Size Control of Mesoscale Aqueous Assemblies of Quantum Dots and Block Copolymers (original) (raw)
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Poly(styrene-b-4-vinylpyridine) diblock copolymers PS404-b-P4VP76 and PS317-b-P4VP76 (the subscripts indicate the degree of polymerization) self-assemble into spherical ‘‘crew-cut’’ micelles with a PS core and P4VP corona when prepared in a mixture of chloroform and 2-propanol. When the micelles are formed in the presence of quantum dots (QDs), the nature of the structures formed depends upon the polymer and the type of QDs. In our previous report [Macromolecules, 2010, 43, 5066–5074], PS404-b-P4VP76 + CdSe QDs formed stable spherical hybrid micelles, but prolonged vigorous stirring of the solutions led to a rearrangement into wormlike networks and loss of photoluminescence (PL) from the QDs. Here we report that PS317-b-P4VP76 + CdSe/ZnS core–shell QDs behave differently. Partial loss of PL intensity occurred upon addition of 2-propanol to the chloroform solution of the components, and the rearrangement to a network structure occurred spontaneously. We describe two strategies for recovery of the PL intensity for the QDs within the network, photo-activation and chemical activation with elemental sulfur.
Journal of the American Chemical Society, 2008
Hierarchical organization of light-absorbing molecules is integral to natural light harvesting complexes and has been mimicked by elegant chemical systems. A challenge is to attain such spatial organization among nanoscale systems. Interactions between nanoscale systems, e.g., conjugated polymers, carbon nanotubes, quantum dots, and so on, are of interest for basic and applied reasons. However, typically the excited-state interactions and dynamics are examined in rather complex blends, such as cast films. A model system with complexity intermediate between a film and a supramolecular system would yield helpful insights into electronic energy and charge transfer. Here, we report a simple and versatile approach to achieving spatially defined organization of colloidal CdSe, CdSe/ZnS core/shell, or PbS nanocrystals (quantum dots) with poly(3-hexylthiophenes) (P3HTs) using micelles of poly(styrene-b-4-vinylpyridine) (PSb-P4VP) as the main structural motif. We compare the characteristics of this system to those of natural light-harvesting complexes. Bulk heterojunction films (and related systems) are characterized by electronic interactions, and therefore dynamics of charge and energy transfer, at interfaces rather than between specific donor-acceptor molecules. Owing to structural disorder, such systems are inherently complex. Therefore, we expect that the spatially defined organization of the active components in the present system provides new opportunities for studying the complicated photophysics intrinsic to blends of nanoscale systems, such as bulk heterojunctions by establishing simplified and better controlled interfaces.
Interfacial Assembly of Nanoparticles in Discrete Block-Copolymer Aggregates
Angewandte Chemie International Edition, 2007
The ability to control the assembly structure of nanoscale materials is critical to understand their collective properties and to develop new materials and devices from nanoscale building blocks. In nature, lipid membranes function as structural scaffolds to organize nanoscale intercellular components. This role of lipids originates from their tendency to self-organize into diverse supramolecular aggregates including micelles, bilayers, vesicles, and liquid-crystalline phases. Amphiphilic block copolymers, man-made analogues of lipids, are becoming increasingly important for the synthesis, manipulation, and assembly of nanoparticles. They have been demonstrated to be effective solubilizing agents to transfer organic-phase nanoparticles into water. Preorganized spherical or cylindrical block-copolymer micelles have been actively explored as a type of confined reactor for the controlled synthesis of nanoparticles. However, in most of the solution-phase studies, the nanoparticles were considered as simple solutes, and the efforts in directing the arrangement of nanoparticles using block copolymers have been largely limited to two-dimensional systems. Herein, we describe a novel solution-phase assembly of CdSe quantum dots (QDs) and a prototypical amphiphilic block copolymer, poly(acrylic acid)-block-polystyrene (PAAb-PS). Importantly, this study demonstrates that the interactions between nanoparticles and block copolymers can drastically alter the morphology of block-copolymer aggregates and can lead to a unique three-dimensional assembly structure of nanoparticles (a nanocavity in the present study) with controllable assembly parameters. Coassemblies of PAA-b-PS and organic-phase nanoparticles, including QDs and magnetic particles, have been prepared previously. However, in those studies, the nanoparticles were passively incorporated into the coassemblies by acting as simple solutes, and they were evenly distributed in block-copolymer micelles. Herein, we show a unique example in which nanoparticles act as active components for the assembly formation. Moreover, we demonstrate that one can control the three-dimensional assembly structure of nanoparticles inside discrete blockcopolymer aggregates by manipulating the interfacial energy of the composite system.
Polymers
A mixed micelle approach is used to produce amphiphilic brush nanoparticles (ABNPs) with cadmium sulfide quantum dot (QD) cores and surface layers of densely grafted (σ = ~1 chain/nm2) and asymmetric (fPS = 0.9) mixed polymer brushes that contain hydrophobic polystyrene (PS) and hydrophilic poly(methyl methacrylate) (PMAA) chains (PS/PMAA-CdS). In aqueous media, the mixed brushes undergo conformational rearrangements that depend strongly on prior salt addition, giving rise to one of two pathways to fluorescent and morphologically disparate QD-polymer colloids. (A) In the absence of salt, centrosymmetric condensation of PS chains forms individual core-shell QD-polymer colloids. (B) In the presence of salt, non-centrosymmetric condensation of PS chains forms Janus particles, which trigger anisotropic interactions and amphiphilic self-assembly into the QD-polymer vesicles. To our knowledge, this is the first example of an ABNP building block that can form either discrete core-shell col...
Water-Soluble Pegylated Quantum Dots: From a Composite Hexagonal Phase to Isolated Micelles
Langmuir, 2006
We present a simple method based on the dispersion of fluorescent quantum dots (QD) into a liquid crystal phase that provides either nanostructured material or isolated QD micelles depending on water concentration. The liquidcrystal phase was obtained by using a gallate amphiphile with a poly(ethylene glycol) chain as the polar headgroup, named I. The hydration of QD/I mixtures resulted in the formation of a composite hexagonal phase identified by small-angle X-ray scattering and by polarized light and fluorescence optical microscopy, showing a homogeneous distribution of fluorescence within hexagonal phase. This composite mesophase can be converted into isolated QD-I micelles by dilution in water. The fluorescent QD-I micelles, purified by size exclusion chromatography, are well monodisperse with a hydrodynamic diameter of 20-30 nm. Moreover, these QD do not show any nonspecific adsorption on lipid or cell membranes. By simply adjusting the water content, the PEG gallate amphiphile I provides a simple method to prepare a self-organized composite phase or pegylated water soluble QD micelles for biological applications.
Controlled organization of self-assembled rod-coil block copolymer micelles
Polymer, 2009
Spherical micelles of a series of poly (styrene-block-(2,5-bis[4-methoxyphenyl]oxycarbonyl)styrene) (PS-b-PMPCS) rod-coil diblock copolymers in a selective solvent can organize into large mono-layered films with a well-ordered hexagonal packing of the spheres after solvent evaporation. Organized domains in the spherical micelle film were observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The core-shell structure of the spherical micelle remained after solvent evaporation. The micelle diameter in the ordered film as observed by TEM and AFM agree. The size of the spherical micelles can be controlled by the length of PMPCS when the length of the PS is fixed. The sphere diameters were varied from several tens of nanometers to more than one hundred nanometers. Solutions of smaller micelle spheres formed less ordered films than those from larger micelle particles. Additionally, monolayer films of cylindrical worm-like micelles were also prepared. Those cylindrical micelles were observed to be end-capped by spherical micelles. The monolayer micelle film from the largest spherical micelles appeared red when observed in optical microscopy in the reflection mode. A broad adsorption peak with a maximum adsorption wavelength of 545 nm was observed via UV-Vis spectroscopy.
The Journal of Physical Chemistry B, 2010
Fluorescent probes, coumarin 153 (C153) and octadecylrhodamine B (ORB), were used to study the selfassembly in water of poly(N-decylacrylamide)-block-poly(N,N-diethylacrylamide), (PDcA 11-block-PDEA 295 ; M n) 40 300 g mol-1 ; M w /M n) 1.01). From the variation of both the fluorescence intensity and the solvatochromic shifts of C153 with polymer concentration, the critical micelle concentration (CMC) was determined as 1.8 (0.1 µM. On the other hand, steady-state anisotropy measurements showed the presence of premicellar aggregates below the CMC. Time-resolved fluorescence anisotropy evidenced that ORB is located in the premicellar aggregates and the micelle core, while C153 is partitioned between the aggregates and the water phase. The micelle core contains both semicrystalline and amorphous regions. In the semicrystalline regions the probes cannot rotate, while in the amorphous regions the rotational correlation times correlate well with the hydrodynamic volume of the probes. The amorphous region of the micelle core is relatively fluid, reflecting the large free-volume accessible to the probes.
Poly(styrene-b-4-vinylpyridine) (PS404-b-P4VP76, where the subscripts refer to the numberaveraged degree of polymerization) forms spherical crew-cut micelles with a PS core and a P4VP corona when 2-propanol is added to a solution of the polymer in chloroform. When the CHCl3 solution also contains CdSe quantum dots, hybrid micelles form with the QDs bound to the corona. Here we report that vigorous magnetic stirring of a solution of these hybrid micelles in a solution containing 73 vol%2-propanol leads to a morphology transformation to form three-dimensional wormlike networks that have structural features similar to that first reported by Jain and Bates [Science 2003, 300, 460-464] for a poly(butadiene-b-ethyleneoxide) block copolymer in water. Under the influence of shear, PS404-b-P4VP76 micelles appear to aggregate and then rearrange to form colloidally stable networks consisting of wormlike micelles, uniform in width, that are present as loops and tails connected by T- and Y-junctions. The wormlike micelles are similar in width to the original hybrid micelles, whereas the tails of the micelles have thicker bulbous end-caps. This morphology transition is extremely sensitive to solvent composition and is also affected by the stirring rate.
Chemical Engineering Journal, 2010
Block copolymer assembled colloidal particles were successfully prepared through electrostatic interactions between cationic polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) and anionic polystyrene-block-poly(acrylic acid) (PS-b-PAA) in a one-pot process. The colloidal particles were prepared by simple mixing of complementary charged block copolymer micelle aqueous solution. This process presents the possibility of very simple and versatile method of mass production of BCM based colloidal particles. Both hairy and crew-cut types of block copolymer micelles (BCMs) showed different electrostatic assembly tendencies as confirmed by Field emission scanning electron microscopy (FE-SEM). These phenomena are mainly caused by the different degrees of electrostatic interdigitation and entanglement between the protonated and deprotonated corona block regions of the micelles.