Alexander B. Pravdin

Random media characterization using the analysis of diffusing light data on the basis of an effective medium model

The transport properties of dense random media such as rutile powder layers and polyball suspensions are analyzed in visible and near infrared on the basis of experimental data on coherent backscattering, diffuse transmittance, and low-coherence interferometry. The developed technique of retrieval of the transport parameters of examined scattering media allows the evaluation of the transport mean free path l* and the effective refractive index n(ef) of the medium without a priori knowledge of the optical properties of the scattering particles. It is found that with decreasing wavelength lambda(0) the value of localization parameter 2pin(ef)l*/lambda(0) of the studied rutile samples abruptly drops and approaches approximately 2.6 at 473 nm. This peculiarity is caused by the very large scattering efficiency of scatterers in the vicinity of the first Mie resonance.

Fluorescence dynamics of human epidermis ( ex vivo ) and skin ( in vivo )

The temporal behavior of autofluorescence of human skin and epidermis under continuous UV-irradiation has been studied. Fluorescence spectra and kinetic curves of fluorescence intensity have been obtained. The fluorescence intensity recovery after dark period also has been examined. The vitiligo skin and epidermis were used for comparing their spectra with reflectance and fluorescence spectra of healthy skin. The epidermal samples were prepared using surface epidermis stripping technique. It has been concluded that fluorophores being undergone the UVA photobleaching are actually present in epidermal layer, and immediate pigment darkening does contribute, no less than a half of magnitude, to the autofluorescence decrease under continuous UVA irradiation.

Dynamics of optical clearing of human skin in vivo

Two ways of improving the skin permeability to clearing agents has been suggested and tested: i. Removal of upper layers of epidermis by glue stripping, ii. Electrophoretic administration of clearing agent into intact skin. Alterations in light scattering by skin under continuous contact with the agent were monitored by the measurement of diffuse reflectance of tissue at 830nm.

ALTERATIONS IN AUTOFLUORESCENCE SIGNAL FROM RAT SKIN EX VIVO UNDER OPTICAL IMMERSION CLEARING

For the first time, the changes in autofluorescence spectra of ex vivo rat skin have been experimentally investigated using the combination of fluorescence spectroscopy and optical immersion clearing. The glucose, glycerol and propylene glycol solutions were used as clearing agents. The optical clearing was performed from the dermal side of skin imitating the in vivo injection of clearing agent under the dermal layers. In this contribution, the common properties of autofluorescence variation during optical immersion clearing were determined. The tendency of autofluorescence signal to decrease with reduction of scattering in tissue was noticed and discussed in detail. However, the differences in the shape of spectral curves under application of different clearing agents showed that optical clearing affects the autofluorescence properties of tissue differently depending on the type of clearing liquid. The results obtained are useful for the understanding of tissue optical clearing mechanisms and for improving techniques such as fluorescence spectroscopy.

Resonant scattering and absorption in the titanate-based nanoplatelet dispersions in near ultraviolet region.

Extinction enhancement and nonlinear near-resonant absorption of potassium polytitanate nanoplatelets were experimentally studied in the near-UV region. Phenomenological models such as the one-oscillator Lorentz model for dielectric function and the two-level model with the depleted ground state were used to interpret the experimental data. The introduced model parameters demonstrate the adequately high sensitivity to variations in nanoplatelet morphology and chemical environment.

Polarization monitoring of structure and optical properties of the heterogeneous birefringent media: application in the study of liquid crystals and biological tissues

The original method of the polarization monitoring of heterogeneous anisotropic media is proposed. In contrast to the known approaches, the proposed method is applicable for the analysis of structure and optical properties of anisotropic media with an azimuthal variation in the local optical axis orientation with respect to the direction of probing. As a consequence, it can be used for studying the media, which contain anisotropic chiral structural fragments. The experimental examples are given, which show the possibility of applying this method in the study of liquid crystals and tissues.

Triplet-triplet energy transfer between luminescent probes bound to albumins

The interaction of polar and nonpolar luminescent probes with human blood serum albumins is studied by absorption and luminescence spectroscopy. It is found that the probes (polar eosin and nonpolar anthracene) can efficiently bind to proteins. The radii of the quenching spheres of energy-donor (eosin) triplet states in the presence of an acceptor (anthracene) in the process of the triplet-triplet energy transfer in proteins are determined for homogeneous and inhomogeneous distributions of acceptor molecules over the solution volume. It is shown that a decrease in the radius of the quenching sphere observed upon the addition of sodium dodecylsulfate surfactant is caused by structural changes in the protein.

Mapping of optical properties of anisotropic biological tissues

A simple polarization method of mapping of tissue structures that allows for a variation in optical properties of medium in the direction of probing is proposed. This method is based on the use of rotationally invariant polarization parameters of sample. The relation between these parameters and Mueller matrix elements is shown.

On the possibility of noninvasive polarimetric determination of glucose content in skin

Based on real structure and optical properties of the dermis, we analyzed the possibility of polarimetric measurement of glucose content in the skin. It was shown that, at physiological concentrations of glucose in the interstitial fluid, the optical activity of glucose is not manifested in the polarization and optical properties of the tissue, since the optical activity of glucose is almost completely suppressed by the linear birefringence of the dermis.

Physical modeling of tissue fluorescence: phantom development

When spectrofluorimetry is applied to the problem of diagnosis, correlation techniques relating spectral features to biotissue status should be used. But the coefficients of correlation equation should account for the relationship of spectral parameters with biochemical and morphological changes associated with the pathology. As a search for these dependencies in actual biotissue is difficult, we offer to employ tissue phantoms -- the physical models mimicking, under conditions of measurement, optical characteristics of the natural object. To model human cervix tissue, we used a three- layer planar structure, with upper 0.3 - 1.0 mm layer simulating epithelium, middle 0.03 - 0.1 mm layer representing basal membrane, and greater than 1.0 mm lower layer modeling subepithelial tissue. As a mechanical base of the structure we used 10% (per weight) gelatin gel. To simulate light scattering by biotissue, the nonabsorbing and nonluminiscent scatters were added to the upper and lower layers. NADH, FAD, and Protoporphyrin IX were added to upper layer. Collagen, as dried thin gelatin film, modeled basal membrane. To reproduce modulation of autofluorescence spectrum by reabsorption within the tissue, we added solution of human hemoglobin to the lower layer. Spectrofluorimetric measurement was performed using various excitation wavelengths (337 nm, 365 plus or minus 20 nm, and 405 plus or minus 20 nm).