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Title: Ion camera development for real–time acquisition of localised pH responses using the CMOS based 64×64–pixel ISFET sensor array technology
Author: Nemeth, Balazs
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2012
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This thesis presents the development and test of an integrated ion camera chip for monitoring highly localised ion fluxes of electrochemical processes using an ion sensitive sensor array. Ionic concentration fluctuations are shown to travel across the sensor array as a result of citric acid injection and the BZ-reaction. The imaging capability of non-equilibrium chemical activities is also demonstrated monitoring self-assembling micrometre sized polyoxometalate tubular and membranous architectures. The sufficient spatial resolution for the visualisation of the 10-60 µm wide growing trajectories is provided by the dense sensor array containing 64×64 pixels. In the case of citric acid injection and the BZ-reaction the ion camera chip is shown to be able to resolve pH differences with resolution as low as the area of one pixel. As a result of the transient and volatile ionic fluxes high time resolution is required, thus the signal capturing can be performed in real.time at the maximum sampling rate of 40 µs per pixel, 10.2 ms per array. The extracted sensor data are reconstructed into ionic images and thus the ionic activities can be displayed as individual figures as well as continuous video recordings. This chip is the first prototype in the envisioned establishment of a fully automated CMOS based ion camera system which would be able to image the invisible activity of ions using a single microchip. In addition the capability of detecting ultra-low level pH oscillations in the extracellular space is demonstrated using cells of the slime mould organism. The detected pH oscillations with extent of ~0.022 pH furthermore raise the potential for observing fluctuations of ion currents in cell based tissue environments. The intrinsic noise of the sensor devices are measured to observe noise effect on the detected low level signals. It is experimentally shown that the used ion sensitive circuits, similarly to CMOS, also demonstrate 1/f noise. In addition the reference bias and pH sensitivity of the measured noise is confirmed. Corresponding to the measurement results the noise contribution is approximated with a 28.2 µV peak-to-peak level and related to the 450 µV +/- 70 µV peak-to-peak oscillations amplitudes of the slime mould. Thus a maximum intrinsic noise contribution of 6.2 +/- 1.2 % is calculated. A H+ flickering hypothesis is also presented that correlates the pH fluctuations on the surface of the device with the intrinsic 1/f noise. The ion camera chip was fabricated in an unmodified 4-metal 0.35 µm CMOS process and the ionic imaging technology was based on a 64×64-pixel ion sensitive field effect transistor (ISFET) array. The high-speed and synchronous operation of the 4096 ISFET sensors occupying 715.8×715.8 µm space provided a spatial resolution as low as one pixel. Each pixel contained 4 transistors with 10.2×10.2 µm layout dimensions and the pixels were separated by a 1 µm separation gap. The ion sensitive silicon nitride based passivation layer was in contact with the floating gates of the ISFET sensors. It allowed the capacitive measurements of localised changes in the ionic concentrations, e.g. pH, pNa, on the surface of the chip. The device showed an average ionic sensitivity of 20 mV/pH and 9 mV/pNa. The packaging and encapsulation was carried out using PGA-100 chip carriers and two-component epoxies. Custom designed printed circuit boards (PCBs) were used to provide interface between the ISFET array chip and the data acquisition system. The data acquisition and extraction part of the developed software system was based on LabVIEW, the data processing was carried out on Matlab platform.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General) ; TK Electrical engineering. Electronics Nuclear engineering