Solid-State Carbon Dots for Local Hotspot Promoted Real-Time Sequential Detection of Mercury and Iodide in Biological Samples via the Fluorescence “ON−OFF−ON” Strategy

Abstract In this work, we propose a cost-effective and equipment-free in situ optical detection approach for quick, selective and ultrasensitive sequential detection of mercury (Hg2+) and iodide (I−) via fluorescence “ON−OFF−ON” principle. The fluorescent probes, nitrogen and sulfur codoped carbon dots (PCDs) were synthesized by solvent and dopant-free environmentally benign solid-state polymerization technique. They were thoroughly characterized for their structure, morphology and optical properties. The studies confirmed presence of heteroatoms containing spherical carbon dots that display strong photoluminescence (PL). They exhibit enhanced photostability, high fluorescence quantum yield and enhanced light absorption capacity acting as efficient “optical nanoheaters”. PCDs demonstrated local hotspot-promoted selective and ultrasensitive nanomolar level detection of Hg2+ via the PL switch-OFF mechanism. Further, a PCDs-Hg2+ nonfluorescing complex was used for subsequent nanomolar detection of iodide ions, free from any interference via PL the switch-ON approach. The detection limits of 11.21 nM for Hg2+ and 2.99 nM for I− was achieved. The PL based detection was further extrapolated to several biological and environmental samples showing excellent translational ability for practical applications. To facilitate this, the PCD-based “in house” paper-based kits were successfully fabricated for in situ and on-site detection of both Hg2+and I−. The exceptional local hotspot effects of PCDs, confirmed by solar absorption capabilities, facilitate enhanced photothermal conversion, generating localized thermal gradients under solar illumination. This in turn accelerates the binding kinetics between the PCDs and analyte, substantially decreasing the interaction time between the analyte and sensor from 12 to 5 min. These findings position PCD “nanoheaters” as promising alternatives to practical sustainable dual-ion sensors for environmental and biomedical samples.