INTRODUCTION
Polymer colloids with metal nanoparticles is a fast-developing field of nanoscience and nanotechnology as encapsulation of metal inside the polymer—or, vice versa, formation of polymer inside the nanocomposite colloids— show a promise for exciting applications: catalytic, optical, magnetic. Metal or semiconductor nanoparticles can be 1) formed inside the polymer colloids (block copolymer micelles, dendrimers, or other functionalized polymer colloids); 2) positioned on the outer surface of the polymer colloids; or 3) polymer layer can be formed or adsorbed on the preformed nanoparticle surface. This entry discusses the above routes to prepare polymer nanocomposite colloids considering possible advantages and disadvantages of different methods. The major methods to characterize composition and structure of these nanomaterials include those used for polymer colloid characterization and metal nanoparticle assessment. Some methods allow gathering information on both polymer and nanoparticle structure; some methods are very specific for nanoparticle characterization. Transmission electron microscopy (TEM) is widely used to characterize both polymer colloids and metal (semiconductor) nanoparticles; the latter especially provide high electron contrast. High-resolution TEM (HRTEM) is used to determine the sizes of small particles and to characterize their inner structure. X-ray diffraction (XRD) is used for crystalline nanoparticles to determine their structure and mean particle size. Scattering techniques, small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS), can be used both for polymer colloid and nanoparticle assessment. The latter is possible if polymer structure does not change after particle formation so scattering from polymer particles can be subtracted. If this is not the case, anomalous SAXS is employed because it allows one to establish nanoparticle size and particle size distribution independently of changes in the nanostructured polymer. Spectroscopic techniques are often used to characterize nanoparticles because properties are dependent on nanoparticle size (Au, Ag, semiconductor nanoparticles). In the sections below, examples of polymer colloid and nanoparticle characterization are presented.
NANOPARTICLE FORMATION INSIDE POLYMER COLLOIDS
Nanoparticle Formation in Block Copolymer Micelles
Amphiphilic block copolymers form micelles in selective solvents (a good solvent only for one block), yet the size and shape of micelles depend on the block chemical structure, molecular weight of each block, and solvent type.[1,2] Block copolymer micelles can be treated as polymer colloids being in dynamic equilibrium with unimers (individual macromolecules in solution) and with each other, which results in exchange between micelles. Although this exchange is a slow process for block co-polymer systems compared to surfactant micelles, this can ensure very useful properties (ability to assemble and disassemble in certain conditions can play very important role for some ”delivery” applications) or demonstrate a disadvantage, as it can facilitate metal-species exchange between polymer colloids.
Depending on the structure of the block copolymer, nanoparticles can be formed both in the micelle core (when the core is functionalized, while corona is not) or in the corona, when the core is not functionalized.
Nanoparticle formation in the micelle core
If the block containing the functional groups (able to react with metal compounds, giving complexes or salts) forms the micelle core, it can be loaded with a corresponding metal compound (by incorporating the metal compound in the block copolymer solution) and can further serve as a nanoreactor for nanoparticle formation. In so doing, because the core-forming block is not soluble in a selective solvent, the micelle core can be treated like a quasi solid, thus additionally stabilizing the nanoparticles. Examples of ”functional” blocks are polyvinylpyridines [P2(4)VP], polymethacrylic and polyacrylic acids (PMAA and PAA), polybutadiene (PB), polyisoprene (PI), and others. A block containing no functional groups but providing solubility and micelle stability in the solution should form the micelle corona. These can be polystyrene (PS), polyethylene oxide) (PEO), polyisobutylene (PIB), etc.
Fig. 1 Electron micrographs of Pd colloids synthesized in PS-b-P4VP block copolymer micelles via reduction with hydrazine (top) and NaBH4 (bottom).
The synthesis of metal or semiconductor nanoparticles in the cores of amphiphilic block copolymer micelles was almost simultaneously reported by several research groups.[3-7] The metal nanoparticle formation in PS-b-P4VP block copolymer micelles demonstrated the strong dependence of nanoparticle morphology on the type of reducing agent. When a sluggish reducing agent is used, one nanoparticle per micelle (”cherry-like” morphology) can be formed if there is no exchange between micelles (e.g., the micelle is cross-linked).[8] Fast reduction leads to the formation of many small particles per micelle (”raspberry-like” morphology), which is considered to be preferable for catalytic applications (Fig. 1).[9] Using block copolymer micelle cores as nanoreactors allows synthesis of monometallic and bimetallic nanoparticles, yet bimetallic particle morphology depends on a metal pair,[10] i.e., on the ability of metal species to be reduced in particular conditions. For the Pd-Au pair, core-shell particles are formed with a gold core and a palladium shell. For Pd-Pt pair, cluster-in-cluster particles are obtained. These different morphologies significantly change the catalytic properties of such systems although the nanoparticle sizes are similar.[10]
Co nanoparticles of different sizes and shapes can be prepared either by incorporation of CoCl2 in the PS-b-P2VP micelles followed by reduction or by thermal decomposition of Co2(CO)8 species embedded in the micelle cores.[11] Stable suspensions of superparamag-netic cobalt nanoparticles were also prepared in poly-(dimethylsiloxane) (PDMS) carrier fluids in the presence of poly[dimethylsiloxane-block-(3-cyanopropyl)methylsi-loxane-block-dimethylsiloxane] (PDMS-b-PCPMS-b-PDMS) triblock copolymers as steric stabilizers.[12] Similar to PS-b-P2VP, these copolymers formed micelles in toluene and served as nanoreactors for thermal decomposition of the Co2(CO)8 precursor. The nitrile groups on the PCPMS central blocks are thought to adsorb onto the particle surface, while the PDMS end blocks protrude into the reaction medium to provide steric stability. Adjusting the cobalt-to-copolymer ratio can control the particle size. Transmission electron microscopy shows nonaggregated cobalt nanoparticles with a narrow size distribution and the particles are evenly surrounded with copolymer covering.
The formation of iron oxide particles in cross-linked block copolymer micelles is described in Ref. [13]. A polyisoprene-block-poly(2-cinnamoylethyl methacrylate)-block-poly(tert-butyl acrylate), PI-b-PCEMA-b-PtBA, forms spherical micelles in THF/hexane mixture with 65% volume fraction of the latter. The micelles consist of a PI corona, a solvent-insoluble PCEMA shell, and a PtBA core. Their structure is locked in by photo-cross-linking the PCEMA shell to yield nanospheres (Fig. 2). Similar to core cross-linking, this approach prevents exchange between micelles. The nanospheres were made water-dispersible by hydroxylating the PI double bonds. The core was made compatible with inorganic species by removing the tert-butyl groups of PtBA. The possibility of using such nanospheres as nanoreactors for inorganic nanoparticle preparation was demonstrated by incorporating iron salt and formation of iron oxide magnetic particles in the cores (Fig. 3).
Fig. 2 Preparation of water-dispersible magnetic nanoparticles. Photolysis cross-links the PCEMA shell (gray to dark). The PI corona chains are made water-soluble by hydroxylating the double bonds (wavy lines to free-hand lines). The core is made inorganic compatible by removing the tert-butyl groups (light gray to gridded pattern). Soaking the nanospheres in aqueous FeCl2 enables proton exchange by Fe2+ (slant to vertical grids) and the Fe2+ ions are precipitated and oxidized to yield cubic g-Fe2O3 magnetic particles using NaOH and H2O2 (last step).
Fig. 3 Transmission electron microscopic images of PI-b-PCEMA-b-PtBA nanospheres at each stage in the synthesis: (a) after PCEMA cross-linking and PI hydroxylation (stained with OsO4 overnight); (b) after removal of tert-butyl groups (stained with OsO4 over a weekend); and (c) after Fe2O3 loading (no staining).
Fig. 4 Transmission electron microscopic image of Pt nanoparticles prepared in PB-b-PEO micelles by NaBH4 reduction.
As seen from the above examples, many amphiphilic block copolymers form micelles with a functionalized core in the organic medium. When aqueous solutions are preferred, the choice of block copolymers is very limited and metal particle formation is normally more complicated as the pH of the medium should be taken into consideration. A few examples of such block copolymers include P2VP-b-PEO and PB-b-PEO,[14,15] yet the former block copolymer micellization depends on the pH val-ue:[16] At pH below 5, P2VP-b-PEO becomes molecularly soluble in water. At the same time, decrease of pH of the P2VP-b-PEO micellar solution after incorporation of metal compounds or metal nanoparticle formation results in no micelle decomposition although the micelle density decreases. In the case of PB-b-PEO, micelles formed in water are very dense, so they successfully fulfill two roles: They serve as nanoreactors for Pd, Pt, and Rh nanoparticle formation (Fig. 4) and as metal-particle-containing templates for mesoporous silica casting.[15]
If the P2VP block is a middle block in PS-b-P2VP-b-PEO triblock copolymer, the ”layered,” well-defined micelles are formed in water with the PS core, P2VP shell, and PEO corona.[17] Here the P2VP shell serves as a nanoreactor for gold nanoparticle formation. As the shell is formed by the pH-sensitive P2VP block, the authors believe that this system can be useful for encapsulation and/or release of active species. However, one should remember that after metal particle formation, this block loses its ability to dissolve at low pH.[14] So this property can be hardly realized in this system if nanoparticles or metal complexes are formed in the P2VP shell.
Fig. 5 Transmission electron microscopic images of spherical aggregates in LCM, with Pd/Pt shadowing (A) and without shadowing (B). The dark particles inside the spheres are CdS nanoparticles.
The formation of spherical assemblies of CdS-containing block copolymer reverse micelles in aqueous solution was reported in Ref. [18]. These stable assemblies were formed by slow addition of water to mixtures of the reverse micelles formed by PS-b-PAA and single PS-b-PAA chains. Large compound micelles (LCMs) with quantum-confined CdS nanoparticles dispersed throughout a spherical PS stabilized in water by a layer of solubilized hydrophilic chain matrix were obtained. The size of the CdS particles (approximately 3 nm) is determined by the ionic block length of the block copolymer forming the reverse micelle (Fig. 5). The formation of LCMs was found to depend on the amount of the added stabilizing copolymer. This method allows transferring the CdS nanoparticles formed in the micelle cores in organic medium to aqueous medium without loss of stability and nanoparticle aggregation.
Block copolymer micelle coronas
Nanoparticles can be synthesized in the corona of amphiphilic block copolymer micelles. However, if the corona is functionalized, addition of a metal salt can result in immediate formation of large aggregates because of the interaction between micelles and their precipitation; thus this method can be used only in very dilute solutions. If the corona does not contain groups able to coordinate with metal compounds, particle stabilization can be ensured because of the hydrophobic interactions with the hydro-phobic core. This feature was used when synthesis of Pd, Pt, Ag, and Au nanoparticles was performed in aqueous solutions of PS-b-PEO and PS-b-PMAA by reduction of the corresponding salts in block copolymer solu-tions.[19,20] However, the stability of such systems, solely provided by the hydrophobic interactions with the PS core, is not satisfactory. On the other hand, accessibility of particles in the micelle coronas can be favorable from the viewpoint of catalytic applications.
Enhanced stabilization in the micelle coronas was achieved when hybrid micelles consisting of PS-b-PEO and surfactants were formed.[21-23] Surfactant hydropho-bic tails were expected to penetrate the PS core while surfactant head groups are located on the micelle core surface or in its vicinity. As shown in Fig. 6, exchange of surfactant counterions for ions of interest would lead to saturation of the core with the given ions. Dynamic light scattering (DLS) and sedimentation in an ultracentrifuge showed that incorporation of positively or negatively charged surfactants results in increase of size and weight of micelles and micellar clusters up to a certain surfactant concentration (which is different for different surfactants). Further increase of surfactant loading (as a rule, above critical micelle concentration for surfactants) results in a moderate decrease of micelle size and weight. Incorporation of surfactant was found to increase the mobility of the PS core and to decrease the mobility of surfactant tails. Both these facts proved comicellization of block copoly-mer molecules and cationic or anionic surfactants. Ion exchange of surfactant counterions in the PS-b-PEO/CPC (cetyl pyridinium chloride) system for PtCl62 _ or PdCl42 _ ions results in saturation of micellar structures with Pt or Pd ions. Subsequent reduction of metal-containing hybrid micellar systems PS-b-PEO/CPC/MX„ with NaBH4 or H2 leads to the formation of metal nanoparticles mainly located within the micelles. The morphology and stability of Pd and Pt nanoparticles synthesized in these systems depends on the metal compound loading and the type of a reducing agent. NaBH4 reduction leads to decomposition of micellar clusters and formation of micelles with embedded nanoparticles. These systems display exceptional stability (for years) if metal salt loading does not exceed 1.24 x 10"2 M. Hydrogen reduction results in metal nanoparticle formation both in micelles and micellar clusters (micelle aggregates), so stability of colloidal solutions is ensured at metal salt concentration of less than 3.36 x 10"3 M. Rh nanoparticles with diameters of 2-3 nm have been obtained in the hybrid micelles formed by PS-b-PEO anionic surfactants: sodium dodecylsulfate (SDS) or sodium dodecylbenzosulfonate (SDBS) using Rh cations [Rh(Py)4Cl2]+ (Fig. 7). As found, nanoparticle size does not depend on the type of reducing agent (contrary to the nanoparticles formed in other block copolymer solu-tions),[4,14] but depends on the type of metal.[21-23] This could be governed by the strong interaction of surfactant head groups with growing nanoparticles.
Fig. 6 Schematic image of the PS-b-PEO/SDS micelle.
Fig. 7 Transmission electron micrographs of Rh nanoparticles formed in the PS-b-PEO/SDS system.
Thus incorporation of surfactants in the block copoly-mer micelles containing no functional groups allows reliable stabilization of metal nanoparticles of 2-6 nm in size. Using both cationic and anionic surfactants allows one to explore an infinite variety of metal ions and to prepare different kinds of nanoparticles. The disadvantage of these systems is a lack of the direct methods to tune the particle size.
