Small molecule- and amino acid-induced aggregation of gold nanoparticles.

To understand which organic molecules are capable of binding to gold nanoparticles and/or inducing nanoparticle aggregation, we investigate the interaction of gold nanoparticles with small molecules and amino acids at variable pH. Dynamic Light Scattering (DLS) and ultraviolet-visible (UV-vis) spectra were measured on mixtures of colloidal gold with small molecules to track the progression of the aggregation of gold nanoparticles. We introduce the 522 to 435 nm UV-vis absorbance ratio as a sensitive method for the detection of colloidal gold aggregation, whereby we delineate the ability of thiol, amine, and carboxylic acid functional groups to bind to the surfaces of gold nanoparticles and investigate how combinations of these functional groups affect colloidal stability. We present models for mechanisms of aggregation of colloidal gold, including surface charge reduction and bridging linkers. For all molecules whose addition leads to the aggregation of gold nanoparticles, the aggregation kinetics were accelerated at acidic pH values. Colloidal gold is maintained only in the presence of anionic carboxyl groups, which are neutralized by protonation at lower pH. The overall reduced charge on the stabilizing carboxyl groups accounts for the accelerated aggregation at lower pH values.

Tip preparation for near-field ablation at mid-infrared wavelengths.

A fabrication method for high-throughput, fiber-based tips for near-field scanning microscopy (NSOM) in the mid-infrared (λ ~ 3 µm) has been developed. Several fiber materials have been investigated and recipes for wet-chemical etching have been varied to produce tips that are physically robust and are capable of low-loss transmission of high-power pulses of mid-infrared light. Ultimately, wet-chemical etching techniques are used on glass fibers to produce tips capable of focusing mid-infrared light to ablate material from sub-micron-sized regions of organic films. The power throughput of the tips is significantly increased by using a novel material, previously unreported for NSOM applications: germanate fibers. The tips produced are mechanically strong and capable of transmitting high light fluence without sustaining physical damage. Here, the development of these tips and their performance are described.

Quantitative analysis of parallel nanowire array assembly by dielectrophoresis.

We describe an assembly technique useful for generating ordered arrays of nanowires (NWs) between electrodes via dielectrophoresis (DEP) and an analysis technique useful for extracting quantitative information about the local electric fields and dielectrophoretic forces from video microscopy data. By tuning the magnitude of the applied electric fields such that the attractive forces on the NWs are of the same order of magnitude as the Brownian forces, and by taking advantage of the inter-NW repulsive forces during DEP, NWs can be assembled into parallel arrays with high reproducibility. By employing a particle-tracking code and analysis of NW motion, we demonstrate a method for quantitative mapping of the dielectrophoretic torques and NW-surface interactions as a function of position on the substrate, which allows a more complete understanding of the dynamics of the assembly and the ability to control these parameters for precise assembly.