Izvestiya of Saratov University. Physics

Background and Objectives: The dynamics of a separate small ensemble and coupled small ensembles of excitable FitzHugh–Nagumo oscillators is studied. Different topologies and types of coupling between elements, as well as external noise and harmonic impact are considered. Models and Methods: The main model is a ring of five locally coupled excitable FitzHugh–Nagumo neurons, into which additional connections and external disturbances are introduced. Also, two such systems are connected via a hub, represented by a single FitzHugh–Nagumo neuron. To assess the influence of various system parameters on the neuronal spike activity, maps of the average firing frequency are constructed in the plane of control parameters, and the critical values of the parameters necessary for the occurrence of spikes are found. Results: It has been shown that a repulsive local coupling can excite spike activity in a network of excitable oscillators without external impact, and the addition of remote coupling expands the range of parameters in which firings are observed. Besides, by introducing anomalous Lévy noise, it is possible to excite oscillations in the system at lower values of the coupling strength between neurons than by utilising normal Gaussian noise. Also, in a system of two ensembles of neurons connected through a common hub, the interlayer coupling leads not only to synchronisation of the firing frequencies of these ensembles, but also to a transition to the spike activity mode even when no firing was observed in individual ensembles. By changing the parameters of the external harmonic impact and the coupling coefficients of the two ensembles with a common hub, it is possible to influence the average firing frequency.

Background and Objectives: Features of photostimulated adsorption of enzymes on a semiconductor substrate, leading to different changes in the sensitivity to glucose and hydrogen peroxide, were studied using the enzymes glucose oxidase and horseradish peroxidase as an example. Materials and Methods: Enzyme molecules were deposited on n-Si and p-Si substrates by photostimulated layer-by-layer adsorption from solution. Glucose oxidase and horseradish peroxidase were used as enzymes. The resulting structures were mounted in an electrochemical cell to measure the capacitance-voltage characteristics of the electrolyte-insulator-semiconductor contact, which were then used to determine the sensitivity of the sensor structures to glucose and hydrogen peroxide. Results: The results were analyzed taking into account photoelectronic processes in n-Si and p-Si semiconductor substrates. An increase in the sensitivity to the analyte from the use of photostimulated adsorption has been found for the structures obtained on the basis of n-Si, regardless of the type of immobilized enzyme. But for glucose oxidase molecules, the effect of photostimulation reaches 200%, and for horseradish peroxidase molecules it does not exceed 30%. The effect of photostimulated adsorption is explained by the charge exchange of surface electronic states at the Si/SiO2 interface upon illumination and the formation of induced dipoles that combine the charge of the enzyme molecule and the opposite charge of the Si/SiO2 interface after the illumination is turned off. Conclusion: The conducted studies can be applied in the development of a capacitive biosensor operating on the field effect, since taking into account the change in the charge state of the immobilized enzyme and the surface of the semiconductor signal converter makes it possible in some cases to significantly increase the sensitivity of the biosensor.

Background and Objectives: The development and functioning of all living beings ends with the inevitable aging process, as a result of which the activity of all organs and the body as a whole is suppressed, which leads to imminent death. Protein glycation is considered to be one of the causes of aging. This process takes place throughout life, but it intensifies with age. Protein glycation is a reaction of covalent coupling of free amino groups of proteins and reducing carbohydrates, which proceeds without the participation of enzymes and leads to disruption of protein functions. This process is unregulated, as it occurs without the participation of biological catalysts. As a result of glycation of proteins in humans, inflammatory processes occur in the body and a number of diseases such as heart attack, stroke, atherosclerosis, cataract, glycemia, Alzheimer’s disease, diabetes mellitus, etc. develop. In the tasks of medical diagnostics, methods of monitoring the state of proteins in the human body are necessary. In this regard, the work is devoted to the study of the processes of interaction of human serum albumin globules (HSA) with globules of human glycated serum albumin (gHSA). Materials and Methods: In conducting a study of the spectral-kinetic characteristics of the eosin luminescent probe in solutions of glycated and non-glycated HSA, as well as in a mixture of glycated and non–glycated HSA, an exponential dependence of the second order was used to approximate the dependencies of DF (delayed fluorescence) and PHOS (phosphorescence), and an anisotropy equation was used to assume the formation of the gHSA-HSA complex. Results: It has been found that the intensity and kinetics of quenching of delayed fluorescence and phosphorescence of the eosin fluorescent probe associated with proteins are sensitive to the ratio of glycated and non-glycated proteins in solution. To explain the increase in the intensity and lifetime of eosin phosphorescence during the transition from a solution of HSA to a mixture of HSA and gHSA, it is assumed that the globules of HSA and gHSA form a complex of the composition of gHSA-HSA, as a result of diffusion encounters. The rotational mobility of this complex is much less than the separate globules of HSA and gHSA. The formation of the complex is confirmed by an increase in the anisotropy of delayed fluorescence and phosphorescence of eosin in a mixture of HSA and gHSA. Conclusion: The obtained results of the work can be used to diagnose the presence of a complex of glycated with non-glycated proteins in human blood plasma. 

Thin films of GaAs1 – x – yNxBiy were deposited on a GaAs (100) substrate by pulsed laser deposition using an argon-nitrogen gas mixture at a pressure ranging from 1 to 60 Pa. The film thickness is found to decrease from 527 to 127 nm as the pressure of the argon-nitrogen gas mixture increased from 20 Pa to 60 Pa due to reflection and scattering of the plasma torch flow on nitrogen and argon atoms. The increase in pressure results in a significant decrease in the size and density of droplets on the film surface. All samples exhibit a polycrystalline structure, and the film obtained at a pressure of 60 Pa exhibits the highest crystalline perfection. The VASP software package was used to calculate theoretically the diffractogram for a (2×2×2) GaAs0.889N0.037Bi0.074 supercell, and it has been observed that the width at half maximum intensity for the GaAsNBi (004) reflection decreases with increasing pressure of the argon-nitrogen gas mixture. The nitrogen concentration in the thin film is found to increase linearly with the increase in the pressure of the argon-nitrogen gas mixture, which was established using X-ray diffraction and photoluminescence methods. The composition of the film obtained at a pressure of 60 Pa is determined to be GaAs0.957N0.012Bi0.021.

Background and Objectives: The article presents pages of the biography of Vladimir N. Shevchik (1923–1980), Doctor of Physical and Mathematical Sciences, Professor, Head of the Department of Electronics, Faculty of Physics, Saratov State University. As a scientist, he made a great contribution to the development of domestic vacuum electronics, including vacuum microelectronics. He is one of the founders of this scientific and technical direction in Saratov (along with Petr V. Golubkov and Venedikt I. Kalinin), scientific director of the authoritative Saratov Electronic School in the 1960s–1970s. The merit of Vladimir N. Shevchik is the organization of a system for training scientific personnel through a permanent scientific seminar at the Department of Electronics, the number of meetings of which exceeded 1300, as well as holding regular winter schools in electronics for young scientists and engineers. For many years Vladimir N. Shevchik headed the Research Institute of Mechanics and Physics at Saratov University. In 1970–1977 he served as rector of Saratov University. Materials and Methods: The article was written on the basis of archival data, analysis of the scientific works by Vladimir N. Shevchik and the memories of his students. Conclusion: It is noted that the scientific heritage of Vladimir N. Shevchik retains its value in the 21st century both in the educational and scientific spheres.