Biomedical subjects

P Etchegoin

Publications and source records attributed to P Etchegoin.

At least 19 recordsLinked to original sources

PubMed2005

SERS in PAH-Os and gold nanoparticle self-assembled multilayers.

We present a detailed structural and surface-enhanced Raman scattering (SERS) study of poly(allylamine) modified with Os(byp)2ClPyCHO (PAH-Os) and gold nanoparticles self-assembled multilayers [PAH-Os+(Au-nanoparticlesPAH-Os)n, n=1 and 5]. Atomic force microscopy and variable-angle spectroscopic ellipsometry measurements indicate that the first nanoparticle layer grows homogenously by partially covering the substrate without clustering. Analyzing the sample thickness and roughness we infer that the growth process advances thereafter by filling with nanoparticles the interstitial spaces between the previously adsorbed nanoparticles. After five immersion steps the multilayers reach a more compact structure. The interaction between plasmons of near-gold nanoparticles provides a new optical absorption around 650 nm which, in addition, allows a more effective SERS process in that spectral region than at the single-plasmon resonance (approximately 530 nm). We compare the electronic resonance Raman and SERS amplification mechanisms in these self-assembled multilayers analyzing Raman resonance scans and Raman intensity micromaps. As a function of nanoparticle coverage we observe large changes in the Raman intensity scans, with maxima that shift from the electronic transitions, to the plasmon resonance, and finally to the coupled-plasmon absorption. The Raman micromaps, on the other hand, evidence huge intensity inhomogeneities which we relate to "hot spots." Numerical discrete dipole approximation calculations including the interaction between gold nanoparticles are presented, providing a qualitative model for the coupled-plasmon absorption and redshifted Raman hot spots in these samples.

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PubMed2004

Stokes/anti-Stokes anomalies under surface enhanced Raman scattering conditions.

The possibility of achieving anti-Stokes stimulation and/or pumping under surface enhanced Raman scattering conditions has been the source of intense controversies in the literature. With the aim of clarifying some of the aspects of this problem, we study theoretically and experimentally the situation in a model system which highlights some of the difficulties in the interpretation of the data. We show that many of the assumptions often presumed in the literature need to be assessed with care in each case. Through careful experiments we show, in particular, that the anti-Stokes/Stokes ratio for a specific mode in the same sample can depend on the chosen laser wavelength. This latter effect is a manifestation of the influence of the internal plasmon resonances (hot spots) in the result. Different possibilities and further research directions are highlighted and discussed.

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PubMed2003

Electromagnetic optimization of light-harvesting proteins.

The problem of electromagnetic optimization of the structure of light-harvesting proteins is studied within a simplified diffraction scheme. It is suggested that evolutionary pressures for protein antennae has resulted in molecular structures that optimize the absorption in the light-sensitive pigments. Bacteriorhodopsin is shown to be an exceptional case in terms of optimization, a fact that can be connected to its extremely high sensitivity as a light detector down to a single photon level.

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PubMed2003

A model for protein-DNA interaction dynamics.

We map a simplified version of the protein-DNA interaction problem into an Ising-model in a random magnetic field. The model includes a "head" which moves along the chain while interacting with the underlying spins. The head moves by using the statistical fluctuations of base openings. A Monte Carlo (MC) simulation of this model reveals the possibility of biased diffusion in one direction, followed by sequence identification and binding. The model provides some insight into the mechanisms used by some repressor proteins to diffuse and bind to specific DNA-binding sites.

Animals↗

PubMed2001

Experimental observation of stochastic resonance in a linear electronic array.

We report the experimental observation of array-enhanced stochastic resonance, spatiotemporal synchronization, and noise-enhanced propagation in a simple coupled linear array of bistable electronic triggers. In addition, we highlight an analogy between charge density wave (CDW) like conductivity and spatiotemporal synchronization in stochastic resonances, several aspects of which are supported by the experimental evidence presented here. This may prove to be important in the understanding of nonlinear conductivity in CDW solids.

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PubMed2000

Photoinduced oxygen dynamics in lyophilized hemoglobin.

Reversible laser induced deoxygenation in the lyophilized phase of hemoglobin is demonstrated by means of resonant Raman scattering, luminescence, and optical transmission. Specific Raman modes, which are both sensitive to the spin states of Fe(II) in the hemes and resonant in the visible, are monitored as a function of time to evaluate the effect of the illuminating laser. These modes act as in-situ markers of the oxygen content of the protein. The reversible photoinduced deoxygenation can be observed through both the Raman spin-markers and the optical transmission experiments. In the former, reversible changes in the intensities of specific Raman modes are observed, while in the latter, the oscillator strength of the two main absorptions of oxyhemoglobin in the visible are seen to vary accordingly. The luminescence in lyophilized hemoglobin is found to have at least two different contributions, (i) a resonant component with the Raman modes and; (ii) a nonresonant contribution, which increases at high input laser powers and eventually masks the Raman signals. The nonresonant contribution is the luminescence of the photoproduct achieved by thermal denaturation of the protein and remains standing as a permanent nonreversible damage in the illuminated spot. Semiempirical electronic calculations of the wavefunction and total energy of the iron porphyrin reveal the underlying physical origin of the laser induced deoxygenation process in the hemes and are also presented.

Animals↗

PubMed2000

Phonon self-energies and phase transitions in a prototype discotic liquid crystal

Specific Raman active vibrations in the discotic liquid crystal nematogen hexakis(6-hexyloxy)triphenylene are shown to be sensitive to either the isotropic<-->columnar or the columnar<-->solid phase transitions. Changes in frequency and/or intensity of vibrations with specific symmetries demonstrate that the method can be used not only to monitor the phase transitions themselves with a microscopic and noninvasive optical technique, but also to gain physical information on the origin of the molecular interactions that produce them.

Journal Article↗

PubMed2000

Coupling between molecular vibrations and liquid crystalline order parameters.

Specific Raman active modes in two prototype cyanobiphenyl liquid crystals are shown to display a temperature dependent softening proportional to either the nematic or smectic order parameters, while other vibrations (like the C identical withN stretch mode) remain unaltered. This selective coupling between intramolecular vibrations and the liquid crystalline order is related to the intrinsic symmetry of the modes. The method provides a simple, microscopic, noninvasive optical technique with which the liquid crystalline order parameters can be qualitatively mapped out.

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PubMed2000

Comment on "Photon transmission technique for studying multiple phase transitions in a liquid crystal".

Contrary to the recently published results by Ozbek et al. [Phys. Rev. E 59, 6798 (1999)], we argue that the transmitted light intensity through an unoriented sample is by no means a good measure of the order parameters of the different phase transitions of a liquid crystal.

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PubMed2000

Blue phases of cholesteric liquid crystals as thermotropic photonic crystals

The study of dye-doped low pitch cholesteric liquid crystals in their blue phases as an example of tunable "weak" photonic crystals is proposed and demonstrated. The presence of the blue phases in cholesterics can be tuned with temperature, and this allows for an easy in situ comparison of the emission and/or absorption of the dyes with or without an enwrapping lattice of disclination lines. The fluorescence emission of the dyes is shown to be affected by the presence of the blue phases. Although unlikely to be suitable for real applications (due to the natural low refractive index contrast), these systems may represent unique examples of tunable photonic crystals. It is proposed that single crystals of dye-doped blue phases should provide a very interesting testing ground for the study of optical emission anisotropies which can, on the other hand, be controlled by an external parameter.

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PubMed1999

Universal low-frequency vibrations of proteins from a simple interaction potential.

A pairwise Born potential connecting the heavy atom sites within a prescribed cutoff, and the equation of motion method (EOM), reproduce the existence of a universal singularity in the low-frequency vibrational density of states of typical globular proteins. This is due to quasilocalization of acoustic waves and an analogy with a similar feature found in glasses is stressed. We explain the dependence of this anomaly with the effective dimensionality of the protein. The EOM method allows for the study of even the largest proteins with a simple personal computer.

Animals↗

PubMed1996