Nugues, S. and Denuault, G.
Scanning electrochemical microscopy: Amperometric probing of diffusional ion fluxes through porous membranes and human dentine
Journal of Electroanalytical Chemistry, 408, (1-2), . (doi:10.1016/0022-0728(96)04523-8).
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The scanning electrochemical microscope (SECM) was used to probe highly localised fluxes of electroactive species diffusing through the channels of porous materials. The validity and feasibility of the approach were demonstrated with the help of model experiments carried out on a track-etched polycarbonate membrane. This sample was chosen for its low porosity (5%) and its quasi-perfect 10 mu m diameter pores. While diffusion of redox species was induced by a large concentration gradient across the sample, other mass transport mechanisms were eliminated. The arrival of redox species, [Fe(CN)(6)](4-), was detected amperometrically by the SECM tip positioned a few micrometres above the mouth of the pores. Two basic SECM techniques were considered. Approach curves (tip current versus tip-substrate distance Z) were recorded to assess the extent of diffusion in the solution close to the substrate and to establish the optimum tip-substrate distance for the detection of the pores. Maps (tip current versus planar coordinates X, Y) were recorded to assess the distribution of diffusional fluxes. Images of localised diffusional fields were obtained and analyzed in terms of single ion fluxes associated with individual pores. More complex experiments were carried out with human dentine samples which had been characterised by atomic force microscopy (AFM). Dentine has a very high porosity (75%) with tubules varying from 2 to 5 mu m in diameter. It was chosen to challenge the ability of the SECM to image the fluxes of electroactive species diffusing through closely spaced pores and because ion transport through dentinal tubules is thought to be one of the major mechanisms for nerve stimulation in dentine hypersensitivity. As expected the diffusion fields from individual channels overlapped and showed the limitation of the SECM. However, the tip response was highly sensitive to the presence of obstruction inside the pores. Spatially resolved low current regions were analyzed in terms of blocked pores. Numerical simulations of diffusion through model porous substrates were used to predict the time dependence and spatial distribution of diffusional patterns in the presence and absence of the SECM tip. The experimental approach reported forms the basis of novel investigations for the assessment of the treatment of dentine hypersensitivity.
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