Quantitative biodistribution of nanoparticles in plants with lanthanide complexes

The inefficient distribution of fertilizers, nutrients, and pesticides on crops is a major challenge in modern agriculture that leads to reduced productivity and environmental pollution. Nanoformulation of agrochemicals is an attractive approach to enable the selective delivery of agents into specific plant organs, their release in those tissues, and improve their efficiency. Already commercialized nanofertilizers utilize the physiochemical properties of metal nanoparticles such as size, charge, and the metal core to overcome biological barriers in plants to reach their target sites. Despite their wide application in human diseases, lipid nanoparticles are rarely used in agricultural applications and a systematic screening approach to identifying efficacious formulations has not been reported. Here, researchers developed a quantitative metal-encoded platform to determine the biodistribution of different lipid nanoparticles in plant tissues. In this platform lanthanide metal complexes were encapsulated into four types of lipid nanoparticles. Researchers' approach was able to successfully quantify payload accumulation for all the lipid formulations across the roots, stem, and leaf of the plant. Lanthanide levels were 20- to 57-fold higher in the leaf and 100- to 10,000-fold higher in the stem for the nanoparticle encapsulated lanthanide complexes compared to the unencapsulated, free lanthanide complex. This system will facilitate the discovery of nanoparticles as delivery carriers for agrochemicals and plant tissue-targeting products.

Introduction

In modern agricultural systems, agrochemicals are critical to support different stages of crop production from seeding to growth and protection. However, it has been estimated that less than 60% of fertilizers and 0.1% of pesticides reach their target sites due to drift, runoff, and degradation. The release of excess agrochemicals can also negatively interfere with inherent nutrient equilibrium in the soil and could lead to water contamination. Nanotechnology has the potential to transform current crop production systems to enable precision farming to maximize output from available resources. Nano-encapsulated agrochemicals are designed to have properties to control the release of agrochemicals at target sites in response to stimuli, thus lowering application frequency and reducing ecotoxicity on the overall environment. When guided by biorecognition ligands, targeted nanomaterials encapsulating payloads including proteins, nucleotides, and other bioactive molecules could enable gene editing and regulation of biological pathways related to diseases origination, nutrition deficiency, and pest control.

The broad potential of nano-agrochemicals can be attributed to their unique physicochemical properties that are compatible with and can help overcome plant barriers. Stomata on the foliar epidermis is the first barrier with pores 10–30 μm long and 3–12 μm wide for gaseous exchange. Nanoparticles can easily pass through these pores when they are open. At locales without the presence of stomata, the cuticle layer is the next barrier with size exclusion limits in the nanometer range. Accumulation of fluorescence-labeled nanoparticles above 50 nm was observed under the epidermis cuticle when 4–100 nm nanoparticles were applied. The last and major barrier is the plant cell wall within the pore size range of 3.5–20 nm. Quantum dots below such ranges were found to easily transverse through the barrier for entry into plant veins. While smaller particles may diffuse through cell walls more freely, interactions between larger nanoparticles at the cell walls may create larger pores to facilitate an active transport process. While size is critical, the uptake process is also dependent on the surface charge of nanoparticles since there is an unequal distribution of negative charges from the hydrophilic and hydrophobic components in the cell wall. One report observed that positively charged CeO nanoparticles adhere to the root surface, while negatively charged counterparts exhibited low root accumulation. Other studies suggest cell walls might promote the internalization of negatively charged gold nanoparticles rather than absorption.

While metal nanoparticles have had some success, non-metal organic nanoparticles made of lipids are rarely explored in agricultural applications. Typically, lipid-based nanoparticles are composed of neutral lipids, cholesterol, an ionizable lipid, and a PEGylated lipid. This multi-lipid system presents unique advantages, such as improved solubility of hydrophobic molecules, tissue-targeting localization, and better safety profiles. Despite the clinical success of lipid-based nanoparticles, only a small number of reports utilize lipid nanoparticles for agrochemical delivery. To date, there are no systematic approaches to determine the biodistribution of nanoparticles in plants. Inductively coupled plasma mass spectrometry (ICP-MS) is a useful approach to determine the biodistribution of nanoparticles. Imaging techniques, such as X-ray, electron microscopy, and computed tomography are helpful to track the accumulation of gold nanoparticles, C70 fullerene, and carbon nanotubes at the cellular and subcellular levels. However, their application suffers from restrictive preparation protocols, analysis of only partial plant organs, and high background signals. ICP-MS, unlike previously mentioned techniques, is a mass spectrometer with the capability to simultaneously quantify multi-element samples with superior sensitivity and accuracy. The uptake of liposomes containing a europium tracer into the leaf at 24 h, 48 h, and 72 h was determined by ICP-MS, suggesting a total of up to 33% of the applied dose was accumulated compared to 1% of free metal without liposome encapsulation. Raliya et al. also utilized ICP-MS to explain the translocation of gold nanoparticles after an aerosolized formulation was applied to watermelon plants. The observation suggested absorption through the stomatal opening, tracked the translocation from the leaf to the root, and determined the factors related to uptake including particle shape, application approach, and plant physiology.

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