Ultrasensitive silver-gold bimetallic interdigitated array sensor for detection of Escherichia coli and Salmonella typhimurium

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.