Dual-signal fluorescence and colorimetric sensor based on N,P co-doped carbon dots for selective detection of doxycycline.

Doxycycline (DOXY) is a broad-spectrum tetracycline antibiotic. It is widely used in the treatment of various infectious diseases. However, the excessive use of doxycycline may result in the emergence of antibiotic-resistant bacteria, environmental contamination, and drug residues in food products. Therefore, developing a sensitive and efficient method is crucial for the accurate detection and monitoring of DOXY. In this study, a novel fluorescent probe based on nitrogen and phosphorus co-doped carbon dots (N,P-CDs) was synthesized with solvothermal processing. The resulting N,P-CDs showed a uniform particle size distribution, excellent water solubility, and high stability, with a fluorescence quantum yield of 34.7 %. The dual-signal sensing system was in the "ON" state when the N,P-CDs exhibited a fluorescence peak at 474 nm (under the optimal excitation wavelength of 380 nm) and a UV-vis absorption peak at 334 nm. Upon the introduction of DOXY, the fluorescence intensity of the N,P-CDs decreased significantly. At the same time, the absorbance increased notably, corresponding an "ON-OFF" transition in fluorescence and an enhanced colorimetric response. Meanwhile, sodium fluoride (NaF) was introduced into the system as a masking agent to suppress ferric ion (Fe) interference, thereby improving the recognition ability toward DOXY. Based on this, a dual-signal fluorescence-colorimetric sensing system using N,P-CDs was constructed for DOXY detection. The fluorescence sensing system exhibited a good linear range from 3.348 to 109.308 μM with a detection limit of 1.5273 μM LOD, S/N = 3. The colorimetric sensing system showed a linear range from 1.609 to 31.448 μM with a detection limit of 0.0295 μMLOD, S/N = 3. This dual-signal sensing platform was successfully applied to detect DOXY in real samples including human serum, Beichuanhe River, laboratory water, milk, and orange juice. The results demonstrated its promising potential for environmental monitoring and bioanalytical applications.