Focal Point: Imaging Optical Near‐field in Plasmonic Nanostructures

International audienceOver the past five years, new developments in the field of plasmonics have emerged with the desire to finely tune a variety of metallic nanostructures to enable a desired function. The use of plasmonics in spectroscopy is of course of great interest due to large local enhancements in the optical near-field confined in the vicinity of a metal nanostructure. For a given metal, such enhancements are dependent on the shape of the structure as well as the optical parameters (wavelength, phase, polarization) of the impinging light, offering a large degree of control over the optical and spatial localization of the plasmon resonance. In this focal point, we highlight recent work that aims at revealing the spatial position of the localized plasmon resonances using a variety of optical and non-optical methods

In search of the hot spot.

Tip-enhanced spectroscopy and its Raman variant, tip-enhanced Raman spectroscopy (TERS), provide the possibility to spatially resolve the vibrations of a single molecule when combined with ultra-high vacuum technology, but this technique suffers from poor time resolution2,3. On the other hand, surface-enhanced Raman spectroscopy (SERS) is a more common technique, and is used for many imaging and bioanalytical applications, still with single molecule sensitivity but, unlike TERS, without spatially resolved molecular imaging capabilities4. In SERS, the roughness and geometric characteristics of metallic surfaces (in general Al, Ag or Au) are exploited to enable high spatial confinement of an impinging electromagnetic field. Since the SERS signal scales with the square power of the electromagnetic fields of both excitation and Raman shift, even a small increase of the local field creates so-called hot spots that yield large Raman intensity. Yet, for low concentrations of analytes deposited onto these surfaces, the collected SERS signal fluctuates with time, and the cause of such signal variation is assigned to the chemical stability of the metal–adsorbate interaction, the dynamics of the molecules in extremely confined fields and the transient nature of the hot spots

Probing mid-infrared plasmon resonances in extended radial fractal structures.

Infrared (IR) antennas made of metallic nanostructures are widely tunable from the near- to the far-IR range. They can be utilized for a variety of applications such as light harvesting and photonic filters, and their structural linear or circular anisotropy can be exploited to further enhance the sensitivity of spectroscopic measurements. Here gold dendritic fractal structures that were optimized to exhibit multiple resonances in the mid-IR range were characterized using a scattering-type scanning near-field optical IR microscope. The spatially resolved IR maps associated with the individual modes serve as a basis to understand the mode evolution between each fractal generation

Deciphering tip-enhanced Raman imaging of carbon nanotubes with deep learning neural networks.

Recent release of open-source machine learning libraries presents opportunities to unify machine learning with nanoscale research, thus improving effectiveness of research methods and characterization protocols. This paper outlines and demonstrates the effectiveness of such a synergy with artificial neural networks to provide for an accelerated and enhanced characterization of individual carbon nanotubes deposited over a surface. Our algorithms provide a rapid diagnosis and analysis of tip-enhanced Raman spectroscopy mappings and the results show an improved spectral assignment of spectral features and spatial contrast of the collected images. Using several examples, we demonstrate the robustness and versatility of our deep learning neural network models. We highlight the use of machine learning and data science in tandem with tip-enhanced Raman spectroscopy technique enables a fast and accurate understanding of experimental data, thus leading to a powerful and comprehensive imaging analysis applied to spectroscopic measurements

Enhanced Rates of Photoinduced Molecular Orientation in a Series of Molecular Glassy Thin Films.

Photoinduced orientation in a series of molecular glasses made of small push-pull azo derivatives is dynamically investigated for the first time. Birefringence measurements at 632.8 nm are conducted with a temporal resolution of 100 ms to probe the fast rate of the azo orientation induced under polarized light and its temporal stability over several consecutive cycles. To better evaluate the influence of the azo chemical substituents and their electronic properties on the orientation of the whole molecule, a series of push-pull azo derivatives involving a triphenylaminoazo core substituted with distinct electron-withdrawing moieties is studied. All resulting thin films are probed using polarization modulation infrared spectroscopy that yields dynamical linear dichroism measurements during a cycle of orientation followed by relaxation. We show here in particular that the orientation rates of small molecule-based azo materials are systematically increased up to 7-fold compared to those of a reference polymer counterpart. For specific compounds, the percentage of remnant orientation is also higher, which makes these materials of great interest and promising alternatives to azobenzene-containing polymers for a variety of applications requiring a fast response and absolute control over the molecular weight

Au nanostructured surfaces for electrochemical and localized surface plasmon resonance-based monitoring of α-synuclein-small molecule interactions.

In this proof-of-concept study, the fabrication of novel Au nanostructured indium tin oxide (Au-ITO) surfaces is described for the development of a dual-detection platform with electrochemical and localized surface plasmon resonance (LSPR)-based biosensing capabilities. Nanosphere lithography (NSL) was applied to fabricate Au-ITO surfaces. Oligomers of α-synuclein (αS) were covalently immobilized to determine the electrochemical and LSPR characteristics of the protein. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed using the redox probe [Fe(CN)6](3-/4-) to detect the binding of Cu(II) ions and (-)-epigallocatechin-3-gallate (EGCG) to αS on the Au-ITO surface. Electrochemical and LSPR data were complemented by Thioflavin-T (ThT) fluorescence, surface plasmon resonance imaging (SPRi), and transmission electron microscopy (TEM) studies. EGCG was shown to induce the formation of amorphous aggregates that decreased the electrochemical signals. However, the binding of EGCG with αS increased the LSPR absorption band with a bathochromic shift of 10-15 nm. The binding of Cu(II) to αS enhanced the DPV peak current intensity. NSL fabricated Au-ITO surfaces provide a promising dual-detection platform to monitor the interaction of small molecules with proteins using electrochemistry and LSPR

Three-color plasmon-mediated reduction of diazonium salts over metasurfaces.

Surface plasmon-mediated chemical reactions are of great interest for a variety of applications ranging from micro- and nanoscale device fabrication to chemical reactions of societal interest for hydrogen production or carbon reduction. In this work, a crosshair-like nanostructure is investigated due to its ability to induce local enhancement of the local electromagnetic field at three distinct wavelengths corresponding to three plasmon resonances. The structures are irradiated in the presence of a solution containing diazonium salts at wavelengths that match the resonance positions at 532 nm, 632.8 nm, and 800 nm. The resulting grafting shows polarization and wavelength-dependent growth patterns at the nanoscale. The plasmon-mediated reactions over arrays of the crosshair structures are further investigated using scanning electron microscopy and supported by finite domain time domain modelling revealing wavelength and polarization specific reactions. Such an approach enables nanoscale molecular printing using light source opening multiplexing applications where different analytes can be grafted under distinct opto-geometric conditions

Tip-enhanced Raman spectroscopy of amyloid β at neuronal spines.

The early stages of Alzheimer\u27s disease pathogenesis are thought to occur at the synapse level, since synapse loss can be directly correlated with memory dysfunction. Considerable evidence has suggested that amyloid beta (Aβ), a secreted proteolytic derivative of amyloid precursor protein, appears to be a critical factor in the early \u27synaptic failure\u27 that is observed in Alzheimer\u27s disease pathogenesis. The identification of Aβ at neuronal spines with high spatial resolution and high surface specificity would facilitate unraveling the intricate effect of Aβ on synapse loss and its effect on neighboring neuronal connections. Here, tip-enhanced Raman spectroscopy was used to map the presence of Aβ aggregations in the vicinity of the spines exposed to Aβ preformed in vitro. Exposure to Aβ was of 1 and 6 hours. The intensity variation of selected vibrational modes of Aβ was mapped by TERS for different exposure times to Aβ. Of interest, we discuss the distinct contributions of the amide modes from Aβ that are enhanced by the TERS process and in particular the suppression of the amide I mode in the context of recently reported observations in the literature

Three-color plasmon-mediated reduction of diazonium salts over metasurfaces.

Surface plasmon-mediated chemical reactions are of great interest for a variety of applications ranging from micro- and nanoscale device fabrication to chemical reactions of societal interest for hydrogen production or carbon reduction. In this work, a crosshair-like nanostructure is investigated due to its ability to induce local enhancement of the local electromagnetic field at three distinct wavelengths corresponding to three plasmon resonances. The structures are irradiated in the presence of a solution containing diazonium salts at wavelengths that match the resonance positions at 532 nm, 632.8 nm, and 800 nm. The resulting grafting shows polarization and wavelength-dependent growth patterns at the nanoscale. The plasmon-mediated reactions over arrays of the crosshair structures are further investigated using scanning electron microscopy and supported by finite domain time domain modelling revealing wavelength and polarization specific reactions. Such an approach enables nanoscale molecular printing using light source opening multiplexing applications where different analytes can be grafted under distinct opto-geometric conditions

Characterization of extracellular vesicles derived from mesenchymal stromal cells by surface-enhanced Raman spectroscopy.

Extracellular vesicles (EVs) are secreted by all cells into bodily fluids and play an important role in intercellular communication through the transfer of proteins and RNA. There is evidence that EVs specifically released from mesenchymal stromal cells (MSCs) are potent cell-free regenerative agents. However, for MSC EVs to be used in therapeutic practices, there must be a standardized and reproducible method for their characterization. The detection and characterization of EVs are a challenge due to their nanoscale size as well as their molecular heterogeneity. To address this challenge, we have fabricated gold nanohole arrays of varying sizes and shapes by electron beam lithography. These platforms have the dual purpose of trapping single EVs and enhancing their vibrational signature in surface-enhanced Raman spectroscopy (SERS). In this paper, we report SERS spectra for MSC EVs derived from pancreatic tissue (Panc-MSC) and bone marrow (BM-MSC). Using principal component analysis (PCA), we determined that the main compositional differences between these two groups are found at 1236, 761, and 1528 c