Colorimetric detection and quantification of Lead(II) in industrial wastewater effluent using gold nanoparticles: A comparative CIE L*a*b*/Yxy colour space and ImageJ digital analysis study

This study reports a citrate-capped gold nanoparticle (AuNP)-based colorimetric sensor for the sensitive and selective detection and quantification of Pb(II) in real industrial wastewater effluent collected over ten consecutive operational days. Pb(II)-induced aggregation of AuNPs, confirmed by UV–Vis spectroscopy and transmission electron microscopy, produced a colour transition from wine-red to blue, accompanied by a decrease in the localised surface plasmon resonance (LSPR) peak at 520 nm and emergence of a new red-shifted peak at 680 nm. Quantification was performed in parallel using UV–Visible (UV–Vis) spectrophotometry and ImageJ digital image analysis, yielding calibration curves of y = 0.0496× + 0.0238 (UV–Vis) and y = 0.79428× + 165.89147 (ImageJ), each with R 2 > 0.999. The UV–Vis method achieved a limit of detection (LOD) of 0.053 mg/L and a limit of quantification (LOQ) of 0.178 mg/L, across a linear range of 0.1–20 mg/L. One-way ANOVA confirmed no statistically significant difference between the two quantification methods (Fcalc < Fcrit; p = 0.739) on the calibration standard dataset; however, a validated calibration equation to convert ImageJ intensity to concentration could not be established for the real wastewater matrix, so ImageJ results for these samples are reported as measured intensity rather than concentration ( Section 3.8 ). CIE L*a*b*/Yxy colour space parameters (colour difference, chroma, hue angle) provided perceptually uniform confirmation of the aggregation-induced colour transition, and quantitative regression models fitted to these parameters (linear for ΔE, R 2 = 0.958; four-parameter logistic for hue angle and chroma, R 2 > 0.98) offered an independent cross-validation channel for Pb(II) quantification. The naked-eye visual detection threshold was 4 mg/L. The sensor showed high selectivity for Pb(II) over six common co-existing cations [Zn(II), Cu(II), Mn(II), Cr(VI), Ni(II), Fe(II)], which produced corrected intensity responses at least four-fold lower than that of Pb(II). Pb(II) was quantitatively detected in seven of the ten wastewater effluent samples (3.52–14.57 mg/L, original wastewater concentration), substantially exceeding both the WHO drinking-water guideline (0.01 mg/L) and the South African industrial discharge limit (0.05–0.10 mg/L), while the remaining three samples showed anomalous (negative) absorbance consistent with assay failure and are reported as not detected. The proposed platform is portable, cost-effective and suitable for on-site heavy metal monitoring in resource-limited environments.