Biogenic carbon dots from adulsa leaves (Adhatoda vasica) for naked-eye and colorimetric sensing of Ag⁺ ions

In this study, we report the green synthesis of carbon dots (CDs) from Adulsa (Adhatoda vasica) leaves via a simple and ecofriendly hydrothermal method. The synthesized CDs were characterized by UV–Vis spectroscopy, fluorescence spectroscopy, FTIR, TEM, and X-ray photoelectron spectroscopy (XPS). A distinct absorption shoulder at around 320–350 nm was attributed to n → π* transitions of C = O functional groups. TEM analysis revealed spherical CDs with an average diameter of 5 nm, while FTIR and XPS confirmed the surface functionality and elemental composition of CDs respectively. The synthesized CDs demonstrated high selectivity and sensitivity towards silver ions, as evidenced by visible colorimetric response. It showed a linear response between 0.5 and 30 µM with a regression equation of y = 0.05243 × and a detection limit of 0.165 µM. The sensing mechanism was attributed to an electron transfer from carbon dots to Ag+ ions, leading to the in situ formation of silver nanoparticles, confirmed by a new absorption broad band at 480 nm and post sensing TEM and SEAD analysis. Furthermore, the applicability of the sensor was validated using Ag+ ions spiked real water samples demonstrated good accuracy and reliability. This work presents a cost-effective, green, and efficient naked-eye optical probe for silver ion detection with promising environmental monitoring potential.