Solvent-mediated in situ growth and assembly of gold nanoparticles@carbon dots for rapid colorimetric nonenzymatic alcohol sensing

The fundamental mechanism underlying solvent-mediated carbon dots (CDs) reduction of HAuCl4 to form gold nanoparticles (AuNPs), and the surface characteristics deciding their dispersion and aggregation, are largely unclear. Monodispersed core-shell AuNPs@CDs(water) (21.9 +/- 2.7 nm) were in situ synthesized in water, while aggregated AuNPs@CDs(ethanol) (102.6 +/- 22.0 nm) were found in ethanol. We propose that the transition is mediated by solvent through changing the solubility and reducing performance of CDs, and interactions among oxidized CDs, AuNPs and solvent. Water with higher polarity promotes CDs to rapidly reduce Au3+ into Au0, and forms a stronger electrical double layer for slow growth of nearly neutral AuNPs. Concurrently, oxidized CDs(water) with higher contents of carboxyl groups coordinate with AuNPs, and stabilize AuNPs by strong electrostatic repulsive forces (zeta potential at -23.5 mV). Red color was obtained in water system. However, ethanol with lower polarity deduces the aggregation of AuNPs. Moreover, oxidized CDs(ethanol) with lower contents of carboxyl groups coordinate with AuNPs, and interact with each other through strong hydrogen bonds to form carboxyl dimers, which further deduces aggregation. Then blue color was achieved in ethanol system. Therefore, a facile AuNPs@CDs-based colorimetric ethanol sensor (linear range, 20.0%-90.0%, v/v, R2 = 0.999) with high selectivity was proposed by measuring the plasmon resonance absorbance change of AuNPs. Real sample analysis of liquors based on the standard addition method showed satisfying accuracy and excellent reproducibility. The proposed method provides new insight into nonenzymatic ethanol detection, and valuable information for the development of AuNPs-based colorimetric sensors.