François Lagugné-Labarthet

Vibrational Circular Dichroism in General Anisotropic Thin Solid Films: Measurement and Theoretical Approach

In this study, the measurement of the true vibrational circular dichroism (VCD) spectrum is considered from an experimental and theoretical approach for any general anisotropic thin solid sample exhibiting linear as well as circular birefringence (LB, CB) and dichroism (LD, CD) properties. For this purpose, we have made use of a simple model α-helix polypeptide, namely, the poly(γ-benzyl-L-glutamate) or PBLG, reference sample possessing a well-known VCD spectrum and giving rise to slightly oriented films by deposition onto a solid substrate. Also, we have used a different Fourier transform infrared modulation of polarization (PM-FTIR) optical setup with two-channel electronic processing in order to record the PM-VLD and PM-VCD spectra for various sample orientations in its film plane. All the corresponding general relations of the expected intensities in these experiments and the related properly designed calibration measurements were established using the Stokes–Mueller formalism; in addition, the residual birefringence of the optical setup and the transmittance anisotropy of the detector were estimated. From a comparative study of the results obtained in solution and in the solid state, we then propose a simple new experimental procedure to extract the true VCD spectrum of an oriented PBLG thin film: its consists of calculating the half-sum of two spectra recorded at θ and at θ ± 90° sample orientations. Moreover, the complete linear and circular birefringence and dichroism properties of the ordered PBLG thin film are estimated in the amide I and amide II vibrational regions. This allows us to establish for any sample orientation various theoretical simulations of the VCD spectra that agree nicely with the observed experimental results; this confirms that the measurement of LD and LB is in this case a prerequisite in simulating the true VCD spectrum of a partly oriented anisotropic sample. This validates our combined experimental and theoretical approach and opens the route to promising future vibrational CD studies on other macroscopic anisotropic thin film samples.

Microfluidic channel with embedded SERS 2D platform for the aptamer detection of ochratoxin A

AbstractA selective aptameric sequence is adsorbed on a two-dimensional nanostructured metallic platform optimized for surface-enhanced Raman spectroscopy (SERS) measurements. Using nanofabrication methods, a metallic nanostructure was prepared by electron-beam lithography onto a glass coverslip surface and embedded within a microfluidic channel made of polydimethylsiloxane, allowing one to monitor in situ SERS fingerprint spectra from the adsorbed molecules on the metallic nanostructures. The gold structure was designed so that its localized surface plasmon resonance matches the excitation wavelength used for the Raman measurement. This optofluidic device is then used to detect the presence of a toxin, namely ochratoxin-A (OTA), in a confined environment, using very small amounts of chemicals, and short data acquisition times, by taking advantage of the optical properties of a SERS platform to magnify the Raman signals of the aptameric monolayer system and avoiding chemical labeling of the aptamer or the OTA target. FigAptamer detection of OTA within a SERS/microfluidic channel

Localized enhancement of electric field in tip-enhanced Raman spectroscopy using radially and linearly polarized light

Finite-Difference Time-Domain (FDTD) calculations are used to characterize the electric field in the vicinity of a sharp silver or gold cone with an apex diameter of 10 nm. The simulations are utilized to predict the intensity and the distribution of the locally enhanced electric field in tip-enhanced Raman spectroscopy (TERS). A side-by-side comparison of the enhanced electric field induced by a radially and a linearly polarized light in both gap-mode and conventional TERS setup is performed. For this purpose, a radially polarized source is introduced and integrated into the FDTD modeling. Additionally, the optical effect of a thin protective layer of alumina on the enhancement of the electric field is investigated.

Label-free mapping of osteopontin adsorption to calcium oxalate monohydrate crystals by tip-enhanced Raman spectroscopy.

In the ectopic biomineralization of calcium oxalate kidney stones, the competition between calcium oxalate monohydrate (COM) formation and its inhibition by the phosphoprotein osteopontin (OPN) plays a key role in COM stone-forming processes. To get more insights into these processes, tip-enhanced Raman spectroscopy (TERS) was used to provide surface-specific information about the adsorption of OPN to faces of COM crystals. In TERS, the surface plasmon resonance of a metallic AFM tip is locally excited when the tip is placed in the optical near-field of a laser focused on the crystal surface. Excitation of this localized surface plasmon resonance allows the enhancement of the Raman signal as well as the improvement of the spatial resolution beyond the diffraction limit of the light. As TERS works label free and noninvasively, it is an excellent technique to study the distribution of adsorbed proteins on crystal faces at the submicrometer scale. In the present work, we generated Raman intensity maps indicating high spatial resolution and a distinct variation in relative peak intensities. The collected TERS spectra show that the OPN preferentially adsorbs to edges and faces at the ends of COM crystals (order: {100}/{121} edge > {100} face > {100}/{010} edge ≈ {121}/{010} edge > {010} face) providing also relevant information on the inhibition of crystal growth. This study demonstrates that TERS is an excellent technique for detailed investigations of biomolecules adsorbed, layered, or assembled to a large variety of surfaces and interfaces.

Surface patterning using plasma-deposited fluorocarbon thin films for single-cell positioning and neural circuit arrangement.

Micropatterning glass substrates with a plasma-deposited fluoropolymer thin film was shown to be an efficient approach to manipulate cell positioning. The glass windows promoted cell adhesion, whereas the surrounding fluoropolymer displays a cell-repelling character. Herein, multiple micropatterned substrates were developed with pattern dimensions sufficient to host solely single-cells. These single-cell arrays would allow analysis of individual cell response to stimulation without interference from cell-cell interactions. Mouse myoblast C2C12 cells and cortical neurons from mice were examined, both for amenability to patterning, as well as success of cell adhesion and cell morphology. Both cell types were found to have optimal adherence and growth on the glass surface, while cell adhesion and function was inhibited on the fluoropolymer. The C2C12 cells conformed to the shape of the pattern, while maintaining a healthy structure. Moreover, the neuron cells followed the hexagonal grid patterns and formed circuits, wherein the complexity of the connections depended on incubation time.

Mechanism of inhibition of calcium oxalate crystal growth by an osteopontin phosphopeptide

Osteopontin (OPN) inhibits the nucleation and/or growth of several biominerals, including hydroxyapatite (HA) and calcium oxalate monohydrate (COM), and is thought to function in the prevention of soft-tissue calcification. In previous studies, pOPAR, a peptide corresponding to amino acids 65–80 of rat bone OPN (pSHDHMDDDDDDDDDGD), was shown to be a potent inhibitor of HA crystal growth. We now show that formation of COM in the presence of this peptide results in plate-shaped crystals with rounded ends and scalloped {100} faces. Measurement of crystal dimensions revealed that the pOPAR inhibits growth of COM faces in the order {100} > {121} > {010}. Crystal growth inhibitors are believed to act by adsorbing to growth steps, sites at which lattice-ion addition is energetically favoured. To test this hypothesis, we performed molecular dynamics (MD) simulations of pOPAR adsorption to {121} steps on a {100} face and {121} steps on an {010} face. In the former case, the peptide adsorbs to the {100} (terrace) plane in preference to the {121} (riser) plane; in the latter, the peptide adsorbs to the {121} (riser) plane in preference to the {010} (terrace) plane. These studies represent the first use of MD to study the interaction between an inhibitor and crystal steps. Contrary to the prevailing belief that crystal growth inhibitors adsorb to both lattice planes of a step, we show that pOPAR interacts preferentially to either the terrace or the riser, depending on which is more cationic.

Remote surface enhanced Raman spectroscopy imaging via a nanostructured optical fiber bundle.

Remote surface enhanced Raman spectroscopy (SERS) imaging of an adsorbed monolayer was demonstrated through a nanostructured array of conical tips inscribed onto the distal face of a 30 cm optical fiber bundle. Despite intense Raman signal from the germanium oxide doped fibers, the Raman signal of an adsorbed monolayer of a reference compound (benzene thiol) was detected in the fingerprint region. This opens up the possibility of local remote imaging through an optical fiber that embeds a SERS active platform.

Imaging the Optical near Field in Plasmonic Nanostructures

Over the past five years, new developments in the field of plasmonics have emerged with the goal of finely tuning 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 properties (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.

Optical erasures and unusual surface reliefs of holographic gratings inscribed on thin films of an azobenzene functionalized polymer

The optical erasing of holographic gratings inscribed on azobenzene homopolymer thin films of p(DR1M) is analyzed for various polarizations of the erasing single beam. Gratings of different diffraction efficiencies are first inscribed using different states of the incident interfering beams, either linearly polarized in the plane of incidence—horizontal direction (p + p)—or linearly polarized at ±45° with respect to the vertical (s) direction (+45°,−45°), or even Right and Left circularly polarized (RCP + LCP), respectively. A single beam of “s”, “RCP” or “p” polarization is then used for optical erasure and the variations in the diffraction efficiencies, as measured on the intensity of the diffracted first order S+1, are monitored over short and long time periods. In addition, changes in the amplitude of the sinusoidal surface relief modulations are checked by atomic force microscopy (AFM) measurements and, in some cases, complex profiles with new modulations parallel and/or perpendicular to the grating vector direction are evidenced for the first time. Finally, kinetics of the various erasing processes are compared and discussed in connection with the more probable constructive interference mechanisms which could take place between the transmitted and diffracted beams. A model for a real retroactive effect is suggested and the necessary conditions to fulfil for using these gratings in phase mask applications are discussed.

Nanoparticle Organization through Photoinduced Bulk Mass Transfer

A series of dipolar triphenylaminoazo derivatives, with largely distinct charge transfer and glass transition temperatures, has been synthesized. Their photomigration capability in the solid state to form surface relief gratings (SRGs) under interferential illumination has been investigated with respect to their photochromic properties and showed a prevailing influence of the bulkiness of the azo substituent. The azo mass transfer was utilized to efficiently photoalign 200 nm polystyrene nanoparticles along the SRG crests, which were initially deposited on nonirradiated azo surfaces. In contrast, nanoparticles spin cast on prestructured surface relief gratings were localized in the troughs of the periodic structures. These distinct locations point out the ability of isotropic and amorphous photochromic thin films to collectively move and organize nano-objects in an ordered fashion through the use of polarized illumination. This versatile approach opens the path to optically aligned ensembles of individual nano-objects over large areas, which can be further combined with metallic conductive or magnetic coating to create novel functional nanostructures.