Synthesis of carbon dot-mediated gold nanostructures: experimental and quantum chemical insights into turn-on fluorescence sensing of N-acetylcysteine
In this work, a green and sustainable hydrothermal approach was employed to synthesize nitrogen-doped carbon dots (NCDs) using lemon juice as a natural precursor. The as-prepared NCDs served as eco-friendly reducing agents and stabilizing scaffolds for the subsequent in situ synthesis of gold nanoparticles (Au@NCDs). The structural and optical properties of the nanocomposite were comprehensively characterized using various analytical techniques. The intrinsic fluorescence of NCDs was significantly quenched upon formation of the composite, mainly due to non-radiative energy transfer processes. Interestingly, the quenched fluorescence could be efficiently restored upon the addition of N-acetylcysteine (NAC), enabling the development of an “off-on” fluorescent sensing platform. The sensing mechanism was systematically elucidated through a combination of spectroscopic studies and density functional theory calculations, revealing that fluorescence recovery originates from a ligand exchange process driven by the stronger binding affinity of NAC toward the gold surface, particularly via Au–S interactions. Under optimized conditions, the assay exhibited a robust linear response for NAC concentrations ranging from 4.0 to 20.0 mg/L, with a detection limit of 0.863 mg/L. These results demonstrate that the Au@NCDs system can function as a sensitive and selective OFF–ON fluorescent probe for NAC detection. This study provides a cost-effective and eco-friendly sensing platform with significant potential for analytical applications in pharmaceutical monitoring.