Ultramicroelectrode (UME) arrays are attractive in the electroanalysis field, as the interesting and useful properties of individual UMEs are retained with the added advantage of an increased current. Spatial variations in the electrical and electrochemical activity of microarray electrodes, fabricated entirely from diamond, have been investigated. The diamond ultamicroelectrode array (DUA) contains ~ 50 μm-diameter boron doped diamond (BDD) disks spaced 250 μm apart (center to center) in insulating intrinsic diamond supports, such that the BDD regions are coplanar with the intrinsic diamond. The DUA was fabricated using several processes from chemical vapour deposition (CVD) for diamond growth to laser ablation and lapidary techniques for diamond patterning (Figure 1).
Figure 1. Schematic illustration of the fabrication procedure used for production of the BDD microdisk array electrodes insulated with coplanar intrinsic diamond.
Conducting Atomic Force Microscopy (C-AFM)
Atomic force microscopy (AFM) imaging of the surface revealed a roughness of no more than 10 nm over the array, with clear visibility of different grains. Each BDD microdisk within the array contains polycrystalline BDD (pBDD) with a variety of different grains exposed. Using C-AFM, the conductivity of the different grains was found to vary within a pBDD microdisk. When applying a tip potential of -2.5 V, the currents were found to vary from grain to grain by ~1 order of magnitude across the surface, with zero current recorded in the insulating diamond areas. While imaging the DUA the tip current is determined by both the intrinsic resistance of the sample and the concentration of charge carriers at the surface of the BDD.
Figure 2. Simultaneously recorded 80 x 80 µm C-AFM (a) height and (b) conductivity images, recorded at a tip potential of -2.5 V with a 10 MΩ current resistor inseries, of a typical BDD electrode in the DUA.
Scanning Electrochemical Microscopy (SECM)
Electrochemical imaging of the electroactivity of the microdisk electrodes using SECM operating in substrate generation-tip collection (SG-TC) mode was performed. With a 25 µm diameter UME, which is large compared to the grain sizes of the pBDD, the tip current measures the responses of many grains within a single DUA electrode simultaneously (Figure 3). This imaging revealed that, under apparently diffusion-limited steady-state conditions, there was only a small variation in the response between electrodes.
Figure 3. A 500 x 500 µm SG-TC SECM scan for the collection of Ru(NH3)62+, electrogenerated from 5 mM Ru(NH3)63+ (in 0.2 M KNO3) at the surface of the DUA.
Higher resolution SECM measreuments were also carried out using a 5 µm diameter Pt UME as the imagining tip. Single electrodes in the DUA were imaged at varying substrate potentials of -0.4 V, -0.3 V and -0.2 V, starting at diffusion-limited reaction rates and subsequently decreasing the rate of reaction (Figure 4). Clear variations in the tip current were observed with only part of the electrode showing metallic behaviour. These variations in electroactivity are most likely linked to the differences in conductivity of the different diamond grains as seen in the C-AFM. The current variation was emphasied further when the substrate potential was decreased to -0.3 V and -0.2 V. The nature and extent of spatial variations in the electron transfer rate varied from electrode to electrode. However the majority of electrodes in the array appeared to show predominantly metallic behaviour when the potential was set to give a diffusion-limited regime.
Figure 4. SG-TC 100 x 100 um images of an electrode in the DUA at substrate potentials (a) -0.4, (b) -0.3 and (c) -0.2 V.
Ag deposition experiments showed that all grains within the microdisk where able to support electron transfer (via the reduction of soluble Ag+ to solid Ag), albeit at different rates (Figure 5). The possibility of using these array electrodes for steady state diffusion-limited measurements in electroanalytical applications is far-reaching.
Figure 5. FE-SEM images of two different electrodes within the DUA after electrodeposition of Ag. Two FE-SEM detectors were used: (i) a conventional SE2 detector and (ii) a high-efficiency in-lens detector.
- A. L. Colley, C. G. Williams, U. D'Haenens Johansson, M. E. Newton, P. R. Unwin, N. R. Wilson, J. V. Macpherson, Anal. Chem. 2006, 78(8), 2539-2548.