The sensitivity of amperometric sensors is typically set by the rate diffusion of the analyte to the electrode surface, so determining diffusion coefficients in various electrolyte solutions is of fundamental interest. It has been theoretically shown and verified that diffusion coefficients of electrochemically generated analytes can be determined using electrochemical time of flight (ETOF), a method that uses an electrochemical array in which one electrode generates a Red/Ox species, and measures the analyte diffusion times to collecting electrodes of differing distances from a stationary generator. ETOF has the potential to greatly simplify the determination of diffusion coefficients as the analyte concentration, the electroactive area, the solution viscosity, and the electron transfer kinetics can remain unknown. Here we demonstrate an alternative data treatment for ETOF in which the electrochemical flight time is measured for a series of different Red/Ox species of known diffusion coefficients at a single distance. We show this a valid application of a method that has existed for almost 30 years, by determining diffusion coefficients for ruthenium (II) hexamine, and diffusion coefficients in solutions of increased viscosity. Diffusion coefficients are important because they set the sensitivity of amperometric sensors and they are a fundamental property both in membrane permeability and in electrochemical measurements. The most common method of determining diffusion coefficients for analytes in bulk solutions or through gels and membranes relies on the rotating disk electrode (RDEs) 1-7 or the rotating ring disk electrode (RRDE).8 This method determines the diffusion coefficients, D, from the slope of a Levich plot constructed by measuring limiting currents, I L , as a function of square root of the rotation rate, w, according to the Levich equation (Equation 1).Accurate values for the area of the electrode, A, the number of electrons transferred, n, the concentration of the molecule, C, and the viscosity of the solution, v, must also be known in order to effectively determine the diffusion coefficient from the slope of a Levich Plot. The diffusion coefficients of molecules through bulk solution can also be determined quantitatively by wall-jet chronoamperometry, 9 or qualitatively by comparing the CV's of different compounds because the shape of the CV is related to the diffusion coefficient of the molecule. [10][11][12] The other primary option for determining diffusion coefficients of a molecule through a membrane coated over an electrode is impedance spectroscopy, [13][14][15][16][17][18] where the diffusion of the molecule through a membrane or polymer is related to the impedance of the polymer or membrane to current flow. As such, the diffusion through the polymer is related directly to the resistance of charge transfer (mobility) through the membrane, which is related to its conductivity and directly correlated to the diffusion coefficient by the Nernst-Einstein equation (Equation 2).Conductance, σ, ca...

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The mass transport or flux of neurochemicals in the brain and how this flux affects chemical measurements and their interpretation is reviewed. For all endogenous neurochemicals found in the brain, the flux of each of these neurochemicals exists between sources that produce them and the sites that consume them all within μm distances. Principles of convective-diffusion are reviewed with a significant emphasis on the tortuous paths and discrete point sources and sinks. The fundamentals of the primary methods of detection, microelectrodes and microdialysis sampling of brain neurochemicals are included in the review. Special attention is paid to the change in the natural flux of the neurochemicals caused by implantation and consumption at microelectrodes and uptake by microdialysis. The detection of oxygen, nitric oxide, glucose, lactate, and glutamate, and catecholamines by both methods are examined and where possible the two techniques (electrochemical vs. microdialysis) are compared. Non-invasive imaging methods: magnetic resonance, isotopic fluorine MRI, electron paramagnetic resonance, and positron emission tomography are also used for different measurements of the above-mentioned solutes and these are briefly reviewed. Although more sophisticated, the imaging techniques are unable to track neurochemical flux on short time scales, and lack spatial resolution. Where possible, determinations of flux using imaging are compared to the more classical techniques of microdialysis and microelectrodes.

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