Lignin-derived red-emitting carbon dots for colorimetric and sensitive fluorometric detection of water in organic solvents

Water contained in organic solvents or products in chemical industries, as contaminants, poses an adverse risk in chemical reaction, life or environmental safety. However, conventional fluorescent water sensing suffers from drawbacks, including limited organic solvents, narrow linear range, lack of visual detection, single detection strategy, and others. Herein, a novel type of red-emitting carbon dots (RCDs) has been created via one-step solvothermal synthesis based on biomass (e.g., lignin) as the carbon source and p-phenylenediamine (PPD) as the nitrogen source. Colorimetric and fluorometric detection of water in organic solvents has been demonstrated. The RCDs showed excitation-independent photoluminescence (PL) in different solvents and solvatochromic behavior, red in water, orange in ethanol, yellow in N,N-dimethyl formamide (DMF), and green in acetone. Remarkably, detection of water content in six organic solvents, including polar solvents (ethanol, acetone, dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and DMF) and apolar solvent (ether), was performed. With increasing water content in solvents, emission colors changed from green to red, or yellow to red, offering qualitative sensing of water. Furthermore, a broad linear detection range (10-90%), low limits of detection (LOD) (e.g., 0.36% for ethanol and 0.082% for acetone), and good generality for various organic solvent systems were realized. Particularly, dual sensing strategies, including PL quenching and shift with water in various solvents, were achieved simultaneously, showing great potential for the development of advanced optical sensors with high performance.