168 research outputs found
Simultaneous measurements of electrophoretic and dielectrophoretic forces using optical tweezers
Herein, charged microbeads handled with optical tweezers are used as a
sensitive probe for simultaneous measurements of electrophoretic and
dielectrophoretic forces. We first determine the electric charge carried by a
single bead by keeping it in a predictable uniform electric field produced by
two parallel planar electrodes, then, we examine same bead's response in
proximity to a tip electrode. In this case, besides electric forces, the bead
simultaneously experiences non-negligible dielectrophoretic forces produced by
the strong electric field gradient. The stochastic and deterministic motions of
the trapped bead are theoretically and experimentally analysed in terms of the
autocorrelation function. By fitting the experimental data, we are able to
extract simultaneously the spatial distribution of electrophoretic and
dielectrophoretic forces around the tip. Our approach can be used for
determining actual, total force components in the presence of high-curvature
electrodes or metal scanning probe tips.Comment: 9 pages, 3 figure
Dark spots along slowly scaling chains of plasmonic nanoparticles
We numerically investigate the optical response of slowly scaling linear
chains of mismatched silver nanoparticles. Hybridized plasmon chain resonances
manifest unusual local field distributions around the nanoparticles that result
from symmetry breaking of the geometry. Importantly, we find localization
patterns characterized by bright hot-spots alternated by what we term
\textit{dark} spots. A dark spot is associated to dark plasmons that have
collinear and antiparallel dipole moments along the chain. As a result, the
field amplification in the dark interjunction gap is extinguished for incident
polarization parallel to the chain axis. Despite the strong plasmonic coupling,
the nanoparticles on the sides of this dark gap experience a dramatic
asymmetric field amplification with amplitude gain contrast .
Remarkably, also for polarization orthogonal to the axis, gap hot-spots form on
resonance.Comment: 6 pages, 2 figure
Enhancement factor statistics of surface enhanced Raman scattering in multiscale heterostructures of nanoparticles
Suitable metal nanostructures may induce surface-enhanced Raman scattering (SERS) enhancement factors (EFs) large-enough to reach single-molecule sensitivity. However, the gap hot-spot EF probability density function (PDF) has the character of a long-tail distribution, which dramatically mines the reproducibility of SERS experiments. Herein, we carry out electrodynamic calculations based on a 3D finite element method of two plasmonic nanostructures, combined with Monte Carlo simulations of the EF statistics under different external conditions. We compare the PDF produced by a homodimer of nanoparticles with that provided by a self-similar trimer. We show that the PDF is sensitive to the spatial distribution of near-field enhancement specifically supported by the nanostructure geometry. Breaking the symmetry of the plasmonic system is responsible for inducing particular modulations of the PDF tail resembling a multiple Poisson distribution. We also study the influence that molecular diffusion towards the hottest hot-spot, or selective hot-spot targeting, might have on the EF PDF. Our results quantitatively assess the possibility of designing the response of a SERS substrate so as to contain the intrinsic EF PDF variance and significantly improving, in principle, the reproducibility of SERS experiments
Surface charge and hydrodynamic coefficient measurements of {\it Bacillus subtilis} spore by Optical Tweezers
In this work we report on the simultaneous measurement of the hydrodynamic
coefficient and the electric charge of single {\it Bacillus subtilis} spores.
The latter has great importance in protein binding to spores and in the
adhesion of spores onto surfaces. The charge and the hydrodynamic coefficient
were measured by an accurate procedure based on the analysis of the motion of
single spores confined by an optical trap. The technique has been validated
using charged spherical polystyrene beads. The excellent agreement of our
results with the expected values demonstrates the quality of our procedure. We
measured the charge of spores of {\it B. subtilis} purified from a wild type
strain and from two isogenic mutants characterized by an altered spore surface.
Our technique is able to discriminate the three spore types used, by their
charge and by their hydrodynamic coefficient which is related to the
hydrophobic properties of the spore surface.Comment: 21 pages 5 figure
Relationship between corneal temperature and i0ntraocular pressure in healthy Individuals. a clinical thermographic analysis
To study the geographical distribution of corneal temperature (CT) and its influence on the intraocular pressure (IOP) of healthy human volunteers. Materials and Methods. Fifteen subjects (7 M, 8 F), 33.8 +/- 17.4 years old, were enrolled in this pilot, cross-sectional study. Measurements of CT were taken after one hour with closed eyelids (CET) or closed eyelids with a cooling mask (cm-CET) and compared to baseline. Results. If compared to baseline, after CET, average CT significantly increased by 0.56 degrees C in the RE and by 0.48 degrees C in the LE (p < 0.001) and IOP concomitantly significantly increased by 1.13 mm Hg and 1.46 mm Hg, respectively, in each eye (p < 0.001). After cm-CET, average CT significantly decreased by 0.11 degrees C and 0.20 degrees C, respectively, in the RE and LE (RE p = 0.04; LE p = 0.024), followed by a significant IOP decrease of 2.19 mm Hg and 1.54 mm Hg, respectively, in each eye (RE p < 0.001; LE = 0.0019). Conclusion. Significant variations of CT occurred after CET and cm-CET and were directly correlated with significant differences of IOP. It can be speculated that both oxidative stress and sympathetic nerve fiber stimulation by temperature oscillations may affect the regulation of AH vortex flow and turnover, thus influencing IOP values
Surface-enhanced Raman imaging of cell membrane by a highly homogeneous and isotropic silver nanostructure
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