Catalysis-Based Fluorometric Method for Semiquantifying Trace Palladium in Sulfur-Containing Compounds and Ibuprofen.

Trace palladium in synthetic materials can be rapidly and inexpensively semiquantified by a catalysis-based fluorometric method that converts resorufin allyl ether to resorufin. However, whether sulfur compounds would interfere with this method has not been systematically studied. Herein, we show that although thiourea in solution interferes with quantification, sulfide, thiol, and thiocarbamate do not. The fluorometric method can also detect palladium bound to sulfur-based scavenger resin and outperform inductively coupled plasma mass spectrometry for detecting trace palladium in ibuprofen.

Role of Ligand Shell Density in the Diffusive Behavior of Nanoparticles in Hydrogels.

The diffusion coefficients of poly(ethylene glycol) methyl ether thiol (PEGSH)-functionalized gold nanoparticles (NPs) with different effective grafting densities were measured in polyacrylamide hydrogels. The NP core size was held constant, and the NPs were functionalized with mixtures of short oligomeric ligands (254 Da PEGSH) and longer (either 1 or 2 kDa PEGSH) ligands. The ratio of short and long ligands was varied such that the grafting density of the high-molecular-weight (MW) ligand ranged from approximately 1 to 100 high-MW ligands/NP. The diffusion coefficients of the NPs were then measured in gels with varying average mesh sizes. The measured diffusion coefficients decreased with higher MW ligand density. Interestingly, the diffusion coefficients for NPs with high effective grafting densities were well-predicted by their hydrodynamic diameters, but the diffusion coefficients for NPs with low effective grafting densities were higher than expected from their hydrodynamic diameters. These results suggest that crowding in the NP ligand shell influences the mechanism of diffusion, with lower grafting densities allowing ligand chain relaxations that facilitate movement through the gel. This work brings new insights into the factors that dictate how NPs move through hydrogels and will inform the development of models for applications such as drug delivery in complex viscoelastic biological materials.

Particle-Driven Effects at the Bacteria Interface: A Nanosilver Investigation of Particle Shape and Dose Metric.

Design criteria for controlling engineered nanomaterial (ENM) antimicrobial performance will enable advances in medical, food production, processing and preservation, and water treatment applications. In pursuit of this goal, better resolution of how specific ENM properties, such as nanoparticle shape, influence antimicrobial activity is needed. This study probes the antimicrobial activity toward a model Gram-negative bacterium, Escherichia coli (E. coli), that results from interfacial interactions with differently shaped silver nanoparticles (AgNPs): cube-, disc-, and pseudospherical-AgNPs. The EC50 value (i.e., the concentration of AgNPs that inactivates 50% of the microbial population) for each shape is identified and presented as a function of mass, surface area, and particle number. Further, shifts in relative potency are identified from the associated dose-response curves (e.g., shifts left, to lower concentrations, indicate greater potency). When using a mass-based dose metric, the disc-AgNPs present the highest antimicrobial activity of the three shapes (EC50: 2.39 ± 0.26 µg/mL for discs, 2.99 ± 0.96 µg/mL for cubes, 116.33 ± 6.43 µg/mL for pseudospheres). When surface area and particle number are used as dose metrics, the cube-AgNPs possess the highest antimicrobial activity (EC50-surface area: 4.70 × 10-5 ± 1.51 × 10-5 m2/mL, EC50-particle: 5.97 × 109 ± 1.92 × 109 particles/mL), such that the relative trend in potency becomes cubes > discs > pseudospheres and cubes ≫ discs ⩾ pseudospheres, respectively. The results reveal that the antimicrobial potency of disc-AgNPs is sensitive to the dose metric, significantly decreasing in potency (∼5-30×) upon conversion from a mass-based concentration to surface area and particle number and influencing the conclusions drawn. The shift in relative particle potency highlights the importance of investigating various dose metrics within the experimental design and signals different particle parameters influencing shape-based antimicrobial activity. To probe shape-dependent behavior, we use a unique empirical approach where the physical and chemical properties (ligand chemistry, surface charge) of the AgNP shapes are carefully controlled, and total available surface area is equivalent across shapes as made through modifications to particle size and concentration. The results herein suggest that surface area alone does not drive antimicrobial activity as the different AgNP shapes at equivalent particle surface area yield significantly different magnitudes of antimicrobial activity (i.e., 100% inactivation for cube-AgNPs, <25% inactivation for disc- and pseudospherical-AgNPs). Further, the particle shapes studied possess different crystal facets, illuminating their potential influence on differentiating interactions between the particle surface and the microbe. Whereas surface area may partly contribute to antimicrobial activity in certain ENM shapes (i.e., disc-AgNPs in relation to the pseudospherical-AgNPs), the different magnitudes of antimicrobial activity across shape provide insight into the likely role of other particle-specific factors, such as crystal facets, driving the antimicrobial activity of other shapes (i.e., cube-AgNPs).

Plate-Like Colloidal Metal Nanoparticles.

The pseudo-two-dimensional (2D) morphology of plate-like metal nanoparticles makes them one of the most anisotropic, mechanistically understood, and tunable structures available. Although well-known for their superior plasmonic properties, recent progress in the 2D growth of various other materials has led to an increasingly diverse family of plate-like metal nanoparticles, giving rise to numerous appealing properties and applications. In this review, we summarize recent progress on the solution-phase growth of colloidal plate-like metal nanoparticles, including plasmonic and other metals, with an emphasis on mechanistic insights for different synthetic strategies, the crystallographic habits of different metals, and the use of nanoplates as scaffolds for the synthesis of other derivative structures. We additionally highlight representative self-assembly techniques and provide a brief overview on the attractive properties and unique versatility benefiting from the 2D morphology. Finally, we share our opinions on the existing challenges and future perspectives for plate-like metal nanomaterials.

Continuous nucleation of metallic nanoparticles via photocatalytic reduction.

Whether in organic synthesis or solar energy conversion, light can be a powerful reagent in chemical reactions and introduce new opportunities for synthetic control including duration, intensity, interval, and energy of irradiation. Here, we report the use of a molecular photosensitizer as a reducing agent in metallic nanoparticle syntheses. Using this approach, we report three key findings. (1) Nanoparticles produced by photocatalytic reduction form via a continuous nucleation mechanism, as opposed to burst and burst-like nucleation processes typically observed in metal nanoparticle syntheses. (2) Because nucleation is continuous, as long as the solution is irradiated (and there remains excess reagents in solution), nanoparticle nucleation can be turned on and off by controlling the timing and duration of irradiation, with no observable particle growth. (3) This synthetic method extends to the formation of bimetallic nanoparticles, which we show also form via a continuous nucleation pathway, and follow predicted patterns of metal incorporation as a function of the magnitude of the difference between the reduction potentials of the two metals. Taken together, these results establish a versatile synthetic method for the formation of multimetallic nanoparticles using visible light.

Relationship between Gel Mesh and Particle Size in Determining Nanoparticle Diffusion in Hydrogel Nanocomposites.

The diffusion of poly(ethylene glycol) methyl ether thiol (PEGSH)-functionalized gold nanoparticles (NPs) was measured in polyacrylamide gels with various cross-linking densities. The molecular weight of the PEGSH ligand and particle core size were both varied to yield particles with hydrodynamic diameters ranging from 7 to 21 nm. The gel mesh size was varied from approximately 36 to 60 nm by controlling the cross-linking density of the gel. Because high-molecular-weight ligands are expected to yield more compressible particles, we expected the diffusion constants of the NPs to depend on their hard/soft ratios (where the hard component of the particle consists of the particle core and the soft component of the particle consists of the ligand shell). However, our measurements revealed that NP diffusion coefficients resulted primarily from changes in the overall hydrodynamic diameter and not the ratio of particle core size to ligand size. Across all particles and gels, we found that the diffusion coefficient was well predicted by the confinement ratio calculated from the diameter of the particle and an estimate of the gel mesh size obtained from the elastic blob model and was well described using a hopping model for nanoparticle diffusion. These results suggest that the elastic blob model provides a reasonable estimate of the mesh size that particles "see" as they diffuse through the gel. This work brings new insights into the factors that dictate how NPs move through polymer gels and will inform the development of hydrogel nanocomposites for applications such as drug delivery in heterogeneous, viscoelastic biological materials.

Role of bacterial motility in differential resistance mechanisms of silver nanoparticles and silver ions.

Unlike conventional antimicrobials, the study of bacterial resistance to silver nanoparticles (AgNPs) remains in its infancy and the mechanism(s) through which it evolves are limited and inconclusive. The central question remains whether bacterial resistance is driven by the AgNPs, released Ag(I) ions or a combination of these and other factors. Here, we show a specific resistance in an Escherichia coli K-12 MG1655 strain to subinhibitory concentrations of AgNPs, and not Ag(I) ions, as indicated by a statistically significant greater-than-twofold increase in the minimum inhibitory concentration occurring after eight repeated passages that was maintained after the AgNPs were removed and reintroduced. Whole-population genome sequencing identified a cusS mutation associated with the heritable resistance that possibly increased silver ion efflux. Finally, we rule out the effect of particle aggregation on resistance and suggest that the mechanism of resistance may be enhanced or mediated by flagellum-based motility.

Connecting Cation Exchange and Metal Deposition Outcomes via Hume-Rothery-Like Design Rules Using Copper Selenide Nanoparticles.

Heterogenous nanomaterials containing various inorganic phases have far-reaching impacts both from the physical phenomena they reveal and the technologies they enable. While the variety and impact of these materials has been demonstrated in many reports, there is critical ambiguity in the factors that lead to major bifurcations in developing these heterostructures, for example, the formation of either mixed metal semiconductors or segregated metal-semiconductor phases. Here, we compare outcomes of independently introducing 5 different metal cations (Au3+, Ag+, Hg2+, Pd2+, and Pt2+) to antifluorite copper selenide (Cu2-xSe) nanoparticles (diameter = 52 ± 5 nm). This suite of metal cations allowed us to control for and evaluate a variety of potentially competing intrinsic system parameters including metal cation size, valency, and reduction potential as well as lattice volume change, lattice formation energy, and lattice mismatch. Upon secondary metal addition, we determined that the transformation of a cubic Cu2-xSe lattice will occur via cation exchange reaction when the change in symmetry to the resulting metal selenide phase(s) preserves mutually orthogonal lattice vectors. However, if the new lattice symmetry would be disrupted further, metal deposition is the likely outcome of secondary metal cation addition, forming metal-semiconductor heterostructures. These results suggest a synthesis design rule that relies on an intrinsic property of the material, not the reaction pathway, and indicates that more such factors may be found in other particle and synthetic systems.

The Design and Science of Polyelemental Nanoparticles.

Polyelemental nanoparticles (PE NPs) containing four or more elements in a single NP have intriguing intrinsic properties compared to their single-element counterparts. The fusion of diverse elements induces synergistic effects including new physical and chemical phenomena. However, conventional methods have not offered effective strategies for the uniform creation of PE NPs with high reproducibility. Recently, with advances in nanoscience, several new methods have been developed using both thermodynamic and kinetic approaches and, often, the interplay between them. In this Perspective, we highlight recent key advances in the design of PE NPs and their underlying formation mechanisms. We discuss the potential applications of PE NPs and the outlook and future directions for this field.

Emerging investigator series: characterization of silver and silver nanoparticle interactions with zinc finger peptides.

In biological systems, chemical and physical transformations of engineered silver nanomaterials (AgENMs) are mediated, in part, by proteins and other biomolecules. Metalloprotein interactions with AgENMs are also central in understanding toxicity and antimicrobial and resistance mechanisms. Despite their readily available thiolate and amine ligands, zinc finger (ZF) peptides have thus far escaped study in reaction with AgENMs and their Ag(I) oxidative dissolution product. We report spectroscopic studies that characterize AgENM and Ag(I) interactions with two ZF peptides that differ in sequence, but not in metal binding ligands: the ZF consensus peptide CP-CCHC and the C-terminal zinc finger domain of HIV-1 nucleocapsid protein p7 (NCp7_C). Both ZF peptides catalyze AgENM (10 and 40 nm, citrate coated) dissolution and agglomeration, two important AgENM transformations that impact bioreactivity. AgENMs and their oxidative dissolution product, Ag(I)(aq), mediate changes to ZF peptide structure and metalation as well. Spectroscopic titrations of Ag(I) into apo-ZF peptides show an Ag(I)-thiolate charge transfer band, indicative of Ag(I)-ZF binding. Fluorescence studies of the Zn(II)-NCp_7 complex indicate that the Ag(I) also effectively competes with the Zn(II) to drive Zn(II) displacement from the ZFs. Upon interaction with AgENMs, Zn(II) bound ZF peptides show a secondary structural change in circular dichroism spectroscopy toward an apo-like structure. The results suggest that Ag(I) and AgENMs may alter ZF protein function within the cell.

Zinc-Adeninate Metal-Organic Framework: A Versatile Photoluminescent Sensor for Rare Earth Elements in Aqueous Systems.

Rare earth elements (REEs) are strategically important for national security and advanced technologies. Consequently, significant effort has been devoted towards increasing REE domestic production, including the extraction of REEs from coal, coal combustion byproducts, and their associated waste streams such as acid mine drainage. Analytical techniques for rapid quantification of REE content in aqueous phases can facilitate REE recovery through rapid identification of high-value waste streams. In this work, we show that BioMOF-100 can be used as a fluorescent-based sensitizer for emissive REE ion detection in water, providing rapid (<10 min) analysis times and sensitive detection (parts-per-billion detection limits) for terbium, dysprosium, samarium, europium, ytterbium, and neodymium, even in the presence of acids or secondary metals.

Plasmon-Enhanced Chemical Conversion Using Copper Selenide Nanoparticles.

The syntheses, properties, and broad utility of noble metal plasmonic nanomaterials are now well-established. To capitalize on this exceptional utility, mitigate its cost, and potentially expand it, non-noble metal plasmonic materials have become a topic of widespread interest. As new plasmonic materials come online, it is important to understand and assess their ability to generate comparable or complementary plasmonic properties to their noble metal counterparts, including as both sensing and photoredox materials. Here, we study plasmon-driven chemistry on degenerately doped copper selenide (Cu2- xSe) nanoparticles. In particular, we observe plasmon-driven dimerization of 4-nitrobenzenethiol to 4,4'-dimercaptoazobenzene on Cu2- xSe surfaces with yields comparable to those observed from noble metal nanoparticles. Overall, our results indicate that doped semiconductor nanoparticles are promising for light-driven chemistry technologies.

Surface Chemistry-Mediated Near-Infrared Emission of Small Coinage Metal Nanoparticles.

From size-dependent luminescence to localized surface plasmon resonances, the optical properties that emerge from common materials with nanoscale dimensions have been revolutionary. As nanomaterials get smaller, they approach molecular electronic structures, and this transition from bulk to molecular electronic properties is a subject of far-reaching impact. One class of nanomaterials that exhibit particularly interesting optoelectronic features at this size transition are coinage metal (i.e., group 11 elements copper, silver, and gold) nanoparticles with core diameters between approximately 1 to 3 nm (∼25-200 atoms). Coinage metal nanoparticles can exhibit red or near-infrared photoluminescence features that are not seen in either their molecular or larger nanoscale counterparts. This emission has been exploited both as a probe of electronic behavior at the nanoscale as well as in critical applications such as biological imaging and chemical sensing. Interestingly, it has been demonstrated that their photoluminescence figures of merit such as emission quantum yield, energy, and lifetime are largely independent of particle diameter. Instead, emission from particles at this size range depends heavily on the particle surface chemistry, which includes both its metallic composition and the capping ligand architecture. The strong influence of surface chemistry on these emergent optoelectronic phenomena has powerful implications for both the study and use of these particles, primarily due to the theoretically limitless possible surface ligand architectures and metallic compositions. In this Account, we highlight recent work that studies and uses surface chemistry-mediated photoluminescence from coinage metal nanoparticles. Specifically, we emphasize the distinct, as well as synergistic, roles of the nanoparticle capping ligand and the nanoparticle core for controlling and/or enhancing their near-infrared photoluminescence. We then discuss the implications of surface chemistry-mediated photoluminescence as it relates to downstream applications such as energy transfer, sensing, and biological imaging. We conclude by discussing current challenges that remain in the field, including opportunities to develop new particle synthetic routes, analytical tools, and physical frameworks with which to understand small nanoparticle emission. Taken together, we anticipate that these materials will be foundational both in understanding the unique transition from molecular to bulk electronic structures and in the development of nanomaterials that leverage this transition.