Nanoparticles Made in Mesoporous Solids

Silver nanoparticle-mesoporous oxide nanocomposite thin films: a platform for spatially homogeneous SERS-active substrates with enhanced stability

We introduce a nanoparticle-mesoporous oxide thin film composite (NP-MOTF) as low-cost and straightforward sensing platforms for surface-enhanced Raman Spectroscopy (SERS). Titania, zirconia, and silica mesoporous matrices templated with Pluronics F-127 were synthesized via evaporation-induced self-assembly and loaded with homogeneously dispersed Ag nanoparticles by soft reduction or photoreduction. Both methods give rise to uniform and reproducible Raman signals using 4-mercaptopyridine as a probe molecule. Details on stability and reproducibility of the Raman enhancement are discussed. Extensions in the design of these composite structures were explored including detection of nonthiolated molecules, such as rhodamine 6-G or salicylic acid, patterning techniques for locating the enhancement regions and bilayered mesoporous structures to provide additional control on the environment, and potential size-selective filtration. These inorganic oxide-metal composites stand as extremely simple, reproducible, and versatile platforms for Raman spectroscopy analysis.

Organized Silica Films Generated by Evaporation-Induced Self-Assembly as Hosts for Iron Oxide Nanoparticles

In this work, we prepared oriented mesoporous thin films of silica on various solid substrates using the pluronic block copolymer P123 as a template. We attempted to insert guest iron oxide (Fe_xO_y) nanoparticles into these films by two different methods: (a) by co-precipitation—where iron precursors are introduced in the synthesis sol before deposition of the silica film—and subsequent oxide production during the film calcination step; (b) by preparing and calcining the silica films first then impregnating them with the iron precursor, obtaining the iron oxide nanoparticles by a second calcination step. We have examined the structural effects of the guest nanoparticles on the silica film structures using grazing incidence X-ray scattering (GISAXS), high-resolution transmission electron spectroscopy (HRTEM), spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), and Raman microscopy. Formation of nanoparticles by co-precipitation may induce substantial changes in the film structure leading, in our adopted process, to the appearance of lamellar ordering in the calcination stage. On the contrary, impregnation-based approaches perturb the film structures much more weakly, but are also less efficient in filling the pores with nanoparticles.

Strategies for the fabrication of porous platinum electrodes

Porous platinum is of high technological importance due to its various applications in fuel cells, sensors, stimulation electrodes, mechanical actuators and catalysis in general. Based on a discussion of the general principles behind the reduction of platinum salts and corresponding deposition processes this article discusses techniques available for platinum electrode fabrication. The numerous, different strategies available to fabricate platinum electrodes are reviewed and discussed in the context of their tuning parameters, strengths and weaknesses. These strategies comprise bottom-up approaches as well as top-down approaches. In bottom-up approaches nanoparticles are synthesized in a fi rst step by chemical, photochemical or sonochemical means followed by an electrode formation step by e.g. thin fi lm technology or network formation to create a contiguous and conducting solid electrode structure. In top-down approaches fabrication starts with an already conductive electrode substrate. Corresponding strategies enable the fabrication of substrate-based electrodes by e.g. electrodeposition or the fabrication of self-supporting electrodes by dealloying. As a further top-down strategy, this review describes methods to decorate porous metals other than platinum with a surface layer of platinum. This way, fabrication methods not performable with platinum can be applied to the fabrication of platinum electrodes with the special benefit of low platinum consumption.

Electron beam synthesis of metal and semiconductor nanoparticles using metal-organic frameworks as ordered precursors

We demonstrate a versatile, bottom-up method of forming metal and semiconducting nanoparticles by exposing precursor metal-organic frameworks (MOFs) to an electron beam. Using a transmission electron microscope to initiate and observe growth, we show that the composition, size, and morphology of the nanoparticles are determined by the chemistry and structure of the MOF, as well as the electron beam properties. Zinc oxide, metallic indium and copper particles were produced with narrow and tunable size distributions comparable to those obtained from state-of-the-art methods. This method represents a first step toward the fabrication of nanoscale heterostructures using the highly controlled environment of the MOF pores as a scaffold or template.

Morphologically Well-defined Gold Nanoparticles Embedded in Thermo-Responsive Hydrogel Matrices

Nanocomposite materials consisting of colloidal gold (Au) nanoparticles embedded in thermo-responsive poly(N-isopropylacrylamide) (PNIPAm) hydrogels are synthesized. Thiol groups that bind to both Au3+ ions and colloidal Au are incorporated into the side-chains of the PNIPAm hydrogels through copolymerization. This report describes formation of morphologically well-defined Au nanoparticles with varying long-term stability inside the hydrogel matrices containing adjustable concentrations of thiols. Compared with the non-Au containing PNIPAm hydrogels, the Au-PNIPAm nanocomposite hydrogels have shown higher degrees of equilibrium swelling and different temperature-triggered phase transitions. It is hypothesized that these remarkable changes in hydrogel bulk properties are related to the different morphologies and sizes, and possibly the amount of surface charges, of the Au nanoparticles.

Electroless growth of silver nanoparticles into mesostructured silica block copolymer films

Silver nanoparticles and silver nanowires have been grown inside mesostructured silica films obtained from block copolymers using two successive reduction steps: the first one involves a sodium borohydride reduction or a photoreduction of silver nitrate contained in the film, and the second one consists of a silver deposit on the primary nanoparticles, carried out by silver ion solution reduction with hydroxylamine chloride. We have demonstrated that the F127 block copolymer ((PEO)(106)(PPO)(70)(PEO)(106)), "F type", mesostructured silica film is a suitable "soft" template for the fabrication of spherical silver nanoparticles arrays. Silver spheres grow from 7 to 11 nm upon the second reduction step. As a consequence, a red shift of the surface plasmon resonance associated with metallic silver has been observed and attributed to plasmonic coupling between particles. Using a P123 block copolymer ((PEO)(20)(PPO)(70)(PEO)(20)), "P type", mesostructured silica film, we have obtained silver nanowires with typical dimension of 10 nm x 100 nm. The corresponding surface plasmon resonance is blue-shifted. The hydroxylamine chloride treatment appears to be efficient only when a previous chemical reduction is performed, assuming that the first sodium borohydride reduction induces a high concentration of silver nuclei in the first layer of the porous silica (film/air interface), which explains their reactivity for further growth.

Integrating structure control over multiple length scales in porous high temperature ceramics with functional platinum nanoparticles

High temperature ceramics with porosity on multiple length scales offer great promise in high temperature catalytic applications for their high surface area and low flow resistance in combination with thermal and chemical stability. We have developed a bottom-up approach to functional, porous, high-temperature ceramics structured on eight distinct length scales integrating functional Pt nanoparticles from the near-atomic to the macroscopic level. Structuring is achieved through a combination of micromolding and multicomponent colloidal self-assembly. The resulting template is filled with a solution containing a solvent, a block copolymer, a ceramic precursor, and a nanoparticle catalyst precursor as well as a radical initiator. Heat treatment results in three-dimensionally interconnected, high-temperature ceramic materials functionalized with well-dispersed 1-2 nm Pt catalyst nanoparticles and very high porosity.

Patterned production of silver-mesoporous titania nanocomposite thin films using lithography-assisted metal reduction

A simple method that allows selective positioning of nanoparticles into mesoporous monolayer or multilayer thin films is presented. This technique applies UV lithography in order to bring about in situ light-induced reduction of silver in templated cavities of TiO2. The nanoparticle lithography presented here provides a novel approach to hierarchical lithography patterning for multifunctional devices.

Controlled deposition of silver nanoparticles in mesoporous single- or multilayer thin films: from tuned pore filling to selective spatial location of nanometric objects

Silver nanoparticle assemblies are embedded within mesoporous oxide thin films by an in situ mild reduction leading to nanoparticle-mesoporous oxide thin-film composites (NP@MOTF). A quantitative method based on X-ray reflectivity is developed and validated with energy dispersive spectroscopy in order to assess pore filling. The use of dilute formaldehyde solutions leads to control over the formation of silver nanoparticles within mesoporous titania films. Inclusion of silver nanoparticles in mesoporous silica requires more drastic conditions. This difference in reactivity can be exploited to selectively synthesize nanoparticles in a predetermined layer of a multilayered mesoporous stack leading to complex 1D-ordered multilayers with precise spatial location of nanometric objects. The metal oxide nanocomposites synthesized have potential applications in catalysis, optical devices, surface-enhanced Raman scattering, and metal enhancement fluorescence.

Water-soluble surface-anchored gold and palladium nanoparticles stabilized by exchange of low molecular weight ligands with biamphiphilic triblock copolymers

A study is presented of the stabilization of gold and palladium nanoparticles (NPs) via a place-exchange reaction. Au and Pd NPs of approximately 3.5 nm were prepared by a conventional method using tetraoctylammonium bromide (TOAB) as the stabilizing agent. The resulting nanoparticles, referred to as Au-TOAB or Pd-TOAB, were later used as templates for the replacement of TOAB ligand with poly(ethylene oxide)- b-polystyrene- b-poly(4-vinylpyridine) (PEO- b-PS- b-P4VP) triblock copolymer. This biamphiphilic triblock copolymer was synthesized by atom transfer radical polymerization (ATRP) with control over the molecular weight and polydispersity. The place-exchange reaction was mediated through strong coordination forces between the 4-vinylpyridine copolymer and the metal species located on the surface of the nanoparticles. In addition, the displacement of the outgoing low molecular weight TOAB ligands by high molecular weight polymers is an entropy-assisted process and is believed to contribute to stabilization. The prepared complex, polymer-NP, exhibits greatly improved stability over the metal-NP complex in common organic solvents for the triblock copolymer. Self-assembly in water after ligand exchange resulted in micellar structures of about approximately 20 nm (electron microscopy) with the metal NP found located on the surface of the micelles. The stability of the nanoparticles in water was shown to depend greatly on the grafting density of the copolymer, with high stability (more than 6 months) at high grafting density and low stability, accompanied with irreversible agglomeration, at relatively low grafting densities. The surprising location of the metal NP (for both Au and Pd) on the surface of the micelles in water is explained by the fact that, upon self-assembly in THF/water system, the most hydrophobic chains (i.e., PS) undergo self-assembly first at low water content forming the core, followed by the P4VP (whether or not associated with the metal) forming a shell, and finally the PEO forming the corona. In lower metal content assemblies, the P4VP chains located in the shell undergo swelling in an acidic medium causing a substantial increase in micellar corona size, as confirmed by dynamic light scattering measurements. The present study offers a simple approach for the stabilization of various metal nanoparticles of catalytic interest, using a unique polymeric support that can be dispersed in organic solvents as well as aqueous solutions.

Clean and facile solution synthesis of iron(III)-entrapped gamma-alumina nanosorbents for arsenic removal

Iro (III)-entrapped gamma-alumina nanosorbents were prepared by an environmentally benign method using an ionothermal process based on an ionic liquid to synthesize the gamma-alumina host and a sonochemical method to entrap the iron(III) guest. The morphology of the alumina depends on the aluminum precursor used, giving aligned bundled and randomly debundled gamma-alumina nanorods as well as wormlike mesoporous alumina. In particular, the rodlike structure shows significantly greater mesoporosity than the wormlike porous gamma-alumina structure. Moreover, entrapment of iron(III) in the gamma-alumina nanosorbents with randomly debundled rodlike structures leads to the greatest AsV removal capacity and the fastest adsorption rate as compared to the other FeIII-entrapped adsorbents, as a result of its larger surface area and pore sizes. Thus, this method provides a clean and effective route to an advanced host-guest adsorbent system for application in the removal of arsenic from drinking water.

Growth of gold nanoparticle arrays in TiO2 mesoporous matrixes

An interconnected Au nanoparticle arrangement is obtained by electrodeposition from Au(III) soluble complexes within the pore system of block-copolymer templated mesoporous titania films. The resulting Au@TiO2 nanocomposites (5 nm Au particles, 5.5 nm amorphous titania walls) have the electrochemical behavior of a gold electrode of high surface area. The attenuation of Au surface plasmon due to -OH electroadsorption and the existence of mixed localized states in these Au@TiO2 nanocomposites are observed by in situ spectroelectrochemistry.