Comparative impacts of myco-synthesized nanoparticles against strains of Aspergillus spp. causing biodeterioration of stored cocoa beans in Nigeria

Silver nanoparticle solutions (AgNPs) of some mushrooms: Pleurotus ostreatus, Agaricus bisporus and Agaricus campestris were prepared and characterized using Transmission Electron Microscopy (TEM), Fourier-Transform Infrared (FTIR) spectroscopy, X-Ray Diffraction (XRD) analysis and Energy Dispersive X-ray (EDX) spectroscopy. Each of the myco-sythesized AgNPs was plated against strains of Aspergillus flavus and A. ochraceous, at 5, 10 and 15% concentrations. Colour change from light yellow to orange, yellowish-brown, and reddish brown was observed after overnight incubation (at 28 °C), in the P. ostreatus, A. bisporus, and A. campestris synthesized AgNPs respectively. TEM analysis showed a spherical shape with an average size of 15.25 to 45.85 nm, 9.22 to 52.60 nm and 10.24 to 17.66 nm in P. ostreatus, A. campestris and A. bisporus AgNPs respectively. EDX spectrum showed absorption peaks of silver in the ranges of 0.8–1.4 keV, 6.2–6.6 keV, and 0.8–1.2 keV, and XRD analysis confirmed the crystalline structure of the biosynthesized AgNPs, while FTIR results revealed O–H, N–H, C=O, and C=N as the prominent functional groups. Mycelial inhibitions against A. flavus strains D28AF and D42AF ranged between 43.86–52.73% and 33.83–57.07% respectively, and were not significantly different (P ≤ 0.05) from the standard (copper sulphate). Inhibitions produced against A. ochraceous strains AOD40 and AOD45 ranged between 34.64–52.36% and 37.43–53.56% respectively and also showed similar trend in relation to the standard. This study showed that the myco-synthesized AgNPs were effective against A. flavus and A. ochraceous infecting cocoa beans at storage. They however need to be further improved for future use in the control of cocoa beans pathogens.