Biomolecule Sensing with Adaptive Plasmonic Nanostructures

One of the challenges of biomolecule sensing with surface-enhanced Raman scattering (SERS) is to preserve all of the advantages of Raman spectroscopy applications for structural biology. There are many examples where Raman spectroscopy provides important information on large, macromolecular structures as a whole and in defining small regions of large complexes through ligand–macromolecule recognition (reviewed in [1, 2, 3, 4, 5, 6]). The Raman scattering process involves interplay between atomic positions, electron distribution, and intermolecular forces. Hence, Raman spectroscopy potentially can be one of the techniques used to reveal correlations between structure and function. It is a common belief that the protein–metal surface interaction may lead to structural changes of proteins and the loss of protein functionality to some extent. To what extent this occurs is a question that needs to be addressed for any particular type of SERS-active substrates. The applicability of SERS to molecular biology has been under extensive study since the 1980s [7]. Despite some unknowns in the SERS process, the molecular mechanisms of biomolecule–metal surface interactions and the distance dependence of the Raman enhancement, SERS has been widely used in biomolecular spectroscopy [8, 9, 10, 11, 12, 13, 14, 15, 16]. In this Chapter we demonstrate several examples of protein sensing with our SERS substrate employing a new, adaptive property of well-known vacuum-evaporated silver films. The deposition of protein solutions on such a film results in the rearrangement of the initial metal nanostructures. Such protein-mediated restructuring leads to the formation of aggregates of metal particles naturally covered and matched with molecules of particular sizes and shapes. This procedure optimizes the SERS signal and, in parallel, stabilizes the metal surface with proteins. Such a substrate, which is referred to as an adaptive silver film (ASF), provides a large SERS enhancement and hence allows protein sensing at monolayer protein surface densities, while enabling the adsorption of proteins without significant changes in their conformational states [17, 18, 19, 20]. For example, we showed that spectral differences in SERS spectra of human insulin and its analog insulin lispro can be detected and assigned to their difference in conformational states. An interesting op-