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Effect of Thin Film Processing on Cavitand Selectivity Devanand K. Shenoy,*,† Elias B. Feresenbet,† Roberta Pinalli,‡ and Enrico Dalcanale‡ Code 6900, Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, D.C. 20375-5320, and Dipartimento di Chimica Organica e Industriale, Universita` di Parma and INSTM, UdR Parma, Viale delle Scienze 17/A, 43100 Parma, Italy Received June 19, 2003. In Final Form: August 26, 2003
Introduction Self-assembled monolayers (SAMs) and thin films continue to find utility in a wide range of applications.1 An important area in which such films are used is in near real-time chemical sensing.2 Because selectivity is a key issue in developing efficient chemical sensors,3 special attention must be paid to all factors affecting it. Typically, the observed selectivity of a sensor is the result of the interplay between three major factors: the transduction scheme adopted (i.e., mass, optical, electrical), the adsorption specificity of the surface-deposited chemical structure, and the film morphology of the coated layer. The use of supramolecular structures has proved to be one of the best approaches to generate new materials with molecular specificity for chemical sensing.4-9 In bulk sensors, like mass sensors, film morphology often jeopardizes selectivity by increasing the weight of nonspecific dispersion interactions, which override the specific analyte-receptor ones.8 To clarify this issue in the case of surface plasmon resonance (SPR) optical transduction, we selected two methylene-bridged cavitands having the same cavity at the upper rim and different alkyl feet. The first, MeCav, presents four undecyl chains at the lower rim, which impart high solubility in common organic solvents for processing and high layer permeability. The second, MeCav-thioether, has four dialkyl sulfide chains, designed for SAMs formation on gold (Figure 1).10 The meniscus-like π-basic cavity of MeCav binds small molecules having acidic hydrogens such as acetonitrile via CH-π interactions, both in solution11 and at the gassolid interface (quartz crystal microbalance, QCM).12 * Corresponding author: e-mail
[email protected]. † Naval Research Laboratory. ‡ Universita ` di Parma and INSTM. (1) Abraham, U. Chem. Rev. 1996, 96, 1533-1554. (2) Christina, B.; Qiuming, Y.; Shengfu, C.; Chi-Ying, L.; Jiri, H.; Sinclair, S. Y.; Shaoyi, J. Sens. Actuators, B 2003, 90, 22-30. (3) Acc. Chem. Res. 1998, 31 (5), Special Issue on Chemical Sensing. (4) Schierbaum, K. D.; Go¨pel, W. Synth. Met. 1993, 61, 37-45. (5) Dickert, F. L.; Haunschild, A. Adv. Mater. 1993, 5, 887-895. (6) Dickert, F. L.; Haunschild, A.; Kuschow, V.; Reif, M.; Stathopulos, H. Anal. Chem. 1996, 68, 1058-1061. (7) Pinalli, R.; Nachtigall, F. F.; Ugozzoli, F.; Dalcanale, E. Angew. Chem., Int. Ed. 1999, 38, 2377-2380. (8) Moore, L. W.; Springer, K. N.; Shi, J. X.; Yang, X. G.; Swanson, B. I.; Li, D. Q. Adv. Mater. 1995, 7, 729-731. (9) Paolesse, R.; Di Natale, C.; Nardis, S.; Macagno, A.; D’Amico, A.; Pinalli, R.; Dalcanale, E. Chem.sEur. J. 2003, 9, in press. (10) Thoden van Velzen, E. U.; Engbersen, J. F. J.; de Lange, P. J.; Mahy, J. W. G.; Reinhoudt, D. N. J. Am. Chem. Soc. 1995, 117, 68536862. (11) Tucker, J.; Knobler, C. B.; Trueblood, K. N.; Cram, D. J. J. Am. Chem. Soc. 1989, 111, 3688-3699. (12) Hartmann, J.; Hauptmann, P.; Levi, S.; Dalcanale, E. Sens. Actuators, B 1996, 35-36, 154-157.
Figure 1. Chemical structures of cavitands.
MeCav-thioether SAMs have shown a high affinity for tetrachloroethylene, both using mass (QCM)13 and optical (SPR)14 transducers. For a meaningful comparison, the monolayer of 1-octadecanethiol (C18-thiol) has also been prepared and tested. In chemical sensing applications, it is important that simple processes be used for coating them onto transducer substrates. Spin coating is a common process that is widely used in industry.15 By spin coating MeCav onto a gold substrate and comparing the selectivity to that of the MeCav-thioether SAMs on gold, the effect of the process on the selectivity may be determined. Herein, we report a comparison of the responses of the two cavitands to six different analytes using SPR as the transduction scheme. Experimental Section Materials. MeCav (1,21,23,25-tetraundecyl-2,20:3,19-dimetheno-1H,21H,23 H,25H-bis[1,3]dioxocino[5,4-i:5′,4′-i′]benzo[1,2d:5,4-d′]bis[1,3]benzodioxocin),16 and MeCav-thioether (1,21,23,25-tetra[1-(decylsulfanyl)undecane-2,20:3,19-dimetho1H,21H,23H,25H-bis[1,3]dioxocino[5,4-i:5′,4′-i′]benzo[1,2-d:5,4d′]bis[1,3]benzodioxocin)17 were synthesized according to wellestablished methods. All chemicals (Aldrich Chemical Co.) were of analytical reagent grade. Substrate Preparation. A cover glass was cleaned prior to gold deposition with hot “piranha” solution (30:70 v/v mixture of H2O2 and H2SO4; warningssolution reacts violently with many organic materials and must be used with extreme caution and (13) Schierbaum, K. D.; Weiss, T.; Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N.; Go¨pel, W. Science 1994, 265, 1413-1415. (14) Huisman, B. H.; Kooyman, R. P. H.; van veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Mater. 1996, 8, 561-564. (15) Critchley, S. M.; Willis, M. R.; Cook, M. J.; McMurdo, J.; Maruyama, Y. J. Mater. Chem. 1992, 2, 157-159. (16) Roma´n, E.; Peinador, C.; Mendoza, S.; Kaifer, A. E. J. Org. Chem. 1999, 64, 2577-2578. (17) Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N. Synthesis 1995, 8, 989-997.
10.1021/la0350857 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/02/2003
Notes should not be stored in sealed containers). An approximately 2-nm-thick chromium adhesion layer was first deposited onto cleaned cover glass (22 × 22 mm, Thomas Scientific) using a vacuum evaporator (Edwards Auto 306). This was followed by evaporation of a nearly 50-nm-thick gold layer for SPR. Evaporation, from a gold coin (Canadian coin, 99.99%), was preformed at a vacuum of 10-6 bar and at a rate of 0.02-0.04 nm s-1 with the thickness of the deposited metal films determined via a crystal oscillator. The sensing layer was spin-coated onto the gold film (model P6700, spin coater) using 20 µL of MeCav (0.38 mM in chloroform) at 4000 rpm for 60 s at room temperature followed by evaporation of the solvent. Also, freshly prepared and cleaned gold-coated substrates were coated with a monolayer of MeCav-thioether and octadecanethiol (C18-thiol). To obtain a monolayer, the gold-coated substrate was immersed into a 1 mM solution of the adsorbate in a mixture of ethanol/chloroform (7:3 v/v). Substrates were left in the C18thiol solution for 6 h for the self-assembly of monolayers on the gold surface. Monolayers of MeCav-thioether were allowed to form for 13 h at approximately 60 °C.10 Subsequently, the solution was allowed to cool to room temperature for about 3 h. The substrate was carefully removed from the solution and submerged into 10 mL of pure dichloromethane to remove any physisorbed material. Finally, the substrate was rinsed with pure ethanol and water. The layers were characterized by Fourier transform infrared (FT-IR) spectroscopy before the SPR experiments were performed. The FT-IR monolayer characterization was in agreement with previously reported results.18-20 Ellipsometric measurements were performed on a multiwavelength ellipsometer (EC110) equipped with a 75-W xenon light source (LPS-300) at an incident angle of 70°. The thickness values were calculated using a software program (WVASE 32, Version 3.337b). Experimental Setup. An in-house SPR experimental arrangement, based on the Kretschmann configuration,21 was used for all the measurements. The glass prism, made of standard BK7 glass (90°, 17-mm high, 22 × 32 mm at the base, refractive index n ) 1.5, Howard Johnson Optical Laboratories), was indexmatched to the cover glass (n ) 1.51) using an index-matching liquid from Cargille, Inc. A semiconductor diode laser (from Lasermax, Inc., operating at 635 nm) was used as the light source. A polarizer was placed in the path of the beam to ensure that p-polarized light was incident on the glass prism. The reflected light was monitored using a photodetector (818 Series, Newport Corp.) calibrated for the chosen wavelength. Variable angles were selected by means of a stepper motor controlled goniometer with a resolution of 0.01°. The data acquisition was completely automated through a computer program, with a typical angular scan (from 40 to 80°) taking about 8 min. To expose organic vapors of precisely known concentrations to cavitands, a flow cell was constructed from a block of Teflon. This was designed to provide an inlet and outlet for the analyte vapors and had a rectangular aperture to hold the cavitand coated glass slide and prism. An O-ring ensured a good seal between the prism/slide and the Teflon chamber. To generate parts per million levels of organic vapors, a diffusion vial (D-5.0 mm capillary, VICI Metronics) was filled with analyte liquid using 5-mL syringe needles (VICI Metronics) and placed in one side of a U-tube. Glass beads where placed on the other side. The U-tube was placed in a temperature-controlled (to within 0.1°) water bath from PolySciences, Inc. Vapors of the organic liquid were diluted with a stream of dinitrogen (carrier gas). The flow of both the carrier gas and the analyte were precisely controlled (accuracy of 1-2%) using mass flow controllers (DFC26, Aalborg, Inc.) and mixed in the appropriate proportions before being introduced into the flow cell for exposure to the cavitands. The SPR angular scan was initiated about 5 min after analyte exposure to the sensing surface. Care was taken to ensure that (18) Marc, D. P.; Thomas, B. B.; David, L. A.; Christopher, E. D. C. J. Am. Chem. Soc. 1987, 109, 3559-3568. (19) John, P. F.; Paul, E. L.; George, M. W. Langmuir 1992, 8, 13301341. (20) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365-385. (21) Kretschmann, E.; Raether, H. Z. Naturforsch. Teil A 1968, 23, 2135-2136.
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Figure 2. Selectivity patterns observed from the interaction of the cavitands MeCav, MeCav-thioether, and C18-thiol with 110 ppm perchloroethylene, toluene, nitromethane acetonitrile, benzene, and n-hexane. The change in the plasmon angle shift is shown on the y axis. Table 1. Measured Resonance Angle Shifts and Calculated Thicknesses of Sensing Layers
film
plasmon angle shift (degrees)
thickness (nm), SPR
thickness (nm), ellipsometry
MeCav MeCav-thioether C18-thiol
0.9 ( 0.2 0.47 ( 0.2 0.42 ( 0.09
3.91 2.04 1.82
3.61 1.73 1.82
the experiment was performed after the vapor had attained equilibrium with respect to its concentration. A software program controlled the flow of the carrier gas as well as the organic vapors.
Results and Discussion To make meaningful comparisons of the selectivities due to the two films processed by spin coating and selfassembly, the film thicknesses are determined. For purposes of clarification, the selectivity here is defined in terms of the relative SPR signal observed for various analyte-sensing layer combinations. We have recently demonstrated the selectivity of other cavitands for aromatic vapors using the same optical transduction scheme.22 Table 1 summarizes thickness data of octadecanethiol and cavitand sensing layers measured using ellipsometry and also extracted from SPR spectroscopy. The plasmon angle shifts were recorded when a clean gold surface was covered with the three different layers. For the C18-thiol monolayer, the thickness of 1.82 nm was observed to be that expected for a monolayer. For the spin-coated cavitands, MeCav, a layer thickness of about 4 ((0.3) nm was obtained, whereas for the self-assembled cavitand, MeCavthioether, the layer thickness was found to be 1.73 ( 0.2. These values correspond to a thickness of approximately two layers of MeCav and one monolayer of MeCavthioether. The selectivity of the response is tested using six different analytes, namely perchloroethylene, nitromethane, acetonitrile, benzene, n-hexane, and toluene by exposing the films to a range of vapor concentrations (02000 ppm). Figure 2 shows the selectivity patterns at a fixed concentration of 110 ppm for six different analytes. It is observed that the C18-thiol monolayer does not show any significant response to any of the vapors, as is expected. The absence of any noticeable plasmon angle (22) Feresenbet, E.; Dalcanale, E.; Dulcey, C.; Shenoy, D. K. Sens. Actuators, B 2003, in press.
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Figure 3. Selectivity patterns of the cavitands MeCav and MeCav-thioether toward perchloroethylene, acetonitrile, toluene, benzene, and nitromethane vapors. The plasmon angle shift, normalized to the film thickness, is shown on the y axis.
shift indicates that there is insignificant physical adsorption of vapors onto the surface of the film. Intercalation, if any, within the well-packed alkyl chains does not influence the plasmon shift. Both cavitands show good selectivity for each of the three analytes, namely, acetonitrile, nitromethane, and perchloroethylene, relative to n-hexane, benzene, and toluene. The insignificant SPR signal observed when n-hexane is exposed to the cavitand sensing layers could be interpreted to mean that either there is no interaction between this specific analyte and the cavitand structure or that any interaction between n-hexane and, for example, the alkyl tails of the cavitands does not give rise to a measurable signal. It is likely that the latter case is true because van der Waals interaction between n-hexane and the alkyl tails may be preferred over interaction within the cavities. With the aromatic vapors, the low selectivity is presumably due to the inability to form guest-host complexes within the small cavities. We have recently shown that aromatic vapors can be selectively complexed with cavitands that have deeper cavities.22 To make a one-to-one comparison of the selectivity arising from the two cavitands (and, therefore, the effect of the two deposition processes on the signal response), plasmon angle shifts normalized to the thickness of the two types of cavitand layers are shown in Figure 3. It is observed that, to within the error bars, the response is
Notes
almost the same. It is assumed that with the spin-coated film, despite the inherent disorder of the film, the analyte molecules presumably diffuse through the ultrathin film to find the cavities to form host-guest complexes. Nevertheless, it is expected that the cavities are more easily accessible for the formation of complexes with the analyte in the case of the self-assembled film. This might be reflected as differences in the kinetics of the hostguest complex formation in the two types of films. In other words, for the static equilibrium case, where sufficient time is allowed for the analyte vapors to interact with the cavitands, the two types of films do not show any difference. Hence, the signal responses, which is controlled by the formation of host-guest complexes between the cavity and the analyte molecule, when normalized to the thickness, are almost the same. It is interesting to note that the effect of the film morphology on the frequency and attenuation of a polymercoated SAW device exposed to organic vapor showed significant differences in the response depending on the process used for coating.23 It was shown that airbrushcoated devices behaved differently from drop-evaporationcoated devices as a result of differences in the film modulus. Differences in the frequency shifts were observed in the two cases. In our case, since SPR is sensitive only to refractive index changes caused by host-guest complexation, differences in the morphology do not seem to affect the observed signal response. This result has broad implications for chemical sensing because it shows that a widely accepted industrial process may be used without the loss of selectivity due to film morphology. This is especially true because the film thickness, even for the disordered film, is only about 4 nm so that the actual orientation of the cavitands on the surface seems to play less of a role in the selectivity. Acknowledgment. We acknowledge funding from the Naval Research Laboratory and the Joint Services. In addition, E.B.F. gratefully acknowledges a postdoctoral fellowship of the National Research Council. We also wish to acknowledge Dr. C. Dulcey for help with some of the measurements. LA0350857 (23) Grate, W. J.; Klusty, M.; McGill, R. A.; Abraham, H. M.; Whiting, G.; Haftvan, J. A. Anal. Chem. 1992, 64, 610-624.
