Biomass based carbon quantum dots in sensor applications: a review (2021-2025).

The past few years have witnessed an exponential rise in the development of carbon quantum dots (CQDs) derived from natural biomasses, rendering them a green and multifunctional platform for sensor applications. This comprehensive review provides a critical assessment of advancements between 2021 and 2025 in the syntheses, physicochemical characterizations, and sensing applications of biomass-derived CQDs. Different precursors-e.g., fruit peels, leaves, and agricultural waste-have been effectively transformed into highly fluorescent CQDs using hydrothermal, microwave-assisted, and pyrolytic routes, with synthesis times as short as 10 min for certain microwave procedures. The structural studies exhibit quasi-spherical morphologies (2-10 nm) and partial graphitization, and the optical studies confirm excitation-dependent fluorescence with quantum yields as high as 30%, particularly in nitrogen- and sulfur-doped systems. Biomass-derived CQDs have shown superb selectivity and sensitivity to a broad spectrum of analytes. For example, CQDs synthesized from Solanum nigrum leaves reported detection limits as low as 8 nM for Fe³⁺, while CQDs synthesized from pitaya peels enabled sensitive detection of antibiotics via aggregation-induced emission effects. Environmental targets such as glyphosate, NH₃, and CH₂O have also been detected at nanomolar levels, employing mechanisms like static/dynamic fluorescence quenching, electron transfer, and FRET. Despite these advances, there are still challenges in large-scale production, standardization of fabrication protocols, and integration of CQDs into real-time sensing platforms. In the future, the work is going to be further developed through the adoption of green synthesis approaches, regulatory standardizations, and integration of CQDs into wearable and portable diagnostic devices. Such developments point to the prospects of biomass-sourced CQDs as green, cost-effective, and ultrasensitive nanomaterials for next-generation chemical, biological, and environmental sensors.