Nanoscale Phase Behavior of Mixed Polymer Ligands on a Gold Nanoparticle Surface

Adaptable Multivalent Hairy Inorganic Nanoparticles

We report a polymer brush-based approach for fabricating multivalent patchy nanoparticles (NPs) with the number of nanodomains (valency) from 6 to 10, potentially from 1 to 10, by exploiting the lateral microphase separation of binary mixed homopolymer brushes grafted on NPs with a radius comparable to the polymer sizes. Well-defined mixed brushes were grown on 20.4 nm silica NPs by two-step surface-initiated reversible deactivation radical polymerizations and microphase separated laterally upon casting from a good solvent, producing multivalent NPs on 2D surfaces. A linear relationship between valency and average core size for the corresponding valency was observed. The mixed brush NPs exhibited abilities to form "bonds" through the overlap of nanodomains and to change the valency when interacting with adjacent NPs. This method could open up a new avenue for studying patchy NPs.

Entropic Effects in Solvent-Free Bidisperse Polymer Brushes Investigated Using Density Functional Theories

Solvent-free polymer-functionalized nanoparticles form a special type of colloid composed of inorganic cores self-suspended by their grafted coronas. In the absence of intervening solvent molecules, the fluidity of the system is provided by these tethered polymers as they fill the space. Here, we study the structure and interaction of neighboring polymer-grafted surfaces in the solvent-free condition using mean-field density functional theories. For opposing flat surfaces, the brush configuration and the associated energy landscape are semianalytically investigated given the incompressibility of the tethered entropic chains. The effect of brush polydispersity (including variations in both chain length and surface grafting density) is considered by two bidisperse models corresponding to different physical scenarios: one for opposing brushes uniformly mixed with two species at a fixed grafting density, and the other for opposing brushes with distinct chain lengths and grafting densities. The space-filling capabilities of the neighboring coronas differ not only by their ratio of radii of gyration for the composing polymers but also by their ratio of grafting densities. We show that the system energy depicts a steric repulsion as the brushes are compressed, which is typical for hairy particles in a solvent. However, as the interwall separation increases, the cooperative stretching of the chains leads to an entropic attraction between them, a unique characteristic of solventless systems. The corresponding brush profiles change from a bell-like shape to a more step-function-like feature as the interwall spacing increases significantly. The interwall separation associated with the overall free energy minimum therefore characterizes the favorable interparticle spacing for solvent-free polymer-functionalized particles. The limiting accessible parameter space of polymer sizes and grafting densities subjected to the space-filling constraint is comprehensively explored for representative interparticle spacing characterizing the compressed, relaxed, and stretched regimes for a given polymer species, respectively. Such information would be useful for guiding the design of experimental solvent-free polymer-functionalized nanoparticles.

Helicoidal Patterning of Gold Nanorods by Phase Separation in Mixed Polymer Brushes

The spatial distribution of polymer ligands on the surface of nanoparticles (NPs) is of great importance because it determines their interactions with each other and with the surrounding environment. Phase separation in mixtures of polymer brushes has been studied for spherical NPs; however, the role of local surface curvature of nonspherical NPs in the surface phase separation of end-grafted polymer ligands remains an open question. Here, we examined phase separation in mixed monolayers of incompatible polystyrene and poly(ethylene glycol) brushes end-capping the surface of gold nanorods in a good solvent. By varying the molar ratio between these polymers, we generated a range of surface patterns, including uniform and nonuniform polystyrene shells, randomly distributed polystyrene surface patches, and, most interestingly, a helicoidal pattern of polystyrene patches wrapping around the nanorods. The helicoidally patterned nanorods exhibited long-term colloidal stability in a good solvent. The helicoidal wrapping of the nanorods was achieved for the mixtures of polymers with different molecular weights and preserved when the quality of the solvent for the polymers was reduced. The helicoidal organization of polymer patches on the surface of nanorods can be used for templating the synthesis or self-assembly of helicoidal multicomponent nanomaterials.

Versatile fabrication of patchy nanoparticles via patterning of grafted diblock copolymers on NP surface

Patchy nanoparticle formation via the patterning of grafted diblock copolymers on NP surface.

Phase separation in mixed polymer brushes on nanoparticle surfaces enables the generation of anisotropic nanoarchitectures

The preparation of nanoparticles and their targeted connection with other functional units is one key challenge in developing nanoscale devices. Herein, we report an experimental strategy toward the development of anisotropic nanoparticle architectures. Our approach is based on phase separation of binary mixed polymer brushes on gold nanoparticle surfaces leading to Janus-type structures, as revealed by scanning transmission electron microscopy and electron energy-loss spectroscopy and, additionally, corroborated by computer simulation. We show that such structures can be used for the site-selective functionalization with additional nanosized entities.

Quantitative 3D determination of self-assembled structures on nanoparticles using small angle neutron scattering

The ligand shell (LS) determines a number of nanoparticles’ properties. Nanoparticles’ cores can be accurately characterized; yet the structure of the LS, when composed of mixture of molecules, can be described only qualitatively (e.g., patchy, Janus, and random). Here we show that quantitative description of the LS’ morphology of monodisperse nanoparticles can be obtained using small-angle neutron scattering (SANS), measured at multiple contrasts, achieved by either ligand or solvent deuteration. Three-dimensional models of the nanoparticles’ core and LS are generated using an ab initio reconstruction method. Characteristic length scales extracted from the models are compared with simulations. We also characterize the evolution of the LS upon thermal annealing, and investigate the LS morphology of mixed-ligand copper and silver nanoparticles as well as gold nanoparticles coated with ternary mixtures. Our results suggest that SANS combined with multiphase modeling is a versatile approach for the characterization of nanoparticles’ LS.

The ligand shell of a nanoparticle remains difficult to resolve, as the available characterization methods provide only qualitative information. Here, the authors introduce an approach based on small-angle neutron scattering that can quantitatively reveal the organization of ligands in mixed-monolayer nanoparticles.

Fluctuation Effects on the Brush Structure of Mixed Brush Nanoparticles in Solution

A potentially attractive way to control nanoparticle assembly is to graft one or more polymers on the nanoparticle, to control the nanoparticle-nanoparticle interactions. When two immiscible polymers are grafted on the nanoparticle, they can microphase separate to form domains at the nanoparticle surface. Here, we computationally investigate the phase behavior of such binary mixed brush nanoparticles in solution, across a large and experimentally relevant parameter space. Specifically, we calculate the mean-field phase diagram, assuming uniform grafting of the two polymers, as a function of the nanoparticle size relative to the length of the grafted chains, the grafting density, the enthalpic repulsion between the grafted chains, and the solvent quality. We find a variety of phases including a Janus phase and phases with varying numbers of striped domains. Using a nonuniform, random distribution of grafting sites on the nanoparticle instead of the uniform distribution leads to the development of defects in the mixed brush structures. Introducing fluctuations as well leads to increasingly defective structures for the striped phases. However, we find that the simple Janus phase is preserved in all calculations, even with the introduction of nonuniform grafting and fluctuations. We conclude that the formation of the Janus phase is more realistic experimentally than is the formation of defect-free multivalent mixed brush nanoparticles.

Janus nanoparticles inside polymeric materials: interfacial arrangement toward functional hybrid materials

Recent advances and successes in interfacial behavior of Janus NPs at interfaces are summarized, with the hope to motivate additional efforts in the studies of Janus NPs in polymer matrix for the design of functional hybrid nanostructures and devices with engineered, desired and tailored properties for real-life applications.

One-pot synthesis of ultrafine amphiphilic Janus gold nanoparticles by toluene/water emulsion reaction

Spontaneous phase separation of capping ligands at emulsion interfaces was used to synthesize amphiphilic Janus gold nanoparticles in batch.

Out of Equilibrium Self-Assembly of Janus Nanoparticles: Steering It from Disordered Amorphous to 2D Patterned Aggregates

Solvent evaporation driven self-assembly of Janus nanoparticles (J-NPs) has been simulated employing lattice-gas models to investigate the possible emergence of new superlattices. Depending on the chemical nature of NP faces (hence solvophilicity and relative interaction strength), zebra-like or check-like patterns and micellar agglomerates can be obtained. Vesicle-like aggregates can be produced by micelle-based corrals during heterogeneous evaporation. Patterns formed during aggregation appear to be robust against changes in evaporation modality (i.e., spinodal or heterogeneous) or interaction strengths, and they are due to a strictly nanoscopic orientation of single J-NPs in all cases. Due to the latter feature, the aggregate size growth law N(t) ∝ t^a has its exponent a markedly depending on the chemical nature of the J-NPs involved in spite of the unvaried growth mechanism. We interpret such a finding as connected to the increasingly stricter orientation pre-requirements for successful (binding) NP landing upon going from isotropic (a ≃ 0.50), to "zebra" (a ≃ 0.38), to "check" (a ≃ 0.23), and finally to "micelle" (a = 0.15-0.17) pattern forming NPs.

Thermoswitchable Janus Gold Nanoparticles with Stimuli-Responsive Hydrophilic Polymer Brushes

Well-defined thermoswitchable Janus gold nanoparticles with stimuli-responsive hydrophilic polymer brushes were fabricated by combining ligand exchange reactions and the Langmuir technique. Stimuli-responsive polydi(ethylene glycol) methyl ether methacrylate was prepared by addition-fragmentation chain-transfer polymerization. The polymer brushes were then anchored onto the nanoparticle surface by interfacial ligand exchange reactions with hexanethiolate-protected gold nanoparticles, leading to the formation of a hydrophilic (polymer) hemisphere and a hydrophobic (hexanethiolate) one. The resulting Janus nanoparticles showed temperature-switchable wettability, hydrophobicity at high temperatures, and hydrophilicity at low temperatures, due to thermally induced conformational transition of the polymer ligands. The results further highlight the importance of interfacial engineering in the deliberate functionalization of nanoparticle materials.