Stephanie L. Brock

Porous Semiconductor Chalcogenide Aerogels

Chalcogenide aerogels based entirely on semiconducting II-VI or IV-VI frameworks have been prepared from a general strategy that involves oxidative aggregation of metal chalcogenide nanoparticle building blocks followed by supercritical solvent removal. The resultant materials are mesoporous, exhibit high surface areas, can be prepared as monoliths, and demonstrate the characteristic quantum-confined optical properties of their nanoparticle components. These materials can be synthesized from a variety of building blocks by chemical or photochemical oxidation, and the properties can be further tuned by heat treatment. Aerogel formation represents a powerful yet facile method for metal chalcogenide nanoparticle assembly and the creation of mesoporous semiconductors.

Efficient Water Oxidation Using CoMnP Nanoparticles.

The development of efficient water oxidation catalysts based on inexpensive and Earth-abundant materials is a prerequisite to enabling water splitting as a feasible source of alternative energy. In this work, we report the synthesis of ternary cobalt manganese phosphide nanoparticles from the solution-phase reaction of manganese and cobalt carbonyl complexes with trioctylphosphine. The CoMnP nanoparticles (ca. 5 nm in diameter) are nearly monodisperse and homogeneous in nature. These CoMnP nanoparticles are capable of catalyzing water oxidation at an overpotential of 0.33 V with a 96% Faradaic efficiency when deposited as an ink with carbon black and Nafion. A slight decrease in activity is observed after 500 cycles, which is ascribed to the etching of P into solution, as well as the oxidation of the surface of the nanoparticles. Manganese-based ternary phosphides represent a promising new system to explore for water oxidation catalysis.

Enhanced Gene and siRNA Delivery by Polycation-Modified Mesoporous Silica Nanoparticles Loaded with Chloroquine

ABSTRACTPurposeTo prepare mesoporous silica-based delivery systems capable of simultaneous delivery of drugs and nucleic acids.MethodsThe surface of mesoporous silica nanoparticles (MSN) was modified with poly(ethylene glycol) (PEG) and poly(2-(dimethylamino)ethylmethacrylate) (PDMAEMA) or poly(2-(diethylamino)ethylmethacrylate) (PDEAEMA). The particles were then loaded with a lysosomotropic agent chloroquine (CQ) and complexed with plasmid DNA or siRNA. The ability of the synthesized particles to deliver combinations of CQ and nucleic acids was evaluated using luciferase plasmid DNA and siRNA targeting luciferase and GAPDH.ResultsThe results show a slow partial MSN dissolution to form hollow silica nanoparticles in aqueous solution. The biological studies show that polycation-modified MSN are able to simultaneously deliver CQ with DNA and siRNA. The co-delivery of CQ and the nucleic acids leads to a significantly increased transfection and silencing activity of the complexes compared with MSN not loaded with CQ.ConclusionPEGylated MSN modified with polycations are promising delivery vectors for combination drug/nucleic acid therapies.

Control of phase in phosphide nanoparticles produced by metal nanoparticle transformation: Fe2P and FeP.

The transformation of Fe nanoparticles by trioctylphosphine (TOP) to phase-pure samples of either Fe(2)P or FeP is reported. Fe nanoparticles were synthesized by the decomposition of Fe(CO)(5) in a mixture of octadecene and oleylamine at 200 degrees C and were subsequently reacted with TOP at temperatures in the region of 350-385 degrees C to yield iron phosphide nanoparticles. Shorter reaction times favored an iron-rich product (Fe(2)P), and longer reaction times favored a phosphorus-rich product (FeP). The reaction temperature was also a crucial factor in determining the phase of the final product, with higher temperatures favoring FeP and lower temperatures Fe(2)P. We also observe the formation of hollow structures in both FeP spherical nanoparticles and Fe(2)P nanorods, which can be attributed to the nanoscale Kirkendall effect. Magnetic measurements conducted on phase-pure samples suggest that approximately 8 x 70 nm Fe(2)P rods are ferromagnetic with a Curie temperature between 215 and 220 K and exhibit a blocking temperature of 179 K, whereas FeP is metamagnetic with a Neel temperature of approximately 120 K. These data agree with the inherent properties of bulk-phase samples and attest to the phase purity that can be achieved by this method.

Expanding the Repertoire of Chalcogenide Nanocrystal Networks: Ag2Se Gels and Aerogels by Cation Exchange Reactions

Cation exchange is shown to be a simple and efficient method to prepare nanostructured Ag(2)Se gels and aerogels from CdSe gel precursors. Treatment of CdSe wet gels with AgNO(3) yields, for the first time, Ag(2)Se gels, and these are transformed to high surface area, nanostructured aerogels by supercritical fluid drying. The striking similarity in crystallite size, morphology, and surface area characteristics between CdSe and the corresponding Ag(2)Se aerogels supports a mechanism in which the bonding within the gel network remains globally unchanged, even as the structural attributes of the primary particle components are undergoing a dramatic transformation (hexagonal CdSe to cubic Ag(2)Se). Intriguingly, the rapid exchange also enables exquisite control of composition on the macroscale; reduced concentrations of Ag(+) lead to two-component (i.e., CdSe inside, Ag(2)Se outside) heterogeneous structures of mm-cm dimensions. Overall, this methodology offers a simple approach for the generation of porous nanocrystalline metal chalcogenide networks of known or new compositions.

Materials: Spontaneous formation of inorganic helices

Attempts to generate films, wires, helices, rings and regular patterns out of inorganic materials have been going on for the past 100 years. We show here that stable helices of porous manganese oxide materials can be formed spontaneously from uniform sols and that they are excellent semiconductors. These inorganic helices contain micropores, can be converted into other structures, and their composition can be varied.

Reversible Gelation of II–VI Nanocrystals: The Nature of Interparticle Bonding and the Origin of Nanocrystal Photochemical Instability

Semiconducting nanocrystals (NCs) with dimensions smaller than the bulk exciton Bohr radius exhibit unique, size-tunable opto-electronic properties due to quantum confinement effects. Accordingly, there has been tremendous interest in the synthesis and characterization of colloidal semiconductor NCs, and they have been investigated for a variety of applications ranging from biological labeling and diagnostics to photovoltaics, photodetectors, sensors, and catalysts. However, the stability of colloidal NCs is a major issue in many of these applications. The most common method of stabilization is by chemically attaching ligands to the surface atoms of the NCs. Consequently, semiconducting NCs are generally synthesized in the presence of coordinating surfactant ligands, for example, trioctylphosphine oxide, which confers dispersibility in non-polar media. For biological applications, dispersibility in polar media (i.e. water) is needed and this is often achieved by substitution of nonpolar surfactant ligands with carboxylate-terminated thiolates by treatment with mercaptoundecanoic acid, mercaptoacetic acid, dihydrolipoic acid, etc., in base. However, the stability of thiolate coated II–VI semiconductor NCs, such as CdSe, is typically poor, often leading to precipitation. In 2001, Peng and co-workers reported a detailed study of the photochemical instability of thiolate-capped CdSe NCs. The photooxidation of the thiolate ligands on the NCs can be catalyzed by the CdSe NCs in the presence of light and O2, producing disulfides and thus, effectively decomplexing the particle en route to aggregation. However, if free thiols are present in the solution, they can replace the thiolates lost as disulfide, dispersing aggregates as they form and prolonging the stability of the sol. We and others have exploited oxidative removal of the thiolates as a means to link particles together into three-dimensional architectures (gels), in which metal chalcogenide NCs are assembled into porous network structures. Importantly, analyses of oxidized CdS sols have shown that the thiols and their oxidized products (disulfides and sulfonates) can be completely removed from the gel network, and are therefore not participating in interparticle bonding, which leads to the conclusion that CdS NCs are physically connected to each other without any organic linkers. The extent of particle interaction in the network (and the related extent of quantum confinement) is found to be a direct function of the dimensionality of the network, itself controlled by the density. Such architectures are of interest for applications requiring maximal transport of charge (through the gel network) and small molecules (through the interconnected pore network), such as sensing and photocatalysis. Here we show that, analogous to the work of Peng et al., thiolates can be employed to break up the gel network into its constituent NCs. Additionally, for the first time, we probe the properties of the interparticle bonding in CdSe gels and aggregates. Dispersion studies were performed principally with CdSe gels, aerogels and xerogels prepared from oxidation of high temperature prepared NCs that were capped with 11-mercaptoundecanoic acid (MUA) in the presence of base (tetramethylammonium hydroxide (TMAH)). Treatment of CdSe wet gels with fresh methanolic solutions of MUA and TMAH (pH 12.0) results in formation of a sol that is visually identical to the precursor NCs (Figure 1) in 2–3 min. Like-

ZnS Nanoparticle Gels for Remediation of Pb2+ and Hg2+ Polluted Water

ZnS nanoparticle (NP) gel networks were used as cation exchange materials for the removal of Pb(2+) and Hg(2+) from aqueous solutions. First, the suitability of the gel as a remediation material was studied by analyzing the mechanism of the cation exchange reaction. ZnS NP gels can exchange with other divalent cations (Pb(2+), Hg(2+)) under mild reaction conditions. The speed of the reaction is influenced by the reduction potential of the incoming cation. The ZnS aerogels can remove Pb(2+) and Hg(2+) from aqueous solutions with a wide range of initial concentrations. For initial Pb(2+) concentrations of 100 ppb, the Pb(2+) concentration can be reduced below the action limit established by the EPA (15 ppb). Under thermodynamically forcing conditions, the water remediation capacity of the ZnS NP aerogels was determined to be 14.2 mmol Pb(2+)/ g ZnS aerogel, which is the highest value reported to date.

Effects of nanoparticle shape on the morphology and properties of porous CdSe assemblies (aerogels)

We demonstrate the effect of differently shaped CdSe nanoscale building blocks (dots, rods, branched nanoparticles, and hyperbranched nanoparticles) on the morphologies, surface characteristics, and optical properties of resultant porous CdSe nanostructured aerogels. Monolithic CdSe aerogels were produced by controlled oxidative removal of surface thiolate ligands from differently shaped CdSe nanoparticles to yield a wet gel, followed by CO(2) supercritical drying. The X-ray diffraction data show that the resultant CdSe aerogels maintain the crystalline phase of the building blocks without significant grain growth. However, the transmission electron microscopy images indicate that the morphology of CdSe aerogels changes from a colloid-type morphology to a polymer-type morphology when the building block changes from dot to rod or the branched nanoparticle. The morphology of the CdSe aerogel assembled from hyperbranched nanoparticles appears to be intermediate between the colloid-type and the polymer-type. Nitrogen physisorption measurements suggest that the surface areas and porosity are a direct function of the shape of the primary building blocks, with aerogels formed from rods or branched particles exhibiting the greatest surface areas (>200 m(2)/g) and those prepared from hyperbranched nanoparticles exhibiting the least (<100 m(2)/g). Band gap measurements and photoluminescence studies show that the as-prepared CdSe aerogels retain to a large extent the intrinsic quantum confinement of the differently shaped building blocks, despite being connected into a 3D network.

Uniform Thin Films of CdSe and CdSe(ZnS) Core(shell) Quantum Dots by Sol-Gel Assembly: Enabling Photoelectrochemical Characterization and Electronic Applications

Optoelectronic properties of quantum dot (QD) films are limited by (1) poor interfacial chemistry and (2) nonradiative recombination due to surface traps. To address these performance issues, sol-gel methods are applied to fabricate thin films of CdSe and core(shell) CdSe(ZnS) QDs. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging with chemical analysis confirms that the surface of the QDs in the sol-gel thin films are chalcogen-rich, consistent with an oxidative-induced gelation mechanism in which connectivity is achieved by formation of dichalcogenide covalent linkages between particles. The ligand removal and assembly process is probed by thermogravimetric, spectroscopic, and microscopic studies. Further enhancement of interparticle coupling via mild thermal annealing, which removes residual ligands and reinforces QD connectivity, results in QD sol-gel thin films with superior charge transport properties, as shown by a dramatic enhancement of electrochemical photocurrent under white light illumination relative to thin films composed of ligand-capped QDs. A more than 2-fold enhancement in photocurrent, and a further increase in photovoltage can be achieved by passivation of surface defects via overcoating with a thin ZnS shell. The ability to tune interfacial and surface characteristics for the optimization of photophysical properties suggests that the sol-gel approach may enable formation of QD thin films suitable for a range of optoelectronic applications.