Nanoparticles are becoming increasingly common in various biomedical applications, such as drug delivery, biomedical imaging, and biosensing, to name a few. The surface properties of these materials play a significant role in their success for these applications, prompting the development of optimized surface functionalization protocols. Likewise, it is pivotal that the surface chemistry and structure of protein fibers used as scaffolds in tissue engineering can be tuned with precision. Thus it is essential that the specific chemistry, the orientation of chemical groups, and their interactions with the surrounding species can be studied in detail at the surface of nanoparticles and protein fibers in biological environments. However, most techniques lack interfacial specificity, require vacuum conditions, or cannot provide information on the chemistry or the orientation of the detected species. Vibrational sum-frequency scattering spectroscopy (SFS), on the other hand, is a label-free method that can be used to gather this information from molecules specifically at the surface of biomaterials in complex biological environments.<br/>Structures that scatter light and include ordered molecular species with vibrational modes that are Raman <i>and</i> infrared active, can produce SFS signals. Protein fibers consist of repeating units that typically exhibit non-isotropic symmetry radially away from and/or along the fiber axis. One example is collagen fibers, for which we have demonstrated a large SFS cross-section. Molecules at the surface of the protein fibers can also yield SFS signals and the scattering pattern in various polarization combinations reveal the average orientation for the adsorbed species.<br/>Similarly, the average orientation for biomolecules at the surface of nanoparticles can be modeled to fit the signal ratio between different polarization combinations. However, for small particles with a diameter of a few tens of nm or less, this gets challenging due to (1) a rapidly decreasing scattering efficiency, (2) a reduced surface area per particle, and (3) canceling scattered E-fields due to a vanishing phase-shift across the particles. For small metal nanoparticles it is therefore an attractive approach to tune the visible laser to the localized surface plasmon resonance to get surface-enhanced SFS. Also, modulation of the non-resonant SFS signals from such particles may be used to quantify the adsorbed species at the surface.

Vibrational sum-frequency scattering spectroscopy reveals the detailed surface chemistry of biomaterials in aqueous solutions

253rd National Meeting (2017)

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