Preparation of Silica Nanoparticles Through Microwave-assisted Acid-catalysis

Microwave-assisted synthetic techniques were used to quickly and reproducibly produce silica nanoparticle sols using an acid catalyst with nanoparticle ...

Oak Ridge Institute for Science and Education

High quantum yield (47%) InP nanocrystals can be prepared without the need for post HF treatment by combining microwave methodologies with the presence of a fluorinated ionic liquid. Growing the InP nanocrystals in the presence of the ionic liquid 1-hexyl-3-methyl-imidazolium tetrafluoroborate (hmim BF4) allows in situ etching to be achieved. The optimization of the PL QY is achieved by balancing growth and etching rates in the reaction.

Microwave induced in-situ active ion etching of growing InP nanocrystals.

The development of ternary nanoscale materials with controlled cross-sectional doping is an important step for the use of chemically prepared quantum dots for nanoscale engineering applications. We report cross-sectional, elemental doping with the formation of an alloyed CdSSe nanocrystal from the thermal decomposition of Li(4)[Cd(10)Se(4)(SPh)(16)]. The sulfur incorporation arises from surface-mediated phenylthiolate degradation on the growing quantum dot surface. In the alloy, we identify a pure CdSe nucleus of approximately 1.5 nm, consistent with the predictions of nucleation theory. As the particle grows, S(2-) incorporation increases until the CdSSe reaches approximately 4 nm, where a marked reduction in phenylthiolate content on the nanocrystal is observed by CP-MAS NMR spectroscopy, implying that rapid decomposition of the phenylthiolate arises with subsequent enhanced S(2-) incorporation at the level of the stoichiometry of the reaction (namely approximately 60%). The use of molecular clusters to allow controlled defect ion incorporation can open new pathways to more complex nanomaterials.

Composition control and localization of S2- in CdSSe quantum dots grown from Li4[Cd10Se4(SPh)16].

Evidence of size-dependent reconstruction in quantum dots leading to changes in bonding is observed through analysis of the (77)Se{(1)H} cross-polarization magic angle spinning and (77)Se spin-echo solid-state NMR for Cd(77)Se quantum dots. The CP-MAS and spin-echo data indicate discrete surface and core (77)Se sites exist with the QD, in which the surface is comprised of numerous reconstructed lattice planes. Due to the nearly 100% enrichment level for (77)Se, efficient spin coupling is observed between the surface (77)Se and sublayer (77)Se sites due to spin diffusion in the Cd(77)Se quantum dots. The observed chemical shift for the discrete (77)Se sites can be correlated to the effective mass approximation via the Ramsey expression, indicating a 1/r(2) size dependence for the change in chemical shift with size, while a plot of chemical shift versus the inverse band gap is linear. The correlation of NMR shift for the discrete sites allows a valence bond theory interpretation of the size-dependent changes in bonding character within the reconstructed QD. The NMR results provide a structural model for the QDs in which global reconstruction occurs below 4 nm in diameter, while an apparent self-limiting reconstruction process occurs above 4 nm.

Size- and site-dependent reconstruction in CdSe QDs evidenced by 77Se{1H} CP-MAS NMR spectroscopy.

Multimodal, biocompatible contrast agents for high magnetic field applications represent a new class of nanomaterials with significant potential for tracking of fluorescence and MR in vitro and vivo. Optimized for high-field MR applications-including biomedical imaging at 21.1 T, the highest magnetic field available for MRI-these nanoparticles capitalize on the improved performance of chelated Dy(3+) with increasing magnetic field coupled to a noncytotoxic Indium Phosphide/Zinc Sulfide (InP/ZnS) quantum dot that provides fluorescence detection, MR responsiveness, and payload delivery. By surface modifying the quantum dot with a cell-penetrating peptide sequence coupled to an MR contrast agent, the bimodal nanomaterial functions as a self-transfecting high-field MR/optical contrast agent for nonspecific intracellular labeling. Fluorescent images confirm sequestration in perinuclear vesicles of labeled cells, with no apparent cytotoxicity. These techniques can be extended to impart cell selectivity or act as a delivery vehicle for genetic or pharmaceutical interventions.

Intracellular bimodal nanoparticles based on quantum dots for high-field MRI at 21.1 T.

The reaction of carbon and CaH2 in a calcium/lithium flux mixture produces crystals of the new compound Ca2LiC3H. This phase forms with a new structure type in tetragonal space group P4/mbm (a = 6.8236(1) Å, c = 3.7518(1) Å, Z = 2, R1 = 0.0151). This is a stuffed variant of the Cs2(NH2)N3 structure, containing hydride anions in octahedral sites; the structure determination by single-crystal X-ray diffraction surprisingly allowed the hydrogen to be detected. The Ca2LiC3H structure also features the rarely seen C3(4-) carbide anion; the protolysis reaction of this compound with ammonium chloride produces C3H4. The electronic properties of Ca2LiC3H were studied by quantum-chemical calculations including band structure and electron localization function (ELF) analysis; the phase is a charge-balanced semiconductor with a calculated band gap of 0.48 eV. This is in agreement with (7)Li, (13)C, and (1)H MAS NMR data, which show resonances in the ionic region instead of the Knight shifted region. ELF analysis of the theoretical nonhydrided Ca2LiC3 structure confirms the ability of these calculations to properly locate hydrides and supports the structural model based on X-ray diffraction data.

Ca2LiC3H: a new complex carbide hydride phase grown in metal flux.

X-ray absorption spectroscopy and ab initio modeling of the experimental spectra have been used to investigate the effects of surface passivation on the unoccupied electronic states of CdSe quantum dots (QDs). Significant differences are observed in the unoccupied electronic structure of the CdSe QDs, which are shown to arise from variations in specific ligand-surface bonding interactions.

Ligand-mediated modification of the electronic structure of CdSe quantum dots.

In this work, we demonstrate the controlled synthesis of silica nanoparticles as small as 30 nm (±5 nm) and as large as 250 nm (±30 nm) in minutes using surfactant free, microwave-assisted synthetic techniques. Proper choice of solvent, silicic acid precursor, catalyst, and microwave irradiation time were the variables used to control nanoparticle size and, ultimately, overcome the previously reported shortcomings of using microwaves for silica nanoparticle synthesis. In these reactions acetone, a low-tan δ solvent, mediates the condensation reaction, while selective absorption of pulsed microwave radiation by the precursor promotes nanoparticle growth. Dynamic light scattering data, scanning electron micrographs, and transmission electron micrographs of the reaction products show that the size, shape, and granularity of the silica nanoparticles are highly dependent on reaction conditions. These microwave methods have utility for mass production of silica nanoparticles or other nanoparticles by flow-through microwave synthetic methods for industrial applications, as well as a facile method for encapsulating or embedding materials with silica for improved functionality and stability.

Controlled microwave-assisted growth of silica nanoparticles under acid catalysis.

Through microwave-assisted techniques, cotton textiles treated with heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane in the presence of high surface area silica nanoparticles create a material capable of repelling bulk liquid challenges while simultaneously adsorbing organic vapors from bulk liquid droplets. Characterizing the contradictory behavior of adsorption of vapors and repellency of liquids is the primary focus of this article. These procedures reveal a quick and simple method for a one-step deposition of a vapor-sorptive, liquid-repellent, Cassie-Baxter surface onto textiles. Packed column breakthrough and single swatch permeation experiments showed that treated materials possess a high affinity for 3-hepten-2-one vapor, while goniometry revealed contact angles in excess of 120° for surface-deposited, 5 μL droplets of several test liquids. Scanning electron micrograph images confirm a lotus-like, nanorough surface, while ATR-FTIR spectra confirm surface fluorocarbon moieties. The performance of so-treated materials lends itself to the application of chemical protective apparel, while the simplicity of the treatment bodes well for potential commercialization.

Derek D. Lovingood

Oak Ridge Institute for Science and Education

Preparation of Silica Nanoparticles Through Microwave-assisted Acid-catalysis