Correspondence Anal. Chem. 1996, 68, 913-917
Selective Vapor Sorption by Polymers and Cavitands on Acoustic Wave Sensors: Is This Molecular Recognition? Jay W. Grate* and Samuel J. Patrash
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352 Michael H. Abraham and Chau My Du
Chemistry Department, University College London, London WCIH OAJ, United Kingdom
Selectivity patterns for the sorption of organic vapors from the gas phase into cavitand monolayers on acoustic wave sensors are very similar to those seen for sorption of the same vapors by amorphous polymers, demonstrating that the vapor/cavitand selectivity patterns are determined primarily by solubility interactions. The amorphous polymers serve as controls demonstrating that the threedimensional structure of a cavitand layer is not primarily responsible for the selectivity observed. Binding and selectivity in the examples cited are governed primarily by general dispersion interactions and not by specific oriented interactions that could lead to molecular recognition.
The process of molecular recognition gives rise to selectivity by discriminating between those molecules that are recognized and those that are not. Lehn has defined molecular recognition as a “process involving both binding and selection of substrate(s)”.1 He noted that, “Mere binding is not recognition, although it is often taken as such.”, and that molecular recognition “implies a structurally well-defined pattern of intermolecular interactions”. The concepts of complementarity and fit are central to successful recognition: two species must complement one another in size, shape, and binding or functionality.2,3 The interactions in molecular recognition and supramolecular chemistry are noncovalent. This mechanism for selectivity could be useful in developing chemical sensors if the recognition process can be successfully transduced into an analytical signal, provided that similar signals are not generated at the same time by other processes. In the last few years, there have been several reports of the application of cavity compounds on acoustic wave sensors for the (1) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89-112. (2) Lehn, J.-M. Science 1985, 227, 849. (3) Rebek, J. Science 1987, 235, 1478-1484. 0003-2700/96/0368-0913$12.00/0
© 1996 American Chemical Society
detection of organic compounds in the gas phase.4-12 In some cases the results are simply reported, and in other cases claims have been made about selectivity, host/guest inclusion, and molecular recognition. In one of the more recent examples, Schierbaum and co-workers described the assembly of a monomolecular film of resorcin[4]arene cavitand molecules tethered to the gold surface of a quartz crystal microbalance (QCM) by long-chain sulfides.6 It was claimed that the “cavitand headgroups... act as molecular recognition sites for small organic molecules with remarkable selectivity for perchloroethylene”, and that “fast and reversible ‘host/guest’ interactions were found by the monitoring of extremely small mass changes with a quartz microbalance oscillator”.6 The pattern of selectivity observed by these authors for five test vapors is shown in Figure 1a. As a control, the authors reported that a simple alkyl sulfide layer did not bind perchloroethylene as tightly as the cavitand-containing layer. However, the pattern of selectivity for the control layer was not shown for comparison. The pattern of selectivity observed for the resorcin[4]arene layer did not appear to be unusual to us, and we have examined this issue using three approaches. First, we used linear solvation (4) Dickert, F. L.; Bruckdorfer, T.; Feigl, H.; Haunschild, A.; Kuschow, v.; Obermeier, E.; Bulst, W. E.; Knauer, U.; Mages, G. Sens. Actuators B 1993, 13-14, 297-301. (5) Schierbaum, K.-D.; Berlach, A.; Bopel, W.; Muller, W. M.; Vogtle, R.; Dominik, A.; Roth, H. J. Fresenius J. Anal. Chem. 1994, 349, 372-379. (6) Schierbaum, K. D.; Weiss, t.; Thoden van Velzen, E. U.; Engbersen, J. R. J.; Reinhoudt, D. N.; Gopel, W. Science 1994, 265, 1413-1415. (7) Nelli, P.; Dalcanale, E.; Faglia, G.; Sberveglieri, G.; Soncini, P. Sens. Actuators B 1993, 13-14, 302-304. (8) Lai, C. S.-I.; Moody, G. J.; Thomas, J. D. R.; Mulligan, D. C.; Stoddart, J. F.; Zarzycki, R. J. Chem. Soc., Perkin Trans. 2 1988, 319-324. (9) Elmosalamy, M. A. F.; Moody, G. J.; Thomas, J. D. R.; Kohnke, F. A.; Stoddart, J. F. Anal. Proc. 1989, 26, 12-15. (10) Dickert, R. L.; Bauer, P. A. Adv. Mater. 1991, 3, 436-438. (11) Dickert, R. L.; Haunschild, A.; Reif, M.; Bulst, W.-E. Adv. Mater. 1993, 5, 277-279. (12) Li, D.; Erdal, B. Characterization, Monitoring, and Sensor Technology Integrated Program (CMST-IP) Technology Summary. DOE/EM-0156T, NTIS DE94-014298, 38-41, 1994.
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sensor surface), ∆fs, the vapor concentration (in g/L), the partition coefficient, and the density of the sorbent phase, F. Therefore, for a set of vapors tested on a single sensor, all the vapors at the same concentration (in g/L), the sensor responses will be proportional to K. If vapor concentrations are expressed in terms of ppm (vol/vol) or partial pressure, which are related to the moles of vapor rather than the mass of the vapor, the vapor molecular weight (MW) must be included. Then, for a set of vapors tested on a single sensor where the vapor concentrations are all identical in units of ppm (vol/vol) or partial pressure, sensor responses will be proportional to K MW.17 Our third approach for examining selectivity patterns was to consider the fundamental interactions that determine the sorption of organic vapors from the gas phase to a condensed phase. The LSER method for calculating polymer/gas partition coefficients is based on eq 3, which expresses log K as a linear
∑R
log K ) c + rR2 + sπH 2 + a
Figure 1. Comparisons of selectivity patterns. (a) Relative sensitivity of the resorcin[4]arene cavitand monolayer on a QCM to perchloroethylene (C2Cl4), trichloroethylene (C2HCl3), carbon tetrachloride (CCl4), chloroform (CHCl3), and toluene (C7H8) vapors, all at partial pressures of 200 Pa and 303 K. (b) and (c) Selectivity patterns predicted by LSER calculations for sorption of the same vapors by PIB- and PVTD-coated acoustic wave sensors at 298 K. (d) Selectivity pattern predicted from measured partition coefficients in hexadecane at 298 K. (e) and (f) Experimentally measured selectivity patterns of PIB- and PVTD-coated QCMs, all at partial pressures of 200 Pa and 303 K. Responses are in hertz for a sensor coated to a polymer thickness giving a 30 kHz frequency shift.
energy relationships (LSER) that we have developed13 to calculate the polymer/gas partition coefficients, K, of the same five vapors into a variety of amorphous polymers. This partition coefficient is the ratio of the concentration of the vapor in the polymer, Cs, to the concentration of the vapor in the gas phase, Cv (both concentrations in g/L).
K ) Cs/Cv
Cv f 0
(1)
Second, we selected two of these polymers for experimental measurements of vapor sorption using QCM sensors. The response of a mass-sensitive acoustic wave sensor (such as the QCM) to absorption of a vapor is related to the partition coefficient as shown in eq 2.14-16 The sensor’s response to the mass of vapor
∆fv ) ∆fsCvK/ρ
(2)
absorbed, a frequency shift denoted by ∆fv , is dependent on the frequency shift due to the deposition of the film material onto the bare sensor (a measure of the amount of polymer on the (13) Abraham, M. H.; Andonian-Haftvan, J.; Du, C. M.; Diart, V.; Whiting, G.; Grate, J. W.; McGill, R. A. J. Chem. Soc., Perkin Trans. 2 1995, 369-368.
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H 2
∑β
+b
H 2
+ l log L16 (3)
combination of terms that represent particular interactions.18-20 H H 16 In this relationship, R2, πH 2 , ΣR2 , Σβ2 , and l log L are solvation parameters that characterize the solubility properties of the vapor,18 where R2 is a calculated excess molar refraction parameter that provides a quantitative indication of polarizable n and p electrons; πH 2 measures the ability of a molecule to stabilize a H neighboring charge or dipole; ΣRH 2 and Σβ2 measure effective hydrogen-bond acidity and basicity, respectively; and log L16 is the liquid/gas partition coefficient of the solute on hexadecane at 298 K (determined by gas/liquid chromatography). The log L16 parameter is a combined measure of exoergic dispersion interactions that increase log L16 and the endoergic cost of creating a cavity in hexadecane leading to a decrease in log L16. The LSER equation for a particular polymer is determined by regressing measured partition coefficients for a diverse set of vapors on that polymer against the solvation parameters of the test vapors. The regression method yields the coefficients (s, r, a, b, l) and the constant (c) in eq 3. These coefficients are related to the properties of the sorbent polymer that are complementary to the vapor properties. The LSER equations determined by this multiple linear regression method are very effective at correlating the sorption of vapors by a polymer to the vapors’ properties and can be used to predict partition coefficients.13 (Correlation coefficients are typically 0.99.) In combination with equations similar to eq 2 above, these LSER equations can be used to esimate the responses of polymer-coated surface acoustic wave (SAW) vapor sensors.21 In addition, these LSER equations have been used to compare (14) Grate, J. W.; Snow, A.; Ballantine, D. S.; Wohltjen, H.; Abraham, M. H.; McGill, R. A.; Sasson, P. Anal. Chem. 1988, 60, 869-875. (15) Grate, J. W.; Martin, S. J.; White, R. M. Anal. Chem. 1993, 65, 940A948A. (16) Grate, J. W.; Martin, S. J.; White, R. M. Anal. Chem. 1993, 65, 987A996A. (17) Grate, J. W.; Abraham, M. H.; Du, C. M.; McGill, R. A.; Shuely, W. J. Langmuir 1995, 11, 2125-2130. (18) Abraham, M. H. Chem. Soc. Rev. 1993, 22, 73-83. (19) Grate, J. W.; Abraham, M. H. Sens. Actuators B 1991, 3, 85-111. (20) Grate, J. W.; Abraham, M. H.; McGill, R. A. In Handbook of Biosensors: Medicine, Food, and the Environment; Kress-Rogers, E., Nicklin, S. Eds.; CRC Press: Boca Raton, FL, in press. (21) Grate, J. W.; Patrash, S. J.; Abraham, M. H. Anal. Chem. 1995, 67, 21622169.
the sorption of vapors by fullerene, graphite, and an assembled fullerene layer on a SAW device with the sorption of vapors by polymeric materials.17 We have determined and published the LSER equations for 14 oligomers and polymers of diverse structures, including strongly hydrogen-bond basic and acidic polymers, as well as a variety of low to moderate polarity polymers.13,21 With these equations in place, and solvation parameters determined for some 2000 compounds,18,22 it is a simple matter to calculate the partition coefficients of hundreds of vapors on each polymer. Using our published LSER equations and solvation parameters, we have calculated the partition coefficients of the five test vapors used in the resorcin[4]arene cavitand study on each of several low polarity polymers.23,24 Figure 1b,c shows the selectivity patterns for two of these polymers, poly(isobutylene) and poly(vinyltetradecanal) (PIB and PVTD, respectively). The resorcin[4]arene data in Figure 1a are for tests against five vapors all at the same concentration in terms of partial pressure (200 Pa);6 therefore, the calculated polymer results are presented as K MW. Clearly these patterns are similar to the pattern observed using the resorcin[4]arene monolayer, with perchloroethylene giving the largest response, followed by toluene and trichloroethylene. The selectivity of the resorcin[4]arene layer for perchloroethylene is not remarkable. For experimental verification, we prepared QCM sensors with thin films of PIB and PVTD and measured the responses of these sensors to the same set of vapors. These results are shown in Figure 1e and 1f. Comparison of Figure 1e, f with 1b,c shows that our predicted patterns are correct, while again emphasizing that the resorcin[4]arene monolayer is not unusual in its sorption properties. The resorcin[4]arene-coated QCM does appear to have slightly greater selectivity for perchloroethylene relative to the other vapors, but the selectivity pattern is not sufficiently different from that of the polymer-coated QCMs to conclude that the resorcin[4]arene monolayer sorbs and selects vapors by a different mechanism. In addition, the polymer layers provide much more sensitive vapor sensors, with signals of 900 Hz for exposure to perchloroethylene at 200 Pa compared to signals of only 60 Hz for the resorcin[4]arene monolayer exposed to perchloroethylene at the same concentration. The LSER calculations and QCM experiments can be regarded as control experiments. Specifically, they test the hypothesis that the three-dimensional structure of the resorcin[4]arene monolayer determines its selectivity. The fact that the “control” amorphous materials show similar selectivity patterns demonstrates conclusively that the three-dimensional structure plays a minor role, if any, in the observed selectivity. LSERs now available for a variety of amorphous polymers13 provide a facile method to calculate controls for such comparisons. Figure 1d shows the pattern for (22) Abraham, M. H.; Andonian-Haftvan, J.; Whiting, G.; Leo, A.; Taft, R. W. J. Chem. Soc., Perkin Trans. 2 1994, 1777-1791. (23) The solvation parameters for vapors in this paper are: vapor, R2, πΣH 2, H 16 ΣRH 2 , Σβ2 , log L ; n-hexane, 0, 0, 0, 0, 2.668; dichloromethane, 0.387, 0.57, 0.1, 0.05, 2.019; chloroform, 0.425, 0.49, 0.15, 0.02, 2.48; carbon tetrachloride, 0.458, 0.38, 0, 0, 2.823; trichloroethylene, 0.52, 0.37, 0.08, 0.03, 2.997; perchloroethylene, 0.639, 0.44, 0, 0, 3.584; acetone, 0.179, 0.7, 0.04, 0.49, 1.696; methanol, 0.278, 0.44, 0.43, 0.47, 0.97; benzene, 0.61, 0.52, 0, 0.14, 2.786; toluene, 0.601, 0.52, 0, 0.14, 3.325; o-xylene, 0.663, 0.56, 0, 0.16, 3.939; p-xylene, 0.613, 0.52, 0, 0.16, 3.839. (24) The LSER coefficients for the polymers in this paper are: polymer, r, s, a, b, l, c; PIB, -0.077, 0.366, 0.180, 0.000, 1.016, -0.766; PVTD, -0.016, 0.736, 2.436, 0.224, 0.919, -0.591; PECH, 0.096, 1.628, 1.450, 0.707, 0.831, -0.749; OV25, 0.177, 1.287, 0.556, 0.440, 0.885, -0.846.
Figure 2. Comparisons of selectivity patterns. The responses (in Hz/mmHg) of an R-cyclodextrin monolayer on a 200 MHz surface acoustic wave device to 11 organic vapors, methanol (CH3OH), n-hexane (C6H14), acetone (C2H6O), chloroform (CHCl3), carbon tetrachloride (CCl4), benzene (C6H6), trichloroethylene (C2HCl3), toluene (C7H8), perchloroethylene (C2Cl4), p-xylene (p-C8H10), and o-xylene (o-C8H10), are compared with the selectivity patterns predicted by LSER calculations for sorption of the same vapors by PECH- and OV25-coated acoustic wave sensors at 298 K. The inset graph shows the correlation of the K(MW) values for the two polymers with the R-cyclodextrin monolayer-coated sensor responses on the x-axis. The linear regression lines shown have correlation coefficients greater than 0.98.
the same vapors sorbed by hexadecane. Since the partition coefficient of a vapor into hexadecane (L16) is a parameter characterizing the vapors, the similarity in overall pattern between panels a and d of Figure 1 shows that the selectivity of the resorcin[4]arene -coated QCM has mainly to do with the vapor properties. When vapor molecules are sorbed by a cavity-containing layer on an acoustic wave device, it is difficult to prove that the observed frequency shift is due specifically to molecules residing in the preformed cavities. Signals will also arise from molecules adsorbed or absorbed at other sites. This fundamental question was recognized in early studies by Kohnke9 and by Dickert.9,10 Polarized infrared external reflectance spectroscopy on the resorcin[4]arene monolayer has shown gauche conformations in the pendant alkyl chains used to tether the layer to the gold surface: these results indicate that the chains are disordered and may be liquidlike.25 Sorption in this region should be expected to occur and should be similar to sorption in hexadecane or the two polymers in Figure 1. Guests residing at sites other than the cavity of a host molecule have been directly observed in the X-ray crystal structures of cavitands with pendant alkyl chains.7,26,27 Guest molecules including acetone, dichloromethane, and fluorobenzene were all found to reside among the alkyl chains. At (25) Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Am. Chem. Soc. 1994, 116, 3597-3598. (26) Dalcanale, E.; Soncini, P.; Bacchilega, G.; Ugozzoli, F. J. Chem. Soc., Chem. Commun. 1989, 500-502. (27) Moran, J. R.; Ericson, J. L.; Dalcanale, E.; Bryant, J. A.; Knobler, C. B.; Cram, D. J. J. Am. Chem. Soc. 1991, 113, 5707-5714.
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Table 1. Calculated Interaction Termsa for Vapor/Polymer Pairs polymer/vapor
polarizability rR2
dipolarity/polarizability sπH 2
hydrogen bonding aRH 2
hydrogen bonding bβH 2
dispersion/cavity l log L16
PIB/perchloroethylene PIB/trichloroethylene PIB/carbon tetrachloride PIB/chloroform PIB/toluene PVTD/perchloroethylene PVTD/trichloroethylene PVTD/carbon tetrachloride PVTD/chloroform PVTD/toluene
-0.05 -0.04 -0.04 -0.03 -0.05 -0.01 -0.01 -0.01 -0.01 -0.01
0.16 0.14 0.14 0.18 0.19 0.32 0.27 0.28 0.36 0.38
0.01 0.03 0.19 0.37 -
0.01 0.03
3.64 3.04 2.87 2.52 3.38 3.29 2.75 2.59 2.28 3.06
a
Dashes indicate terms calculated to be zero.
the test concentration used to generate the pattern in Figure 1a, the observed frequency shift corresponds to over 10 times more perchloroethylene molecules than cavitand molecules on the sensing surface.28 The sensor responses observed in studies where cavity compounds intended as hosts have been applied to the surfaces of acoustic wave devices indicate that such layers sorb many different types of vapors, rather than being specific for particular “guest” molecules.4-12 The promise of compound-specific gas sensors through the use of hosts that will selectively recognize specific guests has not been fulfilled. One of the larger data sets of sensor responses available involves 11 test vapors detected by a SAW device coated with an R-cyclodextrin monolayer.12 These results are plotted in Figure 2 along with the relative sensitivities of two polymers, poly(epichlorohydrin) and a 75% phenyl-25% methyl-substituted polysiloxane (PECH and OV25, respectively), to the same vapors. The polysiloxane is a common gas chromatographic stationary phase. Clearly, the pattern of selectivity observed for the cyclodextrin layer is similar to the patterns for the amorphous polymers, indicating that solubility interactions are determining selectivity patterns rather than size/shape complementarity. The relative importance of vapor properties compared to cavity compound properties can be further emphasized by reexamining data published by Dickert and co-workers.11 These authors reported the responses of a paracyclophane-coated QCM sensor to seven organic vapors and compared these responses to binding energies calculated by molecular modeling methods. They found that the observed sensor responses (in hertz) divided by vapor molecular weight correlated with the calculated binding energies for the vapor in the cavity, and claimed that “This linear correlation proves that host/guest inclusion principles determine the analyte recognition properties of the coatings used”. We find a correlation coefficient R ) 0.976 for their correlation. We also find that partition coefficients (as predicted by LSER equations) for the same vapors in OV25 correlate equally well (R ) 0.976) with the calculated binding energies of the vapors in the paracyclophane cavity. Clearly, a correlation with these calculated binding energies cannot be taken as proof of recognition via host/guest inclusion principles, since OV25 has no preorganized cavities. Rather, the trends in vapor selectivity must depend more on vapor properties than the particular sorbent material. Indeed, we find that these binding energies correlate with the L16 solvation (28) Thoden van Velzen, E. U. Ph.D. Thesis, Unversity of Twente, 1994.
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parameters for the vapors (R ) 0.960). Examining the molecular modeling calculations, the largest term by far is the van der Waals interaction gained when the vapor resides in the cavity.11 Since the vapor is varied while the cavity is common in this series of calculations, it follows logically that trends in these van der Waals interaction terms depend primarily on variations in the properties of the vapors. The particular interactions most important in driving sorption can be examined by calculating the interaction terms in eq 3 for particular vapor/polymer pairs.13,18-20 The results of these calculations for the five cavitand test vapors sorbed by poly(isobutylene) and poly(vinyltetradecanal) are given in Table 1.23,24 The largest term by far is the l log L16 term for all vapor/polymer pairs. This term is a combination of favorable dispersion interactions minus an unfavorable cavity effect. For the particular case of water as a solvent, it is possible to obtain the dispersion interactions and cavity effect separately and to show that dispersion interactions are larger than dipolar or hydrogen-bonding interactions even for solutes such as propanoic acid, benzamide, and pyridine.22 For sorption of organic vapors into phases less polar than water, dispersion interactions will far outweigh all other solubility interactions. We have thus shown that the selectivity pattern of the resorcin[4]arene cavitand layer for the five test vapors follows closely the selectivity patterns of low-polarity amorphous polymers and even liquid hexadecane. The selectivity pattern of a cyclodextrin layer toward 11 test vapors is also very similar to selectivity patterns of amorphous polymers. Calculated binding energies for seven organic vapors in a paracyclophane can be correlated with sorption of those vapors by an amorphous polymer or hexadecane. Our conclusion is that all these patterns of selectivity arise through general dispersion interactions between the vapor and the condensed phase (cavitand or polymer layer) and depend primarily on the vapor’s capacity to gain dispersion interactions on transfer from the gas phase. Therefore, these patterns of selectivity are not indicative of “molecular recognition”. There is a fundamental difficulty in attempting to use host structures to obtain selective gas phase sensors. The main interactions between “guest” molecules and the host structures will be dispersion interactions, and these may take place between the guest and any part of the host structure; they do not depend on residence of the molecule in a preorganized cavity. These conclusions do not preclude the possibility that the use of host structures may perturb the selectivity patterns expected on the
basis of dispersion interactions alone and that this may prove to be useful in sensor development.29 However, claims in the literature for lock-and-key mechanisms and host/guest interactions as a mechanism for organic vapor detection using acoustic wave sensors are exaggerated. For processes that take place in solution, or between one condensed phase and another, dispersion interactions between a solute and a given solvent environment and between the same solute and another solvent environment will mostly cancel out, so that other interactions can predominate. This is why molecular recognition and lock-and-key effects are well-known in solution but have not been established so far for processes such as the sorption of organic vapors by polymers and cavitand layers. (29) One method to assess the usefulness of a structured material is to include it in a sensor array and rigorously evaluate the selectivity of this array compared to a similar sensor array with only amorphous isotropic materials: Patrash, S. J.; Zellers, E. T. Anal. Chim. Acta 1994, 288, 167177.
ACKNOWLEDGMENT We thank Jiri Janata for stimulating discussions on selectivity and molecular recognition and for helpful comments on the manuscript. We are grateful for support from the Office of Technology Development, within the Department of Energy’s Office of Environmental Management, under the Characterization, Monitoring and Sensor Technology Cross-cutting Program, and from the U.S. Army for support under contract DAJA45-93-C-0100. The Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. Received for review May 30, 1995. Accepted December 13, 1995.X AC950518Z X
Abstract published in Advance ACS Abstracts, February 1, 1996.
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