Low-cost pH sensors based on discrete PCB ion-sensitive field effect transistors

Diagnostic technologies will play a critical role in addressing current and future healthcare challenges, with the greatest impact through implementing these technologies at the point of care (PoC). For truly widespread deployment, these PoC technologies should be low-cost and amenable for mass manufacture, even in resource-limited settings, without compromising analytical performance. Discrete, extended gate pH-sensitive field-effect transistors (dEGFETs) fabricated on widely used printed circuit boards (PCBs) are a low-cost, simple to manufacture analytical technology. Electrodeposited iridium oxide (IrOx) films have emerged as a promising pH-sensitive transducer due to their facile deposition. While IrOx is predicted to have a beyond-Nernstian pH sensitivity, the performance measured experimentally is typically lower and variable. This thesis demonstrates a dEGFET pH sensor based on PCB extended gate electrodes and electrodeposited IrOx, which repeatedly displays beyond-Nernstian pH response. Using complementary surface-analysis techniques, it is shown the high pH sensitivity and repeatability is determined both by the chemical composition and critically the uniformity of the IrOx film. Electrochemical polishing of the extended gate electrode prior to electrodeposition enhances IrOx uniformity, leading to a median pH sensitivity of 70.7 ± 5 mV/pH (n=56) compared to 31.3 ± 14 mV/pH (n=31) for non-polished electrodes. The applicability of these devices is demonstrated through the quantification of the β-lactam antibiotic ampicillin, via the pH change that occurs due to hydrolysis catalysed by β-lactamase enzymes. This lays the foundations for a susceptibility assay towards the public health challenge of antimicrobial resistance (AMR). Additionally, this thesis explores the integration of electronically controlled microfluidic valves onto the PCB substrates, towards the development of lab-on-chip systems and PoC diagnostics. The highly sensitive and repeatable dEGFET sensors presented here show great promise as a low-cost diagnostic technology. Moreover, the use of PCB substrates is suitable for manufacture in resource-limited settings, enabling widespread diagnostic testing.