Photocatalytic dye degradation and photoexcited anti-microbial activities of green zinc oxide nanoparticles synthesized via Sargassum muticum extracts

Abstract

Drug-resistant superbugs (DRS) were isolated from hospital sewage waste and confirmed by a 16S rDNA molecular technique as B. filamentosus, B. flexus, P. stutzeri, and A. baumannii. Green nanotechnologies provide a new promising alternative pathway that was found to be much safer, eco-friendly, and has economic benefits over physical/chemical methods. Sargassum muticum (SM) mediated zinc oxide nanoparticles (ZnO-NPs) were proved to be photocatalytic and anti-microbial agents. Anti-microbial action was demonstrated by a maximal growth inhibition activity of 18 mm against A. baumannii and a minimal of 12 mm against B. flexus at 80 μg mL^-1 concentrations. The anti-microbial mechanism of SMZnO-NPs employed a biphasic phenomenon persuaded by an osmotic shock that can attack the DRS bacterial cells directly and lead to death. In addition, photocatalytic activity was investigated by SMZnO-NPs for the degradation of methylene blue (MB) dye under different light conditions. Natural sunlight irradiation shows effective enhancement with the highest efficiencies of 96% being achieved within 60 min compared to UV-light and visible-light. The reusability of SMZnO-NPs provides up to 6 consecutive cycles towards MB decolorization for environmental water cleansing.

Fig. 1. Sample collection areas: (a) Mandapam coastal region in the Gulf of Mannar Marine National Park in the Bay of Bengal, southeast coast of India; (b) behind the Central Electrochemical Research Institute (CECRI) Mandapam coast; (c) and (d) collection of seaweed.

Fig. 2. (a) UV-visible spectroscopy. (b) X-ray diffraction pattern mixed phase of face-centered cubic (fcc) shape. (c) Photoluminescence (PL) spectra of synthesized green SMZnO-NPs. (d) Raman spectra of synthesized green SMZnO-NPs.

Fig. 3. (a) and (b) SEM and Fe-SEM observation of green synthesized SMZnO-NPs (c) EDXS spectrum image of green synthesized SMZnO-NPs. (d) X-ray fluorescence (XRF) spectrum of particle size distribution and of synthesized zinc oxide nanoparticles.

Fig. 4. (a) Green synthesized SMZnO-NPs size and distribution studied by zeta potential analysis. (b) FTIR spectra of Sargassum muticum (SM) extract (alone) and green synthesized SMZnO-NPs. (c) In vitro anti-microbial activity of green synthesized SMZnO-NPs against 16S rDNA confirmed drug-resistant superbugs (DRS).

Fig. 5. (a) MIC growth effects of different concentrations of SMZnO-NPs on the growth of Gram-positive and negative DRS pathogens. (b) Schematic structure representation of green synthesized SMZnO-NPs by the osmotic shock that could attack the cell wall directly and lead to bacterial death. Effect of SMZnO-NPs against DRS pathogen observation by SEM. Inset in control (c) and (d) is a high-resolution Fe-SEM image of bacterial cell death.

Fig. 6. Photocatalytic degradation using green synthesized SMZnO-NPs of MB dyes under the irradiation of (a) visible-light (b) UV-light and (c) natural sunlight.

Fig. 7. Photocatalytic degradation of MB dyes under different regimes of sunlight irradiation: (a)–(c). Calibration plot of ln(C₀/C_t) vs. time for the catalytic degradation of MB dye; (d) XRD patterns results of the photocatalytic stability/cycling test after repetition reaction.