1,721,031 research outputs found
Time-resolved investigation of nanometric cell membrane patches with a mid-infrared laser microscope
Full text linkThe proton pump Bacteriorhodopsin (BR) undergoes repeated photocycles including reversible conformational changes upon visible light illumination. Exploiting the sensitivity of infrared (IR) spectra to the conformation, we have determined the reaction kinetic parameters of the conductive intermediate M for the wild-type protein and for its slow mutant D96N during its photocycle. Time-resolved IR micro-spectroscopy using an in-house developed confocal laser microscope operating in the mid-IR is employed to record absorption changes of 10−4 at wavelengths λ1 = 6.08 μm and λ2 = 6.35 μm, assigned to backbone and retinal structural modifications, respectively. Protein samples were embedded in dried lipid bilayers deposited on ultraflat gold supports to enhance the surface field. The signals were analyzed according to a simplified photocycle model with only two dominant states: the dark-adapted state BR* and the intermediate M. We obtained the excitation and relaxation times of the intermediate M from exponential fits to the absorption change time traces. Our results constitute a first step towards future plasmonic-assisted nanoscale time-resolved mid-IR spectrometers for the characterization of bioelectronic and light-harvesting nanodevices based on BR
Effect of 0.6-THz Continuous-Wave Irradiation on Pathologically Relevant Protein Aggregates
No full textIn this article, we investigated the effect of continuous-wave (CW) radiation at 0.6 THz on pathological protein aggregates in the form of amyloid fibrils, i.e., ordered protein complexes linked to neurodegenerative diseases such as Parkinson's and Frontotemporal Dementia. To monitor the effect of terahertz (THz) irradiation, we exploited mid-infrared (mid-IR) vibrational spectroscopy in the amide-I band range, whose lineshape is known to depend on the protein conformation and on how proteins arrange into ordered supramolecular complexes such as fibrils. We coupled the focused THz beam to two different IR-based spectrometers: a conventional Fourier-transform IR (FTIR) Michelson interferometer where the estimated THz electric field is of the order of similar to 1 V/cm; and an atomic force microscopy-assisted (AFM-IR) near-field spectrometer based on a tunable mid-IR quantum cascade laser, where a much higher electric field (similar to 0.1 kV cm) is mainly achieved thanks to the field enhancement provided by the use of a metallicAFMtip and sample support. In the first case, we interpreted the modification of the amide-I band upon THz irradiation in terms of an increase of the intermolecular forces within fibrils in response to environmental changes induced by THz irradiation (change of hydration). On the other hand, nonthermal effects are observed in the high-THz field experiments performed on isolated fibril agglomerates in dry condition with the AFM-assisted spectrometer. The IR spectral response upon prolonged THz irradiation contains only the protein contribution and we obtain a different trend compared to the FTIR experiments, i.e., a weakening of the intermolecular forces, here directly induced by THz absorption and not mediated by changes of the environmental conditions. One can envision that further increase of the THz field value, such as with pulsed laser, can lead to the disassembly of protein fibrils
Field distribution and quality factor of surface plasmon resonances of metal meshes for mid-infrared sensing
No full textWe studied surface plasmon sensors based on micrometric metal meshes by optical transmission spectroscopy as a function of the angle of incidence. The mesh period was set to 2 mu m for operation at mid-infrared wavelengths. Metal meshes on dielectric substrates were compared to suspended meshes obtained with a lift-off-free fabrication process, which reduces plasmon damping and increases the quality factor up to 25. We have numerically calculated the electric field distribution of "dark" quadrupole-like modes and found that the suspended mesh provides an enhanced interaction volume extending up to hundreds of nanometers in free space. Our sensors have been experimentally tested and they exhibited a sensitivity up to 1.4 center dot 10(-3)nm(-1), at least 1 order of magnitude better than standard mid-infrared absorption spectroscopy
Intrinsic linewidth of the plasmonic resonance in a micrometric metal mesh
No full textThe intrinsic linewidth and angular dispersion of Surface Plasmon Polariton resonance of a micrometric metal mesh have been measured with a collimated mid-infrared beam, provided by an External Cavity tunable Quantum Cascade Laser. We show that the use of a collimated beam yields an observed resonance linewidth gamma = 12 cm(-1) at the resonance frequency nu(0) = 1658 cm-1, better by an order of magnitude than with a non-collimated beam. The extremely narrow plasmon resonance attained by our mesh is then exploited to reconstruct, by varying the QCL angle of incidence theta, the angular intensity distribution f(theta) of a globar at the focal plane of a conventional imaging setup. We thus show that f (theta) is better reproduced by a Gaussian distribution than by a uniform one, in agreement with ray-tracing simulation. (C) 2013 Optical Society of Americ
Protein clustering in chemically stressed HeLa cells studied by infrared nanospectroscopy
Full text linkPhoto-Thermal Induced Resonance (PTIR) nanospectroscopy, tuned towards amide-I absorption, was used to study the distribution of proteic material in 34 different HeLa cells, of which 18 were chemically stressed by oxidative stress with Na3AsO3. The cell nucleus was found to provide a weaker amide-I signal than the surrounding cytoplasm, while the strongest PTIR signal comes from the perinuclear region. AFM topography shows that the cells exposed to oxidative stress undergo a volume reduction with respect to the control cells, through an accumulation of the proteic material around and above the nucleus. This is confirmed by the PTIR maps of the cytoplasm, where the pixels providing a high amide-I signal were identified with a space resolution of ∼300 × 300 nm. By analyzing their distribution with two different statistical procedures we found that the probability to find protein clusters smaller than 0.6 μm in the cytoplasm of stressed HeLa cells is higher by 35% than in the control cells. These results indicate that it is possible to study proteic clustering within single cells by label-free optical nanospectroscopy
Effect of 0.6 THz irradiaton on protein fibrils monitored by mid-infrared nano-spectroscopy
No full textProtein amyloid fibrils relevant for neurodegenerative diseases have been irradiated with continuous wave 0.6 THz radiation for minutes. The effect on protein hydration and on the stability of amyloid fibrils has been monitored with ATR-FTIR (in hydrated environment and low-field condition) and with atomic force microscopy-assisted IR nano-spectroscopy (in dry environment and high-field condition), finding opposite effects on the inter-molecular aggregation state
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Responsivity at 0.27 THz of a heterostructure field effect transistor detector in a quasi-optical package
No full textWe have fabricated AlGaAs/InGaAs/AlGaAs heterostructure field effect transistors (HFET) with integrated on-chip antennas and we have measured their optical responsivity when mounted in a in-house developed quasi-optical package with a silicon substrate lens © 2013 IEEE
Terahertz current oscillations in a gated two-dimensional electron gas with antenna integrated at the channel ends
No full textWe studied terahertz current oscillations induced by a frequency-tunable radiation source in a AlGaAs/InGaAs/AlGaAs heterostructure field effect transistor channel. A planar antenna was integrated on-chip, and a substrate lens was used for broadband coupling of free-space radiation at 0.18-0.72 THz to the channel ends. Through spectral analysis of the detection signal, we identified two different mixing mechanisms: one related to channel current oscillations and the other to modulation of the gate-to-channel potential. Depending on gate bias and radiation frequency, the two mechanisms either compete or cooperate, leading to responsivity up to 300 V/W and noise equivalent power of 1 nW/Hz(0.5) (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4717464
- …
