A smartphone-based portable device for Ag⁺ sensing using eco-synthesized carbon dots from mandarin peels

This study presents a green, cost-effective hydrothermal method for synthesizing carbon dots (CDs) from mandarin peels without toxic solvents or by-products. Spherical CDs (∼4.7 nm) were employed as both reducing and stabilizing agents for the colorimetric detection of Ag⁺ ions via the in situ formation of silver nanoparticles (Ag NPs). A distinct UV-vis absorption peak at 433 nm and a visible color change from light yellow to reddish-brown confirmed the formation of CD-Ag nanocomposites (CD-Ag NCs). The method demonstrated high sensitivity with a detection limit of 0.6 microM and a broad linear range (6-333 microM). A battery-powered Arduino-based portable device was developed for on-site detection, with real-time data transmission to smartphones via a custom app. Post-sensing, the CD-Ag NCs were reused as catalysts for 4-nitrophenol reduction, achieving 90% efficiency within 9 minutes. This integrated platform offers a sustainable approach for environmental monitoring and catalytic applications.