Future MD simulation work

The MD simulation was carried out on an inert diamond surface, where surface atoms have partial charges that cancel each other out, hence there is no overall charge. MD simulation implemented with additional surface charges to mimic a diamond electrode surface is needed. A mixture of surface termination (C- H, C-O-C, C=O) on the same diamond surface, different crystal facets and absorption/desorption processes of dopamine product molecules are of interest to further study the system.

5.5

Conclusions

An all diamond microelectrode was fabricated and firstly characterised electrochemically using an outer sphere fast ET redox mediator, Ru(NH3)63+, to assess the quality of the structure under ambient conditions. Experimental CV was then compared with finite element simulation and good agreement was obtained, indicating the high quality (no pin holes or defects) of the fabricated microelectrode structure.

A pulsed heating technique, as described in chapters 3 and 4, was employed, as a fast controllable heat source. In these measurements, laser pulses of 10 ms duration (990 ms off) were delivered onto a ~ 1 mm spot on the rear side of the BDD microelectrode. Due to the superior thermal diffusivity of diamond, heat propagates to the electrode surface (solution side) rapidly. Average temperature at the electrode/electrolyte interface was obtained, i.e. 47.1 oC (1.2 kW cm-2); 68.4 oC (2.5 kW cm-2); 105.5 oC (3.8 kW cm-2).

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The effect of temperature on the electrochemical response of both an outer sphere (Ru(NH3)63+) and an inner sphere (Fe2+) redox mediator on the BDD microelectrode were studied. Increased ilim’s were observed in both cases, due to the enhanced mass transport rate at elevated temperatures. TPV showed a positive shift in E1/2 for the reduction of Ru(NH3)63+ due to the positive sign of β (+0.68 mV K-1), in accordance with the simulated TPV, whereas a negative shift in E1/2 for the oxidation of Fe2+ was evidenced, which was caused by a combination of the enhanced ET kinetics and the negative sign of β (-1.72 mV K-1). Interestingly, CV and TPVs for the oxidation of Fe2+ exhibited ‘shoulder currents’. This we believe is due to the heterogeneity of the polycrystalline nature of the BDD surface, where highly doped grains exhibits superior ET process compared to less heavily doped grains.

The pulsed laser heating system was then moved to study a well-known surface fouling redox mediator, dopamine. Consecutive CVs at ambient conditions for the oxidation of dopamine on the BDD microelectrode showed decreased ilim (ilim(10) is 52.8 % of ilim(initial)), as an evidence for the surface fouling resulting from the dopamine product. Minimised fouling (only decrease by 4 % in ilim at Pd of 3.8 kW cm-2 over ten cycles) was achieved by TPV, where temperature pulses was applied during potential scans. Thus BDD is promising for high temperature thermoelectrochemical measurements for the surface sensitive (fouling) bio- molecules.

MD simulations on H-terminated and ketone-terminated diamond (100) facet indicate less dopamine molecules absorption under heated conditions, which is in

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good agreement with experimental data and provide guidance for future experiments on examining the surface fouling effect of dopamine oxidation on the polycrystalline BDD with different surface functional groups.

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5.6

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