Ceria nanoparticles ( in diameter) were used as a molecular probe for dissolved oxygen sensing based on fluorescence quenching. Strong inverse correlation was found between the amplitude of the fluorescence emission at 520 nm (from excitation shift at 430 nm) and the dissolved oxygen concentration (between ). The phenomenon employed depends on the concentration, diffusion, and reactivity of the oxygen vacancies in ceria. These vacancies are associated with the conversion of cerium ions from the to states. The Stern–Volmer constant, which is an indication of the sensitivity of gas sensing, was found to be at room temperature. This constant shows good stability between 25°C to 50°C when compared to that of other currently used fluorophores in optical oxygen sensors.