Unravelling mechanism for detecting chromium on functionalized gold nanoparticles via a smartphone and spectrophotometric-based systems supported by CIEL*a*b* colour space and molecular dynamics

Abstract

This paper demonstrates the application of smartphones as well as spectrophotometers as tools to demystify the mechanism leading to colour variation of 1,5-diphenylcarbazide functionalized gold nanoparticles (DPC-AuNPs) due to chromium addition. Colour Grab 3.6.1 app was used to capture and decode colours from samples containing DPC-AuNPs with different concentrations of hexavalent Chromium [Cr(VI)] standards. The presence of Cr(VI) aggregated DPC-AuNPs resulted in a colour change from pink to blue and a decrease in the peak intensity as accompanied by a red shift of the SPR peak to 670 nm.The R colour coordinates decreased as Cr(VI) concentration was increased to 16 µM then a rapid decrease was noted between 18–25 µM and G and B colour coordinates followed the same trend. Colour difference (∆E) increased significantly as the Cr(VI) concentration increased. A rapid decrease was noticed in the hue angle between 16–25 µM while chroma decreased significantly as the Cr(VI) concentration increased. Molecular dynamics calculation of a gold cluster was used to simulate the aggregation process. The calculated radial distribution [g(r)] from cluster models of the Cr-DPC complex was two-fold more than Cr-AuNPs. This was associated with the aggregation of gold nanoparticles leading to the appearance of the blue colour of AuNPs solution which was also supported by the intensity obtained from colour Grab. This study will potentially be applied in the future for the fabrication of a Lovibond photometer for the detection of metal ions in environmental samples.