Electrode nanoelectrode

Nano-electrode arrays can be formed through nano-structuring of the electrocatalyst on an inert electrode support. Indeed, if the current of the analyte reduction (oxidation) on a blank electrode is negligible compared to the activity of the electrocatalyst, the former can be considered as an insulator surface. Hence, for the synthesis of nanoelectrode arrays one has to carry out material nano-structuring. Recently, an elegant approach [140] for the electrosynthesis of mesoporous nano-structured surfaces by depositioning different metals (Pt, Pd, Co, Sn) through lyotropic liquid crystalline phases has been proposed [141-143],... 

X. Zhang, J. Wang, B. Ogorevc, and U.E. Spichiger, Glucose nanosensor based on Prussian-blue modified carbon-fiber cone nanoelectrode and an integrated reference electrode. Electroanalysis 11, 945-949 (1999). 

The addressing of nanoelectronic assemblies metal-molecule (nanocluster)-metal with device-like functions, such as rectifiers, switches, or transistors requires a source and a drain, and one or more localized electronic levels. The roles of source and drain (both as working electrodes WEI and WE2) may be represented by the tip of an STM, combined with an appropriate substrate or, alternatively, a pair of nanoelectrodes see Fig. 3. 

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]. 

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. 

Because the fractional electrode area at the lONEE is lower than at the 30NEE (Table 1), the transition to quasireversible behavior would be expected to occur at even lower scan rates at the lONEE. Voltammograms for RuCNHs) at a lONEE are shown in Eig. 8B. At the lONEE it is impossible to obtain the reversible case, even at a scan rate as low as 5 mV s . The effect of quasireversible electrochemistry is clearly seen in the larger AEp values and in the diminution of the voltammetric peak currents at the lONEE (relative to the 30NEE Fig. 8). This diminution in peak current is characteristic of the quasireversible case at an ensemble of nanoelectrodes [78,81]. These preliminary studies indicate that the response characteristics of the NEEs are in qualitative agreement with theoretical predictions [78,81]. 

We have demonstrated a new method for preparing electrodes with nano-scopic dimensions. We have used this method to prepare nanoelectrode ensembles with individual electrode element diameters as small as 10 nm. This method is simple, inexpensive, and highly reproducible. The reproducibility of this approach for preparing nanoelectrodes is illustrated by the fact that NEEs given to other groups yielded the same general electrochemical results as obtained in our laboratory [84]. These NEEs display cyclic voltammetric detection limits that are as much as 3 orders of magnitude lower than the detection limits achievable at a conventional macroelectrode. 

Figure3.17 (a) and (b) CV measurements in 1 mM ofFe(CN)6 and 1 M KCI with the high-density MWNT nanoelectrode array (2 X lO electrodes/cm ) and low density one (with about 7x 10 electrodes/cm ), respectively, (c) and (d) showthescanningelectron images for the high and low density arrays. Readapted from Ref [4] with permission. Copyright, 2004, RSC.

UPD of Ag onto Au electrodes covered with SAM of alkanethiols has been described by Oyamatsu [319]. Hu et al. [320] have prepared nanoelectrode ensembles by assembling silver colloid and mercaptan on a gold electrode. 

B. Ogorevc, and J. Wang, Solid-State pH Nanoelectrode Based on Polyaniline Thin Film Electrodeposited onto Ion-Beam Etched Carbon Fiber, Anal. Chim. Acta 2002,452, 1. A Zr02 electrode can measure pH up to 300°C [L. W. Niedrach, Electrodes for Potential Measurements in Aqueous Systems at High Temperatures and Pressures, Angew. Chem. 1987,26, 161],... 

The analytical performance of the pH nanoelectrode compares favorably with commercial glass pH electrodes when applied for pH measurements of body fluids (serum, urine) and low ionic strength water samples (rain water, tap water, deionized water) [104],... 

An other interesting strategy is the modification of the surface of the electrodes with multiwalled carbon nanotubes (MWNTs) or single-walled carbon nanotubes (SWNTs) [13,32]. The MWNTs are grown on the electrodes covered with a nickel catalyst film by plasma-enhanced chemical vapour deposition and encapsulated in Si02 dielectrics with only the end exposed at the surface to form an inlaid nanoelectrode array [13]. In the other case, commercial SWNTs are deposited on SPE surface by evaporation [32], The carbon nanotubes are functionalised with ssDNA probes by covalent attachment. This kind of modification shows a very efficient hybridisation and, moreover, the carbon nanotubes improve the analytical signal. 

The above-mentioned values for the critical dimensions should be considered like approximations. In fact, a UME with a critical dimension of 25 pm is not totally different from another of 30 pm. Nevertheless, sometimes the terms UME and, specially, NE are erroneously employed. Today, in keeping with other aspects of nanotechnology and nanoscience, the electrochemical scale of interest is around 100 nm (far from the scale of the nanoelectrodic behaviour). Perhaps, the term ERD (electrodes with reduced dimensions) is more adequate for UMEs and those NEs (i.e. electrodes whose critical dimension is between 25 pm and 10 nm), because their electrochemical behaviours are the same. 

The classical experimental setup developed for fe polarization reversal implies a singledomain fe sample sandwiched between two electrodes [28], While conventional domain inversion techniques use equal sized electrodes covering the polar faces of fe templates, nanodomain inversion occurs under totally different conditions when the bottom electrode is a uniform plate and the upper one is a point contact. Two different kinds of the upper switching mobile nanoelectrodes may be considered afm tip (and/or array of tips) and electron drop formed using electron beam exposure. When a voltage stress is applied to the nanoelectrode, both the electric field intensity and its spatial distribution strongly differ in fe thin films (thin fe crystals) and bulk fe crystals. 

Electrochemical interfaces modified with inorganic NPs behave as nanoelectrode ensembles. In principle, the electroanalytical detection limit at a nanoelectrode ensemble can be much lower than that at an analogous macrosized electrode because the ratio between the faradaic and capacitive currents is higher.19-23... 

Generally, inorganic NPs assembled on electrode surfaces, which behave as nanoelectrode ensembles, are not diffusionally isolated and the overall current... 

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