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Title: Development of CIELAB colour system for colorimetric detection of heavy metals in wastewater using metal nanoparticles
Authors: Shange, Sindisiwe Fortunate 
Issue Date: 2021
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
Thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry in the Faculty of Applied Sciences at Durban University of Technology, 2021.
Appears in Collections:Theses and dissertations (Applied Sciences)

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