Superhydrophobic Polymerized n-Octadecylsilane Surface for BTEX Sensing and Stable Toluene/Water Selective Detection Based on QCM Sensor.

The present study reports a facile and low-cost route to produce a superhydrophobic polymerized n-octadecylsilane surface with micronano hierarchical structure on the surface of quartz crystal microbalance (QCM). The surface is used as a novel functional sensing material to detect benzene, toluene, ethylbenzene, and xylene (BTEX) vapor on the basis of QCM platform. The composites were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and contact angle measurements. The type of solvent used to dissolve N-octadecyltrichlorosilane has a big impact on the morphology, wettability, and sensing performance of the polymer material. Further systematic studies suggest that surface wettability (contact angle) and molecular polarity of the detected analytes are effective factors in selective detection toward BTEX using resonator-type gas sensors. Gas sensing results toward toluene in different relative humidities show that the new-style sensor has stable toluene/water selective detection performance and that the disturbance of water is negligible. Besides, the limit of detection toward toluene of the sensor is lower than the odor threshold value.

Introduction

Industrial and agricultural developments cause not only production of new products but also more usage and emission of harmful chemicals as well as noxious gas.[1−6] As it is well known, volatile organic compounds (VOCs) have a bad health effect on mankind.[7,8] As typical VOCs, which would harm human health, benzene, toluene, ethylbenzene, and xylene (BTEX) are always distributed in the human living environment at low concentrations.[9−12] Thus, several gas sensors, such as semiconductor sensors and quartz crystal microbalance (QCM) transducers, have been developed to detect BTEX and avoid or reduce their harmful effects on human health.[13−17] QCM is used as a transducer for detecting different gases via coating various materials.[18] Although these BTEX sensors possess good sensing ability, most of them have not focused on eliminating the interference of water, which can badly weaken the selectivity of gas sensors. The BTEX detection performance under different relative humidities (RHs) is also unstable.

Materials with a water contact angle of above 150° are usually called superhydrophobic materials.[19] It is well known that superhydrophobic materials can repel water very well.[20] In recent years, the study of superhydrophobic materials has been emerging and many materials have been found to possess superhydrophobic properties, such as poly(vinylidene difluoride) membranes, cotton fabrics, nickel film, poly(dimethyl siloxane), etc.[21−25] To date, these superhydrophobic materials have found many applications like antifouling function, buoyancy improvement, pipeline modification, fabric coating, and microfluidic flow control.[26−30] Nonetheless, the study of superhydrophobic materials as QCM-based sensing materials has not been conducted so far.

n-Octadecylsiloxanes (n-ODS) is a common organic substance, which has been used as a visible photoluminescence and hydrophobic material.[31,32] Meanwhile, polymerized n-octadecylsiloxane (PODS) from hydrolysis and polymerization of n-ODS has also been greatly explored.[33] Currently, PODS is often employed as modified film to make the surface of some objects superhydrophobic.[34] Besides, its composites with other materials, such as SiO2, possess different superhydrophobic properties compared to PODS.[35] It is notable that sponges decorated with PODS show excellent oil/water separation performances between toluene and water, which can only adsorb toluene after being soaked in toluene/water mixed liquor because of their superhydrophobic and superoleophilic properties.[36] It is well known that different hydrophobic chemicals can attract each other and repel water;[20] thus, PODS has potential to detect toluene vapor as a modified sensing material for mass-type sensors on the basis of its theoretical toluene/water separation performance. However, there is still no relevant research about PODS as sensing material so far.

In this work, PODS film has been prepared on the surface of QCM substrate for BTEX detection for the first time. For systematic study, we concentrated our attention to the “sensing factor” between PODS sensing surface and BTEX, so a series of sensing factors have been explored comprehensively, including solvents used to dissolve N-octadecyltrichlorosilane (OTS), contact angle between PODS and the test gases, molecular polarity, etc. The distinguishing individual sensing behavior of target BTEX is attributed to their characteristic difference in molecule structure. It is discovered that superhydrophobic PODS surface would support its ability of toluene/water selective detection. The sensor we designed can detect lower concentrations of toluene than the odor threshold value (34 ppm).

Results and Discussion

Surface Morphology and Wettability

SEM images of PODS treated with air (a), water (b), acetone (c), and ethanol (d) are shown in Figure . The four FTIR spectra all fit well, indicating that they are the same chemicals (Figure S2 and Note S3). The difference in morphology is significant, indicating that the solvent has an effect on the morphology of PODS. When OTS was treated with air (Figure a), amorphous surface with islands can be observed. For water (Figure b), only block structure was found. For acetone, crystalline nanosheets can be observed because of the self-assembly process of hydrolyzed OTS and subsequent polycondensation process (Figure c). For ethanol (Figure d), the morphology of PODS is intestine-like and its crystallinity is not obvious. Correspondingly, the wettability of PODS surface is also different, caused by the solvents. As shown in the results of contact angle test, the contact angles of 1PODS, 2PODS, 3PODS, and 4PODS are 99.7, 136.1, 151.2, and 121.9°, respectively. The contact angle sequence of the four coating materials is as follows: 1PODS < 4PODS < 2PODS < 3PODS. It is obvious that all surfaces are hydrophobic. Notably, the contact angle of 3PODS is larger than 150°, indicating its superhydrophobicity. Therefore, the solvent of OTS influences both morphology and hydrophobicity of its polymer. As shown in Scheme , the polymerization process of PODS was supported by the previous report.[38] Considering the polymerization process and crystallization degree, acetone makes the polymerization process more complete and thus carbon long chains can be well arranged and exposed to the outside. It is well known that carbon long chain is hydrophobic, so the PODS has better hydrophobic property when the solvent is acetone.

Figure 1

SEM images and contact angle test results of (a) 1PODS, (b) 2PODS, (c) 3PODS, and (d) 4PODS.

Scheme 1

Polymerization Process of PODS

Effect of Wettability on Sensing Properties

The contact angles of the four materials (1PODS, 2PODS, 3PODS, and 4PODS) vary widely. Here, all samples were tested in the same testing environment (60% RH) to systematically compare the sensing properties toward BTEX of PODS with different wettabilities. These sensors were allowed to detect benzene, toluene, ethylbenzene, and p-xylene (BTEX) with the same concentration of 400 ppm. Besides, other benzenes also had been tested for more obvious contrast, including nitrobenzene and aniline. The test results are listed in Table S2.

The results from Table S2 revealed that whole PODS coating material shows sensing ability toward six common benzenes. The response of toluene, ethylbenzene, and p-xylene are larger than that of others. With the increase of contact angle, the sensor response toward tested vapor increased. Besides, the response sequence of four coating materials toward different benzenes is almost as follows: 1PODS < 4PODS < 2PODS < 3PODS. This sequence is consistent with their contact angle sequence. The result indicates a correlation between the wettability and BTEX sensing performance of PODS. The response of 3PODS toward six benzenes is larger than 1PODS, 2PODS, and 4PODS, indicating its best BTEX affinity. Thus, 3PODS is selected for the further sensor performance evaluation because of its best sensing ability.

Effect of Molecular Polarity on Sensing Properties

Molecular polarity relation of two substances is an important factor in evaluating their intermiscibility.[39] In the case of superhydrophobic surface, weak polar substances may be adsorbed easily. Xylene has three common isomers, including p-xylene, m-xylene, and o-xylene.[40] Due to different position of the methyl, polarities of the three xylene isomers are also different. It is well known that the polarity sequence of xylene isomers is as follows: o-xylene > m-xylene > p-xylene. Here, we use 3PODS-modified QCM to detect three xylene isomers with various concentrations.

For all xylene isomers, it was observed from Figure that all of the responses increased when the concentration of xylene increased as expected. Notably, the response toward p-xylene is the highest compared to the other two isomers in any tested concentration. The response sequence of the three xylene isomers is as follows: o-xylene < m-xylene < p-xylene. This sequence is the opposite of the sequence of molecular polarity. The results imply that chemicals with weak polarity can be easily adsorbed on a superhydrophobic surface instead of chemicals with high polarity.

Figure 2

Response of 3PODS toward three kinds of xylene isomers with different concentrations.

Sensing Performance of Toluene/Water Selectively Detecting

In the actual detecting process, the interference caused by water molecules in air is inevitable. Thus, toluene under 95% RH was designed to evaluate the humidity resistance property of the sensor. Figure shows the response of PODS-modified QCM sensor toward 400 ppm toluene under 95% RH. For comparison, toluene with 400 ppm concentration under default environment (65% RH) was also studied. For all kinds of PODS, the improvement of response toward 400 ppm toluene under 95% RH rarely increased compared to 400 ppm toluene under default environment. Especially, the response of 3PODS toward 400 ppm toluene keeps well in different RH environments. Besides, the response of 3PODS is larger than others, and these two merits of 3PODS should be attributed to better hydrophobicity than 1PODS, 2PODS, and 4PODS.

Figure 3

Response of four kinds of PODS toward toluene under default and high humidity.

Figure shows the dynamic response–recovery curves of the 3PODS-modified QCM sensor to toluene with the concentration ranging from 100 to 400 ppm at ambient temperature under default 60% RH. As toluenes with continuous increase of concentrations were injected into the chamber, the response increased. Besides, compared to the response of 400 ppm toluene, the response toward 400 ppm toluene under different RHs of 20, 40, 80, and 90% is almost unchanged. This experimental result further indicates its superior toluene/water selective detection performance. The baseline was well reestablished after the toluene was washed off by N2 stream, indicating its ideal reversibility. The response time and recovery time of 3PODS-based QCM sensor to 20 ppm toluene are shown in the inset of Figure . It can be observed that the response time and recovery time of 3PODS-modified QCM sensor were 10 and 9 s, respectively. It is obvious that this sensor is sensitive to 20 ppm toluene, which is lower than the odor threshold value (34 ppm), indicating its practical application potential. To further explore the waterproof toluene detection of 3PODS, we evacuated the sensing test chamber to make it completely dry. Then, pure toluene and mixtures of toluene and water with different concentrations were introduced in the vacuum test chamber. As shown in Figure , the response of 3PODS-based sensor toward 20 ppm toluene mixed with water is very close to that of the pure 20 ppm toluene. For 30 and 40 ppm concentrations of toluene, the results are also consistent, indicating the waterproof property of 3PODS.

Figure 4

Response curves of 3PODS toward toluene under different RH environments. The inset shows the response curve of 3PODS toward 20 ppm toluene.

Figure 5

Response of 3PODS toward mixtures of toluene and water with different concentrations.

The good reversible performance of 3PODS-based sensor should be attributed to its adsorption mode, which is closely relative to the enthalpy change of the sensing material and target molecule. Herein, QCM platform was used to perform temperature-varying experiments for extracting isotherms and enthalpy change. The test method refers to a previous report.[37] Two isotherms of 3PODS toward various toluene concentrations (100, 200, and 400 ppm) at different temperatures (298 and 313 K) should be obtained for calculating enthalpy change value (ΔH) using the Clausius–Clapeyron equation. As shown in Figure a,b, two real-time toluene-sensing curves are recorded, which can obtain the corresponding linear plots. As shown in Figure c, on the basis of the Clausius–Clapeyron equation, the value of ΔH is calculated to be −46.076 kJ mol–1, and the value is moderate between physical adsorption and chemical adsorption. The ΔH value indicates a hydrophilic–hydrophobic interaction between toluene molecule and 3PODS with exothermic property. Considering the adsorption/desorption equilibrium from sensing test and the moderate ΔH value comprehensively, it can be concluded that the adsorption mode between PODS and toluene is reversible chemical adsorption.[41] This adsorption process should be attributed to the “like-dissolves-like” mechanism. Considering the influence of temperature on response ability, the response of the gas sensor based on 3PODS toward 100 ppm toluene is shown in Figure , which was tested at different temperatures ranging from 283 to 313 K. The sensor can detect toluene at all tested temperatures. The response of the sensor decreased with increasing temperature. The result should be attributed to exothermic property during the adsorption process between PODS surface and the toluene molecule.

Figure 6

Sensing curves of 3PODS at 298 K (a) and 313 K (b) to toluene with different concentrations (100, 200, and 400 ppm). On the basis of the temperature-varied microgravimetric curves, plotted isotherms are used to extract the value of ΔH. (c) On the basis of the experimental results of 3PODS in (a) and (b), two isotherms are plotted to calculate the value of ΔH.

Figure 7

Response (hertz) of 3PODS toward 100 ppm toluene at different temperatures.

For all sensors, stability is very important.[42] Superhydrophobic PODS surface avoids being dirty from hydrophilic pollutants, so 3PODS-modified QCM sensors exposed to the air possess excellent stability theoretically. As shown in Figure a, the responses of 4 sensors to 400 ppm toluene were repeated for a month with 16 tests. Before each test, the sensors were cleaned with deionized water and dried at 60 °C to remove adsorbed water molecules. All of the responses showed slight decrease after continuous tests for 31 days, indicating that all of these sensors had good stability. It should be attributed to the self-cleaning ability of the hydrophobic material.[43] As shown in Figure b, impurities adhered to the surface of hydrophobic PODS can be washed with deionized water. Figure S3 shows the SEM image of 3PODS (31st day), which is consistent with Figure c, indicating its structural stability.

Figure 8

(a) Long-term stability of 3PODS-based QCM sensor toward 400 ppm toluene. (b) The reason for long-term stability.

Selectivity, another important property of a gas sensor, was also investigated. In Figure , the contrastive responses of 3PODS are listed upon exposure to four BTEX vapors (400 ppm) and other nine kinds of vapors (400 ppm), including acetone, methane, chloroform, formaldehyde, carbon dioxide, nitrogen dioxide, nitric oxide, ethanol, and water. It is obvious that 3PODS-based QCM sensor possesses a suitable selective detection capacity toward BTEX vapors, such as toluene, p-xylene, ethylbenzene, and benzene. These nine kinds of contrastive gases are common in air and living environment, so 3PODS-based QCM sensor has practical application potential for living BTEX detection.

Figure 9

Responses of 3PODS-based sensor to various kinds of interfering gases compared to BTEX. The concentration of all gases is 400 ppm.

Conclusions

Superhydrophobic PODS surface has been facilely obtained via a one-step preparation. The method is less expensive and applicable to modify QCM electrode for the first time. Accordingly, various gas-sensing tests toward BTEX show that PODS-modified QCM sensor has stable responses to toluene in water-containing environment. Solvent of OTS, contact angle of PODS, and molecular polarity of analytes all affect sensing performance. Potential application of this sensor to toluene detection is anticipated because of its exceptional waterproof property, long-term stability, outstanding selectivity, and satisfying detection limit lower than the odor threshold value.

Experimental Section

Materials

N-Octadecyltrichlorosilane (OTS) (95%) was purchased from Acros. Acetone was purchased from Shanghai Chemical Reagent Co., Ltd. (Shanghai, China). QCM resonators with silver electrode were purchased from Chengdu West Sensor Co., Ltd., China, and were used as substrates. The fundamental frequency of these QCM resonators is 107 Hz (AT-cut). QCM resonators were rinsed with ethanol for 10 min and dried with N2 before being coated with PODS.

Preparation of Modified QCM Sensors

PODS was prepared according to a modified method from a previous work.[34] All QCMs were cleaned with absolute alcohol before use and then taken out and coated with 1 μL OTS. Subsequently, the OTS-coated QCMs were immersed in different solvents (air, water, acetone, and ethanol) for several seconds, dried under room conditions (relative humidity, 65%; temperature, 25 °C), and cured in an oven at 40 °C for 8 h. The PODS films deposited on the QCM substrate using different solvents (air, water, acetone, and ethanol) were named 1PODS, 2PODS, 3PODS, and 4PODS, respectively. The sensors information is presented in Table S1 (see Table S1 in the Supporting Information).

Sensor Fabrication and Sensing Performance Investigation

The characterization instruments of PODS are displayed in Note S1. The sensor fabrication and sensing performance investigation is described in Figure S1 and Note S2.