Impact of Charge and Solubility on Nanoparticle Uptake and Transformations In Planta through Spatially Resolved Chemical Speciation

Plant nanobiotechnology promises transformative solutions to the most vexing problems threatening global food security, e.g. drought, disease, and soil nutrient deficiencies. However, poor understanding of how NP physiochemical properties affect their fate in/on plants and the lack of effective methods to deliver the nanomaterials to where they are needed in a precise plant compartment impedes these technological innovations. This thesis evaluates how three specific NP properties, charge, solubility, and coating, influence plant uptake, metal distribution, and in planta NP transformation, which will provide insight into the design of efficient and safe nano-enabled agrochemicals. The first objective of this work was to evaluate the influence of surface charge on NP uptake by roots, translocation, and distribution in plant tissue. Wheat was hydroponically exposed to CeO₂ NPs functionalized with positively-charged, negatively-charged, or neutral dextran coatings. While the positively-charged NPs adhered significantly more to the roots, the negatively-charged NPs translocated to the shoots most efficiently. Whereas Ce from negatively-charged NP exposed plant was found mostly in the leaf veins, Ce was in the nonvascular leaf tissue of the neutral NP exposed plant. These results demonstrate that different CeO₂ NP surface charges result in different Ce localization in leaves. The second objective of this thesis was to compare the influence of charge on NP uptake and distribution between different types of plants. Experiments with these particles using two monocotyledons (corn and rice) and two dicotyledons (tomato and lettuce) indicated that while total Ce uptake was plant-species dependent, likely due to differences in transpiration rates, Ce distribution in the leaves was driven by NP surface charge and were generalizable across all four plants. Comparing leaf vasculature, Ce was able to move much further outside of the main vasculature in the dicot plants than monocot plants, likely due to the larger airspace volume in dicot leaves compared to monocot leaves. This work clearly demonstrates that tuning NPs coating charge can achieve plant compartment targeting after root uptake.
The third objective of this work was to determine the influence of Cu-based NP 1-h solubility on metal uptake, distribution and speciation over time in wheat. Higher 1-h solubility Cu(OH)₂ NPs provided more uptake of Cu after 1 h of exposure, but the lower 1-h solubility materials (CuO and CuS NPs) were more persistent on/in the roots and continued to slowly deliver Cu to plant leaves over 48 h. The initial NP composition significantly influenced the Cu speciation within the plant roots; the Cu in plants exposed to CuS NPs was mostly reduced and/or sulfidized while the Cu in the CuO NP exposed plants was oxidized and bound to organics. This work demonstrates that tuning initial NP speciation can allow for the delivery of different metal species, resulting in controllable delivery rates and bioavailabilities. The fourth objective of this work was to determine how coating can be modified to increase NP adherence to plant leaf structures, specifically pathogen points of entry. Gold nanoparticles were coated with a biomolecule with affinity for a specific chemical moiety found on guard cells to target leaf stomata. After rinsing, NPs with this coating remained strongly adhered to the stomata on the leaf surface. These results demonstrate, for the first time, active, targeted delivery of NPs to a specific site on live plants via foliar application. Overall, this thesis demonstrates that tuning NP physicochemical properties to achieve specific bioavailability, distribution, targeted delivery in plants is possible. These findings provide key information for the design of nano-enabled agrochemicals that are more targeted, more efficient, and less wasteful.