CC BY
3DOI 10.24412/CL-37135-2023-1-38-41
ASSESSING THE APPLICABILITY OF AEROGELS FOR THE ANALYSIS OF SKIN VAPOR SAMPLES IN TERAHERTZ RADIATION
VIACHESLAV ZASEDATEL1,_YANA TULENEVA2, YURY KISTENEV1
laboratory of Laser Molecular Imaging and Machine Learning, National Research Tomsk State
University, Russia 2Physical Faculty, National Research Tomsk State University, Russia
ABSTRACT
Skin vapor analysis can be used to assess human health conditions using various biological markers. Detection of markers is possible using highly sensitive terahertz radiation and various sorbents, such as aerogel. The study shows that the aerogel is a transparent absorbent for terahertz radiation, and is also capable of accumulating samples within 5-10 minutes after contact with human skin. In this case, the samples are suitable for research up to 78 hours.
INTRODUCTION
Currently, areas of research related to determining the composition of skin vapors using highly sensitive gas analysis are actively developing. The human body in the process of life releases many different substances, including volatile ones. Analysis of biological gas samples, such as exhaled air and skin vapors, can serve as methods for noninvasive analysis of various pathologies and conditions [1]. To determine such substances, non-ionizing, long-wave terahertz radiation is used, which allows the identification of many molecules. In medicine, terahertz spectroscopy is of particular importance because it is an excellent way to characterize a large number of organic molecules. In addition, photons in this radiation range have very low energy, which, compared to existing methods in medicine, gives a tangible advantage to terahertz imaging, since it does not pose a danger of ionization for biological systems. Due to its properties, it allows for rapid non-invasive studies, and the absorption spectra of many organic molecules in the terahertz region have now been obtained. Terahertz spectroscopy makes it possible to analyze interactions both intramolecular and interactions between molecules. Thus, this area contains information about the rotational and low frequency vibrational modes of biological molecules through absorption lines, as well as about the deformations of hydrogen bonds. The vibrational and electronic properties of molecules can be studied over a wide frequency band provided by coherent terahertz spectroscopy in the time domain [2,3].
To analyze samples of skin vapors, methods are needed that allow not only to collect and store samples for a sufficient time, but also to process them with the necessary methods. Various absorbents, in particular aerogels, can be used for this purpose. Aerogels are a porous material, and in the classification of materials, they are classified as mesoporous. Their structure consists of a set of globules, which have a size of the order of several nanometers, connected to each other by a network of air-filled mesopores [4]. It turns out that very thin walls of a few nanometers create a three-dimensional labyrinth of cavities and layers. The cavities have a diameter of about 2-50 nanometers and most often occupy 95% or more of the volume, which indicates that this material consists of 95% or more air. Density is estimated from 1 to 150 kg/m3 [5]. The most common are quartz aerogels. The minimum density of their evacuated version is 1000 times less than the density of water and even 1.2 times less than the density of air. Such aerogels transmit light in soft ultraviolet, visible and infrared radiation, but the latter contains hydroxyl bands at 3500 cm-1 and 1600 cm-1, typical for quartz, which is obtained by dehydrating silica gels.
Since aerogels can be a promising absorbent for analyzing pathological conditions using skin vapors, this study assessed the capabilities of this material for sampling and analyzing samples in terahertz radiation.
MATERIAL AND METHODS
The aerogel used in the study was produced at the Federal Research Center "Institute of Catalysis SB RAS" in Novosibirsk Rissia. It has a density of 0.2 g/cm3 and consist of 80% air and 20% silicon dioxide. The example of aerogel shown on the Figure 1.
Figure 1: the example of silica aerogel
To analyze the collected samples, a T-Spec spectrometer (EXPLA) was used, which allows obtaining data containing information about the structure and spectroscopic characteristics of the samples. This spectrometer allows you to scan various images, both in solid and liquid states, but with a size of no more than 20*20 mm. In this case, the spatial resolution is approximately 1 mm. The appearance and optical design of the T-Spec spectrometer is shown in Figure 2. To generate and record THz radiation, a photoconductive antenna is used, illuminated by very short laser pulses. Pulses are generated by a pump laser with an output power of no more than 100 mW at a wavelength of 1050 ± 40 nm, with a duration of 10 - 150 fs and a repetition rate of about 30 - 100 MHz. The pump beam is divided into two beams, falling on a diaphragm splitter, designated in Figure 4 as BS1, the coefficient for this division is related by the ratio 55:45. Mirrors M1 and M2 direct the beam through the fast delay line to the emitter, and lens L1 focuses the pump beam into a gap on the photoconductive antenna. The slow delay line consists of a corner reflector, which is hollow inside, or a PR4 prism on a stepper motor, as well as a fixed PR3 prism or mirrors. The second beam of the pump beam is directed to the slow delay line. After this, the beam is directed to the detector antenna by mirror M3, and lens L2 focuses the pump beam into the gap on the photoconductive antenna. A slow delay line is only required in some cases where high spectral resolution is required or very thick samples are available.The wavefront of the electric field of THz radiation forms scanning by a fast delay line with a frequency of 10 Hz. The spectral content of THz radiation is given by the Fourier transform taken from the wavefront, and the absorption spectrum of the substance under study is given by a comparison of the spectra with and without a sample along the path of propagation of THz radiation. To obtain a better signal-to-noise ratio, averaging can be performed over 1024 scanned curves.
(a) (b)
Figure 2: (a) spectrometer "T-Spec" (EXPLA) - (b) OPTICAL design of the T-Spec spectrometer
EXPERIMENTAL DESIGN
The experimental technique consisted of dividing an airgel sample into 5mm thick plates, which were sequentially fixed on the forearm using a gauze bandage for a certain time, and then, using a metal holder, they were installed in a spectrometer to study them for transmission (Figure 3).
Figure 3: (a) placing an airgel sample on the forearm - (b) placing the airgel sample in the spectrometer holder
A reference signal of the absorption spectrum was obtained from each plate (at 4 points due to the heterogeneity of the sample), then the sample was placed on the forearm for 5, 10, 15, 20, 25, 30, 35 and 40 minutes. After this, the absorption spectra of each sample were recorded again.
Additionally, spectra of one of the samples (fixed for 5 minutes) were obtained 78 hours after the first measurement. RESULTS
The data obtained for each sample at 4 points was averaged. Figure 4 (a) shows the absorption spectra of the samples for each exposure time and Figure 4 (b) shows absorption spectra of evaporation from the surface of the skin in a sample kept on the forearm for 5 minutes on the day the sample was taken and after 78 hours in comparison with a clean sample.
(a) (b)
Figure 4: (a) absorption spectra of evaporation from the skin surface obtained using different airgel samples for each exposure time - (b) absorption spectra of a clean sample and a sample treated twice - immediately after the experiment
and 78 hours after the experiment
The spectra obtained from the experiment are complex multicomponent spectra of a mixture of various substances; therefore, the principal component analyze (PCA) was used to find the differences in the spectra. The processing results are presented in Figure 5.
(c) (d)
Figure 5: representation of aerogel absorption spectra data in various coordinates of the principal components for different exposure times of images on the forearm compared to the clean sample: (a) exposure times of 5 and 40 minutes - (b) xposure times of 10 and 40 minutes - (c) exposure times of 25 and 40 minutes - (d) clean sample and the sample,
examined after 78 hours
CONCLUSION
When analyzed using the principal component method, it is clear that the studied spectra have sufficient separation in the data, which can serve as confirmation of the possibility of the aerogel for use as an absorbent in the selection of skin evaporations. In this case, a sufficient exposure time on the skin surface is 5-10 minutes, which makes it possible to limit subsequent experiments to this time. Also, analysis of the sample, done 78 hours after the first study, shows that the areas corresponding to the pure spectrum are separated from the areas corresponding to the spectra of the samples maintained on the forearm. At the same time, it can be seen that the areas corresponding to the spectra on the day the sample was taken and after 78 hours are also distant from each other, which may indicate temporary degradation of the sample. However, the separation of areas compared to the pure sample allows the sample to be used for subsequent analysis.
CONTRIBUTION
The research was carried out with the support of a grant under the Decree of the Government of the Russian Federation No. 220 of 09 April 2010 (Agreement No. 075-15-2021-615 of 04 June 2021).
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