Electrodes ranging from 25 nm to500 nm of RuO2 were hydrated in pH 7 buffer for one week toeliminate any effects of ageing (as before).
Electrodes were then equilibratedfor 1 hour at pH 12, and looped from pH 12 to 2 with pH 12 between each step.Here measurement times were reduced form 180s to 90s, since examination of theprevious data showed that electrodes had fully equilibrated within that timeand had begun to drift. It should be noted that this reduction in loop timeshould reduce hysteresis values (compared to the previous data) as less shiftshould be incurred at pH 2 15, which is advantageousfrom an end user point of view. Results presented in table 8reveal that as RuO2 thickness increases there is in-general anincrease in sensitivity, accompanied by a reduction in hysteresis and drift.The increase in sensitivity is approximately linear between 30min and 400 hours;after which the rate of increase slows as sensitivity approaches Nernstian;which agrees with the trend predicted by Bousse (figure 5). Examination of electrodes byscanning electron microscope (SEM) reveals a pronounced change in structure asRuO2 material increases.
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The 30min RuO2 electrode has arelatively flat surface with few features; whilst the 4 hours electrodeexhibits a much more rough and grainy structure. This rough grainy structurewould result in a high number of defects in thecrystal lattice, providing a high density of RuIII sits, allowingthe electrode to attain a stable near-Nernstian response. Hysteresis decreased markedlybetween 30min and 3hours, followed by a gradual decrease as thicknessincreased. This too agrees with the predictions made above. As a higher densityof active sites will make the electrode less sensitive to changes insensitivity from changes in active-site density. There was no observable trendfor drift-rate; a large decrease and then the values were all approximately thesame. It should be noted that the drift rate values calculated here representthe drift in potential over the experimental period; which was only 30 minutes.As a result, they probably do not provide an accurate estimation of theelectrode’s true longer-term drift rate.
Also presented is a calculationfor the precision “a pH senor would hypothetically exhibit if it weremanufactured with RuO2 electrodes of different thicknesses; precisionwas calculated by dividing hysteresis by sensitivity. It can be seen that toobtain a precision of 0.01 pH units a 500nm electrode would be required (if an”ideal” reference electrode was used). However, it can also be seen thatelectrodes with thicknesses between 50 and 300 nm would provide precisionvalues better than 0.
05; which is acceptable for many pH applications.