Nanoelectrodes

Chen S efa/1998 Gold nanoelectrodes of varied size transition to molecular like charging Science 280 2098... 

One of the new trends in chemical analysis appeared in the last decade is that the miniaturization. It becomes apparent in the miniaturization of analytical devices, separation procedures, measuring tools, analyzing samples and as a consequent the term micro have appeared. Further development of this trend have led to transfer from the term micro to nano one (nanoparticles, nanofluides, nanoprobes, nanoelectrodes, nanotubes, nanoscale, nanobarcode, nanoelectrospray, nanoreactors, etc). Thereupon a nanoscale films produced by Langmuir-Blodgett (LB) technique are proposed for modifying of chemical sensors. 

Bates et al. reported the construction and characterization of a gold nanoparticle wire assembled using Mg -dependent RNA-RNA interactions for the future assembly of practical nanocircuits [31]. They used magnesium ion-mediated RNA-RNA loop-receptor interactions, in conjunction with 15 nm or 30 nm gold nanoclusters derivatized with DNA to prepare self-assembled nanowires. A wire was deposited between lithographically fabricated nanoelectrodes and exhibited non-linear activated conduction by electron hopping at 150-300 K (Figure 16). 

Figure 16. Scanning electron micrograph of a pair of nanoelectrodes with gold nanoparticles immobilized into the gap using a mixture of large particles small particles derivatized with RNA phosphate buffer containing NaCl in the presence of Mg. (Reprinted with permission from Ref [31], 2006, American Chemical Society.)...

Figure 17. SEM image of single 20 nm and 30 nm-diameter Au nanoparticles asembled from solution and bridging the two adjacent nanoelectrode junctions. (Reprinted with permission from Ref [32], 2005, Wiley-VCH.)...

On the way to more reliability in device fabrication, Kronholz et al. reported on the reproducible fabrication of protected metal nanoelectrodes on silicon chips with <30nm gap width and their electrochemical characterization [33]. For the fabrication of the chips, an optical lithography step and two electron-beam steps are combined (Figure 18). 

Figure 18. Nanoelectrodes with a 30 nm gap, access window 200 x 200 nm, having a Si02/Si3N4/Si02 (A) or a PMMA based (B) protection layer.

Heller I, Kong J, Heering HA, Williams KA, Lemay SG, Dekker C. 2005. Individual single-waUed carbon nanotubes as nanoelectrodes for electrochemistry. Nano Lett 5 137-142. 

X.J. Zhang, B. Ogorevc, and J. Wang, Solid-state pH nanoelectrode based on polyaniline thin film elec-trodeposited onto ion-beam etched carbon fiber. Anal. Chim. Acta 452, 1-10 (2002). 

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],... 

A.A. Karyakin, E.A. Puganova, I.A. Budashov, I.N. Kurochkin, E.E. Karyakina, V.A. Levchenko, V.N. Matveyenko, and S.D. Varfolomeyev, Prussian Blue based nanoelectrode arrays for H202 detection. Anal Chem. 76, 474-478 (2004). 

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). 

R.S. Chen, W.H. Huang, H. Tong, Z.L. Wang, and J.K. Cheng, Carbon fiber nanoelectrodes modified by single-walled carbon nanotubes. Anal. Chem. 75, 6341-6345 (2003). 

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. 

Fig. 3 Principle of electrolyte gating. Tuning of the Fermi levels of WEI and WE2 relative to the molecular levels enables measuring of current (0-voltage (E) characteristics i vs ( wei -L we2) at fixed wei or we2, i vs wei or we2 at fixed bias Ebias = ( wei -Ewe2> as well as barrier height profiles i vs distance z of tailored molecular junctions in a vertical SPM-based configuration respective horizontal nanoelectrode assembly...

The concept of a molecular wire is a remarkable case in this a conjugated rod-like molecule bridges the gap between nanoelectrodes with molecular dips as crudal components.161... 

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. 

Nanoelectrode ensembles were prepared by electroless deposition of Au within the pores of polycarbonate membrane filters (Poretics). Filters with pore diameters of 10 and 30 nm were used [25]. The pore densities and average center-to-center distances between pores for these membranes are shown in Table 1. Multiplying the pore density (pores cm ) by the cross-... 

A persistent problem with micro- and nanoelectrodes is the sealing of the conductive element to the insulating material that surrounds the element such that solution does not creep into this junction [25,68,75]. This solution creeping is undesirable because it causes the double layer charging currents... 

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. 

Figure 3.16 Different steps in the fabrication of MWNT nanoelectrode arrays, (a) metal film deposition, (b) catalyst deposition, (c) plasma-enhanced chemical vapor deposition for CNT growth, (d) dielectric encapsulation with Si02, (e) planarization with a chemical mechanical polishing to expose the ends of the carbon nanotubes, (f) electrochemical characterization. Readapted from Ref [6].

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