A wide range of semiconductor-assisted photocatalytic nanomaterials (NMs) are currently being considered and investigated as potential photocatalysts in water treatment. The applications of nanocomposites composed of nano-structured titania (nano-TiO2) and multi-walled carbon nanotubes (MWCNTs) nanocomposites is growing markedly on account of enhanced photocatalytic efficiency. However, concurrent with the increasing production and application comes a serious concern of these emerging nanosystems about their potential risks in aquatic systems, and thereby potentially threatening aquatic organisms via toxic mechanisms that are, at present, poorly understood. In the present study, the lethal toxic effect and oxidative stress induced by TiO2/MWCNT-CNF nanocomposite in freshwater Pseudokirchneriella subcapitata were assessed. The growth inhibition and sublethal oxidative stress produced by the nanocomposites were evaluated on green microalgae P. subcapitata after 3 days of exposure at 24 h intervals. Moreover, the nanocomposites were physicochemically characterized using a combination of analytical techniques (XRD, SEM/EDS, HRTEM, TGA, UV-Visible spectroscopy). Evaluation of the hybrid for the photocatalytic degradation of Acid Violet 7 dye indicated an enhanced dye removal performance for TiO2/MWCNT-CNF (96.2%) compared to TiO2 (75.2%) after 2 h of visible light irradiation. While the nanocomposite showed good potential for the degradation of the azo dye, overall, the findings herein indicated that acute exposure of P. subcapitata to various concentrations of TiO2/MWCNT-CNF nanocomposite may cause algal growth inhibition including undesirable sublethal oxidative stress effects. The findings of this study contribute to a better understanding of the potential hazards of the developing nanocomposites materials towards the nano-bioremediation materials to treat wastewaters.

The application of nanocomposite materials fabricated from titanium dioxide nanoparticles (TiO2 NPs) and different carbon (C) allotropes have gained popularity in water treatment applications due to their synergistic properties. Studies to date have focused on simple forms of nanomaterials (NMs), however, with the technology development, there is a dramatic increase in production and application of these complex NMs which could result in toxicological impacts on organisms when released into aquatic environments. This raises serious concerns about their safety and the need to ascertain their potential adverse effects on aquatic organisms. While conjugated TiO2 NPs/carbon-based nanohybrids (TiO2/C-NHs) may exhibit enhanced photocatalytic activity, there is no research in the scientific community regarding their toxicological effects on D. magna, which are indicators of freshwater pollution. In this study, two under-represented TiO2/C-NHs (i.e., TiO2- conjugated carbon nanofiber (CNF), and TiO2-conjugated multi-walled carbon nanotube (CNT)) were investigated for their toxic effects on D. magna, through a series of acute toxicity tests with a set of sublethal biochemical biomarkers of oxidative stress. The lethal toxicity and oxidative stress formation of TiO2/C-NHs over 48 h revealed a concentration-dependant increase in D. magna mortality. The primary mechanism identified was the generation of ROS, which was in line with toxicity results. Light microscopy and CytoViva® images visualized D. magna interaction with the NPs, which accumulated and appeared as dark materials in the lines of the gut tract. The collective results indicate that TiO2/C-NHs have the potential to cause an effect on freshwater organisms when released into the environment. However, the relevance of TiO2/C-NHs effects needs further chronic toxicity studies since they show promise to be used in nano-bioremediation materials to treat wastewaters.

Most developing countries, including South Africa, depend strongly on traditional medicine for a therapeutic outcome and have therefore employed numerous medicinal plants to treat fevers. Therefore, it is imperative that fever-reducing medicinal plants are investigated to establish their efficacy and to determine their potential as sources of new antimicrobials. The incorporation of nanotechnology in antimicrobial research with reference to medicinal plants is a growing domain. The interest in silver nanoparticles (AgNPs) encompasses the tested hypothesis that the chemical combination of silver with medicinal plant extracts results in nanoparticles with enhanced antimicrobial properties in comparison with plant extracts alone. This study investigated the antimicrobial properties from 10 medicinal plants of commercial significance used traditionally for the treatment of fever in South Africa and their potential for enhanced antimicrobial efficacy when incorporated within AgNPs. Plant extracts and AgNPs were tested against fever-related pathogens, i.e., two Gram-positive pathogens; Listeria monocytogenes (ATCC 19111) and Enterococcus faecalis (ATCC 29212) as well as two Gram-negative pathogens; Klebsiella pneumoniae (ATCC 13883) and Acinetobacter baumannii (ATCC 19606) using the broth microdilution method. Chemical characterisation of AgNPs included Ultraviolet–visible (UV–Vis) spectroscopy, dynamic light scattering (DLS), Fourier-transform infrared spectroscopy (FTIR), and Transmission electron microscopy (TEM). The toxicity profiles of the AgNPs were evaluated using the brine-shrimp lethality assay (BSLA). Silver nanoparticles of both Eucomis autumnalis and Sclerocarya birrea display a dramatic increase in antimicrobial activity against the four test pathogens compared to the respective aqueous plant extracts. The greatest difference in antimicrobial activity was observed against E. faecalis where an increase in antimicrobial activity of at least 50-fold when E. autumnalis aqueous sample were compared with the AgNP-counterparts. Cytotoxicity of both AgNP samples from the BSLA emerged at less than 50% mortality, the results obtained in this study justify the use of selected fever-reducing plant extracts with the biosynthesis of AgNPs as promising antibacterial agents with low toxicity.