Introduction: Sustainable, biocompatible fluorescent probes are increasingly needed for clinically relevant analytes. Protamine, the only clinically used antidote for heparin overdose, and uric acid, a key biomarker for metabolic, renal, and cardiovascular disorders, both require reliable quantification in biological fluids for effective clinical management.

Methods: Sulfur- and nitrogen-doped carbon dots (S,N-CDs) were synthesized via a one-step hydrothermal method using Ricinus communis (castor) seeds as a carbon source and thiourea as the nitrogen and sulfur dopant. Finely ground castor seed powder was mixed with thiourea in distilled water and heated in a sealed autoclave at 180 °C for 6 h to obtain S,N-CDs suitable for fluorescence-based sensing.

Results: The S,N-CDs showed uniform spherical morphology (2–8 nm), green emission (λ_em = 520 nm), and good stability toward pH variation, ionic strength, and photobleaching. Surface functional groups enabled a fluorescence turn-off response to both targets. Protamine was quantified over 3.2–7.2 µM (limit of detection 0.45 µM), with recoveries of 98.9–103.1% in spiked human serum. Uric acid was detected over 50–124.8 µM (limit of detection 16.7 µM), with recoveries of 94.9–101.8% in serum and 92.6–95.3% in urine. These results indicate that the probe can operate in complex biological matrices using a simple, low-cost fluorescence readout.

Conclusion: Castor seed-derived S,N-CDs function as an application-focused fluorescent probe for dual detection of protamine and uric acid in biological fluids. This study demonstrates a sustainable synthesis route and links the surface functionality of the CDs to their fluorescence response, supporting their utility for applied biosensing rather than mechanistic photophysical advancement.