Todd Emrick

Nanoparticle polymer composites: Where two small worlds meet

The mixing of polymers and nanoparticles is opening pathways for engineering flexible composites that exhibit advantageous electrical, optical, or mechanical properties. Recent advances reveal routes to exploit both enthalpic and entropic interactions so as to direct the spatial distribution of nanoparticles and thereby control the macroscopic performance of the material. For example, by tailoring the particle coating and size, researchers have created self-healing materials for improved sustainability and self-corralling rods for photovoltaic applications. A challenge for future studies is to create hierarchically structured composites in which each sublayer contributes a distinct function to yield a mechanically integrated, multifunctional material.

Self-directed self-assembly of nanoparticle/copolymer mixtures

The organization of inorganic nanostructures within self-assembled organic or biological templates is receiving the attention of scientists interested in developing functional hybrid materials. Previous efforts have concentrated on using such scaffolds to spatially arrange nanoscopic elements as a strategy for tailoring the electrical, magnetic or photonic properties of the material. Recent theoretical arguments have suggested that synergistic interactions between self-organizing particles and a self-assembling matrix material can lead to hierarchically ordered structures. Here we show that mixtures of diblock copolymers and either cadmium selenide- or ferritin-based nanoparticles exhibit cooperative, coupled self-assembly on the nanoscale. In thin films, the copolymers assemble into cylindrical domains, which dictate the spatial distribution of the nanoparticles; segregation of the particles to the interfaces mediates interfacial interactions and orients the copolymer domains normal to the surface, even when one of the blocks is strongly attracted to the substrate. Organization of both the polymeric and particulate entities is thus achieved without the use of external fields, opening a simple and general route for fabrication of nanostructured materials with hierarchical order.

Self-assembly of nanoparticles at interfaces

Developments in the assembly of nanoparticles at liquid-liquid interfaces are reviewed where the assemblies can be controlled by tuning the size of the nanoparticles and the chemical characteristics of the ligands. Both synthetic and biological nanoparticles are discussed. By controlling the type of ligands, uniform and Janus-type nanoparticles can be produced where, at liquid-liquid interfaces, subsequent reactions of the ligands can be used to generate crosslinked sheets of nanoparticles at the interface that have applications including novel encapsulants, filtration devices with well-defined porosities, and controlled release materials. By controlling the size and volume fraction of the nanoparticles and the chemical nature of the ligands, nanoparticle-polymer composites can be generated where either enthalpy or entropy can be used to control the spatial distribution of the nanoparticles, thereby, producing auto-responsive materials that self-heal, self-corral assemblies of nanoparticles, or self-direct morphologies. Such systems hold great promise for generating novel optical, acoustic, electronic and magnetic materials.

Fulleropyrrolidine interlayers: Tailoring electrodes to raise organic solar cell efficiency

Layering on solar cell power and stability Solar cells made from carbon-based polymers are helpfully flexible. However, theres been a frustrating tradeoff between cell stability and efficiency when converting solar power to electrical power. Page et al. offer a strategy to partially resolve this dilemma by inserting a layer of polar organic compound (a fullerene derivative) between the cathode (the positive pole in the circuit) and the rest of the cell. Aluminum is an efficient cathode material but is prone to oxidative degradation. The easily applied polar layer enables the use of more stable metals, such as silver and copper, for the cathode, while counteracting their tendencies to diminish power conversion efficiency. Science, this issue p. 441 A polar fullerene derivative layered on the cathode of a polymer solar cell fosters simultaneous stability and efficiency. A major challenge in organic solar cell design is the trade-off between oxidative stability and work function of the metal cathode. We found that in single-junction polymer solar cells, this problem can be surmounted by solution-based incorporation of fulleropyrrolidines with amine (C60-N) or zwitterionic (C60-SB) substituents as cathode-independent buffer layers. Specifically, a thin layer of C60-N reduced the effective work function of Ag, Cu, and Au electrodes to 3.65 electron volts. Power conversion efficiency values exceeding 8.5% were obtained for organic photovoltaics independent of the cathode selection (Al, Ag, Cu, or Au). Such high efficiencies did not require precise control over interlayer thickness, as devices prepared with C60-N and C60-SB layers ranging from 5 to 55 nanometers performed with high efficiency.

Hierarchical Helical Assembly of Conjugated Poly(3-hexylthiophene)-block poly(3-triethylene glycol thiophene) Diblock Copolymers

We report on the solution-state assembly of all-conjugated polythiophene diblock copolymers containing nonpolar (hexyl) and polar (triethylene glycol) side chains. The polar substituents provide a large contrast in solubility, enabling formation of stably suspended crystalline fibrils even under very poor solvent conditions for the poly(3-hexylthiophene) block. For appropriate block ratios, complexation of the triethylene glycol side chains with added potassium ions drives the formation of helical nanowires that further bundle into superhelical structures.

Surface Modification of Tobacco Mosaic Virus with “Click” Chemistry

Cu-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC) reaction, renascent of the well-known Huisgen reaction, has recently flourished with applications in organic synthesis, drug discovery, polymer and materials science, and biotechnology. The high reaction yield, simple reaction and purification conditions, wide range of solvent and pH stabilities, and functional group tolerance make the CuAAC reaction a prototypical “click chemistry”, ideal for incorporating functionalities onto desired scaffolds. Over the years it has been widely employed to construct and functionalize polymeric and polyvalent display systems, including polymers, dendrimers, nanoparticles, and surfaces, where an extremely high reaction efficiency for every unit reaction is desirable. In particular, as organic azides and alkynes are almost unreactive with biomolecules and water, CuAAC reactions have been employed in derivatizing biomacromolecules, viruses, and cells with high efficacy under mild reaction conditions. Recently tyrosine residues have been considered as a particularly attractive target for chemoselective modification of proteins because of its subabundant distribution. Francis and coworkers have reported a number of transformations, ranging from the Mannich-type reaction to a transition metal mediated allylation reaction to a diazonium-coupling reaction, which can efficiently target the phenolic group of tyrosine residues at physiological conditions. To overcome the sluggish reactivity with electron-enriched diazonium salts, a sequential reduction/oxidation/Diels–Alder reaction was developed to break the limitation of functionalities being incorporated. In this communication, we report that CuAAC reactions can be combined with a diazonium-coupling reaction to quantitatively functionalize tyrosine residues with a wide array of starting materials. Tobacco Mosaic Virus (TMV) is a classic example of rodlike plant viruses consisting of 2130 identical protein subunits arranged helically around genomic single RNA strand. The length of TMV, that is, 300 nm, is defined by the encapsulated genomic RNA that stabilizes the coat protein assembly. The polar outer and inner surfaces of TMV have been exploited as templates to grow metal or metal oxide nanowires, and conductive polymers have been coated on 1D assembled TMV to produce conductive nanowires. TMV based materials have recently shown great potential with applications in nanoelectronics and energy harvesting devices. In addition, it has been reported that tyrosine residues (Y139) of TMV are viable for chemical ligation using the electrophilic substitution reaction at the ortho position of the phenol ring with diazonium salts. This reaction is very efficient, yet has two distinct disadvantages for broader applications. First, it is difficult to synthesize desired starting materials; and second, the reaction is not compatible with acid-labile functional groups and suitable for electron-deficient anilines only. To embrace the structural diversity of various starting materials, TMV offers an ideal polyvalent display system which allows us to test the efficiency of CuAAC reaction in combining with the tyrosine ligation reaction. As shown in Scheme 1, TMV was first treated with the diazonium salt generated from 3-ethynylaniline 1 in situ adapted from the protocol reported by Francis and co-workers. MALDI-TOF MS analysis indicated that >95% of the capsid monomers were converted into alkyne derivatives 2 (Figure 1) despite the absence of a strong electron withdrawing group in the diazonium reagent. Encouraged by this result, the CuAAC reactions between 2 and azides were explored. For bioconjugation reactions using CuAAC, the Cu catalysts are either generated directly by addition of Cu salts, or in situ from soluble Cu sources and a reducing agent, such as a copper wire, phosphines, thiols, or ascorbate. Multidentate heterocyclic ligands are often required for enhancing the reaction efficiency. Upon screening a series of reaction conditions, we found that the combination of CuSO4/sodium ascorbate (NaAsc) gave the best results. Whereas it is destructive to most other protein complex systems, ascorbate is evidently benign to TMV and has no impact on its structural integrity. 3-Azido-7-hydroxy-coumarin a was first employed as the ACHTUNGTRENNUNGazido counterpart in the reaction, which could be easily monitored by UV-visible absorption at 340 nm (Figure 1B). As a general protocol, 2 (2 mgmL ) and a (3 mm) were added to a solution of CuSO4 (1 mm) and NaAsc (2 mm) in Tris buffer (10 mm, pH 7.8) with 20% DMSO (used to increase the solubility of the azide component). After incubation for 18 h at room temperature, the viral particles were separated from the small molecules by sucrose gradient sedimentation. The integrity of TMV was confirmed by TEM and size-exclusion chromatography (SEC) analysis (data not shown). A strong absorption at 340 nm indicated the successful attachment of coumarin motifs (Figure 1B). MALDI-TOF MS analysis indicated a near quantitative transformation of surface alkynes to triazoles as shown in Figure 1A. [a] M. A. Bruckman, G. Kaur, L. A. Lee, F. Xie, J. Sepulveda, Dr. Q. Wang Department of Chemistry and Biochemistry and Nanocenter University of South Carolina 631 Sumter Street, Columbia, South Carolina 29208 (USA) Fax: (+1)803-777-9521 E-mail : wang@mail.chem.sc.edu [b] R. Breitenkamp, X. Zhang, M. Joralemon, Dr. T. P. Russell, Dr. T. Emrick Polymer Science and Engineering Department, University of Massachusetts Conte Center for Polymer Research, Massachusetts 01003 (USA)

Adsorption Energy of Nano- and Microparticles at Liquid-Liquid Interfaces

We study experimentally the energy of adsorption, DeltaE, of nanoparticles and microparticles at the oil-water interface by monitoring the decrease of interfacial tension as the particles bind. For citrate-stabilized gold nanoparticles assembling on a droplet of octafluoropentyl acrylate, we find DeltaE = -5.1 k(B)T for particle radius R = 2.5 nm and DeltaE proportional, variant R(2) for larger sizes. Gold nanoparticles with (1-mercaptoundec-11-yl)tetra(ethylene glycol) ligand have a much larger binding energy (DeltaE = -60.4 k(B)T) and an energy barrier against adsorption. For polystyrene spheres with R = 1.05 microm, we find DeltaE = -0.9 x 10(6) k(B)T. We also find that the binding energy depends on the composition of the oil phase and can be tuned by the salt concentration of the nanoparticle suspension. These results will be useful for controlling the assembly of nanoparticles at liquid interfaces, and the method reported here should be broadly useful for quantitative measurements of binding energy.

Synthesis and photophysical property of well-defined donor–acceptor diblock copolymer based on regioregular poly(3-hexylthiophene) and fullerene

A new, well-defined diblock copolymer (P3HT-b-C60) based on regioregular poly(3-hexylthiophene) (P3HT) and fullerene was synthesized. First, regioregular P3HT was synthesized through Grignard metathesis polymerization, and then methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) were copolymerized by using an end-functionalized P3HT as a macroinitiator for the atom transfer radical polymerization to yield a diblock copolymer (P3HT-b-P(MMA-r-HEMA)). A fullerene derivative functionalized with carboxylic acid, [6,6]-phenyl-C61-butyric acid (PCBA), was then chemically linked to the HEMA unit in the second block (P(MMA-r-HEMA)) to produce a diblock copolymer with the second block containing fullerenes. Annealing thin films of the copolymer revealed nanometer-scale phase separation, a more suitable morphology for enabling excitons generated in the P3HT domain to more efficiently reach the donor–acceptor interface, relative to simple blends of P3HT and C60. As a result, photoluminescence of the P3HT-b-C60diblock copolymer in the films showed a complete quenching of photoluminescence of P3HT, which is indicative of charge transfer between P3HT and fullerene.

Synthesis of C60-end capped P3HT and its application for high performance of P3HT/PCBM bulk heterojunction solar cells

A new C60-end capped poly(3-hexylthiophene) (P3HT-C60) was synthesized via a simple three-step process, and used as a compatibilizer for P3HT/PCBM composite for the purpose of controlling the morphology of P3HT/PCBM composite film, and thus improving the long-term thermal stability of solar cell performance. When a small amount of P3HT-C60 was added to P3HT/PCBM, the bicontinuous and nanometre-scale film morphology was developed and preserved for 2 h of annealing at 150 °C. Furthermore, the addition of P3HT-C60 as a compatibilizer suppressed large-scale phase separation of P3HT/PCBM composite even after prolonged annealing time (8 days), and as a result, the P3HT/PCBM/P3HT-C60 bulk heterojunction solar cells exhibited the excellent long-term thermal stability of device performance.

Stabilizing liquid drops in nonequilibrium shapes by the interfacial jamming of nanoparticles.

Dynamic Surfactants Surfactants are used to form a stable interface between two nonmiscible liquids, like oil and water, so that droplets of one fluid can be entrained in the other. Cui et al. (p. 460) designed a surfactant based on the association of a hydrophilic nanoparticle with a functionalized oleophilic molecule that self-assembles at a water-oil interface to produce a composite surfactant. Once adsorbed, the nanoparticles tend to remain in place causing them to accumulate and “jam” at the interface. When a drop was deformed, more surfactant could assemble at the surface, allowing droplets of various shapes to be produced. A self-assembling composite surfactant can be used to make stable droplets with complex shapes. Nanoparticles assemble at the interface between two fluids into disordered, liquid-like arrays where the nanoparticles can diffuse laterally at the interface. Using nanoparticles dispersed in water and amine end-capped polymers in oil, nanoparticle surfactants are generated in situ at the interface overcoming the inherent weak forces governing the interfacial adsorption of nanoparticles. When the shape of the liquid domain is deformed by an external field, the surface area increases and more nanoparticles adsorb to the interface. Upon releasing the field, the interfacial area decreases, jamming the nanoparticle surfactants and arresting further shape change. The jammed nanoparticles remain disordered and liquid-like, enabling multiple, consecutive deformation and jamming events. Further stabilization is realized by replacing monofunctional ligands with difunctional versions that cross-link the assemblies. The ability to generate and stabilize liquids with a prescribed shape poses opportunities for reactive liquid systems, packaging, delivery, and storage.