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Selective Picomolar Detection of Hexachlorobenzene in Water Using a Quartz Crystal Microbalance Coated with a Molecularly Imprinted Polymer Thin Film Kanad Das,† Jacques Penelle,*,‡ and Vincent M. Rotello*,† Departments of Chemistry and Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003 Received October 31, 2002. In Final Form: February 11, 2003 A molecular sensor for efficient and rapid recognition of hexachlorobenzene in water has been obtained by careful engineering of a thin, molecularly imprinted polymer thin film attached to a quartz crystal microbalance. Efficient imprinting was obtained in these films by controlling the heterogeneity of crosslinking in the polymer network and using appropriate electron-rich complements to the electron-deficient hexachlorobenzene. A comparison of the signals arising from two sensors, coated with imprinted and blank films respectively, unambiguously distinguishes hexachlorobenzene from small molecules of similar sizes and structures (anisole, benzene, chlorobenzene, and cyclohexane). The investigated sensors demonstrate both high selectivity and sensitivity (down to 10-12 mol L-1), while exhibiting an exceptionally fast response time (∼10 s).
Introduction A major goal of supramolecular chemistry has been the development of molecular sensors that can rapidly detect the presence of specific molecules in air or in solution and match the efficiency displayed by receptors (olfactory and others) used by living organisms to probe their environment.1 The availability of such sensors can be expected to impact many fields of technology from the environment to health science. In particular, real-time sensing of polychlorinated aromatic (PCA) contaminants in water constitutes a challenging and crucial goal in this area. The health hazards2 associated with PCA molecules at very low, environmentally relevant concentrations (∼10-9 mol L-1)3 require sensing technologies for trace contaminant detection that combine efficiency, selectivity, sensitivity, cost-effectiveness, and robustness. Molecularly imprinted polymers (MIPs) are highly cross-linked polymeric matrixes that contain specific recognition sites created during a polymerization in the presence of molecular templates.4-7 Recent progress made by the combination of molecularly imprinted polymers and quartz crystal microbalance (QCM) devices,8-12 which act as a signal transduction element in the sensor, has suggested that MIP-QCM sensors could provide a realistic way to design efficient PCA sensors if three current challenges † ‡
Department of Chemistry. Department of Polymer Science and Engineering.
(1) For a special issue on Chemical Sensors, see: Chem. Rev. 2000, 100. (2) Safe, S. H. Annu. Rev. Pharmacol. Toxicol. 1986, 26, 371-399. (3) Shiu, W. Y.; Wania, F.; Hung, H.; Mackay, D. J. Chem. Eng. Data 1997, 42, 293-297. (4) Shea, K. J. Trends Polym. Sci. 1994, 2, 166-173. (5) Steinke, J.; Sherrington, D. C.; Dunkin, I. R. Adv. Polym. Sci. 1995, 81-90. (6) Wulff, G.; Vesper, W.; Grobe-Einsler, R.; Sarhan, A. Makromol. Chem. 1977, 178, 2799-2816. (7) Shea, K. J. Trends Polym. Sci. 1994, 2, 166-173. (8) Haupt, K.; Mosbach, K. Chem. Rev. 2000, 100, 2495-2504. (9) Peng, H.; Zhang, Y.; Xie, Q.; Nie, L.; Yao, S. Analyst 2001, 126, 189-194. (10) Jenkins, A. L.; Yin, R.; Jensen, J. L. Analyst 2001, 126, 798802. (11) Cao, L.; Zhou, X. C.; Li, S. F. Y. Analyst 2001, 126, 184-188. (12) Haupt, K. Analyst 2001, 126, 747-756.
can be addressed: (1) slow response times, (2) inadequate sensitivity at low concentrations, and (3) poor stability of the final device in aqueous solutions (due to the MIP films rapidly peeling off the metallic surface of the QCM). In this paper, we report a highly sensitive, selective sensor based on a QCM device coupled to molecularly imprinted polymer films, with aromatic/aromatic and hydrophobic/hydrophilic interactions used as key elements in the recognition process. Control of cross-linking heterogeneity within the polymer network, fabrication of a thin film capable of effective adhesion to the gold electrode, and the mediation of molecular interactions by specific substituents introduced into the film proved essential in addressing the challenges mentioned above. Experimental Section General. All chemicals were purchased from Aldrich and used as received. Solvents were purchased from VWR. CH2Cl2 was distilled over CaH2. All reactions were carried out under argon using oven-dried glassware. All compounds were stored in the dark and under refrigeration to avoid photo- or thermal degradation. NMR spectra were recorded using a Bruker 200 MHz spectrometer. Diacrylate 2 (1,5-bis(2-acetamidoacryloyloxy)pentane) was synthesized according to literature procedures.13 Synthesis of Diacrylate 1 (1,4-Diacryolyloxybenzene). To a solution of hydroquinone (1.00 g, 9.1 mmol) and triethylamine (2.80 mL, 20 mmol) in dichloromethane (40 mL), acryloyl chloride (1.60 mL, 20 mmol) was added slowly and stirred overnight. The red solution was dried by rotary evaporation and suspended in ethyl acetate. The organic layer was washed with brine (2 × 100 mL) and saturated sodium bicarbonate (2 × 100 mL). The organic layer was collected and dried over MgSO4, the solvent was removed by rotary evaporation, and the resulting solid was passed through a plug of silica to result in a pure white solid. Yield: 1.54 g, 96%. 1H NMR (CDCl3, 200 MHz): δ ) 6.05 (m, 2H, 11 Hz), δ ) 6.39 (d, 2 H), δ ) 6.59 (d, 2 H), δ ) 7.17 (s, 4 H). Anal. Calcd for C12H10O4: C, 65.90; H, 4.66. Found: C, 66.05; H, 4.62. Preparation of Polymer Films. Imprinted polymers were prepared by spin-coating 20 µL of a CH2Cl2 solution containing 2 (25.9 mg, 0.08 mmol), 3 (0.03 mL, 0.02 mmol, 82/18 mol ratio), 1 (2.1 mg, 0.01 mmol), hexachlorobenzene 4 (2.8 mg, 0.01 mmol), and Irgacure 369 (2-benzyl-2-(dimethylamino)-4-morpholinobu(13) Xie, T.; Penelle, J.; Hsu, S. L.; Stolov, A. A. Green Chem., submitted.
10.1021/la026781u CCC: $25.00 © 2003 American Chemical Society Published on Web 03/19/2003
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Figure 1. Chemical structures of the monomeric components used in the fabrication of the MIP film. tyrophenone) (0.4 mg, 0.1 mmol) onto a 10 MHz QCM chip (International Crystal Manufacturing, Oklahoma City, OK; no. 131284) for 30 s at 7500 rpm. The coating was immediately irradiated for 10 s (Dymax Light Welder 3010EC, 10 mW/cm2) and subsequently placed in distilled CH2Cl2 (10 mL) for 2 h in order to remove hexachlorobenzene. Blank polymers were prepared using the same protocol in the absence of hexachlorobenzene.
Figure 2. Predicted structure of the 2:1 π-stacking complex between 1,4-diacetoxybenzene and hexachlorobenzene within the polymer matrix (Amber force field).
Results and Discussion Fabrication of the Sensors. A common approach to the synthesis of imprinted polymer networks uses noncovalent interactions between host-guest molecules present in the monomers prior to polymerization. It is assumed in this strategy that after polymerization (crosslinking) and guest removal, the resulting highly crosslinked, rigid polymer network maintains a structural “memory” of the electronic and geometric nature of the guest molecule utilized as the template. The fabrication strategy used in this work is based on several molecular components, 1,4-diacryolyloxybenzene 1, 1,5-bis(2-acetylaminoacryloyloxy)pentane 2, and benzyl methacrylate 3 (Figure 1), to make a very thin MIP coating. Diacrylate 1 was readily synthesized by the addition of acryloyl chloride to hydroquinone, and diacrylate 2 was obtained from 1-N-acetamidoacrylic acid and 1,5-dibromopentane.13 A cross-linked polymer film (400 nm) was obtained by polymerizing a spin-coated layer of the above monomers and hexachlorobenzene that had been deposited on top of the surface of a 10 MHz QCM chip. The use of monomer 2 is required as it dramatically improves the adhesion of the thin film to the gold surface, thereby producing a robust MIP coating suitable for recognition in water. In the case of intrinsically lipophilic and aromatic PCAs, both hydrophobic interactions and the electron-poor character of these molecules provide targeted interactions for recognition. Monomer 1 provides an electron-rich host molecule within the imprinted film to accommodate the electronpoor nature of hexachlorobenzene through π-stacking. Utilization of electrostatic charge pairing (Figure 2) has been extensively used in supramolecular chemistry.14 A theoretical model of the hypothetical 2:1 1,4-diacetoxybenzene/hexachlorobenzene complex as obtained by AM1 calculations is presented in Figure 2. Lipophilic benzyl methacrylate (3) was used at a 18 mol % concentration in the starting monomer film to increase the rates of release and uptake of the guest molecule and target analyte. It acts as a reactive diluent to the polymeric matrix and decreases the amount of possible cross-links, resulting in a “looser” matrix which allows for more efficient guest molecule transport.
Fast polymerization (10 s) was readily obtained by a photoinduced cleavage of the commercial benzoin derivative used as a photoinitiator (Irgacure 369). Hexachlorobenzene was removed from the imprinted films by placing the QCM chip in methylene chloride for 2 h. The blank sensing element was synthesized using the same methodology as the one used for an imprinted film, but without 4. Sensor Testing. Aqueous solutions (2.5 × 10-9 mol -1 L ) of hexachlorobenzene (4), cyclohexane (5), benzene (6), chlorobenzene (7), and anisole (8) were prepared and the QCM response was measured as a function of time in order to determine the selectivity of the imprinted films with regard to a family of structurally related molecules. The observed decrease in frequency is directly related to the mass increase in the polymeric film, according to the known mode of action for QCM sensors.15 Figure 3 shows the imprinted polymer response from an equilibrium with water to each of the molecules at the initial concentration (2.5 × 10-9 mol L-1) used in this study. A first feature of interest is the unusually fast response time (∼10 s) exhibited by the sensor. A very sharp decrease in frequency was observed in the first few seconds of exposure, followed by a decrease at a slower rate. This indicates that additional molecules continue to fill the
(14) Goodman, A. J.; Breinlinger, E. C.; McIntosh, C. M.; Grimaldi, L. N.; Rotello, V. M. Org. Lett. 2001, 3, 1531-1534 and references therein.
(15) O’Sullivan, C. K.; Guilbault, G. G. Biosens. Bioelectron. 1999, 14, 633-670.
Figure 3. MIP-QCM sensor response to a set of organic contaminants in water (2.5 × 10-9 mol L-1). The reported frequency shift is based on the signal obtained from sensors equilibrated with deionized water. The data for Blank refer to the observed shift to compound 4 with a nonimprinted polymer.
Detection of Hexachlorobenzene in Water
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matrix; it is not clear yet whether this behavior results from a transport phenomenon or the presence of a family of recognition sites with different affinities in the imprinted film.16,17 In contrast to previous attempts involving binding assays or radiolabeling,18 the sensor obtained here allows for real-time, in situ monitoring of the molecular event. The response time is also much faster than those reported in the literature for MIP-QCM sensors (usually several minutes to hours).8 The large difference in frequency shift observed between the imprint 4 and all other tested molecules indicates a significant degree of selectivity (Figure 3). An affinity factor S (Table 1) based on the relative molar ratios of frequency shifts obtained after 250 s of exposure was calculated from Figure 2 using eq 1 and anisole 8 as a reference:
S ) (∆F(x)/∆F(8))(M(8)/M(x))
(1)
where ∆F is the maximum change in frequency observed and M is the molecular weight of each individual analyte. The use of eq 1 is required by the fact that a QCM is not a balance in the strict meaning of that word in physics and does not directly “weigh” individual molecules. The frequency shift in the QCM response arises from acoustic shear waves propagating at the surface of the quartz crystal.15 In the gas phase, the signal (frequency shift) is directly proportional to the mass of interacting molecules according to Sauerbrey’s equation:
∆f0 ) -
2f02
∆m (Fqµq)1/2 A
(2)
where f0 is the fundamental crystal vibration frequency, A is the surface area, µq and Fq are properties of the piezoelectric crystal (shear density and modulus), and ∆f0 and ∆m are the frequency shifts and mass change, respectively. When liquids interact with the surface, the frequency shift is no longer strictly described by the Sauerbrey equation, though a linear relationship between frequency shift and mass increase is observed when a binding element is attached to the surface of the QCM and a complementary ligand is added to the solution. A binding element can be defined as a receptor that is able to complex with a complementary ligand. The mass of molecules in the film as determined from the Sauerbrey equation, although an admittedly crude approximation, is in the range of 10-13 to 10-12 g. The mass of molecules in the starting solution is several orders of magnitude higher than that, implying that only a very small fraction of the starting analyte is actually in the film. In such an operation mode, a QCM measures binding events by providing a signal proportional to the mass increase associated with the extent of complex formation. The signal in that case depends on the amount of binding elements attached to the surface, the concentration of the ligand in solution, and the association constant between the ligand and the receptor. The molar-based assessment of relative affinities provided by eq 1 allows for a direct comparison of molecular events and the use of free-energy relationships. (16) Umpleby, R. J.; Baxter, S. C.; Chen, Y. Z.; Shah, R. N.; Shimizu, K. D. Anal. Chem. 2001, 73, 4584-4591. (17) Umpleby, R. J. I.; Rushton, G. T.; Shah, R. H.; Rampey, A. M.; Bradshaw, J. C.; Berch, J. K.; Shimizu, K. D. Macromolecules 2001, 34, 8446-8452. (18) Albert, K.; Lewis, N.; Schauer, C.; Sotzing, G.; Stitzel, S.; Vaid, T.; Walt, D. Chem. Rev. 2000, 100, 2595-2626.
Figure 4. Plot of analyte detection versus observed relative affinity for a film imprinted with 4. Table 1. Relative Affinity Factors and Octanol-Water Partition Coefficients (log P) for Analytes 4-8 substrate
log Pa
∆Fmax (Hz)
relative affinity Sb
anisole (8) benzene (6) chlorobenzene (7) cyclohexane (5) hexachlorobenzene (4)
2.11 2.13 2.89 3.44 5.73
-430 -900 -930 -1220 -6570
1c 2.1 2.8 2.9 5.8
a
Reproduced from ref 19. b ∆Fmax/∆Fmax (anisole). c By definition.
The molar selectivity expressed by the raw data, as summarized in Table 1, appears to be in the range obtained in typical imprinted polymer systems.8 However, it must be emphasized that the signal measured by the QCM transducer corresponds to a mass increase of the analyte in the polymer film arising from three mechanisms: (1) selective binding of the analyte to the imprinted sites, (2) nonselective binding to the imprinted sites, and (3) partitioning of the relatively hydrophobic analytes used in this study between water and the lipophilic polymer coating. The third component can be taken into account and eliminated by measuring the signal obtained under identical conditions for nonimprinted “blank” films. A subtraction of the signal due to the imprinted polymer from that of the blank polymer provides the binding-site imprinting selectivity (discrimination between specific and nonspecific binding), which only takes into account binding to the imprinted sites and excludes sensing coming from partitioning between water and the polymer film. Figure 4 is a plot that includes the frequency shift arising from an imprinted and a blank system for each analyte, and the resulting difference in frequency. As shown by a comparison of the black bars in the chart, there is considerable selectivity displayed under the influence of the first two mechanisms. The difference between the frequency shift of a polymer imprinted by hexachlorobenzene (4) and the blank polymer (black bars in Figure 4) is rather small for the unimprinted analytes (80 Hz for anisole (8), 90 Hz for benzene (6), 60 Hz for chlorobenzene (7), and 130 Hz for cyclohexane (5)) and in the range of the error in the measurement (80 Hz). The difference in frequency observed for an imprinted versus blank polymer film for 4 results in an appreciable 4.9 × 103 Hz signal, that is, 38 times higher than the largest signal obtained for the other substrates. The results indicate that by designing a dual-sensor system containing two parallel polymer-coated QCMs, with imprinted and unimprinted films respectively, a selective sensor can be obtained that unambiguously determines whether an aqueous solution contains hexachlorobenzene by a subtraction of the two signals.
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Figure 6. Response of the detector to varying concentrations of hexachlorobenzene (4). Figure 5. Plot of log P values versus observed frequency shifts. The reported frequency shift is based on the signal obtained from sensors equilibrated with deionized water. The data point for Blank refers to the observed shift to compound 4 with a nonimprinted polymer.
As shown in Table 1, the signal for molecules 4-8 increases with the lipophilicity of the substrate, suggesting a common mechanism based primarily on phase transfer of hydrophobic molecules into the lipophilic environment provided by the polymer film. To test this hypothesis, a plot of affinity factors (S) against literature-based19 octanol-water partition coefficients (log P) was obtained (Figure 5). Log P is a standard measure of lipophilicity and is the most used parameter for quantitative assessment of lipophilicity in quantitative structure-activity relationships (QSAR).19 A nearly perfect linear relationship (r2 ) 0.91) can be observed for data points obtained on the unimprinted film (straight line in Figure 5). The data point for hexachlorobenzene on the unimprinted film has been added, the deviation from the correlation line indicating the selectivity arising from the imprinting process. In other words, the affinity of hexachlorobenzene for the unimprinted film can be entirely accounted for by a simple lipophilicity-driven partitioning mechanism as demonstrated by the perfect inclusion of the blank data point in the straight line. The assumption is made in the above analysis that the imprinted and blank films have similar capacities, that is, that the surface area accessible to the analyte is not affected by the presence of the guest molecule (hexachlorobenzene) during the polymerization. Although some of the reaction conditions used in imprinting techniques can be expected to influence the morphology of the cross-linked network obtained after polymerization, such an outcome does not appear very likely at the concentration of guest molecule used in this study. A range of experimental and theoretical studies on the morphology of thin films obtained after fast UV curing conditions support this view,20-24 which is further corroborated by the excellent correlation obtained with the log P parameters. Such a correlation would be very difficult to explain if the two (19) Hansch, C.; Leo, A.; Hoekman, D. Exploring QSAR Hydrophobic, Electronic, and Steric Constants; ACS Professional Reference Book; American Chemical Society: Washington, DC, 1995; Vol. 2. (20) Kannurpatti, A. R.; Bowman, C. N. Macromolecules 1998, 31, 3311-3316. (21) Kannurpatti, A. R.; Anderson, K. J.; Anseth, J. W.; Bowman, C. N. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 2297-2307. (22) Scott, R. A.; Peppas, N. A. Macromolecules 1999, 32, 61396148. (23) Funke, W. Prog. Org. Coat. 1997, 31, 5-9. (24) Funke, W.; Okay, O.; Joos-Muller, B. Microgels - Intramolecularly cross-linked macromolecules with a globular structure. In Microencapsulation, Microgels, Iniferters; Advances in Polymer Science, Vol. 136; Springer: New York, 1998; pp 139-234.
sets of films would indeed have different morphologies (surface area). The sensitivity of the sensor was measured by using successive dilutions of hexachlorobenzene (Figure 6). From these experiments, the limit of detection was determined to be 10-12 mol L-1, which represents a currently unparalleled sensitivity for this type of sensing device. Discussion QCM-based technology has been well established, and sensors based on this methodology are highly effective in a wide range of biotechnological applications.15,25,26 An emerging strategy for QCM sensors uses MIP films as the binding element.8-12,27 Despite the interesting progress already made, the features exhibited by recently described QCM-MIP sensors are not amenable to the design of a commercial sensor for trace analysis. Of particular concern is the absence of a general strategy for the robust attachment of MIP films to the gold or silver electrode that covers the quartz crystal. In experiments performed previously in our laboratory, we have devised a strategy to ensure the polymer film adhesion throughout the entire imprinting process.35 The results obtained in this study indicate that an effective molecular sensor for aqueous hexachlorobenzene can be obtained by the careful design of a MIP-QCM sensor. The three most critical challenges summarized above, that is, inadequate response time, low sensitivity, and poor stability of the device in solution, have been successfully addressed. In addition, a double-sensor technique has been developed that tremendously increases the selectivity by suppression of the nonselective signal arising from partitioning of the substrate molecules between the polymer film and the solution. Two of the three first challenges, response time and device stability, have been dramatically improved by engineering the polymer coating. Imprinted polymers have complex polymerization chemistry, and physicochemical features of the final imprinted systems involve the additive effect of a large number of complex processes that take place at various length scales. In addition, host-guest chemistry is dramatically affected by the local (molecular) environment, while the cross-linking polymerization induces significant stress at the macroscopic level and does not provide an efficient mechanism for heat removal and local temperature control during the exothermic polymerization reaction. As the result of these and other (25) Janshoff, A.; Galla, H. J.; C., S. Angew. Chem., Int. Ed. 2000, 39, 4004-4031. (26) Malitesta, C.; Losito, I.; Zambonin, P. G. Anal. Chem. 1999, 71, 1366-1370. (27) Dickert, F. L.; Tortschanoff, M.; Bulst, W. E.; Fischerauer, G. Anal. Chem. 1999, 71, 4559-4563.
Detection of Hexachlorobenzene in Water
parameters, the final polymer morphology is heterogeneous at length scales only slightly larger that the typical size of the imprinted cavities. This heterogeneous character affects the optimal release and uptake of specific target molecules and certainly influences the postpolymerization binding. We have previously demonstrated that a key element to a successful MIP film is the design of a polymer network in which local, highly cross-linked areas around the imprinted site are surrounded in the films by a continuous phase that allows fast diffusion of the substrate.28 Highly heterogeneous networks are traditionally obtained by free-radical polymerization of the multifunctional monomers, with more heterogeneous networks obtained by fast to ultrafast polymerizations.20-22,29-31 This is the reason a photochemical polymerization at room temperature was used in this study. Further morphological studies on imprinted films synthesized by photopolymerization support this view and will be soon reported.32 An improvement of the response time can be obtained independently from the morphological nature of the film by reducing the film thickness. To the best of our knowledge, no studies have been reported investigating what the optimum film thickness should be for a MIPQCM sensor. The known physical basis for QCM-based sensing suggests that when thick films are used, binding events far away from the transducer surface cannot be efficiently be measured while these binding sites constitute a very efficient barrier that decreases the diffusion rate of substrate molecules across the film.15,25,33,34 On the basis of these two arguments, the most efficient MIP-QCM sensors can be expected to have two key features: low thickness and high network heterogeneity.35 Unfortunately, these two attributes can also be expected to contribute very effectively to poor adhesion of the coating to the QCM chip, in particular the gold electrode that makes up about 40% of the total area covered by the MIP coating. Attempts made to solve this problem by traditional approaches and monomers failed to provide aqueous and organic solvent stability, crucial requirements during the imprinting and testing processes. Exhaustive adaptations of styrenic36 and acrylic37 based formulations proved to peel off of the surface of the QCM chip either during the template removal step or during sensor testing. (28) Duffy, D. J.; Das, K.; Hsu, S. L.; Penelle, J.; Rotello, V. M.; Stidham, H. D. J. Am. Chem. Soc. 2002, 124, 8290-8296. (29) Anseth, K. S.; Anderson, K. J.; Bowman, C. N. Macromol. Chem. Phys. 1996, 197, 833-848. (30) Jager, W. F.; Norder, B. Macromolecules 2000, 33, 8576-8582. (31) Szuromi, E.; Berka, M.; Borbely, J. Macromolecules 2000, 33, 3993-3998. (32) Das, K.; Duffy, D. J.; McKiernan, R.; Gido, S. P.; Hsu, S. L.; Penelle, J.; Rotello, V. M. Manuscript in preparation. (33) Alder, J. F.; McCallum, J. J. Analyst 1983, 108, 1169-1189. (34) Dickert, F. L.; Thierer, S. Adv. Mater. 1996, 8, 987-990. (35) Curing of bisacrylic monomers leads to polymer networks in which highly cross-linked nanodomains are connected by a slightly crosslinked, contiguous matrix [refs 20-24]. Manipulation of the morphology at that length scale is possible by controlling the polymerization conditions, fast polymerizations and low temperatures typically providing the highest contrast in the distribution of cross-linked area. We have shown in a separate study that this strategy can be used to improve effectively the distribution of binding sites in imprinted polymer networks [ref 32].
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We finally solved the problem by using monomer 2 that we have shown previously to be a crucial element in the good adhesion of polymer coatings to metallic substrates, including gold.35 Attachment of all polymer coatings fabricated to metallic surfaces under various conditions proved to be excellent, including under conditions where use of previously reported formulations led to very poor adhesion and rapid peeling of the film. The coating also exhibited excellent long-term stability at very high (pH ) 2) and very low (pH ) 12) acidities. It is difficult to assess whether the selectivity obtained in these experiments is significantly better than in previous reports. The selectivity determined in the single mode, that is, before suppression of the signal coming from the partitioning mechanism, is in the range of what has been demonstrated for other systems.8 It will be important to assess now whether the MIP-QCM analysis performed in the double-sensor mode also dramatically improves the selectivity for other systems. The possibility to generalize this approach would undoubtedly constitute an important step toward the design of robust, affordable molecular sensors. The range of concentrations where a significant signal can be obtained corresponds perfectly to environmentally relevant concentrations known to cause health hazards.2 The recent work by Whitcombe et al., showing that PCAs in nonane can be detected at nanomolar concentrations using radioactive labeling of the target molecule, is a representative example of MIP-based sensing applied to the selective detection of PCAs.38 The nearly instantaneous and environmentally friendly nature of the QCM-based sensor presented here gives distinct advantages over radiolabeling for practical applications. Conclusions In summary, a molecular sensor that efficiently recognizes hexachlorobenzene in water has been obtained by engineering a thin, molecularly imprinted polymer attached to a QCM. Employing π-stacking and a lipophilic environment in a polymeric matrix created a selective QCM-based sensor for hexachlorobenzene in water that can operate down to the picomolar concentration range. It is demonstrated that nonselective binding results from a partitioning of the target molecule between water and the lipophilic film. The investigated sensors demonstrate both high selectivity and sensitivity (down to 10-12 mol L-1), while demonstrating exceptionally fast response times (∼10 s). Acknowledgment. This work was supported by the National Science Foundation (DMR-9809365) and the Brimms Ness Corporation via a SBIR grant. The authors thank Dr. Tao Xie for helpful discussions. LA026781U (36) Wulff, G. Polymeric reagents and catalysts; ACS Symposium Series, Vol. 308; American Chemical Society: Washington, DC, 1986; pp 186-230. (37) Haupt, K.; Noworyta, K.; Kuyner, W. Anal. Commun. 1999, 36, 391-393. (38) Lubke, M.; Whitcombe, M. J.; Vulfson, E. N. J. Am. Chem. Soc. 1998, 120, 13342-13348.
