Improving the reproducibility of hydrogel-coated glutamate microsensors by using an automated dipcoater

Hydrogel-coated microsensors based on carbon fiber electrodes (CFEs) are promising tools for in vivo analysis of endogeneous compounds such as glutamate. However, their construction generally depends on manual fabrication, which often results in poor reproducibility. The aim of this study was to improve the reproducibility and performance of glutamate microsensors. CFEs (10 microm diameter, 300-500 microm long) were coated with a cross-linked redox-polymer hydrogel containing l-glutamate oxidase, horseradish peroxidase and ascorbate oxidase. Various parameters that are likely to influence the reproducibility of the glutamate microsensors were studied. It appeared that the most crucial step in determining the microsensor performance is the manual hydrogel-application procedure. To control this procedure an automated dipcoater was constructed, which allowed mechanical application of the hydrogel on the CFE under standardized conditions. Significant improvements in performance were seen when the CFEs were dipcoated for 10 min at 37 degrees C. Further improvements were obtained when the automated hydrogel application was combined with other cross-link methods, such as electrodeposition and electrostatic complexation. A crucial factor in determining the microsensor performance is the hydrogel thickness. Microscopic observations revealed that, despite the use of an automated dipcoater, the layer thickness was not constant. By combining the automated dipcoat technique with amperometry, the layer thickness could be indirectly monitored and controlled, which resulted in significant improvements of the reproducibility of the sensors.