Langmuir 1998, 14, 2857-2863
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Molecular Imprinting of Azobenzene Carboxylic Acid on a TiO2 Ultrathin Film by the Surface Sol-Gel Process Seung-Woo Lee, Izumi Ichinose, and Toyoki Kunitake* Department of Chemical Science & Technology, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812, Japan Received February 11, 1998. In Final Form: March 16, 1998 Ultrathin films of titanium oxide gel imprinted with 4-(4-propyloxyphenylazo)benzoic acid (C3AzoCO2H) were prepared by repeated immersion of a gold-coated quartz crystal microbalance (QCM) electrode or a quartz plate in solutions of C3AzoCO2H and titanium butoxide (Ti(O-nBu)4) in toluene/ethanol. Regular film growth was confirmed by frequency decrement in QCM measurements and by the appearance of the azobenzene absorption in UV-vis spectroscopy. The template molecule could be completely removed upon treatment with 1% aqueous ammonia. The resulting films showed saturation binding of the template molecule in acetonitrile in less than 1 min. Binding of the template molecule was greatest among aromatic carboxylic acids that are structurally related to the template. A related ester showed a significant reduction in the binding efficiency. The surface sol-gel process provides a powerful tool for molecular imprinting in ultrathin metal oxide films.
Introduction Molecular imprinting is increasingly getting popular as a means to prepare specific binding sites by imprinting the structures of given molecules in appropriate matrices. Its first example is probably the report of Dickey in which silica gel was formed from silica sol in the presence of alkyl oranges.1 The adsorption of a particular alkyl orange was enhanced for a silica gel that had been formed in the presence of that alkyl orange as template. Since that time, a large amount of research has been conducted on molecular imprinting, mostly by using cross-linked polymers as solid matrices.2-4 In spite of the pioneering work of Dickey, inorganic matrices have been rarely employed for molecular imprinting, except for the following studies. Mosbach and co-workers allowed organic silanes to polymerize on the surface of porous silica particles in aqueous solution.5 The resulting polysiloxane copolymers were, when imprinted with dye molecules, superior supports for high-performance liquid chromatography. Morihara and others developed the “footprint” technique, where tailor-made catalysts specific for trans-acylation were designed by imprinting transition-state analogues onto aluminum ion-doped silica gel.6,7 Maier et al. similarly imprinted a transition-state analogue for transesterification on amorphous silicon dioxide.8 Very recently, Pinel, Loisil, and Gallezot took up the imprinting of silica gel again and showed that regiospecificity for cresols was successfully imprinted by using o-cresol as (1) (a) Dickey, F. H. Proc. Natl. Acad. Sci. 1949, 35, 227-229. (b) Dickey, F. H. J. Phys. Chem. 1955, 59, 695-707. (2) (a) Wulff, G.; Sarhan, A. Angew. Chem., Int. Ed. Engl. 1972, 11, 341. (b) Wulff, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1812-1832. (3) Vlatakis, G.; Andersson, L. I.; Mu¨ller, R.; Mosbach, K. Nature 1993, 361, 645-647. (4) Spivak, D.; Gilmore, M. A.; Shea, K. J. J. Am. Chem. Soc. 1997, 119, 4388-4393 and references therein. (5) Glad, M.; No¨rrlow, O.; Sellergren, B.; Siegbahn, N.; Mosbach, K. J. Chromatogr. 1985, 347, 11-23. (6) Morihara, K.; Kurihara, S.; Suzuki, J. Bull. Chem. Soc. Jpn. 1988, 61, 3991-3998. (7) Morihara, K.; Takiguchi, M.; Shimada, T. Bull. Chem. Soc. Jpn. 1994, 67, 1078-1084 and references therein. (8) Heilmann, J.; Maier, W. F. Angew. Chem., Int. Ed. Engl. 1994, 33, 471-473.
template.9 On the other hand, Kodakari, Katada, and Niwa prepared a silica overlayer on tin oxide by chemical vapor deposition using preadsorbed benzoate anion as template.10 The resulting silica overlayer acted as molecular sieve, due to formation of an imprinted cavity. The site of molecular imprinting has to be located in these examples at the surface and/or the crevice close to the surface, in order to secure efficient adsorption of guest molecules. Therefore, the use of ultrathin films should be advantageous, if suitable imprinting techniques can be applied. Organized molecular films such as LangmuirBlodgett multilayers and surface-bound monolayers may appear to be good candidates for this purpose. However, they are not necessarily suitable, since the flexible structural modification that is required for the imprinting process is, in principle, not compatible with the ordered molecular organization. Recently, a novel method to prepare ultrathin layers of metal alkoxide gels by sequential chemisorption and activation was developed independently by our group11 and by Kleinfeld and Ferguson.12 This surface sol-gel process is unique in that it can be used to design individual metal oxide layers with molecular precision and that it is broadly applicable to varied metal alkoxides. Preparation of ultrathin metal oxide layers was feasible from alkoxides of silicon, aluminum, titanium, zirconium, and niobium.11 In a separate study, we prepared alternating layers of titanium oxide and poly(acrylic acid) or poly(vinyl alcohol) and concluded that the hydroxylated surface need not be inorganic.13 This new hybrid methodology should become a powerful technique for preparation of molecular recognition sites, if we can combine it with the imprinting process. Individual metal oxide layers are formed with (9) Pinel, C.; Loisil, P.; Gallezot, P. Adv. Mater. 1997, 9, 582-585. (10) (a) Kodakari, N.; Katada, N.; Niwa, M. J. Chem. Soc., Chem. Commun. 1995, 623-624. (b) Kodakari, N.; Katada, N.; Niwa, M. Chem. Vap. Deposition 1997, 3, 59-66. (11) (a) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Lett. 1996, 831832. (b) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Mater. 1997, 9, 1296-1298. (12) Kleinfeld; E.; Ferguson, G. S. Mater. Res. Soc. Symp. Proc. 1994, 351, 419-424. (13) Kawakami, T.; Senzu, H.; Ichinose, I.; Kunitake, T. Spr. Meet. Proc., Chem. Soc. Jpn. 1997, 1, 71.
S0743-7463(98)00176-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/22/1998
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nanometer precision under the mild preparative conditions typical of the sol-gel process. These features allow us to use thermally labile organic compounds. In this study, we describe application of the surface sol-gel process to molecular imprinting.
Lee et al. Chart 1
Experimental Section All gold-coated quartz crystal microbalance (QCM) resonators used (9 MHz, USI System, Fukuoka) were immersed in an ethanol solution of mercaptoethanol (10 mM) for 12 h, rinsed with ethanol, and dried with nitrogen gas. This surface modification is essential for reproducible formation of imprinted films. QCM experiments were conducted in two modes. In the first mode, frequency shifts due to the surface sol-gel cycle were monitored in air after drying and were transformed into mass changes by using the Sauerbrey equation.14 A frequency decrease of 1 Hz corresponds to a mass increase of 0.9 ng in our system. On the other hand, frequency shifts due to binding were monitored in acetonitrile. In this in situ monitoring, the resonator was placed vertically at a constant depth from the surface of the solution. The details of QCM experiments have been reported in the literature.15,16 Titanium butoxide (Ti(O-nBu)4), benzoic acid, cinnamic acid, and octanoic acid were used as purchased from Kishida Chem., Japan, without further purification. 1-Adamantanol (1-AdOH) was purchased from Aldrich, and anthracene-9-carboxylic acid (9-AnCO2H), anthracene-2-carboxylic acid (2-AnCO2H), and anthracene were obtained from Tokyo Kasei. 4-(4-Hydroxyphenylazo)benzoic acid (AzoCO2H) was prepared according to the procedure reported previously.17 4-(4-Propyloxyphenylazo)benzoic acid (C3AzoCO2H) was obtained by Williamson synthesis of the AzoCO2H and 1-bromopropane. It was allowed to react with SOCl2, and the resulting acid chloride was esterified with methanol and isopropanol to give methyl 4-(4-propyloxyphenylazo)benzoate (C3AzoCO2Me) and isopropyl 4-(4-propyloxyphenylazo)benzoate (C3AzoCO2iPr), respectively. AzoCO2H: reddish brown crystals; mp 283 °C; TLC (silica gel, CHCl3/CH3OH ) 5:1) Rf ) 0.65, single spot. C3AzoCO2H: orange crystals; mp 253 °C; TLC (silica gel, CHCl3/CH3OH ) 10:1) Rf ) 0.71, single spot; 1H-NMR (CDCl ) 1.01 (t, 3H, CH ), 1.83 (m, 2H, CH ), 4.05 (t, 3 3 2 2H, OCH2), 7.14-8.13 (d, m, d, 8H, ArH), 13.07 (s, 1H, OH). C3AzoCO2Me: orange crystals; mp 149 °C; TLC (silica gel, CHCl3/ CH3OH ) 10:1) Rf ) 0.80, single spot; 1H-NMR (CDCl3) 1.07 (t, 3H, CH3), 1.85 (m, 2H, CH2), 3.95 (s, 3H, OCH3), 4.03 (t, 2H, OCH2), 7.02-7.92 (d, m, 8H, ArH). Anal. Calcd for C17N2O3H18: C, 68.44; H, 6.08; N, 9.39. Found: C, 68.47; H, 6.08; N, 9.38. C3AzoCO2iPr: orange crystals; mp 88 °C; TLC (silica gel, CHCl3/ CH3OH ) 10:1) Rf ) 0.76, single spot; 1H-NMR (CDCl3) 1.07 (t, 3H, CH3), 1.40 (s, 6H, OCCH3), 1.85 (m, 2H, CH2), 4.03 (t, 2H, OCH2), 5.28 (s, 1H, CH), 7.02-8.16 (d, m, d, 8H, ArH). Anal. Calcd for C19N2O3H22: C, 69.91; H, 6.79; N, 8.58. Found: C, 69.76; H, 6.79; N, 8.55. Chemical structures of template and guest molecules are shown in Chart 1. UV absorption spectra were obtained by using a JASCO V-570 spectrophotometer. Quartz plates were treated with concentrated H2SO4, washed several times with deionized water, immersed in 1 wt % ethanolic KOH (ethanol/H2O ) 3:2) for a few minutes, rinsed with deionized water, and dried by flushing with N2 gas. FT-IR reflection spectra were obtained by using a Shimadzu FTIR 8100M spectrometer.
Results and Discussion Imprinting Process. In the surface sol-gel process we described previously, metal alkoxides used for adsorption were dissolved in aprotic solvents in order to avoid hydrolysis and subsequent coagulation. They were allowed to react with small amounts of water in case partially condensed species were used for adsorption.11 In the (14) Sauerbrey, G. Z. Phys. 1959, 155, 206-222. (15) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117-6123. (16) Ebara, Y.; Okahata, Y. J. Am. Chem. Soc. 1994, 116, 1120911212. (17) Nakashima, N.; Morimitsu, K.; Kunitake, T. Bull. Chem. Soc. Jpn. 1984, 57, 3253-3257.
present case, the template azobenzene carboxylic acid must be covalently bound to the adsorbing titanium alkoxide. Thus, we allowed the two species to react for certain periods of time prior to the adsorption procedure. Mixtures of 100 mM Ti(O-nBu)4 and 25 mM (or 50 mM) C3AzoCO2H were dissolved in 2:1 (vol/vol) mixtures of toluene and ethanol and stirred at room temperature for more than 12 h. Complexation of Ti(O-nBu)4 and C3AzoCO2H at this stage was confirmed by FT-IR measurement after removal of the solvents. The spectra gave peaks at 1547 and 1410 cm-1 attributable to vibrations of Ticarboxylate complexes.18 Subsequently, water (275 mM for 25 mM template solution and 350 mM for 50 mM template solution) was added (2.75 and 3.5 times, respectively, relative to Ti alkoxide), and the mixtures were aged for one to several hours. The complete reaction of the three components in the 25 mM template solution will produce Ti4O4(OH)4(O-nBu)4C3AzoCO2H. The film composition estimated from QCM experiments and UV spectra is consistent with this chemical formula, as discussed below. Stock solutions (25 mM and 50 mM template solutions) were diluted 20 times by toluene and used as dipping solutions. A gold-coated QCM electrode (9 MHz) modified with mercaptoethanol was immersed in a dipping solution for 1 min at room temperature, washed in toluene for 1 min to remove the physisorbed complex, and dried for 1 min by flushing with N2 gas. It was then attached to the frequency counter and kept in the air for a few minutes until the frequency change probably due to progress of the hydrolysis of the surface alkoxide group became insignificant. These procedures constitute one cycle of chemisorption and activation, as illustrated in Figure 1. (18) Roger, C.; Hampden-Smith, M. J. J. Mater. Chem. 1992, 2, 11111112.
Azobenzene Carboxylic Acid on a TiO2 Ultrathin Film
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Figure 1. Molecular imprinting process by the surface sol-gel process. C and R denote the template molecule and the unhydrolyzed butoxide group of Ti(O-nBu)4.
Figure 2. QCM frequency shifts in the adsorption of the Ti(O-nBu)4-C3AzoCO2H complex. Stock solutions were diluted 20 times with toluene. Stock solution: (a) Ti(O-nBu)4; 100 mM, C3AzoCO2H; 25 mM, H2O; 275 mM in toluene/ethanol ) 2:1; (b) Ti(O-nBu)4; 100 mM, C3AzoCO2H; 50 mM, H2O; 350 mM in toluene/ethanol ) 2:1. The period of adsorption is 1 min at room temperature, followed by washing for 1 min in toluene.
The QCM frequency decrease due to adsorbed mass in each cycle is given in Figure 2 for the 25 mM template solution. It is essentially reproducible up to at least 10 cycles, except for the first cycle, with the average frequency shift of 96 ( 26 Hz within this series. The identical adsorption experiments were repeated independently three times, and regular film growth was observed in each case. The average frequency change in all these runs was 100 ( 30 Hz for the individual layer, corresponding to a thickness of 1.8-2.7 nm using a density of 1.0-1.5. When the concentration of the template was 50 mM, the frequency decrease was again reproducible for 10 adsorption cycles with a 54 ( 18 Hz shift for one cycle. Thus, the regularity of the film growth is maintained in both cases, but the extent of the growth was greater at the
Figure 3. UV-vis absorption spectral change due to adsorption of the Ti(O-nBu)4/C3AzoCO2H complex. The conditions are the same as those of Figure 2a. The inset shows the absorbance change at 358 nm.
lower concentration of the template molecule. Apparently, the organic template slows hydrolysis of the titanium alkoxide, lowering the adsorbed mass. Incorporation of the template molecule and its removal can be confirmed by UV-vis adsorption spectroscopy of the azobenzene moiety. The mixed complex was adsorbed onto the two surfaces of an activated quartz plate, and absorption spectra were measured at every two cycles. The absorbance increased in proportion to the adsorption cycle up to 20 cycles, as shown in Figure 3. Therefore, the azobenzene template is constantly incorporated into the gel film. In order to remove the bound template molecule, surface gel films prepared on quartz plates were treated with 1% aqueous ammonia for 30 min, washed thoroughly with ethanol and ion-exchanged water, and dried by flushing with N2 gas. The UV absorbance at 358 nm
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Table 1. Adsorbed and Desorbed Masses during the Course of Imprinting Procedures
sample
[C3AzoCO2H] in stock solution (mM)
C3AzoCO2H/ Ti(O-nBu)4
adsorption cycles
total adsorbed mass/corresponding frequency shift
desorbed mass of templatea/QCM desorption ratiob
desorbed template from UVc/desorption ratiod
1 2 3 4
25 25 25 50
1/4 1/4 1/4 1/2
2 5 10 10
212 ng/235 Hz 476 ng/529 Hz 907 ng/1008 Hze 486 ng/540 Hz
56 ng/26% 131 ng/28% 190 ng/21% 242 ng/50%
52 ng/25% 132 ng/28% 264 ng/29%
a Estimated from the frequency increase upon NH treatment. b Calculated from frequency shifts before and after desorption. c Mass 3 decrement of the template as estimated from UV-vis data. d Desorbed template (UV) against total adsorbed mass (QCM). e Average frequency change of four separate experiments.
became totally absent upon ammonia treatment, and we can conclude that the template molecule was completely removed. The molecular extinction coefficient of C3AzoCO2H was found to be 1.72 × 104 L/mol‚cm in ethanol at 358 nm. We can then estimate the amount of the template molecule from the difference in absorbance before and after the template removal. Upon correction of the absorbance of the TiO2 film (Abs ) 0.058 at 358 nm), the absorbance increase inserted in Figure 3 is 0.202 for the 20-cycle adsorption (for both sides of the quartz plate). This figure corresponds to a concentration of the template molecule of 1.17 × 10-5 M by using the Lambert-Beer law for a cell length of 1 cm. The number of the template molecules adsorbed in the area of 1 nm2 for both sides of a quartz plate during 20 cycles becomes 70.4, since it is equal to the number of molecules existing in a volume of 1 × 1 × 107 nm3 of the 1.17 × 10-5 M solution. Therefore, the amount of guest molecule introduced in one adsorption cycle onto one side of the plate is determined to be 1.76 molecule/cycle‚nm2. We can also estimate the film composition from QCM frequency shift and UV data. The total frequency shift during 10 adsorption cycles is 1008 Hz for the 25 mM template solution (average of four independent experiments). This shift corresponds to a mass increase of 907 ng. On the other hand, the mass increase due to adsorption of the template molecule onto both sides of the electrode during 10 cycles is calculated as 264 ng from the molecular concentration at the surface (1.76 molecule/cycle‚nm2), the surface area of the electrode (0.159 cm2), and the molecular weight (MW: 284.31). The latter mass increase corresponds to 29 wt % of the total mass increase and is consistent with the film composition of Ti4O4(OH)4(OnBu)4C3AzoCO2H, since the amount of template molecules included in this formula is 31.6 wt %. This agrees with the original mixing ratio in the stock solution. Reflection FT-IR spectra of the 10-cycle film display peaks at 1600 and 1500 cm-1 characteristic of the azobenzene moiety, together with peaks due to titanium carboxylate at 1534 and 1416 cm-1. The νCdO peak of free carboxylic acid (1685.5 cm-1) is not found. It is clear that the structure of the Ti complex in solution is maintained also in the thin film. Table 1 summarizes the relation of the adsorbed and desorbed masses. The total adsorbed mass increases in proportion to the number of the adsorption cycle, as discussed above. The desorbed mass estimated from the frequency increase upon NH3 treatment is also enhanced from 56 to 190 ng corresponding to the adsorption cycle, and the ratio of desorbed mass against total adsorbed mass is in the range 25 ( 4% for samples 1-3. The amount of template molecule incorporated in the film and its desorption ratio against total adsorbed mass are estimated from UV data and are given in the table. The desorbed mass estimated as the QCM desorption ratio and the amount of incorporated template molecule estimated as
the UV desorption ratio agree for each sample within (20%. Thus, we are assured that the template molecule alone is desorbed completely in the ammonia treatment. The C3AzoCO2H/Ti(O-nBu)4 mixing ratio in sample 4 is higher than those of other samples. In this case, the adsorbed mass during the 10 cycles is decreased and the desorbed mass upon ammonia treatment is enhanced to 50% of the total adsorbed mass. Determination of Imprinting Efficiency by in Situ Measurement. We subsequently examined the imprinting efficiency by measuring in situ QCM decrements due to adsorption of guest molecules. The in situ experiment is best suited for determination of weakly-bound guest molecules. Tetrahydrofuran (THF) was selected as solvent for guests, because of its superior dissolution capacity toward a variety of carboxylic acids. The best medium for the adsorption experiment was acetonitrile. Other solvents such as chloroform and ethanol were not suitable, since the imprinted films were not stable in these solvents and/or the measured frequency was not stable. The imprinted films after the desorption treatment were placed in 1 mL of CH3CN with stirring until the frequency reached equilibration. Then, 5 µL of 50 mM guest in THF was added to give a final guest concentration of 0.25 mM. The frequency decrease due to addition of 5 µL of pure THF was 5 Hz and was corrected in the subsequent binding experiment. First, the effect of the film thickness (i.e., the number of the adsorption cycle) on the binding efficiency was tested. A TiO2 gel film obtained with 10 adsorption cycles gave a frequency decrease of 32 Hz when the original template, C3AzoCO2H (0.25 mM), was used as guest. This frequency decrement was reproducible within (1 Hz, as confirmed for four independently prepared films. The corresponding changes were 14 and 20 Hz with films of 2 and 5 adsorption cycles, respectively. The amount of the template bound (frequency decrease) per one cycle of adsorption changed from 7 to 4 to 3.2 Hz with increasing film thickness. This result may indicate that a thicker film is less efficient for accommodating guest molecules than a thinner film because of limited penetration of guest molecules into the inner void, although such small frequency differences are not very reliable. These figures cannot be directly compared with the frequency increase of the desorption process, since ∆F in in situ experiments cannot be directly converted to mass change due to viscoelastic effects of the adsorbed species.19,20 In spite of this experimental limitation, it is clear that the in situ frequency decrease due to readsorption of the template molecule (32 Hz, entry 1, Table 2) is much smaller than the frequency increase due to removal of the template (211 Hz, sample 3, Table 1). This suggests that the binding sites created by the imprinting are not fully (19) Okahata, Y.; Kimura, K.; Ariga, K. J. Am. Chem. Soc. 1989, 111, 9190-9194. (20) Okahata, Y.; Ebuto, H. Anal. Chem. 1989, 61, 2185-2188.
Azobenzene Carboxylic Acid on a TiO2 Ultrathin Film
Figure 4. In situ QCM frequency decreases due to rebinding of the template molecule to the imprinted films of sample 3 (Table 1). Five microliters of 50 mM C3AzoCO2H in THF was added into 1 mL of CH3CN at the time marked with an arrow to give a C3AzoCO2H concentration of 0.25 mM.
utilized or are partially destroyed in the subsequent binding experiment. Stability of the Imprinted Structure. We examined the structural stability of the imprinted film. The in situ QCM experiment was repeated with the identical 10-cycle film (Table 1, sample 3) after complete removal of the template by thorough washing with CH3CN. The film was equilibrated in CH3CN and then immersed in a 0.25 mM solution of the template. The mass increase process was fast and was over within 40-60 s. Then, the QCM plate was washed thoroughly with ethanol and subjected to a second experiment. It was confirmed that the original frequency was recovered by the washing. Figure 4 illustrates that reproducible desorption-adsorption cycles are achieved for at least 3 times. The frequency decrease is highly reproducible, giving a value of 32 ( 1 Hz. In contrast, a similarly prepared TiO2 film without the template shows a much smaller frequency decrease of only 2-3 Hz. It is clear that the rebinding of the C3AzoCO2H molecule is made possible by the preceding imprinting process and that the imprinted structure is maintained during repeated uses under the experimental conditions we used. Molecular Recognition by Imprinted Film. The preceding results assure us that we can use an identical film repeatedly for guest binding experiments. This fact is very advantageous when we make comparisons of the imprinting effect. The property of an imprinted film may change in a subtle manner, due to variation of the extent of hydrolysis of titanium alkoxide during film formation. The use of a single film allows us to avoid such problems. Figure 5 describes in situ experiments of binding of several aromatic acids toward the 10-cycle film. The QCM electrode was washed after each binding experiment with ethanol or methanol to remove the guest molecule. Complete removal of guests was confirmed by observation of the constant frequency. The adsorption process given in Figure 5 became essentially saturated in less than 1 min. The original template molecule of C3AzoCO2H showed a binding efficiency greater than those of other related carboxylic acids. The structurally closest acid, AzoCO2H, was the second best substrate, and benzoic acid and anthracene-9-carboxylic acid (9-AnCO2H) were least effective. Undoubtedly, the original template is recognized most effectively by this ultrathin gel film. A larger variety of compounds was tested for the imprinting efficiency of two kinds of films, M25 and M50, as given in Table 2. These films were prepared by 10 cycles of adsorption from 25 and 50 mM template solutions.
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Figure 5. In situ QCM frequency decreases due to binding of a series of carboxylic acids. Imprinted film, sample 3 in Table 1. Conditions of in situ experiments are identical to those of Figure 4. Table 2. Recognition Efficiency of Imprinted TiO2 Films for Various Guest Molecules
entry
guest molecule (molecular weight)
1 2 3 4 5 6 7 8 9 10 11
C3AzoCO2H (284.31) AzoCO2H (242.32) benzoic acid (122.12) cinnamic acid (148.16) octanoic acid (144.21) 2-AnCO2H (222.24) 9-AnCO2H (222.24) C3AzoCO2Me (298.34) C3AzoCO2iPr (326.41) 1-AdOH (152.24) anthracene (178.23)
in situ frequency relative binding decreasea (Hz) efficiencyb (%) M25
M50
M25
M50
32 24 5 8 5 15 8 6 4 1 2
23 16 3 4 3 9 6
100 88 36 48 31 60 33 18 14 5.8 10
100 82 30 33 26 50 33
a Corrected for the frequeency increment due to solvent THF (5 Hz). b The ratio of in situ frequency changes due to guest binding relative to that of rebinding of the template molecule (C3AzoCO2H). These ratios are corrected for molecular weights of bound species.
The QCM data are corrected by molecular weight. Among the compounds tested, the original template molecule gave the maximum binding for both films. The M25 film shows more efficient binding than the M50 film for all guest molecules. The relative binding efficiency is also given in this table by using the original template as reference. The imprinted films can discriminate the structural difference of the propyl group between C3AzoCO2H and AzoCO2H, since the relative efficiency of the latter guest is 80-90%. Other carboxylic acids (benzoic acid, cinnamic acid, and octanoic acid) are bound even less efficiently, reflecting lessened structural similarity with the template. Anthracenecarboxylic acids show 3060% binding efficiencies. It is interesting that the 2-isomer is a better substrate than the 9-isomer. The 2-isomer appears structurally closer to the original template than the 9-isomer, in terms of the geometrical disposition of carboxylic acid and aromatic moieties. Conversion of the carboxylic acid function to the corresponding ester depresses the binding efficiency, as is clearly illustrated by much lessened binding of the methyl and isopropyl esters. Anthracene itself and 1-adamantanol showed frequency changes of only 1-2 Hz. Thus, the presence of the carboxylic acid moiety is indispensable in the recognition process. Nature of Imprinted Site. Figure 6 contains a graphical presentation of the relative binding efficiency and a schematic illustration of the template-TiO2 gel complex. It is evident from the above results that the
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Figure 6. Relative binding of various guest molecules (a) and a schematic illustration of an imprinted film (b). The data in part a are the same as those listed in Table 2. Rebinding of C3AzoCO2H to the template film is used as reference and is corrected by molecular weight.
guest selectivity is associated with the molecular shape and the functional group. The structural element of this molecular recognition is made of at least three units: one is the carboxylic acid function, the second is an aromatic group juxtaposed to the carboxylic acid, and the third is the shape and size of the aromatic group. FT-IR spectra indicated that the templating azobenzene carboxylic acid reacted with titanium butoxide to form covalent linking in the adsorbing complex as well as in the adsorbed film. Incoming guest molecules can be bound to the TiO2 gel film in a similar manner if they have the carboxylic acid function. However, this could not be confirmed by experiment, and we illustrate in Figure 6b that the carboxylic acid in the guest molecule is bound either through covalent complexation or through noncovalent hydrogen bonding. The aromatic moiety in the guest would be surrounded by the hydrophobic part of the gel composed of the titanium-oxygen network and the unhydrolyzed butoxy group. The inner cavity must be more or less elongated in shape to best fit the azobenzene unit.
It is important to discuss here advantages and disadvantages of the present imprinting process in relation to the past inorganic imprinting technique. When the conventional sol-gel process was used to obtain imprinted cavities, time-consuming procedures of gel formation, pulverization, the extraction were commonly required. The amount of the incorporated guest was quite small.1,9 In order to avoid this problem, surface modification of silica particles was adopted by other research groups,5-7 and satisfactory substrate selectivity has been found. However, the molecular details of these modified surfaces are not clear. Molecular imprinting by chemical vapor deposition10 is unique, in that silicate overlayers are formed in the presence of densely adsorbed benzaldehyde. Well-characterized ultrathin films of silicate can be formed by this procedure. Unfortunately, the CVD condition requires a high temperature, and thermally labile organic compounds cannot be used as templates. We conclude from our experimental results that the surface sol-gel process is superior as a means of molecular imprinting to the past imprinting techniques.
Azobenzene Carboxylic Acid on a TiO2 Ultrathin Film
The substrate selectivitysshape, size, and functionalitysis readily attained, adsorption and desorption of guest molecules are very rapid, and imprinted sites can be created in high density. These advantages characterisitic of ultrathin films, together with versatile templating effects, provide unique possibilities for surface
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sol-gel films as new precision materials for sensing, separation, and catalysis. We believe further elaboration of the imprinting process will give improved structural selectivity. LA9801763
