Green synthesis of copper-based nanoparticles on rice husk ash using cashew apple juice and investigation of its antifungal activity

In this study, copper-based nanoparticles (Cu-based NPs) were green synthesized using cashew apple juice to reduce CuSO4 precursor and deposited on rice husk ash (RHA). Cu-based NPs with 0.5% (w/w) Cu content had a particle size of ∼22 nm as determined by transmission electron microscopy and ∼32 nm as observed by scanning electron microscopy. The characteristics of the nanocomposite were determined by X-ray diffraction and Fourier transform infrared spectroscopy. The main Cu species in the nanocomposite was metallic Cu, accounting for 83.27% as determined by atomic absorption spectroscopy and iodometric titration. In the in vitro experiment, the antifungal effect of Cu-based NPs/RHA against Rhizoctonia solani increased from 58.00% to 95.56% when the concentration increased from 30 to 75 mg/L Cu. In the in vivo experiment, in addition to the direct disease resistance, Cu-based NPs/RHA also stimulated tomato plants to produce the phytohormone salicylic acid, which helps activate the plant's defense system to control harmful fungi with an efficiency of 85.00%–93.33% at Cu concentrations of 45–75 mg/L, respectively. Therefore, Cu-based NPs/RHA have the potential to be used as a disease control agent in agriculture.