Here, we report a simple method to control the location of nanoparticles in colloidal block-copolymer assemblies by using nanoparticles modified with mixed surface ligands. The binary self-assembly of amphiphilic polymers of polystyrene-b-poly(acrylic acid) (PS-b-PAA) and gold nanoparticles (AuNPs) modified with a hydrophobic ligand, dodecanethiol (DT), led to polymer micelles with nanoparticles segregated in the core of polymer micelles. On the other hand, AuNPs modified with mixed ligands of mercaptoundecanol (MUL) and DT were distributed at the PS–PAA interface, reducing the interfacial energy between the two polymers. This result was in good agreement with the prediction by the surface energy calculations. We also showed that the AuNPs with mixed ligands can decorate preformed polymer assemblies by the interfacial self-assembly. Furthermore, we demonstrated the compartmentalization of two different types of nanoparticles in colloidal polymer assemblies based on the strategy.

One- or two-dimensional arrays of iron oxide nanoparticles were formed in colloidal assemblies of amphiphilic polymers. Electron tomography imaging revealed that nanoparticles are arranged into one-dimensional strings in magneto-micelles or two-dimensional sheets in magneto-core/shell assemblies. The distinct directional assembly behavior was attributed to the interparticle interaction relative to the nanoparticle–polymer interaction, which was modulated by varying the cosolvent used for the solution phase self-assembly. Magneto-core/shell assemblies with varying structural parameters were formed with a range of different sized as-synthesized nanoparticles. The transverse magnetic relaxivity rates (r2) of a series of different assemblies were determined to examine the effect of nanoparticle arrangement on the magnetic relaxivity for their potential applications in MRI. The results indicated that the assembly structure of nanoparticles in polymer micelles significantly affects the r2 of surrounding water, providing a way to control magnetic relaxivity.Keywords: amphiphilic; block copolymer; iron oxide; magnetic relaxivity; nanoparticles

Gold nanoshells with varying surface topographies and tunable SPR bands were synthesized in high yields by the templated surfactant-assisted seed growth method. By changing the types and amounts of surfactants and ionic additives in the growth solution, the nanoshell topography was controlled from smooth shells to highly structured nanoshells composed of spherical nanoparticles or sharp spikes of varying aspect ratios. The SPR band of the nanoshells could be tuned over a wide range of wavelengths by varying the nanoshell topography, without significantly changing the amount of gold. Finite-difference time-domain (FDTD) modeling was used to predict and understand the optical properties of nanoshells composed of various subparticles, providing insight into the origins of the tunable SPR band.

Here, we report an unusual oxidation-induced photoluminescence (PL) turn-on response of a poly(3-alkoxythiophene), poly(3-{2-[2-(2-ethoxyethoxy)ethoxy]ethoxy}thiophene) (PEEEET). PEEEET shows a significantly red-shifted absorption spectrum compared to polyalkylthiophenes and is almost nonfluorescent (quantum yield ≪ 1%) in its pristine state. The introduction of sulfonyl defects along the polymer backbone by the oxidation of PEEEET with meta-chloroperbenzoic acid (m-CPBA) increased the emission quantum yield with the intensity increasing with the degree of oxidation. Molecular modeling data indicated that the oxidation-induced PL increase cannot be explained by the nature of monomer units and radiative rate changes. We attributed the enhanced fluorescence to the reduced nonradiative rate caused by the increased band gap, according to the energy gap law, which is consistent with the observed blue shifts in absorption and PL spectra accompanied by the PL increase.

Herein, we report a high-yield click synthesis and self-assembly of conjugated amphiphilic block copolymers of polythiophene (PHT) and polyethylene glycol (PEG) and their superstructures. A series of different length PHTm-b-PEGn with well-defined relative block lengths was synthesized by a click-coupling reaction and self-assembled into uniform and stably suspended nanofibers in selective solvents. The length of nanofibers was controllable by varying the relative block lengths while keeping other dimensions and optical properties unaffected for a broad range of fPHT (0.41 to 0.82), which indicates that the packing of PHT dominates the self-assembly of PHTm-b-PEGn. Furthermore, superstructures of bundled and branched nanofibers were fabricated through the self-assembly of PHTm-b-PEGn and preformed PHT nanofibers. The shape, length, and density of the hierarchical assembly structures can be controlled by varying the solvent quality, polymer lengths, and block copolymer/homopolymer ratio. This work demonstrates that complex superstructures of organic semiconductors can be fabricated through the bottom-up approach using preformed nanofibers as building blocks.Keywords: conjugated block copolymer; rod−coil; self-assembly; supramolecular nanostructures

We report how to control the self-assembly of magnetic nanoparticles and a prototypical amphiphilic block-copolymer composed of poly(acrylic acid) and polystyrene (PAA-b-PS). Three distinct structures were obtained by controlling the solvent−nanoparticle and polymer−nanoparticle interactions: (1) polymersomes densely packed with nanoparticles (magneto-polymersomes), (2) core−shell type polymer assemblies where nanoparticles are radially arranged at the interface between the polymer core and the shell (magneto-core shell), and (3) polymer micelles where nanoparticles are homogeneously incorporated (magneto-micelles). Importantly, we show that the incorporation of nanoparticles drastically affects the self-assembly structure of block-copolymers by modifying the relative volume ratio between the hydrophobic block and the hydrophilic block. As a consequence, the self-assembly of micelle-forming block-copolymers typically produces magneto-polymersomes instead of magneto-micelles. On the other hand, vesicle-forming polymers tend to form magneto-micelles due to the solubilization of nanoparticles in polymer assemblies. The nanoparticle−polymer interaction also controls the nanoparticle arrangement in the polymer matrix. In N,N-dimethylformamide (DMF) where PS is not well-solvated, nanoparticles segregate from PS and form unique radial assemblies. In tetrahydrofuran (THF), which is a good solvent for both nanoparticles and PS, nanoparticles are homogeneously distributed in the polymer matrix. Furthermore, we demonstrated that the morphology of nanoparticle-encapsulating polymer assemblies significantly affects their magnetic relaxation properties, emphasizing the importance of the self-assembly structure and nanoparticle arrangement as well as the size of the assemblies.

Unique radial arrays of quantum dots can be formed by the cooperative self-assembly of quantum dots and amphiphilic block-copolymers. Here, we report the effect of nanoparticle volume fractions as well as the length and concentration of polymers on the self-assembly structure. It was found that the size of the assemblies and the radial position of nanoparticles can be effectively controlled by changing the volume fraction of nanoparticles. Contrary to the typical trend observed for homogeneous incorporation of nanoparticles, increases in nanoparticle volume fractions resulted in decreases in the size and size distribution of the assemblies over a wide nanoparticle volume fraction range. The strong segregation theory calculations indicate that the structural change is due to a balance between the chain stretching energy and the interfacial energy between the two blocks. In addition, the coassemblies became larger with increasing nanoparticle sizes at maximum nanoparticle volume fractions. However, the polymer length did not significantly affect the structural parameters at maximum nanoparticle volume fractions. These findings indicate that the design rules established for the self-assembly of amphiphilic block-copolymers do not directly apply to the coassembly structure with nanoparticles and that the nanoparticles play an active role in the self-assembly.

We report a novel class of amphiphilic conjugated block copolymers composed of poly(3-octylthiophene) and poly(ethylene oxide) (POT-b-PEO) that exhibit highly tunable photoluminescence colors spanning from blue to red. POT-b-PEO self-assembles into various well-defined core/shell-type nanostructures as a result of its amphiphilicity. The self-assembly structure can be readily controlled by altering the solvent composition or by other external stimuli. The color change was completely reversible, demonstrating that the strategy can be used to manipulate the light-emission properties of conjugated polymers in a highly controllable manner without having to synthesize entirely new sets of molecules.

We report a high-yield synthetic method for a new type of metal nanostructure, spiky gold nanoshells, which combine the morphological characteristics of hollow metal nanoshells and nanorods. Our method utilizes block copolymer assemblies and polymer beads as templates for the growth of spiky nanoshells. Various shapes of spiky metal nanoshells were prepared in addition to spherical nanoshells by using block copolymer assemblies such as rod-like micelles, vesicles, and bilayers as templates. Furthermore, spiky gold shells encapsulating magnetic nanoparticles or quantum dots were prepared based on the ability of block copolymers to self-assemble with various types of nanoparticles and molecules. The capability to encapsulate other materials in the core, the shape tunability, and the highly structured surface of spiky nanoshells should benefit a range of imaging, sensing, and medical applications of metal nanostructures.

Nanoparticles can form unique cavity-like structures in core−shell type assemblies of block copolymers through the cooperative self-assembly of nanoparticles and block copolymers. We show that the self-assembly behavior is general for common as-synthesized alkyl-terminated nanoparticles for a range of nanoparticle sizes. We examined various self-assembly conditions such as solvent compositions, nanoparticle coordinating ligands, volume fraction of nanoparticles, and nanoparticle sizes in order to elucidate the mechanism of the radial assembly formation. These experiments along with strong segregation theory calculations indicated that both the enthalpic interaction and the polymer stretching energy are important factors in the coassembly formation. The slightly unfavorable interaction between the hydrophobic segment of polymers and alkyl-terminated nanoparticles causes the accumulation of nanoparticles at the interface between the polymer core and the shell, forming the unique cavity-like structure. The coassemblies were stabilized for a limited range of nanoparticle volume fractions within which the inclusion of nanoparticle layers reduces the polymer stretching. The volume fraction range yielding the well-defined radial coassembly structure was mapped out with varying nanoparticle sizes. The experimental and theoretical phase map provides the guideline for the coassembly formation of as-synthesized alkyl-terminated nanoparticles and amphiphilic block copolymers.

Conjugated polymers have excellent light-emitting properties that are useful for biological imaging and sensing applications. Here, we report a new way to form stable, water-soluble suspensions of conjugated polymers by encapsulating them in amphiphilic block copolymers. It was found that the folding property of conjugated polymers is a critical factor for solubilizing them into typical coil−coil block copolymer micelles. By introducing saturated bonds into conjugated polymers, they were readily encapsulated in block copolymer micelles, resulting in highly fluorescent polymer nanoparticles. The emission wavelength of fluorescent micelles can be tuned by controlling the number of encapsulated light-emitting polymers per micelle as well as changing the exciton delocalization length in conjugated polymers. This strategy has been extended to make multifunctional (i.e., fluorescent and magnetic) nanoparticles by encapsulating iron oxide nanoparticles in light-emitting polymers.

Surface modification of semiconductor quantum dots (QDs) often causes a drastic reduction of photoluminescence quantum yield (QY). Here, we report an efficient way to improve photoluminescence (PL) characteristics of silica-coated QDs using the combination of light-induced PL enhancement (photoenhancement) and the small molecule reducing agent, dithiothreitol (DTT). The photoenhancement process in the absence of a reducing agent is usually accompanied by a blue shift and broadening of the PL spectrum as well as a subsequent rapid PL quenching due to the competitive photo-oxidation process. The addition of DTT augments the degree of photoenhancement and inhibits the spectral shift and broadening. The photoenhancement assisted by DTT reported here should provide a simple and useful means of preparing stable, highly luminescent water-soluble silica-coated QDs that have PL QYs comparable to those exhibited by organic-soluble QDs.

We report a new method to generate families of organic fluorophores with any desirable emission wavelengths based upon the controlled oxidation of the light-emitting conjugated polymer, poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene] (MEH−PPV), with meta-chloroperbenzoic acid (m-CPBA). In this method, m-CPBA reacts with ethylene moieties along the MEH−PPV backbone to create conjugation breaks, which gives rise to a gradual and controllable change in the emission wavelength. By simply adjusting the reaction time, light-emitting polymers possessing emission wavelengths spanning a 470−555 nm wavelength range can be easily prepared. Significantly, fluorescence quantum yields (QYs) of the oxidized polymers were comparable to or greater than that of the pristine polymer, contrary to the products typically resulting from oxidation of MEH-PPV by dioxygen. This new method should provide a simple way to generate color-tunable organic fluorophores with high QYs in a time- and cost-effective manner.

Hollow silica nanoparticles can be spontaneously generated without a template on the basis of the porous nature of silica and the high surface energy on the nanometer scale. We show that solid silica particles synthesized by the Stöber and microemulsion methods initially develop small pores inside the nanoparticles under slightly basic conditions as a result of base-catalyzed etching. With further reaction, those small seed pores merge into a single void to reduce the surface energy of small pores, generating well-defined hollow nanoparticles. This behavior is unique to nanometer-sized porous materials, and the shape evolution is size-dependent, reinforcing the importance of evaluating the reactivity and structural changes of nanomaterials as well as their physical properties in different size ranges. The mechanism described here provides a simple way to generate uniform hollow nanoparticles of porous materials.

Here, we describe novel composite nanoparticles composed of highly emissive fluorophore-doped silica cores and hemispherical gold-coatings, separated by spectroscopically inert silica spacer layers. Photoluminescence (PL) measurements at the single particle level demonstrated that the addition of the spacer layer effectively reduces the core PL quenching by the gold layer, rendering both constituents of the multicomponent nanostructure optically active. The versatility of this approach is further demonstrated by synthesizing analogous gold-coated nanoparticles based upon commercial fluorescent polymer nanoparticles. These multilayered nanoparticles were designed to combine the properties of fluorescent nanoparticles with the useful surface properties of gold, and such species could find utility in a wide range of biosensing applications. We have demonstrated that the hemispherical gold surfaces of these multilayered particles can be readily functionalized with DNA and the DNA-modified hybrids recognize cDNA strands without the problem of nonspecific bindings, showing promise for their use in DNA detection applications.

Gold nanoshells covered with sharp rods called “spiky gold nanoshells” are synthesized by employing a silver-assisted seed-growth method for heterogeneous nanoparticle syntheses at polymer/water interfaces. It is found that silver ions in the growth solution play an important role in forming uniform gold shells as well as regulating the surface morphology. The optical properties of spiky gold nanoshells are investigated by single-particle scattering measurements, single-particle surface-enhanced Raman scattering measurements, and finite-difference time-domain modeling. The scattering intensities from isolated spiky nanoshells are significantly enhanced compared to those of conventional smooth shells. Moreover, due to the abundant hot spots on spiky nanoshells, the SERS signal is readily observed from single spiky shells with a very small intensity variation (35%), whereas there is no detectable signal from isolated smooth shells. These results demonstrate that our synthetic method provides a straightforward way to organize metal nanoparticles into well-defined assemblies with enhanced scattering properties.