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ORIGINAL PAPER

Studies on the preparation and properties of sol-gelmolecularly imprinted polymer based on tetraethoxysilanefor recognizing sulfonamides

Sung-Chuan Lee & Feng-Lin Chuang & Yen-Ling Tsai &

Hui Chen

Received: 12 June 2009 /Accepted: 18 November 2009 /Published online: 3 December 2009# Springer Science + Business Media B.V. 2009

Abstract The synthesis of a molecularly imprinted poly-mer (MIP) composed of tetraethoxysilane (TEOS) for recognizing sulfonamides by sol-gel process is developed.The MIP s preparation conditions, the H 2 O/Si molar ratios(R), pH value of the competition solution, and thecalcination process are discussed. The competition experi-ments of the MIP for template (sulfamethazine, SMZ) andanalogue (sulfamethoxazole, SMO) are performed by High-Performance Liquid Chromatography (HPLC). The resultsshow that the selectivity of the competition solution with pH=7.4 is higher than those with pH=4.6. In addition, theselectivity of SMZ increases, while the calcination time andthe calcination temperatures increase. Moreover, the selec-tivity of the MIP (53.3) is approximately seven timesgreater than that of the non-imprinted polymer (NIP) (8.32)under optimum preparation conditions.

Keywords Molecularly imprinted polymer .Sulfamethazine . Sulfamethoxazole . Sol-gel . HPLC

Introduction

Molecular recognition is a phenomenon that can beenvisaged as the preferential binding of a molecule to a

receptor with high selectivity over its close structuralanalogues. This concept has been translated elegantly intothe technology of molecular imprinting, which allowsspecific recognition sites to be formed in synthetic polymers through the use of various templates [ 1 5].Molecular imprinting technique is used for creating polymers with special recognition ability and predesignedselectivity for the template molecule and structurally relatedcompounds. Molecular imprinting of synthetic polymerswith a specific target molecule can be done if the target resembles the template or imprint molecule used during polymerization and then removed after polymerization.Polymerization occurs when monomers carrying certainfunctional groups interact with the template and arrangethemselves around the template into a frozen position [ 6

8]. Molecular imprinting is becoming increasingly recog-nized as a powerful preparation for synthetic molecularlyimprinted polymers containing specific recognition sites for template molecules.

Imprinting can be achieved in three ways: non-covalent,covalent, and sacrificial spacer. Non-covalent imprintingrelies on the ability of the template molecule to produce oneor more strong intermolecular non-covalent interactionswith the functional monomers, for example, H-bonding,electrostatic, or interactions. The removal of thetemplate results to a cavity, which is complementary insize, shape, and functionality to the template molecule, andwhich contains the recognition site used for recognizingspecific molecules. In covalent imprinting, the template-monomer covalent bond is formed before the polymerizedMIP. Polymerization is conducted in the usual manner, but template removal and rebinding is achieved by a chemicalrather than a physical process [ 9]. In the sacrificial spacer method, the pre-organized or covalent approach employsreversible covalent bonds, usually involving a prior

S.-C. Lee : F.-L. Chuang : H. Chen ( * )Department of Chemical and Materials Engineering, National Central University, No.300, Jhongda Rd.,Jhongli, Taoyuan County 32001, Taiwane-mail: [email protected]

Y.-L. TsaiDepartment of Polymer Materials, Vanung University, No. 1, Van-Nung Rd.,Jhongli, Taoyuan County 32061, Taiwan

J Polym Res (2010) 17:737 744DOI 10.1007/s10965-009-9364-x

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chemical synthesis step to link the precursors to thetemplate or to a structurally similar molecule [ 10]. In other words, monomers are covalently linked to the template viashort spacers. The spacers are eliminated once the imprint-ing process is completed, giving room and suitablefunctional groups for non-covalent interactions with thetarget molecule [ 11].

Many common MIPs are synthesized by organicmethods involving the polymerization of functional mono-mers and a crosslinking monomer such as acrylamide,acrylicacid, ethylene glycol dimethacrylate, and so on. Theapplications of organic MIPs are also restricted to the useof organic solvents for the dissolution of organic polymersor template [ 12]. The preparation of MIPs in aqueoussystem is very challenging task because of the interferenceof water molecules. Non-specific interactions between thetemplate and functional monomers or polymers may also become much weaker or even disappear in aqueoussystem. Thus, it is of great importance to developimprinted materials that can be used in aqueous environ-ment. In this study, our laboratory synthesizes an inorganicMIPs by the sol-gel process that can be used in aqueousenvironment [ 13].

In the sol gel process, inorganic siloxane based poly-mers are formed by acid or alkali catalyzed hydrolysis andcondensation of a series of silane monomers. As theinorganic precursors hydrolyze and condense to formsiloxane bonds, a sol is formed. With time, these colloidal particles aggregate leading to the formation of a porous,three dimensional network the gel. Inclusion of a templatespecies results in the formation of imprinted media [ 14 18].During the past decade, the sol gel process has become aconvenient approach for the preparation of stable host matrices for the development of molecularly imprinted polymers. He et al. [ 19] developed the selective imprintedamino-functionalized silica gel sorbent by combining asurface molecular imprinting technique with a sol-gel process for online solid-phase extraction-HPLC. Themethod was applied to the determination of three tracesulfonamides in pork and chicken muscle samples. Theimprinted functionalized silica gel sorbent showed highaffinity, selectivity, fast kinetics, capacity, and good siteaccessibility for sulfonamides. Su et al. [ 20] synthesized thesulfamethazine-imprinted polymer on the silica surface viaquasiliving radical polymerization. The molecularlyimprinted polymer layer grafted on the silica surface wasconstructed by using sulfamethazine as the template,methacrylic acid as the functional monomer and ethylenedimethacrylate as a cross-linker. The sulfamethazine-imprinted polymer was used successfully for the determi-nation of sulfamethazine in milk by HPLC analysis. Sharonet al. [21] applied the propranolol-imprinted polymers asthin films on glass substrates by using two polymeric

systems, acrylic and hybrid organic-inorganic sol-gels. Themore explored acrylic matrix has been shown to be lessfavorable for molecular imprinting in thin films due to thenontrivial preparation method that reduces the reproduc-ibility and due to the high nonspecific binding. The sol-gelsystem has been shown to be superior in terms of preparation process, faster diffusion times, and reducednonspecific binding.

Sulfamethazine (SMZ) is a widely used antimicrobialagent added to the feed of meat-producing animals to treat infections. It has been reported that SMZ is potentiallycarcinogenic as it can produce thyroid tumors in rodent bioassay [ 22 24]. MIP utilized as an artificial receptor for SMZ may be used for the separation and analysis of SMZand its analogue sulfamethoxazole (SMO), even for theanalysis of SMZ residues in food or water. The EuropeanCommunity has adopted a maximum residue level (MRL)of 100 mg/kg for sulfonamides in foodstuffs of animalorigin [ 25].

The main propose of this article is to prepare a series of MIPs based on tetraethoxysilane (TEOS) by sol-gel processand to systemically investigate the competition experimentsfor the template and analogue. In addition, various H 2 O/Simolar ratios (R), effect of competition experiment pH, andthe effect of the calcination processes on MIPs are alsostudied.

Experimental

Materials

Tetraethoxysilane (TEOS) (Shin-Etsu Chemical Co. Ltd.Tokyo, Japan) as monomer, sulfamethazine (SMZ)(SIGMA Chemical Co. St. Louis, MO) as a template,and sulfamethoxazole (SMO) (SIGMA Chemical Co. St.Louis, MO) as a structure analogous molecule wereused as received. Hydrochloric acid, 37% (ScharlauChemie S.A. Sentmenat, Barcelona) as a menstruum of sulfonamides was used as received. Deionized water (18.2 M cm) as an auxiliary agent of reaction wasused as received.

Preparation of sol-gel molecularly imprinted polymers(MIPs)

Sulfamethazine (SMZ) 0.002 mol and hydrochloric acid(HCl) 760 l were dissolved completely in deionized water at room temperature for 30 min, and then various ratios of tetraethoxysilane (TEOS) was added to above solution.This precursor was stirred at room temperature for 1 dayand then raised the temperature to 60C for 3 days. The

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monoliths were grounded and sieved to obtain particle sizes between 53 and 88 m and the template molecule wasremoved by calcination, and then the MIP was washed bydeionized water. A non-imprinted polymer (NIP) was prepared in parallel and under identical conditions but inthe absence of the template.

High-performance liquid chromatography (HPLC) analysisof MIP

Both the SMZ and the structurally analogous moleculeSMO were dissolved in water to produce a standardsolution containing 100 ppm of SMZ and SMO, respec-tively. The 200 mg MIP was added in 10 ml standardsolution and adsorbed at room temperature for 30 min tocomplete the adsorption. The MIP solution mixtures werethen centrifuged to obtain the residual solution. The abovesolution was injected into HPLC to investigate the residualconcentration of SMZ and SMO. The adsorption andselectivity ( ) of the MIP were determined through acompetition adsorption experiment; the quantity of absorp-tion (Ad SMZ and Ad SMO ) of MIP was obtained by thedifference between the original concentration (100 ppm)and the residual concentration. The selectivity ( ) wasobtained by the comparison of the adsorption of the template(Ad SMZ ) and analogue (Ad SMO ) on MIP. The selectivity wasdefined as =Ad SMZ / AdSMO . The selectivity represents the proportion of adsorption of template and analogue. HPLCanalysis was performed by a Waters 510 HPLC pumpequipped with a SFD UV/vis Detector S32109 (273 nm).Samples were analyzed on a 150 mm4.6 mm 5_ HypersilHS C18 column at room temperature with a flow rate of 1.0 ml/min (mobile phase, water : methanol : acetic acid=69:28: 3, v/v).

Accelerated surface area and porosimetry (ASAP)

BET surface area and pore structure characteristics weredetermined by a nitrogen adsorption-desorption isothermused for analysis. Porosity measurements were performedwith a Micromeritics ASAP 2010 analyzer. All sampleswere degassed at 120C for 24 hours before measurement.BET surface area and pore parameters were measured witha 55-point full analysis at cryogenic temperature (77.35 K).The micropore surface area and micropore pore volumewere obtained by t-plot analysis with the Harkins Juraequation [ 26 29].

Thermogravimetric analysis (TGA) of MIP

The thermal property was examined by a Perkin-Elmer TGA-7 apparatus (MA, USA) at a temperature rangingfrom 50 to 900C in air.

Results and discussion

Mechanism of the sol-gel molecularly imprinted polymer (MIP)

The molecular imprint polymers (MIP) were prepared byadding the sulfamethazine (SMZ) acidic solution dispersedalong with tetraethoxysilane (TEOS) within the sol-gelmixture. The sol gel process resulted in numerous hydroxylgroups produced after the hydrolysis and condensation of the silicon alkoxides. These hydroxyl groups promoted theinteraction (hydrogen-bond) between the imprinted mole-cule and inorganic polymer matrix. During the formation of the MIP, the template (SMZ) was incorporated into theinorganic polymer matrix, and the imprinted cavities andspecific recognition sites of the sol-gel MIP were thenformed after removing the template, water, and methanol.At this time, the water and methanol were removed prior tothe temple, and the hydroxyl groups of matrix interactedwith the template by hydrogen bond to enhance therecognition ability. The schematic diagram of the MIP preparation is shown in Fig. 1.

Effect of the H 2 O/Si molar ratios (R) on MIP

In this study, the adsorption of the template (Ad SMZ ) andthe adsorption of analogue (Ad SMO ) were obtained throughcompetition adsorption experiments. The competitionexperiments using HPLC were measured by various H 2 O/ Si molar ratios (R) to study the effects of R on adsorptionand selectivity. The fundamental properties of the MIP withvarious R are shown in Table 1. The sample codes R5, R10,R15, and R20 represent 5, 10, 15, and 20 of the H 2 O/Simolar ratios in the sol-gel mixture, respectively. The resultsshowed that the competition experiment for Ad SMZ in-creased with an increase in the R. In general, the minimumR value was R=2 when the sol-gel mixture reactant was theTEOS. Therefore, the large amount of R ( R>2), thehydrolysis, and the condensation reaction reacted morecompletely, and the imprinted cavities or specific recogni-tion sites were increased. Accordingly, more cavities were provided by the greater amount of absorption sites, and thusthe Ad SMZ increased with the larger amount of R.

In Table 1, the selectivity ( ) is defined as =Ad SMZ / Ad SMO , and the imprint factor ( f ) is defined as f = Ad SMZ . We expected to achieve high adsorption and highselectivity in these MIPs; hence, the imprint factor isindicative of obtaining higher efficiency from Ad SMZmultiplied by . The and f increased with the increasein the R until R=15, and then decreased gradually. In other words, the better and f were achieved under optimum R=15, but the additional R could decrease the efficiency;hence, the and f had a maximum value at R=15.

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Table 1 The fundamental and pore properties of the molecularly imprinted polymers (MIPs) prepared by various H 2 O/Si molar ratios (R)

Sample codes R (H 2 O/Si) Ad SMZ ( mol/g) Ad SMO ( mol/g) f BET surface Area (m2 /g) Average pore Diameter (nm)

R5 5 0.30 0.38 0.78 0.23 413.2 1.97

R10 10 2.55 1.62 1.57 4.01 549.4 2.01

R15 15 2.91 1.81 1.60 4.66 614.1 2.13

R20 20 2.94 1.88 1.56 4.58 621.8 2.19

* Reacted at 60C for 3 days; the pH value of the competition experiment solution was 4.6

Hydrogen bond

Hydrogen bond

Remove the template by

calcination processes

Template

(SMZ)

Tetraethoxysilane

(TEOS)

Sol-gel process

Fig. 1 Schematic diagram of molecularly imprinted polymer (MIP) preparation


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The BET surface area and the average pore diameter were experimented during the Accelerated Surface Areaand Porosimetry (ASAP). The pore properties of the MIPwith various R are shown in Table 1. The BET surface areaand average pore diameter of the MIP also increased withincrease in R. This was due to the quantity of water playingan important role in the sol-gel process. The water did not only hydrolyze the reactant, but it was likewise a product inthe condensation reaction and played the third role as the porogens. The large amount of water caused the bigger pores and the porous material with the larger BET surfacearea. The BET surface area was only interdependent on thequantity of adsorption, but did not matter with therecognition ability. The more BET surface area providedthe greater amount of absorption sites; thus, the adsorptionof template increased with the larger R. On the other hand,the adsorption of the R15 was rather small; even so, theselectivity of R15 was greater than the R20. However, withthe increasing R, the f was an irregular inflection which hada maximum value of R=15. This indicates that the suitableR ( R=15) can improve the recognition ability of MIP.

Effect of competition experiment pH on MIP

Competition experiments using HPLC were performed onsolutions with different pH values to study the effects of pHon adsorption and selectivity of the MIP. In this study, the pH value of the original sulfonamides solutions was 4.6 because the finding affecting the selectivity was at the levelof deprotonation that occurred for the SMZ and SMO.Therefore, controlling the pH of the competition experi-ment solution to match the pKa could affect the sulfona-mides to change its deprotonate or protonate; furthermore,the level of deprotonation that occurred for the SMZ andSMO affected the adsorption and selectivity.

The fundamental properties of the MIP with diverse pHfor the competition experiment are shown in Table 2. Theresults show that the adsorption of template (Ad SMZ ) andadsorption of analogue (Ad SMO ) decreased with an increasein pH, but the selectivity ( ) and imprint factor ( f ) increasedwith an increase in the pH. Even though the Ad SMZ andAd SMO of MIP were less than that with pH=4.6, the

selectivity was raised to 9.93, and the f was raised to 27.6 at R=15. Consequently, the selectivity and imprint factor for the competition experiment solution at pH=7.4 were morethan the competition experiment solution at pH=4.6. Theseresults indicate that the MIP possesses the recognitionability during the competition experiment at pH=7.4.

The above results are explained as follows. The relativeability for a molecule to give up a proton (deprotonate) ismeasured by the pKa value. A low pKa value indicates that thecompound is acidic and will easily give up its proton to a basein an acid-base reaction. When the pH of a system is equal tothe pKa of the compound within the system, that compound ishalf deprotonated (and half neutral) [ 30]. The pKa of SMOwas 5.9, while the pKa of SMZ was 7.4. The SMZ moleculeswere half deprotonated, while the SMO molecules werealmost deprotonated at pH=7.4. These SMO molecules that were mostly ionized became the amine salt that influencedthe absorption in basic condition [ 31]. Therefore, as theresults with less SMO molecules would be adsorbed, thegreater selectivity and imprint factor would be achieved.

Effect of the calcination temperature on MIP

The silica-based materials are extremely rigid due to thehigh degree of cross-linking found in the SiO 2 network.

Table 2 The fundamental properties of molecularly imprinted polymer (MIP) measured by diverse pH value

Sample codes pH=4.6 pH=7.4

Ad SMZ ( mole/g) Ad SMO ( mole/g) f Ad SMZ ( mole/g) Ad SMO ( mole/g) f

R5 0.30 0.38 0.78 0.23 0.52 0.03 17.3 25.5

R10 2.55 1.62 1.57 4.01 2.19 0.18 12.2 26.7

R15 2.91 1.81 1.60 4.66 2.78 0.28 9.93 27.6

R20 2.94 1.88 1.56 4.58 2.88 0.31 9.29 26.7

0

20

40

60

80

100

120

0 200 400 600 800 1000Temperature ( oC)

W

e i g h

t ( % )

Fig. 2 Thermogravimetric analysis (TGA) diagram of the sulfamethazine


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This property is very important in the design and synthesisof MIP as both the size and shape of the cavities created bythe template must be retained after the removal of thetemplate. The high thermal stability of the sol gel derivedmaterial provides an easy way to remove the templatemolecule using the high temperature calcinations method.

In this section, the removal of the template bycalcination under various elevated temperature processesis studied. The thermogravimetric analysis (TGA) diagramsof SMZ are shown in Fig. 2. The results indicate that thedecomposition weight loss began at 300C. Furthermore,the preparation conditions were expected to produce moreamounts of specific recognition sites formed prior to thetemplate decomposition. For this reason, the second stageof the calcination temperatures were carried out at 250C,275C, 300C, 325C, 350C, 375C, and 400C to obtainthe optimum adsorption and selectivity. The diagrams of thecalcination temperature profiles are shown in Fig. 3.Calcination occurred when the monoliths were exposed toa three-stage temperature profile defined by a fixed first

stage at 200C for 1.5 h; the second stage was from 250C

to 400C for 1.5 h; and the third stage was at 600C for 3 h.The fundamental properties of the MIP with the various

second stage temperatures are shown in Table 3. In thissection, HPLC analysis was carried out on R15 samples(H 2 O/Si molar ratio equals 15, R=15). The results indicatethat the selectivity and the imprint factor of the MIPsincreased with increasing the second stage temperaturesuntil 375C, and the selectivity and the imprint factor thendecreased gradually. In other words, the better selectivityand imprint factor were achieved under optimum secondstage temperature, but the additional second stage temper-ature could have decreased the efficiency. Therefore, thesecond stage temperature was set at 375C for the present MIP.

Effect of the calcination time on MIP

Based on the results and descriptions of the previoussections, the removing template by various calcinationtimes is discussed in this section. The calcination carriedout the first stage at 200C, the second stage at 375C, and

0

100

200

300

400

500

600

0 1 2 3 4 5 6 7 8Time (h)

T e m p e r a

t u r e

( C )

R15-250R15-275R15-300

R15-325R15-350R15-375R15-400

Fig. 3 Diagram of the various calcination temperature processes

Table 3 The fundamental properties of molecularly imprinted polymer (MIP) prepared by various reaction temperature

Samplecodes

Second stagetemperature (C)

Ad SMZ( mol/g)

Ad SMO( mol/g)

f

R15-250 250 2.75 0.31 8.9 24.5

R15-275 275 2.48 0.24 10.3 25.5

R15-300 300 2.78 0.28 9.93 27.6

R15-325 325 2.44 0.11 22.2 54.2

R15-350 350 2.39 0.10 23.9 57.1

R15-375 375 2.50 0.10 25.0 62.5

R15-400 400 2.33 0.09 25.8 60.1

* R=15; reacted at 60C for 3 days; the pH value of the competitionexperiment solution was 7.4

Table 4 The fundamental properties of molecularly imprinted polymer (MIP) removing the template by various calcination times

Samplecodes

200C(h)

375C(h)

Ad SMZ( mol/g)

Ad SMO( mol/g)

f

R15-1-1 1 1 3.85 0.54 7.1 27.3

R15-1.5-1 1.5 1 3.63 0.46 7.9 28.7

R15-2-1 2 1 3.01 0.24 12.5 37.6R15-1-1.5 1 1.5 3.00 0.27 11.1 33.3

R15-1.5-1.5 1.5 1.5 2.50 0.10 25.0 62.5

R15-2-1.5 2 1.5 2.61 0.17 15.4 40.2

R15-1-2 1 2 2.72 0.15 18.1 49.2

R15-1.5-2 1.5 2 2.40 0.07 34.3 82.3

R15-2-2 2 2 2.13 0.04 53.3 113.5

* R=15; reacted at 60C for 3 days; the pH value of the competitionexperiment solution was 7.4

Table 5 The fundamental properties of the molecularly imprinted polymer (MIP) and non-imprinted polymer (NIP)

Samplecodes

Ad SMZ( mol/g)

Ad SMO( mol/g)

f

MIP 2.13 0.04 53.3 113.5

NIP 1.83 0.22 8.32 15.2

* R=15; reacted at 60C for 3 days; the second stage temperature of calcination at 375C; the pH value of the competition experiment solution was 7.4


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the third stage at 600C. The fundamental properties of MIP removing the template by various calcination times areshown in Table 4. The calcination times of the first andsecond stages were changed, and that of the third stage wasfixed to 3 h. For example, the sample R15-1-1 included afirst stage for 1 h, a second stage for 1 h, and a third stagefor 3 h.

The competition experiment results are shown in Table 4,indicating that the MIP adsorption of both the template(Ad SMZ ) and the analogue (Ad SMO ) decreased with theincreasing first stage time, but the selectivity ( ) and theimprint factor ( f ) increased with the increasing first stagetime. It was considered that the calcination was the finalheat treatment step in producing the MIP. This thermaltreatment also affected the characteristic of the MIP. Withincreasing heat treatment, the BET surface area wasdecreased due to sintering [ 32], and the silica surface became hydrophobic due to the condensation of hydroxylgroups to form the siloxane bridges (around 200C) [ 33].Therefore, the adsorption was decreased. Furthermore, thecondensation reaction of hydroxyl groups reacted at 200C.Hence, they easily formed a dense network structure of MIP, and the absorption of template (Ad SMZ ) and analogue(Ad SMO ) were decreased with the increase of the first stagecalcination time. On the other hand, the more specificrecognition sites were formed when the condensationreaction was allowed to continue closer to completion,and the MIP were produced more amounts of specificrecognition sites formed prior to the template decomposi-tion (375C); hence, the selectivity ( ) and the imprint factor ( f ) increased with the increase of the first stage timein those MIPs. Consequently, the results indicate that the best MIP selectivity and imprint factor were obtained at afirst stage temperature for 2 h and a second stagetemperature for 2 h.

Effect of imprint (MIP) and non-imprint (NIP)

In this section, the optimum preparation conditions werecarried out on R=15 and reacted at 60C for 3 days and at the second stage temperature of calcination at 375C. Anon-imprinted polymer (NIP) was prepared in parallel andunder identical conditions but in the absence of thetemplate. The fundamental properties of the MIP and NIPare presented in Table 5. Comparison of the MIP and NIP prepared under optimum conditions showed that the Ad SMZof the MIP was greater than that of the NIP, and the Ad SMOof the MIP was smaller than that of the NIP. Moreover, theselectivity of MIP (53.3) was greater than that of the NIP(8.32), and the imprint factor of MIP (113.5) was greater than that of NIP (15.2). Consequently, the selectivity andimprint factor of MIP was approximately seven timesgreater than that of the NIP.

Conclusions

The experimental results show that the molecularlyimprinted polymers (MIPs) were successfully prepared bysol-gel process during our research. The results indicate that the addition of an opportune amount of water couldeffectively increase the selectivity of the MIP. Moreover,controlling the pH of the competition experiment solutionto match the pKa of the template increased the selectivityand imprint factor of the MIP. In addition, the second stagetemperature of calcination was carried out at 375C. Thefirst stage temperature for 2 h and the second stagetemperature for 2 h were obtained with more efficiencyand high selectivity. Consequently, the most significant results were observed through a comparison of theimprinted MIP prepared under optimum conditions andthe NIP, which showed that the selectivity of the MIP (53.3)was greater than the selectivity of the NIP (8.32), and theselectivity of MIP was approximately seven times greater than that of the NIP. This result indicated that the MIP possessed remarkable recognition ability under optimumconditions.

Acknowledgement The authors gratefully acknowledge financialsupport of this research by the National Science Council, Taipei,Taiwan.

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