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Influential role of fuel to oxidizer ratio (F/O) as the controlling parameter in combustion synthesis of europium doped cerium oxide was studied in terms of defect chemistry, optical property and antioxidant capacity. Europium (5 mol%) doped cerium oxide nanoparticles synthesized by solution combustion in fuel deficient (F/O=0.6, 1.1) and stoichiometric (F/O=1.6) conditions resulted in size ranging from 6 to 25 nm while excess fuel (F/O=2.1) lead to the lower size of 17 nm. Raman spectroscopic analysis showed the formation of intrinsic and europium ion induced extrinsic oxygen vacancies and the defect concentration was found to be decreasing with F/O ratio. Photoluminescence emission was dominated by magnetic dipole transition in F/O=0.6, 1.1 and electrical dipole in F/O=1.6, 2.1 which resulted in a persistent luminescence. Fenton reaction generated hydroxyl radical scavenging activity was influenced by the surface oxygen vacancy concentration and crystallites size. In addition to size and defect, morphology of the nanoparticle plays a significant role in determining the antioxidant efficacy. [ABSTRACT FROM AUTHOR]