Session S42: Assembly of Nanoparticles

Polydots, Soft Nanoparticles, at Membrane Interfaces

Luminescent polymers confined into long lived nano-configurations form dynamic nanoparticles (NPs) or polydots with a potential for new bio imaging markers and targeted drug delivery vehicles. A key step in the use of any NP for therapeutic applications is their translocation across membranes. Here we report the results of all-atom molecular dynamics simulation of a polydot that consists of carboxylate decorated dinonyl poly para phenylene ethynylene, at the interface with a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayer. The polydot size and surface charge are controlled by varying the polymer molecular weight and degree of carboxylation. The polydot structure and its effect on the membrane structure are probed. We find that the polydot remains stable as it transcends the membrane where the initial curvature of the membrane is strongly affected as polydot inserted, but it relaxes with time. The larger the polydots are the less dynamic the DPPC molecules become. Further we find that neutral-surface polydots reside in the center of the bilayer, while increasing the polydot surface charge, the polydot migrates towards the hydrophilic leaflet of the bilayer.   

Surface-Engineered Graphene Quantum Dots for Shape Control of Block Copolymer Particles

Surface-engineered, 10 nm-sized graphene quantum dots (GQDs) are shown to be efficient surfactants for producing poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) particles that feature tunable shapes and internal morphologies. The surface properties of GQDs were modified by grafting different alkyl ligands, such as hexylamine and oleylamine, to generate the surfactant behavior of the GQDs. In stark contrast to the behavior of the unmodified GQDs, hexylamine-grafted GQDs and oleylamine-grafted GQD surfactants were selectively positioned on the PS and P4VP domains, respectively, at the surface of the particles. This positioning effectively tuned the interfacial interaction between two different PS/P4VP domains of the particles and the surrounding water during emulsification and induced a dramatic morphological transition to an unconventional convex lens-shaped particles. Precise and systematic control of interfacial activity of GQD surfactants was also demonstrated by varying the density of the alkyl ligands on the GQDs. The excellent surface tunability of 10 nm-sized GQDs combined with their significant optical and electrical properties highlight their importance as surfactants for producing colloidal particles with novel functions.   

Self-Assembled Soft Porous Particles with Tailored Nano-Porosity.

A series of porous block copolymer (BCP) particles with controlled porosity and nanostructure was fabricated by tuning interfacial hydrodynamics of toluene-in-water emulsion droplets. A synergistic adsorption of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) BCPs and sodium dodecyl sulfate (SDS) to the surface of emulsion particle induced a dramatic decrease in the interfacial tension and generated the interfacial instability at the particle surface, thus producing different types of particles including closed pore particles, open pore particles, capsules and micelles. In particular, the SDS concentration and the P4VP volume fraction of PS-b-P4VP were key parameters in determining the degree of interfacial instability of the emulsion, producing porous particles with tunable pore sizes ranging from 10 to 500 nm. These porous particles could be used as pH responsive carriers, which were demonstrated by combining and releasing of different colored dyes to particles at desired pH conditions.   

Particle-Directed Assembly of Semiflexible Polymer Chains

We use molecular dynamics simulations to investigate several models of semiflexible polymers that exhibit an attractive interaction with spherical particles. The organization of semiflexible polymer chains through attractive interactions with spherical particles occurs in several important processes in nature, such as the winding of DNA around histones and counter-ion condensation of charged polymers. The process is also of technological interest in the packaging of DNA for delivery to cells. In this presentation, we will present data on both the phase behavior and the kinetics of self-assembly as a function of the stiffness of the polymers, the attractive potential between the monomers and the particles, and the relative size of the monomers and particles. Our simulations suggest a transition between globular and rod-like aggregates that changes from a gradual to a sudden transition depending on particle size, and that rod formation is a slow, nucleation dependent process.   

Structure and Entanglement Factors on Dynamics of Polymer Grafted Magnetic Nanoparticles

Magnetic nanoparticles functionalized with long polymer chains at low graft density are interesting systems to study structure-dynamic relationships in polymer nanocomposites since they are shown to aggregate into strings in both solution and melts, and also into spheres and branched aggregates in the presence of free polymer chains. This work investigates the structure, free volume and entanglement effects in composites of polystyrene grafted iron oxide nanoparticles by measuring particle dynamics with x-ray photon correlation spectroscopy technique. Particles of highly ordered strings and aggregated systems follow heterogeneous dynamics commonly observed in jammed soft glassy systems and other nanocomposites. On the other hand, particle dynamics becomes diffusive in branched structures which could be caused by the less penetration of long matrix chains into the brushes. These results show that particle dynamics is dictated through the strong interactions of low graft density chains with the host polymer.   

Selective Permeability of Uranyl Peroxide Nanocages to Different Alkali Ions: Influences from Surface Pores and Hydration Shells

The precise guidance to different ions across the biological channels is essential for many biological processes. An artificial nanopore system will facilitate the study of ion transport mechanism through nanosized channels and offer new views for designing nanodevices. Here we reveal that a 2.5-nm-size, fullerene-shaped molecular cluster Li$_{\mathrm{48+m}}$K$_{\mathrm{12}}$(OH)$_{\mathrm{m}}$[UO$_{\mathrm{2}}$(O$_{\mathrm{2}})$(OH)]$_{\mathrm{60-}}$(H$_{\mathrm{2}}$O)$_{\mathrm{n}}$ (m$\approx $20 and n$\approx $310) (\textbf{U}$_{\mathrm{\mathbf{60}}})$ shows selective permeability to different alkali ions. The sub-nanometer pores on the water-ligand-rich surface of \textbf{U}$_{\mathrm{\mathbf{60}}}$ are able to block Rb$^{\mathrm{+}}$ and Cs$^{\mathrm{+}}$ ions from passing through, while allow Na$^{\mathrm{+}}$ and K$^{\mathrm{+}}$ ions, which possess larger hydrated sizes, to enter the interior space of \textbf{U}$_{\mathrm{\mathbf{60}}}$. An interestingly high entropy gain during the binding process between \textbf{U}$_{\mathrm{\mathbf{60}}}$ and alkali ions suggest that the hydration shells of Na$^{\mathrm{+}}$/K$^{\mathrm{+}}$ and \textbf{U}$_{\mathrm{\mathbf{60}}}$ are damaged during the interaction. The ion selectivity of \textbf{U}$_{\mathrm{\mathbf{60}}}$ is greatly influenced by both the morphologies of surface nanopores and the dynamics of the hydration shells.   

Understanding of DNA directed nanoparticle superlattices in bulk and thin film

Over the years, there have been significant advances in assembling nanoparticles with DNA into superlattices. Since the first reports on DNA directed FCC and BCC superlattices consisting of single type of spherical nanoparticles,[1,2] building blocks for the DNA-nanoparticle superlattices have been extended from a spherical gold nanoparticle to various types of other particles including quantum dots, magnetic, hollow, or polyhedral particles.[3,4] Not only single component, but superlattices of binary[5] and ternary components[6] have also been synthesized. Although still many details are unclear, now there is a general consensus about thermodynamics of this type of assembly, which led us to fabricate thin films of DNA directed nanoparticle superlattices on substrate for applications such as optical materials.[7] Since the structures are formed in aqueous condition, small angle x-ray scattering (SAXS) that does not disturb the system has been a critical tool to determine structural and thermodynamic characteristics of the assemblies. Thus, we have also been improving SAXS instrumentations and computational methods to calculate scattering profiles for the nanoparticle superlattices.[8] In this talk, we will summarize our works with a focus on some structural details of these superlattices and DNA and understanding about the role of DNA in the crystallization processes in bulk and thin film. 1. Park, S. Y. et al, C. A. Nature 2008, 451, 553. 2. Nykypanchuk, D. et al, Nature 2008, 451, 549. 3. Jones, M. R. et al, Nat Mater 2010, 9, 913. 4. O'Brien, M. N. et al, Nat Mater 2015, 14, 833. 5. Macfarlane, R. J. et al, Science 2011, 334, 204. 6. Macfarlane, R. J. et al, Science 2013, 341, 1222. 7. Senesi, A. J.et al, Angew. Chem. Intern. Ed. 2013, 52, 6624. 8. Senesi, A. J. and Lee, B. J. of Appl. Crystallog. 2015, 48, 1172.   

Modeling of water-borne coating: stress relaxation of suspensions of colloids linked by telechelic HEUR polymers.

In water-borne coatings, the rheology of colloidal suspensions is modified by the presence of rheological modifiers, such as Hydrophobic Ethoxylated Urethane (HEUR) polymers. HEUR is a telechelic polymer with two hydrophobic tails (hydrophobes) and a long hydrophilic interblock consisting of poly(ethylene oxide), and its thickening effect is largely determined by the self-association of hydrophobes as well as their adsorption onto latex particles. Here we describe a model that simulates the complex interactions among latex particles due to the formation of bridges or superbridges via model HEURs. We calculate the stress relaxation of the system and identify different relaxation modes. We explore the relaxation time at different latex volume fractions, HEUR concentrations and energies of association between hydrophobes and latex particles, and discuss its relationship with the bridge or latex cluster formation. These results provide important insights for HEUR adsorption and water-borne coating rheology.   

Anisotropic Packing of DNA-Mediated Colloidal Self-Assembly

The self-assembly of DNA-grafted nanoparticles has garnered considerable interest in recent years. However, many efforts focused on the usage of spherical nanoparticles, which limits us to the formation of only a handful of crystal lattices. Recent advances in the synthesis of non-spherical particles have directed attention towards the usage of these anisotropic particles for self-assembly. Here we combine experiments and theory on a series of DNA-grafted nanocubes. Our studies indicate that anisotropy not only directs where DNA linkers graft onto the particle but also affects how they pack and orient within a lattice, giving rise to both a preferential attachment effect and orientation-directed self-assembly. These results emphasize anisotropic self-assembly as a powerful new tool that allows for precise and directed control of nanoparticle self-assembly.   

Giant soft-memory in liquid crystal-nanocomposites

Here, we report a novel way of introducing giant, non-volatile soft-memory in a nanocomposite comprising of amphiphilic polymer functionalized barium titanate (BaTiO$_{\mathrm{3}})$ nanoparticles and~\textit{isotropic}~phase of 5CB liquid crystal. Doping of pure ferroelectric NPs in isotropic phase of 5CB creates nanoscopic domains of highly ordered regions as 5CB molecules arrange themselves around the NPs and we call these regions,~\textit{pseudonematic domains.~}Here, mesogens can~\textit{electromechanically}~rotate the BaTiO$_{\mathrm{3}}$~NPs within the domain, along the direction of applied electric field. These domains are spatially and thermodynamically locked-in and retain their directional orientation and net polarization even after the applied electric field is switched off. We call this net remnant polarization or hysteresis, `\textit{soft memory}'. When NPs are functionalized with amphiphilic block copolymers, self-assembly of mesogens occurs at the interface of polymer tethers and nanoparticles via combination of non-covalent coupling and $\pi $-$\pi $ stacking interaction and this results in multi-fold enhancement in the volume of pseudonematic domains and subsequent increase in the soft memory. This work provides new insight into understanding the interaction of nanoparticles, polymers and liquid crystal and potentially lead to the creation of nanoelectrocmehanical (NEMS) storage device using functionalized nanoparticles.   

Temperature Effects on Soft Polymeric Nanoparticles: Molecular Dynamics Study

Luminescent polymers collapsed into soft nanoparticles or polydots have emerged as the potential candidates for biomedical applications such as drug delivery and biosensing. Here, using fully atomistic molecular dynamics simulation, the temperatures effects on the stability, internal structure and dynamics of polydots formed by substituted and bare dialkyl \textit{para }phenylene ethynylenes (PPEs) will be discussed. We find that with increasing temperature from 300 K to 600K both substituted and bare PPE polydots expand but do not fully unfold and remain in their confined state. As observed visually and by measurement of structure factor S(q), the overall shape of the both type of polydots changes from spherical to elongated with the increase in temperature. These effects are more pronounced for bare PPE polydots which show that interdigitation of side chains in substituted PPE polydots enhances stability. In addition, the side chains are more dynamic than the backbone..   

Spectroscopic Investigations on PVDF-MWCNTs Nanocomposites.

Nanocomposites have been obtained by dispersing Multi Walled Carbon Nanotubes (MWCNTs) within polyvinylidene fluoride. Various samples loaded by 0 to 20 {\%} wt. MWCNTs have been obtained by melt mixing using a Haake RheoMixer, with two counter rotating screws. The effect of the nanofiller concentration on the glass, melting, and crystallization temperatures, as determined from Differential Scanning Calorimetry measurements, is reported. Small shifts towards higher temperatures as the loading with MWCNTs was increased have been noticed. A detailed analysis on the effect of MWCNTs on the degree of crystallinity of PVDF is reported. Raman data obtained by using a Renishaw InVia spectrometer have been used to estimate the stress transfer. Additional information was obtained by FTIR and Wide Angle X-Ray Scattering. The nature of the crystalline phases was determined for each sample. Thermogravimetric data showed a small increase of the thermal stability of the polymeric matrix upon the loading with MWCNTs.   

Investigation of Molecular Interactions between AFM-Tip and Thiol Films

Among various self-assembly processes, the formation of a self-assembled monolayer (SAM) is one of the most elegant ways for making an organic film with specific surface properties. Recently, much effort has been devoted in using AFM-based single-molecule force spectroscopy (SMFS) to understanding the formation of alkanethiol SAMs on gold surfaces. Investigating the factors that affect the AFM tip-SAMs interactions is necessary to clarify the controversial results of these studies. Here, we investigated the interactions between bare AFM-tips and several SAMs thiols-gold surfaces under controlled humidity conditions. Our results demonstrate that the Tip-SAM interactions can be used to precisely determine the length of the thiol chains, the adhesion force between thiols head groups and the AFM tip, and the strength of the thiol-gold contact. Our findings on the dynamics and the structure of the SAMs of alkanethiols on gold are useful for detail understanding of the thermodynamics, kinetics and mechanisms of SAM technology assembly.