Understanding the microscopic origin of gold nanoparticle anisotropic growth from molecular dynamics simulations

Atomistic insights into the morphological dynamics of gold and platinum nanoparticles: MD simulations in vacuum and aqueous media

The thermal response of gold and platinum spherical nanoparticles (NPs) upon cooling is studied through atomistic molecular dynamics simulations. The goal is to identify the morphological transformations occurring in the nanomaterials as well as to quantify their dependence on temperature, chemistry, and NP size. For diameters smaller than 3 nm, the transition temperature from a melted/amorphous to a highly crystalline state varies considerably with NP size. For larger NPs, the transition temperature is almost diameter-independent, yet it differs considerably from the transition temperature of the respective bulk materials. The platinum NPs possess a higher level of crystallinity than the gold counterparts under the same conditions because of the stronger cohesive forces that drive the crystallization process. This observation is also supported by the simulated X-ray powder diffraction patterns of the nanomaterials. The larger NPs have a multifaceted crystal surface, and their shape remains almost constant regardless of temperature variations. The smaller NPs have a smoother and more spherical surface, and their shape varies greatly with temperature. By studying the variation of nano-descriptors commonly employed in QSAR models, a qualitative picture of the NPs' toxicity and reactivity emerges: Small/hot NPs are likely more toxic than their large/cold counterparts. Because of the small size of the NPs considered, the observed structural modifications are challenging to be studied by experimental techniques. The present approach can be readily employed to study other metallic and metal oxide nanomaterials.

Direct visualization of ligands on gold nanoparticles in a liquid environment

The interactions between gold nanoparticles, their surface ligands and the solvent critically influence the properties of these nanoparticles. Although spectroscopic and scattering techniques have been used to investigate their ensemble structure, a comprehensive understanding of these processes at the nanoscale remains challenging. Electron microscopy makes it possible to characterize the local structure and composition but is limited by insufficient contrast, electron beam sensitivity and the requirement for ultrahigh-vacuum conditions, which prevent the investigation of dynamic aspects. Here we show that, by exploiting high-quality graphene liquid cells, we can overcome these limitations and investigate the structure of the ligand shell around gold nanoparticles and at the ligand-gold interface in a liquid environment. Using this graphene liquid cell, we visualize the anisotropy, composition and dynamics of ligand distribution on gold nanorod surfaces. Our results indicate a micellar model for surfactant organization. This work provides a reliable and direct visualization of ligand distribution around colloidal nanoparticles.

Quantitative 3D structural analysis of small colloidal assemblies under native conditions by liquid-cell fast electron tomography

Electron tomography has become a commonly used tool to investigate the three-dimensional (3D) structure of nanomaterials, including colloidal nanoparticle assemblies. However, electron microscopy is typically done under high-vacuum conditions, requiring sample preparation for assemblies obtained by wet colloid chemistry methods. This involves solvent evaporation and deposition on a solid support, which consistently alters the nanoparticle organization. Here, we suggest using electron tomography to study nanoparticle assemblies in their original colloidal liquid environment. To address the challenges related to electron tomography in liquid, we devise a method that combines fast data acquisition in a commercial liquid-cell with a dedicated alignment and reconstruction workflow. We present the advantages of this methodology in accurately characterizing two different systems. 3D reconstructions of assemblies comprising polystyrene-capped Au nanoparticles encapsulated in polymeric shells reveal less compact and more distorted configurations for experiments performed in a liquid medium compared to their dried counterparts. A similar expansion can be observed in quantitative analysis of the surface-to-surface distances of self-assembled Au nanorods in water rather than in a vacuum, in agreement with bulk measurements. This study, therefore, emphasizes the importance of developing high-resolution characterization tools that preserve the native environment of colloidal nanostructures.

Uniform colloidal synthesis of highly branched chiral gold nanoparticles

We present a uniform colloidal synthesis of highly branched gold nanoparticles (GNPs) including nanospheres, nanoplatelets and nanorods by cysteine-assisted seeded growth. The highly branched GNPs show blackbody-like absorption and chirality simultaneously, holding great potential for plasmonic or photothermal applications.

Influence of van der Waals Interactions between the Alkyl Chains of Surface Ligands on the Size and Size Distribution of Nanocrystals Prepared by the Digestive Ripening Process

Thermal heating of polydispersed nanocrystals (NCs) with surface-active organic ligands in a solvent leads to the formation of monodispersed NCs, and this process is known as digestive ripening (DR). Here, by performing DR on Au NCs using different-chain-length amine and thiol ligands, we evidently show that ligands with C12 chain length result in the formation of NCs with narrow size distributions when compared to C8, C16, and C20 chain length ligands. Furthermore, our findings also show that in the case of alkyl thiol, the NC size remains more or less the same, while the size distribution gets altered significantly with the chain length. On the other hand, both size and size distribution are affected significantly when the alkyl amine chain length is varied. Fourier transform infrared (FTIR) studies indicate that the van der Waals (vdW) interactions are weakest when the amine with C12 carbon chain is used as the DR agent, while in the case of thiols, molecules with C8 and C12 chain lengths have nearly the same vdW interactions (with C12 slightly weaker than C8), which are weaker than those of C16 and C20. Molecular dynamics (MD) simulation results corroborate the experimental observations and suggest that due to more defects in the alkyl chain, the C8 and C12 (amine as well as thiol) ligands are disordered and less stable on Au(111) and Au(100) surfaces. This could result in efficient etching and redeposition, making the ligands with C8 and C12 chain lengths the better DR agents.

Composite ligand shells on gold nanoprisms - an ensemble and single particle study

The attachment of thiolated molecules onto gold surfaces is one of the most extensively used and robust ligand exchange approaches to exploit the nanooptical features of nanoscale and nanostructured plasmonic materials. In this work, the impact of thiol adsorption on the optical properties of wet-chemically synthesized gold nanoprisms is studied both at the ensemble and single particle level to investigate the build-up of more complex ligand layers. Two prototypical ligands with different lengths have been investigated ((16-mercaptohexadecyl)trimethylammonium bromide - MTAB and thiolated polyethylene glycol - mPEG-SH). From ensemble experiments it is found that composite ligand layers are obtained by the sequential addition of the two thiols, and an island-like surface accumulation of the molecules can be anticipated. The single particle experiment derived chemical interface damping and resonance energy changes further support this and show additionally that when the two thiols are used simultaneously, a higher density, intermixed layer is formed. Hence, when working with more than a single type of ligand during surface modification, sequential adsorption is preferred for the combination of accessible essential surface functionalities, whereas for high overall loading the simultaneous use of the different ligand types is favourable.

Surfactant Layers on Gold Nanorods

ConspectusGold nanorods (Au NRs) are an exceptionally promising tool in nanotechnology due to three key factors: (i) their strong interaction with electromagnetic radiation, stemming from their plasmonic nature, (ii) the ease with which the resonance frequency of their longitudinal plasmon mode can be tuned from the visible to the near-infrared region of the electromagnetic spectrum based on their aspect ratio, and (iii) their simple and cost-effective preparation through seed-mediated chemical growth. In this synthetic method, surfactants play a critical role in controlling the size, shape, and colloidal stability of Au NRs. For example, surfactants can stabilize specific crystallographic facets during the formation of Au NRs, leading to the formation of NRs with specific morphologies.The process of surfactant adsorption onto the NR surface may result in various assemblies of surfactant molecules, such as spherical micelles, elongated micelles, or bilayers. Again, the assembly mode is critical toward determining the further availability of the Au NR surface to the surrounding medium. Despite its importance and a great deal of research effort, the interaction between Au NPs and surfactants remains insufficiently understood, because the assembly process is influenced by numerous factors, including the chemical nature of the surfactant, the surface morphology of Au NPs, and solution parameters. Therefore, gaining a more comprehensive understanding of these interactions is essential to unlock the full potential of the seed-mediated growth method and the applications of plasmonic NPs. A plethora of characterization techniques have been applied to reach such an understanding, but many open questions remain.In this Account, we review the current knowledge on the interactions between surfactants and Au NRs. We briefly introduce the state-of-the-art methods for synthesizing Au NRs and highlight the crucial role of cationic surfactants during this process. The self-assembly and organization of surfactants on the Au NR surface is then discussed to better understand their role in seed-mediated growth. Subsequently, we provide examples and elucidate how chemical additives can be used to modulate micellar assemblies, in turn allowing for a finer control over the growth of Au NRs, including chiral NRs. Next, we review the main experimental characterization and computational modeling techniques that have been applied to shed light on the arrangement of surfactants on Au NRs and summarize the advantages and disadvantages for each technique. The Account ends with a "Conclusions and Outlook" section, outlining promising future research directions and developments that we consider are still required, mostly related to the application of electron microscopy in liquid and in 3D. Finally, we remark on the potential of exploiting machine learning techniques to predict synthetic routes for NPs with predefined structures and properties.

Evolution, Stability, and Applicability of Surfactant Aggregates in Targeted Delivery

Self-assembly/self-aggregation of surfactant molecules in bulk and the vicinity of a surface has been a topic of interest for decades because of its utilization in numerous modern technical applications. In this article, the results of molecular dynamics simulations are reported to investigate the self-aggregation of sodium dodecyl sulfate (SDS) at an interface of mica and water. SDS molecules starting from lower to higher surface concentrations tend to create distinct aggregated structures in the vicinity of a mica surface. The structural properties, such as density profiles, radial distribution functions, and thermodynamic properties like excess entropy and second virial coefficient, are calculated to address the bits and pieces of the self-aggregation. The change in the free energy for aggregates of varied sizes approaching the surface from the bulk aqueous solution, along with the change in their shapes during the process in terms of change in the radius of gyration and its components, is reported respectively to model a generic pathway for a surfactant-based targeted delivery system.

Theory of Anisotropic Metal Nanostructures

A significant challenge in the development of functional materials is understanding the growth and transformations of anisotropic colloidal metal nanocrystals. Theory and simulations can aid in the development and understanding of anisotropic nanocrystal syntheses. The focus of this review is on how results from first-principles calculations and classical techniques, such as Monte Carlo and molecular dynamics simulations, have been integrated into multiscale theoretical predictions useful in understanding shape-selective nanocrystal syntheses. Also, examples are discussed in which machine learning has been useful in this field. There are many areas at the frontier in condensed matter theory and simulation that are or could be beneficial in this area and these prospects for future progress are discussed.

How a Facet of a Nanocrystal Is Formed: The Concept of the Symmetry Based Kinematic Theory

Conventional nanocrystal (NC) growth mechanisms have overwhelmingly focused on the final exposed facets to explain shape evolution. However, how the final facets are formed from the initial nuclei or seeds, has not been specifically interrogated. In this concept paper, we would like to concentrate on this specific topic, and introduce the symmetry based kinematic theory (SBKT) to explain the formation and evolution of crystal facets. It is a crystallographic theory based on the classical crystal growth concepts developed to illustrate the shape evolution during the NC growth. The most important principles connecting the basic NC growth processes and morphology evolution are the preferential growth directions and the properties of kinematic waves. On the contrary, the final facets are just indications of how the crystal growth terminates, and their formation and evolution rely on the NC growth processes: surface nucleation and layer advancement. Accordingly, the SBKT could even be applied to situations where non-faceted NCs such as spheres are formed.

Colloidal Synthesis of Metal Nanocrystals: From Asymmetrical Growth to Symmetry Breaking

Nanocrystals offer a unique platform for tailoring the physicochemical properties of solid materials to enhance their performances in various applications. While most work on controlling their shapes revolves around symmetrical growth, the introduction of asymmetrical growth and thus symmetry breaking has also emerged as a powerful route to enrich metal nanocrystals with new shapes and complex morphologies as well as unprecedented properties and functionalities. The success of this route critically relies on our ability to lift the confinement on symmetry by the underlying unit cell of the crystal structure and/or the initial seed in a systematic manner. This Review aims to provide an account of recent progress in understanding and controlling asymmetrical growth and symmetry breaking in a colloidal synthesis of noble-metal nanocrystals. With a touch on both the nucleation and growth steps, we discuss a number of methods capable of generating seeds with diverse symmetry while achieving asymmetrical growth for mono-, bi-, and multimetallic systems. We then showcase a variety of symmetry-broken nanocrystals that have been reported, together with insights into their growth mechanisms. We also highlight their properties and applications and conclude with perspectives on future directions in developing this class of nanomaterials. It is hoped that the concepts and existing challenges outlined in this Review will drive further research into understanding and controlling the symmetry breaking process.

Gold Nanostar Characterization by Nanoparticle Tracking Analysis

We demonstrate the application of nanoparticle tracking analysis (NTA) for the quantitative characterization of gold nanostars (GNSs). GNSs were synthesized by the seed-mediated growth method using triblock copolymer (TBP) gold nanoparticles (GNPs). These GNPs (≈ 10 nm) were synthesized from Au³⁺ (≈ 1 mM) in aqueous F127 (w/v 5%) containing the co-reductant ascorbic acid (≈ 2 mM). The GNS tip-to-core aspect ratio (AR) decreased when higher concentrations of GNPs were added to the growth solution. The AR dependency of GNSs on Au³⁺/Au(seed) concentration ratio implies that growth is partly under kinetic control. NTA measured GNS sizes, concentrations, and relative scattering intensities. Molar absorption coefficients ∼ 10⁹-10¹⁰ M^-1 cm^-1 (ε_400 nm) for each batch of GNSs were determined using the combination of extinction spectra and NTA concentrations for heterogeneous samples. NTA in combination with UV-vis was used to derive the linear relationships: (1) hydrodynamic size versus localized surface plasmon peak maxima; (2) ε_400 nm versus localized surface plasmon peak maxima; (3) ε_400 nm versus hydrodynamic size. NTA for quantitative characterization of anisotropic nanoparticles could lead to future applications, including heterogeneous colloidal catalysis.

Facet Selectivity of Cetyltrimethyl Ammonium Bromide Surfactants on Gold Nanoparticles Studied Using Molecular Simulations

We have studied facet selectivity of cetyltrimethyl ammonium bromide (CTAB) surfactants of varying alkyl tail lengths (C₁₇TAB and C₁₀TAB) during their adsorption on a spherical gold metal nanoparticle (MNP) using umbrella sampling and well-tempered metadynamics techniques in molecular simulations. We show that the surfactants strongly adsorb with their alkyl tails wrapped around the MNP. The adsorption morphologies are dictated by the strong preference of the polar head group of the surfactants to adsorb on to the atoms that lie between the facets of the MNP, that is, in the vicinity of low-coordinated gold atoms. The alkyl tails do not display any strong facet preference. Owing to the longer alkyl tails, C₁₇TAB molecules pack together better than the C₁₀TAB molecules in the adsorbed state on the MNP. These findings suggest that the regions near the edges of the facets and low-coordinated atoms are expected to be preferentially covered with the adsorbed surfactants.

Memory in quantum dot blinking

The photoluminescence intermittency (blinking) of quantum dots is interesting because it is an easily measured quantum process whose transition statistics cannot be explained by Fermi's golden rule. Commonly, the transition statistics are power-law distributed, implying that quantum dots possess at least trivial memories. By investigating the temporal correlations in the blinking data, we demonstrate with high statistical confidence that there is nontrivial memory between the on and off brightness duration data of blinking quantum dots. We define nontrivial memory to be statistical complexity greater than one. We show that this memory cannot be discovered using the transition distribution. We show by simulation that this memory does not arise from standard data manipulations. Finally, we conclude that at least three physical mechanisms can explain the measured nontrivial memory: (1) storage of state information in the chemical structure of a quantum dot; (2) the existence of more than two intensity levels in a quantum dot; and (3) the overlap in the intensity distributions of the quantum dot states, which arises from fundamental photon statistics.

Plasmonic All-Frame-Faceted Octahedral Nanoframes with Eight Engraved Y-Shaped Hot Zones

We report the synthesis of all-frame-faceted octahedral nanoframes containing eight Y-shaped hot zones in a single entity where electromagnetic near-field focusing can be maximized. To realize such state-of-the-art complex nanoframes, a series of multiple stepwise bottom-up processes were executed by exploiting Au octahedral nanoparticles as the initial template. By rationally controlling the chemical reactivity of different surface facets (i.e., vertexes, edges, and terraces), the Au octahedral nanoparticles went through controlled shape transformations, leading to Au-engraved nanoparticles wherein 24 edges wrap the octahedral Au nanoparticle core. Those edges were then selectively decorated with Pt, leading to the formation of eight Pt tripods in a single entity. After etching the central Au, 3D Pt tripod frame-faceted octahedral nanoframes were achieved with high integrity. By harnessing the obtained Pt nanoframes as a scaffold, AuAg alloy-based plasmonic all-frame-faceted nanoframes were obtained after the co-reduction of Ag and Au, which generated multiple hot zones within multiple surface intra-nanogaps, creating a single-particle, surface-enhanced Raman spectroscopy enhancer platform.

Determination of Equilibrium Adsorbed Morphologies of Surfactants at Metal-Water Interfaces Using a Modified Umbrella Sampling-Based Methodology

Surfactants adsorb to metal-water interfaces in various morphologies, including self-assembled monolayers (SAMs), cylindrical and spherical micelles, or hemimicelles. Current molecular simulation methods are unable to efficiently sample the formation of these morphologies because of the large diffusive/energetic barriers. We introduce a modified umbrella sampling-based methodology that allows sampling of these morphologies from any initial configuration and provides free energy differences between them. Using this methodology, we have studied adsorption behavior of cationic [quaternary ammonium (quat) of 4 and 12 carbon long alkyl tails], uncharged [decanethiol], and anionic [phosphate monoester] surfactants and their mixtures at a gold-water interface. We find that while Coulombic repulsion between the charged head groups of quat-4 limits their adsorption to a sparse layer, stronger hydrophobic interactions between the alkyl tails of quat-12 promote adsorption resulting in a morphology with adsorbed hemispherical micelles sitting atop a monolayer. Decanethiol molecules adsorb in a densely packed bilayer with the molecules standing-up on the surface in the first layer and lying parallel to the surface in the second layer. Cationic and anionic surfactant mixtures display a synergistic adsorption behavior. These results elucidate the role of molecular characteristics in dictating the nature of adsorbed morphologies of surfactants at metal-water interfaces.

The implication of adsorption preferences of ions and surfactants on the shape control of gold nanoparticles: a microscopic, atomistic perspective

Shape modulation of nanoparticles is crucial for their tailored applications; however, it depends on surfactants, ions, reactants, and other additives present in the growth solution. Here we dissect the role of surfactants, their counterions (halide ions), silver ions, and gold reactant in gold nanoparticle anisotropic growth using polarizable surfaces and nanoseed molecular dynamics simulation models. Our planar surface models predict a 14%-16% increment in cetyltrimethylammonium bromide (CTAB) coverage on Au(111) and Au(100) due to the surface polarization effect. The CTAB micelle adsorbs compactly similar to that observed on non-polarizable surfaces. The cetyltrimethylammonium chloride (CTAC) micelle remains in solution leaving the polarizable gold surfaces unprotected, similar to that observed with the non-polarizable surfaces, which favors isotropic growth. The cetyltrimethylammonium iodide (CTAI) micelle adsorbs with higher surface densities than CTAB on all the surfaces. The surface polarizable penta-twinned nanoseed model predicts the total surface coverage of the cetyltrimethylammonium cation (CTA⁺), Br^- and Ag⁺ to be around two times higher on the side as compared to the tip of the nanoseed, leading to a 2.6 times higher initial rate of adsorption of AuCl₂^- on the tip than on the side. Predicted CTA⁺ surface densities on the tip and the side of the nanoseed are consistent with experimental results. Our simulations explain the growth mechanism of anisotropic nanoparticles and the microscopic origin of their controlled shapes.

On the origin of controlled anisotropic growth of monodisperse gold nanobipyramids

We elucidate the crucial role of the cetyl trimethylammonium bromide (CTAB) surfactant in the anisotropic growth mechanism of gold nano-bipyramids, nano-objects with remarkable optical properties and high tunability. Atomistic molecular dynamics simulations predict different surface coverages of the CTAB (positively charged) heads and their (bromide) counterions as function of the gold exposed surfaces. High concentration of CTAB surfactant promotes formation of gold nanograins in solution that work as precursors for the smooth anisotropic growth of more elongated nano-bipyramidal objects. Nanobipyramids feature higher index facets with respect to nanorods, allowing higher CTAB coverages that stabilize their formation and leading to narrower inter-micelles channels that smooth down their anisotropic growth. Absorption spectroscopy and scanning electron microscopy confirmed the formation of nanograins and demonstrated the importance of surfactant concentration on driving the growth towards nano-bipyramids rather than nanorods. The outcome explains the formation of the monodisperse bipyramidal nano-objects, the origin of their controlled shapes and sizes along with their remarkable stability.

Role of pH in the synthesis and growth of gold nanoparticles using L-asparagine: a combined experimental and simulation study

The use of biomolecules as capping and reducing agents in the synthesis of metallic nanoparticles constitutes a promising framework to achieve desired functional properties with minimal toxicity. The system's complexity and the large number of variables involved represent a challenge for theoretical and experimental investigations aiming at devising precise synthesis protocols. In this work, we use L-asparagine (Asn), an amino acid building block of large biomolecular systems, to synthesise gold nanoparticles (AuNPs) in aqueous solution at controlled pH. The use of Asn offers a primary system that allows us to understand the role of biomolecules in synthesising metallic nanoparticles. Our results indicate that AuNPs synthesised in acidic (pH 6) and basic (pH 9) environments exhibit somewhat different morphologies. We investigate these AuNPs via Raman scattering experiments and classical molecular dynamics simulations of zwitterionic and anionic Asn states adsorbing on (111)-, (100)-, (110)-, and (311)-oriented gold surfaces. A combined analysis suggests that the underlying mechanism controlling AuNPs geometry correlates with amine's preferential adsorption over ammonium groups, enhanced upon increasing pH. Our simulations reveal that Asn (both zwitterionic and anionic) adsorption on gold (111) is essentially different from adsorption on more open surfaces. Water molecules strongly interact with the gold face-centred-cubic lattice and create traps, on the more open surfaces, that prevent the Asn from diffusing. These results indicate that pH is a relevant parameter in green-synthesis protocols with the capability to control the nanoparticle's geometry, and pave the way to computational studies exploring the effect of water monolayers on the adsorption of small molecules on wet gold surfaces.

Mechanism of Producing Metallic Nanoparticles, with an Emphasis on Silver and Gold Nanoparticles, Using Bottom-Up Methods

Bottom-up nanoparticle (NP) formation is assumed to begin with the reduction of the precursor metallic ions to form zero-valent atoms. Studies in which this assumption was made are reviewed. The standard reduction potential for the formation of aqueous metallic atoms-E0(Mn+aq/M0aq)-is significantly lower than the usual standard reduction potential for reducing metallic ions Mn+ in aqueous solution to a metal in solid state. E0(Mn+aq/M0solid). E0(Mn+aq/M0aq) values are negative for many typical metals, including Ag and Au, for which E0(Mn+aq/M0solid) is positive. Therefore, many common moderate reduction agents that do not have significantly high negative reduction standard potentials (e.g., hydrogen, carbon monoxide, citrate, hydroxylamine, formaldehyde, ascorbate, squartic acid, and BH4-), and cannot reduce the metallic cations to zero-valent atoms, indicating that the mechanism of NP production should be reconsidered. Both AgNP and AuNP formations were found to be multi-step processes that begin with the formation of clusters constructed from a skeleton of M+-M+ (M = Ag or Au) bonds that is followed by the reduction of a cation M+ in the cluster to M0, to form Mn0 via the formation of NPs. The plausibility of M+-M+ formation is reviewed. Studies that suggest a revised mechanism for the formation of AgNPs and AuNPs are also reviewed.

Effects of Conformational Variation on Structural Insights from Solution-Phase Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman scattering (SERS) spectra contain information on the chemical structure on nanoparticle surfaces through the position and alignment of molecules with the electromagnetic near field. Time-dependent density functional theory (TDDFT) can provide the Raman tensors needed for a detailed interpretation of SERS spectra. Here, the impact of molecular conformations on SERS spectra is considered. TDDFT calculations of the surfactant cetyltrimethylammonium bromide with five conformers produced more accurate unenhanced Raman spectra than a simple all-trans structure. The calculations and measurements also demonstrated a loss of structural information in the CH2/CH3 scissor vibration band at 1450 cm-1 in the SERS spectra. To study lipid bilayers, TDDFT calculations on conformers of methyl phosphorylcholine and cis-5-decene served as models for the symmetric choline stretch in the lipid headgroup and the C═C stretch in the acyl chains of 1,2-oleoyl-glycero-3-phosphocholine. Conformer considerations enabled a measurement of the distribution of double-bond orientations with an order parameter of SC═C = 0.53.

Solvent Quality-Mediated Regioselective Modification of Gold Nanorods with Thiol-Terminated Polymers

Modification of nanorods (NRs) with functional polymer ligands is of great significance to enhance their surface chemistry and prompt their applications in many fields (e.g., photothermal therapy, bioimaging, and catalysis). However, the regioselective modification of AuNRs still remains a great challenge. Herein, we introduce a facile yet versatile strategy to achieve the regioselective modification of AuNRs through a solvent quality-mediated strategy. By employing a poor solvent of the original ligand cetyltrimethylammonium bromide (CTAB) as the medium in the modification, polymer ligands would selectively graft onto the two ends of AuNRs, while polymer ligands would graft onto the entire surface when employing a good solvent. This strategy demonstrates good reproducibility and is applicable to both hydrophilic and hydrophobic polymer ligand modifications. Moreover, by combing our strategy with the preoccupation route, the two ends and sidewall of AuNRs modified by two different polymers form an "ABA"-type building block, which can further self-assemble into well-ordered superstructures. Our finding provides a new opportunity for multifunctionalization of NRs.

Ligand-Dependent Coalescence Behaviors of Gold Nanoparticles Studied by Multichamber Graphene Liquid Cell Transmission Electron Microscopy

The formation mechanism of colloidal nanoparticles is complex because significant nonclassical pathways coexist with the conventional nucleation and growth processes. Particularly, the coalescence of the growing clusters determines the final morphology and crystallinity of the synthesized nanoparticles. However, the experimental investigation of the coalescence mechanism is a challenge because the process is highly kinetic and correlates with surface ligands that dynamically modify the surface energy and the interparticle interactions of nanoparticles. Here, we employ quantitative in situ TEM with multichamber graphene liquid cell to observe the coalescence processes occurring in the synthesis of gold nanoparticles in different ligand systems, thus affording us an insight into their ligand-dependent coalescence kinetics. The analyses of numerous liquid-phase TEM trajectories of the coalescence and MD simulations of the ligand shells demonstrate that enhanced ligand mobility, employing a heterogeneous ligand mixture, results in the rapid nanoparticle pairing approach and a fast post-merging structural relaxation.

Selective shortening of gold nanorods: when surface functionalization dictates the reactivity of nanostructures

The selective shortening of gold nanorods (NRs) is a directional etching process that has been intensively studied by UV-Vis spectroscopy because of its direct impact on the optical response of these plasmonic nanostructures. Here, liquid-cell transmission electron microscopy is exploited to visualize this peculiar corrosion process at the nanoscale and study the impacts of reaction kinetics on the etching mechanisms. In situ imaging reveals that anisotropic etching requires a chemical environment with a low etching power to make the tips of NRs the only reaction site for the oxidation process. Then, aberration-corrected TEM and atomistic simulations were combined to demonstrate that the disparity between the reactivity of the body and the ends of NRs does not derive from their crystal structure but results from an inhomogeneous surface functionalization. In a general manner, this work highlights the necessity to consider the organic/inorganic natures of nanostructures to understand their chemical reactivity.

Direct Observation of Adsorption Morphologies of Cationic Surfactants at the Gold Metal-Liquid Interface

Understanding interfacial phenomena is important in processes like corrosion, catalysis, and electrochemical reactions. Specifically, in corrosion inhibition, the assembly of adsorbed surfactants at metal-water interfaces in well-packed, ordered layers is desired. We provide direct evidence of the role of alkyl tails of surfactants in the formation of ordered adsorbed layers at metal-water interfaces. We have employed surface-specific sum frequency generation (SFG) spectroscopy to probe the in situ adsorption and self-assembly of cationic surfactants, alkyldimethylbenzyl ammonium bromides of tail lengths n = 4 (C4) and 12 (C12), without any applied potential or stimulus, at the gold-water interface. Our SFG measurements show that C12 Quat adsorbs as an ordered monolayer, whereas C4 Quat adsorbs in a disordered monolayer. All-atom molecular dynamics (MD) simulations of these surfactants corroborate with SFG results. These findings provide new insights on how hydrophobic interactions between alkyl tails of surfactants affect their self-assembly at metal-water interfaces.

Uniform, Anticorrosive, and Antiabrasive Coatings on Metallic Surfaces for Cation-Metal and Cation-π Interactions

Metals are widely used, from daily life to modern industry. Great efforts have been made to protect the metals with various coatings. However, the well-known conventional electrochemical corrosion induced by cations and the ubiquitous nature of the coffee-ring effect make these processes very difficult. Here, a scheme by two bridges of cations and ethylenediamine (EDA) is proposed to overcome the coffee-ring effect and electrochemical corrosion and experimentally achieve uniform, anticorrosive, and antiabrasive coatings on metallic surfaces. Anticorrosive capability reaches about 26 times higher than that without cation-controlled coatings at 12 h in extremely acidic, high-temperature, and high-humidity conditions and still enhances to 2.7 times over a week. Antiabrasive capability also reaches 2.5 times. Theoretical calculations show that the suspended materials are uniformly adsorbed on the surface mediated by complexed cations through strong cation-metal and cation-π interactions. Notably, the well-known conventional electrochemical corrosion induced by cations is avoided by EDA to control cations solubility in different coating processes. These findings provide a new efficient, cost-effective, facile, and scalable method to fabricate protective coatings on metallic materials and a methodology to study metallic nanostructures in solutions, benefitting practical applications including coatings, printing, dyeing, electrochemical protection, and biosensors.

Tuning and tracking the growth of gold nanoparticles synthesized using binary surfactant mixtures

Synthesis of gold nanorods (Au NRs) using surfactant-mediated seeded growth involves the interplay of parameters such as pH, reducing agent, and surfactant among others. The use of binary surfactant mixtures of cetyltrimethylammonium bromide (CTAB) and oleic acid (OA) has been reported by our group previously to obtain other anisotropic shapes. However, there are no reports investigating the growth kinetics and mechanisms of such shapes. Here, we report for the first time a ternary representation for compact visualization of shape transitions of gold nanoparticles (Au NPs) as a function of reaction parameters. Further, using UV-Vis spectrophotometry, the growth kinetics of these shapes was tracked using an in-house developed technique. The interplay between the experimental parameters and the properties of Au NPs was investigated using statistical analysis which showed that the reducing agent and pH were significant in influencing shape and growth kinetics. We further propose a growth mechanism in which the supersaturation of growth units controls the final shapes obtained.

Growth Mechanism of Gold Nanorods: the Effect of Tip-Surface Curvature As Revealed by Molecular Dynamics Simulations

An understanding of the anisotropic growth mechanism of gold nanorods (AuNRs) during colloidal synthesis is critical for controlling the nanocrystal size and shape and thus has implications in tuning the properties for applications in a wide range of research and technology fields. In order to investigate the role of the cetyltrimethylammonium bromide (CTAB) coating in the anisotropic growth mechanism of AuNRs, we used molecular dynamics (MD) simulations and built a computational model that considered explicitly the effect of the curvature of the gold surface on CTAB adsorption and therefore differentiated between the CTAB arrangements on flat and curved surfaces, representing the lateral and tip facets of growing AuNRs, respectively. We verified that on a curved surface, a lower CTAB coverage density and larger intermicellar channels are generated compared to those on a flat surface. Using umbrella sampling simulations, we measured the free energy profile and verified that the environment around a curved surface corresponds to an easier migration from the solution to the gold surface for the [AuBr₂]^- species than does a flat surface. Long unbiased molecular dynamics simulations also corroborated the umbrella sampling results. Therefore, the [AuBr₂]^- diffusion through the environment of the tips is much more favorable than that in the case of lateral facets. This shows that the surface curvature is an essential component of the anisotropic growth mechanism.

Solution synthesis of anisotropic gold microcrystals

Amplification of pentahedrally twinned gold nanorods with Au(i)/AA results in the formation of very well-defined anisotropic microcrystals of gold referred to as gold microrods.

Coating Mechanism of AuNPs onto Sepiolite by Experimental Research and MD Simulation

The amenability of gold nanoparticles (AuNPs) coating on natural and modified (hexadecyl trimethyl ammonium bromide, CTAB) sepiolite surfaces was studied both experimentally and theoretically. The zeta potential experiments and Fourier transform infrared spectrophotometer (FTIR), environmental scanning electron microscope (ESEM), and transmission electron microscopy (TEM) analyses were carried out with the sepiolite samples in the presence of AuNPs. In addition, the adsorption of three gold-nanoparticles on the sepiolite surface (100) in the absence and presence of CTAB was investigated by molecular dynamics (MD) simulations. The AuNPs showed no significant change in the zeta potential of natural sepiolite surfaces due to negative charges of both the sepiolite and AuNPs at natural pH. The surface charge of modified sepiolite decreased with the increase in AuNPs concentration indicating the significance AuNPs adsorption. FTIR, ESEM, and TEM analyses indicated the coating of AuNPs onto the modified sepiolite surface were higher than that of the natural sepiolite surface. The MD simulation results showed that AuNPs can easily adsorb onto the basal surface of the sepiolite due to its hydrophilicity in the presence and absence of CTAB as indicated in the experimental studies. In short, the modification of sepiolite with CTAB made the charge positive, and in turn considerably increased the AuNPs coating on sepiolite surfaces due to electrostatic attraction.

Quantitative Imaging of Organic Ligand Density on Anisotropic Inorganic Nanocrystals

A longstanding challenge in nanoparticle characterization is to understand anisotropic distributions of organic ligands at the surface of inorganic nanoparticles. Here, we show that using electron energy loss spectroscopy in an aberration-corrected scanning transmission electron microscope we can directly visualize and quantify ligand distributions on gold nanorods (AuNRs). These experiments analyze dozens of particles on graphene substrates, providing insight into how ligand binding densities vary within and between individual nanoparticles. We demonstrate that the distribution of cetyltrimethylammonium bromide (CTAB) on AuNRs is anisotropic, with a 30% decrease in ligand density at the poles of the nanoparticles. In contrast, the distribution of (16-mercaptohexadecyl)trimethylammonium bromide (MTAB) is more uniform. These results are consistent with literature reported higher reactivity at the ends of CTAB-coated AuNRs. Our results demonstrate the impact of electron spectroscopy to probe molecular distributions at soft-hard interfaces and how they produce spatially heterogeneous properties in colloidal nanoparticles.

Role of Gluten in Surface Chemistry: Nanometallic Bioconjugation of Hard, Medium, and Soft Wheat Protein

Hard, medium, and soft wheat proteins, based on gluten content, were studied for their important roles in nanometallic surface chemistry. In situ synthesis of Au nanoparticles (NPs) was followed to determine the surface adsorption behavior of wheat protein based on the gluten contents. A greater amount of gluten contents facilitated the nucleation to produce Au NPs. X-ray photoelectron spectroscopy (XPS) surface analysis clearly showed the surface adsorption of protein on nanometallic surfaces which was almost equally prevalent for the hard, medium, and soft wheat proteins. Wheat protein conjugated NPs were highly susceptible to phase transfer from aqueous to organic phase that was entirely related to the amount of gluten contents. The presence of higher gluten content in hard wheat protein readily enabled the hard wheat protein conjugated NPs to move across the aqueous-organic interface followed by medium and soft wheat protein conjugated NPs. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS page) analysis allowed us to determine molar masses of nanometallic surface adsorbed protein fractions. Only two protein fractions of high molar masses (74 and 85 kDa) from SDS solubilized hard, medium, and soft wheat proteins preferred to adsorb on nanometallic surfaces out of more than 15 protein fractions of pure wheat protein. This made the surface adsorption of wheat protein highly selective and closely related to gluten content. Cetyltrimethylammonium bromide (CTAB) solubilized wheat protein conjugated NPs demonstrated their strong antimicrobial activities against both Gram negative and Gram positive bacteria making them suitable for their applications in food industry.

Disconnecting Symmetry Breaking from Seeded Growth for the Reproducible Synthesis of High Quality Gold Nanorods

One of the major difficulties hindering the widespread application of colloidal anisotropic plasmonic nanoparticles is the limited robustness and reproducibility of multistep synthetic methods. We demonstrate herein that the reproducibility and reliability of colloidal gold nanorod (AuNR) synthesis can be greatly improved by disconnecting the symmetry-breaking event from the seeded growth process. We have used a modified silver-assisted seeded growth method in the presence of the surfactant hexadecyltrimethylammonium bromide and n-decanol as a co-surfactant to prepare small AuNRs in high yield, which were then used as seeds for the growth of high quality AuNR colloids. Whereas the use of n-decanol provides a more-rigid micellar system, the growth on anisotropic seeds avoids sources of irreproducibility during the symmetry breaking step, yielding uniform AuNR colloids with narrow plasmon bands, ranging from 600 to 1270 nm, and allowing the fine-tuning of the final dimensions. This method provides a robust route for the preparation of high quality AuNR colloids with tunable morphology, size, and optical response in a reproducible and scalable manner.

Growth Mechanism of Five-Fold Twinned Ag Nanowires from Multiscale Theory and Simulations

Five-fold twinned metal nanowires can be synthesized with high aspect ratios via solution-phase methods. The origins of their anisotropic growth, however, are poorly understood. We combine atomic-scale, mesoscale, and continuum theoretical methods to predict growth morphologies of Ag nanowires from seeds and to demonstrate that high aspect ratio nanowires can originate from anisotropic surface diffusion induced by the strained nanowire structure. Nanowire seeds are similar to Marks decahedra, with {111} "notches" that accelerate diffusion along the nanowire axis to facilitate one-dimensional growth. The strain distribution on the {111} facets induces heterogeneous atom aggregation and leads to atom trapping at the nanowire ends. We predict that decahedral Ag seeds can grow to become nanowires with aspect ratios in the experimental range. Our studies show that there is a complex interplay between atom deposition, diffusion, seed architecture, and nanowire aspect ratio that could be manipulated experimentally to achieve controlled nanowire syntheses.

Understanding Anisotropic Growth of Au Penta-Twinned Nanorods by Liquid Cell Transmission Electron Microscopy

Morphology control of anisotropic nanocrystals is important for tuning shape-dependent physicochemical properties, but the anisotropic growth mechanism remains unclear. Here we investigate the formation of the Au penta-twinned nanorod by liquid cell transmission electron microscopy. It is found that a truncated decahedron forms in the absence of cetyltrimethylammonium bromide (CTAB), whereas in the presence of CTAB, a penta-twinned nanorod forms by producing {100} facets via the reentrant groove and selectively inhibiting atom addition to {100} facets. Density functional theory simulations show that the partial relief of strain energy of the decahedron is caused by the adsorption of Br^- ions. Moreover, the selective adsorption of CTAB lowers the surface energy of {100} facets, thus enhancing the nanorod growth. Our work points out the importance of the synergy of strain and surface energy, which provide an in-depth insight into the anisotropic growth of nanorods and lay foundations for the controlled synthesis of nanomaterials.

AOT Bilayer Adsorption on Gold Surfaces: A Molecular Dynamics Study

A molecular dynamics study was done to reveal the adsorption properties of sodium dioctyl sulfosuccinate (AOT) bilayers on gold Au(111) surfaces. Examining the rotational mobility of AOT molecules, we track that the correlation time of AOT molecules on the adsorbed layer is much higher. The data estimating the diffusive motion of AOT molecule show a substantially lower rate of diffusion (∼10^-10 cm²/s) in the adsorbed layers in comparison to other ones. The results show that an adsorbed layer is more rigid, whereas the outer layers undergo considerable lateral and vertical fluctuations.

Radiative Cooling of Surface-Modified Gold Nanorods upon Pulsed Infrared Photoexcitation

Transient infrared emissions of gold nanorods capped with various materials (AuNR@X, X = CTAB, PSS, mPEG, and SiO₂) upon ∼70-μs pulsed 1064 nm excitation of their longitudinal surface plasmonic bands were collected with a time-resolved step-scan Fourier-transform spectrometer. Comparing the observed emission contours with the blackbody radiation spectra revealed that parts of the additional emission intensity at low wavenumbers (1300-1000 cm^-1) were attributed to the vibrational modes of the capping materials, suggesting that the photothermal energy of AuNRs can be thermalized not only via blackbody radiation but also via radiative and nonradiative processes of the capping materials. In addition, the infrared emission of AuNR@SiO₂ was more prolonged (∼1 ms) than those of the other three (∼300 μs). The photothermal energy can be efficiently randomized to the internal degrees of freedom of the soft molecular capping materials but can be stored by the rigid ones, for example, SiO₂, followed by extended radiative cooling.

The Many "Facets" of Halide Ions in the Chemistry of Colloidal Inorganic Nanocrystals

Over the years, scientists have identified various synthetic "handles" while developing wet chemical protocols for achieving a high level of shape and compositional complexity in colloidal nanomaterials. Halide ions have emerged as one such handle which serve as important surface active species that regulate nanocrystal (NC) growth and concomitant physicochemical properties. Halide ions affect the NC growth kinetics through several means, including selective binding on crystal facets, complexation with the precursors, and oxidative etching. On the other hand, their presence on the surfaces of semiconducting NCs stimulates interesting changes in the intrinsic electronic structure and interparticle communication in the NC solids eventually assembled from them. Then again, halide ions also induce optoelectronic tunability in NCs where they form part of the core, through sheer composition variation. In this review, we describe these roles of halide ions in the growth of nanostructures and the physical changes introduced by them and thereafter demonstrate the commonality of these effects across different classes of nanomaterials.

Prediction of Hydrophobic Reagent for Flotation Process Using Molecular Modeling

The interaction or nonbonded energies of base organic ions and water molecules during the flotation process of minerals have important meanings for organizing hydrophobic and stable collectors. Furthermore, the interaction, cross-term, and valence energies of optimized structures are important for understanding the properties and structures of selective collectors. The simulation of pure scheelite mineral (PSM) surfaces with four different negative ions, using an adsorption locator module is demonstrated. The interaction energies for base organic ions and water molecules were resolved and detected by shaping the best hydrophobic interaction and the most stable suspension over the PSM surface (112) and (101). The adsorption locator results for base organic ions and water molecules on PSM surfaces (112) and (101) using buffer width 0.5 Å and temperature range from 318.15 to 283.15 K confirmed the results obtain from Forcite calculations. The results have demonstrated that the possibilities of using consistent valence force field implemented by Forcite and adsorption locator modules in the selection of flotation reagents are cost saving. Furthermore, hydrophobicity of the main negative ions in soaps were solved by the simulation methods and results are in a good agreement with the experimental methods that proved that mustard soap is more selective on the mineral surfaces than sunflower soap when used as a collector. Increasing the molecular weight of negative ions increases the interaction energy between base collector ions and PSM surfaces (112) and (101) significantly.