Nanoparticles in Block

Copolymer Micelles Block Copolymer Micelle Cores 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, 3]. If the micelle core consists of the block bearing functional groups (able to react with metal compounds, forming complexes or salts), such a core can be filled with a corresponding metal compound (by incorporating the metal compound in the block copoly-mer solution) and further can serve as a nanoreactor for nanoparticle formation. At the same time, the micelle core can be treated like a quasi-solid state, as the core-forming block is not soluble in a selective solvent. As such "functional" blocks, one may use polyvinylpyridines (P2(4)VP), polymethacrylic (PMAA) and PAA acids, PB, PI, and some others. Micelle corona should be formed by a block containing no functional groups but providing solubility and micelle stability in the respective solvent. These can be PS, poly(ethylene oxide) (PEO), polyisobutylene (PIB), etc.

2 I ch3

COOH

Figure 3. Schematic representation of metal nanoparticle-containing PODS. Reprinted with permission from [38], L. M. Bronstein et al., Langmuir 16, 8221 (2000). © 2000, American Chemical Society.

Synthesis of metal or semiconductor nanoparticles in the cores of amphiphilic block copolymer micelles was reported almost simultaneously by several research groups [5, 4043]. The Au, Pd, Pt, and other particle formation in block i n copolymer micelles derived from PS-fo-P4VP block copolymers showed that nanoparticle morphology strongly depends on the type of reducing agent. When a sluggish reducing agent is used, one nanoparticle per micelle ("cherrylike" morphology) can be formed if there is no exchange between micelles (for example, the micelle is cross-linked) [44]. Fast reduction leads to formation of many small particles per micelle ("raspberrylike" morphology) which is considered to be preferable for catalytic applications (Fig. 5) [45]. Using block copolymer micelle cores as nanoreactors allows synthesis of mono- and bimetallic nanoparticles; yet bimetallic particle morphology depends on a metal pair [46].

Co nanoparticles of different size and shape can be prepared either by incorporation of CoCl2 in the PS-fo-P2VP micelles followed by reduction or by thermal decomposition of Co2(CO)8 species embedded in the micelle cores [47]. Stable suspensions of superparamagnetic cobalt nanoparticles were prepared in poly(dimethylsiloxane) (PDMS) carrier fluids in the presence of poly[dimethylsiloxane-Wocfc-(3-cyano-propyl)methylsiloxane-Wocfc-dimethylsiloxane] (PDMS-fo-PCPMS-fo-PDMS) triblock copolymers as steric stabilizers [48].

(CH2)3CN I

(PDMS-b-PCPMS)

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

Figure 5. Electron micrographs of Pd colloids synthesized in PS-6-P4VP block copolymers via reduction with hydrazine (top) and NaBH4 (bottom). Reprinted with permission from [45], M. V. Seregina et al., Chem. Mater. 8, 923 (1997). © 1997, American Chemical Society.

provide steric stability. The particle size can be controlled by adjusting the cobalt to copolymer ratio. TEM shows nonaggregated cobalt nanoparticles with a narrow size distribution and the particles are evenly surrounded with copoly-mer covering.

Formation of iron oxide particles in cross-linked block copolymer micelles is described in [49]. A polyisoprene-b/ock-poly(2-cinnamoylethyl methacrylate)-block-poly(tert-butyl acrylate), PI-fo-PCEMA-fo-PfBA, 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 PfBA core. Their structure is locked in by photo-cross-linking the PCEMA shell to yield nanospheres. 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 PfBA. 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. However, this approach seems to be far too complex for magnetic particle formation as more robust and simple approaches can be used.

All the above examples describe nanoparticle formation in block copolymer micelles in organic solvents. Such examples are numerous, as many amphiphilic block copoly-mers form micelles with functionalized core in the organic medium. The choice of block copolymers for aqueous medium 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-6-PEO and PB-fo-PEO [50, 51]; yet the former block copolymer micellization depends on pH [52]. At pH below 5 P2VP-6-PEO becomes molecularly soluble in water. Nevertheless, lowering the pH of the P2VP-fo-PEO micellar solution after incorporation of metal compounds or metal nanoparticle formation does not result in micelle decomposition, though micelles become less dense. In the case of PB-fo-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 and as metal-particle-containing templates for mesoporous silica casting.

CH II

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 [53]. Here gold nanoparticle formation was carried out in the P2VP shell serving as a nanoreactor. As the shell is formed by the pH-sensitive P2VP block, according to the author's opinion 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 [50]. So this property n m

n n can be used if no nanoparticles providing quasi-cross-linking are formed in the P2VP shell.

An original approach to form spherical assemblies of CdS-containing block copolymer reverse micelles in aqueous solution was reported in [54]. These stable assemblies were formed by slow addition of water to mixtures of the reverse micelles formed by PS-fo-PAA and single PS-fo-PAA chains. The structures are large compound micelles (LCMs) with quantum-confined CdS nanoparticles dispersed throughout a spherical PS matrix, which is stabilized in water by a layer of solubilized hydrophilic chains. The size of the CdS particles (approximately 3 nm) is determined by the ionic block length of the block copolymer forming the reverse micelle. LCM formation is dependent on the amount of added stabilizing copolymer. This method allows one to transfer the CdS nanoparticles formed in the micelle cores in organic medium to aqueous medium without loss of stability and particle aggregation.

Block Copolymer Micelle Coronas Nanoparticle synthesis can be carried out in the corona of amphiphilic block copolymer micelles. However, if the corona is functional-ized, addition of metal salt can result in immediate formation of large aggregates and their precipitation so such a synthesis can be performed only in very diluted solutions. If the corona does not contain groups able to coordinate with metal compounds, particle stabilization is realized solely due to hydrophobic interactions with the hydrophobic core. This scenario was followed when synthesis of Pd, Pt, Ag, and Au nanoparticles was performed in aqueous solutions of PS-fo-PEO and PS-fo-PMAA by reduction of the corresponding salts in block copolymer solutions [55, 56]. As reported, this results in the formation of nanometer sized colloidal particles. As stabilization of these particles is provided due to hydrophobic interactions with the PS core, the stability of such systems cannot be satisfactory. At the same time, accessibility of particles in the micelle coronas can be favorable from the viewpoint of catalytic properties.

To improve stabilization in the micelle coronas, the authors [57-59] suggested using hybrid micelles consisting of PS-fo-PEO and surfactants. It was expected that surfactant hydrophobic tails should be embedded in the PS core while surfactant head groups will be located on the micelle core surface or in its vicinity. As shown in Figure 6, exchange of

Figure 6. Schematic image of the hybrid PS-fo-PEO/SDS micelle. Reprinted with permission from [58], L. M. Bronstein et al., J. Colloid Interface Sci. 230, 140 (2000). © 2000, Academic Press.

surfactant counterions for ions of interest (Pt, Pd, or Rh) ought to allow 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. By 1H nuclear magnetic resonance, incorporation of surfactant leads to strong increase of mobility of the PS core which proves comicellization of block copolymer molecules and cationic or anionic surfactants. Ion exchange of surfactant counterions in the PS-fo-PEO/CPC system for PtCl6- or PdCl2- ions results in saturation of micellar structures with Pt or Pd ions. Subsequent reduction of metal-containing hybrid micellar systems PS-fo-PEO/CPC/MX„ with NaBH4 or H2 results in the formation of metal nanoparticles mainly located within the micelles. 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 nanopar-ticles. 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 so stability of colloidal solutions is provided when metal salt concentration does not exceed 3.36 x 10-3 M. As found, nanoparticle size does not depend on the reducing agent type (contrary to the nanopar-ticles formed in other block copolymer solutions [42, 50]) but depends on the metal type [57-59]. This could be governed by strong interaction of surfactant head groups with growing nanoparticles.

When the targeted metal can be obtained only as a cation, the hybrid micellar system should include anionic surfactant: sodium dodecylsulfate (SDS) or SDBS. For one, using Rh cations [Rh(Py)4Cl2]+, Rh nanoparticles with a diameter of 2-3 nm have been formed in the PS-fo-PEO/SDS solutions. As seen from the above, incorporation of surfactants in the block copolymer micelles containing no functional groups allows reliable stabilization of metal nanoparticles of 2-6 nm in size. The disadvantage of these systems is the lack of direct methods to vary the particle size.

Micelle Formation via Complexation If both blocks of the corresponding block copolymer are soluble in the particular solvent, nanostructurization can occur due to complex-ation of the functional groups of the one block with a metal compound. First this phenomenon was described in [60] for the PS-fo-PB molecular solution in toluene when micelliza-tion occurred due to formation of Pd and Pt n-complexes with double bonds. Another example is PS-fo-P2VP which was molecularly dissolved in a good solvent for both blocks, THF, and subjected to interaction with cadmium ions [61]. It resulted in the formation of aggregates of single micelles called compound micelles. The growth of CdS nanoparti-cles is confined to the core of single micelles after introduction of hydrogen sulfide gas into the solution. UV-visible spectroscopy, fluorescence spectroscopy, and transmission electron microscopy were employed to characterize the CdS nanoparticles. UV-visible absorption spectra show that larger nanoparticles are produced at lower 2VP:Cd2+ molar ratio, i.e. at higher Cd2+ loading. UV-visible absorption spectra and fluorescence spectra both indicate that with decrease of block copolymer concentration in THF, the size of the CdS nanoparticles decreases; that is explained by decrease of the micelle size. Thus, nanoparticle characteristics are dependent on numerous parameters.

If both blocks of a block copolymer are hydrophilic, the block copolymer gives a molecular solution in water. Again, if one of the blocks is inert and another contains functional groups, which are able to interact with metal compounds, their addition leads to micelle formation.

PEO-fo-PEI was employed for synthesis of metal nanoparticles in aqueous medium using the idea described above [62, 63], as PEI easily interacts with metal compounds due to coordination [64]. This results in aggregation of the PEI block and micelle core formation, while PEO forms a corona and ensures colloidal solubility. Such micelles also provide control over particle nucleation and growth (similar to amphiphilic block copolymer micelles), although micellar and metal particle characteristics are more dependent on the reaction conditions: micelle formation is induced solely by metal compounds and micelle core densities are low. Metal ion reduction leads to metal nanoparticle formation. In so doing, PEO-fo-PEI micelles remain intact, while changing their hydrodynamic parameters. This gives evidence of strong interaction of the PEI units with metal nanopar-ticle surface. Metal nanoparticle morphology here can be affected in the same way as in amphiphilic block copolymer micelles by varying the reducing agent type and the metal compound loading along with varying the pH.

The same block copolymer was successfully used for stabilization of CdS nanoparticles [65]. CdS nanoparticles had a well-resolved cubic structure and were monodisperse. The CdS nanoparticles showed a very good resistance against oxidation for months due to their polymer environment. The particle size was controllable in the range between 2 and 4 nm by adjusting the polymer concentration and choice of the solvent.

The ability of PEO-fo-PMAA to control inorganic morphologies was applied recently to synthesis of silver nanowires [66]. Yet here a block copolymer was used both as a directing and reducing agent. The authors suggest that formation of double hydrophilic block copoly-mer complex micelles may play an important role in the directed aggregation growth of the silver nanowires. CdWO4 one-dimensional nanorods with diameters of 2.5 nm were obtained in the presence of a similar double-hydrophilic block copolymer: PEO-fo-PMAA and its partially phospho-nated analog [67]. The CdWO4 species formed in block copolymers showed highly increased fluorescence efficiency.

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