Onofrio M. Maragò

On the SERS depolarization ratio

Abstract The Raman depolarization ratio is a quantity that can be easily measured experimentally and offers unique information on the Raman polarizability tensor of molecular vibrations. In Surface Enhanced Raman Scattering (SERS), molecules are near-field coupled with optical nanoantennas and their scattering properties are strongly affected by the radiation patterns of the nanoantenna. The polarization of the SERS photons is consequently modified, affecting, in a non trivial way, the measured value of the SERS depolarization ratio. In this article we elaborate a model that describes how the SERS depolarization ratio is influenced by the nanoantenna re-radiation properties, suggesting how to retrieve information on the Raman polarizability from SERS experiments.

Nanoscale Discrimination between Toxic and Nontoxic Protein Misfolded Oligomers with Tip-Enhanced Raman Spectroscopy

Highly toxic protein misfolded oligomers associated with neurological disorders such as Alzheimers and Parkinsons diseases are nowadays considered primarily responsible for promoting synaptic failure and neuronal death. Unraveling the relationship between structure and neurotoxicity of protein oligomers appears pivotal in understanding the causes of the pathological process, as well as in designing novel diagnostic and therapeutic strategies tuned toward the earliest and presymptomatic stages of the disease. Here, it is benefited from tip-enhanced Raman spectroscopy (TERS) as a surface-sensitive tool with spatial resolution on the nanoscale, to inspect the spatial organization and surface character of individual protein oligomers from two samples formed by the same polypeptide sequence and different toxicity levels. TERS provides direct assignment of specific amino acid residues that are exposed to a large extent on the surface of toxic species and buried in nontoxic oligomers. These residues, thanks to their outward disposition, might represent structural factors driving the pathogenic behavior exhibited by protein misfolded oligomers, including affecting cell membrane integrity and specific signaling pathways in neurodegenerative conditions.

Polarization Properties of the SERS Radiation Scattered by Linear Nanoantennas with Two Distinct Localized Plasmon Resonances

Playing with the field polarization permits to gain important insight on the near-field interaction between molecules and nanoantennas, as well as to develop new concepts of broadband plasmonic sensors (Fazio B. et al. ACS Nano 5(7):5945–59562, 2011; D’Andrea C. et al. J Phys Chem C 118(16):8571–8580, 2014; D’Andrea C. et al. ACS Nano 7(4):3522–3531, 2013; Foti A. et al. Nanospectroscopy 1:26–32, 2015). By studying the polarization state of the Surface Enhanced Raman Scattering it is, in particular, possible to better understand the re-radiation effects in linear nanoantennas featuring spectrally distinct localized plasmon resonances (LSPR). Here we show that, through the selective excitation of the short- or the long- axis LSPR, it is possible to control and totally rotate (90°) the polarization of the SERS photons of randomly oriented molecules adsorbed on linear nanoantennas. By a proper design of the nanoantenna, this can be accomplished switching the excitation wavelength from the visible to the NIR, while using orthogonal fields polarizations, so to match the two distinct LSPRs. Modeling the nanoantenna enhancement with wavelength-dependent field enhancement tensors we recover the relations ruling the parallel-polarized and unpolarized- SERS intensity dependence found experimentally when selectively driving the two LSPRs. Our results support a physical picture in which the strongly coupled molecule-nanoantenna system behaves as an hybrid structure whose energy levels are determined by the molecule while the far-field radiation properties are determined by the antenna modes.

Spectral dependence of the amplification factor in surface enhanced Raman scattering

Surface Enhanced Raman Scattering (SERS) is characterized by a strong signal amplification (up to 108–10) when both the excitation and the Raman photons frequencies match the localized plasmon resonances (LSPR) of the nanoparticles (NPs). In order to understand if the effective LSPR profile refers to the bare NPs or to the resonance of NPs “dressed” with the probe molecules, we perform multiwavelength (514 nm, 633 nm and 785 nm) SERS experiments using both evaporate gold NPs and gold Nanoantennas produced by electron beam lithography (EBL) as SERS-active substrate on which we deposited Methylene Blue molecules (MB) that yields a resonance energy red-shift and a broadening of LSPR profile.