Chiral Sensing Using a Complementary Metal−Oxide Semiconductor

Anal. Chem. 2009, 81, 9353–9364

Chiral Sensing Using a Complementary Metal-Oxide Semiconductor-Integrated Three-Transducer Microsensor System Petra Kurzawski,† Volker Schurig,‡ and Andreas Hierlemann*,† ETH Zu¨rich, Department of Biosystems Science and Engineering, CH-4058 Basel, Switzerland, and Institute of Organic Chemistry, University of Tu¨bingen, D-72076 Tu¨bingen, Germany Different chiral cyclodextrin derivatives were dissolved in a polysiloxane matrix and have been used as sensitive coatings on a three-transducer microsystem including a calorimetric, a mass-sensitive, and a capacitive chemical sensor. Upon exposure to chiral analytes, such as methyl lactate and methyl-2-chloropropionate, all three transducers showed distinct chiral discrimination of these analytes. The signals were found to constitute a convolution of sorption thermodynamics and transducer-specific contributions, which included, in the case of the capacitive sensor, molecular orientation effects so that even opposite-sign signals for the two enantiomers resulted. The sensor response curves of all three transducers could be explained and fitted by applying a model that essentially implies the superposition of a Langmuir isotherm representing specific interactions, predominant at low concentrations, and a Henry isotherm for nonspecific physisorption. The results disclosed here show that, on the one hand, sensor techniques can be used to reveal details of enantioselective analyte-receptor or analyte-matrix interactions and that, on the other hand, sensors may provide an even more pronounced chiral discrimination (“discrimination enhancement”) with respect to sorptionthermodynamics-determined gas chromatography as a consequence of the transducer-specific signal contributions. The biological activity of many compounds depends on their chirality, so that it is of great importance to know which compound enantiomer is present and to precisely determine the respective enantiomeric purity or enantiomeric composition.1-6 Chiral separation techniques, such as electrophoresis and chromatography, are widely used and established methods.7-10 Chiral sensors for * To whom correspondence should be addressed. Fax: +41 61 387 3992. E-mail: [email protected]. † ETH Zu ¨ rich. ‡ University of Tu ¨ bingen. (1) Stinson, S. C. Chem. Eng. News 2000, 78, 55 ff. (2) Stinson, S. C. Chem. Eng. News 2001, 79, 79 ff. (3) Stinson, S. C. Chem. Eng. News 2001, 79, 45 ff. (4) de Camp, W. H. Chirality 1989, 1, 2–6. (5) Izake, E. L. J. Pharm. Sci. 2007, 96, 1659–1676. (6) Mei, X. F.; Wolf, C. J. Am. Chem. Soc. 2006, 128, 13326–13327. (7) Belder, D.; Ludwig, M. Electrophoresis 2003, 24, 2422–2430. (8) Schreier, P.; Bernreuther, A.; Huffer, M. Analysis of Chiral Organic Molecules; Walter de Gruyter & Co.: New York, 1995. (9) Schurig, V. J. Chromatogr., A 2002, 965, 315–356. 10.1021/ac9017007 CCC: $40.75  2009 American Chemical Society Published on Web 10/20/2009

the liquid phase (e.g., potentiometric sensors11,12) have been extensively studied,11,13-15 whereas there are yet less reports on chiral sensors in the gas phase.13,16 Chiral separation and sensor methods in gas and liquid phase predominantly rely on the usage of matrixes that contain enantioselective receptor structures,7-10,13,17-27 such as cyclodextrins9,13,16,19,22-24,26,28-38 or molecularly imprinted polymers.20,21,25,39,40 Signals of sensors functionalized with chiral recogni(10) Subramanian, G., Ed. A Practical Approach to Chiral Separations by Liquid Chromatography; Verlag Chemie, VCH: Weinheim, Germany, 1994. (11) Aboul-Enein, H. Y.; Stefan, R. I. Crit. Rev. Anal. Chem. 1998, 28, 259–266. (12) Yin, X. L.; Ding, J. J.; Zhang, S.; Kong, J. L. Biosens. Bioelectron. 2006, 21, 2184–2187. (13) Shahgaldian, P.; Pieles, U. Sensors 2006, 6, 593–615. (14) Trojanowicz, M.; Wcislo, M. Anal. Lett. 2005, 38, 523–547. (15) Hofstetter, O.; Hofstetter, H.; Wilchek, M.; Schurig, V.; Green, B. S. Nat. Biotechnol. 1999, 17, 371–374. (16) Bodenho ¨fer, K.; Hierlemann, A.; Juza, N.; Schurig, V.; Go¨pel, W. Anal. Chem. 1997, 69, 4017–4031. (17) Ward, T. J.; Hamburg, D. M. Anal. Chem. 2004, 76, 4635–4644. (18) Diamond, D.; Nolan, K. Anal. Chem. 2001, 73, 22A–29A. (19) Szejtli, J. Chem. Rev. 1998, 98, 1743–1753. (20) Gu ¨ bitz, G.; Schmid, M. G. Mol. Biotechnol. 2006, 32, 159–179. (21) Hillberg, A. L.; Brain, K. R.; Allender, C. J. Adv. Drug Delivery Rev. 2005, 57, 1875–1889. (22) Kieser, B.; Fietzek, C.; Schmidt, R.; Belge, G.; Weimar, U.; Schurig, V.; Gauglitz, G. Anal. Chem. 2002, 74, 3005–3012. (23) Ko ¨hler, J. E. H.; Hohla, M.; Richters, M.; Ko ¨nig, W. A. Angew. Chem. 1992, 31, 319–320. (24) Ko ¨nig, W. A.; Krebber, R.; Mischnick, P. J. High Resolut. Chromatogr. 1989, 12, 732–738. (25) Li, W.; Li, S. J. In Oligomers Polymer Composites Molecular Imprinting; Springer: Berlin, Germany, 2007; Vol. 206, pp 191-210. (26) Ko ¨nig, W. A.; Hochmuth, D. H. J. Chromatogr. Sci. 2004, 42, 423–439. (27) Roussel, C.; Del Rio, A.; Pierrot-Sanders, J.; Piras, P.; Vanthuyne, N. J. Chromatogr., A 2004, 1037, 311–328. (28) Szejtli, J. Pure Appl. Chem. 2004, 76, 1825–1845. (29) Ozoemena, K. I.; Stefan, R. I. Talanta 2005, 66, 501–504. (30) Ko ¨nig, W. A. Gas Chromatographic Enantiomer Separation with Modified Cyclodextrins; Hu ¨ thig Buch-Verlag: Heidelberg, Germany, 1992. (31) Easton, C. J.; Lincoln, S. F. Chem. Soc. Rev. 1996, 25, 163. (32) May, L. P.; Byfield, M. P.; Lindstrom, M.; Wu ¨ nsche, L. F. Chirality 1997, 9, 225–232. (33) Bodenho ¨fer, K.; Hierlemann, A.; Go ¨pel, W.; Juza, M.; Gross, B.; Schurig, V. Chimica Oggi 1998, 16, 56–58. (34) Ko ¨nig, W. A. Chirality 1998, 10, 499–504. (35) Fietzek, C.; Hermle, T.; Rosenstiel, W.; Schurig, V. Fresenius J. Anal. Chem. 2001, 371, 58–63. (36) Juvancz, Z.; Szejtli, J. TrAC, Trends Anal. Chem. 2002, 21, 379–388. (37) Schmitt, U.; Branch, S. K.; Holzgrabe, U. J. Sep. Sci. 2002, 25, 959–974. (38) Shahgaldian, P.; Hegner, M.; Pieles, U. J. Inclusion Phenom. Macrocyclic Chem. 2005, 53, 35–39. (39) Mahony, J. O.; Nolan, K.; Smyth, M. R.; Mizaikoff, B. Anal. Chim. Acta 2005, 534, 31–39. (40) Liu, F.; Liu, X.; Ng, S. C.; Chan, H. S. O. Sens. Actuators, B 2006, 113, 234–240.

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tion structures vary in their response magnitude11-14,16 or, as recently discovered, in their response sign to the different enantiomers.41,42 The chiral recognition is, in most cases, based on the formation of a more stable diastereomeric complex with one of the enantiomers so that a large Gibbs energy difference between the diastereomeric complexes of the two enantiomers of a compound entails a high enantioselectivity. In this article we report on using a multitransducer microsystem for enantiomeric recognition. The multitransducer system has been described in detail earlier43,44 so that we only include little information here: It comprises three polymer-coated transducers, a capacitive, a mass-sensitive, and a calorimetric transducer along with all necessary front-end and signal conditioning circuitry on a 7 by 7 mm2 complementary metal-oxide semiconductor (CMOS) chip.43,45-47 The calorimetric sensor is a thermoelectric sensor based on the Seebeck effect. The thermopile (132 thermocouples in series on a thermally insulated, square membrane) only detect concentration transients, i.e., enthalpy changes produced by the absorption (heat of condensation) or desorption (heat of vaporization) of analyte molecules in a polymer film.48,49 A differential configuration with one thermopile coated with the gas-sensitive polymer and the other being uncoated and acting as a reference has been used.50 The integral of the thermovoltage over time includes the relevant information, which is proportional to the analyte concentration in the gas phase, cA. The sensitivity, S, of the calorimetric sensor is given by44

S)

∆

∫U

th

∆cA

dt

) GCalhKcHsorption

(1)

where GCal includes the geometric properties of the membrane, h denotes the thickness of the polymeric layer, Kc is the partition coefficient describing the analyte enrichment in the sensitive layer, and Hsorption is the heat of condensation or vaporization. The mass-sensitive resonant chemical sensor is based on a thermally actuated 150 µm long and 140 µm wide cantilever featuring a fundamental mechanical frequency of approximately 380 kHz with a quality factor of approximately 1000 in air.43,47 Details on the operation principle can be found in refs 43 and 47. The sensitivity, S, of a polymer-coated cantilever is given by ref 44. (41) Kurzawski, P.; Bogdanski, A.; Schurig, V.; Wimmer, R.; Hierlemann, A. Angew. Chem., Int. Ed. 2008, 47, 913–916. (42) Kurzawski, P.; Bogdanski, A.; Schurig, V.; Wimmer, R.; Hierlemann, A. Anal. Chem. 2009, 81, 1969–1975. (43) Hagleitner, C.; Hierlemann, A.; Lange, D.; Kummer, A.; Kerness, N.; Brand, O.; Baltes, H. Nature 2001, 414, 293–296. (44) Kurzawski, P.; Hagleitner, C.; Hierlemann, A. Anal. Chem. 2006, 78, 6910– 6920. (45) Van Herwaarden, A. W.; Sarro, P. M.; Gardner, J. W.; Bataillard, P. Sens. Actuators, A 1994, A43, 24–30. (46) Hierlemann, A.; Lange, D.; Hagleitner, C.; Kerness, N.; Koll, A.; Brand, O.; Baltes, H. Sens. Actuators, B 2000, 70, 2–11. (47) Hagleitner, C.; Lange, D.; Hierlemann, A.; Brand, O.; Baltes, H. IEEE J. Solid-State Circuits 2002, 37, 1867–1878. (48) Lerchner, J.; Seidel, J.; Wolf, G.; Weber, E. Sens. Actuators, B 1996, 32, 71–75. (49) Bataillard, P.; Steffgen, E.; Haemmerli, S.; Manz, A.; Widmer, H. M. Biosens. Bioelectron. 1993, 8, 89–98. (50) Koll, A.; Kummer, A.; Brand, O.; Baltes, H. Proc. SPIE 1999, 3673, 308– 317.

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S)

∆f0 ) GCanthKcMA ∆cA

(2)

Here, f0 denotes the mechanical resonance frequency of the cantilever, and cA is the analyte concentration in the gas phase. Equation 2 includes a summary term for the mechanical properties of the cantilever, GCant, see also refs 43 and 47. The sensitivity is proportional to the polymer layer thickness, h, and the partition coefficient, Kc, as well as to the molecular mass of the absorbed analyte, MA.51 Swelling effects and analyte-induced changes in the elastic modulus of the polymer have been neglected in a first approximation.52 The capacitive transducer features two sets of interdigitated electrodes that correspond to two capacitor plates.53 The capacitor includes 128 finger pairs, electrode width and spacing are 1.6 µm, and the measurement frequency is 600 kHz. The sensor monitors changes in the dielectric coefficient of the polymer coating upon analyte absorption. The capacitance changes upon analyte absorption are in the attoFarad range and cannot be measured conventionally so that a differential measurement scheme has been applied that produces a frequency signal output (Hz), which is proportional to the occurring capacitance changes.47,53-56 The capacitive sensor response depends on the layer thickness (for a detailed discussion of layer-thickness influence, see ref 53) and is determined by the ratio of the dielectric constants of analyte and polymer and polymer swelling upon analyte uptake.57,58 For thick polymer layers (>2 µm), the sensitivity, S, is the change in capacitance, ∆C, in dependence on the change in the analyte concentration, ∆cA, as given by44

S)

∆C ) GCapKc∆ε ∆cA

(3)

where GCap includes the capacitor geometry. The capacitor sensitivity using thick layers is, in contrast to the other transducers (see eqs 1 and 2), not dependent on the layer thickness, since the electric field has a defined extension for a given electrode spacing, i.e., the probed volume is constant. In case of a thin layer, swelling plays a crucial role and has also to be taken into account, as will be seen later in this article. The partition coefficient, Kc, includes the polymer/analyte interactions, and ∆ε is the change in the dielectric properties of the polymeric matrix upon analyte absorption. More details on (51) Lange, D.; Brand, O.; Baltes, H. CMOS Cantilever Sensor Systems: Atomic Force Microscopy and Gas Sensing Applications; Springer: Berlin, Germany, 2002. (52) Lange, D.; Hagleitner, C.; Hierlemann, A.; Brand, O.; Baltes, H. Anal. Chem. 2002, 74, 3084–3095. (53) Kummer, A. M.; Hierlemann, A.; Baltes, H. Anal. Chem. 2004, 76, 2470– 2477. (54) Hagleitner, C.; Hierlemann, A.; Baltes, H. In Sensors Update; Baltes, H., Fedder, G. K., Korvink, J. G., Eds.; Wiley VCH: Weinheim, Germany, 2003; Vol. 12, pp 51-120. (55) Kummer, A.; Hierlemann, A. IEEE Sens. J. 2006, 6, 3–10. (56) Hagleitner, C.; Hierlemann, A.; Brand, O.; Baltes, H. In Sensors Update; Baltes, H., Fedder, G. K., Korvink, J. G., Eds.; Wiley VCH: Weinheim, Germany, 2002; Vol. 11, pp 101-155. (57) Steiner, F. P.; Hierlemann, A.; Cornila, C.; Noetzel, G.; Ba¨chtold, M.; Korvink, J. G.; Go ¨pel, W.; Baltes, H. Technical Digest of Transducers; Stockholm, Sweden, 1995, pp 814-817. (58) Kummer, A. Ph.D. Thesis, ETH Zurich, Zurich, Switzerland, 2004.

Figure 1. Sensor responses, ∆ysum(c), versus analyte concentration according to the model. Superposition of a Langmuir isotherm, ∆ychiral(c), and a dispersion or Henry isotherm, ∆yachiral(c).16

capacitive sensing and eq 3 can be found in refs 43, 53, 55, and 59. It has recently been reported that capacitive sensors provide opposite-sign signals upon dosage of the two enantiomers of methyl lactate (methyl-2-hydroxypropionate) and methyl-2-chloropropionate.41,42 The sensitive layers consisted of chiral R-, β-, and γ-cyclodextrin-derivatives, dissolved (50% weight fraction) in poly(dimethylsiloxane) matrices. A second sensor chip coated with pure, achiral poly(dimethysiloxane), PDMS, was always used as a reference to monitor the dosed concentrations. For details see the Experimental Section. A sorption-thermodynamic model developed by Bodenho¨fer et al.16 can be used to describe this composite sensitive-layer system. It takes into account three different types of absorption sites and interactions: (i) preferential or enantioselective sorption, (ii) nonspecific sorption at the receptor molecule (cyclodextrin), and (iii) nonspecific sorption within the polymer matrix. Absorption takes place simultaneously at all three different sites (two at the receptor, one in the polymer) so that a superposition of the individual sorption mechanisms results. The sensor response, ∆y, has then been conceptually divided into two principally independent contributions at the different types of absorption sites: ∆ysum ) ∆yspec + ∆ynonspec ) ∆ychiral + ∆ynonchiral

(4)

Here, ∆yspec represents the sensor response due to a specific or enantioselective (∆ychiral) interaction between analyte molecules and the cage-type recognition sites and ∆ynonspec or ∆ynonchiral include the interactions between analyte molecule and nonselective sites at the cyclodextrin receptor and between analyte molecule and polymer matrix. The model does not imply that the two nonspecific interactions are identical. The number of preferential or enantioselective sorption sites at the cyclodextrins is limited (essentially one per cyclodextrin cage), and the site coverage depends on the analyte concentration, c. A Langmuirian isotherm (Figure 1) was consequently used to describe the enantioselective sensor response contribution: ∆ychiral ) Kchiralθ(c) ) Kchiral

K′c 1 + K′c

(5)

(59) Kummer, A. M.; Burg, T. P.; Hierlemann, A. Anal. Chem. 2006, 78, 279– 290.

where K′ ) kad/kde and denotes the ratio of the kinetic constants of the adsorption and desorption process of the chiral molecule at/from the recognition site. K′ determines the initial curvature of the sensor response. Kchiral corresponds to a sensor signal at complete coverage, θ ) 1, of the enantioselective sites. The dimension of K′ here is ppm-1 and is dependent on the analyte concentration unit that has been used. Kchiral has the dimension of the sensor read-out signal, such as Hz for the cantilever and capacitor (frequency conversion of capacitance values) and mV × s for the calorimeter. (It will and has to be discussed in the Results and Discussion to which extent such thermodynamic models can be applied to the responses of the different transducers.) In contrast to the chiral recognition sites, the number of achiral absorption sites is unlimited. Therefore, a Henry-type sorption isotherm has been used (Figure 1). ∆yachiral(c) ) Kachiralc

(6)

Here, Kachiral corresponds to the Henry constant and has been used to describe the linear, nonspecific part of the sensor response, which dominates at higher analyte concentrations.60 The dimension of this constant also depends on the sensor type: Hz/ppm for the mass-sensitive and capacitive device and mV × s/ppm in the case of the calorimeter. Equation 6 includes all nonspecific or nonenantioselective contributions (polymer matrix and nonspecific sites at the cyclodextrin) to the overall sorption. Since both interaction processes happen at the same time and independently from each other, the overall sensor response can be described by superimposing the two different types of isotherms (Figure 1):16 ∆ysum(c) ) ∆ychiral(c) + ∆yachiral(c) ) Kchiral

K′c + Kachiralc 1 + K′c (7)

The sensor response contributions and the superposition of Langmuir and Henry-type sorption are illustrated in Figure 1. The curvature of the superimposed response resulting from the Langmuirian contribution is more or less pronounced, depending on the strength of the preferential or enantioselective interaction of one of the enantiomers with the cyclodextrin recognition site. By means of the fits it is possible to gain access to a maximum achievable chiral discrimination factor, R, through a procedure described in detail in ref 16 with KR′ and KS′ denoting the K′ constant of the (R)- and (S)-enantiomers:

Rmax ) lim R(c) ) cf0

KchiralKS′ + Kachiral KchiralKR′ + Kachiral

(8)

Chiral discrimination factors (ratio of the retention factors of two enantiomers) are routinely used in gas chromatography. For sensors, they depend on the relative concentration ratio of recognition sites (cyclodextrin molecules) in the sensitive layer and analyte molecules in the gas phase. For high analyte concentrations, there is no chiral discrimination (R f 1).16 For (60) Bodenho ¨fer, K.; Hierlemann, A.; Noetzel, G.; Weimar, U.; Go ¨pel, W. Anal. Chem. 1996, 68, 2210–2218.

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more details on the model described here, please consult ref 16. In the following, this model will be applied to the signals of the three transducers in the Results and Discussion, and the applicability of the model to the respective sensor signals and transducers will be discussed. EXPERIMENTAL SECTION Sensitive Layers and Deposition. After completion of the CMOS process and after packaging, the polymer films acting as chemically sensitive layers have been deposited onto the sensor structures by means of airbrush spray-coating (Badger, model 200F, Franklin Park, IL). The airbrush has been fixed at a distance of 10 cm from the chip. A specially designed silicon shadow mask that could be precisely aligned on the packaged chip has been used to shield the circuitry as well as the calorimetric and the capacitive reference sensors. Only the sensing elements of the system were exposed to the spray coating. For determining the influence of the receptor ring size on the selectivity, the measurements were performed with three different cyclodextrins (R-, β-, and γ-CD): hexakis(3-O-butanoyl-2,6-di-O-npentyl)-R-cyclodextrin, heptakis(3-O-butanoyl-2,6-di-O-n-pentyl)-βcyclodextrin, and octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-γ-cyclodextrin.24 The glucose rings of these derivatives exhibit a slightly polar, lipophilic behavior, whereas the side chains have nonpolar properties.61,62 The cyclodextrin derivatives show chiral selectivity for a variety of enantiomers.16,61 The γ-cyclodextrin derivative is commercially available as Lipodex E (Macherey-Nagel, Du¨ren, Germany).24 More information on the modification of cyclodextrins and the influence on the selectivity of the macrocycles toward certain chiral molecules can be found in the literature.28,63,64 The cyclodextrins have been mixed with poly(dimethylsiloxane), PDMS (Silicone GE SE-30, Supelco, Bellefonte, PA), at 50% (w/w) using dichloromethane as a solvent and were then deposited on the transducers. Reference chips to monitor the dosed concentrations have been coated with pure, achiral PDMS (Silicone GE SE-30, Supelco, Bellefonte, PA) and with poly(etherurethane), PEUT (Thermedics, Woburn, MA). After the layer deposition, the polymer coatings were cured in a saturated dichloromethane atmosphere for 2 min so that smooth layers on the sensing elements were obtained. For most experiments, thick polymer layers of approximately 4 µm thickness have been used unless otherwise stated. Gas Manifold. For gas tests, the CMOS chips were mounted on dual-in-line (DIL) packages and then loaded into the measurement chamber of a computer-controlled gas manifold. Measuring physisorption-induced calorimetric transients requires a careful design of the gas manifold so that the signal dynamics reflect the analyte sorption and heat exchange characteristics rather than the gas flow dynamics of the setup. All gas switching processes must be fast in comparison to the analyte sorption dynamics. To this end, a manifold and flow setup was used, the most important features of which include a crossover flow architecture by use of (61) Mele, A.; Raffaini, G.; Ganazzoli, F.; Juza, M.; Schurig, V. Carbohydr. Res. 2003, 338, 625–635. (62) Krebber, R. Ph.D. Thesis, University of Hamburg, Hamburg, Germany, 1991. (63) Khan, A. R.; Forgo, P.; Stine, K. J.; D’Souza, V. T. Chem. Rev. 1998, 98, 1977–1996. (64) Junge, M.; Ko ¨nig, W. A. J. Sep. Sci. 2003, 26, 1607–1614.

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a fast four-way valve, matched flow resistances of the two output gas lines of the four-way valve, and a small tubing volume between the valve and the sensor measurement chamber. This setup, details of which have been described in ref 59, can provide sharp analyte concentration steps. The analyte vapors were generated from specifically developed temperature-controlled (T ) 223-293 K) vaporizers65 using synthetic air as a carrier gas and then diluted as desired using computer-driven mass-flow controllers. The vapor-phase concentrations at the respective temperatures were calculated following the Antoine equation.66 The coefficients for the Antoine equation and the saturation vapor pressures of the chiral analytes were calculated from data published by Bodenho¨fer et al.16 To cover a wide range of analyte concentrations, several measurement series, using different mass-flow controllers, have been performed. The measurements have then been arranged with regard to the analyte concentrations and the overlap helped to set up sensor response plots versus analyte concentrations over a wide concentration range. A photoacoustic detector (infrared light for excitation, 1314 Photoacoustic Multigas Monitor, Innova Airtec Systems, Denmark) has been used as an independent reference to assess the actual analyte gas-phase concentrations. Sensor microsystems featuring enantioselective and achiral coatings have been simultaneously measured in a temperatureregulated flow-through chamber (303 K) at sampling frequencies of 1.5 Hz for cantilever and capacitor and 24 Hz for the calorimetric sensor.59 Both gas streams (pure carrier gas and carrier gas with analyte) were thermostabilized to the measurement chamber temperature before entering the chamber itself. Typical experiments consisted of alternating exposures to pure air and analyteloaded air (20 min exposure intervals to reach steady state or thermodynamic sorption equilibrium states). Capacitive and masssensitive signals have been recorded continuously at rather low temporal resolution, since average values for the baseline (zero analyte) and the equilibrium state (defined analyte concentration) are needed for the evaluation. The calorimetric sensor relies on transients and provides signals exclusively upon concentration changes. Therefore, the calorimetric recording has to be performed at higher temporal resolution (20 Hz) in two short intervals covering both flanks of the concentration signal, i.e., at the maximum gradient of the analyte concentration. Analytes. The selected analytes included, on the one hand, standard organic solvents that were used as purchased from Fluka, Buchs, Switzerland, without further purification (n-octane, propan1-ol, etc.). On the other hand, it included chiral analytes, i.e., both enantiomers of methyl lactate (enantiomeric purity, 98%, Sigma Aldrich AG, Steinheim, Germany), both enantiomers of methyl 2-chloropropionate (enantiomeric purity >99%, Sigma Aldrich AG, Steinheim, Germany), as well as racemic mixtures (Sigma Aldrich AG, Steinheim, Germany) that have also been dosed to the sensors without further purification. RESULTS AND DISCUSSION Chiral Analytes and Achiral Matrix. Two different achiral polymers, the nonpolar PDMS and the slightly polar PEUT, were (65) Bodenho ¨fer, K.; Hierlemann, A.; Schlunk, R.; Go ¨pel, W. Sens. Actuators, B 1997, 45, 259–264. (66) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. Organic Solvents, 4th ed.; Wiley Interscience: New York, 1986.

Table 1. Characteristic Analyte Data and Measured Sensitivities (According to Equations 1-3) and Standard Deviations of Volatile Organic Compounds and Chiral Analytes for the Achiral Polymers PDMS and PEUT calorimeter

cantilever

capacitor

sensitivity [10-1 mV × s/ppm]

sensitivity [10-2 Hz/ppm]

sensitivity [10-1 Hz/ppm]

analyte

saturation vapor pressure at 303 K [kPa]

dielectric constant

PDMS

PEUT

PDMS

PEUT

PDMS

PEUT

n-octane propan-1-ol methyl lactate methyl-2-chloropropionate

2.46 3.77 0.83 1.55

1.9 21.0 31.0 15.0

4.1 ± 0.1 0.4 ± 0.1 3.0 ± 0.3 3.1 ± 0.6

2.1 ± 0.4 3.4 ± 0.4 7.0 ± 0.6 6.6 ± 0.3

5.4 ± 0.8 0.4 ± 0.1 1.6 ± 0.5 4.4 ± 0.1

2.1 ± 0.1 1.8 ± 0.1 7.0 ± 0.2 7.1 ± 0.1

-4.4 ± 0.2 1.7 ± 0.1 10.9 ± 1.6 13.8 ± 0.7

-5.0 ± 0.7 18.6 ± 1.4 65.4 ± 5.4 36.6 ± 0.4

Table 2. Sensitivities (According to Equations 1-3) of Different Enantioselective Coatings to the Volatile Organic Compounds n-Octane and Propan-1-ol calorimeter -1

sensitivity [10

n-octane propan-1-ol

cantilever

mV s/ppm]

-2

sensitivity [10

capacitor

Hz/ppm]

sensitivity [10-1 Hz/ppm]

R-CD/ PDMS

β-CD/ PDMS

γ-CD/ PDMS

R-CD/ PDMS

β-CD/ PDMS

γ-CD/ PDMS

R-CD/ PDMS

β-CD/ PDMS

γ-CD/ PDMS

2.1 ± 0.3 0.6 ± 0.1

2.6 ± 0.4 1.1 ± 0.4

5.7 ± 0.7 4.8 ± 0.4

3.2 ± 0.6 0.3 ± 0.1

3.5 ± 0.9 1.3 ± 0.5

5.8 ± 1.7 2.6 ± 0.4

8.7 ± 3.2 15.6 ± 2.0

12.1 ± 2.1 19.6 ± 0.8

5.9 ± 1.4 17.7 ± 6.8

used as achiral sorption matrixes for comparison with the results of the enantioselective coatings. The two enantiomers of methyl lactate and methyl-2-chloropropionate and the standard organic volatiles, n-octane and propan-1-ol, have been tested. Linear sorption behavior, which is typical for a nonspecific physisorption process, was found for all analytes, so that only sensitivity values according to the simple linear eqs 1-3 are given here. As expected, the sensors showed no discrimination of the enantiomers of the chiral molecules. The measured sensitivities of all three transducers and the respective standard deviations for the different polymer/analyte combinations are listed in Table 1. In an achiral environment, the enantiomers behave like any other analyte undergoing a physisorption process: Polar molecules will always be preferentially absorbed in a polar polymer and nonpolar polymers feature an increased partitioning of nonpolar analyte molecules. It has to be kept in mind that the partitioning is inversely proportional to the saturation vapor pressure, which is 2.46 kPa for n-octane, 3.77 kPa for propan-1-ol, 1.55 kPa for methyl-2-chloropropionate, and 0.83 kPa for methyl lactate at 303 K (see also Table 1). In the case of the capacitive sensor, the dielectric constants of the analytes directly influence the sensor signal, which amount to 1.9 for n-octane, 21 for propan-1-ol, 15 for methyl-2-chloropropionate, and 31 for methyl lactate at 303 K (Table 1). Achiral Analytes and Chiral Matrix. The two standard solvents, n-octane and propan-1-ol, served as control analytes and were tested with the different enantioselectively coated multisensor chips. The enantioselective coatings consisted of a composite of either the R-, β-, or γ-cyclodextrin derivative and of PDMS at a weight ratio of 50% (w/w). All transducers showed, as expected, a linear dependence of the sensor signal on the analyte concentration, which is indicative of, again, a nonselective Henry-type interaction between analytes and the sensitive layer. It was found that the presence of the enantioselective receptors in the polymer increases the overall partitioning of the volatile organic analytes into the composite

sensitive layer. One possible reason for this phenomenon may be attributed to the presence of the bulky supramolecular cyclodextrin molecules, which may increase the free volume within the polymer matrix and which may serve as additional nonspecific sorption sites.16 For the calorimetric and the masssensitive sensor, this assumption is substantiated by an increasing sensitivity toward volatile organic compounds with increasing size of the cyclodextrin molecule (Table 2). The measurements with the capacitive sensor support the assumption that both the increase of the free volume and the presence of the enantioselective receptors that serve as additional nonspecific sites are responsible for the sensitivity increase. For thick layers of the pure rubbery polymers PDMS and PEUT (Table 1), it was shown in refs 53 and 55 that the absorption of analytes featuring a lower dielectric constant (e.g., n-octane) than that of the sensitive polymer leads to a capacitance decrease, whereas analytes with a higher dielectric constant than that of the polymer produce a capacitance increase. Polymers with an open structure and a large free volume, such as ethyl cellulose and poly(epichlorohydrin), however, show a positive signal regardless of the analyte dielectric constant.53,55 For thick layers of the enantioselective coatings, there is also no negative signal. The absorption of n-octane (ε ) 1.95) causes an increase of the layer capacitance, even though the dielectric constant of the enantioselective coating (ε ) 3.1) is higher than that of the analyte. The sensitivities in Table 2 reflect the effects of the cyclodextrins acting as additional nonspecific sites. The capacitive sensor shows the strongest response upon absorption of organic compounds in the β-cyclodextrin derivative/PDMS coating. More details on β-cyclodextrin sorption will be given in the next section. Chiral Analytes and Chiral Matrices. Thick Layers of γ-Cyclodextrins. The relatively large γ-cyclodextrins and their derivatives are the most popular macrocycles for chiral separation.17,20,67 Octakis(3-O-butanoyl-2,6-di-O-pentyl)-γ-cyclodextrin enables the (67) Altria, K. D.; Elder, D. J. Chromatogr., A 2004, 1023, 1–14.

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Figure 2. Measurement results of the calorimetric sensor: Thermovoltage integral versus analyte concentration for (S)- and (R)-methyl lactate (a) and the enantiomers of methyl-2-chloropropionate (b). The respective fits according to the model are displayed as dashed and solid lines.

enantioseparation of a broad range of chemical substances,68 such as the enantiomers of methyl lactate and methyl-2-chloropropionate. The thickness of the enantioselective layer was approximately 4 µm for the experiments here. All measurement curves showed pronounced nonlinearity (see Figures 2-4) so that the absorption model developed by Bodenho¨fer et al.16 described in the introductory material was used to evaluate the results of the multitransducer gas sensor system. The calorimetric sensor detects the release or the absorption of heat induced by the enthalpy changes during the absorption or desorption of analyte molecules in the sensitive layer (see eq 1). Parts a and b of Figure 2 depict the thermovoltage integrals in dependence of the analyte concentrations for the enantiomers of methyl lactate and methyl-2-chloropropionate. The sensor-signalversus-analyte-concentration curves show a strongly curved lowconcentration part, with the curvature being stronger for the more intensely interacting enantiomer, followed by an almost linear high-concentration part.16,41 Linear behavior, observed at higher concentrations, is, as mentioned before, characteristic for nonspecific physisorption processes (Henry-type sorption16), whereas the low-concentration curve of the preferentially sorbed enantiomers or nonpreferentially sorbed enantiomers can be best described as Langmuirian-type as a consequence of the limited number of preferential sorption sites inside the cyclodextrin (68) Ko ¨nig, W. A. J. High Resolut. Chromatogr. 1993, 16, 569–586.

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Figure 3. Measurement results of the mass-sensitive cantilever: Frequency shift versus analyte concentration for (S)- and (R)-methyl lactate (a) and the enantiomers of methyl-2-chloropropionate (b). The respective fits are displayed as dashed and solid lines. The dashed/ dotted line through the origin marks the slope of the corresponding linear, nonspecific sorption contribution.

cages.16 The absorption and desorption characteristics of the calorimetric signals of the chiral analytes suggest that not only the thermodynamic partitioning but also the different enantiomer/ receptor-complexation heats significantly contribute to the sensor signal. The sensors show larger signals for the preferentially incorporated (R)-methyl lactate or (S)-methyl-2-chloropropionate which is in accordance with the literature.16,69,70 The larger signals of the preferred enantiomer are probably due to two effects: (i) a larger enrichment in the composite matrix and (ii) the release of more heat upon formation of the stronger diastereomeric complex. In comparison to the physisorption of standard organic compounds (see preceding section), both enantiomers generally generate larger and, evidently, nonlinear signals, and the respective thermal transient durations are longer. Measurements with the mass-sensitive cantilever, i.e., the experimental data and the respective curve fits, are depicted in Figure 3. The negative frequency shift of the resonating cantilever is a consequence of the increase of the oscillating mass. Since the molecular weight of both methyl lactate enantiomers is identical, the higher signal for the preferentially absorbed enan(69) Hierlemann, A.; Ricco, A. J.; Bodenho ¨fer, K.; Go ¨pel, W. Anal. Chem. 1999, 71, 3022–3035. (70) Bodenho ¨fer, K.; Hierlemann, A.; Seemann, J.; Gauglitz, G.; Christian, B.; Koppenhoefer, B.; Go ¨pel, W. Anal. Chem. 1997, 69, 3058–3068.

tiomer can only result from a larger partitioning (see eq 2). Again, a preferential sorption of the (R)-methyl lactate and the (S)-methyl2-chloropropionate is evident. From Figure 3 it is also obvious that unspecific absorption becomes predominant at higher concentrations so that the chiral discrimination factor, R, equals unity (R f 1) at very high concentrations. This means that the (S)and (R)-analyte signal adopt the same linear sorption isotherm for very high concentrations. A tendency to convergence of the chloropropionate response curves can be seen in Figure 3b, but the applied concentrations, in accordance with the literature,16 are still too low to see the isotherm coincidence. The third capacitive transducer detects changes in the dielectric constant of the composite sensitive layer upon analyte absorption. Though the measurement principle seems to be rather simple, the resulting experimental data look different than expected and are the most difficult to interpret, as we will see below. The sensor responses upon dosage of the two enantiomers of a specific analyte are vastly different, although the dielectric constant of both enantiomers is identical. The dielectric constants of the chiral analytes are comparably high: Methyl-2-chloropropionate has a value of ε ) 15 and methyl lactate features an ε of 31. For the pure cyclodextrins, the dielectric coefficient values amount to approximately 4. The β-cyclodextrin with an uneven number of seven glucose units in the ring has the highest dielectric constant of ε ) 4.15, followed by R-cyclodextrin (ε ) 4.12) and γ-cyclodextrin (ε ) 4.07). Because of the lower dielectric constant of PDMS (ε ) 2.8), the mixing of the respective cyclodextrins with polysiloxane at 50% (w/w) reduces the composite dielectric constant to ε ) 3.1-3.2. Figure 4 displays the capacitor responses to the enantiomers of methyl lactate and methyl-2-chloropropionate. Whereas the absorption of (S)-methyl lactate always generates a capacitance increase, the preferentially sorbed enantiomer (R)-methyl lactate causes a signal decrease up to a concentration of about 80-100 ppm. Then, the sensor signal returns to positive frequency shifts. Similar behavior is observed for methyl-2-chloropropionate with the (S)-enantiomer showing initially negative signals. As reported on previously,41,42 the two enantiomers of methyl lactate or methyl-2-chloropropionate evoke opposite signal signs at low concentrations for capacitive chemical sensors with negative signals for the preferentially sorbed enantiomers (R)-methyl lactate and (S)-methyl-2-chloropropionate. The negative signals upon dosage of the (R)-enantiomer of methyl lactate or the (S)enantiomer of methyl-2-chloropropionate occur despite the fact that methyl lactate and methyl-2-chloropropionate exhibit dielectric coefficients of 31 and 15, which are considerably higher than that of the cyclodextrin/PDMS mixture (ε ) 3.1). This behavior has been explained with molecular orientation effects as a consequence of the stronger interaction of the methyl lactate (R)enantiomer and the chloropropionate (S)-enantiomer with the modified cyclodextrin.41 These orientation effects have been assumed to produce a local compensation of partial charges in the receptor/enantiomer complex, which entails a capacitance decrease. The strange shape of the sensor-signal-versus-analyte-concentration curves of the preferentially sorbed enantiomers ((S)-methyl2-chloropropionate, (R)-methyl lactate) has been explained by the fact that the number of preferentially sorbed analyte molecules

Figure 4. Experimental results and model fits of the capacitive sensor responses with dependence of the analyte concentration for (S)- and (R)-methyl lactate (a) and the enantiomers of methyl-2chloropropionate (b). The model fits are displayed as dashed and solid lines.41

supersedes that of the cyclodextrin molecules upon increasing analyte concentration (1:1 complexation at approximately 100 ppm, see Figure 4).41,42 The signal contribution of the orientation effect (dielectric coefficient decrease) is, therefore, more and more counterbalanced by the dielectric coefficient increase originating from analyte molecules, for which no more cyclodextrin cavities are available so that they nonspecifically absorb somewhere in the cyclodextrin/polymer matrix.41 Finally, at large concentrations, a capacitance increase is observed. The signal heights, however, are lower than that produced by the same concentration of the less intensely interacting enantiomer, since a fraction of the preferentially sorbed analyte molecules is still trapped within the cyclodextrin cavities so that a certain number of nonspecifically absorbed molecules is needed to counterbalance the capacitancelowering effect of the trapped ones. The less intensely interacting enantiomers do not enter into close enough contact with the cyclodextrin torus to produce significant orientation effects (for details, see ref 41). It is very much evident that the capacitive sensor signals reported on here represent a convolution of (i) the analyte sorption thermodynamics and (ii) transducer-specific analyte-induced effects (molecular orientation effects) on the dielectric properties of the overall layer. Since the sensor responses obtained with all three transducers feature “similar” shapes, i.e., a nonlinear low-concentration region and a prevailingly linear high-concentration region, it was attempted to apply the model of Bodenho¨fer et al.,16 detailed in the Analytical Chemistry, Vol. 81, No. 22, November 15, 2009

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Table 3. Characteristic Parameters of the Fits for the Enantiomers of Methyl Lactate and Methyl-2-Chloropropionate: K′, Kchiral, Kachiral, and Average Relative Errora calorimeter K′ [ppm-1] Kchiral [mV × s] Kachiral [mV × s/ppm] average relative error [%]

(S)-methyl lactate

(R)-methyl lactate

(S)-methyl-2-chloropropionate

(R)-methyl-2-chloropropionate

6.4 × 10-3 2060

8.8 × 10-3 2260

10.1 × 10-3 2790

3.2 × 10-3 2800

0.5

0.6

0.58

0.53

0.5

0.4

mass-sensitive cantilever (S)-methyl lactate -1

K′ [ppm ] Kchiral [Hz] Kachiral [Hz/ppm] average relative error [%]

(R)-methyl lactate

-3

6.6 × 10 -2.8

-19 -4.2 × 10-2

2.2 × 10

-2

-2.5

(S)-methyl-2-chloropropionate 1.0 × 10

(R)-methyl-2-chloropropionate

-2

-1.7

-34 -2.9 × 10-2

1.9 × 10-3 -1.9

capacitor K′ [ppm-1] Kchiral [Hz] Kachiral [Hz/ppm] average relative error [%]

(S)-methyl lactate

(R)-methyl lactate

(S)-methyl-2-chloropropionate

(R)-methyl-2-chloropropionate

4.0 × 10-3 550

2.2 × 10-2 -1130

2.0 × 10-2 -2370

7.0 × 10-4 2880

-2.1

-1.5

5.9 3.4

5.4 0.7

a

The values have been determined with the different sensor types coated with octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-γ-cyclodextrin/ PDMS (50% (w/w)).

introductory material (eqs 4-8 and Figure 1), to the responses of all three transducers. The results are given in Table 3. The evaluation of the data and the extraction of the fitting parameters were done with a MatLab program. The parameters K′, Kchiral, and Kachiral were extracted from the measured signals using least-mean-square fits: (a) a combined fit to the curves of both enantiomers or (b) separate fits to the two curves invoked by the two different enantiomers under the prerequisite that the behavior of the enantiomers of a certain analyte in a nonchiral environment, i.e., Kachiral is identical. In the case of the calorimetric and capacitive sensors, Kchiral and Kachiral include also sorption heat terms and dielectric terms. The fit procedure and its applicability will be elaborated in more detail below for each transducer. In taking a look at Table 3, it has to be discussed which processes and parameters contribute to signals of the different transducers (eqs 1-3). These considerations are needed to assess, how and to which extent the different sensor/transducer signals can be described with the above-mentioned model and what the meaning of the different parameters is. It is obvious that the chiral receptor/analyte interaction and the underlying absorption and partitioning thermodynamics are certainly identical for all three sensors. The easiest case is probably the mass-sensitive sensor. In looking at eq 2, the sensor response depends, besides a transducerspecific term, GCant, and the layer thickness, h, on the extent of gas-phase/matrix partitioning, Kc, and the analyte molecular mass. The analyte molecular mass is the same for the two enantiomers, so that only the difference in partitioning, i.e., the sorption thermodynamics (Kc) of the enantiomers influence the sensor signal and the enantiomeric discrimination. Therefore, the model should be applicable without major alterations. 9360

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The partition coefficient, Kc, in eq 2, which is only appropriate for linear characteristics, has to be replaced by a more complex function sketched in eq 7, see ref 16. A contribution, not accounted for in eq 2, may arise from polymer modulus changes due to analyte sorption (plasticization). This contribution is of minor importance in the thickness-shear-mode resonator used by Bodenho¨fer,16 but it may influence the stiffness and the resonance behavior of the polymer-coated cantilever as the polymer layer is comparably thick (4 µm) and is severely deformed by cantilever motion.52 In a first approximation, it has been assumed that the curve shapes obtained with the quartz microbalance as reported in ref 16 and also those obtained with the cantilever in Figure 3 represent, in a first approximation, the thermodynamic sorption characteristics. This assumption is supported by the fact that the curve shapes of the cantilever results here and those reported in ref 16 are very similar and that the ratios of Kchiral/Kachiral are also very similar (values around 500-1000; to quickly assess the contributions of specific sorption and nonspecific sorption to the respective sensor response curves, the values of Kachiral given in Table 3 can be multiplied with 100 ppm, which is the concentration value, at which the specific sorption reaches its maximum or saturation owing to the limited number of receptor sites. The obtained values can then directly be compared to the Kchiral values). Kchiral of the mass-sensitive sensors is, according to the model in ref 16, the same for both enantiomers, since it reflects full occupation of all cyclodextrin sites with molecules of the same mass (Table 3). Kachiral is anyway the same, as both enantiomers are absorbed to the same extent in a nonchiral environment at the nonspecific sorption sites. In the case of the calorimetric sensor, there is the heat of sorption that contributes to the sensor signal besides the transducerspecific term GCal, the layer thickness, h, and the sorption thermodynamics (Kc). Kc in eq 1 can be replaced by a more

complex function similar to that in eq 7. The relative heat-ofsorption contribution to the sensor response is larger at low concentrations (until the cyclodextrin cages are occupied) due to the strong specific interactions between analyte and cyclodextrin that produce significant quantities of heat. This particularly holds in comparison to high concentrations, where nonspecific physisorption is dominant, which produces less heat. This is also reflected in the ratio of Kchiral/Kachiral, which amount to between 3000 and 8000 for the calorimeter, indicating a pronounced dominance of the specific sorption in the measured heat or temperature changes. As a consequence, different sorption heat terms for the specific and for the nonspecific interaction come into play. The calorimetric sensor signals, hence, represent a convolution of the sorption thermodynamics and the thermal transducer function, as can be seen Figures 2 and 3: The signal slope at higher concentrations in Figure 2 is considerably smaller than that in Figure 3, a phenomenon also visible in the results of other authors.71,72 Moreover, the transition to linear Henry-type behavior seems to occur at considerably larger concentrations. The sorption heat and, consequently, the sensor signal at complete coverage of the enantioselective sites, is, in contrast to mass-sensitive sensors (same molar mass), clearly different for the two enantiomers of a certain analyte, since one enantiomer features stronger interaction with the cyclodextrin (more sorption heat). The nonspecific part, however, has to be identical for both enantiomers. This is reflected in Table 3 by having two different Kchiral for the enantiomers of the same analyte, while having the same Kachiral. In the case of the capacitive sensor, the situation becomes even more complex, though eq 3 seems to be simple and includes the transducer-specific term, GCap, the sorption thermodynamics, Kc, to be replaced by a more complex function in analogy to eq 7, and the change in the dielectric constant. The change in dielectric constant, however, is not directly correlated with sorption or thermodynamic processes and differs for specific and nonspecific sorption. To render the situation more complex, there is even a large difference (antipodal signals) between the changes in the dielectric properties invoked by the enantiomers of the same compound at low concentrations, which is a consequence of the already described molecular orientation processes.41 This also leads to a pronounced difference in the specific contribution (Kchiral) that is reflected in opposite signs, whereas the nonspecific part (Kachiral), again, must be identical. From the table and the graphs it is also evident that the ratio Kchiral/Kachiral is considerably lower than that of the mass-sensitive transducers, which means that there is not such a pronounced curvature in the low-concentration region. This holds particularly true for the nonpreferentially absorbed enantiomer, where the lowconcentration region is almost linear (Figure 4). To better show the low-concentration behavior, the Langmuirian or specific contribution to the capacitive sensor signals has been extracted using the fit parameters and is displayed for both chiral analytes (frequency shift versus analyte concentration) in Figure 5. The response curves of the nonpreferred enantiomers are only slightly bent. (71) Lerchner, J.; Kirchner, R.; Seidel, J.; Wa¨hlisch, D.; Wolf, G. Thermochim. Acta 2004, 415, 27–34. (72) Lerchner, J.; Kirchner, R.; Seidel, J.; Wa¨hlisch, D.; Wolf, G.; Ko ¨nig, W. A. Thermochim. Acta 2006, 445, 98–103.

Figure 5. Specific contribution to the capacitive sensor response extracted from the model and the fits to the sensor data. The (S)enantiomers of methyl lactate and methyl-2-chloropropionate are displayed as dashed lines, the (R)-enantiomers as solid lines.

Since the mass-sensitive responses, which most closely reflect the sorption thermodynamics, however, indicate a strong nonlinear enrichment of analyte molecules in the polymer matrix (strong curvature), there must be a contribution to capacitive sensor responses, which counterbalances the sorption-induced effects on the overall sensor signal. On the basis of results of earlier papers41,42 and this article, we assume that weak molecular orientation effects between cyclodextrin and the analyte enantiomers exist even for the nonpreferred enantiomer and thus reduce the overall sensor signal in the low-concentration region so that almost linear characteristics result. Of course, the orientation effects are much stronger for the preferred enantiomer and then drive the response curve even into negative territory (Figures 4 and 5). With the summary of the above considerations, it can be said that a purely thermodynamic model as proposed in ref 16 and a combined fit for chiral and nonchiral contribution can only be directly applied to the signals of the mass-sensitive cantilever, whereas, in the case of the calorimetric and the capacitive sensors, the sensor signals represent a convolution of sorption thermodynamics and transducer-specific signal contributions, the latter of which are different for the two enantiomers of a compound. Analytical Chemistry, Vol. 81, No. 22, November 15, 2009

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Therefore, individual fits of the sensor response curves of the enantiomers are more appropriate, which have to be done under the prerequisite that the nonchiral contribution (Henry part) has to be identical for both enantiomers. From the sensor and fit data (Figures 2-5) it is also evident that, e.g., capacitive sensors may provide even better chiral discrimination as defined or limited by sorption thermodynamics. This is due to the fact that there is a strong transducer-specific contribution resulting from the transduction principle (dielectric properties) that is superimposed to the sorption thermodynamics (specific nonlinear and nonspecific linear contributions). This “sensitivity enhancement” can, however, not be expressed in terms of the chiral discrimination factor, R, commonly employed in gas chromatography (GC), as a consequence of the occurrence of negative and positive sensor signal signs. Thin Layers of γ-Cyclodextrins. In the next step, thin composite layers were used on all three transducers. The multisensor systems were coated with an approximately 1 µm-thick layer of the modified γ-cyclodextrin/PDMS mixture (50% (w/w)) and then exposed to different concentrations of the enantiomers of methyl lactate and methyl-2-chloropropionate. In the case of the cantilever and the calorimeter, the signal heights dropped due to the reduction of the sensitive-layer volume, but the general characteristics of the curves were the same as those displayed in Figures 2 and 3. Therefore, we do not want to further elaborate on these results here. The capacitive sensor, however, produced signal characteristics that were significantly different from those of thick layers displayed in Figure 4. The sensor response of a capacitor with a thin sensitive layer upon exposure to different concentrations of the various enantiomers is depicted in parts a and b of Figure 6. It is obvious that there is always a capacitance increase and that there are no negative signals, which were pronounced for the two preferentially sorbed enantiomers at low concentrations in thick layers (Figure 4). The sensor signal increases in proportion to the increasing analyte concentration as indicated by the solid and dashed lines in Figure 6a,b. The graphs of the two enantiomers of methyl lactate and methyl-2-chloropropionate are very similar at low concentrations and diverge somewhat for higher concentrations. The capacitor yields larger responses for both nonpreferentially absorbed enantiomers, (S)-methyl lactate and (R)-methyl2-chloropropionate. This is striking, in particular, since the partitioning of the less-preferred enantiomers in the enantioselective layer is significantly lower in comparison to that of the preferentially absorbed enantiomers (methyl lactate partition coefficients, 31 000 for (S) and 43 000 for (R); methyl-2-chloropropionate partition coefficients, 19 000 for (R) and 70 000 for (S)), as accessible through mass-sensitive measurements conducted on the same chip or calculated from the results of discrete thickness-shear mode resonators.16 The responses of masssensitive devices show strong curvatures at low concentrations regardless of the layer thickness (see also Figure 3).16 As discussed previously, the signals or isotherms of mass-sensitive devices reproduce the sorption thermodynamics, whereas capacitive sensor signals constitute a convolution of sorption thermodynamics and analyte-induced effects on the dielectric properties of the overall layer. The two major effects producing capacitance changes include (a) swelling and (b) a change of the dielectric 9362

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Figure 6. Frequency shift of a capacitor coated with a 1 µm-thick enantioselective coating upon absorption of (a) (S)- and (R)-methyl lactate and (b) the two enantiomers of methyl-2-chloropropionate.

constant of the receptor/analyte composite.58 In the case of thick polymer layers (thicker than half the periodicity of the electrodes), which have been used to produce the results reported on so far (Figures 4 and 5), the swelling occurs outside the reach of the field lines so that the capacitive sensor response is determined by the dielectric constant of the analyte, that of the sorption matrix alone, and molecular orientation effects in the receptor/analyte complex, which influence the dielectric properties of the composite layer. For thin layers, i.e., layers thinner than the vertical extension of the electric field lines (3.2 µm), however, the amount of polarizable material in the sensitive region of the capacitor increases due to the so-called matrix swelling upon analyte incorporation, which, in most cases, results in a capacitance increase regardless of the dielectric constants of analyte and sorption matrix.58 The capacitance increase is due to the fact that air (ε ) 1) within the reach of the electrical field lines is replaced by the layer/analyte composite featuring larger dielectric coefficients (ε ) 3.1). For low concentrations of the analyte within the sensitive layer, the resulting relative increase of the layer thickness is directly proportional to the volume fraction of the analyte in the composite matrix, and the change of capacitance can be assumed to be proportional to the volume increase upon analyte absorption.58

Considering the arguments elaborated above, an explanation of the shapes of the curves for thin layers, as displayed in Figure 6, will be attempted here. On the basis of mass-sensitive sensor evidence in this study and in previous investigations,16 the thermodynamic sorption isotherm is nonlinear and strongly convex for low concentrations of the sorbed analytes. For thin layers on capacitors, we do not observe a significant curvature at low concentrations and we definitely do not observe any negative signal or capacitance decrease. It is reasonable to assume that the intimate interaction between preferentially sorbed enantiomers and cyclodextrin produces the described orientation effects and the associated capacitance decrease also in the case of thin layers. It seems however that the swelling of the composite layer upon analyte uptake, the only effect which can produce positive capacitance changes in the case of the preferentially sorbed enantiomers, outbalances any molecule-orientation-induced capacitance decrease. The measured concentration range is identical in Figures 4 and 6. The slope of the curves of the nonpreferentially sorbed enantiomers, however, is approximately 30% steeper in the case of the thin layers (Figure 6), so that thin layers produce larger signals, even though there is less sorptive matrix to incorporate analyte molecules. This indicates, in our opinion, that the swelling indeed very significantly and prominently contributes to the overall capacitive sensor signals at small layer thickness and determines the respective signal characteristics. It is interesting to note that the capacitive signals of the less preferred enantiomers are, again, somewhat larger (compare Figure 4). Smaller Cyclodextrin Derivatives. The three investigated enantioselective coatings were identical in their compositions (cyclodextrin derivative and PDMS at 50% (w/w)) but differed in the size of the dissolved cyclodextrin ring: R-cyclodextrin, including six R-D-glucopyranoside units, the β-cyclodextrin with seven units, and the γ-cyclodextrin with an eight-unit torus. Even though R-cyclodextrins are known to show little enantioselectivity30,62 and are, therefore, rarely used for chiral separation, a sensor chip was also coated with a 4 µm-thick layer of hexakis(3-O-butanoyl-2,6di-O-pentyl)-R-cyclodextrin/PDMS (50% (w/w). The measurements were conducted for comparison with the other enantioselective coatings. All three sensor types showed approximately linear response characteristics upon exposure to the chiral analytes methyl lactate and methyl-2-chloropropionate, which indicates no or only very little specific interaction between the enantioselective cyclodextrin and the chiral analytes. A slight preference of (R)-methyl lactate over the (S)-enantiomer was found, however, no identifiable preference for any of the enantiomers of methyl-2-chloropropionate. The results are in agreement with gas chromatography measurements performed in this study: A chiral discrimination factor of 1.03 was found for methyl lactate, whereas the discrimination of the enantiomers of the methy-2-chloropropionate was not possible (R ) 1). The native β-cyclodextrin and, particularly, its derivatives are often used for chiral separation and discrimination in chromatography. They are suitable materials for the separation of chiral amines, amino acids, ketones, and lactones.17,62 The multisensor system chip was coated with a sensitive layer of heptakis(3-Obutanoyl-2,6-di-O-pentyl)-β-cyclodextrin/PDMS (50% (w/w) and exposed to different concentrations of the enantiomers of methyl-

Figure 7. Sensor response versus analyte concentration of the capacitive sensor upon exposure to (S)- and (R)-methyl-2-chloropropionate. The coating is a 4 µm-thick β-cyclodextrin-derivative/PDMS (50% (w/w)) matrix. The plotted lines serve as guides to the eye.

2-chloropropionate. The sensor responses to the chiral analytes showed a strong response curvature as well as enantioselective interaction. Since the response curves of the calorimeter and cantilever resemble those in Figures 2 and 3, only the response curves of the capacitor, which shows the most pronounced enantiomeric discrimination, are depicted in Figure 7. All three transducers showed a preferential absorption of (S)methyl-2-chloropropionate in comparison to the (R)-enantiomer. This result coincides with NMR measurements from the literature that describe a stronger interaction with (S)-methyl-2-chloropropionate using a similar β-cyclodextrin derivative (heptakis(3-Oacetyl-2,6-di-O-pentyl)-β-cyclodextrin).23 The capacitor signals feature a very flat initial slope for the nonpreferred enantiomer in comparison to the γ-cyclodextrin-derivative shown in Figure 4, providing more evidence that the nonpreferred enantiomer also induces molecular ordering effects, which are, however, less pronounced than those induced by the preferred enantiomer. With comparison of the sensor responses using the three different cyclodextrin coatings, it was found that the enantioselectivity as well as the sensitivity to the organic compounds of the calorimetric and the mass-sensitive transducer increased with the size of the cyclodextrin. The capacitive sensor, however, shows the largest sensor responses to volatile organic compounds (Table 2) and the most pronounced chiral discrimination effects for the β-cyclodextrin derivative. The short-range-ordering seems to be particularly strong for the β-cyclodextrin. However, it is unclear whether this is a consequence of the smaller cavity (assuming inclusion of the analyte molecule) or of the arrangement of the glucose units and side chains. A decrease in enantioselectivity of the cyclodextrin/polymer layers over time was noticeable for all three transducers. A short thermal treatment (80 °C) restored the original sensitivity so that it can be assumed that molecular reorganizations lower the accessibility of the cyclodextrin units over time. Moreover, the loss in sensitivity is not uncommon for resonating cantilevers: Fractions of the polymer volume shift from the tip of the cantilever toward the base due to the cantilever oscillation and the elasticity of the polymer coating, which causes a decrease of the cantilever sensitivity. Analytical Chemistry, Vol. 81, No. 22, November 15, 2009

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In summary it can be said that chiral cyclodextrin-based sensitive coatings applied to a three-transducer microsystem (calorimetric, mass-sensitive, and capacitive chemical sensor) showed distinct chiral discrimination of the investigated analytes. The signals were found to constitute a convolution of sorption thermodynamics and transducer-specific contributions, which included, in the case of the capacitive sensor, molecular orientation effects, so that even opposite-sign signals for the two enantiomers were observed. The sensor response curves of all three transducers could be explained and fitted by applying a model that essentially implies the superposition of a Langmuir isotherm representing specific interactions, predominant at low concentrations, and a Henry isotherm for nonspecific physisorption. The

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results disclosed here show that sensors may provide an even more pronounced chiral discrimination with respect to sorptionthermodynamics-based gas chromatography as a consequence of additional transducer-specific signal contributions. ACKNOWLEDGMENT Financial support was provided within the framework of a European Network of Excellence, GOSPEL, Grant FP6-IST-507610, by the Swiss Bundesamt fu¨r Bildung und Wissenschaft. Received for review July 30, 2009. Accepted September 29, 2009. AC9017007