Electron Transfer Kinetics at Oxide Films on Metallic Biomaterials

Abstract

A native oxide film on the biphasic alloy Ti6Al4V was studied with scanning electrochemical microscopy (SECM). This alloy is commonly used for biomedical applications due to its biocompatibility and mechanical properties. The heterogeneously composed, n-semiconducting oxide film is of particular interest as biological systems are in contact only with these oxides and immunological rejection mechanisms may be connected to their electrochemical properties. Auger electron spectroscopy showed that the elemental composition of the oxide films on the α and β phase differ from each other. Approach curves were recorded above individual grains of the α phase in the feedback mode with several redox mediators. They were selected to cover a wide range of redox potentials for a better understanding of the surface kinetics of the oxide layer. The electron-transfer kinetics changed strongly depending on the redox potential of the mediator with respect to the energetic position of the bandgap of the oxide film. Predictions about the value of the flatband potential on an individual phase were derived from these experiments. Furthermore, SECM images were recorded to laterally resolve different electrochemical properties of the oxide film originating from the heterogeneous composition of the oxide on both phases.

Sensing application in the precursor region of localized corrosion by scanning electrochemical microscopy

“…Their use is based on the formation of thin and dense oxide layers that form spontaneously in air and passivate the metals surface, thereby diminishing the release of potentially toxic ions into surrounding tissue. Pust et al 81,82 The results pointed out that depending on the formal potential of the applied mediator, the b phase showed a higher electrochemical activity (as is shown in Fig. 9) and pitting nucleation easily occurs as it allows higher electron transfer rates during the regeneration of the mediator.…”

Developments in Titanium Research and Applications in Germany

“…These are (i) vanadium and aluminium as potentially cytotoxic or problematic alloying elements and (ii) the alloy specific semiconductor properties of the oxide layers. In case of high electronic conductivity this may result in redox reactions with adsorbed proteins with the danger of unfavourable immunological reactions to adsorbed proteins in non-native conformation [12,13].…”

Investigation of Localized Catalytic and Electrocatalytic Processes and Corrosion Reactions with Scanning Electrochemical Microscopy (SECM)

“…This was necessary to achieve an adequate lateral resolution for the visualization of the two phases. Furthermore, approach curves were recorded above the α phase at different substrate potentials (Figure 25c), visualizing the semiconducting properties of the passive film [205]. With increasing overvoltage at the substrate, the kinetics of the mediator regeneration normally shift from almost no regeneration to quite significant reaction rates, but the quantitative behavior differs for each investigated mediator.…”

“…It complements other scanning probe techniques such as the scanning reference electrode technique (SRET) [178,179], conductive scanning force microcopy (CSFM), electrochemical scanning tunneling microscopy (ECSTM), and scanning Kelvin probe techniques which are popular methods for the investigation of functional materials [180]. Basic experimental approaches include the imaging of the permeability of applied protective coatings [181][182][183][184][185][186][187][188][189][190][191][192][193], the imaging of regions with distinctly higher electron transfer rates which may be precursor sites for pitting corrosion [29,57,[194][195][196][197][198][199][200][201][202][203][204][205][206][207], the initiation of pitting corrosion by local generation of aggressive species at the UME [208,209] and the detection of active corrosion by collecting released species [55,58,60,104,[210][211][212][213][214]…”

“…The grain-resolved electrochemical activity of the alloy Ti6Al4V was investigated by Pust et al [204,205] in the FB mode with several redox mediators covering a broad range of formal potentials around the flat band potential of the semiconducting passive film. Ti6Al4V, which is used as biomedical material for load-bearing implants, has a well-defined phase structure with the V-rich phase being more conductive than the Al-rich α phase [204].…”