1. Introduction

Liquid crystal (LC) that can reversibly change their refractive index and molecule alignment in response to an external stimulus, which is suited for application in tunable optical devices like sensors [1]. The LC based sensors have received special interest and attention in recent years because of their highly sensitive to the changes of external environmental conditions [2], such as temperature [3,4], pressure [5], electromagnetic fields [6,7], light [8] and chemical environments [9] etc.

Cholesteric liquid crystal (CLC) with self-assembly periodic structure can be regarded as a kind of one-dimensional photonics crystals [10]. The CLC can reflect the circularly polarized light with the same handedness due to the Bragg reflection. The central wavelength of reflection photonics bandgap (PBG) can be expressed as: λ=n×p, where n is the mean refractive index of the materials, and p is the pitch of the CLC. As reported previously, the gas sensors based on CLC that is acted as a functional material, have the characteristics of high sensitivity, selectivity, fast response, and low energy consumption [11–24]. Generally, the pitch and refraction index of the CLC can be modulated by some VOCs vapors, such as ethanol, methanol, acetone, toluene, dioxane, etc., resulting in the change of the central wavelength of reflection PBG. Compared with CLC, the PCLC is known to provide a higher stability and more reliable for sensor applications, largely because the polymer monomers align with the LC and form a scaffold after polymerization, which can restrict the flow of the LC [12,22]. However, most of the reported PCLC based VOC vapor sensors have only discussed the red-shift of the PBG. In fact, both the red- and blue-shifts of the PBG can appear successively after absorption of VOCs vapors, which is nevertheless unclarified, seriously restricting extension and application of the technology. As the concentration of VOC gases is very high in organic chemical reactions, in order to ensure the safety and accuracy of the experiment, it is very important to develop a gas sensor with a wide detection range. As well known, optical fiber sensors hold many advantages over other competing sensor technologies, including compactness, real-time response, low cost, high sensitivity, and natural immunity to external electromagnetic interference [25,26].

In this paper, we demonstrated a high-robustness PCLC based fiber sensor for VOCs vapors detection using a novel pre-compressed technique. The PCLC films doped with different polymer concentrations are used to detect different concentrations of acetone vapor. The shift law of the PBG position has been studied comprehensively in the detection of VOC vapor. As the increase of the detected gas concentration, the intractable problem is that the PBG position of PCLC exhibits red- and blue-shifts successively. Particularly, the pre-compressed technique is highly important for development of a high-performance PCLC based fiber probe, which is crucial for effectively solving the bottleneck problem mentioned. Therefore, the proposed technique not only effectively enhances the stability and robustness of the PCLC fiber probe for VOCs vapors detection, but also improves its sensitivity and detection range. This a new VOCs vapor sensor has high stability and wide detection range great than 50×10⁴ ppm. The stability here mainly means that the PBG position has a stable shift direction after absorbing VOC vapor. Sensors with an ultra-wide detection range can be used to monitor process of organic chemical reactions etc.

2. Experiments

2.1 Materials

In our experiment, the liquid crystal E7 (Cleaning point T_c= 59°C), chiral dopant R5011, polymer monomer RM257 and liquid crystal HTW1001-00 (Cleaning point T_c= 110°C) are purchased from HCCH Co. Ltd. The photo-initiator Irgacure 651 is purchased from BSF Co. The azobenzene sulphonic dye SD1 is purchased from DIC Inc. The acetone is purchased from Shenzhen Huashi technique Co. Ltd. The MMF with core size of 62.5 µm is purchased from Yangtze Soton Laser Co. Ltd.

2.2 Fabrication of the PCLC film

The polyimide used for alignment is coated on the glass by spin coating and then annealed. The CLC mixture is sandwiched in a cell, which is assembled by two pieces glass with parallel alignment. The cell thickness of 30 µm. The LC cell is exposed under UV light for 1 minute at an intensity of 100 mW/cm². Finally, the PCLC is obtained by removing glass substrates.

2.3 Preparation of the PCLC based fiber probe

Figure 1(a) depicts the fabrication process of the proposed fiber sensor based on PCLC film using the LC sucked out technique (LCSO). The LCSO technique is to suck out part of the LC from the LC device. (i) Firstly, a clear multi-mode fiber (MMF) with cutting angle of 0° is cleaned by an ultraviolet ozone cleaner, and then spin-coated with an azobenzene sulphonic dye SD1 (DIC Inc.) onto the fiber end face. (ii) The fiber coated with SD1 is annealed in an oven of 100 °C for 10 min, giving a 10 nm thick solid film. (iii) The SD1 is used as a photo-alignment material to orientate LC molecules. The LC orientated process and the chemical structure of SD1 monomers are shown in the inset of the Fig. 1(a). A linearly polarized ultraviolet light with central wavelength of 365 nm, and power density of 20 mW/cm² is used to expose the SD1 layer for 200 s, and the polarization orientation is recorded by the photo-alignment layer. (iv) A CLC mixture consisting of 72 wt% liquid crystal HTW10900-100, 2 wt% chiral dopant R5011, 25 wt% polymer monomer RM257 and 1 wt% photo initiator Irgacure 651, is precisely controlled to the end face of the fiber by using the preparation method in Ref. [22]. A certain amount of liquid crystal is stuck and dipped by a clean optical fiber-A. A microscope is used to observe optical fiber and fiber alignment. The optical fiber-A is slowly moved close to the optical fiber end face that has been oriented by the SD1. The liquid crystal comes into contact with the optical fiber end face, and then remove the optical fiber-A. Then another clean optical fiber-B is used to constantly dip the liquid crystal from the optical fiber probe, so as to repeat the operation until the liquid crystal thickness of the end face reaches the thickness required by the experiment. Finally, the optical fiber probe coated liquid crystal mixture is polymerized to form PCLC based fiber probe. (v) The fiber coated with CLC onto the end face is cured by UV light with central wavelength of 295 nm, and power density of 100 mW/cm² for 120 s in nitrogen environment to form a PCLC based probe. (vi) The LCSO technique plays the most critical role in the entire fiber probe preparation process. The probe is placed on the dust-free cloth, and the LC is sucked out by the dust-free cloth through the contact point between the dust-free cloth and the end face of the probe. The thickness of PCLC is compressed as the LC molecules are sucked out by the dust-free cloth. Note that the liquid crystal inside the PCLC film is only partially sucked out. In this process, the spectrometer is used to display the PBG position of PCLC in real time. The probe preparation is completed when the PBG position is blue-shifted to target point. Moreover, the optical fiber gas detection system based on PCLC is introduced as follows. A setup for VOC vapor sensor consisting of a light source with spectral range from 350 nm to 1500 nm (Thorlabs), a Type-Y optical fiber, an air chamber, and an optical fiber spectrometer is used to collect the spectra data, as shown in the Fig. 1(b). A light beam passes through the blue marked fiber and then reflected by PCLC on the fiber end face. The reflection light is returned by the red marked fiber and collected by optical fiber spectrometer. The air chamber with volume of 500 mL is used as a gas generator. So far, the preparation of the optical fiber gas detection system based on PCLC is completed.

 

Fig. 1. (a) The fabrication process of the proposed fiber sensor based on PCLC film with the LCSO technique. Inset shows the LC orientated process and the chemical structure of SD1 monomers. (b) The setup of the VOC vapor sensor.

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3. Results and discussed

3.1 Mechanism and law of photonic bandgap (PBG) shift after absorption of volatile organic compounds (VOCs) vapors

For the VOC vapor sensor based on CLC, the red or blue-shift of the PBG position is observed as reported in many literatures [27,19,28]. However, the PBG position shift of the VOC vapor sensor based on CLC has not been analyzed in detail. It is known that the central wavelength of the PBG is mainly determined by the period of the CLC and the refractive index of materials, according to the formula of the reflection PBG position: λ=n×p, where the n is mean refractive index of the materials, and the p is pitch of the CLC. The CLC’ volume is going to expand as absorption of VOC vapor, which leads to a longer pitch of the CLC and a red-shift of the PBG position. Usually, the refractive index of the VOC is lower than that of LC mixture. The refractive index of the CLC mixture decreases when the VOC vapor is absorbed by the CLC, resulting in a blue-shift of the PBG position. In the experiment, it is found that the shift law of the PBG is mainly related to the concentrations of detection vapor and doped polymer.

With the change of polymer monomer doping concentration, the images of the CLC film after absorption of VOC are shown in Fig. 2(a). The preparation of PCLC film refers to the method in the published literature [29]. In the Fig. 2(a), some representative polymer doping concentrations are selected for demonstration. It can be seen that the color of the PCLC film changes with increasing the concentration of acetone vapor. The color of the PCLC film with polymer doping concentration of less than 15 wt%, shows a red shift and a blue-shift successively within the concentration of the acetone vapor ranging from 0 ppm to 40 × 10⁴ ppm. For better understanding, the vapor concentration corresponding to the suddenly change from red-shift to blue-shift is called demarcation point concentration (DPC). For the polymer doping concentration of less than 15 wt%, the DPC is at 10 × 10⁴ ppm. The DPC is 10 × 10⁴ ppm when the polymer doping concentration reaches 25 wt%. However, the color of the PCLC film with a polymer doping concentration of 35 wt%, shows only blue-shift when the detected concentration of acetone vapor increases from 0 ppm to 40 × 10⁴ ppm, indicating that the change in the refractive index of the LC mixture is more dominant in the modulation process compared to the change in the PCLC’s pitch. However, the color change of PCLC film shows red- and blue-shift again, when the polymer doping concentration reaches 50 wt%, and the corresponding DPC is at 10 × 10⁴ ppm. Moreover, when the polymer doping concentration is greater than 75 wt%, the PCLC film’s color is only red-shifted with the concentration of the acetone vapor increased from 0 ppm to 40 × 10⁴ ppm, revealing that the change in PCLC’s pitch plays a more dominant role in the modulation process. In addition, the central wavelength shift of reflection PBG as a function of acetone vapor concentrations is shown in Fig. 2(b). For the polymer doping concentration of 10 wt%, the central wavelength of reflection PBG has a red-shift of 42 nm firstly and then shows a blue-shift of 30 nm, and the DPC of the VOCs detection is at 20 × 10⁴ ppm. The corresponding reflection PBG positions are first red-shifted by 22 nm, 7 nm, 0 nm, 4 nm, 3 nm, 17 nm, and 23 nm, and then blue-shifted by 38 nm, 36 nm, 36 nm, 19 nm, 4 nm, 0 nm and 0 nm, respectively, with polymer doping concentrations of 25 wt%, 35 wt%, 50 wt%, 65 wt%, 75 wt% and 85 wt%.

 

Fig. 2. Acetone vapor sensors based on PCLC films. (a) The images of the PCLC film after absorbing acetone vapor, as the change of polymer monomer doping concentration. (b)The central wavelength shift of reflection PBG as a function of acetone vapor concentration. (c) The effect of polymer doping concentration on the DPC. (d) Summary of the response of PCLC film to acetone vapor and the application scenarios with different concentration of the polymer.

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The effect of polymer doping concentration on the DPC of the PCLC based VOC sensor is summarized in Fig. 2(c). The DPC decreases from 20 × 10⁴ ppm to zero with the polymer doping concentration increasing from 10 wt% to 35 wt%. While the DPC keeps increasing as the polymer doping concentration increases from 35 wt% to 85 wt%. In particular, the DPC cannot be detected since the gas detection concentration is not high enough yet when the polymer doping concentration is greater than 75 wt%. For the polymer doping concentration of 35 wt%, the PBG of the CLC film gets only blue-shift with the vapor concentration change from 0 ppm to 40 × 10⁴ ppm. The blue-shifted region of the PBG position at different polymer doping concentration is estimated by a nonlinear fitting method, as shown in the blue area Fig. 2(c). In the table of Fig. 2(d), the response of above PCLC film to acetone vapor is summarized, and giving the application scenarios of PCLC films doped with different concentration of the polymer. For example, the polymer doping concentration is in the range less than 65 wt%, which is applied to the detection of VOC vapor with low concentration. The PCLC with a polymer doping concentration of 35 wt% has only a blue shift in the PBG after absorbing VOC, which can be applied to a wide range of gas detection. The effect of VOCs vapor on the CLC film is very small in the polymer doping concentration ranging from 65 wt% to 85 wt%, which can be used as an ink in the field of printing. The PCLC with polymer doping concentrations of 35 wt% and 85 wt% can be used to detect VOCs with high concentration range. When the testing range of acetone vapor is from 0 ppm to 40 × 10⁴ ppm, the PBG displacement value of the CLC film decreases (from 70 nm to 20 nm) with the polymer doping concentration increasing, which indicates that the sensitivity of vapor detection is reduced. However, the PBG position is moving with an unstable shift direction (red- and blue-shift) after the absorption of VOCs vapors, which brings serious challenges to the practical application of the devices.

3.2 Polymer CLC film using pre-compressed technique

The spring-like theory is proposed for the preparation of the PCLC films, to solve the problems of unstable shift direction of the PBG position, narrow detection range and low sensitivity. For springs of the same material, the shorter the pitch, the greater the force required to stretch the same length. When the spring is used in the sensor field, lower hardness and larger pitch are usually selected to prepare the detector, which makes it easier to deform the spring. For example, a force of F can stretch the spring by L₁. If the spring is given a downward pressure (F₁) in advance, after compression length of L₂, the spring will have an upward restoring force. A pre-compressed spring needs a small force (-F₁) to initiate the spring tension action than an uncompressed spring. For the compressed spring, the overall tensile length of the spring becomes L₁+ L₂ when the F force is applied, as shown in Fig. 3(a).

 

Fig. 3. (a) Schematic diagram of spring theoretical model. (b) According to the spring theory, the PCLC films are compressed using LCSO technique.

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The greater the pitch variation achieved, the larger the elongated range of the spring presented. The compressed spring operation can not only reduce the minimum detection limit, but also increase the detection range of the sensor. Similarly, the PCLCs also have a mechanical effect as springs. The hardness of PCLCs with polymer-rich phase is greater than the polymer-poor phase, as shown in Fig. 3(b). According to the spring theory, the PCLCs with higher polymer concentration and smaller pitch for the VOCs vapors detection, is going to reduce the sensitivity of the sensor. The selection of more low concentration polymer and CLC with larger pitch is beneficial to the comprehensive performance of the PCLC based sensors. However, it will tend to increase the sensor uncertainty due to the LC flow induced by the low concentration of polymer doped. If the PCLC can be pre-compressed like a spring, it can make the sensor more sensitive with a lower detection limit, and offer a wider detection range. Here, LCSO is used to reduce the thickness of CLC film, so as to achieve a pre-compression effect similar to that of spring. It is assumed that the thickness of the PCLC film without using LCSO technique can be increased by H₁ after absorption of a certain amount of VOC vapor. If the thickness of PCLC film is compressed by H₂ after adopting the LCSO technique, the change range of PCLC film thickness reaches H₁+ H₂, which is larger than that before sucking LC out of PCLC in the case of the same external stimulation. Therefore, the PCLC film using LCSO technique has a spring-like restoring force, which broadens the detection range, increases the sensitivity of the PCLC based sensor, and effectively avoids the phenomenon of unstable moving direction of the PBG position. It provides a new technique for the application of the PCLC in the sensor field.

Figure 4(a) shows the reflection spectral curves of the fiber probe based on PCLC before and after the photo-polymerization, and after LC sucked out, respectively. Black curve represents the reflection spectrum before polymerization, and we can see that the PBG position is centered at wavelength of 1000 nm. While the red and blue curves represent the reflection spectra after polymerization and LC sucked out, respectively. The central wavelength of the PBG is located at 765 nm, which has a blue-shift of 235 nm compared with that before polymerization, resulting from the PCLC’s pitch shrinkage by the reaction of polymerization and crosslinking. As the LC is sucked out by a dust-free cloth, the central wavelength of the PBG continues to blue-shift from 765 nm to 645 nm, which provides a pre-elastic potential energy for PCLC. The optical images of the fiber probe are illustrated in Fig. 4(b), corresponding to the optical fiber probe before and after CLC coated (Upper), after polymerization and LC sucked out (Lower). After polymerization, polymer-rich phase and polymer-poor phase are formed in regions corresponding to near or far from the end face of the fiber, since the UV light is output from fiber and distributed non-uniformly in the CLC. There is no obvious change of the thickness of PCLC after polymerization. However, after LC sucked out, the thickness of the PCLC has been reduced to 33 µm from the original thickness of 45 µm.

 

Fig. 4. (a) The reflection spectral curves of the fiber probe based on PCLC before and after the photo-polymerization, and after LC sucked out. (b) Photographs of the sample at fabrication process of before and after CLC coated (Upper), after polymerization and LC sucked out (Lower).

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To date, many optical fiber gas sensors based on PCLC require the PBG to change with a stable moving direction, ultra-wide detection range and sensitive to the external environment. Sensors with an ultra-wide detection range can be used in applications such as monitoring organic chemical reactions. Therefore, we further investigate the performance of the PCLC based VOC fiber sensors using LCSO technique. The acetone vapor is generated in the chamber described in the Fig. 1(b). Figure 5(a) shows the reflection spectra of the sample versus acetone vapor concentrations. The central wavelength of reflection spectrum has a red-shift of 115 nm, ranging from 761 nm to 646 nm as the acetone vapor concentration increases from 0 ppm to 50 × 10⁴ ppm. Correspondingly, the central wavelength as a function of acetone vapor concentration is shown in Fig. 5(b). In the Fig. 5(b), the device has been measured several times, where the wavelength shift was always the same and the error bar was less than 1 nm. There is a linear fit between the acetone vapor concentration and the central wavelength with a mean sensitivity of 0.23 pm/ppm. Meantime, the minimum detection limit of acetone vapor using the fiber probe based on PCLC with LCSO technique is 600 ppm, and the detection range is greater than 50 × 10⁴ ppm. Figure 5(c) plots the dynamic response of the central wavelength to the acetone vapor concentrations of 10 × 10⁴ ppm, 14 × 10⁴ ppm and 18 × 10⁴ ppm, respectively. Firstly, acetone vapor with concentration of 10 × 10⁴ ppm is generated by air chamber, and then absorbed by the PCLC-based fiber optic probe. During the vapor absorption process, the central wavelength is red-shifted by 14 nm, and the corresponding response time is 30 s. After that, a certain amount of dry air is filled into the air chamber until the reflection spectrum of the fiber probe returns to its original position with a restoring time of 100 s. Then repeat the above steps for the following two vapor concentrations of 14 × 10⁴ ppm and 18 × 10⁴ ppm, respectively. The central wavelengths of the reflection spectra are red-shifted by 24 nm and 34 nm with the corresponding response time of 42 s and 45 s, and the gas release time of 160 s and 180 s, respectively. According to the data statistics, the response time and recovery time of the optical fiber probe increase with the increasing vapor concentration. After several repeated experiments, the PBG position of the PCLC can be restored to the original wavelength after the release of VOC gas. In our previous report [30], we proposed that the recovery time can be greatly shorten for LC based fiber sensor by using a heating method. The temperature effect on the PCLC based fiber probe is studied using a hot stage (INSTEC, MK2000). Figure 5(d) shows the reflection spectrum of the PCLC based fiber probe versus temperature. The central wavelength of reflection spectrum has a red-shift of 4 nm within the temperature ranging from 20 °C to 80 °C. The phase transition cannot occur for the PCLC, since the cleaning point of LC HTW1001-00 is as high as 110 °C. This negligible red-shift could be attributed to the slight pitch increase induced by thermal expansion. Particularly, it is worth mentioning here that when the temperature varies from 20 °C to 40 °C, the position of the reflection spectrum is completely unchanged, due to the reasonable selection and matching of materials.

 

Fig. 5. (a) The reflective spectra of acetone vapor sensor measured at different vapor concentrations with polymer doping concentration of 25 wt%. (b) The summarized relationship of central wavelength versus different acetone vapor concentration. (c) The dynamic response of the sensor’s central wavelength when detecting acetone vapor at concentrations of 10 × 10⁴ ppm, 14 × 10⁴ ppm and 18 × 10⁴ ppm. (d) Temperature effect on central wavelength of the PBG. Inset shows the reflective spectra of acetone vapor sensor measured at different temperatures of 20 °C, 40 °C, 60 °C, and 80 °C.

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For comparison, the PCLC based fiber probe without the use of LCSO technique is prepared by the same process as the above experiments. As shown in Fig. 6, it is obvious that the PCLC based fiber probe has a completely different change law compared with that using LCSO technique. The central wavelength of reflection spectrum has a red-shift 18 nm within the acetone vapor concentration ranging from 0 ppm to 4 × 10⁴ ppm, and then gets a blue-shift of 29 nm as the acetone vapor concentration increases from 4 × 10⁴ ppm to 18 × 10⁴ ppm. Consequently, the mean sensitivity is about 0.16 pm/ppm with a detection range less than 20 × 10⁴ ppm (lower than 0.23 pm/ppm of the LCSO technique based fiber probe). In addition, the spectral change law of the PCLC based fiber probe without LCSO technique in the gas detection process is basically consistent with that in Fig. 2. However, the unstable moving direction of the PBG position (coexistence of red-shift and blue-shift) brings severe challenges to the practical application of the device, which further validates the feasibility and effectiveness of our approach of developing a spring-like theory. In addition, the PCLC pre-compressed has a spring-like restoring force by using LCSO technique, which not only effectively avoids the instability of the moving direction of the PBG position, but also improves the robustness, sensitivity and the detection range of the PCLC based fiber sensor.

 

Fig. 6. (a) The measured reflective spectra of acetone vapor sensor without using LCSO technique at different vapor concentration in case of polymer doping concentration of 25 wt%. (b) The summarized relationship of centra wavelength versus different acetone vapor concentration.

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4. Conclusions

In summary, the PBG moving law of the PCLC after absorption of VOC vapor, is revealed by analyzing the effect of detected vapor concentration on the PBG position as different concentrations of the polymer doped. The results show that the PBG position shifts with an unstable moving direction when the gas concentration range is from 0 ppm to 40 × 10⁴ ppm, which makes it difficult for PCLC to apply in wide-range and high sensitivity gas sensors. Therefore, we demonstrate VOC vapor fiber sensor based on PCLC with a LCSO technique, which show excellent performance compared with that without using LCSO technique. For the polymer doping concentration of 25 wt%, the central wavelength is red-shifted from 646 nm to 761 nm when the acetone vapor concentration increases from 0 ppm to 50 × 10⁴ ppm. The minimum detection limit of acetone vapor using the PCLC based fiber probe is 600 ppm, and the detection range is greater than 50 × 10⁴ ppm. In particular, the spring-like theory further validates the feasibility and effectiveness of our approach by comparing with the PCLC based fiber probe without using LCSO technique. Therefore, it is a breakthrough that the LCSO technique which not only effectively enhances the stability and robustness of the PCLC based fiber probe for VOC vapor detection, but also improves its sensitivity and expands the detection range. Sensors with an ultra-wide detection range can be used in applications such as monitoring organic chemical reactions. The performances of this LC based VOC sensor including the sensitivity, the limit of detection, and specificity can be optimized in the future work by modifying material component and improving fabrication processes.