A 20 nm-thin layer of Ti or Cr, as intermediary adhesion layers, followed by 100 nm-layers of Pt or Au, respectively, is deposited by e-beam evaporation ( Figure 3c) as previously reported for a standard photolithographic fabrication process. The wafer is spin-coated with a photoresist and exposed to UV light through a mask containing the pattern for the electrodes and interconnects ( Figure 3b). This metal layer is defined by photolithography as follows. A 1,000 nm-thick thermal oxide layer is firstly grown on the silicon wafer in order to avoid short-circuits with the subsequent metal layer ( Figure 3a). A 4-inch diameter silicon wafer is used as substrate. A scheme of the fabrication process is depicted in Figure 3. Īu and Pt ultramicroelectrode arrays with different geometries have been fabricated according to standard photolithographic techniques using Si/SiO 2/metal structures. suggested that separation distances should be at least 10 times the diameter of an individual microelectrode. It has been assessed that loosely packed arrays where the inter-electrode distance d ≫ 2 r ( r being the radius of a single microelectrode) yield the expected current signal ( m times amplified) whereas closely packed arrays, where d ≈ 2 r, behave as a macroelectrode having a current that is proportional to the total geometric area of the microelectrodes in the array. One of them is related to the packaging density of microelectrodes. In practice, several requirements should be met in order to attain this signal amplification. ![]() Ideally, these arrays should yield a current amplification by a factor of m relative to a single microelectrode. The term ultramicroelectrode array (UMEA) is referred to a device formed by m identical microelectrodes. However, due to the very small currents measured with these devices, ensembles of ultramicroelectrodes have been designed as a means of increasing the magnitude of the current while retaining those advantages of a single UME mentioned above. Ultramicroelectrodes of planar configuration in a range of configurations, from discs to bands, have been developed using microfabrication technologies. When this happens, the electrode voltammetric performance is dramatically enhanced because of: a) Improved mass transport towards the transducer (radial diffusion dominates) B) reduced double-layer capacitance and thus greatly enhanced faradaic to capacitive current ratios and C) very small iR drop. Ultramicroelectrode (UME) is a term used to describe microelectrodes where at least one of their dimensions is smaller than the thickness of the target analyte diffusion layer. Likewise, their small size is a key factor in specific applications such as single brain cells or capillary chromatography detectors. As a consequence UMEs can be applied in highly resistive media and in very fast scan-rate voltammetric experiments, oxygen does not interfere in the electrochemical experiments and the signal is less dependent on convection. Such an equilibrium is not possible at bigger electrodes, where the amount of species exchanging electrons at their surface is higher than those diffusing from the bulk of the solution, thus attaining transient currents. This feature is related with the equilibrium established between the diffusion of electroactive species that arrive to the electrode surface due to the concentration gradient and the electroactive species that exchange electrons at this surface. ![]() It is the possibility of achieving responses in the steady-state what makes UMEs highly attractive for specific applications. ![]() ![]() Furthermore the use of UMEs expands the time scale for carrying out measurements by several orders of magnitude, which is particularly useful to study rapid homogeneous or heterogeneous reactions. Such critical dimension makes the electrochemical response differ from that of a conventional electrode and opens up new possibilities for studying electrode reactions, which includes dynamic measurements in solutions with low electrolyte concentration, in non-polar solvents or low conducting media, and even in the solid phase or gas state. Since then, the concept of UME has been extended in the literature and can be defined taking into account that only one of the electrode characteristic dimensions, given by the geometry, must be in the order of some micrometers. One decade later the first comprehensive survey of the special properties and perspectives of the so-called micro-voltammetric electrodes was provided. The changes in mass transport conditions bring about extremely high current densities at UMEs, whereas the current themselves become very small. In the 1970s the discovery, although non elucidation, of the unusual properties of ultramicroelectrodes (UMEs) opened new possibilities of analyzing electrode processes.
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