Easy-to-manufacture sustainable films based on chitosan/HPMC containing biosynthesized silver nanoparticles obtained via green chemistry for the potential antimicrobial control of Moniliophthora roreri in cocoa pods

Abstract

The productivity of cocoa crops worldwide is preponderantly affected by the phytopathogenic fungus Moniliophthora roreri, which can cause total crop loss. In this study, four different plant extracts from Aloe vera (A), Eucalyptus camaldulensis (E), Gliricidia sepium (M) and Azadirachta indica (N) were used to obtain, via green chemistry, biosynthesized silver nanoparticles (AgNPs), which were then characterized and analyzed. Apart from this, seven (7) distinct chitosan (Chi)/hydroxypropylmethylcellulose (Chi/HPMC)-based films containing the different biosynthesized AgNPs were developed by the casting methodology at room temperature (25 °C) containing glycerol as a plasticizer, and were subsequently analyzed in structural, thermal, physicochemical, morphological and antimicrobial terms. Said materials were proposed to mitigate the aforementioned issue. The results demonstrated that M-biosynthesized AgNPs were the NP system with the highest antimicrobial activity against M. roreri at 1000 ppm, and a dose-dependent effect was also denoted for this AgNP system. Referred antimicrobial activity of M-biosynthesized AgNPs could be related to the potential anthraquinones superficially adhered to the surface of the AgNPs. This latter also led to the obtaining of Chi/HPMC-based films containing the AgNPs biosynthesized from M with the highest antimicrobial activity against M. roreri compared to the Chi/HPMC-based film. Unfortunately, none of the Chi/HPMC-based film systems containing the different biosynthesized AgNPs achieved to overcome the antimicrobial activity of the Chi film against M. roreri. Notwithstanding, to the best of the authors’ knowledge, this is the first study revealing the antimicrobial activity of biosynthesized AgNPs and Chi against M. roreri (main pathogen contaminating cocoa pods). By last, apparently, the carbohydrates and polyphenols coating and stabilizing the AgNPs biosynthesized from E and N interacted more efficiently with the Chi section in the Chi/HPMC-based films through hydrogen (H)-bond interactions, thus incrementing the spacing between the Chi chains, and as a result, said NP systems acted as a plasticizer on the Chi section.