Optimization of photodegradation of crystal violet dye and biomedical applications of greenly synthesized NiO nanoparticles†

Metal oxide nanoparticles (NPs) are considered suitable candidates for photocatalytic applications because of their large surface area, easy generation of electron–hole pairs for redox reactions and tunable optical properties. Additionally, the successful capping of NP surfaces by bioactive species of plant extracts can further improve their size, shape and bandgap. Inspired by the green synthesis approach, the first time synthesis of nickel oxide NPs (CMFE@NiO NPs) using an aqueous extract of C. macrocarpa fruit (natal plum) is reported herein for the photodegradation of crystal violet (CV) dye. The synthesized NPs were characterized by PXRD, UV-vis spectra, FTIR, HR-TEM, EDX, DLS, ZP and TGA. After characterization, CMFE@NiO NPs were evaluated for the degradation of CV dye under sunlight for 120 min. The effect of various reaction parameters, such as pH, temperature, catalyst dose and initial dye concentration, were studied, and reaction conditions were optimized by applying mathematical and statistical tools, i.e., RSM/BBD design. Maximum degradation (99%) of 10 ppm CV solution was observed at a catalyst dose of 50 mg, 358 K and pH 7 with a rate constant value of 3.81 × 10⁻² min⁻¹. The effect of radical scavengers was studied to determine major ROS involved and propose a reaction mechanism. Moreover, the antibacterial activity of the NPs was evaluated against Gram-positive and Gram-negative strains. CMFE@NiO NPs showed good inhibition of all bacterial strains with inhibition diameters of 15 ± 1.5 mm, 14 ± 1.2 mm, 22 ± 2.0 mm and 24 ± 2.2 mm for S. aureus, B. subtilis, E. coli and P. multocida, respectively. CMFE@NiO NPs were found to be more noxious against Gram-negative bacterial strains. The antioxidant potential of CMFE@NiO NPs also showed good reduction potential to reduce DPPH˙ with an IC₅₀ value of 32.9 ± 2.4 μg mL⁻¹, which is better than that of the extract (IC₅₀ = 39.3 ± 2.1 μg mL⁻¹).