Anode Catalysts in Anion-Exchange-Membrane Electrolysis without Supporting Electrolyte: Conductivity, Dynamics, and Ionomer Degradation.

Anion-exchange-membrane water electrolyzers (AEMWEs) in principle operate without soluble electrolyte using earth-abundant catalysts and cell materials and thus lower the cost of green H2 . Current systems lack competitive performance and the durability needed for commercialization. One critical issue is a poor understanding of catalyst-specific degradation processes in the electrolyzer. While non-platinum-group-metal (non-PGM) oxygen-evolution catalysts show excellent performance and durability in strongly alkaline electrolyte, this has not transferred directly to pure-water AEMWEs. Here, AEMWEs with five non-PGM anode catalysts are built and the catalysts' structural stability and interactions with the alkaline ionomer are characterized during electrolyzer operation and post-mortem. The results show catalyst electrical conductivity is one key to obtaining high-performing systems and that many non-PGM catalysts restructure during operation. Dynamic Fe sites correlate with enhanced degradation rates, as does the addition of soluble Fe impurities. In contrast, electronically conductive Co3 O4 nanoparticles (without Fe in the crystal structure) yield AEMWEs from simple, standard preparation methods, with performance and stability comparable to IrO2 . These results reveal the fundamental dynamic catalytic processes resulting in AEMWE device failure under relevant conditions, demonstrate a viable non-PGM catalyst for AEMWE operation, and illustrate underlying design rules for engineering anode catalyst/ionomer layers with higher performance and durability.

Three-Electrode Study of Electrochemical Ionomer Degradation Relevant to Anion-Exchange-Membrane Water Electrolyzers.

Among existing water electrolysis (WE) technologies, anion-exchange-membrane water electrolyzers (AEMWEs) show promise for low-cost operation enabled by the basic solid-polymer electrolyte used to conduct hydroxide ions. The basic environment within the electrolyzer, in principle, allows the use of non-platinum-group metal catalysts and less-expensive cell components compared to acidic-membrane systems. Nevertheless, AEMWEs are still underdeveloped, and the degradation and failure modes are not well understood. To improve performance and durability, supporting electrolytes such as KOH and K2CO3 are often added to the water feed. The effect of the anion interactions with the ionomer membrane (particularly other than OH-), however, remains poorly understood. We studied three commercial anion-exchange ionomers (Aemion, Sustainion, and PiperION) during oxygen evolution (OER) at oxidizing potentials in several supporting electrolytes and characterized their chemical stability with surface-sensitive techniques. We analyzed factors including the ionomer conductivity, redox potential, and pH tolerance to determine what governs ionomer stability during OER. Specifically, we discovered that the oxidation of Aemion at the electrode surface is favored in the presence of CO32-/HCO3- anions perhaps due to the poor conductivity of that ionomer in the carbonate/bicarbonate form. Sustainion tends to lose its charge-carrying groups as a result of electrochemical degradation favored in basic electrolytes. PiperION seems to be similarly negatively affected by a pH drop and low carbonate/bicarbonate conductivity under the applied oxidizing potential. The insight into the interactions of the supporting electrolyte anions with the ionomer/membrane helps shed light on some of the degradation pathways possible inside of the AEMWE and enables the informed design of materials for water electrolysis.

Reaction Atmospheres and Surface Ligation Control Surface Reactivity and Morphology of Cerium Oxide Nanocrystals during Continuous Addition Synthesis.

Cerium oxide nanocrystals have size- and shape-dependent properties that are potentially useful in a variety of applications if these structural attributes can be controlled through synthesis. Various syntheses have been developed in attempts to access different sizes and shapes, but little is known about selecting reaction conditions to predictably control the growth, and therefore properties, of the nanocrystals. Here, we investigate the role of cerium precursor oxidation states, reaction atmospheres, and acetic acid ligation on the size and shape of cerium oxide nanocrystals. A continuous addition synthesis allowed us to vary individual reaction parameters to better understand how each affects growth and morphology. Under N2, the synthesis leads to either irregular shapes or nanoribbons, whereas the same synthesis under air leads to size-tunable nanocubes. To determine whether air might be oxidizing the cerium precursor and changing its reactivity, we synthesized Ce(IV)-rich and Ce(III) oleate precursors and found that the oxidation state of the precursor has little effect on the resulting nanocrystals. In fact, we found that Ce(IV) oleate is readily reduced to Ce(III) at at temperatures above 100 °C in the reaction medium. The significant role of air during synthesis therefore suggests that oxygen is altering the surface reactivity of the nanocrystals, as opposed to the precursor. We investigated the origin of nanoribbon formation and found that the presence of acetate ligands is responsible for nanoribbon formation in syntheses under N2, with more acetate leading to longer nanoribbons. These insights were used to identify conditions to predictably grow various sizes and shapes of nanocubes and nanoribbons. The findings elucidate the effects that various synthetic parameters can have on cerium oxide nanocrystal synthesis and suggest that redox reactivity may influence growth and properties in other syntheses where changes in the oxidation state occur for the precursor or the nanocrystal surface.

Zinc oxide-induced changes to sunscreen ingredient efficacy and toxicity under UV irradiation.

Sunscreen safety and efficacy is generally evaluated based upon the properties of the individual chemicals in a formulation. However, the photostability of sunscreens has been shown to be highly dependent on the mixture of chemicals present. To better understand how sunscreen formulation influences stability, and to establish a foundation for probing the influence of zinc oxide additives, we formulated five different small-molecule based ultraviolet-filter (UV-filter) mixtures with a Sun Protection Factor (SPF) of 15. These mixtures contained active ingredients approved in either the United States or European Union and were designed to represent formulations of actual products on the market. We evaluated the photostability and toxicity of these mixtures in the absence and presence of zinc oxide after UV exposure for two hours. Changes in UV absorbance were minimal for all five small-molecule-based mixtures without zinc oxide. The presence of either micro- or nano-sized zinc oxide caused significant small-molecule photodegradation and the degraded mixtures exhibited higher levels of toxicity in embryonic zebrafish assays. This study suggests that caution must be taken when formulating sunscreens containing both zinc oxide and small-molecule UV-filters to avoid unintended consequences during use.

Understanding the Effects of Iron Precursor Ligation and Oxidation State Leads to Improved Synthetic Control for Spinel Iron Oxide Nanocrystals.

Iron oxide nanocrystals have the potential for use in a wide variety of applications if we can finely control and tune the diverse structural attributes that lead to specific, desired properties. At the high temperatures utilized for thermal decomposition based syntheses, commonly used Fe(III) alkylcarboxylate precursors are inadvertently reduced and produce wüstite (FeO), which is paramagnetic, as opposed to the desired ferrimagnetic spinel phases of magnetite (Fe3O4) and maghemite (γ-Fe2O3). To circumvent this issue, we carried out syntheses at lower temperatures (∼230 °C) using an esterification-mediated approach. Under these conditions, formation of the FeO phase can be avoided. However, we found that the precursor oxidation state and ligation had a surprisingly strong influence on the morphologies of the resulting nanocrystals. To investigate the cause of these morphological effects, we carried out analogous nanocrystal syntheses with a series of precursors. The use of Fe(III) oleate precursors yielded highly crystalline, largely twin-free nanocrystals; however, small amounts of acetylacetonate ligation yielded nanocrystals with morphologies characteristic of twin defects. During synthesis at 230 °C, the Fe(III) oleate precursor is partially reduced, providing sufficient quantities of Fe(II) that are needed to grow the Fe3O4 nanocrystals (wherein one-third of the iron atoms are in the Fe(II) state) without twinning. Our investigations suggest that the acetylacetonate ligands prevent reduction of Fe(III) to Fe(II), leading to twinned structures during synthesis. Harnessing this insight, we identified conditions to predictably and continuously grow octahedral, spinel nanocrystals as well as conditions to synthesize highly twinned nanocrystals. These findings also help explain observations in the thermal decomposition synthesis literature which suggest that iron oxide nanocrystals produced from Fe(acac)3 are less prone to FeO contamination in comparison to those produced from Fe(III) alkylcarboxylates.

Evolution of Atomic-Level Structure in Sub-10 Nanometer Iron Oxide Nanocrystals: Influence on Cation Occupancy and Growth Rates.

Spinel iron oxide nanocrystals (NCs) have been reported to have atomic-level core and surface structural features that differ from those of the bulk material. Recent advances in a continuous growth synthesis of metal oxide NCs make it possible to prepare a series of NCs with subnanometer control of size with diameters below 10 nm that are well-suited for investigating size-dependent structure and reactivity. Here, we study the evolution of size-dependent structure in spinel iron oxide and determine how nanoscale structure influences the growth of NCs. We synthesized spinel iron oxide NCs via a continuous growth method that permits layer-by-layer control of size in order to monitor nanoscale structure over 16 core sizes between 3 and 10 nm. X-ray total scattering data were collected and analyzed with pair distribution function (PDF) analysis in order to refine quantitative structural features including cation occupancies that could be used to detect changes both in the oxidation state and the presence of tetrahedrally coordinated cation vacancies in the NCs. We find that the average iron oxidation state increases as core diameters decrease from 8 down to 3 nm. The trend in iron oxidation state can be explained by the oxidation of surface layers in the NCs. For samples exposed to air for several weeks, oxidation appears to cease when a volume equivalent to that of an ∼1.3 nm shell is converted to the more oxidized maghemite. The number of tetrahedrally coordinated cation vacancies also increases as the NC core size decreases. The correlation between the number of these vacancies and the faster growth for smaller NCs suggests that these reactive vacancies may be responsible for the rapid growth observed for nanocrystals with diameters smaller than 8 nm.

Systematic determination of the relationship between nanoparticle core diameter and toxicity for a series of structurally analogous gold nanoparticles in zebrafish.

Predictive models for the impact of nanomaterials on biological systems remain elusive. Although there is agreement that physicochemical properties (particle diameter, shape, surface chemistry, and core material) influence toxicity, there are limited and often contradictory, data relating structure to toxicity, even for core diameter. Given the importance of size in determining nanoscale properties, we aimed to address this data gap by examining the biological effects of a defined series of gold nanoparticles (AuNPs) on zebrafish embryos. Five AuNPs samples with narrowly spaced core diameters (0.8-5.8 nm) were synthesized and functionalized with positively charged N,N,N-trimethylammonium ethanethiol (TMAT) ligands. We assessed the bioactivity of these NPs in a high-throughput developmental zebrafish assay at eight concentrations (0.5-50 µg/mL) and observed core diameter-dependent bioactivity. The smaller diameter AuNPs were the most toxic when expressing exposures based on an equal mass. However, when expressing exposures based on total surface area, toxicity was independent of the core diameter. When holding the number of nanoparticles per volume constant (at 6.71 × 1013/mL) in the exposure medium across AuNPs diameters, only the 5.8 nm AuNPs exhibited toxic effects. Under these exposure conditions, the uptake of AuNPs in zebrafish was only weakly associated with core diameter, suggesting that differential uptake of TMAT-AuNPs was not responsible for toxicity associated with the 5.8 nm core diameter. Our results indicate that larger NPs may be the most toxic on a per particle basis and highlight the importance of using particle number and surface area, in addition to mass, when evaluating the size-dependent bioactivity of NPs.

Synergistic Toxicity Produced by Mixtures of Biocompatible Gold Nanoparticles and Widely Used Surfactants.

Nanoparticle safety is usually determined using solutions of individual particles that are free of additives. However, the size-dependent properties of nanoparticles can be readily altered through interactions with other components in a mixture. In applications, nanoparticles are commonly combined with surfactants or other additives to increase dispersion or to enhance product performance. Surfactants might also influence the biological activity of nanoparticles; however, little is known about such effects. We investigated the influence of surfactants on nanoparticle biocompatibility by studying mixtures of ligand-stabilized gold nanoparticles and Polysorbate 20 in embryonic zebrafish. These mixtures produced synergistic toxicity at concentrations where the individual components were benign. We examined the structural basis for this synergy using solution-phase analytical techniques. Spectroscopic and X-ray scattering studies suggest that the Polysorbate 20 does not affect the nanoparticle core structure. DOSY NMR showed that the hydrodynamic size of the nanoparticles increased, suggesting that Polysorbate 20 assembles on the nanoparticle surfaces. Mass spectrometry showed that these assemblies have both increased uptake and increased toxicity in zebrafish, as compared to the gold nanoparticles alone. We probed the generality of this synergy by performing toxicity assays with two other common surfactants, Polysorbate 80 and sodium dodecyl sulfate. These surfactants also caused synergistic toxicity, although the extent and time frame of the response depends upon the surfactant structure. These results demonstrate a need for additional, foundational studies to understand the effects of surfactants on nanoparticle biocompatibility and challenge traditional models of nanoparticle safety where the matrix is assumed to have only additive effects on nanoparticle toxicity.

Radial Dopant Placement for Tuning Plasmonic Properties in Metal Oxide Nanocrystals.

Doped metal oxide nanocrystals that exhibit tunable localized surface plasmon resonances (LSPRs) represent an intriguing class of nanomaterials that show promise for a variety of applications from spectroscopy to sensing. LSPRs arise in these materials through the introduction of aliovalent dopants and lattice oxygen vacancies. Tuning the LSPR shape and energy is generally accomplished through controlling the concentration or identity of dopants in a nanocrystal, but the lack of finer synthetic control leaves several fundamental questions unanswered regarding the effects of radial dopant placement, size, and nanocrystalline architecture on the LSPR energy and damping. Here, we present a layer-by-layer synthetic method for core/shell nanocrystals that permits exquisite and independent control over radial dopant placement, absolute dopant concentration, and nanocrystal size. Using Sn-doped In2O3 (ITO) as a model LSPR system, we synthesized ITO/In2O3 core/shell as well as In2O3/ITO core/shell nanocrystals with varying shell thickness, and investigated the resulting optical properties. We observed profound influence of radial dopant placement on the energy and linewidth of the LSPR response, noting (among other findings) that core-localized dopants produce the highest values for LSPR energies per dopant concentration, and display the lowest damping in comparison to nanocrystals with shell-localized or homogeneously distributed dopants. Inactive Sn dopants present on ITO nanocrystal surfaces are activated upon the addition of a subnanometer thick undoped In2O3 shell. We show how LSPR energy can be tuned fully independent of dopant concentration, relying solely on core/shell architecture. Finally, the impacts of radial dopant placement on damping, independent of LSPR energy, are explored.

Synthesis of an Aluminum Hydroxide Octamer through a Simple Dissolution Method.

Multimeric oxo-hydroxo Al clusters function as models for common mineral structures and reactions. Cluster research, however, is often slowed by a lack of methods to prepare clusters in pure form and in large amounts. Herein, we report a facile synthesis of the little known cluster Al8 (OH)14 (H2 O)18 (SO4 )5 (Al8 ) through a simple dissolution method. We confirm its structure by single-crystal X-ray diffraction and show by 27 Al NMR spectroscopy, electrospray-ionization mass spectrometry, and small- and wide-angle X-ray scattering that it also exists in solution. We speculate that Al8 may form in natural water systems through the dissolution of aluminum-containing minerals in acidic sulfate solutions, such as those that could result from acid rain or mine drainage. Additionally, the dissolution method produces a discrete Al cluster on a scale suitable for studies and applications in materials science.

Single-Step Synthesis of Small, Azide-Functionalized Gold Nanoparticles: Versatile, Water-Dispersible Reagents for Click Chemistry.

Nanoparticles possessing functional groups that can be readily conjugated (e.g., through click chemistry) are important precursors for the preparation of customized nanohybrid products. Such nanoparticles, if they are stable against agglomeration, are easily dispersible and have well-defined surface chemistry and size. As click-ready reagents, they can be stored until their time of use and then simply dispersed and reacted with an appropriate substrate. Gold nanoparticles (AuNPs) are excellent candidates for this purpose, and some clickable gold nanoparticles have been developed; however, AuNPs for use in aqueous systems are often prepared through difficult multistep processes and/or can be poorly dispersible in water. Here we report a single-step synthesis of clickable, water-dispersible AuNPs. The synthesis yields uniform, 3.5 nm diameter cores coated with a well-defined molecular ligand shell that makes the AuNPs stable and dispersible in water. The AuNP mixed ligand shell consists of hydroxyl-terminated ethylene glycol-based ligands to promote dispersion in water and a small number of azide-terminated ligands that readily undergo click reactions with alkynes. The use of a mesofluidic reactor affords fine control over the core size and ligand shell composition and ensures reproducible results (e.g., less than 0.1 nm variation in core diameter between batches). The purified reagents were successfully coupled to a variety of alkyne-containing substrates using both Cu-catalyzed and strain-promoted click reactions. Particle size, morphology, stability, and surface composition were thoroughly characterized using small-angle X-ray scattering, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis, and 1H NMR before and after the click reactions. Both the parent nanoparticles and their click chemistry products are stable during storage and remained dispersible for over a year in water, suggesting their potential for environmental, biological, and biomedical applications.

UV-Visible Spectroscopy-Based Quantification of Unlabeled DNA Bound to Gold Nanoparticles.

DNA-functionalized gold nanoparticles have been increasingly applied as sensitive and selective analytical probes and biosensors. The DNA ligands bound to a nanoparticle dictate its reactivity, making it essential to know the type and number of DNA strands bound to the nanoparticle surface. Existing methods used to determine the number of DNA strands per gold nanoparticle (AuNP) require that the sequences be fluorophore-labeled, which may affect the DNA surface coverage and reactivity of the nanoparticle and/or require specialized equipment and other fluorophore-containing reagents. We report a UV-visible-based method to conveniently and inexpensively determine the number of DNA strands attached to AuNPs of different core sizes. When this method is used in tandem with a fluorescence dye assay, it is possible to determine the ratio of two unlabeled sequences of different lengths bound to AuNPs. Two sizes of citrate-stabilized AuNPs (5 and 12 nm) were functionalized with mixtures of short (5 base) and long (32 base) disulfide-terminated DNA sequences, and the ratios of sequences bound to the AuNPs were determined using the new method. The long DNA sequence was present as a lower proportion of the ligand shell than in the ligand exchange mixture, suggesting it had a lower propensity to bind the AuNPs than the short DNA sequence. The ratio of DNA sequences bound to the AuNPs was not the same for the large and small AuNPs, which suggests that the radius of curvature had a significant influence on the assembly of DNA strands onto the AuNPs.

Small Gold Nanoparticles Interfaced to Electrodes through Molecular Linkers: A Platform to Enhance Electron Transfer and Increase Electrochemically Active Surface Area.

For the smallest nanostructures (<5 nm), small changes in structure can lead to significant changes in properties and reactivity. In the case of nanoparticle (NP)-functionalized electrodes, NP structure and composition, and the nature of the NP-electrode interface have a strong influence upon electrochemical properties that are critical in applications such as amperometric sensing, photocatalysis and electrocatalysis. Existing methods to fabricate NP-functionalized electrodes do not allow for precise control over all these variables, especially the NP-electrode interface, making it difficult to understand and predict how structural changes influence NP activity. We investigated the electrochemical properties of small (dcore < 2.5 nm) gold nanoparticles (AuNPs) on boron doped diamond electrodes using three different electrode fabrication techniques with varying degrees of nanoparticle-electrode interface definition. Two methods to attach AuNPs to the electrode through a covalently bound molecular linker were developed and compared to NP-functionalized electrodes fabricated using solution deposition methods (drop-casting and physiadsorption of a monolayer). In each case, a ferrocene redox probe was tethered to the AuNP surface to evaluate electron transfer through the AuNPs. The AuNPs that were molecularly interfaced with the electrode exhibited nearly ideal, reproducible electrochemical behavior with narrow redox peaks and small peak separations, whereas the solution deposited NPs had broader redox peaks with large peak separations. These data suggest that the molecular tether facilitates AuNP-mediated electron transfer. Interestingly, the molecularly tethered NPs also had significantly more electrochemically active surface area than the solution deposited NPs. The enhanced electrochemical behavior of the molecularly interfaced NPs demonstrates the significant influence of the interface on NP-mediated electron transfer and suggests that similar modified electrodes can serve as versatile platforms for studies and applications of nanoparticles.

Continuous Growth of Metal Oxide Nanocrystals: Enhanced Control of Nanocrystal Size and Radial Dopant Distribution.

The ability to precisely control the composition of nanocrystals, similar to the way organic chemists control the structure of small molecules, remains an important challenge in nanoscience. Rather than dictating nanocrystal size through the nucleation event, growth of nanocrystals through continuous precursor addition would allow fine structural control. Herein, we present a method of growth for indium oxide nanocrystals that relies on the slow addition of an indium carboxylate precursor into hot oleyl alcohol. Nanocrystal size and structure can be governed at a subnanometer scale, and it is possible to precisely control core size over a range of three to at least 22 nm with dispersities as low as 7%. Growth can be stopped and restarted repeatedly without aggregation or passivation. We show that the volume of the nanocrystal core (and thus molecular weight) increases linearly with added monomer and the number of nanocrystals remains constant throughout the growth process, yielding an extremely predictable approach to size control. It is also possible to place metal oxide shells (e.g., Sn-doped In2O3 (ITO)) at various radial positions within the nanocrystal, and we use this approach to synthesize ITO/In2O3 core/shell nanocrystals as well as In2O3/ITO/In2O3 core/shell/shell nanocrystals.

Potential Environmental Impacts and Antimicrobial Efficacy of Silver- and Nanosilver-Containing Textiles.

For textiles containing nanosilver, we assessed benefit (antimicrobial efficacy) in parallel with potential to release nanosilver (impact) during multiple life cycle stages. The silver loading and method of silver attachment to the textile highly influenced the silver release during washing. Multiple sequential simulated household washing experiments for fabric swatches in deionized water with or without detergent showed a range of silver release. The toxicity of washing experiment supernatants to zebrafish (Danio rerio) embryos was negligible, with the exception of the very highest Ag releases (∼1 mg/L Ag). In fact, toxicity tests indicated that residual detergent exhibited greater adverse response than the released silver. Although washing the fabrics did release silver, it did not affect their antimicrobial efficacy, as demonstrated by >99.9% inhibition of E. coli growth on the textiles, even for textiles that retained as little as 2 µg/g Ag after washing. This suggests that very little nanosilver is required to control bacterial growth in textiles. Visible light irradiation of the fabrics reduced the extent of Ag release for textiles during subsequent washings. End-of-life experiments using simulated landfill conditions showed that silver remaining on the textile is likely to continue leaching from textiles after disposal in a landfill.

Influence of Ligand Shell Composition upon Interparticle Interactions in Multifunctional Nanoparticles.

The interactions of nanoparticles with biomolecules, surfaces, or other nanostructures are dictated by the nanoparticle's surface chemistry. Thus, far, shortcomings of syntheses of nanoparticles with defined ligand shell architectures have limited our ability to understand how changes in their surface composition influence reactivity and assembly. We report new synthetic approaches to systematically control the number (polyvalency), length, and steric interactions of omega-functionalized (targeting) ligands within an otherwise passivating (diluent) ligand shell. A mesofluidic reactor was used to prepare nanoparticles with the same core diameter for each of the designed ligand architectures. When the targeting ligands are malonamide groups, the nanoparticles assemble via cross-linking in the presence of trivalent lanthanides. We examined the influence of ligand composition on assembly by monitoring the differences in optical properties of the cross-linked and free nanoparticles. Infrared spectroscopy, electron microscopy, and solution small-angle X-ray scattering provided additional insight into the assembly behavior. Lower (less than 33%) malonamide ligand densities (where the binding group extends beyond the periphery of diluent ethylene glycol ligands) produce the strongest optical responses and largest assemblies. Surprisingly, nanoparticles containing a higher surface number of targeting ligand did not produce an optical response or assemble, underscoring the importance of an informed mixed ligand strategy for highest nanoparticle performance.

Subnanometer Control of Mean Core Size during Mesofluidic Synthesis of Small (D(core) < 10 nm) Water-Soluble, Ligand-Stabilized Gold Nanoparticles.

A convenient, single-step synthesis is reported that produces ligand-stabilized, water-soluble gold nanoparticles (AuNPs) with subnanometer-level precision of the mean core diameter over a range of 2-9 nm for a series of desired surface chemistries. The synthesis involves the reduction of a Au(III) species with sodium borohydride in the presence of a functionalized alkyl thiosulfate (Bunte salt) to yield thiolate-protected AuNPs. A key advantage of this synthesis is that simply adjusting the pH of the gold salt solution leads to control over the AuNP core size. The speciation of Au(III), and therefore the kinetics for its reduction and the core size produced, depends upon pH. The use of pH as the sole variable to control core size is a more reliable and convenient method than traditional approaches that rely on adjusting the concentrations and ratios of ligand, metal salt, and reducing agent. The average core size increased as the pH was raised for each ligand studied. Because the influence of pH was different for each of the ligands, working curves were plotted for each ligand to identify conditions to synthesize particles with specific, targeted core diameters. Using this approach, reaction conditions can be rapidly optimized using a combination of a mesofluidic reactor and small-angle X-ray scattering (SAXS) size analysis. The use of the mesofluidic reactor was needed to ensure fast mixing given the rapid kinetics for core formation. Using the reactor, it is possible to obtain reproducible sizes across multiple syntheses (<1-2% core size variation) and subnanometer control of the mean core dimensions. The synthetic method demonstrated here provides an attractive alternative to two-step syntheses involving ligand exchange because it is more efficient and eliminates the possibility of nanoparticle core size changes during exchange steps. This approach enables the development of "size ladders" of particles with the same surface chemistry for investigations of structure-function relationships.

Designing nanomaterials to maximize performance and minimize undesirable implications guided by the Principles of Green Chemistry.

The Twelve Principles of Green Chemistry were first published in 1998 and provide a framework that has been adopted not only by chemists, but also by design practitioners and decision-makers (e.g., materials scientists and regulators). The development of the Principles was initially motivated by the need to address decades of unintended environmental pollution and human health impacts from the production and use of hazardous chemicals. Yet, for over a decade now, the Principles have been applied to the synthesis and production of engineered nanomaterials (ENMs) and the products they enable. While the combined efforts of the global scientific community have led to promising advances in the field of nanotechnology, there remain significant research gaps and the opportunity to leverage the potential global economic, societal and environmental benefits of ENMs safely and sustainably. As such, this tutorial review benchmarks the successes to date and identifies critical research gaps to be considered as future opportunities for the community to address. A sustainable material design framework is proposed that emphasizes the importance of establishing structure-property-function (SPF) and structure-property-hazard (SPH) relationships to guide the rational design of ENMs. The goal is to achieve or exceed the functional performance of current materials and the technologies they enable, while minimizing inherent hazard to avoid risk to human health and the environment at all stages of the life cycle.

Removal of thiol ligands from surface-confined nanoparticles without particle growth or desorption.

Size-dependent properties of surface-confined inorganic nanostructures are of interest for applications ranging from sensing to catalysis and energy production. Ligand-stabilized nanoparticles are attractive precursors for producing such nanostructures because the stabilizing ligands may be used to direct assembly of thoroughly characterized nanoparticles on the surface. Upon assembly; however, the ligands block the active surface of the nanoparticle. Methods used to remove these ligands typically result in release of nanoparticles from the surface or cause undesired growth of the nanoparticle core. Here, we demonstrate that mild chemical oxidation (50 ppm of ozone in nitrogen) oxidizes the thiolate headgroups, lowering the ligand's affinity for the gold nanoparticle surface and permitting the removal of the ligands at room temperature by rinsing with water. XPS and TEM measurements, performed using a custom planar analysis platform that permits detailed imaging and chemical analysis, provide insight into the mechanism of ligand removal and show that the particles retain their core size and remain tethered on the surface core during treatment. By varying the ozone exposure time, it is possible to control the amount of ligand removed. Catalytic carbon monoxide oxidation was used as a functional assay to demonstrate ligand removal from the gold surface for nanoparticles assembled on a high surface area support (fumed silica).

Structurally similar triphenylphosphine-stabilized undecagolds, Au11(PPh3)7Cl3 and [Au11(PPh3)8Cl2]Cl, exhibit distinct ligand exchange pathways with glutathione.

Ligand exchange is frequently used to introduce new functional groups on the surface of inorganic nanoparticles or clusters while preserving the core size. For one of the smallest clusters, triphenylphosphine (TPP)-stabilized undecagold, there are conflicting reports in the literature regarding whether core size is retained or significant growth occurs during exchange with thiol ligands. During an investigation of these differences in reactivity, two distinct forms of undecagold were isolated. The X-ray structures of the two forms, Au11(PPh3)7Cl3 and [Au11(PPh3)8Cl2]Cl, differ only in the number of TPP ligands bound to the core. Syntheses were developed to produce each of the two forms, and their spectroscopic features correlated with the structures. Ligand exchange on [Au11(PPh3)8Cl2]Cl yields only small clusters, whereas exchange on Au11(PPh3)7Cl3 (or mixtures of the two forms) yields the larger Au25 cluster. The distinctive features in the optical spectra of the two forms made it possible to evaluate which of the cluster forms were used in the previously published papers and clarify the origin of the differences in reactivity that had been reported. The results confirm that reactions of clusters and nanoparticles may be influenced by small variations in the arrangement of ligands and suggest that the role of the ligand shell in stabilizing intermediates during ligand exchange may be essential to preventing particle growth or coalescence.