Nanoscopic Electrodes and Ensembles

Electrochemistry at electrodes with microscopic dimensions (e.g., a disk of 10 j,m diameter) and nanoscopic dimensions (e.g., a disk of 100 nm diameter) constitutes one of the most important frontiers in modern electrochemical science [25]. Such micro- and nanoscopic electrodes allow for electrochemical experiments that are impossible at electrodes of macroscopic dimensions (e.g., disks of mm diameter we call such electrodes macroelectrodes ). Examples of unique opportunities afforded by micro- and nanoscopic electrodes include the possibility of doing electrochemistry in highly resistive media and the possibility of investigating the kinetics of redox processes that are too fast to study at electrodes of conventional dimensions (both are discussed in detail below). In addition, microscopic electrodes have proven extremely useful for in vivo electrochemistry [62]. 

The small size of nanoelectrodes also makes possible the detection of discrete electron transfer events. Fan and Bard have recently shown cou-lombic staircase response using electrodes of nanometer dimensions [63], Ingram and co-workers have also shown coulombic staircase response, in their case while studying colloids and collections of colloids [64]. Fan and Bard have also applied nanoelectrodes to achieve high-resolution electrochemical imaging and single-molecule detection [65]. 

Electrochemistry of proteins is another case where electrode size affects the electrochemical results. Direct adsorption of proteins, such as enzymes, onto bulk metal surfaces frequently results in denaturation of the 

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In order to explore the effects of small electrode size, we have used the template method to prepare ensembles of disk-shaped nanoelectrodes with diameters as small as 10 nm. We have shown that these nanoelectrode ensembles (NEEs) demonstrate dramatically lower electroanalytical detection limits compared to analogous macroelectrodes. The experimental methods used to prepare these ensembles and some recent results are reviewed below. 

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One of the earliest applications of the template method was to prepare ensanble miCTo-scopic (7, 18) and nanoscopic electrodes (116, 141). Such electrodes were prepared by electrochemically depositing noble metals within the pores of the commercially available polymeric filtration membranes. The fabrication of a microelectrode ensemble based on the electrochemical deposition of platinum into the pores of a track-etched microporous polycarbonate host membrane was first shown in 1987 by Charles Martin (7). The word ensemble was used to describe the final device because the elements in the device are not evenly spaced. The procedure is simple, and requires only routine and inexpensive electrochemical instrumentation. It was ultimately found that electroless plating allowed for more uniform metal deposition (116). Both plating methods are important for the fabrication of the array, and further considerations continue in the following. 

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