A High-Performance Raspberry Pi-Based Interface for Ion Imaging Using ISFET Arrays

This paper presents a standalone system based on the Raspberry Pi miniature computer capable of ion-imaging and visualisation using a CMOS chemical sensing array. The sensory part utilises a $64\times 200$ ISFET-based array biased in velocity saturation that achieves high linearity in its input-output characteristics (pH-current). The system consists of a Raspberry Pi and custom PCB board and allows high portability at a low overall cost. As a result, it constitutes a portable platform capable of real-time ion sensing and visualisation. The achieved frame rate is 10FPS maximum or limited to 5FPS with real-time visualisation of the 12,800 pixel array. Moreover, we show the simultaneous and real-time calibration and readout of the ISFET array, reducing the mismatches between pixels due to uneven trapped charge, without any impact on the performance. Demonstrations further show that the system can detect changes in pH and capture visuals of dynamic ion concentration changes in a solution. Ion imaging captured at 5FPS shows HCl diffusion in a de-ionized water solution showcasing the spatial changes in the ion concentration. This constitutes a promising approach for the realisation of ion imaging platforms and portable Point-of-Care diagnostic systems using ISFET arrays.