Redox-Enabled Microscale Opto-Electronically Transduced Electrodes (ReMOTEs)

We present the elements of a pH-measuring autonomous microsystem, a Redox-Enabled Microscale Opto-Electronically Transduced Electrode (ReMOTE). The ReMOTE contains two electrodes of different materials, where the nearby $\mathbf{pH}$ dictates the voltage between the two electrodes. The ReMOTE is a tetherless, all-optical microsystem which is powered and is transmitting its data optically. The ReMOTE is designed in a 180 nm CMOS SOI process, occupies merely $\mathbf{417}\ \mu \mathbf{m}\times \mathbf{48}\ \mu \mathbf{m}$, and it only consumes $\mathbf{2}\ \mu\mathbf{W}$. For the functional verification, we have conducted pH measurements using a thin film titanium nitride working electrode and a Ag/AgCl reference electrode, of which the voltage per pH sensitivity was 42.53 mV/pH.

I. Introduction

There is a growing interest in microscale electro-chemical measurements for cellular studies, especially in pH - higher levels of cellular activities in cancer cells lead to a more acidic extracellular pH [1] and a similar observation has been made in the extracellular pH of the cells infected with a flu virus [2]. Moreover, the rapidly growing field of organoid research can also benefit from microscale sensing systems [3]. An ideal microscale pH-sensing system should be tetherless and minimal in size, for which optical links for power and data telemetry are favorable compared to other tetherless power and data telemetry methods such as radio-frequency electromagnetic and ultrasonic link [4]. We present such micro-system called Redox-Enabled Microscale Opto-Electronically Transduced Electrodes (ReMOTEs).