Cells respond to forces, and their quantification can potentially inform on the role of mechanics in cell development, differentiation, tissue repair and homeostasis. Other force sensitive processes include cancer cell metastasis, heart development in embryos driven by fluid forces, and individual cell response to tension by enhancing microtubule growth and connections. Development of current mechano-sensing approaches has not yielded many options, especially in directional force measurement. We present a sharpened fiber-based approach for uniaxial forces. An upconversion nanoparticle (UCNP) is mounted on the tip of the fiber and optically accessed through the fiber, which is manipulated as a probe. In UCNPs, the modification of the crystal field via mechanical forces result in changes in emission intensity, spectral shifts, upconversion luminescence (UCL) lifetime and ratiometric UCL response. We report on a discernably large peak shift of between 5-10 nm, and an apparent phase transition, with increasing amount of applied force in the micro Newton regime, in a single direction. Moreover, the peak shift is linear to the applied compression force. We investigate the influence of the UCNP force sensing process using Raman spectroscopy.
Resonance Raman offers a significant increase in Raman signal levels. We show how this can be used to select a specific molecule within a complex biosystem to study, in our case to determine if hemoglobin survives in ancient fossils. Key to this ability is the fact that the vibration must be on the same molecule as the absorption. Further, we show that the Raman fingerprint, or changes to it, can provide further selectivity or identify changes in that molecule based upon the particular sample. In our case, we find that the iron in the hemoglobin has oxidized into FeOOH, but still attached to both its porphyrin-like heme group and the protein network that gives the hemoglobin absorption. Very narrow Raman resonances are found in molecules with symmetry-forbidden, phonon-allowed absorptions. We show several in biologically relevant materials including that methylated-DNA (m-DNA) can be distinguished from non-methylated (n-DNA) with nano-bowtie- and resonance-enhanced Raman spectra. These efiects are retained when plasmon resonances are used to enhance a local region of the sample, but find that the overall signal from a uniformly distributed specimen is not increased significantly by the enhancement of a small region, so is not recommended unless the sample can be concentrated into that region.
Methylation in DNA is a controlling factor in gene expression, embryonic development, and has been found to be important in infections and cancer. From a basic biology point of view, great heterogeneity has been found in methylation levels within tissues, so questions arises as to how and why. We show that methylated-DNA (m-DNA) can be distinguished from non-methylated (n-DNA) with nano-bowtie- and resonance- enhanced Raman spectra. By tuning the bowtie antenna to the resonance wavelength, both gains can be realized. Two additional Raman peaks in the 1200 – 1700 cm-1 band appear with methylation: one at 1239 cm-1 and the other at 1639 cm-1; a weak peak near 1000 cm-1 also appears with methylation. We also find that the two spectral features, although the latter with slight modification, can be used to distinguish the methylation state even when the DNA is denatured, as we show when we induce crystallization of the salts in the solution with increased excitation power, or allow it to happen naturally via solvent evaporation, and the DNA is trapped within the salt crystals. A comparison between liquid/solution to dried/denatured state m-DNA shows a general broadening of the larger lines and a transfer of spectral weight from the ~1470 cm-1 vibration to two higher energy lines. The applicability of the resonance-Raman in these spectra is shown by demonstrating that the Raman spectral characteristics hardly change as the Raman resonance in excitation wavelength is approached. Finally, we comment on real signal gain in this double-resonance system.
The discovery of soft structures in dinosaur bone with the morphological and molecular characteristics of blood vessels in extant vertebrates was both surprising and controversial. Mounting evidence suggests that these soft tissues are blood vessels, their preservation driven in part by reactive oxygen species derived from hemoglobin degradation. More data are needed to support this hypothesis. Raman spectroscopy, and resonance Raman in particular, can provide detailed information as to the chemical makeup of these samples. We used two different excitation wavelengths in microscale Raman measurements to look for lines characteristic of degraded heme molecules, both in ancient vessels and modern analogues taken from semi-fossilized, hemoglobin-soaked ostrich bones. In both samples, we observed two regimes: dark colored, stiff regions and more transparent, elastic regions. We discovered that the two apparent regimes in the samples had different strengths of Raman returns, and that resonance effects greatly affected the Raman intensity. In all cases, there was some evidence of degraded heme spectra, though the increased returns indicated that the dark regimes had reacted more strongly with the heme specie. The modern vessels displayed a resonance Raman intensity consistent with hemoglobin molecular structures, which indicated resonance spectra would provide understanding of the ancient heme molecule. To investigate the two regimes more thoroughly, we acquired Raman spectra over areas where the sample transitioned from one regime to another. Variable wavelength resonance Raman measurements over the whole sample were used to give more information about the heme species present, in both ancient and modern samples.
The optical scattering from laser beams propagating through atmospheric aerosols has been shown to be very useful in
describing air pollution aerosol properties. This research explores and extends that capability to particulate matter. The
optical properties of Arizona Road Dust (ARD) samples are measured in a chamber that simulates the particle dispersal
of dust aerosols in the atmospheric environment. Visible, near infrared, and long wave infrared lasers are used. Optical
scattering measurements show the expected dependence of laser wavelength and particle size on the extinction of laser
beams. The extinction at long wavelengths demonstrates reduced scattering, but chemical absorption of dust species
must be considered. The extinction and depolarization of laser wavelengths interacting with several size cuts of ARD are
examined. The measurements include studies of different size distributions, and their evolution over time is recorded by
an Aerodynamic Particle Sizer. We analyze the size-dependent extinction and depolarization of ARD. We present a
method of predicting extinction for an arbitrary ARD size distribution. These studies provide new insights for
understanding the optical propagation of laser beams through airborne particulate matter.
Lidar is a powerful tool for measuring the vertical profiles of aerosols in the atmosphere using Rayleigh and Raman lidar
techniques. Bistatic lidar can be used to obtain the angular structure of the scattered light. When the aerosols are
uniformly distributed, this information can be analyzed to provide particle size distribution information. However, dusts
tend to be irregularly shaped particles with varied composition. We investigate the impact of the irregular shape using
optical scattering at several wavelengths, scanning electron microscopy, and T-matrix calculations. In particular, we
study the rapid loss of Mie scattering resonances as the particle shape departs from spherical. Different size distributions
produced by different size-cuts of Arizona Road Dust (ARD) are studied.
Lidar is a powerful tool for measuring the vertical profiles of aerosols. Dusts are irregularly-shaped particles with varied
composition and strong index of refraction variations in the LWIR. We measure dust indices using ellipsometry and
transmission through KBr pellets. Milling makes the ellipsometry data less dependent on incidence angle, and the results
of measurements on milled materials agree with those from transmission measurements. Measurements show that the
spectrum of a milled Arizona Road Dust (ARD) approaches that of pure quartz, indicating a decrease of absorption
efficiency for particles larger than the absorption length. These indices of refraction will be used in the future to simulate
extinction for the beam of a LWIR lidar.
Aerosol optical scattering experiments are often large, expensive, and provide poor control of dust uniformity and size
distribution. The size distribution of such suspended atmospheric aerosols varies rapidly in time, since larger particles
settle quickly. Even in large chambers, 10 micron particles settle in tens of seconds. We describe lab-scale experiments
with stable particle distributions. A viscous colloidal solution can stabilize the particles for sufficient time to measure
optical scattering properties. Colloids with different concentrations or size distributions enable nearly time independent
studies of prepared distributions. We perform laser aureole scattering from a colloid containing a few percent by volume
of Arizona Road Dust (ARD) in mineral oil and glycerin, and 1-micron polystyrene spheres in water. We discuss aureole
analysis, the differences expected in scattering properties due to the index of refraction of the mineral oil medium versus
air, and the impact of non-spherical shape on the scattering. This research demonstrates that particles suspended in a
viscous medium can be used to simulate aerosol optical scattering in air, while enabling signal averaging, offering
reproducibility, and easing problems resulting from parameter variations in studies of dust properties.
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