J. Phys. Chem. B 2006, 110, 17161-17166
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A Substrate-Independent Approach for Cyclodextrin Functionalized Surfaces W. C. E. Schofield, J. D. McGettrick, and J. P. S. Badyal* Department of Chemistry, Science Laboratories, Durham UniVersity, Durham DH1 3LE, England, UK ReceiVed: May 16, 2006; In Final Form: June 26, 2006
6-Amino-6-deoxy-β-cyclodextrin can be immobilized onto a range of solid surfaces via reaction with a predeposited pulsed plasma poly(glycidyl methacrylate) layer. X-ray photoelectron spectroscopy, infrared spectroscopy, and quartz crystal microbalance measurements have been employed to monitor guest-host interactions between N,N-dimethylformamide or cholic acid and surface-tethered 6-amino-6-deoxy-βcyclodextrin barrels.
1. Introduction Small molecule detection in solution by solid surface bound receptor molecules is an important goal in chemosensor and biosensor development, drug delivery, chromatography, solubility enhancement, enzyme modeling, artificial catalysts, and the selective removal of undesired substances.1,2 Robust supramolecular structures exhibiting a distinct affinity for small molecules by forming inclusion complexes are considered to be among the most versatile chemical strategies for this purpose. In particular, naturally occurring cyclodextrins have gained widespread attention as affinity molecules over the last few decades.3-6 Alternative surface methods for detecting small molecules include using hydrogels,7,8 chromatographic materials,9-11 and patterned molecular arrays. 12,13 Unfortunately, such approaches suffer from drawbacks such as substrate specificity, low throughputs, and can involve complex and expensive multistep fabrication methods. Cyclodextrins are cyclic (R-1,4)-linked oligosaccharides composed of at least six R-D-glucopyronose subunits forming a toroidal or conical cavity,14 Structure 1. The primary hydroxyl
groups (narrow side of the cavity) and the secondary hydroxyl groups (wider mouth of the cavity) form a complex network of intramolecular hydrogen bonds to give a hydrophobic interior and a hydrophilic external surface.15 This means that the hydrophobic cavity of the cyclodextrin host is capable of forming inclusion complexes with small hydrophobic guest molecules. Binding specificity depends predominantly on the guest molecule size and geometry and is principally noncovalent in nature (e.g., van der Waals forces, hydrogen bonding, or hydrophobic interactions).1,16 Consequently, cyclodextrin and its derivatives are suitable for a wide variety of potential uses, e.g., for stabilization, masking, or controlled release of hydrophobic * To whom correspondence should be addressed. E-mail: j.p.badyal@ durham.ac.uk.
substances,17 new HPLC stationary phases,18 future electronic devices,19 high-density data storage20 on the nanometer scale, and drug delivery.21 In the case of efficiently utilizing cyclodextrins as sensor elements supported on solid surfaces, the prerequisites are suitable orientation and ease of accessibility into the hydrophobic barrel. Previous attempts to form oriented solid supported monolayers of cyclodextrins include Langmuir-Blodgett films22,23 and selfassembled monolayers (SAMS) of thiolated cyclodextrin derivatives on gold substrates.24-28 These approaches have gained limited success due to their inherent complexities and requirement for specific solid substrates. In this article we describe a simple two-step substrate-independent method for the immobilization of 6-amino-6-deoxy-β-cyclodextrin molecules onto solid surfaces. This entails pulsed plasma deposition of poly(glycidyl methacrylate) thin films,29 followed by reacting the primary amine groups of 6-amino-6-deoxy-β-cyclodextrin with epoxide centers on the surface, Scheme 1. The major advantage of pulsed plasma polymerization is that it is a straightforward and effective method for functionalizing solid surfaces (single-step, solventless, and substrate independent). This constitutes the generation of active sites (predominantly radicals) at the surface and in the electrical discharge during the duty cycle on-period, followed by conventional polymerization reaction pathways proceeding during each extinction period. Typical time scales are of the order of µs and ms, respectively. The level of surface functionalization can be tailored by simply preprogramming the pulsed plasma duty cycle. Functional films containing high levels of anhydride,30 carboxylic acid,31 cyano,32 epoxide,29 hydroxyl,33 furfuryl,34 thiol,35 perfluoroalkyl,36 perfluoromethylene,37 or trifluoromethyl38 groups have been successfully prepared in the past using this methodology. Here we describe guest-host interactions between small molecules (e.g., N,N-dimethylformamide and cholic acid) and cyclodextrin barrels immobilized onto glycidyl methacrylate pulsed plasma polymer films. These guest molecules have been chosen for their appropriate size, geometry, structural features, and binding constants, which are necessary for easy host-guest inclusion complex formation with cyclodextrins.39-42 The resulting inclusion complexes have been examined by X-ray photoelectron spectroscopy (XPS), infrared spectroscopy, and quartz crystal microbalance (QCM). 2. Experimental Section Pulsed plasma polymerization of glycidyl methacrylate (Aldrich, +97%, purified using several freeze-pump-thaw cycles)
10.1021/jp0629801 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/09/2006
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SCHEME 1: Immobilization of 6-Amino-6-deoxy-β-cyclodextrin onto Pulsed Plasma Deposited Poly(glycidyl methacrylate) via Amine Functionalities on the Cyclodextrin Barrel
(Some amine groups on the barrel remain unreacted because they are facing away from the surface.)
was carried out in an electrodeless cylindrical glass reactor (5 cm diameter, 520 cm3 volume, base pressure of 1 × 10-3 mbar, and with a leak rate better than 1.8 × 10-9 kg s-1) enclosed in a Faraday cage. The chamber was fitted with a gas inlet, a Pirani pressure gauge, a 30 L min-1 two-stage rotary pump attached to a liquid nitrogen cold trap, and an externally wound copper coil (4 mm diameter, 9 turns, spanning 8-15 cm from the gas inlet). All joints were grease free. An L-C network was used to match the output impedance of a 13.56 MHz radio frequency (RF) power generator to the partially ionized gas load. The RF supply was triggered by a signal generator and the pulse shape monitored with an oscilloscope. Prior to each experiment, the reactor was cleaned by scrubbing with detergent, rinsing in water and propan-2-ol, followed by oven drying. The system was then reassembled and evacuated. Further cleaning consisted of running an air plasma at 0.2 mbar pressure and 40 W power for 30 min. Next, a polished silicon (100) wafer (MEMC Electronics Materials, cleaned ultrasonically in a 50/50 propan-2-ol/cyclohexane solvent mixture) was inserted into the center of the reactor, and the chamber pumped back down to base pressure. At this stage, glycidyl methacrylate monomer vapor was introduced at a pressure of 0.2 mbar for 5 min prior to ignition of the electrical discharge. The optimum conditions corresponded to a peak power of 40 W, a duty cycle pulse on-time of 20 µs, a pulse off-time equal to 20 ms, and deposition allowed to proceed for 10 min to yield 150 ( 5 nm thick films. Reaction of the epoxide functionalized surfaces with 6-amino6-deoxy-β-cyclodextrin (Carbomer Inc.) entailed immersion of the coated substrate into various concentration solutions (230 µM) of 6-amino-6-deoxy-β-cyclodextrin in high purity water. This gave rise to a range of packing densities at the surface. After incubation for 24 h at 60% humidity, the samples were thoroughly rinsed in high purity water, methanol, and propan2-ol to remove any remaining unreacted 6-amino-6-deoxy-βcyclodextrin. Inclusion complexes between guest N,N-dimethylformamide (Aldrich 99.5%) or cholic acid (Aldrich 98%) molecules with surface immobilized 6-amino-6-deoxy-β-cyclodextrin were prepared by exposing the surface to 100 µM aqueous solutions of the respective guest molecule for 24 h. Subsequent washing with high purity water, methanol, and propan-2-ol removed any nonbound guest molecules. Film thickness measurements were carried out using an nkd6000 spectrophotometer (Aquila Instruments Ltd). The acquired transmittance-reflectance curves (350-1000 nm wavelength range) were fitted to a Cauchy model for dielectric materials using a modified Levenburg-Marquardt method.43
Figure 1. Infrared spectra of: (a) glycidyl methacrylate pulsed plasma polymer layer (Pcw ) 40 W; ton ) 20 ms; toff ) 20 ms; 10 min), (b) 6-amino-6-deoxy-β-cyclodextrin, and (c) 20 µM solution of 6-amino6-deoxy-β-cyclodextrin reacted with glycidyl methacrylate pulsed plasma polymer.
X-ray photoelectron spectroscopy (XPS) analysis of the films was undertaken on a VG ESCALAB system. The instrument was equipped with an unmonochromated Mg KR X-ray source (1253.6 eV) and a hemispherical analyzer operating in the constant analyzer energy mode (CAE, pass energy ) 20 eV). XPS core level spectra were fitted using Marquardt minimization computer software assuming a linear background and equal full width at half-maximum (fwhm) for all Gaussian component peaks.44 Elemental concentrations were calculated using instrument sensitivity (multiplication) factors determined from chemical standards, C(1s)/O (1s)/N (1s) ) 1.0:0.45:0.67. The absence of any Si(2p) signal from the underlying silicon substrate was taken as being indicative of a pinhole free plasma polymer film coverage of a thickness exceeding the XPS sampling depth (2-5 nm).45,46 Fourier transform infrared (FTIR) analysis of the films at each stage of reaction was carried out using a Perkin-Elmer Spectrum One spectrometer equipped with a liquid nitrogen cooled MCT detector operating across the 700-4000 cm-1 range. Reflection-absorption (RAIRS) measurements were performed using a variable angle accessory (Specac) set at 66° with a KRS-5 polarizer fitted to remove the s-polarized component. All spectra were averaged over 256 scans at a resolution of 4 cm-1. Finally, real-time in situ guest-host interactions were followed by exposure of dimethylformamide vapor at 0.2 mbar pressure for 3 min to a quartz crystal detector (Varian model 985-7013 using a 5 MHz AT-cut quartz crystal with a diameter of 13 mm), which had been coated with pulsed plasma poly(glycidyl methacrylate) and 6-amino-6-deoxy-β-cyclodextrin. Mass readings were taken every 7.5 s during exposure and for 1 min thereafter. 3. Results 3.1. Surface Immobilization of 6-Amino-6-deoxy-β-cyclodextrin. Infrared spectroscopy was used to probe the molecular structure of the glycidyl methacrylate pulsed plasma polymer layer and subsequent probe molecule attachment, Figure 1. The dominant features for the pulsed plasma poly(glycidyl meth-
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TABLE 1: XPS Atomic Percentages for Surface Functionalized Glycidyl Methacrylate Pulsed Plasma Polymer Films (pppGMA)* composition (%) sample
%C
theoretical GMA 70.0 pppGMA 72.9 ( 0.1 theoretical 6-amino-6-deoxy-β-CD 54.5 monolayer pppGMA/6-amino-6-deoxy-β-CD 55.4 ( 0.1 (20 µM) pppGMA/6-amino-6-deoxy-β-CD/ 63.8 ( 0.1 cholic acid
%O
%N
30.0 27.1 ( 0.1 36.4
9.1
35.2 ( 0.1
8.6 ( 0.5
31.9 ( 0.1
4.3 ( 0.5
*N,N-dimethylformamide desorbs under vacuum, hence no data.
acrylate) film were assigned as follows:47 epoxide ring C-H stretching (3063 cm-1), C-H stretching (3000-2900 cm-1), carbonyl stretching (1738 cm-1), epoxide ring breathing (1253 cm-1), antisymmetric epoxide ring deformation (908 cm-1), and symmetric ring deformation (842 cm-1). This verifies the high level of epoxide group retention at the surface.29 Derivatization with 6-amino-6-deoxy-β-cyclodextrin gave rise to the appearance of several new bands at 1582, 1160, 1085, 1045, 840, and 764 cm-1 associated with 6-amino-6-deoxy-β-cyclodextrin.48 The carbonyl stretching mode (1738 cm-1) of the pulsed plasma polymer was noted to have shifted to 1728 cm-1 and become broader. Also, the 6-amino-6-deoxy-β-cyclodextrin amine NH2 scissoring and wagging deformation bands (1582 and 840 cm-1, respectively) were significantly reduced (by 60% relative to the 6-amino-6-deoxy-β-cyclodextrin CH2 deformation (764 cm-1)) as a consequence of reaction (irrespective of the concentration of 6-amino-6-deoxy-β-cyclodextrin employed). This attenuation in primary amine groups corresponds to their consumption during link formation between individual 6-amino-6-deoxy-βcyclodextrin barrels and the epoxide functionalized surface. The remaining NH2 scissoring and wagging deformation bands, following reaction, correspond to the unreacted NH2 groups on the immobilized cyclodextrin barrels that were facing away from the surface. The broad band centered around 3500 cm-1 corresponds to O-H and N-H stretching from the cyclodextrin molecules. XPS analysis of the pulsed plasma poly(glycidyl methacrylate) films before and after 6-amino-6-deoxy-β-cyclodextrin derivatization confirmed the presence of carbon, oxygen, and nitrogen at the surface (with no Si(2p) signal showing through from the underlying substrate), Table 1. The C(1s) XPS envelope of the pulsed plasma poly(glycidyl methacrylate) film was peak fitted in accordance with reference spectra previously obtained for conventional solution phase polymerized glycidyl methacrylate:49 CxHy (285.0 eV), C(CH3)(CdO)O (285.7 eV), O-CH2-CO (286.7 eV), epoxide carbons (287.2 eV), C(dO)O (289.1 eV), Figure 2. Reaction with 6-amino-6-deoxy-β-cyclodextrin gave rise to an enhancement of the C-O (286.7 eV) component,50 an attenuation of the hydrocarbon (285.0 eV) and methacrylate ester group (289.1 eV) C(1s) signals, as well as an increase in surface oxygen and nitrogen concentrations, Figure 2 and Table 1. The N(1s) peak at 399.8 eV can be taken as being unequivocal confirmation of 6-amino-6-deoxy-β-cyclodextrin attachment to the glycidyl methacrylate pulsed plasma polymer surface via epoxide ring opening. The packing density of the 6-amino-6-deoxy-β-cyclodextrin barrels at the surface could be varied by solution dilution, Figure 3. Concentrations exceeding 20 µM (of the 6-amino-6-deoxy-β-cyclodextrin) yielded a surface saturation level (5.34 × 1013 molecules/cm2) which is equivalent to 95% of the calculated theoretical monolayer coverage (5.65 × 1013 molecules/cm2), Table 1. The theo-
Figure 2. C(1s) XPS spectra of: (a) glycidyl methacrylate pulsed plasma polymer layer (Pcw ) 40 W; ton ) 20 ms; toff ) 20 ms; 10 min), (b) 5 µM solution of 6-amino-6-deoxy-β-cyclodextrin reacted with glycidyl methacrylate pulsed plasma polymer, and (c) 20 µM solution of 6-amino-6-deoxy-β-cyclodextrin reacted with glycidyl methacrylate pulsed plasma polymer.
Figure 3. Nitrogen concentration (% N) at the surface of the glycidyl methacrylate plasma polymer layer following 6-amino-6-deoxy-βcyclodextrin functionalization as a function of solution concentration.
retical monolayer coverage level is based on using a β-cyclodextrin surface area footprint of 1.77 nm,2,51 with the wide side of the barrel aligned parallel to the surface so as to facilitate guest-host molecule interactions, Scheme 1. Dilutions below 20 µM correspond to submonolayer coverage and tie in with a reappearance of the methacrylate group peak (C(dO)O at 289.1 eV) in the C(1s) XPS envelope, Figure 2. 3.2. N,N-Dimethylformamide Probe Molecule Detection. Exposure of 6-amino-6-deoxy-β-cyclodextrin functionalized surfaces to N,N-dimethylformamide (DMF) gave rise to several new prominent peaks in the infrared spectra at 1667, 1376, 1257, and 1092 cm-1, Figure 4. These correlate to characteristic N,Ndimethylformamide molecule absorbances at 1676 cm-1 (amide CdO), 1387 cm-1 (aldehyde HCO rocking), 1257 cm-1 (C-N stretching (amide III)), and 1092 cm-1 (antisymmetric NC3 stretching), respectively.45 The slight shift toward lower wavenumbers for the 1676 and 1387 cm-1 peaks upon binding to 6-amino-6-deoxy-β-cyclodextrin signify the impact of intermolecular H-bond formation upon the carbonyl stretch and the HCO rocking deformation within the N,N-dimethylformamide molecule,52 respectively. Also, within the fingerprint region of the 6-amino-6-deoxy-β-cyclodextrin molecule, two of the major
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Figure 4. Infrared spectra of (a) N,N-dimethylformamide, (b) 20 µM solution of 6-amino-6-deoxy-β-cyclodextrin onto glycidyl methacrylate pulsed plasma polymer, and (c) following exposure of (b) to 100 µM solution of N,N-dimethylformamide.
Schofield et al.
Figure 6. Infrared spectrum of (a) 6-amino-6-deoxy-β-cyclodextrin tethered onto glycidyl methacrylate pulsed plasma polymer, (b) 6-amino-6-deoxy-β-cyclodextrin tethered onto glycidyl methacrylate pulsed plasma polymer and exposed to a 100 µM solution of cholic acid, and (c) cholic acid.
by surface immobilized 6-amino-6-deoxy-β-cyclodextrin rings in real time, Figure 5. The mass detected by the QCM increased rapidly upon exposure of N,N-dimethylformamide to the glycidyl methacrylate-6-amino-6-deoxy-β-cyclodextrin functionalized surface, reaching saturation at approximately t ) 55 s. Upon termination of the N,N-dimethylformamide feed, followed by evacuation, the observed drop in mass reading corresponds to a loss of N,N-dimethylformamide molecules from the 6-amino6-deoxy-β-cyclodextrin rings under vacuum. A control experiment using the underivatized glycidyl methacrylate plasma polymer surface displayed very little interaction with the N,Ndimethylformamide probe molecule. A small increase in mass was detected, but this was easily lost upon evacuation. Based on the full monolayer coverage of 6-amino-6-deoxy-β-cyclodextrin (5.34 × 1013 molecules/cm2) determined by XPS, the quartz crystal microbalance measurements indicated that approximately 4.75 × 1013 6-amino-6-deoxy-β-cyclodextrin molecules/cm2 (approximately 89%) on the surface are capable of hosting N,N-dimethylformamide molecules. 3.3. Cholic Acid Inclusion Complex. The infrared spectrum of cholic acid, Structure 2, displays two characteristic regions: Figure 5. Quartz crystal microbalance measurements of: (a) glycidyl methacrylate pulsed plasma polymer layer (Pcw ) 40 W; ton ) 20 ms; toff ) 20 ms; 10 min) and (b) 6-amino-6-deoxy-β-cyclodextrin functionalized onto glycidyl methacrylate pulsed plasma polymer exposed to N,N-dimethylformamide vapor at t ) 30 s.
C-O ether linkage absorbances centered at 1085 and 1045 cm-1 have dropped in intensity. This can be explained on the basis of the fact that when a N,N-dimethylformamide guest molecule enters the cyclodextrin barrel, new intermolecular hydrogen bonds are formed with the C-O-C centers contained in the barrel,53 accompanied by a partial disruption of the complex network of cyclodextrin intramolecular H-bonds. The changes seen within the fingerprint region (1500-700 cm-1) are a manifestation of inclusion complex formation.54 Quartz crystal microbalance (QCM) measurements were able to track the entrapment of N,N-dimethylformamide molecules
the carbonyl (CdO) stretching mode of the cholate group (1712 cm-1), and a complicated fingerprint region 1500-700 cm-1, Figure 6. These bands changed upon complex formation with surface tethered 6-amino-6-deoxy-β-cyclodextrin. The carbonyl stretching mode of the cholic acid splits between 1712 and 1736 cm-1. The latter has previously been reported for the hydrophilic side group of the cholic acid inserting into the 6-amino-6-deoxy-
Approach for Cyclodextrin Functionalized Surfaces β-cyclodextrin cavity, while the remainder of the cholic acid molecule (hydrophobic portion) remains outside the cavity, Structure 3.55-57 Within the infrared fingerprint region, several
characteristic bands of the 6-amino-6-deoxy-β-cyclodextrin derivatized pulsed plasma poly(glycidyl methacrylate) film are also prominent; these include carbonyl groups at 1728 cm-1 and the ether linkages (C-O-C) at 1060 and 1045 cm-1. Other evidence for cholic acid 6-amino-6-deoxy-β-cyclodextrin complexation on the surface was noted from a decrease in XPS oxygen concentration, Table 1. It is of interest to note that the cholic acid guest molecule could only be removed from the inclusion complex by washing with a 20 µM aqueous solution of β-cyclodextrin. 4. Discussion Derivatization of epoxide functionalized surfaces with nucleophilic reagents29 (e.g., carboxylic acids, amines, alcohols, etc.) typically occurs via electrophilic attack leading to the epoxide ring opening.58 In the case of 6-amino-6-deoxy-β-cyclodextrin, reaction with the less substituted epoxide carbon center is predicted to yield the secondary alcohol,59 Scheme 1. The number of 6-amino-6-deoxy-β-cyclodextrin molecules immobilized onto the glycidyl methacrylate pulsed plasma polymer layer (as seen by XPS) corresponds to 95% monolayer coverage based upon theoretical surface packing, Table 1 and Figure 3. This should be contrasted with the average attachment of only 4.2 out of the 7 6-amino-6-deoxy-β-cyclodextrin primary amine centers (approximately 60%) on each barrel to the pulsed plasma poly(glycidyl methacrylate) surface calculated from infrared measurements, Figure 1. This disparity can in part be ascribed to the large difference in size between 6-amino-6-deoxy-βcyclodextrin (approximately 1.77 nm2)51 and the glycidyl methacrylate units (approx 0.2 nm2), leading to a steric mismatch.60 Another factor may be that some of the glycidyl methacrylate units reside in the subsurface and are therefore chemically inaccessible. However, the high surface packing density denoted by XPS analysis is confirmative proof that the cyclodextrin barrels are well tethered to the substrate. Although less than the theoretical maximum number of linkages (as seen by infrared spectroscopy) are indicated, in fact the shortfall will contribute toward an inherent steric flexibility at the surface, thereby making guest molecule accessibility into the immobilized 6-amino-6-deoxy-β-cyclodextrin barrels far easier in comparison to previous gold-thiol self-assembled monolayer systems.28,61 Namely, it is a matter of steric accessibility: the incomplete attachment (4.2 out of a possible 7 linkages) and the inherent flexibility of the polymeric layer allow for a greater range of surface orientations, whereas the rigidity of thiol systems caters to only one possible orientation.
J. Phys. Chem. B, Vol. 110, No. 34, 2006 17165 The modus operandi of such pulsed plasma poly(glycidyl methacrylate) immobilized cyclodextrin molecules as hosts for guest molecules has been exemplified using N,N-dimethylformamide, Figures 4 and 5. Selectivity for detection of small molecules is dependent upon the guest molecule size, geometry, structural features, and binding constants.39,40 Docking comprises the N,N-dimethylformamide molecule undergoing hydrogen bonding via its aldehyde hydrogen atom to the glycoside oxygen centers present in 6-amino-6-deoxy-β-cyclodextrin62 as well as hydrogen bonding taking place between the aldehyde oxygen atom in N,N-dimethylformamide and hydroxyl groups belonging to 6-amino-6-deoxy-β-cyclodextrin. In the case of cholic acid, a detailed knowledge of the binding mechanism is unclear63 because the cholate group is able to enter and bind to the asymmetric 6-amino-6-deoxy-β-cyclodextrin from both ends (hydrophobic and hydrophilic sides). For instance, it is known that bile acids and salts such as cholic acid enter into the cavity of β-cyclodextrin either with the A-ring of the steroid nucleus64 or with the carboxylate group (tail).65-68 The enthalpy (∆H) values for both types of inclusion complex are quite similar and therefore each is just as likely.2,68 Finally, the general applicability of the outlined approach was exemplified by successfully repeating these experiments using a range of substrate materials and geometries (e.g., polyethylene film, polystyrene microbeads, cotton fabric, and glass). 5. Conclusions 6-Amino-6-deoxy-β-cyclodextrin can be tethered to solid surfaces coated with glycidyl methacrylate pulsed plasma polymer thin films. These structures are shown to be highly efficient at hosting guest molecules such as N,N-dimethylformamide and cholic acid. A distinct advantage of this approach is that this methodology is applicable to a whole range of substrate materials and geometries. Acknowledgment. We thank Professor D. Parker FRS for helpful discussions. J.P.S. Badyal is grateful to the EPSRC for an Advanced Research Fellowship. References and Notes (1) Wenz, G. Angew. Chem., Int. Ed. Engl. 1994, 22, 803. (2) Szejtli, J. Supramol. Chem. 1995, 6, 217. (3) Bodenhofer, K.; Hierlamann, A.; Juza, M.; Schurig, V.; Gopel, W. Anal. Chem. 1997, 69, 4017. (4) Hierlamann, A.; Ricco, A. J.; Bodenhofer, K.; Juza, M.; Gopel, W. Anal. Chem. 1999, 71, 3022. (5) Kataky, R.; Parker, D. Electrochem. Soc. Proc. 1997, 19, 524. (6) Lucklum, R.; Henning, B.; Hauptmann, P.; Schierbaum, K. D.; Vaihinger, S.; Gopel, W. Sens. Actuators, A 1991, 27, 705. (7) Kim, K.; Nayak, S.; Lyon, L. A. J. Am. Chem. Soc. 2005, 127, 9588. (8) Datta, A.; Das, S.; Mandal, D.; Pal, S. K.; Bhattacharyya, K. Langmuir 1997, 13, 6922. (9) Sibrian-Vazquez, M.; Spivak, D. A. J. Am. Chem. Soc. 2004, 126, 7827. (10) Jaraniec, C. P.; Gilpin, R. K.; Jaraniec, M. J. Phys. Chem. B 1997, 101, 6861. (11) Fois, E.; Gamba, A. J. Phys. Chem. B 1997, 101, 4487. (12) Hergenrother, P. J.; Depew, K. M.; Schreiber, S. L. J. Am. Chem. Soc. 2000, 122, 7849. (13) Burmeister, J. J. Gerhardt, G. A. Anal. Chem. 2001, 73, 1037. (14) Kellersberger, K. A.; Dejsupa, C.; Liang, Y.; Pope, R. M.; Dearden, D. V. Int. J. Mass Spectrom. 1999, 199, 181. (15) Szejtli, J. Cyclodextrin Technology; Kluwer Academic: Dordrecht, The Netherlands, 1988. (16) Li, S.; Purdy, W. C. Chem. ReV. 1992, 92, 1457. (17) Parrish, M. A. Spec. Chem. 1987, 7, 366. (18) Janus, L.; Carbonnier, B.; Deratani, A.; Bacquet, M.; Crini, G.; Laureyns, J.; Morcellet, M. New J. Chem. 2003, 27, 307. (19) Gittins, D. I.; Bethell, D.; Schiffrin, D. J.; Nichols, R. J. Nature 2000, 408, 67.
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