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    • br Introduction Studying the temperature dynamics of

    2018-10-26


    Introduction Studying the temperature dynamics of the mechanical properties of toll like receptor is important for solving the problems of cryobiology, a field concerned with the behavior of cells and tissues at low temperatures [1,2]. These properties of the plasma membrane influence the mechanism behind cell destruction through dehydration as a result of freezing. The mechanical properties of the membrane need to be studied for selecting the optimal conditions for freezing and thawing biological objects, as well as for storing them. The cell is a difficult object for studying the temperature dynamics of the mechanical properties of the membrane, as measurement results are influenced by various processes including crystallization of ice. Therefore, before proceeding to investigate the mechanical properties of the cells, it is preferable to first carry out model studies in order to identify and separate the contributions that different phenomena and processes make into the mechanical properties of the cell membrane under freezing. For such a model we have chosen the polylysine biopolymer which is a small natural homopolymer of the essential L-lysine amino acid.
    The procedure The optical system for detecting the position of the cantilever 4 (Fig. 1a) in the cryogenic AFM is based on interferometric principles. It consists of a single-mode fiber 6, an infrared semiconductor laser 5 with the wavelength of 1330nm, and a photodetector. A Fabry–Pérot interferometer is located between the probe microscope 3 and 4 and the polished tip 7 of fiber 6. Thus, the optical fiber serves a dual purpose: on the one hand, it delivers laser light to the probe (approximately 96% of the radiation passes through the polished tip of the optical fiber and is partly reflected by the probe), and on the other hand forms a part of the interferometer (the rest of the radiation is reflected from the optical fiber/air interface). The laser beam reflected from the probe is partially transmitted back to the optical fiber and interferes with the beam reflected from the optical fiber/air interface. If the probe deflects from its initial position, the size of the interferometer cavity changes, which corresponds to the change in the level of the interferometric signal and, in turn, leads to a change in the signal received by the photodetector. The process of adjusting the optical system which is sensitive to the deflection of the cantilever consists in registering the dependence of the output signal from the interferometer on the size of its cavity, i.e., the distance between cantilever 4 and optical fiber 6. The size of the interferometer cavity can be changed by two methods: either using a piezoelectric element located directly under the probe (designed for the AFM operating in dynamic modes [3]), or during a scan, when the AFM probe tip is in contact with the sample\'s surface. The probe does not interact with the surface during optical system calibration, and the size of the interferometer cavity is changed by means of the piezoelectric element. A typical interferogram obtained during the calibration of the optical system is shown in Fig. 2a. The voltage U applied to the piezoelectric element is plotted on the abscissa; as a result, the piezoelectric element changes its size dX and moves the probe toward the optical fiber. The ordinate represents the level of the signal dV from the interferometer output. The distance between the peaks in the interferogram corresponds to half the wavelength (λ/2) of the semiconductor laser of the optical detection system. The signal level dV on the ordinate signal can be converted to the value of the probe tip displacement dZ by the following formula: where ΔV is the difference between the maximal and the minimal transmission of the interferometer (see Fig. 2а). The approach and retraction curves for measuring the mechanical properties of the sample are recorded as follows: sample 1 is mounted on piezoscanner 2 of the AFM (see Fig. 1a), the piezoscanner moves towards probe 3 and 4, as probe tip 3 contacts the surface of sample 1, elastic forces start acting on the probe; this leads to bending of cantilever 4 and the corresponding change in the interferometer cavity. The typical form of the approach curve obtained by this method is shown in Fig. 2b. The abscissa represents the piezoscanner movement dZ, and the ordinate represents the interferometer transmission (signal level) dV. Region I corresponds to the situation when the probe does not interact with the sample surface and the signal from the interferometer is the highest; Region II describes the change in the cantilever bending due to the Van der Waals forces acting on the probe; Region III represents the contact interaction when the probe tip is in contact with the sample surface and the cantilever bending balances the elastic forces acting on the probe from the sample. It is Region III that is used to calculate Young\'s modulus of the sample.