We are interested in a range of different topics ranging from the plasmonic properties of nanoparticles to their use in heterogeneous catalysis. There are a number of different synergistic projects.
Localized surface plasmon resonances (LSPRs) are coherent oscillations of the conduction electrons inside a metal particle. They can be excited by visible light, and their wavelength-dependent absorption and scattering leads to bright colors. The sensitivity of the plasmon resonance frequency to the density of the medium surrounding the particle makes them excellent sensors for events such as binding and bulk refractive index changes. Applications of LSPR sensing have shown that a variety of chemically and biologically relevant molecules can be detected ranging from biomarkers of Alzheimer's disease and anthrax to the direct detection of glucose and chemical warfare agents. LSPR detects the target's refractive index, so the sensors made using this technology are label free sensors. This is in contrast to the vast majority of biological sensors, which often rely on chemical ligands to generate and amplify the target signal.
Plasmonic properties share many attributes of other nanoscale phenomena: they are hard to predict because of their acute dependence on particle size, shape, and composition. Structure-function relationships are important to elucidate, however, experimentally, they are limited by the typically heterogeneous reaction products. Single particle studies surmount this difficulty and allow us to quantitatively probe structure-function relationships in a variety of particles.
In the Marks group, we are interested understanding the effect of shape on plasmonic properties. We can detect single particle scattering ("color") by using dark field optical microscopy, and characterize the size, shape, and composition with electron microscopy. An extensive understanding of the structure of nanoparticles, along with information on how they correlate to plasmonic properties, gives us quantitative understanding on how to fine-tune these properties for a variety of applications. One project is on the experimental analyses of structure-function correlation of gold and silver nanoparticles. We are collaborating with Van Duyne and Schatz groups on plasmon characterization techniques for these particles. We have recently reported on the plasmonic properties of gold and silver nanocubes on different substrates.
Another project is building up an analytical model that predicts nanoparticle growth behavior, taking into consideration surface energy effects as well as elastic strain within the particle. Some experimental TEM studies to corroborate this model are underway; we are currently inspecting seed growth and shape evolution. Also by being a part of MRSEC IRG-3, an interdisciplinary research group that includes eight faculty members and many different projects, further collaborations and new directions are made possible.