High-Performance Ethylene Glycol Sensor Based on Imine Covalent Organic Frameworks

Abstract

The colorless and odorless ethylene glycol is prone to unknowingly causing poisoning, making preventive monitoring of ethylene glycol necessary. In this paper, scandium (III) trifluoromethanesulfonate was used as a catalyst to successfully prepare covalent organic framework (COF) nanospheres linked by imines at room temperature. The COF nanospheres were characterized by XRD, SEM, TEM, FT-IR, UV-Vis and BET. The results show that COF nanospheres have rough surfaces and a large number of mesoporous structures, which greatly increase the active sites on the surface of the sensing material and enhance the gas sensing performance. The sensing results showed that the prepared imine-conjugated COF nanospheres exhibited a good response–recovery ability for 10 consecutive response–recovery cycles for ethylene glycol at room temperature and had a theoretical detection limit of 40 ppb. In addition, the responses of COF nanospheres to nearly 20 interfering gases, including HCl, HNO₃, phenol, formaldehyde and aniline, are relatively low compared to the response to ethylene glycol, indicating that the COF nanospheres have high selectivity towards ethylene glycol. The COF nanospheres show good sensitivity and selectivity for the detection of ethylene glycol, which should be attributed to the large specific surface area, hydrogen bonding interactions, and high defects. This work provides an effective method for the detection of ethylene glycol and expands the application field of COF materials.

1. Introduction

Ethylene glycol (C₂H₆O₂) is a widely used organic chemical, often used as an organic solvent in automotive antifreeze and in paints and coatings. C₂H₆O₂ is highly toxic and lethal, mainly because of its colorless and odorless nature, which can lead to unintentional inhalation and subsequent difficulty in determining the cause of poisoning [1]. If left untreated after inhalation, C₂H₆O₂ is metabolized to glycolic acid and oxalic acid, leading to metabolic acidosis, acute renal failure and death [2,3]. In addition, C₂H₆O₂ is highly flammable, and its vapors readily ignite or detonate when exposed to open flames [4], making the development of high-performance C₂H₆O₂ sensors critical to protecting individuals from its hazards.

Therefore, preventive detection of C₂H₆O₂ is necessary and important for the prevention of poisoning and combustion. Currently, commonly used methods for C₂H₆O₂ detection include gas chromatography [5], spectrophotometry [6] and flame ionization detection [7]. However, due to the complexity of these methods and the inability to detect C₂H₆O₂ in real time, they can only be used as a means of chemical analysis and detection for gas detection and monitoring [8,9]. As an important device for detecting and measuring the type and concentration of gases in the environment, gas sensors are characterized by simple operation and short detection time compared to other gas detection methods [10,11,12]. For example, a variety of gas sensors have been developed for the detection of C₂H₆O₂: Liu et al. prepared ZnO/ZnCo₂O₄ composites using a one-step hydrothermal method, which showed a high response to, and excellent selectivity for, C₂H₆O₂ at 160 °C [13]. Ding et al. prepared ZnO/rGO nanosheets using chemical precipitation and hydrothermal methods and obtained the best gas sensitivity through high-temperature annealing treatment at 220 °C. The best gas sensitivity was obtained at 220 °C [14]. However, these C₂H₆O₂ sensors need to be at high temperatures for optimal performance [15], which itself carries a certain degree of danger [4]. Therefore, there is an urgent need for an ambient-temperature C₂H₆O₂ sensor.

Covalent organic frameworks (COFs) are a new class of crystalline porous materials consisting of light elements bonded by strong covalent bonds [16,17]. COFs have many unique properties, such as π-π conjugated structure, good electrical conductivity and large specific surface area, and the various functional groups and chemical bonds present in the backbones of the COFs provide a rich variety of active sites. Recently, COFs have been widely used in many fields [18,19]. COFs are also used for gas-sensitive detection; for example, Choi et al. combined COFs with rGO to achieve selective detection of NO₂ [20]. Krishnaveni et al. hybridized Pd NPs with imine-based covalent organ skeletal nanosheets (ePd@TpPa-SO₃H COFs) to achieve high-performance detection of H₂ [21], which greatly expanded the application areas of COFs. However, there are no COFs available for C₂H₆O₂ detection.

Preventive detection of C₂H₆O₂ is important as it is a VOC that can easily cause poisoning. In this work, COF nanospheres with rough surfaces and a large number of pore structures were successfully prepared by using scandium (III) trifluoromethanesulfonate (Sc(OTf)₃) as a catalyst to promote the synthesis of imine bonding between monomers via a dehydration reaction under room-temperature conditions. The high sensitivity and good selectivity of the COF nanosphere sensors for C₂H₆O₂ at room temperature were attributed to their large specific surface area, pore structure and abundant functional groups, which added a large number of active sites for gas adsorption.

2. Materials and Methods

2.1. Materials and Reagents

The materials and reagents involved in this work are described in the Supplementary Materials.

2.2. Preparation of Materials

First, 70.3 mg of 1,3,5-tris(4-aminophenyl) benzene (TAPB) and 63.06 mg of 4,4′-biphenyldicarbaldehyde (BPDA) were added to an 8 mL mixture of 1,4-dioxane and 1,3,5-trimethylbenzene (4/1, v/v) in a centrifuge tube and sonicated immediately until the monomers were completely dissolved. This mix resulted in the formation of a yellow solution. Next, 17.7 mg of catalyst Sc(OTf)₃ was weighed and added to the resulting mixed solution, which was immediately covered and shaken vigorously to thoroughly mix the catalyst and mixed solution, producing a large amount of red precipitate during shaking. The mixture was incubated at room temperature for 72 h, during which the red precipitate gradually formed a spongy shape and deepened in color. The precipitate was collected by centrifugation and repeatedly washed and centrifuged with methanol (CH₃OH) 5 times to remove unreacted monomers and catalysts. Finally, the precipitate was dried overnight in a vacuum drying oven at 70 °C. Eventually, a red COF powder was obtained, hereafter referred to as COF_TB.

2.3. Sensor Preparation and Gas-Sensitive Testing

Sensor preparation: The previously prepared COF_TB was put into a mortar and ground into powder. A certain amount of acetonitrile was added to the mix and sonicated for 5 min. After waiting for the excess COF_TB to precipitate, the light-red solution of the upper layer was collected. The solution was dripped onto the silver interdigital electrodes and dried, forming a reddish COF_TB film on the electrode. The interdigital electrodes have a 13 mm × 7 mm × 0.5 mm alumina ceramic substrate with 5 pairs of silver fork fingers with a spacing of 200 μm between two fingers. After the sensor was prepared, it was stored at room temperature for about 24 h before use.

Gas sensitivity test: The gas sensitivity test was performed using a static test method [22], using a multi-function probe station (CGS-MT, Beijing, China), with a test voltage of 4 V. In order to minimize the interference of external environmental influences on the tests, the following methods were used to prepare the experimental gases: two clean experimental vessels of the same volume were filled with the same air to obtain the same initial environment, after which one vessel was sealed directly as the comparison air, and the other was added with the test liquid to be used as the target gas after the liquid was evaporated. The response is defined as response = (I_gas − I_air)/I_air × 100%, where I_gas and I_air are the current of the sensor in the comparison air and target gas, respectively. The response time is defined as the time to reach 90% of the stable response value, and the recovery time is defined as the time to reach within 10% of the initial response value.

2.4. Characterization of Materials

The morphology of the samples was characterized by scanning electron microscopy (SEM, Thermo Fisher, Quattro S, Waltham, MA, USA) and transmission electron microscopy (TEM, JEOL, JEM 2100 F, Tokyo, Japan). The structure and composition of the samples were characterized using X-ray diffraction (XRD, Ultima, UltimaIV, Tokyo, Japan) and Fourier-transform infrared spectroscopy (FT-IR, Bruker, VERTEX 70 RAMI, Ettlingen, Germany). The absorbance of the samples was determined using ultraviolet–visible absorption spectroscopy (UV-Vis, PerkinElmer, Lambda 650, Waltham, MA, USA). Specific surface area was measured by using a multi-station extended automatic surface area and porosity analyzer (Micromeritics, ASAP 2460, Norcross, GA, USA).

3. Results and Discussion

3.1. Structure and Composition of the COF_TB Sample

For the synthesis of COF_TB, the first step is to weigh 0.1 mmol of the two monomers and dissolve them in a mixture of 1,4-dioxane and trimethylbenzene (4/1, v/v). During this step, it is necessary to pay attention to the inside of the container at all times to prevent the monomers from forming a precipitate in solution, and to sonicate the two monomers until they are completely dissolved immediately upon their addition to the mixture. Errors in this step can cause inconsistency in the color of the synthesized material. Next, the catalyst was added to the mixture, and the container was immediately capped and shaken vigorously to fully mix the catalyst and the mixture solution, which was then incubated at room temperature for 72 h. Sc(OTf)₃ acts as a Lewis acid to catalyze the synthesis of imine bonds during the reaction and has a higher catalytic efficiency compared to the use of acetic acid as a catalyst for the synthesis of COFs, and an excess of Lewis acid inhibits the exchange of imine bonds [23,24]. At the end of the reaction, the products were separated by centrifugation, and the precipitate was washed with methanol five times in order to remove unreacted monomer and catalyst that could interfere with subsequent characterization and testing. To prepare the sensor, the synthesized material was dissolved in acetonitrile solution, and the upper layer of the solution was collected and drop coated to ensure that the COF film on the sensor was of uniform thickness. Finally, the sensor was left for a period of time before use (Figure 1).

The XRD spectrum of COF_TB is shown in Figure 2a, in which a few diffraction peaks can be observed, including a diffraction peak corresponding to the (200) crystalline plane. This peak indicates that the crystallite of the synthesized material is poor [25], suggesting that the rapid and large generation of the imine bond during the synthesis process leads to the disordered structure of the material. The functional groups of COF_TB were next analyzed using FT-IR (Figure 2b), and the stretched vibrational band located at 1617 cm⁻¹ [26] was attributed to C=N, confirming the successful synthesis of the imine bond. There were no characteristic peaks of N-H observed in the range of 3100–3400 cm⁻¹ [27] in the infrared spectra, suggesting that the obtained COF_TB lacked the presence of unreacted amino groups at the edge. Furthermore, a weak vibration of C=O was observed at 1697 cm⁻¹ [28], which was attributed to the presence of an unreacted aldehyde end group at the edge of COF_TB.

Next, to gain a more comprehensive understanding of the specific surface area and pore size distribution of the specimen, N₂ adsorption/desorption experiments were conducted on the material at a temperature of 77 K. The Brunauer–Emmett–Teller (BET) curve (Figure 2c) of the material is a typical type II curve, reflecting that the adsorption process of COF_TB is a physical adsorption process of non-porous or microporous adsorbent. This indicates that COF_TB has a large pore size, which may be due to the pleats on the surface of the COF_TB nanosphere and the pore holes formed by the nanosphere stacking [29]. The NLDFT/GCMC method was then used to analyze the pore size distribution curve of the sample (Figure 2d). It can be seen that the sample has a wide range of pore size distribution, with a large number of mesoporous and microporous structures. The calculated specific surface area of COF_TB was 10.04 m²g⁻¹ and the average pore size was 19.8 nm.

In order to understand the effect of material morphology on gas-sensitive properties, the morphological structure of the samples was analyzed using SEM and TEM. The SEM images of the material are shown in Figure 3a–c, demonstrating that COF_TB is composed of nanoparticle agglomerates with a particle size of about 500 nm, which form many channel structures. Further magnification shows that COF_TB has a rough surface which increases the reaction area of the material, is conducive to the adsorption of the gas molecules on the surface and improves the gas sensing performance [30,31,32]. Figure 3d–f show the TEM images of the material, in which it can be seen that the COF_TB material is a solid spherical structure and there are multiple spheres stacked together. Figure 3g shows that the COF_TB material has a highly disordered texture, which suggests that the material has an amorphous structure. This same conclusion was derived from the XRD results [33].

3.2. Gas Sensitivity of COF_TB

Next, the gas-sensitive properties of COF_TB were investigated. The response value, response time and recovery time of COF_TB to C₂H₆O₂ and 19 other gases (including DMSO, NMP, HCl, HNO₃, C₆H₆O, CH₂O, C₆H₇N, CH₄O, NH₃, C₃H₆O, C₂H₆O, C₇H₈, C₂H₃N, C₇H₆O, C₆H₄O₂, C₉H₁₂, O₃, C₄H₈O₂, H₂O₂) at room temperature were compared. Figure 4a shows a histogram of the response size of COF_TB to different gases, showing that the response value of C₂H₆O₂ is 13.9k%, or more than 7.8-times that of other gases, reflecting the better selectivity of the COF_TB gas sensor. Figure 4b shows a histogram of the response time and recovery time: the response time of C₂H₆O₂ is 71 s, which is slower due to the fact that the adsorbed oxygen on the surface is not enough to oxidize the adsorbed C₂H₆O₂, while the recovery time is 13.7 s. Figure S2 shows three response cycles of the COF_TB sensor for 20 different atmospheres, including C₂H₆O₂. In summary, it can be seen that the COF_TB sensor has good selectivity in detecting C₂H₆O₂. As shown in Figure S2, when the sensor is exposed to a reducing gas (such as CH₄O, NH₃ and C₃H₆O), the current rises rapidly, indicating a decrease in the resistance of the sensor. Reducing gases provide electrons to the COF_TB sample, and the decrease in resistance when the sensor is exposed to reducing gases suggests that COF_TB has n-type semiconducting properties [25].

In order to further evaluate the theoretical limit of detection (LoD) of the samples, the sensing curves were tested for different concentrations of C₂H₆O₂ (Figure 5a), and a histogram of averages and error bars is illustrated in Figure S3. It can be seen that the magnitude of the response of C₂H₆O₂ is positively correlated with the concentration of C₂H₆O₂. The linear relationship between the response of the samples and the concentration is shown in Figure 5b. The response value of COF_TB is linear with the magnitude of the concentration of C₂H₆O₂ at concentrations ranging from 1 to 5 ppm. This is based on LoD = 3S_D/m, where S_D is the standard deviation of the noise in the response curve with a magnitude of 0.00507, and m is the slope of the linearly fitted curve with a magnitude of 0.381. Based on these calculations, the LoD of the COF_TB is about 0.04 ppm, which indicates that the COF_TB has a high sensitivity to C₂H₆O₂. The insert in Figure 5a shows the response–recovery time of COF_TB for 1 ppm C₂H₆O₂: the response time is 21 s and recovery time is 1 s. Figure 5c shows 10 consecutive response–recovery cycles of COF_TB for 500 ppm C₂H₆O₂, which demonstrates the high reproducibility of the COF_TB-based sensor under 500 ppm C₂H₆O₂ conditions. Figure 5d shows a line graph of the fluctuation in 10 stable response–recovery cycles, and it can be seen that the fluctuation in 10 response cycles is small, again emphasizing the good experimental reproducibility. These results further demonstrate the potential of COF_TB in C₂H₆O₂ detection applications.

Ambient humidity is also an important factor affecting the performance of gas sensors. The response of COF_TB to different relative humidity (RH) was tested, as shown in Figure S4. With the increase in RH, the response of COF_TB to RH shows an increasing trend and reaches 1.73k% at 95% RH, which is about 12.6% of the response to 500 ppm C₂H₆O₂. Meanwhile, according to the test method in Figure 5e, we also tested the response curve of the sensor under common humidity (33% RH and 65% RH), as in Figure 5f. The response value decreased by 8.5% at 65% RH compared to 33% RH, but it did not have much effect on the ability of COF_TB to detect C₂H₆O₂ under common humidity.

Long-term stability is crucial for the lifetime of gas-sensitive materials, and the response curves of COF_TB to 500 ppm C₂H₆O₂ at 0, 25, and 50 days are shown in Figure S5. Figure S5 shows that there is a decreasing trend in the response value of the samples to C₂H₆O₂, which decreased by 7.4% after 25 days and 14.9% after 50 days, but this had little effect on the detection performance of the samples. Hydrolysis of the imine bonds in the samples over time may be the cause of this phenomenon, where the breakage of the imine bonds causes changes in the internal structure of the material, leading to the destruction of the original conductive structure [34] and, thereby, increasing resistance. Moreover, the hydrophilic groups in the materials make the COF materials hydrophilic, which may also accelerate the hydrolysis of imine bonds [35]. It may be possible to improve the long-term stability of COFs and their resistance to humidity by adding hydrophobic functional groups [36].

Table 1. Various glycol sensors reported in recent literature.

Materials	Concentration (ppm)	Response	Preparation Method	LoD (ppb)	Temp (°C)
ErFeO3 [32]	100	15.8 b	Electrostatic spinning	35	230
ZnO/ZnCo2O4 [13]	100	15.63 b	Hydrothermal method	1590	160
ZnO/rGO [14]	100	277 b	Hydrothermal method	1000	200
CuO/Co3O4 [4]	100	6.3 b	Hydrothermal method	-	130
SmFeO3 [37]	100	18.19 b	Electrostatic spinning and calcination	-	240
NTO [38]	100	160.72 b	Chemical vapor deposition	472	125
G-NiO-ZnO [39]	100	142 b	Hydrothermal method	-	140
La-doped ZnSnO3 [40]	100	1488.79 b	Hydrothermal method	200	140
(SEMCs)/SnO2 [41]	100	132 b	Carbonization and activation	4.8	160
This work	500	13,880 a	Normal temperature catalyst synthesis	40	RT

^a (R_gas − R_air)/R_gas × 100%; ^b R_gas/R_air.

summarizes the recent studies of various C₂H₆O₂ sensors. By comparing the different metrics, it can be seen that COF_TB has obvious advantages in the detection of C₂H₆O₂: COF_TB was synthesized under milder conditions, showed excellent selectivity and response to C₂H₆O₂ at room temperature, and had a low detection limit, demonstrating a high sensitivity. These comprehensive indices prove the potential application value of COFs as gas-sensitive materials.

3.3. Analysis of Sensing Mechanism

As shown in Figure S6, the linear current–voltage (I–V) indicates ohmic contact between the COF_TB and the electrode [42,43]. The main factors affecting the sensitivity and selectivity of COF_TB materials with n-type semiconductor properties should be attributed to the large specific surface area, hydrogen bonding interactions and high defects [25,33,44,45].

Firstly, COF_TB has a specific surface area of 10.04 m²g⁻¹ and a spherical rough surface, which provides a large number of active sites for adsorption. The average pore size of COF_TB is 19.8 nm, which provides a large number of channels for the diffusion and transport of gas molecules [46]. Numerous pores allow the COF_TB sample to better bind with gas molecules, which improves the sensing response.

Secondly, the presence of a large number of imine bonds and amino functional groups in COF_TB may make it easier for different gas molecules to be adsorbed onto the surface of COF_TB nanospheres through hydrogen bonding (Figure 6). The electron depletion layer (L) is positively related to the oxygen ion concentration (N_t) on the surface of sensing materials and inversely related to the charge carrier concentration (N_d) of the sensing material, as shown in Equation (1). The change of L causes a change in the resistance of the sensing material, and the greater the change of L, the better the gas sensing performance of the sensing material.

$$L\propto \sqrt{\frac{N_t^2}{N_d^2}}=\frac{N_t}{N_d}$$

(1)

When the material is exposed to ambient air, hydrogen bonds are formed between the large amount of N-H exposed on the surface of the COF_TB and the O₂ in the air, thus adsorbing the O₂ on the surface (Figure 6). Due to the strong electronegative nature of oxygen atoms, oxygen molecules capture electrons from the surface of the COF_TB material, as shown in Equation (2), which is converted into chemisorbed oxygen-negative ions at room temperature. Because the temperature is lower than 100 °C, oxygen molecules which capture an electron are then converted into O₂⁻, as shown in Equation (3) [33].

O₂ (gas) → O₂ (ads)

(2)

O₂ (ads) + e⁻ → O₂⁻ (ads) (<100 °C)

(3)

Due to the adsorption of O₂ molecules, the electrons of the COF_TB material were taken away, the carrier concentration decreased, and the L increased [34]. The increasing L hinders the electron transport in the material and causes the resistance of the material to rise [35]. When the COF_TB material is exposed to C₂H₆O₂ vapor, the N atoms of the COF_TB material form hydrogen bonds with the -OH of C₂H₆O₂, which makes it easier for C₂H₆O₂ molecules to adsorb onto the surface of the COF_TB material. The C₂H₆O₂ molecules and O₂⁻ on the surface of the COF_TB material undergo the reaction shown in Equation (4). Electrons are released and return to the conduction band of COF_TB material; the L and the resistance of the material decrease [17]. At the same time, C₂H₆O₂ has a stronger electron-donating ability than other gases [4], which also makes the COF_TB material have a higher response to C₂H₆O₂.

2 C₂H₆O₂ + 5 O₂⁻ → 4 CO₂ + 6 H₂O + 5e⁻

(4)

Finally, the disordered structure formed during the synthesis of COF_TB improves the conductivity of the material. Crystalline COFs have poor gas-sensitive properties due to the presence of many crystal boundaries, which prevent the migration of carriers. Conversely, disordered COFs form a three-dimensional conductive network due to the lack of such boundaries, which facilitates the transfer of carriers through the material and contributes to the improvement in the gas-sensitive properties of the material [25]. The reason for the poor crystallinity of COF_TB is related to the amount of catalyst and the mechanism of synthesis of imine COFs. The synthesis of imine COFs is a dynamic and reversible process. Firstly, a large number of amorphous structures are formed rapidly by dehydration reactions between monomers, followed by a slow reorganization into crystalline structures by reversible reactions of imine bonds [34], in which appropriate amounts of catalyst and water are required. However, a large amount of catalyst was added at one time during our experiments, which led to the rapid formation of many disordered amorphous structures. The excess catalyst and less water used inhibited the reversible reaction of imine bonding [23], which made it difficult for the material to be transformed into a crystalline material, so that the COF_TB material with disordered structure and small specific surface area was obtained [47] as indicated by the results of the XRD, TEM and BET of the material. In conclusion, the synergies of large specific surface area, hydrogen bonding interactions and high defects determine the high selectivity and sensitivity of COF_TB towards C₂H₆O₂.

4. Conclusions

In this work, COFs with highly defective amorphous structure were synthesized using catalysts at room temperature, and the COF_TB nanospheres with rough surfaces had high specific surface area. The rapid synthesis method using catalysts greatly increased defects, improved the electrical conductivity of COF materials and enhanced the sensitivity and selectivity of COF_TB to C₂H₆O₂ at room temperature, with a theoretical LoD of 40 ppb. Moreover, COF_TB maintained good sensitivity to C₂H₆O₂ vapor at room temperature under common humidity environments, showing high sensing stability. This study expands the application of COF materials and provides a C₂H₆O₂ sensor that is functional at room temperature. However, the sensing performances of COF_TB material still need to be improved. For example, COF_TB is susceptible to high humidity, and it may be possible to improve the humidity resistance of the COF_TB material by adding hydrophobic functional groups to further expand the range of humidity at which the COF_TB material can be applied.