This study investigates the potential of post-harvest immersion in plasma-activated water (PAW) and water containing 1 mg/L selenium nanoparticles (Se-NP solution) to extend the shelf life of mandarin fruit using a two-factor factorial experiment with three replications in a completely randomized design. Freshly harvested mandarins were divided into six groups: (1) control (immersed in distilled water), (2) and (3) treated with PAW for 2 and 4 min, respectively, (4) treated with aqueous solution of Se-NP for 5 min, and (5) and (6) treated sequentially with the Se-NP solution for 5 min combined by PAW for 2 and 4 min, respectively under different storage time (At harvest, 15, 30 and 45 days). The samples were stored at 5 ± 1 °C and 85–90% relative humidity. Biochemical parameters, including chilling injury percentage, pH, TSS, vitamin C, electrolyte leakage, lipid peroxidation, hydrogen peroxide content, antioxidant enzyme activity, and selenium concentration, were measured after 15, 30, and 45 days of storage. The results showed that in the control group, the pH, chilling injury, lipid peroxidation, hydrogen peroxide (H₂O₂) content, and electrolyte leakage developed significantly during the storage. Applying immersion treatment in an aqueous Se-NP solution for 5 min followed by immersion in PAW for 2 min during 30 and 45 days of storage resulted in minimal chilling injury in mandarins under storage conditions, leading to a respective 35% and 20% reduction in damaged fruits compared to the control treatment. However, immersion treatments utilizing Se-NP solution and PAW mitigated these detrimental effects in other treated groups. It was found that Sequential treatment with Se-NP solution and PAW increased TSS (2.8%), vitamin C (7.14%), and enzyme activities (9.35%) in mandarins during storage compared to the control. The 5-minute Se-NP immersion followed by a 2-minute PAW immersion was the most effective treatment for extending shelf life and preserving biochemical characteristics. These findings suggest that combining selenium nanoparticles and plasma-activated water is a promising approach for expanding citrus fruit shelf life and improving quality and marketability.
