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

Abstract

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.

Keywords: aggregation agent; colorimetry; fluoride ion detection; gold nanoparticles; quercetin; real samples.

Scheme 1

Synthesis strategy for QT-AuNPs at RT.

Figure 1

(a) UV–Vis absorption spectra of QT-AuNPs realized under different gold salt concentrations; the inset shows the phot images. (b) Plot of A₅₂₀/A₆₅₀ against gold salt concentrations. (c) FTIR of the QT-green line and QT-AuNPs-red line and (d) TEM image of QT-AuNPs produced under 0.523 mM gold salt concentration as the optimum condition.

Figure 2

(a) UV–Vis absorption spectra of a—QT-AuNPs, b—Al³⁺ charging on QT-AuNPs, and c—Al³⁺/F⁻ pre-mixed before QT-AuNPs addition; Al³⁺ = 150 µM and F⁻ = 600 µM. (b) Photo images of varying concentrations of Al³⁺ (0–400 µM) on QT-AuNPs.

Figure 3

(a) Photo images of the QT-AuNPs/Al³⁺ colorimetric probe under F⁻ ion charging from 0 to 600 µM. (b) UV–Vis absorption spectra of QT-AuNPs/Al³⁺ detection probe corresponding to the photo images obtained in F⁻ detection from 0 to 600 µM. (c) Plot of (A₅₂₀/A₆₅₀) against F⁻ concentration from 0 to 600 µM. Plot of (A₅₂₀/A₆₅₀) against F⁻ concentration from (d) 25–250 µM, and (e) 250–600 µM. The concentration of Al³⁺ = 150 µM.

Scheme 2

Mechanism of the QT-AuNPs/Al³⁺-based detection system for F⁻.

Figure 4

(a–c) TEM images of the QT-AuNPs/Al³⁺ colorimetric probe under F⁻ ion charging at 0, 150, and 500 µM. (d–f) Hydrodynamic diameter from DLS corresponding to (a–c) of TEM and (g) zeta potential under F⁻ concentration charging at a = 0, b = 150, and c = 500 μM. The concentration of Al³⁺ = 150 µM.

Figure 5

UV–Vis absorption spectra of QT-AuNPs/Al³⁺ for repeated F⁻ detection at (a) 150 and (b) 500 µM; the inset shows the photo images of the blank and the tested F⁻, for n = 8. (c) Photo images of F⁻ and other possible interfering anions, where 1 = Br⁻, 2 = Cl⁻, 3 = I⁻, 4 = NO₂⁻, 5 = NO₃⁻, 6 = SO₄²⁻, 7 = S₂O₃⁻, 8 = SO₃²⁻, 9 = HCO₃⁻, 10 = CO₃²⁻, 11 = SCN⁻, 12 = H₂PO₄⁻, 13 = HPO₄²⁻, 14 = F, 15 = ALL, F⁻ = 300 µM, and the others were 3000 µM. The concentration of Al³⁺ = 150 µM, (d) UV–Vis absorption spectra corresponding to the shown images in (c,e). Plot of (A₅₂₀/A₆₅₀) against tested anions and F⁻.

Figure 5

UV–Vis absorption spectra of QT-AuNPs/Al³⁺ for repeated F⁻ detection at (a) 150 and (b) 500 µM; the inset shows the photo images of the blank and the tested F⁻, for n = 8. (c) Photo images of F⁻ and other possible interfering anions, where 1 = Br⁻, 2 = Cl⁻, 3 = I⁻, 4 = NO₂⁻, 5 = NO₃⁻, 6 = SO₄²⁻, 7 = S₂O₃⁻, 8 = SO₃²⁻, 9 = HCO₃⁻, 10 = CO₃²⁻, 11 = SCN⁻, 12 = H₂PO₄⁻, 13 = HPO₄²⁻, 14 = F, 15 = ALL, F⁻ = 300 µM, and the others were 3000 µM. The concentration of Al³⁺ = 150 µM, (d) UV–Vis absorption spectra corresponding to the shown images in (c,e). Plot of (A₅₂₀/A₆₅₀) against tested anions and F⁻.

Figure 6

UV–Vis absorption spectra of QT-AuNPs/Al³⁺ for real samples F⁻ detection, including (a) tap water, (b) toothpaste, and (c) fluoride-rich mouthwash; the insets show the photo images corresponding to the blank sample, unpiked sample, and standard F⁻-spiked real samples at two concentrations, c and d. Al³⁺ concentration was at 150 µM.