Spray-Coating As a Versatile Tool for Electrode Surface Modification – from Nanostructured Electrodes to Enzyme Based Sensors

Abstract

Electrochemical biodevices (e.g. biosensors and biofuel cells) do not only rely on the general stability and activity of the utilized biocatalyst but also on the architecture of the entire electrode. In particular, the choice of suitable electrode surface structuring, enzyme immobilization tech­nique, as well as the co-immobilization of electron-transfer mediators is crucial for highly accu­rate sensing systems. Recently, we presented the fabrication of a membrane-free glucose/O2 powered enzymatic biosupercapacitor based on transparent nanostructured indium tin oxide (ITO) electrodes prepared by means of a spray-coating process.1,2 Compared to ITO nanoparticle (NP) based electrodes obtained by drop-casting, the performance and stability of the glucose/oxygen biosupercapacitor was significantly improved by utilizing spray-coated ITO-NP based electrodes. Optimization of the spray-parameters led to a high electrochemically active surface area with substantially increased porosity, thus enhancing the catalytic currents due to higher biocatalyst loading. In this work, we used a specifically designed spray-coating system as a tool for electrode surface modification. The automated process ensures the formation of homogeneous and reproducible electrode surfaces. Moreover, the system cannot only be used for the preparation of nano-struc­tured electrode surfaces, such as ITO-NP based electrodes, but also allows spraying of enzyme and polymer solutions. By layered spraying of glucose oxidizing enzymes, embedded in an Os complex modified polymer onto the porous surface, so prepared electrodes were utilized in a self-powered glucose biosensor operated by charge/discharge of a Nernstian biosupercapacitor. Addi­tional surface modification has been achieved by the development and optimization of meso- and macro-porous ITO-NP based electrodes. A mixture of ITO nanoparticles and spherical, monodis­perse polymer beads acting as template for pore formation was sprayed onto flat electrodes. The polymer beads were removed by calcination of the material which generates macro-pores of the size of the used polymer beads (see sketch). Based on the porous 3D surface structure, an effective wiring of a subsequently immobilized enzyme redox polymer hydrogel is achieved which minimizes limitations caused by a slow electron transfer within the polymer. Our results demonstrate the high versatility of the developed spray-coater not only for surface structuring, but also for whole modification of entire electrode designs with enzymes wired in hydrogels with applications in fuel cells, bio-supercapacitors and self-powered biosensors.