Green synthesis of carbon dots from spent coffee grounds via ball-milling: Application in fluorescent chemosensors

Carbon dots (CDs) are nanoparticles that have gained widespread attention due to their unique properties including multicolored fluorescence, photochemical stability, and biocompatibility. However, the conventional approaches for the preparation of CDs typically require high energy input and toxic reagents. Herein, a sus-tainable, high throughput approach is reported for producing uniform CDs. Specifically, spent coffee grounds (SCGs) are used to produce coffee ground-derived carbon dots (CFCDs) via a simple wet ball-milling process. The versatility of our approach can be extended to the synthesis of heteroatom-doped CDs and the preparation of CDs from various SCG sources. Also, the functional groups of the CFCDs are manipulated by the addition of citric acids during the ball-milling process to produce carboxylic acid-functionalized carbon dots (CA-CFCDs). CA-CFCDs exhibit highly selective and sensitive detection of Fe3+ ions in aqueous solution via fluorescence quenching, and the relevant quenching mechanism is investigated. Finally, we demonstrate the fluorescent microgel sensor exhibiting a rapid, ratiometric fluorescence change upon the addition of Fe3+ ion by encapsu-lating the CA-CFCDs within the microgel matrix.