Sensors and Actuators B 220 (2015) 222–226
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Cellulose photonic crystal ﬁlm sensor for alcohols Fengyan Wang a , Zuogang Zhu b , Min Xue a,∗ , Fei Xue a , Qiuhong Wang a , Zihui Meng a,∗ , Wei Lu a , Wei Chen a , Fenglian Qi a , Zequn Yan a a b
School of Chemical Engineering & Environment, Beijing Institute of Technology, Beijing 100081, PR China Beijing Municipal Institute of Labor Protection, Beijing 100054, PR China
a r t i c l e
i n f o
Article history: Received 30 January 2015 Received in revised form 7 May 2015 Accepted 19 May 2015 Available online 3 June 2015 Keywords: Volatile organic compounds Colloidal array Cellulose Photonic crystal
a b s t r a c t A novel photonic crystal sensor, a cellulose ﬁlm with a three dimensional (3D) colloidal array embedded inside, was fabricated by inﬁltrating the voids of a 3D poly methyl methacrylate (PMMA) colloidal array with methyl cellulose aqueous solution, followed by thermal curing. When the obtained cellulose photonic crystal ﬁlm sensor (CPCFS) was immersed in alcohols, including ethanol, n-propanol, isopropanol and n-butanol, its lattice constant and mean effective refractive index increased, which led to the redshift of the reﬂection of incident light. The redshift of this sensor had linear response to the concentration of alcohol vapors, while its structural color changed from blue to green visually. This CPCFS demonstrates promising potential as an on-site monitoring sensor for alcohols and an inexpensive and minimally invasive breathalyzer in the future. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, with the fast growth in economy and civilization, people pay more attention to the health issues which are adversely affected by serious environmental pollutions . Among all the pollutants, volatile organic compounds (VOCs) have attracted more and more concerns in recent years. VOCs have high vapor pressure at room temperature and are difﬁcult to disperse, which makes people easily be exposed to them for long term and eventually is harmful to human health. Alcohols, including ethanol, n-propanol, isopropanol and n-butanol, are the common compounds as VOCs. Ethanol is an important raw material in chemical industry, but long-term exposure to ethanol vapors leads to nausea, headache, dizziness and cancer [2–4]. Isopropanol is widely used in the manufacture of paints and printing, but the emission of isopropanol can be photochemical oxidants which cause “summer smog” and destroy the body’s respiratory system and eyes. The serious toxicities of alcohols mean that it’s in an urgent demand to develop sensor techniques to monitor them on site. Monkawa et al. developed a high sensitive sensor with wide dynamic range localized surface plasma resonance for VOCs . Deo et al. reported an ultrasonically sprayed nanostructured CdSnO3 thin ﬁlm to detect isopropanol vapors . However, almost all conventional sensors involve high-cost and complicated process, and
∗ Corresponding author. Tel./fax: +86 10 68913065. E-mail addresses: [email protected]
(M. Xue), m [email protected]
(Z. Meng). http://dx.doi.org/10.1016/j.snb.2015.05.057 0925-4005/© 2015 Elsevier B.V. All rights reserved.
the response cannot be visually detected. Recently, Winther et al. developed a novel biosensor which detected alcohol vapor sensitively by immobilizing enzymes on a breathable electrode, but its output showed unstable with conventional long-term storage . An easy-to-operate, stable and economical sensor technique is in an urgent need for the sensing of alcohols. Moreover, to control the drunken driving on the road, it is also pressing to develop a sensor to monitor ethanol in human body [8,9]. Colloidal arrays with periodically arranged 3D structures can be assembled from monodispersed colloidal particles [10,11]. In a perfect colloidal array, only the light of a certain wavelength can propagate through it, which suggests the existence of photonic band gap (PBG) [12,13]. If the PBG is located in the visible-light region, the light reﬂected by the colloidal array is visible to the naked eyes [12–15]. Sensors that utilize the optical properties and superior sensitivity of colloidal array have already been achieved. Asher’s group developed an ammonia sensitive material by coupling the Berthelot reaction to the polymerized crystalline colloidal array (PCCA), which can be used as a point-of-care device for the detection of blood NH3 concentration . Recently, colloidal arrays incorporated with hydrogel have been reported for the sensing of alcohols [10,17,18]. For example, Zeng et al. developed a polyacrylamide inverse opal hydrogel to detect liquid alcohols . However, the inverse opal hydrogels were synthesized by the templated hydrogel preparation, which uses colloidal array as nanoscale template at ﬁrst, and then etches away the template to obtain an interconnected porous structure. Such procedure is time-consuming, and the composition of conventional hydrogel is
F. Wang et al. / Sensors and Actuators B 220 (2015) 222–226
not environmental friendly. In addition, the usability of hydrogel materials after long term storage cannot be guaranteed either. As one of the most versatile biopolymers in nature, cellulose nanocrystals (CNCs) extracted from wood pulp or cotton were assembled into a chiral nematic liquid crystal in water [20,21]. The obtained order can selectively reﬂect circularly polarized light that has a wavelength matching the pitch . Some researchers have already fabricated free-standing ﬁlms with mesoporous structures using CNCs as templates for the applications including catalysis, separation and sensing [23,24]. Simultaneously, other cellulose derivatives were doped into the microstructured optical ﬁbers to increase the sensitivity of sensor probe or for imaging in vivo [25–27]. Sahcear et al. reported a new strategy to build large-scale solvent-responsive elastomeric opal ﬁlms, in which hard-soft coreinterlayer-shell (CIS) beads were used to prepare paper-supported elastomeric opal ﬁlms with remarkably distinct iridescent reﬂection colors. Due to the high porosity of the paper-sheets used, these composite ﬁlms could be easily swelled by various solvents. The swelling changed the crystalline lattice of the opals which provoked a tremendous photonic band gap shift and also enhanced the brilliance of these colors. This approach became the basis of a whole family of polymer-based soft sensors with a fascinating optical response . Herein, we constructed a sensor by embedding a 3D colloidal array inside the cellulose ﬁlm, and investigated its response to VOCs, such as alcohols, formaldehyde, toluene and acetone. Compared with the conventional photonic crystal with 3D inverse opal structure, the preparation process of this cellulose photonic crystal ﬁlm sensor (CPCFS) was easy-to-operate and cost-efﬁcient, which only needs self-assembling and following curing. This cellulosebased photonic crystal sensor is expected to become a promising sensing material for the in situ detection of alcohols by the naked eye. 2. Experimental 2.1. Materials Methyl methacrylate (MMA) was purchased from J&K Scientiﬁc Ltd and treated with Al2 O3 . Potassium peroxydisulfate (KPS) was obtained from Xilong Chemical Co. Ltd. Methyl cellulose (MC) was obtained from Aladdin Industrial Inc. Methanol, ethanol, n-propanol, isopropanol, n-butanol, toluene, trioxymethylene, acetone and sulfuric acid (98%) were purchased from Beijing Chemical Plant, and hydrogen peroxide (30% water solution) was purchased from Tianjin Fuyu Fine Chemical Co. Ltd. Glass slides (20 × 20 mm) were obtained from Weiss Experiment Products Co. Ltd. and washed with H2 SO4 /H2 O2 (7/3, v/v) solution for 12 h, followed by being rinsed with ultra pure water in an ultrasonic bath for three times before usage .
light source and a FC-UV600-2-SR ﬁber optic reﬂection probe. The assembly of colloidal array was carried out within a Safe HWS150 incubator (Haishu). The heating process was carried out in a DNP-9022 electro-thermal incubator (Jinghong). SEM images were obtained from a QUANTA FEG 250 ﬁeld emission scanning electron microscope (FEI). The Z-average diameter was measured using a Zetasizer dynamic light scattering (DLS) device (Malvern). 2.3. Fabrication of CPCFS The monodispersed PMMA colloidal particles were prepared according to the literature . MMA (6 ml) and ultra pure water (139 ml) were mixed and heated in a four-neck round-bottom ﬂask which was equipped with a nitrogen inlet tube, a water cooled reﬂux condenser, a mechanical stirrer and a digital thermometer. The initiator KPS (0.3 g dissolved in 5 ml ultrapure water) was added into the mixture at 80 ◦ C. The stirring rate was kept at 300 rpm and nitrogen was bubbling to remove oxygen throughout the reaction. Monodispersed PMMA colloidal particles obtained after 45 min. The suspension was separated by centrifugation at 6000 rpm for 5 min with discharging the supernatant. Particles were cleaned with ultra pure water for three times. The process of CPCFS fabrication was shown in Scheme 1. After self-assembly of PMMA particles on the surface of glass slides, the obtained opal was inﬁltrated with methyl cellulose aqueous solution (3% g/ml) and was subsequently incubated in an incubator at 60 ◦ C for 3 h. 2.4. Detection of VOCs by CPCFS The obtained CPCFS was cut into 7 × 7 mm in size. To investigate the response of CPCFS to liquid alcohols, we ﬁrst recorded the original reﬂection wavelength of CPCFS by optical ﬁber spectrometer, and then ﬁxed it at the bottom of weighing bottle (50 mm × 30 mm). After that, alcohols were added and the reﬂection wavelength was measured. All of the experiments were repeated for three times. To respond to the saturated alcohol vapors, the CPCFS was attached to the inner side of the lid of a weighing bottle (50 mm × 30 mm), which was sealed after the injection of 1 ml solvents. When the bottle was saturated by vapors, the reﬂection of the cellulose ﬁlm was recorded. In order to detect vapors of different concentrations, the CPCFS was ﬁxed inside an air bag (0.5 l). After that, the air bag was ﬁlled with nitrogen to the speciﬁc volume, and different volumes of alcohols were injected into the air bag through an injection port. After the complete evaporation of alcohols, the CPCFS was taken out from the air bag and its reﬂection wavelength was recorded immediately. 3. Results and discussions
3.1. Characterization of CPCFS
The reﬂection spectra were recorded using an Avaspec-3648TEC optical ﬁber spectrometer (Avantes) with an AvaLight-DH-S-BAL
The SEM image (Fig. 1(a and b)) shows a periodicity and uniformity in the prepared CPCFS. The mean diameter of PMMA particles
Scheme 1. Fabrication of CPCFS.
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Fig. 1. SEM image of CPCFS: (a) top; (b) cross-section; (c) the reﬂectance spectrum and structural color of CPCFS.
is 180 ± 5 nm. The CPCFS has a maximum reﬂection at 461 nm and a blue structural color (Fig. 1c). For the PMMA colloidal array, the reﬂection follows Bragg and Snell’s law (1) [30–32]: mpeak = 2d111 (n2eff − sin2 )
where m is the order of reﬂection, peak is the wavelength of the reﬂected light, neff is the mean effective refractive index (RI), is the angle between the incident light and the normal to the reﬂection plane, and d111 is the distance between array planes that can be expressed as a function of the diameter D for PMMA colloidal array :
2 D 3
For the colloidal array and CPCFS, the effective refractive indexes accordingly are: n2eff = 0.74n2p + 0.26n2air
Fig. 2. The redshift and structural color of the CPCFS in different alcohols.
and n2eff-CPCF = 0.74n2p + fn2c + (0.26 − f )n2air
where np , nair and nc are the refractive indexes of the particles, air and cellulose ﬁlm, respectively. The values 0.74 and 0.26 are the ﬁlling factors for the host material (colloidal) and air, and f is the ﬁlling factor for CPCFS [34,35]. The reﬂection wavelength can be shifted by altering the lattice parameters or the mean effective refractive index. 3.2. The response of the CPCFS to alcohols The CPCFS was immersed in ethanol, n-propanol, isopropanol and n-butanol respectively. The original structural color of the CPCFS was blue (Fig. 1c), and it turned to green when immersed in alcohols (Fig. 2). To analyze the response mechanism of CPCFS, we ﬁrst immersed a cellulose ﬁlm without colloidal array in alcohols. The size of cellulose ﬁlm remained the same, which meant that cellulose ﬁlm itself would not swell or shrink in alcohols. Thus according to the Bragg and Snell’s equation, the redshift of the CPCFS is caused by the change of the refractive index of solvents or the swelling of PMMA particles. If the refractive index is the only factor to shift the reﬂection wavelength, the magnitude of redshift should be identical to the order of refractive index, which is n-butanol (1.3993) > n-propanol (1.3856) > isopropanol (1.3775) > ethanol (1.3614). However, the magnitude of redshift follows: n-propanol (76 nm) > ethanol (73 nm) > n-butanol (63 nm) > isopropanol (56 nm). The reasonable explanation for this inconsistency is that PMMA colloidal particles swelled in alcohols
Fig. 3. The response of the CPCFS to the saturated vapor of alcohols.
and the swelling also shifted the wavelength of reﬂection. To conﬁrm the explanation, PMMA colloidal particles were dispersed in alcohols and then their Z-average diameters were measured with DLS. The swelling ratio of PMMA colloidal particles in different alcohols was calculated (Table 1). Since the magnitude of the redshift is not completely consistent with the swelling ratio either, we conclude that the change of reﬂection wavelength is affected by the swelling of PMMA colloidal particles and the refractive index of solvents together. Table 1 Z-average diameters and the swelling ratio of PMMA particles in alcohols.
Z-average (nm) Swelling ratio (D/D0 )
D0 (180 nm) is the original average diameter of PMMA particles.
F. Wang et al. / Sensors and Actuators B 220 (2015) 222–226
We also investigated the response of CPCFS to the saturated vapor of alcohols at 20 ◦ C. The saturated vapor pressure follows: ethanol (5.671 kPa) > isopropanol (4.32 kPa) > n-propanol (1.917 kPa) > n-butanol (0.59 kPa). We found that the magnitude of redshift of the CPCFS in the saturated vapors follows: ethanol (39 nm) > n-propanol (34 nm) > isopropanol (32 nm) > n-butanol (23 nm) (Fig. 3). This is different from that in liquid alcohols, and the redshift in the saturated vapor of ethanol was the biggest. The saturated vapor pressure of the solvents becomes a main factor to change the reﬂection wavelength. The saturated vapor can affect the amount of alcohols absorbed by the CPCFS and eventually affect the swelling of PMMA colloidal particles. Under the experiment conditions, the CPCFS in the saturated ethanol vapor absorbed the biggest amount of analytes, leading to the biggest swelling of PMMA colloidal particles and the biggest redshift. The CPCFS was also immersed in alcohol vapors with different concentrations. As described in the experiment section, the whole process was conducted in an air bag, and different amount of alcohols were injected through a sample needle and evaporated at the point of the needle. Fig. 4 shows the response of CPCFS to a series of vapors of various concentrations for ethanol, n-propanol, isopropanol and n-butanol. For alcohol vapors, the magnitude of redshift for the CPCFS is identical to the order of the refractive index of alcohols. The refractive index of alcohols is the key factor to change the redshift of reﬂection. We also observed that the vapor concentration was correlated to the magnitude of redshift almost linearly (Table 2), which might
Table 2 The linear response of the CPCFS to the alcohol vapors.
Ethanol n-Propanol Isopropanol 1-Butanol
y = 0.9499 + 0.02822x y = 1.5000 + 0.05385x y = 1.7950 + 0.02775x y = 1.2810 + 0.09234x
0.9502 0.9430 0.9772 0.9643
x represents vapor concentration, y represents magnitude of redshift and r2 is linear correlation coefﬁcient.
be used at least in the semi-quantitative analysis. This property makes the applications of the CPCFS in breathalyzers for ethanol possible. Just as shown in Fig. 4, the color of CPCFS changed from blue to green, which offered an opportunity for the visual detection. We also investigated the responses of the CPCFS to other VOCs, such as cyclohexane, formaldehyde, toluene and acetone. However, our sensor did not respond to cyclohexane or formaldehyde sensitively. The other organic solvents can dissolve PMMA colloidal array, and the structural color of CPCFS changed from blue to transparent. Thus, the CPCFS responds to alcohols speciﬁcally. In our ongoing work, we are replacing PMMA colloidal particles by silica colloidal particles, which cannot be destroyed by toluene or acetone, as templates to expand the application. Although the CPCFS cannot be reused without deterioration currently, it is still highly responsive without degradation of optical properties or mechanical robustness after stored at stable room temperature for 6 months.
Fig. 4. The CPCFS responded to vapors: (a) ethanol, (b) n-propanol, (c) isopropanol, (d) n-butanol.
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4. Conclusions We developed a novel alcohol-sensitive photonic sensor which is low-cost, biofriendly, easy to prepare and handle by implanting a close-packed PMMA colloidal array inside a methyl cellulose ﬁlm. This CPCFS can be used to monitor alcohols in both forms of liquid and vapor. When exposed to alcohols, the structural color of CPCFS changed from blue to green and the reﬂection wavelength redshifted signiﬁcantly. Therefore, it can be used for in-situ monitoring of alcohols by the naked eye for environmental protection and drunken driving test in the future. Acknowledgement Financial support from NSFC (21375009), and the scientiﬁc advisement from Professor Sanford Asher from University of Pittsburgh are appreciated. References  F. Xu, S. Guo, Y.L. Luo, Novel THTBN/MWNTs-OH polyurethane conducting composite thin ﬁlms for applications in detection of volatile organic compounds, Mater. Chem. Phys. 145 (2014) 222–231.  M. Kampa, E. Castanas, Human health effects of air pollution, Environ. Pollut. 151 (2008) 362–367.  Bouchaala, Volatile Organic Compounds Removal Methods: A Review, Am. J. Biochem. Biotechnol. 8 (2012) 220–229.  M.L. Boeglin, D. Wessels, D. Henshel, An investigation of the relationship between air emissions of volatile organic compounds and the incidence of cancer in Indiana counties, Environ. Res. 100 (2006) 242–254.  A. Monkawa, T. Nakagawa, H. Sugimori, E. Kazawa, K. Sibamoto, T. Takei, M. Harutab, With high sensitivity and with wide-dynamic-range localized surfaceplasmon resonance sensor for volatile organic compounds, Sens. Actuators B 196 (2014) 1–9.  V.V. Deo, D.M. Patil, L.A. Patil, M.P. Kaushik, Ultrasonically sprayed nanostructured CdSnO3 thin ﬁlms for the detection of VOC’s, Sens. Actuators B 196 (2014) 489–494.  O. Winther-Jensen, R. Kerr, B. Winther-Jensen, Alcohol vapour detection at the three phase interface using enzyme-conducting polymer composites, Biosens. Bioelectron. 52 (2014) 143–146.  M. Gamella, S. Campuzano, J. Manso, G.G. de Rivera, F. Lopez-Colino, A.J. Reviejo, J.M. Pingarron, A novel non-invasive electrochemical biosensing device for in situ determination of the alcohol content in blood by monitoring ethanol in sweat, Anal. Chim. Acta 806 (2014) 1–7.  M. Santonico, P. Pittia, G. Pennazza, E. Martinelli, M. Bernabei, R. Paolesse, A. D’Amico, D. Compagnone, C. Di Natale, Study of the aroma of artiﬁcially ﬂavoured custards by chemical sensor array ﬁngerprinting, Sens. Actuators B 133 (2008) 345–351.  R. Pernice, G. Adamo, S. Stivala, A. Parisi, A.C. Busacca, D. Spigolon, M.A. Sabatino, L. D’Acquisto, C. Dispenza, Opals inﬁltrated with a stimuli-responsive hydrogel for ethanol vapor sensing, Opt. Mater. Express 3 (2013) 1820–1833.  F. Li, D.P. Josephson, A. Stein, Colloidal assembly: the road from particles to colloidal molecules and crystals, Angew. Chem. Int. Ed. Engl. 50 (2011) 360–388.  S. John, Strong localization of photons in certain disordered dielectric superlattices, Phys. Rev. Lett. 58 (1987) 2486–2489.  E. Yablonovitch, Inhibited spontaneous emission in solid-state physics and electronics, Phys. Rev. Lett. 58 (1987) 2059–2062.  J. Ge, Y. Yin, Responsive photonic crystals, Angew. Chem. Int. Ed. Engl. 50 (2011) 1492–1522.  C. Fenzl, T. Hirsch, O.S. Wolfbeis, Photonic crystals for chemical sensing and biosensing, Angew. Chem. Int. Ed. Engl. 53 (2014) 3318–3335.  K.W. Kimble, J.P. Walker, D.N. Finegold, S.A. Asher, Progress toward the development of a point-of-care photonic crystal ammonia sensor, Anal. Bioanal. Chem. 385 (2006) 678–685.  M. Xu, A.V. Goponenko, S.A. Asher, Polymerized polyHEMA photonic crystals: pH and ethanol sensor materials, J. Am. Chem. Soc. 130 (2008) 3113–3119.  C. Fenzl, T. Hirsch, O.S. Wolfbeis, Photonic crystal based sensor for organic solvents and for solvent-water mixtures, Sensors 12 (2012) 16954–16963.  Z. Pan, J. Ma, J. Yan, M. Zhou, J. Gao, Response of inverse-opal hydrogels to alcohols, J. Mater. Chem. 22 (2012) 2018.  J.A. Kelly, A.M. Shukaliak, C.C. Cheung, K.E. Shopsowitz, W.Y. Hamad, M.J. MacLachlan, Responsive photonic hydrogels based on nanocrystalline cellulose, Angew. Chem. Int. Ed. Engl. 52 (2013) 8912–8916.
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Biographies Fengyan Wang is a graduate student of Beijing Institute of Technology in China. She obtained bachelor’s degree from Beijing Institute of Technology in 2012. Her research aims at fabrication of functional photonic crystal sensor. Zuogang Zhu is a professor of Beijing Municipal Institute of Labor Protection. His research work focuses on detection of environmental pollutant vapors. Min Xue received her Ph.D. degree from University of Fukui. Currently, she is a lecturer of Beijing Institute of Technology and focuses on chromatography research. Fei Xue is a Ph.D. candidate of Beijing Institute of Technology. His research work was about fabrication and application of photonic crystals of different dimensions. Qiuhong Wang obtained her bachelor’s degree from China Agricultural University in 2007, and then received her Master’s degree from Beijing Institute of Technology in 2014. Zihui Meng received his Ph.D. degree from Dalian Institute of Chemical Physics, Chinese Academy of Sciences in 1998 and now is a professor of Beijing Institute of Technology. His research interests include photonic crystal, molecular recognition, and chromatography. Wei Lu is a Ph.D. candidate of Beijing Institute of Technology. Her research interest is to detect explosive by combining molecularly imprinted technology with photonic crystal. Wei Chen is a Ph.D. candidate of Beijing Institute of Technology. Her work is about fabrication of photonic crystal and nanoparticles. Fenglian Qi received her bachelor’s degree from Tsingtao University of Science & Technology and now is a graduate student of Beijing Institute of Technology. Zequn Yan obtained her bachelor’s degree from Beijing Institute of Technology in 2014 and now is a graduate student of Beijing Institute of Technology.