Measurement of Lateral Charge Propagation in [Os(bpy)₂(PVP)_nCl]Cl Thin Films: A Scanning Electrochemical Microscopy Approach

Abstract

The use of scanning electrochemical microscopy (SECM) to measure charge transport diffusion constants (D CT ) in metallopolymers of the type [Os(bpy) 2 (PVP) n Cl]Cl, bpy ) 2,2′-bipyridyl and PVP ) poly(4vinylpyridine), is described. In this approach, a triple potential step technique is employed in which the ultramicroelectrode (UME) tip of the SECM is used to electrogenerate a solution phase oxidant, Ru(CN) 6 3-, in an initial potential step, via the oxidation of Ru(CN) 6 4-. This moiety diffuses from the tip to the underlying polymer film where electron transfer occurs, causing the local oxidation of the polymer-bound complex of Os II to Os III . The form of the current-time characteristic in this step provides information on the kinetics of the ET process between the solution species and the polymer-bound moiety, as well as the concentration of redox-active species in the polymer film. This process creates lateral concentration gradients of Os II and Os III along the film. After the first potential step, a waiting period is introduced in which Ru(CN) 6 3is converted back to Ru(CN) 6 4at the UME and Os II can recover in concentration by electron self-exchange between Os III and Os II moieties. After a defined time, the potential of the UME is switched again to cause the generation of the solution-phase oxidant, Ru(CN) 6 3-. The current-time behavior associated with this step is influenced significantly by the extent of lateral electron hopping in the waiting period. It is shown that SECM is capable of measuring D CT values as low as 10 -10 cm 2 s -1 with good precision. We report experimental measurements on spin-coated films of [Os(bpy) 2 (PVP) n Cl]Cl, where n ) 5 or 10, which indicate that D CT is affected significantly by redox site loading and film structure (as determined by atomic force microscopy).

An Electroactive Versatile Matrix for the Construction of Sensors

“…The electrochemistry of thick drop cast films electrodes similar to the one presented in this work shows a D eh in the order of 10 À11 cm 2 s À1 [14], while for thin films its value is in the order of 10 À9 cm 2 s À1 [11,15].Taking into account this two situations, we found that using the diffusion coefficient obtained by Calvo et al [11] is possible to reproduce with a great accuracy the experimental results (Figure 2), showing an efficient electron hopping behavior and the fact that the observed behavior is a consequence of the thickness of the film. This value was also used for Flexer et al [12] to simulate a self-assembled system using a similar electroactive polyelectrolyte.…”

Evaluation of the Chemical Reactions from Two Electrogenerated Species in Picoliter Volumes by Scanning Electrochemical Microscopy

“…Measurable effects on this electrochemical signal are only possible when the probe and the substrate are close to each other, for example, to a distance shorter than about five times the UME radius where their diffusion layers can interact; a typical UME has a radius close to 10 mm or smaller. The electrogenerated species may be reactive towards chemical species at the substrate; thus, the feedback mode has been applied to study the heterogeneous reactivity, for instance, from mediators that "micro titrate" chemisorbed species at electrodes, [5,6] that chemically etch metals [7] involved in the study of the effects of mediator surface diffusion, [8][9][10] lateral charge propagation in polymer films, [11,12] and diffusion in monolayers, [13] as well as used in the detection of reactive species in living cells. [14] In these examples, the SECM probe acts both as a generator of the reactive species and as a transducing element as the feedback current indicates the existence and extent of chemical reaction.…”

Charge Transport in Redox Polyelectrolyte Multilayer Films: The Dramatic Effects of Outmost Layer and Solution Ionic Strength

“…[62] A value of 6-8 10 À10 cm 2 s À1 has been reported for [Os(bpy) 2 -(PVI) 10 Cl] (PVI: polyvinyl-imidazol) (from CV). [63] The charge diffusion coefficients for an osmium-bipyridine redox couple embedded in Nafion and PSS has been reported to be 0.7 10 À10 [26] and 5 10 À9 cm 2 s À1 , [64] respectively.…”

“…The large spread of D app values reported for similar systems can be ascribed to the measuring technique. [62] Stationary techniques (such as EIS) yield larger D app values than techniques involving complete oxidation/reduction (i.e. CV) because they do not require ion fluxes and/or polymer rearrangements.…”