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Colorimetric detection of volatile organic compounds using a colloidal crystal-based chemical sensor for environmental applications
Sensors and Actuators B 125 (2007) 589–595
Colorimetric detection of volatile organic compounds using a colloidal crystal-based chemical sensor for environmental applications Tatsuro Endo a,b,∗ , Yasuko Yanagida a,b , Takeshi Hatsuzawa a,b a
Department of Mechano-Micro Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan b Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan Received 8 November 2006; received in revised form 28 February 2007; accepted 5 March 2007 Available online 12 March 2007
Abstract The detection of pollutants such as volatile organic compounds (VOCs) is of significant importance for environmental protection. However, conventional monitoring methods are often time consuming and require expensive equipments. In this study, a colloidal crystal-based colorimetric chemical sensor was developed for environmental applications. The device consists of a glass substrate with a three-dimensional colloidal crystal and poly(dimethylsiloxane) (PDMS) elastomer. Such a colloidal crystal was generated by infiltrating the voids within an opaline lattice of polystyrene nanoparticles with a liquid prepolymer to PDMS, followed by thermal curing. When a sample solution such as benzene, toluene, or xylene, capable of swelling the elastomer matrix, was applied to the surface of this crystal, the lattice constant and thus the wavelength of Bragg diffracted light was increased. On the basis of this mechanism, we demonstrated the colorimetric detection of VOCs. As a result, the colloidal crystal-based chemical sensor could be used to specifically determine VOC concentrations. Additionally, using this colloidal crystal-based chemical sensor, the change in the optical characteristics could be observed with the naked eye. Therefore, this chemical sensor can be applicable to on-site monitoring for environmental applications. © 2007 Elsevier B.V. All rights reserved. Keywords: Colloidal crystal; Nanoparticles; Chemical sensor; Poly(dimethylsiloxane); Environmental application; Volatile organic compounds (VOCs)
1. Introduction During recent decades, volatile organic compounds (VOCs) have been commonly found in the environment, in the soil, groundwater, and atmosphere [1,2]. Therefore, humans are easily exposed to these chemicals through the skin, by breathing, and by eating, and even at low concentrations this exposure can present long-term health risks. Among the VOCs, benzene and its derivatives such as benzene, toluene, and xylene (BTX), were confirmed to be human carcinogens, and could cause diversiform cancers, for example, lymphatic and hematopoietic cancers. Furthermore, ingestion of drinking water containing VOCs may lead to liver and kidney damage, and disorders of the immune system, nervous system, and reproductive system as well as several ∗
Corresponding author at: Department of Mechano-Micro Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan. Tel.: +81 45 924 5088; fax: +81 45 924 5088. E-mail address: [email protected] (T. Endo). 0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.03.003
types of cancers [3]. The fact that these VOCs have very different toxicities has meant that society has dealt with each one of them differently. From the viewpoint of environmental protection, it has become a matter of high priority to address the issue of the detection of these compounds. An important and non-trivial first step is to identify sites of pollution, and perform on-field monitoring, which requires a simple, economical, and rapid test for the detection of these compounds in samples taken from industrial processing, soils, human and animal tissues, and foodstuffs. This fact stimulates the development of measurement methods and techniques for environmental pollutant monitoring. With this background, there is an increasing interest in the development of a sensor for VOC detection [4–7]. There are many fields in which these kinds of sensors can be used, such as environmental applications, electronic noses, and in chemical industries, and in fact, many sensors have already been developed. However, the present sensors used for environmental applications, such as gas chromatography/mass spectrometry (GC/MS), require a relatively long assay time that involves troublesome liquid handling and many expensive reagents and apparatus [8,9].
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To overcome these disadvantages, several groups have developed different detection methods for sensors in environmental applications. Mitsubayashi et al. reported the development of an enzyme-based formaldehyde biosensor that detected low concentrations of acetaldehyde using an electrochemical detection method [10]. Mascini et al. developed a dioxin biosensor based on a piezoelectric sensor [11]. In addition, a microfluidic environmental sensor fabricated using microelectromechanical system (MEMS) technology has also been reported [12–14]. However, almost all conventional sensors for environmental applications involve high-cost apparatus that are difficult to use. Therefore, there is a demand for a more convenient and costeffective sensor system. Hence, we targeted the development of the colloidal crystal-based chemical sensor towards this requirement. Colloidal crystals are three-dimensionally periodic lattices of mono-dispersed, spherical colloids such as polystyrene and silica. The periodic lattices of the colloidal crystal will diffract incident light in accordance with Bragg’s law [15]. The reflected light in the visible spectrum is called the structural color. Additionally, the optical characteristics of colloidal crystals are related to their lattice constants. A millimeter-sized periodic structure can act as a colloidal crystal in millimeter wavelengths; a submicrometer-sized structure, in a visible light region; and the nanometer-sized ones, in the X-ray region. Hence, the resulting colloidal crystals diffract UV, visible, or near-IR light, depending on their lattice constants. A colloidal crystal diffracts light according to the Bragg equation (Fig. 1(a)): 1/2
mλpeak = 2 d1 1 1 (n2eff − sin2 θ)
(1)
where m denotes the order of diffraction; λpeak the wavelength of the diffraction peak; d1 1 1 the spacing between (1 1 1) planes; θ the angle between the incident light and the normal to the diffraction planes; and neff is the mean refractive index of the crystalline lattice. Based on these features, the colloidal crystal is an ideal candidate for fabricating optical sensors that can be used to determine environmental changes in terms of color changes. By using these features, several sensors based on colloidal crystals were fabricated by embedding colloidal crystals in appropriate polymers or hydrogels. These colloidal crystalbased sensors exhibit brilliant colors, which change in response to temperature, pH, ionic species, and glucose levels (Fig. 1(b)) [16,17]. Asher et al. reported the development of a colloidal crystal-based sensor for the detection of creatinine [18] and glucose [19] for medical applications. In all of these studies, the lattice constant, and thus the color exhibited by the colloidal crystal, as determined by the Bragg Eq. (1), varied in response to the environmental changes. In a number of related studies, the change in the refractive index was also demonstrated as a means to detect variations in the environment. In this study, poly(dimethylsiloxane) (PDMS) was used for the fabrication of colloidal crystal-based chemical sensors for environmental applications. PDMS swells in non-polar organic solvents. In addition, its swelling kinetics differ according to the polarity of the solvent [20]. Using this material, we fabricated a simple, convenient, and cost-effective colloidal crystal-based
Fig. 1. (a) Theoretical schematic illustration of the colloidal crystal-based chemical sensor. (b) Detection principle of the colloidal crystal-based chemical sensor.
chemical sensor for the colorimetric detection of VOCs for environmental applications. Previously, for environmental applications, similar detection principles or materials have been used for detection of the various vapors [21,22]. In addition, using this colloidal crystal-based chemical sensor for detection of VOCs, more simplified detection systems can be established. In this report, we fabricated a large-area (up to mm2 ), wellordered, three-dimensional colloidal crystal-based chemical sensor using polystyrene nanoparticles by using the dry-up process of its colloidal dispersion. The optical characteristics of the three-dimensional colloidal crystal-based chemical sensor were evaluated by using a UV–vis spectrometer, and the surface analysis was performed by using atomic force microscopy (AFM). Simultaneously, we performed the evaluation of the selectivity and calibration characteristics of this colloidal crystal-based chemical sensor for BTX. 2. Experimental 2.1. Materials Polystyrene nanoparticles (diameter: 202 nm, 2.57%, w/v) used for preparing the colloidal crystals were purchased from
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Polysciences Inc. (Warrington, PA). Slide glass substrates (S1111, 76 mm × 26 mm, thickness: 0.8–1.0 mm) were purchased from Matsunami Glass Ind., Ltd. (Osaka, Japan). The PDMS prepolymer (SILPOT 184 silicone elasotmer kit) was purchased from Dow Corning Asia (Tokyo, Japan). Organic solvents (ethanol, methanol, 2-propanol, acetone, benzene, toluene, and xylene) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). Milli-Q water (18.3 M cm) from Millipore (Billerica, USA) was used for all experiments. 2.2. Apparatus A spectrophotometer (DU® 800 wavelength range: 190– 1100 nm) used for the evaluation of the optical characteristics of the colloidal crystal-based chemical sensor was purchased from Beckman Coulter (CA, USA). AFM for the surface analysis of the colloidal crystal sensor was performed using a commercial AFM unit (SPA-400, Seiko Instruments, Inc., Chiba, Japan) with a calibrated 20 m xy-scan and 10 m z-scan range PZTscanner. 2.3. Formation of the colloidal crystal using polystyrene nanoparticles Slide glass substrates, after the pretreatment of their surfaces, were used as substrates for the formation of the colloidal crystal. The slide glass substrates were cut to 10 mm × 26 mm. All substrates were cleaned using a washing process involving ultrasonic cleaning in acetone for 30 min, followed by thorough rinsing with Milli-Q water (18.3 M cm) and drying at room temperature (RT). Finally, it was stored in a desiccator before use. Polystyrene nanoparticles (particle diameter: 202 nm) were used to form the colloidal crystals. The first step involved the fabrication of colloidal crystals by drying aqueous dispersions of polystyrene nanoparticles on the glass substrates. In this experiment, 20 l of the polystyrene nanoparticle dispersion (2.0%, w/v) was introduced on the surface of a slide glass substrate to form a thin layer of liquid film. Finally, this sample was placed under ambient laboratory conditions to allow the water to evaporate. The polystyrene nanoparticles were driven into a long-range ordered, opaline lattice by the attractive capillary forces generated during water evaporation. 2.4. Fabrication of the colloidal crystal-based chemical sensor using PDMS The fabrication procedure of the colloidal crystal-based chemical sensor using nanoparticles of different diameters is shown in Fig. 2(a). After the dry-up process of the nanoparticle dispersion, the PDMS solution (50 l) was then distributed on the top of the colloidal crystal, and the voids between the polystyrene nanoparticles were completely filled with the premixed elastomer of PDMS through capillary action. The elastomer was then cured at room temperature overnight, followed by additional baking at 60 ◦ C for 1 h.
Fig. 2. (a) Fabrication procedure of the colloidal crystal-based chemical sensor using polystyrene nanoparticles. (b) Experimental setup for the evaluation of the optical characteristics of the colloidal crystal-based chemical sensor.
In this study, in order to fabricate of the colloidal crystalbased chemical sensor using PDMS, a curing agent and the PDMS prepolymer were mixed in a 1:10 ratio by weight. The PDMS solution was degassed in desiccators with a mechanical vacuum pump to remove any air bubbles in the solution and ensure complete mixing between the two parts. Thus, these fabrication procedures resulted in the fabrication of a colloidal crystal-based chemical sensor. To evaluate the optical characteristics of the colloidal crystalbased chemical sensor, the experimental setup used was as shown in Fig. 2(b). All absorbance spectra were taken from 400 to 800 nm on the UV–vis spectrometer at RT. Under these experimental conditions, the optical characteristics of the colloidal crystal-based chemical sensor were investigated. 2.5. Surface analysis of the colloidal crystal surface using AFM To analyze the surface quality of the colloidal crystal in terms of particle density and periodicity, AFM was carried out in tapping mode using silicon tips and cantilevers with a nominal spring constant of 13 N/m for scanning in air. All reported images were acquired at scan rates in the range 0.25–0.50 Hz. 2.6. Colorimetric detection of non-polar organic solvents using the colloidal crystal-based chemical sensor To evaluate the sensing capability of this sensor, different kinds of organic solvents (ethanol, methanol, 2-propanol, acetone, benzene, toluene, and xylene) and ultra pure water were introduced onto its surface and its optical characteristics were
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measured. Simultaneously, we measured the change in its optical characteristics with time (0, 1, 2, 5, and 10 min) due to the swelling of the PDMS and evaluated its calibration characteristics by introducing different concentrations of non-polar organic solvents. 3. Results and discussions 3.1. Characteristics of colloidal crystal-based chemical sensor Fig. 3(a) shows the changes in the absorbance of the colloidal crystal-based chemical sensor from visible (violet color) to nearIR wavelengths. The structural color due to Bragg reflection could also be observed (Fig. 3(b)). In addition, the dependency of absorbance on the polystyrene nanoparticle concentration (for 2.0, 1.0, 0.5, and 0.1%, w/v) was monitored at 552 nm (green color). On the basis of these results, it was considered that the polystyrene nanoparticle concentrations affected the absorbance because of Bragg reflection. As a result, in this study, a 2.0% (w/v) polystyrene nanoparticle solution was used for the fabrication of the colloidal crystal-based chemical sensor. Additionally, surface imaging using AFM was carried out for the observation of the colloidal crystal, formed on the surface of the slide glass substrate, in order to evaluate its periodicity and coverage on the slide glass substrate surface. The AFM images of the colloidal crystal surface using 202 nm polystyrene nanoparticles are shown in Fig. 4. These images clearly illustrated that
Fig. 4. AFM images of the colloidal crystal-based chemical sensor surface.
colloidal crystals having a face-centered cubic (FCC) lattice structure are formed. It was observed that this FCC structure had been formed over a large area. Therefore, it was confirmed that sufficient crystal structure could be formed by this crystallization method using capillary forces. 3.2. Colorimetric detection of organic solvents using the colloidal crystal-based chemical sensor
Fig. 3. (a) Optical characteristics of the colloidal crystal-based chemical sensor fabricated using different concentrations of polystyrene nanoparticles. (b) Photograph of the colloidal crystal-based chemical sensor fabricated using 202 nm polystyrene nanoparticles.
The colorimetric detection of different kinds of organic solvents using colloidal crystal-based chemical sensors was carried out. Five hundred microliters of different organic solvents was introduced onto the sensor surface, and the changes in its optical characteristics were investigated. Fig. 5 shows the changes in the optical characteristics with time of the colloidal crystal-based chemical sensors in the visible region (400–800 nm). When ultra pure water (Fig. 5(a)), ethanol (Fig. 5(b)), acetone (Fig. 5(c)), and several kinds of VOCs were introduced onto the chemical sensor surfaces, a change in its
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Fig. 6. Selectivity for different kinds of organic solvents using the colloidal crystal-based chemical sensor.
Fig. 5. The change in the optical characteristics due to the introduction of different organic solvents: (a) ultra pure water; (b) methanol; (c) acetone.
optical characteristics could be observed (Fig. 6). These peak shifts were affected by only the polarity the organic solvent, and the refractive indexes of these organic solvents did not affect the swelling ratio. The peak wavelength of the colloidal
crystal-based chemical sensor was approximately 550 nm. In addition, when the chemical sensor was immersed in a non-polar organic solvent such as acetone, the structural color immediately changed from green to red (Fig. 7). The peak wavelength shifted to 743 nm after the introduction of acetone. Similarly, structural color changes could also be observed on the introduction of BTX. Thus, the color was tuned according to the swelling of the PDMS matrix in non-polar organic solvents. Additionally, when this chemical sensor was removed from the organic solvent and completely dried in air, its peak wavelength returned to the original position due to the shrinking of the PDMS matrix. For example, when acetone was evaporated from the swollen chemical sensor, the required shrinking time was just over 5 min. Thus, the colloidal crystal-based chemical sensor was perfectly reversible and could be used over 10 times repeatedly (data not shown). These results confirmed that the swelling ratio of the PDMS depended only on the polarities of the organic solvents. In particular, non-polar organic solvents such as BTX caused the PDMS to swell significantly. Hence, the colloidal crystal-based chemical sensor is applicable to detecting BTX in environmental applications. While studying this colloidal crystal-based chemical sensor for environmental applications, different concentrations of BTX solutions were introduced onto the colloidal crystal-based chemical sensor in order to evaluate its calibration characteristics. In the experiments, different concentration of BTX solutions diluted with methanol were introduced onto the sensor for 5 min, and subsequently, its optical characteristics were evaluated. The relationship between the peak shift and the BTX concentrations could be observed, as illustrated in Fig. 8. Furthermore, the detection limits of this sensor were found to be dependent on the polarities of the solvents (benzene: 10 ng/ml (Fig. 8(a)), toluene: 1 ng/ml (Fig. 8(b)), xylene: 10 pg/ml (Fig. 8(c))). From these characteristics, it can be seen that this chemical sensor has a possibility for use in the detection of non-polar organic solvents in the form of a simplified test kit. However, the detection limit of this sensor for BTX was not sufficient to satisfy the environmental quality criteria for ground water, gasses, and foodstuffs. Therefore, further investigations into the details of its fabrication conditions are required.
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Fig. 7. Color change of the colloidal crystal-based chemical sensor due to the introduction of acetone.
4. Conclusions We developed a colloidal crystal-based chemical sensor with a reversibly tunable structural color for environmental applications. The color change caused by swelling and shrinking of the matrix was rapid and could be easily judged with the naked eye. Furthermore, it is possible to determine the concentration of the target molecules by measuring the observed color variation. Hence, this chemical sensor has a potential application in intelligent (or smart) sensors that detect BTX without using a detecting device. Acknowledgements This study was supported by a Grant-in-Aid for Young Scientists (KAKENHI WAKATE (start-up) 18810012) from the Japanese Ministry of Education, Culture, Science, Sports and Technology (MEXT). The authors wish to thank Michiko Banno from the Japan Advanced Institute of Science and Technology (JAIST) for her valuable advice during the fabrication of the colloidal crystal. References
Fig. 8. Calibration characteristics for BTX solutions using the colloidal crystalbased chemical sensor: (a) benzene (b) toluene (c) xylene.
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Biographies Tatsuro Endo received his PhD degree from the Japan Advanced Institute of Science and Technology (JAIST), Biotechnology Laboratory in the Department of Biological Science and Biotechnology, in 2006. His current field of interest includes biochip and nanobiotechnology. Yasuko Yanagida holds a position as associate professor at Tokyo Institute of Technology (Tokyo Tech), Tokyo, Japan from 2003. She received her PhD degree from the Tokyo Tech in 1995. Her current interests are nanobiotechnology, cell engineering, and biochips. Takeshi Hatsuzawa holds a position as professor at Tokyo Institute of Technology (Tokyo Tech), Tokyo, Japan from 2002. He received his PhD degree from the Tokyo Tech in 1983. His current interests are microelectromechanical system (MEMS), nanobiotechnology, and biochips.
Colorimetric detection of volatile organic compounds using a colloidal crystal-based chemical sensor for environmental applications Tatsuro Endo a,b,∗ , Yasuko Yanagida a,b , Takeshi Hatsuzawa a,b a
Department of Mechano-Micro Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan b Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan Received 8 November 2006; received in revised form 28 February 2007; accepted 5 March 2007 Available online 12 March 2007
Abstract The detection of pollutants such as volatile organic compounds (VOCs) is of significant importance for environmental protection. However, conventional monitoring methods are often time consuming and require expensive equipments. In this study, a colloidal crystal-based colorimetric chemical sensor was developed for environmental applications. The device consists of a glass substrate with a three-dimensional colloidal crystal and poly(dimethylsiloxane) (PDMS) elastomer. Such a colloidal crystal was generated by infiltrating the voids within an opaline lattice of polystyrene nanoparticles with a liquid prepolymer to PDMS, followed by thermal curing. When a sample solution such as benzene, toluene, or xylene, capable of swelling the elastomer matrix, was applied to the surface of this crystal, the lattice constant and thus the wavelength of Bragg diffracted light was increased. On the basis of this mechanism, we demonstrated the colorimetric detection of VOCs. As a result, the colloidal crystal-based chemical sensor could be used to specifically determine VOC concentrations. Additionally, using this colloidal crystal-based chemical sensor, the change in the optical characteristics could be observed with the naked eye. Therefore, this chemical sensor can be applicable to on-site monitoring for environmental applications. © 2007 Elsevier B.V. All rights reserved. Keywords: Colloidal crystal; Nanoparticles; Chemical sensor; Poly(dimethylsiloxane); Environmental application; Volatile organic compounds (VOCs)
1. Introduction During recent decades, volatile organic compounds (VOCs) have been commonly found in the environment, in the soil, groundwater, and atmosphere [1,2]. Therefore, humans are easily exposed to these chemicals through the skin, by breathing, and by eating, and even at low concentrations this exposure can present long-term health risks. Among the VOCs, benzene and its derivatives such as benzene, toluene, and xylene (BTX), were confirmed to be human carcinogens, and could cause diversiform cancers, for example, lymphatic and hematopoietic cancers. Furthermore, ingestion of drinking water containing VOCs may lead to liver and kidney damage, and disorders of the immune system, nervous system, and reproductive system as well as several ∗
Corresponding author at: Department of Mechano-Micro Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan. Tel.: +81 45 924 5088; fax: +81 45 924 5088. E-mail address: [email protected] (T. Endo). 0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.03.003
types of cancers [3]. The fact that these VOCs have very different toxicities has meant that society has dealt with each one of them differently. From the viewpoint of environmental protection, it has become a matter of high priority to address the issue of the detection of these compounds. An important and non-trivial first step is to identify sites of pollution, and perform on-field monitoring, which requires a simple, economical, and rapid test for the detection of these compounds in samples taken from industrial processing, soils, human and animal tissues, and foodstuffs. This fact stimulates the development of measurement methods and techniques for environmental pollutant monitoring. With this background, there is an increasing interest in the development of a sensor for VOC detection [4–7]. There are many fields in which these kinds of sensors can be used, such as environmental applications, electronic noses, and in chemical industries, and in fact, many sensors have already been developed. However, the present sensors used for environmental applications, such as gas chromatography/mass spectrometry (GC/MS), require a relatively long assay time that involves troublesome liquid handling and many expensive reagents and apparatus [8,9].
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To overcome these disadvantages, several groups have developed different detection methods for sensors in environmental applications. Mitsubayashi et al. reported the development of an enzyme-based formaldehyde biosensor that detected low concentrations of acetaldehyde using an electrochemical detection method [10]. Mascini et al. developed a dioxin biosensor based on a piezoelectric sensor [11]. In addition, a microfluidic environmental sensor fabricated using microelectromechanical system (MEMS) technology has also been reported [12–14]. However, almost all conventional sensors for environmental applications involve high-cost apparatus that are difficult to use. Therefore, there is a demand for a more convenient and costeffective sensor system. Hence, we targeted the development of the colloidal crystal-based chemical sensor towards this requirement. Colloidal crystals are three-dimensionally periodic lattices of mono-dispersed, spherical colloids such as polystyrene and silica. The periodic lattices of the colloidal crystal will diffract incident light in accordance with Bragg’s law [15]. The reflected light in the visible spectrum is called the structural color. Additionally, the optical characteristics of colloidal crystals are related to their lattice constants. A millimeter-sized periodic structure can act as a colloidal crystal in millimeter wavelengths; a submicrometer-sized structure, in a visible light region; and the nanometer-sized ones, in the X-ray region. Hence, the resulting colloidal crystals diffract UV, visible, or near-IR light, depending on their lattice constants. A colloidal crystal diffracts light according to the Bragg equation (Fig. 1(a)): 1/2
mλpeak = 2 d1 1 1 (n2eff − sin2 θ)
(1)
where m denotes the order of diffraction; λpeak the wavelength of the diffraction peak; d1 1 1 the spacing between (1 1 1) planes; θ the angle between the incident light and the normal to the diffraction planes; and neff is the mean refractive index of the crystalline lattice. Based on these features, the colloidal crystal is an ideal candidate for fabricating optical sensors that can be used to determine environmental changes in terms of color changes. By using these features, several sensors based on colloidal crystals were fabricated by embedding colloidal crystals in appropriate polymers or hydrogels. These colloidal crystalbased sensors exhibit brilliant colors, which change in response to temperature, pH, ionic species, and glucose levels (Fig. 1(b)) [16,17]. Asher et al. reported the development of a colloidal crystal-based sensor for the detection of creatinine [18] and glucose [19] for medical applications. In all of these studies, the lattice constant, and thus the color exhibited by the colloidal crystal, as determined by the Bragg Eq. (1), varied in response to the environmental changes. In a number of related studies, the change in the refractive index was also demonstrated as a means to detect variations in the environment. In this study, poly(dimethylsiloxane) (PDMS) was used for the fabrication of colloidal crystal-based chemical sensors for environmental applications. PDMS swells in non-polar organic solvents. In addition, its swelling kinetics differ according to the polarity of the solvent [20]. Using this material, we fabricated a simple, convenient, and cost-effective colloidal crystal-based
Fig. 1. (a) Theoretical schematic illustration of the colloidal crystal-based chemical sensor. (b) Detection principle of the colloidal crystal-based chemical sensor.
chemical sensor for the colorimetric detection of VOCs for environmental applications. Previously, for environmental applications, similar detection principles or materials have been used for detection of the various vapors [21,22]. In addition, using this colloidal crystal-based chemical sensor for detection of VOCs, more simplified detection systems can be established. In this report, we fabricated a large-area (up to mm2 ), wellordered, three-dimensional colloidal crystal-based chemical sensor using polystyrene nanoparticles by using the dry-up process of its colloidal dispersion. The optical characteristics of the three-dimensional colloidal crystal-based chemical sensor were evaluated by using a UV–vis spectrometer, and the surface analysis was performed by using atomic force microscopy (AFM). Simultaneously, we performed the evaluation of the selectivity and calibration characteristics of this colloidal crystal-based chemical sensor for BTX. 2. Experimental 2.1. Materials Polystyrene nanoparticles (diameter: 202 nm, 2.57%, w/v) used for preparing the colloidal crystals were purchased from
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Polysciences Inc. (Warrington, PA). Slide glass substrates (S1111, 76 mm × 26 mm, thickness: 0.8–1.0 mm) were purchased from Matsunami Glass Ind., Ltd. (Osaka, Japan). The PDMS prepolymer (SILPOT 184 silicone elasotmer kit) was purchased from Dow Corning Asia (Tokyo, Japan). Organic solvents (ethanol, methanol, 2-propanol, acetone, benzene, toluene, and xylene) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). Milli-Q water (18.3 M cm) from Millipore (Billerica, USA) was used for all experiments. 2.2. Apparatus A spectrophotometer (DU® 800 wavelength range: 190– 1100 nm) used for the evaluation of the optical characteristics of the colloidal crystal-based chemical sensor was purchased from Beckman Coulter (CA, USA). AFM for the surface analysis of the colloidal crystal sensor was performed using a commercial AFM unit (SPA-400, Seiko Instruments, Inc., Chiba, Japan) with a calibrated 20 m xy-scan and 10 m z-scan range PZTscanner. 2.3. Formation of the colloidal crystal using polystyrene nanoparticles Slide glass substrates, after the pretreatment of their surfaces, were used as substrates for the formation of the colloidal crystal. The slide glass substrates were cut to 10 mm × 26 mm. All substrates were cleaned using a washing process involving ultrasonic cleaning in acetone for 30 min, followed by thorough rinsing with Milli-Q water (18.3 M cm) and drying at room temperature (RT). Finally, it was stored in a desiccator before use. Polystyrene nanoparticles (particle diameter: 202 nm) were used to form the colloidal crystals. The first step involved the fabrication of colloidal crystals by drying aqueous dispersions of polystyrene nanoparticles on the glass substrates. In this experiment, 20 l of the polystyrene nanoparticle dispersion (2.0%, w/v) was introduced on the surface of a slide glass substrate to form a thin layer of liquid film. Finally, this sample was placed under ambient laboratory conditions to allow the water to evaporate. The polystyrene nanoparticles were driven into a long-range ordered, opaline lattice by the attractive capillary forces generated during water evaporation. 2.4. Fabrication of the colloidal crystal-based chemical sensor using PDMS The fabrication procedure of the colloidal crystal-based chemical sensor using nanoparticles of different diameters is shown in Fig. 2(a). After the dry-up process of the nanoparticle dispersion, the PDMS solution (50 l) was then distributed on the top of the colloidal crystal, and the voids between the polystyrene nanoparticles were completely filled with the premixed elastomer of PDMS through capillary action. The elastomer was then cured at room temperature overnight, followed by additional baking at 60 ◦ C for 1 h.
Fig. 2. (a) Fabrication procedure of the colloidal crystal-based chemical sensor using polystyrene nanoparticles. (b) Experimental setup for the evaluation of the optical characteristics of the colloidal crystal-based chemical sensor.
In this study, in order to fabricate of the colloidal crystalbased chemical sensor using PDMS, a curing agent and the PDMS prepolymer were mixed in a 1:10 ratio by weight. The PDMS solution was degassed in desiccators with a mechanical vacuum pump to remove any air bubbles in the solution and ensure complete mixing between the two parts. Thus, these fabrication procedures resulted in the fabrication of a colloidal crystal-based chemical sensor. To evaluate the optical characteristics of the colloidal crystalbased chemical sensor, the experimental setup used was as shown in Fig. 2(b). All absorbance spectra were taken from 400 to 800 nm on the UV–vis spectrometer at RT. Under these experimental conditions, the optical characteristics of the colloidal crystal-based chemical sensor were investigated. 2.5. Surface analysis of the colloidal crystal surface using AFM To analyze the surface quality of the colloidal crystal in terms of particle density and periodicity, AFM was carried out in tapping mode using silicon tips and cantilevers with a nominal spring constant of 13 N/m for scanning in air. All reported images were acquired at scan rates in the range 0.25–0.50 Hz. 2.6. Colorimetric detection of non-polar organic solvents using the colloidal crystal-based chemical sensor To evaluate the sensing capability of this sensor, different kinds of organic solvents (ethanol, methanol, 2-propanol, acetone, benzene, toluene, and xylene) and ultra pure water were introduced onto its surface and its optical characteristics were
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measured. Simultaneously, we measured the change in its optical characteristics with time (0, 1, 2, 5, and 10 min) due to the swelling of the PDMS and evaluated its calibration characteristics by introducing different concentrations of non-polar organic solvents. 3. Results and discussions 3.1. Characteristics of colloidal crystal-based chemical sensor Fig. 3(a) shows the changes in the absorbance of the colloidal crystal-based chemical sensor from visible (violet color) to nearIR wavelengths. The structural color due to Bragg reflection could also be observed (Fig. 3(b)). In addition, the dependency of absorbance on the polystyrene nanoparticle concentration (for 2.0, 1.0, 0.5, and 0.1%, w/v) was monitored at 552 nm (green color). On the basis of these results, it was considered that the polystyrene nanoparticle concentrations affected the absorbance because of Bragg reflection. As a result, in this study, a 2.0% (w/v) polystyrene nanoparticle solution was used for the fabrication of the colloidal crystal-based chemical sensor. Additionally, surface imaging using AFM was carried out for the observation of the colloidal crystal, formed on the surface of the slide glass substrate, in order to evaluate its periodicity and coverage on the slide glass substrate surface. The AFM images of the colloidal crystal surface using 202 nm polystyrene nanoparticles are shown in Fig. 4. These images clearly illustrated that
Fig. 4. AFM images of the colloidal crystal-based chemical sensor surface.
colloidal crystals having a face-centered cubic (FCC) lattice structure are formed. It was observed that this FCC structure had been formed over a large area. Therefore, it was confirmed that sufficient crystal structure could be formed by this crystallization method using capillary forces. 3.2. Colorimetric detection of organic solvents using the colloidal crystal-based chemical sensor
Fig. 3. (a) Optical characteristics of the colloidal crystal-based chemical sensor fabricated using different concentrations of polystyrene nanoparticles. (b) Photograph of the colloidal crystal-based chemical sensor fabricated using 202 nm polystyrene nanoparticles.
The colorimetric detection of different kinds of organic solvents using colloidal crystal-based chemical sensors was carried out. Five hundred microliters of different organic solvents was introduced onto the sensor surface, and the changes in its optical characteristics were investigated. Fig. 5 shows the changes in the optical characteristics with time of the colloidal crystal-based chemical sensors in the visible region (400–800 nm). When ultra pure water (Fig. 5(a)), ethanol (Fig. 5(b)), acetone (Fig. 5(c)), and several kinds of VOCs were introduced onto the chemical sensor surfaces, a change in its
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Fig. 6. Selectivity for different kinds of organic solvents using the colloidal crystal-based chemical sensor.
Fig. 5. The change in the optical characteristics due to the introduction of different organic solvents: (a) ultra pure water; (b) methanol; (c) acetone.
optical characteristics could be observed (Fig. 6). These peak shifts were affected by only the polarity the organic solvent, and the refractive indexes of these organic solvents did not affect the swelling ratio. The peak wavelength of the colloidal
crystal-based chemical sensor was approximately 550 nm. In addition, when the chemical sensor was immersed in a non-polar organic solvent such as acetone, the structural color immediately changed from green to red (Fig. 7). The peak wavelength shifted to 743 nm after the introduction of acetone. Similarly, structural color changes could also be observed on the introduction of BTX. Thus, the color was tuned according to the swelling of the PDMS matrix in non-polar organic solvents. Additionally, when this chemical sensor was removed from the organic solvent and completely dried in air, its peak wavelength returned to the original position due to the shrinking of the PDMS matrix. For example, when acetone was evaporated from the swollen chemical sensor, the required shrinking time was just over 5 min. Thus, the colloidal crystal-based chemical sensor was perfectly reversible and could be used over 10 times repeatedly (data not shown). These results confirmed that the swelling ratio of the PDMS depended only on the polarities of the organic solvents. In particular, non-polar organic solvents such as BTX caused the PDMS to swell significantly. Hence, the colloidal crystal-based chemical sensor is applicable to detecting BTX in environmental applications. While studying this colloidal crystal-based chemical sensor for environmental applications, different concentrations of BTX solutions were introduced onto the colloidal crystal-based chemical sensor in order to evaluate its calibration characteristics. In the experiments, different concentration of BTX solutions diluted with methanol were introduced onto the sensor for 5 min, and subsequently, its optical characteristics were evaluated. The relationship between the peak shift and the BTX concentrations could be observed, as illustrated in Fig. 8. Furthermore, the detection limits of this sensor were found to be dependent on the polarities of the solvents (benzene: 10 ng/ml (Fig. 8(a)), toluene: 1 ng/ml (Fig. 8(b)), xylene: 10 pg/ml (Fig. 8(c))). From these characteristics, it can be seen that this chemical sensor has a possibility for use in the detection of non-polar organic solvents in the form of a simplified test kit. However, the detection limit of this sensor for BTX was not sufficient to satisfy the environmental quality criteria for ground water, gasses, and foodstuffs. Therefore, further investigations into the details of its fabrication conditions are required.
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Fig. 7. Color change of the colloidal crystal-based chemical sensor due to the introduction of acetone.
4. Conclusions We developed a colloidal crystal-based chemical sensor with a reversibly tunable structural color for environmental applications. The color change caused by swelling and shrinking of the matrix was rapid and could be easily judged with the naked eye. Furthermore, it is possible to determine the concentration of the target molecules by measuring the observed color variation. Hence, this chemical sensor has a potential application in intelligent (or smart) sensors that detect BTX without using a detecting device. Acknowledgements This study was supported by a Grant-in-Aid for Young Scientists (KAKENHI WAKATE (start-up) 18810012) from the Japanese Ministry of Education, Culture, Science, Sports and Technology (MEXT). The authors wish to thank Michiko Banno from the Japan Advanced Institute of Science and Technology (JAIST) for her valuable advice during the fabrication of the colloidal crystal. References
Fig. 8. Calibration characteristics for BTX solutions using the colloidal crystalbased chemical sensor: (a) benzene (b) toluene (c) xylene.
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Biographies Tatsuro Endo received his PhD degree from the Japan Advanced Institute of Science and Technology (JAIST), Biotechnology Laboratory in the Department of Biological Science and Biotechnology, in 2006. His current field of interest includes biochip and nanobiotechnology. Yasuko Yanagida holds a position as associate professor at Tokyo Institute of Technology (Tokyo Tech), Tokyo, Japan from 2003. She received her PhD degree from the Tokyo Tech in 1995. Her current interests are nanobiotechnology, cell engineering, and biochips. Takeshi Hatsuzawa holds a position as professor at Tokyo Institute of Technology (Tokyo Tech), Tokyo, Japan from 2002. He received his PhD degree from the Tokyo Tech in 1983. His current interests are microelectromechanical system (MEMS), nanobiotechnology, and biochips.