In Vitro and In Vivo Testing of Microbe Growth on Antimicrobial Nursing Scrubs.

Around 5% to 10% of hospitalized patients develop a hospital-acquired infection (HAI). Scrubs are a potential vector of HAIs. To compare the antimicrobial characteristics of scrubs with and without an antimicrobial fabric coating, as tested in the laboratory (in vitro) and hospital (in vivo) environments. Two protocols were conducted to address the purpose. The in vitro protocol was a laboratory study that involved observing the microbe growth after inoculating coated and uncoated scrub fabric swatches with S. aureus and then processing them in moist and dry environments. The in vivo protocol was a clinical trial that measured microbe growth on coated and uncoated scrubs prior to and following nursing staff completing a 12-hr shift on an acute care unit, as measured by colony forming units (CFUs). For high-humidity environments, the in vitro study indicated that swatches treated with an antimicrobial coating exhibited minimal microbe growth, while untreated swatches exhibited significant microbe growth. For low-humidity environments, coated and uncoated swatches were all found to exhibit minimal microbe growth. In the in vivo study, the CFUs increased on scrubs worn by nurses over a 12-hr shift with no significant difference in CFUs for coated and uncoated scrubs. For bacteria in a warm and moist environment, the antimicrobial coating was found to be important for inhibiting growth. For bacteria in a warm and dry environment, both coated and uncoated fabrics performed similarly as measured at 24 hr, with minimal bacterial growth observed. In a hospital environment, microbe growth was observed, but no significant difference was detected when comparing coated and uncoated scrubs. This may have been due to the short time between exposure and culturing the scrubs for analysis immediately at the end of the shift not allowing for enough time to kill or inhibit growth. Contact time between the bacteria and scrub fabric (coated or uncoated) in the in vivo study more directly correlated with the 0-hr observations for the in vitro study, suggesting that the ineffectiveness of the treated scrubs in the clinical results may be due in part to short residence times before collection.

The Role of Oxidation during the Synthesis of Silver-Glutathione Monolayer-Protected Clusters.

The synthesis of metal monolayer-protected clusters (MPCs) is still not well understood. It was recently shown that the mechanism of MPC formation involves sequential growth, wherein small MPCs form first and then grow into progressively larger sizes. The sequential growth model does not entirely explain all experimental observations, however. For example, the evolution of MPC product sizes is found to be a non-monotonic function of reaction kinetics, whereas the sequential growth model predicts monotonic behavior. Size evolution of MPCs is studied during synthetic reactions for a wide range of kinetics and it is found that all syntheses began with the sequential growth of MPCs but also found that growth transitioned to degradation if reduction kinetics are fast enough to give way to ambient oxidation. It is identified that MPCs can degrade via oxidation during syntheses and in a manner that is opposite to sequential growth, namely by forming smaller known MPC species from larger MPC species. This sequential degradation process therefore played an important role in determining final MPC products for reactions with fast reduction kinetics. Together, complementary oxidative and reductive processes provide a more complete description of MPC synthesis as well as new tools for controlling metal MPC synthesis.

Sequential Growth as a Mechanism of Silver-Glutathione Monolayer-Protected Cluster Formation.

Silver monolayer-protected clusters (MPCs) are an important new class of small metal nanoparticles with discrete sizes and unique properties that are eminently tunable; however, a fundamental understanding of the mechanisms of MPC formation is still lacking. Here, the basic mechanism by which silver-glutathione MPCs form is established by using real-time in situ optical measurements and ex situ solution-phase analyses to track MPC populations in the reaction mixture. These measurements identify that MPCs grow systematically, increasing in size sequentially as they transform from one known species to another, in contrast to existing models. In the new sequential growth model of MPC formation, the relative stability of each species in the series results in thermodynamic preferences for certain species as well as kinetic barriers to transformations between stable sizes. This model is shown to correctly predict the outcome of silver MPC synthetic reactions. Simple analytic expressions and simulations of rate equations are used to further validate the model and study its nature. The sequential growth model provides insights into how reactions may be directed, based on the interplay between relative MPC stabilities and reaction kinetics, providing tools for the synthesis of particular MPCs in high yield.

Interactions between Ultrastable Na4Ag44(SR)30 Nanoclusters and Coordinating Solvents: Uncovering the Atomic-Scale Mechanism.

Recently, silver nanoclusters have garnered considerable attention after the high-yield synthesis and crystallization of a thiolate-protected silver nanocluster, Na4Ag44(SR)30 (SR, protecting thiolate ligand). One intriguing feature of Na4Ag44(SR)30 is its outstanding stability and resistance to chemical reactions, in striking difference from other silver nanostructures whose susceptibility to oxidation (tarnishing) has been commonly observed and thus limits their applications in nanotechnology. Herein, we report the mechanism on the ultrahigh stability of Na4Ag44(SR)30 by uncovering how coordinating solvents interact with the Na4Ag44(SR)30 nanocluster at the atomic scale. Through synchrotron X-ray experiments and theoretical calculations, it was found that strongly coordinating aprotic solvents interact with surface Ag atoms, particularly between ligand bundles, which compresses the Ag core and relaxes surface metal-ligand interactions. Furthermore, water was used as a cosolvent to demonstrate that semiaqueous conditions play an important role in protecting exposed surface regions and can further influence the local structure of the silver nanocluster itself. Notably, under semiaqueous conditions, aprotic coordinating solvent molecules preferentially remain on the metal surface while water molecules interact with ligands, and ligand bundling persisted across the varied solvation conditions. This work offers an atomic level mechanism on the ultrahigh stability of the Na4Ag44(SR)30 nanoclusters from the nanocluster-coordinating solvent interaction perspective, and implies that nanocluster-solvent interactions should be carefully considered moving forward for silver nanoclusters, as they can influence the electronic/chemical properties of the nanocluster as well as the surface accessibility of small molecules for potential catalytic and biomedical applications.

Chemistry and Structure of Silver Molecular Nanoparticles.

Silver and gold molecular nanoparticles (mNPs) are a relatively new class of molecular materials of fundamental interest. They are high-nuclearity metal-organic compounds, with ligated metal cores, where the different character of bonding in the ligand shell and metal core gives rise to many of the unique properties of these materials. Research has primarily focused on gold mNPs, due to their good stability and the ease with which they may be synthesized and processed. To understand these materials as a general class, however, it will be necessary to broaden research efforts to other metals. Gold and silver are isoelectronic and have the same atomic radius, making the comparison of gold and silver mNPs attractive. The optical and chemical differences of the two metals provide useful contrasts, however, as well as a means to access a wider range of properties. In this Account, we focus on the synthesis, structure, and reactivity of silver mNPs. First, we review the origins and history of the field, from the ill-defined gas-phase metal clusters of the 1980s to the precisely defined mNPs of 1996 and onward. Next, we discuss the role of silver as a complement to gold mNPs in the effort to generalize lessons learned from either material and extend them into new metals. The synthesis of silver mNPs is covered in some detail, noting the choices made as the chemistry and the materials were developed. The importance of coordinating solvents and thermodynamic stability are also noted. The need to reduce solvent use is discussed and a new approach to achieving this goal is presented. Next, the structures of silver mNPs are discussed, including the Ag44 and Ag17 archetypes, and focusing on the successful de novo structure prediction of the latter. Structure and prediction of ligand shell motifs are also discussed. Finally, the postsynthetic chemistry and reactivity of silver mNPs are presented, including some of the first efforts to elucidate reaction mechanisms, beginning in 2012. Silver nanoparticles are gaining in popularity, particularly compared with gold, as the potential for silver to make a technological and economic impact is recognized. The superior optical properties of silver already make it a valuable material for plasmonics, but this may also translate to molecular species for nonlinear optics, sensors, and optoelectronics. The higher reactivity may also lead to a greater diversity of chemistry for silver compared to gold, including as an important broad-spectrum antimicrobial. Conversely, the "ultrastability" of the Ag44 archetype has already enabled unprecedented scale up with molecular precision, and may lead to the first industrial-scale production of metal mNPs. Clearly, silver mNPs are one of the most promising and significant new materials being studied today.

M4Au12Ag32(p-MBA)30 (M = Na, Cs) bimetallic monolayer-protected clusters: synthesis and structure.

Crystals of M4Au12Ag32(p-MBA)30 bimetallic monolayer-protected clusters (MPCs), where p-MBA is p-mercapto-benzoic acid and M+ is a counter-cation (M = Na, Cs) have been grown and their structure determined. The mol-ecular structure of triacontakis[(4-carboxylatophenyl)sulfanido]dodecagolddotriacontasilver, Au12Ag32(C7H5O2S)30 or C210H150Ag32Au12O60S30, exhib-its point group symmetry at 100 K. The overall diameter of the MPC is approximately 28 Å, while the diameter of the Au12Ag20 metallic core is 9 Å. The structure displays ligand bundling and inter-molecular hydrogen bonding, which gives rise to a framework structure with 52% solvent-filled void space. The positions of the M+ cations and the DMF solvent mol-ecules within the void space of the crystal could not be determined. Three out of the five crystallographically independent ligands in the asymmetric unit cell are disordered over two sets of sites. Comparisons are made to the all-silver M4Ag44(p-MBA)30 MPCs and to expectations based on density functional theory.

Linear and Nonlinear Optical Response in Silver Nanoclusters: Insight from a Computational Investigation.

We report a density functional theory (DFT) and time-dependent DFT (TDDFT) investigation of the thiolated silver nanoclusters [Ag44(SR)30](4-), Ag14(SR)12(PR'3)8, Ag31(SG)19, Ag32(SG)19, and Ag15(SG)11, which were synthesized and for which one-photon absorption (OPA) characterization is available. Our computational investigation based on careful examination of the exchange-correlation functional used in DFT geometry optimization and for the linear optical properties predictions by TDDFT, demonstrated good agreement with the measured linear absorption spectra, however dependent on the applied functional. Following the benchmarking, we evaluated the two-photon absorption (TPA) response using TDDFT, noting that accurate prediction of OPA is important for suppositions on the spectral range for TPA enhancement because of the sensitivity to the excitation energies. Although the TPA cross-section results are complicated by resonance effects and quantifying TPA cross sections for these systems is difficult, our results indicate that the nanoclusters Ag15 and Ag31/32 are likely to have large TPA cross sections. The spherical symmetry of the Ag44 and Ag14 nanoclusters leads to applicability of superatom theory, while it is not as useful for the more oblate geometries of the Ag15 and Ag31/32 systems.

Confirmation of a de novo structure prediction for an atomically precise monolayer-coated silver nanoparticle.

Fathoming the principles underpinning the structures of monolayer-coated molecular metal nanoparticles remains an enduring challenge. Notwithstanding recent x-ray determinations, coveted veritable de novo structural predictions are scarce. Building on recent syntheses and de novo structure predictions of M3Au x Ag17-x (TBBT)12, where M is a countercation, x = 0 or 1, and TBBT is 4-tert-butylbenzenethiol, we report an x-ray-determined structure that authenticates an a priori prediction and, in conjunction with first-principles theoretical analysis, lends force to the underlying forecasting methodology. The predicted and verified Ag(SR)3 monomer, together with the recently discovered Ag2(SR)5 dimer and Ag3(SR)6 trimer, establishes a family of unique mount motifs for silver thiolate nanoparticles, expanding knowledge beyond the earlier-known Au-S staples in thiol-capped gold nanoclusters. These findings demonstrate key principles underlying ligand-shell anchoring to the metal core, as well as unique T-like benzene dimer and cyclic benzene trimer ligand bundling configurations, opening vistas for rational design of metal and alloy nanoparticles.

M3Ag17(SPh)12 Nanoparticles and Their Structure Prediction.

Although silver nanoparticles are of great fundamental and practical interest, only one structure has been determined thus far: M4Ag44(SPh)30, where M is a monocation, and SPh is an aromatic thiolate ligand. This is in part due to the fact that no other molecular silver nanoparticles have been synthesized with aromatic thiolate ligands. Here we report the synthesis of M3Ag17(4-tert-butylbenzene-thiol)12, which has good stability and an unusual optical spectrum. We also present a rational strategy for predicting the structure of this molecule. First-principles calculations support the structural model, predict a HOMO-LUMO energy gap of 1.77 eV, and predict a new "monomer mount" capping motif, Ag(SR)3, for Ag nanoparticles. The calculated optical absorption spectrum is in good correspondence with the measured spectrum. Heteroatom substitution was also used as a structural probe. First-principles calculations based on the structural model predicted a strong preference for a single Au atom substitution in agreement with experiment.

Critical island size, scaling, and ordering in colloidal nanoparticle self-assembly.

In order to obtain a better understanding of short-range (SR) and long-range (LR) nanoparticle (NP) interactions during the self-assembly of dodecanethiol-coated Au NPs in toluene via drop drying, we have investigated the dependence of the island density, scaled island-size distribution (ISD), and scaled capture-zone distribution (CZD) on coverage, deposition flux, and NP size. Our results indicate that, while the critical island size is larger than 1 for all NP sizes studied, due to the increase in the strength of the SR attraction between NPs with increasing NP size, both the exponent describing the dependence of the island density on deposition flux and the critical island-size decrease with increasing NP size. We also find that, despite the existence of significant cluster diffusion and coalescence, the ISD is sharply peaked as in epitaxial growth. In particular, for large NP size, we find good agreement between the scaled ISD and epitaxial growth models as well as good agreement between the scaled CZD and scaled ISD. However, for smaller NPs the scaled ISD is less sharply peaked despite the fact that the critical island size is larger. This latter result suggests that in this case additional effects such as enhanced island coalescence or NP detachment from large islands may play an important role. Results for the ordering of NP islands are also presented which indicate the existence of LR repulsive interactions. One possible mechanism for such an interaction is the existence of a small dipole moment on each NP which arises as a result of an asymmetry, driven by surface tension, in the thiol distribution for NPs adsorbed at the toluene-air interface. Consistent with this mechanism, we find good agreement between experimental results for the nearest-neighbor island-distance distribution and simulations which include dipole repulsion.

Model for the phase transfer of nanoparticles using ionic surfactants.

Ionic surfactants are widely used for the phase transfer of nanoparticles from aqueous to organic phases; however, a model that can be used to select ionic surfactants based on the nanoparticle solution properties has yet to be established. Here, we have studied the phase transfer of a variety of nanoparticles and have identified hydrophobicity, steric repulsion, and interfacial tension as key factors in determining whether or not phase transfer will occur. Based on these studies, we have developed a simple model for phase transfer wherein the success of the surfactant depends only on three criteria. The phase transfer agents must (i) efficiently load onto or cross the interface, (ii) solubilize the nanoparticles in the receiving phase, and (iii) sterically stabilize the nanoparticles to prevent aggregation due to van der Waals forces between the inorganic cores. Using these criteria, the effectiveness of ionic surfactants could be predicted based on their molecular geometry and the properties of the nanoparticle solutions. These rules provide a basis for choosing surfactants for phase transfer of spherical nanoparticles up to 16 nm in diameter and advances the development of a general model of nanoparticle phase transfer, which would include all nanoparticle shapes, sizes, and solvents.

Ultrastable silver nanoparticles.

Noble-metal nanoparticles have had a substantial impact across a diverse range of fields, including catalysis, sensing, photochemistry, optoelectronics, energy conversion and medicine. Although silver has very desirable physical properties, good relative abundance and low cost, gold nanoparticles have been widely favoured owing to their proved stability and ease of use. Unlike gold, silver is notorious for its susceptibility to oxidation (tarnishing), which has limited the development of important silver-based nanomaterials. Despite two decades of synthetic efforts, silver nanoparticles that are inert or have long-term stability remain unrealized. Here we report a simple synthetic protocol for producing ultrastable silver nanoparticles, yielding a single-sized molecular product in very large quantities with quantitative yield and without the need for size sorting. The stability, purity and yield are substantially better than those for other metal nanoparticles, including gold, owing to an effective stabilization mechanism. The particular size and stoichiometry of the product were found to be insensitive to variations in synthesis parameters. The chemical stability and structural, electronic and optical properties can be understood using first-principles electronic structure theory based on an experimental single-crystal X-ray structure. Although several structures have been determined for protected gold nanoclusters, none has been reported so far for silver nanoparticles. The total structure of a thiolate-protected silver nanocluster reported here uncovers the unique structure of the silver thiolate protecting layer, consisting of Ag2S5 capping structures. The outstanding stability of the nanoparticle is attributed to a closed-shell 18-electron configuration with a large energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, an ultrastable 32-silver-atom excavated-dodecahedral core consisting of a hollow 12-silver-atom icosahedron encapsulated by a 20-silver-atom dodecahedron, and the choice of protective coordinating ligands. The straightforward synthesis of large quantities of pure molecular product promises to make this class of materials widely available for further research and technology development.

Temporal stability of magic-number metal clusters: beyond the shell closing model.

The anomalous stability of magic-number metal clusters has been associated with closed geometric and electronic shells and the opening of HOMO-LUMO gaps. Despite this enhanced stability, magic-number clusters are known to decay and react in the condensed phase to form other products. Improving our understanding of their decay mechanisms and developing strategies to control or eliminate cluster instability is a priority, to develop a more complete theory of their stability, to avoid studying mixtures of clusters produced by the decay of purified materials, and to enable technology development. Silver clusters are sufficiently reactive to facilitate the study of the ambient temporal stability of magic-number metal clusters and to begin to understand their decay mechanisms. Here, the solution phase stability of a series of silver:glutathione (Ag:SG) clusters was studied as a function of size, pH and chemical environment. Cluster stability was found to be a non-monotonic function of size. Electrophoretic separations showed that the dominant mechanism involved the redistribution of mass toward smaller sizes, where the products were almost exclusively previously known cluster sizes. Optical absorption spectra showed that the smaller clusters evolved toward the two most stable cluster sizes. The net surface charge was found to play an important role in cluster stabilization although charge screening had no effect on stability, contrary to DLVO theory. The decay mechanism was found to involve the loss of Ag(+) ions and silver glutathionates. Clusters could be stabilized by the addition of Ag(+) ions and destabilized by either the addition of glutathione or the removal of Ag(+) ions. Clusters were also found to be most stable in near neutral pH, where they had a net negative surface charge. These results provide new mechanistic insights into the control of post-synthesis stability and chemical decay of magic-number metal clusters, which could be used to develop design principles for synthesizing specific cluster species.

Mass spectrometric identification of silver nanoparticles: the case of Ag32(SG)19.

Mass spectrometry has played a key role in identifying the members of a series of gold clusters, which has enabled the development of magic-number cluster theory. The successes of the gold cluster system have yet to be repeated in another metal cluster system, however. Silver clusters in particular have proven to be challenging due to their relative instability compared with gold clusters. Using the well-characterized gold nanocluster, Au(25)(SG)(18), we present optimized electrospray ionization mass spectrometry (ESI-MS) instrumental parameters for the maximal transmission of the intact cluster. Parameters shown to have the largest effect on intact cluster transmission/detection include trap and transfer collision energy, source temperature, and cone gas flow rate. Herein we describe a general strategy to acquire mass spectra of fragile metal clusters with reliable mass assignments. By also optimizing sample solution conditions, high-quality ESI mass spectra of a prototypical silver:glutathione (Ag:SG) cluster were obtained without significant fragmentation. By using gentle conditions and solution conditions designed to stabilize the clusters, fragmentation was dramatically reduced and mass spectra with isotopic resolution were measured. Using this strategy, we have made the first formula assignment for a ligand-protected Ag cluster of Ag(32)(SG)(19).

Glutathione-stabilized magic-number silver cluster compounds.

Magic-number theories, developed to explain the anomalous stability of clusters in the gas phase, are being successfully applied to explain the stability of families of condensed phase Au clusters. To test the generalizability of these theories, we have synthesized a family of magic-numbered Ag clusters. Silver clusters ligated with glutathione (GSH) were synthesized by reduction of silver glutathiolate in water and then separated by polyacrylamide gel electrophoresis (PAGE). The raw synthetic product consisted of a family of discrete Ag:SG clusters, each forming a band in the PAGE gel. Varying reaction conditions changed the relative abundance of the family members but not their positions and colors within the gel, indicating the molecular precision of magic-number clusters. Absorption onsets for the most abundant clusters monotonically decreased with increasing cluster size, and spectra contained a small number of peaks that corresponded to single electron transitions. Although these Ag:SG clusters are related to Au:SG clusters, the distribution of cluster sizes and the optical absorption spectra were markedly different for the two families. This suggests that the Ag:SG clusters are not a simple extension of the Au:SG system, possibly due to differences in Au and Ag chemistry. Alternatively, condensed-phase magic-number cluster theories may need to be more complex than currently believed.

Tris(1,10-phenanthroline-kappa2N,N')cobalt(III) tris(trifluoromethanesulfonate) dihydrate.

The title compound, [Co(C(12)H(8)N(2))(3)](CF(3)SO(3))(3) x 2 H(2)O, crystallizes to form infinite chains of complex cations that are connected through offset face-to-face and edge-to-face interactions between their phenanthroline ligands. The chains are themselves interconnected through weak offset face-to-face ligand interactions. The three trifluoromethanesulfonate anions of the asymmetric unit are connected with one another through the two water molecules by hydrogen bonds. One of the trifluoromethanesulfonate anions is described by a disorder over three positions, with occupancies of 0.35, 0.35 and 0.3 in the refined model.

Vertically aligned dense carbon nanotube growth with diameter control by block copolymer micelle catalyst templates.

We have grown a dense array of vertically aligned carbon nanotubes (CNTs) with a controlled distribution of diameters by using block copolymer micelles to form and pattern catalyst particles. The block copolymer poly(styrene-block-acrylic acid) (PS16500-PAA4500) was dissolved in toluene to form micelles and then loaded with FeCl3. The metal-loaded micelles were spin-coated on Si and then thermally treated to remove the polymer. Using this process, we produced surfaces patterned with iron oxide catalyst particles with particle densities ranging from 1400 microm(-2) to 3800 microm(-2) and a size distribution of (6.9 +/- 0.8) nm. CNT growth by thermal chemical vapor deposition was then performed on these samples. The low-density catalyst sample produced unaligned, low-density CNTs, whereas the high-density catalyst sample produced vertically aligned, dense CNTs about 10 microm in length. Transmission electron microscopy revealed that the CNTs typically had double and triple graphitic layers with normally distributed diameters of (4.5 +/- 1.1) nm. For comparison, CNTs grown from the standard approach of blanket Fe films had a wide distribution of diameters between 6 and 21 nm. This catalyst preparation approach dramatically sharpens the size distribution of CNTs, compared to standard approaches, and provides a simple means of controlling the areal density of CNTs.

Kinetically driven self assembly of highly ordered nanoparticle monolayers.

When a drop of a colloidal solution of nanoparticles dries on a surface, it leaves behind coffee-stain-like rings of material with lace-like patterns or clumps of particles in the interior. These non-uniform mass distributions are manifestations of far-from-equilibrium effects, such as fluid flows and solvent fluctuations during late-stage drying. However, recently a strikingly different drying regime promising highly uniform, long-range-ordered nanocrystal monolayers has been found. Here we make direct, real-time and real-space observations of nanocrystal self-assembly to reveal the mechanism. We show how the morphology of drop-deposited nanoparticle films is controlled by evaporation kinetics and particle interactions with the liquid-air interface. In the presence of an attractive particle-interface interaction, rapid early-stage evaporation dynamically produces a two-dimensional solution of nanoparticles at the liquid-air interface, from which nanoparticle islands nucleate and grow. This self-assembly mechanism produces monolayers with exceptional long-range ordering that are compact over macroscopic areas, despite the far-from-equilibrium evaporation process. This new drop-drying regime is simple, robust and scalable, is insensitive to the substrate material and topography, and has a strong preference for forming monolayer films. As such, it stands out as an excellent candidate for the fabrication of technologically important ultra thin film materials for sensors, optical devices and magnetic storage media.