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A micro optofluidic system for toluene detection application
Microelectronic Engineering 222 (2020) 111204
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Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Research paper
A micro optofluidic system for toluene detection application a,⁎
a
T
a
Mohammad Ramezannezhad , Alireza Nikfarjam , Hassan Hajghassem , Mohammad Makhdoumi Akrama, Meisam Gazmehb a b
MEMS & NEMS Laboratory, Faculty of New Sciences & Technologies, University of Tehran, 1439957131 Tehran, Iran Laser and Plasma Research Institute, Shahid Beheshti University, G.C.Evin, Tehran,Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: PMMA Gas detection UV spectroscopy Toluene
A new application of metal deposited poly (methyl methacrylate) (PMMA) microchannel for optical detection of toluene in the UV spectrum region is presented in this study. Laser cutting and hole formation in initial PMMA sheets, micro-milling for groove shaping, aluminium deposition on two caps using DC magnetron sputtering for increasing optical path length, and bonding process using thermally-solvent method are the main steps for the microchannel fabrication. The dimensions and length of microchannel are 370 μm × 370 μm and 5 cm, respectively. The SEM and leakage tests are implemented to investigate the quality of the microchannel. Besides, the characterization results indicate that Al deposition has no adverse influence on the bonding process. Our simulation results indicate that the optical path length enhanced by 2.2% and the loss decreased by 48.9% when Al is deposited on the surfaces of the microchannel. Also, the effect of microchannel length on optical path length and loss is investigated using simulation. The optical detection tests are performed in a single step using UV spectroscopy based on absorption for toluene in concentration range within 20–800 ppm. The tests are repeated several times for each toluene concentration. Our experimental observation indicate that the results obtained from this optofluidic system is repeatable with linear gas detection and fast response and perfect selectivity which make it a good candidate for toluene detection applications.
1. Introduction The ability of detecting different types of pollutant gas and controlling their concentration are considered to be an important issue in recent years. This attention comes from their vital role in human's life and the environmental condition. Among different pollutant gases, the BTX group (Benzene, Toluene, and Xylene) are the environmental pollutants, which are categorized as Volatile Organic Compounds (VOCs). Detection importance of this group is due to their harmful effects on human's health and environment. Among various BTX detection methods, the optical method is one of the oldest and the most important technique which has attracted researchers' focus [1–6]. This interest is due to its considerable advantages such as suitable accuracy and sensitivity, perfect selectivity, appropriate stability, fast response, and integration ability with the other measurement instruments [7–11]. However, the enormous size, non-portability, and high price of conventional optical setups are the disadvantages of these measurement systems. According to the considerable advantages of optical measurement methods, miniaturization has been one of the most prominent aspects for overcoming their
⁎
limitations. Using a microfluidic instrument with the measurement system can be a reliable solution for miniaturization purpose. Microfluidic system has some advantages such as portability, fast analyzing time, low sample volume consumption, laminar flow rate and high sensitivity which enable it to be a suitable candidate for using in measurement systems [9,12–15]. Furthermore, UV-VIS spectroscopy based on absorption can be a suitable choice for BTX detection due to their absorption wavelength range which is between 230 and 290 nm [16]. It should be noticed that several limit detections are presented for this group due to different standards [1,6,17]. Several studies have been carried out on the detection methods in recent years. Ueno et al. introduced an optical system for BTX detection and developed it through the years [1,2,9]. The system was based on two steps detection, including pre-concentration and detection cells. The detection process was performed by UV spectroscopy based on absorption. Moreover, the cells were fabricated by Pyrex through the anodic bonding process, and the outcoming limit detection was 4 ppm. In following, Camou et al. worked on BTX extraction from the liquid and its improvement [18,19]. Most of the improvements were
Corresponding author. E-mail address: [email protected] (A. Nikfarjam).
https://doi.org/10.1016/j.mee.2019.111204 Received 21 October 2019; Received in revised form 3 December 2019; Accepted 27 December 2019 Available online 31 December 2019 0167-9317/ © 2019 Elsevier B.V. All rights reserved.
Microelectronic Engineering 222 (2020) 111204
M. Ramezannezhad, et al.
Fig. 1. Main steps for microchannel fabrication including laser cutting for size and hole formation, micro-milling for groove shaping, Al deposition on two caps using DC magnetron sputtering, and bonding process by the thermally-solvent method.
2. Experimental methods
attributed to the pre-concentration cell, such as optimal design and using more efficient absorbent materials. Other studies for optical detection of BTX were implemented a few years later [20,21]. In these researches, the gases were detected using an optical waveguide with sensitive material on a glass substrate. Measurement procedure was carried out based on light transmission variations after chemical absorption between a target gas and sensitive material, which helped for achieving 8 ppm limit detection. Although, mentioned studies revealed great results including magnificent limit detection, some limitative parameters such as complexity of the sensing systems(e.g., two step detection), and using fragile and hard-machining material(Pyrex and glass) still existed. The optofluidic system, which is the integration of the microfluidic and optical systems, contains a waveguide which enables the interaction between the light and gas [22]. It provides some unique advantages compared to bulk detection systems such as smaller sample volumes, faster response time, portability, and lower cost. Among different types of waveguide [22–25], using metal coating within the microchannel can be an appropriate method to fabricate micro waveguides, especially for gas detection purpose. When microchannel is used for both guiding light and fluid, proper metal deposition may be used to increase the optical path length, enhancing the interaction between fluid and light, and obtaining more favorable sensitivity due to the consecutive reflections within the microchannel. Choosing suitable material as a metal coating depends on the range of working wavelength since each metal has particular reflectivity in different segments of the electromagnetic spectrum. In the UV region, aluminium can be a suitable choice to be deposited within the microchannel [26]. Among the different types of polymer used for microfluidic device fabrication, poly(methyl methacrylate) (PMMA) is a perfect choice due to its unique features such as suitable mechanical stability, capability for performing micromachining process, low weight, gas impermeability and low price [15]. Moreover, in fabricating PMMA microchannels, several distinct methods can be used for bonding. Among them, a thermally-solvent method is a simple and appropriate choice due to its well-known advantages [27]. Although PMMA is a beneficial material to be used in optofluidic devices based on the mentioned properties of it, it has had limited number of usages in the optofluidic area [28,29] which all of them were used without metal deposition. In this study, a new and high throughput optical toluene detection system using a low cost optofluidic microchip fabricated through a simple method is introduced. The microchannel fabrication processes include channel formation with micro-milling method, aluminium deposition on all surfaces, and bonding process. Using Al deposition within the microchannel facilitates the optical path length increment and improves the gas detection condition, which is also investigated using simulation process. It is noteworthy to say that there is no report regarding Al deposition on PMMA microchannel surfaces for optical gas detection purposes. Moreover, the microfluidic system has the benefit of a small device size during fabrication and testing. The quality of the fabricated microchannel is investigated with SEM characterization and leakage test. Eventually, it is tested in an optofluidic system for toluene gas detection with concentrations between 20 and 800 ppm.
The experimental procedures of the current study are performed in different stages which are explained in the following subsections. 2.1. Microchannel fabrication Based on the standard for PMMA bonding [27], transparent PMMA with 2 mm thickness(Year Long Co) was chosen as a substrate material for microchannel fabrication. PMMA is more flexible and stress-resistant material compared to common microfluidic materials such as Pyrex or silicon. Also, its cutting and engraving are more straightforward. Fig. 1 shows the implemented stages for microchannel fabrication. At first, the PMMA sheet was cut by CO2 laser in 2 cm × 5 cm dimensions. Afterwards, a 500 μm hole was created in the center of upper microchannel cap for gas injection purpose. Subsequently, for 370 μm × 370 μm microfluidic groove formation, micro-milling process was implemented on the lower cap using CNC machine. This method is a very cost effective and fast way for the fabrication of the microfluidic devices [30]. After the mentioned stages, DC magnetron sputtering for Al deposition on PMMA surface was adopted. An HNO3 solution (65%) was used as an Al etchant for locally etching the extra Al segments. Photolithography process was then used for a final Al patterning on the surfaces. Al layer was formed just in the middle of the surfaces. A final step in microchannel fabrication was bonding process for upper and lower caps. Generally, there are various bonding methods for PMMA such as thermal bonding [31], UV assistant bonding [32], solvent bonding [27], microwave bonding [33], and friction spot welding [34]. However, a final microchannel in this work was made using thermally-solvent process without any deformation and imperfection. This method is a simple and inexpensive bonding technique, without need of any individual and expensive instruments. The intended thermally-solvent bonding method was previously reported [27]. Briefly, for PMMA-PMMA bonding, surfaces were first accurately rinsed with water and ordinary detergent to eliminate oil and dust. Afterwards, deionized water and Isopropyl alcohol (IPA) were separately used for the next step of washing and cleaning of the surfaces respectively. Each sample was shaken and kept in deionized water and IPA for 3 min. As surface cleaning is an essential part of the process, this step was repeated for three times. Nitrogen gas was used for surfaces drying at the end of each cleaning part. After cleaning and drying, 70% Isopropyl alcohol solution was prepared and poured on one of the surfaces using a syringe. The other surface was precisely aligned and placed on the other one. Subsequently, they were fixed by the paper clamps. The assembled surfaces were then placed in a fan-assisted oven at 68 °C for 10 min. Eventually, the sample was ready to be used after gradual cooling down to room temperature and inspection of the bonding quality. 2.2. Test setup The experimental setup for an optical gas detection test is shown in Fig. 2. The working region in the electromagnetic spectrum contained UV wavelengths; therefore, DT-Mini-2-GS (Ocean Optics Co) light 2
Microelectronic Engineering 222 (2020) 111204
M. Ramezannezhad, et al.
Fig. 2. (a) The schematic and (b) image of the experimental test setup for optical gas detection.
angles between 0 to ± 12 degree with a step of ± 3 degree. The results are shown in Table 1. According to Table 1, the optical path length in the microchannel with the Al coating increases by 2.2% compared to no Al coating. It may be due to an enhancement of consecutive reflections inside the Al deposited microchannel. More optical path length can cause more interaction between the light and gas during the gas detection, which can in turn improve the detection system performance. Furthermore, the loss inside the Al deposited microchannel dramatically decreased by 48.9%. Al deposition can also improve guiding of the light inside the microchannel and prevent the escape of the light through its path in the waveguide. The results clearly indicate that metal deposition inside the microchannel has significant effect on the improvement of the detection situation. Moreover, Table 2 represents the effect of microchannel length on loss and optical path length. Simulation condition was kept the same as the one which was already investigated. All microchannels were considered with Al deposition. According to Table 2, the optical path length increases with the microchannel length however, this also increases the loss as well. Therefore, 5 cm channel length could be a suitable choice considering both parameters.
source and HR 4000(Ocean Optics Co) spectrometer were used in the setup, which both of them were connected to a fiber optic at their outputs. According to the current discrete design for light guiding, two 74-UV collimating lenses (Ocean Optics Co) were used for a better light coupling in an experimental setup. The light source output and lenses were placed on the fixed stages. However, the microchannel and spectrometer outputs were put on three axes manual stages for more accurate aligning and guiding light through the waveguide. The microchannel was accurately placed in the focal length of lenses for more efficient testing. Also, a well-sealed chamber for liquid toluene evaporation and a Hamilton syringe for a gas injection to the microchannel were utilized for detection purposes. A target gas was injected form the upper hole of the microchannel, whereas it exited from its two ends. At the same time, the light was focused inside the microchannel through the lenses and reached the spectrometer after interaction with gas molecules inside the microchannel. 3. Results The results, which were obtained through the simulation, characterization, and testing are discussed in following subsections.
3.2. Microchannel characterization 3.1. Simulation results Several aspects of the microchannel, which are related to our present work, are investigated in the following sections. Fig. 4(a) and (b) represent the SEM image of the microchannel on the PMMA substrate before and after Al deposition(before the bonding process). It demonstrates that micro- milling process was accurately performed and produced uniform and regular edges of the groove. Moreover, it can be seen that Al deposition has appropriate quality and uniformity within the microchannel. Fig. 4(c) shows a cross section image of the microchannel before the bonding process. This image
The influence of the reflective layer on optical path length and the loss of the microchannel which are the key parameters in optical gas detection was simulated and is shown in Fig. 3. The simulation was performed using COMSOL software and ray optic physics at the wavelength of 267 nm. The dimensions and length of the simulated microchannel were the same as the experimental device. Two microchannels, one with an Al coating of 93% reflectivity and the other without Al coating were investigated. The light source was applied with the tilt 3
Microelectronic Engineering 222 (2020) 111204
M. Ramezannezhad, et al.
Fig. 3. The effect of metal deposition on optical path length and loss by simulation for microchannel (a) without Al deposition (b) with Al deposition.
same. Other characterizations of the intended PMMA microchannel bonding method were previously investigated [27].
Table 1 Simulation results about the effect of Al deposition on optical path length and loss for 5 cm microchannel. Al deposition
Optical path length (mm)
Loss(%)
No Yes
50 51.1
88.9 40
3.3. Detection results Gas detection was performed by an optical method based on absorption. In absorption state, gas detection is achieved after gas molecules interaction with light beams and causing the absorption of light by gas molecules which can be investigated by a spectrophotometry technique. According to Beer-Lambert law, which is attributed to the absorption state of optical measurement, the optical path length is a determining factor in an optical gas detection based on absorption. It demonstrates that optical path length has a direct relation with received light intensity to a detector after interaction with gas molecules. This also has an effect on the final absorption peak spectrum. In an optofluidic device, it is possible to use wall reflections to increase an optical path length. Wall reflection can be performed by metal deposition within the microchannel, which can cause more wall reflections and more appropriate light guidance in the light path. As it was mentioned previously, Al was deposited on PMMA microchannel surfaces. However, it should be noticed that physical optical path length enhancement is somewhat restricted since in the practical situation, physical space is limited for a test setup. Furthermore, large optical path length can also cause more losses along the microchannel, which can reduce the detection quality. As a result, having an optimum microchannel length can improve a limit of detection of the device. In addition, simulation using Comsol software was used to obtain the suitable microchannel length. As the simulation results show, the length of 5 cm can be an appropriate choice. It was determined regarding both optical path length and loss as the critical factors in an optical instrument. 8 cm length microchannel has the largest optical path length with considerable loss(84%). Also, the fabrication process for 8 cm length microchannel is more difficult compared to shorter microchannels. Therefore, according to the mentioned factors and experimental observations, the final length of microchannels for our experimental proposes was 5 cm. In the first step of the experimental test, a certain volume of the liquid toluene as a sample was injected into a well-sealed chamber with a specified volume. After that, the chamber was heated for toluene evaporation resulting in a specified concentration of toluene in a gas phase. Subsequently, the gas was injected by a Hamiltonian syringe to a microchannel through the upper hole of the chamber. As a result, the detection process was started by gas injection into the microchannel
Table 2 Simulation results about the effect of microchannel length on optical path length and loss. Microchannel length (mm)
Optical path length (mm)
Loss(%)
20 50 80
20 51.1 84.6
22.2 40 84.4
provides an opportunity to compare two dimensions of the microchannel before and after the bonding process. In addition, a cross section image of the microchannel after the bonding process is observable in Fig. 4(d). It depicts that microchannel bonding process was successfully performed. Furthermore, by comparing the mentioned images, it can be confirmed that there are no considerable changes in the horizontal and vertical dimensions of the microchannel after the bonding process. Moreover, no microcracks and dimension deformation was observed all over the microchannel. Consequently, these observations indicate the quality of the fabricated microchannel with deposition of Al layer. Although two different types of leakage tests were performed and demonstrated previously [27], a simple leakage test was implemented in the present study in order to ensure that deposition layer does not disturb the quality of microchannel bonding. The leakage test was performed through the compressed dry air capsule and the fabricated microchannel. As it was intended to use inlet and outlet of the microchannel in this test, the upper hole of the microchannel was utterly blocked and sealed. Afterwards, an entrance was connected to the compressed air through the fluidic connectors, and the sample was immersed in a bowl of water (Fig. 5). The compressed air was then injected into the microchannel at the constant pressure of 5 bars. It was observed that the bubbles just came out from the outlet of the microchannel and there were not any exited bubbles from other sections (bubbles were pointed by arrows). In order to prevent the leakage during the test process, all of the connectors were carefully sealed. This process was tested under several pressures, and the results were the 4
Microelectronic Engineering 222 (2020) 111204
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Fig. 4. (a) Top view SEM image of the microchannel on PMMA substrate before and (b) after Al deposition. (c) Cross section SEM image of the microchannel before and (d) after the bonding process.
seen in Fig. 6, the absorption spectrum of toluene is in agreement with the normalized reference spectrum [16]. Generally, it is considered that toluene has a characteristic wavelength at 267 nm due to its absorption peak [22]. Thus, it is possible to use the amplitude of the absorption peak in this wavelength for comparing different concentrations of toluene (Fig. 7). According to Fig. 7, as the concentration of an injected gas increases, the height of an absorption peak increases as well. This is due to the fact that increased gas molecules can absorb more light photons. The tests were implemented in the single step for the toluene concentrations between 20 and 800 ppm. Furthermore, the limit detection of the experimental test was obtained 20 ppm. Moreover, the device shows a linear behavior for the tested gas concentrations. It demonstrates that the device has a predictable performance when it is exposed to different concentrations of toluene gas. Moreover, the error during the test process at each concentration can be seen in Fig. 7. The experimental tests were repeated for three times at different concentrations to evaluate their repeatability. This device shows suitable repeatability according to the determined error for each concentration. Furthermore, the fast response time of the device is another privilege of it compare to other detection devices such as metal oxide sensors. Fast, simple, and reliable method using inexpensive material for microchannel fabrication and bonding, using Al deposition within the PMMA microchannel was successfully performed in this work. It was shown that a repeatable and linear behavior for different concentrations of the toluene could be obtained in a single step detection. This device can be a suitable candidate to be used in practical applications. The main advantage of the present work against prior ones is using PMMA with Al deposition layer as a microchannel which results in lower cost and fragility, faster fabrication process, and more optical path length for gas detection. The effect of Al deposition on microchannel loss and optical path length is also investigated through the simulation process.
Fig. 5. Investigation of the leakage test for the microchannel.
while also the light was exposed along the microchannel. Eventually, we were able to observe the results and graphs due to interaction of gas molecules with light photons using the spectrometer, which was connected to a computer. All of the obtained results and graphs for different gas concentrations were collected as numerical data which enabled us to analyze them in various ways. Fig. 6 shows the absorption spectrum for 350 ppm toluene. Each substance has its own absorption peak spectrum based on its internal structure. As it is a unique characteristic of a substance in various concentrations, absorbance peak can be used as a fingerprint for detection purposes [8]. High selectivity is the primary advantage of the optical gas detection method over other detection methods. As can be 5
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Fig. 6. The toluene absorption spectrum in the concentration of 350 ppm and the normalized toluene reference absorption spectrum.
detection system such as metal oxide sensors, have an excellent intrinsic selectivity and fast response. Declaration of Competing Interest The authors don't have any financial relationship that might be construed as a conflict of interest significant enough to influence the results or interpretation of article accepted for publication. References [1] T. Horiuchi, Y. Ueno, S. Camou, T. Haga, A. Tate, Portable aromatic VOC gas sensor for onsite continuous air monitoring with 10-ppb benzene detection capability, NTT Tech. Rev. 4 (1) (2006) 30–37. [2] Y. Ueno, et al., Separate detection of BTX mixture gas by a microfluidic device using a function of nanosized pores of mesoporous silica adsorbent, Anal. Chem. 74 (20) (2002) 5257–5262. [3] H. Ablat, A. Yimit, M. Mahmut, K. Itoh, Nafion film / K + −exchanged glass optical waveguide sensor for BTX detection, Anal. Chem. 80 (20) (2008) 7678–7683. [4] J. Hue, et al., Benzene detection by absorbance in the range of 20 ppb-100 ppb application: quality of indoor air, Procedia Eng. 47 (2012) 232–235. [5] S. Camou, A. Shimizu, T. Horiuchi, T. Haga, Ppb-level detection of benzene diluted in water by bubbling extraction system and UV spectroscopy based measurements, TRANSDUCERS EUROSENSORS ‘07 - 4th Int. Conf. Solid-State Sensors, Actuators Microsystems, vol. 132, 2007, pp. 261–264. [6] K. Hamdi, M. Hébrant, P. Martin, B. Galland, M. Etienne, Mesoporous silica nanoparticle film as sorbent for in situ and real-time monitoring of volatile BTX (benzene, toluene and xylenes), Sensors Actuators B Chem. 223 (2016) 904–913. [7] H. Qazi, A. Mohammad, M. Akram, Recent Progress in optical chemical sensors, Sensors 12 (12) (2012) 16522–16556. [8] J. Hodgkinson, R.P. Tatam, Optical gas sensing: a review, Meas. Sci. Technol. 24 (1) (2013) 012004. [9] Y. Ueno, T. Horiuchi, T. Morimoto, O. Niwa, Microfluidic device for airborne BTEX detection, Anal. Chem. 73 (19) (Oct. 2001) 4688–4693. [10] H. Manap, K. Suzalina, M.S. Najib, A potential development of breathing gas sensor using an open path fibre technique, Microelectron. Eng. 164 (2016) 59–62. [11] H. Steffes, A. Schleunitz, U. Gernert, R. Chabicovsky, E. Obermeier, A novel optical gas sensor based on sputtered InxOyNz films with gold-nano-dots, Microelectron. Eng. 83 (4–9 SPEC. ISS) (2006) 1197–1200. [12] M. Masrie, B.Y. Majlis, J. Yunas, Fabrication of multilayer-PDMS based microfluidic device for bio-particles concentration detection, Biomed. Mater. Eng. 24 (6) (2014) 1951–1958. [13] H.-F. Li, J.-M. Lin, Applications of microfluidic systems in environmental analysis, Anal. Bioanal. Chem. 393 (2) (2009) 555–567. [14] J. Zhou, A.V. Ellis, N.H. Voelcker, Recent developments in PDMS surface modification for microfluidic devices, Electrophoresis 31 (1) (2010) 2–16. [15] P.N. Nge, C.I. Rogers, A.T. Woolley, Advances in microfluidic materials, functions, integration, and applications, Chem. Rev. 113 (2013) 2550–2583. [16] S. Camou, E. Tamechika, T. Horiuchi, Portable sensor for determining benzene concentration from airborne/ liquid samples with high accuracy, NTT Tech. Rev. 10 (2) (2012) 10–12.
Fig. 7. Absorbance of different toluene concentrations at 267 nm wavelength.
4. Conclusion In this study, a new optofluidic system for optical detection application of toluene in the UV region was introduced. Low cost PMMA was used as a substrate material which reduces the fabrication cost. In addition, PMMA makes it possible to use diverse techniques for microchannel fabrication. Using microfluidic chips in this work for channel fabrication had different advantages such as small size and low fabrication cost. The inside of microchannel was deposited by Al to increase the optical path length and causing better detection response. Furthermore, the effect of metal deposition on optical path length and loss were investigated by simulation study, where 2.2% enhencment in path length and 48.9% reduction in loss was obtained. Different microchannel characterizations revealed that Al deposition did not disturb the bonding process. The Optical detection for toluene in concentrations between 20 and 800 ppm indicated that this optofluidic system can be an appropriate candidate for toluene detection purposes due to its linearity, repeatability, appropriate response, and meeting a low limit of detection for toluene. Also, these kind of devices, unlike other 6
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719. [27] A. Bamshad, A. Nikfarjam, H. Khaleghi, A new simple and fast thermally-solvent assisted method to bond PMMA–PMMA in micro-fluidics devices, J. Micromech. Microeng. 26 (6) (2016) 065017. [28] M. Fleger, D. Siepe, A. Neyer, Microfabricated polymer analysis chip for optical detection, IEE Proc. Nanobiotechnol. 151 (4) (2004) 159–161. [29] F. Pujol-Vila, et al., Portable and miniaturized optofluidic analysis system with ambient light correction for fast in situ determination of environmental pollution, Sensors Actuators B Chem. 222 (2016) 55–62. [30] P.C. Chen, C.W. Pan, W.C. Lee, K.M. Li, An experimental study of micromilling parameters to manufacture microchannels on a PMMA substrate, Int. J. Adv. Manuf. Technol. 71 (9–12) (2014) 1623–1630. [31] X. Zhu, Æ. Liu, Æ. Guo, Study of PMMA thermal bonding, Microsyst. Technol. 13 (2007) 403–407. [32] H.H. Tran, W. Wu, N.Y. Lee, Ethanol and UV-assisted instantaneous bonding of PMMA assemblies and tuning in bonding reversibility, Sensors Actuators B Chem. 181 (2013) 955–962. [33] K. Fong, S. Ahsan, N. Budraa, W.J. Li, J.D. Mai, Microwave bonding of polymerbased substrates for potential encapsulated micro / nanofluidic device fabrication, Sensors Actuators A Phys. 114 (2004) 340–346. [34] W.S. Junior, et al., Friction spot welding of PMMA with PMMA / silica and PMMA / silica- g -PMMA nanocomposites functionalized via ATRP, Polymer. 55 (20) (2014) 5146–5159.
[17] A. Allouch, S. Le Calvé, C.A. Serra, Portable, miniature, fast and high sensitive realtime analyzers: BTEX detection, Sensors Actuators B Chem. 182 (2013) 446–452. [18] S. Camou, A. Shimizu, T. Horiuchi, T. Haga, ppb-Level detection of benzene diluted in water with portable device based on bubbling extraction and UV spectroscopy, Sensors Actuators B Chem. 132 (2) (2008) 601–607. [19] S. Camou, A. Shimizu, T. Horiuchi, T. Haga, Selective aqueous benzene detection at ppb level with portable sensor based on pervaporation extraction and UV-spectroscopy, Procedia Chem. 1 (1) (2009) 1495–1498. [20] H. Ablat, M. Mahmut, K. Itoh, Optical waveguide BTX gas sensor, Environ. Sci. Technol. 43 (13) (2009) 5113–5116. [21] P. Nizamidin, A. Yimit, I. Nurulla, K. Itoh, Optical waveguide BTX gas sensor based on yttrium-doped lithium iron phosphate thin film, ISRN Spectrosc. 2012 (2012) 1–6. [22] G. Testa, G. Persichetti, L. Zeni, P.M. Sarro, R. Bernini, Optofluidics: a new tool for sensing, Proc. SPIE. 8794 (2013) 879402. [23] C.W. Wu, G.C. Gong, Fabrication of PDMS-based nitrite sensors using teflon AF coating microchannels, IEEE Sensors J. 8 (5) (2008) 465–469. [24] C.A. Barrios, et al., Demonstration of slot-waveguide structures on silicon nitride/ silicon oxide platform, Opt. Express 15 (11) (2007) 6846–6856. [25] V. Korampally, et al., Development of a miniaturized liquid core waveguide system with nanoporous dielectric cladding - a potential biosensing platform, IEEE Sensors J. 9 (12) (2009) 1711–1718. [26] G. Hass, J.E. Waylonis, Optical constants and reflectance and transmittance of evaporated aluminum in the visible and ultraviolet, J. Opt. Soc. Am. 51 (7) (1961)
7
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Research paper
A micro optofluidic system for toluene detection application a,⁎
a
T
a
Mohammad Ramezannezhad , Alireza Nikfarjam , Hassan Hajghassem , Mohammad Makhdoumi Akrama, Meisam Gazmehb a b
MEMS & NEMS Laboratory, Faculty of New Sciences & Technologies, University of Tehran, 1439957131 Tehran, Iran Laser and Plasma Research Institute, Shahid Beheshti University, G.C.Evin, Tehran,Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: PMMA Gas detection UV spectroscopy Toluene
A new application of metal deposited poly (methyl methacrylate) (PMMA) microchannel for optical detection of toluene in the UV spectrum region is presented in this study. Laser cutting and hole formation in initial PMMA sheets, micro-milling for groove shaping, aluminium deposition on two caps using DC magnetron sputtering for increasing optical path length, and bonding process using thermally-solvent method are the main steps for the microchannel fabrication. The dimensions and length of microchannel are 370 μm × 370 μm and 5 cm, respectively. The SEM and leakage tests are implemented to investigate the quality of the microchannel. Besides, the characterization results indicate that Al deposition has no adverse influence on the bonding process. Our simulation results indicate that the optical path length enhanced by 2.2% and the loss decreased by 48.9% when Al is deposited on the surfaces of the microchannel. Also, the effect of microchannel length on optical path length and loss is investigated using simulation. The optical detection tests are performed in a single step using UV spectroscopy based on absorption for toluene in concentration range within 20–800 ppm. The tests are repeated several times for each toluene concentration. Our experimental observation indicate that the results obtained from this optofluidic system is repeatable with linear gas detection and fast response and perfect selectivity which make it a good candidate for toluene detection applications.
1. Introduction The ability of detecting different types of pollutant gas and controlling their concentration are considered to be an important issue in recent years. This attention comes from their vital role in human's life and the environmental condition. Among different pollutant gases, the BTX group (Benzene, Toluene, and Xylene) are the environmental pollutants, which are categorized as Volatile Organic Compounds (VOCs). Detection importance of this group is due to their harmful effects on human's health and environment. Among various BTX detection methods, the optical method is one of the oldest and the most important technique which has attracted researchers' focus [1–6]. This interest is due to its considerable advantages such as suitable accuracy and sensitivity, perfect selectivity, appropriate stability, fast response, and integration ability with the other measurement instruments [7–11]. However, the enormous size, non-portability, and high price of conventional optical setups are the disadvantages of these measurement systems. According to the considerable advantages of optical measurement methods, miniaturization has been one of the most prominent aspects for overcoming their
⁎
limitations. Using a microfluidic instrument with the measurement system can be a reliable solution for miniaturization purpose. Microfluidic system has some advantages such as portability, fast analyzing time, low sample volume consumption, laminar flow rate and high sensitivity which enable it to be a suitable candidate for using in measurement systems [9,12–15]. Furthermore, UV-VIS spectroscopy based on absorption can be a suitable choice for BTX detection due to their absorption wavelength range which is between 230 and 290 nm [16]. It should be noticed that several limit detections are presented for this group due to different standards [1,6,17]. Several studies have been carried out on the detection methods in recent years. Ueno et al. introduced an optical system for BTX detection and developed it through the years [1,2,9]. The system was based on two steps detection, including pre-concentration and detection cells. The detection process was performed by UV spectroscopy based on absorption. Moreover, the cells were fabricated by Pyrex through the anodic bonding process, and the outcoming limit detection was 4 ppm. In following, Camou et al. worked on BTX extraction from the liquid and its improvement [18,19]. Most of the improvements were
Corresponding author. E-mail address: [email protected] (A. Nikfarjam).
https://doi.org/10.1016/j.mee.2019.111204 Received 21 October 2019; Received in revised form 3 December 2019; Accepted 27 December 2019 Available online 31 December 2019 0167-9317/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Main steps for microchannel fabrication including laser cutting for size and hole formation, micro-milling for groove shaping, Al deposition on two caps using DC magnetron sputtering, and bonding process by the thermally-solvent method.
2. Experimental methods
attributed to the pre-concentration cell, such as optimal design and using more efficient absorbent materials. Other studies for optical detection of BTX were implemented a few years later [20,21]. In these researches, the gases were detected using an optical waveguide with sensitive material on a glass substrate. Measurement procedure was carried out based on light transmission variations after chemical absorption between a target gas and sensitive material, which helped for achieving 8 ppm limit detection. Although, mentioned studies revealed great results including magnificent limit detection, some limitative parameters such as complexity of the sensing systems(e.g., two step detection), and using fragile and hard-machining material(Pyrex and glass) still existed. The optofluidic system, which is the integration of the microfluidic and optical systems, contains a waveguide which enables the interaction between the light and gas [22]. It provides some unique advantages compared to bulk detection systems such as smaller sample volumes, faster response time, portability, and lower cost. Among different types of waveguide [22–25], using metal coating within the microchannel can be an appropriate method to fabricate micro waveguides, especially for gas detection purpose. When microchannel is used for both guiding light and fluid, proper metal deposition may be used to increase the optical path length, enhancing the interaction between fluid and light, and obtaining more favorable sensitivity due to the consecutive reflections within the microchannel. Choosing suitable material as a metal coating depends on the range of working wavelength since each metal has particular reflectivity in different segments of the electromagnetic spectrum. In the UV region, aluminium can be a suitable choice to be deposited within the microchannel [26]. Among the different types of polymer used for microfluidic device fabrication, poly(methyl methacrylate) (PMMA) is a perfect choice due to its unique features such as suitable mechanical stability, capability for performing micromachining process, low weight, gas impermeability and low price [15]. Moreover, in fabricating PMMA microchannels, several distinct methods can be used for bonding. Among them, a thermally-solvent method is a simple and appropriate choice due to its well-known advantages [27]. Although PMMA is a beneficial material to be used in optofluidic devices based on the mentioned properties of it, it has had limited number of usages in the optofluidic area [28,29] which all of them were used without metal deposition. In this study, a new and high throughput optical toluene detection system using a low cost optofluidic microchip fabricated through a simple method is introduced. The microchannel fabrication processes include channel formation with micro-milling method, aluminium deposition on all surfaces, and bonding process. Using Al deposition within the microchannel facilitates the optical path length increment and improves the gas detection condition, which is also investigated using simulation process. It is noteworthy to say that there is no report regarding Al deposition on PMMA microchannel surfaces for optical gas detection purposes. Moreover, the microfluidic system has the benefit of a small device size during fabrication and testing. The quality of the fabricated microchannel is investigated with SEM characterization and leakage test. Eventually, it is tested in an optofluidic system for toluene gas detection with concentrations between 20 and 800 ppm.
The experimental procedures of the current study are performed in different stages which are explained in the following subsections. 2.1. Microchannel fabrication Based on the standard for PMMA bonding [27], transparent PMMA with 2 mm thickness(Year Long Co) was chosen as a substrate material for microchannel fabrication. PMMA is more flexible and stress-resistant material compared to common microfluidic materials such as Pyrex or silicon. Also, its cutting and engraving are more straightforward. Fig. 1 shows the implemented stages for microchannel fabrication. At first, the PMMA sheet was cut by CO2 laser in 2 cm × 5 cm dimensions. Afterwards, a 500 μm hole was created in the center of upper microchannel cap for gas injection purpose. Subsequently, for 370 μm × 370 μm microfluidic groove formation, micro-milling process was implemented on the lower cap using CNC machine. This method is a very cost effective and fast way for the fabrication of the microfluidic devices [30]. After the mentioned stages, DC magnetron sputtering for Al deposition on PMMA surface was adopted. An HNO3 solution (65%) was used as an Al etchant for locally etching the extra Al segments. Photolithography process was then used for a final Al patterning on the surfaces. Al layer was formed just in the middle of the surfaces. A final step in microchannel fabrication was bonding process for upper and lower caps. Generally, there are various bonding methods for PMMA such as thermal bonding [31], UV assistant bonding [32], solvent bonding [27], microwave bonding [33], and friction spot welding [34]. However, a final microchannel in this work was made using thermally-solvent process without any deformation and imperfection. This method is a simple and inexpensive bonding technique, without need of any individual and expensive instruments. The intended thermally-solvent bonding method was previously reported [27]. Briefly, for PMMA-PMMA bonding, surfaces were first accurately rinsed with water and ordinary detergent to eliminate oil and dust. Afterwards, deionized water and Isopropyl alcohol (IPA) were separately used for the next step of washing and cleaning of the surfaces respectively. Each sample was shaken and kept in deionized water and IPA for 3 min. As surface cleaning is an essential part of the process, this step was repeated for three times. Nitrogen gas was used for surfaces drying at the end of each cleaning part. After cleaning and drying, 70% Isopropyl alcohol solution was prepared and poured on one of the surfaces using a syringe. The other surface was precisely aligned and placed on the other one. Subsequently, they were fixed by the paper clamps. The assembled surfaces were then placed in a fan-assisted oven at 68 °C for 10 min. Eventually, the sample was ready to be used after gradual cooling down to room temperature and inspection of the bonding quality. 2.2. Test setup The experimental setup for an optical gas detection test is shown in Fig. 2. The working region in the electromagnetic spectrum contained UV wavelengths; therefore, DT-Mini-2-GS (Ocean Optics Co) light 2
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Fig. 2. (a) The schematic and (b) image of the experimental test setup for optical gas detection.
angles between 0 to ± 12 degree with a step of ± 3 degree. The results are shown in Table 1. According to Table 1, the optical path length in the microchannel with the Al coating increases by 2.2% compared to no Al coating. It may be due to an enhancement of consecutive reflections inside the Al deposited microchannel. More optical path length can cause more interaction between the light and gas during the gas detection, which can in turn improve the detection system performance. Furthermore, the loss inside the Al deposited microchannel dramatically decreased by 48.9%. Al deposition can also improve guiding of the light inside the microchannel and prevent the escape of the light through its path in the waveguide. The results clearly indicate that metal deposition inside the microchannel has significant effect on the improvement of the detection situation. Moreover, Table 2 represents the effect of microchannel length on loss and optical path length. Simulation condition was kept the same as the one which was already investigated. All microchannels were considered with Al deposition. According to Table 2, the optical path length increases with the microchannel length however, this also increases the loss as well. Therefore, 5 cm channel length could be a suitable choice considering both parameters.
source and HR 4000(Ocean Optics Co) spectrometer were used in the setup, which both of them were connected to a fiber optic at their outputs. According to the current discrete design for light guiding, two 74-UV collimating lenses (Ocean Optics Co) were used for a better light coupling in an experimental setup. The light source output and lenses were placed on the fixed stages. However, the microchannel and spectrometer outputs were put on three axes manual stages for more accurate aligning and guiding light through the waveguide. The microchannel was accurately placed in the focal length of lenses for more efficient testing. Also, a well-sealed chamber for liquid toluene evaporation and a Hamilton syringe for a gas injection to the microchannel were utilized for detection purposes. A target gas was injected form the upper hole of the microchannel, whereas it exited from its two ends. At the same time, the light was focused inside the microchannel through the lenses and reached the spectrometer after interaction with gas molecules inside the microchannel. 3. Results The results, which were obtained through the simulation, characterization, and testing are discussed in following subsections.
3.2. Microchannel characterization 3.1. Simulation results Several aspects of the microchannel, which are related to our present work, are investigated in the following sections. Fig. 4(a) and (b) represent the SEM image of the microchannel on the PMMA substrate before and after Al deposition(before the bonding process). It demonstrates that micro- milling process was accurately performed and produced uniform and regular edges of the groove. Moreover, it can be seen that Al deposition has appropriate quality and uniformity within the microchannel. Fig. 4(c) shows a cross section image of the microchannel before the bonding process. This image
The influence of the reflective layer on optical path length and the loss of the microchannel which are the key parameters in optical gas detection was simulated and is shown in Fig. 3. The simulation was performed using COMSOL software and ray optic physics at the wavelength of 267 nm. The dimensions and length of the simulated microchannel were the same as the experimental device. Two microchannels, one with an Al coating of 93% reflectivity and the other without Al coating were investigated. The light source was applied with the tilt 3
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Fig. 3. The effect of metal deposition on optical path length and loss by simulation for microchannel (a) without Al deposition (b) with Al deposition.
same. Other characterizations of the intended PMMA microchannel bonding method were previously investigated [27].
Table 1 Simulation results about the effect of Al deposition on optical path length and loss for 5 cm microchannel. Al deposition
Optical path length (mm)
Loss(%)
No Yes
50 51.1
88.9 40
3.3. Detection results Gas detection was performed by an optical method based on absorption. In absorption state, gas detection is achieved after gas molecules interaction with light beams and causing the absorption of light by gas molecules which can be investigated by a spectrophotometry technique. According to Beer-Lambert law, which is attributed to the absorption state of optical measurement, the optical path length is a determining factor in an optical gas detection based on absorption. It demonstrates that optical path length has a direct relation with received light intensity to a detector after interaction with gas molecules. This also has an effect on the final absorption peak spectrum. In an optofluidic device, it is possible to use wall reflections to increase an optical path length. Wall reflection can be performed by metal deposition within the microchannel, which can cause more wall reflections and more appropriate light guidance in the light path. As it was mentioned previously, Al was deposited on PMMA microchannel surfaces. However, it should be noticed that physical optical path length enhancement is somewhat restricted since in the practical situation, physical space is limited for a test setup. Furthermore, large optical path length can also cause more losses along the microchannel, which can reduce the detection quality. As a result, having an optimum microchannel length can improve a limit of detection of the device. In addition, simulation using Comsol software was used to obtain the suitable microchannel length. As the simulation results show, the length of 5 cm can be an appropriate choice. It was determined regarding both optical path length and loss as the critical factors in an optical instrument. 8 cm length microchannel has the largest optical path length with considerable loss(84%). Also, the fabrication process for 8 cm length microchannel is more difficult compared to shorter microchannels. Therefore, according to the mentioned factors and experimental observations, the final length of microchannels for our experimental proposes was 5 cm. In the first step of the experimental test, a certain volume of the liquid toluene as a sample was injected into a well-sealed chamber with a specified volume. After that, the chamber was heated for toluene evaporation resulting in a specified concentration of toluene in a gas phase. Subsequently, the gas was injected by a Hamiltonian syringe to a microchannel through the upper hole of the chamber. As a result, the detection process was started by gas injection into the microchannel
Table 2 Simulation results about the effect of microchannel length on optical path length and loss. Microchannel length (mm)
Optical path length (mm)
Loss(%)
20 50 80
20 51.1 84.6
22.2 40 84.4
provides an opportunity to compare two dimensions of the microchannel before and after the bonding process. In addition, a cross section image of the microchannel after the bonding process is observable in Fig. 4(d). It depicts that microchannel bonding process was successfully performed. Furthermore, by comparing the mentioned images, it can be confirmed that there are no considerable changes in the horizontal and vertical dimensions of the microchannel after the bonding process. Moreover, no microcracks and dimension deformation was observed all over the microchannel. Consequently, these observations indicate the quality of the fabricated microchannel with deposition of Al layer. Although two different types of leakage tests were performed and demonstrated previously [27], a simple leakage test was implemented in the present study in order to ensure that deposition layer does not disturb the quality of microchannel bonding. The leakage test was performed through the compressed dry air capsule and the fabricated microchannel. As it was intended to use inlet and outlet of the microchannel in this test, the upper hole of the microchannel was utterly blocked and sealed. Afterwards, an entrance was connected to the compressed air through the fluidic connectors, and the sample was immersed in a bowl of water (Fig. 5). The compressed air was then injected into the microchannel at the constant pressure of 5 bars. It was observed that the bubbles just came out from the outlet of the microchannel and there were not any exited bubbles from other sections (bubbles were pointed by arrows). In order to prevent the leakage during the test process, all of the connectors were carefully sealed. This process was tested under several pressures, and the results were the 4
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Fig. 4. (a) Top view SEM image of the microchannel on PMMA substrate before and (b) after Al deposition. (c) Cross section SEM image of the microchannel before and (d) after the bonding process.
seen in Fig. 6, the absorption spectrum of toluene is in agreement with the normalized reference spectrum [16]. Generally, it is considered that toluene has a characteristic wavelength at 267 nm due to its absorption peak [22]. Thus, it is possible to use the amplitude of the absorption peak in this wavelength for comparing different concentrations of toluene (Fig. 7). According to Fig. 7, as the concentration of an injected gas increases, the height of an absorption peak increases as well. This is due to the fact that increased gas molecules can absorb more light photons. The tests were implemented in the single step for the toluene concentrations between 20 and 800 ppm. Furthermore, the limit detection of the experimental test was obtained 20 ppm. Moreover, the device shows a linear behavior for the tested gas concentrations. It demonstrates that the device has a predictable performance when it is exposed to different concentrations of toluene gas. Moreover, the error during the test process at each concentration can be seen in Fig. 7. The experimental tests were repeated for three times at different concentrations to evaluate their repeatability. This device shows suitable repeatability according to the determined error for each concentration. Furthermore, the fast response time of the device is another privilege of it compare to other detection devices such as metal oxide sensors. Fast, simple, and reliable method using inexpensive material for microchannel fabrication and bonding, using Al deposition within the PMMA microchannel was successfully performed in this work. It was shown that a repeatable and linear behavior for different concentrations of the toluene could be obtained in a single step detection. This device can be a suitable candidate to be used in practical applications. The main advantage of the present work against prior ones is using PMMA with Al deposition layer as a microchannel which results in lower cost and fragility, faster fabrication process, and more optical path length for gas detection. The effect of Al deposition on microchannel loss and optical path length is also investigated through the simulation process.
Fig. 5. Investigation of the leakage test for the microchannel.
while also the light was exposed along the microchannel. Eventually, we were able to observe the results and graphs due to interaction of gas molecules with light photons using the spectrometer, which was connected to a computer. All of the obtained results and graphs for different gas concentrations were collected as numerical data which enabled us to analyze them in various ways. Fig. 6 shows the absorption spectrum for 350 ppm toluene. Each substance has its own absorption peak spectrum based on its internal structure. As it is a unique characteristic of a substance in various concentrations, absorbance peak can be used as a fingerprint for detection purposes [8]. High selectivity is the primary advantage of the optical gas detection method over other detection methods. As can be 5
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Fig. 6. The toluene absorption spectrum in the concentration of 350 ppm and the normalized toluene reference absorption spectrum.
detection system such as metal oxide sensors, have an excellent intrinsic selectivity and fast response. Declaration of Competing Interest The authors don't have any financial relationship that might be construed as a conflict of interest significant enough to influence the results or interpretation of article accepted for publication. References [1] T. Horiuchi, Y. Ueno, S. Camou, T. Haga, A. Tate, Portable aromatic VOC gas sensor for onsite continuous air monitoring with 10-ppb benzene detection capability, NTT Tech. Rev. 4 (1) (2006) 30–37. [2] Y. Ueno, et al., Separate detection of BTX mixture gas by a microfluidic device using a function of nanosized pores of mesoporous silica adsorbent, Anal. Chem. 74 (20) (2002) 5257–5262. [3] H. Ablat, A. Yimit, M. Mahmut, K. Itoh, Nafion film / K + −exchanged glass optical waveguide sensor for BTX detection, Anal. Chem. 80 (20) (2008) 7678–7683. [4] J. Hue, et al., Benzene detection by absorbance in the range of 20 ppb-100 ppb application: quality of indoor air, Procedia Eng. 47 (2012) 232–235. [5] S. Camou, A. Shimizu, T. Horiuchi, T. Haga, Ppb-level detection of benzene diluted in water by bubbling extraction system and UV spectroscopy based measurements, TRANSDUCERS EUROSENSORS ‘07 - 4th Int. Conf. Solid-State Sensors, Actuators Microsystems, vol. 132, 2007, pp. 261–264. [6] K. Hamdi, M. Hébrant, P. Martin, B. Galland, M. Etienne, Mesoporous silica nanoparticle film as sorbent for in situ and real-time monitoring of volatile BTX (benzene, toluene and xylenes), Sensors Actuators B Chem. 223 (2016) 904–913. [7] H. Qazi, A. Mohammad, M. Akram, Recent Progress in optical chemical sensors, Sensors 12 (12) (2012) 16522–16556. [8] J. Hodgkinson, R.P. Tatam, Optical gas sensing: a review, Meas. Sci. Technol. 24 (1) (2013) 012004. [9] Y. Ueno, T. Horiuchi, T. Morimoto, O. Niwa, Microfluidic device for airborne BTEX detection, Anal. Chem. 73 (19) (Oct. 2001) 4688–4693. [10] H. Manap, K. Suzalina, M.S. Najib, A potential development of breathing gas sensor using an open path fibre technique, Microelectron. Eng. 164 (2016) 59–62. [11] H. Steffes, A. Schleunitz, U. Gernert, R. Chabicovsky, E. Obermeier, A novel optical gas sensor based on sputtered InxOyNz films with gold-nano-dots, Microelectron. Eng. 83 (4–9 SPEC. ISS) (2006) 1197–1200. [12] M. Masrie, B.Y. Majlis, J. Yunas, Fabrication of multilayer-PDMS based microfluidic device for bio-particles concentration detection, Biomed. Mater. Eng. 24 (6) (2014) 1951–1958. [13] H.-F. Li, J.-M. Lin, Applications of microfluidic systems in environmental analysis, Anal. Bioanal. Chem. 393 (2) (2009) 555–567. [14] J. Zhou, A.V. Ellis, N.H. Voelcker, Recent developments in PDMS surface modification for microfluidic devices, Electrophoresis 31 (1) (2010) 2–16. [15] P.N. Nge, C.I. Rogers, A.T. Woolley, Advances in microfluidic materials, functions, integration, and applications, Chem. Rev. 113 (2013) 2550–2583. [16] S. Camou, E. Tamechika, T. Horiuchi, Portable sensor for determining benzene concentration from airborne/ liquid samples with high accuracy, NTT Tech. Rev. 10 (2) (2012) 10–12.
Fig. 7. Absorbance of different toluene concentrations at 267 nm wavelength.
4. Conclusion In this study, a new optofluidic system for optical detection application of toluene in the UV region was introduced. Low cost PMMA was used as a substrate material which reduces the fabrication cost. In addition, PMMA makes it possible to use diverse techniques for microchannel fabrication. Using microfluidic chips in this work for channel fabrication had different advantages such as small size and low fabrication cost. The inside of microchannel was deposited by Al to increase the optical path length and causing better detection response. Furthermore, the effect of metal deposition on optical path length and loss were investigated by simulation study, where 2.2% enhencment in path length and 48.9% reduction in loss was obtained. Different microchannel characterizations revealed that Al deposition did not disturb the bonding process. The Optical detection for toluene in concentrations between 20 and 800 ppm indicated that this optofluidic system can be an appropriate candidate for toluene detection purposes due to its linearity, repeatability, appropriate response, and meeting a low limit of detection for toluene. Also, these kind of devices, unlike other 6
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