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Organic Vapor Sensing with Ionic Liquids Entrapped in Alumina Nanopores on Quartz Crystal Resonators Ilchat Goubaidoulline,† Gabriele Vidrich, and Diethelm Johannsmann*
Institute of Physical Chemistry, TU Clausthal, Arnold-Sommerfeld-Strasse 4, D-38678 Clausthal-Zellerfeld, Germany
We report on a concept for vapor sensing with the quartz crystal microbalance where the vapor phase is absorbed into small droplets of an ionic liquid. The liquid is contained in the pores of a nanoporous alumina layer, created on the front electrode of the quartz crystal by anodization. Ionic liquids are attractive for vapor sensing becausesbeing liquidssthey equilibrate very fast, while at the same time having negligible vapor pressure. Containing the ionic liquids inside cylindrical cavities of a solid matrix removes all problems related to the liquid’s softness as well as the possibility of dewetting and flow. The absence of viscoelastic effects is evidenced by the fact that the bandwidth of the resonance remains unchanged during the uptake of solvent vapor. The Henry constants for a number of solvents have been measured. The quartz crystal microbalance (QCM) is a well-known tool for the determination of film thickness.1 For thin films, the Sauerbrey relation holds, stating that the shift in frequency is proportional to the areal mass density of the film. One has
δf ) - (2fo/Zq)δm
(1)
where δf is the frequency shift, f0 is the frequency of the fundamental, Zq ) 8.8 × 106 kg m-2 s-1 is the acoustic impedance of the quartz, and δm is the areal mass density. Due to the high Q factor of the resonance, shifts of frequency are easily determined with an accuracy below 1 Hz, which results in monolayer sensitivity. The high sensitivity and ease of operation have turned the QCM into a routine tool for adsorption measurements. Vapor sensing with acoustic wave devices based on the uptake of vapor by a film has been recently reviewed by Grate.2-4 In most cases, a sensing layer is deposited on the electrodes of the resonator, which specifically absorbs the vapor. The sensing layer must be nonvolatile and must allow for rapid vapor diffusion throughout * To whom correspondence should be addressed. Phone: +49-5323-72-3768. Fax: +49-5323-72-4835. E-mail:
[email protected]. † Present address: Deutsches Kunststoff Institut, Schlossgartenstr. 6, 64289 Darmstadt, Germany. (1) Sauerbrey, G. Z. Phys. 1959, 155, 206. (2) Grate, J. W. Chem. Rev. 2000, 100, 2627. (3) Grate, J. W.; Abraham, M. H. Sens. Actuators, B 1991, 3, 85. (4) Grate, J. W.; Frye, G. C. In Sensors Update; Baltes, H., Goepel, W., Hesse, J., Eds.; VHS: Weinheim, 1996; pp 51-78. 10.1021/ac048436a CCC: $30.25 Published on Web 12/13/2004
© 2005 American Chemical Society
the film. A wide variety of sensing materials has been tested, including chromatographic stationary phases, polymers, lipids, proteins, and dendrimers.2 The sensing material must have a glass temperature Tg below the operating temperature. Otherwise the diffusion kinetics is slow and there is poor reversibility of swelling in the vapor phase. In principle, liquids would be attractive sensing materials because they are in thermodynamic equilibrium and diffusion is fast. On the other hand, the layer must not dewet from the sensor surface, which usually precludes the use of ordinary liquids. Evaporation is another problem with the use of liquids. Substituted polysiloxane polymers constitute a certain compromise because they have a low Tg, on one hand, and a high viscosity as well as a small vapor pressure, on the other. A problem with polysiloxanes is a potential change in shear modulus induced by the update of solvent. viscoelasticity does affect the sensor response if the layer thickness is comparable to the wavelength of sound.5-7 Viscoelastic effects can amplify the sensor response8 but they complicate the quantitative analysis as well. Recently, Dai reported on the usage of ionic liquids as sensing materials for QCM-based vapor-sensing devices.9 Ionic liquids have vanishing vapor pressure because of the strong intermolecular interaction.10 Ionic liquids with a broad range of functionalities in the side groups are now being synthesized.11 In ref 9, the problem of dewetting has been circumvented by using thick droplets (rather than thin films) and basing the analysis on the liquid’s viscosity induced by the uptake of solvent vapor, rather than the mass uptake. As was shown by Kanazawa, the frequency shift of the QCM is a measure of viscosity for films thicker than the penetration depth of acoustic shear waves.12,13 However, use of the viscosity as the primary observable entails some disadvantages. For instance, there is no a priori way to relate the viscosity to the solvent volume fraction without calibration. Also, the viscosity is strongly temperature-dependent. The use of trapped liquid avoids all problems with dewetting and viscoelasticity. (5) Granstaff, V. E.; Martin, S. J. J. Appl. Phys. 1994, 75, 1319. (6) Johannsmann, D. Macromol. Chem. Phys. 1999, 200, 501. (7) Martin, S. J.; Frye, G. C. Proc. IEEE Ultrason. Symp. 1991, 293. (8) Lucklum, R.; Hauptmann, P. Sens. Actuators, B 2000, 70, 30. (9) Liang, C.; Yuan, C.-Y.; Warmack, R. J.; Barnes, C. E.; Dai, S. Anal. Chem. 2002, 74, 2172. (10) Weldon, T. Chem. Rev. 1999, 99, 2071. (11) Wasserscheid, P., Welton, T., Eds. Ionic Liquids in Synthesis; Wiley VCH: Weinheim, 2002. (12) Kanazawa, K. K.; Gordon, J. C.; J. Anal. Chim. Acta 1985, 175, 99. (13) Borovikov, A. P. Instrum. Exp. Tech. 1976, 19, 223.
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Figure 1. Chemical structures of the ionic liquids IL I and IL II.
Porous alumina layers grown by anodization of aluminum in diluted polyprotic acids have been known for half of a century.14 They have found numerous applications, for instance, as templates for the fabrication of nanostructured materials.15 Two examples, where alumina nanopores have been employed for sensing are given in refs 16 and 17. The alumina layer mainly consists of amorphous aluminum oxide, which is a chemically and mechanically stable material. The size of the pores can be adjusted in the range of 5-300 nm by the parameters of the anodization process. The depth of the pores can be tuned via anodization time. In the context of this work, the system of pores constitutes a rigid assembly of containers. When the pores are about half-filled with the ionic liquid, the liquid does not experience internal shear and therefore does not dissipate energy, even if the liquid volume increases during sorption. Any uptake of vapor therefore only increases the mass, which can be quantitatively analyzed with the Sauerbrey equation (eq 1). Evidently, the concept of containing the sorbent in array of nanocontainers is by no means limited to the use of ionic liquids. Functionalized polysiloxanes, for instance, can be filled into the pores of an anodized alumina surface as well. EXPERIMENTAL SECTION Materials. Organic solvents with at least 99.5% purity were obtained from Aldrich and used without further purification. Quartz crystals (1-in. diameter) with a fundamental resonance frequency of 5 MHz were purchased from Maxtek Inc. (Santa Fe Springs, CA). The ionic liquids shown in Figure 1 were synthesized by Merck KgAA and kindly provided by F. Endres. These ionic liquids were selected because they are not hygroscopic. This circumvents the need to use a dry chamber. Aluminum wire of 99.99% purity for evaporation was obtained from Goodfellow. The acids for the anodizing process were obtained from Aldrich and used as received. Preparation of Nanopores. After cleaning of the bare quartz blanks in surfactant solution (Hellmanex, Hellma, Germany), an aluminum electrode of 1200-nm thickness was thermally evaporated onto the crystal surface. Conventional chromium/gold (30/ (14) Keller, F.; Hunter, M. S.; Robinson, D. L. J. Electrochem. Soc. 1953, 100, 411. (15) Schmid, G. J. Mater. Chem. 2002, 12, 1231. (16) Varghese, O. K.; Gong, D. W.; Dreschel, W. R.; Ong, K. G.; Grimes, C. A. Sens. Actuators, B 2003, 94, 27. (17) Varghese, O. K.; Grime, C. A. Nanosci. J.; Nanotechnol. 2003, 3, 277.
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Figure 2. Sketch of the experimental setup. The activity of the solvent is adjusted by injecting known amounts of liquid into a closed compartment. The use of filter paper increases the rate of evaporation.
300 nm) electrodes were deposited on the back of the crystal. The aluminum was anodized in an aqueous 0.4 M H3PO4 solution at room temperature and 80 V.18 This set of parameters leads to an average pore diameter of 120 nm. The anodization time required to reach the desired pore depth was determined experimentally on glass substrates coated with aluminum layers of the same thickness. The optimum time was ∼85% of the time needed for complete anodization of the entire layer. Since the pore growth starts under the natural oxide layer, the alumina surface is quite smooth after anodization. The dynamic instability giving rise to the pores develops a few nanometers underneath the surface. For the details of the growth process, we refer the reader to ref 15. In order to remove the cover layer, the samples were treated with a solution of chromic acid (0.2 M) and phosphoric acid (0.4 M) at 60 °C for 2 min. Ionic liquids were spin-cast onto the resonator surface at a speed of 2000 rpm from 10% solutions in acetonitrile. A solution volume of 5 µL was dropped onto the rotating substrate, and the rotation was continued for ∼1 min to allow for complete evaporation of the solvent. Residual solvent was removed by blowing with dry nitrogen. The crystals can be reused, where cleaning is achieved by rinsing with acetonitrile and blowing with nitrogen gas. Data Acquisition. The experimental setup is schematically depicted in Figure 2. The system was operated at room temperature. The vapor concentration in the cell was adjusted by injecting known volumes of solvent onto a filter paper suspended inside the cell. The air in the cell was stirred with propeller blade. The activity of the vapor was calculated from the injected amount. Frequency shift, δf, and the shift of the half-band-half-width, δΓ, were measured by means of a network analyzer (E5100A, Agilent). The analyzer sweeps the frequency around the resonance frequency and determines the spectra of the conductance G(ω) and the susceptance B(ω). Resonance frequency and bandwidth are determined by fitting resonance curves to the admittance spectra. The amount of ionic liquid inside the pores was determined from the frequency shift with regard to the bare crystal. (18) O’Sullivan, J. P.; Wood, G. C. Proc. R. Soc. London A 1970, 317, 511.
Figure 4. Kinetics of removal of THF from pores filled with IL I. The kinetics has is long time tail, related to the fact that the driving force for desorption is small for small vapor concentrations.
Figure 3. (a) Shifts of frequency and bandwidth on the third harmonic for partially filled pores. When solvent vapor is admitted to the chamber, the frequency decreases, whereas the bandwidth remains unchanged. (b) Comparison of frequency shifts on different overtones. The steps correspond to increases of the solvent activity. The ratio δ×a6/n (n the overtone order) is the same on all harmonics, proving that effects of finite elasticity can be ignored. (c) For completely filled pores, the bandwidth increases during vapor uptake as well, indicating that some of the material spills over and experiences internal shear, leading to viscous dissipation.
For the measurement of the vapor uptake, the resonance frequency of quartz crystal loaded with ionic liquid was used as the reference. RESULTS AND DISCUSSION Figure 3 addresses the issue of dissipation and viscoelastic contribution to the sensor response. Generally speaking, the bandwidth of the quartz crystal-nanopore assembly is in the range expected for the dry resonator even when the pores are filled with liquid. Importantly, the bandwidth does not change when acetonitrile vapor is admitted (Figure 3a). In this experiment, the pores had been filled to a level of 60% with the material IL I. Upon admission of solvent vapor, the frequency decreases. Figure 3b shows the same experiment, where the normalized frequency shift δf/n is displayed for the overtone orders 3-13. The data can hardly be distinguished: δf/n (n, the overtone order) is the same on all harmonics. A scaling of the frequency shift δf with overtone order, n, is characteristic of the Sauerbrey equation. The Sauerbrey relation in a rather simplistic way predicts that the fractional decrease in frequency is equal to the fractional increase in mass. This ignores all viscoelastic effects. Finding the Sauerbrey relation being obeyed in the experiment, one may conclude that viscoelastic effects are indeed negligible. In the context of sensing, this is a highly desirable situation. Figure 3c shows a counterexample. In this case, the pores have been completely filled. In this case, the bandwidth does increase with the uptake of the vapor. This is explained by the fact that the material spills over. The excess liquid not contained in the
pores shears under its own inertia, thereby increasing the bandwidth. One may wonder how wide the pores can be before the liquid sloshing around inside the pores starts to dissipate energy. This question is addressed in the Appendix. It is interesting to compare the adsorbed mass not only to the amount of liquid but also to the total mass of the porous layer. For the convenience of the comparison, we convert the total mass to an equivalent frequency. This frequency shift is 10 kHz on the fundamental. The mass uptake (equivalent to δf ∼ 100 Hz) therefore amounts to ∼1% of the total mass of the entire assembly. From the fact the mass of the vapor is ∼5% of the mass of the liquid (see Figure 6 below), one infers that the fractional void volume of the structure is in the range of 40% (where the difference in density has been accounted for). Note that a frequency shift of 100 Hz would correspond to a film thickness of 20 nm, if the mass increase were caused by adsorption of the vapor to a flat sensor surface. Since this is much more than a monolayer (even considering that the alumina layer has a high surface area), the frequency shift must be caused by a bulk effect, as opposed to surface adsorption. Such a surface adsorption may be problematic when sensing polar liquids, which adsorb well to the hydrophilic alumina surface. Deactivation of the surface with self-assembled monolayers19 probably is advisable in such a case. Figure 4 shows the desorption kinetics. Tetrahydrofuran (THF) had been admitted to the chamber at the level of a ) 0.096, (a the activity; see below) resulting in a solvent content in the liquid of 3 wt %. At t ) 0, the sensor was removed from the chamber and exposed to ambient air (and zero solvent vapor activity, in particular). The initial loss of weight is faster than the data acquisition rate of 1 Hz. There is a slow tail, which is due to the limited rate of desorption. If the speed of response is a performance criterion, desorption can be accelerated by the use of wider and less deep pores. Figure 5 documents the noise level. The fluctuations of normalized frequency shift δf/n are less than 1 Hz, corresponding to a mass uptake of ∆m < 2 × 10-7 kg/m2 or a solvent weight fraction of ∆w < 0.05 wt %. The step was generated by injecting 5 µL of THF into the cell, increasing the activity from 0 to a ) 7 (19) Ma¨ge, I.; Ja¨hne, E.; Ch. Bram, Ch. Jung, Stratmann, M.; Adler, H. P.; Macromol. Symp. 1997, 126, 7.
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Table 1. Henry Constants in IL I and Detection Limits
Figure 5. Noise level compared to the frequency change upon the vapor uptake at THF activity of a ≈ 7 × 10-4.
Figure 6. Weight fraction of solvent inside the pores as a function of the solvent activity. The straight lines are linear fits.
Figure 7. Sketch of liquid material moving inside a pore under the influence of inertia.
× 10-4. The sensitivity can be increased (at the expense of speed) by using deeper pores. Figure 6 shows measurements of the Henry constants of a few common solvents in IL I. The lines are linear fits. Clearly, the liquid is selective in the sense that it dissolves polar solvents much better than nonpolar ones. The weight fraction w was calculated as w ) δf/(δf + δf0), where δf is the frequency shift induced by the solvent and δf0 is the frequency shift caused by filling the pores with the liquid. The solvent activity was calculated according to the following:20 ai ) (pi/pis) exp(-B11(pis - pi)/RT), where pi and psi are the partial pressure and the saturated vapor pressure, (20) Bonner, D. C.; Prausnitz, J. M. J. Polym. Sci. Polym. Phys. Ed. 1974, 12, 51.
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solvent
Henry’s constant/Pa
detection limit/mg/m3
acetonitrile cyclohexane isooctane methanol THF toluene
9 300 760 000 400 000 14 000 7 400 46 000
1232 1875 7634 1429 321 411
respectively, B11 is the second virial coefficient, R is the gas constant, and T is the temperature. The tabulated values of B11 are -1.67 × 10-3, -4.04 × 10-3, -1.43 × 10-3, and -2.25 × 10-3 for cyclohexane, isooctane, methanol, and toluene, respectively.16,21 For THF and acetonitrile, B11 was not available and the activity was assumed to be equal to the pi/pis. The correction with the second virial coefficient is small in any case. The sensor response is linear in a broad range of solvent activities. The nonpolar solvents isooctane and cyclohexane have low solubility in IL I, whereas THF and acetonitrile are much more soluble. At equilibrium and low concentration of the volatile component, the partial pressure of a vapor, pi, is related to the molar fraction of solvent in the liquid, xi, by Henry’s law: pi ) Hixi. The parameter Hi is Henry’s constant. Hi has the dimension of a pressure and is specific to a solvent-solute pair. The molar concentrations were calculated from the weight fractions and molar masses according to xi ) wiMIL/(wiMIL - wiMi + Mi), where wi is the weight fraction of organic solvent in IL and MIL and Mi are the molar masses of ionic liquid and the solvent, respectively. Henry’s constants of six organic solvents in IL I were determined by fitting the experimental data with straight lines (Figure 6). The results are given in Table 1. With Henry’s constants, the sensitivity of the instrument in hertz can be converted to sensitivity in molar percent of organic solvent in the air. This conversion has been done in the last column of Table 1. Finally, we have compared the solubility of isooctane in IL I and IL II. The ionic liquid IL II has three octyl substituent groups, which interact favorably with isooctane. As a consequence, the solubility is much better. The Henry’s constant of isooctane in ILII is 2000, whereas it is 400 000 in ILI. This comparison shows that a set of ionic liquids can be found that can be used in an array of sensors producing “fingerprints”. CONCLUSIONS By containing ionic liquids in alumina nanopores, one can build a gravimetric sensor for the concentration of gases that uses liquids as the transducing layers. All problems related to viscoelasticity and dewetting are overcome by using the pores as a rigid matrix. Evaporation is avoided by using ionic liquids that have no detectable vapor pressure. The sensitivity, the speed, and the selectivity of the instrument were demonstrated. ACKNOWLEDGMENT The ionic liquids were synthesized by Merck KgaA and kindly provided by Prof. Frank Endres, Institute of Metallurgy, TUClausthal. (21) Tables of physical and chemical constants and some mathematical functions, 15 ed.; (originally compiled by G. W. C. Kaye and T. H. Laby) Longman: London, 1986.
APPENDIX: ESTIMATION OF A CRITICAL MAXIMUM PORE SIZE BEYOND WHICH THE MOVEMENT OF THE LIQUID INSIDE THE CONTAINER CONTRIBUTES TO THE DISSIPATION OF ENERGY Consider a column filled with liquid undergoing an oscillatory lateral motion (Figure 7). Inertia will change the shape of the liquid surface, allowing for a movement of the liquid inside the pore (dashed line in Figure 7). This movement will be counterbalanced by a capillary pressure. Whereas the original surface has the same curvature 1/R everywhere, the new surface has a somewhat lower curvature on the left-hand side and a correspondingly increased curvature on the right-hand side. The difference in curvature causes a difference in Laplace pressure, which acts as a restoring force. In the following, we provide an estimate of this restoring force as well as the inertial force. From the force balance, we calculate a characteristic frequency of the movement of the liquid surface. As long as this frequency is much larger than the frequency of oscillation, the liquid will instantaneously follow the motion of the container and therefore dissipate little energy. The force of inertia is given by Fin ∼ Fω2d3Ra, where F is the density of the fluid, d3 is about the volume of moving liquid (d the diameter of the pore), a is the amplitude of the lateral movement, and R 1 a is small numerical coefficient indicating that
the amplitude of displacement of the liquid is small compared to the amplitude of motion of the entire container. From simple geometric considerations, one can see that the curvature of the dashed line in Figure 7 differs from the curvature of the full line by an amount of the order of Ra/R2. The difference in the Laplace pressure ∆p between the right-hand and the lefthand side of the container therefore is of the order of ∆p ∼ Raγ/ R2, which leads to a force of about F ∼ ∆pd2 ∼ Raγd2/R2 ∼ Raγ. The radius of curvature, R, was approximated by the pore diameter, d, in the last transformation. The spring constant κ ) dF/da therefore is κ ) Rγ. The force balance leads to the characteristic frequency ω0 ∼ (κ/m)1/2 ∼ [γ/(Fd3)]1/2. Inserting γ ∼ 100 × 10-3 J/m2, F ∼ 103 kg/m3, and d ∼ 100 nm, we arrive at ω0 ∼ 300 MHz. This is larger than the frequency of the measurement, albeit not by a large margin. The fact that we do not observe increased dissipation when filling the columns with liquid indicates that the above estimation was conservative.
Received for review October 22, 2004. Accepted October 25, 2004. AC048436A
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