Development of CIELAB colour system for colorimetric detection of heavy metals in wastewater using metal nanoparticles
Authors:   , Shange Sindisiwe Fortunate 
Published: January 2022

This study presents a simple colorimetric assay that was used to develop gold nanoparticles (AuNPs) enabled optical sensor. The sensor was fabricated using 3-(p-tolyl)-2,3- dihydropyrazolo[3,4-b] indole-1(4H)-carbothioamide (TRPIDA_CH3) complex synthesized through one pot reaction of toluidine, thio-semi-carbazide and indole in the presence of indium chloride as a catalyst under reflux. The attained product was then characterized fully by Fourier Transform Infrared (FT-IR), 1H and 13C Nuclear Magnetic Resonance (NMR), Time of flight (Mass Spectroscopy (TOF-MS) and elemental analysis for selective detection of hexavalent chromium (Cr(VI)). Two well-separated Surface Plasmon Resonance (SPR) peaks were observed in the spectra at 520 nm and 645 nm, respectively. The introduction of Cr(VI) into TRPIDA_CH3-AuNPs solution resulted in a decrease of SPR intensity at 520 nm with an increase of the peak at 645 nm. ImageJ software version 1.8.0_172 was used to measure the colour dynamics between the reaction of TRPIDA_CH3-AuNPs and Cr(VI) for image processing. The CIE L*a*b* colour system was utilized to analyse the digital images obtained which were converted to CIE Yxy chromaticity diagram. The chromaticity diagram of gold nanoparticle TRPIDA_CH3 complex was in agreement with colour change as observed from RGB values after addition of different concentration of the chromium standards. The obtained recoveries for both tap and river water which was spiked with chromium ranged from 72 to 101 % with a limit of detection (LOD) and limit of quantification (LOQ) of 0.14 and 0.47 µM, respectively. Nine possible interfering ions (Cr, Cu, Fe, Ni, Zn, Pb and Mn) were investigated and showed low detection, thus, indicating low interference with the analyte of choice. Additionally, a significant feature of this method is that it involves a simple technique exhibiting high selectivity to Cr(VI) over other heavy metal ions that were tested. The application of CIE L*a*b*/Yxy colour space based on the TRPIDA_CH3-AuNPs aggregation to quantify Cr(VI) in wastewater effluent was studied. The colorimetric sensor showed an excellent linear range of 0.01-100.0 µM (R2=0.9856). Additionally, the residual plot showed that residual errors were randomly distributed, meaning we should accept the results of a linear regression. The wastewater effluent samples were collected over a period of 10 days and each sample was analysed in triplicate for statistical purposes. The concentration in the collected wastewater effluent samples were in the range of 0.5-25.0 µM. Furthermore, the measured concentrations of Cr(VI) in wastewater effluent samples using the proposed colorimetric method agreed with those obtained when using the traditional 1,5-diphenyl carbazide (DPC) method. The DPC method also showed an excellent linear range of 10.0-100.0 µg/L (R2=0.9955) with a residual plot showing random distribution of residual errors. The RGB colour coordinates of the TRPIDA_CH3-AuNPs with wastewater effluent were compared with one without TRPIDA_CH3-AuNPs to determine the effect of the TRPIDA_CH3-AuNPs on the water samples. Development of a Smartphone and spectrophotometric based systems for colorimetric detection of Cr(VI) using functionalized AuNPs supported by CIEL*a*b*/Yxy colour space and molecular dynamics was also conducted in this study. This demonstrated the comparative study of the application of smartphone as well as spectrophotometer as tools to detect colour variation of functionalized DPC-AuNPs. These tools were demonstrated for their potential use as a colorimetric device for detecting Cr(VI) in wastewater. Color Grab 3.6.1 app was used to capture and recognize colours from samples containing gold nanoparticles with different concentrations of chromium standards. The RGB values obtained were compared with those obtained from spectrophotometer. It was observed that DPC-AuNPs aggregated in the presence of Cr(VI), with clear colour change from pink to blue due SPR of AuNPs. This resulted in a decrease in the intensity of the SPR band at 520 nm and the formation of a new red-shifted band at 670 nm and a colour change from red to blue from UV-Visible spectra. The R colour coordinates decreased as Cr(VI) concentration was increased to 16 µM then a rapid decreased 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 was increased. A rapid decrease was noticed in hue angle between 16-25 µM while chroma decreased significantly as the Cr(VI) concentration increased. Molecular dynamics using gold cluster was used to simulate the aggregation process. The radial distribution [g(r)] was calculated from cluster models. The radial distribution of Cr-DPC complex was more than twofold than for Cr-AuNPs. This was associated with the aggregation of AuNPs leading to the appearance of blue colour of AuNPs solution which was also supported by the intensity obtained from Color Grab. The other case study presented herein is on the development of a simple and facile colorimetric method for the detection of lead (Pb(II)) ions using silver nanoparticles (AgNPs) functionalized with 1-methyl-6-phenyl-6, 7-dihydro-5H-indolo [3, 2-c] [1, 8] naphthyridine (TRPIDB_H) ligand. The synthesized AgNPs were characterized by UV-Vis, TEM and Dynamic Light Scattering (DLS). The UV–Vis spectrum displayed a local surface Plasmon resonance (LSPR) absorption with a peak maximum at 410 nm and TEM results image showed that the synthesised AgNPs were well dispersed in aqueous solution. The TRPIDB_H complex was synthesized through Povarov reaction [4+2] cycloaddition and yielded 80% of the product which was characterized by FT-IR, 1H and 13C NMR, TOF-MS. The Pb(II) ions induced aggregation of the TRPIDB_H-AgNPs in solution from 60-100 mg/L. This resulted in a colour change from yellow to reddish brown which was accompanied by the appearance of the second surface plasmon absorption peak at 505 nm. Moreover, further study reported herein focus on the development of a rapid and efficient colorimetric method for the detection of Mn(II) with high selectivity and sensitivity using 3- (4-hydroxy-3-methoxyphenyl)-2, 3-dihydropyrazolo [3, 4-b] indole-1(4H)-carbothioamide (TRPIDA_V) modified gold nanoparticles (TRPIDA_V-AuNPs). The TRPIDA_V-AuNPs aggregated upon the introduction of 2 mg/L, this led to a change in colour of the dispersed TRPIDA_V-AuNPs from red to blue and a decrease of the surface plasmon absorption intensity at 520 nm. The TRPIDA_V-AuNPs aggregated between 2-10 mg/L resulting in the formation of a second peak at 655 nm. The colorimetric detection showed high selectivity to Mn(II) ions and was not selective to other investigated metal ions as there was no aggregation induced upon addition of 2 mg/L of other metal ions. Furthermore, only Mn(II) ion resulted in colour change from wine red to blue and forming a second absorption peak at 655 nm. Additionally, the colorimetric detection system yielded a detection limit of 0.00691 mg/L showing excellent sensitivity towards Mn(II). The results obtained on the spiked river and tap water samples further confirmed that the TRPIDA_V-AuNPs colorimetric detection system is applicable for Mn(II).