Alexandr Jonáš

Thermal fluctuations of grafted microtubules provide evidence of a length-dependent persistence length

We use single-particle tracking to study the elastic properties of single microtubules grafted to a substrate. Thermal fluctuations of the free microtubule’s end are recorded, in order to measure position distribution functions from which we calculate the persistence length of microtubules with contour lengths between 2.6 and 48 μm. We find the persistence length to vary by more than a factor of 20 over the total range of contour lengths. Our results support the hypothesis that shearing between protofilaments contributes significantly to the mechanics of microtubules. PACS numbers: 87.15.Ya, 87.15.La, 87.16.Ka, 36.20.Ey ∗ Corresponding author. FP and GL have equally contributed to this work.

Three-dimensional tracking of fluorescent nanoparticles with subnanometer precision by use of off-focus imaging

We show that the position of a fluorescent nanoparticle can be measured in three dimensions with subnanometer precision and 100-ms temporal resolution by use of standard epifluorescence video imaging in off-focus mode. The particle can be tracked without feedback in a volume of at least 40 microm x 60 microm x 3 microm. With the technique presented, the structure-mobility relationship of 216-nm latex particle in a porous polymer network was studied in three dimensions.

Light at work: The use of optical forces for particle manipulation, sorting, and analysis

We review the combinations of optical micro‐manipulation with other techniques and their classical and emerging applications to non‐contact optical separation and sorting of micro‐ and nanoparticle suspensions, compositional and structural analysis of specimens, and quantification of force interactions at the microscopic scale. The review aims at inspiring researchers, especially those working outside the optical micro‐manipulation field, to find new and interesting applications of these methods.

Raman Microspectroscopy of Individual Algal Cells: Sensing Unsaturation of Storage Lipids in vivo

Algae are becoming a strategic source of fuels, food, feedstocks, and biologically active compounds. This potential has stimulated the development of innovative analytical methods focused on these microorganisms. Algal lipids are among the most promising potential products for fuels as well as for nutrition. The crucial parameter characterizing the algal lipids is the degree of unsaturation of the constituent fatty acids quantified by the iodine value. Here we demonstrate the capacity of the spatially resolved Raman microspectroscopy to determine the effective iodine value in lipid storage bodies of individual living algal cells. The Raman spectra were collected from three selected algal species immobilized in an agarose gel. Prior to immobilization, the algae were cultivated in the stationary phase inducing an overproduction of lipids. We employed the characteristic peaks in the Raman scattering spectra at 1,656 cm−1 (cis C═C stretching mode) and 1,445 cm−1 (CH2 scissoring mode) as the markers defining the ratio of unsaturated-to-saturated carbon-carbon bonds of the fatty acids in the algal lipids. These spectral features were first quantified for pure fatty acids of known iodine value. The resultant calibration curve was then used to calculate the effective iodine value of storage lipids in the living algal cells from their Raman spectra. We demonstrated that the iodine value differs significantly for the three studied algal species. Our spectroscopic estimations of the iodine value were validated using GC-MS measurements and an excellent agreement was found for the Trachydiscus minutus species. A good agreement was also found with the earlier published data on Botryococcus braunii. Thus, we propose that Raman microspectroscopy can become technique of choice in the rapidly expanding field of algal biotechnology.

Optical trapping of Rayleigh particles using a Gaussian standing wave

Abstract We suggest a modification of a single beam optical trap which enables more effective axial trapping of nanoparticles. We employed interference of an incident wave and the wave which is reflected by the bottom of the trapping cell to create a standing wave trap. The scattering force is strongly suppressed for a highly reflective surface in this configuration and consequently the axial force is represented only by the axial gradient force. The main advantage of the standing wave set-up is that it produces a much stronger axial gradient force than the single beam trap, even without high N.A. focusing optics. The trap is less than four times deeper than the single beam one produced by a laser of the same power so that smaller particles could be trapped in the vicinity of an array of stable positions separated by λ/2 along the beam axis. Even the axial trap stiffness is several orders higher than in the single beam trap.

Simplified description of optical forces acting on a nanoparticle in the Gaussian standing wave

We study the axial force acting on dielectric spherical particles smaller than the trapping wavelength that are placed in the Gaussian standing wave. We derive analytical formulas for immersed particles with relative refractive indices close to unity and compare them with the numerical results obtained by generalized Lorenz-Mie theory (GLMT). We show that the axial optical force depends periodically on the particle size and that the equilibrium position of the particle alternates between the standing-wave antinodes and nodes. For certain particle sizes, gradient forces from the neighboring antinodes cancel each other and disable particle confinement. Using the GLMT we compare maximum axial trapping forces provided by the Gaussian standing-wave trap (SWT) and single-beam trap (SBT) as a function of particle size, refractive index, and beam waist size. We show that the SWT produces axial forces at least ten times stronger and permits particle confinement in a wider range of refractive indices and beam waists compared with those of the SBT.

Theoretical comparison of optical traps created by standing wave and single beam

Abstract We used generalised Lorenz–Mie scattering theory (GLMT) to compare submicron-sized particle optical trapping in a single focused beam and a standing wave. We focus especially on the study of maximal axial trapping force, minimal laser power necessary for confinement, axial trap position, and axial trap stiffness in dependency on trapped sphere radius, refractive index, and Gaussian beam waist size. In the single beam trap (SBT), the range of refractive indices which enable stable trapping depends strongly on the beam waist size (it grows with decreasing waist). On the contrary to the SBT, there are certain sphere sizes (non-trapping radii) that disable sphere confinement in standing wave trap (SWT) for arbitrary value of refractive index. For other sphere radii we show that the SWT enables confinement of high refractive index particle in wider laser beams and provides axial trap stiffness and maximal axial trapping force at least by two orders and one order bigger than in SBT, respectively.

Preparation and characterization of superhydrophobic surfaces based on hexamethyldisilazane-modified nanoporous alumina

Superhydrophobic nanoporous anodic aluminum oxide (alumina) surfaces were prepared using treatment with vapor-phase hexamethyldisilazane (HMDS). Nanoporous alumina substrates were first made using a two-step anodization process. Subsequently, a repeated modification procedure was employed for efficient incorporation of the terminal methyl groups of HMDS to the alumina surface. Morphology of the surfaces was characterized by scanning electron microscopy, showing hexagonally ordered circular nanopores with approximately 250 nm in diameter and 300 nm of interpore distances. Fourier transform infrared spectroscopy-attenuated total reflectance analysis showed the presence of chemically bound methyl groups on the HMDS-modified nanoporous alumina surfaces. Wetting properties of these surfaces were characterized by measurements of the water contact angle which was found to reach 153.2 ± 2°. The contact angle values on HMDS-modified nanoporous alumina surfaces were found to be significantly larger than the average water contact angle of 82.9 ± 3° on smooth thin film alumina surfaces that underwent the same HMDS modification steps. The difference between the two cases was explained by the Cassie-Baxter theory of rough surface wetting.

Single-beam trapping in front of reflective surfaces

We show that the optical trapping of dielectric particles by a single focused beam in front of a weakly reflective surface is considerably affected by interference of the incident and reflected beams, which creates a standing-wave component of the total field. We use the two-photon-excited fluorescence from a trapped dyed probe to detect changes in the distance between the trapped beam focus as the focus approaches the reflective surface. This procedure enables us to determine the relative strengths of the single-beam and the standing-wave trapping forces. We demonstrate that, even for reflection from a glass-water interface, standing-wave trapping dominates, as far as 5 mum from the surface.

Surface delivery of a single nanoparticle under moving evanescent standing-wave illumination

We study the delivery of a submicrometre-sized spherical dielectric particle suspended in water and confined in an evanescent field in the proximity of a glass–water interface. When illuminated by a single evanescent wave, the particle is propelled along the glass surface by the radiation pressure. Illumination by two counter-propagating and coherent evanescent waves leads to the formation of a surface-bound evanescent standing wave serving as a one-dimensional array of optical traps for the stable confinement of the particle. These traps can be translated simultaneously along the surface by shifting the phase of one of the two interfering evanescent waves, carrying the confined particle along in an optical conveyor belt (OCB). However, due to the thermal activation, the particle jumps between neighboring optical traps, and its delivery conditions in the OCB are thus more complex than in the case of the single evanescent wave propulsion. We analyze the delivery speed of a single particle confined in the OCB moving with different speeds and formed by optical traps of different depths. We present a theoretical description of the particle delivery speed in the OCB and compare it with the delivery speed in the single evanescent wave. We support our theoretical conclusions by experimental observations and demonstrate that especially particles having diameters smaller than ~220 nm are delivered faster in the OCB using the same total optical power.