Our research uses Raman and surface-enhanced Raman scattering (SERS) spectroscopies to to investigate the following

• How do adsorbed molecules interact with surfaces?
• How do changing the chemical and physical properties of the surfaces affect the surface-adsorbate interaction?
• How can SERS be used to better understand conformational changes in biochemical systems, especially enzyme-substrate and enzyme-inhibitor systems?

These questions are important in a variety of fields, from fundamental physical chemistry, to analytical applications and biochemistry. The following briefly describes each area.

Surface mediated chemistry
Early in the development of SERS, theoreticians recognized that the mechanisms (chemical and electromagnetic enhancement) that produce huge Raman scattering signals for molecules adsorbed to roughened metal surfaces can also enhance other physical and chemical processes. One example is the photochemical reduction of p-nitrobenzoic acid (PNBA).

When is adsorbed to roughened silver surfaces and illuminated with visible laser radiation, it undergos a photochemical reduction to form p,p’-diazobenzoic acid (PPDABA). This reaction does not occur unless PNBA is adsorbed to roughened Ag surfaces and it is not photothermal. The photoreduction is well documented in the literature, but no systemmatic study of the chemical and electromagnetic contributions have been made.

We investigate these contributions by making simple chemical changes to the surfaces. In one type of experiment, the roughness of the silver surfaces is controlled by depositing a silver coating on a preformed Au nanoparticle surface. As the coating thickens, the roughness of the surface decreases, and a decrease in the rate of the photoreduction is observed.

Another way to study the reaction is by changing the chemical composition of the nanoparticles. Instead of using pure Ag nanoparticles, either Ag/Au core-shell or alloy nanoparticles can be use. The composition of these nanoparticles is easily controlled by simple wet chemical techniques.

The SERS spectrum of PNBA is monitored as a function of laser illumination time to determine the rate of the photoreaction on the surfaces described above.

SERS Spectroscopy

Most of the projects we are working on involve the use of SERS spectroscopy. SERS has two great advantages – it is extremely sensitive and it provides a vibrational spectrum (similar to an IR absorption spectrum) that can be used to identify chemical species. Molecules must adsorb to roughened metal surfaces for SERS, so not all molecules exhibit a strong SERS spectrum.

Currently we are working on characterizing the SERS spectra of p-(dimethylamino)cinnamaldehyde (DMAC) and Kojic Acid on silver nanoparticle colloids. These projects involve obtaining SERS spectra, assigning the vibrational bands, and determining how the adsorbate interacts with the surface (chemi- or physisorption, what functional group is invloved), and the basic geometry of the adsorbate relative to the surface.

Biochemical Applications

The sensitivity of SERS, along with the information rich vibrational spectrum make it an ideal technique for use in biochemical applications. We are currently using SERS to characterize the enzyme polyphenyloxidase (PPO) and its complexes with catechol (the substrate), kojic acid, p-hydroxybenzoic acid and ascorbic acid (all inhibitors).

The PPO-catechol system is a familiar enzyme catalyzed reaction; anyone who has seen browning of fruit or guacamole has observed it. If you have squirted some lemon juice on a fruit salad, you have taken advantage of the inhibition of the reaction by ascorbic acid.

We are using SERS to understand conformational changes in PPO that occur when either the substrate, or inhibitors bind to the enzyme. Changes in the structure of the enzyme should be apparent in the SERS spectrum, which is sensitive to the secondary and tertiary structure of the protein.

Students interested in working on any of these projects should contact Dr. Gilbert.