Photochemical Imprint of Molecular Recognition Sites in Two

Langmuir 2001, 17, 7387-7395

7387

Photochemical Imprint of Molecular Recognition Sites in Two-Dimensional Monolayers Assembled on Au Electrodes: Effects of the Monolayer Structures on the Binding Affinities and Association Kinetics to the Imprinted Interfaces1 Michal Lahav, Eugenii Katz, and Itamar Willner* Institute of Chemistry, The Farkas Center for Light-Induced Processes, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received July 25, 2001 Photochemical imprint of molecular recognition sites for phenoxynaphthacene quinone (1a) in twodimensional monolayers assembled on Au surfaces is accomplished. The photochemical imprint of the molecular recognition sites involves several steps: The primary step includes the assembly of the transphenoxynaphthacene quinone monolayer (2a), followed by the rigidification of the monolayer with longchain alkanethiols that generates a densely packed quinone monolayer. The second process involves the photoisomerization of the monolayer to the ana-quinone state (2b), followed by the nucleophilic displacement of the quinone with butylamine. The association of 1a to the imprinted sites and the dissociation of the host substrate from the sites are followed by electrochemical means as well as by microgravimetric quartzcrystal-microbalance measurements. The binding of 1a to the imprinted recognition sites reveals selectivity, and structurally related substrates do not associate to the imprinted sites. The kinetics of association of 1a to the imprinted sites and of the dissociation of 1a from the sites is affected by the length of the rigidifying alkanethiols. As the alkanethiol is longer, the association of 1a to the sites and the dissociation of 1a from the recognition sites is slower. The slower association of 1a to perforated monolayers with a long-chain alkanethiol (C18H37SH) is attributed to the blocking of the imprinted sites by the flexible alkanethiol chains. The retardation of the dissociation of 1a from the imprinted sites rigidified by the long-chain alkanethiols is attributed to the capping of the substrate by the long-chain thiols. The selectivity of the imprinted recognition sites is attributed to a structural fit of 1a to the imprinted hydrophobic contour and the synergetic stabilization of 1a in the site by complementary H-bonds.

Introduction The functionalization of electrodes with redox-active or photoactive monolayers attracts substantial recent research activities for the development of new electronic2 and optoelectronic3 devices. The assembly of ordered redoxactive components on electrodes was used to organize electronic elements,4 that is, rectifiers or amplifiers, or to develop sensing interfaces.5 Photochemical activation of redox functionalities in monolayer assemblies on electrodes was used for the electronic transduction of photonic * To whom correspondence should be addressed. Tel: 972-26585272. Fax: 972-2-6527715. E-mail: [email protected]. (1) For a preliminary report, see: Lahav, M.; Katz, E.; Doron, A.; Patolsky, F.; Willner, I. J. Am. Chem. Soc. 1999, 121, 862-863. (2) (a) Gitting, D. I.; Bethell, D.; Schiffrin, D. J.; Nichols, R. J. Nature 2000, 408, 67-69. (b) Paardo-Yissar, V.; Katz, E.; Willner, I.; Kotlyar, A. B.; Sanders, C.; Lill, H. Faraday Discuss. 2000, 116, 119-134. (3) (a) Motesharei, K.; Myles, D. C. J. Am. Chem. Soc. 1998, 120, 7328-7336. (b) Willner, I.; Willner, B. J. Mater. Chem. 1998, 8, 25432556. (c) Willner, I.; Willner, B. Adv. Mater. 1997, 9, 351-355. (d) Ayadim, M.; Jiwan, J. L. H.; DeSilva, A. P.; Soumillion, J. P. Tetrahedron Lett. 1996, 37, 7039-7049. (e) Potyrailo, R. A.; Conrad, R. C.; Ellington, A. D.; Hieftje, G. M. Anal. Chem. 1998, 70, 3419-3425. (f) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Chem. Commun. 1999, 22292230. (4) (a) Katz, E.; Heleg-Shabtai, V.; Willner, I.; Rau, H. K.; Haehnel, W. Angew. Chem., Int. Ed. Engl. 1998, 37, 3253-3256. (b) Willner, I.; Heleg-Shabtai, V.; Katz, E.; Rau, H. K.; Haehnel, W. J. Am. Chem. Soc. 1999, 121, 6455-6468. (5) (a) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Mater. 2000, 12, 1315-1328. (b) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688-691. (c) Lahav, M.; Katz, E.; Willner, I. Electroanalysis 1998, 10, 1159-1162. (d) Beulen, M. W. J.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Chem. Commun. 1999, 503-504. (e) Zhang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1993, 9, 786-791. (f) Liu, Y.; Zhao, M.; Bergbreiter, D. E.; Crook, R. M. J. Am. Chem. Soc. 1997, 119, 8720-8721.

signals.6-8 Molecular architectures that duplicate functions of logic gates6 or machinery functionalities8 were developed. Immobilization of functional monolayers on electronic transducers provides a means to tailor specific sensing interfaces and sensor devices. Assembly of molecular receptors on electrodes such as calixarenes,9 crown ethers,10 cyclodextrins,11 or π-acceptors12 was used to concentrate specific analytes at the receptor sites and for the electronic transduction of the respective sensing processes. Charged monolayer interfaces were used to electrostatically attract or repel charged redox species to (6) (a) Collier, C. P.; Mattersteig, G.; Wong, E.; Luo, Y.; Beverly, K.; Sampaio, J.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science 2000, 289, 1172-1175. (b) Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391-394. (c) Wong, E. W.; Collier, C. P.; Behloradsky, M.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. J. Am. Chem. Soc. 2000, 122, 5831-5840. (d) Doron, A.; Portnoy, M.; Lion-Dagan, M.; Katz, E.; Willner, I. J. Am. Chem. Soc. 1996, 118, 8937-8944. (e) Doron, A.; Katz, E.; Portnoy, M.; Willner, I. Angew. Chem., Int. Ed. Engl. 1996, 35, 15351537. (7) (a) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658-660. (b) Nakashima, N.; Nakanishi, T.; Nakatani, A.; Deguchi, Y.; Murakami, H.; Sagara, T.; Irie, M. Chem. Lett. 1997, 591-592. (c) Cai, S.-M.; Inokuchi, H.; Fujishima, A.; Liu, Z.-F. Langmuir 1996, 12, 28432848. (d) Doron, A.; Katz, E.; Tao, G.; Willner, I. Langmuir 1997, 13, 1783-1790. (8) (a) Willner, I.; Pardo-Yissar, V.; Katz, E.; Ranjit, K. R. J. Electroanal. Chem. 2001, 497, 172-177. (b) Shipway, A. N.; Willner, I. Acc. Chem. Res. 2001, 34, 421-432. (c) Bissell, R. A.; Cordova, E.; Kaifer, A. E.; Stoddart, J. F. Nature 1994, 369, 133-137. (9) (a) Zhang, L. T.; Godinez, L. A.; Lu, T.; Gokal, G. W.; Kaifer, A. E. Angew. Chem., Int. Ed. Engl. 1995, 34, 235-238. (b) Dermody, D. L.; Crooks, R. M.; Kim, T. J. Am. Chem. Soc. 1996, 118, 11912-11917. (c) Dermody, D. L.; Lee, Y.; Kim, T.; Crooks, R. M. Langmuir 1999, 15, 8435-8440. (d) Cygan, M. T.; Collins, G. E.; Dunbar, T. D.; Allara, D. L.; Gibbs, C. G.; Gutsche, C. D. Anal. Chem. 1999, 71, 142-148.

10.1021/la011172s CCC: $20.00 © 2001 American Chemical Society Published on Web 10/18/2001

7388

Langmuir, Vol. 17, No. 23, 2001

and from the electrode, thus enabling the selective electrochemical detection of the attracted redox component.13,14 Molecular imprinting15,16 provides a general method to tailor specific recognition sites in organic polymers17 or inorganic matrixes.18 The method involves the polymerization of matrixes around a template target molecule, followed by the depletion of the template from the respective matrixes to leave selective recognition sites of the template contours. Molecular imprinted polymers were used for chromatographic separations,19 including chiral separations,20 as well as for the specific sensing of imprinted substrates.21 The use of imprinted polymer matrixes for the selective sensing of the imprinted substrate suffers from diffusion limitation of the analyte through the polymer matrixes, leading to long response times. Thus, the imprint of the recognition sites in thin films or even monolayers would reveal a significant advance in sensing processes. Recently, imprinted recognition sites in thin-film assemblies associated with the gate interface of field-effect transistors were employed for the selective sensing of chloroaromatic acids.22 The use of two-dimensional imprinted monolayers assembled on electronic transducers seems to be the optimal nano(10) (a) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Phys. Chem. B 1999, 103, 6515-6520. (b) Flink, S.; Boukamp, B. A.; van den Berg, A.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 120, 4652-4657. (11) (a) Lee, J.-Y.; Park, S.-M. J. Phys. Chem. B 1998, 102, 99409945. (b) Lahav, M.; Ranjit, K. T.; Katz, E.; Willner, I. Chem. Commun. 1997, 259-260. (c) Rojas, M. T.; Ko¨niger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336-343. (12) (a) Shipway, A. N.; Lahav, M.; Blonder, R.; Willner, I. Chem. Mater. 1999, 11, 13-15. (b) Lahav, M.; Shipway, A. N.; Willner, I. J. Chem. Soc., Perkin Trans. 2 1999, 1925-1931. (c) Shipway, A. N.; Lahav, M.; Willner, I. Adv. Mater. 2000, 12, 993-998. (d) Lahav, M.; Shipway, A. N.; Willner, I.; Nielsen, M. B.; Stoddart, J. F. J. Electroanal. Chem. 2000, 482, 217-221. (e) Kharitonov, A. B.; Shipway, A. N.; Willner, I. Anal. Chem. 2001, 71, 5441-5443. (13) (a) Malem, F.; Mandler, D. Anal. Chem. 1993, 65, 37-41. (b) Doron, A.; Katz, E.; Tao, G.; Willner, I. Langmuir 1997, 13, 1783-1790. (14) Yissar-Pardo, V.; Katz, E.; Lioubashevski, O.; Willner, I. Langmuir 2001, 17, 1110-1118. (15) (a) Haupt, K.; Mosbach, K. Chem. Rev. 2000, 100, 2495-2504. (b) Sellergren, B. Trends Anal. Chem. 1997, 6, 310-320. (c) Shea, K. J. Trends Polym. Sci. 2 1994, 166-173. (d) Wulff, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1812-1832. (16) (a) Ramstro¨m, O.; Ansell, R. J. Chirality 1998, 10, 195-209. (b) Brady, A. P.; Sanders, J. K. M. Chem. Soc. Rev. 1997, 26, 327-336. (c) Mosbach, K. Trends Biochem. Sci. 1994, 9-14. (d) Wulff, G. In Polymeric Reagents and Catalysis; Ford, W. T., Ed.; American Chemical Society: Washington, DC, 1986; pp 186-230. (17) (a) Mosbach, K.; Ramstro¨m, O. Bio/Technology 1996, 14, 163170. (b) Klein, J. U.; Whitcombe, M. J.; Mulholland, F.; Vulfson, E. N.; Evgeny, N. Angew. Chem., Int. Ed. Engl. 1999, 38, 2057-2060. (c) Sellergren, B.; Shea, K. J. J. Chromatogr. 1993, 635, 31-49. (d) O’Shannessy, D. J.; Ekberg, B.; Mosbach, K. Anal. Biochem. 1989, 177, 144-159. (e) Sellergren, B.; Lepisto¨, M.; Mosbach, K. J. Am. Chem. Soc. 1988, 110, 5852-5860. (18) (a) Sakata, K.; Kunitake, T. Chem. Lett. 1989, 2159-2162. (b) Beckett, A. H.; Anderson, P. Nature 1957, 179, 1074-1075. (c) Lee, S. W.; Ichinose, I.; Kunitake, T. Langmuir 1998, 14, 2857-2863. (d) Ishinose, I.; Senzu, H.; Kunitake, T. Chem. Mater. 1997, 9, 1296-1298. (e) Makote, R.; Collison, M. M. Chem. Mater. 1998, 10, 2440-2445. (19) (a) Sellergren, B. J. Chromatogr., A 1994, 673, 133-141. (b) Schweitz, L.; Andersson, L. I.; Nilsson, S. Anal. Chem. 1997, 69, 133141. (c) Nilsson, S.; Schweitz, L.; Andersson, L. I. Electrophoresis 1997, 18, 884-890. (d) Bru¨ggemann, O.; Freitage, R.; Whitcombe, M. J.; Vulfson, E. N. J. Chromatogr., A 1997, 781, 43-53. (20) (a) Matsui, J.; Miyoshil, Y.; Matsui, R.; Takeuchi, T. Anal. Sci. 1995, 11, 1017-1019. (b) Sellergren, B. In A Practical Approach to Chiral Separation by Liquid Chromatography; Subramanian, G., Ed.; VCH: Weinheim, 1994; pp 69-93. (c) Bru¨ggemann, O.; Haupt, K.; Ye, L.; Yilmaz, E.; Mosbach, K. J. Chromatogr., A 2000, 889, 15-24. (21) (a) Wang, W.; Gao, S.; Wang, B. Org. Lett. 1999, 8, 1209-1212. (b) Malitesta, C.; Losito, I.; Zambonin, P. G. Anal. Chem. 1999, 71, 1366-1370. (c) Liang, C.; Peng, H.; Bao, X.; Nie, L.; Yao, S. Analyst 1999, 124, 1781-1785. (d) Ji, H. S.; McNiven, S.; Ikebukuro, K.; Karube, I. Anal. Chim. Acta 1999, 390, 93-100. (22) Lahav, M.; Kharitonov, A. B.; Katz, O.; Kunitake, T.; Willner, I. Anal. Chem. 2001, 73, 720-723.

Lahav et al.

scale architecture to overcome diffusion barriers. The imprinting of molecular recognition sites in molecular assemblies would enable programming of molecular contours and binding sites for any guest molecule. The reduced dimensionality of the interface would eliminate diffusion barriers thus, facilitating the association of the guest to the imprinted sites. This would result in fast response times of the sensing interface. Previous studies have shown that perforated monolayers with predesigned molecular channels act as specific sensing interfaces for target compounds.23 In a recent preliminary communication,1 we reported on the photochemical imprint of specific recognition sites for 6-[(4-carboxymethyl)phenoxy]-5,12naphthacene quinone (1a) in monolayers assembled on Au electrodes. Following this study, a different approach to generate molecular recognition sites in monolayer interfaces, by the use of molecular spider templates, was reported.24 Here, we present a comprehensive study on the photochemical imprint of molecular recognition sites for 1a in alkanethiol monolayer assemblies. We address the effects of the monolayer structure on the kinetics of association or dissociation of the imprinted substrate to and from the molecular recognition sites. We also discuss the relation between the monolayer structure and the binding affinities of the substrate to the imprinted sites. Experimental Section Materials. 6-[(4-Carboxymethyl)phenoxy]-5,12-naphthacene quinone25 (1a) was synthesized according to the literature;25c mp 220-224 °C (dec). H NMR (200 MHz CDCl3) δ (ppm): 3.4 (s, 2H), 6.78 (d, J ) 7.3 Hz, 2H), 7.13 (d, J ) 7.3 Hz, 2H), 7.70-7.94 (m, 4H), 8.07 (dm, J ) 7 Hz, 2H), 8.23 (m, 1H), 8.40 (d, J ) 7.5 Hz, 1H), 8.84 (s, 1H). Anal. Calcd: C, 69.96; H, 3.36. Found: C, 70.26; H,3.60.2-Chloro-3-{[2-(dimethylbutylammoniumbromide)ethyl]amino}-1,4-naphthoquinone (4) was prepared according to the literature.26 All other materials were of commercial sources (Aldrich, Sigma, or Fluka) and were used with no further purification. Ultrapure water from a Serapur PRO90CN system was used throughout the work. Instruments and Methods. Absorption spectra were recorded on a Uvicon-860 (Kontron) spectrophotometer. NMR spectra were recorded with a Bruker AMX 400 or a Bruker WP200 spectrometer. Chemical shifts were referenced to tetramethylsilane. Electrochemical measurements were performed using a potentiostat (EG&G, VersaStat) connected to a personal computer (EG&G electrochemical software model 270/250). All measurements were carried out in a three-compartment electrochemical cell consisting of the chemically modified electrode as the working electrode, a glassy carbon auxiliary electrode isolated by a glass frit, and a saturated calomel electrode (SCE) connected to the working volume with a Luggin capillary. All potentials are reported with respect to the SCE. Argon bubbling was used to remove oxygen from the electrolyte solutions in the electrochemical cell. The background electrolyte solution, 0.01 M phosphate buffer, pH ) 7.0, containing 0.1 M Na2SO4, was used in all of the measurements. Isomerization of monolayers was achieved by subjecting the electrodes to irradiation at appropriate wavelengths. The electrodes were illuminated in the air outside the electrochemical (23) (a) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92-98. (b) Chailapakul, O.; Crooks, R. M. Langmuir 1993, 9, 884-888. (24) Mirsky, V. M.; Hirsch, T.; Piletsky, S. A.; Wolfbeis, O. S. Angew. Chem., Int. Ed. Engl. 1999, 38, 1108-1110. (25) (a) Zelichenok, A.; Buchholtz, F.; Fischer, E.; Ratner, J.; Krongauz, V.; Anneser, H.; Bra¨euchle, C. J. Photochem. Photobiol., A 1993, 76, 135-141. (b) Fang, Z.; Wang, S.; Yang, Z.; Chen, B.; Li, F.; Wang, J.; Xu, S.; Jiang, Z.; Fang, T. J. Photochem. Photobiol., A 1995, 88, 23-30. (c) Buchholtz, F.; Zelichenok, A.; Krongauz, V. Macromolecules 1993, 26 (6), 906-910. (26) Calabrese, G. S.; Buchanan, R. M.; Wrighton, M. S. J. Am. Chem. Soc. 1983, 105, 5594-5600.

Imprint of Molecular Recognition Sites

Langmuir, Vol. 17, No. 23, 2001 7389

Scheme 1. Preparation of the trans-Quinone (2a)/CH3(CH2)13SH Mixed Monolayer on a Au Electrode and Its Photoisomerization

cell. A 150 W xenon arc lamp equipped with a Schott fitter (λ > 475 nm) and an IR CuSO4 filter was used as the light source for generation of the trans-quinone state (1a). An 18 W mercury pencil lamp source (Oriel-6042) with a long-wave filter (320 nm < λ < 380 nm) was used to generate the ana-quinone state (1b). The electrodes were protected from room light during the electrochemical measurements. Electrodes and Their Modification. Au electrodes (0.5 mm diameter Au wire, geometrical area ca. 0.2 cm2, roughness factor ca. 1.2) were used for the modifications. The Au electrodes were cleaned by boiling in 2 M KOH for 1 h followed by rinsing with water. The electrodes were stored in concentrated sulfuric acid. Prior to the modification, the electrodes were rinsed with water, soaked for 10 min in concentrated nitric acid, and rinsed again with water. A cyclic voltammogram recorded in 1 M H2SO4 was used to determine the purity and roughness of the electrode surface just before modification.27 The Au electrodes were soaked in a solution of 0.05 M cystamine (2,2′-dithio-bis(ethanamine), Aldrich) in water for 1 h and then rinsed thoroughly with water to remove the physically adsorbed cystamine. The cystaminemodified Au electrodes were incubated for 1 h in a 0.5 mM solution of 1a in 0.01 M HEPES aqueous/ethanolic (1:1 v/v) buffer, pH ) 7.3, in the presence of 10 mM EDC (1-ethyl-3(3-dimethylaminopropyl) carbodiimide, Aldrich). For further modification, the 2a-modified electrodes was treated with 1 mM of 1-decanethiol, 1-tetradecanethiol, or 1-octadecanethiol (Fluka) in ethanol for 30 min and rinsed with ethanol and then with water.

the broad voltammetric response. Treatment of the trans monolayer with long-chain alkanethiols (C10H21SH, C14H29SH, or C18H37SH) results in a densely packed monolayer where the trans-quinone units are stretched and aligned in the monolayer assembly, Scheme 1. The alignment of the quinone units is reflected by a quasi-reversible cyclic voltammogram of the quinone units. Figure 1 (curve b) shows the cyclic voltammogram of the trans-quinone units aligned with C14H29SH, Epc - Epa ≈ 60 mV. The anodic or cathodic peak currents reveal a linear dependence as a function of scan rate, consistent with the presence of a surface-confined redox-active component. By the coulometric analysis of the cathodic or anodic redox waves of the trans-phenoxynaphthacene quinone, we estimate the surface coverage of the quinone units to be ca. 2.0 × 10-10 mol cm-2. Quartz-crystal-microbalance (QCM) analysis enabled us to determine the ratio between 2a and the supporting alkanethiol units. The coupling of 1a to the base cystamine monolayer associated with an Au/quartz crystal results in a frequency change of ∆f ) -16 ( 2 Hz.

Results and Discussion Scheme 1 outlines the method to assemble a transphenoxynaphthacene quinone, 2a, monolayer on a Au electrode: a primary cystamine monolayer was assembled on the Au surface, and 1a was covalently coupled to this base monolayer. Figure 1 (curve a) shows the cyclic voltammogram of the resulting trans-quinone monolayer electrode. An ill-defined amperometric response is observed. This ill-defined redox wave is attributed to the formation of a nondensely packed quinone monolayer of variable orientations of the electroactive component relative to the electrode surface.28,29 The different orientations of the quinone adsorbate on the surface result in different interfacial electron-transfer rate constants and (27) Woods, R. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1980; Vol. 9, p 1.

Figure 1. Cyclic voltammograms of (a) the trans-quinone (2a) in a nondensely packed monolayer, (b) trans-quinone (2a) in a rigidified configuration monolayer generated by the treatment of the electrode with 10 mM ethanolic solution of C14H29SH for 30 min, (c) the rigidified ana-quinone (2b) monolayer generated by irradiation at 320 nm < λ < 380 nm, and (d) the imprinted monolayer produced by treatment of the ana-quinone monolayer (2b) with 1-butylamine, 10 mM, for 5 min followed by rinsing the electrode with ethanol and water. All experiments were performed in 0.01 M phosphate buffer, pH ) 7.0, and 0.1 M sodium sulfate, under argon, at 25 °C and a potential scan rate of 50 mV s-1.

7390


Lahav et al.

Scheme 2. Photoinduced Formation of the Molecular Recognition Sites for 1a in the Monolayer Assembly and the Association and Dissociation of trans-Quinone (1a) to and from the Recognition Sites, Respectively

This translates30 to a surface coverage of 2a that corresponds to (2.4 ( 0.3) × 10-10 mol cm-2. Treatment of the surface with C14H29SH results in a further frequency change of ca. ∆f ) -700 ( 5 Hz which translates to a surface coverage of 1.6 × 10-8 mol cm-2. Using these values, the ratio of 2a/C14H29SH in the monolayer is estimated to be 1:60. Similar QCM analyses for the generation of the 2a/C18H37SH and 2a/C10H21SH monolayers reveal that the ratio of the components in the mixed monolayers is 1:50 and 1:55, respectively. It should be noted, however, that in all of the three systems the surface coverage of 2a is identical, within the experimental error ((5%). The cyclic photochemical isomerization of the phenoxynaphthacene quinone monolayer assembled on an Au electrode was previously studied6d,e and used for the amperometric transduction of recorded photonic signals. Photoirradiation of the trans-quinone 2a monolayer, 320 nm < λ < 380 nm, results in the ana-quinone monolayer, 2b (Figure 1 (curve c)), whereas irradiation of the anaquinone monolayer, 2b, λ > 475 nm, restores the transquinone 2a monolayer, eq 1. Previous studies31 have

indicated that the ana-state, 2b, is susceptible to a Michael type addition of amines, to yield 3, eq 2. This property of

the photochemical imprint of molecular recognition sites for the phenoxynaphthacene quinone skeleton, as outlined in Scheme 2. Reaction of the photogenerated 2b monolayer with butylamine is anticipated to cleave off the phenoxyquinone units from the mixed monolayer, leading to molecular imprinted sites in the two-dimensional monolayer array, Scheme 2. Specific assembly is selected as a model system to develop the concept of photochemical imprint of molecular recognition sites due to the availability of the reaction sequence for the generation of the sites. Figure 1 (curve d) shows the cyclic voltammogram of the monolayer-functionalized electrode after treatment of the 2b monolayer with butylamine for 5 min. The resulting monolayer lacks photoactivity, and upon irradiation, λ > 475 nm, the redox properties of the 2a monolayer electrode are not restored, indicating that the quinone units are depleted from the surface. Depletion of the quinone units from the monolayer assembly in the presence of butylamine presumably generated the molecular imprinted sites for the quinone units (vide infra). The kinetics of the formation of the molecular imprinted sites was followed by the interaction of the 2b monolayer for different time intervals with butylamine. At variable time intervals of reaction, the functionalized monolayer was reisomerized to the 2a monolayer configuration. The redox response of the monolayer provides a quantitative measure for the residual surface coverage of the quinone units associated with the electrode. We find that upon reaction of the 2b monolayer with butylamine, 10 mM for 5 min, all of the ana-quinone units are removed from the monolayer interface, Figure 2. Assuming that all of the quinone units are dissociated from the monolayer, we (28) Katz, E.; Itzhak, N.; Willner, I. J. Electroanal. Chem. 1992, 336, 357-362. (29) Katz, E.; Itzhak, N.; Willner, I. Langmuir 1993, 9, 1392-1396. (30) The number of recognition sites was calculated from eq i that correlates the mass changes on the modified piezoelectric crystal with its frequency changes. ∆f ) -Cf∆m

the ana-quinone (2b) monolayer provides the basis for

(Cf ) 1.83 × 108 Hz/g)

(i)

The saturated value of the frequency changes, ∆f, was translated into the mass change, ∆m, occurring on the crystal as a result of the association of 1a. The value was translated to the respective surface coverage value of the imprinted sites, assuming that all of the recognition sites are accommodated by 1a. (31) Gerasimenko, Y. E.; Parshutkin, A. A.; Poteleshenko, N. T.; Poteleshenko, V. P.; Ropmanov, V. V. Zh. Prikl. Spektrosk. (in Russian) 1979, 30, 954-956.


Figure 2. Cyclic voltammograms corresponding to the timedependent functionalization of the 2a/C14H29SH monolayer with imprinted sites: (a) the 2a-functionalized electrode prior to the imprinting and (b-d) the electrodes isomerized to the 2b state, λ > 475 nm. The cyclic voltammograms correspond to the nondissociated quinone. (b) After reaction with butylamine for 100 seconds. (c) After reaction with butylamine for 200 s. (c) After reaction with butylamine for 300 s. Voltammograms were recorded by the conditions detailed in Figure 1.

Figure 3. Cyclic voltammograms corresponding to the timedependent association of 1a, 1 × 10-4 M, to the imprinted monolayer C14H29SH electrode: (a) 0 min, (b) 5 min, (c) 10 min, (d) 15 min, (e) 20 min, and (f) 25 min. The cyclic voltammogram (g) corresponds to the 2a/C14H29SH monolayer electrode prior to the photochemical imprint of the recognition sites. Data were recorded by the conditions outlined in Figure 1. Inset: Cathodic peak currents of the imprinted monolayer electrode upon interaction with 1a, 1 × 10-4 M, for different time intervals.

estimate the surface coverage of the imprinted sites to be also ca. 2 × 10-10 mol cm-2. Figure 3 shows the cyclic voltammograms of the imprinted monolayer functionalized electrode upon the interaction with the trans-quinone (1a) for different time intervals. As the time of incubation is longer, the electrical response of the quinone increases until it reaches a saturation value and the saturated amperometric response of 1a is identical to the initial current value of 2a before the chemical cleavage from the monolayer. Coulometric analysis of the resulting redox wave indicates that the surface density of quinone units associated with the electrode corresponds to ca. 2 × 10-10 mol cm-2, a value identical to the value of imprinted holes in the monolayer. The reversible electrical response of 1a suggests that all associated quinone units are structurally aligned in the monolayer array. Control experiments revealed that in the presence of a C14H29SH-modified electrode, no electrical response of 1a is observed. Thus, the C14H29SH monolayer blocks the electrical contact between the electrode and 1a. Also, in the presence of the bare Au electrode, 1a exhibits an ill-defined redox process, ∆Ep ≈ 400 mV,

Langmuir, Vol. 17, No. 23, 2001 7391

Figure 4. Cyclic voltammograms corresponding to the timedependent dissociation of 1a from the monolayer-imprinted electrode: (a) the imprinted monolayer electrode (C14H29SH) saturated with 1a and corresponding to the experiment outlined in Figure 3, curve f; (b-f) after immersing the electrode in the phosphate buffer solution, pH ) 7.0, for 5, 10, 20, 30, and 37 min, respectively. Data were recorded under conditions outlined in the caption of Figure 1. Inset: Cathodic peak currents of the 1a-saturated imprinted monolayer electrode upon immersion in the buffer solution for different time intervals.

characterized with very broad redox waves. Thus, the reversible electrical response of 1a associated with the imprinted monolayer clearly indicates the formation of rigidified aligned assembly between the trans-quinone (1a) and the monolayer-imprinted recognition sites. The timedependent increase of the electrical response of 1a represents the dynamics of association of the quinone to the imprinted recognition sites. Figure 3 (inset) shows the kinetics of the binding of 1a to the imprinted sites. The association of 1a to the imprinted sites follows a pseudo-first-order kinetics, kobs ) 5 × 10-2 min-1 at 25 °C (for a detailed kinetic analysis, vide infra). Insertion of the imprinted monolayer electrode that includes the bound quinone units 1a to the recognition sites into a buffer solution results in the dissociation of 1a from the imprinted sites. Figure 4 shows the gradual decrease of the electrical response of the quinone units associated with the monolayer as a result of its dissociation from the imprinted sites. Figure 4 (inset) shows the kinetics of dissociation of 1a from the imprinted sites. It follows a first-order kinetics, kdiss ) 3.85 × 10-2 min-1 at 25 °C. The association of 1a to the imprinted sites and the dissociation of 1a from the binding sites reveals reversibility, and the electrochemical response of 1a as a result of binding or dissociation can be cycled for at least 10 times. The association and dissociation of 1a to and from the monolayer-imprinted recognition sites was further confirmed by microgravimetric QCM measurements. A Auquartz crystal was functionalized with the mixed monolayer consisting of 1a and C14H29SH, and the imprinted sites were generated in the monolayer as shown in Scheme 2. Figure 5 (curve a) shows the frequency changes of the quartz crystal as a result of the association of 1a to an imprinted monolayer assembled onto Au electrodes associated with the quartz crystal (AT-cut, 9 MHz). The frequency decreases for ca. 400 s and then levels off to a constant value. The frequency decrease (12 ( 1 Hz) implies that an increase of mass occurs on the crystal as a result of the association of 1a, and this reaches a constant value as a result of saturation of the imprinted binding sites.

7392


Lahav et al.

Figure 5. Microgravimetric quartz-crystal-microbalance analyses corresponding to (a) the association of 1a, 1 × 10-4 M, to the Au-quartz crystal functionalized with the imprinted C14H29SH monolayer; (b) the dissociation of 1a from the saturated 1a/C14H29SH monolayer functionalized Au-quartz crystal (the zero frequency represents the final crystal frequency observed in (a), ∆f ) -12 Hz); (c) the interaction of 7, 1 × 10-4 M, with the imprinted C14H29SH monolayer Au-quartz crystal.

From the frequency change, the surface coverage of 1a, or the number of recognition sites, is estimated to be (1.85 ( 0.20) × 10-10 mol cm-2, consistent with the value determined by the electrochemical results.30 Figure 5 (curve b) shows the frequency changes of the quartz crystal that includes 1a associated to the recognition sites upon treatment in a pure buffer solution. The increase of the crystal frequency indicates the release of 1a from the recognition sites, and the crystal frequency is stabilized to almost its original value prior to the noncovalent association of 1a. The formation of the imprinted sites is attributed to the formation of “hydrophobic pockets” in the monolayer by the rigidifying alkyl chains. Cleavage of the quinone from the monolayer leaves a site with molecular contours of the cleaved quinone, and the residual phenol function (cf. Scheme 2) may synergetically bind the quinone (1a) to the imprinted hydrophobic sites. Thus, the structural contours of the imprinted sites and their binding capacity should be controlled by the length of the alkyl chain that rigidifies the monolayer assembly. Furthermore, since the cleavage of the naphthacene quinone units generates a perforation (or “hole”) in the monolayer, the alkyl chains are anticipated to bend and perturb the imprinted binding site. That is, for long alkyl chains we expect to observe a kinetic barrier for the association of 1a to the imprinted sites. Once the guest substrate binds to the recognition site, the long chains cap the incorporated quinone and its dissociation is hindered. Thus, the kinetics of binding or dissociation of 1a to or from the imprinted sites as well as the thermodynamic association constants of 1a to the imprinted sites are expected to be controlled by the length of the alkyl chain that is incorporated in the functional mixed monolayer on the electrode. Accordingly, we studied the kinetics of binding and dissociation of 1a to and from the imprinted sites and of thermodynamic properties of molecular recognition sites for 1a in the presence of C10H21SH, C14H29SH, and C18H37SH as rigidifying alkyl chains. For alkyl chains shorter than C10H21SH, no imprinted recognition sites could be detected upon following the perforation protocol outlined in Scheme 2. This suggests that the shortest alkyl chain which enables the structural perforation of the monolayer consists of 10 carbons. The effect of C10H21SH and C18H37SH on the electrical response of 1a is similar to that described for C14H29SH. Thus, the

Figure 6. (A-C) Cyclic voltammograms corresponding to the association of 1a, 1 × 10-4 M, at variable time intervals, to the different imprinted CnH2n+1SH monolayer electrodes. (A) Association of 1a to the C10H21SH “imprinted” monolayer electrode (a) prior to interaction with 1a, (b) after interaction for 5 min, and (c) after interaction for 30 min. (B) Association of 1a to the C14H29SH imprinted monolayer electrode (a) prior to the interaction of 1a, (b) after 5 min, (c) after 10 min, (d) after 15 min, (e) after 20 min, and (f) after 25 min. (C) Association of 1a to the C18H37SH imprinted monolayer electrode (a) before interaction with 1a, (b) after 10 min, (c) after 20 min, (d) after 35 min, (e) after 50 min, and (f) after 90 min. All data are recorded at conditions outlined in the caption of Figure 1, at 25 °C. (D) Charge associated with the reduction of 1a bound to the different imprinted monolayer electrodes at variable time intervals: (a) association of 1a to the imprinted C10H21SH monolayer electrode, (b) association of 1a to the imprinted C14H29SH monolayer electrode, and (c) association of 1a to the imprinted C18H37SH monolayer electrode.

effects of the different chain lengths on the rigidification of the trans-phenoxynaphthacene quinone units in the two-dimensional monolayer are similar. We observe similar amperometric responses of the trans-quinone units for the monolayers that include the different rigidifying alkyl chains. Figure 6 shows the cyclic voltammograms that correspond to the association of 1a at different time intervals to the imprinted binding sites, generated in monolayers composed of C10H21SH (Figure 6A), C14H29SH (Figure 6B), and C18H37SH (Figure 6C). This kinetic analysis is performed at 25 °C. It is evident that the association of 1a to the imprinted sites reaches a saturation (equilibrium) value that is controlled by the respective association constants. While the imprinted sites in the monolayers consisting of C14H29SH and C18H37SH are almost fully


Langmuir, Vol. 17, No. 23, 2001 7393

Figure 7. (A-C) Cyclic voltammograms corresponding to the dissociation of 1a from the 1a-saturated CnH2n+1SH monolayer electrodes. (A) Dissociation of 1a from the C10H21SH monolayer electrode upon interaction of the saturated interface with a 0.01 M buffer solution, pH ) 7.0, for (a) 1 min, (b) 2 min, and (c) 5 min. (B) Dissociation of 1a from the C14H29SH monolayer electrode: (a) the saturated 1a-C14H29SH monolayer electrode, (b) after interaction for 5 min in the 0.01 M buffer solution, pH ) 7.0, (c) after 10 min, (d) after 20 min, (e) after 30 min, and (f) after 37 min. (C) Dissociation of 1a from the 1a-saturated C18H37SH monolayer electrode: (a) the 1a-saturated interface, (b) after 20 min, (c) after 40 min, (d) after 60 min, (e) after 80 min, (f) after 100 minutes, and (g) after 120 min. Data were recorded at 25 °C under the conditions specified in the caption of Figure 1. (D) Charge associated with the reduction of 1a upon dissociation from the 1a-saturated monolayer electrodes at variable time intervals: (a) from the C10H29SH monolayer electrode, (b) from the C14H29SH monolayer electrode, and (c) from the C18H37SH monolayer electrode.

occupied with 1a, only ca. 40% of the sites generated in the C10H21SH-rigidified monolayer bind the host substrate. These results are consistent with the fact that the C10H21SH monolayer exhibits an inefficient hydrophobic contour to associate 1a, resulting in a low association constant. The longer alkyl chains yield perforated hydrophobic contours that enable the effective association of 1a. Figure 6D shows the time-dependent binding of 1a to the imprinted sites of the different monolayers. Interestingly,

the rate of association of 1a to the C14H29SH monolayer, kobs ) (5.0 ( 0.2) × 10-2 min-1 is 2.2-fold higher than the rate of binding of 1a to the C18H37SH imprinted monolayer, kobs ) (2.25 ( 0.20) × 10-2 min-1. Note that the C14H29SH and C18H37SH imprinted monolayers are almost similarly occupied with 1a after 25 and 90 min, respectively. These results are in agreement with the fact that for the longchain alkanethiol partial blocking of the imprinted-site opening inhibits the association of the guest substrate to the imprinted sites, as schematically presented in Figure 6B,C. The dissociation of 1a from the respective imprinted monolayer electrodes was similarly studied by following the electrical responses of the respective 1a-saturated electrodes at time intervals of incubation in a buffer solution at 25 °C, Figure 7. The rate of dissociation of the bound substrate strongly depends on the nature of the “hydrophobic pockets” in the perforated monolayers. The quinone 1a is rapidly depleted from the C10H21SH monolayer imprinted sites, Figure 7A and Figure 7D, curve a, indicating low affinity of the trans-quinone to the imprinted sites. Interestingly, the C14H29SH and the C18H37SH imprinted monolayer assemblies reveal a substantially different kinetics for the dissociation of 1a, kdiss is (3.85 ( 0.20) × 10-2 and (1.87 ( 0.20) × 10-2 min-1 for the C14H29SH and C18H37SH monolayers, respectively. Note, for example, that ca. 60% of the bound 1a dissociates from the C14H29SH monolayer after ca. 30 min while the same value of dissociated 1a at the C18H37SH monolayer is observed only after ca. 60 min. The slower dissociation of 1a from the C18H37SH imprinted monolayer is attributed to the capping of the noncovalently linked quinone by the long alkyl chains (see schematic configurations in parts B and C of Figure 7). Capping of the bound substrate introduces a barrier for the dissociation of 1a, resulting in the lower rate constant. From the respective rate constants corresponding to the association and dissociation of 1a to and from the imprinted monolayers, the association constants Ka (Ka ) kin/kdis) for binding of 1a to the different imprinted sites were calculated, Table 1. As the chain length of the supporting hydrophobic monolayer increases, the imprinted sites reveal a higher association constant for 1a. This is consistent with the improved binding capacities of imprinted sites exhibiting enhanced hydrophobic perforation and the capping phenomenon of longchain alkanethiols that assist synergetically in the binding of 1a to the imprinted sites. Realizing that the imprinted monolayers that include long-chain thiols as supporting units reveal kinetic barriers for the association or dissociation of 1a to and from the imprinted sites, we attempted to characterize activation energies related to the association and dissociation of 1a to and from the imprinted C14H29SH and C18H37SH monolayers. Figure 8A,B exemplifies the effect of temperature on the rate of association of 1a to the imprinted sites in the C14H29SH monolayer assembly, whereas Figure 8C,D shows the temperature effect on the dissociation of 1a from the perforated C14H29SH monolayer electrode. These transformations were examined at four temperatures corresponding to 25, 35, 45, and 55 °C. For the monolayers consisting of C14H29SHand C18H37SH-imprinted interfaces, Arrhenius plots of

Table 1. Kinetic and Thermodynamic Properties for the Interaction of 1a with Different Imprinted Monolayer Electrodes type of supporting monolayer

kin(25 °C) M-1 min-1

Eain kcal mol

kdis(25 °C) min-1

C14H29SH C18H37SH

(5.0 ( 0.20) × 102 (2.25 ( 0.20) × 102

8.7 ( 0.1 10.5 ( 0.1

(3.85 ( 0.20) × 10-2 (1.87 ( 0.20) × 10-2

Eaout kcal mol 18 ( 0.5 22 ( 0.5

Ka(25 °C) M-1 (1.3 ( 0.1) × 104 (1.2 ( 0.1) × 104

7394


Figure 8. Cyclic voltammograms exemplifying the temperature-dependent association of 1a to the imprinted C14H29SH monolayer electrodes (A and B) and the dissociation of 1a from the saturated 1a-C14H29SH monolayer electrode (C and D). (A) Interaction of imprinted C14H29SH monolayer electrode with 1a, 1 × 10-4 M, at 25 °C: (a) at time 0, (b) after 5 min, (c) after 10 min, (d) after 15 min, (e) after 20 min, and (f) after 25 min. (B) Interaction of the imprinted C14H29SH monolayer electrode with 1a, 1 × 10-4 M, at 55 °C: (a) at time 0 and (b-f) after 0.5, 1.0, 1.5, 2.0, and 3 min, respectively. (C) Dissociation of 1a from the 1a-saturated monolayer electrode at 25 °C upon incubation in the 0.01 M buffer solution, pH ) 7.0: (a) at time 0 and (b-f) after 5, 10, 20, 30, and 37 min, respectively. (D) Dissociation of 1a from the saturated 1a-C14H29SH monolayer electrode at 45 °C: (a) at time 0 and (b-f) after 2, 4, 6, 8, and 10 min, respectively. All data were recorded under the conditions specified in Figure 1.

the activation energy for the binding of 1a to the imprinted sites, Eain(C14H29SH) ) 8.7 ( 0.1 kcal mol-1 and Eain(C18H37SH) ) 10.5 ( 0.1 kcal mol-1, and the activation energies for the dissociation of 1a from the imprinted sites, Eadis(C14H29SH) ) 18 ( 0.5 kcal mol-1 and Eadis(C18H37SH) ) 22 ( 0.5 kcal mol-1, were derived. The higher activation energies for the association of 1a to the imprinted C18H37SH monolayer and for the dissociation of the guest from the same monolayer are attributed to the blocking of the recognition sites and capping of the bound substrate in the imprinted sites by the long alkyl chains, respectively. A further aspect that was examined relates to the specificity of the imprinted recognition sites for the target substrate. To examine this aspect, the C14H29SH-rigidified phenoxynaphthacene quinone imprinted monolayer was challenged with the redox-active compounds 2-chloro-3[[2-(dimethylbutylammoniumbromine)ethyl]amino]-1,4naphthaquinone (4), anthraquinone-2-sulfonic acid (5), N,N′-dimethyl-4,4′-bipyridinium (6), and 2-chloro-3[amino(4-benzoic acid)]-1,4-naphthaquinone (7), and the possible association of these compounds to the imprinted sites was studied electrochemically and by microgravimetric quartz-crystal-microbalance measurements. All of the compounds 4-7 reveal at a bare Au electrode reversible redox processes. In the presence of the phenoxynaphthacene quinone imprinted monolayer, all the redox compounds 4-6 lack any electrical responses, implying that the imprinted interface blocks the interfacial electron transfer to these redox compounds. The interaction of the

Lahav et al.

imprinted monolayer functionalized electrode with a mixture of 1a and 4, 5, or 6 (the concentration of each of the components was 1 × 10-4 M) yields an electrochemical response only for 1a. Only compound 7 reveals an illdefined redox response. Figure 9, curve a, shows the illdefined cyclic voltammogram of 7 upon interaction with the C14H29SH phenoxynaphthacene imprinted monolayer electrode, whereas curve b shows the quasi-reversible cyclic voltammogram of the functionalized monolayer upon the association of 1a to the imprinted sites. The lack of a reversible redox response of compounds 4-7 indicates that these substrates do not bind to the imprinted sites associated with the monolayer interface. Figure 5, curve c, shows the time-dependent frequency of an Au-quartz crystal functionalized with the 1a-imprinted C14H29SH monolayer interface upon the interaction with 7. No frequency changes are observed, implying that 7 does not bind (even nonspecifically) to the monolayer interface. Similarly, no frequency changes of the imprinted Auquartz crystal are observed upon interaction of the C14H29SH-imprinted Au-quartz crystal with 4, 5, or 6. The reference compound 5, that is anticipated to have hydrophobic features very similar to those of 1a, does not bind to the perforated interface. This suggests that the recognition of 1a by the imprinted sites is governed not by simple hydrophobic interactions but by a structural fit in the molecular contour and possible H-bonds between 1a and the residual phenol function at the base of the imprinted site. The formation of specific recognition sites for 1a in the monolayer assembly is further supported by following the temperature effect on the imprinted molecular recognition sites. The densely packed monolayer represents a twodimensional semicrystalline array that melts at a characteristic temperature. We expect that under melting conditions the monolayer components exchange, a process that ruins the specific recognition sites.32 Indeed, heating of the C14H29SH-imprinted monolayer up to 60 °C did not influence the imprinted sites, and the electrode revealed affinity for 1a. At 65 ( 2 °C, a sharp deactivation of the (32) (a) Ulman, A. Adv. Mater. 1991, 3, 298-303. (b) Badia, A.; Back, R.; Lennox, R. B. Angew. Chem., Int. Ed. Engl. 1994, 33, 2332-2335. (c) McCarley, R. L.; Dunaway, D. J.; Willicut, R. J. Langmuir 1993, 9, 2775-2777.


Figure 9. Cyclic voltammograms corresponding to the interaction of the 1a-imprinted C14H29SH monolayer electrode with (a) 7, 1 × 10-4 M, for 25 min and (b) 1a, 1 × 10-4 M, for 25 min. Data were recorded under the conditions specified in the caption of Figure 1.

electrode toward the association of 1a to the monolayer was observed. Cooling of the electrode after its heating to 65 °C does not restore the recognition sites for 1a. This irreversible destruction of the recognition sites is attributed to the melting of the monolayer, resulting in the exchange of the monolayer components and the destruction of the molecular association sites. The melting process of the monolayer does not desorb the monolayer assembly, and the melted monolayer electrode after cooling reveals interfacial blocking of electron transfer to the redox labels 1a and 4-7 in the solution. Conclusions The present study describes a novel method to functionalize monolayers on electrode supports by perforation

Langmuir, Vol. 17, No. 23, 2001 7395

of the monolayer with imprinted molecular recognition sites. The imprint of specific molecular recognition sites in two-dimensional monolayers represents a novel method to tailor sensing interfaces on electrode supports or to design imprinted monolayer functionalized particulate systems for selective separation processes. The fact that the imprinted sites are generated in a monolayer interface excludes diffusion limitations that are experienced in thick imprinted membranes, and thus the response time of the monolayer interfaces is expected to be high. By studying the kinetic and thermodynamic features of the imprinted recognition sites with 1a as a function of the chain length of the supporting alkanethiol, we were able to elucidate molecular functions that control the binding properties of the imprinted sites. Barriers for the entry of the guest to the imprinted sites and capping of the bound guest substrate by the neighboring alkanethiol chains were discovered. The method to perforate the mixed monolayer includes the photochemical cleavage of the imprinted substrate. Various photoprotective groups for chemical functionalities are known in organic chemistry.33 The use of these photoprotective groups as a building block for the generation of mixed monolayers of functional substrates/ alkanethiols may provide a general strategy for the photoinduced formation of imprinted recognition sites in monolayers. Acknowledgment. This study is supported by the National Science Foundation administered by the Israel Academy of Sciences and Humanities. The support of The Israel Clore Foundation Scholars Program (M. Lahav) is gratefully acknowledged. LA011172S (33) (a) Corrie, J. E. T.; Trentham, D. R. In Biological Applications of Photochemical Switches; Morrison, H., Ed.; Wiley: New York, 1993; pp 243-305. (b) Pillai, V. N. R. In Organic Photochemistry; Padwa, A., Ed.; Marcel Dekker: New York, 1987; Vol. 9, pp 225-323.

Photochemical Imprint of Molecular Recognition Sites in Two

Recommend Documents