Practical considerations for DNA sensing using Faradaic electrochemical impedance spectroscopy on ENIG PCB electrodes

Abstract

Electroless nickel immersion gold (ENIG) finish printed circuit board (PCB) substrates offer a cost-effective solution for electrochemical sensing of biomolecules. Electrochemical impedance spectroscopy (EIS) is highly sensitive and can differentiate between bioelectrochemical circuit elements over a wide frequency range. Traditional Faradaic EIS measurements often focus on charge transfer resistance $\left(R_{ct}\right)$ to determine analyte concentration. However, the long measurement time required to perform such EIS measurements down to very low frequencies (typically 0.1 Hz) can adversely affect the ENIG PCB due to gold layer degradation and copper corrosion. To overcome these issues, we propose using the constant phase element (CPE) in the electrical equivalent circuit in EIS measurements for DNA sensing. Our approach employs ENIG PCB electrodes functionalized with thiolated single-stranded DNA probes targeting the uidA gene of E. coli. We evaluate the specificity of this sensing scheme to a 166 bp complementary amplicon from E. coli against non-complementary amplicons of different lengths from E. coli and bacteriophage Phi6. By operating at higher frequencies ($>$10 Hz), CPE analysis reduces measurement times and minimizes the risk of PCB degradation. Preliminary findings indicate that the CPE impedance exhibits concentration-dependent changes with increasing target DNA concentrations. We also present preliminary results for utilizing this sensing mechanism to detect E. coli from wastewater samples.