Electron-transfer kinetics and impedance at the electrode-solution interface affect biosensor performance. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are used to understand the reversibility of electron transfer and impedance at the electrode-solution interface, respectively. Effective surface areas calculated based on the Randles-Sevcik equation for a bare screen-printed carbon electrode (SPCE), a graphene oxide (GO)-poly (3,4-ethylenedioxythiophene):polystyrenesulfonic acid (PEDOT:PSS)-modified electrode (GO-PEDOT:PSS/SPCE), a Engineering main heading: Conducting polymers

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Cyclic voltammetry (CV) and electrical impedance spectroscopy (EIS) are electrochemical techniques to characterize reversibility of electron transfer and impedance at the electrode-solution interface, respectively. Reduced graphene oxide (rGO) and conductive polymer PEDOT:PSS are often used to enhance electron transfer at electrode surface. This chapter provides a step-by-step methodology of CV and EIS conducted on screenprinted carbon electrode (SPCE) modified with rGO-PEDOT:PSS and brief discussion on the CV and EIS results. The CV of rGO-PEDOT:PSS shows a reversible electron transfer in comparison to SPCE modified with PEDOT:PSS. For EIS, rGO-PEDOT:PSS was found to reduce the Warburg impedance, suggesting enhanced electrode conductivity. These results suggest that rGO-PEDOT:PSS is a suitable material for biosensing purpose.

Immunosensors have been widely developed to use antibodies to detect a pathogen of interest; it is interesting to look at the effect of nonspecific antibody binding to E. coli using electrochemical methods. IgG antibody not specific to E. coli O157:H7 was crosslinked onto a screen-printed carbon electrode. The presence of E. coli at 4, 4 × 102, 4 × 105, and 4 × 108 CFU/mL on the electrode surface was detected via linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Current transfer at both electrodes was reduced as the concentration of bacteria increased; however, the calibration of number of cells to decreased current was nonlinear for IgG-modified electrodes. The nonlinearity is confirmed by EIS measurements which showed highest impedance at 4 CFU/mL E. coli when impedance should be the lowest. FESEM images showed higher binding of cells when IgG is present compared to electrodes with reduced graphene oxide (rGO) alone. Electrodes with rGO alone show less attachment of E. coli, with EIS showing a linear calibration profile, while LSV shows not much difference in current values for all concentrations aside from the highest concentration. These results suggest that nonspecific binding can provide false signals in electrochemical measurements, and it is crucial to provide proper controls. 

Screen-printed carbon electrodes (SPCEs), modified with graphene oxide and poly(3,4ethylenedioxythiophene):polystyrenesulfonic acid (GO-PEDOT:PSS/SPCEs), and SPCEs modified with partially reduced GO and PEDOT:PSS (prGO-PEDOT:PSS/SPCEs) were studied for electrochemical transduction. Randles-Sevcik analysis showed that the prGO-PEDOT:PSS/SPCE has a higher effective surface area of 219.3 µm 2 in comparison to the unreduced GO-PEDOT:PSS/SPCE (87.0 µm 2) and the bare SPCE (71.7 µm 2). Using electrical impedance spectroscopy (EIS), we determined that the prGO-PEDOT:PSS/SPCE has a lower charge-transfer resistance (Rct) of 163.82 Ω in comparison to the GO-PEDOT:PSS/SPCE (427.87 Ω) and the bare SPCE (13.31 kΩ). Glucose oxidase (GOx) was immobilized on all electrode types, including GO/SPCE as additional control and tested with low (0.2 mM), intermediate (0.6 mM), and high (1 mM) glucose concentrations. GOx/GO/SPCEs showed the largest change in anodic peak current (Ipa), 8.5, 7.5 and 4.9 µA for low, intermediate, and high glucose concentrations, respectively. Interestingly, GOx/prGO-PEDOT:PSS/ SPCEs have no change in both anodic and cathodic peak current, although they exhibit better redox capability, while GOx/SPCEs have very low Ipa. The results show that the high effective surface area and low charge-transfer resistance (Rct) of prGO-PEDOT:PSS/ SPCEs do not necessarily result in a sensitive glucose sensor in cases where immobilizatio n of enzymes on the material can affect electron transfer.