Ficin-functionalized gold nanoparticles for selective UV-Vis colorimetric detection of Hg²⁺, Cu²⁺, and Cr⁶⁺: optimization and RGB image-based quantification

Nowadays heavy metal pollution poses a critical environmental and public health challenge making affordable detection methods for water quality monitoring urgently needed. To address this issue, a novel colorimetric detection system based on ficin-functionalized gold nanoparticles (F-AuNPs) was developed and evaluated using UV-Vis spectrophotometry and smartphone-based RGB image analysis for the detection of Hg²⁺, Cu²⁺, and Cr⁶⁺. Response Surface Methodology using Box-Behnken Design optimized the functionalization conditions of Ficin to AuNPs resulting in an excellent model fit (R² = 0.9818, adequate precision = 26.992). The optimized F-AuNPs exhibited selective aggregation with Hg²⁺, Cu²⁺, and Cr⁶⁺ causing a visible color transition from red to blue gray but remained dispersed in the presence of nine other tested metal ions. UV-Vis spectrophotometry demonstrated strong linearity with LOD/LOQ values of 0.02/0.06 ppm for Hg²⁺, 0.05/0.17 ppm for Cu²⁺, and 0.10/0.33 ppm for Cr⁶⁺. RGB image analysis yielded comparable sensitivity across all three metals, with LOD/LOQ values of 0.02/0.06 ppm for Hg²⁺, 0.06/0.17 ppm for Cu²⁺, and 0.11/0.33 ppm for Cr⁶⁺. Spike-recovery tests in tap and lake water confirmed feasibility of the F-AuNPs with spiked samples validating the method’s applicability for environmental monitoring. Thus, the F-AuNPs colorimetric platform provides a low-cost operation-minimal sample preparation that functions as practical field-ready tool for future near real time monitoring of these heavy metals.

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