Toward Real-World Tetracycline Sensing: Label-Free Carbon Quantum Dot Fluorescence From Molecular Design to Matrix-Driven Challenges.

Carbon quantum dot (CQD)-based label-free fluorescent sensing platforms have emerged as promising analytical approaches for the detection of tetracycline residues in food and environmental samples. This review summarizes recent advances in CQD design, fluorescence sensing mechanisms, and practical analytical applications, with an emphasis on precursor engineering, heteroatom doping, and surface-state modulation. Among antibiotic contaminants, tetracyclines represent an important model target due to their extensive use, environmental persistence, and characteristic molecular properties that enable diverse interactions with CQD surfaces. Recognition-assisted CQD biosensing architectures incorporating selective elements such as aptamers, antibodies, and molecularly imprinted polymers represent an important complementary strategy; however, this review specifically focuses on label-free systems in which tetracycline-induced modulation of CQD photoluminescence provides the sensing response. Reported label-free CQD sensors demonstrate improved analytical performance and successful application in complex matrices, including milk, honey, serum, and water samples. Nevertheless, challenges related to fluorescence mechanism ambiguity, synthesis variability, insufficient standardization, matrix interference, and limited long-term validation remain major barriers to practical deployment. Emerging trends, including smartphone-assisted detection, paper-based platforms, and sustainable biomass-derived CQDs, indicate progress toward portable sensing technologies. Future development requires rational CQD design, standardized evaluation frameworks, and robust field-validation strategies for reliable tetracycline monitoring.