Smartphone-based colorimetric detection of milk adulteration via an AgNP/AuNP/TMB-AuGel sensor array and multivariate...

In this study, a novel colorimetric sensor array composed of three nanomaterial-based sensing elements-silver nanoprisms (Ag Prism), gold nanoparticles (Au NPs), and a TMB-Au hydrogel (TMB-Au Gel)-was developed to identify and discriminate between three major milk adulterants: hypochlorite (ClO^-), hydrogen peroxide (H₂O₂), and dichromate (Cr(vi)). The sensing elements were optimized in terms of nanoparticle volume ratios, reagent concentrations, and reaction time to achieve distinct and reproducible color responses. Smartphone-based imaging captured these changes, and RGB values were extracted for analysis. The sensor responses exhibited good reproducibility, with relative standard deviations below 5% across five replicate measurements (Cr(vi): 2.8%, ClO^-: 4.3%, H₂O₂: 1.2%). The limits of detection were 0.62, 0.02, and 0.03 mmol L^-1 for H₂O₂, ClO^-, and Cr(vi), respectively. Recovery tests in spiked milk samples confirmed the reliability of the system (H₂O₂: 100-106%, ClO^-: 99-112%, Cr(vi): 103-115%). Pattern recognition analysis using linear discriminant analysis and hierarchical cluster analysis revealed excellent discrimination between the three adulterants, with the first two discriminant functions explaining 100% of variance (LD1: 56%, LD2: 44%) and producing clearly separated clusters. Leave-one-out cross-validation yielded an overall classification accuracy of 81%, with sensitivities ranging from 0.74% to 0.94%, specificities from 0.84% to 0.97%, and precisions from 0.75% to 0.93%. Analysis of loading plots highlighted the significance of the green channel in Au NPs and the blue channel in TMB-Au Gel for differentiating adulterants. These findings demonstrate that by integrating smartphone-based imaging for RGB extraction, the method allows for on-site, real-time analysis without specialized equipment. This capability enhances practical food monitoring, enables early detection of contaminants, and offers a user-friendly platform to improve food safety and support public health initiatives.