A smartphone-based application for the colorimetric detection of lead in wastewater using gold nanoparticles

Abstract

This study was conducted to develop and validate a straightforward, selective, and efficient colorimetric optical sensor that relies on gold nanoparticles for the detection and quantification of Pb(II) in wastewater systems. Gold nanoparticles were synthesized by chemical reduction of gold(III) chloride trihydrate, with trisodium citrate as a stabilizing agent to prevent aggregation. The synthesized AuNPs were characterized by ultraviolet-visible (UV-Vis) spectroscopy and transmission electron microscopy (TEM), confirming a spherical morphology with an average diameter of approximately 20 nm and a characteristic surface plasmon resonance (SPR) peak at 520 nm. The detection mechanism was based on Pb(II)-induced nanoparticle aggregation, resulting in a visible color change from red to blue. Quantitative analysis was conducted using both UVVis spectroscopy and digital image processing in ImageJ. The method was further validated using spiked river and tap water samples across a concentration range of 0.1–20 mg/L. Selectivity was assessed in the presence of potential interfering ions, while colorimetric response was analysed using the CIE Lab* color space. The presence of Pb(II) resulted in a red shift in the UV-Vis spectra, with the identification of two distinct SPR peaks at 520 nm and 680 nm, confirming nanoparticle aggregation. The colorimetric sensor demonstrated a strong linear response over the concentration range of 0.0520.0 mg/L (R² = 0.9901), with a detection limit of 0.01 mg/L. The recovery studies yielded values of 93%-99% for the ImageJ method and 88%-98% for the UV-Vis method. The Analysis of wastewater samples revealed Pb(II) concentrations ranging from 0.0 to 0.0379 mg/L. Then, minimal interference from other metal ions (Cr, Cu, Fe, Ni, Zn, Mn) was observed, indicating high selectivity. Colour analysis showed a systematic transition from red to green in the CIE Lab* space, with increasing Pb(II) concentration strongly correlating with measurable colour variation. The colorimetric sensor based on AuNPs developed provides a swift, cost-efficient, and exceptionally selective means for detecting Pb(II) in wastewater. The robust alignment between ImageJ and UV-Vis results indicates that smartphone image analysis can serve as a reliable alternative to standard spectrophotometric techniques. This strategy holds significant potential for on-site environmental assessments and in settings with limited resources.