Nitrogen-doped red carbon dots: A dual-functional nanoplatform for visual detection of Ag⁺ and photothermal antimicrobial applications

The development of multifunctional nanomaterials for efficient detection of hazardous metal ions and effective antibacterial treatment is of great significance in environmental monitoring and public health protection. In this study, nitrogen-doped red carbon dots (R-CDs) with a fluorescence emission peak of 633 nm were synthesized through a facile one-pot hydrothermal approach using p -phenylenediamine and D-sorbitol as precursors. The obtained R-CDs possess outstanding photostability and water dispersibility. Based on a combined mechanism of electron transfer and static quenching, the R-CDs serve as a highly sensitive fluorescent probe for Ag + detection, achieving a detection limit as low as 0.43 μM. To extend their practical utility, a portable smartphone-assisted sensing platform coupled with RGB color analysis was constructed, allowing direct capture and analysis of fluorescence images for rapid on-site visual quantification of Ag + with a detection limit of 2.95 μM. Moreover, the R-CDs exhibit a remarkable photothermal conversion efficiency of 38.86 % under 808 nm laser irradiation. Benefiting from this property, the material achieves an antibacterial efficiency of up to 99.91 % against E. coli through photothermal ablation. This work not only offers a reliable and field-deployable strategy for Ag + monitoring but also demonstrates the great potential of R-CDs as an integrated nanoplatform for environmental safety and biomedical applications.