Advanced miniaturised electrochemical detection for flow-based analytical systems

Electrochemical (EC) detection has grown in popularity across the chemical sciences over the last three decades, leading to significant advances in EC techniques for the detection of a wide range of analytes in the liquid phase. EC techniques are being constantly improved, and pulsed amperometric detection (PAD) is considered one of the most advanced EC technique to-date for flow-based analysis. However, a conventional EC system—incorporating a heavy potentiostat with large flow cells (FCs), large electrodes or microelectrodes with complex modification techniques and limited stability, and a long pulsed waveform is typically required to perform PAD for the sensitive detection of most target analytes. Herein, a miniaturised EC detector, rapidly modified disposable electrodes with high stability, and a fast-pulsed waveform have been studied to understand their capabilities and potential to perform advanced PAD within a miniaturised format for flow-based analytical systems. Initially, a miniaturised gap-FC was demonstrated as a replacement for commonly used larger scale EC detectors (i.e., wall-jet (WJ) or thin-layer (TL) FCs), and its capability for PAD within a miniaturised format explored. The gap-FC overcame the difficulties in miniaturisation of the EC system, i.e., the rigid construction of the WJ- and TL-FCs, and the fact that the conventional system was incapable of being coupled with analytical systems operating at nano- to microlitre per minute flow rates. The gap-FC provides a low gap distance (30 μm from capillary outlet to electrode) for the formation of a very thin layer of electrolyte on the electrode surface, achieving high EC conversion efficiency and low effective cell volume. The gap-FC demonstrated the highest efficiency (ca. 16-times than reported in [10]) and lowest effective cell volume (35 nL) to-date. With the use of the gap-FC with LC, the sensitivity of the detector for test solutes (namely, ascorbic acid, 2,3-dihydroxybenzoic acid, pyrocatechol and dopamine) was greater than the value reported in the literature (ca. 4 to 25-times higher than UV and 2-times higher than previous EC detection). This work provides a comprehensive EC characterisation of miniaturised gap-FC for flow-based analytical systems, which will be beneficial for the advancement of the PAD technique. Disposable electrodes, rapidly modified with nanoparticles (i.e., ZrO\(_2\) NPs, modification time approximately 10 min), were investigated as a replacement for the commonly applied metal electrodes. The existing electrodes used with the PAD are very sensitive to electroactive analytes but require time-consuming complex modification techniques, and have limited stability. Furthermore, the effective surface area of the electrode, EC reversibility, and stability of the modified electrodes in continuous flow have not been reported in the literature. In this study, the electrodes modified with ZrO\(_2\) NPs showed a 100% increase of effective surface area as compared to bare electrodes, due to the formation of a porous electrode surface. In addition, a 95 to 180-times improvement of current was achieved due to the formation of the large effective surface area and the minimisation of the capacitance of the double layer. Also, the modified electrodes showed high stability (8.5 hr during continuous-flow and 45 days during intermittent use in flow-through systems) and ca. 2-times higher sensitivity to the above-mentioned test solutes to that reported in the literature. The use of ZrO\(_2\) NPs facilitates electrocatalytic properties, stability, and good electrical conductivity. This work has also provided a comprehensive EC characterisation of both the bare and modified electrodes for flow-based analytical systems, which will further advance the PAD technique. Finally, a fast-pulsed waveform (2-steps at +400 mV and -400 mV, 10 ms each, and 50 Hz cycle repetition) was developed, and the first miniaturised PAD was performed using gap-FC-incorporating modified electrodes in the liquid phase. The technique allowed miniaturised EC detection coupled with fast flow-through systems (such as FIA, LC, and IC in this study) for the first time, and overcame limitations such as the difficulties with data acquisition at high frequency (50 Hz), and the loss of useful data during a long cleaning step (e.g., 560 ms for the removal of surface oxide. The developed high frequency waveforms (15 to 45-times fast than existing waveforms result in an improved electroactivity of the electrode and facilitated the rapid establishment of baseline stability, higher current, and high detection sensitivity to electroactive solutes (such as obtaining ca. 100-times lower LOD for hydrazine than the lowest reported value in the literature). The analytical application of the developed waveform was demonstrated by monitoring the consumption of reducing agents (hydrazine and ascorbic acid) during the graphene oxide (GO) reduction process. In conclusion, a new miniaturised EC detector (gap-FC), using highly stable disposable electrodes rapidly modified with NPs (ZrO\(_2\)), and a fast-pulsed waveform (2-step, 20 ms, and 50 Hz) has been developed and resultant capabilities and potential to perform advanced miniaturised EC detection for flow-based analytical systems has been demonstrated