High resolution imaging of the distribution and permeability of methyl viologen dication in bovine articular cartilage using scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) has been used in the induced transfer (SECMIT) mode to image the permeability of a probe cation, methyl viologen (MV2+), in samples of articular cartilage. An ultramicroelectrode (UME), scanned just above the surface of a sample, is used to amperometrically detect the probe solute. The resulting depletion of MV2+ in solution induces the transfer of this cation from the sample into the solution for detection at the UME. The current provides quantitative information on local permeability, provided that the sample-UME distance is known. It is shown that the necessary topographical information can be obtained using the amperometric response for the oxidation of Ru(CN)(6)(4-), which does not permeate into the cartilage matrix. This procedure was validated by marking samples in situ, after electrochemical imaging, with subsequent examination by ex situ interferometry and optical microscopy. Wide variations in the permeability of MV2+ have been detected by SECMIT. These observations represent the first demonstration of the inhomogeneous permeability of a cation in cartilage on a micrometre scale. The permeability maps show similar features to the proteoglycan distribution, identified by toluidine blue staining, and it is likely that proteoglycans are the main determinant of MV2+ permeability in articular cartilage. (C) 2000 Elsevier Science B.V. All rights reserved.