Anal. Chem. 2003, 75, 5387-5393
Quartz Crystal Microbalance Sensor for Organic Vapor Detection Based on Molecularly Imprinted Polymers Yi Fu and Harry O. Finklea*
Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506
Molecularly imprinted polymers on quartz crystal microbalances (QCM) are examined for their ability to detect vapors of small organic molecules with greater sensitivity and selectivity than the traditional amorphous polymer coatings. Hydroquinone and phenol serve as noncovalently bound templates that generate shape-selective cavities in a poly(acrylic) or poly(methacrylic) polymer matrix. The imprinted polymers are immobilized on the piezoelectric crystal surface via a precoated poly(isobutylene) layer. The behavior of the imprinted polymer films is characterized by the dynamic and steady-state response of the QCM frequency to pulses of organic vapors in dry air. The apparent partition coefficients are determined for imprinted and nonimprinted polymers prepared by two synthetic methods and for varying mole ratios of template to monomer. The hydroquinoneimprinted polymers and, to a lesser extent, the phenolimprinted polymers exhibit greater sensitivity and higher selectivity than the nonimprinted polymers toward organic vapors that are structurally related to the templates. These results indicate that molecularly imprinted polymers are promising for the development of selective piezoelectric sensors for organic vapor detection. Chemical sensors are of growing importance in real-time analysis, especially in monitoring air quality.1 Although the traditional assay technologies such as gas chromatography and solid-phase extraction have high sensitivity and selectivity, they are expensive and are not customarily used for real-time measurements in the field. Chemical sensors have the potential of providing real-time measurements along with the conveniences of portability, low cost, and ease of operation. The general sensor technologies that have shown promise for this kind of use are acoustic oscillator devices,2 optical waveguides,3 and chemically sensitive field effect transistors.4 All of these sensor technologies use thin sorbent layers of chemically selective material to collect and concentrate targeted analytes at the interface of the coated * Corresponding author. E-mail:
[email protected]. (1) Cammann, K.; Lemke, U.; Rihen, R.; Sander, J. Angew. Chem., Int. Ed. Engl. 1991, 30, 516. (2) Grate, J. W.; Patrash, S. J.; Abraham, M. H. Anal. Chem. 1995, 67, 2162. (3) Alarie, J. P.; VoDinh, T. Talanta 1991, 38 (5), 529. (4) Hedberg, E.; Winquist, F.; Lundstrom, I. Sens. Actuators, A 1993, 37/38, 796. 10.1021/ac034523b CCC: $25.00 Published on Web 09/09/2003
© 2003 American Chemical Society
transducer. Physical or chemical property changes at the interface of the sorbent layer are monitored by the transducer. Selective response is one of the most crucial properties of a sensor. In the environmental field, the selectivity of sensor is often achieved by using biological antibodies covalently bound to a suitable support.5 However, most biomolecules suffer from poor stability and a complicated preparation scheme, which limit their applications in real-time analyses. The tedious procedure and cost of biological molecules have led to attempts to synthesize antibody mimics in the chemistry laboratory. Molecular imprinting is an emerging technique for the preparation of materials with highly selective adsorption properties. In this technique, functional monomers, organized around a template molecule by noncovalent interactions (such as hydrogen bond or ion-pair interactions) or reversible covalent interactions, are copolymerized with crosslinking monomers to form a highly cross-linked network polymer.6 Extraction of the template leaves sites in the polymer that are complementary to the size, shape, and chemical functionality of the template molecule. Molecularly imprinted polymers (MIPs) have been successfully used as stationary phases in chromatography,7,8 solid-phase extraction,9 and ligand-binding assays.10,11 The advantage of chemical stability aids the application of MIP-based sensors in harsh chemical environments, such as organic solvents or at high temperature. Their use in electrochemical,12 optical,13 and surface acoustic wave14 sensors has been demonstrated. The goal of this project is to explore molecularly imprinted polymers for selective vapor adsorption/desorption and to incorporate these polymers as stationary phases into preconcentrators in gas-phase sensors. As an initial test, the detection and quantification of small aromatic (e.g., benzene, toluene) vapors is chosen. Consequently, the cavities inside the polymer matrix are produced by using hydrogen-bonding analogues (hydro(5) Altstein, M.; Bronshtein, A.; Glasttstein, B.; Zeichner, A.; Tamiri, T.; Almog, J. Anal. Chem. 2001, 73, 2461-2467. (6) Wulff, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1812. (7) Fischer, L.; Mueller, R.; Ekberg, B.; Mosbach, K. J. Am. Chem. Soc. 1991, 113, 9358. (8) Nicholls, I. A.; Ramstrom, O.; Mosbach, K. J. Chromatogr. 1995, 691, 349. (9) Mudoon, M. T.; Stanker, L. H. Anal. Chem. 1997, 69, 803. (10) Anderson, L. I. Anal. Chem. 1996, 68, 111. (11) Vlatakis, G.; Anderson, L. I.; Muller, R.; Mosbach, K. Nature 1993, 361, 645. (12) Kriz, D.; Mosbach, K. Anal. Chim. Acta. 1995, 300, 71. (13) Turkwitcsch, P.; Wandelt, B.; Darling, G. D.; Powell, W. S. Anal. Chem. 1995, 67, 2142. (14) Peng, H.; Liang, C. D.; He, D. I.; Nie, L. H.; Yao, S. Z. Anal. Lett. 2000, 33, 793.
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quinone, phenol) as the templates. Monofunctional monomers containing acidic groups (acrylic acid, methacrylic acid) are used to allow hydrogen bonding with the templates during polymerization. The selective response of the imprinted polymers to small volatile organic compounds is characterized by quartz crystal microbalance (QCM) measurements. The chemical sensing films are coated onto the QCM surface to monitor the mass change induced by the analyte adsorbed into the imprinted polymer. The QCM measurements yield an apparent partition coefficient and rise and decay times for vapor uptake and release. The partition coefficient K is used to compare sensitivity and selectivity for imprinted versus nonimprinted polymers (control polymer) and for several protocols of polymer preparation. The partition coefficient K is defined as
K ) Cf/Cg
(1)
where Cg is the analyte concentration in the air and Cf is the analyte concentration in the film. At steady state, the amount of sorbed gas molecules onto/into the sensing film is determined from the interaction between gas molecules and the film material. The partition coefficient is independent of the kind of piezoelectric devices and resonance frequency and is constant in the range of low gas concentrations over which Henry’s law is valid. Substitution of eq 1 into the general form of the Sauerbrey equation leads to the following equation:15
K ) ∆fvF/∆fsCv
(2)
where ∆fv is the frequency shift due to the vapor adsorption, ∆fs is the frequency shift due to the deposition of the film itself, Cv is the vapor concentration, and F is the density of the film (assumed to be 1 g/cm3 for the polymers in this study). This equation is based on the assumption that all frequency changes are due to changes in the mass of the film. Several variables in molecularly imprinted polymerization are examined for their effect on the sensitivity and selectivity of the MIPs. One protocol generates solid blocks of polymer, and the other generates microspheres. Microsphere synthesis produces particles with uniform sizes and avoids the necessity of grinding of the block polymer prior to template extraction. The effects of template identity and template concentration are also investigated. EXPERIMENTAL SECTION Polymer Synthesis. Block Imprinted Polymer (BIP). A typical bulk polymer preparation scheme was used to prepare the molecularly imprinted polymer particles with irregular shapes.16 In a typical preparation, 2 mmol of a template such as hydroquinone (HQ) was dissolved in 5 mL of dimethyl sulfoxide and mixed with 5 mmol of cross-linker ethylene glycol dimethacrylate (EGDMA) and 5 mmol of monomer, either acrylic acid or methacrylic acid (MAA). The solution was deoxygenated by bubbling argon for 15 min, and 15 mg of initiator azobis(isobutyronitrile) (AIBN, >98%) was added. The solution was sealed in a test tube and immersed in water bath at 50 °C overnight. The resulting block polymer was ground in a mortar (15) Grate, J.; Kaganove, S.; Bhethananbotla, V. Anal. Chem. 1998, 70, 199. (16) Dunkin, I. R.; Lenfeld, J.; Sherrington, D. C. Polymer 1993, 34 (1), 77.
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and pestle to form fine particles. The polymer imprinted with phenol (P) was synthesized using the same procedure. The control polymer (CP) was synthesized using the same procedure without any template. Imprinted Microsphere Polymer (IMP). In a typical synthesis,17 2 mmol of a template such as HQ was dissolved in 40 mL of acetonitrile with the functional monomer MAA (5 mmol) and the cross-linker EGDMA (5 mmol). The solution was deoxygenated with argon for 15 min, the initiator AIBN added, and the tube was sealed and placed in a 60 °C water bath overnight. The microsphere particles were then collected by centrifugation. Template Extraction. Template extraction of polymers creates the cavities capable of specific sorption of the template and may also remove other materials from the polymer, e.g., residual monomers or oligomers and initiator fragments. The fine particles from ground block polymer or the microspheres were placed into a round-bottom flask with a condenser, and 40 mL of methanol/ acetic acid (v:v ) 3:1) solution was added. The slurry was stirred at 50 °C for 4 h. The extracting solvent mixture in the flask was removed, and this extraction procedure was repeated with fresh solvent mixture 3 more times. The particles were isolated and dried under vacuum at 70 °C. Vapor Generation. The vapor generator used dry air obtained via a large zeolite (13X) drying column. Part of the dry air passed through a model 491M Kintek unit containing permeation tubes in temperature-controlled ovens. The concentration of the analyte vapor was manipulated by changing the flow rate and the oven temperature. Vapors with different molecular shapes and sizees, toluene (TOL), benzene (BEZ), trichloroethylene (TCE), carbon tetrachloride (CCl4), and heptanes (HEP), were selected as the target vapors for the sorption/desorption tests. Toluene, benzene, and trichloroethylene are planar molecules similar in size and shape to the templates hydroquinone and phenol. QCM Adsorption Measurement. The QCM device included an aluminum block that housed both the sample crystal and reference crystal. The output ports of the locally built oscillator circuitry provided sample, reference, and beat frequencies.18 The beat frequency is defined as the difference in frequency between the sample crystal and the reference crystal. AT-cut disk-shaped quartz crystals (International Crystal Manufacturing Co.) with a resonant frequency of 10 MHz and gold pad electrodes were chosen so that the reference crystal had a higher base frequency than the sample crystal. An increase in the loading of the sample crystal resulted in an increase of the beat frequency. Prior to a measurement, the QCM sensors were exposed to a dry air stream until a stable baseline ((1 Hz/min) was obtained. During a measurement, the reference dry air stream flowed over the sample crystal to obtain a baseline response. Then the valve was switched to introduce the analyte vapor. The QCM sample crystal was exposed to the analyte stream at a constant flow rate until a stable response was obtained. After exposure, organic-free dry air was reintroduced into the sampling chamber to reestablish the baseline. (17) Ye, L.; Weiss, R.; Mosbach, K. Macromolecules 2000, 33, 8239. (18) Finklea, H. O.; Phillippi, M. A.; Lompert, E.; Grate, J. W. Anal. Chem. 1998, 70, 1268.
Crystal Coating. Poly(isobutylene) (PIB) served as the adhesive for binding the imprinted polymer particles to the QCM. A two-step procedure was used. A solution of 1 mg/mL PIB in TCE was placed on one side of the QCM, and the crystal was spun at high speed to create a uniform PIB coating.19 Typical frequency changes associated with the deposited PIB film after one application were ∼2 kHz. The bulk polymer powders were suspended in acetone and allowed to settle for 4 h. The particles that remained suspended in the acetone were collected by centrifugation and used for QCM coating. A drop of a suspension of polymer powder in ethanol (1 mg/mL) was then spread on the PIB layer. Evaporation of the ethanol at room temperature yielded the MIP coating on the QCM. The frequency baseline became stable ((1 Hz/min) after 1-2 h conditioning period in a dry air stream. RESULTS AND DISCUSSION Formation of MIP Coatings on QCM. Stable adherent coatings of soft polymers on QCM crystal surfaces are prepared by spray- or spin-coating a solution of the polymer in a volatile solvent. However, the molecularly imprinted polymers are rigid glassy materials and are insoluble in all solvents. When deposited by evaporation of a suspension, they do not adhere well to QCM surfaces as shown by small frequency changes (
