Carbon Quantum Dot-Based Sensors: Photochemical Principles and Multimodal Applications

Carbon quantum dots (CQDs) are inherently photochemically active nanomaterials, exhibiting excitation-dependent emission, proton-responsive surface states, and modifiable redox properties, enabling various sensing applications across fluorescence, electrochemistry, and electrochemiluminescence (ECL) modalities. This comprehensive review elucidates their methodologies, including PET-driven "turn-off/on" fluorescence, ratiometric pH sensing, electrocatalytic currents, and co-reactant-amplified ECL, achieving low detection limits for metal ions, biomolecules, and environmental analytes. Surface-mediated responsiveness is essential to CQD performance, offering exceptional sensitivity while also conferring inherent cross-reactivity. Meta-analysis was conducted using data extracted from previously published studies on CQDs for the detection property, in which the failure ratio was computed as the number of unsuccessful detections divided by the total number of tests reported in each study. Additionally, critical examination reveals inconsistencies in the limit of detection (LOD) metrics and mechanistic uncertainties, as well as strategies for enhancing selectivity through rational doping and molecular recognition hybrids.