Colorimetric and Label-Free Optical Detection of Pb2+ Ions via Colloidal Gold Nanoparticles

Abstract

The detection of the lead heavy metal (Pb) in water is crucial in many chemical processes, as it is associated with serious health hazards. Here, we report the selective and precise colorimetric detection of Pb²⁺ ions in water, exploiting the aggregation and self-assembly mechanisms of glutathione (GSH)-functionalized gold nanoparticles (GNPs). The carboxyl functional groups are able to create coordination complexes with Pb²⁺, inducing aggregation amongst the GSH-GNPs in the presence of Pb²⁺ due to the chelation of the GSH ligands. The resulting aggregation amongst the GSH-GNPs in the presence of Pb²⁺ increases the aggregate size depending on the available Pb²⁺ ions, affecting the plasmonic coupling. This causes a substantial shift in the plasmon wavelength to a longer wavelength side with increasing Pb²⁺ concentration, resulting in a red-to-blue colorimetric or visual change, enabling the instant determination of lead content in water.

Keywords: calorimetric detection of metal ions; gold nanoparticles; surface plasmons.

Figure 1

UV-Vis spectra and pictures of GSH-GNPs in the (A) absence and (B) presence of 0.05 mM Pb²⁺ of supernatant solutions at pH 8 and GSH-GNPs with and without addition of 5 mM Pb²⁺. The inset in (A) shows the colorimetric display with addition of 0.05 mM Pb²⁺ of supernatant solutions compared to the control. (C) UV-Vis spectra of 83 µL of 19 mM GSH added to GNPs to make up the control investigated over 42 min and (D) UV-Vis spectra of 0.05 mM Pb²⁺ added to GSH-GNPs over a 42 min timeframe.

Figure 2

Images of colloidal solutions of GSH-GNPs at (a) 0 min and (b) 60 min of Pb²⁺ detection at various concentrations. Dynamic light scattering (DLS) data showing particle size distribution of control (Au NPs) (c) before and (d) after the addition of 120 µL of 0.05 mM Pb²⁺. (e) FESEM image of GSH-GNP aggregation induced by 50 µM Pb²⁺, and (f) enlarge scale of GSH-GNP aggregation induced by 50 µM Pb²⁺. Inset of (c) shows the FESEM image of Au NPs and inset of (f) shows the GSH-GNP aggregation induced by Pb²⁺ ions.

Figure 3

(a) Plots of the time-dependent absorption ratio (A_660/500) over 10 min for various volumes of 0.05 mM Pb²⁺ added to the control. (b) A_660/500 vs. various concentrations of Pb²⁺ in the range of 0.004–0.05 mM. (c) Images depicting the presence of decreasing volumes of 0.05 mM Pb²⁺ solution at (i) 0 min, (ii) 2 min, (iii) 4 min, (iv) 6 min, (v) 8 min, and (vi) 10 min. From left to right: control, 120 µL, 110 µL, 105 µL, 100 µL, 90 µL, 80 µL, 70 µL, 60 µL, 50 µL, and 40 µL of a 0.05 mM Pb²⁺ solution.

Figure 4

FTIR absorbance spectra of the glutathione peptide as a function of the accelerated heat stress exposure time. Reprinted from ref. [24].

Figure 5

Detection of various ions at (a) 0 min and (b) 10 min. (b) From left to right: 100 µM Fe²⁺, Zn²⁺, Ba²⁺, Ni²⁺, Ca²⁺, Cr³⁺, Mg²⁺, Mn²⁺, Cu²⁺, Cd²⁺, 50 µM Pb²⁺, and the control. (c) Absorption spectra comparing control with 100 µL of various metal ions vs. 50 µM Pb²⁺ (all metals were incubated with the control for 10 min). (d) The absorbance values (A_660/500) of the control upon addition of 100 µM of various metal ions vs. 50 µM Pb²⁺ ions and (e) schematic representation of the progression of GNPs after functionalization with GSH and the detection of Pb²⁺ through the formation of a coordination complex.