Interactions and Attachment Pathways between Functionalized Gold Nanorods

Intra-nanoparticle plasmonic nanogap based spatial-confinement SERS analysis of polypeptides

Sensing and characterizing water-soluble polypeptides are essential in various biological applications. However, detecting polypeptides using Surface-Enhanced Raman Scattering (SERS) remains a challenge due to the dominance of aromatic amino acid residues and backbones in the signal, which hinders the detection of non-aromatic amino acid residues. Herein, intra-nanoparticle plasmonic nanogap were designed by etching the Ag shell in Au@AgNPs (i.e., obtaining AuAg cores) with chlorauric acid under mild conditions, at the same time forming the outermost Au shell and the void between the AuAg cores and the Au shell (AuAg@void@Au). By varying the Ag to added chloroauric acid molar ratios, we pioneered a simple, controllable, and general synthetic strategy to form interlayer-free nanoparticles with tunable Au shell thickness, achieving precise regulation of electric field enhancement within the intra-nanogap. As validation, two polypeptide molecules, bacitracin and insulin B, were successfully synchronously encapsulated and spatial-confined in the intra-nanogap for sensing. Compared with concentrated 50 nm AuNPs and Au@AgNPs as SERS substrates, our simultaneous detection method improved the sensitivity of the assay while benefiting to obtain more comprehensive characteristic peaks of polypeptides. The synthetic strategy of confining analytes while fabricating plasmonic nanostructures enables the diffusion of target molecules into the nanogap in a highly specific and sensitive manner, providing the majority of the functionality required to achieve peptide detection or sequencing without the hassle of labeling.

Cysteamine Chemisorption at Mercury-Solution Interfaces in the Context of Redox and Microdissociation Equilibria

The redox behavior and chemisorption of cysteamine (CA) at a charged mercury surface are described, with an emphasis on its acid-base properties supported by molecular dynamics and quantum mechanical calculations. It was found that CA forms chemisorbed layers on the surface of the mercury electrode. The formation of Hg-CA complexes is connected to mercury disproportionation, as reflected in peaks SII and SI at potentials higher than the electrode potential of zero charge (p.z.c.). Both the process of chemisorption of CA and its consequent redox transformation are proton-dependent. Also, depending on the protonation of CA, the formation of typical populations of chemisorbed conformers can be observed. In addition, cystamine (CA disulfide dimer) can be reduced on the mercury surface. Between the potentials of this reduction and peak SI, the p.z.c. of the electrode used can be found. Furthermore, CA can serve as an LMW catalyst for hydrogen evolution. The mechanistic insights presented here can be used for follow-up research on CA chemisorption and targeted modification of other metallic surfaces.

Thiols Modulated Gold Nanorods Self-Assembly: Indirect Hydrophobic Effects Instead of Direct Electrostatic/Hydrogen Bonds Attraction

For nanocrystals (NCs) self-assembly, understanding the chemical and supramolecular interactions among building blocks is significant for both fundamental scientific interests and rational nanosuperstructure construction. However, it has remained an extreme challenge for many self-assembly systems due to the lack of appropriately quantitative approaches for the corresponding exploration. Herein, by combination of the proposed colorimetric method for cationic surfactant quantitation and all-atom simulations, we manage to present a clear chemical picture for the thiol molecules modulated self-assembly of gold nanorods (GNRs), one of the earliest and most convenient methods for the fabrication of freestanding GNR self-assemblies. It is revealed that the self-assembly of GNRs is driven by the hydrophobic effects of the alkyl chains of the modified cationic surfactants, as their bilayer structure is destroyed by the added thiol molecules. In other words, the actual roles of the thiol molecules for causing GNRs assembly are indirectly inductive effects instead of the previously believed direct electrostatic attraction and/or hydrogen-bond linking effects of the binding thiol molecules. Furthermore, the GNRs exhibit diameter-dependent assembly behaviors: thicker GNRs tend to adopt the end-to-end assembly mode, while thin ones prefer the side-by-side assembly mode, further demonstrating that hydrophobic effects among the build blocks are the driving force for the GNRs assembly.

Observing the Evolution of Metal Oxides in Liquids

Metal oxides with diverse compositions and structures have garnered considerable interest from researchers in various reactions, which benefits from transmission electron microscopy (TEM) in determining their morphologies, phase, structural and chemical information. Recent breakthroughs have made liquid-phase TEM a promising imaging platform for tracking the dynamic structure, morphology, and composition evolution of metal oxides in solution under work conditions. Herein, this review introduces the recent advances in liquid cells, especially closed liquid cell chips. Subsequently, the recent progress including particle growth, phase transformation, self-assembly, core-shell nanostructure growth, and chemical etching are introduced. With the late technical advances in TEM and liquid cells, liquid-phase TEM is used to characterize many fundamental processes of metal oxides for CO2 reduction and water-splitting reactions. Finally, the outlook and challenges in this research field are discussed. It is believed this compilation inspires and stimulates more efforts in developing and utilizing in situ liquid-phase TEM for metal oxides at the atomic scale for different applications.

Nanoparticle Size Influences the Self-Assembly of Gold Nanorods Using Flexible Streptavidin-Biotin Linkages

The self-assembly of colloidal nanocrystals remains of robust interest due to its potential in creating hierarchical nanomaterials that have advanced function. For gold nanocrystals, junctions between nanoparticles yield large enhancements in local electric fields under resonant illumination, which is suitable for surface-enhanced spectroscopies for molecular sensors. Gold nanorods can provide such plasmonic fields at near-infrared wavelengths of light for longitudinal excitation. Through the use of careful concentration and stoichiometric control, a method is reported herein for selective biotinylation of the ends of gold nanorods for simple, consistent, and high-yielding self-assembly upon addition of the biotin-binding protein streptavidin. This method was applied to four different sized nanorods of similar aspect ratio and analyzed through UV-vis spectroscopy for qualitative confirmation of self-assembly and transmission electron microscopy to determine the degree of self-assembly in end-linked nanorods. The yield of end-linked assemblies approaches 90% for the largest nanorods and approaches 0% for the smallest nanorods. The number of nanorods linked in one chain also increases with an increased nanoparticle size. The results support the notion that the lower ligand density at the ends of the larger nanorods yields preferential substitution reactions at those ends and hence preferential end-to-end assembly, while the smallest nanorods have a relatively uniform ligand density across their surfaces, leading to spatially random substitution reactions.

Capping Layer Determined Self-assembly of Au-Ag Bimetallic Janus Nanoparticles at An Oil/Water Interface by Molecular Dynamics Simulations

Bimetallic Janus nanoparticles (BJNPs) have gained more attention due to their unique catalytic and optical properties. The self-assembly of BJNPs is expected as an effective way to fabricate metamaterials suitable for different potential applications. However, the self-assembly dynamic process of BJNPs, which is key to achieving a controllable synthesis, is limited in both experimental and theoretical investigations. Herein, all-atom molecular dynamics (MD) simulations were employed to investigate the self-assembly process of 1-dodecanethiol (DDT)-decorated Au-Ag BJNPs at an oil-water interface. We demonstrate that DDT's van der Waals (vdW) interaction dominates the self-assembly process. BJNPs form close-packed structures at both fast and slow evaporation rates. Au-Ag BJNPs exhibit relatively larger rotations at a low evaporation rate than those at a high evaporation rate, suggesting that the evaporation rate influences the orientation of the Au-Ag BJNPs. BJNPs tend to orient their electric dipole moments toward the external electric field, according to the ab initio MD simulation results. Based on the energy comparison and model analysis, it is found that the parallel array is more stable than the antiparallel one for the Au-Ag BJNP arrays. The dipole-dipole interaction difference between the parallel and antiparallel BJNP arrays obtained according to dipole moment obtained from ab initio calculation is qualitatively consistent with that obtained from MD simulations, indicating that the dipole plays a decisive role in determining the orientation of the BJNP array. This work uncovers the self-assembly dynamic process of BJNPs, paving the way for future applications.

Necklace-Like Nanostructures: From Fabrication, Properties to Applications

The shape-controlled synthesis of nanocrystals remains a hot research topic in nanotechnology. Particularly, the fabrication of 1D structures such as wires, rods, belts, and tubes has been an interesting and important subject within nanoscience in the last few decades. 1D necklace-like micro/nanostructures are a sophisticated geometry that has attracted increasing attention due to their anisotropic and periodic structure, intrinsic high surface area, abundant transport channels, exposure of each component to the surface, and multiscale roughness of the surface. These characteristics enable their unique electrical, optical, and catalytic properties. This review provides a comprehensive summary of the advanced research progress on the fabrication strategies, novel properties, and various applications of necklace-like structures. It begins with the main fabrication methods of necklace-like structures and subsequently details a variety of their properties and applications. It concludes with the authors' perspectives on future research and development of the necklace-like structures.

Microalgae Bioactive Carbohydrates as a Novel Sustainable and Eco-Friendly Source of Prebiotics: Emerging Health Functionality and Recent Technologies for Extraction and Detection

There is a global interest in the novel consumption, nutritional trends, and the market of new prebiotic sources and their potential functional impacts. Commercially available nutritional supplements based on microalgae that are approved to be edible by FDA, like Arthrospira platensis (Cyanobacteria) and Chlorella vulgaris (Chlorophyta) become widely attractive. Microalgae are rich in carbohydrates, proteins, and polyunsaturated fatty acids that have high bioactivity. Recently, scientists are studying the microalgae polysaccharides (PS) or their derivatives (as dietary fibers) for their potential action as a novel prebiotic source for functional foods. Besides, the microalgae prebiotic polysaccharides are used for medication due to their antioxidant, anticancer, and antihypertensive bioactivities. This review provides an overview of microalgae prebiotics and other macromolecules’ health benefits. The phytochemistry of various species as alternative future sources of novel polysaccharides were mentioned. The application as well as the production constraints and multidisciplinary approaches for evaluating microalgae phytochemistry were discussed. Additionally, the association between this potential of combining techniques like spectroscopic, chromatographic, and electrochemical analyses for microalgae sensation and analysis novelty compared to the chemical methods was emphasized.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

Synthesis, Assembly, Optical Properties, and Sensing Applications of Plasmonic Gap Nanostructures

Plasmonic gap nanostructures (PGNs) have been extensively investigated mainly because of their strongly enhanced optical responses, which stem from the high intensity of the localized field in the nanogap. The recently developed methods for the preparation of versatile nanogap structures open new avenues for the exploration of unprecedented optical properties and development of sensing applications relying on the amplification of various optical signals. However, the reproducible and controlled preparation of highly uniform plasmonic nanogaps and the prediction, understanding, and control of their optical properties, especially for nanogaps in the nanometer or sub-nanometer range, remain challenging. This is because subtle changes in the nanogap significantly affect the plasmonic response and are of paramount importance to the desired optical performance and further applications. Here, recent advances in the synthesis, assembly, and fabrication strategies, prediction and control of optical properties, and sensing applications of PGNs are discussed, and perspectives toward addressing these challenging issues and the future research directions are presented.

Revealing Reactions between the Electron Beam and Nanoparticle Capping Ligands with Correlative Fluorescence and Liquid-Phase Electron Microscopy

Liquid-phase transmission electron microscopy (LP-TEM) enables real-time imaging of nanoparticle self-assembly, formation, and etching with single nanometer resolution. Despite the importance of organic nanoparticle capping ligands in these processes, the effect of electron beam irradiation on surface-bound and soluble capping ligands during LP-TEM imaging has not been investigated. Here, we use correlative LP-TEM and fluorescence microscopy (FM) to demonstrate that polymeric nanoparticle ligands undergo competing crosslinking and chain scission reactions that nonmonotonically modify ligand coverage over time. Branched polyethylenimine (BPEI)-coated silver nanoparticles were imaged with dose-controlled LP-TEM followed by labeling their primary amine groups with fluorophores to visualize the local thickness of adsorbed capping ligands. FM images showed that free ligands crosslinked in the LP-TEM image area over imaging times of tens of seconds, enhancing local capping ligand coverage on nanoparticles and silicon nitride membranes. Nanoparticle surface ligands underwent chain scission over irradiation times of minutes to tens of minutes, which depleted surface ligands from the nanoparticle and silicon nitride surface. Conversely, solutions of only soluble capping ligand underwent successive crosslinking reactions with no chain scission, suggesting that nanoparticles enhanced the chain scission reactions by acting as radiolysis hotspots. The addition of a hydroxyl radical scavenger, tert-butanol, eliminated chain scission reactions and slowed the progression of crosslinking reactions. These experiments have important implications for performing controlled and reproducible LP-TEM nanoparticle imaging as they demonstrate that the electron beam can significantly alter ligand coverage on nanoparticles in a nonintuitive manner. They emphasize the need to understand and control the electron beam radiation chemistry of a given sample to avoid significant perturbations to the nanoparticle capping ligand chemistry, which are invisible in electron micrographs.

Programmable Self-Assembly of Gold Nanoarrows via Regioselective Adsorption

Programing the self-assembly of colloidal nanoparticles into predetermined superstructures represents an attractive strategy to realize functional assemblies and novel nanodevices, but it remains a challenge. Herein, gold nanoarrows (GNAs) showing a distinct convex-concave structure were employed as unique building blocks for programmable self-assembly involving multiple assembly modes. Regioselective adsorption of 1,10-decanedithiol on the vertexes, edges, and facets of GNAs allowed for programmable self-assembly of GNAs with five distinct assembly modes, and regioselective blocking with 1-dodecanethiol followed by adsorption of 1,10-decanedithiol gave rise to programmable self-assembly with six assembly modes including three novel wing-engaged modes. The assembly mode was essentially determined by regioselective adsorption of the dithiol linker dictated by the local curvature together with the shape complementarity of GNAs. This approach reveals how the geometric morphology of nanoparticles affects their regioselective functionalization and drives their self-assembly.

Janus bimetallic nanorod clusters-poly(aniline) nanocomposites with temperature-responsiveness for Raman scattering-based biosensing

Herein, Janus bimetallic nanorod clusters-poly(aniline) nanocomposites (JRCPCs) with gold nanorod clusters (GNRCs) in side-by-side (SBS) or end-to-end (ETE) configuration are synthesized, and applied to surface-enhanced Raman scattering (SERS)-based biosensing of carcinoembryonic antigen (CEA). Taking advantage of their geometrical and chemical anisotropy, GNRCs in both SBS and ETE configurations are prepared by addition of negatively charged citrate anions and poly(acrylic acid)-block-poly(N-isopropylacrylamide) (PAAc-b-PNIPAM), respectively, to electrostatically interact with cationic cetyltrimethylammonium bromide surfactant on the side of the gold nanorods (GNRs). Subsequently, the JRCPCs are prepared by unidirectional growth of polyaniline and additional growth of Ag onto these GNRCs. JRCPCs with GNRCs in either the SBS or the ETE configuration show strong enhancement of electromagnetic field at both GNR aggregates and GNRC core-Ag shell gaps of bimetallic nanorod cluster components. In particular, because temperature-responsive PAAc-b-PNIPAM of JRCPCs is embedded at GNR junctions, interparticle gaps generated in GNRCs in ETE configuration are controlled via temperature-triggered hydration-dehydration of the PAAc-b-PNIPAM chains such that optical properties are largely changed. With distinct surface functionalities from JRCPCs, SERS-based quantitative analysis of CEA is achieved using JRCPCs as SERS nanoprobes. This work presents the great potential of advanced Janus nanocomposites for SERS-based biosensing applications.

Ligand-Induced Motion and Self-Assembly Pathways between Nanocubes

Nanoparticle motion and self-assembly have been regarded as a promising pathway for forming ordered nanostructures. However, the detailed dynamics processes induced by ligand involvement remained poorly understood. Here, we used in situ liquid-cell electron microscopy technology to image the formation of face-to-face Pt cube ordered structures: pairs, linear chains, and squares. The van der Waals interaction between the two neighboring cubes was quantified in real time. Interestingly, the two different formation processes of the square phase were achieved via a rotational and translational method. It is found that the space between two neighboring cubes was the same as the ex-TEM results. The density functional theory calculation demonstrated that it was attributed to the DMF ligand interactions of the cubes that promoted their face-to-face attachment.

Atomic imaging of the motion and transformation of Pt3Ni nanoparticles in liquids

In this work, we used liquid cell TEM to observe the motion and transformation pathways of Pt₃Ni nanoparticles in solution by systematically changing the electron beam dose rate.

In Situ Tracking of Colloidally Stable and Ordered Assemblies of Gold Nanorods

Solution-phase self-assembly of anisotropic nanoparticles into complex 2D and 3D assemblies is one of the most promising strategies toward obtaining nanoparticle-based materials and devices with unique optical properties at the macroscale. However, controlling this process with single-particle precision is highly demanding, mostly due to insufficient understanding of the self-assembly process at the nanoscale. We report the use of in situ environmental scanning transmission electron microscopy (WetSTEM), combined with UV/vis spectroscopy, small-angle X-ray diffraction (SAXRD) and multiscale modeling, to draw a detailed picture of the dynamics of vertically aligned assemblies of gold nanorods. Detailed understanding of the self-assembly/disassembly mechanisms is obtained from real-time observations, which provide direct evidence of the colloidal stability of side-to-side nanorod clusters. Structural details and the forces governing the disassembly process are revealed with single particle resolution as well as in bulk samples, by combined experimental and theoretical modeling. In particular, this study provides unique information on the evolution of the orientational order of nanorods within side-to-side 2D assemblies and shows that both electrostatic (at the nanoscale) and thermal (in bulk) stimuli can be used to drive the process. These results not only give insight into the interactions between nanorods and the stability of their assemblies, thereby assisting the design of ordered, anisotropic nanomaterials but also broaden the available toolbox for in situ tracking of nanoparticle behavior at the single-particle level.

Tracking the Effects of Ligands on Oxidative Etching of Gold Nanorods in Graphene Liquid Cell Electron Microscopy

Surface ligands impact the properties and chemistry of nanocrystals, but observing ligand binding locations and their effect on nanocrystal shape transformations is challenging. Using graphene liquid cell electron microscopy and the controllable, oxidative etching of gold nanocrystals, the effect of different ligands on nanocrystal etching can be tracked with nanometer spatial resolution. The chemical environment of liquids irradiated with high-energy electrons is complex and potentially harsh, yet it is possible to observe clear evidence for differential binding properties of specific ligands to the nanorods' surface. Exchanging CTAB ligands for PEG-alkanethiol ligands causes the nanorods to etch at a different, constant rate while still maintaining their aspect ratio. Adding cysteine ligands that bind preferentially to nanorod tips induces etching predominantly on the sides of the rods. This etching at the sides leads to Rayleigh instabilities and eventually breaks apart the nanorod into two separate nanoparticles. The shape transformation is controlled by the interplay between atom removal and diffusion of surface atoms and ligands. These in situ observations are confirmed with ex situ colloidal etching reactions of gold nanorods in solution. The ability to monitor the effect of ligands on nanocrystal shape transformations will enable future in situ studies of nanocrystals surfaces and ligand binding positions.

Effect of chemisorbed thiophenols with an electron donating group on surface-enhanced Raman scattering of gold nanorods

Surface-enhanced Raman scattering (SERS) is a powerful technique to amplify the weak Raman scattering intensity by molecules chemisorbed on a metallic surface. Herein, we present the interfacial electronic effect of para-substituted aromatic thiophenols (TPs) with an electron donating group (EDG) on SERS of anisotropic gold nanorods (AuNRs) under resonant conditions. Probe molecules with an EDG showed great SERS enhancement in AuNRs at the resonant excitation wavelength. We found that the SERS enhancement with an EDG is caused by the formation of aggregates through intermolecular interactions among probe molecules, such as dimerization with hydrogen bonding via an amino group (-NH2) of p-aminothiophenol (p-ATP) and hydroxyl group (-OH) of p-mercaptophenol (p-MP), resulting in hot-spots between AuNRs. Furthermore, SERS having a stronger EDG (-NH2, p-ATP) with the Hammett constant of -0.66 exhibited greater enhancement than p-MP having hydroxyl (-OH) groups with the Hammett constant of -0.37. We found that the greater enhancement is ascribed to the temporary formation of a positively charged electron withdrawing group (-NH3+) in p-ATP, unlike p-MP, via the interaction of the lone pair of the amino group (-NH2) with ethanol. Therefore, this investigation provides new insightful experimental observations on SERS enhancement of probe molecules with an EDG.

Distinct mechanisms in the heteroaggregation of silver nanoparticles with mineral and microbial colloids

Attachment to solids is an important process for determining nanomaterial transport and their fate in environments. Here we revealed distinct behaviours in the attachment of silver nanoparticles (AgNPs) to kaolin and bacterial cells. We found preferential attachment of AgNPs to the edges of kaolin. Decreasing pH or adding metal ions promoted AgNP-kaolin attachment due to the increase of positive charge on kaolin's surfaces. Multivalent cations (Mg²⁺ and Ca²⁺) induced stronger enhancement than monovalent cations (Na⁺, K⁺ and Ag⁺), which demonstrated the positive role of electrostatic interaction in AgNP-kaolin attachment. However, the presence of metal ions inhibited AgNP binding to bacterial cells. The inhibitive effect was significantly correlated with solubility product of metal ions, which implied a chemical reaction mechanism in AgNP-cell attachment. In kaolin system, humic acid (HA) can considerably inhibit AgNP attachment and diminish the enhanced effects induced by metal ions. In contrast, in bacterial cell system, HA reduced the inhibitive effect of metal ions for AgNP adsorption, although HA itself had negligible effect on AgNP-cell attachment. Taken together, our results demonstrated the contribution of electrostatic attraction versus chemical interaction to the attachment of AgNPs to kaolin or bacterial cells, providing fundamental support to understand the attachment of nanomaterials to inorganic and organic solids in the environments.

Nanoparticle Characterization: What to Measure?

What to measure? is a key question in nanoscience, and it is not straightforward to address as different physicochemical properties define a nanoparticle sample. Most prominent among these properties are size, shape, surface charge, and porosity. Today researchers have an unprecedented variety of measurement techniques at their disposal to assign precise numerical values to those parameters. However, methods based on different physical principles probe different aspects, not only of the particles themselves, but also of their preparation history and their environment at the time of measurement. Understanding these connections can be of great value for interpreting characterization results and ultimately controlling the nanoparticle structure-function relationship. Here, the current techniques that enable the precise measurement of these fundamental nanoparticle properties are presented and their practical advantages and disadvantages are discussed. Some recommendations of how the physicochemical parameters of nanoparticles should be investigated and how to fully characterize these properties in different environments according to the intended nanoparticle use are proposed. The intention is to improve comparability of nanoparticle properties and performance to ensure the successful transfer of scientific knowledge to industrial real-world applications.

In Situ Observations of Shell Growth and Oxidative Etching Behaviors of Pd Nanoparticles in Solutions by Liquid Cell Transmission Electron Microscopy

It is demonstrated that the redox reaction behaviors of Pd nanoparticles in HAuCl₄ solutions can be substantially modified by the introduction of hexadecyl trimethyl ammonium bromide (CTAB) agents through systematic liquid cell transmission electron microscopy (LCTEM) investigations. The gradual dissolution of Pd nanoparticles is observed when HAuCl₄ solution is pumped into liquid flow cells, the etching characteristics of which are depended on both HAuCl₄ concentrations and incident electron doses. In comparison, with the presence of CTAB agents, the dominated phenomenon appears to be the precipitation of Au species and incorporation onto the surface of Pd seeds. It is also observed that the rapid growth of Au on Pd seeds occurs by loading Pd and HAuCl₄ solutions into static liquid cells. The resultant Au shells exhibit rather sparse structural configurations and are formed possibly by homogeneous nucleation/coalescence of Au species as well as monomer attachments. The observed Au-shell growth instead of Pd dissolution is attributed to the presence of the residual regents, which may be also responsible for the initially already existing small Au adsorptions at the corner/edge sites of Pd seeds. The study provides a useful reference for the convenient fabrication of complex nanostructures and functional nanomaterials in a controllable way.

Gold Nanocrystal Etching as a Means of Probing the Dynamic Chemical Environment in Graphene Liquid Cell Electron Microscopy

Graphene liquid cell electron microscopy has the necessary temporal and spatial resolution to enable the in situ observation of nanoscale dynamics in solution. However, the chemistry of the solution in the liquid cell during imaging is as yet poorly understood due to the generation of a complex mixture of radiolysis products by the electron beam. In this work, the etching trajectories of nanocrystals were used as a probe to determine the effect of the electron beam dose rate and preloaded etchant, FeCl₃, on the chemistry of the liquid cell. Initially, illuminating the sample at a low electron beam dose rate generates hydrogen bubbles, providing a reservoir of sacrificial reductant. Increasing the electron beam dose rate leads to a constant etching rate that varies linearly with the electron beam dose rate. Comparing these results with the oxidation potentials of the species in solution, the electron beam likely controls the total concentration of oxidative species in solution and FeCl₃ likely controls the relative ratio of oxidative species, independently determining the etching rate and chemical potential of the reaction, respectively. Correlating these liquid cell etching results with the ex situ oxidative etching of gold nanocrystals using FeCl₃ provides further insight into the liquid cell chemistry while corroborating the liquid cell dynamics with ex situ synthetic behavior. This understanding of the chemistry in the liquid cell will allow researchers to better control the liquid cell electron microscopy environment, allowing new nanoscale materials science experiments to be conducted systematically in a reproducible manner.

Plasmonic Optical and Chiroptical Response of Self-Assembled Au Nanorod Equilateral Trimers

Assembling metamolecules from anisotropic, shape-engineered nanocrystals provides the opportunity to orchestrate distinct optical responses one nanocrystal at a time. The Au nanorod has long been a structural archetype in plasmonics, but nanorod assemblies have largely been limited to end-to-end or side-to-side arrangements, accessing only a subset of potential metamolecule structures. Here, we employ triangular templates to direct the assembly of Au nanorods along the edges of an equilateral triangle. Using spatially resolved, dark-field scattering spectroscopy in concert with numerical simulation of individual metamolecules, we map the evolution in surface plasmon resonances as we add one, two, and three nanorods to construct triangular nanorod assemblies. The assemblies exhibit rotation- and polarization-dependent hybridized plasmon modes, which are sensitive to variations in nanorod size, position, and orientation that lead to geometrical symmetry breaking. The triangular arrangement of nanorods supports magnetic plasmon modes where electric fields are directed along the perimeter of the triangle, and the magnetic field intensity within the triangle's open interior is enhanced. Circumferential displacements of the nanorods within the templates impart either a left- or right-handed sense of rotation to the structure, which generates a chiroptical response under unidirectional oblique illumination. Our results represent an important step in realizing and characterizing metamaterial assemblies with "open" structures utilizing anisotropic plasmonic building blocks, with implications for optical magnetic field enhancement and chiral plasmonics.

In situliquid cell TEM observation of solution-mediated interaction behaviour of Au/CdS nanoclusters

Near a thicker liquid region, droplets grow and become overlap-like, liquid fronts push forward to facilitate NC coalescence. In a thin liquid region, e-beam induces bubble formation, dissolution of CdS, and deformation of the Au/CdS composite.

Mechanistic Understanding of the Growth Kinetics and Dynamics of Nanoparticle Superlattices by Coupling Interparticle Forces from Real-Time Measurements

Superlattice structures formed by nanoparticle (NP) self-assembly have attracted increasing attention due to their potential as a class of nanomaterials with enhanced physicochemical properties tailored by the assembly structure. However, many key questions remain regarding the correlation between the dynamics of individual NPs and emerging superlattice patterns. Here we investigated the self-assembly of gold NPs by employing in situ transmission electron microscopy equipped with direct detection camera capabilities, which enabled us to track the rapid motion of individual nanoparticles in real time. By calculating the contributions of Brownian, van der Waals, hydrodynamic, and steric hindrance forces, we obtained a quantitative evaluation of the competitive interactions that drive the assembly process. Such competition between forces over various separations is critical for the kinetics of cluster growth, leading to the superlattice formation. Brownian motion resulted in the formation of small-sized clusters, whose growth dynamics was characterized as reaction-limited aggregation. Subsequently, at relative short-range particle separations, van der Waals force overrode the Brownian force and dominantly drove the assembly process. When the particles were in close proximity, a delicate balance between van der Waals and steric hindrance forces led to an unexpected dynamic nature of the assembled superlattice. Our study provides a fundamental understanding of coupling energetics and dynamics of NPs involved in the assembly process, enabling the control and design of the structure of nanoparticle superlattices.

Anisotropic functionalization of upconversion nanoparticles

Despite significant advances toward accurate tuning of the size and shape of colloidal nanoparticles, the precise control of the surface chemistry thereof remains a grand challenge. It is desirable to conjugate functional bio-molecules onto the selected facets of nanoparticles owing to the versatile capabilities rendered by the molecules. We report here facet-selective conjugation of DNA molecules onto upconversion nanoparticles via ligand competition reaction. Different binding strengths of phosphodiester bonds and phosphate groups on DNA and the surfactant molecules allow one to create heterogeneous bio-chemistry surface for upconversion nanoparticles. The tailored surface properties lead to the formation of distinct self-assembly structures. Our findings provide insight into the interactions between biomolecules and nanoparticles, unveiling the potential of using nanoparticles as fundamental building blocks for creating self-assembled nano-architectures.

Bacteria-like mesoporous silica-coated gold nanorods for positron emission tomography and photoacoustic imaging-guided chemo-photothermal combined therapy

Mesoporous silica nanoshell (MSN) coating has been demonstrated as a versatile surface modification strategy for various kinds of inorganic functional nanoparticles, such as gold nanorods (GNRs), to achieve not only improved nanoparticle stability but also concomitant drug loading capability. However, limited drug loading capacity and low tumor accumulation rate in vivo are two major challenges for the biomedical applications of MSN-coated GNRs (GNR@MSN). In this study, by coating uniformly sized GNRs with MSN in an oil-water biphase reaction system, we have successfully synthesized a new bacteria-like GNR@MSN (i.e., bGNR@MSN) with a significantly enlarged pore size (4-8 nm) and surface area (470 m²/g). After PEGylation and highly efficient loading of doxorubicin (DOX, 40.9%, w/w), bGNR@MSN were used for positron emission tomography (PET, via facile and chelator-free ⁸⁹Zr-labeling) and photoacoustic imaging-guided chemo-photothermal cancer therapy in vivo. PET imaging showed that ⁸⁹Zr-labeled bGNR@MSN(DOX)-PEG can passively target to the 4T1 murine breast cancer-bearing mice with high efficiency (∼10 %ID/g), based on enhanced permeability and retention effect. Significantly enhanced chemo-photothermal combination therapy was also achieved due to excellent photothermal effect and near-infrared-light-triggered drug release by bGNR@MSN(DOX)-PEG at the tumor site. The promising results indicate great potential of bGNR@MSN-PEG nanoplatforms for future cancer diagnosis and therapy.

DNA-mediated anisotropic silica coating of upconversion nanoparticles

We report a facile approach of using DNA molecules as switches to selectively activate silica coating onto specific facets of upconversion nanoparticles.

Detecting patchy nanoparticle assembly at the single-particle level

Patchy colloidal particles offer a unique platform to explore and investigate spatially inhomogeneous colloidal interactions. In the present work we provide direct, in situ experimental observation at the single particle level on the colloidal interaction driven formation of a heterodimer, consisting of a patchy nanorod and a sphere. The gold nanorods employed during the experiments feature positively charged tip regions, while the side of the rods is covered by surface grafted PEG. The driving force of the assembly is the electric double layer interaction between the nanorod and the nanosphere possessing opposite surface charge. The great advantage of using gold heterodimers is that their optical scattering spectrum characteristics are known to be very sensitive to the actual structure of the assembly due to plasmon hybridization. This was exploited to determine the structure of the heterodimer in situ upon formation in aqueous medium. The results indicate, that despite the attractive patches being located at the nanorod tips, for the given particle pairs the spheres readily bind to the side region of the nanorods in the aqueous medium during the assembly. Finite element simulations of the electric double layer interaction reveal that this is the energetically favorable configuration for the given heterodimer. This work demonstrates the potential of in situ experiments performed at single particle level for the characterization of self-assembling structures, which can contribute to the development of rationally designed nanoscale building blocks.

Direct Observation of Interactions between Nanoparticles and Nanoparticle Self-Assembly in Solution

Hierarchically organized nanoparticles (NPs) possess unique properties and are relevant to various technological applications. An important "bottom-up" strategy for building such hierarchical nanostructures is to guide the individual NPs into ordered nanoarchitectures using intermolecular interactions and external forces. However, our current understanding of the nanoscale interactions that govern such self-assembly processes usually relies on post-synthesis/assembly or indirect characterization. Theoretical models that can derive these interactions are presently constrained to systems with only a few particles or on short time scales. Hence, except for a number of special cases, a description that captures the detailed mechanisms of NP self-assembly still eludes us. By imaging the assembly of NPs in solution with subnanometer resolution and in real-time, in situ liquid cell transmission electron microscopy (LC-TEM) can identify previously unknown intermediate stages and improve our understanding of such processes. Here, we review recent studies where we explored NP self-assembly at different organization length scales using LC-TEM: (1) we followed the transformation of atoms into crystalline NPs in solution, (2) we highlighted the role of solvation forces on interaction dynamics between NPs, and (3) we described the assembly dynamics of NPs in solution. In the case of nanocrystal nucleation, we identified the existence of three distinct steps that lead to the formation of crystalline nuclei in solution. These steps are spinodal decomposition of the precursor solution into solute-rich and solute-poor liquid phases, nucleation of amorphous clusters within the solute-rich liquid phase, followed by crystallization of these amorphous clusters into crystalline NPs. The next question we ask is how NPs interact in solution once they form. It turns out that the hydration layer surrounding each NP acts as a repulsive barrier that prevents NPs from readily attaching to each other due to attractive vdW forces. Consequently, two interacting NPs form a metastable pair separated by their one water molecule thick hydration shell and they undergo attachment only when this water between them is drained. Next, we explore the self-assembly of many NP systems where the formation of linear chains from spherical NPs or nanorods (NRs) is mediated by linker molecules. At low linker concentration, both spherical NPs and NRs tend to form linear chains because of the need to reduce electrostatic repulsion between NP building blocks. When the concentration of linkers is increased, the attachment of NPs is no longer linear. For example, we find that two NRs undergo side-to-side assembly due to decreased electrostatic repulsion and the anisotropic distribution of linkers on NR surfaces at high linker concentration. Lastly, we look at the formation of NP nanorings directed by ethylenediaminetetraacetic acid (EDTA) nanodroplets in water. Our study shows that nanoring assemblies form via sequential attachment of NPs to binding sites located along the circumference of the EDTA nanodroplet, followed by rearrangement and reorientation of the attached NPs. Our approach based on real-time visualization of nanoscale processes not only reveals all the intermediate steps of NP assembly, but also provides quantitative description on the interactions between nanoscale objects in solution.

Quantifying the Self-Assembly Behavior of Anisotropic Nanoparticles Using Liquid-Phase Transmission Electron Microscopy

For decades, one of the overarching objectives of self-assembly science has been to define the rules necessary to build functional, artificial materials with rich and adaptive phase behavior from the bottom-up. To this end, the computational and experimental efforts of chemists, physicists, materials scientists, and biologists alike have built a body of knowledge that spans both disciplines and length scales. Indeed, today control of self-assembly is extending even to supramolecular and molecular levels, where crystal engineering and design of porous materials are becoming exciting areas of exploration. Nevertheless, at least at the nanoscale, there are many stones yet to be turned. While recent breakthroughs in nanoparticle (NP) synthesis have amassed a vast library of nanoscale building blocks, NP-NP interactions in situ remain poorly quantified, in large part due to technical and theoretical impediments. While increasingly many applications for self-assembled architectures are being demonstrated, it remains difficult to predict-and therefore engineer-the pathways by which these structures form. Here, we describe how investigations using liquid-phase transmission electron microscopy (TEM) have begun to play a role in pursuing some of these long-standing questions of fundamental and far-reaching interest. Liquid-phase TEM is unique in its ability to resolve the motions and trajectories of single NPs in solution, making it a powerful tool for studying the dynamics of NP self-assembly. Since 2012, liquid-phase TEM has been used to investigate the self-assembly behavior of a variety of simple, metallic NPs. In this Account, however, we focus on our work with anisotropic NPs, which we show to have very different self-assembly behavior, and especially on how analysis methods we and others in the field are developing can be used to convert their motions and trajectories revealed by liquid-phase TEM into quantitative understanding of underlying interactions and dynamics. In general, liquid-phase TEM studies may help bridge enduring gaps in the understanding and control of self-assembly at the nanoscale. For one, quantification of NP-NP interactions and self-assembly dynamics will inform both computational and statistical mechanical models used to describe nanoscale phenomena. Such understanding will also lay the groundwork for establishing new and generalizable thermodynamic and kinetic design rules for NP self-assembly. Synergies with NP synthesis will enable investigations of building blocks with novel, perhaps even evolving or active behavior. Moreover, in the long run, we foresee the possibility of applying the guidelines and models of fundamental nanoscale interactions which are uncovered under liquid-phase TEM to biological and biomimetic systems at similar dimensions.