This paper describes an experimental technique for measuring the extinction cross-section of anisotropic nanomaterials. The experiments are performed by focusing a laser beam to a diffraction-limited spot under an optical microscope, and using a photoelastic modulator (PEM) to rotate the polarization of the light beam. Monitoring the transmitted beam with a lock-in amplifier referenced to twice the resonant frequency of the PEM yields the difference in extinction for light polarized parallel and perpendicular to the optical axis of the nanostructure. These experiments are the single particle analog of polarization modulation microscopy (PMM). Experimental results for gold nanorods and CdSe nanowires are presented that demonstrate the sensitivity and properties of this technique. In particular, we show that by collecting images at two laser polarizations separated by 45°it is possible to construct an extinction cross-section image. The advantages and disadvantages of the PMM technique compared to existing ways of measuring the extinction of nanoparticles (photothermal heterodyne imaging and spatial modulation spectroscopy) are discussed. Results from experiments where we collected simultaneous absorption and emission images are also presented for different shaped CdSe nanowires. These measurements provide insight into how the structure of the nanowire affects its photophysics. † Part of the special issue "Protected Metallic Clusters, Quantum Wells, and Metallic Nanocrystal Molecules".

As the use of nanoparticles as biological and electronic platforms increases, research must be conducted to determine the kinetics of varying types of particles. In this paper, we determine the forward heterogeneous electrontransfer rate constant (k f ) of water-soluble monolayerprotected gold nanoparticles using a scanning electrochemical microscope (SECM). Using SECM approach curves, we were able to determine the electron-transfer rates of water-soluble nanoparticles protected with a variety of ligands: tiopronin, glutathione, and trimethylammonium undecenyl mercaptan. Our results show the electron-transfer rate is largely dependent on the charge presented by the ligand shell. Fixed charges on ligands inhibit the tunneling of the electron through the protecting monolayer, making ligand charge a dominant factor in electron-transfer rates. By changing the ligand charge on tioproninprotected gold nanoparticles through pH, we show the electron-transfer rate is inversely proportional with pH and decreases dramatically as the ligands move from an uncharged to a negatively charged species.

Shewanella oneidensis is an electroactive bacterium that has the ability to harvest electrons from insoluble metals in its environment for biological processes via dissimilatory metal reduction (DMR). There are 4 mechanisms by which S. oneidensis accomplishes these reductions: direct contact of the outer cell membrane to the metals, protein “nanowires” that extend from the cell surface, metal chelators that solubilize the metals, and small molecule electron shuttles. While evidence suggests S. oneidensis predominately utilizes soluble electron shuttles for DMR, it has yet to be directly proven. If the exact mechanism or mechanisms could be elucidated, S. oneidensis could be employed in a fuel cell that rids polluted water of metal contaminants in the process of generating electricity. For this research, scanning electrochemical microscopy (SECM) was used to investigate the DMR processes of S. oneidensis. By coupling SECM to an inverted microscope, we optically imaged the bacteria during two-dimensional electrochemical measurements. Combined with advances in carbon ultra-micro electrode fabrication, this optical-SECM was used for high resolution scans of S. oneidensis biofilms, as well as planktonic bacteria.