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European International Journal of Science and Technology Vol. 5 No. 9 December 2016
47
GENERATION OF MOLECULARLY IMPRINTED ELECTROSPUN
NANOFIBERS FOR ADSORPTION OF 4-NITROPHENOL WITH
QUARTZ CRYSTAL MICROBALANCE METHOD
Sibel Emir Diltemiz 1*
and RabiaBerna Demirel 2
1,2 Department of Chemistry, Anadolu University University, Eskişehir, Turkey
*Correspondence Author:
Sibel Emir Diltemiz
Anadolu Üniversitesi, Fen Fakültesi, KimyaBölümü,
YunusEmreKampüsü, 26470 Eskişehir, Turkey
E-mail : semir@anadolu.edu.tr
ABSTRACT
In this study, it has targeted determination of 4-Nitrophenol (4-NP), which is a toxic and carcinogenic
compound at high selectivity. For this purpose, firstly molecularly imprinted nanoparticles was synthesis by
mini-emulsion polymerization by using 4-NF as template. Then nanofiber systems containing 4-NP
imprinted nanoparticles have been developed by electro spinning technology. 4-NP imprinted nanoparticles
containing nanofiber systems is coated onto Quartz Crystal Microbalance (QCM) electrode. The electrode
was used for determination of 4-NF from aqueous solution. Scatchard, Langmuir, Freundlich and Temkin
adsorption models were applied to describe the adsorption isotherms.
Keywords: 4-Nitrophenol, QuartzCrystalMicrobalance, MolecularlyImprintedPolymers, Electrospinning
1. INTRODUCTION
Polymer based nanomaterials, due to their chemical and mechanical properties can be used in a number of
different applications such as adsorption of various metal and organic chemicals [1], catalyst (Günter Wulff,
2011), chromatography (Svec & Kurganov, 2008), development of sensors and electrodes (Chen, Wen, &
Teng, 2003). Also polymeric nanomaterials offer a wide opportunity of modulating their adsorption capacity
through the control of both morphology and chemical composition (Kang, Chen, Zhang, Liu, & Gu, 2008;
Khajeh, Laurent, & Dastafkan, 2013) Among the different synthetic strategies, molecular imprinting is
probably the most straightforward for the purpose of producing nano and micro structured materials that
have predesigned molecular recognition capabilities(Conrad et al., 2003; H. C. Huang, Lin, Joseph, & Lee,
2004; Yang et al., 2004)
European International Journal of Science and Technology ISSN: 2304-9693 www.eijst.org.uk
48
The technique of molecular imprinting creates specific molecular recognition sites in solid materials by
using template molecules. In this process, a complex of a functional monomer with a template is
copolymerized with a cross-linking agent and removal of the template from the polymerized matrix
generates the binding sites that are compatible to the template (Kim & Chang, 2011). Three different
methods to prepare MIPs have been reported, depending on the bond that is established between the
template molecule and the polymer (covalent, noncovalent, and semicovalent approaches) are reported
(Turiel & Martín-Esteban, 2010). A noncovalent approach has been used in this work, because relatively
weak noncovalent intermolecular interactions, between the template molecule 4-NPand the functional
monomers served to form the molecular assemblies. To use MIPs in applications such as sensing or affinity
separation, it is often required to immobilize the polymer onto solid surfaces. Several research studies have
been carried out by combining the amazing properties of MIPs and nanofibers to obtain molecularly
imprinted polymer nanofibers (MIP-NFs)(Zaidi, 2015)(Li, 2012). Furthermore, nanofiber sorbents not only
possesses a great specific surface area, which allows a more efficient adsorption, but also the possibility to
remove them by solution more easily(Piperno, Tse, Bui, Haupt, & Gheber, 2011).
The electrospinning process produces continuous polymer nanofibers with diameters ranging from a few
nanometers to several micrometers (usually between 50 and 500 nm) through the action of an external
electric field imposed on a polymer solution or melt(Chronakis, Milosevic, Frenot, & Ye, 2006; Z.-M.
Huang, Zhang, Kotaki, & Ramakrishna, 2003; Subbiah, Bhat, Tock, Parameswaran, & Ramkumar, 2005).
Electrospun nanofibers have very high surface area, mechanical strength, and flexibility. When collected on
a plain target plate, they can easily form defined three-dimensional mats with well-controlled pore sizes.
This makes electrospun nanofiber matsideal candidates for industrial filtration purposes. It is highly
desirable for selective molecular recognition sites to be able to be introduced into nanofiber materials as
they may provide highly efficient compound separation involving only a simple filtration step(Gibson,
Schreuder-Gibson, & Rivin, 2001). In this study, we report on the entrapping of MIP nanoparticles in
electrospun polymer nanofibers, thus creating a nonwoven mat of sensing sites with a high surface area and
accessibility.
2. EXPERIMENTAL
2.1. Materials
4-NP, methacrylic acid (MAA), Ethylene glycol dimethacrylate (EDMA), hexadecane, 2,2'-azobis(2-
methylpropionitrile) (AIBN), polyvinyl alcohol (PVA, MN ~ 72.000), potassium bromide (FTIR grade) and
sodium dodecyl sulphate (SDS) were supplied by SIGMA-ALDRICH (Milwaukee, WI, USA). All
glassware was extensively washed with diluted HNO3 before use. All other chemicals were of analytical
grade purity and purchased from Merck AG (Darmstadt, Germany). All water used in the experiments was
purified using a Barnstead (Dubuque, IA) ROpure LP® reverse osmosis unit with a high flow cellulose
acetate membrane (Barnstead D2731) followed by a Barnstead D3804 NANO pure® organic/colloid
removal and ion exchange packed-bed system.
2.2. Apparatus
House-made electrospun system was built and used for fiber production. A custom-made high voltage DC
converter (EMCO 4300) was biased with a DC power supply. FTIR spectroscopy was used in the 4000-400
cm-1 range for the characterization of functional monomer, pre-organized complex, and imprinted beads in
the solid state (FTIR 100 series, Perkin Elmer, USA). The size of the particles was determined by dynamic
light scattering on a Malvern Instruments Zeta Sizer. The imprinted beads and nanofibers were characterized
European International Journal of Science and Technology Vol. 5 No. 9 December 2016
49
by field emission gun scanning electron microscope (FESEM, ZEISS Ultraplus), SONOPULS HD 2070,
BANDELIN, (Germany) model homogenizer and MPW-251, MPW Med-Instruments (Poland) centrifuge
were used for homogenization and centrifuge processes.
2.3. Synthesis of 4-NP imprinted nanoparticles
MIP nanoparticles were prepared with mini emulsion polymerization method which used by Belmont et. al.
(Belmont et al., 2007). Therefore, the organic phase that contained 1.72 mmol of MAA, 6.9 mmol of
EDMA, 80 μL of hexadecane and 30 mg of AIBN was prepared and then ultrasonicated for 1 min. The
aqueous phase was prepared by dissolving of 0.5 mmol4-NP and 38.5 mg of SDS in 18 mL of water. The
organic phase was slowly added to the aqueous phase. In order to obtain mini-emulsion, the mixture was
homogenized at 25.000 rpm by a homogenizer (SONOPULS HD 2070, BANDELIN, Germany). Polymer
mixture was stirred and held in 70ºC water bath for 18 hours to obtain nanoparticles. The 4-Np imprinted
nanoparticles were washed three times 5h against water and water/ ethylalcohol mixtures in order to remove
unreacted monomers, surfac tantand initiator. To removing imprinted molecule, the mixture was washed
with methanol: acetic acid (4:1) solution three times for two hours.
FT-IR, Zeta-sizer and SEM analyzes were performed to nanoparticles characterization. Particle size was
measured by Malvern Instrument Zeta-sizer device. 30µg/ml water suspension was prepared for this
purpose. The imprinted 4-NP and nonimprinted MIP nanoparticles were examined and imaged by Scanning
Electron Microscope.4-NPimprinted and nonimprinted MIP nanoparticles were mixed with 98 mg
potassium bromide and grounded until turn into fine powder form then pressed to pellet for FTIR
measurement.
2.4. The coating of QCM sensors with 4-NP imprinted polymer encapsulated nanofibers and their
characterization.
Chronakis et al.’s method was used for preparation of 4-NP imprinted polymer encapsulated nanofibers
(Yoshimatsu, Ye, Lindberg, & Chronakis, 2008). 3 ml Dichloromethane (DCM), 3 ml trifluoroacetic acid
(TFA) were mixed then 0,5 gr poly ethylene terephthalate (PET), 0,05 gr polyvinyl alcohol (PVA), NaCl
were added to this mixture and stirred for two hours.The prepared mixture was filled in plastic syringe and
placed to syringe pump (KDS-200, Focus Co., Ltd., USA) for electrospinning. The positive side of the high
voltage power supply was attached to the syringe’s 0,8 mm diameter metal needle tip. The solution feeding
rate was adjusted to 1,5 mL/h. Gold coated quartz crystals were used as collector electrode and placed
horizontally which has a 130 mm distance away from the needle tip. Before electrospinning, the QCM
crystal surface was cleaned in piranha solution (H2SO4/30% H2O2 (3:1, v/v)) for 5 min, rinsed thoroughly
with Milli-Q water, and dried with a stream of nitrogengas. The quartz portion of the QCM electrode was
covered with aluminum folio to prevent unwanted interaction with nanofibers. System voltage was adjusted
as 10kV in room temperature. The fibers were deposited on the quartz crystal (Figure 2.1a) and left to dry
after deposition to remove trace amount of solvent. 4-NP nonimprinted polymer encapsulated nanofibers
preparation steps were remained same. For this purpose 4-NP nonimprinted particles were added to the
PET-PVA mixture and stirred for one hour to obtain homogenous mixture. The same parameters and
conditions were applied during electrospinning.
European International Journal of Science and Technology ISSN: 2304-9693 www.eijst.org.uk
50
(a)
(b)
Figure 2.1.(a) Theschematicview of nanofibercreation on QCM electrodewithelectrospinningprocessand (b)
formation of Taylor cone
2.5. The sensor measurements of nanofibers coated QCM electrode
The QCM sensors consist of a disk-shaped, AT-cut piezoelectric quartz crystal, coated with gold thin layer
on both sides and operate at a frequency of 5 MHz. The resonance frequencies were measured by using a
QCM digital controller (MAXTEK RQCM, Research Quartz Crystal Microbalance Monitor). For this
purpose electrode was attached to the holder, rinsed with water via perilstatic pump at room temperature for
stabilization then measured constant resonance frequency (F0).0.5 ppm 4-NP solution was passed on
electrode and sensor frequency observed until stable value (F1) The frequency shift of all 4-NP
concentrations (0.5-100 ppm were calculated from ΔF=F0–F1 equation. The electrodes were rinsed with
H2O and 0.1 mMglisin/10 mMHCl mixture until sensor frequency come close to F0 value to remove
template after every concentration measurement step. The binding value of nano sensor to 4-NP molecules
is statistically calculated from three values of blank solution.
European International Journal of Science and Technology Vol. 5 No. 9 December 2016
51
Figure 2.2. QCM measurement with 4-NP imprinted polymer encapsulated nanofibers coated QCM sensor
3. RESULTS
3.1. 4-NP imprinted nanoparticles characterization
4-NP imprinted nanoparticles were prepared by two-phase emulsion polymerization method. In this
technique, polymerization process performed by passing from stabile oily phase to water phase. The FTIR
characterization of 4-NP imprinted and nonimprinted polymers were carried out by FTIR spectrometer. The
dried polymer mixed with KBr and shaped to tablet form to obtain FTIR spectrum. The FTIR spectrum of 4-
NP imprinted and nonimprinted polymers can be shown in Figure 3.1A.As seen from FTIR spectra specific
bands of the polymeric structure have been detected. In the FTIR spectrum characteristic peaks of 4-NP
imprinted nanoparticles are carboxylic acid O-H band at 3568 cm-1
, C-H stretching at 2980 cm-1
and
carbonyl peak (C=O) at 1728 cm-1
. When FTIR spectrums of imprinted and nonimprinted polymers are
compared, the peak is observed at 1637 cm-1
in 4-NP imprinted polymers. It is believed that this peak should
be come from benzene ring’s C=C bending band. For Zeta-Sizer measurements 4-NP imprinted
nanoparticles were suspended in deionized water with 30 µg/ml ratio to measure particle size. Zeta-Sizer is
benefit from light scattering techniques and is capable of hydrodynamic sizing of nanoparticles, determining
of zeta potential and molecular weight. Zeta sizer analysis result can be shown in Figure 3.1B. The obtained
nanoparticles size is found about 200 nm. 4-NP imprinted nanoparticle’s size and morphology were revealed
with SEM analysis. Nanoparticles were dried and coated with gold before SEM surface analysis. SEM view
of 4-NP imprinted nanoparticles is given in Figure 3.1C.
(a) (b)
(A)
NIP B.Csız_1
Name
Sample 493 By Analyst Date Friday, July 29 2016
Description
4000 4003500 3000 2500 2000 1500 1000 500
89
58
60
62
64
66
68
70
72
74
76
78
80
82
84
86
88
cm-1
%T
1728.73cm-1
1159.51cm-1
1263.65cm-1
1452.68cm-1
2987.92cm-1
1389.32cm-1
954.89cm-1
3568.59cm-1
4NF-MIP_1
Name
Sample 495 By Analyst Date Friday, July 29 2016
Description
4000 4003500 3000 2500 2000 1500 1000 500
105
25
30
40
50
60
70
80
90
100
cm-1
%T
1728.68cm-1
1159.80cm-1
1260.40cm-1
2959.07cm-1
1453.86cm-1
1047.47cm-13518.35cm-1
1389.37cm-1
958.01cm-1
1637.23cm-1
879.35cm-1
752.53cm-1
520.32cm-1
European International Journal of Science and Technology ISSN: 2304-9693 www.eijst.org.uk
52
(B)
(C)
Figure 3.1.(A) The FTIR spectrum of 4-NP imprinted (a) and nonimprinted polymers
(B) Zeta-Size analysis result of 4-NP imprinted nanoparticles
(C) SEM photograph of 4-NP imprinted nanoparticles
3.2. 4-NP imprinted nanoparticle encapsulated nanofibers characterization
4-NP imprinted nanoparticle encapsulated nanofiber’s surfaces were also characterized with SEM. 4-NP
imprinted nanoparticle encapsulated nanofibers were dried, coated with gold before SEM images were
taken. The SEM view of nanofibers that were not containing 4-NP imprintedp articles can be shown in
Figure 3.2- a. 1 mg 4-NP imprinted nanoparticles were added into the polymer mixture and nanofiber
creation process was repeated. The obtained nanofiber’s SEM view is given in Figure 3.2-b.
(a)
European International Journal of Science and Technology Vol. 5 No. 9 December 2016
53
(b)
Figure 3.2. SEM images of nanofibers (a) PET/PVA (b) 4-Nitrophenol imprinted nanoparticle/PET/PVA
3.3.4-NP imprinted polymer encapsulated nanoparticle coated QCM sensor’s binding interactions.
When target molecules bind to the 4-NP imprinted nanofibers, the crystal’s mass changes (Δm) that effects
to frequency. The relation between Δm and frequency shift (ΔF) is expressed with following equation.
∆𝐹 = 𝐶𝑥∆𝑚
Where,
ΔF : frequency shift
Δm: the amount of matter that binds on quartz
C: 56,6 Hzcm2 µg
-1 for 5 mHz crystal
The 4-NP imprinted polymer is expected to bind the 4-NP sensing. The frequency of the sensor increased
after pumping the 4-NP solution. These frequency changes strongly indicated that the 4-NP molecules were
bounded to the imprinted polymer on the quartz crystal.4-NP solutions with different concentration between
0.5 to 100 ppm were pumped to 4-NP imprinted electrode (Figure 3.3). LOD and LOQ values were
calculated as 0,395 nM and 1,2 mM respectively.
Figure 3.3.Change in ΔF and time of 4-NP imprintedparticleencapsulatednanofibercoated sensor
-0.003
-0.002
-0.001
0
0 20 40 60 80 100
Df
(Hz)
Zaman (dk)
European International Journal of Science and Technology ISSN: 2304-9693 www.eijst.org.uk
54
The binding interactions between imprinted nanofibers and 4-NP template is obtained from Scatchard
analysis method. Following equation is used in Scatchard analysis method (Atay, 2016).
∆𝑚
[∁]= −
∆𝑚
𝐾𝐷+∆𝑚𝑚𝑎𝑥
𝐾𝐷
Where,
Δm: The amount of matter that binds on quartz,
C: Free 4-NP concentration
KD : Equilibrium constant in reverse direction
Δmmax: Maximum mass increasing in QCM sensor unit area
Figure 3.4.4-NP imprinted particle encapsulated nanofiber coated QCM sensor’s Scatchardcurve
∆𝑚
∁= −2 × 106∆𝑚 + 11,76
Using with this equation, the binding constant and number of ligand changing sites are found as (KA) 2x106
M-1
and Qmax 11,76 respectively. The founded KA value indicates the binding site’s has strong affinity. KA
value of 4-NP imprinted QCM sensor proves that the developed system is nearly as selective as biological
sensors (Diltemiz, 2010), (Amanda, 2004).
The binding interactions-equations between imprinted polymer encapsulated nanofibers and 4-NP are also
examined with Langmuir, Freundlich and Temkin analysis method.
4-NP imprinted polymer’s Langmuir curve is obtained by following equation and KL and Δmmax values are
found from equation as 2,33x105 M and 5,8x10
-5 respectively. The founded KL value indicates the binding
site’s has strong affinity to target molecule 4-NP.
∆𝑚 =∆𝑚𝑚𝑎𝑥 𝑥 𝐶
𝐾𝐿+ 𝐶
In this equation;
KL: Langmuir constant for a homogeny adsorbent
C: 4-NP free concentration
Δmmax: Maximum mass increasing in unit area of QCM sensor
Δm: Mass increasing amount in unit area of QCM sensor
y = -2E+06x + 11.76R² = 0.946
0
0.5
1
1.5
2
4.80E-06 5.20E-06 5.60E-06 6.00E-06
Δm
(µ
g)
/ C
(M
)
Δm (µg)
European International Journal of Science and Technology Vol. 5 No. 9 December 2016
55
Figure 3.5.4-NP imprinted polymer’s Langmuir curve
4-NP imprinted polymer’s Freundlich curve is plotted from following equation (Atay, 2016).KF and n
values are found from equation as 5,39x10-12
M and 66,6 respectively.
ℓ𝓃∆𝑚 =1
𝑛ℓ𝓃𝐶 + ℓ𝓃𝐾𝐹
Where,
KF Freundlich constant for heterogenic adsorbent
C: 4-NP free concentration
Δm: Mass increasing amount in unit area of QCM sensor
n: Relation between adsorption force magnitude and energy distribution of adsorbent area
162000
171000
180000
189000
198000
0.00E+00 9.00E+04 1.80E+05 2.70E+05 3.60E+05
1/
Δm
(µ
g)
1/C (M)
y = 0,074x + 17271R² = 0,988
y = 0,219x + 16996R² = 0,684
European International Journal of Science and Technology ISSN: 2304-9693 www.eijst.org.uk
56
Figure 3.6.4-NP imprinted polymer’s Freundlich curve
4-NP imprinted polymer’s Temkin curve is obtained by following equation
∆𝑚 = 𝐵ℓ𝑛𝐾𝑇 + 𝐵ℓ𝑛𝐶𝑒
Where,
KT : Binding constant of Temkinizoterm (L/gr)
Ce :4-NP free concentration
Δm : Mass increasing amount in unit area of QCM sensor
B : Adsorption constant related with heat (J/mol)
Figure 3.7.4-NP imprinted polymer’s Temkin curve
-12.24
-12.15
-12.06
-11.97
0 2 4 6 8
ln Δ
m
ln C (mM)
y = 0,039x - 12,20R² = 0,852
y = 0,015x - 12,13R² = 0,894
European International Journal of Science and Technology Vol. 5 No. 9 December 2016
57
Table 3.1. The comparison of 4-NP imprinted QCM sensor’s Langmuir, Freundlich and Temkin analysis.
Scatchard Langmuir Freundlich Temkin
R2 0,946 R
2 0,988 R
2 0,894 R
2 0,95
KD 5x10-7
Δmmax 5,80x10-5
1/n 0,015 B 8x10-8
KA 2x106 KL 2,33x10
5 KF 5,39x10
-12 KT 1,4x10
27
CONCLUSION
4-NP imprinted polymer encapsulated nanofibers were created firstly to remove 4-NP in this study.PVA and
PET polymers that are capable of water bending are used in this stage. As can be understood from SEM
images nanofibers are created in randomly distributed, uniform morphology and dense forms. Accessibility
to target molecule and appearance of large part of the nanoparticle surface is accomplished by trapping of
MIP particles with larger diameter than PET/PVA fibers to the fiber interspaces. The binding amount of 4-
NP imprinted nanofibers to 4-NP molecule, calculated from 3 statistical data of blank solution. LOD and
LOQ values are found as 0,027 ppm and 0,081 ppm respectively.QCM nanosensor is developed to
determining 4-NP with high selectivity. 4-NP imprinted nanoparticles encapsulated nanofibers are coated on
QCM electrode. The binding interactions between 4-NP imprinted polymer encapsulated nanofibers and 4-
NP are examined with Scatchard, Langmuir, Freundlich and Temkin analysis methods. The binding constant
(KA) for binding 4-NP molecules to 4-NP imprinted particle-encapsulated nanofibers coated QCM sensor is
found as 2x106 M
-1. This value indicates that the binding sites have strong affinity. Linear character of
plotted curve shows that adsorption process is more suitable to matched with Langmuir. Moreover when
comparing Langmuir, Freundlich and Temkin curves, Langmuir curve has highest correlation coefficient.
This situation indicates single layer and homogeny adsorption exists on QCM sensor. The binding amount
of nanosensor to 4-NP molecule, calculated from 3 statistical data of blank solution. LOD and LOQ values
are found as 0,395 nm and 1,2 mM respectively. The calculated values of observability and determinity
limits are given in table 5.1. The writers of this study couldn’t find a study about determining of 4-NP using
with QCM in literature. Therefore 4-NP determining with using QCM technique could be accomplished
firstly reported in literature with this study.
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