Holographic Detection of Hydrocarbon Gases and Other Volatile

Sep 13, 2010 - University of Cambridge, Department of Chemical Engineering and Biotechnology, Tennis Court Rd, CB2 1QT,. Cambridge, United Kingdom...
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Holographic Detection of Hydrocarbon Gases and Other Volatile Organic Compounds J. L. Martı´ nez-Hurtado, C. A. B. Davidson, J. Blyth, and C. R. Lowe* University of Cambridge, Department of Chemical Engineering and Biotechnology, Tennis Court Rd, CB2 1QT, Cambridge, United Kingdom Received July 5, 2010. Revised Manuscript Received August 31, 2010 There is a need to develop sensors for real-time monitoring of volatile organic compounds (VOCs) and hydrocarbon gases in both external and indoor environments, since these compounds are of growing concern in human health and welfare. Current measurement technology for VOCs requires sophisticated equipment and lacks the prospect for rapid real-time monitoring. Holographic sensors can give a direct reading of the analyte concentration as a color change. We report a technique for recording holographic sensors by laser ablation of silver particles formed in situ by diffusion. This technique allows a readily available hydrophobic silicone elastomer to be transformed into an effective sensor for hydrocarbon gases and other volatile compounds. The intermolecular interactions present between the polymer and molecules are used to predict the sensor performance. The hydrophobicity of this material allows the sensor to operate without interference from water and other atmospheric gases and thus makes the sensor suitable for biomedical, industrial, or environmental analysis.

Introduction Measuring molecules in the environment, particularly identifying gas molecules such as hydrocarbons or volatile organic compounds (VOCs), has been of growing concern in recent years.1-3 Hydrocarbon gases, widely used as combustibles, and VOCs are released into the atmosphere from antropogenic sources, road transport, and other industrial processes. These molecules have attracted interest as potentially harmful substances to human health as indoor air pollutants originating from building or furnishing materials, paints, adhesives, and other consumer products.4 Current technology for real-time sensing of gases or VOCs is classified according to how the sensor operates, for example, categorizing gaseous analytes nonspecifically as flammables or toxics.3 Other more accurate techniques such as gas chromatography coupled with mass spectroscopy or flame ionization detectors have been hampered by the need for expensive and sophisticated equipment and a high degree of operator expertise. Although these technologies have proven applicability in many industrial processes, the need for real-time sensors capable of distinguishing molecular differences in gases, locally and remotely, is, as yet, unfilfilled. Hydrophobic polymer films are known to interact with hydrocarbons5,6 and show affinity for organic compounds with chemical similarities.7 However, there is still a requirement to incorporate these polymers into novel sensor configurations in order to exploit fully the gas polymer molecular interactions as signal transducers, thereby converting them into readable measurements.3 *To whom correspondence should be addressed. E-mail: crl1@biotech. cam.ac.uk. Phone: þ44(0)1223 334157. Fax: þ44(0)1223 334162. (1) Turner, A. P. F. Science 2000, 290, 1315–1317. (2) Comini, E.; Faglia, G.; Sberveglieri, G. Solid State Gas Sensing; Springer: Berlin, 2009. (3) Yamazoe, N. Sens. Actuators, B 2005, 108, 2–14. (4) Guo, H.; Lee, S. C.; Chan, L. Y.; Li, W. M. Environ. Res. 2004, 94, 57–66. (5) Tres, M. V.; Mohr, S.; Corazza, M. L.; Luccio, M. D.; Oliveira, J. V. J. Membr. Sci. 2009, 333, 141–146. (6) Jiang, X.; Kumar, A. J. Membr. Sci. 2006, 286, 285–292. (7) Fried, J. Conformation, Solutions, and Molecular Weight. In Polymer Science and Technology; Fried, J., Ed.; Prentice Hall: Upper Saddle River, NJ, 2003.

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We have introduced a method for fabricating holographic sensors which report analyte concentrations as changes in reflected color or intensity.8 However, while this technique is capable of sensing a plethora of analytes,9-14 it is limited by the conventional hydrophilic materials used to record the holograms. This restricts the versatility of the system and reduces the possibility of measuring a wide range of other molecules, particularly hydrophobic gases and volatiles. Here, however, we show that, by using a novel diffusion method to form nanoparticles in situ, followed by laser ablation, it is possible to produce holographic sensors in materials in which volume holograms have never previously been recorded. We exemplify this approach by recording holograms in hydrophobic films of poly(dimethylsiloxane) (PDMS) and using these to monitor hydrocarbons and other volatile compounds. PDMS films are water-repellent and physiologically inert and thus are suitable for applications in biomedicine, personal care, and industry.15 Traditionally, as in photography, a holographic film is made in a gel containing photosensitive silver salts. When exposed to a coherent light source, the silver salts are transformed into their metallic form (Ag0).16 The hologram reflects light at a particular wavelength on illumination with white light due to the periodicity, spacing, and refractive index of the holographic gratings. At that scale, only certain wavelengths are allowed to be reflected.17,18 (8) Blyth, J.; Millington, R. B.; Mayes, A. G.; Frears, E. R.; Lowe, C. R. Anal. Chem. 1996, 68, 1089–1094. (9) Mayes, A.; Blyth, J.; Millington, R.; Lowe, C. Anal. Chem. 2002, 74, 3649– 3657. (10) Gonzalez, B. M.; Christie, G.; Davidson, C. A. B.; Blyth, J.; Lowe, C. R. Anal. Chim. Acta 2005, 528, 219–228. (11) Mayes, A.; Blyth, J.; Kyrolainen-Reay, M.; Millington, R.; Lowe, C. Anal. Chem. 1999, 71, 3390–3396. (12) Sartain, F.; Yang, X.; Lowe, C. Anal. Chem. 2006, 78, 5664. (13) Marshall, A.; Young, D.; Blyth, J.; Kabilan, S.; Lowe, C. Anal. Chem. 2004, 76, 1518–1523. (14) Millington, R.; Mayes, A.; Blyth, J.; Lowe, C. Anal. Chem. 1995, 67, 4229– 4233. (15) Kuo, A. C. Poly(dimethylsiloxane). In Polymer Science and Technology; Mark, J. E., Ed.; Oxford University Press: Oxford, 1999. (16) Denisyuk, Y. J. Appl. Spectrosc. 1980, 33, 901–915. (17) Yablonovitch, E. J. Opt. Soc. Am. B 1993, 10, 283–295. (18) Pen, E. F.; Rodionov, M. Y.; Shelkovnikov, V. V. Russ. Phys. J. 2001, 44, 1074–1080.

Published on Web 09/13/2010

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Figure 1. (A) Holographic recording setup. An Nd:YAG 532 nm 300 mJ pulsed laser beam deviated with dichroic mirrors (M1, M2, M3) and spreader lens (SL) and reflected from a reflective surface (R) was used to form standing waves with a constructive interference pattern, ablating silver particles in hydrophobic holographic films (H). The drawings are not to scale. (B) Photographs of a typical holographic film response in a cuvette when exposed to different concentrations of 1-butyne at 22 °C. The hologram is illuminated with a white light source showing a color transition from green to red. No color adjustments were performed on these images.

In these structures, also called Bragg reflectors, certain wavelengths incur a maximum reflectivity when satisfying the so-called Bragg condition,19-22 i.e., cos θ = K/2nk0 where K = 2π/∂ is a grating vector perpendicular to the interference planes, θ is the Bragg angle of incidence, n the average refractive index of the medium, ∂ the grating period or spacing, and k0 = 2π/λ. This can be simplified to λ = 2n∂ cos θ. If we consider small variations in the refractive indices and small angles of incidence, then the main contribution to a change in wavelength would be provided mainly by a change in ∂, the fringe spacing (Figure 1A).

Experimental Section Materials. Silicon elastomer poly(dimethylsiloxane) (PDMS), Dow Corning curing agent, and prepolymer base PDMS kit Sylgard 184, purchased from Farnell, U.K. Butane (99.5%), 1-butene (99%), propane (99.5%), propene (99.5%), ethane (99.5%), isobutane (99.5%), and oxygen (99.99%) lecture bottles were purchased from CK Gas Products, U.K. 1-Butyne (98%) and 1-propyne (97%) were from Intergas, U.K. Nitrogen (99.99%) and carbon dioxide (99.8%) were from BOC, U.K. Silver pentafluoropropionate salt (AgPFP, 98%) from SigmaAldrich, U.K. Hydroquinone (HQ, 99%) from Acros Organics and tetrahydrofuran (99.9%) were purchased from Fisher Scientific, U.K. Except where noted otherwise, all other solvents and materials were obtained from Sigma-Aldrich and were of analytical grade or higher. Hydrophobic Supports for Holograms. We formed thin films of PDMS (Sylgard184, Dow Corning) as indicated by the manufacturer by mixing thoroughly PDMS elastomer and curing agent containing a Pt based catalyst with a 10:1 (v/v) ratio. The (19) Joannopoulos, J.; Johnson, S.; Winn, J.; Meade, R. Photonic crystals: molding the flow of light; Princeton University Press: Princeton, 2008. (20) Hariharan, P. Basics of holography; Cambridge University Press: Cambridge, 2002. (21) Kogelnik, H. The Bell System Technical Journal 1969, 48, 2909–2947. (22) Paschotta, R. Encyclopedia of laser physics and technology; Wiley-VCH: Weinheim, 2008.

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mixture was used to coat clean glass slides using a Mayer rod no. 14 to form a 32 μm thin film23 whose thickness was confirmed by optical microscopy in transverse sections. The polymer films were cured (cross-linked) in a preheated oven at 70 °C for 2 h. For the in situ formation of silver nanoparticles, we deposited 200 μL aliquots of a solution containing 0.2 M silver pentafluoropropionate (AgPFP) in tetrahydrofuran (THF) and 200 μL of a 0.1 M solution of reducing agent hydroquinone (HQ) in THF on top of the cured PDMS film. The solutions were applied simultaneously on the surface of the PDMS films spreading the liquids using a pipet tip. The films were dried with hot air for 60 s to evaporate remnants of solvent. Finally, the films were rinsed thoroughly with deionized water to remove excess particles on the surface and dried again under an air flow. Holographic Signal Measurement. A portion (8 mm  25 mm) of the hologram, cut with a diamond pen, was fixed to the transparent wall of a 3 mL glass cuvette that was then sealed with a flowthrough cap. The cuvette was co-located in a temperaturecontrolled cuvette holder at the required angle of reflection to capture the holographic signal as changes in wavelength. We used a lens ended torch (E10  12 lamp, 3.7 V, 300 mA, RS - UK) as white incident light source and a CCD reflectometer to measure the changes in reflected wavelength. Andor MCD software configured to capture the percentage of reflectance was used to pick maximum reflectivity peaks for wavelengths in the 450-820 nm interval. The spectrophotometer was calibrated by subtracting the background with the torch switched off in the dark and a reference with the torch switched on over a hologram surface at an angle with no holographic reflections. The holographic response was measured as a function of time; it usually took from 3 to 9 s to reach a stable maximum response (equilibrium) at the flow rates used. When the spacing ∂ changes, as a consequence of the molecular interactions between the polymer and the tested molecules, the reflected wavelength λ at the Bragg angle also changes.

(23) MacLeod, D. M. Wire-Wound Rod Coating. In Coatings Technology: Fundamentals, Testing and Processing Techniques; Tracton, A. A., Ed.; CRC Press: Boca Raton, 2006; pp 19-1-19-8.

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Martı´nez-Hurtado et al. 4 °C were completed for each gas. The response of the hologram in air to changes in temperature was also recorded while cooling and heating the cuvette holder with no significant variations detected (0.27 nm/°C). For liquid substances like water and other organic compounds, the cuvette was filled with a sufficient volume of liquid to cover the hologram completely; the measurements were recorded every second as for the gaseous species.

Figure 2. (A) Typical example of the repeatability of the holographic response. Arrows at the bottom indicate insertion of the gas, in this case, maximum concentration of isobutane (99.5% (v/v)); and arrows at the top purging of air into the cuvette. (B) Wavelength changes for different concentrations in % (v/v) of n-butane continuously measured; air was used to wash the cuvette chamber. Therefore, a direct relation between a color change and the exposure to individual molecules can be observed. Gas Exposure. High-purity hydrocarbon gases used in this study were purchased in lecture bottles (>97%); impurities were commonly from other hydrocarbons. The concentrations displayed in the results below are calculated as volume percentage from the known values for gases in the lecture bottles and air. For the determination of the performance of the holographic sensor, the gases were pumped into the sealed cuvette using a 60 mL plastic syringe; the gas was collected from the lecture bottle into the syringe by using a pressure regulator (4 bar) and a valve. After establishing a stable baseline for 5 s, the gas was pumped at 12 mL/s for 5 s through the cuvette; the signal was collected every second for 30 s. In a similar manner, to measure the decay of the signal, a flow of 6 mL/s was pumped into the cuvette and stopped after 10 s when the signal reached equilibrium, whence one of the cuvette orifices was opened to the atmosphere removing the syringe connections; the data were collected every 3 s for 17 min while the gas diffused out. For the concentration dependence plots, the different gases from the lecture bottles were mixed with air at different volume percentages. The mixture was pumped from the containing syringes into the cuvette at 10 mL/s until a maximum response and equilibrium were achieved and then washed with a vigorous airflow; this procedure was repeated several times, recording values for the different gas species. Oxygen, nitrogen, carbon dioxide, water vapor, and air were measured in the same manner; no change was detected (see Supporting Information Figure S2). The temperature on the surface of the hologram was controlled using a self-controlled water bath connected with plastic tubing to the metallic cuvette holder. It was left on for 30 min to reach equilibrium before the experiment started. The temperature was measured by inserting a thermometer into the cuvette holder. Measurements using the hologram at 40, 22, and 15696 DOI: 10.1021/la102693m

Results and Discussion Holographic Recordings in Hydrophobic Supports. Spreading solutions of silver salts in solvents on the surface of PDMS films allowed the silver salts and solvents to perfuse into the polymer matrix24 and be reduced by the HQ forming nucleation sites for the growth of nanoparticles.25 When the solvent evaporated completely and there was no more diffusion into the film, the interactions of the AgPFP with the reducing agent HQ, and thus the nanoparticle growth, stopped. Upon solvent evaporation and thorough washing with water to remove the excess of silver, the transparent PDMS films turned into semitranslucent dark olive green films, indicative of the formation of the metallic silver particles.26 Contrary to typical hologram recording protocols, we ablated these previously formed silver particles27 inside the polymer matrix by using a frequency doubled Nd:YAG (Ndyttrium-aluminum-garnet) pulsed laser (532 nm, 300 mJ). The setup is shown in Figure 1A; the laser beam was diverted using dichroic mirrors (M1, M2, M3) and a spreader lens (SL) to strike the recording material (H) with a 1-cm-diameter spot. The films containing silver particles were placed with a 7° inclination from a mirrored backing in the path of the laser beam, which was reflected by the reflective surface (R) forming standing waves and an interference pattern with nodes and antinodes. In the antinodes, the constructive interference provided sufficient energy to ablate the previously formed silver particles, generating a refractive index periodicity with a grating period (∂) caused by the distribution of silver particles (Figure 1A). In contrast with traditional holography, the bright fringes were generated in the crests, where the particles were ablated. The distance from node to node is equivalent to half of the laser wavelength. The small angle used for recording allowed us to have a different angle for the reflection of incident white light and holographic reflections satisfying the Bragg’s condition. Before ablation, microscopy showed that there is a broad distribution of particle sizes. The hologram was shot twice with a laser spot of 1 cm diameter; the second shot ensures that the laser radiation is reflected after the silver particles are ablated into smaller ones;27 analysis of electron microscopy images showed an average size distribution of 19.2 nm (15.5 nm. This new way of making holograms required only a few nanoseconds of exposure to the high-energy laser beam. Holographic Response. It was possible to measure a direct real-time response of the hologram to differing concentrations of hydrocarbon gases flowing through the cuvette. Figure 1B shows a typical change in replay color when exposed to a hydrocarbon gas, in this example, 1-butyne. A difference in coloration only appears obvious to the “naked eye” after a 50 nm wavelength shift (73.5% v/v) due to the processing of the wavelengths to colors;28 however, the spectrophotometer is more sensitive and can measure differences up to 0.3 nm in wavelength. Figure 2 shows the sensitivity of the hologram when exposed to repeated cycles of (24) Lee, J.; Park, C.; Whitesides, G. Anal. Chem. 2003, 75, 6544–6554. (25) Schmid, G. Chem. Rev. 1992, 92, 1709–1727. (26) Liz-Marzan, L. Mater. Today 2004, 7, 26–31. (27) Hajiesmaeilbaigi, F.; Mohammadalipour, A.; Sabbaghzadeh, J.; Hoseinkhani, S.; Fallah, H. Laser Phys. Lett. 2005, 3, 252–256. (28) Bohren, C.; Clothiaux, E. In Fundamentals of atmospheric radiation; WileyVCH: Weinheim, 2006; Chapter 4, p 212.

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Figure 3. (A) Individual recordings of the holographic response for different hydrocarbon gases at the highest concentration. The maximum response is reached after only 5 s of exposure. (B) Release kinetics of the hydrocarbon gas without perturbation of the system with external airflow.

additions of 99.5% (v/v) isobutane followed by purging with air; the arrows at the bottom represent the addition of the gas, the arrows at the top when the cuvette was purged with air. The sensor response was freely reversible over multiple cycles. Figure 2B shows wavelength changes for different concentrations of n-butane in % (v/v) continuously added and purged between additions with air to regenerate the cuvette chamber. Similar results were obtained for all the other gaseous hydrocarbons. It is difficult to detect changes below 5% (v/v) due to the relatively small changes in fringe spacing ∂ and the wavelength sensitivity of the spectrophotometer; however, the hologram accurately measures concentrations up to 100% (v/v) which are required for some industrial applications. Figure 3 shows a more detailed view of the kinetic process depicted for the various gaseous hydrocarbons in order to evaluate the performance of the sensor described above. A maximum response was reached after 5 s of exposure to the hydrocarbon gas, and the signal stayed constant thereafter when equilibrium was achieved (Figure 3A). Likewise, Figure 3B shows the decay of the response was also recorded for the gas diffusing out of the cuvette chamber. The replay wavelength of the holographic response, without purging, returned to its initial state (Δλ = 0) after 300 s for all the gases; smaller molecules took less time to wash out, and unsaturated hydrocarbons displayed a faster initial decay in the replay wavelength. Purging the hologram with air rapidly recovered its initial replay wavelength. High molecular weight hydrocarbons in the liquid state, such as pentane, hexane, heptane, octane, and decane, and other high molecular weight ketones and alcohols were also detected by the sensor, although the replay wavelength response extended rapidly beyond the visual range of the spectrum.29 The holographic sensor was responsive neither to common atmospheric gases like O2, N2, or CO2, nor to water vapor or liquid water. This observation makes the sensor suitable to operate under different atmospheric conditions, in humid environments, or in aqueous solutions. Moreover, the sensor could be coupled with biomedical devices that operate in breath or biological fluids or with industrial processes that require remote sensing and monitoring, since optical sensors offer this advantage. Furthermore, we found that the holographic response, detected as a change in reflected wavelength, can be recorded continuously and is dependent on the concentration of the analyte gas, as shown in Figure 4. Slight variations were found when the sensor was operated at different temperatures: For example, the response appeared to increase significantly for the higher concentrations at colder temperatures, as in the case for butyne at 8 °C (Figure 4A), in which the response exceeded the visual range. However, the (29) Cacciari, I.; Righini, G. Optical Gas Sensing. In Solid State Gas Sensing; Comini, E., Faglia, G., Sberveglieri, G., Eds.; Springer: New York, 2009.

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Figure 4. Holographic response (replay wavelength shift, Δλ) to various hydrocarbon gas concentrations in the range 0-100% (v/v). Measurements at (A) 8 °C, (B) 22 °C, and (C) 40 °C.

relative profiles of the responses were similar even at different temperatures, with a higher wavelength change being observed for higher molecular weight hydrocarbons (Figure 4A,B,C). In general, the magnitude of the Δλ increased for low temperatures and decreased for high temperatures, as shown in Figure 5. The holographic response appears to be closely related to the molecular structure of the tested molecules. One physical property that can be related to the geometrical structure of the molecules and can be correlated to the magnitude of the holographic response is DOI: 10.1021/la102693m

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Figure 5. Changes in the magnitude of the holographic response (replay wavelength shift, Δλ) and performance of the holographic sensor at different temperatures when exposed to the maximum concentration of gaseous hydrocarbons.

the boiling point at standard temperature and pressure (Figure 6A). Another similar physical property which relates to changes in polarity caused by differences in the atomic conformations and bonds within the molecule is that of partitioning between two immiscible solvents. Figure 6B illustrates a comparison of the sensor replay wavelength response as a function of the octanolwater partition coefficients (Kow). The wavelength shift increases coherently as the number of carbons in the molecular structure increases for the homologous series of alkanes, alkenes, and alkynes. Thus, the holographic response relates to the molecular structure of the analyte molecules, although the relation between molecular weight and boiling points for hydrocarbons does not always apply for other volatile compounds, particularly in the liquid state. Putative Mechanisms. In general, the holographic response or reflected wavelength change (Δλ) process is driven by the affinity of the hydrocarbon gas molecules for the PDMS matrix. The affinity of any substance for certain polymers is related to the components of the intramolecular forces present in the polymer and within the molecules, i.e., van der Waals forces,30 which together contribute to the free energy required for the expansion of the polymer film, and thus, for a change in ∂ and reflected wavelength, i.e., holographic response, Δλ. Moreover, the strength of these attractive forces or interactions can be determined by the energy required to separate the molecules completely, as in a perfect gas. In other words, the energy required to break all the interactions should be proportional to the total number of interactions present and their strength. Thermodynamically, these attractive forces are expressed as cohesive energy densities (c) in terms of the enthalpies of vaporization (ΔHv), which can be calculated from experimental data. This relationship is expressed as c = (ΔHv - RT)/Vm, where R is the ideal gas constant, T the absolute temperature, and Vm the molar volume; c is commonly expressed as a solubility parameter δ ¼

pffiffiffi c ¼ ½ΔHv - RTÞ=Vm 1=2 ½MPa1=2 

Interactions of small molecules with themselves allow a better understanding their interactions with other molecules, since molecules with similar δ values have more affinity to each other or display a higher propensity to mix. Thus, the intramolecular attractive interactions, and hence the cohesive energy densities, (30) Belmares, M.; Blanco, M.; Goddard, W.; Ross, R.; Caldwell, G.; Chou, S.; Pham, J.; Olofson, P.; Thomas, C. J. Comput. Chem. 2004, 25, 1814–1826.

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Figure 6. (A) Holographic response (replay wavelength shift, Δλ) compared with boiling points of hydrocarbons: 1, ethyne; 2, ethylene; 3, ethane; 4, propyne; 5, propylene; 6, propane; 7, isobutane; 8, 1-butene; 9, butane; 10, 1-butyne. (B) Log10[replay wavelength shift, Δλ] as a function of octanol-water partition coefficients (Kow) for three homologous series of saturated and unsaturated hydrocarbon gases.

can be related to the affinity of the tested molecules for the PDMS polymer chains and thereby to the holographic response of the sensor, as shown in Figure 7. Other volatile organic compounds tested with the sensor showing a reversible response are included in Figure 7; as the values of δ in the plot were closer to those of PDMS (δ = 14.9-7.5 MPa1/2, list of values in Supporting Information Table S1), the holographic response increased. Substances with dissimilar δ values to that of PDMS, like water or glycerol, produced no holographic response. Likewise, molecules with very similar values of δ to those of PDMS exhibited greater responses. The attractive forces in our sensor system were mainly attributed to apolar hydrophobic interactions, i.e., London dispersion forces, and outweigh others. Thus, this parameter can be used to predict the sensor response to large numbers of organic molecules, as depicted in Figure 7, with relatively good precision. Any interaction of the analytes with the Ag particles would appear as a change in refractive index and consequently a change in diffraction efficiency, which was not observed; therefore, we deduce that the change in wavelength is due primarily to swelling of the polymer chains. Mixtures could also be tested for particular applications, such as differences in fuels, VOCs in furnishing materials, and organic contaminants in water. However, the selectivity is not sufficient and needs to be improved to detect different molecules in a mixture where all the species interact with the sensor; turning the sensor into a more selective and sensitive system could be achieved by incorporation of specific ligands and reactive compounds and tailoring cross-linking into the PDMS matrix for particular target applications. Langmuir 2010, 26(19), 15694–15699

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Figure 7. Holographic response of the sensor and square root of the cohesive energy densities of hydrocarbon gases and certain volatile organic compounds expressed as the solubility parameter (δ) reported in MPa1/2. The chemical formulas of the molecules are shown in the legend: 1, ethyne; 2, ethene; 3, ethane; 4, propene; 5, propane; 6, isobutane; 7, 1-butene; 8, butane; 9, tert-butanol; 10, butanone; 11, propanone; 12, isobutanol; 13, propan-2-ol; 14, isopentanol; 15, propan-1-ol; 16, butan-1-ol; 17, ethanal; 18, pentan-1-ol; 19, 1-butyne; 20, ethanol; 21, methanol; 22, propyne/propadiene stabilized; 23, ethylene glycol; 24, glycerol; 25, water. Changes in the holographic response (Δλ, [nm]) are related to molecular interactions between those structures and PDMS chains; δ for PDMS has been reported to have different values within the range 14.9-7.5 MPa1/2 depending on the cross-linking, chain length, and so forth.

Conclusions This report demonstrates not only a novel laser ablation protocol for producing volume holographic gratings in polymeric matrices, but also the fabrication of volume holograms for the first time in hydrophobic materials such as poly(dimethylsiloxane) (PDMS). This new technique allows us to fabricate rapidly responding holographic sensors for hydrocarbon gases and other VOCs in hydrophobic polymers. The response of the holograms, observed as a bright-colored reflection, is explicable in terms of the apolar molecular interactions between the polymer and hydrocarbons or other organic compounds and expressed as a molecular thermodynamic parameter (δ). These interactions are detected as a change in the grating periodicity (∂) of the hologram and hence a reply wavelength change (Δλ). The hydrophobicity of the hologram material confers stability and a noninterfering response from water, water vapor, and other atmospheric gases, creating the possibility of using the sensor in high water activity environments. We have shown that we can detect different concentrations of hydrocarbon gases as changes in reflected wavelength, which are

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readable by eye. The demonstrated color changes in response to the presence of hydrocarbon gases and VOCs, and the advantages conferred by the polymeric material, open up the possibility of applying holographic gas sensors in devices where physiologically compatible materials or remote sensing is required, making them especially suitable for applications in the built and natural environment and in other biomedical and industrial scenarios. Acknowledgment. The authors thank the Technology Strategy Board (TSB) and CONACYT (Grant No. 182820) for the funding. Supporting Information Available: Table of values for holographic response, cohesive energy, boiling point, and octanol-water partition coefficient for the molecules tested. Plot of the sensor response rates and equilibrium times for the hydrocarbon gases tested in this study and other volatile organic compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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