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Journal of Industrial and Engineering Chemistry 19 (2013) 1788–1792
Silan based paraoxon memories onto QCM electrodes
Ebru Birlik Ozkutuk a,*, Sibel Emir Diltemiz b, Elif Ozalp a, Arzu Ersoz b, Rıdvan Say b
a Department of Chemistry, Eskis ehir Osmangazi University, Eskis ehir, Turkeyb Department of Chemistry Anadolu University, Eskis ehir, Turkey
A R T I C L E I N F O
Article history:
Received 2 January 2013
Accepted 19 February 2013
Available online 28 February 2013
Keywords:
QCM
Paraoxon
Sensor
MIP
A B S T R A C T
In this study, a novel quartz crystal microbalance (QCM) based on the modification of paraoxon-
imprinted polymer onto a QCM with a high selectivity and sensitivity has been developed for the
determination of paraoxon. The QCM sensor has characterized using AFM and ellipsometer. The
performance of the QCM sensor has evaluated and presented good reproducibility, shorter response time
(20 min), wider linear range (0.02–10 mM) and low detection limit (0.02 mM). The results have shown
that the selectivity of QCM sensor has found as being very high in the presence of parathion which is
similar in structure with paraoxon.
� 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights
reserved.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry
jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec
1. Introduction
Organophosphates (OPs) are widely used in the agriculturearound the world as pesticides. These neurotoxic compounds,which are structurally similar to the nerve gases soman and sarin,irreversibly inhibit AChE resulting in the accumulation of AChwhich interferes with muscular responses and in vital organsproduces serious symptoms and eventually death [1,2]. For humanhealth protection and environmental control, it is important todevelop a selective and sensitive method for the detection oforganophosphate pesticides in water, plants, soil and foodstuff, etc.Classical analytical techniques for OPs determination are gaschromatography (GC), high performance liquid chromatography(HPLC) and thin layer chromatography (TLC) coupled withdifferent detectors and spectral techniques [3–5].
The development of chemical sensors that can be tailored forspecific analytes is important for many fields of study includingenvironmental testing, chemical manufacturing, therapeutics, andorganic synthesis. More recently, sensor arrays based on syntheticreceptors have yielded even higher levels of accuracy and have theadvantage that they can be targeted to specific analytes [6–8].However, a major challenge to the sensor array approach hasbeen synthesizing sufficient numbers of recognition elementswith unique selectivity patterns. The preparation of individual
* Corresponding author at: Eskis ehir Osmangazi Universitesi, Fen-Edebiyat
Fakultesi, Kimya Bolumu, Eskis ehir, Turkiye. Tel.: +90 222 239 35 78;
fax: +90 222 239 35 78.
E-mail address: [email protected] (E.B. Ozkutuk).
1226-086X/$ – see front matter � 2013 The Korean Society of Industrial and Engineer
http://dx.doi.org/10.1016/j.jiec.2013.02.021
biological or synthetic molecular receptors is a time and resourceintensive process and the synthesis of an array of receptors onlymultiplies the difficulty of this task. One solution is to usemolecularly imprinted polymers (MIPs) as the recognitionelements in sensor arrays. MIPs are crosslinked polymers thatare formed in the presence of a template molecule [9–12]. Removalof the template creates binding cavities with affinity andselectivity for the template molecule. This templated imprintingprocess enables the rapid preparation of an array of polymers withdifferent binding selectivities via the use of different templates inthe imprinting process.
The fabrication of MIP films to detect certain compounds via aQCM transducer has been accomplished in recent years [13–17].AT-cut thickness-shear mode quartz crystal microbalance (QCM)sensors are commonly used as gravimetric elements due to theirgood mass resolution at comparatively low operation frequency.The QCM is a simple, cost effective, high-resolution mass sensingtechnique, which has been favorably adopted for analyticalchemistry, and electrochemistry applications due to its sensitivesolution–surface interface measurement capability [18]. The QCMhas also been broadly applied in biology [19–21], environmentalassays [22], analytical chemistry [23–25], life sciences [20,26], andpharmaceutical sciences [27,28].
In this study, we proposed a novel phosphotriesterase mimicksurface imprinted polymeric QCM sensor for selective determina-tion of the nerve agent using N-(2-aminoethyl)-3-aminopropyl-trimethoxysilan–Cu(II) (AAPTS–Cu(II)) as a new metal–chelatingmonomer via metal coordination–chelation interactions andparaoxon (diethyl-4-nitrophenyl phosphate, target nevre agent)templates. We have combined molecular imprinting with the
ing Chemistry. Published by Elsevier B.V. All rights reserved.
Fig. 1. AFM image of the (a) gold surface of the plain QCM electrode, (b) surface
paraoxon imprinted QCM electrode. Scanning mode: tapping; scanning area:
1.0 � 1.0 mm.
E.B. Ozkutuk et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1788–1792 1789
ability of AAPTS–Cu(II) to chelate metal ions to create activecentrum sites of phosphotriesterase on microbead surfaces.Covalent molecular imprinting polymers were synthesized usingAAPTS–Cu(II), paraoxon and tetraethyl orthosilicate (TEOS) as afunctional monomer (metal-chelate monomer), template and across-linking agent, respectively.
2. Experimental
2.1. Chemicals
AAPTS was supplied from Aldrich Chemical (USA) (www.sig-maaldrich.com). TEOS was purchased from Acros (Belgium)(www.acros.com). All other chemicals (NaOH, HNO3, formaldhyde)were analytical reagent grade and purchased from Merck AG(Darmstadt, Germany) (www.merck-chemicals.com) All glasswarewas extensively washed with dilute HNO3 before use. All waterused in the experiments was purified using a Barnstead (Dubuque,IA) ROpure LP1 reverse osmosis unit with a high flow celluloseacetate membrane (Barnstead D2731) followed by a BarnsteadD3804 NANO pure1 organic/colloid removal and ion exchangepacked-bed system.
2.2. Instrumentation
Fourier transform infrared (FT-IR) spectrum was recorded usingKBr plate on Perkin-Elmer Spectrum 2000 Spectrometer betweenthe range of 4000–400 cm�1. Cu(II) concentration of each samplewas determined by using Inductive Coupled Plasma Optic EmissionSpectrometer (ICP-OES), Optima 4000 Series of Perkin-Elmer.
Binding events were followed using a Research quartz crystalmicrobalance (RQCM, INFICON Acquires Maxtek Inc., NY, USA)with phaselock oscillator, Kynar crystal holder, and 1-in., Ti/Au/Si,AT-cut, polished, 5-MHz quartz crystals, all purchased fromMaxtek, Inc. The holder was mounted with crystal face positioned908 to ground to minimize gravity precipitation onto the surface.The RQCM phase-lock oscillator provided loading resistancemeasurements and allowed for the examination of crystal dampingresistance during frequency measurements. All measurementswere made at room temperature. Sensitivity is known to be56.6 Hzcm2 mg�1 for a 5-MHz crystal.
2.3. Synthesis of AAPTS–Cu(II) metal-chelate monomer
In a typical synthesis of imprinted sol–gel silica, CuCl2�2H2O(0.002 mol) was dissolved in 4 mL of MeOH. An appropriateamount of AAPTS ligand (0.004 mol) was added into this solutionand stirred for 4 days at room temparature. At the end of thisprocess, the blue copper complex (Cu(AAPTS)2
2+) was formed.FT-IR spectra of AAPTS–Cu(II) is given below: FT-IR (KBr, cm�1):
3432 cm�1 (N–H band), 2945 cm�1 (C–H band), 1384 cm�1 (C–Nband). When the possible interactions between Cu(II) and ‘‘N’’atoms were considered, it has been concluded that Cu(II) ion hasmainly coordinated to the ‘‘N’’ atom of the NH2 groups of AAPTS;because the considerable changes in the infrared frequencies wereobserved only for those bands containing NH2 groups.
2.4. Preparation of the paraoxon imprinted QCM sensors
QCM electrode surfaces were cleaned in a piranha solution for1 h (1:3 30% H2O2/concentrated H2SO4) before coating. As seen inthe Fig. 1a, the surface roughness and deepness of the plainelectrode were determined as 8.0 nm and 14.6 nm, respectively.For the polymerization, the crystals were immersed in thereaction mixture containing the [AAPTS–Cu(II)] metal-chelatemonomer (0.01 mmol), paraoxon (0.01 mmol) template and TEOS
crosslinking monomer (0.5 mmol). Pure nitrogen gas was purgedinto the cell for 5 min to evacuate the air completely since thepresence of oxygen would prohibit the polymerization. Thepolymerization was carried out at room temperature applyingUV light irradiation for 4 h under nitrogen atmosphere. The non-imprinted polymer-coated QCM sensors, as a reference, were alsoprepared in a similar manner using metal-chelate monomer[AAPTS–Cu(II)] without paraoxon. The electrodes were finallywashed with 0.5 M formaldehyde for the template extraction. TheAFM image of [AAPTS–Cu(II)–paraoxon] coated electrodes (Fig. 1b)represented that the surface roughness and deepness (10.0 nm and23.8 nm, respectively) has been increased because of thepolymerization on the QCM electrode. The roughness of surfaceis well distributed through whole surface of the electrode. Thisresult indicates that paraoxon imprinting on the QCM electrodehas been homogeneously achieved. This property is one of theimportant parameters controlling the specificity, selectivity andrecognition rate of the sensor.
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
0 10 20 30 40
Time (min.)
Fre
qu
ency
Sh
ift
(Hz)
MIP
NIP
Fig. 2. QCM response of paraoxon imprinted and non-imprinted films (C: 2.5 mM,
pH 7.0, T: 25 8C).
y = -0,0305x + 0 ,4343
R2 = 0 ,9414
0
0,1
0,2
0,3
0,4
0,5
0 2 4 6 8 10 12
Q(nmol)
Q/C
(nm
ol/
μM
)
Fig. 3. Scathard plot of paraoxon imprinted sensor.
E.B. Ozkutuk et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1788–17921790
2.5. Monitoring the response of paraoxon imprinted QCM sensor
The paraoxon imprinted QCM sensor was used for realtimedetection of paraoxon. The different concentrations of paraoxon(0.1–100 mM) was prepared in 2-propanol. The frequency of thesensor was monitored until it became stable and the frequencyshift for each concentration of paraoxon was determined and theevaluation was performed in triplicate. The desorption was doneusing 10 mL of 1 M formaldhyde solution. After the desorptionstep, the paraoxon imprinted QCM sensor was washed withdeionized water and equilibration buffer. For each paraoxonsample application, adsorption–desorption–cleaning steps wererepeated.
2.6. Selectivity of MIP coated QCM sensor
The selectivity of the prepared sensor for paraoxon wasestimated using parathion which is similar in chemical structureto paraoxon. The QCM sensor was treated with these competitivemolecules. After the equilibrium, the frequency shift of parathionmeasured by prepared paraoxon imprinted sensor and the Dm andQ (nmol) values were calculated according to Eq. (1).
DF ¼ �2F20 ðrqmqÞ�1=2Dm
A(1)
where DF is the measured frequency shift due to the added mass inHz, F0 is the fundamental oscillation frequency of the dry crystal, m
is the surface mass loading in grams, mq is the density of quartz(2.65 g cm�3), rq is the shear modulus (2.95 � 1011 dyncm�2), andA is the electrode area (0.19 � 0.01 cm2). For the 5 MHz quartzcrystals used in this work, Eq. (1) predicted that a frequency change of1 Hz corresponds to a mass increase of 1.03 ngcm�2 on the electrode.
The selectivity coefficient (k) for the binding of paraoxon in thepresence of competitor species can be obtained from equilibriumbinding data consistent to k = Qtemplat molecule/Qcompetitor species. Therelative selectivity coefficient (k0 = kimprinted/knon-imprinted) result-ing from the comparison of the k values of the imprinted polymerwith nonimprinted polymers allows an estimation of the effect ofimprinting on selectivity.
3. Results and discussion
3.1. Measurement of binding interaction of molecularly imprinted
QCM sensor via ligand interaction
The binding of paraoxon to the methacryloyl-based polymer ongold quartz crystals, causes a mass change, Dm, that is reflected inthe crystal frequency. The relationship between Dm and thefrequency shift (Df) can be expressed by the Sauerbrey’s equation[29].
The imprinted [AAPTS–Cu(II)–paraoxon] polymer is expected tobind the paraoxon. The frequency of the sensor decreased after theaddition of paraoxon solution, then reached to constant value in20 min (Fig. 2). It can be seen that the reaction reached equlibriumquickly and these frequency changes strongly indicated that theparaoxon molecules were bound to the imprinted polymer on thequartz crystal. But, when the non-imprinted polymer was used, theparaoxon binding to non-imprinted polymer was much weaker.
There is no any study about pesticide in literature using MIP-QCM technique. Because of this, it is difficult to compare this resultwith the literature. Marx and Zaltsman [30] have investigated themolecular imprinting of sol gel polymers for the detection ofparaoxon in water. The kinetic profile of paraoxon binding to thepolymer matrix has calculated, and the saturation has reacted afterca. 2 h.
Say [31] has investigated the creation of recognition sites fororganophosphate esters based on charge transfer and ligandexchange imprinting methods. The adsorption time was reportedwithin 40 min for molecular imprinted polymers based on chargetransfer (MIP-CT) and around 25 min for molecular imprintedpolymer based on ligand-exchange (MIP-LE).
Ozkutuk et al. [32] have studied the ligand exchange basedparaoxon imprinted QCM sensor. The paraoxon imprinted polymerwas expected to bind the paraoxon sensing. The frequency of thesensor decreased after adding the paraoxon solution, then reachedthe constant value in 30 min.
In our study, response time of paraoxon imprinted sensor is20 min. It is relatively quick compared to above studies.
The binding interaction and equilibrium information betweenthe imprinted polymer and the paraoxon template can be obtainedby Scathard analysis [33–35] (Fig. 3). This analysis employs thefollowing equation:
Q
C¼ Qmax
KD� Q
KD(2)
where Q is the amount of paraoxon bound to the polymer, ascalculated by the mass frequency variation upon the addition ofanalyte and C is the concentration of free paraoxon. Qmax
represents the apparent maximum number of binding sites, andKD is the equilibrium dissociation constant of the metal–chelate
-600
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-100
0
0 20 40 60 80 100 120
Concentra tion (μM)
Fre
qu
ency
Sh
ift
Parao xon
Parat hion
Fig. 5. Selectivity of paraoxon imprinted sensor (C: 1.0–100 mM, pH 7.0, T: 25 8C).
E.B. Ozkutuk et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1788–1792 1791
copolymer based on ligand exchange. The obtained Scathardregression equation for Fig. 3 is
Q
C¼ �0:0305Q þ 0:4343 (3)
So, the association constant (Ka) for the binding of paraoxon toAAPTS–Cu(II)-based MIP sensor is 3.05 � 104 M�1 and the maxi-mum number of ligand-exchange interaction site, Qmax, is14.23 mmol. The value of Ka suggests that the affinity of thebinding sites is very strong.
3.2. Effect of pH on frequency shift
Molecular imprinting using paraoxon template gives a cavitythat is selective for paraoxon. The paraoxon can simultaneouslychelate to metal ion and fit into the shape-selective cavity.
So, this interaction between Cu(II) ion and free coordinationspheres has an effect on the binding ability of the quartz sensor. Webelieve that the pH value of the detection medium also plays animportant role for the binding ability of the quartz crystal sensor.The sensor responses of paraoxon imprinted crystal for 3.0 mMparaoxon at different pH values (pH 6.0–9.0) are shown in Fig. 4. Asit seen in the figure, the frecuency shift for the imprinted polymerdecreased with increasing pH. The apparent binding affinity wasobserved at pH 7.
Alizadeh [36] has developed the paraoxon-imprinted volta-metric sensor. The results of experiment have showed that in thepH range of 3–7, the paraoxon related electrochemical signals werehigh and attained about fixed amounts.
3.3. Recognition selectivity of imprinted polymer on QCM sensor
The adsorption of parathion that is similar in structure withparaoxon on the imprinted quartz crystal sensor was investigatedfor the better understanding of the specificity of the interactionsbetween the binding sites of the MIP–QCM sensors and templatemolecules. The ability of this imprinted polymer to recognise theparaoxon molecules is higher than parathion. The initial slopes inthe sorption isotherms (Fig. 5) indicate that the frequency changesare directly proportional to the concentration of biomolecules.
In the sorption isotherms, the initial slope for parathion is�1184 Hz mmol L�1, while the initial slope of the curve is�5206 Hz mmol L�1 for paraoxon. The selectivity coefficients ofthe paraoxon imprinted QCM with respect to parathion is 4.39(5206/1184). From this point; we can get the following conclusion:The [AAPTS–Cu(II)–paraoxon] monomer has been incorporatedinto a polymer matrix in order to increase the selectivity ofparaoxon recognition on the QCM sensors. The selectivity
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-20
-10
0
0 2 4 6 8 10
pH
Fre
qu
ency
Sh
ift
Fig. 4. The effect of pH on binding ability (C: 3.0 mM, pH 7.0, T: 25 8C).
coefficients of paraoxon with respect to parathion is high, becauseparaoxon can simultaneously fit into the shape-selective cavity.
Ozkutuk et al. [32] have studied the ligand exchange basedparaoxon imprinted QCM sensor. The paraoxon imprinted polymerwas expected to bind the paraoxon sensing. Selectivity coefficientsof the paraoxon imprinted QCM sensor was reported 3.57. In thisstudy, the selectivity coefficients of the paraoxon imprinted QCMwith respect to parathion is 4.39. It is relatively high compared toabove studies.
3.4. Analytical performances
Fig. 6 shows the calibration curve obtained plotting thefrequency shifts versus the concentration of paraoxon (0.1–5 mM) imprinted quartz crystal. A linear curve was obtainedbetween 0.02 and 5 mM paraoxon. As shown in the figure, whenthe concentration of paraoxon increased, the frequency of theQCM also increased. The sensitivity was calculated as the slopeat the linear part of low concentration range and found to be29 Hz mmol�1. The detection limit, defined as the concentrationof analyte giving frequency shift equivalent to three standarddeviation of the blank plus the net blank frequecy shift,was 0.02 mM. The experiments were performed in replicatesof three and the samples were analyzed in replicates of three aswell.
Ozkutuk et al. [32] have studied the ligand exchange basedparaoxon imprinted QCM sensor. The paraoxon imprinted polymerwas expected to bind the paraoxon sensing. Dedection limit wasreported to 0.06 mM. In this study, the dedection limit of paraoxonimprinted sensor is 0.02 mM. It is relatively low compared to abovestudies.
y = 2 9,25 7x - 0,791 7
R2 = 0 ,9981
-20
0
20
40
60
80
100
120
140
160
0 1 2 3 4 5 6
Concentration (μM)
Fre
qu
ency
Sh
ift
(Hz)
Fig. 6. Calibration curve of paraoxon (C: 0.1–5.0 mM, pH 7.0, T: 25 8C).
E.B. Ozkutuk et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1788–17921792
3.5. Reproducibility of the sensor
In order to check the reproducibility of the sensor, after eachinjection of paraoxon and the attainment of equilibrium, theparaoxon imprinted sensor was recovered by sequential washeswith 1 M formaldehyde solution and deionized water until thefrequency of QCM reached a steady value. There was a littledifference in the frequency shift vs. the same paraoxonconcentration. The sensor can be stored in deionized water atroom temperature when not in use. The slight difference can betolerated. The results indicated that the sensor had asatisfactory reproducibility most likely due to the stabilityand reversible recognition of the molecularly imprintedpolymers.
4. Conclusions
In the present work, a paraoxon imprinted sensor has beendeveloped for the determination of paraoxon based on themodification of paraoxon imprinted film onto a quartz crystalcombining the advantages of high selectivity of the piezoelectricmicrogravimetry using MIP films technique and high sensitivity ofQCM detection. The paraoxon imprinted (MIP) polymer wasprepared by the emulsion polymerization reaction of AAPTS–Cu(II) and TEOS in the presence of paraoxon and characterized byFTIR measurements. The paraoxon imprinted polymer wasattached by dropping of polymer solution to gold surface andthen, dried at 25 8C for 4 h. Paraoxon imprinted sensor wascharacterized with AFM and ellipsometer. The detection limit wasfound to be 0.02 (M. The frequency shift for the paraoxon-imprinted and non-imprinted QCM decreased with increasing pH.The value of Ka (3.05 � 104 M�1) that was obtained using Scathardgraph suggests that the affinity of the binding sites is strong. Inaddition, the selectivity experiments showed that the selectivitycoefficients of the AAPTS–Cu(II)–paraoxon complex with respectto parathion which is similar in structure with paraoxon is 4.39. Byusing the paraoxon MIP/QCM detection system as the recognitionmaterial, a novel paraoxon imprinted sensor with good reproduc-ibility, short response time, wide linear range, low detection limitand high selectivity has been developed for the determination ofparaoxon.
Acknowledgments
This work has been supported by Eskis ehir OsmangaziUniversity, Commission of Scientific Research Projects (ProjectNo: 200619004).
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