Synthesis of Gold Nanoparticles (AuNPs) Using Salt-Soluble Wheat Flour Proteins and Their Use in the Fluorescence-based Sensing of Folic Acid

Protein-stabilized metal nanoparticles have been receiving significant attention because their optical properties could be used for sensing applications. This study describes the synthesis of gold nanoparticles (AuNPs) stabilized with salt-soluble proteins extracted from wheat flour, and their use in the fluorescence-based detection and quantification of folic acid. The salt-soluble protein extract from defatted wheat flour contained at least eight components with molecular weights ranging from 7.26 to 57.0 kDa. The extract was made alkaline and then mixed with a HAuCl4 solution to synthesize the AuNPs. The wheat-AuNPs were red in color and had a surface plasmon resonance (SPR) peak at 519 nm. Based on the UV-visible spectrum, the AuNPs may have ~3.4 nm average size. The AuNPs exhibited fluorescence emission at 734 nm upon excitation at 365 nm. The fluorescence quenching of the AuNPs observed upon incremental addition of folic acid (0 to 10 μg mL-1) was fitted to the Stern-Volmer equation. From the Stern-Volmer plot, the limits of linearity (LOL), detection (LOD), and quantitation (LOQ) were determined as 22.0 μg mL-1, 0.92 μg mL-1, and 3.1 μg mL-1, respectively. Several substances, including some metal ions, were tested for their effects on the AuNPs’ fluorescence. Moreover, a proposed fluorescence-based folic acid quantification was evaluated by determining the folic acid content of several pharmaceutical formulations.