Application of mesoporous silica nanoparticles for biocide delivery to plants to prevent pre-harvest losses

The ever-increasing human population, expected to reach over 9 billion people by 2050, is creating a growing need for new technologies that increase agricultural productivity and yields. The use of mesoporous silica nanoparticles (MSNPs) as biocide delivery vehicles in agriculture has only recently started receiving attention, as these nanoparticles have the potential to revolutionise agriculture. MSNPs are biocompatible, biodegradable, have a low-cost synthesis and their surface can be easily functionalised or modified according to the desired application. For this reason, they represent a promising approach to encapsulate antimicrobials which enable longer-lasting, more targeted and controlled delivery. The encapsulation of biocides into MSNPs has the benefit of protecting the compounds from degradation or evaporation whilst enhancing the antimicrobial activity by providing a heightened and more localised killing effect precisely where it is required. The reduction of the quantities of biocides needed to obtain an efficient antimicrobial effect consequently has the added benefit of reducing the amount of chemical released into the environment and so any potential hazardous unwanted consequences. This thesis aims to evaluate the application of MSNPs as delivery vehicles to encapsulate natural antimicrobial agents such as essential oils to address bacterial diseases affecting agriculture. Forty-one essential oils were evaluated for their efficacy and specificity against three bacterial species, including two plant pathogens. The most effective antimicrobials were encapsulated into MSNPs, which prevented their rapid volatilisation and degradation whilst maintaining a slow-release and prolonged effect of the biocide. The biocide-loaded MSNPs were demonstrated to be effective in vitro against plant pathogens. Particularly, cinnamaldehyde-MSNPs were shown to decrease bacterial counts (CFUs) by up to 99.9% and the encapsulation increased the antimicrobial activity by 10-fold, compared to free cinnamon essential oil. The loaded nanoparticles were also incorporated into an alginate seed coating to evaluate the use of a seed treatment to prevent bacterial infection of plants. Different alginate formulations were assessed and the beneficial effects of the seed coat were demonstrated, resulting in taller plants (42.2%-57.6% taller) that germinated and developed faster (25%-62.5% more germinated plants by the third day after sowing) than the controls. To determine the protective effect of the alginate seed treatment containing essential oil-loaded MSNPs, in planta experiments were carried out. As a proof of concept, this study focused on phytopathogen Pseudomonas syringae pv. pisi, the causative agent of pea bacterial blight. The number of symptomless plants after treatment increased by 143.58% compared to the control, demonstrating the efficacy of this treatment at decreasing the incidence of bacterial diseases in plants. This study demonstrates the efficacy of mesoporous silicates as delivery vehicles for volatile compounds. The encapsulation of highly antimicrobial essential oils into MSNPs prevented the rapid loss or degradation of the biocides and enabled a controlled slow-release of the compounds achieving targeted delivery towards the seed-borne pathogens commonly present on the seed coat. The use of MSNPs also permitted very low concentrations of essential oils, up to 90,000-fold lower, to be effective antimicrobials. The results of this study demonstrate the enormous potential of MSNP in agriculture for stealth delivery of killing agents for reducing crop losses and increasing agricultural yields whilst reducing the unwanted consequences of pesticide release in the environment.