Reliability of Multi-Emissive Carbon Quantum Dots for Multiplexing; Assessing the Figures of Merit

Incredible properties of quantum dots (QDs) have once again been acclaimed with this year's (2023) Nobel prize in chemistry. On the other hand, the invention of multicolour molecular imaging of cell surface receptors for tumour diagnosis by Koyama and group has opened up a new era in diagnostics. Among them carbon quantum dots (CQDs) are interesting class of fluorescent nanomaterials, superior in terms of low toxicity, high solubility and biocompatibility along with simple and cost-effective synthesis processes unlike the traditional metal chalcogenide or perovskite quantum dots. Multi emissive fluorescence property of these carbon quantum dots are very useful in multiplex sensing. Their excellent biocompatibility and low toxicity have attracted researchers to use them extensively for biosensing and imaging of multiple analytes at a time. Core state emission from π-domains and surface state emissions of functional groups surrounding CQDs play a major role in achieving the multicolour emissions and this review discusses the various strategies used to achieve desired multi colour emissions, yet preserving their stability, non-interactive emissive states and quantum yields. Their fine tuning via variation in temperature, pH, time, and heteroatom doping has been comprehensively discussed. A thorough history compared to a list of characteristics for creating effective multicolour CQDs will point us in the proper route. This minireview also assesses the electronic band structure of these multicolour CQDs, their stability with respect to multi emissions, photoluminescence quantum yields, approaches employed for tunability of their optical band gaps, and also enhancement of carrier lifetimes, to arrive at conclusions on the reliability of these materials for multiplexing. The mechanisms namely chemical coupling, FRET, On-Off, Ab-antigen interactions involved in sensing mechanisms involving these materials are analysed in depth. Ultimately, the present obstacles and future directions for the use of these CQDs in sensing applications are discussed.