Design of pH sensor for liquid cell Transmission electron microscopy

Abstract

Electron microscopy has enabled us to visualize objects that are not observable with a light microscope. With Transmission electron microscope observation up to subatomic level is possible. Generally, the sample is under vacuum in an electron microscope. But in real life sample is under influence of environmental conditions like liquid, gas, temperature. Development in Microelectromechanical systems has made it possible to make nanocell, which encloses sample and provide different stimuli like gas, liquid, voltage, temperature.
Imaging in liquid environment contains encapsulated liquid, which allows high energy electron beam passes through thin windows. While the electron beam passing through the liquid, the interaction of both splits water and generates radiolysis products. These radiolysis products contain gases, ions, radicals, and other chemical compositions. These products affect the chemistry in liquid and therefore the observations in the experiment. So it is important to quantify these generated species. One of the species that is generated due to radiolysis is Hydrogen ion. Because of this, the pH of the liquid changes. Numerical studies are available to quantify this pH change but species generation data is interpolated to several orders of magnitude and very limited experimental work is available.
The goal of this thesis was to measure pH change due to radiolysis. For this, two problems need to tackle simultaneously, making micro size pH sensor on-chip and trying to find out what species due to radiolysis has the potential to affect pH measurements. Radiolysis generates 15 species for pure water and characterizing all of them is difficult. So here approach is taken to measure species effect just outside the electron beam area, which reduces to only 5 species to consider. Species generation and its effect on the platinum electrode are analyzed. For Ultrapure water, the measured voltage shift was negative and for acidic solutions, it was positive with an increase in dose rate. The reason for this could be the generation of excess Hydrogen peroxide for acidic solutions and generation of excess Superoxide for neutral pH solutions. So it is important to shield the effect of these two species for successful pH measurement just outside the beam region. The optimization code is written to decide the place and thickness of two pH measuring electrodes. From literature, it is found that Iridium oxide has a selectivity of 0.0001 for Hydrogen peroxide. Results from the optimization code conclude that the sensing layer should have minimum selectivity for Superoxide of 0.1, to measure any meaningful pH change outside the beam area.
pH sensing characteristics of the Iridium oxide layer reveals maximum error is 0.4 pH. However, according to simulations outside the beam area, the maximum pH change is 1.34 pH. From the optimization code dimensions of two pH sensing electrodes are decided. Considering maximum error from pH measurement experiments and error from species interference from simulations, an electrode configuration has been designed. The design allows pH measurement for the dose rate 50000000 Gy/s and above for initial pH of solution ranging from 6 to 8.