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We report on a novel class of optical materials for ethanol vapor sensing, based on polystyrene opals infiltrated with an innovative stimuli-responsive hydrogel. We describe the fabrication process of the bare polystyrene opals and their subsequent infiltration. The optical characterization of the photonic crystal templates was performed to prove the good quality of the samples. Measurements on the infiltrated opals showed that the transmission spectra in the visible range strongly change at varying concentrations of ethanol vapor. The fabricated structures show a linear optical response in the visible range, for high values of ethanol concentration.
© 2013 Optical Society of America
1. Introduction
Optically based chemical sensing has been under extensive research all over the world during last decades because of the increasing applications in industry, environmental monitoring, medicine, biomedicine and chemical analysis [1]. Optical sensors can be based on various optical principles, such as absorbance, reflectance or transmittance, luminescence and fluorescence, covering different regions of the spectrum (UV, visible, IR, NIR) [2]. They have several advantages over conventional electricity-based sensors, in terms of selectivity, immunity to electromagnetic interference, higher sensitivity, and they are also relatively inexpensive and minimally invasive [3].
A wide class of optical chemical sensors is based on Photonic Crystals (PCs) i.e., regular arrays of materials with different refractive indices [4]. In particular, they are artificial structures with a periodic dielectric function. Their use as sensors is possible thanks to their well-defined physical properties, such as reflectance and transmittance and to their superior levels of sensitivity [5].
Opals are among the simplest and most accessible artificial 3D PCs; their template consists of closely packed spheres, which can be formed by self-assembly of monodisperse colloids using several techniques, such as evaporation-induced vertical deposition [6,7].
Opals can be easily fabricated from colloidal suspensions of commercially available silica or polystyrene (PS) microspheres [8]. They can be used as a tool to test materials and concepts and, in particular, when infiltrated by a responsive hydrogel, they are suitable structures for optical sensors [5–13]. In fact, a common detection mechanism is based on a reversible volume change of polymer hydrogels in response to external stimuli [11]. Reversible volume changes may occur as a response to an external environmental stimulus and they may occur either continuously or discontinuously [14].
Hydrogel based photonic crystals have been first investigated in the pioneering work of the Asher’s group [15–17]. They created functional periodic structures through polymerization of responsive hydrogels inside colloidal crystals (Polymerized Colloidal Crystal Arrays - PCCAs). These structures consist of a crystalline colloidal array of spheres that Bragg diffracts light at ultraviolet, visible and near-infrared light, depending on the lattice spacing. The hydrogel swells and shrinks reversibly in the presence of certain analytes. The swelling/deswelling process changes the mean separation between the colloidal, thus shifting the Bragg peak wavelength of the diffracted light. The reversibility of this process makes PCCAs very useful for commercial applications.
Since then, photonic crystals infiltrated by stimuli-responsive hydrogels have been extensively used in literature [5–20] as chemical sensors and biosensors, in particular for pH and liquid ethanol sensing. Instead, little research has been conducted on applications of opals as ethanol vapor optical sensors [12,13].
In this paper, a photonic crystal structure that is responsive to ethanol vapor by Bragg diffracting light in the visible region of the spectrum is described for the first time. In particular, the hydrogel is constituted by a blend of three monomers, 2-hydroxyethyl methacrylate, acrylic acid and polyethylene glycol methacrylate. 2-Hydroxyethyl methacrylate (HEMA) was used as main building block of the network for its known favorable Flory-Huggins mixing parameter with ethanol; acrylic acid (AA) was introduced as co-monomer for its affinity toward water and its contribution to hydrogel network mechanical properties, due to the establishment of further crosslinking through strong secondary interactions; finally poly-ethylene glycol-200 dimethacrylate (PEG200DMA) was used as crosslinking agent. The concentration of the different monomers and initiator was determined on the account of literature information on similar systems, since HEMA based hydrogels are widely used for contact lenses [21,22], and evaluated through dynamic mechanical analysis and swelling studies carried out on the macrogel analogues. In particular, the swelling behavior of the material, when exposed to liquid ethanol, methanol and acetone, was studied (data here not reported for brevity). These preliminary investigations allowed selecting a formulation that swells significantly in water, up to about 50% of its dry weight and up to further 50% when, after equilibration in water, it is exposed to ethanol. Conversely, when the water-swollen macrogel is immersed in acetone, it undergoes deswelling, while exposure to methanol causes only a 20% further weight increase. These results suggest potential selectivity of the active material towards ethanol as targeted analyte. Furthermore, the selected hydrogel formulation was proved to withstand also repeated cycles of swelling and deswelling, by alternate exposition to ethanol vapor-saturated and ethanol vapor-free atmospheres, without significant detrimental effects on its mechanical properties. Altogether these properties make the chosen poly(HEMA-co-acrylic acid) system a promising candidate as stimuli-responsive material for ethanol vapor sensing [23].
The selected hydrogel formulation precursors were employed to fill the interstitial spaces of a polystyrene opal to confer stimuli-responsiveness to the periodic structure. At the best of our knowledge, there are no other reported studies on the optical response of inverse hydrogel opals to ethanol vapors present in air and in the presence of water vapors. This first work focuses on the optical properties of the hydrogel-infiltrated opal, with its polystyrene template, when exposed to gaseous atmospheres of different compositions. In a follow up of this research, the inverse hydrogel opal properties, after template removal, will be investigated. Our optical measurements on the poly(HEMA-co-acrylic acid) hydrogel infiltrating a PS colloidal crystal, yet still preliminary, prove that this structure has a good sensitivity and a high dynamic range and, hence, it could be used, in the future, as an ethanol vapor sensor.
2. Fabrication
A natural method to obtain 3D PCs in the optical range is based on the self-assembly of colloidal spheres, and it was firstly used by Astratov et al. [24]. In literature, a number of different techniques have been developed to obtain self-assembled structures (such as sedimentation, cell confinement, vertical deposition, Langmuir-Blodgett, shear induced, motor drawing, air-water interface, spin-coating, wedge-cell) [25]. Among them, the most widely used is the vertical deposition method, that, although is a relatively slow process (1-2 days to complete), provides the highest optical quality. It is based on the evaporation of the liquid forcing the spheres to arrange in the meniscus formed between a vertical substrate, the suspension and the air. The main advantages of this method are a precise control over the thickness and a superior crystalline quality of the structures. Despite the fact that this approach is affected by concentration gradients, which can lead to disturbances of the structures, such as growth defects and disordered domains, it has the advantage to be very simple and straightforward and it yields to significant portions of the casted film with fairly regularly packed nanoparticles [26]. However, for large-scale preparation (wafer size), the spin-coating method should be used instead, because it allows the growth of planar artificial opals on large surfaces in a few minutes [25]. This technique creates structures having a lower crystalline quality if compared to vertical deposition, but it offers other advantages (large surfaces and rapid process) that might be important in industrial mass production. For the purpose of this first exploratory study, in order to obtain the highest optical quality for the fabricated PCs, among the above-described techniques the vertical-deposition method was selected and applied.
In order to fabricate the infiltrated opals we followed a two-step process. We first obtained a crystalline colloidal array through self-assembly of monodisperse polystyrene nanoparticles to form a face centered cubic (fcc) lattice (i.e., bare opal). More in detail, a commercial polystyrene nanoparticle dispersion (Polyscience, 0.3 wt%) with an average diameter of 220 nm ( ± 5%) was used to obtain a 3D close-packed array of polymer nanoparticles onto Piranha solution pre-etched soda-lime glass slides, through a solvent evaporation-induced vertical deposition method (Fig. 1(a)). The final bare opal, depicted in Fig. 1(b), is 1 cm large and 1.2 cm long, while its thickness is about 650 nm. Figure 1(c) displays a scanning electron micrograph of the fabricated bare opal, showing the good homogeneity and regularity of the structure.
Fig. 1 (a) Sketch of the evaporation-induced vertical deposition. (b) Photograph of the fabricated bare opal. (c) Scanning electron microphotograph of the periodic structure of the sample.
The obtained bare opal was subsequently infiltrated with the ethanol-responsive hydrogel precursors and the latter were then crosslinked via UV photo-polymerization. The hydrogel is composed by three monomers: 2-hydroxyethyl methacrylate (HEMA, Sigma Aldrich), acrylic acid (AA, Sigma Aldrich) and poly-ethylene glycol-200 dimethacrylate (PEG200DMA, Polysciences). While all the other monomers were used as received, HEMA was distilled under vacuum prior to the use. Azobisisobutyronitrile (AIBN, Sigma Aldrich) was employed as initiator. The composition of the formulation was 66:12:0.8:21.2 (HEMA:AA:PEG200DMA:distilled water) % vol and AIBN was added at 0.15% wt/vol. Micromolar amounts of this solution were dropped on the top-edge of the deposit, which was inclined at a fixed angle of 15° with respect to the horizontal plane. The infiltration process was followed by the color change of the deposit from white to colorless. When infiltration was completed, a quartz slide was placed on top of the deposit to reduce uncontrolled accumulation of material on the surface and to limit water loss during photo-polymerization. Irradiation was carried out for 2 hours using a UV irradiator from Helios Italquartz “Polymer 125 UV”, equipped with a high-pressure Hg lamp “Zp-type” (2mW/cm2). The temperature inside the chamber was maintained at 25 ± 1 °C. After photo-curing, a thermal post-curing treatment was carried out in an oven at 60°C for further 1 hour. After removal of the top-cover, samples were equilibrated in bidistilled water at room temperature for 24 hours.
3. Measurement set-up
In order to perform the optical characterization of the samples, the set-up depicted in Fig. 2 was used. The white light coming from a Xenon lamp was spectrally filtered by a monochromator (Horiba Jobin Yvon FHR 1000) and focused onto the opal through a 50 mm focal length lens. The monochromator possesses a spectral resolution of 0.008 nm when the slits width is set to 10 μm. Optical transmission measurements were performed in the 400-700 nm range with a wavelength step of 1 nm, dividing the transmitted power by the incident one. A Silicon Photomultiplier (SiPM) [27] was used as a detector and it was connected to a lock-in amplifier that, together with a chopper, helped to reduce noise. The sample, the lens and the SiPM were enclosed in a black box during the measurements, in order to isolate the detection stage from external light and electromagnetic noise.
To obtain the optical characterization of the infiltrated opals at different ethanol vapor concentrations, the basic set-up was modified as follows. A 100 cm3 sealed polyethylene box, possessing two fused silica windows (to allow the passage of light) and a small hole, connected to the outside through a hose (to introduce liquids), was placed inside the metallic black box, between the lens and the SiPM.
The sample was positioned inside the box, hung from its top, so that transmission measurements could be performed while the sample was exposed to a controlled vapor atmosphere created by a layer of liquid, filling the bottom of the box. Particular care was taken to avoid any contact between the sample and the liquid phase. The temperature of the black box was monitored by a PT100 thermistor, to ensure that it was constant during all the measurements.
4. Experimental results
4.1 Bare opals
In order to test the quality and uniformity of the fabricated template, transmission measurements on bare opals in function of their position and of the incident angle were performed.
The chosen fabrication process provides a good uniformity on the horizontal axis (Fig. 1(b)). On the contrary, due to the progressive modification of nanoparticles dispersion concentration upon solvent evaporation, there may be variations in the quality of the deposited film along the vertical axis. For this reason, a first series of transmission measurements (here not shown) was performed at normal incidence by varying the vertical position from the top edge of the opal (first deposited part), with a step of 0.5 mm. For each measured transmission spectrum, the Bragg peak wavelength, the Full Width at Half Maximum (FWHM) and the depth of the gap were estimated. It was observed that the Bragg resonance was centered around 508 nm ( ± 3 nm) for each position. Small variations also occurred on the FWHM (42 ± 6 nm) and depth of the gap. From these results, it can be claimed that the sample shows a good homogeneity and exhibits its best optical properties (i.e., smaller FWHM and deeper gap) in the first deposited layers of the fabricated bare opal, as it would be expected, being only minimally affected by changes in dispersion concentration upon solvent evaporation.
Subsequently, a second series of measurements was carried out in correspondence of the vertical opal position that showed the best optical properties, by varying the angle of incidence (from 4° to 41.5°). These measurements have a double goal: they represent a further test on the optical quality of the opal, and they also provide an indirect measurement of the PS sphere diameter. The latter is an important parameter, useful to achieve the effective index of the infiltrated opal, as illustrated in the following section.
The results are shown in Fig. 3: at increasing angles of incidence, the Bragg peak is blue-shifted, from λB = 510 nm at θ = 4° to λB = 457 nm at θ = 41.5°. It is worth noticing that, at angles greater than 40°, a change in the shape in the spectrum can be observed. This is related to the interplay of the stop band and of the excitation of the photonic modes observed in the density of states, which takes place for large angles of incidence [28]. Moreover, scattering becomes relevant, causing a strong reduction of the transmitted light intensity.
Our measurements are in good agreement with the theoretical behavior of PCs. In fact, as known, because of the Bragg diffraction, the transmission spectrum of an opal presents a minimum peak, whose position is described by the following general relation that combines Snell’s and Bragg’s laws [29,31]:
where m is the diffraction order (1 for the first Bragg peak), θ is the angle between the incident beam and the normal to the plane, and d111 is the distance between two diffracting (111) planes, that can be expressed as a function of the PS sphere diameter D [32]:
The effective index can be calculated taking into account the refractive indices of polystyrene (nPS) and air (nair) as follows [12, 33]:
where 0.74 and 0.26 are the volume filling fraction for spherical balls and air, respectively. Wavelength dependent polystyrene refractive index was provided by the Sellmeier equation [34].
Our experimental data were compared with the theoretical position of the Bragg wavelength given by Eq. (1), using D as the only fit parameter (see Fig. 4). The best fit value for the diameter of the nanoparticles is D = 213 nm, that falls within the range provided by the nanoparticles supplier.
Fig. 4 Bragg peak wavelength versus angle of incidence: theoretical values (red curve) and experimental data (black circles). Blue lines are the error bars ( ± 1 nm).
As it can be seen, the agreement between the theoretical curve and the experimental data is very good. Error bars in figure (about 2 nm) are mainly due to the spectral resolution of the monochromator, since, in order to have enough optical power, its slits aperture was set to 1 mm.
4.2 Infiltrated opals
Optical transmission measurements on the hydrogel-infiltrated opals were carried out on samples equilibrated in air at a temperature of 23.8°C and relative humidity of 45% for minimum 24 h. In order to verify that the infiltration process did not alter the periodic structure of the bare opal, measurements at varying vertical positions and angles of incidence were performed. The Bragg resonance at normal incidence was now centered at 548 ( ± 6) nm, with a red-shift of 40 nm with respect to the bare opal. The performed measurements at varying vertical positions (here not shown) indicate that the periodic structure was preserved with a fairly good uniformity. Similarly to the bare opal, in correspondence to the position that showed the best optical properties (again in the first deposited layers near to the up edge of the fabricated opal), measurements at varying angles of incidence were taken. Bragg peak is blue-shifted at the increase of the angle of incidence, in accordance to Eq. (1), similarly to the direct bare opal.
From the measurement at θ = 0° (λB = 543 nm) the effective index of the infiltrated opal was estimated, as the inverse formula of Eq. (1):
An effective index neff,i = 1.561 was obtained, that is obviously greater than the effective index of the bare opal (neff, b = 1.469), calculated using Eq. (3) at λ = 508 nm.
For the infiltrated opal, Eq. (3) can be modified as follows:
where f is the filling factor for infiltrated opals [35].We employed a fully-automated refractometer (Metricon Model 2010/M Prism Coupler), providing high accuracy measurement of refractive index. The refractive index of the hydrogel equilibrated in air was measured, obtaining a value of nhydr = 1.513. Substituting this value in Eq. (5) and considering the effective index obtained by the measured λB through Eq. (4), the filling factor for the infiltrated opal was calculated: f = 0.225. Refractive index dispersion for polystyrene and hydrogel has been taken into account.
Subsequently, a series of measurements was executed to test the potential of the infiltrated opal as active material for ethanol vapor sensing.
The hydrogel used to infiltrate the bare opals is expectedly swollen by water, retaining some hydration when exposed to the air, but - more interestingly - it is further swollen when exposed to ethanol vapor. This property is especially interesting in view of the potential application of this sensing material in breathalyzers, where water vapor is one of the main components of the analyte, in addition to ethanol and a number of others components present in lower concentrations. For this reason, in order to analyze the optical response of the infiltrated opal at varying concentrations of ethanol vapor, it is of outmost importance to first study its behavior in the presence of water vapor.
The sample, in equilibrium with air at a temperature of 23.8°C and relative humidity of 45%, was placed within the sealed box. Then, distilled water was inserted into the box through the hose and transmission measurements were taken at normal incidence, approximately every 10 minutes. During the whole process, the temperature inside the black box was continuously measured, resulting approximately constant (about 24°C).
In the chosen experimental conditions, the time required for establishing the liquid-vapor equilibrium inside the small box is expectedly much lower than the time required for the sample to reach the equilibrium swelling conditions in the new environment. Therefore, vapor phase composition changes can be neglected.
As shown by the transmission spectra depicted in Fig. 5(a), the Bragg peak wavelength was red-shifted (from 543 nm at t = 0 min to 576 nm at t = 220 min), while the transmittance decreased. After 220-250 minutes, there were no more significant variations of the spectra, so it can be assumed that the infiltrated hydrogel reached its equilibrium swelling condition. The increase of the Bragg peak wavelength as a function of the time (Fig. 5(b)) can be explained by the progressive increase of the relative distance among polystyrene beads in the periodic structure of the infiltrated opal, as a result of further swelling of the hydrogel in the water-vapor saturated atmosphere.
Fig. 5 Response of the opal to water vapor: (a) Transmission spectra and (b) Bragg peak wavelength versus time.
By using the above-mentioned refractometer, the refractive index of the hydrogel exposed to water vapor was measured, obtaining the value nhydr,water = 1.468. Therefore, the refractive index decreases when the hydrogel absorbs water vapor. This leads to a dielectric contrast increase for the photonic crystal (being the refractive index of the polystyrene independent on the water concentration). As known [32,36], both the FWHM of the stop band and the peak depth increase with higher dielectric contrasts, thus explaining the behavior of the curves in Fig. 5(a).
In addition, an overall decrease of the transmission is observed, even far from the stop band. This can be explained as follows: the infiltrated opal maintains an unwanted, although very thin, layer of non-structured hydrogel on its surface. When exposing the sample to water vapor, the hydrogel noteworthy increases its surface roughness, which produces a stronger light scattering. The latter, together with the finite area of the employed photodetector, explains the trend shown in Fig. 5(a).
The transmission spectrum of the water-saturated infiltrated opal was used as a reference for subsequent measurements carried out in the presence of both water and ethanol vapors.
Liquid ethanol was progressively added to water into the bottom of the box. At any step of addition, a new series of transmission measurements was taken, approximately every 10 minutes, until the new equilibrium swelling condition for the sample was reached. This procedure was repeated four times at gradually increasing values of ethanol concentration in water (for each solution we reset the initial time, t = 0 min). Measurement conditions are summarized in Table 1.
Table 1. Measurement conditions
Figure 6(a) displays the transmission spectra at the mole fraction of 9.61 × 10−3 taken at different time intervals after the addition of ethanol. It can be observed (i) an initial significant decrease of the transmittance curve, which is partially restored with time; (ii) a progressive increase of the Bragg peak band depth and (iii) a concomitant red-shift of the band.
Fig. 6 (a) Transmission spectra and (b) Bragg peak wavelength versus time at a mole fraction of 9.61 × 10−3. RS in the legend stands for “reference spectrum”, corresponding to the water-saturated opal.
The reduction in transmittance, also in this case, is due to an increase of the roughness of the thin hydrogel layer present on the opal surface, when the sample is exposed to ethanol vapor. The main difference, if compared to the case of water vapor exposition, is that such a roughness disappears in less time, thus explaining the partial recovery of transmittance in Fig. 6(a).
Simultaneously, the Bragg peak shifts toward higher wavelengths, due to the further swelling of the interstitial hydrogel operated by the absorbed ethanol, which progressively increases the distance between the embedded polystyrene beads and the depth of the infiltrated opal swollen by the ethanol/water mixture. In Fig. 6(b) the Bragg peak wavelength shift as a function of the time is shown: it can be noticed that a steady-state condition is approached after approximately 1 hour.
The response time (τ90) of the sensor, determined according to the definition of τ90 given in [37,38], i.e., the time that it takes the sensor to reach 90% of its steady-state value after the introduction of the analyte, and measured when passing from a condition of equilibrium with water-vapor saturated air to an atmosphere constituted by air/water/ethanol at 9.61 × 10−3 mole fraction of ethanol. A response time of about 47 minutes was obtained. This is quite high, but it should be considered that it actually refers to the response of a hydrogel infiltrated in the interstitial spaces of a polystyrene photonic crystal, which is indeed impermeable to ethanol. Therefore, the attainment of the steady-state conditions requires sorption and diffusion of the analyte through the confined hydrogel and the cooperative dynamic rearrangements of the macromolecular segments between crosslinks of the latter. The removal of the polystyrene template is expected to lead to significant reductions in response time.
For the other concentrations a similar trend is observed: the Bragg peak wavelength increases with time because of the further swelling of the hydrogel at the increase of ethanol concentration, while the transmission at the Bragg wavelength first strongly drops down, then it is partially recovered.
Figure 7 summarizes the Bragg wavelength shift versus time for the four different analyzed mole fractions. It is worth noticing that the Bragg peak wavelength increases not only with time for each concentration, but also with the concentration of ethanol in the vapor phase.
The behavior of the structure based on the infiltrated opal is now described by comparing the steady values obtained during the different measurements. In particular, transmission curves at the steady state and at varying mole fractions are depicted in Fig. 8(a). The Bragg peak is shifted towards higher wavelengths at the increase of ethanol mole fraction in the vapor phase, thus proving the potential of the infiltrated opal as active material for ethanol vapor sensing. In detail, as shown in Fig. 8(b), the Bragg peak wavelength is shifted, almost linearly, from 576 nm (water-saturated opal) to 669 nm (mole fraction = 24.7 × 10−3), in a wide concentration range. The inset in Fig. 8(a) shows a photograph of the infiltrated opal immediately after the measurement at ethanol vapor mole fraction of 24.7 × 10−3. A visual color change, in the PC structure, was observed from green (sample in equilibrium with air at a temperature of 23.8°C and relative humidity of 45%, as in Fig. 1(b)) to red.
Fig. 8 (a) Transmission spectra and (b) Bragg peak wavelength at varying ethanol vapor concentrations after that the steady value has been reached. The inset in (a) shows a photograph of the infiltrated opal immediately after the measurement at ethanol vapor mole fraction of 24.7 × 10−3. The dashed blue line in (b) is the reference, i.e., the Bragg peak wavelength for the water-saturated opal.
In order to better explain such a wavelength shift, Eqs. (1-2) can be modified by adding a factor, Ds/D0, called swelling degree [6,7]:
From Eq. (6) the red-shift could be ascribed to an increase of both the opal effective index and of the swelling degree.
In order to disentangle both effects in the case of ethanol vapor absorption, we decided to consider a “worst-case condition”, by exposing the hydrogel to liquid ethanol until saturation. In fact, we think that hydrogel exposure to ethanol vapor would change less the refractive index. The latter decreased from nhydr,water = 1.468 to nhydr,ethanol = 1.435, so the opal effective index diminished from neff,i,water ≈1.552 to neff,i,ethanol ≈1.542. This would cause a blue-shift in the diffracted Bragg peak of about 4 nm, as already reported in [6], and the shift would be in the opposite direction with respect to what experimentally observed.
Hence, the change in refractive index plays a marginal role if compared to the variation of the swelling degree, the latter representing the increased distance of polystyrene beads in the periodic structure, as a result of ethanol vapor-induced swelling of the hydrogel.
Assuming Ds/D0 = 1 for the infiltrated opal in its initial conditions and neglecting the effective refractive index variations with the ethanol concentration (neff,i = 1.561), the swelling degree was determined for each measurement from Eq. (6). The results are depicted in Fig. 9.
Fig. 9 Swelling degree at varying ethanol vapor concentrations. Dashed blue line refers to the water-saturated case.
The swelling degree increases from 1.00 to 1.06 when the opal becomes water-saturated, while it rises from 1.06 to 1.23 at higher ethanol vapor concentrations.
Finally, the sensitivity of the infiltrated opal was determined. As known, the sensitivity of a sensor can be defined as the change of its output with respect to the input (i.e., the measured quantity) [39]. In our case, we can define the sensitivity as the ratio of the variation of the Bragg peak wavelength and the variation of the ethanol vapor mole fraction.
As previously discussed, the Bragg peak wavelength is shifted almost linearly at increasing ethanol vapor mole fractions. Therefore, by means of a linear interpolation of the experimental data in Fig. 8(b), we found a value of sensitivity (i.e., the slope of the measured data) equal to 3799.2 nm/mole fraction, or, equivalently, 2.3983 × 10−3 nm/ppm. This value is not too far from what found in literature for other chemical sensors [40,41] and from other ethanol vapor sensors [42,43]. The response of our photonic crystal presents a range of linearity (more than 20000 ppm) at high values of ethanol concentrations. We also qualitatively compared our results with those reported in [9,44], where the behavior of photonic crystal structures with liquid ethanol (not ethanol vapor) has been studied. In particular, we found that our wavelength shift is linear and comparable to that reported in [44], while it is smaller than in [9], even though in [9] the shift is not linear.
The described processes (change in dielectric contrast, swelling and Bragg shift) are reversible. In fact, transmission spectra obtained after water and ethanol evaporation, drying and re-equilibration in air provided the same curves shown by the original opal. The time required by the structure to reach the normal base line after exposure to ethanol vapor was measured and it resulted to be less than 1 minute.
5. Conclusion
In this paper, we presented a novel class of optical materials based on polystyrene opals infiltrated with an innovative stimuli-responsive hydrogel. The chemical composition of the hydrogel network was specifically engineered to be able to show significant swelling when exposed to ethanol vapor, from an initial condition of equilibrium swelling with water vapor pressure. In fact, the above-mentioned hydrogel also swells in the presence of water, retaining some hydration when exposed to the air, and maintains its optical and mechanical properties after repeated cycles of swelling and deswelling, by alternate exposition to ethanol vapor-saturated and ethanol vapor-free atmospheres. These properties make this hydrogel an ideal sensing material to be applied into inexpensive and minimally invasive breathalyzers.
The optical characterization of the hydrogel-infiltrated opals shows that, at increasing mole fractions of ethanol vapor, the Bragg peak wavelength is red-shifted from 576 nm (water-saturated opal) to 669 nm (ethanol mole fraction of 24.7 × 10−3). According to these results, the fabricated nanostructured film can be used as a sensing material, with a linear behavior (more than 20000 ppm) at high values of ethanol concentrations and a good sensitivity (2.4 × 10−3 nm/ppm). As a matter of the fact, our infiltrated opals represent the first photonic crystal based structures for ethanol vapor sensing in the visible region of the spectrum: their remarkable Bragg peak shift make it possible to visually notice, through a change in the opals coloration, the variations in ethanol vapor concentration. Moreover, they present a linear optical response, at high values of concentrations, especially if compared to other kinds of already existing ethanol vapor sensors [43].
More detailed values of the effective refractive index can be obtained by investigating the dispersion properties of the infiltrated opal. These measurements, as well as an investigation on the performance of the material with pulsed vapor injection and high energy optical response [45,46] will be addressed in the near future. Moreover, the inverse hydrogel opal properties, after template removal, will be investigated.
Acknowledgments
The activity is partially funded by National Ministry of University with the project PRIN 2008 “Studio di fattibilità e sviluppo prototipale di sensori elettro-ottici per la misura di BrAC”, prot. 2008F7P4MX.
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