High-Speed Imaging of 2-D Ionic Diffusion Using a 16 $,\times,$ 16 Pixel CMOS ISFET Array on the Microfluidic Scale

We present an electrochemical high-speed measurement for direct observation of the mixing of ions in solution over a sensor surface. The method is applied to the determination of time-varying mixing and diffusion rates in microfluidic systems. The system generates high-speed videos of the analytes under investigation as they mix, without the need for any preliminary staining or labeling and has a pixel-to-pixel pitch of 14 μm. We demonstrate ion displacement phenomena on the surface of a Si₃N₄-passivated complementary metal oxide semiconductor ion-sensitive field effect transistor array device and also monitor the pH change induced by the addition of sulfuric acid to normal saline. These chemical videos are recorded with a frame rate of up to 333 frames/s. By evaluating the time-varying change in ionic concentrations across the surface of the chip, we calculate time-varying estimates of diffusivity coefficient values for the mixing analytes.