Electrochemistry of Electroactive Surface-Immobilized Nanoparticles

The lure of new physical phenomena and new patterns of chemical reactivity has driven a tremendous surge in the study of nanoscale materials. This activity spans many areas of chemistry. In the specific field of electrochemistry, much of the activity has focused on several areas (a) electrocatalysis with nanoparticles (NPs) of metals supported on various substrates, for example, fuel-cell catalysts comprising Pt or Ag NPs supported on carbon [1,2], (b) the fundamental electrochemical behavior of NPs of noble metals, for example, quantized double-layer charging of thiol-capped Au NPs [3-5], (c) the electrochemical and photoelectrochemical behavior of semiconductor NPs [4, 6-8], and (d) biosensor applications of nanoparticles [9, 10]. These topics have received much attention, and relatively recent reviews of these areas are cited. Considerably less has been reported on the fundamental electrochemical behavior of electroactive NPs that do not fall within these categories. In particular, work is only beginning in the area of the electrochemistry of discrete, electroactive NPs. That is the topic of this review, which discusses the synthesis, interfacial immobilization and electrochemical behavior of electroactive NPs. The review is not intended to be an exhaustive treatment of the area, but rather to give a flavor of the types of systems that have been examined and the types of phenomena that can influence the electrochemical behavior of electroactive NPs. 

Examples of electroactive NP materials discussed in the review include Ti02, Mn02, iron oxides, other metal oxides, hydroxides and oxyhydroxides and Prussian Blue. We use the term electroactive N Ps to refer to the faradaic electroactivity in such materials and to distinguish them from NPs comprised of metals (such as Au, Ag, Pt, Co, etc.) or semiconductors (such as CdS, CdSe, etc.). This distinction is based on the ability of many electroactive NPs to undergo faradaic oxidation or reduction of all of the metal (redox) centers in the NP. This is in contrast to the behavior of many metal and semiconductor NPs for which oxidation or reduction is fundamentally an interfacial, double-layer process. This deflnition is somewhat arbitrary, since the smallest metal and semiconductor NPs behave molecularly, blurring the distinction 

This review focuses on nanoparticles, namely objects that are roughly spherical. We use the commonly accepted definition for nanoscale objects of having a dimension below 100 nm, and so identify nanoparticles as objects with a diameter of 100 nm or smaller. The review does not focus on larger aspect ratio nanoscale materials such as nanotubes and nanorods, though they are mentioned in some cases. 

As mentioned above, the electroactive NPs discussed here differ from typical semiconductor NPs or capped noble-metal NPs (e.g., Au-monolayer-protected dusters) in an important way. Specifically, the reduction or oxidation of the metal centers in 

In many cases, the electroactive NPs of interest here can be considered to be colloidal materials. Thus, many of the approaches to their synthesis rely on reaction types and 

Advances in Electrochemical Science and Engineering Vol. 11 Chemically Modified Electrodes. Edited by Richard C. Alkire, Dieter M. Kolb, Jacek Lipkowski and Philip N. Ross Copyright 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31420-1 

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