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Polymer-infiltrated SiO2 inverse opal photonic crystals for colorimetrically selective detection of xylene vapors
Sensors & Actuators: B. Chemical 291 (2019) 67–73
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Polymer-infiltrated SiO2 inverse opal photonic crystals for colorimetrically selective detection of xylene vapors
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Yuqi Zhanga, Yimin Suna, Jiaqi Liua, Pu Guoa, Zhongyu Caib, , Ji-Jiang Wanga a
Key Laboratory of New Energy & New Functional Materials, Shaanxi Key Laboratory of Chemical Reaction Engineering, College of Chemistry and Chemical Engineering, Yan’an University, Yan’an, Shaanxi, 716000, PR China b Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Photonic crystals Inverse opal Functional polymer Colorimetric sensing Selective xylene vapor detection
Xylenes are common aromatic volatile organic compounds, which can cause severe environmental pollution and healthy issues. The current instrumental detection methods are generally expensive, time-consuming, complex, and unable to be used for on-site detection. It is thus highly desirable to develop an efficient method for the detection of xylenes. In this study, we report the development of a polymer infiltrated SiO2 inverse opal photonic crystal (IOPC) for selective detection of xylene vapors. Poly(4-vinylbenzyl chloride-co-methyl methacrylate) (P (VBC-co-MMA)), which shows strong affinity towards xylenes because of their similar solubility parameters, was synthesized and infiltrated into SiO2 IOPC (P(VBC-co-MMA)-SiO2 IOPC). The resulting P(VBC-co-MMA)-SiO2 IOPC sensor shows color change from green to red upon exposure to xylene vapors as the diffusion and adsorption of gaseous xylenes increased the effective refractive index of the IOPC. It shows good sensitivity with a limit of detection of 0.51, 0.41 and 0.17 μg mL−1 for o-xylene, m-xylene, and p-xylene, respectively. The sensor also shows excellent selectivity for xylenes over other organic vapors. In addition, the P(VBC-co-MMA)-SiO2 IOPC sensor demonstrates quick response and good reversibility. This polymer infiltrated IOPC sensor provides a universal strategy for the detection of organic vapors through building a sensor using polymers with similar solubility parameters to targeting vapors.
1. Introduction Xylenes are one kind of common aromatic volatile organic compounds (VOCs), which have been extensively used in fuel, adhesives, additives, and industrial products including synthetic fiber, rubber and plastic. Xylenes are flammable, volatile and harmful, which can cause air pollution and thus damage human health. In recent years, many studies have shown that harmful xylenes can lead to headache, dizziness, dermatitis, anemia, and even cause lung cancer upon long-term exposure [1]. Therefore, it is highly desirable to develop effective methods for the detection of xylenes. Highly sensitive, precise and reliable detection techniques including gas chromatography [2], surface enhanced Raman scattering [3], and surface Plasmon resonance [4] have been reported to detect xylenes. These methods are generally expensive, time-consuming, complex, and unable to be used for on-site detection. Recently, intense efforts have also been devoted to develop different sensors using a variety of sensing materials to detect xylenes, such as polymer-coated quartz crystal microbalance (QCM) sensors [5,6], metal oxide semiconductor
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chemiresistors [7,8], AIE (Aggregation-Induced Emission)-based fluorometric sensors [9] and polydiacetylene paper-based colorimetric sensors [10]. Among these techniques, colorimetric sensing is one of the most convenient methods. This method can provide a straightforward signal for monitoring, which even can be observed by naked eyes. Photonic crystals (PCs) with periodic dielectric structures offer a new opportunity for the development of colorimetric sensors due to the tunable photonic stopband, whose position determines the color of PCs along normal incidence [11–35]. By altering the effective refractive index and/or the diffracting plane spacing distance, various colorimetric sensors based on PCs have been developed for the detection of pH [16–18], metal ions [19,20], anions [21,22], biomolecules [23,24], pesticides [25] and gas/VOCs [26–34]. For example, Lova et al. [26] reported a one-dimensional PC colorimetric sensor that fabricated from poly(p-phenylene oxide) and cellulose acetate to detect VOCs including benzene, 1,2-dichlorobenzene, carbon tetrachloride and toluene. Porous silicon PCs were also developed and used to detect organic vapors including isopropanol, heptane, cyclohexane and ethanol according to the shift of the PC reflectance peak [27,28]. In particular,
Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Cai).
https://doi.org/10.1016/j.snb.2019.04.036 Received 30 January 2019; Received in revised form 20 March 2019; Accepted 7 April 2019 Available online 08 April 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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three-dimensional inverse opal PCs (IOPC), as colorimetric sensing platforms, have attracted much attention due to their interconnected macroporous structures and relatively high specific surface area [29,27–34]. These properties are useful to facilitate the diffusion and adsorption of vapor analytes. In addition, the combination of IOPC with responsive small organic molecules or polymers provides an excellent platform for constructing PC colorimetric sensors. The high specific surface area of IOPC, and the reaction activity or adsorption affinity of responsive molecules toward vapor analytes can greatly improve the sensitivity and selectivity of the resulting sensors when sensing is performed by changing the refractive index of IOPC. For example, an amine-functionalized inverse opal film has been developed for colorimetric detection of CO2 over a wide concentration range [30]. Our group developed several hexaphenylsilole, tetraphenylethene polymer, and poly(2-hydroxyethyl methacrylate-comethyl acrylate) infiltrated SiO2 IOPCs for efficient and visual detection of diethyl ether/petroleum [31], tetrahydrofuran/acetone [32] and volatile alcohols [33], respectively. However, it remains a challenge to construct a highly ordered macroporous structure inverse opal for colorimetric sensing of aromatic VOCs such as xylenes. Here, we report a poly(4-vinylbenzyl chloride-co-methyl methacrylate) (P(VBC-co-MMA))-infiltrated SiO2 IOPC (P(VBC-co-MMA)-SiO2 IOPC) as a colorimetric sensor to detect xylene vapors. The as-constructed IOPC sensor changed from green to red upon exposure to xylene vapors as the diffusion and adsorption of gaseous xylenes increased the effective refractive index of the IOPC. The sensor can recover to its initial color after following exposure to air. This P(VBC-co-MMA)-SiO2 IOPC sensor also showed good sensitivity, excellent selectivity and facile reusability. These results indicate that our IOPC film can be a potential candidate for colorimetric sensing of xylenes.
Scheme 1. Synthesis of P(VBC-co-MMA).
of frozen petroleum ether under vigorous stirring and an insoluble white solid appeared. After stirring for 2 h, the white precipitates were collected through vacuum filtration, followed by washing with petroleum ether for 5 times and subsequently drying overnight under vacuum at 50 ℃. The IR spectrum of P(VBC-co-MMA) (Fig. S1) confirmed the successful synthesis according to the reported literature [6]. 2.3. Fabrication of P(VBC-co-MMA)-SiO2 IOPC The fabrication of P(VBC-co-MMA)-SiO2 IOPC included two steps: preparation of SiO2 IOPC and infiltration of P(VBC-co-MMA) molecules. First, SiO2 IOPC was obtained by using a sacrificial template method [31,32]. As shown in Scheme 2, 20 g of ∼ 0.2 wt% PS microsphere latex was mixed with 0.1 mL of SiO2 colloidal sol, followed by ultrasonication for 30 min. SiO2 sol was prepared by stepwise dropping HCl (0.10 M) into the absolute ethanol solution of tetraethyl orthosilicate (TEOS, 28 wt%) and stirring for 1 h at room temperature (TEOS:C2H5OH:HCl = 1:2:1, weight ratio). A clean glass slide (2.0 cm × 1.0 cm) was vertically placed into the mixture. The PS microspheres self-assembled on the glass surface and SiO2 sol was infiltrated into their interstices upon evaporation at 65 °C for 48 h. The resulting coassembled PC film, denoted as PS-SiO2 PC, was sintered at 90 °C for 1 h and then immersed in THF for 2 h. The PS templates were dissolved and a green SiO2 IOPC formed. Then, 60 μL of 0.5 wt%–2.0 wt% THF solution containing P(VBC-coMMA) was dripped onto the surface of the resulting SiO2 IOPC (size:1.0 cm × 0.65 cm). P(VBC-co-MMA) molecules were infiltrated into the voids of the SiO2 IOPC due to capillary forces. After drying in air, a P(VBC-co-MMA)-SiO2 IOPC sensor for xylenes detection was obtained. A PMMA infiltrated SiO2 IOPC (PMMA-SiO2 IOPC) was also fabricated by dripping 60 μL of 1.5 wt% THF solution containing PMMA onto the surface of the SiO2 IOPC (2.0 cm × 1.0 cm) and used as a control.
2. Experimental 2.1. Chemicals and instruments 4-Vinylbenzyl chloride (VBC, 90%) was purchased from SigmaAldrich and purified with neutral Al2O3 column. Methyl methacrylate (MMA, 99%) was supplied by Sigma-Aldrich and purified by distillation. The initiator, a,a’-azodiisobutyronitrile (AIBN, 99%, J&K Scientific Ltd, Beijing, China) was recrystallized from methanol. Monodisperse polystyrene (PS) colloidal particles with a diameter of 350 nm were acquired from Shanghai Huge Biotechnology Co., Ltd, China. Poly (methyl methacrylate) (PMMA, Mw = 35 000) was purchased from Sinopharm Chemical Reagent Beijing Co. Ltd. All the other analytical reagents were purchased commercially and used as received. Nanopure MilliQ water (18.2 mΩ cm) was utilized for all experiments. A field-emission scanning electron microscope (FESEM, SU8010, Hitachi, Japan) was used to characterize the morphology of the PCs after the samples were sputter-coated with a thin layer of gold. The IR spectrum of P(VBC-co-MMA) was obtained from a FT-IR spectrometer (IR Prestige-21, Shimadzu, Japan). In situ reflectance spectra were recorded with a fiber optic spectrophotometer (PG-2000-Pro-Ex, Ideaoptics Instrument Co., Ltd, China) along normal incidence to the (111) plane of the PCs.
2.4. In situ reflectance spectra measurement The response of the P(VBC-co-MMA)-SiO2 IOPC sensor was allowed to proceed at room temperature. The sensor was fixed in a gas chamber, which has a pinhole. The fiber-optics probe was perpendicular to the surface of the PC sensors. A certain volume of volatile solvents was injected into the gas chamber through the pinhole by a microliter syringe. The in situ reflectance spectra along normal incidence to the (111) plane of the PCs were recorded when the sample was in the vaporous environments of the solvents. SiO2 IOPC, as a control sensor, was also tested when exposed to different vaporous environments. The SiO2 IOPC film was alternately placed in air and saturated p-xylene vapor and the reflectance spectra were recorded to study the reusability of our sensors. The in situ reflectance spectra of PMMA-SiO2 IOPC were also measured upon exposure to p-xylene vapor.
2.2. Synthesis of P(VBC-co-MMA) Scheme 1 shows the synthesis route of P(VBC-co-MMA). P(VBC-coMMA) was synthesized through a free radical polymerization method in which VBC and MMA were used as monomers and AIBN was used as an initiator. In a typical process, 30.3 mg of AIBN was added to a 60 mL of tetrahydrofuran (THF) solution containing 4.5 mL of VBC and 2.7 mL of MMA, followed by magnetic stirring and nitrogen bubbling for 30 min. The solution was allowed to react at 62 ℃ for 48 h under nitrogen bubbling. The resulting reaction mixture was cooled down to room temperature. Then the resulting mixture was dropwise added to 400 mL
3. Results and discussion 3.1. Fabrication and sensing mechanism of P(VBC-co-MMA)-SiO2 IOPC sensor The P(VBC-co-MMA)-SiO2 IOPC sensor consists of polymeric 68
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Scheme 2. Fabrication and sensing mechanism of P(VBC-co-MMA)-SiO2 IOPC.
refractive index, which decrease the neff of PC. After infiltration of P (VBC-co-MMA), the stopband shows a red-shift of 17 nm because small quantities of P(VBC-co-MMA) induces a tiny variation in neff of PC.
molecules of P(VBC-co-MMA) and SiO2 IOPC structure. Scheme 2 shows its fabrication and sensing mechanism. The PS particles latex and SiO2 colloidal sol were mixed and coassembled by vertical deposition at 65 °C to prepare the PS-SiO2 PC films. The co-assembled PC films were immersed in THF to selectively remove the PS particles and then SiO2 IOPC were formed. A THF solution containing P(VBC-co-MMA) was dropped on the SiO2 IOPC. After air drying, P(VBC-co-MMA) molecules infiltrated into the pores and adsorbed on the pore walls of the SiO2 IOPC via capillary forces. P(VBCco-MMA)-SiO2 IOPC sensor was then obtained. The P(VBC-co-MMA)-SiO2 IOPC film can be used to detect xylenes. Upon exposure to the xylene vapors, the reflectance peak of the IOPC undergoes a red-shift due to an increase in the effective refractive index (neff ) of the IOPC. The increase of neff can be attributed to the diffusion and adsorption of xylene vapors in the voids of the IOPC (Scheme 2). This is because xylenes, which have higher refractive index, replace air in the interconnected macroporous structure of the IOPC. P(VBC-coMMA) molecules show a strong affinity toward xylenes due to their similar solubility parameter [6], which will be explained in detail hereinafter. This infiltration of organic vapors causes an increase in the effective refractive index, and thus induces a red-shift of the PC stopband [36,37]. Simultaneously, the color of P(VBC-co-MMA)-SiO2 IOPC changes from green to red upon exposure to xylene vapors. In addition, the sensor can be recycled through exposing it to air due to the volatilization and desorption of xylene vapors away from the P(VBC-coMMA)-SiO2 IOPC. Therefore, our P(VBC-co-MMA)-SiO2 IOPC can be utilized as a reusable and visual sensor for the detection of xylenes.
P(VBC-co-MMA)-SiO2 IOPC sensors containing various infiltration quantities of P(VBC-co-MMA) were fabricated by infiltrating 60 μL of polymer THF solutions at different concentrations. Upon exposure to a saturated vapor of p-xylene, the reflectance spectra of these sensors show different wavelength changes. As shown in Fig. 2, the photonic stopbands of P(VBC-co-MMA)-SiO2 IOPC sensors with different amounts of polymer infilatration in air are at 562 nm. After exposing to p-xylene vapor, their photonic stopbands shift to 589 nm, 597 nm and 614 nm, respectively, with increasing the concentrations of the infiltration solutions from 0.5 wt% to 1.5 wt%. The stopband red-shifts of these sensors are 27 nm, 35 nm and 52 nm, respectively. The results indicate that a higher infiltration quantity of the polymer is benificial to diffusion and adsorption of p-xylene, which induce a larger red-shift. However, a red-shift of 23 nm is observed when further increase the infiltrated polymer solution concentration to 2.0 wt%. This is because too much infiltration of polymer decreases the pore volume of the IOPC. Obviously, the P(VBC-co-MMA)-SiO2 IOPC sensor fabricated from 1.5 wt% polymer solution shows the largest stopband red-shift of 52 nm. Thus, we chose this IOPC as a sensor candidate for the following studies. Here the infiltration quantity of P(VBC-co-MMA) is 0.85 mg.
3.2. Morphology and stopband characterization of P(VBC-co-MMA)-SiO2 IOPC
3.4. Time dependence of reflectance spectra and reversibility of the IOPC sensor
Fig. 1a shows an SEM image of co-assembled PS–SiO2 PC. The PS colloidal spheres within the PS–SiO2 PC arrange orderly with a hexagonally close-packed plane (111) oriented parallel to the substrate. Fig. 1b exhibits an SEM image of the resulting P(VBC-co-MMA)–SiO2 IOPC. An ordered macroporous SiO2 scaffold can be clearly observed. Air pores with a diameter of ∼ 285 nm replace the PS spheres and remain hexagonally close-packed structure. The morphology of P(VBCco-MMA)–SiO2 IOPC does not change significantly compared with that of the SiO2 IOPC (Fig. S2) due to a small quantity of the polymer infiltration, which does not affect the periodic structure. A cross-sectional SEM image (Fig. 1c) of P(VBC-co-MMA)–SiO2 IOPC demonstrates that the air pores in each layer are also highly ordered. The thickness of the sensor film is ∼3.6 μm. Reflectance spectra of the PCs are shown in Fig. 1d. The photonic stopbands of PS-SiO2 PC, SiO2 IOPC and P(VBC-co-MMA)-SiO2 IOPC occur at 764 nm, 546 nm and 562 nm, respectively. Apparently, there is a remarkable stopband blue-shift for SiO2 IOPC compared to the PSSiO2 PC. This is because the PS spheres are replaced by air with a lower
Fig. 3a exhibits that the reflectance spectra of the P(VBC-co-MMA)SiO2 IOPC sensor undergo a stepwise red-shift as the exposure time increases, resulting from continuous increase in the effective refractive index of the pores during diffusion and adsorption of p-xylene. The stopband peak position shifts from 562 nm to 580 nm upon exposure to p-xylene vapor within 1 min. The fast response of this sensor can be attributed to the IOPC’s interconnected macroporous structure with high specific surface area. This facilitates the diffusion and adsorption of gaseous compounds. The stopband shows a maximum red-shift from 562 nm to 612 nm after exposure to p-xylene vapor for 8 min. Subsequently, the sensor was placed in air. A quick blue-shift occurs for the reflectance peak due to the fast volatilization of p-xylene in air (Fig. 3b). The responses of our sensor to o-xylene and m-xylene are similar to that of p-xylene vapor. The maximum stopband red-shifts for o-xylene and m-xylene are 55 nm and 48 nm, respectively (Fig. S3). The reflectance spectra of the senor also return to the initial position upon exposure to air. We alternately place the P(VBC-co-MMA)-SiO2 IOPC in the
3.3. Dependence of stopband red-shift on the infiltration quantity of P(VBCco-MMA)
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Fig. 1. Top view SEM images of (a) PS-SiO2 PC and (b) P(VBC-co-MMA)-SiO2 IOPC; (c) cross-sectional SEM image of P(VBC-co-MMA)-SiO2 IOPC; and (d) the reflectance spectra of the PS-SiO2 PC, SiO2 IOPC and P(VBC-co-MMA)-SiO2 IOPC fabricated from 1.5 wt% THF solution containing P(VBC-co-MMA).
sensor over cycles of p-xylene exposures is shown in Fig. S5. The photonic stopband shift of our sensor accompanies the color change of the P (VBC-co-MMA)-SiO2 IOPC film. Fig. 4b shows the optical photographs of the color changes of our sensor during a period of the response for pxylene vapor. The initial green of P(VBC-co-MMA)-SiO2 IOPC sensor in air converts to red upon exposure to p-xylene vapor. The color change is also reversible, which implies that the P(VBC-co-MMA)-SiO2 IOPC is a good candidate for visual sensing of xylenes. 3.5. Detection sensitivity of xylene vapors The on-line recorded reflectance spectra of the P(VBC-co-MMA)SiO2 IOPC sensor (Fig. 5a) show that larger stopband red-shift occurs at higher concentration of p-xylene vapor. This is due to the larger neff of the IOPC, resulting from diffusion and adsorption of more vapor molecules at higher p-xylene vapor concentrations. A 17 nm stopband redshift is obtained at the concentration of 10.3 μg mL−1. The red-shift in stopband position increases to 45 nm at 34.4 μg mL−1 and reaches to its maximum of 52 nm at 43.1 μg mL−1. The stopband position shifts of our P(VBC-co-MMA)-SiO2 IOPC sensor are plotted as a function of p-xylene vapor concentrations, as shown in Fig. 5b. A very good linear correlation with R2 = 0.9975 is obtained. The limit of detection (LOD) for pxylene is 0.17 μg mL−1 (39 ppm), based on 3 SD/S. Here SD is the standard deviation of the response and S is the slope of the calibration curve. The LOD of our P(VBC-co-MMA)-SiO2 IOPC sensor (LOD = 39 ppm) is comparable with the QCM sensor reported earlier (LOD = 54 ppm) [6]. But our sensor is slightly more sensitive and much cheaper than that of the QCM sensor, which can be ascribed to the unique structure of IOPC and its ease of fabrication. The reflectance spectra and stopband position shifts of the sensor exposed to different concentrations of o-xylene (LOD: 0.51 μg mL−1, 117 ppm) and m-xylene (LOD:
Fig. 2. Reflectance spectra of P(VBC-co-MMA)-SiO2 IOPCs fabricated from different concentrations of THF solutions containing P(VBC-co-MMA) upon exposure to air and the saturated vapor of p-xylene, respectively.
atmosphere of xylenes and air for 10 cycles. The stopband position changes are shown in Fig. 4a (p-xylene vapor) and Fig. S4 (o-xylene and m-xylene vapor) in Supplementary Information. The stopband occurs at ∼ 614 nm upon exposure to xylene vapors, and shifts to ∼ 562 nm in air. For xylene vapors, entering into and escaping out of the IOPC pores of the sensor changes the neff of the IOPC, and the photonic stopband shifts correspondingly. The stopband position change is reversible, which implies that our P(VBC-co-MMA)-SiO2 IOPC has an excellent stability and can be used repeatedly. Reflectance spectra of the IOPC 70
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Fig. 3. Time dependence of reflectance spectra for P(VBC-co-MMA)-SiO2 IOPC sensor upon exposure to (a) p-xylene saturated vapor and (b) the air subsequently. Fig. 5. Dependence of (a) reflectance spectra and (b) stopband shifts of the P (VBC-co-MMA)-SiO2 IOPC on concentrations of p-xylene vapor.
0.41 μg mL−1, 94 ppm) vapors are shown in Fig. S6. Regarding to its practical application for environmental monitoring, our P(VBC-coMMA)-SiO2 IOPC sensor still needs to be optimized in the future.
investigate the selectivity of our sensor toward xylenes. The spectra responses of blank SiO2 IOPC were also monitored as a control. As shown in Fig. 6, both P(VBC-co-MMA)-SiO2 IOPC and SiO2 IOPC demonstrate stopband red-shifts for all the detected vapors. This is because the vapors of those solvents with higher refractive indices replace
3.6. Selectivity of the sensor toward xylenes The photonic stopband shifts of the P(VBC-co-MMA)-SiO2 IOPC sensor exposed to saturated vapors of various solvents were recorded to
Fig. 4. (a) Reversible changes of maximum stopband position for the P(VBC-co-MMA)-SiO2 IOPC when alternately exposed to saturated p-xylene vapor and air, and (b) the corresponding optical photographs of color changes. 71
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Therefore, we can design and synthesize specific polymers whose solubility parameters can be adjusted via copolymerization with various monomers to detect targeting organic vapors. In this study, we also fabricated a PMMA-infiltrated SiO2 IOPC as a control to detect xylene vapors. The in situ reflectance spectra in the pxylene saturated vapor are shown in Fig. S7. The photonic stopband displays a maximum red-shift of 22 nm, which is much smaller than that of the P(VBC-co-MMA)-SiO2 IOPC (52 nm) at saturated vapor concentration. This can be attributed to the obvious difference in solubility parameters between PMMA (21.7 (J/cm3)1/2) [38] and p-xylene (18.0(J/cm3)1/2). Thus, P(VBC-co-MMA) has a stronger affinity toward p-xylene than that of PMMA and is prone to adsorb more p-xylene molecules, resulting in a larger increase in neff of the IOPC. The results imply that P(VBC-co-MMA) plays a key role in the selective sensing of xylenes. 4. Conclusions We developed a polymer P(VBC-co-MMA) infiltrated SiO2 IOPC sensor for colorimetric detection of xylene vapors. Upon exposure to xylene vapors, the sensor undergoes a photonic stopband red-shift due to the diffusion and adsorption of aromatic organic vapors. This method is based on the strong affinity of the polymer toward xylenes because of their similar solubility parameters. The color of the sensor apparently changes from green to red, in which the stopband red-shifts over 50 nm. The constructed IOPC sensor is fast responsive and reusable by simply exposing the sensor to air. It also shows good sensitivity with a limit of detection of 0.51, 0.41 and 0.17 μg mL−1 for o-xylene, m-xylene, and pxylene, respectively. The sensor shows excellent selectivity over other organic vapors as well. This polymer infiltrated IOPC sensor provides a novel and universal strategy to detect organic vapors by synthesizing polymers with similar solubility parameters to the pre-selected vapors.
Fig. 6. The stopband shifts of P(VBC-co-MMA)-SiO2 IOPC and SiO2 IOPC upon exposure to different solvent saturated vapors.
the air in the IOPC pores, which induces an increase in neff of the IOPC. Notably, stopband red-shifts ∼ 50 nm for P(VBC-co-MMA)-SiO2 IOPC upon exposure to xylene vapors, while it shifts less than 25 nm for other solvents regardless of the IOPCs with or without P(VBC-co-MMA). The results indicate that our sensor is selective toward xylenes. The larger stopband red-shift of the P(VBC-co-MMA)-SiO2 IOPC toward xylenes can be ascribed to the stronger affinity of the polymer to xylenes and higher refractive index of xylenes. For a given polymerinfiltrated IOPC, the photonic stopband is determined by the refractive index of its pores. This is related to the diffusion of the vapor within the IOPC’s pores, the adsorption of the vapor on the pore walls and the refractive index of the vapor within the pores. The adsorption of these organic solvents vapor is strongly affected by the affinity of the polymer toward the solvent, which depends on their solubility parameters. According to the Hildebrand equation, a solvent whose solubility parameter is similar to a polymer usually has a good compatibility with the polymer because of the decreased enthalpy of mixing. Thus, the polymer with a strong affinity toward the solvent is favorable to increase adsorption of the solvent on the pore walls of the IOPC. Larger adsorption and higher refractive index are beneficial to increase neff of the IOPC, inducing a larger stopband red-shift of PC sensors. For this sensor, P(VBC-co-MMA) was introduced into the IOPC. Its solubility parameter is ∼ 18.6 (J/cm3)1/2 [6], which is similar to those of aromatic solvents including benzene, toluene and xylenes. Their solubility parameters are 18.8 (J/cm3)1/2, 18.2(J/cm3)1/2 and 18.0 ∼ 18.5(J/cm3)1/2, respectively. As discussed, all of these aromatic solvents might have strong affinities toward P(VBC-co-MMA). Furthermore, these aromatic solvents have higher refractive indices compared to the other solvents in Fig. 6. Their refractive indices are also close to each other. Table S1 lists the refractive indices and solubility parameters of various solvents. In theory, there might be large stopband redshifts for P(VBC-co-MMA)-SiO2 IOPC sensors upon exposure to any of these aromatic vapors. Actually, only xylenes show the apparent redshifts of ∼ 50 nm, which induces the color change of the sensor. A redshift of ∼ 25 nm occurs for benzene and toluene. This is because xylenes have higher electron density in the benzene ring due to the methyl groups as electron donors [6]. That might induce stronger interactions between P(VBC-co-MMA) and xylenes. For the other solvent vapors, both the large difference of solubility parameter and low refractive index cause the small red-shifts, for example, 9 nm of red-shift for water. As previously discussed, the polymer infiltrated in the IOPC would be sensitive to the solvent if their solubility parameters are close.
Declaration of interest None. Acknowledgements This work was supported by National Natural Science Foundation of China (grant number 21663032); and Natural Science Fundamental Research Program Key Projects of Shaanxi Province (grant number 2016JZ005). Zhongyu Cai is grateful for the support from the joint French-Singaporean MERLION program under Grant No. R-279-000334-133. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.04.036. References [1] R. Kandyala, S.P. Raghavendra, S. Rajasekharan, Xylene: an overview of its health hazards and preventive measures, J. Oral Maxillofac. Pathol. 14 (2010) 1–5, https://doi.org/10.4103/0973-029X.64299. [2] B. Horstkotte, N.L.D. Atochero, P. Solich, Lab-In-Syringe automation of stirringassisted room-temperature headspace extraction coupled online to gas chromatography with flame ionization detection for determination of benzene, toluene, ethylbenzene, and xylenes in surface waters, J. Chromatogr. A 1555 (2018) 1–9, https://doi.org/10.1016/j.chroma.2018.04.055. [3] P. Hoang, N.M. Khashab, Non-resonant large format surface enhanced Raman scattering substrates for selective detection and quantification of xylene isomers, Chem. Mater. 29 (2017) 1994–1998, https://doi.org/10.1021/acs.chemmater. 6b05431. [4] L. Brigo, N. Michieli, L. Artiglia, C. Scian, G.A. Rizzi, G. Granozzi, G. Mattei, A. Martucci, G. Brusatin, Silver nanoprism arrays coupled to functional hybrid films for localized surface Plasmon resonance-based detection of aromatic hydrocarbons, ACS Appl. Mater. Interfaces 6 (2014) 7773–7781, https://doi.org/10.1021/
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Yuqi Zhang received her PhD degree in Physical Chemistry from University of Chinese Academy of Sciences. She has been appointed as a Professor at College of Chemistry and Chemical Engineering, Yan’an University since 2015. Her current research interests include the design and construction photonic crystal chemical and biological sensors. Yimin Sun is a graduate student major in chemistry. Jiaqi Liu is a graduate student major in chemistry. Pu Guo is a graduate student major in chemistry. Zhongyu Cai was a research assistant professor at University of Pittsburgh. Dr. Cai received his PhD degree in 2012 from Department of Chemical and Biomolecular Engineering, National University of Singapore. His research mainly focuses on photonic crystals and their applications in chemical sensing, biosensing, tunable photonic devices and environmental remediation. Ji-Jiang Wang received his PhD degree in Inorganic Chemistry from Northwest University. He has been appointed as a Professor at College of Chemistry and Chemical Engineering, Yan’an University since 2014. His current research interests include the design and construction functional materials and gas sensors.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Polymer-infiltrated SiO2 inverse opal photonic crystals for colorimetrically selective detection of xylene vapors
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Yuqi Zhanga, Yimin Suna, Jiaqi Liua, Pu Guoa, Zhongyu Caib, , Ji-Jiang Wanga a
Key Laboratory of New Energy & New Functional Materials, Shaanxi Key Laboratory of Chemical Reaction Engineering, College of Chemistry and Chemical Engineering, Yan’an University, Yan’an, Shaanxi, 716000, PR China b Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Photonic crystals Inverse opal Functional polymer Colorimetric sensing Selective xylene vapor detection
Xylenes are common aromatic volatile organic compounds, which can cause severe environmental pollution and healthy issues. The current instrumental detection methods are generally expensive, time-consuming, complex, and unable to be used for on-site detection. It is thus highly desirable to develop an efficient method for the detection of xylenes. In this study, we report the development of a polymer infiltrated SiO2 inverse opal photonic crystal (IOPC) for selective detection of xylene vapors. Poly(4-vinylbenzyl chloride-co-methyl methacrylate) (P (VBC-co-MMA)), which shows strong affinity towards xylenes because of their similar solubility parameters, was synthesized and infiltrated into SiO2 IOPC (P(VBC-co-MMA)-SiO2 IOPC). The resulting P(VBC-co-MMA)-SiO2 IOPC sensor shows color change from green to red upon exposure to xylene vapors as the diffusion and adsorption of gaseous xylenes increased the effective refractive index of the IOPC. It shows good sensitivity with a limit of detection of 0.51, 0.41 and 0.17 μg mL−1 for o-xylene, m-xylene, and p-xylene, respectively. The sensor also shows excellent selectivity for xylenes over other organic vapors. In addition, the P(VBC-co-MMA)-SiO2 IOPC sensor demonstrates quick response and good reversibility. This polymer infiltrated IOPC sensor provides a universal strategy for the detection of organic vapors through building a sensor using polymers with similar solubility parameters to targeting vapors.
1. Introduction Xylenes are one kind of common aromatic volatile organic compounds (VOCs), which have been extensively used in fuel, adhesives, additives, and industrial products including synthetic fiber, rubber and plastic. Xylenes are flammable, volatile and harmful, which can cause air pollution and thus damage human health. In recent years, many studies have shown that harmful xylenes can lead to headache, dizziness, dermatitis, anemia, and even cause lung cancer upon long-term exposure [1]. Therefore, it is highly desirable to develop effective methods for the detection of xylenes. Highly sensitive, precise and reliable detection techniques including gas chromatography [2], surface enhanced Raman scattering [3], and surface Plasmon resonance [4] have been reported to detect xylenes. These methods are generally expensive, time-consuming, complex, and unable to be used for on-site detection. Recently, intense efforts have also been devoted to develop different sensors using a variety of sensing materials to detect xylenes, such as polymer-coated quartz crystal microbalance (QCM) sensors [5,6], metal oxide semiconductor
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chemiresistors [7,8], AIE (Aggregation-Induced Emission)-based fluorometric sensors [9] and polydiacetylene paper-based colorimetric sensors [10]. Among these techniques, colorimetric sensing is one of the most convenient methods. This method can provide a straightforward signal for monitoring, which even can be observed by naked eyes. Photonic crystals (PCs) with periodic dielectric structures offer a new opportunity for the development of colorimetric sensors due to the tunable photonic stopband, whose position determines the color of PCs along normal incidence [11–35]. By altering the effective refractive index and/or the diffracting plane spacing distance, various colorimetric sensors based on PCs have been developed for the detection of pH [16–18], metal ions [19,20], anions [21,22], biomolecules [23,24], pesticides [25] and gas/VOCs [26–34]. For example, Lova et al. [26] reported a one-dimensional PC colorimetric sensor that fabricated from poly(p-phenylene oxide) and cellulose acetate to detect VOCs including benzene, 1,2-dichlorobenzene, carbon tetrachloride and toluene. Porous silicon PCs were also developed and used to detect organic vapors including isopropanol, heptane, cyclohexane and ethanol according to the shift of the PC reflectance peak [27,28]. In particular,
Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Cai).
https://doi.org/10.1016/j.snb.2019.04.036 Received 30 January 2019; Received in revised form 20 March 2019; Accepted 7 April 2019 Available online 08 April 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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three-dimensional inverse opal PCs (IOPC), as colorimetric sensing platforms, have attracted much attention due to their interconnected macroporous structures and relatively high specific surface area [29,27–34]. These properties are useful to facilitate the diffusion and adsorption of vapor analytes. In addition, the combination of IOPC with responsive small organic molecules or polymers provides an excellent platform for constructing PC colorimetric sensors. The high specific surface area of IOPC, and the reaction activity or adsorption affinity of responsive molecules toward vapor analytes can greatly improve the sensitivity and selectivity of the resulting sensors when sensing is performed by changing the refractive index of IOPC. For example, an amine-functionalized inverse opal film has been developed for colorimetric detection of CO2 over a wide concentration range [30]. Our group developed several hexaphenylsilole, tetraphenylethene polymer, and poly(2-hydroxyethyl methacrylate-comethyl acrylate) infiltrated SiO2 IOPCs for efficient and visual detection of diethyl ether/petroleum [31], tetrahydrofuran/acetone [32] and volatile alcohols [33], respectively. However, it remains a challenge to construct a highly ordered macroporous structure inverse opal for colorimetric sensing of aromatic VOCs such as xylenes. Here, we report a poly(4-vinylbenzyl chloride-co-methyl methacrylate) (P(VBC-co-MMA))-infiltrated SiO2 IOPC (P(VBC-co-MMA)-SiO2 IOPC) as a colorimetric sensor to detect xylene vapors. The as-constructed IOPC sensor changed from green to red upon exposure to xylene vapors as the diffusion and adsorption of gaseous xylenes increased the effective refractive index of the IOPC. The sensor can recover to its initial color after following exposure to air. This P(VBC-co-MMA)-SiO2 IOPC sensor also showed good sensitivity, excellent selectivity and facile reusability. These results indicate that our IOPC film can be a potential candidate for colorimetric sensing of xylenes.
Scheme 1. Synthesis of P(VBC-co-MMA).
of frozen petroleum ether under vigorous stirring and an insoluble white solid appeared. After stirring for 2 h, the white precipitates were collected through vacuum filtration, followed by washing with petroleum ether for 5 times and subsequently drying overnight under vacuum at 50 ℃. The IR spectrum of P(VBC-co-MMA) (Fig. S1) confirmed the successful synthesis according to the reported literature [6]. 2.3. Fabrication of P(VBC-co-MMA)-SiO2 IOPC The fabrication of P(VBC-co-MMA)-SiO2 IOPC included two steps: preparation of SiO2 IOPC and infiltration of P(VBC-co-MMA) molecules. First, SiO2 IOPC was obtained by using a sacrificial template method [31,32]. As shown in Scheme 2, 20 g of ∼ 0.2 wt% PS microsphere latex was mixed with 0.1 mL of SiO2 colloidal sol, followed by ultrasonication for 30 min. SiO2 sol was prepared by stepwise dropping HCl (0.10 M) into the absolute ethanol solution of tetraethyl orthosilicate (TEOS, 28 wt%) and stirring for 1 h at room temperature (TEOS:C2H5OH:HCl = 1:2:1, weight ratio). A clean glass slide (2.0 cm × 1.0 cm) was vertically placed into the mixture. The PS microspheres self-assembled on the glass surface and SiO2 sol was infiltrated into their interstices upon evaporation at 65 °C for 48 h. The resulting coassembled PC film, denoted as PS-SiO2 PC, was sintered at 90 °C for 1 h and then immersed in THF for 2 h. The PS templates were dissolved and a green SiO2 IOPC formed. Then, 60 μL of 0.5 wt%–2.0 wt% THF solution containing P(VBC-coMMA) was dripped onto the surface of the resulting SiO2 IOPC (size:1.0 cm × 0.65 cm). P(VBC-co-MMA) molecules were infiltrated into the voids of the SiO2 IOPC due to capillary forces. After drying in air, a P(VBC-co-MMA)-SiO2 IOPC sensor for xylenes detection was obtained. A PMMA infiltrated SiO2 IOPC (PMMA-SiO2 IOPC) was also fabricated by dripping 60 μL of 1.5 wt% THF solution containing PMMA onto the surface of the SiO2 IOPC (2.0 cm × 1.0 cm) and used as a control.
2. Experimental 2.1. Chemicals and instruments 4-Vinylbenzyl chloride (VBC, 90%) was purchased from SigmaAldrich and purified with neutral Al2O3 column. Methyl methacrylate (MMA, 99%) was supplied by Sigma-Aldrich and purified by distillation. The initiator, a,a’-azodiisobutyronitrile (AIBN, 99%, J&K Scientific Ltd, Beijing, China) was recrystallized from methanol. Monodisperse polystyrene (PS) colloidal particles with a diameter of 350 nm were acquired from Shanghai Huge Biotechnology Co., Ltd, China. Poly (methyl methacrylate) (PMMA, Mw = 35 000) was purchased from Sinopharm Chemical Reagent Beijing Co. Ltd. All the other analytical reagents were purchased commercially and used as received. Nanopure MilliQ water (18.2 mΩ cm) was utilized for all experiments. A field-emission scanning electron microscope (FESEM, SU8010, Hitachi, Japan) was used to characterize the morphology of the PCs after the samples were sputter-coated with a thin layer of gold. The IR spectrum of P(VBC-co-MMA) was obtained from a FT-IR spectrometer (IR Prestige-21, Shimadzu, Japan). In situ reflectance spectra were recorded with a fiber optic spectrophotometer (PG-2000-Pro-Ex, Ideaoptics Instrument Co., Ltd, China) along normal incidence to the (111) plane of the PCs.
2.4. In situ reflectance spectra measurement The response of the P(VBC-co-MMA)-SiO2 IOPC sensor was allowed to proceed at room temperature. The sensor was fixed in a gas chamber, which has a pinhole. The fiber-optics probe was perpendicular to the surface of the PC sensors. A certain volume of volatile solvents was injected into the gas chamber through the pinhole by a microliter syringe. The in situ reflectance spectra along normal incidence to the (111) plane of the PCs were recorded when the sample was in the vaporous environments of the solvents. SiO2 IOPC, as a control sensor, was also tested when exposed to different vaporous environments. The SiO2 IOPC film was alternately placed in air and saturated p-xylene vapor and the reflectance spectra were recorded to study the reusability of our sensors. The in situ reflectance spectra of PMMA-SiO2 IOPC were also measured upon exposure to p-xylene vapor.
2.2. Synthesis of P(VBC-co-MMA) Scheme 1 shows the synthesis route of P(VBC-co-MMA). P(VBC-coMMA) was synthesized through a free radical polymerization method in which VBC and MMA were used as monomers and AIBN was used as an initiator. In a typical process, 30.3 mg of AIBN was added to a 60 mL of tetrahydrofuran (THF) solution containing 4.5 mL of VBC and 2.7 mL of MMA, followed by magnetic stirring and nitrogen bubbling for 30 min. The solution was allowed to react at 62 ℃ for 48 h under nitrogen bubbling. The resulting reaction mixture was cooled down to room temperature. Then the resulting mixture was dropwise added to 400 mL
3. Results and discussion 3.1. Fabrication and sensing mechanism of P(VBC-co-MMA)-SiO2 IOPC sensor The P(VBC-co-MMA)-SiO2 IOPC sensor consists of polymeric 68
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Scheme 2. Fabrication and sensing mechanism of P(VBC-co-MMA)-SiO2 IOPC.
refractive index, which decrease the neff of PC. After infiltration of P (VBC-co-MMA), the stopband shows a red-shift of 17 nm because small quantities of P(VBC-co-MMA) induces a tiny variation in neff of PC.
molecules of P(VBC-co-MMA) and SiO2 IOPC structure. Scheme 2 shows its fabrication and sensing mechanism. The PS particles latex and SiO2 colloidal sol were mixed and coassembled by vertical deposition at 65 °C to prepare the PS-SiO2 PC films. The co-assembled PC films were immersed in THF to selectively remove the PS particles and then SiO2 IOPC were formed. A THF solution containing P(VBC-co-MMA) was dropped on the SiO2 IOPC. After air drying, P(VBC-co-MMA) molecules infiltrated into the pores and adsorbed on the pore walls of the SiO2 IOPC via capillary forces. P(VBCco-MMA)-SiO2 IOPC sensor was then obtained. The P(VBC-co-MMA)-SiO2 IOPC film can be used to detect xylenes. Upon exposure to the xylene vapors, the reflectance peak of the IOPC undergoes a red-shift due to an increase in the effective refractive index (neff ) of the IOPC. The increase of neff can be attributed to the diffusion and adsorption of xylene vapors in the voids of the IOPC (Scheme 2). This is because xylenes, which have higher refractive index, replace air in the interconnected macroporous structure of the IOPC. P(VBC-coMMA) molecules show a strong affinity toward xylenes due to their similar solubility parameter [6], which will be explained in detail hereinafter. This infiltration of organic vapors causes an increase in the effective refractive index, and thus induces a red-shift of the PC stopband [36,37]. Simultaneously, the color of P(VBC-co-MMA)-SiO2 IOPC changes from green to red upon exposure to xylene vapors. In addition, the sensor can be recycled through exposing it to air due to the volatilization and desorption of xylene vapors away from the P(VBC-coMMA)-SiO2 IOPC. Therefore, our P(VBC-co-MMA)-SiO2 IOPC can be utilized as a reusable and visual sensor for the detection of xylenes.
P(VBC-co-MMA)-SiO2 IOPC sensors containing various infiltration quantities of P(VBC-co-MMA) were fabricated by infiltrating 60 μL of polymer THF solutions at different concentrations. Upon exposure to a saturated vapor of p-xylene, the reflectance spectra of these sensors show different wavelength changes. As shown in Fig. 2, the photonic stopbands of P(VBC-co-MMA)-SiO2 IOPC sensors with different amounts of polymer infilatration in air are at 562 nm. After exposing to p-xylene vapor, their photonic stopbands shift to 589 nm, 597 nm and 614 nm, respectively, with increasing the concentrations of the infiltration solutions from 0.5 wt% to 1.5 wt%. The stopband red-shifts of these sensors are 27 nm, 35 nm and 52 nm, respectively. The results indicate that a higher infiltration quantity of the polymer is benificial to diffusion and adsorption of p-xylene, which induce a larger red-shift. However, a red-shift of 23 nm is observed when further increase the infiltrated polymer solution concentration to 2.0 wt%. This is because too much infiltration of polymer decreases the pore volume of the IOPC. Obviously, the P(VBC-co-MMA)-SiO2 IOPC sensor fabricated from 1.5 wt% polymer solution shows the largest stopband red-shift of 52 nm. Thus, we chose this IOPC as a sensor candidate for the following studies. Here the infiltration quantity of P(VBC-co-MMA) is 0.85 mg.
3.2. Morphology and stopband characterization of P(VBC-co-MMA)-SiO2 IOPC
3.4. Time dependence of reflectance spectra and reversibility of the IOPC sensor
Fig. 1a shows an SEM image of co-assembled PS–SiO2 PC. The PS colloidal spheres within the PS–SiO2 PC arrange orderly with a hexagonally close-packed plane (111) oriented parallel to the substrate. Fig. 1b exhibits an SEM image of the resulting P(VBC-co-MMA)–SiO2 IOPC. An ordered macroporous SiO2 scaffold can be clearly observed. Air pores with a diameter of ∼ 285 nm replace the PS spheres and remain hexagonally close-packed structure. The morphology of P(VBCco-MMA)–SiO2 IOPC does not change significantly compared with that of the SiO2 IOPC (Fig. S2) due to a small quantity of the polymer infiltration, which does not affect the periodic structure. A cross-sectional SEM image (Fig. 1c) of P(VBC-co-MMA)–SiO2 IOPC demonstrates that the air pores in each layer are also highly ordered. The thickness of the sensor film is ∼3.6 μm. Reflectance spectra of the PCs are shown in Fig. 1d. The photonic stopbands of PS-SiO2 PC, SiO2 IOPC and P(VBC-co-MMA)-SiO2 IOPC occur at 764 nm, 546 nm and 562 nm, respectively. Apparently, there is a remarkable stopband blue-shift for SiO2 IOPC compared to the PSSiO2 PC. This is because the PS spheres are replaced by air with a lower
Fig. 3a exhibits that the reflectance spectra of the P(VBC-co-MMA)SiO2 IOPC sensor undergo a stepwise red-shift as the exposure time increases, resulting from continuous increase in the effective refractive index of the pores during diffusion and adsorption of p-xylene. The stopband peak position shifts from 562 nm to 580 nm upon exposure to p-xylene vapor within 1 min. The fast response of this sensor can be attributed to the IOPC’s interconnected macroporous structure with high specific surface area. This facilitates the diffusion and adsorption of gaseous compounds. The stopband shows a maximum red-shift from 562 nm to 612 nm after exposure to p-xylene vapor for 8 min. Subsequently, the sensor was placed in air. A quick blue-shift occurs for the reflectance peak due to the fast volatilization of p-xylene in air (Fig. 3b). The responses of our sensor to o-xylene and m-xylene are similar to that of p-xylene vapor. The maximum stopband red-shifts for o-xylene and m-xylene are 55 nm and 48 nm, respectively (Fig. S3). The reflectance spectra of the senor also return to the initial position upon exposure to air. We alternately place the P(VBC-co-MMA)-SiO2 IOPC in the
3.3. Dependence of stopband red-shift on the infiltration quantity of P(VBCco-MMA)
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Fig. 1. Top view SEM images of (a) PS-SiO2 PC and (b) P(VBC-co-MMA)-SiO2 IOPC; (c) cross-sectional SEM image of P(VBC-co-MMA)-SiO2 IOPC; and (d) the reflectance spectra of the PS-SiO2 PC, SiO2 IOPC and P(VBC-co-MMA)-SiO2 IOPC fabricated from 1.5 wt% THF solution containing P(VBC-co-MMA).
sensor over cycles of p-xylene exposures is shown in Fig. S5. The photonic stopband shift of our sensor accompanies the color change of the P (VBC-co-MMA)-SiO2 IOPC film. Fig. 4b shows the optical photographs of the color changes of our sensor during a period of the response for pxylene vapor. The initial green of P(VBC-co-MMA)-SiO2 IOPC sensor in air converts to red upon exposure to p-xylene vapor. The color change is also reversible, which implies that the P(VBC-co-MMA)-SiO2 IOPC is a good candidate for visual sensing of xylenes. 3.5. Detection sensitivity of xylene vapors The on-line recorded reflectance spectra of the P(VBC-co-MMA)SiO2 IOPC sensor (Fig. 5a) show that larger stopband red-shift occurs at higher concentration of p-xylene vapor. This is due to the larger neff of the IOPC, resulting from diffusion and adsorption of more vapor molecules at higher p-xylene vapor concentrations. A 17 nm stopband redshift is obtained at the concentration of 10.3 μg mL−1. The red-shift in stopband position increases to 45 nm at 34.4 μg mL−1 and reaches to its maximum of 52 nm at 43.1 μg mL−1. The stopband position shifts of our P(VBC-co-MMA)-SiO2 IOPC sensor are plotted as a function of p-xylene vapor concentrations, as shown in Fig. 5b. A very good linear correlation with R2 = 0.9975 is obtained. The limit of detection (LOD) for pxylene is 0.17 μg mL−1 (39 ppm), based on 3 SD/S. Here SD is the standard deviation of the response and S is the slope of the calibration curve. The LOD of our P(VBC-co-MMA)-SiO2 IOPC sensor (LOD = 39 ppm) is comparable with the QCM sensor reported earlier (LOD = 54 ppm) [6]. But our sensor is slightly more sensitive and much cheaper than that of the QCM sensor, which can be ascribed to the unique structure of IOPC and its ease of fabrication. The reflectance spectra and stopband position shifts of the sensor exposed to different concentrations of o-xylene (LOD: 0.51 μg mL−1, 117 ppm) and m-xylene (LOD:
Fig. 2. Reflectance spectra of P(VBC-co-MMA)-SiO2 IOPCs fabricated from different concentrations of THF solutions containing P(VBC-co-MMA) upon exposure to air and the saturated vapor of p-xylene, respectively.
atmosphere of xylenes and air for 10 cycles. The stopband position changes are shown in Fig. 4a (p-xylene vapor) and Fig. S4 (o-xylene and m-xylene vapor) in Supplementary Information. The stopband occurs at ∼ 614 nm upon exposure to xylene vapors, and shifts to ∼ 562 nm in air. For xylene vapors, entering into and escaping out of the IOPC pores of the sensor changes the neff of the IOPC, and the photonic stopband shifts correspondingly. The stopband position change is reversible, which implies that our P(VBC-co-MMA)-SiO2 IOPC has an excellent stability and can be used repeatedly. Reflectance spectra of the IOPC 70
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Fig. 3. Time dependence of reflectance spectra for P(VBC-co-MMA)-SiO2 IOPC sensor upon exposure to (a) p-xylene saturated vapor and (b) the air subsequently. Fig. 5. Dependence of (a) reflectance spectra and (b) stopband shifts of the P (VBC-co-MMA)-SiO2 IOPC on concentrations of p-xylene vapor.
0.41 μg mL−1, 94 ppm) vapors are shown in Fig. S6. Regarding to its practical application for environmental monitoring, our P(VBC-coMMA)-SiO2 IOPC sensor still needs to be optimized in the future.
investigate the selectivity of our sensor toward xylenes. The spectra responses of blank SiO2 IOPC were also monitored as a control. As shown in Fig. 6, both P(VBC-co-MMA)-SiO2 IOPC and SiO2 IOPC demonstrate stopband red-shifts for all the detected vapors. This is because the vapors of those solvents with higher refractive indices replace
3.6. Selectivity of the sensor toward xylenes The photonic stopband shifts of the P(VBC-co-MMA)-SiO2 IOPC sensor exposed to saturated vapors of various solvents were recorded to
Fig. 4. (a) Reversible changes of maximum stopband position for the P(VBC-co-MMA)-SiO2 IOPC when alternately exposed to saturated p-xylene vapor and air, and (b) the corresponding optical photographs of color changes. 71
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Therefore, we can design and synthesize specific polymers whose solubility parameters can be adjusted via copolymerization with various monomers to detect targeting organic vapors. In this study, we also fabricated a PMMA-infiltrated SiO2 IOPC as a control to detect xylene vapors. The in situ reflectance spectra in the pxylene saturated vapor are shown in Fig. S7. The photonic stopband displays a maximum red-shift of 22 nm, which is much smaller than that of the P(VBC-co-MMA)-SiO2 IOPC (52 nm) at saturated vapor concentration. This can be attributed to the obvious difference in solubility parameters between PMMA (21.7 (J/cm3)1/2) [38] and p-xylene (18.0(J/cm3)1/2). Thus, P(VBC-co-MMA) has a stronger affinity toward p-xylene than that of PMMA and is prone to adsorb more p-xylene molecules, resulting in a larger increase in neff of the IOPC. The results imply that P(VBC-co-MMA) plays a key role in the selective sensing of xylenes. 4. Conclusions We developed a polymer P(VBC-co-MMA) infiltrated SiO2 IOPC sensor for colorimetric detection of xylene vapors. Upon exposure to xylene vapors, the sensor undergoes a photonic stopband red-shift due to the diffusion and adsorption of aromatic organic vapors. This method is based on the strong affinity of the polymer toward xylenes because of their similar solubility parameters. The color of the sensor apparently changes from green to red, in which the stopband red-shifts over 50 nm. The constructed IOPC sensor is fast responsive and reusable by simply exposing the sensor to air. It also shows good sensitivity with a limit of detection of 0.51, 0.41 and 0.17 μg mL−1 for o-xylene, m-xylene, and pxylene, respectively. The sensor shows excellent selectivity over other organic vapors as well. This polymer infiltrated IOPC sensor provides a novel and universal strategy to detect organic vapors by synthesizing polymers with similar solubility parameters to the pre-selected vapors.
Fig. 6. The stopband shifts of P(VBC-co-MMA)-SiO2 IOPC and SiO2 IOPC upon exposure to different solvent saturated vapors.
the air in the IOPC pores, which induces an increase in neff of the IOPC. Notably, stopband red-shifts ∼ 50 nm for P(VBC-co-MMA)-SiO2 IOPC upon exposure to xylene vapors, while it shifts less than 25 nm for other solvents regardless of the IOPCs with or without P(VBC-co-MMA). The results indicate that our sensor is selective toward xylenes. The larger stopband red-shift of the P(VBC-co-MMA)-SiO2 IOPC toward xylenes can be ascribed to the stronger affinity of the polymer to xylenes and higher refractive index of xylenes. For a given polymerinfiltrated IOPC, the photonic stopband is determined by the refractive index of its pores. This is related to the diffusion of the vapor within the IOPC’s pores, the adsorption of the vapor on the pore walls and the refractive index of the vapor within the pores. The adsorption of these organic solvents vapor is strongly affected by the affinity of the polymer toward the solvent, which depends on their solubility parameters. According to the Hildebrand equation, a solvent whose solubility parameter is similar to a polymer usually has a good compatibility with the polymer because of the decreased enthalpy of mixing. Thus, the polymer with a strong affinity toward the solvent is favorable to increase adsorption of the solvent on the pore walls of the IOPC. Larger adsorption and higher refractive index are beneficial to increase neff of the IOPC, inducing a larger stopband red-shift of PC sensors. For this sensor, P(VBC-co-MMA) was introduced into the IOPC. Its solubility parameter is ∼ 18.6 (J/cm3)1/2 [6], which is similar to those of aromatic solvents including benzene, toluene and xylenes. Their solubility parameters are 18.8 (J/cm3)1/2, 18.2(J/cm3)1/2 and 18.0 ∼ 18.5(J/cm3)1/2, respectively. As discussed, all of these aromatic solvents might have strong affinities toward P(VBC-co-MMA). Furthermore, these aromatic solvents have higher refractive indices compared to the other solvents in Fig. 6. Their refractive indices are also close to each other. Table S1 lists the refractive indices and solubility parameters of various solvents. In theory, there might be large stopband redshifts for P(VBC-co-MMA)-SiO2 IOPC sensors upon exposure to any of these aromatic vapors. Actually, only xylenes show the apparent redshifts of ∼ 50 nm, which induces the color change of the sensor. A redshift of ∼ 25 nm occurs for benzene and toluene. This is because xylenes have higher electron density in the benzene ring due to the methyl groups as electron donors [6]. That might induce stronger interactions between P(VBC-co-MMA) and xylenes. For the other solvent vapors, both the large difference of solubility parameter and low refractive index cause the small red-shifts, for example, 9 nm of red-shift for water. As previously discussed, the polymer infiltrated in the IOPC would be sensitive to the solvent if their solubility parameters are close.
Declaration of interest None. Acknowledgements This work was supported by National Natural Science Foundation of China (grant number 21663032); and Natural Science Fundamental Research Program Key Projects of Shaanxi Province (grant number 2016JZ005). Zhongyu Cai is grateful for the support from the joint French-Singaporean MERLION program under Grant No. R-279-000334-133. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.04.036. References [1] R. Kandyala, S.P. Raghavendra, S. Rajasekharan, Xylene: an overview of its health hazards and preventive measures, J. Oral Maxillofac. Pathol. 14 (2010) 1–5, https://doi.org/10.4103/0973-029X.64299. [2] B. Horstkotte, N.L.D. Atochero, P. Solich, Lab-In-Syringe automation of stirringassisted room-temperature headspace extraction coupled online to gas chromatography with flame ionization detection for determination of benzene, toluene, ethylbenzene, and xylenes in surface waters, J. Chromatogr. A 1555 (2018) 1–9, https://doi.org/10.1016/j.chroma.2018.04.055. [3] P. Hoang, N.M. Khashab, Non-resonant large format surface enhanced Raman scattering substrates for selective detection and quantification of xylene isomers, Chem. Mater. 29 (2017) 1994–1998, https://doi.org/10.1021/acs.chemmater. 6b05431. [4] L. Brigo, N. Michieli, L. Artiglia, C. Scian, G.A. Rizzi, G. Granozzi, G. Mattei, A. Martucci, G. Brusatin, Silver nanoprism arrays coupled to functional hybrid films for localized surface Plasmon resonance-based detection of aromatic hydrocarbons, ACS Appl. Mater. Interfaces 6 (2014) 7773–7781, https://doi.org/10.1021/
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Yuqi Zhang received her PhD degree in Physical Chemistry from University of Chinese Academy of Sciences. She has been appointed as a Professor at College of Chemistry and Chemical Engineering, Yan’an University since 2015. Her current research interests include the design and construction photonic crystal chemical and biological sensors. Yimin Sun is a graduate student major in chemistry. Jiaqi Liu is a graduate student major in chemistry. Pu Guo is a graduate student major in chemistry. Zhongyu Cai was a research assistant professor at University of Pittsburgh. Dr. Cai received his PhD degree in 2012 from Department of Chemical and Biomolecular Engineering, National University of Singapore. His research mainly focuses on photonic crystals and their applications in chemical sensing, biosensing, tunable photonic devices and environmental remediation. Ji-Jiang Wang received his PhD degree in Inorganic Chemistry from Northwest University. He has been appointed as a Professor at College of Chemistry and Chemical Engineering, Yan’an University since 2014. His current research interests include the design and construction functional materials and gas sensors.
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