Micelle-Encapsulated Carbon Nanotubes: A Route to Nanotube Composites

We report a general approach toward dispersing single-walled carbon nanotubes (SWNTs) in solvents and polymer materials, by encapsulating SWNTs within cross-linked micelles. Micelles made from polystyrene-block-poly(acrylic acid) (PS-b-PAA), an amphiphilic block copolymer, are first assembled around SWNTs by gradually adding H2O to a suspension of nanotubes in dimethylformamide. The hydrophilic, outer shells of these micelles are then chemically cross-linked with a difunctional linker molecule. Pure encapsulated SWNTs (e-SWNTs) can then be separated from empty cross-linked micelles by consecutive cycles of centrifugation and redispersion. Atomic force and transmission electron microscopies of the resulting nanostructures demonstrate that individual nanotubes (rather than bundles) have been completely encased in polymer shells whose thickness is slightly larger than that of empty micelles. e-SWNTs encapsulated in PS-b-PAA can be permanently redispersed in H2O, in organic solvents, and in both hydrophobic and hydrophilic polymer matrices with minimal sonication. Micelle encapsulation could improve the compositing of SWNTs in a wide variety of polymer materials for structural, electronic, and thermal applications

Controlling Shell Thickness in Core−Shell Gold Nanoparticles via Surface-Templated Adsorption of Block Copolymer Surfactants

When Au nanoparticles are encapsulated within shells of cross-linked, block copolymer amphiphiles, the structure of the shells is determined by the initial interaction between the amphiphile and the nanoparticle surface. In the case of small nanoparticles, for which particle size is comparable to the dimension of the block copolymer (ρAu/Rg ≈ 1), particles act like solutes that are dissolved within polystyrene-block-poly(acrylic acid) (PS-b-PAA) micelle cores. In the case of larger nanoparticles (ρAu/Rg > 1), PS-b-PAA adsorption is templated by the particle surface, and a concentric core−shell structure is formed. The thickness of this shell can be predicted from theoretical models of polymer adsorption onto highly curved surfaces and controlled by varying the ratio of polymer to available nanoparticle surface area. We anticipate that these rules will illustrate how cross-linked copolymer shells with predetermined thickness can be used to stabilize and functionalize a variety of nanoparticle materials

Multicomponent Nanoparticles via Self-Assembly with Cross-Linked Block Copolymer Surfactants

We describe a simple and versatile protocol to prepare water-soluble multifunctional nanostructures by encapsulation of different nanoparticles in shell cross-linked, block copolymer micelles. This method permits simultaneous incorporation of different nanoparticle properties within a nanoscale micellar container. We have demonstrated the co-encapsulation of magnetic (γ-Fe2O3 and Fe3O4), semiconductor (CdSe/ZnS), and metal (Au) nanoparticles in different combinations to form multicomponent micelles that retain the precursor particles' distinct properties. Because these multifunctional hybrid nanostructures spontaneously assemble from solution by simultaneous desolvation of nanoparticles and amphiphilic block copolymer components, we anticipate that this can be used as a general protocol for preparing multifunctional nanostructures without explicit multimaterial synthesis or surface functionalization of nanoparticles

Alignment of Liquid-Crystalline Polymers by Field-Oriented, Carbon Nanotube Directors

Bulk liquid-crystalline polymer (LCP) material can be macroscopically aligned by seeding the growth of LCP domains with preoriented, single-wall carbon nanotubes (SWNTs). SWNT seeds dispersed in a columnar polyoxazoline melt were first oriented by applying an ac electric field across the molten material. Then, cooling the material below the liquid-crystalline transition temperature yielded domains that were oriented in the direction of the applied field. The orientation of these domains was characterized by temperature-controlled polarized light microscopy, static birefringence, and wide-angle X-ray scattering. These experiments demonstrate that domains can be oriented at field strengths that are orders of magnitude lower than those used to pole anisotropic polymers alone, and that the seeded domains can grow together to form homogeneously oriented bulk material. Because the nanotubes act only as nucleants, very little SWNT material is required. We anticipate that this general concept could aid the fabrication of monolithic objects with tailored anisotropic properties from SWNT−polymer composites

Oligothiol Graft-Copolymer Coatings Stabilize Gold Nanoparticles against Harsh Experimental Conditions

We report that poly­(l-lysine)-<i>graft</i>-poly­(ethylene glycol) (PLL-<i>g</i>-PEG) copolymers that bear multiple thiol groups on the polymer backbone are exceptional ligands for gold nanoparticles (AuNPs). In general, these graft copolymer ligands stabilize AuNPs against environments that would ordinarily lead to particle aggregation. To characterize the effect of copolymer structure on AuNP stability, we synthesized thiolated PLL-<i>g</i>-PEGs (PLL-<i>g</i>-[PEG:SH]) with different backbone lengths, PEG grafting densities, and number of thiols per polymer chain. AuNPs were then combined with these polymer ligands, and the stabilities of the resulting AuNP@PLL-<i>g</i>-[PEG:SH] particles against high temperature, oxidants, and competing thiol ligands were characterized using dynamic light scattering, visible absorption spectroscopy, and fluorescence spectrophotometry. Our observations indicate that thiolated PLL-<i>g</i>-PEG ligands combine thermodynamic stabilization via multiple Au–S bonds and steric stabilization by PEG grafts, and the best graft copolymer ligands balance these two effects. We hope that this new ligand system enables AuNPs to be applied to biotechnological applications that require harsh experimental conditions

Two-Color Labeling of Oligonucleotide Arrays via Size-Selective Scattering of Nanoparticle Probes

Two-Color Labeling of Oligonucleotide Arrays via Size-Selective Scattering of Nanoparticle Probe

Plasmonic Nanoparticle Chains via a Morphological, Sphere-to-String Transition

Au nanoparticles encapsulated within polystyrene-block-poly(acrylic acid) (PS-b-PAA) micelles assemble into regular, one-dimensional arrays when they are exposed to solvent conditions that relax interfacial curvature in the micellar shell. Nanoparticle chaining was induced by adding salt, acid, or cationic carbodiimide to the suspension of purified encapsulated Au nanoparticles (Au@PS-b-PAA). The resulting assemblies were characterized by scanning and transmission electron microscopies, by dark-field optical microscopy, and by visible absorption spectroscopy. The length of the chains was modulated by varying the concentration of additive. More importantly, the spacing between Au nanoparticles was dictated entirely by the shell thickness of the Au@PS-b-PAA starting material. Far-field polarization microspectroscopy demonstrated directional surface plasmon coupling in a straightened nanoparticle chain, which is a basic requirement for the use of these assemblies as plasmon waveguides

Encapsulated Magnetic Nanoparticles as Supports for Proteins and Recyclable Biocatalysts

This paper describes the bioconjugation of histidine-tagged enzymes and other proteins to the surface of composite “magnetomicelles” consisting of magnetic γ-Fe2O3 nanoparticles encapsulated within cross-linked polystyrene-block-polyacrylate copolymer micelle shells. Free carboxylic acid groups on the magnetomicelle surface were converted to Cu2+-iminodiacetic acid (IDA) for protein capture. The conjugation of T4 DNA ligase and enhanced green fluorescent protein to magnetomicelles revealed that proteins were captured with a high surface density and could be magnetically separated from reaction mixtures and subsequently released from the nanoparticle surface. Additionally, bioconjugation of T7 RNA polymerase yielded a functional enzyme that maintained its biological activity and could be recycled for up to three subsequent transcription reactions. We propose that protein−magnetomicelle bioconjugates are effective for protein bioseparation and enzymatic recycling and further strengthen the idea that nanoparticle surfaces have utility in protein immobilization

Enhanced Stability and Bioconjugation of Photo-Cross-Linked Polystyrene-Shell, Au-Core Nanoparticles

Encapsulating Au nanoparticles within a shell of photo-cross-linked block copolymer surfactant dramatically improves the physical and chemical stability of the nanoparticles, particularly when they are applied as bioconjugates. Photo-cross-linkable block copolymer amphiphiles [polystyrene-co-poly(4-vinyl benzophenone)]-block-poly(acrylic acid) [(PS-co-PVBP)-b-PAA] and [poly(styrene)-co-poly(4-vinyl benzophenone)]-block-poly(ethylene oxide) [(PS-co-PVBP)-b-PEO] were assembled around Au nanoparticles ranging from 12 to 108 nm in diameter. UV irradiation cross-linked the PVBP groups on the polymer to yield particles that withstood extremes of temperature, ionic strength, and chemical etching. Streptavidin was attached to [PS-co-PVBP]-b-PAA-coated particles using the same noncovalent and covalent conjugation protocols used to bind biomolecules to divinylbenzene-cross-linked PS microspheres. We expect that these particles will be useful as plasmonic, highly light-scattering and light-absorbing analogs to fluorescently labeled PS nanospheres

Homogeneous, Coaxial Liquid Crystal Domain Growth from Carbon Nanotube Seeds

We have developed a general method for aligning anisotropic materials by using carbon nanotubes to influence order in the surrounding material. Specifically, we have shown that carbon nanotubes seed the formation of oriented domains in a liquid crystalline polymer (LCP). Using polarized light microscopy, we have observed that the molecular alignment in these large (10−100 μm long) domains is homogeneous and controlled by the direction of the nanotube nucleus. The kinetic nature of this nucleation process was verified by differential scanning calorimetry. The coupling of preferential nucleation and controlled seed orientation may allow bulk LCP materials to be aligned by simply preorganizing a small number of dispersed nanotube seeds. We expect that this work will aid in the development and application of macroscopically ordered nanostructured composite materials