In 1962, Clark and Lyons proposed incorporating the enzyme glucose oxidase in the construction of an electrochemical sensor for glucose in blood plasma. In their application, Clark and Lyons describe an electrode in which a membrane permeable to glucose traps a small volume of solution containing the enzyme adjacent to a pH electrode, and the presence of glucose is detected by the change in the electrode potential that occurs when glucose reacts with the enzyme in this volume of solution. Although described nearly 50 years ago, this seminal development provides the general structure for constructing electrochemical glucose sensors that is still used today. Despite the maturity of the field, new developments that explore solutions to the fundamental limitations of electrochemical glucose sensors continue to emerge. Here we discuss two developments of the last 15 years; confining the enzyme and a redox mediator to a very thin molecular films at electrode surfaces by electrostatic assembly, and the use of electrodes modified by carbon nanotubes (CNTs) to leverage the electrocatalytic effect of the CNTs to reduce the oxidation overpotential of the electrode reaction or for the direct electron transport to the enzyme.

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This paper reports the EPR spectroscopic characterization of a recently developed magnetically oriented spin-labeled model membrane system. The oriented membrane system is composed of a mixture of a bilayer forming phospholipid and a short chain phospholipid that breaks up the extended bilayers into bilayered micelles or bicelles that are highly hydrated (approximately 75% aqueous). Paramagnetic lanthanide ions (Tm 3+ ) were added to align the bicelles such that the bilayer normal is collinear with the direction of the static magnetic field. Optimal bicelle alignment was obtained when the temperature was increased slowly (approximately 15 min) from 298 K (gel phase) to 318 K (L R phase) at 0.64 T. The nitroxide spin probe 3β-doxyl-5R-cholestane (cholestane) was used to demonstrate the effects of macroscopic bilayer alignment through the measurement of orientational dependent hyperfine splittings that were close to A yy . The EPR signals of cholestane inserted into oriented and randomly dispersed DMPC-rich bilayers have been investigated over the temperature range 298-348 K. Also, the time dependence of the loss of orientation upon cessation of the magnetic field has been characterized. Power saturation EPR experiments indicate that for the sample compositions described here, the lanthanide ions do not induce spectral line broadening of the cholestane EPR signal in DMPC-rich lipid bilayers. Recently, there has been a great deal of excitement over the use of magnetically oriented systems for both solution and solid-state NMR spectroscopy. This study demonstrates the feasibility of conducting bicelle experiments in the relatively low magnetic field of a conventional EPR spectrometer. The system offers the opportunity to carry out EPR studies using a well-oriented highly hydrated model membrane system whose preparation is amenable to the reconstitution of labile membrane components such as integral membrane proteins.

Interfacial assemblies capable of determining glutamate by redox catalysis are prepared by electrostatic assembly of alternating layers of ferrocene poly(allylamine) polymer and glutamate oxidase on a gold electrode. Deposition of the polymer was confirmed in cyclic voltammetry measurements by the presence of a surface wave corresponding to the oxidation of the ferrocene group. In the presence of glutamate in the adjacent electrolyte solution, the current increases and approaches a pseudosteady state, consistent with redox catalysis. Electrodes modified with glutamate oxidase had a linear response to glutamate up to 0.0045 M with sensitivity of 20 mA/cm 2 and a limit of detection of 31.4 mM glutamate. An apparent Michaelis -Menten constant of 0.40(AE 0.13) mM for the confined glutamate oxidase was determined for this assembly. When used in flow-injection experiments, glucose oxidase modified electrodes responded to transient zones of glucose; however, the detection limits of the assemblies to the flowing stream were substantially higher than found for measurements on static solutions.