Detection of the organic solvent vapors by the optical gas sensors based on polyaminoarenes

Stepan Gzhytskyi National University of Veterinary Medicine and Biotechnologies Lviv, ekarska Str., 50, Lviv, 79010, Ukraine. Tel.: +38-032-239-26-35 E-mail: tsizhb@ukr.net Aksimentyeva, O. I., Tsizh, B. R., Horbenko, Yu. Yu., & Stepura, A. L. (2021). Detection of the organic solvent vapors by the optical gas sensors based on polyaminoarenes. Scientific Messenger of Lviv National University of Veterinary Medicine and Biotechnologies. Series: Food Technologies, 23(95), 20–24. doi: 10.32718/nvlvet-f9504


Introduction
Due to environmental degradation situations in Ukraine and in the world an important problem is the development of effective methods for monitoring of toxic gases in the atmosphere and industrial premises. Special devices (sensors) allow detecting harmful factors below the limit of their perception by human senses organs (Dong et al., 2016;Aksimentieva et al., 2018;Aksimentyeva et al., 2019;Tsizh & Aksimentyeva, 2020).
The key elements of sensors, regardless of their design, are an indicator substance -a sensory medium that changes its properties (optical, electrical, etc.) in response to an external action. Most known gas sensors have high operating temperatures, which creates certain difficulties in their operation, so more and more attention is paid to sensors based on conjugated electrically conductive polymers, in particular, polyaminoarenes operating at room temperatures. The interesting optical phenomena, which are inherent in conjugated polyaminoarenes and are part of their specific properties, are the ability to optical transformations, namely -changes in the spectra of optical absorption under the influence of external factors, including gas adsorption (Bai et al., 2007;Hu et al., 2017;Aksimentieva et al., 2018). Of interest are the optical properties of conjugated polymer systems, which are detected both in the visible range of the spectrum and in the near-IR and UV regions.
Among the sensor materials based on conducting polymers the polyaniline is mostly studied (Dong et al., 2016;Tsizh & Aksimentyeva, 2020) while its derivatives are under-researched. Our attention is devoted to polytoluidine (PoTI) and polyanisidine (PoA) due its high sensitivity to influence of external chemical and physical factors, especially adsorption of gas molecules (Savale et al., 2007;Horbenko & Aksimentyeva, 2013;Khan et al., 2015;Tsizh et al., 2017;Aksimentieva et al., 2018). In result of absorption of inorganic polar gases such as ammonia, nitrogen oxide, hydrogen chloride and others the significant changes in conductivity, optical absorption and morphology of polyaminoarene films are observed (Bai et al., 2007;Aksimentieva et al., 2018). This may be used for creation the sensitive elements for both resistive and optical type of sensors. However, the effect of the vapors of organic solvents on the optical spectra of PoTI and PoA films for today is poorly studied. In the present work we investigated influence of the adsorption of organic solvents -chloroform, tetrahydrofuran (THF), dimethylformamide (DMF), toluene and benzene, etc. on the optical spectra of PoTI and PoA thin films on the transparent SnO 2 surface.

Material and methods
The thin films of PoTI and PoA were obtained by the method of "in situ" electrochemical polymerization of otoluidine and o-anisidine on the transparent surface of the conductive SnO 2 coated glass as described in (Tsizh et al., 2017). Electrosynthesis was realized under cyclic sweeping potential in the range from 0 to 1.0 V (Ag/AgCl) at sweep rate of 50 mV/s and the number of cycles N = 50. As a power source used the potentiostat PI-50 paired with a digital controller MTech CON-PI-50-2. Optical absorption spectra of the obtained films were recorded in quartz cells using a SF-46 spectrophotometer (spectral range 200-1100 nm) and a KFK-3 photoelectrocolorimeter. Operating wavelength range -from 400 to 900 nm. All measurements were carried out at room temperature. Thickness of the films measured with microinterferomer MII-4 was 360 ± 20 nm.

Results and discussion
The optical absorption spectra of polyaminoarenes typically are characterized by three bands. The first peak at a wavelength of 370-420 nm corresponds to π-π* transition in band gap, second peak of absorption at the 620 -650 nm can be attributed to the n-π* transition in amino-quinoid fragments of the polymer system and third band at λ > 800 nm due to the absorption of delocalized charge carriers in polaron -bipolaron zone (Aksimentieva et al., 2018;Hu et al., 2017).
The optical absorption spectra investigated PoA film was characterized by two band with maximum near 360-410 nm (π-π* transition) and broad band in the range of 620-950 nm which is a superposition of the second and third bands ( fig. 1, curve 1). Influence of organic vapors causes the changes in films coloration that corresponds to spectral changes in the visible and NIR range of spectrum. Under action of organic vapors, the strong changes of optical absorption are observed in the spectra of PoTI and PoA films. However, a character of these variations is depended on the nature of polyaminoarenes and organic solvents. As can be seen from fig. 1 and 2, the dynamics of changes in optical absorption under the action of organic solvent vapors consists in a decrease of absorption in the NIR range and an increase in absorption in the n-π* transition in amino-quinoid fragments. Adsorption of polar organic solvents on the PoA or PoTI surface decreases an electron delocalization in conjugated polymer chain and may change a microstructure and morphology of the polymer films in accord with a picture of inhomogeneous order in these materials (Aksimentieva et al., 2018;Hu et al., 2017) as one can see from fig. 3, a, b. A B Fig. 3. Microphotos of the PoTi (a) and PoA (b) films electrodeposited on the SnO 2 surface

A B
The relative change in optical absorption (ΔD) at a certain wavelength (λ, nm) referred to the initial value of A in the absence of gas, was chosen to evaluate the sensitivity of the films to various gases presented in the Table. The maximum of sensitivity to the organic vapors for PoTI films in all cases is observed at λ > 550 nm and extends to near-infra-red area indicating a formation of free charge carriers of polaron type (Bai et al., 2007;Khan et al., 2015). Nonpolar solvent vapors insignificantly affect the optical properties of polyaminoarene films (table 1).
Therefore, it is established that the maximum sensitivity of PoTi to vapors of organic solvents at 550 nm increases in a number of chloroforms, DMF, THF. The maximum optical response to the adsorption of unipolar aromatic solvents, such as toluene and nitrobenzene, is observed at a shorter wavelength, namely at 400 nm.
An important indicator of the success of the sensor element is its response speed. For the PoTI films the time of response on DMF and chloroform action is near 60 s ( fig. 3) at both wavelength 550 nm and 810 (850) nm. As can be seen from fig. 4, the stationary value of the PoA optical response on the DMF action is reached within 120-180 s only for λ = 620 nm. At the λ = 860 nm a decrease in optical density is observed during 10 min and more. In the case of THF vapors action on PoA film a stationary value of D achieves more quickly in the time 60-90 s. Fig. 4. Temporal dependence of the optical density of PoA film in the action of (а) DMF and (b) THF vapors; P = 7,2 kPa Different character in the changes of optical absorption in PoTI and PoA films found for organic solvents vapors may be useful for selective detection of polar and nonpolar solvents in technological processes.

Conclusions
Based on the spectral dependence of the optical absorption of polyortotoluidine and polyorthoanisidine films, the presence of the main bands in the region of 400-450, 550-570 and 650-900 nm due to electronic transitions in the conjugated polyaminoarene system and the presence of polaron and bipolaron type carriers has been established.
The spectral sensitivity of polymer films to the action of organic solvents (dimethylformamide, tetrahydrofuran, chloroform, etc.) has been established. The proposed approaches to the creation of sensors for detecting vapors of organic solvents can be the basis for the development of new selective and fast-acting devices for monitoring gaseous media in the chemical, medical and cosmetic industries.
This work was supported by the project of Ministry of Education and Science of Ukraine "Development of composite organo-inorganic heterostructures for reversible gas sensors" (state registration number 012990U101998).

Prospects for further research.
For the first time, the sensitivity of thin films of polytoluidine and polyanisidine to the action of vapors of organic solvents was established. This sensitivity provides prospects for the development of optical gas sensors to detect harmful fumes of organic substances used in various industries and medicine. Based on the obtained results, models of optical sensors for monitoring gaseous media in production facilities with high selectivity will be proposed. The next step is to study the reversibility of sensor films to the action of organic vapors and optimization of the sensor design.