offering advantages such as rapid response, cost efficiency, and ease of use.