Membrane-covered working electrodes

The experimental setup is shown in Figure 9.23. The Pt-black catalyst film also served as the working electrode in a Nafion 117 solid polymer electrolyte cell. The Pt-covered side of the Nafion 117 membrane was exposed to the flowing H2-02 mixture and the other side was in contact with a 0.1 M KOH aqueous solution with an immersed Pt counterelectrode. The Pt catalyst-working electrode potential, Urhe (=Uwr)> was measured with respect to a reversible reference H2 electrode (RHE) via a Luggin capillary in contact with the Pt-free side of the Nafion membrane. 

A second surface modification has been reported by Yamamoto et al. These workers added stearic acid to their carbon paste mixture. This produced an electrode which was relatively insensitive to ascorbic acid and DOPAC relative to dopamine. It is theorized that this electrode works because of electrostatic repulsion of the anionic ascorbate and DOPAC by surface stearate groups. Ionic repulsion has also been employed by covering the surface of the working electrode with an anionic polymer membrane. Gerhardt et al. used Nafion, a hydrophobic sulfonated perfluoro-polymer, to make a dopamine selective electrode. This electrode exhibited selectivity coefficients as large as 250 1 for dopamine and norepinephrine over ascorbic acid, uric acid, and DOPAC. 

A microsensor system for simultaneous amperometric determination of H2O2 and 02 consists of two working electrodes, one Pt counterelectrode and one WO3 reference electrode. The working electrodes are GCE covered with HRP immobilized in an elec-trodeposited membrane of polypyrrole the working electrode for 02 has in addition a layer of immobilized superoxide dismutase. Operation with this system is at pH 5.1, setting both working electrodes at —60 mV. Other operating modes are also possible. ... 

An important parameter in a number of fields is the study of inorganic phosphate. Recently, Kwan et al. [206,207] have reported on a screen-printed phosphate biosensor based on immobilised pyruvate oxidase (PyOD) for monitoring phosphate concentrations in a sequencing batch reactor system [206] and in human saliva [207]. The enzyme was immobilised by drop-coating a Nation solution onto the working electrode surface this was then covered by a poly(carbamoyl) sulfonate (PCS) hydrogel membrane. 

Another type of thin film device consists of four 0.5 x 0.5 mm2 platinum working electrodes and a Ag/AgCl reference electrode made on a 0.3 mm glass carrier. The enzymes were immobilized onto the working electrodes in a photo crosslinked pHEMA membrane and additionally covered by a diffusion limiting and a catalase containing top layer [77]. 

Acetylcholineesterase Miniaturized multichannel transduc-tor with planar Au electrode which was first covered with a choline-selective liquid membrane made from 66% PVC-polyvinyl acetate (PVA), 33% 2-nitrophenyl octyl ether plasticizer and 1% ion-pair choline phosphotungstate. A second layer of 2% AChE in the PVA-polyethylene dispersion was spread on the top. The electrode was used as working electrode versus Ag/AgCl for potentiometric measurement of Ch and ACh in 0.1 M Tris buffer at 7.4. Optimum pH range for the sensor was 7-9. The calibration graph was linear from 0.02-10 mm ACh and detection limit was 5 pM. Response time was 3-5 min. Sensor was suitable for determination of ACh in biological fluids. [86]... 

Fabrication procedure of gold nanodisk electrodes (NEEs) is schematically shown in Fig. 3.14 (Menon and Martin 1995 Pereira et al. 2006). Step I A piece of the Au/Au-PC/Au membrane is first affixed to a piece of adhesive aluminum foil tape (Fig. 3.14a). Step II A rectangular strip of a copper foil, with a conductive adhesive, is then affixed to the upper Au-coated surface of the Au/Au-PC/Au membrane (Fig. 3.14b). This Cu foil tape acts as a current collector and working electrode lead for the NEE. Step III The upper Au surface layer from the portion of the Au/Au-PC/Au membrane not covered by the Cu foil tape is then removed by simply applying and then removing a strip of Scotch tape. Removal of the Au surface layer exposes the disk-shaped ends of the Au nanowires within the pores of the membrane (Fig. 3.14c). These nanodisks will become the active electrode elements. Step IV The NEE assembly is heat treated at 150°C for 15 min. This produces a water-tight seal between the Au nanowires and the pore walls. Finally, strips of strapping tape are applied to the lower and upper surfaces of the assembly to insulate the Al and Cu foil tapes (Fig. 3.14d). 

Assolant-Vinet and Coulet (1986) used a preactivated Biodyne Im-munoaffinity Membrane (Pall, USA) to cover two electrodes working in differential mode. Immobilization was performed by simply dropping the enzyme solution onto the membrane, i.e., the biosensor-users may prepare the enzyme membranes themselves. Good results were obtained with GOD, lactate oxidase, and oxalate oxidase (Coulet, 1987). 

Nafion membranes have been used in gas sensors, e.g. (i) to determine CO gas (CO gas sensor) by setting the working electrode and counter electrode on a glass substrate and by covering the electrodes with a Nafion recast membrane because Nafion polymer is relatively permeable to 02, CO and C02 (working electrode ... 

Gas-sensing membrane electrodes use a gas-permeable membrane that allows measured species (as a dissolved gas) to pass through and be measured within the electrode. These probes are very selective and sensitive for gases such as ammonia and carbon dioxide. For carbon dioxide, the working electrode is usually a modified glass pH electrode covered in a teflon membrane. Carbon dioxide in solution forms carbonic acid which lowers the pH. 

The polarographic oxygen-measuring cell, covered with membranes and working on the so-called Clark principle, consists of two electrodes arranged so as to be insulated, which are joined by a liquid or pastelike electrolyte. The electrode compartment is separated from the... 

Membrane covered amperometric sensors are also suitable for determining gases. If a disc-shaped working electrode radius is less than about 90 membrane thicknesses, then the edge current causes more than 1% exhibition of the one-dimensional (1-D, planar) component of the current, which is proportional to the electrode area [122]. The 1-D mass flux, JiD, can be calculated from ... 

To fabricate NO sensors, chemically modified electrodes are commonly covered with an additional membrane layer. This increases the selectivity for NO by cutting off other easily oxidized and interfering species. A variety of membranes (e.g., cellulose acetate (14, 15), Nafion (16)) have been used to modify the surface of working electrodes via electropolymerization or classic dip coating methods. 

In this Clark-type NO microsensor, a PTFE gas permeable membrane was used for selectivity specific to NO. A capillary structure with the bottom end simply covered with a thin PTFE gas permeable membrane ( 30 pm thick) was used as an outer sleeve of the sensor. A platinized Pt working electrode with a Ag wire reference electrode coiled around its glass sheath was inserted into the outer sleeve filled with an internal solution (aqueous 30 mM NaCl and 0.3 mM HCl solution as recommended by Shibuki) to optimize kinetics of NO oxidation at the platinum working electrode of the NO gas sensor (20). The Pt... 

---