Organic Semiconductor Material Featuring 4-Ethylbenzoyl-3-(4- ethynylbenzonitrile-phenyl)-thiourea (CYT-1) of Donor-π-acceptor for Chemiresistive Carbon Dioxide (CO2) Sensing

In this work, a new member of acetylide-thiourea namely 4-ethylbenzoyl3-(4-ethynylbenzonitrile-phenyl)-thiourea (CYT-1) was successfully designed, synthesised, and characterised by nuclear magnetic resonance (NMR) analysis. In turn, CYT-1 acting as sensor was fabricated onto interdigitated electrodes (IDEs) by drop-casting method. The performance of CYT-1 on reaction response and sensitivity characteristics towards carbon dioxide (CO2) as chosen gas was measured at different CO2 concentration (10–1000 ppm) for 30 min of time exposure. The investigation was carried out at room temperature via studying the changes in resistivity measurement before and upon CO2 exposure. The unique molecular characteristic of CYT-1 is due to the presence of two potential active sites of –NH-C=O and acetylide -C≡Cfor H-bonding and π-π interaction to take place between molecule and CO2 analyte.


INTRODUCTION
Carbon dioxide (CO 2 ) is one of the dominant greenhouse gas emissions which takes up around 77% of total gas in air. 1 The most comprehensive research conducted stated that a safe level of CO 2 concentration in a closed environment should not exceed 350 ppm. 2 A concern on global warming has motivated researchers to conduct intense research in the aspects of sense, capture and CO 2 storage. 3,4 Conventionally, active sensing materials that are commercially used nowadays are based on inorganic and conductive polymeric materials, with ideal sensitivity at high operating temperatures. 5−9 However, the disadvantages of these readily available materials are that they exhibit low ability of reversibility, less stability performances, and highly sensitive in ambient atmosphere. 10 Hence, in the present work, a new derivative of acetylide-thiourea featuring 4-ethylbenzoyl-3-(4-ethynylbenzonitrile-phenyl)-thiourea (CYT-1) as illustrated in Figure 1 becomes an ideal molecular candidate to act as semiconductor molecular system in the implementation of fundamental importance of the interaction with CO 2 at highly stable response. It also works at ambient atmosphere with high reversibility performance. The derivative of CYT-1 is having two reactive sites of carbonyl amide (-NH-C=O) and acetylide (C≡C) regions which make it distinctive from other typical thiourea derivatives. By observing its molecular framework, CYT-1 has gained much importance due to the presence of electron withdrawing group, cyano (CN) which has the ability to form highly uniform and compactly adhered film on interdigitated electrodes (IDEs). In this contribution, we discuss the resistive-type semiconductor properties based study on sensing ability of CYT-1 towards CO 2 gas. CYT-1 is modelled for CO 2 gas sensing and our findings reveal that CYT-1 gave high sensitivity towards high CO 2 concentration (1000 ppm) with 25.8%.

Synthesis of 4-Ethylbenzoyl-3-(4-ethynylbenzonitrile-phenyl) Thiourea (CYT-1)
A 4-ethylbenzoyl chloride solution (2.29 mmol) was slowly added into ammonium thiocyanate (2.29 mmol) solution in 15 ml acetone. The reaction mixture was stirred for 10 min to give pale yellow solution in presence of white precipitate of ammonium chloride. Then, a solution of equimolar 4[(4-aminophenyl) ethynylbenzonitrile] in 10 ml acetone was added dropwise into the reaction mixture and continuously put at reflux for another 8 h. The colour of the solution changed to bright yellow. The reaction was adjudged completion via thin layer chromatography (TLC) (hexane:ethylacetate:4:1). Yellow precipitate was formed once the solution mixture was poured into several small ice blocks. The obtained precipitate was filtered, washed with little cold water, and recrystallised from acetonitrile to yield yellowish crystalline solids of CYT-1 (0.6 g, 67% yield).

CO 2 Gas Sensing System
The gas sensing evaluation was performed in a tightly enclosed stainless steel chamber to achieve controllable inert, temperature, and humidity conditions during experimental tasks. The fabricated gas sensing electrode with 100 μm thickness of layered CYT-1 on IDEs was placed (hanging) inside the chamber near the gas inlet valve in a highly pure, nitrogenous environment with a controlled humidity between 65−70% RH for 1−2 h to equilibrate the chamber environment. The gas flow rate was controlled by a gas flow meter. The changes in resistivity of the electrode before and after exposure with CO 2 gas were recorded using HIOKI IM3570-LCR-Hi tester analyser at constant voltage of 1 V. Nitrogen gas was purged over sensing module after every exposure cycle. The changes in resistivity measurement of the sensing material, CYT-1 successive to exposure with gas were evaluated by the formula as stated in Equation 1: where Rg is the responsive resistivity of the sensing material upon exposure with CO 2 gas and Ra is the responsive resistivity of the sensing material without CO 2 in the high purity of nitrogen gas flow. Response time (τ res ) and recovery time (τ rec ) were evaluated as time required for 90% of maximum response of CYT-1 with CO 2 gas, and time needed for recovering 90% of the original resistivity in the inert environment respectively.

Spectroscopic and Characterisation of CYT-1
For structural elucidation, both 1 H and 13 C NMR spectra of CYT-1 are consistent with the proposed molecular structure. Two singlet proton resonances were deshielded at two distinctive environments δ H 9.09 ppm and δ H 12.86 ppm respectively, in which they were assigned for NH-C=O and NH-C=S moieties due to the anisotropic effect and intramolecular hydrogen bonding in the transcis conformation.

Evaluation of CO 2 Sensing Performance
The dynamic response of the gas sensor and its sensitivity for different concentrations of CO 2 gas at room temperature have been carried out with an applied bias of 1 V as shown in Figure 2. The designing principle for this sensor was to exploit the chemical interaction of CO 2 with the amide moiety (NH-C=O; NH-C=S) as well as acetylide -C≡C-functional groups that present in CYT-1 which in turn changed the signal resistivity before and upon exposure with CO 2 . CYT-1 based CO 2 gas sensor showed weak to moderate response at room temperature, which was observed to increase upon the increase in CO 2 concentrations from 10 ppm to 1000 ppm. 11 CYT-1 exhibited high response of 8.24 × 10 6 Ω at the CO 2 concentration of 1000 ppm from 6.55 × 10 6 Ω (resistivity in air) with 25.8% of Figure 2: Response and recovery curve of CYT-1 towards various CO 2 concentrations (10-1000 ppm) between 65%-70% RH at room temperature (1V DC, 2 kHz). sensitivity in N 2 environment. The response and recovery times for 10-1000 ppm concentrations of the CO 2 gas to be approximately 1 min and 3 min respectively. Besides, Figure 3 shows the linearity graph of CYT-1 towards 10-1000 ppm CO 2 gas with the efficiency of R 2 = 0.9873. Referring to the linear graph in Figure 3, limit of detection (LOD) of CYT-1 towards CO 2 gas was examined to investigate the efficiency and sensitivity of the sensor for CO 2 gas detection which was calculated using Equation 2: where s is the standard deviation and σ is the slope from the linearity graph. The LOD for CYT-1 sensor for CO 2 detection is the lowest with 249 ppm. According to World Health Organisation (WHO), the maximum permissible CO 2 concentration in closed working environment for 8 h of exposure is about 350 ppm. 2 Indeed, in this contribution, LOD value of CYT-1 exhibited to be lower than the permissible CO 2 concentration which was good indication for CYT-1 to be applied further as chemiresistive CO 2 sensor.

CONCLUSION
A new member of organic semiconductor featuring acetylide-thiourea, CYT-1 has been successfully designed, synthesised and characterised as chemiresistivetype CO 2 sensor. Its properties and behaviours were investigated via electrical properties characterisation which revealed good sensitivity response of 25.8% with 1 min response time and 3 min recovery time operated at room temperature. CYT-1 revealed possible interaction towards CO 2 due to the H-bonding interaction occurred at NH-C=O and π-π interaction occurred at C≡C site.