Abstract: MXene quantum dots (MQDs) hold great potential in fluorescence sensing for antibiotic detection, but remain limited by poor phase purity, environmental instability, and biosafety concerns. Due to the intrinsic chemical inertness and biocompatibility of tantalum (Ta), Ta-based MQDs offer a rational pathway to overcome these challenges. In this work, a novel low-temperature molten salt etching strategy was developed to synthesize phase-pure and structurally intact Ta₂C MXene, overcoming long-standing synthetic difficulties. The obtained Ta₂C exhibits exceptional aqueous stability and the highest biocompatibility reported among MXenes to date. Then, Ta₂C MXene was hydrothermally cut into Ta₂C QDs while being simultaneously aminated, constructing highly sensitive fluorescent probes for oxytetracycline (OTC). This sensing platform operates via static quenching followed by an inner filter effect, enabling highly selective detection even against closely related tetracycline analogs, addressing a long-standing challenge in OTC detection. It also affords a broad linear detection range from 0.5 to 400 µM, suitable for both trace and high concentrations of OTC. Combined with the simplicity and immediacy of fluorescence detection, Ta₂C QDs enable rapid and efficient detection of OTC in real samples within 1 min, including tap water, river water, and milk. Thus, this work not only develops an ultrastable and biocompatible Ta₂C MXene but also paves the way for its integration into biosensing platforms for real-world environmental and food safety monitoring.

Key words: MQDs, Tantalum carbide, MXene, Antibiotic detection, Oxytetracycline