Precise Adjustment of Nanometric-Scale Diffusion Layers within a Redox Dendrimer Molecule by Ultrafast Cyclic Voltammetry: An Electrochemical Nanometric Microtome

Abstract

Performing cyclic voltammetry at scan rates into the megavolt per second range allows the exploration of the nanosecond time scale as well as the creation of nanometric diffusion layers adjacent to the electrode surface. This latter property is used here to adjust precisely the diffusion layer width within the outer shell of a fourth-generation dendrimer molecule decorated by 64 [Ru(II)(tpy)2] redox centers (tpy = terpyridine). Thus the shape of the dendrimer molecule adsorbed onto the ultramicroelectrode surface can be explored voltammetrically in a way reminiscent of an analysis with a nanometric microtome. The quantitative analysis developed here applied to the experimental voltammograms demonstrates that in agreement with previous scanning tunneling microscopy (STM) studies the adsorbed dendrimer molecules are no more spherical as they are in solution but resemble more closely hemispheres resting onto the electrode surface on their diametrical planes. The same quantitative analysis gives access to the apparent diffusion coefficient featuring electron hopping between the [Ru(II)/ Ru(III)(tpy)2] redox centers distributed on the dendrimer surface. Based on the electron hopping rate constant thus measured and on a Smoluchowski-type model developed here to take into account viscosity effects during the displacement of the [Ru(II)/Ru(III)(tpy)2] redox centers around their equilibrium positions, it is shown that the [Ru(II)/Ru(III)(tpy)2] redox centers are extremely labile in their potential wells so that they may cross-talk considerably more easily than they would do in solution at an equivalent concentration.

An Integrated Dual Ultramicroelectrode with Lower Solution Resistance Applied in Ultrafast Cyclic Voltammetry

“…In fact, if this electrode system is applied to ultrafast cyclic voltammetry, it also provides a tool to monitor quick long-range electron transfer and electron hopping within molecules. 25,26 Theoretical studies and its applications in singlecell determination are in progress.…”

When Voltammetry Reaches Nanoseconds

“…From analysis of voltammograms up to 2.5 MV/s (27,28), we determined that the dendrimer, which is spherical in solution, gets "squashed" when it adsorbs and comes to resemble a hemisphere (Figure 4a). We also estimated D hop to be~5 ϫ 10 for the would-be homogeneous rate of electron transfer between Ru II and Ru…”

“…Because the radius of one ruthenium center is ~0.7 nm, a pair of nearby centers should move cumulatively by 0.6 nm from their equilibrium positions to reach close contact without a high energy penalty or severe viscous drag. A high expenditure of energy or severe drag would be immediately reflected in the k 2 E D T value (27,28). At first glance, this strong interaction of the electroactive centers is certainly counterintuitive because in a dendrimer, a Ru(tpy) 2 center is linked to a radial chain, which ought to rub against its neighbors with any of the motion of the Ru(tpy) 2 center.…”

Vinylferrocene Homopolymer and Copolymers: An Electrochemical Comparison

“…This is analogous to the Dahms-Ruff electron shuttling mechanism observed in fourth generation PAMAM dendrimers recently reported by Amatore and coworkers. 29,30 Copolymerization of the VFc with a second monomer has widened the analytical scope with further sensors being developed. [31][32][33][34] In these cases VFc was polymerized with 2-(methylthio)ethyl methacrylate, 31 2-hydroxyethyl methacrylate 32 and vinylanthracene.…”