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Ultrasensitive silver-gold bimetallic interdigitated array sensor for detection of Escherichia coli and Salmonella typhimurium
Author(s)
Duya, Calleb Otieno
Date Issued
2023
Type
Thesis
Publisher
Cape Peninsula University of Technology
Abstract
The main objective of this study was to optimize, develop and validate bimetallic silver (Ag) -
gold (Au) interdigitated nano-array sensor for ultra-sensitive detections of Escherichia coli (E.
coli) and Salmonella typhimurium (S. typhimurium). The bimetallic Ag-Au (1:2) nanoparticles
(NPs) were successfully synthesized via chemical and electrochemical methods as confirmed by
the presented spectroscopic and electrochemical data. The electrochemical properties of
chemically and electrochemically synthesized NPs films formed by drop-coating and electrodeposition
methods, respectively were studied via cyclic (CV) and differential pulse voltammetry
(DPV). Chemically modified GCE/Ag-Au (1:2) NPs film showed enhanced electron transfer
coefficient (α), heterogeneous rate constant (ks), electrode surface coverage (Γ) and diffusion
coefficient (D) of 0.02, 0.01 s-1, 7.00 × 10-11 molescm-2and 7.34 × 10-7 cm2s-1, respectively
compared to the electro-deposited NPs films. The optical and morphological properties of
chemically synthesized NPs were investigated via UV-visible, FT-IR, XRD and SEM techniques.
The zeta potential (mV) of pure bimetallic Ag-Au (1:2) NPs, untreated bacteria and their
complexes with E. coli and S. typhimurium ranged between -4.34 ± 0.71 for the bimetallic NPs and
-17.30 ± 0.89 for S. typhimurium - bimetallic Ag-Au (1:2) NPs complex. The electrochemical
interactions between chemically synthesized NPs and each of the bacterium on GCE and
interdigitated array electrode (IDAE) were sequentially carried out in 0.1 M PBS (pH 7.4) for
sensor fabrication. The bacterium - NPs interaction studies were investigated using UV-visible,
CV, DPV and EIS techniques and found to be mainly dependent on bacterial concentrations,
bacteria-NPs interaction time and NPs concentrations. The UV-visible and electrochemical
experiments showed blue-and negative-shifts, respectively compared to responses from the NPs,
an indicative of complex formation. The electrochemical kinetic parameters determined for GCE -
bacteria - NPs interactions were D (4.90 × 10-15 cm2s-1) and ks (1.55 × 10-11 s-1) for E. coli and D
(9.13 × 10-4 cm2s-1) and ks (1.54 s-1) for S. typhimurium. The IDAE-bacteria-NPs interactions
kinetic parameters determined were D (4.71 × 10-15 cm2s-1) and ks (1.53 × 10-11) for E. coli while
those for S. typhimurium were D (5.12 ×10-2 cm2s-1) and ks (0.88). Sensor‟s methodology was
optimized for applied potential, AC amplitude, supporting electrolyte concentration and the choice
of electro-analytical technique which was based on sensitivity. The optimum conditions chosen for
nano-impact detections of E. coli using IDAE were: NPs volume (500 μL), NPs - E. coli
incubation period (4 minutes), PBS concentration (0.1 M), AC amplitude (5 mV) and applied potential (+0.1 V) while those obtained for S. typhimurium included: NPs volume (600 μL), NPs -
E. coli incubation period (5 minutes), PBS concentration (0.1 M), AC amplitude (10 mV) and
applied potential (+0.5 V). The IDAE nano-impact sensor integrated with EIS technique showed
enhanced sensitivity (μLὨ-1cell-1) for detections of E. coli (1.64 × 10-5) and S. typhimurium (1.37
× 10-5) compared to DPV technique. The IDAE nano-impact sensors for both bacteria had
significantly lower LOD (101 cells μL-1) and LOQ (102 cells μL-1) for E. coli, and LOD of (101
cells μL-1) and LOQ (103 cells μL-1) for S. typhimurium. These low LOD and LOQs coupled with
higher % recoveries (95.18 ± 2.50 - 100.00 ± 0.76) and (100.60 ± 0.58 - 116.02 ± 0.15) for E. coli
and S. typhimurium, respectively showed that the developed nano-impact sensors for the bacteria
were accurate and precise.
gold (Au) interdigitated nano-array sensor for ultra-sensitive detections of Escherichia coli (E.
coli) and Salmonella typhimurium (S. typhimurium). The bimetallic Ag-Au (1:2) nanoparticles
(NPs) were successfully synthesized via chemical and electrochemical methods as confirmed by
the presented spectroscopic and electrochemical data. The electrochemical properties of
chemically and electrochemically synthesized NPs films formed by drop-coating and electrodeposition
methods, respectively were studied via cyclic (CV) and differential pulse voltammetry
(DPV). Chemically modified GCE/Ag-Au (1:2) NPs film showed enhanced electron transfer
coefficient (α), heterogeneous rate constant (ks), electrode surface coverage (Γ) and diffusion
coefficient (D) of 0.02, 0.01 s-1, 7.00 × 10-11 molescm-2and 7.34 × 10-7 cm2s-1, respectively
compared to the electro-deposited NPs films. The optical and morphological properties of
chemically synthesized NPs were investigated via UV-visible, FT-IR, XRD and SEM techniques.
The zeta potential (mV) of pure bimetallic Ag-Au (1:2) NPs, untreated bacteria and their
complexes with E. coli and S. typhimurium ranged between -4.34 ± 0.71 for the bimetallic NPs and
-17.30 ± 0.89 for S. typhimurium - bimetallic Ag-Au (1:2) NPs complex. The electrochemical
interactions between chemically synthesized NPs and each of the bacterium on GCE and
interdigitated array electrode (IDAE) were sequentially carried out in 0.1 M PBS (pH 7.4) for
sensor fabrication. The bacterium - NPs interaction studies were investigated using UV-visible,
CV, DPV and EIS techniques and found to be mainly dependent on bacterial concentrations,
bacteria-NPs interaction time and NPs concentrations. The UV-visible and electrochemical
experiments showed blue-and negative-shifts, respectively compared to responses from the NPs,
an indicative of complex formation. The electrochemical kinetic parameters determined for GCE -
bacteria - NPs interactions were D (4.90 × 10-15 cm2s-1) and ks (1.55 × 10-11 s-1) for E. coli and D
(9.13 × 10-4 cm2s-1) and ks (1.54 s-1) for S. typhimurium. The IDAE-bacteria-NPs interactions
kinetic parameters determined were D (4.71 × 10-15 cm2s-1) and ks (1.53 × 10-11) for E. coli while
those for S. typhimurium were D (5.12 ×10-2 cm2s-1) and ks (0.88). Sensor‟s methodology was
optimized for applied potential, AC amplitude, supporting electrolyte concentration and the choice
of electro-analytical technique which was based on sensitivity. The optimum conditions chosen for
nano-impact detections of E. coli using IDAE were: NPs volume (500 μL), NPs - E. coli
incubation period (4 minutes), PBS concentration (0.1 M), AC amplitude (5 mV) and applied potential (+0.1 V) while those obtained for S. typhimurium included: NPs volume (600 μL), NPs -
E. coli incubation period (5 minutes), PBS concentration (0.1 M), AC amplitude (10 mV) and
applied potential (+0.5 V). The IDAE nano-impact sensor integrated with EIS technique showed
enhanced sensitivity (μLὨ-1cell-1) for detections of E. coli (1.64 × 10-5) and S. typhimurium (1.37
× 10-5) compared to DPV technique. The IDAE nano-impact sensors for both bacteria had
significantly lower LOD (101 cells μL-1) and LOQ (102 cells μL-1) for E. coli, and LOD of (101
cells μL-1) and LOQ (103 cells μL-1) for S. typhimurium. These low LOD and LOQs coupled with
higher % recoveries (95.18 ± 2.50 - 100.00 ± 0.76) and (100.60 ± 0.58 - 116.02 ± 0.15) for E. coli
and S. typhimurium, respectively showed that the developed nano-impact sensors for the bacteria
were accurate and precise.
Additional information
Thesis (DPhil (Chemistry))--Cape Peninsula University of Technology, 2023
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