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Controlled-release materials are desirable for many delivery applications and have been used to improve the efficiency of fertilizers and pesticides in crop management. Due to their potential to reduce application of toxic chemicals while prolonging exposure to active agents, controlled-release nanomaterials are currently being investigated for increasing agricultural production and preventing overfertilization. Hydrogels are underexplored as controlled-release nanomaterials and can deliver many types of cargo, from metal ions to small molecules. Alginate-based hydrogels are biocompatible and their internal carboxylic acids coordinate agriculturally valuable micronutrients like Cu2+, Zn2+, and Ca2+. Hydrogels comprising ionic and nonionic polymers can coordinate agriculturally valuable micronutrients, and the combination of ionic and nonionic polymers results in hydrogels with tunable release profiles. Alginate, for example, contains carboxylates that ionically cross-link with divalent cations like Cu2+, Zn2+, and Ca2+, while polar moieties on chitin enable nonionic coordination. To our knowledge, soft-material copper-loaded nanoparticles have not yet been applied as controlled-release materials for foliar delivery. In this work, we present the synthesis and micronutrient release characteristics of hydrogel nanoparticles containing Cu2+, which is coordinated by ionic and nonionic polymers. Hydrogel nanoparticles (HNPs) were prepared by liquid–liquid emulsion techniques and cross-linked with Cu2+ to form double-network hydrogels made from alginate and non-cross-linking chitin. Nanoparticles (100–300 nm in diameter) were characterized by cryogenic electron microscopy, nanoparticle tracking analysis, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The copper release profiles of HNPs with different polymer compositions were compared. HNPs containing both chitin and alginate released 8–20 times more copper than HNPs with alginate alone, suggesting that the presence of non-cross-linking polymers improves copper release. Thus, HNP delivery characteristics can be tuned by manipulating intraparticle bond dynamics in the hydrogel polymer matrix. 

ConspectusGold nanoparticles (AuNPs) exhibit unique size- and shape-dependent properties not obtainable at the macroscale. Gold nanorods (AuNRs), with their morphology-dependent optical properties, ability to convert light to heat, and high surface-to-volume ratios, are of great interest for biosensing, medicine, and catalysis. While the gold core provides many fascinating properties, this Account focuses on AuNP soft surface coatings, which govern the interactions of nanoparticles with the local environments. Postmodification of AuNP surface chemistry can greatly alter NP colloidal stability, nano-bio interactions, and functionality. Polyelectrolyte coatings provide controllable surface-coating thickness and charge, which impact the composition of the acquired corona in biological settings. Covalent modification, in which covalently bound ligands replace the original capping layer, is often performed with thiols and disulfides due to their ability to replace native coatings. N-heterocyclic carbenes and looped peptides expand the possible functionalities of the ligand layer.The characterization of surface ligands bound to AuNPs, in terms of ligand density and dynamics, remains a challenge. Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for understanding molecular structures and dynamics. Our recent NMR work on AuNPs demonstrated that NMR data were obtainable for ligands on NPs with diameters up to 25 nm for the first time. This was facilitated by the strong proton NMR signals of the trimethylammonium headgroup, which are present in a distinct regime from other ligand protons’ signals. Ligand density analyses showed that the smallest AuNPs (below 4 nm) had the largest ligand densities, yet spin–spin T2 measurements revealed that these smallest NPs also had the most mobile ligand headgroups. Molecular dynamics simulations were able to reconcile these seemingly contradictory results.While NMR spectroscopy provides ligand information averaged over many NPs, the ligand distribution on individual particles’ surfaces must also be probed to fully understand the surface coating. Taking advantage of improvements in electron energy loss spectroscopy (EELS) detectors employed with scanning transmission electron microscopy (STEM), a single-layer graphene substrate was used to calibrate the carbon K-edge EELS signal, allowing quantitative imaging of the carbon atom densities on AuNRs with sub-nanometer spatial resolution. In collaboration with others, we revealed that the mean value for surfactant-bilayer-coated AuNRs had 10–30% reduced ligand density at the ends of the rods compared to the sides, confirming prior indirect evidence for spatially distinct ligand densities.Recent work has found that surface ligands on nanoparticles can, somewhat surprisingly, enhance the selectivity and efficiency of the electrocatalytic reduction of CO2 by controlling access to the active site, tuning its electronic and chemical environment, or denying entry to impurities that poison the nanoparticle surface to facilitate reduction. Looking to the future, while NMR and EELS are powerful and complementary techniques for investigating surface coatings on AuNPs, the frontier of this field includes the development of methods to probe the surface ligands of individual NPs in a high-throughput manner, to monitor nano-bio interactions within complex matrices, and to study structure–property relationships of AuNPs in biological systems. 

Surface patterning of inorganic nanoparticles through site-selective functionalization with mixed-ligand shells or additional inorganic material is an intriguing approach to developing tailored nanomaterials with potentially novel and/or multifunctional properties. The unique physicochemical properties of such nanoparticles are likely to impact their behavior and functionality in biological environments, catalytic systems, and electronics applications, making it vital to understand how we can achieve and characterize such regioselective surface functionalization. This Feature Article will review methods by which chemists have selectively modified the surface of colloidal nanoparticles to obtain both two-sided Janus particles and nanoparticles with patchy or stripey mixed-ligand shells, as well as to achieve directed growth of mesoporous oxide materials and metals onto existing nanoparticle templates in a spatially and compositionally controlled manner. The advantages and drawbacks of various techniques used to characterize the regiospecificity of anisotropic surface coatings are discussed, as well as areas for improvement, and future directions for this field.