Spiers Memorial Lecture. Introductory lecture: Hot-electron science and microscopic processes in plasmonics and catalysis.

In these introductory remarks we discuss the generation of nonequilibrium electrons in metals, their properties, and how they can be utilized in two emerging applications: for extending the capabilities of photodetection, and for photocatalysis, lowering the barriers of chemical reactions. Because direct illumination of noble/coinage metal nanoparticles results in the excitation of their localized surface plasmons, these nanostructures can serve as active optical antennas, central to the goal of efficient hot electron generation to drive these processes. Currently, noble/coinage metal nanoparticles are being supplemented by earth-abundant, sustainable alternatives. Herein, we discuss how active optical antennas can expand the wavelength accessibility and alter the properties of traditional photoconductive detectors in new ways. We also examine how active optical antennas, when combined with conventional catalytic nanoparticles in an integrated manner, can convert catalysts into photocatalysts to change chemical product specificities and even alter chemical reaction pathways.

Bethe-hole polarization analyser for the magnetic vector of light.

The nature of light as an electromagnetic wave with transverse components has been confirmed using optical polarizers, which are sensitive to the orientation of the electric field. Recent advances in nanoscale optical technologies demand their magnetic counterpart, which can sense the orientation of the optical magnetic field. Here we report that subwavelength metallic apertures on infinite plane predominantly sense the magnetic field of light, establishing the orientation of the magnetic component of light as a separate entity from its electric counterpart. A subwavelength aperture combined with a tapered optical fibre probe can also serve as a nanoscale polarization analyser for the optical magnetic field, analogous to a nanoparticle sensing the local electric polarization. As proof of its functionality, we demonstrate the measurement of a magnetic field orientation that is parallel to the electric field, as well as a circularly polarized magnetic field in the presence of a linearly polarized electric field.

Optical spectroscopy of conductive junctions in plasmonic cavities.

The optical properties of a nanoparticle dimer bridged by a conductive junction depend strongly on the junction conductivity. As the conductivity increases, the bonding dimer plasmon blueshifts and broadens. For large conductance, a low energy charge transfer plasmon also appears in the spectra with a line width that decreases with increasing conductance. A simple physical model for the understanding of the spectral feature is presented. Our finding of a strong influence of junction conductivity on the optical spectrum suggests that plasmonic cavities might serve as probes of molecular conductance at elevated frequencies not accessible through electrical measurements.

Multimodal optical molecular image reconstruction with frequency domain measurements.

Multimodality molecular imaging is becoming more and more important to understand both the structural and the functional characteristics of tissue, organs and tumors. So far, invasive nuclear methods utilizing ionizing radiation have been the "gold standard" of molecular imaging. We investigate non-contact, non-invasive, patient-tolerant and inexpensive near infrared (NIR) frequency domain optical tomography (FDOT) as a functional complement to structural X-ray computed tomography (CT) data. We show a novel multifrequency NIR FDOT approach both in transmission and reflectance mode and employ radiative transport equation (RTE) for 3D reconstruction of a target with novel fluorescent gold nanoshell indocyanine green (NS ICG) in an ex vivo nude mouse. The results demonstrate that gold NS ICG with multifrequency NIR FDOT is a promising fluorophore for multimodal optical molecular image reconstruction.

Electronic and optical properties of electromigrated molecular junctions.

Electromigrated nanoscale junctions have proven very useful for studying electronic transport at the single-molecule scale. However, confirming that conduction is through precisely the molecule of interest and not some contaminant or metal nanoparticle has remained a persistent challenge, typically requiring a statistical analysis of many devices. We review how transport mechanisms in both electronic and optical measurements can be used to infer information about the nanoscale junction configuration. The electronic response to optical excitation is particularly revealing. We briefly discuss surface-enhanced Raman spectroscopy on such junctions, and present new results showing that currents due to optical rectification can provide a means of estimating the local electric field at the junction due to illumination.

Plasmons in the metallic nanoparticle-film system as a tunable impurity problem.

We show that the plasmon resonances of a metallic nanoparticle interacting with the surface plasmons of a metallic film is an electromagnetic analogue of the spinless Anderson-Fano model. This is the same model used to describe the interaction of a localized electronic state with a continuous band of electronic states. The three characteristic regimes of this model are realized here, where the energy of the nanoparticle plasmon resonance lies above, within, or below the energy band of surface plasmon states. These three interaction regimes are controlled by film thickness. The latter regime is experimentally observed and identified.

Independent Optical Control of Microfluidic Valves Formed from Optomechanically Responsive Nanocomposite Hydrogels.

Independent optical control of microfluidic valves formed from optomechanically responsive nanocomposite hydrogels is achieved using strongly absorbing Au nanoparticles or nanoshells embedded within a thermally responsive polymer. Valves formed from composites with different nanoparticles could be independently controlled by changing the illumination wavelength.

Surface-enhanced Raman scattering on tunable plasmonic nanoparticle substrates.

Au and Ag nanoshells are investigated as substrates for surface-enhanced Raman scattering (SERS). We find that SERS enhancements on nanoshell films are dramatically different from those observed on colloidal aggregates, specifically that the Raman enhancement follows the plasmon resonance of the individual nanoparticles. Comparative finite difference time domain calculations of fields at the surface of smooth and roughened nanoshells reveal that surface roughness contributes only slightly to the total enhancement. SERS enhancements as large as 2.5 x 10(10) on Ag nanoshell films for the nonresonant molecule p-mercaptoaniline are measured.

Nanoshell-mediated near-infrared thermal therapy of tumors under magnetic resonance guidance.

Metal nanoshells are a class of nanoparticles with tunable optical resonances. In this article, an application of this technology to thermal ablative therapy for cancer is described. By tuning the nanoshells to strongly absorb light in the near infrared, where optical transmission through tissue is optimal, a distribution of nanoshells at depth in tissue can be used to deliver a therapeutic dose of heat by using moderately low exposures of extracorporeally applied near-infrared (NIR) light. Human breast carcinoma cells incubated with nanoshells in vitro were found to have undergone photothermally induced morbidity on exposure to NIR light (820 nm, 35 W/cm2), as determined by using a fluorescent viability stain. Cells without nanoshells displayed no loss in viability after the same periods and conditions of NIR illumination. Likewise, in vivo studies under magnetic resonance guidance revealed that exposure to low doses of NIR light (820 nm, 4 W/cm2) in solid tumors treated with metal nanoshells reached average maximum temperatures capable of inducing irreversible tissue damage (DeltaT = 37.4 +/- 6.6 degrees C) within 4-6 min. Controls treated without nanoshells demonstrated significantly lower average temperatures on exposure to NIR light (DeltaT < 10 degrees C). These findings demonstrated good correlation with histological findings. Tissues heated above the thermal damage threshold displayed coagulation, cell shrinkage, and loss of nuclear staining, which are indicators of irreversible thermal damage. Control tissues appeared undamaged.

A hybridization model for the plasmon response of complex nanostructures.

We present a simple and intuitive picture, an electromagnetic analog of molecular orbital theory, that describes the plasmon response of complex nanostructures of arbitrary shape. Our model can be understood as the interaction or "hybridization" of elementary plasmons supported by nanostructures of elementary geometries. As an example, the approach is applied to the important case of a four-layer concentric nanoshell, where the hybridization of the plasmons of the inner and outer nanoshells determines the resonant frequencies of the multilayer nanostructure.

A whole blood immunoassay using gold nanoshells.

A rapid immunoassay capable of detecting analyte within complex biological media without any sample preparation is described. This was accomplished using gold nanoshells, layered dielectric-metal nanoparticles whose optical resonance is a function of the relative size of its constituent layers. Aggregation of antibody/nanoshell conjugates with extinction spectra in the near-infrared was monitored spectroscopically in the presence of analyte. Successful detection of immunoglobulins was achieved in saline, serum, and whole blood. This system constitutes a simple immunoassay capable of detecting sub-nanogram-per-milliliter quantities of various analytes in different media within 10-30 min.

From the cover: nanoscale imaging of chemical interactions: fluorine on graphite.

Using C60-functionalized scanning tunneling microscope tips, we have investigated the adsorption of fluorine on graphite. Based on characteristics of the accompanying electron standing waves, we are able to distinguish the fluorine adatoms that have bonded ionically to the graphite surface from those that have formed covalent bonds with the surface. This result permits determination of the ratio of ionic to covalent C-F bonds on graphite obtained by gas phase fluorination, which seems to be temperature-independent between 200 and 300 degrees C under the reaction conditions used.

Temperature-sensitive polymer-nanoshell composites for photothermally modulated drug delivery.

Composites of thermally sensitive hydrogels and optically active nanoparticles have been developed for the purpose of photothermally modulated drug delivery. Copolymers of N-isopropylacrylamide (NIPAAm) and acrylamide (AAm) exhibit a lower critical solution temperature (LCST) that is slightly above body temperature. When the temperature of the copolymer exceeds the LCST, the hydrogel collapses, causing a burst release of any soluble material held within the hydrogel matrix. Gold-gold sulfide nanoshells, a new class of nanoparticles designed to strongly absorb near-infrared light, have been incorporated into poly(NIPAAm-co-AAm) hydrogels for the purpose of initiating a temperature change with light; light at wavelengths between 800 and 1200 nm is transmitted through tissue with relatively little attenuation, absorbed by the nanoparticles, and converted to heat. Significantly enhanced drug release from composite hydrogels has been achieved in response to irradiation by light at 1064 nm. We have investigated the release of methylene blue and proteins of varying molecular weight. Additionally, the nanoshell-composite hydrogels can release multiple bursts of protein in response to repeated near-IR irradiation.

Applications of nanotechnology to biotechnology commentary.

The ability to systematically modify the properties of nanostructures by controlling their structure and their surface properties at a nanoscale level makes them extremely attractive candidates for use in biological contexts, from fundamental scientific studies to commercially viable technologies.