Quercetin Reduced and Stabilized Gold Nanoparticle/Al3+: A Rapid, Sensitive Optical Detection Nanoplatform for Fluoride Ion

In this contribution, facile synthesis of gold nanoparticles (AuNPs) at ambient conditions is reported based on the use of the polyphenolic compound quercetin (QT) as the reducing and stabilizing agent at room temperature (RT). Under alkali-induced pH adjustment of QT solution and stirring conditions at RT, QT could quickly reduce gold salt (Au³⁺) into its nanoparticle form (Au⁰), resulting in the formation of a sparkling red color colloidal solution (AuNPs) with an absorption maximum at 520 nm. Further, Fourier transform infrared spectroscopy (FTIR) was employed to showcase the role of QT in the nanomaterial's synthesis process. The formed QT-AuNPs responded swiftly to Al³⁺ charging with color perturbation from red to grayish-purple, coupled with an absorption spectra red shift, owing to Al³⁺-induced aggregation of QT-AuNPs. However, when fluoride ion (F^-) was pre-mixed with an optimized Al³⁺ concentration, reversed color changes from grayish-purple to red were observed with a blue shift in the absorption spectra. Simply put, F^- formed a complex with Al³⁺, thus preventing Al³⁺-induced aggregation of QT-AuNPs. The analytical response A₅₂₀/A₆₅₀ was linear with F^- concentration ranging from 25.0 to 250.0 µM and 250.0-600.0 µM, with a detection limit of 7.5 µM. The developed QT-AuNPs/Al³⁺ detection probe was selective to only F^- charging, in comparison with other possible interfering anions. Real sample potentiality of the developed sensor was demonstrated on tap water samples, toothpaste, and fluoride-rich mouthwash, with reliable accuracy.