Macroscopic and tunable nanoparticle superlattices

Nanocrystal Assemblies: Current Advances and Open Problems

We explore the potential of nanocrystals (a term used equivalently to nanoparticles) as building blocks for nanomaterials, and the current advances and open challenges for fundamental science developments and applications. Nanocrystal assemblies are inherently multiscale, and the generation of revolutionary material properties requires a precise understanding of the relationship between structure and function, the former being determined by classical effects and the latter often by quantum effects. With an emphasis on theory and computation, we discuss challenges that hamper current assembly strategies and to what extent nanocrystal assemblies represent thermodynamic equilibrium or kinetically trapped metastable states. We also examine dynamic effects and optimization of assembly protocols. Finally, we discuss promising material functions and examples of their realization with nanocrystal assemblies.

Assembling PNIPAM-Capped Gold Nanoparticles in Aqueous Solutions

Employing small-angle X-ray scattering (SAXS), we explore the conditions under which assembly of gold nanoparticles (AuNPs) grafted with the thermosensitive polymer poly(N-isopropylacrylamide) (PNIPAM) emerges. We find that short-range order assembly emerges by combining the addition of electrolytes or polyelectrolytes with raising the temperature of the suspensions above the lower-critical solution temperature (LCST) of PNIPAM. Our results show that the longer the PNIPAM chain is, the better organization in the assembled clusters. Interestingly, without added electrolytes, there is no evidence of AuNPs assembly as a function of temperature, although untethered PNIPAM is known to undergo a coil-to-globule transition above its LCST. This study demonstrates another approach to assembling potential thermosensitive nanostructures for devices by leveraging the unique properties of PNIPAM.

Two-dimensional assembly of gold nanoparticles grafted with charged-end-group polymers

HYPOTHESIS

Introducing charged terminal groups to polymers that graft nanoparticles enable Coulombic control over their assembly by tuning the pH and salinity of their aqueous suspensions.

EXPERIMENTS

Gold nanoparticles (AuNPs) are grafted with poly (ethylene glycol) (PEG) terminated with (charge-neutral), (negatively charged) or groups (positively charged), and characterized with dynamic light scattering, ζ-potential, and thermal gravimetric analysis. Liquid surface X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS) are used to determine the density profile and in-plane structure of the AuNPs assembly at the aqueous surface.

FINDINGS

Assembly of PEG-AuNPs at the liquid/vapor interface is tunable by adjusting pH or salinity for COOH but less for terminals. The distinct assembly behaviors are attributed to the overall charge of PEG-AuNPs as well as PEG conformation. COOH-PEG corona is more compact than those of the other terminal groups, leading to a crystalline structure with a smaller superlattice. The net charge per particle depends not only on the PEG terminal groups but also on the cation sequestration of PEG and the intrinsic negative charge of the AuNP surface. [1] The closeness to overall charge neutrality, and hydrogen bonding in play, brought by -PEG, drive -PEG-AuNPs to assembly and crystallinity without additives to the suspensions.

Hydrogen Bond Network Disruption by Hydration Layers in Water Solutions with Salt and Hydrogen-Bonding Polymers (PEO)

A mean field theory model describing the interaction of ion hydration layers with the network of hydrogen bonds of both water and the nonionic polymer poly(ethylene oxide) (PEO) is presented. The predictions of the model for types and statistics of hydrogen bonds, the number of water molecules bound to PEO, or their dependence on temperature are successfully verified from all-atom simulations at different NaCl and PEO concentrations. Furthermore, our simulations show that the binding of cations to PEO increases monotonically with salt concentration, in agreement with recent experimental results, through a mechanism in which the sum of the number of bound water and cations is independent of salt concentration. The model introduced is general and can describe any salt or hydrogen-bond-forming polymer.

Effect of rhodamine 6G dye molecular interactions on counterintuitive self-assembly of noble metal nanorods

HYPOTHESIS

Self-assembled nanostructures with highly ordered and diversified patterns can be obtained by adding additives that directionally control the interparticle interactions. However, due to the complex non-covalent weak interactions in the self-assembly process, the active mechanism of additives is not fully understood, resulting in the limitation of obtaining the nano-superstructures. The introduction of rhodamine 6G (R6G) enables gold nanorods (GNRs) self-assembled into a counterintuitive tetragonal superlattice, during which the exploration of the influence of R6G molecular interactions on the GNRs self-assembly is of importance.

EXPERIMENTS

We present the detailed investigations of spacial configuration, binding modes, and aggregated degree of R6G molecule on formation of the tetragonal GNRs superlattices by combining the experimental and simulated results.

FINDINGS

By analyzing the peak position and peak intensity in the fluorescent spectra of assembled samples and pure R6G samples, H-dimer is verified as the main cause for inducing the tetragonal superstructures. Molecular dynamics simulations reveal that 2-3 H-dimers adsorbed obliquely in a zigzag chain manner on the surface of GNRs is the most stable state of the self-assembly. This work would contribute to a deeper understanding of the complex colloidal nanoparticle self-assemblies and push forward the development of the bottom-up nanoscale superstructures.

Compact Peptoid Molecular Brushes for Nanoparticle Stabilization

Controlling the interfaces and interactions of colloidal nanoparticles (NPs) via tethered molecular moieties is crucial for NP applications in engineered nanomaterials, optics, catalysis, and nanomedicine. Despite a broad range of molecular types explored, there is a need for a flexible approach to rationally vary the chemistry and structure of these interfacial molecules for controlling NP stability in diverse environments, while maintaining a small size of the NP molecular shell. Here, we demonstrate that low-molecular-weight, bifunctional comb-shaped, and sequence-defined peptoids can effectively stabilize gold NPs (AuNPs). The generality of this robust functionalization strategy was also demonstrated by coating of silver, platinum, and iron oxide NPs with designed peptoids. Each peptoid (PE) is designed with varied arrangements of a multivalent AuNP-binding domain and a solvation domain consisting of oligo-ethylene glycol (EG) branches. Among designs, a peptoid (PE5) with a diblock structure is demonstrated to provide a superior nanocolloidal stability in diverse aqueous solutions while forming a compact shell (∼1.5 nm) on the AuNP surface. We demonstrate by experiments and molecular dynamics simulations that PE5-coated AuNPs (PE5/AuNPs) are stable in select organic solvents owing to the strong PE5 (amine)-Au binding and solubility of the oligo-EG motifs. At the vapor-aqueous interface, we show that PE5/AuNPs remain stable and can self-assemble into ordered 2D lattices. The NP films exhibit strong near-field plasmonic coupling when transferred to solid substrates.

Binary Superlattices of Gold Nanoparticles in Two Dimensions

We have created two-dimensional (2D) binary superlattices by cocrystallizing gold nanoparticles (AuNPs) of two distinct sizes into √3 × √3 and 2 × 2 complex binary superlattices, derived from the hexagonal structures of the single components. The building blocks of these binary systems are AuNPs that are functionalized with different chain lengths of poly(ethylene glycol) (PEG). The assembly of these functionalized NPs at the air-water interface is driven by the presence of salt, causing PEG-AuNPs to migrate to the aqueous surface and assemble into a crystalline lattice. We have used liquid surface X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS) to examine the assembly and crystallization at the liquid interface.

Effect of mono- and multi-valent ionic environments on the in-lattice nanoparticle-grafted single-stranded DNA

Polyelectrolyte (PE) chains respond in a complex manner to multivalent salt environments, and this behavior depends on pH, temperature, and the presence of specific counter ions. Although much work has been done to understand the behaviour of free PE chains, it is important to reveal their behaviour on a nanoparticle's surface, where surface constraints, particle geometry, and multi-chain environment can affect their behaviour and contribute to particles' assembly states. Our work investigates, using in situ small-angle X-ray scattering (SAXS), the morphology of PE (single-stranded DNA) chains grafted onto the surface of spherical gold nanoparticles assembled in a lattice in the presence of monovalent, divalent and trivalent salts. For divalent salts, the DNA brush length was found to decrease at a faster rate with salt concentration than in the monovalent salt environment, while trivalent salts led to chain collapse. Using a power law analysis and the modified Daoud-Cotton model, we have obtained insight into the mechanism of a nanoparticle-grafted chain's response to ionic environments. Our analysis suggests that the decrease in brush length is due to the conventional electrostatic screening for monovalent systems, whereas for divalent systems both electrostatic screening and divalent ion bridging must be considered.

Gold nanoparticles endowed with low-temperature colloidal stability by cyclic polyethylene glycol in ethanol

The colloidal stability of metal nanoparticles is tremendously dependent on the thermal behavior of polymer brushes. Neat polyethylene glycol (PEG) presents an unconventional upper critical solution temperature in ethanol, where phase segregation and crystallization coexist. This thermal behavior translated to a PEG brush has serious consequences on the colloidal stability in ethanol of gold nanoparticles (AuNPs) modified with PEG brushes upon cooling. We observed that AuNPs (13 nm diameter) stabilized with conventional linear PEG brushes (M_n = 6 and 11 kg mol^-1) in ethanol suffer from reversible phase separation upon a temperature drop over the course of a few hours. However, the use of a polymer brush with cyclic topology as a stabilizer prevents sedimentation, ensuring the colloidal stability in ethanol at -25 °C for, at least, four months. We postulate that temperature-driven collapse of chain brushes promotes the interpenetration of linear chains, causing progressive AuNP sedimentation, a process that is unfavorable for cyclic polymer brushes whose topology prevents chain interpenetration. This study reinforces the notion about the importance of polymer topology on the colloidal stability of AuNPs.

Effect of Polymer Chain Length on the Superlattice Assembly of Functionalized Gold Nanoparticles

We report on the assembly of gold nanoparticle (AuNPs) superlattices at the liquid/vapor interface and in the bulk of their suspensions. Interparticle distances in the assemblies are achieved on multiple length scales by varying chain lengths of surface grafted AuNPs by polyethylene glycol (PEG) with molecular weights in the range 2000-40,000 Da. Crystal structures and lattice constants in both 2D and 3D assemblies are determined by synchrotron-based surface-sensitive and small-angle X-ray scattering. Assuming knowledge of grafting density, we show that experimentally determined interparticle distances are adequately modeled by spherical brushes close to the θ point (Flory-Huggins parameter, χ≈12) for 2D superlattices at a liquid interface and a nonsolvent (χ = ∞) for the 3D dry superlattices.

Distinct thermoresponsive behaviour of oligo- and poly-ethylene glycol protected gold nanoparticles in concentrated salt solutions

In this work, the methoxy terminated oligo- and polyethylene glycol of different chain lengths (EGn, n = 7, 18, 45 and 136) is grafted on AuNP surfaces under conditions where they attain maximum grafting densities. These EGn-AuNPs gain stability relative to the pristine c-AuNPs in aqueous solutions and in a wide temperature interval and they form stable suspensions in solutions of high NaCl concentrations. To show the thermoresponsive properties of these EGn-AuNPs, temperature titration experiments are carried out in the presence of increasing amounts of salts. The concentrations of NaCl are chosen by checking the stability of EGn-AuNPs at room temperature and choosing the highest concentrations that allow them to form stable suspensions. The analysis of the temperature titration experiments monitored by UV-visible spectroscopy and dynamic light scattering allows us to establish the existence of transitions from individual to assembled nanoparticles, the reversibility of the temperature transitions and hysteretic behaviour in these systems. While EG7-AuNPs only show reversible temperature transitions in the presence of 5 mM NaCl, EG18-AuNPs do up to 1 M NaCl, becoming only partially reversible in 2 M NaCl. The titrations of EG45-AuNPs in 3 and 5 M NaCl show irreversible temperature transitions. Finally, EG136-AuNPs present a complex and interesting behaviour with two temperature transitions, the first one showing hysteresis and the second being reversible.

Morphologically Diverse Micro- and Macrostructures Created via Solvent Evaporation-Induced Assembly of Fluorescent Spherical Particles in the Presence of Polyethylene Glycol Derivatives

The creation of fluorescent micro- and macrostructures with the desired morphologies and sizes is of considerable importance due to their intrinsic functions and performance. However, it is still challenging to modulate the morphology of fluorescent organic materials and to obtain insight into the factors governing the morphological evolution. We present a facile bottom-up approach to constructing diverse micro- and macrostructures by connecting fluorescent spherical particles (SPs), which are generated via the spherical assembly of photoisomerizable azobenzene-based propeller-shaped chromophores, only with the help of commercially available polyethylene glycol (PEG) derivatives. Without any extra additives, solvent evaporation created a slow morphological evolution of the SPs from short linear chains (with a length of a few micrometers) to larger, interconnected networks and sheet structures (ranging from tens to >100 µm) at the air–liquid interface. Their morphologies and sizes were significantly dependent on the fraction and length of the PEG. Our experimental results suggest that noncovalent interactions (such as hydrophobic forces and hydrogen bonding) between the amphiphilic PEG chains and the relatively hydrophobic SPs were weak in aqueous solutions, but play a crucial role in creating the morphologically diverse micro- and macrostructures. Moreover, short-term irradiation with visible light caused fast morphological crumpling and fluorescence switching of the obtained structures.

Effects of polyethylene glycol on the surface of nanoparticles for targeted drug delivery

The rapid development of drug nanocarriers has benefited from the surface hydrophilic polymers of particles, which has improved the pharmacokinetics of the drugs. Polyethylene glycol (PEG) is a kind of polymeric material with unique hydrophilicity and electrical neutrality. PEG coating is a crucial factor to improve the biophysical and chemical properties of nanoparticles and is widely studied. Protein adherence and macrophage removal are effectively relieved due to the existence of PEG on the particles. This review discusses the PEGylation methods of nanoparticles and related techniques that have been used to detect the PEG coverage density and thickness on the surface of the nanoparticles in recent years. The molecular weight (MW) and coverage density of the PEG coating on the surface of nanoparticles are then described to explain the effects on the biophysical and chemical properties of nanoparticles.

The Effects of Temperature on the Assembly of Gold Nanoparticle by Interpolymer Complexation

Using synchrotron-based small-angle X-ray scattering techniques, we demonstrate that poly(ethylene glycol)-functionalized gold nanoparticles (PEG-AuNPs) are assembled into close-packed structures that include short-range order with face-centered cubic structure, where crystalline qualities are varied by controlling the electrolyte concentration, pH, and temperature of the suspensions. We show that interpolymer complexation with poly(acrylic acid) (PAA) is induced by lowering the pH level of the PEG-AuNPs suspensions, and furthermore, increasing the temperature of the suspension strengthens interparticle attraction, leading to improved supercrystal structures. Our results indicate that this strategy creates robust nanoparticle superlattices with high thermal stability. The effects of PAA and PEG chain lengths on the assemblies are also investigated, and their optimal conditions for creating improved superlattices are discussed.

Poly(N-isopropylacrylamide)-grafted gold nanoparticles at the vapor/water interface

HYPOTHESIS

Grafting nanoparticles surfaces with water-soluble polymers modify interparticle interactions that are pivotal for assembling them into ordered phases. By manipulating salt concentrations of gold nanoparticles (AuNPs) that are grafted with poly(N-isopropylacrylamide) (PNIPAM-AuNPs), we hypothesize that various aggregated phases form at the suspension/vapor interface or in the bulk that depend on the molecular weight (MW) of PNIPAM and on salt concentrations.

EXPERIMENTS

AuNPs are grafted with thiolated PNIPAM of molecular weights of 3 or 6 kDa, and grafting is confirmed by dynamic light scattering. Liquid-surfaces X-ray reflectivity and grazing incidence small-angle X-ray scattering are used to determine the density profiles of the suspension/vapor interface and their inplane structure as salt is added to the suspensions.

FINDINGS

We find that surface enrichment is induced by adding NaCl to the suspensions, and that at low salt concentrations, the monoparticle layer formed is dispersed, and above a threshold salt concentration, depending on MW of PNIPAM, the PNIPAM-AuNPs order in a hexagonal structure. We show that the lattice constant of the two-dimensional hexagonal structure varies with salt concentration, and more significantly with MW of PNIPAM.

Cyclic Polyethylene Glycol as Nanoparticle Surface Ligand

Cyclic polymers behave different than linear polymers due to the lack of end groups and smaller coil dimensions. Herein, we demonstrate that cyclic polyethylene glycol (PEG) can be used as an alternative of classical linear PEG ligands for gold nanoparticle (AuNP) stabilization. We observed that the brush height of cyclic PEG on AuNPs of diameter 4.4 and 13.2 nm increases identically as that of linear brushes with (Nσ^1/2)^0.7 (N, number of monomers in a chain and σ, grafting density) and that cyclic brushes are more stretched than their linear analogues when compared to their unperturbed dimensions. Such structural effect and the reduced footprint diameter in cyclic brushes with the entire chain in a concentrated polymer brush regime explains the distinct response of NPs to ionic strength and temperature, respectively, compared to linear analogues. These experiments are an important step in understanding the effect of polymer brush topology on colloidal properties.

Enhancing the stability of single-stranded DNA on gold nanoparticles as molecular machines through salt and acid regulation

DNA-functionalized gold nanoparticles (DNA-AuNPs) have shown great potential and exciting opportunities for constructing machine-like nanodevices. Nonthiolated DNA can be grafted onto gold surfaces via DNA bases, such as polyadenine (polyA)-DNA. The colloidal stability of polyA-DNA-AuNPs has a significant dependency on salt and pH that affects the assembly of AuNPs and their application in polyA-DNA molecular machines. High salt and low pH value contribute to the stabilization of polyA-DNA-AuNPs. In acid conditions, adenine can be protonated and becomes positively-charged, thus enhancing the adsorption of polyA-DNA onto the gold surface by electrostatic interactions; coordination of multiple interactions achieves a high DNA grafting density and colloidal stability. In addition, the length of adenine has an important effect on the efficiency of the DNA machine, while the length of thymine has little effect when the thymine length is less than or equal to seven. The assembly of AuNPs driven by dynamic polyA-DNA molecular machines was successfully accomplished with A5-DNA and A9-DNA. A moderate concentration of catalyst oligomer (50 nM) could improve the DNA hybridization efficiency. The A9-DNA based molecular machine is more efficient than the A5-DNA based one because of the larger amount of A9-DNA on the AuNPs, which increases the probability of collisions between complementary DNA strands. Therefore, polyA-DNA functionalized nanoparticles can be used as a basic unit to construct assembly-ordering structures and achieve dynamic molecular machines to be applied in the molecular diagnostics field.

Assembly of nanocrystal clusters by solvent evaporation: icosahedral order and the breakdown of the Maxwell regime

We carry out molecular dynamics simulations of N gold alkylthiolated nanocrystals (0 ≤ N ≤ 29) contained in liquid droplets of octane, nonane and decane coexisting with its vapor. The equilibrium structures that result when all the solvent dries up consist of highly symmetric nanocrystal clusters with different degrees of icosahedral order that are thoroughly characterized. We show that the relaxation times follow two regimes, a first for small nanocrystal packing fraction, dominated by the diffusion of vapor molecules (Maxwell regime, relaxation times independent of N) and another, for larger packing fractions, where the solvent diffuses through the cluster (with relaxation times growing like N2/3). We discuss the connection to the assembly of superlattices, prediction of lattice constants and evaporation models.

Unusual Effect of Iodine Ions on the Self-Assembly of Poly(ethylene glycol)-Capped Gold Nanoparticles

We use synchrotron X-ray reflectivity and grazing incidence small-angle X-ray scattering to investigate the surface assembly of the poly(ethylene glycol) (PEG)-grafted gold nanoparticles (PEG-AuNPs) induced by different salts. We find that NaCl and CsCl behave as many other electrolytes, namely, drive the PEG-AuNPs to the vapor/suspension interface to form a layer of single-particle depth and organize them into very high-quality two-dimensional (2D) hexagonal crystals. By contrast, NaI induces the migration of PEG-AuNPs to the aqueous surface at much higher surface densities than the other salts (at similar concentrations). The resulting 2D ordering at moderate NaI concentrations is very short ranged, and at a higher NaI concentration, the high-density monolayer is amorphous. Considering NaCl, CsCl and the majority of salts behave similarly, this implicates the anomaly of iodine ion (I^-) in this unusual surface population. We argue that the influence of most electrolytes on the PEG corona preserves the polymer in the θ-point with sufficient flexibility to settle into a highly ordered state, whereas I^- has a much more severe effect on the corona by collapsing it. The collapsed PEG renders the grafted AuNP a nonspherical shaped complex that, although packs at high density, cannot organize into a 2D ordered arrangement.

Salt Mediated Self-Assembly of Poly(ethylene glycol)-Functionalized Gold Nanorods

Although challenging, assembling and orienting non-spherical nanomaterials into two- and three-dimensional (2D and 3D) ordered arrays can facilitate versatile collective properties by virtue of their shape-dependent properties that cannot be realized with their spherical counterparts. Here, we report on the self-assembly of gold nanorods (AuNRs) into 2D films at the vapor/liquid interface facilitated by grafting them with poly(ethylene glycol) (PEG). Using surface sensitive synchrotron grazing incidence small angle X-ray scattering (GISAXS) and specular X-ray reflectivity (XRR), we show that PEG-AuNRs in aqueous suspensions migrate to the vapor/liquid interface in the presence of salt, forming a uniform monolayer with planar-to-surface orientation. Furthermore, the 2D assembled PEG functionalized AuNRs exhibit short range order into rectangular symmetry with side-by-side and tail-to-tail nearest-neighbor packing. The effect of PEG chain length and salt concentration on the 2D assembly are also reported.

Superlattice assembly by interpolymer complexation

We present a coarse grained model for a system where nanocrystals are functionalized with a polymer that is a hydrogen bond acceptor, such as polyethylene glycol (PEG), and are dispersed in a solution with a polymer whose monomers consist of a hydrogen bond donor, such as polyacrylic acid (PAA) at low pH (interpolymer complexation). We determine the minimum concentration of the polymer donor to induce aggregation and the structure and dynamics of the induced (fcc) superlattice. Our results are compared to previous and new experiments.

In situ structure and force characterization of 2D nano-colloids at the air/water interface

The control of self-assembly and the related interactions among nanoparticles (NPs) at liquid surfaces and interfaces represents a stimulating experimental challenge to fully understand the behaviour of nano-colloids confined in a 2D asymmetric environment, in turn prompting the building of novel NP-based functional monolayers. Here, we first investigate the structural evolution of a model mixed surfactant/NP monolayer as a function of the surfactant/NP bulk ratio finding that, at ratios lower than 20, the adsorption at the air/water interface of surfactant-decorated NPs is dominant. We then employed these 2D nano-colloidal monolayers as model systems for grazing incidence small angle X-ray scattering measurements, performed using synchrotron radiation, while compressing the monolayers in a Langmuir trough. The simultaneous determination of the compression work and the related reduction of the inter-particle distance at the interface enabled, for the first time, the quantitative characterization of the forces acting between adsorbed NPs, as well as their dispersion law with the inter-particle distance. Distinct surfactant reorganization processes are proposed to interpret the measured forces and the characteristic inter-particle distances.

Assembly by solvent evaporation: equilibrium structures and relaxation times

We present a study describing the dynamics and equilibrium of the assembly of nanostructures by solvent evaporation. We first consider N nanocrystals stabilized by capping ligands in a spherical droplet of liquid solvent coexisting with its gas and show that, as the liquid solvent evaporates slowly, NCs crystallize into clusters of high symmetry based on tetrahedral and octahedral units: tetrahedron (N = 4), octahedron (N = 6), icosahedron (N = 13), Archimedean truncated tetrahedron (N = 16) and Z₂₀ (N = 21). We derive explicit formulas for the process and rigorously compute relaxation times, which drastically increase when the packing parameter reaches the hard-sphere liquid-solid transition η = 0.49. This result shows that contrary to what occurs in an evaporation of a single component system, the relaxation times are not determined by the diffusion constant of the vapor, but rather, are dominated by the residence time of solvent molecules trapped within the capping ligands. Our theory provides a number of predictions that enable the design of new structures while improving the control and quality of their assembly.

Self-assembly and characterization of 2D plasmene nanosheets

Freestanding plasmonic nanoparticle (NP) superlattice sheets are novel 2D nanomaterials with tailorable properties that enable their use for broad applications in sensing, anticounterfeit measures, ionic gating, nanophotonics and flat lenses. We recently developed a robust, yet general, two-step drying-mediated approach to produce freestanding monolayer, plasmonic NP superlattice sheets, which are typically held together by holey grids with minimal solid support. Within these superlattices, NP building blocks are closely packed and have strong plasmonic coupling interactions; hence, we termed such freestanding materials 'plasmene nanosheets'. Using the desired NP building blocks as starting material, we describe the detailed fabrication protocol, including NP surface functionalization by thiolated polystyrene and the self-assembly of NPs at the air-water interface. We also discuss various characterization approaches for checking the quality and optical properties of the as-obtained plasmene nanosheets: optical microscopy, spectrophotometry, transmission/scanning electron microscopy (TEM/SEM) and atomic force microscopy (AFM). With regard to different constituent building blocks, the key experimental parameters, including NP concentration and volume, are summarized to guide the successful fabrication of specific types of plasmene nanosheets. This protocol, from initial NP synthesis to the final fabrication and characterization, takes ~33.5 h.

Structural and Physicochemical Properties and Biocompatibility of Linear and Looped Polymer-Capped Gold Nanoparticles

Various polymer brushes with linear and looped conformations have gained considerable attention in the application of biomaterials and nanotechnology. In this work, the linear and looped polymer brush shells based on PEG-SH and SH-PEG-SH chains binding to gold nanoparticles (AuNPs) are synthesized. The structure and topology of the PEGylated AuNPs are systematically investigated. The basic physicochemical parameters of these PEGylated AuNPs such as hydrodynamic size, grafting density, hydrophilicity, colloidal stability, and biocompatibility are determined intensively. The looped polymer topology can remarkably alter physicochemical properties of polymer brushes compared with the linear counterparts. When the molecular weight of PEG is low (1 and 5 kDa), the looped polymer shells have smaller hydrodynamic size and lower grafting density than their linear analogues; when the molecular weight of PEG is high (10 kDa), the looped shells are much thicker and denser than the linear ones. Interestingly, the looped PEGs on AuNPs are more stable in a high-salt environment and have better hydrophilicity, which endow excellent biocompatibility, including protein resistance and cell viability. These results provide a simple approach to design polymer brushes with different topologies on AuNPs, improve the biocompatibility of hybrid AuNPs, and acquire the potential application in the biomedical field.

A non-sacrificial method for the quantification of poly(ethylene glycol) grafting density on gold nanoparticles for applications in nanomedicine

The grafting density of poly(ethylene glycol) (PEG) on nanoparticle (NP) surfaces is the most important parameter determining the interaction of nanoparticles with serum proteins, the subsequent sequestration of the nanoparticle from the bloodstream by the mononuclear phagocyte system, and the eventual delivery efficiency to tumor tissues. However, the majority of in vivo studies do not characterize or report the grafting density of PEG on nanoparticles due to a lack of feasible characterization methods, making it difficult to evaluate the published studies and reconcile apparent conflicting results. Herein, we develop a facile and non-sacrificial 1H NMR analytical approach for the quantitative characterization of grafting density of thiol-terminated PEG (HS-PEG) on gold NPs (GNPs). A multi-Lorentzian-splitting algorithm is used to distinguish the NMR signal of free PEG from those of the grafted ones, therefore allowing in situ monitoring of the grafting process to study the effects of GNP sizes, PEG molecular weights and NP capping ligands on grafting rates and grafting densities. The main advantage of this method is that it is not limited by the types of terminal functional groups on PEG, surface chemistry of the nanoparticles or their composition. It also provides a set of critical and standard guides for characterization of the PEG grafting density on nanoparticles for in vivo biological and biomedical studies.

Effect of (Poly)electrolytes on the Interfacial Assembly of Poly(ethylene glycol)-Functionalized Gold Nanoparticles

We report on the effect of interpolymer complexes (IPCs) of poly(acrylic acid) (PAA) with poly(ethylene glycol)-functionalized Au nanoparticles (PEG-AuNPs) as they assemble at the vapor-liquid interface, using surface-sensitive synchrotron X-ray scattering techniques. Depending on the suspension pH, PAA functions both as a weak polyelectrolyte and a hydrogen bond donor, and these two roles affect the interfacial assembly of PEG-AuNPs differently. Above its isoelectric point, we find that PAA leads to the formation of a PEG-AuNP monolayer at the interface with a hexagonal structure. In the presence of high concentration of HCl (i.e., below the isoelectric point), at which PAA forms IPCs with PEG, the hexagonal structure at the interface appears to deteriorate, concurrent with aggregation in the bulk. Thus, while the electrolytic behavior of PAA induces interfacial assembly, the hydrogen bonding behavior, as PAA becomes neutral, favors the formation of 3D assemblies. For comparison, we also report on the formation of PEG-AuNP monolayers (in the absence of PAA) with strong electrolytes such as HCl, H₂SO₄, and NaOH that lead to a high degree of crystallinity.

Pressure-Stimulated Supercrystal Formation in Nanoparticle Suspensions

Nanoparticles can self-organize into "supercrystals" with many potential applications. Different paths can lead to nanoparticle self-organization into such periodic arrangements. An essential step is the transition from an amorphous state to the crystalline one. We investigate how pressure can induce a phase transition of a nanoparticle model system in water from the disordered liquid state to highly ordered supercrystals. We observe reversible pressure-induced supercrystal formation in concentrated solutions of gold nanoparticles by means of small-angle X-ray scattering. The supercrystal formation occurs only at high salt concentrations in the aqueous solution. The pressure dependence of the structural parameters of the resulting crystal lattices is determined. The observed transition can be reasoned with the combined effect of salt and pressure on the solubility of the organic PEG shell that passivates the nanoparticles.

The Daoud and Cotton blob model and the interaction of star-shaped polymers

Since it was first proposed in 1982, the Daoud and Cotton (DC) model for star-shaped polymers was intensively used also for self-assembled copolymers and small colloids grafted with long polymers. We try to clarify the position of the DC model and focus on the star partition function which plays a central role in self-assembly and gives access to the star-star interaction. While the predicted star-star interaction agrees with scattering data by Likos et al. (Phys. Rev. Lett. 80, 4450 (1998)), an extensive simulation by Hsu et al. (Macromolecules, 37, 4658 (2004)) does not recover the prediction for the partition function. We try to reconcile this seemingly conflicting results. We discuss star-star interactions, star free energy in θ -solvents, mixing of A/B branches in copolymer stars, within or beyond the Daoud and Cotton blob model.

Two-Dimensional Crystallization of Poly( N-isopropylacrylamide)-Capped Gold Nanoparticles

Surface-sensitive X-ray reflectivity and grazing incidence small-angle X-ray scattering reveal the structure of polymer-capped-gold nanoparticles (AuNPs that are grafted with poly( N-isopropylacrylamide); PNIPAM-AuNPs) as they self-assemble and crystallize at the aqueous suspension/vapor interface. Citrate-stabilized AuNPs (5 and 10 nm in nominal diameter) are ligand-exchanged by 6 kDa PNIPAM-thiol to form corona brushes around the AuNPs that are highly stable and dispersed in aqueous suspensions. Surprisingly, no clear evidence of thermosensitive effect on surface enrichment or self-assembly of the PNIPAM-AuNPs is observed in the 10-35 °C temperature range. However, addition of simple salts (in this case, NaCl) to the suspension induces migration of the PNIPAM-AuNPs to the aqueous surface, and above a threshold salt concentration, two-dimensional crystals are formed. The 10 nm PNIPAM-AuNPs form a highly ordered single layer with in-plane triangular structure, whereas the 5 nm capped NPs form short-range triangular structure that gradually becomes denser as salt concentration increases.

Many Body Effects and Icosahedral Order in Superlattice Self-Assembly

We elucidate how nanocrystals "bond" to form ordered structures. For that purpose we consider nanocrystal configurations consisting of regular polygons and polyhedra, which are the motifs that constitute single component and binary nanocrystal superlattices, and simulate them using united atom models. We compute the free energy and quantify many body effects, i.e., those that cannot be accounted for by pair potential (two-body) interactions, further showing that they arise from coalescing vortices of capping ligands. We find that such vortex textures exist for configurations with local coordination number ≤6. For higher coordination numbers, vortices are expelled and nanocrystals arrange in configurations with tetrahedral or icosahedral order. We provide explicit formulas for the optimal separations between nanocrystals, which correspond to the minima of the free energies. Our results quantitatively explain the structure of superlattice nanocrystals as reported in experiments and reveal how packing arguments, extended to include soft components, predict ordered nanocrystal aggregation.

Liquid interfaces with pH-switchable nanoparticle arrays

Stimuli-responsive 2D nanoscale systems offer intriguing opportunities for creating switchable interfaces. At liquid interfaces, such systems can provide control over interfacial energies, surface structure, and rheological and transport characteristics, which is relevant, for example, to bio- and chemical reactors, microfluidic devices, and soft robotics. Here, we explore the formation of a pH-responsive membrane formed from gold nanoparticles grafted with DNA (DNA-NPs) at a liquid-vapor interface. A DNA-NP 2D hexagonal lattice can be reversibly switched by pH modulation between an expanded state of non-connected nanoparticles at neutral pH and a contracted state of linked nanoparticles at acidic pH due to the AH+-H+A base pairing between A-motifs. Our in situ surface X-ray scattering studies reveal that the reversible lattice contraction can be tuned by the length of pH-activated linkers, with up to ∼71% change in surface area.

Nanoparticle Superlattices: The Roles of Soft Ligands

Nanoparticle superlattices are periodic arrays of nanoscale inorganic building blocks including metal nanoparticles, quantum dots and magnetic nanoparticles. Such assemblies can exhibit exciting new collective properties different from those of individual nanoparticle or corresponding bulk materials. However, fabrication of nanoparticle superlattices is nontrivial because nanoparticles are notoriously difficult to manipulate due to complex nanoscale forces among them. An effective way to manipulate these nanoscale forces is to use soft ligands, which can prevent nanoparticles from disordered aggregation, fine-tune the interparticle potential as well as program lattice structures and interparticle distances - the two key parameters governing superlattice properties. This article aims to review the up-to-date advances of superlattices from the viewpoint of soft ligands. We first describe the theories and design principles of soft-ligand-based approach and then thoroughly cover experimental techniques developed from soft ligands such as molecules, polymer and DNA. Finally, we discuss the remaining challenges and future perspectives in nanoparticle superlattices.

Capping Ligand Vortices as "Atomic Orbitals" in Nanocrystal Self-Assembly

We present a detailed analysis of the interaction between two nanocrystals capped with ligands consisting of hydrocarbon chains by united atom molecular dynamics simulations. We show that the bonding of two nanocrystals is characterized by ligand textures in the form of vortices. These results are generalized to nanocrystals of different types (differing core and ligand sizes) where the structure of the vortices depends on the softness asymmetry. We provide rigorous calculations for the binding free energy, show that these energies are independent of the chemical composition of the cores, and derive analytical formulas for the equilibrium separation. We discuss the implications of our results for the self-assembly of single-component and binary nanoparticle superlattices. Overall, our results show that the structure of the ligands completely determines the bonding of nanocrystals, fully supporting the predictions of the recently proposed Orbifold topological model.

Generalized skew-symmetric interfacial probability distribution in reflectivity and small-angle scattering analysis

Generalized skew-symmetric probability density functions are proposed to model asymmetric interfacial density distributions for the parameterization of any arbitrary density profiles in the `effective-density model'. The penetration of the densities into adjacent layers can be selectively controlled and parameterized. A continuous density profile is generated and discretized into many independent slices of very thin thickness with constant density values and sharp interfaces. The discretized profile can be used to calculate reflectivitiesviaParratt's recursive formula, or small-angle scatteringviathe concentric onion model that is also developed in this work.

Interfacial Self-Assembly of Polyelectrolyte-Capped Gold Nanoparticles

We report on pH- and salt-responsive assembly of nanoparticles capped with polyelectrolytes at vapor-liquid interfaces. Two types of alkylthiol-terminated poly(acrylic acid) (PAAs, varying in length) are synthesized and used to functionalize gold nanoparticles (AuNPs) to mimic similar assembly effects of single-stranded DNA-capped AuNPs using synthetic polyelectrolytes. Using surface-sensitive X-ray scattering techniques, including grazing incidence small-angle X-ray scattering (GISAXS) and X-ray reflectivity (XRR), we demonstrate that PAA-AuNPs spontaneously migrate to the vapor-liquid interfaces and form Gibbs monolayers by decreasing the pH of the suspension. The Gibbs monoalyers show chainlike structures of monoparticle thickness. The pH-induced self-assembly is attributed to the protonation of carboxyl groups and to hydrogen bonding between the neighboring PAA-AuNPs. In addition, we show that adding MgCl₂ to PAA-AuNP suspensions also induces adsorption at the interface and that the high affinity between magnesium ions and carboxyl groups leads to two- and three-dimensional clusters that yield partial surface coverage and poorer ordering of NPs at the interface. We also examine the assembly of PAA-AuNPs in the presence of a positively charged Langmuir monolayer that promotes the attraction of the negatively charged capped NPs by electrostatic forces. Our results show that synthetic polyelectrolyte-functionalized nanoparticles exhibit interfacial self-assembly behavior similar to that of DNA-functionalized nanoparticles, providing a pathway for nanoparticle assembly in general.

Nanoparticle Superlattices as Quasi-Frank-Kasper Phases

I show that all phases reported experimentally in binary nanoparticle superlattices can be described as networks of disclinations in an ideal lattice of regular tetrahedra. A set of simple rules is provided to identify the different disclination types from the Voronoi construction, and it is shown that those disclinations completely screen the positive curvature of the ideal tetrahedral lattice. In this way, this study provides a generalization of the well-known Frank-Kasper phases to binary systems consisting of two types of particles, and with a more general type of disclinations, i.e., quasi-Frank-Kasper phases. The study comprises all strategies in nanoparticle self-assembly, whether driven by DNA or hydrocarbon ligands, and establishes the universal tendency of superlattices to develop icosahedral order, which is facilitated by the asymmetry of the particles. Besides its interest in predicting nanoparticle self-assembly, I discuss the implications for models of the glass transition, micelles of diblock polymers, and dendritic molecules, among many others.

Assembling and ordering polymer-grafted nanoparticles in three dimensions

Taking advantage of the aqueous biphasic behavior of polyethylene glycol (PEG)/salts, recent experiments have demonstrated self-assembly and crystallization of PEG-grafted gold nanoparticles (PEG-AuNPs) into tunable two-dimensional (2D) supercrystals by adjusting salt concentration (for instance, K₂CO₃). In those studies, combined experimental evidence and theoretical analysis have pointed out the possibility that similar strategies can lead to three-dimensional (3D) formation of ordered nanoparticle precipitates. Indeed, a detailed small-angle X-ray scattering (SAXS) study reported herein reveals the spontaneous formation of PEG-AuNPs assemblies in high-concentration salt solutions that exhibit short-range 3D order compatible with fcc symmetry. We argue that the assembly into fcc crystals is driven by partnering nearest-neighbors to minimize an effective surface-tension gradient at the boundary between the polymer shell and the high-salt media. We report SAXS and other results on PEG-AuNPs of various Au core diameters in the range of 10 to 50 nm and analyze them in the framework of brush-polymer theory revealing a systematic prediction of the nearest-neighbor distance in the 3D assemblies.

Soft Skyrmions, Spontaneous Valence and Selection Rules in Nanoparticle Superlattices

A number of bewildering paradoxes arise in the field of nanoparticle self-assembly: nominal low density superlattices, strong stability of low coordination sites, and a clear but imperfect correlation between lattice stability and the maximum of hard sphere packing, despite the fact that that nanocrystals themselves are, through their ligands, very much compressible. In this study, I show that by regarding nanocrystals as pseudotopological objects ("soft skyrmions"), it is possible to identify and classify the ligand textures that determine their bonding. These textures consist of interacting vortices, where the total vorticity defines a spontaneous valence (coordination). Furthermore, skyrmion interactions are governed by two simple assumptions, which lead to a set of selection rules for superlattice structure. Besides resolving all the above paradoxes, the predictions are completely supported by more than one hundred sixty experiments gathered from the literature, including a wide range of nanocrystal cores and ligands (saturated or unsaturated hydrocarbons, amines, polystyrene, etc.). How those results can be used for addressing more complex structures and guiding future experiments is also addressed.