Plasmonics - Enhanced Vibrational Circular Dichroism

The ability to detect the chirality of molecules is of paramount importance in biology, medicine, and pharmacology. DNA and proteins, on which life is based, are chiral and most drugs interact differently with the human body depending on their chirality. Since left-handed molecules cannot be distinguished from right-handed ones by their chemical composition, optical means are often employed for chiral recognition. A typical experiment is based on circular dichroism (CD), i.e. the difference in absorption between left and right circularly polarized light. While electronic CD addresses molecular excitations in the UV and visible spectral range, which bring little specificity, vibrational CD (VCD) targets mid-IR vibrational resonances, which represent a true ‘fingerprint’ of the molecule. By doing so, VCD uniquely provides chiral recognition with the ability to detect specific molecules and address their conformational state.

Unfortunately, VCD effects are extremely weak, so far limiting their routine application. Improving existing molecular spectroscopy techniques to enhance their capability of sensing chirality is very appealing for basic applications in chemistry and molecular physics and in the long run for life science as well. Over the last decade, two independent approaches have been proposed to (i) increase the interaction between light and molecules in the mid-IR and (ii) increase the sensitivity of circularly polarized light to chiral molecules. On the one side, plasmonic nanoantennas have been developed to boost IR absorption. With this approach, signal enhancements exceeding the three orders of magnitude have been demonstrated for the vibrational fingerprints of specific molecules. On the other side, the so-called superchirality has been unveiled, where engineering of the electric and magnetic optical fields allows increasing the CD signal by up to two orders of magnitude for a given molecule. While the first attempts to merge these two approaches at visible wavelengths have been discussed in the last few years, so far the very promising combination of plasmonics and superchirality has never been applied to VCD in the mid-IR. More significantly, at present not even one of the two individual approaches has found its way towards VCD techniques.

In this project we aim at increasing the signal-to noise ratio of VCD by two orders of magnitude by a combination of plasmonic enhancement and superchiral fields. We will exploit state-of-the-art electromagnetic and quantum-mechanical modeling to draw a precise theoretical framework, electron-beam lithography to realize nanostructured substrates, and polarization-modulation IR spectroscopy to benchmark the new designs against standard VCD. The achievement of our goals would represent a real breakthrough by making substrates for enhanced VCD spectroscopy readily available and favoring a pervasive use of VCD techniques in biochemistry and pharmacology research.


Francesco Mattioli

Michele Ortolani

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