Nitrogen-doped SiQDs and their PVA fluorescent films for rapid fluorescence detection of chlorogenic acid in green coffee beans.

Nitrogen-doped silicon quantum dots (N-SiQDs) were prepared via a one-pot hydrothermal route, and subsequently utilized to establish a fluorescent sensing system for the rapid determination of chlorogenic acid (CGA). Mechanistic studies revealed a mixed quenching mechanism. The inner filter effect arises from the strong spectral overlap between CGA absorption and N-SiQD excitation. Meanwhile, CGA forms a non-fluorescent ground-state complex with N-SiQDs via hydrogen bonding and/or electrostatic interactions, as confirmed by a new absorption peak and a progressive emission red-shift. Such complexation neutralizes surface trap states, resulting in a surprising extension of fluorescence lifetime from 1.84 to 2.71 ns. At elevated CGA concentrations, dynamic collision quenching further contributes, as evidenced by temperature-dependent Stern-Volmer plots. Under optimized conditions, a linear detection window from 0.5 to 12 μg mL-1 and a limit of detection (LOD) of 0.14 μg mL-1 were obtained for the solution-phase sensor, along with a response time of just 30 s. A portable N-SiQDs/PVA fluorescent film was also fabricated, exhibiting optimal excitation/emission at 360/450 nm due to hydrogen bonding-induced surface state lowering. The film sensor exhibited a linear response ranging from 1 to 12 μg mL-1, along with the LOD of 0.28 μg mL-1 and a response time of 2 min. It also demonstrated satisfactory stability and the ability for visual detection. Both approaches were effectively employed to quantify CGA in normal and decaffeinated green coffee beans, and the obtained data were in good agreement with those from HPLC-UV. Compared with other methods, this work offers advantages in fast analysis speed and good device portability, providing a practical tool for CGA detection in the coffee industry.