Balázs Csóka

Investigation of concentration profiles inside operating biocatalytic sensors with scanning electrochemical microscopy (SECM)

Scanning electrochemical microscopy (SECM) with amperometric or potentiometric measuring tips was used to investigate biocatalytic reactions inside the enzyme layer of a biosensor during its operation. The well known glucose oxidase catalyzed oxidation of glucose has been selected for the studies. Local, instantaneous concentration of dissolved oxygen and hydrogen peroxide was studied observing the amperometric current while miniaturized potentiometric tip served for local pH measurements. Liquid enzyme layer immobilized with Cellophane membrane or cross linked polyacrilamide gel membrane containing entrapped enzyme served for biocatalytic media in the SECM imaging. Local maximum of H(2)O(2) and minimum of O(2) profiles were found at approximately 200 microm far from the substrate/enzyme layer boundary. From the experimental findings guidelines to design well functioning biocatalytic sensors could be concluded. The concentration profiles obtained with SECM techniques were compared with the results of simple model calculations carried out with the method of finite changes. Most of earlier made SECM studies dealing with enzyme reactions imaged the electrolyte being in contact with the immobilized enzyme. The data in our investigation, however, were collected inside the working catalytic layer.

Periodically interrupted amperometry at membrane coated electrodes: A simplified pulsed amperometry

Amperometric detection combined with separation technique or with selective molecular recognition step can be very effective solving quantitative analytical tasks. When the amperometric working electrode surface needs cleaning or reactivation, pulsed amperometric technique can be the choice. Coating working electrodes with different sensitizing or protecting layer is quite common in the practice of voltammetric analysis. In these studies the behavior of coated electrodes using a simplified pulsed amperometric working program which can be named periodically interrupted amperometric (PIA) detection has been investigated. Rotating platinum, and carbon paste electrodes coated with dialysis film or porcine intestinal membrane were used in the experiments. The signal in case of electrochemical oxidation of hydrogen peroxide and ascorbic acid at convective conditions has been evaluated. The signal, obtained with conventional amperometry has been compared with signal collected with a periodically interrupted amperometric measuring program, allowing time for the diffusion to reload the diffusion layer at the electrode surface. The sensitivity and the lower limit of detection (4.5 x 10(-7)M for ascorbic acid and 2 x 10(-6)M for H2O2) proved superior in case of the periodically interrupted amperometry.

Lowering the Detection Limit of Solvent Polymeric Ion-Selective Membrane Electrodes. An Experimental Study with Calcium-Selective Micropipette Electrodes

Abstract Ion-selective microelectrodes (ISMEs) in the range of 2–90 µm were investigated using different composition of inner reference electrolytes (IRE). The electrodes prepared were PVC-free solvent polymeric based calcium-selective (Ca-ISMEs) micropipette ones. Efforts were done to find an optimal composition of the membrane, IRE, and pipette size in order to reduce the apparent super-Nerstian part of the calibration curves for Ca2+-measurements in the submicromolar range. As a result, we could find an optimal composition, which was linear down to about 10−8 M.