Polymer-tethered Nanoparticles Research Articles

Self-Organization of Polymer-Tethered Gold Nanoparticles into Hybrid Polyhedral Clusters Confined in Soft Emulsion Droplets

Self-Organization of Polymer-Tethered Gold Nanoparticles into Hybrid Polyhedral Clusters Confined in Soft Emulsion Droplets

Quaternization-Assisted Assembly of Polymer-Tethered Gold Nanoparticles into Superlattices with a Tunable Structure

Programmable organization of functional inorganic nanoparticles (NPs) into well-defined hierarchical superstructures has raised considerable concern due to the enhanced magnetic, electronic, and optical properties arising from the strong plasmonic coupling effect. However, the spontaneous self-assembly of inorganic NPs into uniform superlattices with the desired or tunable shape remains a formidable challenge. Herein, we report the quaternization-assisted hierarchical assembly of gold nanoparticles (AuNPs) tethered with polystyrene (PS) and quaternized poly(2-vinylpyridine) (P2VP) ligands into controllable superstructures in emulsion droplets. The quaternization interaction between bromoalkyl molecular additives (e.g., CnH2n+1Br) and P2VP ligands results in the increase of the hydrophilicity degree of quaternized P2VP ligands and triggers the interfacial instability of emulsion droplets, achieving the continuous transformation of generated superstructures from solid assemblies to hollow colloidosomes and then to the multilayer sheet-like superlattices. Furthermore, the degree of quaternization can be readily tuned by the added content of additives and the length of the alkyl chain, providing a unique and promising approach to prepare contrivable superstructures and functional materials.

Dynamic Assembly of Polymer-Tethered Gold Nanoparticles into a 2D Superlattice at the Air–Liquid Interface: Influence of the Polymer Structure and Solvent Vapor

The two-dimensional (2D) superlattice obtained by self-assembly of gold nanoparticle (NPs) at the air–liquid interface shows a wide range of potential applications in catalysis, sensing, electronic storage, and molecular recognition. However, the dynamic assembly process of the gold NPs at the air–liquid interface is still unclear. In this study, gold NPs grafted with thiol-terminated polystyrene homopolymers (AuNPs@PS) were used as the assembly unit, and the 2D superlattice of AuNPs@PS was prepared by spreading and evaporating the AuNPs@PS toluene suspension at the air/diethylene glycol interface. The effects of polymer chain length, grafting density, and toluene vapor atmosphere on the microstructure of the superlattice were investigated as well. The microstructural evolution process of the AuNPs@PS superlattice at the air/diethylene glycol interface was monitored by the in situ grazing incident small-angle X-ray scattering (GISAXS) technique. The results demonstrate that the transition of long-chain polymer-tethered NPs from random packing to face-centered-cubic packed superlattice followed by two-dimensional hexagonal packing can be observed under a toluene vapor atmosphere. Additionally, by varying the polymer chain length and solvent vapor atmosphere, the spreading rate and superlattice interparticle spacing can be well mediated. This work provides a useful guide to prepare an ordered 2D superlattice with polymer-tethered inorganic NPs.

Encapsulation of Nanoparticles into Preformed Block Copolymer Micelles Driven by Competitive Solvation: Experimental Studies and Molecular Dynamic Simulations

We investigate the encapsulation of polymer-tethered nanoparticles (NPs) inside block copolymer (BCP) micelles, which turns to the mutual stabilization of BCP micelles and NPs by each other. It has been demonstrated, for the first time, that in the presence of cosolvent, which is selective for the core-forming block, the encapsulation of polymer-tethered NPs into the core of preformed BCP micelles is feasible. Furthermore, interestingly, in a certain range of cosolvent volume fraction, NP/BCP hybrid micelles comprising a single NP in the micelle core could be obtained. Significantly, in the presence of cosolvent the NP-loaded hybrid micelles remain intact, whereas the NP-free (empty) micelles undergo further transformations. The experimental observations are additionally supported by the results obtained from the molecular dynamic simulation. In particular, our modeling recognizes mechanisms for the formation of NP/BCP hybrids, which include a one-by-one adsorption of individual BCP chains onto the NP surface, fusion of NP with the preformed BCP micelles, or a combination of both mechanisms. The present work provides new opportunities for the fabrication of BCP-based hybrid nanomaterials having potential applications in drug delivery, diagnostics, catalysis, energy harvesting materials, optical devices, etc.

Theoretical Design of a Janus-Nanoparticle-Based Sandwich Assay for Nucleic Acids.

Nanoparticles exhibit diverse self-assembly attributes and are expected to be applicable under unique settings. For instance, biomolecules can be sandwiched between dimer nanoparticles and detected by surface-enhanced Raman scattering. Controlling the gap between extremely close dimers and stably capturing the target molecule in the gap are crucial aspects of this strategy. Therefore, polymer-tethered nanoparticles (PTNPs), which show promise as high-performance materials that exhibit the attractive features of both NPs and polymers, were targeted in this study to achieve stable biomolecule sensing. Using coarse-grained molecular dynamics simulations, the dependence of the PTNP interactions on the length of the grafted polymer, graft density, and coverage ratio of a hydrophobic tether were examined. The results indicated that the smaller the tether length and graft density, the smaller was the distance between the PTNP surfaces (). In contrast, decreased as the coverage ratio of the hydrophobic surface () increased. The sandwiching probability of the sensing target increased in proportion to the coverage ratio. At high values, the PTNPs aggregated into three or more particles, which hindered their sensing attributes. These results provide fundamental insight into the sensing applications of NPs and demonstrate the usefulness of PTNPs in sensing biomolecules.

Shape Transformations and Self-Assembly of Hairy Particles under Confinement.

Molecular dynamics simulations are used to investigate the behavior of polymer-tethered nanoparticles between two inert or attractive walls. The confinement in pores creates new possibilities for controlling the shape transformation of individual hairy particles and their self-organization. We introduce a minimalistic model of the system; only chain-wall interactions are assumed to be attractive, while the others are softly repulsive. We show how the shape of isolated particles can be controlled by changing the wall separation and the strength of the interaction with the surfaces. For attractive walls, we found two types of structures, “bridges” and “mounds”. The first structures are similar to flanged spools in which the chains are connected with both walls and form bridges between them. We observed various bridges, symmetrical and asymmetrical spools, hourglasses, and pillars. The bridge-like structures can be “nano-oscillators” in which the cores jump from one wall to the other. We also study the self-assembly of a dense fluid of hairy particles in slit-like pores and analyze how the system morphology depends on interactions with the surfaces and the wall separation. The hairy particles form layers parallel to the walls. Different ordered structures, resembling two-dimensional crystalline lattices, are reported. We demonstrate that hairy particles are a versatile soft component forming a variety of structures in the slits.

Polymer-Tethered Nanoparticles: From Surface Engineering to Directional Self-Assembly.

Current interest in nanoparticle ensembles is motivated by their collective synergetic properties that are distinct from or better than those of individual nanoparticles and their bulk counterparts. These new advanced optical, electronic, magnetic, and catalytic properties can find applications in advanced nanomaterials and functional devices, if control is achieved over nanoparticle organization. Self-assembly offers a cost-efficient approach to produce ensembles of nanoparticles with well-defined and predictable structures. Nanoparticles functionalized with polymer molecules are promising building blocks for self-assembled nanostructures, due to the comparable dimensions of macromolecules and nanoparticles, the ability to synthesize polymers with various compositions, degrees of polymerization, and structures, and the ability of polymers to self-assemble in their own right. Moreover, polymer ligands can endow additional functionalities to nanoparticle assemblies, thus broadening the range of their applications.In this Account, we describe recent progress of our research groups in the development of new strategies for the self-assembly of nanoparticles tethered to macromolecules. At the beginning of our journey, we developed a new approach to patchy nanoparticles and their self-assembly. In a thermodynamically driven strategy, we used poor solvency conditions to induce homopolymer surface segregation in pinned micelles (patches). Patchy nanoparticles underwent self-assembly in a well-defined and controlled manner. Following this work, we overcame the limitation of low yield of the generation of patchy nanoparticles, by using block copolymer ligands. For block copolymer-capped nanoparticles, patch formation and self-assembly were "staged" by using distinct stimuli for each process. We expanded this work to the generation of patchy nanoparticles via dynamic exchange of block copolymer molecules between the nanoparticle surface and micelles in the solution. The scope of our work was further extended to a series of strategies that utilized the change in the configuration of block copolymer ligands during nanoparticle interactions. To this end, we explored the amphiphilicity of block copolymer-tethered nanoparticles and complementary interactions between reactive block copolymer ligands. Both approaches enabled exquisite control over directional and self-limiting self-assembly of complex hierarchical nanostructures. Next, we focused on the self-assembly of chiral nanostructures. To enable this goal, we attached chiral molecules to the surface of nanoparticles and organized these hybrid building blocks in ensembles with excellent chiroptical properties. In summary, our work enables surface engineering of polymer-capped nanoparticles and their controllable and predictable self-assembly. Future research in the field of nanoparticle self-assembly will include the development of effective characterization techniques, the synthesis of new functional polymers, and the development of environmentally responsive self-assembly of polymer-capped nanoparticles for the fabrication of nanomaterials with tailored functionalities.

Hierarchical Colloidosomes and Superlattices via Confined Assembly of Polymer-Tethered Inorganic Nanoparticles

Self-assembly of polymer-tethered metal nanoparticle (NP) building blocks is studied via an emulsion interface self-assembly strategy. The results show that the hollow colloidosomes with a highly ordered hexagonal arrangement of metal NP building blocks are obtained. The interparticle distance between the adjacent metal NPs within the hollow colloidosomes can be effectively tuned by adjusting the molecular weight of the polymeric ligands. Interestingly, with an increase in the volume fraction of the internal hexadecane phase, the particular sheet-like superlattices can be formed. Moreover, the prepared colloidosomes can serve as an excellent substrate for the surface-enhanced Raman scattering and have potential application in photothermal therapy. We believe that this study can provide a promising way to fabricate hierarchical ordered metal superstructures, which have potential applications in photothermal therapy, chemical sensors, and biological optical-imaging.

Effect of chemical design of grafted polymers on the self-assembled morphology of polymer-tethered nanoparticles in nanotubes

There is a clear relationship between the self-assembling architecture of nanoparticles (NPs) and their physical properties, and they are currently used in a variety of applications, including optical sensors. Polymer-tethered NPs, which are created by grafting polymers onto NPs to control the self-assembly of NPs, have attracted considerable attention. Recent synthetic techniques have made it possible to synthesize a wide variety of polymers and thereby create NPs with many types of surfaces. However, self-assembled structures have not been systematically classified because of the large number of tuning parameters such as the polymer length and graft density. In this study, by using coarse-grained molecular simulation, we investigated the changes in the self-assembled structure of polymer-tethered NP solutions confined in nanotubes due to the chemical properties of polymers. Three types of tethered polymer NP models were examined: homo hydrophilic, diblock hydrophilic–hydrophobic (HI–HO), and diblock hydrophobic–hydrophilic. Under strong confinement, the NPs were dispersed in single file at low axial pressure. As the pressure increased, multilayered lamellar was observed in the HI–HO model. In contrast, under weak confinement, the difference in the pressure at which the phases emerge, depending on the model, was significant. By changing the chemical properties of the grafted polymer, the thermodynamic conditions (the axial pressure in this study) under which the phases appear is altered, although the coordination of NPs remains almost unchanged. Our simulation offers a theoretical guide for controlling the morphologies of self-assembled polymer-tethered NPs, a novel system that may find applications in nanooptical devices or for nanopatterning.

Self-assembly of polymer-tethered nanoparticles with uniform and Janus surfaces in nanotubes.

In this study, a coarse-grained molecular simulation was performed to investigate the morphologies and phase diagrams of self-assembled polymer-tethered nanoparticles (NPs) confined in nanotubes (NTs). Unlike ordinary NPs, polymer-tethered NPs have two distinct characteristic lengths, which are key factors that determine their self-assembly. Herein, two distinct types of NT walls and three types of polymer-tethered NPs were considered: hydrophilic and hydrophobic walls, and hydrophilic, hydrophobic, and Janus surfaces. First, the qualitative phase diagrams of the axial pressure, Pz, versus the ratio of the NT radius to the NP radius, L, were derived. The results revealed that diverse self-assembled morphologies, which are not formed in non-tethered NPs, were observed in the polymer-tethered NPs. For example, three types of ordered structures with different structural characteristic lengths, depending on Pz, were obtained. In addition, the effect of the chemical nature of the polymer-tethered NP surface on the self-assembled morphology confined in NTs was investigated. Clusters of water molecules were formed, particularly in the hydrophobic polymer-tethered NPs, and these clusters caused the structural distortion of the NP. Moreover, in the polymer-tethered NPs with the Janus amphiphilic surface, the hydrophobic and hydrophilic polymer tethered NPs assembled in the axial direction to form an ordered structure, and a double-helix structure was formed at L = 3.0 in the hydrophobic NT. The results of these simulations indicate that the self-assembly behaviours of polymer-tethered NPs can be qualitatively predicted based on the chemical nature of the NT walls and the surface design of the polymer-tethered NP.

Effects of Geometric Confinement on Caging and Dynamics of Polymer-Tethered Nanoparticle Suspensions

Well-dispersed polymer-tethered silica nanoparticles exhibit soft glassy rheology and caging behaviors due to chain interpenetration driven by entropic attraction and geometric confinement. In this...

Effects of Tethered Polymers on Dynamics of Nanoparticles in Unentangled Polymer Melts.

Polymer-tethered nanoparticles (NPs) are commonly added to a polymer matrix to improve material properties. Critical to the fabrication and processing of such composites is the mobility of the tethered NPs. Here we study the motion of tethered-NPs in unentangled polymer melts using molecular dynamics simulations, which offer a precise control of the grafted chain length N g and the number z of grafted chains per particle. As N g increases, there is a crossover from particle-dominated to tethered-chain-dominated terminal diffusion of NPs with the same z. The mean squared displacement of loosely tethered NPs in the case of tethered-chain dominated terminal diffusion exhibits two sub-diffusive regimes at intermediate time scales for small z. The first one at shorter time scales arises from the dynamical coupling of the particle and matrix chains, while the one at longer time scales is due to the participation of the particle in the dynamics of the tethered chains. The friction of loosely grafted chains in unentangled melts scales linearly with the total number of monomers in the chains, as the frictions of individual monomers are additive in the absence of hydrodynamic coupling. As more chains are grafted to a particle, hydrodynamic interactions between grafted chains emerge. As a result, there is a non-draining layer of hydrodynamically coupled chain segments surrounding the bare particle. Outside the non-draining layer is a free-draining layer of grafted chain segments with no hydrodynamic coupling. The boundary of the two layers is the stick surface where the shear stress due to the relative melt flow is balanced by the friction between grafted and melt chains in the interpenetration layer. The stick surface is located further away from the bare surface of the particle with higher grafting density.

Soft Confined Assembly of Polymer-Tethered Inorganic Nanoparticles in Cylindrical Micelles

Cylindrical block copolymer (BCP) micelles could be considered as two-dimensional (2D) soft confined space for assembly of inorganic nanoparticles (NPs). However, the deformable boundary of the mic...

Flow hydrodynamics-dependent assembly of polymer-tethered gold nanoparticles in microfluidic channels

Recently, self-assembly of polymer-tethered inorganic nanoparticles (NPs) in microfluidic chips has been employed as an effective approach to fabricate novel assemblies.

Chain-length effect on binary superlattices of polymer-tethered nanoparticles

The co-assembly behavior of polymer-tethered NPs is determined by the chain-length of the polymer ligand on the two sized NPs, and exhibits three different models, where each one has its own key factor that determines the crystalline structure.

Polymer tethered graphene oxide influences miscibility and cooperative relaxation in LCST blends

The miscibility of a classical lower critical solution temperature (LCST) system; PMMA/SAN was probed by different rheological fingerprints such as isochronal temperature ramps, and isothermal frequency sweeps, in presence of graphene oxide (GO) and PMMA tethered GO. The chain length of the grafted polymer was varied to study its effect on miscibility, volume cooperativity, entanglement density and localization in the blends. The PMMA tethered GO alters the chain dynamics both at local and cooperative length scale through entropic and enthalpic interactions. It was perceived that the thermodynamic miscibility, cooperative relaxations and concentration fluctuation in PMMA/SAN blends are controlled by the spatial distribution of GO in the blend. By varying the chain length, we were able to understand that shorter tethered chains lead to early phase separation whereas the tethered chain length similar to that of the host matrix delays phase separation. Interestingly, irrespective of the length of the tethered polymer chain, the grafted GO sheets were not localized in the thermodynamically preferred phase (here PMMA). These findings are of great importance to further help us understand the theories and simulations which pertain to chain dynamics of PMMA/SAN blends containing polymer-tethered nanoparticles with large specific surface area like GO.

Mobility of Polymer-Tethered Nanoparticles in Unentangled Polymer Melts.

A scaling theory is developed for the motion of a polymer-tethered nanoparticle (NP) in an unentangled polymer melt. We identify two types of scaling regimes depending on the NP diameter d and the size of a grafted polymer chain (tail) R tail . In one type of regimes, the tethered NP motion is dominated by the bare NP, as the friction coefficient of the tails is lower than that of the less mobile particle. The time dependence of the mean square displacement (MSD) of the tethered NP ⟨Δr 2(t)⟩ in the particle-dominated regime can be approximated by ⟨Δr 2(t)⟩ bare for the bare NP. In the other type of regimes, the tethered NP motion is dominated by the tails when the friction coefficient of the tails surpasses that of the particle at times longer than the crossover time τ ∗. In a tail-dominated regime, the MSD ⟨Δr 2(t)⟩ ≈ ⟨Δr 2(t)⟩ bare only for t < τ ∗. ⟨Δr 2(t)⟩ of a single-tail NP for t > τ ∗ is approximated as the MSD ⟨Δr 2(t)⟩ tail of monomers in a free tail, whereas ⟨Δr 2(t)⟩ of a multi-tail NP for t > τ ∗ is approximated as the MSD ⟨Δr 2(t)⟩ star of the branch point of a star polymer. The time dependence of ⟨Δr 2(t)⟩ in a tail-dominated regime exhibits two qualitatively different sub-diffusive regimes. The first sub-diffusive regime for t < τ ∗ arises from the dynamical coupling between the particle and the melt chains. The second sub-diffusive regime for t > τ ∗ occurs as the particle participates in the dynamics of the tails. For NPs with loosely grafted chains, there is a Gaussian brush region surrounding the NP, where the chain strands in Gaussian conformations undergo Rouse dynamics with no hydrodynamic coupling. The crossover time τ ∗ for loosely grafted multi-tail NPs in a tail-dominated regime decreases as the number of tails increases. For NPs with densely grafted chains, the tails are hydrodynamically coupled to each other. The hydrodynamic radii for the diffusion of densely grafted multi-tail NPs are approximated by the sum of the particle and tail sizes.

3D confined assembly of polymer-tethered gold nanoparticles into size-segregated structures

A mixture of binary polystyrene (PS)-tethered gold nanoparticles (GNPs) with different sizes can co-assemble into size-segregated core–shell structures under 3D confinement. The location and arrangement of GNPs can be effectively tuned by varying the chain length of the modified PS.

Self-Assembly of Giant Amphiphiles Based on Polymer-Tethered Nanoparticle in Selective Solvents

We study the self-assembly and formation process of vesicles of giant molecular shape amphiphiles in a selective solvent using the Brownian dynamics approach. Each amphiphile is composed of one hydrophilic nanoparticle tethered with one to five hydrophobic polymer tail(s), and the number of coarse-grained beads in each polymer tail is comparable to the number of repeating units in shape amphiphile used in the experiments. The effects of various parameters, such as the number of polymer tails, the length of each tail, the concentration of amphiphile beads, the size of the nanoparticle, and the temperature of the system on the self-assembled aggregate morphologies, are investigated. Morphological phase diagrams are constructed in different parameter spaces, and multiple morphological transitions are predicted and explained based on packing parameter. The formation pathways of vesicles are examined systematically, and mechanism II is identified for the first time in such shape amphiphilic systems. Transition...

Polystyrene-Grafted Silica Nanoparticles: Investigating the Molecular Weight Dependence of Glass Transition and Fragility Behavior

Polymer-tethered nanoparticles provide a strategy to improve particle dispersion in polymer nanocomposites and as materials themselves can exhibit self-healing behavior and enhanced mechanical properties. The few studies that previously characterized the glass transition temperature (Tg) behavior of neat polymer-grafted nanoparticles in the absence of a polymer matrix largely focused on average Tg response. We synthesized polystyrene-grafted silica nanoparticles (Si-PS) via ARGET ATRP, achieving the densely grafted state. Using differential scanning calorimetry, we investigated the brush molecular weight (MW) dependence of Tg, Tg breadth, heat capacity jump (ΔCp), and fragility from 12 to 98 kg/mol. Compared with free PS chains of the same MW, brush Tg increases by 1–2 °C, brush Tg breadth remains unchanged within error down to 36 kg/mol and increases by 3–4 °C at brush MWs of 12 and 13 kg/mol, and brush ΔCp and fragility remain unchanged within error down to 52 kg/mol and then decrease with decreasing MW...