A colorimetric reagent in the form of nanoparticles, composite nanoparticles, and nanoparticle coatings, including methods of use, methods of preparation, deposition, and assembly of related devices and specific applications. The colorimetric reagent comprises cerium oxide nanoparticles which are used in solution or immobilized on a solid support, either alone or in conjunction with oxidase enzymes, to form an active colorimetric component that reacts with an analyte to form a colored complex. The rate of color change and the intensity of the color are proportional to the amount of analyte present in the sample. Also described is the use of ceria and doped ceria nanoparticles as an oxygen storage/delivery vehicle for oxidase enzymes and applications in biocatalytic processes in anaerobic conditions of interest in biomedicine and bioanalysis. Further described are a variety of related applications of the disclosed technology including clinical diagnosis, in vivo implantable devices, food safety, and fermentation control.