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

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

Transformations during Sintering of Small (<i>D</i>_core < 2 nm) Ligand-Stabilized Gold Nanoparticles: Influence of Ligand Functionality and Core Size

Ligand-stabilized gold nanoparticles have been investigated as both discrete entities with size-dependent properties and as precursor inks for low-temperature deposition of thin films and patterns. In the first instance it is important to preserve the nanoparticle core, whereas for thin film applications it is desirable for the nanoparticles to sinter at relatively low temperatures. In each case, a detailed understanding of the factors that govern nanoparticle sintering will lead to improved nanomaterial design. An investigation of the sintering behavior of ∼1.4 and ∼0.9 nm gold nanoparticle cores, each passivated with two different ligands, by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) illustrates a clear size and ligand dependency on the sintering process. TGA reveals that free ligand volatilizes at lower temperatures than when bound to the nanoparticle core. Ligands of the same chain length with different terminal functionality show distinctly different volatilities and rates of ligand loss, revealing that volatility is derived from composition rather than merely ligand chain length. Conducting TGA and DSC measurements on nanoparticles of the same ligand passivation but of different core size shows that larger nanoparticles lose ligands and sinter more readily than smaller nanoparticles, suggesting a greater stability of the ligand shell on smaller nanoparticles. TGA, DSC, and X-ray photoelectron spectroscopy (XPS) analyses show that sintering is triggered by a very small amount of ligand loss. Once initiated, the sintering process rapidly excludes ligand from the gold surface, forming a porous film, as shown by scanning electron microscopy (SEM). These studies suggest that both the nanoparticle core size and ligand identity need to be considered together when selecting nanoparticles to either prevent or promote nanoparticle sintering

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 (<i>d</i>_core < 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

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)

Subnanometer Control of Mean Core Size during Mesofluidic Synthesis of Small (<i>D</i>_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

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

Generation of Metal Nanoparticles from Silver and Copper Objects: Nanoparticle Dynamics on Surfaces and Potential Sources of Nanoparticles in the Environment

The use of silver nanoparticles (AgNPs) in antimicrobial applications, including a wide range of consumer goods and apparel, has attracted attention because of the unknown health and environmental risks associated with these emerging materials. Of particular concern is whether there are new risks that are a direct consequence of their nanoscale size. Identifying those risks associated with nanoscale structure has been difficult due to the fundamental challenge of detecting and monitoring nanoparticles in products or the environment. Here, we introduce a new strategy to directly monitor nanoparticles and their transformations under a variety of environmental conditions. These studies reveal unprecedented dynamic behavior of AgNPs on surfaces. Most notably, under ambient conditions at relative humidities greater than 50%, new silver nanoparticles form in the vicinity of the parent particles. This humidity-dependent formation of new particles was broadly observed for a variety of AgNPs and substrate surface coatings. We hypothesize that nanoparticle production occurs through a process involving three stages: (i) oxidation and dissolution of silver from the surface of the particle, (ii) diffusion of silver ion across the surface in an adsorbed water layer, and (iii) formation of new, smaller particles by chemical and/or photoreduction. Guided by these findings, we investigated non-nanoscale sources of silver such as wire, jewelry, and eating utensils that are placed in contact with surfaces and found that they also formed new nanoparticles. Copper objects display similar reactivity, suggesting that this phenomenon may be more general. These findings challenge conventional thinking about nanoparticle reactivity and imply that the production of new nanoparticles is an intrinsic property of the material that is not strongly size dependent. The discovery that AgNPs and CuNPs are generated spontaneously from manmade objects implies that humans have long been in direct contact with these nanomaterials and that macroscale objects represent a potential source of incidental nanoparticles in the environment

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

Synthesis of Ligand-Stabilized Metal Oxide Nanocrystals and Epitaxial Core/Shell Nanocrystals <i>via</i> a Lower-Temperature Esterification Process

The properties of metal oxide nanocrystals can be tuned by incorporating mixtures of matrix metal elements, adding metal ion dopants, or constructing core/shell structures. However, high-temperature conditions required to synthesize these nanocrystals make it difficult to achieve the desired compositions, doping levels, and structural control. We present a lower temperature synthesis of ligand-stabilized metal oxide nanocrystals that produces crystalline, monodisperse nanocrystals at temperatures well below the thermal decomposition point of the precursors. Slow injection (0.2 mL/min) of an oleic acid solution of the metal oleate complex into an oleyl alcohol solvent at 230 °C results in a rapid esterification reaction and the production of metal oxide nanocrystals. The approach produces high yields of crystalline, monodisperse metal oxide nanoparticles containing manganese, iron, cobalt, zinc, and indium within 20 min. Synthesis of tin-doped indium oxide (ITO) can be accomplished with good control of the tin doping levels. Finally, the method makes it possible to perform epitaxial growth of shells onto nanocrystal cores to produce core/shell nanocrystals