Mode-filtered light methane gas sensor based on cryptophane A

Mode-filtered light methane gas sensor based on cryptophane A

Analytica Chimica Acta 633 (2009) 238–243 Contents lists available at ScienceDirect Analytica Chimica Acta journal homepage: www.elsevier.com/locate...

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Analytica Chimica Acta 633 (2009) 238–243

Contents lists available at ScienceDirect

Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

Mode-filtered light methane gas sensor based on cryptophane A Suozhu Wu a , Yan Zhang a , Zhongping Li a , Shaomin Shuang a,∗ , Chuan Dong a , Martin M.F. Choi b,∗∗ a b

Research Center of Environmental Science and Engineering, School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China

a r t i c l e

i n f o

Article history: Received 10 July 2008 Received in revised form 24 October 2008 Accepted 21 November 2008 Available online 30 November 2008 Keywords: Mode-filtered light Methane Cryptophane A

a b s t r a c t A mode-filtered light sensor has been developed for methane (CH4 ) gas determination at ambient conditions. The proposed chemosensor consisted of an annular column which was constructed by inserting an optical fiber coated with a thin silicone cladding of cryptophane A into a fused-silica capillary. When CH4 was introduced to the sensor, selective inclusion of CH4 into the silicone layer would cause a change in the local refractive index of the cladding, resulting in the change of mode-filtered light that emanated from the fiber. Three detection windows were set alongside the capillary to propagate the light to a charge-coupled device (CCD). The changes of mode-filtered light on exposure to various concentrations of CH4 were thus simultaneously monitored. The mode-filtered light intensity decreased with the increase in concentration of CH4 . The dynamic concentration range of the sensor for CH4 was 0.0–16.0% v/v with a detection limit of 0.15% v/v. The highest sensitivity was found at the channel furthest away from the excitation light source. The response time (t95% ) was about 5 min. The reproducibility was good with a relative standard deviation (RSD) of less than 7% from evaluating six cryptophane A-coated fibers. Oxygen, hydrogen and carbon dioxide showed very little interference on detection but interferences from dichloromethane and carbon tetrachloride were observed. The proposed mode-filtered light sensor has been successfully applied to determine CH4 samples and the accuracy was good. Our work offers a promising approach for CH4 detection. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Methane (CH4 ) detection continues to attract much attention because of its highly explosive property and its contribution to global warming. Several techniques have been developed for CH4 detection including infrared (IR) spectroscopy [1,2], electrochemical method [3], metal oxide semiconductor detectors [4], and fiber-optic sensors [2,5,6]. Among these techniques, fiber-optic sensors possess the advantages of small size, low cost in fabrication and operation, possibility of remote monitoring and selective monitoring, safe operation in hazardous environments, and immunity to electromagnetic field interference. Indeed, fiber-optic sensors are very promising tools for real applications. However, most of these sensors have high background transmitted lights which lead to lower signal-to-noise ratio (S/N). By contrast, other fiber-optic sensors based on mode-filtered light detection were firstly designed by Synovec and co-workers [7–9]. The mode-filtered light that emanated from the fiber was collected by a detector positioned alongside the fiber. Comparatively, this type of sensors substantially

∗ Corresponding author. Tel.: +86 351 7018842; fax: +86 351 7018613. ∗∗ Corresponding author. Tel.: +852 34117839; fax: +852 34117348. E-mail addresses: [email protected] (S. Shuang), [email protected] (M.M.F. Choi). 0003-2670/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2008.11.051

increases the S/N. Since the inception of this sensing technique, it has been widely applied to various liquid samples, such as aromatic and chlorinated hydrocarbons [7–10], ethanol [11,12], glucose and glycerol [13], acetic acid [14], and amino acids [15,16]. So far, only a few gas samples were studied at steady-state concentrations by heated annular column [9]. Maybe it is mainly due to the fact that gas medium is a thousand-fold more dilute than liquid medium, making it much more difficult to monitor the trace gaseous components. As such, the successful fabrication of analyte-sensitive cladding materials becomes very crucial in enhancing the chemical selectivity and sensitivity for detection of a chemical compound or a class of compounds [7]. Since CH4 is a fairly inert chemical compound, it is not easy to identify appropriate cladding materials to fabricate mode-filtered light CH4 sensors. It has been reported that cryptophanes can complex with neutral molecules like halogenomethanes in organic solvents and water [17]. Cryptophanes are molecular hosts of primary importance for the transport of apolar guests as well as for understanding the interactions between a host and a neutral guest in solution [18]. Among the cryptophane family, cryptophane A as shown in Fig. 1 exhibits an amazing affinity towards CH4 in organic solution [19]. In addition, cryptophane A has been deposited onto an optical fiber to fabricate an evanescent wave CH4 sensor. Unfortunately, the detection limit for this sensor is still relatively high (2% v/v CH4 ) [6].

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Fig. 1. Chemical structure of cryptophane A.

In this work, we report the use of cryptophane A for modefiltered light sensing CH4 gas at ambient conditions. An annular column CH4 sensor was constructed from an optical fiber coated with a thin silicone cladding of cryptophane A and a fused-silica capillary. Our proposed mode-filtered light sensor shows good sensitivity to CH4 and lower limit of detection. It has been successfully applied to the determination of CH4 in gas samples.

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Fig. 2. Schematic diagram of the mode-filtered light methane sensor. I0 : incidence light beam from laser, IT : transmitted light, IF : mode-filtered light beam detected by CCD, and 1–3: three selected detection windows.

2. Experimental

were evaluated and captured by a LEO 1530 field emission scanning microscope equipped with an energy dispersive X-ray spectrometer (LEO Elektronenmikroskopie, GmbH, Oberkochen, Germany). Finally, the cryptophane A-coated fiber was inserted into a 21cm fused-silica capillary to construct an annular column for modefiltered light sensing. Three small sections (each 4 mm long and 30 mm apart) of protective layer of the capillary were removed to setup three detection channels.

2.1. Materials and chemicals

2.4. Mode-filtered light sensor

An optical fiber (core diameter of 300 ␮m and numerical aperture of 0.37) with protective polymer jacket and a fused-silica capillary (inner diameter of 530 ␮m) were bought from Polymicro Technologies (Phoenix, AZ, USA). Cryptophane A was synthesized according to a literature method [6]. Siloprene K1000 and Siloprene Crosslinking Agent K-11 were bought from Fluka Chemie GmbH (Buchs, Switzerland). High purity nitrogen (N2 , 99.99% v/v), oxygen (O2 , 99.99% v/v), hydrogen (H2 , 99.99% v/v), carbon dioxide (CO2 , 99.99% v/v) and various standards of CH4 gases (0.25, 1.0, 3.0, 5.0, 10.0, and 16.0% v/v) balanced with N2 were purchased from Taiyuan Steel Factory (Taiyuan, Shanxi, China). Samples of CH4 gases containing various interferents (i.e., CH2 Cl2 or CCl4 ) were prepared by mixing one stream of CH4 in N2 with another stream of the respective interferent in N2 at specific ratios. All other chemicals of analytical reagent grade were used without further purification.

A schematic diagram of the mode-filtered light sensor is depicted in Fig. 2. A helium–neon laser light source (1.5 mW, stock No. 111-346, RS Components, Hong Kong, China) with wave output at 635 nm was used to couple light into the flat end of the fiber that was supported by an optical micro-adjustor. The three detection windows were 97 mm, 127 mm and 157 mm away from the flat end of the fiber where the incidence light beam was focused. Channel 1 was the closest to the laser source and channel 3 was the farthest. Three plastic optical fibers (core diameter of 1.00 mm and overall diameter of 2.25 mm from RS Components) were set beside the detection windows to collect the mode-filtered lights emanating from the capillary. The plastic fibers of the three channels waveguided the mode-filtered lights to a linear array charge-coupled device (CCD) (Toshiba, Tokyo, Japan). The signals collected were processed by a data acquisition system (LHL Corporation, Tianjin, China) and a personal computer. The CCD has a maximum signal intensity of 4096 units and contains 2160 elements. The whole detection system was housed in a laboratory-made dark box to minimize stray light.

2.2. Preparation of silicone coating solution The silicone coating solution used in this work was prepared according to the Benounis et al.’s method with slight modification [6]. In brief, 10 mg cryptophane A solid powder was dissolved in a 0.60-mL mixture solution of tetrahydrofuran and dichloromethane (CH2 Cl2 ) (v/v 1:1) and the mixture was shaken until a single-phase solution was obtained. Then it was mixed with 0.10 mL Siloprene K1000 and 0.015 mL Siloprene Crosslinking Agent K-11. 2.3. Preparation of annular column A 25-cm long optical fiber with its protective polymer jacket removed was dipped into a 3.0% w/v potassium dichromate/sulfuric acid solution for 48 h at room temperature to remove the cladding. It was then cleansed and washed with copious amounts of distilled water, alcohol and acetone successively and dried at 100 ◦ C for 15 min. The fiber was dip-coated with a thin layer of cryptophane A/silicone at a withdrawal speed of 12 cm min−1 [6]. About 20 cm of the fiber was coated with cryptophane A/silicone. Scanning electron micrographs (SEM) and energy dispersive X-ray spectra (EDS) of the fibers with and without cryptophane A/silicone coating

2.5. Detection The laser beam irradiated the exposed end of the fiber. The optimal incidence angle was set at 6◦ . Various concentrations of standard CH4 (0.0–16.0% v/v) gas were introduced to the annular column from low to high at a fixed flow rate of 0.25 L h−1 . At the same time, the mode-filtered light signals from each detection channel were captured and stored in the computer for data processing. 3. Results and discussion 3.1. Characterization of the silicone cladding with cryptophane A by SEM, EDS and IR spectroscopy Small sections of fibers with and without cryptophane A/silicone coating were cut and assessed by SEM and EDS as shown in Fig. 3. It is clear that silicon and oxygen present as silicon dioxide (SiO2 )

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Fig. 3. SEM images and EDS of (a) bare fiber and (b) fiber coated with cryptophane A/silicone. Insets display the elemental compositions of the bare fiber and coating, respectively. Note that hydrogen was not included in the calculation as EDS cannot accurately measure its signal.

in the bare fiber (Fig. 3a). By contrast, a thin layer of coating having the composition of 36.98% carbon, 24.90% oxygen and 38.12% silicon w/w was coated on the fiber as depicted in Fig. 3b. Note that hydrogen was not included in the calculation as EDS cannot accurately measure its signal. The carbon content was mainly contributed by cryptophane A in the silicone coating. The EDS confirms the presence of cryptophane A (as carbon)/silicone (as oxygen and silicon) on the fiber. In addition, the IR absorption spectra of a bare glass and a glass coated with silicone cladding/cryptophane A were recorded. Fig. 4 displays the differential IR absorption spectrum of cryptophane A in silicone cladding after subtraction of its background (i.e., IR absorption of bare glass surfaces). Strong absorption peaks of cryptophane A at 2870, 2964 and 2982 cm−1 were observed which are consistent with literature results [18]. The shoulder absorption peak at 2998 cm−1 was assigned to the phenyl hydrogen of cryptophane A. These results indicated that the cladding with cryptophane A could be coated onto an optical fiber.

is introduced to the mode-filtered light sensor, selective partitioning of analyte into the cladding will cause a local change in the critical angle at the core/clad interface, and thus light initially propagating down an optical fiber will be mode-filtered from that fiber.

3.2. Response of the sensor to methane The theory for mode-filtered light detection was discussed in detail by Synovec and co-workers [7–9]. In essence, when a sample

Fig. 4. Differential infrared spectrum of the silicone cladding with cryptophane A coated on a glass surface.

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Fig. 6. Calibration curves of the mode-filtered light sensor at the three detection channels. Fig. 5. Response curves of the mode-filtered light methane sensor at the three detection channels: (a) 0.0, (b) 1.0, (c) 5.0, (d) 10.0, and (e) 16.0% v/v methane.

Although some background signal is scattered from the fiber core, the signal intensity is the change in the amount of light that can be mode-filtered with respect to the analyte concentration. The theoretical equation for the change of the mode-filtered light intensity IF is formulated as

   ˛K Cv,m  IF =  d  n2 (n − n2 ) n NA

(1)

1

where ˛ is a proportionality constant, Kd is the partition coefficient of the analyte (in here, CH4 ), Cv,m is the volume fraction of the analyte in the mobile phase at the point of detection, n1 is the refractive index of the fiber core, NA is the numerical aperture of the fiber optic, n2 is the refractive index of cladding (in here, cryptophane A/silicone coating), and n is the refractive index of the interacting analyte species. Eq. (1) shows that the change of mode-filtered light intensity IF is related to the refractive index of the cladding n2 which is modulated by the partition of analyte with the refractive index of n when ˛, Kd , Cv,m , n1 and NA are constant. When CH4 partitions in the silicone cladding with cryptophane A (n2 ≈ 1.42), the refractive index of the silicone cladding which is larger than that of CH4 (n ≈ 1.00) [20] will increase, resulting in the decrease in [n2 (n − n2 )] value [6]. In other words, when the mode-filtered light sensor is exposed to CH4 , the mode-filtered light intensity will decrease. Thus, the change of mode-filtered light intensity can be employed to determine CH4 . Generally speaking, the CCD is a detector which can record the response at any position along the whole optical fiber and has great potential in the development of multi-channel optical sensing systems. Owing to limitations of the software in processing the signals and the size of the CCD, only a few detection channels are selected [12]. In our work, there are three detection channels for collecting the mode-filtered light intensity. Fig. 5 depicts the typical response of the mode-filtered light sensor to various concentrations of CH4 at the three detection channels. The incidence angle was at 6◦ and the sample gas flow rate was 0.25 L h−1 (vide infra). The modefiltered light intensity at each detection channel decreased with the increase in CH4 concentration. Furthermore, the signal intensities of the three channels decreased successively from channel 1 to channel 3 even at same concentration of CH4 . The channel furthermost from the laser source received the lowest light intensity. Channel 1 received the highest signal because the intensity of the incidence light beam propagating along the fiber diminished at each internal reflection on the interface between the fiber and cladding. Each channel showed very similar response behavior to CH4 . When the

fiber was coated only with silicone cladding, no response to CH4 was observed. Obviously, the response of the sensor to CH4 was mainly attributed to the local change in the critical angle at the core/clad interface [7] caused by the selective inclusion of CH4 into the silicone cladding/cryptophane A. The response of the sensor to CH4 is arbitrarily defined as [12,13] R=

IN2 ICH4

(2)

where IN2 and ICH4 are the mode-filtered signal obtained at N2 and various concentrations of CH4 , respectively. Fig. 6 displays the calibration curves by plotting R against concentration of CH4 at the three detection channels. All the calibration curves exhibit similar response pattern showing fair linearity at the range 5–16% v/v CH4 . Channel 3 shows the highest sensitivity to CH4 as it was furthest away from the laser source and had the lowest background. The dynamic concentration range for CH4 was 0.0–16.0% v/v. Fig. 7 displays the real-time response curve of the sensor on exposure to a low concentration of CH4 (0.25% v/v). The limit of detection calculated from Fig. 7 was approximately 0.15% v/v according to a S/N ratio of 3. 3.3. Effect of the incidence angle The incidence angle of the laser beam can affect the detection as sufficient mode-filtered light intensity has to be provided for the CCD. As such, the selection of an optimal incidence angle is

Fig. 7. Time-dependent signal trace of the sensor from no methane to a low concentration of methane: (1) N2 and (2) 0.25% v/v methane.

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Fig. 8. Effect of the incidence angle on the mode-filtered light intensity at the three detection channels.

crucial. Fig. 8 presents the effect of the incidence angle of the laser beam on the signal intensity at the three detection channels under a stream of N2 . Too small or large incidence angles cause drastic drop in the signal intensity at each detection channel. When the incidence angle is very small, the number of the mode of the light beam propagating in the fiber will be very small and less light beam can reach each channel, leading to a decrease in the signal. As the incidence angle increases, more incidences light will propagate in the fiber leading to a concomitant increase in the signal. On the other hand, when the incidence angle is too large, the number of total internal reflections will increase excessively before the light beam reaches each channel. An optimal incidence angle of 6◦ , which produced the highest signal at each channel, was chosen for this work according to a literature method [14]. 3.4. Effect of flow rate It is known that stability of the optical fiber in the capillary is very essential to achieve a lower noise level and higher detection sensitivity for mode-filtered light detection. Therefore, an appropriate flow rate should be selected to prevent the fiber from vibration. When the flow rate of the gas sample was higher than 0.45 L h−1 , the noise level would be higher. When the flow rate was too low, the response time would become longer. Therefore, the optimal flow rate of 0.25 L h−1 was chosen as it produced the lowest noise and reasonable response time in our detection. In addition, the time taken for CH4 to dissolve in the silicone coating and diffuse to cryptophane A to form an inclusion complex of cryptophane A–CH4 determines the response rate of the sensor. This complex is stabilized both enthalpically (H◦ = –6.7 kJ mol−1 ) and entropically (S◦ = +17 K mol−1 K−1 ) and it is likely that the host spacer bridges adopt the most stable gauche conformation [21]. As a result, the total time taken to reach a steady signal is relatively long and the response time (t95% ) defined as the 95% signal intensity change of the detection was about 5 min. 3.5. Reproducibility of the sensor To test the reproducibility of the sensor, six fibers were coated with silicone cladding/cryptophane A under the same experimental conditions. N2 and 3.0% v/v CH4 were introduced to the sensor to determine the R value. The mean R value was 1.07 at channel 1 with a relative standard deviation (RSD) of 4.7%, 1.08 at channel 2 with a RSD of 6.2%, and 1.09 at channel 3 with a RSD of 3.9%. These results indicate that the reproducibility of our mode-filtered light sensor is good.

Fig. 9. Storage stability of the fiber coated with silicone/cryptophane A. Plot of modefiltered light intensity against incidence angle of light beam. (a) Freshly prepared coated fiber and (b) coated fiber after 6-month storage at ambient conditions.

3.6. Storage stability of the silicone/cryptophane A-coated fiber The aging of the silicone cladding/cryptophane A-coated fiber was studied by comparing the figures of the mode-filtered light intensity versus the incidence angle of a new fiber and of the same after 6-month storage at ambient conditions (Fig. 9). The modefiltered light intensity did not change much and ambient humidity and temperature did not show any effect on the response to CH4 after 6-month of storage. The results demonstrate that the proposed sensor shows excellent shelf-life and stability. 3.7. Interference test The selectivity of the sensor was determined by exposing it to O2 , H2 and CO2 under optimal experimental conditions. The results are summarized in Table 1. The change in the signal was calculated as the CH4 concentration equivalence (i.e., the concentration of CH4 that can produce the same change in the signal as the interferents). It was found that common potential interferents such as O2 , H2 and CO2 did not interfere significantly the response of the sensor. However, CH2 Cl2 and CCl4 interfere the response of the sensor (Table 1). This is probably due to the fact that CH2 Cl2 (n = 1.4244) and CCl4 (n = 1.4607) have higher refractive indices than that of cryptophane A/silicone cladding (n2 ≈ 1.42), producing a positive [n2 (n − n2 )] value which in turn increases the mode-filtered light intensity according to Eq. (1). Fortunately, this effect is not significant as far as the concentrations of these vapors are no larger than 10% v/v. 3.8. Analysis of CH4 sample Known concentrations of CH4 sample gases with or without interferents were applied to the mode-filtered light sensor and Table 1 Effect of potential interferents on the sensor. Interferent (% v/v)

Signal change equivalent to [CH4 ] (% v/v)a

RSD (%)

O2 (99.99) H2 (99.99) CO2 (99.99) CH2 Cl2 (30.15) CCl4 (13.16)

−0.10 0.29 −0.30 −2.80 −2.37

4.0 5.0 2.6 5.9 4.3

a An average of three determinations. A negative value means an increase in the mode-filtered light intensity of the sensor.

S. Wu et al. / Analytica Chimica Acta 633 (2009) 238–243 Table 2 Determination of methane samples with and without interferents using the modefiltered light sensor. Sample CH4 concentration (% v/v)

Interferent added

CH4 found (% v/v)a

RSD (%)

3.0

– 10% v/v H2 97% v/v air 10% v/v CH2 Cl2 10% v/v CCl4

3.15 2.58 2.71 −0.61 −1.82

3.7 0.4 1.9 2.1 1.2

5.0

– 10% v/v H2 10% v/v CH2 Cl2 10% v/v CCl4

4.90 4.50 −0.32 −0.64

8.9 1.6 3.6 0.7

a An average of three determinations. A negative value means an increase in the mode-filtered light intensity of the sensor.

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proposed method allows flexibility in choosing different detection points of various sensitivities. The analytical feature and characteristic of our proposed method as compared to an evanescent wave sensor [6] using cryptophane A is summarized in Table 3. In essence, our proposed sensor is reproducible, accurate and has long shelflife. There is almost no interference from O2 , H2 and CO2 on the detection but CH2 Cl2 and CCl4 can cause some interferences. The analytical performance of the sensor makes it promising for CH4 detection in coal mine samples. Our results are encouraging and strongly suggest that other claddings incorporated with suitable host compounds can also be applied to determine other neutral gases. Acknowledgements

Characteristics

Mode-filtered light sensor

Evanescent wave sensor [6]

This work was supported by the Key Project of National Natural Science Foundation of China (50534100), the Project of Taiyuan Science and Technology Bureau of Shanxi Province and the Foundation for Returned Overseas Chinese Scholars of Shanxi Province. We express our sincere thanks to Mr. Tommy W. H. Poon for taking the SEM and EDS of the fibers.

Light intensity monitored Change of signal with CH4 concentration Response time (min) Detection limit (% v/v)

IF Decrease

IT Increase

References

ca. 5 0.15

2–3 2

Table 3 Comparison of the analytical feature and characteristic between the mode-filtered light sensor and an evanescent wave sensor.

their concentrations were determined and depicted in Table 2. The results demonstrate that the mode-filtered light sensor offers an excellent, accurate and precise method for the determination of gaseous CH4 sample with almost no interferences from common gases such as O2 , H2 and CO2 . However, when a larger amount of CH2 Cl2 or CCl4 (≥10% v/v) are present in the CH4 sample, they can cause significant effect on the determination of CH4 . Fortunately, these vapors are not normally found in coal mine gaseous samples; thus, this should not induce any potential interference on CH4 detection. 4. Conclusion A simple mode-filtered light sensor has been developed for CH4 gas sensing at ambient conditions. The response of the sensor to CH4 was determined by the light mode-filtered from the fiber. It was found that the mode-filtered light intensity decreased with the increase in CH4 concentration. Although the use of cryptophane A for CH4 sensing has been reported in the literature [6], our proposed method still possesses certain attractive features. First, our sensor has lower limit of detection. Second, our method uses lower baseline intensity which means better sensitivity of detection. Third, our

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