Green Synthesis of Biomass-Derived Carbon Quantum Dots from Syzygium aromaticum via Carbonization-Assisted Ultrasonication for a Selective Colorimetric Sensor of Ag⁺ in Environmental Waters

Carbon quantum dots (CQDs) were successfully synthesized from Syzygium aromaticum biomass using a carbonization-assisted ultrasonication approach. The resulting SA-CQDs exhibited a particle size of 3.31 nm, a quantum yield of 12.61%, and strong blue emission at 440 nm under 350 nm excitation. Spectroscopic analysis (FTIR, XPS, Raman) confirmed abundant oxygen- and nitrogen-containing functional groups, contributing to excellent aqueous dispersibility and optical responsiveness. The CQDs demonstrated high selectivity and sensitivity toward Ag⁺ ions, with a distinct color change and an absorbance peak at 431 nm. A linear response was obtained in the 10–500 µM range (R² = 0.95579) with a low detection limit of 0.103 µM. Stability assessments revealed excellent fluorescence and absorbance retention across a broad range of pH, temperature, storage time, and light exposure. Recovery tests in tap and underground water showed 96.94–100.08% with RSD < 1%. This study supports the application of SA-CQDs as a green, cost-effective, and sensitive colorimetric probe for Ag⁺ detection.