Biomass-derived carbon quantum dot: "On–off-on" fluorescent sensor for rapid detection of multi-metal ions and green photocatalytic CO2 reduction in water.

We have developed carbon quantum dots (CQDs) with excellent photoluminescence (PL) properties from macaúba (Acrocomia aculeate) fibers; a widely available cellulosic biomass species of palm trees in South America. As-prepared CQDs showed quasi-spherical morphology with high aqueous solubility and strong excitation-dependent fluorescence behaviour. Interestingly, the CQDs display fluorescence 'turn-off' response with excellent sensitivity toward multi-metal ions including Fe³⁺, Cu²⁺ and Hg²⁺ with very low detection limits of 0.69 μM, 0.99 μM, 0.25 μM, respectively. Notably, ascorbic acid (AA) induced a change in the (turn-off) fluorescence of Fe³⁺-CQDs, which caused an almost 70% revival of fluorescence (turn-on) by displacing Fe³⁺ ions. We have also harnessed CQDs as the visible-light-induced photocatalyst to reduce CO₂ in water. Especially, the CQDs efficiently promote the photocatalytic reduction of CO₂ into methane (CH₄) with an evolution rate of 99.8 nmol/g at 436 nm in aqueous conditions. This indicates that the CQDs provide abundant active sites for CO₂ adsorption and thus enhance the separation and migration of photo-induced charge carriers that efficiently reduce CO₂ into CH₄ without any co-catalyst in 100% water. [ABSTRACT FROM AUTHOR]