Mid-infrared resonant nanostructures for in-vitro monitoring of polypeptides

Abstract

Infrared vibrational spectroscopy is a technique based on the molecular vibrations, that is, the oscillation of individual atoms with respect to each other. Each of these vibrations has a characteristic resonance frequency which leads to the distinct vibrational fingerprint of a molecule and thus enables a label-free, non-destructive, and chemically specific detection of molecular species.

The infrared absorption cross-sections, which characterize the optical interaction strength, are relatively small. This is of minor importance for conventional spectroscopy, where large ensembles of molecules can be measured and thus contribute to the overall signal. However, the small infrared absorption hampers detection of molecules at low concentrations, which is of large importance for medical diagnostics, for instance, where the determination of the secondary structures of proteins is crucial due to their role in many incurable diseases. A key to overcome this limitation is to utilize plasmonic nanostructures, which confine the electromagnetic radiation on the nanometer scale and allow higher overall absorption.

In this dissertation, it is demonstrated that even a monolayer of proteins can be detected using mid-infrared resonant gold nanostructures. We use polypeptides as a model system and were able to investigate the secondary structure of molecular monolayers in-vitro. Applying different external stimuli, we are able to induce structural changes of polypeptides in aqueous environments. In addition to a mid-infrared resonant nanoantenna, nanoslits (or inverse antennas) can also enhance the optical response of polypeptides, which allowed us to detect the secondary structure of minicollagen monolayers. Both nanostructure designs provided the possibility even to monitor reversible conformational transitions of molecular monolayers.

Scaling this approach down to a single nanostructure allows to detect only a few thousands of polypeptides in liquid environments. The demonstrated concept could lead to integrated chip-level technology for biological and even medical applications, where biosamples with minute concentrations are investigated. With further advances, it could be possible to scale the process to a few or single proteins and observe the structural changes of individual entities.