Molecularly Imprinted Polymers as Synthetic Catalysts

9 Molecularly Imprinted Polymers as Synthetic Catalysts Piyush Sindhu Sharma1, Agnieszka Wojnarowicz1, Wlodzimierz Kutner1, 2, Francis D’Souza3 1 DE PARTME NT OF P HY SICAL CHEM ISTRY OF SUPRAMOLE CUL AR COM PL EXE S, INSTITUTE OF PHYSICAL C HEMISTRY, POLISH A CADEMY OF SCIENCES, W ARSAW, POLAND; 2 FACULTY OF MATHEMATICS AND NATUR AL SCI ENCES, SCH OOL OF SCIENCE, CARDINAL STEFAN WYSZYN SKI UNIVERSITY IN WARSAW, WARSAW, POLAND; 3 DE PARTMENT OF CHEMISTRY, UNIVERSITY OF NORTH TEXAS, DENTON, TX, USA

1. Introduction In a catalyzed reaction, a catalyst increases the rate of a chemical reaction without being consumed or changing the reaction equilibrium. Catalysis is selective when a catalyst recognizes a substrate rather than interferences and, moreover, it adopts the desired reaction route. In natural systems, different enzymes, antibodies, and microorganisms are responsible for selective recognition (1, 2). For instance, the use of enzymes results in specific sensing systems for chosen analytes (3). A three-dimensional (3-D) structure of these biocatalytic macromolecules forms a distinct microenvironment featuring functionalities complementarily fitting the target molecule exactly. One challenge in making reusable enzyme-based biosensors is to ensure that the immobilized enzyme molecules maintain this specific microenvironment over time. Besides the loss of the enzyme microenvironment structure, the decrease in enzyme mobility upon immobilization has a role in the loss of enzymatic activity (4). Moreover, the use of solutions of high pH or organic solvent solutions disturbs this specific microenvironment (5). Over the last three decades, efforts were initiated to prepare synthetic recognition systems by molecular imprinting, which would operate like enzymes without losing their selectivity under harsh reaction conditions. Currently, molecular imprinting is a wellestablished procedure for preparing such artificial recognition systems (6–9). This procedure involves imprinting molecular templates, most often in a polymer matrix. Subsequent removal of this template, e.g., by extraction with a suitable solvent, leaves in the polymer molecular cavities of the shape and size matching those of the template molecules. Moreover, orientation and position of recognizing functionalities of these cavities are complementary to binding functionalities of the selected template molecule. Molecularly Imprinted Catalysts. http://dx.doi.org/10.1016/B978-0-12-801301-4.00009-8 Copyright © 2016 Elsevier Inc. All rights reserved.

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MOLECULARLY IMPRINTED CATALYSTS

To prepare a highly selective MIP, initially formed a prepolymerization complex of a functional monomer with a template in solution must be stable. This complexation is afforded by self-assembling owing to (1) noncovalent interactions such as hydrogen bonding, ion-pairing, or hydrophobic, van der Waals, or dipole–dipole interactions; or (2) reversible covalent bonding (6, 10) (Scheme 1). Noticeably, imprinting requires two types of monomers. That is, besides a functional monomer participating in these interactions (Scheme 1), another one required is a cross-linking monomer. This latter monomer provides rigidity to the MIP after polymerization of the complex. Until now, different acrylic functional monomers, mainly commercial, and a range of conducting polymers (11) have been used for imprinting (12). Moreover, organically modified silanes (13) and self-assembled monolayers (SAMs) of thiols (14) were often used for that purpose. Current commercial availability of many different functional monomers and procedures developed for MIP preparation resulted in successful imprinting of several different analytes ranging from small (9, 15–23) to macromolecular analytes, such as proteins (7, 24), deoxyribonucleic acids (25–27), and even whole cells (28, 29). Initially, MIPs were prepared as blocks in bulk solution. In this form, after grinding, they were suitable as column packing materials for solid-phase extraction and liquid chromatography (30–33). However, their subsequent involvement in other areas, such as sensing, (11) prompted researchers to develop MIPs in the form of thin films or membranes (9, 11, 12, 34, 35). Apparently, morphology is an important criterion for a given MIP application. Since the first report on MIPs, several different synthetic strategies have

SCHEME 1 Consecutive steps of preparation, by different template reversible binding, of an MIP. Adapted with permission from Ref. (38). Copyright 2013 Elsevier.

Chapter 9 • Molecularly Imprinted Polymers as Synthetic Catalysts 185

been proposed (36). To date, most efforts have been directed toward preparing uniformly sized spherical MIP particles in the micrometer range (37). In these MIP materials, the surface-to-volume ratio was clearly maximized. Hence, accessibility of the imprinted cavities buried inside the polymer matrix was enhanced. Apart from much-studied sensing and separating, another important application of MIPs includes their use in catalysis (39–42). For this application, MIPs are cast around either a transition state analogue (TSA) or an intermediate analogue of a chemical reaction (43). These analogues have the role of templates. Imprinted cavities prepared that way are able to catalyze corresponding reactions (44). A closer inspection of the literature reveals few earlier uses of MIPs dedicated to application as either a catalyst (45) or a material for separation of racemic product mixtures (46, 47). For catalytic applications, attempts were made to exactly mimic the arrangement of amino acids encountered in active centers of enzymes to design molecular cavities. To that aim, functional monomers featuring both the desired recognizing functionality and polymerizable end groups were synthesized (16, 41, 42, 48, 49).

2. Generation of Catalytic Cavities in MIPs Before presenting examples of the use of MIPs as catalysts, it is important to understand how a natural catalyst such as an enzyme operates. One of the most frequently encountered mechanisms of enzyme catalysis is the Michaelis–Menten mechanism, presented in Eqn (1): Kd

kcat

E þ S % ES / E þ P

(1)

To convert a substrate (S) to a product (P), S must attain the transition state of higher energy. The activation energy required to reach this state is a barrier to the progress of the reaction. Here, an enzyme (E) decreases this barrier and consequently increases the reaction rate. An enzyme and its substrate first reversibly combine to give an enzyme–substrate complex (ES). For a simple enzymatic reaction, the dissociation constant, Kd, of the ES complex is equal to the Michaelis–Menten constant, Km. It describes the affinity of the enzyme for the substrate. In a second step, an irreversible chemical reaction then proceeds with a rate constant kcat, called the turnover number, which is the maximum number of substrate molecules converted to a product per active site of the enzyme per time unit. Enzymes are proteins composed of different amino acids. Because of different interaction sites on amino acids, enzymes can form 3-D microcavities with shapes matching specific parts of substrates. Initially, weak interactions between an enzyme and a substrate bring these two components together, and then they induce rapid conformational changes in the enzyme structure that result in strengthening enzyme– substrate binding. These changes in enzyme conformation decrease the activation energy of the transition state, thus stabilizing this state.

186

MOLECULARLY IMPRINTED CATALYSTS

From the beginning of the research on imprinting (47, 50, 51), MIPs were considered synthetic receptors that can mimic the function of enzymes not only for selective sensing but also for selective catalysis (41, 52). These MIP applications were made possible because of the selective microenvironment during polymerization formed by suitable functional monomers. An enzyme guides a substrate to attain a transition state. For this catalytic purpose, MIPs were synthesized in the presence of analogues of the substrate TSAs. A TSA resembles a transition state compound of the substrate in an enzyme catalyzed reaction. Therefore, a TSA of the substrate is chosen to serve as the template. For instance, 1,4-nitrophenyl methylphosphonate, a TSA of the substrate, was imprinted with polyvinylimidazole and cobalt chloride for catalytic hydrolysis of the 1,4-nitrophenyl acetate substrate (Scheme 2) (45). Here, 1,4-dibromobutane was used as the cross-linking monomer. The MIP prepared in that way enhanced acetate hydrolysis. Most significantly, this hydrolysis enhancement was specifically inhibited by the presence of TSA (45). A similar approach using acrylic-type cross-linking provided an MIP for paraoxon hydrolysis with a kcat value of 5.6  105 s1 under optimized conditions (53). Unfortunately, this value was much lower than that of the phosphotriesterase-catalyzed hydrolysis of paraoxon, which was w2200 s1. Cavity shaping with the TSA alone does not necessarily lead to a high catalytic effect. It appeared that additional dedicated recognizing functionality in the cavity was required for stronger TSA binding (Scheme 3). Moreover, molecular cavities should be distributed homogeneously in the MIP matrix (54). In that respect, a functional monomer of the functionality similar to that of the arginine amino acid was synthesized for alkaline ester hydrolysis. That was because of the presence of the guanidine moieties in it (Scheme 3). A TSA of phosphonic acid monoester was then used as the template for subsequent hydrolysis of the ester substrate (Scheme 3) (54–57). Strong ionic interactions between the amidine moiety of the functional monomer and the phosphonic monoester TSA enabled the designed functional monomer to form a stable complex. In addition, the amidine moiety played an important role in this catalytic reaction. In effect, the rate of this hydrolysis was increased by approximately 100 times (54). Moreover, the advantage NO2

NO2

NO2 NO2 O

O O H3C

1,4-Nitrophenyl acetate

O H3C

O

+ HO

-

O

CH3 O

-

O

P

-

OH

H3C

O

1,4-Nitrophenyl methylphosphonate

SCHEME 2 Consecutive steps of hydrolysis of 1,4-nitrophenyl acetate. 1,4-Nitrophenyl methylphosphonate was used as the reaction intermediate for molecular imprinting. Reprinted with permission from Ref. (45). Copyright 1989 Royal Society of Chemistry.

Chapter 9 • Molecularly Imprinted Polymers as Synthetic Catalysts 187

CH3 NH

H3C

H3C

O

NH O + NH O

Removal of the transition state analogue template

CH3 + NH

-

CH3 H3C

N

N

O

CH3 NH

CH3

P

NH

O

H3C CH3 O

CH3

HO

Substrate intake

O O CH3

4-[2-(3,5-Dimethylphenoxy)-2-oxoethyl]benzoic acid

O

OH

OH

+ H3C

CH3

Substrate hydrolysis

CH3

H3C

OH H3C

O

3,5-Dimethylphenol

CH3

NH NH O + NH O

O

+ NH

-

O

O

CH3

-

4-(Carboxymethyl)benzoic acid CH3

SCHEME 3 Consecutive steps of preparation of a molecular cavity imprinted by polymerization of bis(amidinium) and the TSA template. Reprinted with permission from Ref. (54). Copyright 1997 Wiley-VCH Verlag GmbH & Co.

of this stoichiometric noncovalent interaction between the functional monomer moiety and the diphenyl phosphate templating molecule was that it introduced the suspension polymerization method of preparing the MIP beads (56). Other than mimicking phosphotriester hydrolysis biocatalyzed by phosphotriesterase, MIPs were designed for carbonate hydrolysis (Scheme 4) by mimicking enzyme carboxypeptidase A function (41, 48, 49, 58–62). This single-peptide–chain enzyme binds to the Zn metal ion. This characteristic metal ion is located within the active center of the enzyme along with the four amino acid residues involved in the substrate binding. These

O

R1 O

R2 O

-

OH

O

R1 O

R2 O

R2

R1 OH + HO

Transition state

SCHEME 4 The mechanism of hydrolysis of carbonate. R1 and R2 stand for either the C or N atoms.

+ CO2

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MOLECULARLY IMPRINTED CATALYSTS

Table 1 Comparison of Michaelis–Menten Kinetics of Carbonate Hydrolysis with the Molecularly Imprinted and Nonimprinted Catalyst (48) Catalyst

kcat minL1

Km mM

kcat/kuncat

kcat/Km minL1 ML1

MIP-Cu-FM(a)-TSA(c) NIP-Cu-FM(a) MIP-Cu-FM(b)-TSA(c) NIP-Cu-FM(b)

28.0 0.37 10.2 0.35

0.58 6.10 0.36 4.16

110,000 1450 413,000 4520

48,200 61 292,000 276

FM(a), functional monomer (a); TSA(c), transition state analogue (c) in Scheme 5.

amino acids mainly include arginine, tyrosine, glutamic acid, and asparagine. To mimic the active center of carboxypeptidase, arginine-like functional monomers such as amidine derivatives were designed. A review discusses syntheses and suitability of these functional monomers in detail (41). Any functional monomer forms a complex with the template through weak interactions. To increase the number of these interactions, one may use an excess of the functional monomer. However, the monomer molecules may then be distributed randomly within the polymer, thus decreasing its selectivity. Therefore, functional monomers of strong complex formation ability were designed and synthesized instead. Several studies used these designed functional monomers containing an amidinium moiety (N,N0 -diethyl-4-vinylphenylamidine) for hydrolyzing esters, carbonates, and carbamates (48, 57). The amidinium moiety activity as a transition state binding site was similar to the catalytic activity of Arg 127 in carboxypeptidase A, resulting in a hydrolysis rate increase by 100–3000 times (Table 1).

3. Molecularly Imprinted Polymer-Based Catalysts for Degradation of Pollutants The rapidly growing use of pesticides to protect crops in agriculture, along with the increasing human population, has resulted in the accumulation of a wide variety of toxic CH2

CH2

O

CH2

OH

P

N

O

O

TSA(c) CH3

CH3 N

NH

N

H2N FM(a)

NH2

N

CH3 NH

N

NH

N

O

N O

N O

H2N FM(b)

(d)

SCHEME 5 Structural formulas of the designed amidine moiety containing functional monomers (a) N-{2-[bis(2-aminoethyl)amino]-}N0 -ethyl-(4-vinylbenzamidine) and (b) N,N-[N00 -(2-aminoethyl)-1,5(3-azapentylen)]-bis[(N0 -ethyl)-4-vinylbenzamidine]. (c) The transition state analogue, diphenyl phosphate, and (d) the substrate of the carbonate hydrolysis, di-(2-pyridyl)carbonate.

Chapter 9 • Molecularly Imprinted Polymers as Synthetic Catalysts 189

chemicals in natural aquatic systems. Therefore, great effort had been made to implement new technologies for either the complete elimination or at least significantly decrease the limit of these contaminants in the environment. Toward this aim, MIPs combined with TiO2 were applied to degrade selectively highly toxic compounds in the presence of a large excess of less toxic substances.

3.1

Degradation of Pollutants by MIPs Combined with TiO2-Based Photocatalysts

Titanium dioxide (TiO2) occurs in nature in the well-known rutile, anatase, and brookite mineral forms. In particular, semiconducting TiO2 in the anatase form generates photocurrent in a photoelectrochemical system if irradiated with light of energy matching its energy band gap (63–66). In this system, current flows through an external circuit from the TiO2 photoanode to the Pt cathode. This current direction indicates that oxidation occurs at the TiO2 photoanode (oxygen evolution) and reduction at the Pt cathode (hydrogen evolution). That is, this system reveals conversion of photoenergy into chemical energy by water splitting. This splitting is an important application of TiO2-assisted photocatalysis because hydrogen is a highly energetic fuel for electric energy production in fuel cells without generating CO2 (33). The UV light irradiation initiates the generation of the electron–hole pairs in TiO2 (Scheme 6). Water is then oxidized to the hydroxyl radical and dioxygen is reduced to the superoxide radical. Apart from photocatalytic water splitting, this irradiation can similarly activate other organic compounds adsorbed onto TiO2 for their transformations (67). Because of this advantage, photocatalysis is currently in use to degrade nonbiodegradable toxic contaminants into their nontoxic degradation products. As such, this process is nonselective. However, selectivity can easily be introduced by modifying the O2 Conduction band

+e–

- electron

.–

O2 Superoxide

UV light

Band gap (Eg)

H2O + hole Valence band

-e– .

OH Hydroxyl radical

SCHEME 6 Mechanism of photocatalytic water splitting on TiO2.

190

MOLECULARLY IMPRINTED CATALYSTS

surface of the TiO2 catalyst with selective recognition units. To this aim, molecular imprinting provides the desired selectivity. For selective recognition, the surface of spherical TiO2 particles was used to grow an MIP film for the selective degradation of 4-chlorophenol (4-CP) and 2-chlorophenol (2-CP) (68) (Scheme 7). Ultraviolet light was used to polymerize the 1,2-diphenylamine functional monomer. The TiO2 sphere, which had zero dimensionality with a high specific surface area, provided a high rate of photocatalytic decomposition of organic pollutants. 2-CP, 4-CP, and phenol were photodegraded over the photocatalysts according to the pseudo–first-order reaction kinetics. The determined apparent rate constant (kcat) values and their ratios were used to evaluate the influence of the MIPs on the rate of degradation of pollutants and the selectivity of different photocatalysts. Compared with the control unmodified TiO2 core, TiO2-MIP-(4-CP) increased the rate constant of degradation of 4-CP from 0.02449 to 0.03494 min1. However, the kcat for degradation of 2-CP was much lower, equaling 0.00810 min1. Similarly, when 2-CP was used as the template, the kcat value for the 2-CP was 0.02579 min1, whereas this value was 0.02032 min1 for the control TiO2. Apparently, the MIP film-coated TiO2 enhanced photodegradation of the toxins in the presence of their close structural analogue (68). A similar approach was used to imprint 2-nitrophenol (2-NP) and 4-nitrophenols (4-NPs) (69). The rate constant for photodegradation of the 2-NP and 4-NP over the corresponding MIP-coated TiO2 was 10.73  103 and 7.06  103 min1, 2.46 and 4.61 times higher than that for TiO2, which was 4.36  103 and 1.53  103 min1, respectively.

(a)

(b)

Complexation with 4-CP

1.0 0.8

NH-H H

O

NH

CI NH-H

C/C0

TiO

Polymerization on TiO

Template extraction NH-H

0.0

TiO2 NH

0.4 0.2

NH-H TiO

0.6

NH-H

NH

H

O

0

10

20 30 40 50 Illumination time, min

60

CI

SCHEME 7 (a) Consecutive steps of MIP preparation using a conducting polymer. (b) Degradation kinetics for 4-chlorophenol (4-CP) (triangles) and phenol (circles) in their mixtures (C0 ¼ 2 mg/L is the initial concentration of 4-CP and C is actual 4-CP concentration) under UV light illumination in the absence (empty symbols) and presence (filled symbols) of the TiO2-(4-CP) photocatalyst. Reprinted with permission from Ref. (68). Copyright 2007 Royal Society of Chemistry.

Chapter 9 • Molecularly Imprinted Polymers as Synthetic Catalysts 191

The MIP film deposited on TiO2 had a polyaniline backbone, which was much more resistant to photodegradation than the adsorbed organic toxins. However, the selectivity of the MIP film-coated TiO2 slowly decreased over a long period of UV light illumination in the absence of organic pollutants in solution. To overcome this, a film of inorganic MIP was proposed to coat the TiO2 photocatalyst to obtain a material with higher photochemical stability under UV irradiation conditions (70). This all-inorganic MIP film was doped with Al3þ to generate a selective molecular cavity for the phthalate ester toxic pollutants. The Lewis acid nature of the Al3þ dopant favored formation of a stable prepolymerization complex during imprinting. The photocatalytic degradation of these pollutants over differently prepared photocatalysts obeyed the pseudo–firstorder reaction kinetics. The apparent rate constant was, kcat ¼ 0.12 min1 for photodecomposition over (SiO2-Al3þ)-TiO2-MIP, thus 9.2, 6.5, and 2.5 times that over the control SiO2-TiO2 (0.013 min1), (SiO2-Al3þ)-TiO2-NIP (0.018 min1), and TiO2 (0.049 min1), respectively. As an advantage, the (SiO2-Al3þ)-TiO2-MIP material was stable over long exposure to UV light with only a minor decrease in the catalytic rate constant (0.098  0.015 min1) (70). As expected, the increase in effective surface area resulting from the presence of the MIP coat was the rationale for enhancing the degradation rate. To that aim, in one study, the degradation rate was three to four times enhanced although the difference in surface area of different photocatalytic materials was not high (20–30%) (71). Conclusively, this enhancement was only caused by the preconcentration effect of the MIP coats. A comparative study was performed to determine the rate of degradation of the saxitoxin over two different imprinted matrices (72). One MIP was prepared by direct imprinting of the saxitoxin template with 2-hydroxyethanesulfonic acid (HEA) over TiO2 beads; the other MIP was prepared in a two-step process. The first involved modification of the TiO2 surface with vinyltrimethoxysilane (VMS). In the second the initiator was used to polymerize the surface-exposed vinyl groups in the template’s presence. Interestingly, there was no saxitoxin photodegradation on the bare TiO2 whereas saxitoxin readily decomposed at the MIP film-coated TiO2. In addition, the rates of saxitoxin degradation were higher on the TiO2-MIP-HEA and TiO2-MIP-VMS photocatalysts. These different nanosized TiO2-modified materials were efficient in photocatalysis. However, they were difficult to separate and reuse. Therefore, effort was put into preparing a photocatalytic system that could overcome these deficiencies. With this purpose, MIP films enveloping Fe3O4 magnetic cores were devised (73). That is, over these cores, first an additional amino-terminated silane film was synthesized. This film provided stability of the Fe3O4 core. Then, an MIP film templated with 4-NP was deposited using the 1,2-phenylenediamine functional monomer. The resulting magnetic MIP particles were assembled over the TiO2 photocatalytic surface for final application. Degradation of 4-NP obeyed the pseudo–first-order kinetics with the rate constant, kcat ¼ 5.9  103 min1. This value was about 3.7 times that for NIP (1.6  103 min1), which indicated that the MIP/Fe3O4 film deposited on the TiO2 nanotubes enhanced the rate of photodegradation (73).

192

MOLECULARLY IMPRINTED CATALYSTS

Apart from this approach, a TiO2 photocatalyst in the form of a thin film was used (74). For that, a liquid phase procedure of a TiO2 film deposition was developed to prepare a salicylic acid (SA) imprinted TiO2 film. For that, (NH4)2TiF4 and boric acid were used as precursors. Such inorganic film materials were expected to be highly stable (74) compared with the photocatalysts coated with conducting MIP films (68). Because of the high stability of the imprinted matrix, different extraction procedures were introduced to study the effect of the template extraction conditions on performance of the SA imprinted TiO2 film (MIP-SA) (Scheme 8). The nature of the TiO2 structural geometry, such as the nanotube or fiber, significantly influences TiO2 photocatalytic performance (75). High development of the TiO2 surface results in a high surface-to-volume ratio, thus facilitating efficient interfacial charge transfer in the redox hole–electron recombination and favoring enhancement of photocatalytic reactions (76). In that respect, different pollutants, including tetracycline (77) and anthracene-9-carboxylic acid (78), were sol–gel imprinted in the pre-prepared array of TiO2 nanotubes. This array was prepared by anodic oxidation of a titanium

SCHEME 8 Scanning electron microscopy (SEM) images of the salicylic acid (SA) imprinted TiO2 (MIP-SA) film with the template removed by (a) extraction, (b) photodegradation, and (c) calcination. (d) Rate constants for degradation of SA over different MIPs and NIPs (kSA) determined for the different template-releasing conditions including (I) extraction, (II) photodegradation, and (III) calcination. (e) Semilogarithmic dependence of a reactive change of the analyte concentration on illumination time for (1) direct photolysis and photocatalytic degradation of SA over (2) bare glass, (3) NIP, and (4) MIP. C0, initial concentration of SA; C, actual concentration of SA. Reprinted with permission from Ref. (74). Copyright 2009 Royal Society of Chemistry.

Chapter 9 • Molecularly Imprinted Polymers as Synthetic Catalysts 193

sheet in an electrolyte solution containing sodium fluoride (75). Uniform TiO2 nanotubes with defined sizes were obtained by controlling the anodic potential. In one study, before condensation of TiO2 for imprinting, anthracene-9-carboxylic acid (9-AcCOOH) was covalently bound to TiO2 (78). This approach resulted in the formation of homogeneous molecular cavities revealing increased binding efficiency of the resulting material (Scheme 9). Importantly, these TiO2 photocatalysts were active under UV light excitation conditions because of the high energy band gap of 3.2 eV in their anatase form. To extend this gap, TiO2 was doped with Cl anions (79) or metal cations (80). The response to visible light as well as photoseparation of the holes and electrons largely increased for the resulting photocatalysts. In one study, the degradation ratio of the ciprofloxacin antibiotic reached 70.9% in 60 min (79). Another alternative method was proposed to extend this range of the photoexcitation band gap. It consisted in preparing composite materials of conducting polymers and TiO2. These materials were efficient in charge separating upon light irradiation. Almost since the beginning of the MIP research, monomers for the preparation of conducting polymers have been used for imprinting, mainly in sensing applications (11). Recently, this type of imprinting has been applied to photocatalysis as well. For instance, a

SCHEME 9 Top view SEM images of (a) unmodified TiO2 nanotubes, (b) MIP film-coated TiO2 nanotubes, (c) thicker MIP-film coated TiO2 nanotubes, (d) adsorption isotherm for 9-AnCOOH on (1) unmodified TiO2 nanotubes, (2) thin MIP film-modified TiO2, and (3) thicker MIP film-modified TiO2, as well as (4) nonimprinted polymer film-coated TiO2 nanotubes. (e) Dependence of ln(C0/C) (C0 and C stand for the initial and actual 9-AnCOOH concentrations, respectively) on degradation time in these processes involving direct photolytic process (10 ) without the photocatalyst (20 ) with the unmodified TiO2 nanotube photocatalyst, (30 ) with a thin TiO2-MIP nanotube photocatalyst, and (40 ) with a thin TiO2-MIP nanotube photocatalyst at 0.50 V versus SCE. Reprinted with permission from Ref. (78). Copyright 2010 Elsevier.

194

MOLECULARLY IMPRINTED CATALYSTS

polypyrrole (PPy) MIP film coating the TiO2 surface was reported (81). Analyte adsorption capability and selectivity of the TiO2-(MIP-PPy) nanocomposites were higher than those of the TiO2-(NIP-PPy) nanocomposites, and photocatalytic activity of the TiO2(MIP-PPy) was twice that of the TiO2-(NIP-PPy) owing to the presence of the imprinted cavities (81).

3.2

Photodegradation of Pollutants by MIPs Deposited on Other Photoactive Materials

As an alternative to the MIP film-coated TiO2 used for the degradation of toxic compounds, an MIP containing an organic photosensitizer was prepared (82). For this, red bengal, a well-known photosensitizer, was covalently bound to the surface of the acrylic acid–based MIP. This photosensitizer produced singlet oxygen (1O2) by an energy transfer from the photoexcited sensitizer to O2 (83). This singlet oxygen degraded an organic toxin adsorbed on the MIP surface, which served as an acceptor. The extent of conversion of red bengal alone exceeded 30% for all toxins without selectivity. The selective conversion of chlorophenols was pronounced when MIP was introduced in combination with this photosensitizer (Scheme 10) (82). Biodegradation and detoxification of phosphotriesters are slow. To enhance the hydrolysis of paraoxon insecticide, an MIP-based catalyst was proposed (84). This MIP was grafted onto the surface of multiwall carbon nanotubes using the paraoxon 4-NP hydrolysis product as the template, 4-vinyl pyridine as the functional monomer, and divinylbenzene as the cross-linking monomer. 4-NP is the product of paraoxon hydrolysis. This hydrolysis was reversible. However, the presence of the MIP selective to 4-NP disturbed this reversibility by absorbing this product, thus enhancing degradation of the paraoxon toxin (84).

3.3

Degradation of Pollutants by MIP Alone

Phosphotriesterase is a frequently used enzyme for the degradation of phosphotriesters (85). An X-ray crystal structure determination and site-directed mutagenesis revealed the HO

CI CI

OH

O H

HO

O

O

CI HO

O

O H

I

CI CI

OH

CI

NaO

O

H O

1

CI O

NaO O2

I O

O I

I

HO O2

Red bengal

CI

SCHEME 10 Sketch of the mechanism for selective 1O2 oxidation of chlorophenols catalyzed by the red bengal photosensitizer-modified MIP. Reprinted with permission from Ref. (82). Copyright 2010 Royal Society of Chemistry.

Chapter 9 • Molecularly Imprinted Polymers as Synthetic Catalysts 195

presence of two Zn bivalent metal ions and the histidine moiety in the active center of this enzyme. To mimic the enzyme microcavity structure, a synthetic receptor was prepared using 4-vinylimidazole and CoCl2 (53). During polymerization, the imidazole together with the template coordinated cobalt. This complexation was evidenced by the 1.5 times higher catalytic performance of MIP over NIP. Although this synthetic polymer attempted to mimic the coordination similar to that of the active center of phosphotriesterase, the Co2þ-imidazole moiety was not homogeneously distributed in it. This inhomogeneity resulted in much lower catalytic activity toward the paraoxon hydrolysis than that of phosphotriesterase. The catalytic effect was enhanced by 30% by replacing the cobalt atom with that of zinc and introducing methacrylic acid (MAA) as another functional monomer for paraoxon imprinting (86). A turnover constant of this MIP catalyst toward the hydrolysis of paraoxon was kcat ¼ 7.4  102 s1. Subsequently, a range of studies tested the ability of the methacryloyl histidine-based functional monomer to chelate differently divalent metal ions, such as Cu2þ (87), Co2þ, Ni2þ, and Zn2þ (88), to mimic the active center of phosphotriesterase. The rate of paraoxon hydrolysis was increased by the designed imprinted versus nonimprinted polymers. Moreover, this increase was much higher compared with that of noncatalyzed hydrolysis. All histidine coordinated metal ion (Co2þ, Ni2þ, Zn2þ, and Cu2þ) centers were able to catalyze the hydrolysis of paraoxon. The Km value varied from 0.25 to 1.36 mM for all MIPs and NIPs. Although the metal ion nature brought some variation to the hydrolysis rate, the close Km values indicated that the nature of metal ions present in the catalytically active center did not alter the affinity much (88). Another MIP catalyst was designed using a similar functional monomer, with N,N0 -methylenebisacrylamide as the hydrophilic cross-linker for higher performance in aqueous solutions (89). The value of Km determined for this catalyst was 0.3 mM. In a different study, imidazole and amidine functionalities were introduced to construct an active center for the catalytic hydrolysis of 1,4-nitrophenyl methyl carbonate ester (90). The Km value of catalysis afforded with the resulting MIP was 1.06 mM at pH ¼ 7.0. The rate of the hydrolysis catalyzed by this MIP was 60 times higher than that of the noncatalyzed reaction and twice that in the presence of NIP. Notably, the main degradation product of this catalysis was 4-NP, a toxic pollutant. Therefore, a dual-functioning MIP catalyst was recently introduced. This catalyst efficiently catalyzed paraoxon degradation and the 4-NP product was adsorbed on it selectively (91). However, the choice of the functional monomer was different from that in the above mentioned reports. That is, the Zn dimethacrylate (MAA-Zn) functional monomer with the divinylbenzene (DVB) cross-linking monomer was used for 4-NP imprinting. The core–shell microsphere of the catalyst was prepared by precipitation polymerization. That is, the as-synthesized core–shell microsphere imprinted with 4-NP was used with no further modification for the MAA-Zn and DVB precipitation copolymerization in the presence of the paraoxon template to result in the dual-template MIP microspheres. The paraoxon-imprinted outer shell acted as the layer for paraoxon catalysis and the (4-NP)-imprinted inner shell as the layer for 4-NP adsorption (91).

196

MOLECULARLY IMPRINTED CATALYSTS

4. Dedicated MIP-Catalyzed Reactions 4.1

MIP-Catalyzed Diels–Alder Reactions

The Diels–Alder reaction is important synthetically because of its ability to form new carbon–carbon bonds. It involves the addition of a concentrated conjugated diene to an olefin to produce a cyclohexane derivative. The entropy barrier for this bimolecular reaction is high, with activation entropy typically in the range of 1.25 to 1.67 kJ mol1 K1 (92, 93). Surprisingly, there are no documented examples of enzyme-catalyzed pericyclic cycloadditions. In contrast to most enzyme-catalyzed reactions, this reaction is believed to proceed typically through a concerted transition state involving the simultaneous formation of carbon–carbon bonds within a cyclic array of interacting orbitals (Scheme 11). To catalyze this reaction, MIPs were generated with selective molecular cavities of the structure mimicking the pericyclic transition state of the Diels–Alder reaction (Scheme 11) (94, 95). That way, the entropy of activation of this reaction was lowered because of binding of both the diene and the dienophile in the imprinted cavity. The TSA, chlorendic anhydride (CA) was imprinted as the template using the MAA functional monomer and the EGDMA cross-linking monomer. The Km and kcat values of this MIP-catalyzed Diels–Alder cycloaddition in acetonitrile were 42.5 mM and 3.82  102 min1, respectively (94). Typically, this way of polymerization results in a bulk polymer material with nonhomogeneous distribution of imprinted cavities. With this in mind, a novel procedure of imprinting of homogeneously distributed cavities in an MIP was introduced (96, 97). In this procedure, a template molecule was covalently attached to a sacrificial silica support. Then, the immobilized TSA template was imprinted using the same functional and cross-linking monomers. The final MIP catalyst was obtained by dissolving the support. As expected, performance of this catalytic MIP was higher than that of the MIP produced via a classical procedure of polymerization applied for similar cycloaddition of hexachlorocyclopentadiene and maleic acid. The determined Km value was 5.8 mM with the effective reaction rate constant keff ¼ 1.1  103 s1 (96).

Cl

Cl

O

O

Cl

Cl SO2

Cl

O

+

S

O

S O

Cl

O O

Cl

O

O

Cl

Cl Cl

O

O Hexachloro cyclopentadiene

Maleic anhydride

Transition state

Cl Cl

Cl

O

Cl

O

O Cl

Cl

Cl

O

Cl O

4,5,6,7-Tetrachloro2-benzofuran-1,3-dione

Chlorendic anhydride transition state analogue

SCHEME 11 Consecutive steps of Diels–Alder reaction mechanism. In this example, the transition state analogue is similar to the product. Reprinted with permission from Ref. (94). Copyright 1997 Wiley.

Chapter 9 • Molecularly Imprinted Polymers as Synthetic Catalysts 197

Studies have proposed the temperature and cross-linker–dependent catalytic activity of MIPs. For that, a series of catalytic MIPs were designed and synthesized to increase the rate of the Diels–Alder cycloaddition of 1,3-butadiene carbamic acid benzyl ester and N,N-dimethylacrylamide to yield corresponding endo and exo products (98, 99). The TSAs for the endo and exo reaction pathways were used as templates. These designed catalysts increased the catalysis rate up to 20 times compared with that of the catalysis in solution at room temperature. The MIP catalytic performance was lower at an elevated temperature. Binding of the analyte on MIP, prepared with the EGDMA cross-linker, was higher than that prepared with the DVB cross-linker. Interestingly, higher binding reached in the batch binding studies did not lead to higher reaction rates. Although the EGDMA-based MIP catalyst binding of the analyte was higher, the DVB-based MIP revealed a more pronounced catalytic effect. Higher binding of the EGDMA-based MIP, ascribed to competitive binding of different functionalities of the functional and cross-linking monomers, led to a situation in which additional functional groups of the cross-linking monomer contributed to nonspecific binding (98).

4.2

MIP-Catalyzed b-Elimination Reactions

The b-elimination reaction involves the cleavage of a s bond and the formation of a p bond. The b-elimination selected to study the MIP catalysis was dehydrohalogenation of 4-fluoro-4-(1,4-nitrophenyl)butan-2-one. The catalytic site for substrate reaction was designed by imprinting benzylmalonic acid, the analogue of the substrate, with an N(2-aminoethyl)-methacrylamide functional monomer (15). The resulting polymer was prepared using the DMF solution of the EGDMA and methyl methacrylate (MMA) crosslinkers. Importantly, the template-assisted polymerization allowed orientation of recognizing functionalities in the catalytic cavity to be adjusted by using template molecules with binding sites at different positions. Interestingly, the catalytic effect of the MIP prepared in the presence of the template bearing the 1,3-dicarboxylic acid substituent was appreciable compared with two other MIPs bearing dicarboxylic acid groups in other positions (Scheme 12). Moreover, not only the position but also the mutual orientation of two carboxylic functionalities in the template molecule decided on the catalytic performance. That is, the MIP prepared with the template with the 1,3-dicarboxylic acid substituent with the carboxyl groups oriented in opposite directions performed similarly to NIP. Evidently, only the template with these groups directed at amino recognizing groups of the cavity participated in the dehydrohalogenation. The Km and kcat values for MIP prepared with the template with 1,3-dicarboxylic acid of properly oriented carboxyl groups were 27 mM and 1.1  102 minl, respectively (15). Another interesting catalytic b-elimination used bovine serum albumin (BSA) as the matrix to imprint a structural analogue of a substrate as the template (102). The template molecule changed the protein molecule conformation during freeze-drying,

198

MOLECULARLY IMPRINTED CATALYSTS

O

-

NH

H 3N +

O

O O O

-

CH3

H 2C

H 3N +

EGDMA, MMA, AIBN

O

-

+

H3N O O

Polymerization

-+

O H3N NH H 2C O CH3 Bezylmalonate template removal

Template-(functional monomer) assembly

O

H2N CH3

F

H2N Reactant intake

H

H2N

H2N

NO2 4-Fluoro-4-(1,4-nitrophenyl)butane-2-one

SCHEME 12 Sketch of the mechanism of imprinting a molecular cavity for catalytic dehydrohalogenation of 4-fluoro-4-(1,4-nitrophenyl)butane-2-one. EGDMA, ethyleneglycoldimethacrylate; MMA, methyl methacrylate; AIBN, azobisisobutyronitrile. Benzylmalonate and N-(2-aminoethyl)methacrylamide served as the template and functional monomer, respectively. Reprinted with permission from Ref. (15). Copyright 1994 American Chemical Society.

which resulted in the fabrication of a catalyst for dehydrohalogenation of 4-fluoro-4(1,4-nitrophenyl)-2-butane (Table 2). The MIP-BSA catalytic b-elimination was faster than that of the NIP-BSA. However, this catalytic effect disappeared after dissolution of MIP-BAS in water. The Km and kcat values of dehydrofluorination were 189 mM and 267 min1, respectively (102). Similarly, another structural analogue of the substrate, N-(1,4-nitrobenyl)-isopropylamine (Table 2), was imprinted in the papain and blactoglobulin biomacromolecular proteins (103). Interestingly, the catalytic effect was lower when catalytic cavities were generated in the presence of either the substrate or the product of the dehydrohalogenation. To apply the developed procedures to MIP catalysis in continuously driven reactors, an MIP catalyst was devised in the form of a membrane (100). This membrane was fabricated by wetting a filter paper with a solution for polymerization. The template was imprinted using MAA and EGDMA as the functional and crosslinking monomer, respectively (Table 2). This MIP was catalytically active even in aqueous solutions.

Chapter 9 • Molecularly Imprinted Polymers as Synthetic Catalysts 199

Table 2 Relative Catalytic Effects of Dehydrofluorination of 4-Fluoro-4(1,4-nitrophenyl)-2-butanone Catalyzed by Different Molecularly Imprinted Polymers (100) Substrate Analogue Imprinted as Template

(Functional Monomer)/ (Cross-linking Monomer)

Solvent

Catalytic Effecta

References

N-Benzyl-isopropylamine Benzylmalonic acid N-Methyl-N-(4-nitrobenzyl)d-aminovaleric acid N-(1,4-Nitrobenzyl)isopropylamine N-Benzyl-isopropylamine

MAA/EGDMA MMA/EGDMA BSA

Acetonitrile Benzene Ethyl acetate

2.4 3.2 3.3

(101) (15) (102)

b-Lactoglobulin

Acetonitrile

3.27

(103)

MAA/EGDMA

Acetonitrile/ water (1:1, v:v)

5.97

(100)

MAA, methacrylic acid; MMA, methylmethacrylate; EGDMA, ethyleneglycoldimethacrylate; BSA, bovine serum albumin. a Catalytic effect, (catalytic effect of MIP)/(catalytic effect of NIP).

4.3

MIPs as Catalysts for Aldol Condensation Reaction

The aldol reaction has long been recognized as one of the most useful tools for a synthetic organic chemist. This reaction is able to form carbon–carbon bonds and generate up to two new stereogenic centers. The aldolase enzymes are capable of catalyzing this reaction. There are two different types of aldolases depending on the mechanism of the enzymatic catalysis. Type I aldolases have a lysine residue as an active center. This center is involved in formation of a Schiff base with a molecule of the donor substrate. Type II aldolases require a metal ion, prevailingly Zn2þ, as the cofactor for reactivity. For instance, an MIP catalyst mimicking type II aldolase was prepared to build a C–C bond between acetophenone and benzaldehyde to produce chalcone. For that, a complex of dibenzoylmethane with the cobalt(II) ion was imprinted in a 4-vinylpyridine-styrene-divinylbenzene matrix (Scheme 13) (44, 104). This imprinting led to catalytic cavity formation in the following way. The two-(oxygen atom) center was accommodated in two of the four coordination sites of the tetrahedrally configured Co2þ. In addition, this cation was coordinated by two nitrogen atoms, each of different pyridinyl moieties. Besides Co2þ coordination, these moieties provided the basic environment necessary for the transformation of acetophenone into enolate. Moreover, the styrene and divinylbenzene cross-linking monomers provided p–p stacking and van der Waals interactions to aid-in defining the recognition site topography in the resultant MIP (Scheme 13). This MIP demonstrated a substrate selective turnover and rate enhancement when used to catalyze the formation of an entropically unfavorable C–C bond between acetophenone and benzaldehyde. This MIP-catalyzed aldol reaction rate was eightfold higher than that of the noncatalyzed reaction and twice that of the NIP catalyzed reaction. The determined value of Km was 1.23 mM (104).

200

MOLECULARLY IMPRINTED CATALYSTS

(a)

O CH3

+

Acetophenone

(b)

O

O

O

O

+ H20

H

Dibenzoylmethane

Benzaldehyde

H2C

Chalcone

CH2 CH2

CH2 N

Polymerization

N

N

N

2+

Co

2+

CH2

Co O

H2C

O

O

(i)

H2C

O

CH2

Removal of template

N

2+

Co O

N

N -

O

N

2+

(iv)

Co O

N

(iii)

(ii)

2+

Co

N

O

H

CH3 H

SCHEME 13 (a) A sketch of the mechanism of aldol condensation of acetophenone and benzaldehyde to yield chalcone. (b) Consecutive steps of preparation of an MIP for catalysis of the aldol reaction, including (i) complexation of functional monomers and dibenzoylmethane, (ii) removal of template, (iii) ingress of reactants, and (iv) enolization and nucleophilic addition yielding an MIP-stabilized reactive intermediate, followed by proton transfer to this intermediate, and then dehydration affording chalcone. Reprinted with permission from Ref. (104). Copyright 1996 American Chemical Society.

Afterward, an MIP mimicking aldolases type I was prepared (105). A polymerizable proline derivative and the diketone was used as the functional monomer and template, respectively, to form a molecular cavity dedicated to imitating the intermediate of the aldol reaction. To overcome heterogeneity of cavity distribution in the MIP, covalent imprinting was introduced using a derivatized functional monomer. More importantly, the MIP for this catalysis was prepared in a form of the nanogel of beads comparable in

Chapter 9 • Molecularly Imprinted Polymers as Synthetic Catalysts 201

B

(a)

(b)

H N O

Benzisoxazole

N

+B

N O

O

-

2-Cyanophenol

NH

N CH2

Indole

4-Vinylpyridine

SCHEME 14 (a) Sketch of the Kemp elimination mechanism involving base (B) catalyzed proton abstraction from benzisoxazole. (b) Possible interaction of the indole template with the 4-vinylpyridine functional monomer. Reprinted with permission from Ref. (106). Copyright 1998 Wiley.

size to enzymes. Although active site titration confirmed a similar concentration of the proline active sites in MIP and in NIP, there was a pronounced difference in their catalytic activity. This difference confirmed the presence of catalytic cavities in the MIP. The ratio of the catalytic rate constants for the MIP and NIP was appreciable, kcat,MIP/ kcat,NIP ¼ 18.8 for kcat,MIP at 0.25  102 min1 (105).

4.4

MIPs as Catalysts for Kemp Elimination

The Kemp elimination reaction involves base-induced proton abstraction from benzisoxazole (Scheme 14). This elimination results in a single chromophoric 2-cyanophenol product. For that, the first MIP report described bulk imprinting of indole, a structural analogue of the reaction intermediate, with the 4-vinylpyridine functional monomer (106). Tuning the catalytic microenvironment with different amine bases increasingly stabilized the transition state. The catalyst, in a form of the bulk MIP, provided the Km and kcat value equal to 0.484 mM and 0.2 min1, respectively. The elimination rate was markedly increased, up to 4  104 times, compared with that of the noncatalyzed reaction. A similar approach but involving the MIP in a form of the nanogel particles comparable in size to that of enzymes was reported for further increase of the rate of catalytic Kemp elimination (Scheme 14) (107, 108). These systematic studies showed the effect of the concentration of the monomer and initiator on the resulting MIP nanogel performance. For the MIP nanogel prepared under optimized conditions, the Km and kcat values were 0.41 mM and 1.34 min1, respectively (107, 108).

4.5

Metal Ion-Based MIPs as Catalysts for Hydrogenation

The introduction of a catalytic metal center during the formation of an imprinted cavity results in an environment similar to that of a (chiral metal)-containing complex. In this cavity, a substrate is catalytically transformed to a product at the metal center whereas stereochemistry of this transformation is controlled by the chiral environment surrounding the metal. For instance, molecular imprinting with the use of an SiO2supported metal complex provided a promising approach to a shape-selective catalysis (Scheme 15). Several reviews extensively described how the efficient Rh ion-based

202

MOLECULARLY IMPRINTED CATALYSTS

Metal complex precursor Ligand Functional group

Binding site Template ligand

M M

Metal center

Oxide surface Metal complex attachment on oxide surface

M

Surface-attached metal complex

Coordination of the template to surface-attached metal complex

Integrated catalytic system Surface matrix overlayers

Molecular binding site

M

Stacking of inorganic/organic matrix overlayers around the supported metal complex

Imprinted cavity for shape selective catalysis

M

Removal of the template ligand

Surface matrix overlayers for stabilization of catalyst

Catalytically active metal center Molecularly-imprinted metal complex catalyst on oxide surface

SCHEME 15 Consecutive steps of preparation of an MIP catalyst on an oxide surface. Reprinted with permission from Ref. (109). Copyright 2013 American Chemical Society.

MIP catalysts for hydrogenation of ketones and alkenes were designed and prepared (109–111). That is, an Rh monomer and Rh dimer, attached to silica supports, were used to prepare selective cavities in MIPs by imprinting reaction intermediates. Importantly, this procedure afforded surface imprinting. For that, first, a complex of a metal ion with the reaction intermediate was immobilized on the oxide surface. Then, the template was covalently immobilized by a ligand exchange. In one study, a different polymerization matrix, such as acrylate photo-copolymerized with 2-hydroxyethyl methacrylate, a vapor-deposited copolymer of styrene and DVB, and a polymer of MMA and EGDMA as well as by condensation of Si(OCH3)4, was prepared to yield polymer materials operating in aqueous solutions (111). The catalytic efficiency of the surface-confined photopolymerized copolymer of acrylate and 2-hydroxymethacrylate was higher than that of the hydrophilic SiO2 matrix. The hydrophilicity of these overlayers was proposed to be a main reason for the decrease in the catalytic activity of the molecularly imprinted Ru-complex catalyst. Asymmetric hydrogenation of olefins is important in practical applications. For this hydrogenation, a new class of heterogeneous catalysts was designed (Scheme 16). The activity of these catalysts combined transition-metal catalysis and molecular imprinting (112). For that, the nitrogen-based chiral bis(oxazolines) and MAA were used as the functional monomers. Asymmetric hydrogenation of enamides was selected as a model reaction because of its significance in the pharmaceutical industry. The MIP prepared

Chapter 9 • Molecularly Imprinted Polymers as Synthetic Catalysts 203

H2C

CH2 O

O

O

N

1. Cross-linking 2. Polymerization 3. Template extraction

+

N

N Rh O Cl

O +

-N

Rh Cl

Cl

OR1 R2

HO

NH O

CH2

HO O

O

CH3

SCHEME 16 Preparation of rhodium-containing asymmetric heterogeneous catalyst by molecular imprinting. Reprinted with permission from Ref. (112). Copyright 2009 Elsevier.

using the bis(oxazoline)–Cu(II) complexes and the methyl-(Z)-a-N-acetamidocinnamate template significantly increased enantioselectivity compared with that of the corresponding NIP, with an impressive 82% excess of the D-enantiomer product. The use of this MIP resulted in an 78% increase in enantioselectivity compared with that of the NIP (112).

5. Enzyme Mimicking for MIP-Based Sensing Beside catalytic applications, enzyme-like cavities were designed in MIPs for selective sensing purposes (113, 114). In one study, an MIP cavity was designed by mimicking the active center of the tyrosinase enzyme for the selective determination of phenols (Scheme 17) (113). The MIP in the form of a membrane operated as the recognition unit

(a)

(b) NH N 2+ OH Cu

NH N

Cu2+ N OH N N NH H

HIS 38

4.78

2.

61

2.7

2

Cu(II)

3.28

HIS 194 Cu(II) HIS 190

HIS 54 O2

HIS 216

HIS 63 SCHEME 17 (a) Proposed simplified structure of a molecularly imprinted cavity with the tyrosinase-like activity. (b) Structure of active site of Streptomyces castaneoglobisporus tyrosinase; HIS, histidine. Reprinted with permission from Ref. (113). Copyright 2010 Elsevier.

204

MOLECULARLY IMPRINTED CATALYSTS

of an oxygen electrode. Under optimized conditions, the chemosensor determined phenol in water with the limit of detection as low as 0.063 mM. This tyrosinase activity mimicking MIP converted phenol to di-one with a concomitant reduction of molecular oxygen to water. A similar MIP cavity was designed to sense dopamine and catechol (114). The resulting MIP chemosensor revealed the Michaelis–Menten kinetics for the oxidation of both dopamine and catechol. The Km value for catechol and dopamine was 0.049 and 0.093 mM, respectively.

6. Conclusions The number of examples of imprinting of different reactants and analytes and their applications are steadily growing. Experts from different research fields are attracted to produce dedicated imprinted materials for a wide range of applications. To date, their efforts have largely contributed to the enzyme-like Michaelis-Menten kinetics for MIPs. However, high analyte selectivity and high turnover numbers similar to those revealed by enzymes are still challenging. Although MIP cavities are designed by mimicking the active centers of enzymes, their catalytic performance is still inferior to that of enzymes. A possible reason might be the heterogeneity of molecular cavities. Several reports have described this problem in detail (41, 54). Preparation of MIPs in different forms, such as nanogels and hydrogels, largely improved the catalytic effect. However, further improvement is in demand. Nevertheless, undoubtedly, MIP reusability and survival under harsh operating conditions are advantages as novel catalytic materials compared with their natural counterparts.

Acknowledgment The current research was financially supported by the Polish National Science Center (NCN, Grant No. 2011/03/D/ST4/02596 to P.S.S.), the European Regional Development Fund (ERDF, POIG.01.01.02-00-008/08 2007–2013 to W.K.) co-financed from the European Regional Development Fund within the Innovative Economy Operational Program “Grants for Innovations,” the European Union 7.FP (Grant REGPOT-CT-2011-285949-NOBLESSE to W.K.), the Foundation of Polish Science (Project MPD/2009/1/styp 15 to W.K.), and the United States National Science Foundation (Grant CHE-1401188 to F.D.).

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Chapter 9 • Molecularly Imprinted Polymers as Synthetic Catalysts 205

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