Microwave-Assisted Synthesis of Hydrophilic Carbon Dots in a Recyclable, Hydrophobic Deep Eutectic Solvent

Deep eutectic solvents (DESs) offer a green, nontoxic alternative for nanomaterial synthesis. Here, we report a rapid microwave-assisted method for synthesizing hydrophilic carbon dots (CDs) in a hydrophobic menthol/decanoic acid (2:1) DES. The process mimics reverse micelle synthesis by dispersing aqueous reactants in the hydrophobic DES, but unlike conventional reverse micelle methods, it avoids hazardous organic solvents. Nitrogen-doped CDs (N-CDs) prepared by this method exhibited spectroscopic properties comparable to those obtained via hydrothermal and water-based domestic microwave routes. Notably, the DES-based method is faster than the hydrothermal approach and yields a narrower size distribution than the domestic microwave method. The N-CDs served as fluorescence-quenching probes for p-nitrophenol, achieving a quantitation limit of 0.39 μM. To demonstrate versatility, N,S-doped CDs and polyethylenimine (PEI)-modified CDs were also synthesized in the same DES. The N,S-CDs selectively detected Fe(III) and Hg(II), while the PEI-CDs─applied for the first time in bacterial detection─showed linearity between fluorescence intensity and Staphylococcus aureus concentration (1.6 × 105–1.0 × 107 CFU/mL). The DES was reusable for at least 10 cycles with consistent fluorescence properties, underscoring its sustainability.