Carbon dots, an emerging class within the carbon allotrope family, have gained significant attention largely due to their versatile and tunable physico-chemical and optical properties.

Carbon dots (CDs) are nanoparticles with tunable physicochemical and optical properties. Their resistance to photobleaching and relatively low toxicity render them attractive alternatives to fluorescent dyes and heavy metal-based quantum dots in the fields of bioimaging, sensing, catalysis, solar cells, and light-emitting diodes, among others. Moreover, they have garnered considerable attention as they lend themselves to green synthesis methods. Increasingly, one-pot syntheses comprising exclusively of renewable raw materials or renewable refined compounds are gaining favor over traditional approaches that rely on harsh chemicals and energy intensive conditions. The field of green CD synthesis is developing rapidly; however, challenges persist in ensuring the consistency of their properties (e.g., fluorescence quantum yield) relative to conventional preparation methods. This has mostly limited their use to sensing and bioimaging, leaving opportunities for development in optoelectronic applications. Herein, we discuss the most common green CD synthesis and purification methods reported in the literature and the renewable precursors used. The physical, chemical, and optical properties of the resulting green-synthesized CDs are critically reviewed, followed by a detailed description of their applications in sensing, bioimaging, biomedicine, inks, and catalysis. We conclude with an outlook on the future of green CD synthesis. Future research efforts should address the broad knowledge gap between CDs synthesized from renewable versus non-renewable precursors, focusing on discrepancies in their physical, chemical, and optical properties. The development of cost effective, safe, and sustainable green CDs with tunable properties will broaden their implementation in largely untapped applications, which include drug delivery, photovoltaics, catalysis, and more.

Passivation of carbon dots, via nitrogen-doping, enhances their photoluminescent properties yet the impact of the chemical composition of the passivating molecule remains poorly understood. In this work, we seek to clarify the role of nitrogen-doping in the observed fluorescence enhancement of carbon dots. Starting with citric acid, and using microwave-assisted synthesis, we systematically passivate carbon dots with a series of aliphatic amines with increasing nitrogen content. Significant changes in their surface chemistry are observed with increased N-doping as evidenced by infrared and X-ray photoelectron spectroscopies that clearly show a progressive decrease in the oxygen : carbon ratio and a concomitant increase in the nitrogen : carbon counterpart. Spectroscopic analysis reveals that surface passivation results in excitation-independent fluorescence with fluorescence quantum yields reaching 56.7%. We note that not all amines passivate carbon dot surfaces equally, with triamines offering the highest fluorescence quantum yields. Our results clearly demonstrate that the chemical makeup of the passivating agent is critical in tailoring the physico-chemical and optical properties of carbon dots.

Classical methods for the detection of heavy metal ions in water are tedious and time consuming. Thus, there exists a need to develop novel sensing technologies for more sensitive and rapid detection. While optical-based assays using fluorescent nanomaterials have been reported, they are typically dependent on changes to the signal intensity, which brings about several disadvantages for potential environmental monitoring. Moreover, most of these systems only demonstrate selectivity and sensitivity to one cation. Herein, we report the use of a novel dual absorbing/fluorescing carbon dot system and develop a ratiometric assay to detect heavy metal ions such as Co²⁺, Fe³⁺, Hg²⁺, and Pb²⁺. These carbon dots simultaneously absorb and fluoresce in both blue and red regions of the spectrum. Their optical absorbance signature differs depending on the metallic cation present in solution and the absorbance pattern also varies as a function of the metal cation concentration. This effect is also seen in fluorescence assays where selective quenching of the red fluorescence is observed. This quenching behavior is ascribed to the interaction of the metallic cations with the fluorescent surface states of the carbon dot. Using either absorbance or fluorescence quenching assays, a linear response in the nanomolar range (1–961 nM) is observed with detection limits of 96.8, 61.7, 39.5, 37.1 nM for Co²⁺, Fe³⁺, Hg²⁺, and Pb²⁺, respectively.

Highly sensitive non-invasive temperature sensing is critical for studying fundamental biological processes and applications in medical diagnostics. Nanoscale-based thermometers are promising non-invasive probes for precise temperature sensing with subcellular resolution. However, many of these systems have limitations as they rely on fluorescence intensity changes, deconvolution of peaks, or the use of hybrid systems to measure thermal events. To address this, we developed a fluorescence-based ratiometric temperature sensing approach using carbon dots prepared via microwave synthesis. These dots possess dual fluorescence signatures in the blue and red regions of the spectrum. We observed a linear response as a function of temperature in the range of 5–60 °C with a thermal resolution of 0.048 K⁻¹ and thermal sensitivity of 1.97% C⁻¹. Temperature-dependent fluorescence was also observed in HeLa cancer cells over a range of 32–42 °C by monitoring changes in the red-to-blue fluorescence signatures. We demonstrate that the ratiometric approach is superior to intensity-based thermal sensing because it is independent of the intracellular concentration of the optical probe. These findings suggest that dual-emitting carbon dots can be an effective tool for in vitro and possibly in vivo fluorescence nanothermometry.

Carbon dots have garnered significant attention owing to their versatile and highly tunable optical properties; however, the origins and the underlying mechanism remains a subject of debate especially for dual fluorescent systems. Here, we have prepared carbon dots from glutathione and formamide precursors via a one-pot solvothermal synthesis. Steady state and dynamic techniques indicate that these dual fluorescent dots possess distinct emissive carbon-core and a molecular states, which are responsible for the blue and red optical signatures, respectively. To further glean information into the fluorescence mechanism, electrochemical analysis was used to measure the bandgaps of the two fluorescent states, while femtosecond transient absorption spectroscopy evidenced the two-state model based on the observed heterogeneity and bimodal spectral distribution. Our findings provide novel and fundamental insights on the optical properties of dual fluorescent dots, which can translate to more effective and targeted application development particularly in bioimaging, multiplexed sensing and photocatalysis.