The green synthesis of zinc oxide nanoparticles (ZnO NPs) using a diverse range of plant species has been extensively reported. Despite the success achieved by biogenic synthesis, there are problems with the control and prediction of the properties of ZnO NPs, due to phytochemical diversity between plant species. In this sense, the main objective of our work was to investigate the effect of the antioxidant activity (AA) of plant extracts on the physicochemical characteristics of ZnO NPs (production yield, chemical composition, polydispersity index (PDI), surface charge (ζ-potential) and average particle size). In order to accomplish this objective, four plant extract with different antioxidant activities were used: Galega officinalis, Buddleja globosa, Eucalyptus globulus, and Aristotelia chilensis. Phytochemical screening, quantitative analysis of phenolic compounds and antioxidant activity determination of the different extracts were carried out. Chemical species such as catechin, malvidin, quercetin, caffeic acid, and ellagic acid were the dominant components, found in the extracts studied. The A. chilensis extract showed the highest value of total phenolic compounds (TPC) and AA, followed by E. globulus, B. globosa and G. officinalis. Zetasizer, Fourier-transform infrared (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) data show that plant extracts with lower AA leads to a decrease in the yield of ZnO NPs and an increase in the amount of residual organic extract that remains on the particles. The latter caused an increase in the average particle size, PDI and ζ-potential as a consequence of agglomeration and particle coarsening. Our result suggest that it is possible to use the AA as an indicator of the potential reducing capacity of plant extracts. In this way it is possible to guarantee the reproducibility of the synthesis process as well as ensure the formation of ZnO NPs with desired characteristics.

Copper oxide nanoparticles (NCuO) have emerged as an alternative to pesticides due to their antifungal effect against various phytopathogens. Combining them with fungicides represents an advantageous strategy for reducing the necessary amount of both agents to inhibit fungal growth, simultaneously reducing their environmental release. This study aimed to evaluate the antifungal activity of NCuO combined with three fungicide models separately: Iprodione (IPR), Tebuconazole (TEB), and Pyrimethanil (PYR) against two phytopathogenic fungi: Botrytis cinerea and Fusarium oxysporum. The fractional inhibitory concentration (FIC) was calculated as a synergism indicator (FIC ≤ 0.5). The NCuO interacted synergistically with TEB against both fungi and with IPR only against B. cinerea. The interaction with PYR was additive against both fungi (FIC > 0.5). The B. cinerea biomass was inhibited by 80.9% and 93% using 20 mg L⁻¹ NCuO + 1.56 mg L⁻¹ TEB, and 40 mg L⁻¹ NCuO + 12 µg L⁻¹ IPR, respectively, without significant differences compared to the inhibition provoked by 160 mg L⁻¹ NCuO. Additionally, the protein leakage and nucleic acid release were also evaluated as mechanisms associated with the synergistic effect. The results obtained in this study revealed that combining nanoparticles with fungicides can be an adequate strategy to significantly reduce the release of metals and agrochemicals into the environment after being used as antifungals.

The biological synthesis of silver nanoparticles (AgNPs) for medical, environmental, and industrial applications is considered an alternative to chemical synthesis methods. Additionally, the reducing, capping, and stabilizing molecules produced by the organisms can play a key role in the further activity of AgNPs. In this work, we evaluated the synthesis of AgNPs by four molecular weight fractions (S1: 50 kDa) of mycelia-free aqueous extract produced by the white-rot fungus Stereum hirsutum and their effect on the antimicrobial activity against Pseudomonas syringae and photocatalytic decolorization of nine synthetic dyes exposed to sunlight radiation. All synthesis assay fractions showed the characteristic surface plasmon resonance (SPR) with 403 to 421 nm peaks. TEM analysis of synthesized AgNPs showed different sizes: the whole mycelia-free extracts S0 (13.8 nm), S1 (9.06 nm), S2 (10.47 nm), S3 (22.48 nm), and S4 (16.92 nm) fractions. The results of disk diffusion assays showed an inverse relation between antimicrobial activity and the molecular weight of compounds present in the mycelia-free aqueous extract used to synthesize AgNPs. The AgNPs synthesized by S0 (14.3 mm) and S1(14.2 mm) generated the highest inhibition diameter of P. syringae growth. By contrast, in the photocatalytic assays, the AgNPs synthesized by the S2 fraction showed the highest discoloration in all the dyes tested, reaching 100% of the discoloration of basic dyes after 2 h of sunlight exposure. The maximum discoloration observed in reactive and acid dyes was 53.2% and 65.3%, respectively. This differentiation in the antimicrobial and photocatalytic activity of AgNPs could be attributed to the capping effect of the molecules present in the extract fractions. Therefore, the molecular separation of synthesis extract enables the specific activities of the AgNPs to be enhanced.

The antimicrobial activity of metal nanoparticles obtained by biogenic routes has been extensively reported. However, their combined use with other antimicrobial formulations, such as essential oils, remains scarcely explored. In this work, a manganese-ferrite/silver nanocomposite (MnFe₂O₄/Ag-NC) was synthesized in a two-step procedure: first, MnFe₂O₄ nanoparticles were produced by a coprecipitation method, followed by in situ biogenic reduction of silver ions using Galega officinalis. MnFe₂O₄/Ag-NC was characterized using transmission electron microscopy (TEM), scanning electron microscopy equipped with an energy dispersive X-ray analyzer (SEM-EDX), and a vibrating sample magnetometer (VSM-SQUID). The antibacterial activity if MnFe₂O₄/Ag-NC was evaluated against Pseudomonas syringae by determining its minimum inhibitory concentration (MIC) in the presence of two essential oils: eucalyptus oil (EO) and garlic oil (GO). The fractional inhibitory concentration (FIC) was also calculated to determine the interaction between MnFe₂O₄/Ag-NC and each oil. The MIC of MnFe₂O₄/Ag-NC was eightfold reduced with the two essential oils (from 20 to 2.5 µg mL⁻¹). However, the interaction with EO was synergistic (FIC: 0.5), whereas the interaction with GO was additive (FIC: 0.75). Additionally, a time-kill curve analysis was performed, wherein the MIC of the combination of MnFe₂O₄/Ag-NC and EO provoked a rapid bactericidal effect, corroborating a strong synergism. These findings suggest that by combining MnFe₂O₄/Ag-NC with essential oils, the necessary ratio of the nanocomposite to control phytopathogens can be reduced, thus minimizing the environmental release of silver.