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In a recent report, we have presented the layer-by-layer (LBL) assembly of a biomimetic nanostructured composite from Na⁺-montmorillonite clay nanosheets and poly(diallylmethylammonium chloride) (Tang, Z.; Kotov, N.; Magonov, S.; Ozturk, B. Nat. Mater. 2003, 2, 413). The structure, deformation mechanism, and mechanical properties of the material are very similar to those of natural nacre and lamellar bones. This fact prompts further investigation of these composites as potential bone implants. LBL assembly affords preparation of multifunctional composites, and here we demonstrate that not only mechanical strength, but also antibacterial activity, can be introduced in these implantable materials by alternating clay layers with starch-stabilized silver nanoparticles. The resulting composite showed excellent structural stability with no detectable levels of silver lost over a 1 month period. Evaluation of the antibacterial properties showed almost complete growth inhibition of E. coli over an 18 h period. The amount of silver eluted from the LBL composite over a 1 month period was determined to be only 0.5−3.0 μg/L. This concentration of silver did not prevent the growth of the mammalian tissue cultures. The LBL composite has shown biocompatibility with the human osteoblast cell line.

Cellulose nanocrystals are promising as a new class of reinforcing material for the preparation of nanostructured composites. We report here the preparation of cellulose nanocrystal multilayer composites with poly(diallyldimethylammonium chloride) using layer-by-layer assembly (LBL) technique. The LBL assembly was characterized with UV−Vis spectroscopy and ellipsometry. The average thickness of a single bilayer was found to be 11 nm. AFM and SEM characterization revealed uniform coverage and densely packed cellulose crystal surface.

Preparation of well-ordered three-dimensional (3D) scaffolds is of great importance for various aspects of tissue engineering and fundamental research in cell biology. A versatile technique, floating self-assembly, is reported here for the first time to prepare 3D colloidal crystals from polystyrene particles of various diameters (10−240 μm). Ethylene glycol was used as the solvent in the self-assembly process of colloidal crystals. Polymer colloidal particles float on the surface of ethylene glycol and self-assemble into a hexagonal crystal lattice structure. The slow evaporation of ethylene glycol left solvent-free 3D colloidal crystals. Na2SiO3 sol−gel solution was infiltrated into the interstitial space of the prepared colloidal crystals, taking advantage of the capillary effect, and an inverse opal with a highly ordered system of spherical cavities interconnected by channels was obtained by calcining the colloidal particles at high temperature. As-prepared inverse opals were used as 3D cell scaffolds which possess a high degree of order compared to existing foamlike and fibrous cell scaffolds and afford tight control over the scaffolds' porosity and tissue organization. Their preparation by this technique is much simpler than that by free-form fabrication techniques. The biocompatibility of the prepared cell scaffolds is demonstrated through human hepatocellular carcinoma HEP G2 and human bone marrow HS-5 cell cultures.

Oriented SWNTs in polymer composites have shown dramatic improvements in the physical properties of a composite because of the anisotropic shape and properties of SWNTs. Controlled alignment of SWNTs during composite fabrication implies better material function performance. This letter reports a new fabrication technique whereby aligned SWNTs and robust SWNT−polymer composites can be made using a fusion method of SWNT combing and layer-by-layer (LBL) assembly. As we previously reported, LBL assembly demonstrated exceptional processing ability in constructing the uniform distribution of a SWNT−polymer composite. Combined with this uniformity, this SWNT combing technique endows controlled alignment of single-stranded SWNTs in a SWNT−polymer composite system. SWNT combing employs air−water interfacial forces to change the molecular topography from the random adsorption state to the stretched alignment of SWNTs. More specifically, air−water interfacial forces are associated with an excess viscous drag force and an intrinsic dewetting rate along SWNTs. Moreover, the alignment efficiency of SWNTs is high enough to construct a multilayered LBL film with horizontal−linear weaving structures. This simple method also can be applied for aligning other nanowire materials because it utilizes simple geometric features of SWNTs.

Layer-by-layer films have been constructed from a cationic diazoresin and poly(sodium 4-styrenesulfonate)-functionalized single-wall carbon nanotubes (SWNT). Raman and near-IR absorption spectra indicate that some diazonium ions reacted with SWNT during film construction. The ionic bonds in the film between diazonium ions and sulfonate ions converted to covalent bonds upon UV irradiation as shown by the disappearance of diazonium ion peaks in the UV and IR spectra and appearance of an arenesulfonate peak in the IR spectra. The resistance of the film toward etching by polar solvents increased significantly after irradiation.

Semiconductor CdTe nanowires were coated with a coaxial layer of silica in aqueous solutions by the hydrolysis of tetraethyl orthosilicate. The structure and morphology of the nanowires before and after coating were studied by UV−vis spectroscopy, atomic force microscopy, and transmission electron microscopy. Interestingly, silica deposition on the nanowire ends was retarded. For sufficiently thin layers, this effect makes possible the preparation of coaxial insulated nanowires with electrical access to the semiconductor core. Insulating properties of the silica layer were tested by applying voltage to a conductive AFM tip placed directly on the sidewall surface of the nanowires. These experiments demonstrated that the silica layers had strong insulating properties for coatings as thin as 10 nm. Similar measurements done with naked nanowires showed that transversal conductance took place through the middle band-gap states.

Layer-by-layer sequential adsorption technique (LBL) can be utilized as a deposition method for nanostructured coatings on highly curved micrometer to millimeter scale surfaces, which is demonstrated in this work on a model films of strongly luminescent CdTe nanoparticles. Unlike LBL on colloids and flat surfaces, such procedure opens the door for the preparation of conformal nanostructured coatings for a variety of photonics and lab-on-the chip devices with predictable technological importance. Confocal microscopy data showed that prepared CdTe/PDDA coatings were found to be uniform and continuous. Strong coupling of the nanoparticle luminescence into the optical fiber attributed to high refractive index of CdTe/PDDA composite was obtained. As applied to nanoparticle coatings, two unanticipated effects were observed:  (1) gradual red shift of the luminescence spectrum as the layer thickness increases; (2) two-stage luminescence transient including the significant enhancement of the luminescence during first 100 min of UV illumination. Similar results were obtained for the coatings made in the glass tube interior, which can be considered as simple models of microfluidic devices with channels of complex geometry.

Citrate-stabilized CdSe or CdSe/CdS core−shell nanoparticles (NPs) were adsorbed on the standard silicon wafers bearing either a short-chain covalently bound adsorption promoter (3-aminopropyl)triethoxylsilane (APTES) or macromolecular adsorption promoters&sbd;polyethylenimine (PEI) or poly(diallydimethylammonium) chloride (PDDA). The aim of this study is 2-fold:  (1) to compare different methods of NP processing into thin films and (2) to elucidate the effect of the long-chain dynamics on the NP film structure. Systematic atomic force microscopy study of the films revealed that both types of NPs produced densely packed films on PDDA, while rarified films with significant clustering formed on PEI and APTES. The difference in NP layer morphologies was rationalized on the basis of intermolecular NP−polyelectrolyte interactions. Importantly, we observed that the adsorption layer of the weak polyelectrolyte PEI could alter its chain distribution by partial wrapping around the NPs, while no disturbance in APTES and PDDA monolayer by NP was observed. This was attributed to more labile binding of PEI to the solid substrate than for other adsorption promoters.

Complementary bioconjugates based on antibody−antigen interactions were synthesized from luminescent CdTe nanoparticles (NPs). Antigen (bovine serum albumin) was conjugated to red-emitting CdTe NPs, while green-emitting NPs were attached to the corresponding anti-BSA antibody (IgG). The NP bioconjugates were characterized by native and SDS−PAGE electrophoresis, gel-permeation HPLC, and circular dichroism. Antigen−antibody binding affinity was evaluated by enzyme-linked immunosorbent assay (ELISA). The formation of BSA−IgG immunocomplex resulted in the Förster resonance energy transfer (FRET) between the two different NPs:  the luminescence of green-emitting NPs was quenched whereas the emission of the red-emitting NPs was enhanced. The luminescence recovered when the immunocomplex was exposed to an unlabeled antigen. The immunocomplexes can be considered as a prototype of NP superstructures based on biospecific ligands, while the competitive FRET inhibition can be used in an immunoassay protocol.

Layer-by-layer (LBL) films of a fulleropyrrolidinium−androstane−ferrocene dyad revealed a surprisingly high level of intermolecular organization. In particular, in every adsorption layer, the films self-assemble into linear nanowire superstructures. Given the inherent intramolecular charge transfer performance of the dyads, such a film organization facilitates the charge diffusion from the photoactive centers to the electrodes, due to extensive interpenetration of the layers, which leads to multiple contacts between the fullerene−ferrocene nanowires.

Gold nanoparticles (NPs) were prepared by reduction with salicylic acid in aqueous solution. The resulting dispersions were found to contain a mixture of flat triangular/hexagonal and smaller close-to-spherical NPs. As expected from theoretical considerations, such nanocolloids display two clearly differentiated surface plasmon bands at 540 and 860 nm associated with spherical and anisotropic triangular/hexagonal NPs, respectively. Layer-by-layer (LBL) assembly was used to deposit thin films of the Au colloids. UV−visible data indicate preferential adsorption of the flat particles on polyelectrolyte films. Importantly, a new band developed at 650 nm as the number of the Au NPs bilayers increased. This finding indicates that there exists a strong interaction between the NPs in adjacent layers, resulting in the surface plasmon absorption at a new wavelength. The insertion of extra polyelectrolyte or montmorillonite layers between the Au bilayers was shown to gradually reduce the interlayer interaction and resulted in the NP composite films with absorption spectra virtually identical to those of the original dispersion. The bilayer deposition sequence in LBL assembly, i.e. multilayer architecture, can be used to control the strength of NP−NP coupling in the layered composites.

Stable aqueous colloids of 2−3 nm In2S3 nanocrystals have been prepared by using the classical method of nanoparticle stabilization by low molecular weight thiols. TEM crystal lattice spacing, X-ray diffraction, EDAX data, and electron diffraction indicate that the nanoparticles are predominantly in β-In2S3 form. They exhibit relatively strong excitonic emission at 360−380 nm with a quantum yield of 1.5%. The excitonic radiative lifetime is 350 ns, which indicates that a direct allowed electronic transition is responsible for this emission. The NMR lines of the stabilizer are strongly broadened and shifted as a result of deshielding induced by electron withdrawing by positively charged metal ions. This effect quickly wears off as the carbon chain becomes longer and the separation between the hydrogen atoms of the stabilizer and the semiconductor surface increases. The broadening is attributed to the reduced mobility of the stabilizer in the nanoparticle shell. For CdS nanoparticles of the same size, this effect was found to be substantially stronger than for In2S3. The lower density of metal centers in In2S3 than in CdS, which serve as anchor points for the stabilizer, promotes greater mobility of the stabilizer moieties.

Conjugates of bovine serum albumin and CdTe nanoparticles capped with l-cysteine have been synthesized via a one-pot glutaric dialdehyde cross-linking procedure. Diads (1:1) with some amount of 2:1 albumin−nanoparticle assemblies preferably form in this reaction, as evidenced by gel electrophoresis. Circular dichroism spectroscopy demonstrates that the tertiary structure of the protein remains largely intact after the conjugation. Attachment of protein moieties result in a significant increase of CdTe emission, which is attributed to the resonance energy transfer from the tryptophan moieties of albumin to CdTe nanoparticles acting as receptors for the protein antennae.

CdTe nanocrystals capped with 1-mercapto-2,3-propandiol, CdSe nanocrystals capped with sodium citrate, and core−shell CdSe/CdS nanocrystals capped with sodium citrate were synthesized in aqueous solutions, and their surface was modified by 3-mercaptopropyltrimethoxysilane (MPS) in water−ethanol mixtures. By addition of sodium silicate, “raisin bun”-type composite particles were formed, with either CdTe, CdSe, or CdSe/CdS nanocrystals being homogeneously incorporated as multiple cores into silica spheres of 40−80 nm size, accompanied by some alteration of optical properties of the nanoparticles and, in particular, the reduction of the luminescence quantum yield. Further, growth of larger silica spheres (100−700 nm) can be performed by the Stöber technique using either MPS-modified semiconductor nanocrystals or “raisin bun”-type composite particles as seeds, which gives semiconductor-doped silica globules of desirable sizes in the submicrometer range. The composite spheres can be used as building blocks for 3D colloidal crystals, prepared in this study for CdS/CdSe-doped 250 nm silica colloid. The shift of the photonic band gap to the red was observed in photonic crystals made of nanoparticles-doped silica due to the high refractive index of the semiconductors.

A new technique for preparation of free-standing ultrathin films is presented. These films were made by layer-by-layer (LBL) deposition process, which is utilized for coatings on solid substrates and colloids. A film composed of alternating layers of magnetite nanoparticles and poly(diallyldimethylammonium bromide) was assembled on cellulose acetate, which was subsequently dissolved in acetone. From the suspended state, the LBL film can be transferred onto any solid or porous support. The strength of the film was observed to significantly increase when every other layer of magnetite was replaced with a layer of alumosilicate sheets serving as a molecular framework for the assembly.

Hybrid thin films are prepared from 8 to 10 nm Fe3O4 nanoparticles and exfoliated montmorillonite clay by using layer-by-layer assembly on poly(diallyldimethylammonium bromide), PDDA. Distinct stratification of the Fe3O4/PDDA/clay films is obtained due to the sheetlike structure of the clay particles. This feature distinguishes these assemblies from their polyelectrolyte−polyelectrolyte analogues, where the layers of individual polyelectrolytes are strongly interdigitated. Being adsorbed on PDDA strictly parallel to the substrate surface, montmorillonite produces a dense layer of overlapping alumosilicate sheets, which virtually flawlessly separates one magnetite layer from another. The difference in magnetic properties between assemblies of various architectures is attributed to the insulation effect of clay layers inserted between magnetic layers. The montmorillonite sheets disrupt the electron exchange interactions between the magnetite nanoparticles in adjacent layers, thereby limiting the magnetization reversal to two dimensions. Some optical properties of Fe3O4/PDDA films are investigated as well. When they are deposited on thin plastic substrate, oscillations of optical density were observed in the red part of the UV−vis spectrum. This effect, which was never been observed for conventional, thick substrates such as glass slides, stems from the interference of the light beams passed through and reflected off of the assembled film.

Silver nanoparticles coated with a uniform, thin shell of titanium dioxide are synthesized via a remarkably simple one-pot route, where the reduction of Ag⁺ to Ag⁰ and the controlled polymerization of TiO2 on the surface of silver crystallites take place simultaneously. The prepared dispersions of coated nanoparticles display a surface plasmon band, which is significantly red-shifted with respect to that of bare Ag. High-quality ultrathin films of the core−shell clusters are prepared via layer-by-layer assembly. The nanoparticles are arranged in closely packed layers interlaced with polyelectrolyte producing a stratified core−shell hybrid material with unique structure and catalytic and electron-transport properties.

Modification with thiomolybdate results in the attachment of spatially separated MoS4^2- groups to the surface of CdS. This produces the red shift of excitonic absorption and emission bands and splitting of the trapped emission band. Because of the spatial gap between the thiomolybdate molecules of the modifying layer, the interaction of individual groups of the modifier with the CdS core must be considered. This was accomplished by using the density functional theory (DFT) description of a MoS4^2-−CdS model cluster. Geometry, dipole moment, and localization of calculated molecular orbitals allow for the consistent description of optical effects observed for thiomolybdate-modified CdS nanoparticles, such as the red shift of adsorption and emission bands, enhancement of excitonic emission, and splitting of the trapped emission band. A good agreement between DFT calculated and experimental FTIR spectra has been obtained.

CdS nanoparticles (NP), 22 Å, have been synthesized from cadmium 2-ethylhexanoate in DMSO as a uniformly sized dispersion. After ripening, CdS exhibits a sharp excitonic emission peak at 402 nm, while in freshly prepared dispersions a broad trapped emission at 510 nm dominates. By using one- and two-dimensional NMR spectroscopy, the conformation of the stabilizer adsorbed to the NP has been determined. The long hexyl chain of 2-ethylhexanoate ions spreads over the surface of NP, whereas the short ethyl end is primarily surrounded by DMSO. Surface modification of CdS with nucleophiles such as 4,4-bipyridine, thiophene, trimethylamine, and thiomolybdate anion results in a partial replacement of the stabilizer and reorientation of the hexyl chain away from the surface. The difference in the degree of replacement and/or conformational changes of 2-ethylhexanoate ion depends on the electron donor activity of the modifier.

A close packed layer of Fe3O4 and SiO2-coated Fe3O4 nanoparticles can be assembled on silicon and glass substrates modified with polyelectrolytes following the principle of the layer-by-layer self-assembly (Science 1997, 277, 1232). Microwave (MW) treatment of the poly(dimethyldiallylammonium chloride) layer prior to the nanoparticle adsorption results in substantial reduction of the surface roughness of the particulate films. This effect is attributed to reduction of the length of partially desorbed segments of macromolecules protruding into the aqueous phase at a distance of about 70 nm as estimated from force−distance curves. Aggregation of nanoparticles on these segments is responsible for a relatively high degree of disorder in layer-by-layer self-assembled films. A brief MW exposure results in cross-linking of polyelectrolyte chains. This substantially reduces the number of loose segments, and improves 2D packing of nanoparticles. For optimized conditions, the rms roughness, R, of magnetite self-assembled films can be as low as 1.5−3.5 nm. For silica-coated magnetite, initial R of adsorbed films is typically 10−14 nm. It can be reduced to 5.5 nm following the MW treatment of the polyelectrolyte; however, this does not completely prevent the formation of multiparticle 3D aggregates. Further reduction of R to 3.5 nm can be achieved by a brief ultrasonication of the nanoparticulate film, which removes weakly attached particles.

Hydrothermal treatment of albite, labradorite, and orthoclase in mixtures with dolomite has revealed that at PH2O = 1000 kg/cm2 and a residence time of 24-72 hr at T = 200-400° C, mainly calcite and trioctahedral layered silicates are formed. At 300-500° we observe the following formation sequence: montmorillonite → (Mg, Al) serpentine → chlorite from plagioclases and montmorillonite → a disordered mixed-layer phase of the mica-montmorillonite type → phlogopite from orthoclase. At 500-600° there is a tendency for formation of framework aluminosilicates (nepheline, kalsilite) at high Na and K concentrations in the initial mixtures, and nesosilicates (monticellite, forsterite) at high Ca and Mg concentrations. It is shown that as the residence time is increased synthetic and natural montmorillonites with AlIV are not converted to ordered 1:1 mixed-layer phases, in contrast to those observed in systems with kaolinite. This is probably due to the statistically uniform distribution of A1 in a structure of beideliite-type montmorillonites synthesized from feldspars, whereas beidellites with polar distribution of AlIV over the (Si, Al-O) tetrahedral networks are apparently developed from kaolinite. When the albite structure is converted to tie pyroxene structure, a considerable part of the tetrahedra is inherited by the latter; this follows from retention of the values of the chemical shift and quadrupole splitting typical of tetrahedral Fe57 (III). The experimental data can be used to reconstruct the primary mineral composition of metamorphic pararocks. —Authors.

Capture of argon by pyroxene, amphibole, biotite, albite and beryl from argon-enriched atmosphere in autoclave, in quantities exceeding the natural A-content of these minerals, is demonstrated experimentally. It occurs almost instantaneously in pyroxene in which there are at least three positions where the captured gas is held.--V. P. Sokoloff.