Cerium oxide (IV) nanoparticles offer a high redox ability, while maintaining nontoxicity and high stability. Thus, dispersed nanoceria is a promising candidate as antioxidant material for human cells. In this work, we report on a fast and simple one-pot process that yield a final nanoparticle size of 2-4 nm in polar solvents such as water and alcohols. High boiling point solvents, namely, benzyl alcohol and triethylene glycol, are used to obtain high crystalline nanoparticles by thermal and microwave activation. Transmission electron microscopy investigations prove the narrow size distribution of the CeO2 nanoparticles and show that the shape can be tuned from spherical to cubic using an appropriate precursor. The main objective of this work is to produce nanoparticles, which are well-defined, biocompatible, and stable in highly concentrated colloidal solutions (up to 90 mM) for a long period of time to study their behavior as antioxidant agents in human cells under oxidative stress.