Laser reduced graphene oxide-based interdigitated microelectrodes were functionalized with TiO<sub>2</sub> nanoparticles towards sensor applications. Two kinds of interdigitated microelectrodes were prepared by laser direct writing using graphene oxide (GO) and TiO<sub>2</sub> nanoparticles. One is a TiO<sub>2</sub> nanoparticle-deposited interdigitated microelectrode consisting of GO and laser-induced reduced graphene oxide (rGO), where the rGO/GO/rGO structure was prepared by laser direct writing on a GO-coated PET film and then a TiO<sub>2</sub> sol solution was drop-casted on the electrode. Another is a TiO<sub>2</sub>/rGO hybrid interdigitated microelectrode prepared by laser direct writing on a TiO<sub>2</sub> nanoparticle-GO hybrid film. The UV light sensitivity of the TiO<sub>2</sub> nanoparticle-deposited rGO/GO/rGO interdigitated microelectrode and the oxygen quenching behavior were applied to oxygen sensing. The output voltage from the TiO<sub>2</sub> nanoparticle-deposited rGO/GO/rGO structure in the AC detection mode under 369 nm LED irradiation showed clear relationship with the degree of vacuum. The sensing behavior was based on the photo-generated carrier quenching by oxygen. The irradiation of a 405 nm blue violet laser to a TiO<sub>2</sub> nanoparticle-GO hybrid film caused the crystal phase transition from anatase to rutile TiO<sub>2</sub> accompanying the melting of anatase nanoparticles. The TiO<sub>2</sub>/rGO hybrid interdigitated microelectrode consisting of anatase TiO<sub>2</sub>, rutile TiO<sub>2</sub>, and rGO was prepared by laser direct writing. The TiO<sub>2</sub>/rGO hybrid interdigitated microelectrode showed the response to visible light irradiation.
The electronic interconnection based on solution processes using nanomaterials is one of the key technologies in the printed electronics towards flexible and wearable devices in IoT. The laser direct writing of three kinds of conductive micropatterns and the device applications were studied. The first one is a Cu micro-grid structure using a Cu nanoparticle ink and the application to a strain sensor. The second one is a reduced graphene oxide (rGO) interdigitated electrode prepared by laser-induced reduction of graphene oxide (GO) and the application of an rGO/GO/rGO interdigitated microelectrode to a humidity sensor. The third one is a carbon interdigitated electrode prepared by laser carbonization of a polyimide (PI) film and the application to an in-plane micro-supercapacitor (MSC). One of the important features in wearable devices is the stability and reliability under bending conditions. The influences of bending in the electric properties were studied for the Cu micro-grid and an rGO/GO/rGO interdigitated electrode. Remarkable resistance change of the Cu micro-grid structure was observed in bending experiment. An unusual large resistance change was explained by a nanostructure remaining even after sintering. On the other hand, the rGO/GO/rGO structure showed excellent stability of the output signal in humidity sensing experiments against bending. Such a stable electronic properties against bending stress was attributed to the electronic conductivity based on π−π interaction between graphene planes. A crystalline layered structure of graphene planes were clearly observed in TEM images for an rGO film.