Understanding plasmon-enhanced electromagnetic hot spots for surface-enhanced spectroscopies

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With this award, the Chemical Measurement and Imaging Program is funding the research of Katherine Willets at the University of Texas to develop new techniques to study the detailed structure of surfaces. These techniques will improve the functioning of a variety of chemical sensors, in particular a type based on a phenomenon known as surface-enhanced Raman scattering, or SERS. While SERS is an intensely promising sensing technology, its commercial utilization has been limited by problems that appear to be due to non-uniform surfaces that are not well-characterized or controlled. The current research seeks to improve the functioning of devices based on SERS technology by studying the signals that come from the surface when small particles of gold and silver are added. When molecules come into contact with surfaces treated with these small bits of gold and silver, the resulting SERS signal is enhanced, sometimes by more than a million-fold. The investigators are bringing a variety of analysis techniques to bear on this system in order to understand how this enhancement occurs and, more importantly, how it can be controlled. The work is having a broad impact on the development of new chemical sensing technologies. The long-term goal of the work is to develop new SERS probes that will have greater commercialization opportunities, thus exploiting the ease, portability and relative cheapness of this type of sensor. It is having a further broad impact on the training of the next generation of scientists through the involvement of students at all levels, including high school, in the research investigations.This project is focused on understanding how molecules interact with gold and silver nanoparticles that support localized surface plasmons in SERS. Excitation of plasmons leads to strongly enhanced electromagnetic fields at the surface of the nanoparticles, and by placing molecules into these enhanced fields at the surface, optical signals from the molecules can be increased. In particular, Raman scattering, which provides a molecular 'fingerprint' and is useful for a variety of chemical sensing applications, is strongly enhanced by these nanoparticles. Most work on understanding plasmons and SERS has focused on how excitation fields are enhanced by the nanoparticles, but has neglected the role of emission by the molecule, despite its importance in generating the measured signals. This project probes the role of the molecule interacting with plasmonic nanoparticles, in order to better understand the factors that lead to the strongest possible signals from molecular targets of interest. To more precisely understand how molecules couple to plasmonic nanoparticles, both spectral and spatial overlap between the molecules and the nanoparticles are being investigated. Electrogenerated chemiluminescence is used to probe plasmon-coupled emission in the absence of plasmon-enhanced optical excitation. Super-resolution optical imaging is also being used to probe how the location of molecules on the surface of plasmonic nanoparticles influences how and where the emission is coupled into the far-field with sub-10 nm resolution. A wavelength-resolved version of super-resolution imaging is used to achieve simultaneous spectral and spatial resolution in order to show how different Raman modes are coupled into the far-field via wavelength-dependent plasmon coupling. These studies will provide an improved understanding of how accounting for the molecule can potentially redefine the conventional picture of a 'hot spot' region of strongly enhanced local electromagnetic fields.