Supramolecular interaction of 2,4-dichlorophenol and β-cyclodextrin functionalized ionic liquid and its preliminary study in sensor application

Article abstract of DOI:10.1016/j.molliq.2015.10.044

The supramolecular interaction of 2,4-dichlorophenol (2,4-DCP) and β-cyclodextrin functionalized ionic liquid, a mono-6-deoxy-6-(3-benzylimidazolium)-β-cyclodextrin tosylate (β-CD-BIMOTs) has been investigated. The inclusion complex of β-CD-BIMOTs-2,4-DCP complex was analyzed with conventional spectroscopic techniques and compared with native β-CD-2,4-DCP complex. The mechanism of the inclusion too was studied and discussed based on FTIR, ¹H NMR and 2D NOESY analysis. β-CD-BIMOTs has a stronger binding affinity towards 2,4-DCP compared to native β-CD based on the formation constant value calculated. In this work, modified electrodes were fabricated by mixing β-CD-BIMOTs and carbon paste electrode (CPE) for determination of 2,4-DCP. The electrochemical behavior of 2,4-DCP at β-CD-BIMOTs/CPE exhibits higher electrochemical performance than that of native β-CD/CPE and CPE, revealing that CPE modified β-CD-BIMOTs not only displays good electrical properties of the ionic liquid but also exhibits high supramolecular recognition properties of β-CD-BIMOTs through the formation of an inclusion complex between β-CD-BIMOTs and 2,4-DCP molecules. This preliminarily study shows that β-CD-BIMOTs can act as an inexpensive electrochemical sensor that is suitable for the determination of 2,4-DCP.

Full text of DOI:10.1016/j.molliq.2015.10.044

Journal of Molecular Liquids 212 (2015) 850–856
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Supramolecular interaction of 2,4-dichlorophenol and β-cyclodextrin
functionalized ionic liquid and its preliminary study in sensor application
a,
a
a
a
Sharifah Mohamad ⁎, Kumuthini Chandrasekaram , Fairuz Liyana Mohd Rasdi , Ninie Suhana Abdul Manan ,
b
a
a
Muggundha Raoov , Nadiah Sidek , Syed Fariq Fathullah
a
Department of Chemistry, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia
Cluster Integrative Medicine, Advanced Medical & Dental Institute, Universiti Sains Malaysia, No 1-8 (Lot 8), Persiaran Seksyen 4/1, Bandar Putra Bertam, Kepala Batas, Pulau Pinang 13200, Malaysia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The supramolecular interaction of 2,4-dichlorophenol (2,4-DCP) and β-cyclodextrin functionalized ionic liquid, a
mono-6-deoxy-6-(3-benzylimidazolium)-β-cyclodextrin tosylate (β-CD-BIMOTs) has been investigated. The in-
clusion complex of β-CD-BIMOTs-2,4-DCP complex was analyzed with conventional spectroscopic techniques
and compared with native β-CD-2,4-DCP complex. The mechanism of the inclusion too was studied and
Received 3 August 2015
Received in revised form 15 October 2015
Accepted 20 October 2015
Available online xxxx
1
discussed based on FTIR, H NMR and 2D NOESY analysis. β-CD-BIMOTs has a stronger binding affinity towards
2,4-DCP compared to native β-CD based on the formation constant value calculated. In this work, modified elec-
Keywords:
trodes were fabricated by mixing β-CD-BIMOTs and carbon paste electrode (CPE) for determination of 2,4-DCP.
The electrochemical behavior of 2,4-DCP at β-CD-BIMOTs/CPE exhibits higher electrochemical performance than
that of native β-CD/CPE and CPE, revealing that CPE modified β-CD-BIMOTs not only displays good electrical
properties of the ionic liquid but also exhibits high supramolecular recognition properties of β-CD-BIMOTs
through the formation of an inclusion complex between β-CD-BIMOTs and 2,4-DCP molecules. This preliminarily
study shows that β-CD-BIMOTs can act as an inexpensive electrochemical sensor that is suitable for the determi-
nation of 2,4-DCP.
β-cyclodextrin
Ionic liquid
Inclusion complex
2,4-Dichlorophenol
Sensor
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
chemical modifications to improve their complexing ability [1] makes
them invaluable.
Supramolecular chemistry is an interesting discipline of chemistry
A room temperature ionic liquid (RTIL), or molten salt, is defined as a
material containing only ionic species without any neutral molecules
and having a melting point lower than room temperature [6]. Typically,
an ionic liquid (IL) consists of a main organic cation and a corresponding
inorganic or organic anion. The composition and properties of ILs are de-
termined by the cation and anion involved. The cations strongly influ-
ence characteristic properties of the IL while the chemical properties
and functionality are controlled by the anions [7]. RTIL provides many
unique electrochemical properties such as higher ionic conductivity,
wide electrochemical windows and acts as ion exchanger to increase
analytical signals as well as to improve sensitivity [8,9]. A few investiga-
tions on electroanalytical characteristics and possibilities of ionic liquid
composite sensors have been described in several papers with ionic liq-
uids being able to form conductive composite materials on modified
working electrodes [10,11].
Since both β-CD [12–14] and IL [15–18] have such a profound effect
individually, forging them together could provide new frontiers. Thus
sparked we delved into preparing a new generation of materials namely
IL functionalized β-CD. The functionalization of β-CD with various pre-
cursors basically alters the physical properties of β-CD in order to
make it more suitable for a wide range of applications [19,20]. It can
be supposed that the multi-functionality of β-CD-IL obtained from
involving all intermolecular interactions where no covalent interactions
are established between the interacting species either molecules, ions
or radicals [1]. Most of these interactions are the host–guest type and
cyclodextrins (CDs) seem to be the most promising among all the po-
tential hosts. This is due to the fact that they are semi-natural products
produced from a renewable natural material: starch, by a relatively sim-
ple enzymatic conversion. The doughnut-shaped cyclic oligosaccha-
rides, cyclodextrins produced from the degradation of starch by
bacterial enzyme are well known supramolecules because of their abil-
ity to encapsulate foreign molecules [2]. The three major types of cyclo-
dextrins are α-, β- and γ-CD comprising six, seven and eight glucose
units respectively connected via α-1,4-glycosidic linkages [3]. Every
glucose unit has three different reactivating hydroxyl groups. The sec-
ondary hydroxyl group is situated at the wider cavity while the primary
hydroxyl group is located at the relatively smaller cavity [4]. Because of
the presence of this hydrophobic cavity, cyclodextrins are able to form
inclusion complexes with a wide variety of guest molecules [5]. That
combined with the fact that cyclodextrins are pretty malleable allowing
⁎
Corresponding author.
E-mail address: sharifahm@um.edu.my (S. Mohamad).
http://dx.doi.org/10.1016/j.molliq.2015.10.044
167-7322/© 2015 Elsevier B.V. All rights reserved.
0

S. Mohamad et al. / Journal of Molecular Liquids 212 (2015) 850–856
851
both cyclodextrin and ionic liquids provides large surface area, high
2.2. Instrumentation
ionic conductivity and improved supramolecular recognition essential
to our purpose [21].
The Fourier Transform Infrared (FTIR) spectra were recorded on a
Perkin-Elmer Spectrum 400 FTIR spectrometer at room temperature.
All the spectra were recorded with characteristic peaks in the range of
400–4000 cm . The Proton Nuclear Magnetic Resonance ( H NMR)
spectra were recorded on Lambda JEOL 400 MHz FT-NMR
spectrometer while the Two Dimensional Nuclear Overhauser Effect
Spectroscopy (2D NOESY) spectrum was recorded on AVN600 spec-
trometer (600 MHz) both at room temperature. All the samples were
Chlorinated phenolic compounds are a common class of industrial
pollutant. Some of its analogs are listed as priority pollutants by the
U.S. Environmental Protection Agency due to their toxicity and persis-
tence in the environment. Chlorophenols are present widely in pesti-
cides, disinfectants, wood preservatives, personal care formulations,
and many other products. Chlorophenols too are a substantial by-
product of wood pulp bleaching with chlorine [22]. The interest in mon-
itoring pollutants of the halocarbon family is increasing due to its avail-
ability in water bodies and its known toxic and carcinogenic effects [23].
−
1
1
6
dissolved in dimethyl sulfoxide (DMSO-d ). Tetramethylsilane (TMS)
was used as an internal reference for the calibration of proton chemical
shift. Voltammetric study was accomplished using Metrohm Autolab
(Model AUT101, Utrecht, Netherlands) with conventional three-
electrode cell.
2
,4-Dichlorophenol (2,4-DCP) is a key intermediate in the synthesis of
the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) [6]. It is also
used extensively in the manufacture of pesticides and herbicides.
To study the encapsulating ability, β-CD-BIMOTs (Fig. 1(a)) was
reacted with 2,4-DCP (Fig. 1(b)) to form an inclusion complex (β-
CDBIMOTs-2,4-DCP). β-CD-BIMOTs is very selective towards anionic
analytes such as 2,4-DCP since itpossesses a positive charge due to the
presence of cations from ionic liquid. The synthesized β-CD-BIMOTs
and β-CD-BIMOTs-2,4-DCP complex was analyzed with spectroscopic
techniques and compared with native β-CD-2,4-DCP complex. The
strong binding affinity of βCD-BIMOTs towards 2,4-DCP sets the plat-
form for our sensor study. β-CD-BIMOTs modified carbon paste elec-
trode (β-CD-BIMOTs/CPE) has been fabricated and utilized for the
determination of 2,4-DCP compound.
2.3. Synthesis of βCD-IL complex
2.3.1. Preparation of mono-6-deoxy-6-(3-benzylimidazolium)-β-cyclodex-
trin tosylate (β-CD-BIMOTs)
The synthesis of mono-6-deoxy-6-(3-benzylimidazolium)-β-cyclo-
dextrin tosylate (β-CD-BIMOTs) was accomplished based on reported
literary works with primary reference to Raoov et al. (2013). A brief
pathway is shown in Fig. 2.
2.3.2. Preparation of inclusion complex by kneading method
2
. Experimental
2 g of β-CD was mixed with ethanol in a mortar and kneaded to form
a paste. Then, an equimolar amount of 2,4-DCP (0.2869 g, 6.91 ×
−
4
2
.1. Reagent and solution
10
mol) was added. The sample was kneaded for approximately
6
0 min and dried to a constant mass. The final product obtained was a
β-cyclodextrin (β-CD) is commercially available and was pur-
chased from Acros (Hungary) (99%). 1-benzylimidazole (BIM) and
,4-dichlorophenol (2,4-DCP) was purchased from Sigma Aldrich
white powdery precipitate. The process was repeated for the prepara-
tion of βCD-BIMOTs-2,4-DCP inclusion complex with 1 g of β-CD-
BIMOTs and an equimolar amount of 2,4-DCP (0.1126 g, 6.91 ×
2
−
4
while N,N-dimethylformamide (DMF) was purchased from Merck.
Graphite was purchased from HmbG Chemicals. Other reagents and
chemicals were of analytical reagent grade and were used as re-
ceived without further purification. Phosphate buffer solution PBS,
pH 7.2 was prepared by mixing sodium dihydrogen phosphate,
disodium monohydrogen phosphate and sodium hydroxide,
10 mol).
2.4. Determination of formation constant
The concentration of 2,4-DCP was held constant at 0.05 mM, mean-
while the concentration of β-CD was varied at range 0–0.001 M. This
procedure was triplicated to obtain three absorbance values for each
of the β-CD concentration studied. The same procedure was then repli-
cated replacing β-CD with βCD-BIMOTs. All the absorbance was mea-
sured at 285 nm (pH 7.0).
−
1
−1
−1
0
.1 mol L respectively. 0.01 mol L 2,4-DCP in 0.1 mol L phos-
phate buffer stock solution was prepared with PBS pH 7.2. The work-
ing solution was prepared freshly by diluting the stock solution just
before taking the electrochemical measurement. Double distilled
water was used throughout the experiment.
Fig. 1. (a) β-cyclodextrin functionalized ionic liquid (mono-6-deoxy-6-(3-benzylimidazolium)-β-cyclodextrin tosylate-β-CD-BIMOTs) and (b) 2,4-dichlorophenol (2,4-DCP).

852
S. Mohamad et al. / Journal of Molecular Liquids 212 (2015) 850–856
Fig. 2. Synthesis pathway of (a) 6-O-monotosyl-6-deoxy-β-cyclodextrin (β-CD-OTs) and (b) Mono-6-deoxy-6-(3-benzylimidazolium)-β-cyclodextrin tosylate (β-CD-BIMOTs).
2
.5. Electrochemical measurement
3. Results and discussion
The conventional three electrode system used consists of bare CPE or
modified CPE as the working electrode, Ag|AgCl|3 M KCl (BASi MF-
063) as the reference electrode, and Pt wire (BASi model MW-1032)
3.1. FTIR analysis
2
Table 1 shows the interpretation of β-CD-2,4-DCP and βCD-BIMOTs-
2,4-DCP inclusion complex. As reported by Li et al. (2007), the FTIR
spectra of inclusion complexes were similar to those of pure β-CDs, a
major characteristic for host–guest inclusion complexes of β-CDs [24].
According to Zarif et al. (2012), structurally, β-CD is huge in size as it
is built up from seven repeating monomer units and therefore has sig-
nificant control on the FTIR profile of the complex [25].
as the counter electrode.
β-CD/CPE and β-CD-BIMOTs/CPE were prepared according to the
following procedure: 0.09 g of β-CD or β-CD-BIMOTs sample, 100 μL
of liquid paraffin and 0.21 g of graphite powder were hand-mixed in
a glass mortar and ground carefully. The prepared homogeneous
slurry was packed firmly into a syringe (2.0 mm id syringe tip),
with copper wire as the electrical contact. Prior to use, the modified
electrode was dried at 60 °C for 2 h and the electrode surface was
polished.
The existence of characteristic bands of 2,4-DCP in the inclusion
complex such as O-H, C_C aromatic stretching and C–O functional
−
1
−1
−1
groups at 3202.06 cm , 1479.17 cm and 1058.83 cm respectively
Table 1
FTIR interpretation of inclusion complex β-CD-2,4-DCP and βCD-BIMOTs-2,4-DCP.
Wavenumber (cm−1)
Functional group
β-CD
β-CD-BIMOTs
2,4-DCP
β-CD-2,4-DCP
βCD-BIMOTs-2,4-DCP
3
2
1
1
–
–
–
–
–
–
–
–
299.00
924.22
152.53
022.52
3297.00
2922.00
1153.00
1024.80
1652.00
1153.00
1366.00
1153.00
–
–
–
–
–
–
–
–
–
3202.06
2924.32
1153.25
1021.25
–
–
–
–
3202.06
1479.17
1058.83
–
3294.22
2925.00
1153.35
1024.27
1653.37
1153.35
1366.77
1153.35
3294.22
–
O–H
C–H
C–O–C
C–O
C_C
C_N
SO
SO
2
(asymmetric)
(symmetric)
2
3400.06
1583.06, 1473.20
1051.25
722.42
O–H
–
–
–
C_C
C–O
C–Cl
–
–

S. Mohamad et al. / Journal of Molecular Liquids 212 (2015) 850–856
853
Fig. 3. (a) β-CD, (b) β-CD-BIMOTs, (c) DCP, (d) β-CD-BIMOTs-2,4-DCP, (e) β-CD-2,4-DCP.
in β-CD-2,4-DCP supports the interaction between β-CD and 2,4-DCP.
The absence of C–Cl functional group from 2,4-DCP suggests the encap-
sulation of C–Cl in the β-CD cavity. The observation on the change in in-
tensity and wavenumbers too provide support for the inclusion
complex formation as the changes could have resulted from the restric-
tion of its stretching vibrations upon inclusion within the β-CD cavity.
These occurrences are quite naturally observed whenever there is a
weakening of the interatomic bonds as a consequence of an altered en-
vironment around these bonds upon complexation [25].
the 2,4-DCP stretching vibration was restricted upon the encapsulation
into the βCD-BIMOTs cavity. These phenomena can be explained in
terms of the breakdown of intermolecular hydrogen bonding within
2,4-DCP molecule and the establishment of a less weak force in the com-
plex system as reported recently by Eid et al. (2011) [1]. Fig. 3 provides
the comparison viewing of all the FTIR spectra.
1
3.2. H NMR analysis
As with β-CD, the FTIR pattern of the complex was dominated by βCD-
BIMOTs vibrational bands and shifts in the intensity and wavenumbers
are observed in the inclusion complex. The obvious presence of character-
istic bands of βCD-BIMOTs and 2,4-DCP in the spectrum supports the for-
mation of βCD-BIMOTs-2,4-DCP [26]. Furthermore, the disappearance of
major characteristic bands of 2,4-DCP such as C_C aromatic ring stretch
NMR spectroscopy is a quintessential tool on inclusion complex
studies. NMR allows the observation of key changes in the spectroscopic
pattern of the formed complex [27]. According to a recent study by Yuan
et al. (2012), if a guest molecule is incorporated into the β-CD cavity, the
screening constants of the β-CD protons inside the cavity (H3 and H5)
should be sensitive to the changed environment, but that of the outside
protons (H1, H2, and H4) should not. Therefore, the presence of a guest
−
1
−1
−1
at 1583.06 cm
and 1473.20 cm , C–Cl stretch at 722.42 cm
as
−
1
well as C–O stretching of phenol group at 1051.25 cm indicates that
Table 2
The chemical shift changes (Δδ, ppm) for βCD-BIMOTs-2,4-DCP and β-CD-2,4-DCP.
Proton
δ, ppm
β-CD
Δδ, ppm
δ, ppm
Δδ, ppm
2,4-DCP
β-CD-2,4-DCP
β-CD-BIMOTs
2,4-DCP
β-CD-BIMOTs-2,4-DCP
H1
H2
H3
H4
H5
H6
4.8118
3.2973
3.6115
3.3406
3.5383
3.6298
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
4.8972
3.3744
3.7002
3.4183
3.6500
3.7126
−
−
−
−
−
−
−
7.5051
7.0500
7.2500
+0.0854
+0.0771
+0.0887
+0.0777
+0.1117
+0.0828
−
−
−
−
−
4.8330
3.3030
3.5450
3.3268
3.3980
3.6298
7.4597
7.7450
7.4914
8.2100
7.9458
9.2031
5.1880
−
−
−
−
−
−
−
−
−
−
−
−
−
4.8345
3.3102
3.6231
3.3239
3.5567
3.6359
7.4829
7.7454
7.4957
8.1502
7.9480
9.2226
5.2032
7.4300
6.9765
7.1135
+0.0015
+0.0072
+0.0781
−0.0029
+0.1587
+0.0061
+0.0232
+0.0004
+0.0043
−0.0598
+0.0026
+0.0195
+0.0152
−0.0034
−0.0493
−0.0960
H
H
H
H
H
H
H
a
b
c
d
e
f
−
−
−
−
−
−
g
−
−
Ha-p
Hb-p
Hc-p
7.4334
7.0258
7.2095
+0.0717
+0.0242
+0.0405
7.4334
7.0258
7.2095
−
−
−

854
S. Mohamad et al. / Journal of Molecular Liquids 212 (2015) 850–856
Fig. 4. 2D NOESY of βCD-BIMOTS-2,4-DCP inclusion complex.
molecule should result in chemical shift changes of the interior protons
2].
[
β-CD-2,4-DCP inclusion complex observes a significant downfield
shift of H3 and H5 protons suggesting the encapsulation of 2,4-DCP mol-
ecule in the hydrophobic cavity of β-CD. The phenyl ring of 2,4-DCP
shifts the signal downfield. By contrast, the chemical shifts of H1, H2,
H4, and H6ab, on the outer surface of β-CD and the narrow opening of
β-CD respectively were only slightly affected upon addition of the
guest molecule [3].
The greater downfield shifts are observed for the protons in the
inner cavity of β-CD, H3 and H5 protons. Hence, this observation strong-
ly confirms that the inclusion complex has been formed between β-CD
and 2,4-DCP.
Similarly as observed in the β-CD-2,4-DCP spectra, βCD-BIMOTs-2,4-
DCP inclusion complex too observes a strong downfield shift at H3 and
H5 protons. The deshielding of protons in βCD-BIMOTs-2,4-DCP indicates
that the 2,4-DCP molecule had created an anisotropic effect in the interior
cavity due to the weak interactions of Van der Waals forces with the inter-
nal hydrogen atoms. A similar phenomenon has also been observed by
Vianna et al. (1998) when they complexed dexamethasone acetate
(DMA) with β-CD and hydroxypropyl-β-cyclodextrin (HP-β-CD) respec-
tively [28]. In addition, the H5 proton signal of β-CD-BIMOTs too changed
from doublet to singlet upon formation of inclusion complex.
With reference to Gao et al. (2006), it is well known that β-CD mol-
ecules adopt the torus conformation where H3 and H5 protons are lo-
cated inside the cavity, while H2 and H4 are outside the torus. Though
the H6 protons of the primary alcohol group are on the narrow side,
the shift is not as obvious as that of H5. On the other hand, the H2 and
H4 of β-CD are located outside the cavity, and their signals remain un-
changed upon the addition of 2,4-DCP. These results indicate that the
2
,4-DCP molecules were included into the cavity of β-CD and no inter-
action occurs on the external surface of β-CD [29].
The 2,4-DCP molecule can be either totally or partially enclosed in
1
the β-CD cavity. From the H NMR spectrum, it can be deduced that
the 2,4-DCP molecule entered fully into the cavity due to the obvious
Fig. 5. Double reciprocal plot of the a) β-CD–2,4-DCP complex and the b) βCD-BIMOTs-
upfield shift of Hb-p and Hc-p protons of 2,4-DCP.
2,4-DCP inclusion complex.

S. Mohamad et al. / Journal of Molecular Liquids 212 (2015) 850–856
855
Fig. 6. Proposed structure of β-CD-BIMOTs-2,4-DCP inclusion complex based on 1H NMR, 2D NOESY and FTIR.
Table 2 compares the chemical shift changes (Δδ, ppm) for βCD-
BIMOTs-2,4-DCP and β-CD-2,4-DCP. From this comparison, the chemi-
cal shift changes of H5 proton for β-CD-BIMOTs-2,4-DCP were greater
than for β-CD-2,4-DCP. Moreover, the chemical shift of Hc-p proton of
formation constant value is then derived from a 1/absorbance (A) vs
1/(concentration (C)) graph as shown in Fig. 5 with the aid of the mod-
ified Hildebrand–Benesi equation [29]. In this study the apparent for-
4
−1
mation constant determined for β-CD-2,4-DCP is 2.45 × 10
M
4
−1
2
2
,4-DCP in β-CD-BIMOTs-2,4-DCP complex was larger than in β-CD-
,4-DCP as well. These phenomena suggest that the modification of β-
while for β-CD-BIMOTs-2,4-DCP it is 4.45 × 10 M ; both were obtain-
ed from the slope and intercept of the double reciprocal plots shown in
Fig. 5. The two double reciprocal plots show good linearity with correla-
tion coefficients of 0.9214 for β-CD and 0.9720 for βCD-BIMOTs. Thus, it
can be summarized that both β-CD and βCD-BIMOTs have formed inclu-
sion complexes with 2,4-DCP at a ratio of 1:1. In addition, βCD-BIMOTs-
CD with ionic liquid improves its complexation ability with guest mole-
cules since it has high affinity towards anionic analytes such as 2,4-DCP.
3
.3. 2D NOESY NMR analysis
2,4-DCP registers a higher formation constant value thereby supporting
The formation of the inclusion complex was further proved by 2D
the assumption that β-CD- BIMOTs has a stronger binding affinity to-
wards 2,4-DCP compared to β-CD. The stronger binding affinity of the
βCD-BIMOTs-2,4-DCP towards 2,4-DCP provides a promising premise
for sensor application.
NOESY analysis, a powerful tool for investigating intermolecular inter-
action and to gain more information on the conformation of the inclu-
sion complex [30]. 2D NOESY experiments also provide an upper limit
(ca. 5 Å) on the distance between protons' cross peaks under favorable
conditions. Inclusion complex analysis between modified β-CD (βCD-
BIMOTs) and 2,4-DCP is very important in this work, since the cavity
of β-CD was maintained during the functionalization process. The
cross-peaks in the spectra indicated in Fig. 4 represents the interaction
of the protons of 2,4-DCP and βCD-BIMOTs. The cross peaks of βCD-
BIMOTs (3.5–3.6 ppm, H-3, H-5) and 2,4-DCP (7.4–6.9 ppm, Ha-p, Hb-
p, Hc-p) demonstrate strong intensity. Hence, from the 2D NOESY spec-
tra it can be concluded that the aromatic ring of 2,4-DCP has been ac-
commodated in the β-CD cavity. In addition, the cross-peaks around
3.5. The proposed mechanism of inclusion complex
The proposed mechanism was deduced stepwise supported by ana-
lytical data throughout the experiment. The initial interpretation of the
1
FTIR and H NMR data gave birth to the idea that the 2,4-DCP molecule
was encapsulated in the βCD-BIMOTs' cavity. The obvious downfield
1
shift of H3 and H5 protons in H NMR spectrum of βCD-BIMOTs-2,4-
DCP suggested that there is an interaction within the β-CD cavity.
Furthermore, the upfield shift of Hb-p and Hc-p protons and the disap-
pearance of major characteristic bands suggested that 2,4-DCP has suc-
cessfully entered the cavity. The completion and analysis of 2D NOESY
however provided support for an additional interaction between the
imidazolium ring and 2,4-DCP as well. The proposed mechanism for
the βCD-BIMOTs-2,4-DCP inclusion complex studied in this work is
shown in Fig. 6.
7–8 ppm belonging to imidazolium ring of BIMOTs and 2,4-DCP show
a promising complex formation via π–π interaction and this observation
will be discussed clearly in Section 3.5.
3
.4. Formation constant
The formation constant remains an important parameter on the
study of inclusion behaviors of β-CD [31]. In this study, the formation
constant was determined by observing the change in the intensity of
absorption of both β-CD-2,4-DCP and βCD-BIMOTs-2,4-DCP. The
3.6. Cyclic voltammograms of 2-4-dichlorophenol on CPE, β-CD/CPE and
β-CDBIMOTs/CPE
Electrochemical behavior of 2,4-dichlorophenol (2,4-DCP) on the
surface of the three different carbon paste electrodes was investigated
by cyclic voltammetry. Fig. 7 shows the cyclic voltammograms of 2,4-
DCP on the surface of bare CPE, β-CD/CPE and β-CDBIMOTs/CPE in
−
1
−1
0
.1 mol L PBS pH 7.2 at scan rates of 100 mVs . Only one oxidation
−
peak is observed attributed to the oxidation of OH group. On the re-
verse sweep, no distinct reduction wave was observed indicating that
2,4- DCP is irreversibly oxidized at the carbon paste electrode in the po-
tential range of 0.50 V to 1.10 V. On bare CPE, 2,4-DCP had an oxidation
peak potential located at 676 mV, with the oxidation peak current of
about 0.78 μA (curve a). On the surface of β-CD/CPE, 2,4-DCP underwent
oxidation at 708 mV with the oxidation peak current of 1.24 μA (curve
b), while on the surface of β-CDBIMOTs/CPE, 2,4-DCP had oxidation
peaks located at 720 mV with the oxidation peak current at 1.28 μA
(
curve c). The maximum response current was observed at curve c
where β-CDBIMOTs was responsible for the better enhancement of
the oxidation peak current compared to native β-CD. The results indi-
cated that the presence of β-CD and ionic liquid on the CPE surface
Fig. 7. Cyclic voltammograms of (a) CPE, (b) β-CDCPE, (c) β-CDBIMOTsCPE in the presence
of 60 × 10− mol L 2,4-DCP at pH 7.2, respectively with scan rate of 100 mV s−1
6
−1
.

856
S. Mohamad et al. / Journal of Molecular Liquids 212 (2015) 850–856
[
[
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improved the electrochemical response and the presence of IL could ef-
fectively improve the electron transfer rate between the electrode sur-
face and 2,4-DCP. This single and irreversible oxidation peak of 2,4-
DCP with β-CDBIMOTs promises potential application prospect, espe-
cially with its simple preparation and economical cost.
[
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4
. Conclusion
[
4
The presence of ionic liquids significantly improves the inclusion
[14] D. Lu, S. Lin, L. Wang, X.Z. Shi, C.M. Wang, Y. Zhang, Electrochim. Acta 85 (2012)
complex forming ability of β-CD namely with 2,4-DCP. With reference
to 2D NMR NOESY spectroscopy, it can be deduced that 2,4-DCP mole-
cules interact with both the β-CD cavity and the imidazolium side-
chain of β-CD-BIMOTs. Thus is can be concluded that β-CD-BIMOTs
has stronger binding affinity towards 2,4-DCP and as such more effec-
tive than native β-CD. The electrochemical results of β-CD-BIMOTs/
CPE indicates good performance for future development of 2,4-DCP
sensors.
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