Self-assembly and imprinting of macrocyclic molecules in layer-by-layered TiO2 ultrathin films

Analytica Chimica Acta 779 (2013) 72–81

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Self-assembly and imprinting of macrocyclic molecules in layer-by-layered TiO2 ultrathin films Kazuma Araki, Do-Hyeon Yang, Tao Wang, Roman Selyanchyn, Seung-Woo Lee ∗ , Toyoki Kunitake Graduate School of Environmental Engineering, The University of Kitakyushu, 1-1 Hibikino, Wakamatsu-ku, Kitakyushu 808-0135, Japan

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Porphyrine/TiO2

nanocomposite films achieved via layer-by-layer assembly. • Essentially selective imprinted sites to native templates over similar molecules. • Competitive binding equilibrium constants calculated prove binding specificity.

a r t i c l e

i n f o

Article history: Received 29 November 2012 Received in revised form 28 February 2013 Accepted 7 April 2013 Available online 17 April 2013 Keywords: Porphyrin Phthalocyanine Molecular imprinting Surface sol–gel process

a b s t r a c t Alternate TiO2 gel ultrathin films assembled with a macrocyclic carboxylic acids of tetrakis-4carboxyphenyl porphine (TCPP) or tetra-4-carboxylphthalocyanine cobalt (II) (Co-TCPc) were prepared by the surface sol–gel process. To confirm the film growth and imprinting effect, quartz crystal microbalance (QCM) and UV–vis spectroscopy measurements were employed. The binding of TCPP was 1.2–14.3 times more selective compared to structurally related macrocyclic guest molecules. Among other findings, tetrakis-4-carboxymethyloxyphenyl porphine (TCMOPP) that has a spacer ( O CH2 ) between the phenyl rings and carboxylic acid moieties of TCPP showed a significantly lower binding efficiency equal to 0.07, regardless of its similar molecular structure to the template molecule. Structural difference of porphyrin and phthalocyanine analogs could be also selectively discriminated: the TCPP imprinted film showed ca. 13 times higher selectivity for recognition of TCPP itself from the mixture of TCPP and Co-TCPc. Characterization by AFM demonstrated that the TiO2 /TCPP film has highly uniform surface and ultrathin thickness, while both TEM and SEM studies confirmed the immobilized structures of TCPP inside the film. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Molecular imprinting (MI) technique offers a powerful synthetic process for fabrication of artificial receptors that mimic biological molecular recognition phenomena in nature. Generally, MI in organic matrices is based on the assembly of several components

∗ Corresponding author. Tel.: +81 93 695 3293; fax: +81 93 695 3384. E-mail address: [email protected] (S.-W. Lee). 0003-2670/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2013.04.009

such as template, functional monomer, and cross-linking molecules to create a polymer matrix, while in inorganic MI metal oxides can play a cross-linking role and simultaneously provide functional moieties for interaction with template. The acquired cavities after template removal are complementary to the shape, size and functionality of the respective template molecules. Nowadays MI technique attracts the considerable academic research and already has a wide range of practical applications in separation science, catalysis, enzyme-mimicking, chemical and biosensing, etc. [1–7]. Although MI matrices using organic polymers for cross-linking are dominant in this field, inorganic matrices such as silica [8]

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and titania [10] have attracted practical interests as well because of their robustness and easy control. In our previous studies, MI TiO2 thin films showed good properties for molecular recognition with high sensitivity and selectivity to a variety of target analytes [10–16]. Imprinting approach based on the surface sol–gel process [9,10,16] could be performed using two different methods, namely complexation [13–15] and layer-by-layer assembly [12,16]. The first approach involves a complexation process of template molecules with metal alkoxides in organic solvents as a first step of MI material fabrication. Second, layer-by-layer assembly approach consists of alternate adsorption of template molecules and metal alkoxides. It is particularly useful for organic species that can be self-assembled on the metal oxide surface, such as water-soluble amino acids, peptides, and saccharides [11,16]. Porphyrins are a class of hetero-macrocyclic compounds based on four pyrrole units interconnected via methine bridges ( CH ). Due to the possession of highly conjugated ␲-electrons, they show very intensive light absorption in the visible region. An additional feature is their possibility to form specific aggregated structures due to their various intermolecular interactions [17–23]. Thus, considerable interests to porphyrins in various areas have been attracted due to their structural, optical, and electrical properties [24–26]. Zimmerman et al. demonstrated porphyrin imprinted materials for the first time, in which highly selective binding sites for porphyrin derivatives were created in the center of the crosslinked dendrimers [27]. Since then MI of porphyrin compounds was investigated by many researchers in order get rid of their useful properties e.g. carbohydrate recognition [28], for creation of three-dimensional cavity in cross-linked polymers for capillary chromatography [29], modulation of porosity of solid materials [30], etc. Recently, Takeuchi et al. reported a novel approach for porphyrin imprinting using a photoresponsive functional monomer composed of diaminopyridine and azobenzene moieties [31]. However, organic synthetic approaches still require some improvement from the viewpoints of complexity of synthesis and binding speed of guest molecules. Combination of porphyrin or phthalocyanine derivatives with TiO2 has also attracted increasing scientific and technical interests in dye-sensitized solar cells in order to improve light absorption in the visible range [32,33]. In this work, we explored a new approach for MI of macrocyclic porphyrin and phthalocyanine derivatives in alternate TiO2 ultrathin layers [34]. The current approach would provide potential direction for not only creation of novel sensing materials but also for preparation of the elements of photovoltaic solar cells.

2. Experimental 2.1. Materials Titanium (IV)-n-butoxide (Ti(O-n Bu)4 ), was purchased from Kishida Chem., Japan. Tetrakis-4-carboxyphenyl porphine (TCPP), hemin, tetrakis-4-carboxymethyloxyphenyl porphine (TCMOPP), tetrakis-4-aminophenyl porphine (TAPP), tetrakis4-hydroxyphenyl porphine (THPP), 2-anthracenecarboxylic acid (2-AnCO2 H) and 2-mercaptoethanol were obtained from Tokyo Kasei. All of these chemicals were guaranteed reagents and used as purchased without further purification. Tetra-4carboxylphthalocyanine cobalt (II) (Co-TCPc) was synthesized according to the procedure reported previously [35]. All other chemicals used as solvents were of analytical grade purity and obtained from commercial sources. Deionized pure water (18.3  cm−1 ) was obtained by reverse osmosis followed by ion exchange and filtration (Millipore, Direct-QTM). Chemical structures used in this study are shown in Scheme 1.

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2.2. Preparation of imprinted TiO2 films Quartz crystal microbalance (QCM, 9 MHz manufactured by USI System, Japan) was used for monitoring film assembly. Prior to film deposition, a gold-coated QCM resonator was treated with a piranha solution (96.0% sulfuric acid/30.0–35.5% hydrogen peroxide, 3:1, v/v), rinsed with pure water, and dried in a stream of N2 gas. Then, the electrode was treated in an ethanol solution of 2-mercaptoethanol (10 mM) for 12 h, followed by rinsing with ethanol and drying with N2 gas in order to hydroxylate the electrode surface. QCM frequency was measured with a Hewlett-Packard 53131 A counter (255 MHz). Stable frequency shifts due to alternate deposition cycles were recorded in air after drying and transformed into mass increase by using the Sauerbrey equation [36]. In our system, a frequency decrease of 1 Hz corresponds to a mass increase of ca. 0.9 ng [10,37]. In the current work, Ti(O-n Bu)4 was used as a precursor of the TiO2 matrix and TCPP and Co-TCPc were used as template molecules. First, a hydroxylated QCM electrode was immersed in 100 mM solution of Ti(O-n Bu)4 in toluene/ethanol (1:1, v/v) for about 3 min at 25 ◦ C, rinsed thoroughly with ethanol to remove the physically adsorbed Ti(O-n Bu)4 , and then subjected to hydrolysis in water for 1 min and N2 gas flushing. Subsequently, the TiO2 gel-deposited substrate was immersed into a 0.5 mM TCPP solution in ethanol or a 0.5 mM Co-TCPc solution in dimethylformamide (DMF) for 10 min at 30 ◦ C, followed by thorough ethanol or DMF rinsing to remove the physically adsorbed template molecules and dried by flushing with N2 gas. This process was repeated to introduce certain amounts of template molecules, where one cycle is considered to be a combined TiO2 /TCPP (or CoTCPc). For template removal, immersion into 1 wt% aqueous ammonia for 10 min with subsequent water washing was used. This treatment led to successful template removal from the film, providing imprinted sites. UV–vis measurements by a JASCO-570 spectrophotometer were also conducted for monitoring the film assembly, template removal and guest binding. Prior to film deposition, a quartz plate was cleaned with concentrated sulfuric acid and thoroughly washed with pure water. Then, the substrate was treated with 1 wt% KOH in ethanol/water (3:2, v/v) for 30 min, and finally washed in ethanol and pure water followed by drying with N2 gas. 2.3. Imprinting effect and guest selectivity For binding tests of template and structurally related guest molecules, a 5-cycle TiO2 /template film covered with a TiO2 outermost layer was employed. After template removal, the imprinted film was immersed in a 0.5 mM guest solution in DMF for 10 min at 30 ◦ C, rinsed with DMF and ethanol, and dried with nitrogen gas. Then bound guest molecules were removed by immersing the film into 5 mL of 1 wt% ammonia solution for 10 min. Subsequently, UV–vis absorption of the resulting ammonia solution was measured to estimate the amount of the bound guest molecules using a calibration curve of the respective analyte used for guest binding. For competition binding tests, each imprinted film was immersed in a mixture of the same concentration TCPP and Co-TCPc in the range from 0.2 ␮M to 0.1 mM in DMF, rinsed with DMF and ethanol, and dried with N2 gas. 2.4. Film morphology Imprinted TiO2 films were scratched off from the quartz substrate, transferred onto a 200-mesh carbon-coated copper grid, dried overnight, and then observed by a transmission electron

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Scheme 1. Chemical structures of the templates (a) TCPP, (b) CoTCPc; guest molecules (c) TCMOPP, (d) TAPP, (e) hemin, (f) THPP, (g) 2-AnCO2 H, and (h) matrix precursor Ti(O-n Bu)4 used in this study.

microscopy (TEM) using JEOL JEM-3010 instrument. Scanning electron microscopy (SEM) measurements were carried out by using a Hitachi S-5200 instrument. A 3 nm thick platinum layer was deposited on all specimens by using a Hitachi E-1030 ion sputter at a current of 15 mA and pressure of 10 Pa in order to prevent the electrical charge up. The surface morphology and thickness of film were studied with a JEOL JSPM-5200 atomic force microscope (AFM) working in non-contact mode using a MicroMash NSC12/TiPt/15 silicon cantilever (curvature tip radius <40 nm, tip length 15–20 ␮m).

3. Results and discussion 3.1. Film assembly Fig. 1a and b shows QCM frequency shifts and UV–vis absorption spectral changes due to the alternate deposition of Ti(O-n Bu)4 and TCPP, respectively. The QCM frequency for the TiO2 /TCPP alternate film uniformly decreased up to at least 10 cycles, indicating regular film growth (Fig. 1a). Average frequency changes for the adsorption of Ti(O-n Bu)4 and TCPP are 25 ± 6 and 16 ± 2 Hz per

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Fig. 1. (a) QCM frequency shifts and (b and c) UV–vis absorption spectral changes due to the alternate adsorption of TCPP and CoTCPc with Ti(O-n Bu)4 , respectively.

deposition cycle, respectively. Additionally, the Soret band of TCPP at 420 nm regularly increased in proportion to the number of deposition cycles with the average absorbance change per cycle of deposition (Abs) equal to 0.037 ± 0.003 (Fig. 1b). However,

the absorbance of TCPP is reduced after deposition of Ti(O-n Bu)4 with average decrease per cycle for Soret band at 420 nm equal to 0.025 ± 0.002 (see inset of Fig. 1b). This indicates partial desorption of the TCPP molecules immobilized on the TiO2 gel layer along with

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the deposition of Ti(O-n Bu)4 , ca. 67% of the adsorbed molecules at every step. Therefore, the substantial absorbance change for TCPP band at 420 nm can be recalculated to be 0.013 ± 0.002 per cycle and the average frequency changes for Ti(O-n Bu)4 and TCPP are reestimated to be 35 ± 7 and 5 ± 1 Hz, respectively. Adsorption density of TCPP can be calculated from the QCM results considering the adsorbed mass corrected for the molecular weight (Mw : 790.77) of the template and the both sides surface area of the QCM electrode (0.32 cm2 ). The assembled film shows an adsorption density of 0.12 molecules nm−2 (8.3 nm2 molecule−1 ), corresponding to ca. 27% of the theoretical adsorption density of 0.44 molecules nm−2 , which was estimated from the side length (ca. 1.5 nm) of the square planar TCPP [38]. Subsequently, the remaining area of 73% and the adsorbed TCPP molecules are covered with a TiO2 gel layer. The TiO2 /Co-TCPc alternate film was also regularly assembled up to 10 cycles. Compound specific absorbance regularly increased in proportion to the number of the deposition cycle of Co-TCPc. The average absorbance change per cycle of Co-TCPc deposition measured at 680 nm was 0.004 ± 0.001 (see Fig. 1c). Similar to the case of TCPP, the adsorbed Co-TCPc molecules are partially desorbed along with the deposition of Ti(O-n Bu)4 . The substantial absorbance change at 680 nm for Co-TCPc is estimated to be 0.003 ± 0.001 per cycle, as shown in the inset of Fig. 1c. 3.2. Removal and rebinding of template molecules Fig. 2a shows UV–vis absorption spectra of the (TiO2 /TCPP)5 film before and after template removal and after TCPP rebinding. Treating with ammonia is the most common way for removing the bound template from the imprinted films and especially favorable for carboxylic templates [10,13,16] used in the current study. The Soret band of TCPP at 420 nm quickly disappeared along with 1 wt% ammonia treatment and it is evident that the TCPP template incorporated in the TiO2 gel matrix could be completely removed. The film after TCPP removal showed only TiO2 gel absorbance in the UV and near visible regions. On the other hand, the rebinding of TCPP showed very fast saturation within approximately 5 min (see Fig. 2b). The absorbance change at 420 nm due to the TCPP rebinding (Absbinding ) is 1.18 times larger as compared to the absorbance change due to the template removal (Absremoval ): 0.0897 for Absremoval and 0.106 for Absbinding . This indicates that the imprinted cavities are fully occupied with TCPP and additionally non-specific binding onto the TiO2 gel outermost surface occurs. In order to investigate the optimized film thickness, alternate films with different number of deposition cycles were tested for template rebinding. As can be seen from the inset of Fig. 2b, the rebinding of TCPP is optimized with a 5-cycle deposition film, showing a rebinding efficiency close to almost 100%. On the other hand, the thinner films (1 and 3 cycles) show relatively high rebinding efficiencies compared to that of the 5-cycle deposition film, obviously evidencing relatively higher non-specific surface adsorption, whereas the TCPP binding to the thicker 9-cycle film is suppressed to less than 40%, evidencing hiding of the binding sites in the thicker titania film. The obtained result suggests that the optimized specific template rebinding is achieved with the 5 cycle-deposition film. Thus, we selected the 5-cycle TiO2 /TCPP and TiO2 /Co-TCPc films for further imprinting tests. 3.3. Film morphology For the precise information about the surface morphology and film thickness, AFM measurements were carried out with a 10-cycle TiO2 /TCPP film that was deposited on a silicon wafer. Fig. 3a shows

an AFM image of the surface morphology of the film before template removal. The film is very uniform over the whole area, showing a RMS (root mean squared) roughness equal to 0.68 nm. Usually, AFM provides much more precise information about height on surface rather than width. To estimate the thickness of the prepared film, a small place (1 ␮m2 ) of the film was continuously scratched with a silicon cantilever in contact mode. Film thickness was estimated by comparison the height in the scratched area with the ordinary (not scratched) surface. For the reliability of the data, several places were chosen, as shown in Fig. 3b. As a result 10cycle TiO2 /TCPP film demonstrates thickness of 10.6 ± 1.1 nm and thus one deposition cycle is estimated to be ca. 1 nm thick. On the other hand, the thickness of the film can be also estimated from the total frequency change (ca. 410 Hz) due to the alternate deposition of Ti(O-n Bu)4 and TCPP using the modified Sauerbrey Eq. (1), already accounting the parameters of the used QCM [10,37]: 2d (Å ) = −

F 1.832

(1)

where  is the film density and F is the frequency shift of the QCM. According to the calculation using Eq. (1) the thickness (d) of the adsorbed film on one side of the resonator is estimated to be ca. 7 nm, with the average film density assumed to be ca. 1.6 g cm−3 that was calculated taking to account the frequency ratios and density of each component (1.7 g cm−3 for TiO2 dried gel [39] and 1.40 g cm−3 for TCPP [38]). The difference in the film thicknesses obtained from the AFM and QCM results may be attributed to the fact that the density of the prepared film is lower than that used in the above calculation. Probably, the intrinsic film porosity is even more improved after template removal. Combination of both AFM and QCM methods for thickness estimation becomes thus a valuable tool to assess the film porosity. On the basis of the QCM and AFM results, we can conclude that the optimized film thickness for guest binding is considered to be ca. 3–5 nm thick, insuring the rapid binding/extraction of guest molecules. 3.4. Guest selectivity The guest binding and selectivity of the imprinted TiO2 films were confirmed by UV–vis measurements, as described in the Experimental section. Fig. S1 shows UV–vis absorption spectra of 4 selected analytes. The absorption coefficient (ε) of each analyte used for binding study was estimated from its concentration dependence curve: 5.24 × 105 , 3.81 × 105 , 1.53 × 105 , 1.58 × 105 , 1.10 × 105 , 5.22 × 104 , and 8.75 × 104 (L mol−1 cm−1 ) in 1 wt% aqueous ammonia for TCMOPP, TCPP, Co-TCPc, THPP, TAPP, hemin, and 2-AnCO2 H, respectively (see Table 1). These absorption coefficient values were used to estimate the concentration of each analyte extracted by ammonia solution from the film. Supplementary material related to this article found, in the online version, at http://dx.doi.org/10.1016/j.aca.2013.04.009. Fig. 4 shows difference UV–vis spectra of the TCPP and CoTCPc imprinted films after binding of TCPP and Co-TCPc and their extracted solutions are as well compared in the inset of Fig. 4. Other guest molecules were applied to the essentially same guest binding process and the binding efficiency of each analyte is defined as a molar ratio of the bound guest and template molecules (guest /template ). The binding results of all guest molecules are summarized in Table 1. The TCPP-imprinted film reveals higher selectivity toward TCPP, and lower bindings to other guest molecules. The relative binding efficiency of the TCPPimprinted film decreases in the order of Co-TCPc, hemin, 2-AnCO2 H, THPP, TAPP, and TCMOPP. The binding efficiency for Co-TCPc is the following of the TCPP template and was estimated to be 0.81. Among the guest molecules, TCMOPP that is structurally close to TCPP and also possesses four carboxylic groups in a molecule

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Fig. 2. (a) UV–vis absorption spectra of the 5-cycle TiO2 /TCPP film before and after template removal and after TCPP rebinding. (b) UV–vis absorbance changes at 420 nm due to the removal () and rebinding (䊉) of TCPP.

showed the lowest relative binding efficiency of 0.07. Perhaps, this result may be attributed to the fact that the size of imprinted cavity of TCPP is not proper for TCMOPP binding because it has a spacer ( O CH2 ) between the phenyl ring and carboxylic acid groups. The binding efficiency for THPP and TAPP in the TCPP-imprinted film was 0.30 and 0.11, respectively. It appears that the presence of hydroxyl functional groups in the guest molecule is more efficient for guest binding rather than that of amino moieties. The same binding experiments were also conducted with the Co-TCPc imprinted film and Table 1 summarizes the binding

efficiency for all guest molecules. Similar binding results to the TCPP-imprinted film were obtained, showing higher selectivity toward Co-TCPc than other guest molecules. The relative binding efficiency decreases in the order of hemin, 2-AnCO2 H, TCPP, TAPP, THPP, and TCMOPP. Hemin and 2-AnCO2 H give relatively high binding efficiencies of 0.90 and 0.60, respectively. Perhaps, the higher binding efficiency of hemin will be due to its molecular structure as a metalloporphyrin close to the Co-TCPc template. A graphical comparison based on the amount of bound guest molecules is given in Fig. 5. Interestingly, binding features in both

Table 1 Relative binding efficiency of the imprinted TiO2 films. Guest

Mw

εa (m2 mol−1 )

Binding density (mol nm−2 ) IFTCPP

TCPP TAPP THPP TCMOPP Hemin 2-AnCO2 H Co-TCPc a b

Absorption coefficient. IF means imprinted film.

790.77 674.79 678.73 910.88 651.94 222.24 743.54

381,567 109,665 157,634 524,188 52,220 87,450 152,739

b

4.60 × 10−18 5.27 × 10−19 1.37 × 10−18 3.20 × 10−19 2.16 × 10−18 1.81 × 10−18 3.71 × 10−18

Relative binding efficiency

IFCo-TCPc

IFTCPP

IFCo-TCPc

1.01 × 10−18 3.65 × 10−19 3.04 × 10−19 2.46 × 10−19 2.97 × 10−18 1.98 × 10−18 3.30 × 10−18

1.00 0.11 0.30 0.07 0.47 0.39 0.81

0.31 0.11 0.09 0.07 0.90 0.60 1.00

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Fig. 3. (a) 3D AFM image of the 1 ␮m × 1 ␮m surface of the 10-cycle TiO2 /TCPP film deposited on the silicon substrate and (b) top surface AFM image with scratched area and respective surface profile used to estimate the film thickness.

cases are slightly different. It appears that additional factors other than the shape, size and functionality of the template molecules can be considered in the imprinting process. The binding of both template molecules in their imprinted films can be considered as follows: the carboxylic groups can be covalently bound to the titanium atoms of TiO2 gel (Ti O OC R, where R means a molecular frame of TCPP or Co-TCPc) with together noncovalent (hydrogen) bonding between the Ti OH and R COOH moieties. After ammonia treatment, the Ti O OC R moiety can be decomposed to Ti OH and RCOO− NH4 + species by the aid of H2 O. Additionally, the hydrogen bonding between the Ti OH and R COOH moieties can be broken by the obstruction of water. Opposite reactions may be possible for template rebinding, forming the Ti O OC R bond due to the condensation reaction and the non-

Fig. 4. Difference UV–vis spectra of TCPP (red lines) and Co-TCPc (blue lines) bound to the TCPP (straight lines) and Co-TCPc (dotted lines) imprinted films, respectively: the inset shows UV–vis absorption spectra of the extracted solutions in 1 wt% aqueous ammonia from the corresponding films. Conditions for binding: 0.5 mM in DMF for 10 min at 30 ◦ C for each template. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

covalent hydrogen bonding between both Ti OH and R COOH functional groups, as schematically illustrated in Fig. 6. 3.5. Competition binding experiments To more clearly understand the imprinting effect of both imprinted TiO2 films, competition binding experiments using a mixture of TCPP and Co-TCPc were studied. Fig. 7 shows difference UV–vis spectra of the 3-cycle TCPP and 5-cycle Co-TCPc imprinted films before and after competition binding experiments. The deposition cycle of the TiO2 /TCPP film was adjusted to take valence of UV–vis absorption of TCPP in both imprinted films. As can be seen from Fig. 7a, at the sufficiently low template concentration of 0.5 ␮M the TCPP imprinted film could definitely recognize TCPP in the mixture. On the other hand, the response of the Co-TCPc imprinted film is also higher to TCPP than to Co-TCPc at the same concentration (Fig. 7b). However, the TCPP binding in the TCPPimprinted film is very fast and saturated within 5 min, whereas it reaches equilibrium after 1 h in the Co-TCPc-imprinted film. With

Fig. 5. Comparison of the bound amount of the guest molecules in the TCPP and Co-TCPc-imprinted TiO2 films. The inset shows side and top views of space-filling models of TCPP and Co-TCPc.

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Fig. 6. Schematic illustration of template binding to the imprinted binding site.

further increase of the concentration of the mixture (2.5 ␮M), the selectivity of both imprinted films becomes more competitive, and their initial difference almost disappears, as shown in Fig. 7c and d. Details of the competition binding of each analyte on both imprinted films were examined by the Benesi–Hildebrand method [40]. Benesi–Hildebrand plots gave binding constants of 1.25 × 106 and 7.62 × 105 M−1 for TCPP and 1.00 × 105 and 2.99 × 105 M−1 for Co-TCPc in the TCPP-imprinted and Co-TCPc-imprinted films, respectively (see Table 2), where the concentration of the TCPP and Co-TCPc mixture for the binding test was adjusted in the range from 0.2 ␮M to 0.1 mM. Generally, the binding of TCPP in both imprinted films is superior to that of Co-TCPc. The guest selectivity of the TCPP-imprinted film toward TCPP, Ka (template)/Ka (guest), is sufficiently high and estimated to be 12.5, whereas the Co-TCPc-imprinted film shows a small value of 0.39 for its own

template over TCPP. On the other hand, the film selectivity, which is defined as a ratio of the Ka values of both imprinted films for the individual template, is estimated to be 1.64 and 2.99 for TCPP and Co-TCPc, respectively. This result indicates that Co-TCPc can be more selectively recognized by the Co-TCPc-imprinted film than by the TCPP-imprinted film, although its binding is much slower than that of TCPP. 3.6. Influence of assembled template structures on selectivity Fig. 8a shows a scanning electron microscopy (SEM) image of the surface of the 5-cycle TiO2 /TCPP film on a quartz plate before template removal. The surface of the film is very smooth and uniform over large area. Usually, porphyrins can form specific aggregates using their various intermolecular interactions, which cause optical

Fig. 7. UV–vis absorption spectra changes of the 3-cycle TCPP (a and c) and 5-cycle Co-TCPc (b and d) imprinted films after guest binding using TCPP and Co-TCPc mixtures, respectively: 0.5 ␮M (a and b) and 2.5 ␮M (c and d) for each analyte.

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Table 2 Binding constants of TCPP and Co-TCPc on their imprinted films. Film

TCPP-imprinted Co-TCPc-imprinted Film selectivityc a b c

Binding constant, Ka (×104 M−1 )

Guest selectivityb

TCPP

Co-TCPc

TCPP/Co-TCPc

Co-TCPc/TCPP

125 (0.989)a 76.2 (0.999) 1.64

10.0 (0.996) 29.9 (0.999) 2.99

12.5 2.55 –

0.08 0.39 –

Correlation coefficient (R2 ). Defined as a ratio of the Ka values of the individual imprinted film for both analytes (templates). Defined as a ratio of the Ka values of both imprinted films for the individual template.

changes in their UV–vis spectra. To confirm the structural features of TCPP inside the film, the film was scratched off from the substrate for transmission electron microscopy (TEM) observation. Fig. 8b shows a TEM image of the film, including several types of specific

structures such as rod, tube and globule. Interestingly, similar structures that were found in the TEM image are observed when a TCPP solution (0.5 mM) in EtOH was dropped onto carbon-coated copper grid, as shown in Fig. 8c.

Fig. 8. (a) SEM image of the 5-cycle TiO2 /TCPP film surface, (b) TEM image of the scratched 5-cycle TCPP-imprinted film, (c) SEM image of TCPP structures after dropping the TCPP ethanol solution on a TEM grid, (d) comparison of UV–vis absorption spectra of TCPP bound to its imprinted film and TCPP extracted in 1 wt% aqueous ammonia, and (e and f) magnified SEM images of the 5-cycle TiO2 /TCPP film surface before and after template removal, respectively.

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Obviously, the Soret band of the bound TCPP in the film is broad and relatively red-shifted to 420 nm (as protonated form), whereas the Soret band (413 nm, as monomeric form) of the extracted TCPP in 1 wt% aqueous ammonia from the film is narrow and sharp, as can be seen from Fig. 8d. This result suggests that mixed forms of TCPP, mainly monomeric and H-aggregated structures, exist in the TiO2 film. Perhaps, the obtained binding sites reflect the initial structures of TCPP immobilized inside the film. This fact may explain the fast and selective binding of TCPP to its imprinted film (see Table 2). In comparison with TCPP, Co-TCPc is considered to be individually deposited on the TiO2 gel layer because of its electrostatic repulsion between the molecules. Consequently, it is concluded that the presence of self-assembled structures of TCPP inside the film led to its higher binding efficiency to TCPP than to Co-TCPc. Magnified SEM images of the 5-cycle TiO2 /TCPP film surface before and after template removal given in Fig. 8e and f additionally demonstrate that after organic component was removed, surface charge-up by electron beam significantly decreased which leads to much clearer image and possibility to observe the pores, plausibly left by aggregated TCPP structures. 4. Conclusions In this study, we explored a new approach for MI of a macrocyclic porphyrin or phthalocyanine based on layer-by-layered TiO2 ultrathin films. The imprinted films showed higher selectivity toward their macrocyclic template molecules over other guest molecules similar in structure. In particular, the TCPP-imprinted film showed ca. 13 times higher selectivity to own TCPP, which could be achieved by the synergic effect of MI and intermolecular interaction in ultrathin layers of TiO2 . We believe that the current approach would provide an attractive tool for a range of applications in sensing and analysis and inspire the development of precisely controlled inorganic imprinted materials for molecular recognition.

References [1] [2] [3] [4] [5] [6] [7]

[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

Acknowledgements This work was supported by MEXT via 2nd Kitakyush Knowledge-based Cluster Project (Regional Innovation Cluster Program (Global Type)). R. Selyanchyn acknowledges the Research Fellowship of the Japan Society for the Promotion of Science for Young scientists (JSPS). Authors would like to thank Dr. Sergiy Korposh for his helpful discussions on the calculation of binding constants.

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[32] [33] [34] [35] [36] [37] [38] [39] [40]

K. Haupt, K. Mosbach, Chem. Rev. 100 (2000) 2495. G. Wulff, Angew. Chem. Int. Ed. Engl. 34 (1995) 1812. V.B. Kandimalla, H. Ju, Anal. Bioanal. Chem. 380 (2004) 587. G. Wulff, S. Schauhoff, J. Org. Chem. 56 (1991) 395. J.D. Marty, M. Mauzac, Adv. Polym. Sci. 172 (2005) 1. W. Li, S. Li, Adv. Polym. Sci. 206 (2007) 191. ˜ G. Díaz-Díaz, D. Antuna-Jiménez, M. Carmen Blanco-López, M. Jesús Lobo˜ ˜ Castanón, A.J. Miranda-Ordieres, P. Tunón-Blanco, Trends Anal. Chem. 33 (2012) 68. C.D. Ki, C. Oh, S.G. Oh, J.Y. Chang, J. Am. Chem. Soc. 124 (2002) 14838. I. Ichinose, H. Senzu, T. Kunitake, Chem. Mater. 9 (1997) 1296. S.W. Lee, I. Ichinose, T. Kunitake, Langmuir 14 (1998) 2857. S.W. Lee, I. Ichinose, T. Kunitake, Chem. Lett. (1998) 1193. S.W. Lee, I. Ichinose, T. Kunitake, Chem. Lett. 31 (2002) 678. S.W. Lee, D.H. Yang, T. Kunitake, Sens. Actuators B: Chem. 104 (2005) 35. D.H. Yang, S.W. Lee, T. Kunitake, Chem. Lett. 34 (2005) 1686. N. Mizutani, D.H. Yang, R. Selyanchyn, S. Korposh, S.W. Lee, T. Kunitake, Anal. Chim. Acta 694 (2011) 142. D.H. Yang, N. Takahara, S.W. Lee, T. Kunitake, Sens. Actuators B: Chem. 130 (2008) 379. S. Okada, H. Segawa, J. Am. Chem. Soc. 125 (2003) 2792. A.S.R. Koti, N. Periasamy, Chem. Mater. 15 (2003) 369. J.S. Hu, Y.G. Guo, H.P. Liang, L.J. Wan, L. Jiang, J. Am. Chem. Soc. 127 (2005) 17090. H. Matsui, R. MacCuspie, Nano Lett. 1 (2001) 671. A.V. Udal’tsov, L.A. Kazarin, A.A. Sweshnikov, J. Mol. Struct. 562 (2001) 227. S.B. Lei, C. Wang, S.X. Yin, H.N. Wang, F. Xi, H.W. Liu, B. Xu, L.J. Wan, C.L. Bai, J. Phys. Chem. B 105 (2001) 10838. J. Otsuki, E. Nagamine, T. Kondo, K. Iwasaki, M. Asakawa, K. Miyake, J. Am. Chem. Soc. 127 (2005) 10400. G.K. Boschloo, A. Goossens, J. Phys. Chem. 100 (1996) 19489. R.B.M. Koehorst, G.K. Boschloo, T.J. Savenije, A. Goossens, T.J. Schaafsma, J. Phys. Chem. B 104 (2000) 2371. T. Hasobe, H. Imahori, P.V. Kamat, S. Fukuzumi, J. Am. Chem. Soc. 125 (2003) 14962. S.C. Zimmerman, I. Zharov, M.S. Wendland, N.A. Rakow, K.S. Suslick, J. Am. Chem. Soc. 125 (2003) 13504. J.D. Lee, N.T. Greene, G.T. Rushton, K.D. Shimizu, J.I. Hong, Org. Lett. 7 (2005) 963. J. Matsui, M. Higashi, T. Takeuchi, J. Am. Chem. Soc. 122 (2000) 5218. K.S. Suslick, P. Bhyrappa, J.H. Chou, M.E. Kosal, S. Nakagaki, D.W. Smithenry, S.R. Wilson, Acc. Chem. Res. 38 (2005) 283. T. Takeuchi, K. Akeda, S. Murakami, H. Shinmori, S. Inoue, W.S. Lee, T. Hishiya, Org. Biomol. Chem. 5 (2007) 2368. B. O’Regan, M. Grätzel, Nature 353 (1991) 737. G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 6 (2006) 215. I. Ichinose, T. Kawakami, T. Kunitake, Adv. Mater. 10 (1998) 535. J.H. Schutten, J. Zwart, J. Mol. Catal. 5 (1979) 109. G. Sauerbrey, Z. Phys. 155 (1959) 206. S.W. Lee, N. Takahara, S. Korposh, D.H. Yang, K. Toko, T. Kunltake, Anal. Chem. 82 (2010) 2228. S. Cherian, C.C. Wamser, J. Phys. Chem. B 104 (2000) 3624. I. Ichinose, H. Senzu, T. Kunitake, Chem. Lett. (1996) 831. H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703.