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Selective electrochemical molecular recognition of benzenediol isomers using molecularly imprinted TiO2 film electrodes
Analytica Chimica Acta 506 (2004) 31–39
Selective electrochemical molecular recognition of benzenediol isomers using molecularly imprinted TiO2 film electrodes Shuangyan Huan a , Hui Chu a , Chenxu Jiao a , Guangming Zeng b , Guohe Huang b , Guoli Shen a,∗ , Ruqin Yu a a
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China b Department of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China Received 13 May 2003; received in revised form 28 October 2003; accepted 29 October 2003
Abstract In this paper, selective recognition of benzenediol isomers was studied using molecularly imprinted TiO2 film formed on a graphitic electrode. The imprinting process was investigated in detail using IR. The electrode process for p-hydroquinone follows a Er Cr mechanism. The cavities formed by p-phthalic acid have good selectivity toward p-hydroquinone among the isomers. The complex ratio between p-hydroquinone and binding sites was estimated to be 1:2 with an apparent equilibrium constant of 2.98 × 106 l2 mol−2 . For o-hydroquinone and m-benzenediol, the ratio decreased to 1:1 with an apparent equilibrium constant of 1.20 × 103 and 1.35 × 103 l mol−1 . The apparent complexing ratio and equilibrium constants could shed some insight on the nature of isomeric selectivity of the recognition sites with respect to different isomers of benzenediol. The cavities designed by o-phthalic acid present good selectivity toward o-hydroquinone. © 2003 Elsevier B.V. All rights reserved. Keywords: Molecular imprinting; Titanium dioxide; Isomeric selectivity; Complex
1. Introduction In recent years there has been considerable interest in the development of molecularly imprinted materials that open up new opportunities in separation, enzyme-like catalysis, and sensing technology [1–4]. Molecular imprinting has been proposed as one of the most promising techniques for preparing artificially generated molecular recognition materials, which might circumvent some difficulties associated with the use of biomolecules [5,6] and be useful for analytical separations, in particular chiral separation [7]. Traditionally, imprinted materials are synthesized through three-dimensional co-polymerization of functional and cross-linking monomers in the presence of a target analyte which serves as a template [8,9]. The functional groups in the monomers are spatially arranged either by covalent or by non-covalent interaction with template molecules during ∗ Corresponding author. Tel.: +86-731-8821355; fax: +86-731-8821818. E-mail address: [email protected] (G. Shen).
0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2003.10.082
the synthesis process. The subsequent removal of the template molecules leaves behind designed recognition sites that are ideally suited for template molecules. This approach suffers, however, from some limitations such as slow mass transfer, moderate sensitivity and selectivity, incomplete template removal, and broad guest affinity [10–12]. Heterogeneity in the imprinted sites is also a difficulty when the imprinting synthesis used non-covalent interactions [13]. Recently, the generation of imprinted sites in monolayer or thin-film assemblies was suggested as a way to eliminate the diffusion barriers for the approach of target analytes toward the recognition sites and improve electrical communication between recognition sites and transducer [14,15]. Imprinting methodology, that involves covalent interactions within bulk silica reported by Katz and Davis, was proposed to circumvent the heterogeneity in the imprinted sites formed [11]. Zimmerman et al. [13] reported an alternative strategy described as “dynamic imprinting” of dendritic macromolecules with a porphyrin template to yield synthetic host molecule containing one binding site each. This approach ensured nearly homogeneous binding sites and
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quantitative template removal. These kinds of recognition materials, while conceptually idea for recognition, are difficult to synthesize experimentally. Lahav et al. described the imprinting of chloroaromatic acids and chiral carboxylic acids using TiO2 thin films [16,17]. As an ion-sensitive field-effect transistor, the sensor exhibited an impressive selectivity toward the imprinted substrate, with a response time of about 5 min. The hydrolysis of titanium alkoxides, Ti(OR)4 , is one of the standard routes to prepare the fine particles of TiO2 , according to the following reaction: Ti(OR)4 + 4H2 O → TiO2 + 4ROH Several synthetic chemical routes have been investigated to prepare TiO2 composed of fine particles [18–21]. In this paper, an imprinting technology that involves coordination interactions is proposed to form recognition sites in the TiO2 film. The imprinting process, together with the removal of the template molecules, was investigated by IR.
Electrochemical methods were used to characterize the analytical behavior and isomeric selectivity of the molecularly imprinted TiO2 film formed on a graphitic electrode. The apparent complexing ratio and equilibrium constants were also evaluated. 2. Experimental 2.1. Apparatus and material Amperometric measurements were carried out using Potentiostat/Galvanostat Model 273 (EG&G Princeton Applied Research) controlled by the M270 software. A threeelectrode configuration containing a homemade graphitic working electrode (s = 0.5 cm2 ), a SCE reference electrode and a Pt counter electrode was used for electrochemical measurements. For the infrared measurements a Fourier infrared spectrophotometer, Model AQS-20 (Analect Instruments, USA) was used.
Fig. 1. Schematic preparation procedures used to create the imprinted TiO2 film for recognition of p-hydroquinone.
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p-Phthalic acid, o-phthalic acid, p-hydroquinone, o-hydroquinone and m-benzenediol were of analytical grade and used as received. Doubly distilled water was employed to prepare all solutions.
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ability investigation, the TiO2 film imprinted by p-phthalic acid and o-phthalic acid formed on the graphitic electrodes were immersed in the mixture solution 0.10 mM in p-, o- and m-benzenediol for 30 min and then cycled between +0.85 and −0.50 V in blank phosphate buffer solution.
2.2. The modification of the graphitic electrode Under constant stirring, 10 ml of 0.085 M p-phthalic acid in 2-propanol was slowly added to 10 ml of 0.34 M titanium(IV) isopropoxide also dissolved in 2-propanol. After addition of two drops of diethylene glycol, the resulting solution was slowly hydrolyzed by drop wise addition of 4 ml of ethanol containing 0.50 ml of water at the rate of 1 drop/10 s under constant stirring at room temperature. The fine suspension of TiO2 sol became a yellowish sol when gradually heated to 85 ◦ C for 4 h under constant stirring. A polished graphitic electrode was immersed in the solution for 20 min and dried in air and then heated in oven at 85 ◦ C overnight. The electrode was then treated with 1% (v/v) ammonia solution for 20 min and cleaned with doubly distilled water. For comparison, similar electrodes were prepared by using o-phthalic acid as template or using benzene as a “dummy” molecule. Fig. 1 outlines the procedure for preparing the electrode based on molecularly imprinted TiO2 film. Infrared adsorption spectra of the imprinted particles before (‘b’ in Fig. 2) and after (‘c’ in Fig. 2) the treatment with ammonia solution were compared with pure p-phthalic acid (‘a’ in Fig. 2) and TiO2 (‘d’ in Fig. 2). 2.3. Electrochemical measurements The graphitic electrode modified with imprinted TiO2 film was immersed in a phosphate buffer solution of pH 7.95, and cycled between +0.85 and −0.50 V at 30 mV s−1 . Different volumes of p-, o- or m-benzenediol were added and the response was recorded with the blank corrected. The response on the bare electrode, imprinted electrode and “dummy” molecularly imprinted electrode were studied. For uptake a 1678cm -1
c 1400cm-1 1633cm-1 d -1
1638cm
4000
3000
2000
1500
3.1. Preparation of the imprinted TiO2 film Lahav et al. [16,17] used chloroaromatic acids and chiral carboxylic acids as templates to prepare imprinted TiO2 films. They claimed that titanium(IV) butoxide reacted with carboxylic acid to form a mixture that included the titanium(IV) butoxide–carboxylate complex and the sol–gel polymerization of the mixture would result in formation of the TiO2 film with embedded carboxylate. Treatment of the film with ammonia solution resulted in elimination of the carboxylate templates and formation of the recognition sites. In the present paper, instead of titanium(IV) butoxide and chloroaromatic acids we used titanium(IV) isopropoxide and phthalic acids, respectively. Phthalic acid was chosen as template for its two equivalent carboxylic groups, which make it possible to characterize the isomeric selectivity of the cavities formed with respect to para-, ortho- and meta-isomers of benzenediol. One could assume that, as illustrated by Fig. 1, when titanium(IV) isopropoxide is mixed with p-phthalic acid as a template, reactions take place between the two carboxylic groups and the isopropoxy groups connected with two Ti(IV) atoms. Then a 2:1 titanium(IV) isopropoxide–carboxylate complex is formed. After hydrolysis the templates are embedded in the titanium sol, which will be cast onto a graphite surface as a film. The treatment of the film with ammonia solution results in elimination of the templates and formation of cavities of appropriate size and shape in the TiO2 film. The cavities formed will exactly match the template molecules. As phthalic acid is a dicarboxylic acid with three isomers, one would expect that the imprinted TiO2 film could be tailor-made for one of these three isomers by using corresponding para-, orthoor meta-isomers. 3.2. IR characterization
1540cm-1 1407cm-1
b
%T
1284cm-1
3. Results and discussion
1000
450.
cm-1
Fig. 2. IR adsorption spectra of (a) pure p-phthalic acid, (b) TiO2 imprinted by p-phthalic acid before washing with ammonia solution, (c) TiO2 imprinted by p-phthalic acid after washing with ammonia solution, (d) pure TiO2 .
Lahav et al. postulated the formation of titanium(IV) butoxide–carboxylate complex by chemical reasoning. In a study of the sol–gel process of titanium alkoxide with acetate anion, Murakam et al. [20] investigated the coordination reaction using IR and UV spectra. The UV spectra seem not to provide much information. In the present paper, the coordination between titanium(IV) isopropoxide and p-phthalic acid was investigated by recording the IR spectra. From Fig. 2 one can see that compared with pure p-phthalic acid, new adsorption bands appears in the infrared adsorption spectrum of the imprinted particles (b). The ch-
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0.6 Epa 0.4 Potential (V)
aracteristics of the new adsorption bands observed are quite different from those for monodentate in Ti(IV)–carboxylate complexes observed at 1295 (symmetric) and 1720 cm−1 (antisymmetric), but similar to those for bidentate of Ti(IV)– carboxylate complexes observed at 1430–1470 (symmetric) and 1550–1590 cm−1 (antisymmetric) [20]. The observed bands are therefore attributed to the COO− stretching bands for bidentate of the titanium oxide–p-phthalic acid complex. This is direct experimental evidence indicating that during the imprinting process p-phthalic acid reacted with titanium(IV) isopropoxide to form a complex and was embedded in the TiO2 film. After the treatment of the imprinted particles with ammonia the adsorption bands attributed to COO− and benzene ring stretching disappear (‘c’ in Fig. 2). The adsorption bands corresponding to Ti–O–Ti stretching are observed at 600–900 cm−1 , which indicated the formation of titanium dioxide. The small bands at about 1633 and 1400 cm−1 belong to the adsorption band of water that adsorbed on the titanium dioxide surface. Therefore, the treatment of the electrode with ammonia would destroy the bidentate of the titanium oxide–p-phthalic acid complex and result in the formation of recognition sites in the titanium dioxide film. Using the proposed procedure almost quantitative template removal could be obtained. One would propose that the cavities observed would be formed by multiple template molecules as each template molecule could form two bonds with the central metal of the matrix and experiments showed that the template species could not be removed by water or organic solvent.
Epc
-0.2
2
4
6
(a)
(in acidic media)
C6 H4 O2 + 2e− + 2H2 O C6 H4 (OH)2 + 2OH− (in basic media) With pH value increasing, p-hydroquinone becomes easy to be oxidized, so oxidation potential decreases more significantly than reduction potential. The difference between reduction potential (Epc ) and oxidation potential (Epa ) decreases indicating increased reversibility of the electrode reaction. The currents in weak basic solution are all higher than the currents in acid or strong basic solutions as one can see in Fig. 3b. The reduction peak currents have a maximum value at pH 8. When pH values are higher than 8, the current response decreases. Therefore, the major electrochemical experiments are conducted at about pH 7.96.
10
12
Ipc 30
20
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Ipa
-10
The effect of pH on the peak potential and current due to p-hydroquinone shown in Fig. 3a and b was recorded with the modified electrodes in Britton–Robinson buffer solutions. The potential of oxidation and reduction peaks moves negatively with increasing pH values (Fig. 3a). Compared with Epa , Epc appears to have significantly lower slope. It is thought that the redox reaction conducted as following:
8 pH
40
3.3. Effect of pH
C6 H4 O2 + 2e− + 2H+ C6 H4 (OH)2
0.2
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Peak current (µA)
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-20
-30 0 (b)
2
4
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pH
Fig. 3. Effect of the solution pH on the (a) reduction (Epc ) and oxidation (Epa ) peak potentials, (b) on the reduction (Ipc ) and oxidation (Ipa ) peak currents.
Electrochemical response could be observed on the imprinted electrode means that the cavities are contacted with electrode surface. This may originate either from very thin TiO2 layers on the porous graphitic support, or from a high density of cavities that intercommunicate one with the other. In our experiment, fine response is observed with sub smooth graphitic surface and diluted TiO2 suspension solution for coating. So the former reason is more reasonable. 3.4. The electrode process mechanism The relationship between peak current and scan rate is shown in Fig. 4. Both the reduction and oxidation peak currents increase linearly with increasing scan rate, with
S. Huan et al. / Analytica Chimica Acta 506 (2004) 31–39 80
35
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60 100
Ipc
Current(µA)
Peak current (µA)
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Fig. 4. The plot of peak current (Ipc and Ipa ) vs. scan rate (ν).
correlative coefficients of 0.9977 and 0.9996, respectively, indicating a surface-controlled electrode process. The slope of the line for reduction is 1.8-fold the slope of the line for oxidation. It is thought that the recognition sites formed in the TiO2 film have higher affinity toward reduction product than oxidation product, and the adsorption of reduction product will also influence the electrode process. So the reduction current slope is higher than the oxidation current. Two electrons are involved in the charge transfer reaction. The current function (ϕ) was defined by Nicholson and Shain as follows [22]:
-0.2
-0.4
p-hydroquinone and 0.1 mM o-hydroquinone broad oxidation and reduction peaks are observed on bare electrode (‘a’ and ‘b’ in Fig. 5) with similar current intensity. Little response observed in 0.1 mM m-benzenediol (‘c’ in Fig. 5). On the imprinted electrode, the peaks for p-hydroquinone present more nicely shaped redox peaks (Fig. 6). But the current intensity for p-hydroquinone is about two times the other isomers. As electrode imprinted by “dummy” molecule
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This correlation is easy to obtain by plotting Ip /ν1/2 versus ν. In our experiment, the current function increases gradually with increasing scan rate. The ratio of anodic to cathodic peak current approaches 1 at very low scan rate and decreases with increasing scan rate. From the ϕ–ν graph and the graph of Ipa /Ipc versus ν in [22], we propose that the electrode process is a traditional Er Cr mechanism:
O∗ O
0.4
Fig. 5. The cyclic voltammograms for (a) 0.10 mM p-hydroquinone, (b) 0.10 mM o-hydroquinone and (c) 0.10 mM m-benzenediol on the bare graphitic electrode in phosphate buffer solution of pH 7.95. Scan rate: 30 mV s−1 .
i nFν 1/2 C RT nFAD1/2
R O∗ + ne−
0.6
Potential
100 a Current (µA)
ϕ=
0.8
120
b
50
0
(charge transfer)
(reversible chemical reaction)
-50
c
O∗
R represents reduced species, represents direct electrode reaction product and O represents final oxidized product. -100 0.8
3.5. The isomeric selectivity of the imprinted film The cyclic voltammograms for individual isomers on the bare and imprinted electrode in phosphate buffer solutions (pH = 7.95) were recorded (Figs. 5 and 6). In 0.1 mM
0.6
0.4
0.2
0.0
-0.2
-0.4
Potential (V)
Fig. 6. The cyclic voltammograms for (a) 0.10 mM p-hydroquinone, (b) 0.10 mM o-hydroquinone and (c) 0.10 mM m-benzenediol on the electrode imprinted by p-phthalic acid in phosphate buffer solution of pH 7.95. Scan rate: 30 mV s−1 .
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12
60
a
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Current (µA)
Peak current (µA)
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Concentration (mM)
Fig. 7. The concentration-dependent peak current after blank correction for (a) p-hydroquinone, (b) o-hydroquinone and (c) m-benzenediol.
benzene is concerned, no response is observed (not shown). Benzene could not form desired cavities through coordination interaction as p-phthalic acid. The TiO2 film formed is very compact, and there are almost no channels for the benzenediol to approach the electrode surface, so no electrochemical response is observed. The redox peaks for p-hydroquinone are observed at −0.070 (reduction) and +0.012 V (oxidation). The redox peaks due to o-hydroquinone appear at +0.050 (reduction) and +0.108 V (oxidation). Only one oxidation peak is observed for m-benzenediol at +0.50 V. The peak current changes for each analyte after blank correction are plotted versus the concentration of the corresponding analytes in Fig. 7. The peak currents increase linearly with increasing concentration of p-hydroquinone and o-hydroquinone (‘a’ and ‘b’ in Fig. 7), while the relationship between peak current and m-benzenediol concentrations is not linear (‘c’ in Fig. 7). The sensitivity of the response for p-hydroquinone is significantly higher than that for the other two isomers. The difference in response sensitivity of the three isomers is clearly associated with the shape-selective property of the imprinted TiO2 film formed on the electrode. The effect of electrolyte background was studied as shown in Fig. 8. The reduction and oxidation peaks were recorded on the imprinted electrode in Britton–Robinson buffer solutions (pH = 7.96) and in phosphate buffer solutions (pH = 7.95) both containing p-hydroquinone of the same concentration. The Britton–Robinson buffer gave peaks shifted toward more negative potentials with higher peak current and better reversibility. Similar phenomena were also being observed in case of the mixture of isomers (not shown). Boric acid can form negatively charged complex ion with polyhydroxylate compounds in weak basic medium. One would propose that boric acid in Britton–Robinson buffer
Fig. 8. The voltammograms of 0.1 mM p-hydroquinone on the imprinted electrode (a) in Britton–Robinson buffer solution (pH = 7.96) and (b) in phosphate buffer solution (pH = 7.95). Scan rate: 30 mV s−1 .
could influence the bonding between the phenolic hydroxyl and the hydroxyl group on the titanium matrix. On the other hand, in phosphate buffer solutions the difference in peak potentials of p-hydroquinone and o-hydroquinone increases to 0.22 V. Detection of p-hydroquinone in the presence of 0.40 mM o-hydroquinone and m-benzenediol can also be carried out in phosphate buffer solutions (not shown). Most of the experiments are conducted in phosphate buffer solutions. 3.6. The complexing ratio of benzenediol and binding sites The formation of the designed cavities in the imprinted film made use of coordination complex formation as discussed in previous section, seemed to depend on the interaction between the hydroxyl groups of benzenediol and the hydroxyl groups (binding sites) on the film matrix. One can write: mA + nN (membrane) Am Nn
(1)
A and N represent benzenediol and binding sites (Ti–OH) in the cavities, respectively. According to the law of mass action the corresponding apparent equilibrium constant K can be expressed as the following: [Am Nn ] K= (2) [A]m [N]n A parameter α is defined as the ratio of free and total number of recognition sites: [N]f Ilim − I α= = (3) [N]t Ilim − I0 I0 is the peak current of the imprinted film in blank solution and Ilim is the saturated peak current when all the
S. Huan et al. / Analytica Chimica Acta 506 (2004) 31–39
recognition sites are all complexed by benzenediol. I is the peak current of the imprinted film actually measured when contacting with benzenediol of different concentrations. The relationship between α and benzenediol concentration [A] can be expressed as α 1 = n−1 1−α nK[N] [A]m
37
librium constant K. The curve fitting for different isomers are shown in Fig. 9. At low concentration p-hydroquinone yields a fairly good fit to curve 2 (Fig. 9a). However, for o-hydroquinone and m-benzenediol, the experimental data points are best fitted to curve 3 in Fig. 9b and c. This suggests a 1:2 p-hydroquinone/binding sites complexing ratio with a reasonable apparent equilibrium constant of 2.98 × 106 l2 mol−2 . So far as o-hydroquinone and m-benzenediol are concerned, the experimental data are best fitted to the curves of 1:1 complex with an apparent equilibrium constant of 1.20 × 103 and 1.35 × 103 l mol−1 , respectively.
(4)
The functional relationship between α and the concentration of benzenediol is governed by different m, n and K. The experimental data were fitted to Eq. (4) by changing the ratio of m:n and adjusting the overall apparent equi-
1.0
1.0
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12345
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12345
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α
α
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α
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0.0 -6
(c)
-4
-2
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Log [m-benzenediol]
Fig. 9. The parameter values (α) as a function of log[benzenediol]. The curves fitting the experimental data were calculated from Eq. (4). The circles are experimental observed data points. (a) For p-hydroquinone, (1) m:n = 1:3, K = 1.51×109 l3 mol−3 ; (2) m:n = 1:2, K = 2.98×106 l2 mol−2 ; (3) m:n = 1:1, K = 1.01 × 103 l mol−1 ; (4) m:n = 2:1, K = 1.01 × 106 l2 mol−2 ; (5) m:n = 3:1, K = 9.11 × 108 l3 mol−3 . (b) For o-hydroquinone, (1) m:n = 1:3, K = 1.98 × 109 l3 mol−3 ; (2) m:n = 1:2, K = 1.30 × 106 l2 mol−2 ; (3) m:n = 1:1, K = 1.20 × 103 l mol−1 ; (4) m:n = 2:1, K = 1.62 × 106 l2 mol−2 ; (5) m:n = 3:1, K = 1.90 × 109 l3 mol−3 . (c) For m-benzenediol, (1) m:n = 1:3, K = 1.86 × 109 l3 mol−3 ; (2) m:n = 1:2, K = 1.43 × 106 l2 mol−2 ; (3) m:n = 1:1, K = 1.35 × 103 l mol−1 ; (4) m:n = 2:1, K = 1.86 × 106 l2 mol−2 ; (5) m:n = 3:1, K = 2.38 × 109 l3 mol−3 .
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signed by o-phthalic acid present good selectivity toward ohydroquinone.
100
4. Conclusions Current (µA)
50
0
-50
a b
-100 0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
Potential (V)
Fig. 10. The voltammograms recorded on the electrodes imprinted by (a) p-phthalic acid and (b) o-phthalic acid in blank phosphate buffer solution after contacting with solution containing 0.10 mM p-hydroquinone, 0.10 mM o-hydroquinone and 0.10 mM m-benzenediol for 30 min. Scan rate: 30 mV s−1 .
The aforementioned apparent complexing ratio and equilibrium constants could shed some insight on the nature of isomeric selectivity of the recognition sites with respect to different isomers of benzenediol. The binding sites formed by p-phthalic acid as a template have relatively high binding affinity toward p-hydroquinone, which is well shape-matched to the recognition sites. While o-hydroquinone and m-benzenediol, which are not shape-matched, can only form the 1:1 complex with the recognition sites with low affinity. Therefore, the TiO2 films imprinted by p-phthalic acid have relatively higher selectivity toward p-hydroquinone than o-hydroquinone and m-benzenediol. 3.7. Selective uptake ability for different isomers After contacting with the mixture solution containing 0.20 mM p-hydroquinone, o-hydroquinone and m-benzenediol each for 30 min, the redox peaks of p-hydroquinone are observed obviously on the electrode imprinted by p-phthalic acid, while the redox peaks of o-hydroquinone are observed on the electrode imprinted by o-phthalic acid (Fig. 10). This confirms that imprinting methodology that involves covalent interactions within TiO2 film is promising for designing shape-selective cavities for recognition. Although we have to use benzenediol rather than the template molecules phthalic acids themselves simply because the later are electrochemically unactive, the molecular structure and the nature of bonding atoms involved for benzenediol and phthalic acids are the same or quite similar. So the cavities spatially organized by p-phthalic acid have good selectivity toward p-hydroquinone, while the cavities de-
The imprinting methodology that involves covalent interactions has been proved promising for designing shapeselective cavities in the TiO2 film. The template could be removed quantitatively by treating with ammonia solution. The cavities spatially organized by p-phthalic acid have good selectivity toward p-hydroquinone, while the cavities designed by o-phthalic acid present good selectivity toward o-hydroquinone. The TiO2 films prepared by the proposed imprinting methodology have pre-defined selectivity and affinity toward specified analytes. The proposed method could be extended to separation and analysis of some enantiomers if coupling with direct spectroscopic determination.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 20075006, 70171055 and 50179011) and the Foundation for Ph.D. Thesis Research (No. 20010532008), and the National 863 High Technologies Research Foundation of China (No. 2001AA644020). References [1] K. Haupt, K. Mosbach, Chem. Rev. 100 (2000) 2495. [2] G. Wulff, Chem. Rev. 102 (2002) 1. [3] F.L. Dickert, M. Tortschanoff, W.E. Bulst, G. Fischerauer, Anal. Chem. 71 (1999) 4559. [4] K. Haupt, K. Noworyta, W. Kutner, Anal. Commun. 36 (1999) 391. [5] S. Kröger, A.F.P. Turner, K. Mosbach, K. Haupt, Anal. Chem. 71 (1999) 3698. [6] D. Spivak, M.A. Gilmove, K.J. Shea, J. Am. Chem. Soc. 119 (1997) 4388. [7] E. Bellacchio, R. Lauceri, S. Gurrieri, L.M. Scolaro, A. Romeo, R. Purrello, J. Am. Chem. Soc. 120 (1998) 12353. [8] J.V. Beach, K.J. Shea, J. Am. Chem. Soc. 116 (1994) 379. [9] C. Yu, K. Mosbach, J. Org. Chem. 62 (1997) 4057. [10] N. Sallacan, M. Zayats, T. Bourenko, A.B. Kharitonov, I. Willner, Anal. Chem. 74 (2002) 702. [11] A. Katz, M. Davis, Nature 403 (2000) 286. [12] G. Vlatakis, L.I. Andersson, R. Muller, K. Mosbach, Nature 361 (1993) 645. [13] S.C. Zimmerman, M.S. Wendland, N.A. Rakow, I. Zharov, K.S. Suslick, Nature 418 (2002) 399. [14] M. Lahav, E. Katz, A. Doron, F. Patolsky, I. Willner, J. Am. Chem. Soc. 121 (1999) 862. [15] V.M. Mirsky, T. Hirsch, S.A. Piletsky, O.S.A. Wolfbeis, Angew. Chem. Int. Ed. 38 (1999) 1108. [16] M. Lahav, A.B. Kharitonov, O. Katz, T. Kunitake, I. Willner, Anal. Chem. 73 (2001) 720. [17] M. Lahav, A.B. Kharitonov, I. Willner, Chem.-A Eur. J. 7 (2001) 3992.
S. Huan et al. / Analytica Chimica Acta 506 (2004) 31–39 [18] L. Znaidi, R. Seraphimova, J.F. Bocquet, C. Colbeau-Justin, C.A. Pommier, Mater. Res. Bull. 36 (2001) 811. [19] K.L. Gnanasekar, V. Subramanian, J. Robinson, J.C. Jiang, F.E. Posey, B. Rambabu, J. Mater. Res. 17 (2002) 1507. [20] Y. Murakami, T. Matsumoto, Y. Takasu, J. Phys. Chem. B103 (1999) 1836.
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[21] Y. Hamasaki, S. Ohkubo, K. Murakami, H. Sei, G. Nogami, J. Electrochem. Soc. 141 (1994) 660. [22] R.S. Nicholson, I. Shain, Anal. Chem. 36 (1964) 706.
Selective electrochemical molecular recognition of benzenediol isomers using molecularly imprinted TiO2 film electrodes Shuangyan Huan a , Hui Chu a , Chenxu Jiao a , Guangming Zeng b , Guohe Huang b , Guoli Shen a,∗ , Ruqin Yu a a
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China b Department of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China Received 13 May 2003; received in revised form 28 October 2003; accepted 29 October 2003
Abstract In this paper, selective recognition of benzenediol isomers was studied using molecularly imprinted TiO2 film formed on a graphitic electrode. The imprinting process was investigated in detail using IR. The electrode process for p-hydroquinone follows a Er Cr mechanism. The cavities formed by p-phthalic acid have good selectivity toward p-hydroquinone among the isomers. The complex ratio between p-hydroquinone and binding sites was estimated to be 1:2 with an apparent equilibrium constant of 2.98 × 106 l2 mol−2 . For o-hydroquinone and m-benzenediol, the ratio decreased to 1:1 with an apparent equilibrium constant of 1.20 × 103 and 1.35 × 103 l mol−1 . The apparent complexing ratio and equilibrium constants could shed some insight on the nature of isomeric selectivity of the recognition sites with respect to different isomers of benzenediol. The cavities designed by o-phthalic acid present good selectivity toward o-hydroquinone. © 2003 Elsevier B.V. All rights reserved. Keywords: Molecular imprinting; Titanium dioxide; Isomeric selectivity; Complex
1. Introduction In recent years there has been considerable interest in the development of molecularly imprinted materials that open up new opportunities in separation, enzyme-like catalysis, and sensing technology [1–4]. Molecular imprinting has been proposed as one of the most promising techniques for preparing artificially generated molecular recognition materials, which might circumvent some difficulties associated with the use of biomolecules [5,6] and be useful for analytical separations, in particular chiral separation [7]. Traditionally, imprinted materials are synthesized through three-dimensional co-polymerization of functional and cross-linking monomers in the presence of a target analyte which serves as a template [8,9]. The functional groups in the monomers are spatially arranged either by covalent or by non-covalent interaction with template molecules during ∗ Corresponding author. Tel.: +86-731-8821355; fax: +86-731-8821818. E-mail address: [email protected] (G. Shen).
0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2003.10.082
the synthesis process. The subsequent removal of the template molecules leaves behind designed recognition sites that are ideally suited for template molecules. This approach suffers, however, from some limitations such as slow mass transfer, moderate sensitivity and selectivity, incomplete template removal, and broad guest affinity [10–12]. Heterogeneity in the imprinted sites is also a difficulty when the imprinting synthesis used non-covalent interactions [13]. Recently, the generation of imprinted sites in monolayer or thin-film assemblies was suggested as a way to eliminate the diffusion barriers for the approach of target analytes toward the recognition sites and improve electrical communication between recognition sites and transducer [14,15]. Imprinting methodology, that involves covalent interactions within bulk silica reported by Katz and Davis, was proposed to circumvent the heterogeneity in the imprinted sites formed [11]. Zimmerman et al. [13] reported an alternative strategy described as “dynamic imprinting” of dendritic macromolecules with a porphyrin template to yield synthetic host molecule containing one binding site each. This approach ensured nearly homogeneous binding sites and
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quantitative template removal. These kinds of recognition materials, while conceptually idea for recognition, are difficult to synthesize experimentally. Lahav et al. described the imprinting of chloroaromatic acids and chiral carboxylic acids using TiO2 thin films [16,17]. As an ion-sensitive field-effect transistor, the sensor exhibited an impressive selectivity toward the imprinted substrate, with a response time of about 5 min. The hydrolysis of titanium alkoxides, Ti(OR)4 , is one of the standard routes to prepare the fine particles of TiO2 , according to the following reaction: Ti(OR)4 + 4H2 O → TiO2 + 4ROH Several synthetic chemical routes have been investigated to prepare TiO2 composed of fine particles [18–21]. In this paper, an imprinting technology that involves coordination interactions is proposed to form recognition sites in the TiO2 film. The imprinting process, together with the removal of the template molecules, was investigated by IR.
Electrochemical methods were used to characterize the analytical behavior and isomeric selectivity of the molecularly imprinted TiO2 film formed on a graphitic electrode. The apparent complexing ratio and equilibrium constants were also evaluated. 2. Experimental 2.1. Apparatus and material Amperometric measurements were carried out using Potentiostat/Galvanostat Model 273 (EG&G Princeton Applied Research) controlled by the M270 software. A threeelectrode configuration containing a homemade graphitic working electrode (s = 0.5 cm2 ), a SCE reference electrode and a Pt counter electrode was used for electrochemical measurements. For the infrared measurements a Fourier infrared spectrophotometer, Model AQS-20 (Analect Instruments, USA) was used.
Fig. 1. Schematic preparation procedures used to create the imprinted TiO2 film for recognition of p-hydroquinone.
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p-Phthalic acid, o-phthalic acid, p-hydroquinone, o-hydroquinone and m-benzenediol were of analytical grade and used as received. Doubly distilled water was employed to prepare all solutions.
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ability investigation, the TiO2 film imprinted by p-phthalic acid and o-phthalic acid formed on the graphitic electrodes were immersed in the mixture solution 0.10 mM in p-, o- and m-benzenediol for 30 min and then cycled between +0.85 and −0.50 V in blank phosphate buffer solution.
2.2. The modification of the graphitic electrode Under constant stirring, 10 ml of 0.085 M p-phthalic acid in 2-propanol was slowly added to 10 ml of 0.34 M titanium(IV) isopropoxide also dissolved in 2-propanol. After addition of two drops of diethylene glycol, the resulting solution was slowly hydrolyzed by drop wise addition of 4 ml of ethanol containing 0.50 ml of water at the rate of 1 drop/10 s under constant stirring at room temperature. The fine suspension of TiO2 sol became a yellowish sol when gradually heated to 85 ◦ C for 4 h under constant stirring. A polished graphitic electrode was immersed in the solution for 20 min and dried in air and then heated in oven at 85 ◦ C overnight. The electrode was then treated with 1% (v/v) ammonia solution for 20 min and cleaned with doubly distilled water. For comparison, similar electrodes were prepared by using o-phthalic acid as template or using benzene as a “dummy” molecule. Fig. 1 outlines the procedure for preparing the electrode based on molecularly imprinted TiO2 film. Infrared adsorption spectra of the imprinted particles before (‘b’ in Fig. 2) and after (‘c’ in Fig. 2) the treatment with ammonia solution were compared with pure p-phthalic acid (‘a’ in Fig. 2) and TiO2 (‘d’ in Fig. 2). 2.3. Electrochemical measurements The graphitic electrode modified with imprinted TiO2 film was immersed in a phosphate buffer solution of pH 7.95, and cycled between +0.85 and −0.50 V at 30 mV s−1 . Different volumes of p-, o- or m-benzenediol were added and the response was recorded with the blank corrected. The response on the bare electrode, imprinted electrode and “dummy” molecularly imprinted electrode were studied. For uptake a 1678cm -1
c 1400cm-1 1633cm-1 d -1
1638cm
4000
3000
2000
1500
3.1. Preparation of the imprinted TiO2 film Lahav et al. [16,17] used chloroaromatic acids and chiral carboxylic acids as templates to prepare imprinted TiO2 films. They claimed that titanium(IV) butoxide reacted with carboxylic acid to form a mixture that included the titanium(IV) butoxide–carboxylate complex and the sol–gel polymerization of the mixture would result in formation of the TiO2 film with embedded carboxylate. Treatment of the film with ammonia solution resulted in elimination of the carboxylate templates and formation of the recognition sites. In the present paper, instead of titanium(IV) butoxide and chloroaromatic acids we used titanium(IV) isopropoxide and phthalic acids, respectively. Phthalic acid was chosen as template for its two equivalent carboxylic groups, which make it possible to characterize the isomeric selectivity of the cavities formed with respect to para-, ortho- and meta-isomers of benzenediol. One could assume that, as illustrated by Fig. 1, when titanium(IV) isopropoxide is mixed with p-phthalic acid as a template, reactions take place between the two carboxylic groups and the isopropoxy groups connected with two Ti(IV) atoms. Then a 2:1 titanium(IV) isopropoxide–carboxylate complex is formed. After hydrolysis the templates are embedded in the titanium sol, which will be cast onto a graphite surface as a film. The treatment of the film with ammonia solution results in elimination of the templates and formation of cavities of appropriate size and shape in the TiO2 film. The cavities formed will exactly match the template molecules. As phthalic acid is a dicarboxylic acid with three isomers, one would expect that the imprinted TiO2 film could be tailor-made for one of these three isomers by using corresponding para-, orthoor meta-isomers. 3.2. IR characterization
1540cm-1 1407cm-1
b
%T
1284cm-1
3. Results and discussion
1000
450.
cm-1
Fig. 2. IR adsorption spectra of (a) pure p-phthalic acid, (b) TiO2 imprinted by p-phthalic acid before washing with ammonia solution, (c) TiO2 imprinted by p-phthalic acid after washing with ammonia solution, (d) pure TiO2 .
Lahav et al. postulated the formation of titanium(IV) butoxide–carboxylate complex by chemical reasoning. In a study of the sol–gel process of titanium alkoxide with acetate anion, Murakam et al. [20] investigated the coordination reaction using IR and UV spectra. The UV spectra seem not to provide much information. In the present paper, the coordination between titanium(IV) isopropoxide and p-phthalic acid was investigated by recording the IR spectra. From Fig. 2 one can see that compared with pure p-phthalic acid, new adsorption bands appears in the infrared adsorption spectrum of the imprinted particles (b). The ch-
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0.6 Epa 0.4 Potential (V)
aracteristics of the new adsorption bands observed are quite different from those for monodentate in Ti(IV)–carboxylate complexes observed at 1295 (symmetric) and 1720 cm−1 (antisymmetric), but similar to those for bidentate of Ti(IV)– carboxylate complexes observed at 1430–1470 (symmetric) and 1550–1590 cm−1 (antisymmetric) [20]. The observed bands are therefore attributed to the COO− stretching bands for bidentate of the titanium oxide–p-phthalic acid complex. This is direct experimental evidence indicating that during the imprinting process p-phthalic acid reacted with titanium(IV) isopropoxide to form a complex and was embedded in the TiO2 film. After the treatment of the imprinted particles with ammonia the adsorption bands attributed to COO− and benzene ring stretching disappear (‘c’ in Fig. 2). The adsorption bands corresponding to Ti–O–Ti stretching are observed at 600–900 cm−1 , which indicated the formation of titanium dioxide. The small bands at about 1633 and 1400 cm−1 belong to the adsorption band of water that adsorbed on the titanium dioxide surface. Therefore, the treatment of the electrode with ammonia would destroy the bidentate of the titanium oxide–p-phthalic acid complex and result in the formation of recognition sites in the titanium dioxide film. Using the proposed procedure almost quantitative template removal could be obtained. One would propose that the cavities observed would be formed by multiple template molecules as each template molecule could form two bonds with the central metal of the matrix and experiments showed that the template species could not be removed by water or organic solvent.
Epc
-0.2
2
4
6
(a)
(in acidic media)
C6 H4 O2 + 2e− + 2H2 O C6 H4 (OH)2 + 2OH− (in basic media) With pH value increasing, p-hydroquinone becomes easy to be oxidized, so oxidation potential decreases more significantly than reduction potential. The difference between reduction potential (Epc ) and oxidation potential (Epa ) decreases indicating increased reversibility of the electrode reaction. The currents in weak basic solution are all higher than the currents in acid or strong basic solutions as one can see in Fig. 3b. The reduction peak currents have a maximum value at pH 8. When pH values are higher than 8, the current response decreases. Therefore, the major electrochemical experiments are conducted at about pH 7.96.
10
12
Ipc 30
20
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Ipa
-10
The effect of pH on the peak potential and current due to p-hydroquinone shown in Fig. 3a and b was recorded with the modified electrodes in Britton–Robinson buffer solutions. The potential of oxidation and reduction peaks moves negatively with increasing pH values (Fig. 3a). Compared with Epa , Epc appears to have significantly lower slope. It is thought that the redox reaction conducted as following:
8 pH
40
3.3. Effect of pH
C6 H4 O2 + 2e− + 2H+ C6 H4 (OH)2
0.2
0.0
Peak current (µA)
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-20
-30 0 (b)
2
4
6
8
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pH
Fig. 3. Effect of the solution pH on the (a) reduction (Epc ) and oxidation (Epa ) peak potentials, (b) on the reduction (Ipc ) and oxidation (Ipa ) peak currents.
Electrochemical response could be observed on the imprinted electrode means that the cavities are contacted with electrode surface. This may originate either from very thin TiO2 layers on the porous graphitic support, or from a high density of cavities that intercommunicate one with the other. In our experiment, fine response is observed with sub smooth graphitic surface and diluted TiO2 suspension solution for coating. So the former reason is more reasonable. 3.4. The electrode process mechanism The relationship between peak current and scan rate is shown in Fig. 4. Both the reduction and oxidation peak currents increase linearly with increasing scan rate, with
S. Huan et al. / Analytica Chimica Acta 506 (2004) 31–39 80
35
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60 100
Ipc
Current(µA)
Peak current (µA)
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Fig. 4. The plot of peak current (Ipc and Ipa ) vs. scan rate (ν).
correlative coefficients of 0.9977 and 0.9996, respectively, indicating a surface-controlled electrode process. The slope of the line for reduction is 1.8-fold the slope of the line for oxidation. It is thought that the recognition sites formed in the TiO2 film have higher affinity toward reduction product than oxidation product, and the adsorption of reduction product will also influence the electrode process. So the reduction current slope is higher than the oxidation current. Two electrons are involved in the charge transfer reaction. The current function (ϕ) was defined by Nicholson and Shain as follows [22]:
-0.2
-0.4
p-hydroquinone and 0.1 mM o-hydroquinone broad oxidation and reduction peaks are observed on bare electrode (‘a’ and ‘b’ in Fig. 5) with similar current intensity. Little response observed in 0.1 mM m-benzenediol (‘c’ in Fig. 5). On the imprinted electrode, the peaks for p-hydroquinone present more nicely shaped redox peaks (Fig. 6). But the current intensity for p-hydroquinone is about two times the other isomers. As electrode imprinted by “dummy” molecule
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This correlation is easy to obtain by plotting Ip /ν1/2 versus ν. In our experiment, the current function increases gradually with increasing scan rate. The ratio of anodic to cathodic peak current approaches 1 at very low scan rate and decreases with increasing scan rate. From the ϕ–ν graph and the graph of Ipa /Ipc versus ν in [22], we propose that the electrode process is a traditional Er Cr mechanism:
O∗ O
0.4
Fig. 5. The cyclic voltammograms for (a) 0.10 mM p-hydroquinone, (b) 0.10 mM o-hydroquinone and (c) 0.10 mM m-benzenediol on the bare graphitic electrode in phosphate buffer solution of pH 7.95. Scan rate: 30 mV s−1 .
i nFν 1/2 C RT nFAD1/2
R O∗ + ne−
0.6
Potential
100 a Current (µA)
ϕ=
0.8
120
b
50
0
(charge transfer)
(reversible chemical reaction)
-50
c
O∗
R represents reduced species, represents direct electrode reaction product and O represents final oxidized product. -100 0.8
3.5. The isomeric selectivity of the imprinted film The cyclic voltammograms for individual isomers on the bare and imprinted electrode in phosphate buffer solutions (pH = 7.95) were recorded (Figs. 5 and 6). In 0.1 mM
0.6
0.4
0.2
0.0
-0.2
-0.4
Potential (V)
Fig. 6. The cyclic voltammograms for (a) 0.10 mM p-hydroquinone, (b) 0.10 mM o-hydroquinone and (c) 0.10 mM m-benzenediol on the electrode imprinted by p-phthalic acid in phosphate buffer solution of pH 7.95. Scan rate: 30 mV s−1 .
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12
60
a
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Current (µA)
Peak current (µA)
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Concentration (mM)
Fig. 7. The concentration-dependent peak current after blank correction for (a) p-hydroquinone, (b) o-hydroquinone and (c) m-benzenediol.
benzene is concerned, no response is observed (not shown). Benzene could not form desired cavities through coordination interaction as p-phthalic acid. The TiO2 film formed is very compact, and there are almost no channels for the benzenediol to approach the electrode surface, so no electrochemical response is observed. The redox peaks for p-hydroquinone are observed at −0.070 (reduction) and +0.012 V (oxidation). The redox peaks due to o-hydroquinone appear at +0.050 (reduction) and +0.108 V (oxidation). Only one oxidation peak is observed for m-benzenediol at +0.50 V. The peak current changes for each analyte after blank correction are plotted versus the concentration of the corresponding analytes in Fig. 7. The peak currents increase linearly with increasing concentration of p-hydroquinone and o-hydroquinone (‘a’ and ‘b’ in Fig. 7), while the relationship between peak current and m-benzenediol concentrations is not linear (‘c’ in Fig. 7). The sensitivity of the response for p-hydroquinone is significantly higher than that for the other two isomers. The difference in response sensitivity of the three isomers is clearly associated with the shape-selective property of the imprinted TiO2 film formed on the electrode. The effect of electrolyte background was studied as shown in Fig. 8. The reduction and oxidation peaks were recorded on the imprinted electrode in Britton–Robinson buffer solutions (pH = 7.96) and in phosphate buffer solutions (pH = 7.95) both containing p-hydroquinone of the same concentration. The Britton–Robinson buffer gave peaks shifted toward more negative potentials with higher peak current and better reversibility. Similar phenomena were also being observed in case of the mixture of isomers (not shown). Boric acid can form negatively charged complex ion with polyhydroxylate compounds in weak basic medium. One would propose that boric acid in Britton–Robinson buffer
Fig. 8. The voltammograms of 0.1 mM p-hydroquinone on the imprinted electrode (a) in Britton–Robinson buffer solution (pH = 7.96) and (b) in phosphate buffer solution (pH = 7.95). Scan rate: 30 mV s−1 .
could influence the bonding between the phenolic hydroxyl and the hydroxyl group on the titanium matrix. On the other hand, in phosphate buffer solutions the difference in peak potentials of p-hydroquinone and o-hydroquinone increases to 0.22 V. Detection of p-hydroquinone in the presence of 0.40 mM o-hydroquinone and m-benzenediol can also be carried out in phosphate buffer solutions (not shown). Most of the experiments are conducted in phosphate buffer solutions. 3.6. The complexing ratio of benzenediol and binding sites The formation of the designed cavities in the imprinted film made use of coordination complex formation as discussed in previous section, seemed to depend on the interaction between the hydroxyl groups of benzenediol and the hydroxyl groups (binding sites) on the film matrix. One can write: mA + nN (membrane) Am Nn
(1)
A and N represent benzenediol and binding sites (Ti–OH) in the cavities, respectively. According to the law of mass action the corresponding apparent equilibrium constant K can be expressed as the following: [Am Nn ] K= (2) [A]m [N]n A parameter α is defined as the ratio of free and total number of recognition sites: [N]f Ilim − I α= = (3) [N]t Ilim − I0 I0 is the peak current of the imprinted film in blank solution and Ilim is the saturated peak current when all the
S. Huan et al. / Analytica Chimica Acta 506 (2004) 31–39
recognition sites are all complexed by benzenediol. I is the peak current of the imprinted film actually measured when contacting with benzenediol of different concentrations. The relationship between α and benzenediol concentration [A] can be expressed as α 1 = n−1 1−α nK[N] [A]m
37
librium constant K. The curve fitting for different isomers are shown in Fig. 9. At low concentration p-hydroquinone yields a fairly good fit to curve 2 (Fig. 9a). However, for o-hydroquinone and m-benzenediol, the experimental data points are best fitted to curve 3 in Fig. 9b and c. This suggests a 1:2 p-hydroquinone/binding sites complexing ratio with a reasonable apparent equilibrium constant of 2.98 × 106 l2 mol−2 . So far as o-hydroquinone and m-benzenediol are concerned, the experimental data are best fitted to the curves of 1:1 complex with an apparent equilibrium constant of 1.20 × 103 and 1.35 × 103 l mol−1 , respectively.
(4)
The functional relationship between α and the concentration of benzenediol is governed by different m, n and K. The experimental data were fitted to Eq. (4) by changing the ratio of m:n and adjusting the overall apparent equi-
1.0
1.0
0.8
12345
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12345
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α
α
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α
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0.0 -6
(c)
-4
-2
0
2
Log [m-benzenediol]
Fig. 9. The parameter values (α) as a function of log[benzenediol]. The curves fitting the experimental data were calculated from Eq. (4). The circles are experimental observed data points. (a) For p-hydroquinone, (1) m:n = 1:3, K = 1.51×109 l3 mol−3 ; (2) m:n = 1:2, K = 2.98×106 l2 mol−2 ; (3) m:n = 1:1, K = 1.01 × 103 l mol−1 ; (4) m:n = 2:1, K = 1.01 × 106 l2 mol−2 ; (5) m:n = 3:1, K = 9.11 × 108 l3 mol−3 . (b) For o-hydroquinone, (1) m:n = 1:3, K = 1.98 × 109 l3 mol−3 ; (2) m:n = 1:2, K = 1.30 × 106 l2 mol−2 ; (3) m:n = 1:1, K = 1.20 × 103 l mol−1 ; (4) m:n = 2:1, K = 1.62 × 106 l2 mol−2 ; (5) m:n = 3:1, K = 1.90 × 109 l3 mol−3 . (c) For m-benzenediol, (1) m:n = 1:3, K = 1.86 × 109 l3 mol−3 ; (2) m:n = 1:2, K = 1.43 × 106 l2 mol−2 ; (3) m:n = 1:1, K = 1.35 × 103 l mol−1 ; (4) m:n = 2:1, K = 1.86 × 106 l2 mol−2 ; (5) m:n = 3:1, K = 2.38 × 109 l3 mol−3 .
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signed by o-phthalic acid present good selectivity toward ohydroquinone.
100
4. Conclusions Current (µA)
50
0
-50
a b
-100 0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
Potential (V)
Fig. 10. The voltammograms recorded on the electrodes imprinted by (a) p-phthalic acid and (b) o-phthalic acid in blank phosphate buffer solution after contacting with solution containing 0.10 mM p-hydroquinone, 0.10 mM o-hydroquinone and 0.10 mM m-benzenediol for 30 min. Scan rate: 30 mV s−1 .
The aforementioned apparent complexing ratio and equilibrium constants could shed some insight on the nature of isomeric selectivity of the recognition sites with respect to different isomers of benzenediol. The binding sites formed by p-phthalic acid as a template have relatively high binding affinity toward p-hydroquinone, which is well shape-matched to the recognition sites. While o-hydroquinone and m-benzenediol, which are not shape-matched, can only form the 1:1 complex with the recognition sites with low affinity. Therefore, the TiO2 films imprinted by p-phthalic acid have relatively higher selectivity toward p-hydroquinone than o-hydroquinone and m-benzenediol. 3.7. Selective uptake ability for different isomers After contacting with the mixture solution containing 0.20 mM p-hydroquinone, o-hydroquinone and m-benzenediol each for 30 min, the redox peaks of p-hydroquinone are observed obviously on the electrode imprinted by p-phthalic acid, while the redox peaks of o-hydroquinone are observed on the electrode imprinted by o-phthalic acid (Fig. 10). This confirms that imprinting methodology that involves covalent interactions within TiO2 film is promising for designing shape-selective cavities for recognition. Although we have to use benzenediol rather than the template molecules phthalic acids themselves simply because the later are electrochemically unactive, the molecular structure and the nature of bonding atoms involved for benzenediol and phthalic acids are the same or quite similar. So the cavities spatially organized by p-phthalic acid have good selectivity toward p-hydroquinone, while the cavities de-
The imprinting methodology that involves covalent interactions has been proved promising for designing shapeselective cavities in the TiO2 film. The template could be removed quantitatively by treating with ammonia solution. The cavities spatially organized by p-phthalic acid have good selectivity toward p-hydroquinone, while the cavities designed by o-phthalic acid present good selectivity toward o-hydroquinone. The TiO2 films prepared by the proposed imprinting methodology have pre-defined selectivity and affinity toward specified analytes. The proposed method could be extended to separation and analysis of some enantiomers if coupling with direct spectroscopic determination.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 20075006, 70171055 and 50179011) and the Foundation for Ph.D. Thesis Research (No. 20010532008), and the National 863 High Technologies Research Foundation of China (No. 2001AA644020). References [1] K. Haupt, K. Mosbach, Chem. Rev. 100 (2000) 2495. [2] G. Wulff, Chem. Rev. 102 (2002) 1. [3] F.L. Dickert, M. Tortschanoff, W.E. Bulst, G. Fischerauer, Anal. Chem. 71 (1999) 4559. [4] K. Haupt, K. Noworyta, W. Kutner, Anal. Commun. 36 (1999) 391. [5] S. Kröger, A.F.P. Turner, K. Mosbach, K. Haupt, Anal. Chem. 71 (1999) 3698. [6] D. Spivak, M.A. Gilmove, K.J. Shea, J. Am. Chem. Soc. 119 (1997) 4388. [7] E. Bellacchio, R. Lauceri, S. Gurrieri, L.M. Scolaro, A. Romeo, R. Purrello, J. Am. Chem. Soc. 120 (1998) 12353. [8] J.V. Beach, K.J. Shea, J. Am. Chem. Soc. 116 (1994) 379. [9] C. Yu, K. Mosbach, J. Org. Chem. 62 (1997) 4057. [10] N. Sallacan, M. Zayats, T. Bourenko, A.B. Kharitonov, I. Willner, Anal. Chem. 74 (2002) 702. [11] A. Katz, M. Davis, Nature 403 (2000) 286. [12] G. Vlatakis, L.I. Andersson, R. Muller, K. Mosbach, Nature 361 (1993) 645. [13] S.C. Zimmerman, M.S. Wendland, N.A. Rakow, I. Zharov, K.S. Suslick, Nature 418 (2002) 399. [14] M. Lahav, E. Katz, A. Doron, F. Patolsky, I. Willner, J. Am. Chem. Soc. 121 (1999) 862. [15] V.M. Mirsky, T. Hirsch, S.A. Piletsky, O.S.A. Wolfbeis, Angew. Chem. Int. Ed. 38 (1999) 1108. [16] M. Lahav, A.B. Kharitonov, O. Katz, T. Kunitake, I. Willner, Anal. Chem. 73 (2001) 720. [17] M. Lahav, A.B. Kharitonov, I. Willner, Chem.-A Eur. J. 7 (2001) 3992.
S. Huan et al. / Analytica Chimica Acta 506 (2004) 31–39 [18] L. Znaidi, R. Seraphimova, J.F. Bocquet, C. Colbeau-Justin, C.A. Pommier, Mater. Res. Bull. 36 (2001) 811. [19] K.L. Gnanasekar, V. Subramanian, J. Robinson, J.C. Jiang, F.E. Posey, B. Rambabu, J. Mater. Res. 17 (2002) 1507. [20] Y. Murakami, T. Matsumoto, Y. Takasu, J. Phys. Chem. B103 (1999) 1836.
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[21] Y. Hamasaki, S. Ohkubo, K. Murakami, H. Sei, G. Nogami, J. Electrochem. Soc. 141 (1994) 660. [22] R.S. Nicholson, I. Shain, Anal. Chem. 36 (1964) 706.