Flexible pH Sensors based on Electrodeposited Iridium Oxide

Abstract

Due to mounting global water scarcity and source contamination concerns, significant water monitoring efforts are required, though current data suggests significant gaps in monitoring coverage and a lack of cost effective and versatile sensing devices. A complete system is proposed and demonstrated herein which monitors a water quality indicator of high importance, solution pH. Therefore, this system is also suitable for biomedical applications with minor modifications, thus it was designed and tested with both in mind. The sensors possess high pH sensitivity and stability of iridium oxide (IrOx) with the efficiency of electrodeposition and conformability and biocompatibility of polyimide substrates; additionally, the device was designed to communicate wirelessly and make use of both Bluetooth-Low-Energy (BLE) transmission and frequency-based load modulation (FM) transmission with wireless power transfer (WPT) via inductive coupling. The probes were fabricated by traditional lithography and a cyclic voltammetry method using an Iridium-based, oxalate-containing solution. The IrOx film and surface characterization was performed by voltammogram investigation, scanning electron microscopy (SEM), profilometry, electron dispersive x-ray spectroscopy (EDS/EDX), x-ray photoelectron spectroscopy (XPS), and optical and laser microscopy. Reference electrodes were fabricated by application of silver chloride (AgCl) ink. Gold (Au), copper (Cu), and silver (Ag) substrates were evaluated, Cu being discarded as a viable substrate. Probe performance was characterized in terms of pH sensitivity, sensitivity with respect to surface area, linearity, response time, lower limit of detection, cationic selectivity, and response to mechanical stress. Some attempts were made to programmatically and electrically reduce noise and programmatically characterize experimental results via Matlab (Mathworks, MA). Applicability is demonstrated by both a review of previous packaging work and current wireless schemes. Wireless schemes based around amplitude modulation (AM), frequency modulation (FM), and Bluetooth communication are demonstrated. FM communication associated with WPT was demonstrated as the most appropriate for the widest variety of applications. Finally, future work is discussed in terms of a completed patch design, possible surface material modifications, packaging improvements, and monitoring vehicle concepts.