The diffusion-controlled transport of ions and molecules through polymer membranes utilized in chemical and biosensors is often the key factor determining the response characteristics of these sensors. In this paper, a simple voltammetric method is described for the determination of diffusion coefficients of redox molecules in resistive polymer membranes using a planar electrochemical cell (PEC) with a 5 μm radius carbon fiber as working electrode. In the proposed method, the diffusion coefficients are assessed from the scan rate dependence of the peak (or limiting) currents in linear sweep or cyclic voltammograms. The accuracy of the method is estimated through simulations using the method of Nicholson and Shain describing the quantitative relationship between the measured peak current (i) and the square root of the scan rate (v). The proposed method has been used for measuring the diffusion coefficients of ferrocene derivatives in highly resistive aqueous solutions, organic solvents, plasticizers, and plasticized PVC membranes. The measured diffusion coefficients are in agreement with theoretical models and previously reported values.

Diffusion coefficients are important in the design, optimization, and selection of membranes used in many applications including chemical and biosensors. Since the diffusion‐controlled transport of the analyte and interfering ions and molecules through polymer membranes may affect the rate of response, detection limit, selectivity, lifetime, etc. of chemical and biosensors, having a rapid and accurate method for the determination of diffusion coefficients of different compounds in a variety of membranes is uniquely important. In our contribution, we present a simple protocol for the rapid determination of the diffusion coefficients of electrochemically active analytes by linear sweep voltammetry. The expected errors in the estimated coefficients have been analyzed through modeling and the correlation between the magnitude of the unknown diffusion coefficients and the method's related maximum error is presented as a contour plot for different working electrode dimensions. It is also shown that our analysis protocol can be used to estimate the diffusion coefficient in complex membrane matrices and non‐ideal experimental conditions with a maximum systematic error of 14 % even without any experimental parameter optimization.

For feedback controlled monitoring of the popular anesthetic drug propofol we used an organic membrane-coated voltammetric sensor.1 The membrane coating prevents electrode fouling, and provides outstanding detection limit and selectivity for the voltametric working electrode that is adequate for continuous monitoring of propofol in whole blood in the presence of physiologically relevant interferences. The detection limit of the voltametric sensor is controlled by diffusion coefficient (D) and the concentration of the analyte in the membrane (cM), where cM is a function of membrane/solution partition coefficient (P). The selectivity of the membrane-coated sensor, on the other hand, depends on the partition coefficients of the analyte and interfering compounds. To assess the attainable detection limit and selectivity of the membrane coated sensor the diffusion and the partition coefficients have to be known. For the determination of the diffusion coefficients in the membrane it has been loaded with the analyte and the steady state current of a membrane coated planar electrochemical cell with a carbon fiber ultra microelectrode has been recorded in air (D=i/(4nFcMr)). By placing the membrane-coated planar electrochemical cell into a solution the concentration of the analyte in the membrane will be determined by its partition coefficients. From cyclic voltammetric experiments with the membrane-coated sensor in an analyte solutions one can calculate the partition coefficient of the analyte. In our contribution we discuss the unique advantages of the voltammetric method and report the partition coefficients of a variety of compounds (e.g., propofol, ascorbic acid, p-acetamido phenol) between plasticized PVC membranes and aqueous electrolytes as well as the diffusion coefficients of these compounds in the membranes. 1. Kivlehan, F.; Garay, F.; Guo, J. D.; Chaum, E.; Lindner, E., Anal. Chem. 2012, 84 (18), 7670-7676.