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Highly sensitive supra-molecular thin films for gravimetric detection of methane
Sensors and Actuators B 161 (2012) 954–960
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Highly sensitive supra-molecular thin films for gravimetric detection of methane Amir H. Khoshaman a , Paul C.H. Li b , Nabyl Merbouh b , Behraad Bahreyni a,∗ a b
School of Engineering Science, Simon Fraser University, Surrey, BC, Canada Department of Chemistry, Simon Fraser University, Burnaby, BC, Canada
a r t i c l e
i n f o
Article history: Received 30 August 2011 Received in revised form 16 November 2011 Accepted 26 November 2011 Available online 8 December 2011 Keywords: Methane Gravimetric sensors Supra-molecular Cryptophanes Electrospinning Spin coating
a b s t r a c t This report discusses two deposition methods for the preparation and deposition of thin films of Cryptophane A for gravimetric detection of methane. The first method relies on electrospinning of Cryptophane A with carrier polymers. The second technique is based on spin coating of a mixture of Cryptophane A, tetrahydrofuran, and SU8-3000 followed by curing. The required Cryptophane A molecules were synthesized using an improved 2-step trimerization method. Both methods offer good control over the uniformity and thickness of the deposited layers. Moreover, both techniques improved the performance of the gravimetric sensor due to the higher possible quality factors of the resonant sensors. The responses of methane sensors based on both methods were characterized using various concentrations of methane. The prototype sensors had an estimated detection limit of about 3 ppm with a sensitivity of 80 mHz/ppm. Both parameters show improvement over the previously reported works. Our research demonstrates that electrospinning of Cryptophane A films generally results in a higher quality factor for the gravimetric sensors, and thus, improved sensor resolution. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Measurement of methane concentration is important in several applications. Methane is the main ingredient of natural gas which is ubiquitously used in industry and residences for heating purposes. Furthermore, methane is abundantly produced by oxidation of organic compounds in nature or landfills. Methane is a significant contributor to greenhouse effect whose adverse effects can be as much as 20 times worse than carbon dioxide [1]. Methane is an odorless, colorless gas with a Lower Explosive Limit (LEL) concentration of about 5% by volume in air. Therefore, early detection of methane accumulation is particularly important in closed environments such as mines or landfills. Consequently, long-term measurement of methane concentration is crucial for health and safety reasons, ecosystem monitoring, and in industrial applications. A common method for quantification of methane concentration is gas chromatography [2]. However, this method is expensive and unsuitable for in situ monitoring which is essential in most cases. On the other hand, electrochemical sensing of methane faces many challenges due to the inertness of methane molecule. The anodic potential necessary for direct oxidation of methane is higher than the value of oxidation potential for water making the direct
∗ Corresponding author. Tel.: +1 778 7828694; fax: +1 778 7827514. E-mail address: [email protected] (B. Bahreyni). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.11.071
amperometric sensing of methane troublesome [3]. In order to circumvent this problem, Otagawa et al. use an organic solvent and Pt-Teflon electrode which allows application of high anodic potentials which is not compatible with aqueous solutions, making humidity a significant source of background noise [4]. Optical methane sensors offer advantages such as increased safety, possibility of remote access, and the feasibility of in situ measurements [5,6]. However, one of the main challenges with optical sensors is finding a suitable light source in the infrared range where the main absorption bands for most gases occur. Consequently, most of these optical sensors scan the near infrared range where the absorption bands of gases such as methane, carbon monoxide and carbon dioxide lay. Other optical methods for detection of methane rely on the change in material properties due to methane absorption. Metal oxides have also been used for the sensing of methane [8–10]. In these sensors, palladium or platinum are used to catalyze the oxidation of methane at high temperatures (about 400 ◦ C). The detection limit of these sensors is about 0.5% [11]. However, the elevated temperatures that metal oxide sensors require make them unsuitable for environments with a risk of explosion. Moreover, high operating temperatures lead to higher power consumption and short battery life for portable/remote applications [12,13]. Detection of methane molecules at room temperature is possible by taking advantage of supramolecular interactions between a host and methane [14,15]. Nicolas et al. showed that Cryptophane A (CrypA) exhibits affinity towards methane molecules.
A.H. Khoshaman et al. / Sensors and Actuators B 161 (2012) 954–960
However, the sensitivity of the sensors was greatly influenced by the employed deposition technique [16]. Benounis et al. covered the core of an optical fibre with a transparent polymer that contained a methane sensitive substance. As methane was absorbed, the effective refraction index of the polymer changed which was detected using evanescent wave optical fibre sensor with a detection limit of about 2% [7]. Sun et al. employed the supramolecular interaction to fabricate a methane sensor by directly electrospraying a CrypA solution onto the surface of a quartz crystal [17]. They managed to detect methane at concentrations as low as 0.05%. Quartz Crystal Microbalances (QCMs) provide fast, reliable responses which are suitable for in situ monitoring of analyte species. They are cost-effective and eliminate the need for time-consuming sample preparation. Some of their other benefits are high-sensitivity, operation at room temperature, simple packaging requirements, and low power consumption. Nonetheless, more research has to be done to reduce the amount of drift and improve the reproducibility in order to make them suitable for long-term applications in environment conditions. One of the most effective ways to increase their sensitivity and stability is to employ techniques with good control over the uniformity of the properties of layers of sensing material with better surface properties. An indicator of the mechanical and morphological properties of a deposited film is the quality factor of the resultant QCM, which ultimately determines the detection limit of the sensor as well. In this paper, we propose two new techniques for the deposition of thin films for gravimetric detection of methane. The first method is based on electrospinning of a mixture of CrypA and a polymer solution, which resulted in highly porous, nano-structured thin films. The second method is based on spin-coating of a mixture of CrypA and a photosensitive polymer. Both methods resulted in improved sensitivity and minimum detection limits compared to prior art. 2. Materials and methods Three different methods were employed for deposition of CrypA on the surface of the quartz crystal microbalances (QCM). The crystals were purchased from International Crystal MFG and had blank diameters of 0.5 in. with a resonant frequency of 10.000 MHz. Both sides of the crystals were polished to a smooth surface. Circular electrodes with diameters of 0.2 in. were patterned on each side of the crystals from a deposited layer of Ti/Au. During the sensor operation, the additional mass of the gas molecules modifies the resonant frequency of the QCM according to the Sauerbrey’s equation for a quartz crystal [18]: f = −2.26 × 10−6 f02
m A
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Fig. 1. Procedure used for synthesis of CrypA in this work, starting with vanillin: ((i) BrCH2 CH2 Br, NaOH/EtOH, (ii) NaBH4 , EtOH, and (iii) HCOOH).
3.1. Electrospinning and electrospraying Electrospinning is one of the most versatile methods for fabrication of thin films of nanofibres. The films produced by this method offer high surface to volume ratio, high porosity, superior mechanical properties, flexibility, and stability in surface properties. These properties make electrospinning a prime candidate for the deposition of sensitive films for sensing applications [21,22]. Electrospraying is a similar process where the deposited film is composed of nanoparticles instead of nano-fibres. A basic electrospraying/electrospinning setup is shown in Fig. 2. The properties of the deposited films may be controlled through several parameters including the molecular weight, polarity, and concentration of the polymer as well as the electrical conductivity of the solvent, surface tension of the solution, and dielectric constant of the solvent. Nonchemical parameters such as the flow rate of the syringe pump, the applied voltage, distance between collector and needle tip, gauge of the needle, the geometry and composition of the collector, and
(1)
where f is the change in frequency and f0 is the resonant frequency of crystal both measured in Hz, m is the mass of the film (in grams), that is deposited on the crystal and A is the surface area of the circular electrode in cm2 . For all of the experiments, the film thickness was selected such that the initial change in the resonant frequency of the crystals was about 50 kHz. 3. Synthesis of CrypA In 1981, Gabard and Collet proposed a method for the synthesis of cryptophanes starting with a substituted meta-methoxy benzyl alcohol derivative [19]. Canceill and Collet improved this method by first synthesizing a dimer of vanillinyl alcohol using a dialkyl halide which was then trimerized in hot formic acid to yield CrypA [20]. We synthesized CrypA from vanillin in three steps following the method by Benounis et al. and had a 3.7% overall yield (Fig. 1) [7].
Fig. 2. Schematic diagram of the electrospraying set-up used for fabrication of thin films.
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Fig. 3. SEM image of electrospun CrypA/siloprene film at (a) 2000× (b) 35,000× magnification.
finally, the duration of electrospinning process also strongly affects the film properties [22,23]. It can therefore be seen that optimization of the electrospinning or electrospraying process is critically important. Following the work of Sun et al., a thin film of electrosprayed CrypA was deposited on a quartz crystal (Crystal A) for reference [17]. A solution of 0.1601% (m/) of CrypA in tetrahydrofuran (THF) was used in the electrospraying process (Fig. 2). The distance between the needle tip and the crystal surface was set to 7 cm, the DC voltage was 13 kV, and the flow-rate of the syringe pump was fixed at 0.50 ml/h. The electrospraying process was continued until an approximate frequency change of 40 kHz was observed. Compared to simple drop-casting, electrospraying avoids clustering of CrypA and results in formation of a more uniform layer, leading to a sensor with higher quality factor. Siloprene K1000 was mixed with siloprene cross-linking agent K11 (both purchased from Sigma–Aldrich) at a ratio of 5:1 for this method. CrypA was completely dissolved in THF before being mixed with the polymers. To each unit volume of the polymeric mixture, 30 unit volumes of 0.1601% (m/) of CrypA in THF was added. A uniform nanofibrous film of CrypA/polymer was electrospun on a quartz crystal (Crystal B) with the following parameters. The distance between the needle tip and the crystal surface was set to 5 cm with a flow rate of 1 ml/h and an applied voltage of 15 kV. The electrospraying process was continued until a frequency change of about 40 kHz was observed. Measurement of the morphological properties (e.g. film thickness or roughness) of dense electrosprayed or electrospun thin film’s thickness is challenging using a regular profilometer or even an atomic force microscope as the probe tip can deform, stretch, or scratch the film. Moreover, a shadow mask affects the deposition parameters and a step in film thickness as required for such techniques may not be realized accurately. We therefore relied on Sauerbrey’s Eq. (1) to estimate the equivalent thicknesses of the electrosprayed and electrospun films (i.e. assuming the film was made from solid matter) as 600 nm and 650 nm, respectively. The scanning electron microscope (SEM) images of this film are provided in Fig. 3, showing the formation of nanofibers which gives rise to higher surface area and uniformity of the resulting film. The average diameter of the nano-fibres is estimated to be 9 ± 2 nm by processing SEM images of the films in image processing software. The fill-factor of the electrospun film (when mapped into a plane) was estimated to be 61 ± 5% by comparing SEM images from different regions of the same film.
4. Spin coating SU-8 is a negative photoresist originally developed by IBM [24] which is sensitive to wavelengths shorter than 360 nm [25]. When this photoresist is exposed to ultraviolet (UV) sources, a strong acidic group is formed. This step is followed by epoxy cross-linking which is acid-catalyzed, as well as, thermally driven during the post exposure bake. It is possible to accurately control the properties of the final film (e.g. its thickness and mechanical strength) through the spinning speed and the solvent concentration. A mixture of CrypA solution and SU-8 was prepared and used to spin-coat the surface of quartz crystals. The resultant film was flood exposed to UV light and baked. A solution containing 1:1 vol ratio of 0.1601% (m/) of CrypA and SU8-3005 was spin-coated at 5000 rpm for 20 s on a quartz crystal (Crystal C). The quartz crystals were baked for 1 min at 95 ◦ C followed by an UV exposure dose of around 100 mJ/cm2 . The film was baked again at 95◦ C for 1 min and immersed in SU8 developer followed by rinsing in isopropyl alcohol and drying. The sample was finally cured at 150 ◦ C for 3 min on the hot-plate. The observed frequency change was 52 kHz. Using a surface profilometer, the film thickness was measured to be around 720 nm with a mean surface roughness value of 4 nm. The SEM image of the resultant film is shown in Fig. 4. 5. Results and discussion 5.1. Characterization of Cryptophane A 1 H-NMR
spectrum of the synthesized CrypA was recorded on a Bruker 4000 mHz spectrometer and was referenced to the residual solvent peak. The NMR results are (see Fig. 5): 1 H-NMR (CDCl3 , 400 MHz) ı = 6.74 (s, 6H), 6.65(s, 6H), 4.58 (d, J = 13.7 Hz, 6H), 4.14 (m, 12H), 3.77 (s, 18H), 3.39 (d, J = 13.8 Hz, 6H) ppm. 6. Device characterization Reference methane and nitrogen gas mixtures were used to test the performance of the sensors. A custom-made test chamber with multiple flow-meters was set up in our lab to control the methane concentration of gases (Fig. 6). Nitrogen was used as the purging gas. The quartz crystals were placed in an oscillator circuit loop whose output frequency was measured using an Agilent 53132A frequency meter and recorded by a computer.
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Fig. 4. SEM image of SU-8/CrypA film at (a) 6510× and (b) 9980× magnification. The surface deformations are due to creation of gas bubbles during the curing process of the polymer.
An important property of resonant devices, such as the crystals discussed here, is their quality factor (Q). Quality factor is inversely proportional to the amount of damping mechanisms that affect the device around its resonant frequency, including air friction, internal material losses, support losses, and surface losses due to the existence of a thin film. In general, higher quality factors are desired in order to improve the oscillator stability and the noise performance of the sensor. The quality factor of the crystals was measured using a Rhode and Schwarz ZVB4 vector network analyzer. The measured quality factor of a crystal with a drop-casted layer of CrypA was about 80. The quality factor of Crystal A was measured
Fig. 5.
1
to be around 700. The quality factor of Crystal B was measured to be 3100, which shows a significant improvement over the case of electrospraying with no polymer for roughly the same amount of CrypA. Crystal C had a quality factor of about 1600 which shows significant improvement over Crystal A but is still lower than the quality factor of electrospun film on Crystal B. The normalized frequency responses of Crystals A, B, and C are compared in Fig. 7. The difference in quality factors of these crystals is due to the losses at their surfaces. It can therefore be concluded that electrospun films contributed the least to the overall damping, and therefore, electrospinning is preferred method to obtain high quality factors.
HNMR spectrum of the synthesized CrypA.
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Fig. 9. Response of Crystal C coated with SU-8/cryptophane film to methane.
Fig. 6. Block diagram of the set-up used in gas-sensing experiments of this work. FM, flow-meter.
Fig. 10. Response of Crystals B and C to different concentrations of methane.
6.1. Sensor performance evaluation
Fig. 7. Frequency response of crystals coated with three different methods. The curves are centred around the resonant frequency, f0 , for easier comparison.
The set-up in Fig. 6 was used to evaluate the sensor responses at room temperature to different concentrations of methane. Fig. 8 shows the diagram of frequency change, f, of the oscillator circuit with Crystal B when the film was subjected to a sequence of pulses of pure nitrogen and a mixture of nitrogen and 2.5% concentration of methane. For reference, the resonant frequency of Crystal A varied by about 200 Hz for the same concentration of methane [17]. The response of the Crystal C to the methane at several concentrations is represented in Fig. 9. As it can be seen, this response is superior to that of Crystal B for these concentrations.
Fig. 8. Response of Crystal B coated with CrypA/siloprene film to 2.5% methane concentration.
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7. Conclusions
Fig. 11. Frequency change of crystals coated with siloprene and CrypA/siloprene (Crystal B), when subjected to different levels of humidity.
The responses of the Crystals B and C were studied using several other concentrations of methane at room temperature (see Fig. 10). It is evident that the response of both crystals is proportional to the gas concentration at low methane concentrations. Electrospun films possess a large surface to volume ratio compared to a spin-coated film. Therefore, it is reasonable to expect that electrospun films bind with methane molecules more readily than the spin-coated films. Furthermore, as discussed earlier, electrospinning leads to a higher quality factor for the coated crystals. These two phenomena will result in higher sensitivity and better resolution (i.e. lower detection limit) for sensors based on electrospun films (i.e. Crystal B). Both sensors B and C start to display saturation at methane concentrations of about 0.5% with the response of spincoated films (Crystal C) surpassing that of the electrospun films at high methane concentrations. This could be due to the fact that the concentration of Cryptophane A molecules per volume is higher in spin-coated films than in the electrospun layers. The minimum detectable concentration of methane was estimated by monitoring the oscillator signal over an extended period under stable conditions. The short-term stability of the sensor signal was then calculated using the Allan variance of the oscillator frequency with a coated crystal in the loop. The oscillator frequency was recorded over time until 10,000 data points collected with an integration time of 0.1 s. It was concluded that the oscillator signal was stable within 62 mHz for an integration time of about 20 s. Using the data from Fig. 10 on the sensitivity of the sensors and assuming a signal to noise ratio of 3:1, the theoretical minimum detectable concentration of methane was determined to be 3 ppm and 7 ppm for Crystals B and C, respectively. Since these sensors are designed for operation under ambient conditions, the effect of humidity on these sensors was studied as well. It was observed that humidity in fact affected the sensor response. For example, the response of Crystal B to a constant methane concentration of 2.5% but different humidity levels of 0, 20% and 30% was 460 Hz, 530 Hz, and 580 Hz, respectively. Another crystal was electrospun with the same ratio of siloprene solution without CrypA to the same film thickness. The response of this crystal and that of Crystal B to different levels of humidity are illustrated in Fig. 11. It can be seen that the response of this crystal to humidity is nearly identical to that of Crystal B. The effect of humidity on sensor response can be alleviated in different ways. For example, it is possible to calibrate the device at different humidity levels and correct the sensor reading based on the level of humidity in the environment. Alternatively, one could use a device which is coated with a similar electrospun polymer film without the Cryptophane A molecules as a reference device. The two devices have similar responses to most of the environmental disturbances (including humidity) but only the device whose film contains Cryptophane A exhibits sensitivity to methane. The difference in the signals between the two devices is a direct function of methane concentration only. The response of the reference device can additionally be used to measure the humidity levels in the environment.
Two new methods were employed to deposit films containing CrypA on the surface of quartz resonators for methane sensing purposes. The response of these sensors to different concentrations of methane was investigated. Both methods exhibited significant improvements in sensor response over the prior art in terms of crystal quality factor, sensitivity, and minimum detectable concentrations of methane. The sensors can measure methane concentrations from a few ppm up to about the lower explosion limit (5%) of methane at room temperature. The response of the crystals to different levels of relative humidity was also investigated and it was concluded that a differential measurement would remove the interference from humidity variations.
Acknowledgments The authors would like to acknowledge the financial support of Natural Sciences and Engineering Research Council of Canada (NSERC) and Simon Fraser University (SFU). The first author would like to thank Mr. Alborz Amini for his assistance in fabrication of the devices.
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Biographies Amir HosseinKhoshaman received his BSc degree from Sharif University of Technology, Iran, and his M.A.Sc from Simon Fraser University, Canada, in 2009 and 2011, respectively. He is currently a PhD student at the Electrical and Computer Engineering of University of British Columbia (UBC), Vancouver, Canada. His research interests involve sensitive materials and field emission from carbon nanotubes. NabylMerbouh received his B.Sc. in Chemistry from the University of Tours in 1998, and subsequently attended the IRCOF-INSA at the University of Rouen where he was awarded a M.Sc. in 1999. He then joined the University of Connecticut in Storrs, USA, where he received his PhD under the supervision of Profs Christian Brückner and James M. Bobbitt developing novel oxidation methods for carbohydrates. In 2004 he joined the Burnham Institute in San Diego, USA, as a Post-Doctoral fellow under Prof. Peter Seeberger working on carbohydrate syntheses, and the development of traceless linkers. In 2005 he accepted a position as a lecturer in organic chemistry
and spectroscopy at Simon Fraser University, Burnaby, Canada, where he is still very active in the area of heterocyclic chemistry. Paul C.H. Li received his PhD (chemistry) in 1995 from the University of Toronto. After postdoctoral fellowship at the University of Alberta, he was assistant professor at City University of Hong Kong. He joined the Department of Chemistry Simon Fraser University in 1999, and became professor in 2010. He was visiting professor at the Institute of Toxicology of Johannes Gutenberg Universitat-Mainz, Germany in 2006. His research interests are chemical/biosensor, microfluidic biochip for singlecell drug resistance study and microarray-based pathogen detection. He wrote 2 monographs entitled “Microfluidic lab-on-a-chip for Chemical and Biological Analysis and Discovery, CRC press 2006”, and “Fundamentals to Microfluidics and Lab on a Chip for Biological Analysis and Discovery, CRC press 2010”. Behraad Bahreyni is an Assistant Professor and the Director of the Integrated Multi-Transducer Systems laboratory (IMuTS Lab) at Simon Fraser University, Surrey, Canada. He received his BSc in Electronics Engineering from Sharif University of Technology, Iran, and MSc and PhD degrees in Electrical Engineering from the University of Manitoba, Canada, in 1999, 2001, and 2006, respectively. He then spent a year as a post-doctoral researcher with the NanoSicence Centre at Cambridge University, UK, where he conducted research on interface circuit design for microresonators. In 2008, he joined the School of Engineering Science at Simon Fraser University, BC, Canada. His research has concentrated on the design and fabrication of silicon and polymer-based micro- and nano-sensors, RF MEMS, and the design of interface electronics for microdevices.Dr Bahreyni is the author of more than 50 technical publications including a book on the fabrication and design of resonant microdevices.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Highly sensitive supra-molecular thin films for gravimetric detection of methane Amir H. Khoshaman a , Paul C.H. Li b , Nabyl Merbouh b , Behraad Bahreyni a,∗ a b
School of Engineering Science, Simon Fraser University, Surrey, BC, Canada Department of Chemistry, Simon Fraser University, Burnaby, BC, Canada
a r t i c l e
i n f o
Article history: Received 30 August 2011 Received in revised form 16 November 2011 Accepted 26 November 2011 Available online 8 December 2011 Keywords: Methane Gravimetric sensors Supra-molecular Cryptophanes Electrospinning Spin coating
a b s t r a c t This report discusses two deposition methods for the preparation and deposition of thin films of Cryptophane A for gravimetric detection of methane. The first method relies on electrospinning of Cryptophane A with carrier polymers. The second technique is based on spin coating of a mixture of Cryptophane A, tetrahydrofuran, and SU8-3000 followed by curing. The required Cryptophane A molecules were synthesized using an improved 2-step trimerization method. Both methods offer good control over the uniformity and thickness of the deposited layers. Moreover, both techniques improved the performance of the gravimetric sensor due to the higher possible quality factors of the resonant sensors. The responses of methane sensors based on both methods were characterized using various concentrations of methane. The prototype sensors had an estimated detection limit of about 3 ppm with a sensitivity of 80 mHz/ppm. Both parameters show improvement over the previously reported works. Our research demonstrates that electrospinning of Cryptophane A films generally results in a higher quality factor for the gravimetric sensors, and thus, improved sensor resolution. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Measurement of methane concentration is important in several applications. Methane is the main ingredient of natural gas which is ubiquitously used in industry and residences for heating purposes. Furthermore, methane is abundantly produced by oxidation of organic compounds in nature or landfills. Methane is a significant contributor to greenhouse effect whose adverse effects can be as much as 20 times worse than carbon dioxide [1]. Methane is an odorless, colorless gas with a Lower Explosive Limit (LEL) concentration of about 5% by volume in air. Therefore, early detection of methane accumulation is particularly important in closed environments such as mines or landfills. Consequently, long-term measurement of methane concentration is crucial for health and safety reasons, ecosystem monitoring, and in industrial applications. A common method for quantification of methane concentration is gas chromatography [2]. However, this method is expensive and unsuitable for in situ monitoring which is essential in most cases. On the other hand, electrochemical sensing of methane faces many challenges due to the inertness of methane molecule. The anodic potential necessary for direct oxidation of methane is higher than the value of oxidation potential for water making the direct
∗ Corresponding author. Tel.: +1 778 7828694; fax: +1 778 7827514. E-mail address: [email protected] (B. Bahreyni). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.11.071
amperometric sensing of methane troublesome [3]. In order to circumvent this problem, Otagawa et al. use an organic solvent and Pt-Teflon electrode which allows application of high anodic potentials which is not compatible with aqueous solutions, making humidity a significant source of background noise [4]. Optical methane sensors offer advantages such as increased safety, possibility of remote access, and the feasibility of in situ measurements [5,6]. However, one of the main challenges with optical sensors is finding a suitable light source in the infrared range where the main absorption bands for most gases occur. Consequently, most of these optical sensors scan the near infrared range where the absorption bands of gases such as methane, carbon monoxide and carbon dioxide lay. Other optical methods for detection of methane rely on the change in material properties due to methane absorption. Metal oxides have also been used for the sensing of methane [8–10]. In these sensors, palladium or platinum are used to catalyze the oxidation of methane at high temperatures (about 400 ◦ C). The detection limit of these sensors is about 0.5% [11]. However, the elevated temperatures that metal oxide sensors require make them unsuitable for environments with a risk of explosion. Moreover, high operating temperatures lead to higher power consumption and short battery life for portable/remote applications [12,13]. Detection of methane molecules at room temperature is possible by taking advantage of supramolecular interactions between a host and methane [14,15]. Nicolas et al. showed that Cryptophane A (CrypA) exhibits affinity towards methane molecules.
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However, the sensitivity of the sensors was greatly influenced by the employed deposition technique [16]. Benounis et al. covered the core of an optical fibre with a transparent polymer that contained a methane sensitive substance. As methane was absorbed, the effective refraction index of the polymer changed which was detected using evanescent wave optical fibre sensor with a detection limit of about 2% [7]. Sun et al. employed the supramolecular interaction to fabricate a methane sensor by directly electrospraying a CrypA solution onto the surface of a quartz crystal [17]. They managed to detect methane at concentrations as low as 0.05%. Quartz Crystal Microbalances (QCMs) provide fast, reliable responses which are suitable for in situ monitoring of analyte species. They are cost-effective and eliminate the need for time-consuming sample preparation. Some of their other benefits are high-sensitivity, operation at room temperature, simple packaging requirements, and low power consumption. Nonetheless, more research has to be done to reduce the amount of drift and improve the reproducibility in order to make them suitable for long-term applications in environment conditions. One of the most effective ways to increase their sensitivity and stability is to employ techniques with good control over the uniformity of the properties of layers of sensing material with better surface properties. An indicator of the mechanical and morphological properties of a deposited film is the quality factor of the resultant QCM, which ultimately determines the detection limit of the sensor as well. In this paper, we propose two new techniques for the deposition of thin films for gravimetric detection of methane. The first method is based on electrospinning of a mixture of CrypA and a polymer solution, which resulted in highly porous, nano-structured thin films. The second method is based on spin-coating of a mixture of CrypA and a photosensitive polymer. Both methods resulted in improved sensitivity and minimum detection limits compared to prior art. 2. Materials and methods Three different methods were employed for deposition of CrypA on the surface of the quartz crystal microbalances (QCM). The crystals were purchased from International Crystal MFG and had blank diameters of 0.5 in. with a resonant frequency of 10.000 MHz. Both sides of the crystals were polished to a smooth surface. Circular electrodes with diameters of 0.2 in. were patterned on each side of the crystals from a deposited layer of Ti/Au. During the sensor operation, the additional mass of the gas molecules modifies the resonant frequency of the QCM according to the Sauerbrey’s equation for a quartz crystal [18]: f = −2.26 × 10−6 f02
m A
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Fig. 1. Procedure used for synthesis of CrypA in this work, starting with vanillin: ((i) BrCH2 CH2 Br, NaOH/EtOH, (ii) NaBH4 , EtOH, and (iii) HCOOH).
3.1. Electrospinning and electrospraying Electrospinning is one of the most versatile methods for fabrication of thin films of nanofibres. The films produced by this method offer high surface to volume ratio, high porosity, superior mechanical properties, flexibility, and stability in surface properties. These properties make electrospinning a prime candidate for the deposition of sensitive films for sensing applications [21,22]. Electrospraying is a similar process where the deposited film is composed of nanoparticles instead of nano-fibres. A basic electrospraying/electrospinning setup is shown in Fig. 2. The properties of the deposited films may be controlled through several parameters including the molecular weight, polarity, and concentration of the polymer as well as the electrical conductivity of the solvent, surface tension of the solution, and dielectric constant of the solvent. Nonchemical parameters such as the flow rate of the syringe pump, the applied voltage, distance between collector and needle tip, gauge of the needle, the geometry and composition of the collector, and
(1)
where f is the change in frequency and f0 is the resonant frequency of crystal both measured in Hz, m is the mass of the film (in grams), that is deposited on the crystal and A is the surface area of the circular electrode in cm2 . For all of the experiments, the film thickness was selected such that the initial change in the resonant frequency of the crystals was about 50 kHz. 3. Synthesis of CrypA In 1981, Gabard and Collet proposed a method for the synthesis of cryptophanes starting with a substituted meta-methoxy benzyl alcohol derivative [19]. Canceill and Collet improved this method by first synthesizing a dimer of vanillinyl alcohol using a dialkyl halide which was then trimerized in hot formic acid to yield CrypA [20]. We synthesized CrypA from vanillin in three steps following the method by Benounis et al. and had a 3.7% overall yield (Fig. 1) [7].
Fig. 2. Schematic diagram of the electrospraying set-up used for fabrication of thin films.
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Fig. 3. SEM image of electrospun CrypA/siloprene film at (a) 2000× (b) 35,000× magnification.
finally, the duration of electrospinning process also strongly affects the film properties [22,23]. It can therefore be seen that optimization of the electrospinning or electrospraying process is critically important. Following the work of Sun et al., a thin film of electrosprayed CrypA was deposited on a quartz crystal (Crystal A) for reference [17]. A solution of 0.1601% (m/) of CrypA in tetrahydrofuran (THF) was used in the electrospraying process (Fig. 2). The distance between the needle tip and the crystal surface was set to 7 cm, the DC voltage was 13 kV, and the flow-rate of the syringe pump was fixed at 0.50 ml/h. The electrospraying process was continued until an approximate frequency change of 40 kHz was observed. Compared to simple drop-casting, electrospraying avoids clustering of CrypA and results in formation of a more uniform layer, leading to a sensor with higher quality factor. Siloprene K1000 was mixed with siloprene cross-linking agent K11 (both purchased from Sigma–Aldrich) at a ratio of 5:1 for this method. CrypA was completely dissolved in THF before being mixed with the polymers. To each unit volume of the polymeric mixture, 30 unit volumes of 0.1601% (m/) of CrypA in THF was added. A uniform nanofibrous film of CrypA/polymer was electrospun on a quartz crystal (Crystal B) with the following parameters. The distance between the needle tip and the crystal surface was set to 5 cm with a flow rate of 1 ml/h and an applied voltage of 15 kV. The electrospraying process was continued until a frequency change of about 40 kHz was observed. Measurement of the morphological properties (e.g. film thickness or roughness) of dense electrosprayed or electrospun thin film’s thickness is challenging using a regular profilometer or even an atomic force microscope as the probe tip can deform, stretch, or scratch the film. Moreover, a shadow mask affects the deposition parameters and a step in film thickness as required for such techniques may not be realized accurately. We therefore relied on Sauerbrey’s Eq. (1) to estimate the equivalent thicknesses of the electrosprayed and electrospun films (i.e. assuming the film was made from solid matter) as 600 nm and 650 nm, respectively. The scanning electron microscope (SEM) images of this film are provided in Fig. 3, showing the formation of nanofibers which gives rise to higher surface area and uniformity of the resulting film. The average diameter of the nano-fibres is estimated to be 9 ± 2 nm by processing SEM images of the films in image processing software. The fill-factor of the electrospun film (when mapped into a plane) was estimated to be 61 ± 5% by comparing SEM images from different regions of the same film.
4. Spin coating SU-8 is a negative photoresist originally developed by IBM [24] which is sensitive to wavelengths shorter than 360 nm [25]. When this photoresist is exposed to ultraviolet (UV) sources, a strong acidic group is formed. This step is followed by epoxy cross-linking which is acid-catalyzed, as well as, thermally driven during the post exposure bake. It is possible to accurately control the properties of the final film (e.g. its thickness and mechanical strength) through the spinning speed and the solvent concentration. A mixture of CrypA solution and SU-8 was prepared and used to spin-coat the surface of quartz crystals. The resultant film was flood exposed to UV light and baked. A solution containing 1:1 vol ratio of 0.1601% (m/) of CrypA and SU8-3005 was spin-coated at 5000 rpm for 20 s on a quartz crystal (Crystal C). The quartz crystals were baked for 1 min at 95 ◦ C followed by an UV exposure dose of around 100 mJ/cm2 . The film was baked again at 95◦ C for 1 min and immersed in SU8 developer followed by rinsing in isopropyl alcohol and drying. The sample was finally cured at 150 ◦ C for 3 min on the hot-plate. The observed frequency change was 52 kHz. Using a surface profilometer, the film thickness was measured to be around 720 nm with a mean surface roughness value of 4 nm. The SEM image of the resultant film is shown in Fig. 4. 5. Results and discussion 5.1. Characterization of Cryptophane A 1 H-NMR
spectrum of the synthesized CrypA was recorded on a Bruker 4000 mHz spectrometer and was referenced to the residual solvent peak. The NMR results are (see Fig. 5): 1 H-NMR (CDCl3 , 400 MHz) ı = 6.74 (s, 6H), 6.65(s, 6H), 4.58 (d, J = 13.7 Hz, 6H), 4.14 (m, 12H), 3.77 (s, 18H), 3.39 (d, J = 13.8 Hz, 6H) ppm. 6. Device characterization Reference methane and nitrogen gas mixtures were used to test the performance of the sensors. A custom-made test chamber with multiple flow-meters was set up in our lab to control the methane concentration of gases (Fig. 6). Nitrogen was used as the purging gas. The quartz crystals were placed in an oscillator circuit loop whose output frequency was measured using an Agilent 53132A frequency meter and recorded by a computer.
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Fig. 4. SEM image of SU-8/CrypA film at (a) 6510× and (b) 9980× magnification. The surface deformations are due to creation of gas bubbles during the curing process of the polymer.
An important property of resonant devices, such as the crystals discussed here, is their quality factor (Q). Quality factor is inversely proportional to the amount of damping mechanisms that affect the device around its resonant frequency, including air friction, internal material losses, support losses, and surface losses due to the existence of a thin film. In general, higher quality factors are desired in order to improve the oscillator stability and the noise performance of the sensor. The quality factor of the crystals was measured using a Rhode and Schwarz ZVB4 vector network analyzer. The measured quality factor of a crystal with a drop-casted layer of CrypA was about 80. The quality factor of Crystal A was measured
Fig. 5.
1
to be around 700. The quality factor of Crystal B was measured to be 3100, which shows a significant improvement over the case of electrospraying with no polymer for roughly the same amount of CrypA. Crystal C had a quality factor of about 1600 which shows significant improvement over Crystal A but is still lower than the quality factor of electrospun film on Crystal B. The normalized frequency responses of Crystals A, B, and C are compared in Fig. 7. The difference in quality factors of these crystals is due to the losses at their surfaces. It can therefore be concluded that electrospun films contributed the least to the overall damping, and therefore, electrospinning is preferred method to obtain high quality factors.
HNMR spectrum of the synthesized CrypA.
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Fig. 9. Response of Crystal C coated with SU-8/cryptophane film to methane.
Fig. 6. Block diagram of the set-up used in gas-sensing experiments of this work. FM, flow-meter.
Fig. 10. Response of Crystals B and C to different concentrations of methane.
6.1. Sensor performance evaluation
Fig. 7. Frequency response of crystals coated with three different methods. The curves are centred around the resonant frequency, f0 , for easier comparison.
The set-up in Fig. 6 was used to evaluate the sensor responses at room temperature to different concentrations of methane. Fig. 8 shows the diagram of frequency change, f, of the oscillator circuit with Crystal B when the film was subjected to a sequence of pulses of pure nitrogen and a mixture of nitrogen and 2.5% concentration of methane. For reference, the resonant frequency of Crystal A varied by about 200 Hz for the same concentration of methane [17]. The response of the Crystal C to the methane at several concentrations is represented in Fig. 9. As it can be seen, this response is superior to that of Crystal B for these concentrations.
Fig. 8. Response of Crystal B coated with CrypA/siloprene film to 2.5% methane concentration.
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7. Conclusions
Fig. 11. Frequency change of crystals coated with siloprene and CrypA/siloprene (Crystal B), when subjected to different levels of humidity.
The responses of the Crystals B and C were studied using several other concentrations of methane at room temperature (see Fig. 10). It is evident that the response of both crystals is proportional to the gas concentration at low methane concentrations. Electrospun films possess a large surface to volume ratio compared to a spin-coated film. Therefore, it is reasonable to expect that electrospun films bind with methane molecules more readily than the spin-coated films. Furthermore, as discussed earlier, electrospinning leads to a higher quality factor for the coated crystals. These two phenomena will result in higher sensitivity and better resolution (i.e. lower detection limit) for sensors based on electrospun films (i.e. Crystal B). Both sensors B and C start to display saturation at methane concentrations of about 0.5% with the response of spincoated films (Crystal C) surpassing that of the electrospun films at high methane concentrations. This could be due to the fact that the concentration of Cryptophane A molecules per volume is higher in spin-coated films than in the electrospun layers. The minimum detectable concentration of methane was estimated by monitoring the oscillator signal over an extended period under stable conditions. The short-term stability of the sensor signal was then calculated using the Allan variance of the oscillator frequency with a coated crystal in the loop. The oscillator frequency was recorded over time until 10,000 data points collected with an integration time of 0.1 s. It was concluded that the oscillator signal was stable within 62 mHz for an integration time of about 20 s. Using the data from Fig. 10 on the sensitivity of the sensors and assuming a signal to noise ratio of 3:1, the theoretical minimum detectable concentration of methane was determined to be 3 ppm and 7 ppm for Crystals B and C, respectively. Since these sensors are designed for operation under ambient conditions, the effect of humidity on these sensors was studied as well. It was observed that humidity in fact affected the sensor response. For example, the response of Crystal B to a constant methane concentration of 2.5% but different humidity levels of 0, 20% and 30% was 460 Hz, 530 Hz, and 580 Hz, respectively. Another crystal was electrospun with the same ratio of siloprene solution without CrypA to the same film thickness. The response of this crystal and that of Crystal B to different levels of humidity are illustrated in Fig. 11. It can be seen that the response of this crystal to humidity is nearly identical to that of Crystal B. The effect of humidity on sensor response can be alleviated in different ways. For example, it is possible to calibrate the device at different humidity levels and correct the sensor reading based on the level of humidity in the environment. Alternatively, one could use a device which is coated with a similar electrospun polymer film without the Cryptophane A molecules as a reference device. The two devices have similar responses to most of the environmental disturbances (including humidity) but only the device whose film contains Cryptophane A exhibits sensitivity to methane. The difference in the signals between the two devices is a direct function of methane concentration only. The response of the reference device can additionally be used to measure the humidity levels in the environment.
Two new methods were employed to deposit films containing CrypA on the surface of quartz resonators for methane sensing purposes. The response of these sensors to different concentrations of methane was investigated. Both methods exhibited significant improvements in sensor response over the prior art in terms of crystal quality factor, sensitivity, and minimum detectable concentrations of methane. The sensors can measure methane concentrations from a few ppm up to about the lower explosion limit (5%) of methane at room temperature. The response of the crystals to different levels of relative humidity was also investigated and it was concluded that a differential measurement would remove the interference from humidity variations.
Acknowledgments The authors would like to acknowledge the financial support of Natural Sciences and Engineering Research Council of Canada (NSERC) and Simon Fraser University (SFU). The first author would like to thank Mr. Alborz Amini for his assistance in fabrication of the devices.
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Biographies Amir HosseinKhoshaman received his BSc degree from Sharif University of Technology, Iran, and his M.A.Sc from Simon Fraser University, Canada, in 2009 and 2011, respectively. He is currently a PhD student at the Electrical and Computer Engineering of University of British Columbia (UBC), Vancouver, Canada. His research interests involve sensitive materials and field emission from carbon nanotubes. NabylMerbouh received his B.Sc. in Chemistry from the University of Tours in 1998, and subsequently attended the IRCOF-INSA at the University of Rouen where he was awarded a M.Sc. in 1999. He then joined the University of Connecticut in Storrs, USA, where he received his PhD under the supervision of Profs Christian Brückner and James M. Bobbitt developing novel oxidation methods for carbohydrates. In 2004 he joined the Burnham Institute in San Diego, USA, as a Post-Doctoral fellow under Prof. Peter Seeberger working on carbohydrate syntheses, and the development of traceless linkers. In 2005 he accepted a position as a lecturer in organic chemistry
and spectroscopy at Simon Fraser University, Burnaby, Canada, where he is still very active in the area of heterocyclic chemistry. Paul C.H. Li received his PhD (chemistry) in 1995 from the University of Toronto. After postdoctoral fellowship at the University of Alberta, he was assistant professor at City University of Hong Kong. He joined the Department of Chemistry Simon Fraser University in 1999, and became professor in 2010. He was visiting professor at the Institute of Toxicology of Johannes Gutenberg Universitat-Mainz, Germany in 2006. His research interests are chemical/biosensor, microfluidic biochip for singlecell drug resistance study and microarray-based pathogen detection. He wrote 2 monographs entitled “Microfluidic lab-on-a-chip for Chemical and Biological Analysis and Discovery, CRC press 2006”, and “Fundamentals to Microfluidics and Lab on a Chip for Biological Analysis and Discovery, CRC press 2010”. Behraad Bahreyni is an Assistant Professor and the Director of the Integrated Multi-Transducer Systems laboratory (IMuTS Lab) at Simon Fraser University, Surrey, Canada. He received his BSc in Electronics Engineering from Sharif University of Technology, Iran, and MSc and PhD degrees in Electrical Engineering from the University of Manitoba, Canada, in 1999, 2001, and 2006, respectively. He then spent a year as a post-doctoral researcher with the NanoSicence Centre at Cambridge University, UK, where he conducted research on interface circuit design for microresonators. In 2008, he joined the School of Engineering Science at Simon Fraser University, BC, Canada. His research has concentrated on the design and fabrication of silicon and polymer-based micro- and nano-sensors, RF MEMS, and the design of interface electronics for microdevices.Dr Bahreyni is the author of more than 50 technical publications including a book on the fabrication and design of resonant microdevices.