In this review, we describe dielectric properties of biological tissues and liquids in the context of terahertz (THz) biophotonics. We discuss a model of the THz dielectric permittivity of water and ...water-containing media, which yields analysis of the relaxation and damped resonant molecules modes. We briefly describe modern techniques of THz spectroscopy and imaging employed in biophotonics with a strong emphasize on a THz time-domain spectroscopy. Furthermore, we consider the methods of sub-wavelength resolution THz imaging and the problem of THz wave delivery to hard to access tissues and internal organs. We consider the THz dielectric properties of biological solutions and liquids. Although strong absorption by water molecules prevents THz-waves from penetration of hydrated tissues and probing biological molecules in aqueous solutions, we discuss approaches for overcoming these drawbacks – novel techniques of freezing and temporal dehydration by application of hyperosmotic agents which have a potential for cancer detection. We review recent applications of THz technology in diagnosis of malignancies and aiding histology paying particular attention to the origin of contrast observed between healthy and pathological tissues. We consider recent applications of THz reflectometry in sensing the thinning dynamics of human pre-corneal tear film. Modern modalities of THz imaging, which relies on the concepts of multi-spectral and multi-temporal domains and employing the principles of color vision, phase analysis and tomography are discussed. Novel methods of THz spectra analysis based on machine learning, pattern recognition, chemical imaging and the revealing of the spatial distribution of various substances in a tissue, are analyzed. Advanced thermal model describing biological object irradiated by THz waves and phantoms mimicking the optical properties of tissues at THz frequencies are presented. Finally, application of the high-resolution THz spectroscopy in analytic chemistry, biology and medicine are described.
A new approach to terahertz (THz) diagnostics of the human psychoemotional state is proposed, based on the analysis of the THz contribution to the total signal while simultaneously recording the IR ...and THz emissions from the human body (hereinafter referred to as the IR - THz image). The developed image-processing algorithm allows extraction of the informative contribution determined by the THz radiation from the total signal perceived by the recording system. Simultaneous registration of the IR - THz images of the human face and the psychophysiological indicators are carried out in situations of physical stress (short-term intense physical exercise - squats functional test), electrical stimulation, and information stress (cognitive load - simple arithmetic mental calculations). The obtained data are compared with those of similar measurements at rest. It is shown that using cluster analysis of IR - THz images it is possible to divide the test subjects into classes according to the type of reaction of the circulatory system under stressful conditions.
The processes occurring during photobleaching of a new nonlinear optical co-crystal 2,6-diaminopyridine-4-nitrophenol-4 nitrophenolate are investigated. Using infrared spectroscopy of co-crystals, ...destruction of an aminopyridine-nitrophenol molecular complex by photoactinic radiation without breakage of aminopyridine and nitrophenol molecules integrity is shown. Differential scanning calorimetry and thermogravimetric analysis of the co-crystals showed a decrease in temperature of transitions of exposed co-crystals due to structural changes. Intensities of the second harmonic of the co-crystals under study were measured before and after photoactinic exposure; the results showed the decrease in the intensities of the second harmonic during prolonged illumination due to the structural changes in volume of the co-crystals confirming the breakage of the aminopyridine-nitrophenol molecular complex with preservance of the integrity of the molecules forming the molecular complex. Methods of terahertz absorption spectroscopy showed changes in absorption and a refractive index of a co-crystal in the range of 0.3-2.5 THz after the exposure to the photoactinic radiation, which can be interpreted as a result of destruction of the crystal structure. All the results obtained clearly showed that a co-crystal structure is disordered and the aminopyridine-nitrophenol bond is broken on the exposure to the photoactinic radiation, while the molecules of both components do not collapse, which leads mainly to disappearance of nonlinear optical properties. The investigated process is suitable for creating nonlinear optical lattices in the co-crystals using the photolithography method.
Using the effective medium model, the complex permittivity of the ocular surface system in the THz frequency range is considered. The penetration depth of THz radiation and the sensitivity of the ...reflected component to the dynamics of evaporation of the tear film are discussed. The results of in vivo experiments aimed to determine the reflection coefficient of the ocular surface using continuous THz reflectometers are well described using the developed model.
We report the results of experimental studies of the photoelectric properties of a
p–i–n
GaAs photodiode with InAs/GaAs(001) double asymmetric quantum dots (DAQDs) grown by self-assembling in the ...metal–organic vapor-phase epitaxy process. Three peaks were observed in the dependence of the photocurrent on the reverse bias measured at monochromatic photoexcitation of the DAQDs at the wavelength corresponding to the energy of interband optical transitions between the ground hole and electron states in the larger quantum dots (QDs). These peaks were related to the tunneling of the photoexcited electrons between the QDs, including a dissipative one (with emission and absorption of optical phonons). The experimental results agree qualitatively with the theoretical field dependence of the probability of 1D dissipative tunneling between QDs.
The analysis of the effect of ivermectin on phytopathogenic strains of
Fusarium graminearum
(F‑55644, F-55748) and
Fusarium oxysporum
f. sp.
lycopersici
(F-52897, F-55547) was carried out; as a ...result, its concentrations were established at which a fungistatic effect on the growth of colonies of the specified strains was observed (2 and 3 mg/mL). It was found that
F. oxysporum
strains were more susceptible in general to ivermectin than
F. graminearum
strains. Since it is known that ivermectin is able to interact with β-tubulin (causing a stabilization of microtubules), to explain the obtained results, a 3-dimensional model of the complex of this compound with
Fusarium
β-tubulin was developed and ivermectin-induced changes in the conformation of β-tubulin were determined, including, particularly, the stabilization and spiralization of the M‑loop of the β-tubulin molecule. This structural element of β-tubulin plays an important role in the lateral contacts between tubulin subunits of adjacent protofilaments within the microtubule. Since the M-loop stabilization reflects a very important feature of microtubule stabilizing agents' binding to the taxane site of β-tubulin, it can be supposed, that ivermectin possesses the same effect on
Fusarium
microtubules. The results obtained allow for considering ivermectin or its derivatives as potential compounds with fungicidal activity.
This paper briefly examines methods for the generation of pulsed terahertz radiation and principles of pulsed terahertz spectroscopy, an advanced informative method for studies of complex biological ...and nanostructured systems. Some of its practical applications are described. Using a number of steroid hormones as examples, we demonstrate that terahertz spectroscopy in combination with molecular dynamics methods and computer simulation allows one to gain information about the structure of molecules in crystals. A 'terahertz colour vision' method is proposed for analysis of pulsed terahertz signals reflected from biological tissues and it is shown that this method can be effectively used to analyse the properties of biological tissues and for early skin cancer diagnosis.
We propose a new approach to non-contact recording of respiratory function based on the analysis of a sequence of Infrared-terahertz images of the human face, allowing the processes that occur during ...breathing to be visualized. To obtain quantitative estimates of the respiratory function, two methods are proposed. The first one utilizes a probe which implements the function of spatial differentiation and provides high sensitivity, but requires an increased accuracy of positioning the probe at the nasal opening and scaling in accordance with the camera angle. The other one is a histogram method for obtaining quantitative estimates of the external respiratory function, which is scale invariant and does not require precise positioning, but has a lower sensitivity compared to the first one. The methods proposed have made it possible to remotely evaluate the respiratory rate, which correlates well with the data obtained by a contact method of respiratory function registration.
We demonstrate the possibility of using a multi-frequency terahertz source to identify substances basing on the analysis of relative amplitudes of the terahertz waves scattered by the object. The ...results of studying experimentally the scattering of quasi-monochromatic radiation generated by a two-frequency terahertz quantum-cascade laser by the surface of the samples containing inclusions of absorbing substances are presented. It is shown that the spectral features of absorption of these substances within the terahertz frequency range manifest themselves in variations of the amplitudes of the waves at frequencies of 3.0 and 3.7 THz, which are scattered by the samples under consideration.
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