Over the past decade, carbon dots have ignited a burst of interest in many different fields, including nanomedicine, solar energy, optoelectronics, energy storage, and sensing applications, owing to their excellent photoluminescence properties and the easiness to modify their optical properties through doping and functionalization. In this review, the synthesis, structural and optical properties, as well as photoluminescence mechanisms of carbon dots are first reviewed and summarized. Then, we describe a series of designs for carbon dot-based sensors and the different sensing mechanisms associated with them. Thereafter, we elaborate on recent research advances on carbon dot-based sensors for the selective and sensitive detection of a wide range of analytes, including heavy metals, cations, anions, biomolecules, biomarkers, nitroaromatic explosives, pollutants, vitamins, and drugs. Lastly, we provide a concluding perspective on the overall status, challenges, and future directions for the use of carbon dots in real-life sensing.

This work reports the synthesis of nitrogen and sulfur co-doped carbon dots (NS-CDs) via a one-step facile microwave-assisted pyrolysis using citric acid and thiourea as the carbon precursor and dopant, respectively. The resultant NS-CDs were found to be uniform in size and can be dispersed in aqueous solution. With an average size of 4.73 ± 0.65 nm, the NS-CDs exhibit excitation-dependent and pH-dependent photoluminescence properties in the visible range. Moreover, the NS-CDs also possessed a relatively high photoluminescence quantum yield of 28.9% and strong photostability in a high ionic strength environment. The NS-CDs were highly selective for Fe³⁺ detection in a water sample, achieving a limit of detection (LOD) of 0.16 μM. On top of this, the NS-CDs also demonstrated high detection sensitivity with a LOD of 0.17 μM and improved selectivity to Fe³⁺ in an acidic environment (pH 2). Specifically, the interference effect from Cu²⁺, Co²⁺, and Ag⁺ was lowered by 15%, 18%, and 64% respectively, highlighting an additional merit for the as-synthesized NS-CDs. This concept of pH tuning can be implemented to exploit the detection capability in both neutral and acidic environments to create specific analytical responses correlating to a specific targeted metal ion. Finally, we also demonstrated the versatility of the NS-CDs by employing them as a low-toxicity fluorescent label and for semi-quantitative Fe³⁺ sensing in cancer cells.