Diffusion restricting outer membranes for greatly extended linearity measurements with glucose oxidase enzyme electrodes

A series of microporous and homogeneous membranes have been employed as outer membranes in an amperometric glucose oxidase enzyme electrode for the determination of glucose at concentrations substantially higher than the enzyme Km. Reduction of porosity by use of low pore diameter (0.01 μm) microporous polycarbonate showed only a minor enhancement of glucose linearity compared to dialysis membrane, but treatment with liquid phase lipid (tripalmitin) increased linearity to 100mM. Specially manufactured low pore density membrane increased linearity to 200 mM and with silane treatment to 500 mM. Glucose flux was generally lower through homogeneous membranes. Cellulose acetate gave linearity to 200 mM and with incorporation of surfactant, flux was modified such that sensitivity was enhanced whilst linearity increased to 500 mM. Relative glucose : oxygen diffusion through the outer membrane is discussed as a critical factor in determining linearity and sensitivity. Linearity with unplasticized PVC as outer membrane was 300 mM, but the highest linearity of 2000 mM was achieved with a mixed PVC/polycarbonate membrane. Glucose : ascorbate selectivity was also superior for the cellulose acetate and PVC based membranes. Such membranes may be used as dual purpose linearising/ascorbate rejecting membranes for sensing in samples with extreme glucose concentrations and significant ascorbate content such as fruit juices. Membrane effective diffusion coefficients were determined by amperometry and by diffusion chamber for selected membranes and generally reflected observed sensor linearity and ascorbate rejection.