Simultaneous In Situ Monitoring of Trimethoxysilane Hydrolysis Reactions Using Raman, Infrared, and Nuclear Magnetic Resonance (NMR) Spectroscopy Aided by Chemometrics and Ab Initio Calculations.

Sol-gels are found in many different scientific fields and have very broad applications. They are often prepared by the hydrolysis and condensation of alkoxysilanes such as trimethoxysilanes, which are commonly used as precursors in the preparation of silsequioxanes via the sol-gel process. The reaction rates of such reactions are influenced by a wide range of experimental factors such as temperature, pH, catalyst, etc. In this study, we combined multiple in situ spectroscopic techniques to monitor the hydrolysis and partial condensation reactions of methyltrimethoxysilane and phenyltrimethoxysilane. A rich set of kinetics information on intermediate species of the hydrolysis reactions were obtained and used for kinetics modeling. Raman and nuclear magnetic resonance (NMR) spectroscopy provided the most information about hydrolysis and NMR provided the most information about condensation. A quantitative method based on Raman spectra to quantify the various transient intermediate hydrolysis products was developed using NMR as the primary method, which can be deployed in the field where it is impractical to carry out NMR measurements.

Electronic Structure and Potential Reactivity of Silaaromatic Molecules.

Silicon-based materials are crucial for conventional electronics. The fascinating properties of the new two-dimensional material silicene, the silicon analogue of graphene (one atom-thick silicon sheets), offer a potential bridge between conventional and molecular electronics. The ground-state configuration of silicene is buckled, which compromises optimal constructive overlap of p orbitals. Because silicene is not planar like graphene, it has a lower intrinsic electron/hole mobility than graphene. This motivates a search for improved, alternative, planar materials. Miniaturization of silicene/graphene hybrid monolayers affords diverse silicon-organic and -inorganic molecules, whose potential as building blocks for molecular electronics is unexplored. Additionally, hybridization of pure silicon rings (or sheets) is a versatile way to control the geometrical and electronic characteristics of the aromatic ring. In this work we systematically investigate, computationally, architectures and electronic structures of a series of hybrid silaaromatic monomers and fused-ring oligomers. This includes the thermochemistry of representative reactions: hydrogenation and oxidation. The effect of various skeletal substituents of interest is elucidated as well. We show that the specific location of carbon and silicon atoms, and their relative populations in the rings are crucial factors controlling the molecular geometry and the quasi-particle gap. Furthermore, we suggest that electron-withdrawing substituents such as CN, F, and CF3 are promising candidates to promote the air-stability of silaaromatics. Finally, on the basis of the analysis of benzene-like silaaromatic molecules, we discuss a set of alternative, prototype ring molecules that feature planarity and delocalized π bonds. These motifs may be useful for designing new extended materials.

Tunable SERS in gold nanorod dimers through strain control on an elastomeric substrate.

In this Letter we provide experimental verification of the interparticle distance dependence of the SERS enhancement factor in 1 µm gold gapped nanorods. Au dimers are fabricated from electrochemically grown heterogeneous Au-Ag-Au nanorods and deposited on a stretchable elastomer film which allows active and reversible tuning of the interparticle gap on the sub-5-nm level. Significantly, this technique allows the distance dependence to be tracked using a single dimer, thereby avoiding enhancement factor reproducibility issues arising from morphological differences in disparate nanoparticle pairs.

Cylinders vs. spheres: biofluid shear thinning in driven nanoparticle transport.

Increasingly, the research community applies magnetophoresis to micro and nanoscale particles for drug delivery applications and the nanoscale rheological characterization of complex biological materials. Of particular interest is the design and transport of these magnetic particles through entangled polymeric fluids commonly found in biological systems. We report the magnetophoretic transport of spherical and rod-shaped particles through viscoelastic, entangled solutions using lambda-phage DNA (λ-DNA) as a model system. In order to understand and predict the observed phenomena, we fully characterize three fundamental components: the magnetic field and field gradient, the shape and magnetic properties of the probe particles, and the macroscopic rheology of the solution. Particle velocities obtained in Newtonian solutions correspond to macroscale rheology, with forces calculated via Stokes Law. In λ-DNA solutions, nanorod velocities are 100 times larger than predicted by measured zero-shear viscosity. These results are consistent with particles experiencing transport through a shear thinning fluid, indicating magnetically driven transport in shear thinning may be especially effective and favor narrow diameter, high aspect ratio particles. A complete framework for designing single-particle magnetic-based delivery systems results when we combine a quantified magnetic system with qualified particles embedded in a characterized viscoelastic medium.

Selective functionalization of arbitrary nanowires.

We report the selective functionalization of uniform and heterostructured nanowires with self-assembled monolayers (SAMs) of (3-mercaptopropyl)trimethoxysilane (MPTMS). The wires were grown electrochemically in anodic aluminum oxide (AAO) templates. Selective deposition and removal of SAMs during nanowire growth permits the decoration of specific regions of the surface along the length of the nanowires. This technique presents a facile method for the tailored functionalization of nanowires, but does not rely on the intrinsic chemical properties of the nanowires as previous methods have.