Surface enhanced Raman spectroscopy (SERS) is a field enhanced vibrational spectroscopy that shows great promise for the rapid spectroscopic identification of pathogenic microorganisms. To realize its full potential, SERS substrates are required that generate large and reproducible electromagnetic field enhancements. In this project fabrication technologies are developed that provide control over the complex electromagnetic response of two-dimensional plasmonic structures. To achieve this aim chemical assembly procedures and lithographic nanofabrication approaches will be combined to generate nanoscale noble metal structures at defined locations over micron length scales. Reliable fabrication methods for locations of giant electromagnetic field enhancement - so called "hot spots" - will allow a rational fabrication of optimized SERS substrates which combine large field enhancement and high reproducibility. The intellectual merit of the effort is that it will improve our understanding of the complex electromagnetic interactions in periodic and aperiodic plasmonic structures with multiple length scales. The broader impact of the project is that the optimized SERS substrates will allow a rapid and reliable identification of bacterial pathogens through SERS. This is highly relevant, for instance, in the clinical setting where it could enable more effective treatment strategies of acute infections. SERS substrates with improved reliability and reproducibility will also lead to entirely new applications of SERS in challenging biological and biomedical sensing applications. In addition to the scientific impact of this proposal, there are clear educational and outreach impacts. The project will offer students and junior researchers the opportunity to participate in a collaborative research and education program. Because both the topic and technique are of great interest to the general population, this effort will enable a substantial outreach program. One important component will be the Boston University Nanocamp, organized by the PI in collaboration with the Boston University's learning network resource (LERNet). The nanocamp will attract and familiarize students from inner-city high schools - which typically have a higher fraction of underrepresented groups - to the research outlined in this proposal.
Nanostructured noble metal surfaces can focus incident light into discrete spots with nanoscale volumes. This light focusing effect leads to a strong local enhancement of the electromagnetic field and provides new opportunities for improving the sensitivity of surface enhanced spectroscopies in biosensing. Raman spectroscopy, for instance, is a vibrational spectroscopy whose signal is greatly enhanced in the vicinity of a nanostructured metal surface. This signal amplification facilitates the application of surface enhanced Raman spectroscopy (SERS) in novel sensing strategies, for instance, for recording Raman spectra of bacterial surfaces in direct contact with the metal surface. Since the recorded vibrational spectra provide a wealth of molecular information, SERS can provide vibrational "fingerprints" that are of significant interest for the development of rapid spectroscopic bacterial identification modalities. A successful realization of these technologies would greatly aid the treatment of bacterial infections with narrow band antibiotics and, thus, help to avoid the spread of bacterial resistances to antibiotics. One important requirement for the application of SERS in bacterial biosensing is, however, the ability to fabricate nano-scale metal surfaces that generate strong signal enhancements reliably. In response to this challenging need, this project developed novel fabrication methods that enable the reproducible formation of noble metal nanoparticle clusters of defined size and composition at pre-defined lattice sites in two-dimensional arrays of defined morphology. These fabrication tools made it possible to realize SERS substrates in which the signal enhancement of the constitutive nanoparticle clusters is further boosted through synergistic electromagnetic interactions in the array. The template guided self-assembly approach used for the fabrication of the NCAs was demonstrated to provide rational control over the size, shape, and position of the generated nanoparticle clusters. The resulting SERS substrates were shown to provide an excellent compromise between signal amplification and reproducibility. The NCA's ability to identify different bacterial species was tested with a test panel of bacteria, including Escherichia coli, Staphylococcus aureus and Bacillus cereus. Pure samples of the individual species were successfully differentiated through SERS on the rationally designed metal substrates. Optimized NCAs demonstrated sufficient signal enhancement to facilitate the acquisition of single cell spectra. The latter is an important step towards a rapid identification of bacterial pathogens in samples that contain mixtures of different bacterial species. The project also systematically investigated the fundamental electromagnetic coupling mechanisms in two-dimensional arrays of noble metal nanoparticles in which the individual building blocks interact on multiple length scales. This research revealed the basic design criteria for optimizing a cascaded electromagnetic field enhancement in multiscale SERS substrates. Besides the intellectual merits described above, the project created significant broader impact. The research formed the foundation of two PhD theses and provided a platform to educate high-school students and undergraduate researchers about nanotechnology and biosensing.
