Synthesis and characterization of 3-(p-tolyl)-2,3-dihydropyrazolo[3,4-b]indole-1(4H)-carbothioamide functionalized gold nanoparticles for the CIEL∗a∗b∗/Yxy colorimetric detection of Cr(VI)

Abstract

In this study we present a simple colorimetric assay that was used to develop a gold nanoparticles (AuNPs) enabled optical sensor. The colorimetric sensor was fabricated using the ligand 3-(p-tolyl)-2,3-dihydropyrazolo[3,4-b]indole-1(4H)-carbothiomide (TRPIDA_CH3) which was synthesized through the multicomponent reaction of toluidine, thio-semi-carbazide and indole. The synthesized ligand was fully characterized by 1H NMR, 13C NMR, 2D NMR, TOF-MS and elemental analysis. Spherical gold nanoparticles depicted two well-separated surface plasmon resonance (SPR) peaks that were observed at 520 nm and 645 nm. The addition of Cr (VI) into TRPIDA_CH3-AuNPs solution resulted in a decrease of SPR intensity at 520 nm whilst there was an increase in intensity at 645 nm. ImageJ software was used to measure the CIEL∗a∗b∗/Yxy colour dynamics between the reaction of TRPIDA_CH3-AuNPs and Cr(VI). The chromaticity diagram of gold nanoparticle complex with TRPIDA_CH3 correlated with the colour change as observed from RGB values after addition of different concentration of the chromium standards. Furthermore, recoveries for both tap and river water, after spiked with chromium, ranged from 72 to 101% with a limit of detection and limit of quantification of 0.14 and 0.47 μM, respectively. Seven interfering ions, viz., Cr, Cu, Fe, Ni, Zn, Pb and Mn were investigated: low detection limits were observed suggesting minimum interference with the desired analyte occurred. This colorimetric assay is simple and demonstrates high selectivity to Cr (VI) over other heavy interfering metal ions that were tested.