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Amine Functionalized polyaniline grafted to exfoliated graphite oxide: Synthesis, characterization and multi-element sensor studies
Functionalized polyaniline grafted to exfoliated graphite oxide: Synthesis, characterization and multi-element sensor studies Alex Joseph, Sankaran Subramanian, Praveen C. Ramamurthy, Srinivasan Sampath, R. Vasant Kumar, Carsten Schwandt PII: DOI: Reference:
S1572-6657(15)30111-9 doi: 10.1016/j.jelechem.2015.09.015 JEAC 2278
To appear in:
Journal of Electroanalytical Chemistry
Received date: Revised date: Accepted date:
6 February 2015 9 September 2015 11 September 2015
Please cite this article as: Alex Joseph, Sankaran Subramanian, Praveen C. Ramamurthy, Srinivasan Sampath, R. Vasant Kumar, Carsten Schwandt, Functionalized polyaniline grafted to exfoliated graphite oxide: Synthesis, characterization and multi-element sensor studies, Journal of Electroanalytical Chemistry (2015), doi: 10.1016/j.jelechem.2015.09.015
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ACCEPTED MANUSCRIPT Functionalized polyaniline grafted to exfoliated graphite oxide: Synthesis, characterization and multi-element sensor studies [a]
, Sankaran Subramanian
[b]
[e]
[c]
, Srinivasan Sampath [d],
and Carsten Schwandt
[f]
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R. Vasant Kumar
, Praveen C. Ramamurthy
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Alex Joseph
[a] Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India & Department of Chemistry, Newman College, Thodupuzha, Kerala, 685585, India. Email: [email protected]
Email: [email protected]
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[b] Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India.
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[c] Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. Email: [email protected]
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[d] Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore
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560012, India. Email: [email protected] [e] Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK. Email: [email protected] [f] Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK. Email: [email protected]
Correspondence author: S. Subramanian Professor Materials Engineering Department, Indian Institute of Science, Bangalore-560012, India Phone +91-80-22932261 Fax +91-80-23600472 Email [email protected]
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ACCEPTED MANUSCRIPT Abstract: A functionalized polyaniline derivative (AMPANI) has been grafted onto exfoliated graphite oxide (EGO). The synthesis involved the in-situ chemical oxidative polymerization of
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functionalized aniline monomer in the presence of EGO, with diaminobenzene acting as a bridging ligand, to yield EGAMPANI. The synthesized compound was characterized by FT-IR and
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FT-Raman spectroscopy as well as thermogravimetric and X-ray diffraction analysis.
The
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EGAMPANI was then used to modify a carbon paste electrode (CPE), which was applied for multi-elemental sensing of Pb2+, Cd2+ and Hg2+ ions using differential pulse anodic stripping voltammetry. The limits of detection achieved using the EGAMPANI modified CPE were 2.2×10-6 M for Hg2+ ion, 1.2×10-6 M for Cd2+ ion, and 9.8×10-7 M for Pb2+ ion.
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Keywords:
Exfoliated graphite oxide, Surface functionalization, Graft polymerization, Differential pulse
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anodic stripping voltammetry, Multi-elemental sensor.
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Acronyms:
EG - Exfoliated graphite, EGO - Exfoliated graphite oxide, DAB - 1,4-diaminobenzene, EGO-
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DAB - exfoliated graphite oxide modified with 1,4-diaminobenzene, AMPANI - Amine functionalized polyaniline, EGAMPANI - Amine functionalized polyaniline grafted onto exfoliated graphite oxide, DCC - N,N’-dicyclohexylcarbodiimide, DMAP - dimethylaminopyridine, APS - Ammonium persulfate.
1.
Introduction Exfoliated graphite oxide (EGO) is generally regarded as a key material for the
preparation of functional materials for diverse applications.
EGO carries various oxygen
containing functional groups, such as the carboxyl, hydroxyl and epoxy groups, on its surface [1]
, which are distributed on the basal planes and edges. These groups facilitate the chemical
functionalization of the EGO with both simple compounds polymers
[3,4,5]
.
[2]
and high molecular weight
Another important property of EGO is its ability to adsorb simple and
macromolecules through non-covalent electronic interactions. Depending upon the nature of 2
ACCEPTED MANUSCRIPT functionalization, modified EGO exhibits different physical and chemical properties which in turn determine its applications for electronic devices and electrochemical sensors modified electrodes
[10]
[6,7,8,9]
, surface
, and others. Studies have also been reported on the functionalization of
EGO intended to make it more lyophilic in nature
[11,12]
. Chemical functionalization has the
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advantage of very good polymer layer adhesion and long term performance
compared to
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physical deposition by non-covalent interaction.
[13]
EGO in its native form has already been reported for use in metal ion sensors [14]. EGO
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which is surface modified with an amine is expected to show sensor properties that are different from those of pure EGO. A convenient way of achieving amine carrying EGO surfaces is to graft amine containing polymers, such as functionalized polyaniline, onto them. It has been reported in an earlier investigation that functionalized polyaniline possesses an improved metal [15]
. It is therefore likely that this property can be
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uptake capacity compared to its native form
effectively used in metal ion sensors consisting of an EGO substrate and surface grafted
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polyaniline. A distinct advantage of using EGO as the substrate for metal ion sensors is its surface functional groups which will provide extra metal chelation in addition to the polymer.
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In this investigation, the synthesis and characterization of EGO modified with amine functionalized polyaniline (EGAMPANI) have been performed, and its application in a multi-
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elemental electrochemical sensor for selected metal ions has been assessed. Initially, the EGO has been synthesized by following a modified procedure Hummers and Offeman
[17]
[16]
of that originally reported by
, and then the EGO surface has been enriched with diaminobenzene
(DAB) to obtain EGO-DAB. Subsequently, the amine functionalized polyaniline (AMPANI) has been grafted onto the EGO-DAB surface by in-situ chemical oxidative polymerization, yielding EGAMPANI as the reaction product. In each step of the synthesis, the structure of the intermediary product formed has been confirmed by using NMR or FT-IR and FT-Raman techniques.
The synthesized EGAMPANI has then been characterized with regard to its
compositional, thermal, structural and topographic properties by means of CHNO analysis and TGA, XRD, SEM and microanalytical techniques.
The differential pulse anodic stripping
voltammetry (DPASV) technique has been adopted for Pb2+, Cd2+ and Hg2+ metal ion sensing with an EGAMPANI modified carbon paste electrode (CPE).
3
ACCEPTED MANUSCRIPT 2.
Experimental
2.1.
Chemical syntheses and analytical methods The chemical synthesis work involved several parts, (i) the preparation of the amine
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functionalized aniline monomer starting from 2-hydroxyaniline, (ii) the preparation of exfoliated
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graphite oxide (EGO) from crushed commercial graphite, and (iii) the formation of the desired EGAMPANI by in-situ chemical oxidative graft polymerization of the AMPANI monomer onto the
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EGO surface with 1,4-diaminobenzene (p-phenylenediamine) (DAB) as the bridging ligand. Details of the individual synthesis protocols, the results from the NMR analysis of the intermediary products, and the suppliers of the chemicals used are provided in the Supporting Information.
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NMR spectra were recorded using a Bruker 400 MHz spectrometer. Molecular mass of compounds was determined with a Waters Q-TOF micro mass spectrometer.
For Fourier
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transform infrared (FT-IR) studies, samples were pressed into pellets using KBr and scanned in the range of 4000 to 400 cm-1 in a Perkin Elmer FT-IR spectrometer. FT-Raman spectra were
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acquired in a LabRAM HR spectrometer with Ar ion laser of wavelength 514 nm as source at magnification 100x and using a CCD camera ranging from 0 to 3000 cm -1 as detector. CHNO
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contents were determined using an EA Flash 1112 series instrument. Thermograms of the samples were recorded using a Netzsch STA 409 instrument at heating rate 2 °C min-1. X-ray diffraction (XRD) patterns of powdered samples were recorded with a PANalytical X-ray diffractometer ranging from 6 to 70° with Cu anode K-α radiation of wavelength 1.54 Å as source. For the morphological studies, an FEI Quanta 200 scanning electron microscope was used. Xray photoelectron studies (XPS) were carried out using a SPECS GmbH Phoibos 100 MCD Energy Analyzer with AlK radiation of energy 1486.6 eV. The electrochemical studies were carried out using a CHI 660D electrochemical workstation. 2.2.
Preparation of EGAMPANI modified carbon paste electrode (CPE) and differential pulse anodic stripping voltammetry (DPASV) test procedure A carbon past electrode (CPE) modified with EGAMPANI was prepared by mixing 5
wt% of EGAMPANI to a blend of 4:1 ratio of carbon black and paraffin oil. This paste was then filled into a glass tube of inner diameter 4 mm, into which a copper wire was inserted for electrical contact. All stripping voltammetric analyses were conducted in a three terminal cell, 4
ACCEPTED MANUSCRIPT which contained a CPE as the working electrode, a calomel electrode as the reference electrode, and a platinum wire of 1 mm diameter as the counter electrode. Individual solutions of Pb 2+, Cd2+ and Hg2+ ions as well as mixed equimolar solutions of these metal ions were used for analysis. All solutions were prepared from the respective nitrate salts, using an acetate buffer to
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maintain a constant pH of 5. Volumes of 10 ml were used in each experiment, and the solutions were purged with argon to remove dissolved oxygen prior to investigation. The deposition of
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metal onto the CPE surface was performed at a constant potential of -1.3 V for about 10 min,
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and the stripping of the deposit from the electrode surface was achieved by sweeping from -1.1 to +1.0 V with a scan rate of 25 mV s-1. The stripping analysis was carried out with step potential of 0.004 V, amplitude of 0.05 V, pulse period of 0.5 s, pulse width of 0.2 s and sampling width of 0.0167 s. A neat CPE was prepared and used under the same conditions in
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order to distinguish the response of the EGAMPANI modified CPE from that of the neat one. All electrochemical studies were carried out under nitrogen atmosphere. The limit of detection (LoD) has been determined according to its definition given in the literature
[18]
, using the
ED
relation LoD = LoB + 1.645 SD (low concentration sample), where LoB = Mean (blank) + 1.645 SD (blank) and
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SD is the standard deviation of three determinations.
Results and discussion
3.1.
Synthesis of EGAMPANI
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3.
The synthesis of the aniline monomer is depicted in Scheme 1. The synthesis was initiated from 2-hydroxyaniline, in which the amine functional group was protected by reacting it with acetic anhydride to form the corresponding acetanilide (1). This was followed by the reaction with epichlorohydrin in the presence of a weak base to produce compound (2). The epoxide ring introduced in step 2 was then reacted with dimethylamine so that compound (3) was formed.
Finally, the acetanilide moiety in compound (3) was deprotected to obtain the
monomer (4). This was later used for chemical oxidative grafting. The further reaction pathway to EGAMPANI is illustrated in Scheme 2. EGO prepared according to Hummers’ method was used as the starting material. As it is well known, EGO is a good adsorbing agent for organic molecules and macromolecules by π-π stack interactions
[19]
and, beyond this, the oxygen containing functional groups on the EGO surface enable grafting 5
ACCEPTED MANUSCRIPT polymers onto it through the formation of covalent bonds
[1,20,21]
. Taking advantage of both
these properties of EGO, the EGO surface has been enriched with DAB. Chemical linking was achieved by amide bond formation in the presence of DCC as the coupling agent and DMAP as a base. Grafting of AMPANI was then accomplished through reaction of EGO-DAB with
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monomer (4) by in-situ chemical oxidative polymerization using APS as an oxidizing agent. This produced the desired EGAMPANI. One of the advantages of using AMPANI in the
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synthesis of EGAMPANI is that it is highly water soluble at all pH, which facilitates washing
NH2 OH
NHCOCH3
(CH3CO) 2O
OH
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off any ungrafted excess AMPANI that may have formed during the in-situ polymerization.
Cl
O
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70 C, 3 h
(1)
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CH3
CH3
N
O
CH3
Reflux, 12 h
O
Reflux, 24 h
NHCOCH3
CH3
O
K2CO3, Dry THF
o
HN
NHCOCH3
(2) +
-
NH3 Cl
1:2 Con HCl/EtOH
O
Reflux 24 h
OH
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HO
(3)
+
NH Cl H3C CH3
-
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2-oxy (1-dimethylamine-2-hydroxy propyl) aniline hydrochloride Monomer (4)
Scheme 1: Synthesis of the monomeric unit 2-oxy (1-dimethylamine-2-hydroxypropyl) aniline hydrochloride (4) starting from 2-hydroxyaniline.
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ACCEPTED MANUSCRIPT H2N O
HOOC
OH
H N
H2N
OH
C
O
NH2
O
OH
OH
DCC/DMAP O
H2N
O
NH
reflux H2 N
COOH
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2O 8
4) 2
CH3
N
+
HO
-
NH4 Cl
(N H
H3C
O
H3C
CH3
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OH O
N
CH3
NH
N
CH3
O
+
HO
H3C
NH O
NH2 H3C
N
CH3
HO
HO
N
NH Cl CH3
-
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HO
O
EGO-DAB
S
EGO
NH2
C
H3C
N
CH3
O
O H3C
N
H3C
NH
O
NH
O
HO
CH3 H3C
H N
H2N C
N
NH
OH
OH
O
O
NH
O NH2
HO
H N
O
HN O N NH
N
OH
CH3
N
EGAMPANI
CH3
ED
H3C
CH3
N H3C
H3C
O
HO
OH
OH
O N
CH3
H2N
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HO
H3C
N
CH3
O
H3C
O
N
HN
Scheme 2: Synthesis of EGO-DAB and in-situ chemical oxidative polymerization grafting
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of monomer (4) onto EGO-DAB to form EGAMPANI. (The tetramer shown is a
3.2.
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representative of the type of polymer formed.)
Characterization of EGAMPANI To study the type of bonding of the AMPANI to the EGO surface, the synthesized
EGAMPANI and its intermediate compounds were analyzed using FT-IR spectroscopy. The spectra recorded for EGO, EGO-DAB and EGAMPANI are compiled in Fig. 1.
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ACCEPTED MANUSCRIPT A 1234
B
3456
2857
1632 1503 1709 1654
1064
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2926 3453
C
832
2994
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Transmittance
1457
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1733 1622
1710
3444 3003
4000
3500
3000
1509 1578
2500
2000
1500
1000
500
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-1 Wavenumber (cm )
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Fig. 1: FT-IR spectra of a) EGO b) EGO-DAB and c) EGAMPANI.
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The spectrum of EGO (Fig. 1a) shows major bands at 1064, 1234, 1457, 1622 and 1733 cm-1, which correspond respectively to the vibrational modes of C–O stretching, C–OH
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stretching of phenolic group, C=C phenyl ring stretching, and carbonyl group stretching at the edge of layer planes or in conjugate functional groups. The intense band observed at 3456 cm -1 is due to O–H stretching vibration of graphite oxide. These results confirm that EGO has a large number of oxygen containing functional groups on its surface. At both 2926 and 2857 cm-1 the C–H stretching vibrations of substituted aromatic compounds of EGO are observed, which is in line with earlier findings
[3]
. After the reaction of EGO with DAB to EGO-DAB,
the carboxylic stretching frequency previously observed at 1733 cm -1 for EGO is shifted to 1709 cm-1 (Fig. 1b), indicating the formation of an amide bond. The amide bond is further confirmed by the stretching vibrations observed at 1654 and 1632 cm-1 and the bending vibration at 1503 cm-1. The presence of the phenylene moiety is confirmed by the new intense band observed at 2994 cm-1, which is due to the C–H stretching vibration of phenylic group from the DAB. The formation of EGAMPANI by AMPANI grafting onto the EGO-DAB surface is confirmed by the formation of new characteristic intense bands corresponding to the polyaniline backbone (Fig. 1c). The stretching vibrations observed at 1578, 1509 and 832 cm -1 are respectively due to the quininoid, benzenoid and 1,4-disubstituted phenylic stretching 8
[22]
of
ACCEPTED MANUSCRIPT the polyaniline backbone present in the AMPANI. Furthermore, the increased intensity of the C–H stretching frequency at 3003 cm-1 is due to the phenylic group present at the AMPANI backbone, and thus attests to the AMPANI grafting onto the EGO-DAB.
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Further evidence for the AMPANI grafting onto the EGO-DAB is obtained from the CHNO elemental analysis. The elemental compositions of EGO, EGO-DAB and EGAMPANI,
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expressed in atomic percentages, are as shown in Table 1.
Table 1: Elemental compositions of EGO, EGO-DAB and EGAMPANI. C (%)
EGO
41.6
EGO-DAB
44.7
EGAMPANI
52.0
H (%)
N (%)
O (%)**
C/N
2.6
1.0*
54.8
41.6
2.8
4.6
47.9
9.72
3.8
5.5
38.7
9.45
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Compound
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* The small content of nitrogen is either from the concentrated acid used for exfoliation or from impurities. ** The oxygen content is calculated by subtracting the other elemental percentages from 100%.
The significant increase in the content of nitrogen in EGO-DAB compared to EGO is due to DAB absorption on the EGO surface by π-π stack interaction as well as amide bond formation. A further additional increase in the content of nitrogen in EGAMPANI suggests that the AMPANI polymer has been successfully grafted onto the EGO-DAB surface. As has been shown in a recent study
[23]
, XPS analysis is a suitable technique for the analysis of a
polyaniline backbone on an EGO substrate. However, in the present study, attempts to obtain further information on the structure of the EGAMPANI by means of XPS analysis were not successful as only a faint peak for nitrogen was obtained. This observation can be explained by combining the CHNO results of EGO-DAB and EGAMPANI with the following assumptions. Firstly, the adsorption of AMPANI onto the EGO-DAB surface is unfavorable as both AMPANI and EGO-DAB carry free amino groups and hence acquire a positive charge in acidic 9
ACCEPTED MANUSCRIPT media and tend to repel each other electrostatically. This will favor the formation of chemically grafted AMPANI rather than physically adsorbed AMPANI.
Secondly, the high water
solubility of AMPANI facilitates the complete removal of any ungrafted AMPANI that may still be present immediately after the chemical polymerization step. This will leave behind only
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the chemically grafted AMPANI. Finally, the AMPANI formed will only have a low degree of polymerization due to its bulky ortho substituent. Altogether, the above will lead to a rather
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low amount of additional nitrogen on the EGAMPANI surface compared to that on the EGO-
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DAB.
FT-Raman spectra can provide additional evidence for the structural modification of the EGO as a result of its interaction with the AMPANI grafted onto it. It is presumed that this interaction will introduce changes in the vibrational properties of the EGO carbon skeleton.
D
G
Intensity
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Fig. 2 presents the Raman spectra obtained for EGO, EGO-DAB and EGAMPANI.
EGO
EGO-DAB
EGAMPANI
1000
1200
1400
1600
1800
-1 Raman shift (cm )
Fig. 2: FT-Raman spectra of EGO, EGO-DAB and EGAMPANI.
The Raman spectrum of EGO exhibits two distinct peaks at 1590 and 1350 cm -1, which are attributed to the tangential mode (G-band) and disorder mode (D-band) respectively. These
10
ACCEPTED MANUSCRIPT two peaks are correlated with the ordered and disordered skeletal structures of the graphite layers present in the EGO. The ratios of the G-band to D-band intensities (IG/ID) measured for EGO, EGO-DAB and EGAMPANI are shown in Table 2. It is evident that the intensity ratio decreases successively as the EGO changes to EGO-DAB and then further to EGAMPANI.
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This is indicative of the formation of new chemical bonds impacting on the EGO carbon skeletal vibration frequencies. These interactions provide evidence in support of the formation
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of an amide bond as well as the grafting of AMPANI onto the EGO surface.
Table 2: G-band to D-band intensity ratio (IG/ID) of EGO, EGO-DAB and EGAMPANI
Material
IG/ID
EGO
0.996
ED
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calculated from Raman spectra.
0.920
EGAMPANI
0.894
The
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EGO-DAB
thermal
properties
of
the
compounds
synthesized
were
assessed
using
thermogravimetric analysis carried out under inert atmosphere. Fig. 3 displays the thermograms of EGO, EGO-DAB and EGAMPANI.
11
ACCEPTED MANUSCRIPT
100
EGO-DAB
70
EGAMPANI
50
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60
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Mass (%)
80
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90
EGO
40 200
400
600
800
1000
Temperature ( C)
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0
ED
Fig. 3: Thermograms of EGO, EGO-DAB and EGAMPANI recorded under inert atmosphere.
All materials were analyzed in the heating range from 30 to 1000 °C by increasing the
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temperature at 2 °C min-1, and the onset temperatures of decomposition were determined from the corresponding derivative curves. The EGO exhibits a weight loss of 26% up to around 100
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°C which is caused by the release of physically absorbed water. An additional weight loss of 19% is observed at around 190 °C which is mainly due to the pyrolysis of the labile oxygen [24]
containing functional groups, such as –OH and –COOH, yielding CO, CO2 and H2O
.
Thereafter the EGO residue formed remains stable up to around 870 °C. Both the EGO-DAB and the EGAMPANI only exhibit a comparatively small weight loss up to around 120 °C caused by the liberation of physically bound water. Beyond that, both the EGO-DAB and the EGAMPANI undergo a slower and more gradual weight loss than the EGO, which points to their improved thermal stability due to the presence of the relatively stable amide bond
[25]
. In
case of the EGO-DAB this gradual weight loss extends up to around 700 °C, whereafter it accelerates because of the breakage of the amide bond and the loss of the DAB entity
[25]
. In
case of the EGAMPANI, there is a similar gradual weight loss up to around 840 °C, whereafter the curve shows a downward deflection due to the decomposition of the AMPANI side chains. Overall, the EGO-DAB displays a larger weight loss up to 1000 °C than the EGAMPANI, because the decomposition of the DAB entity leads to some volatile entities while the surface
12
ACCEPTED MANUSCRIPT grafted polyaniline leaves behind a mainly nonvolatile carbonaceous residue
[26]
. The trend of
decomposition for the AMPANI in the EGAMPANI is similar to that of polyaniline reported earlier
[27]
. It should also be noted that the comparatively low plateau for the EGO at high
temperatures is simply a consequence of its high initial water content and is not an indication of
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lower thermal stability.
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X-ray powder diffraction was used to investigate the crystallinity of the pristine graphite and the compounds synthesized from it. Fig. 4 displays the XRD patterns of pristine graphite,
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Graphite
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Intensity
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EGO and EGAMPANI.
10
EGO
EGAMPANI 20
30
40
50
60
70
2 Theta
Fig. 4: X-Ray diffraction patterns of graphite, EGO and EGAMPANI.
For pristine graphite the characteristic peak corresponding to the (002) plane appears at 2θ = 26.4°
[5]
along with other less intense peaks at higher angles. In the case of EGO the
characteristic peak is observed at 2θ = 11.5°. This indicates that the graphite oxide layers stack with a larger interlayer separation due to steric hindrance
[3]
, which may readily be attributed to
the presence of oxygen containing functional groups within the material. The interlayer spacing of the EGO is calculated to be around 0.8 nm, which is significantly larger than that of the
13
ACCEPTED MANUSCRIPT pristine graphite of 0.338 nm. The grafting of AMPANI onto the EGO surfaces results in the disappearance of the sharp peak observed at 11.5° for EGO, which demonstrates that the grafting reaction was successful and led to an AMPANI assisted expansion of the EGO layers [5]
. The AMPANI intervening between the EGO layers will bring out a steric effect which
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explains the lack of the hitherto orderly arrangement of the graphite oxide layers in the stacking direction. A scanning electron microscopic image of an EGAMPANI film prepared from a
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dispersion of EGAMPANI in deionized water is shown in Fig. 5. This puts weightage to the
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ED
MA
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same observation as there is a clearly visible disorder in the morphological arrangements.
Fig. 5: Scanning electron micrograph of EGAMPANI film.
3.3.
Differential pulse anodic stripping voltammetric (DPASV) studies of EGAMPANI modified CPE: sensitivity and cross-sensitivity for metal ions The application of EGAMPANI as an active material for metal ion sensing was carried
out using differential pulse anodic stripping voltammetric (DPASV) analysis. A CPE modified with 5 wt% of EGAMPANI has been used to investigate the sensor ability for Pb 2+, Cd2+ and Hg2+, and the results were compared to those of a neat CPE.
14
ACCEPTED MANUSCRIPT Fig. 6 shows the anodic stripping currents recorded for individual metal ion solutions using both the EGAMPANI modified CPE and the neat CPE. It is seen that the anodic stripping current for Pb2+ ions on the EGAMPANI modified CPE is significantly increased in comparison to that of the neat CPE. The improvement in stripping current for Cd 2+ ions is rather marginal
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and the observed current remains significantly below that of Pb 2+. The stripping current for Hg2+ ions using the EGAMPANI modified CPE is found to be lower than that of the neat CPE.
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These results imply that the EGAMPANI modified CPE shows a higher sensitivity for Pb2+
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compared to the other two metals. The large stripping current for the Pb 2+ ion can be attributed to the high binding affinity of the EGAMPANI to the Pb 2+ ion, which enhances its surface concentration on the electrode during the deposition step prior to the stripping process. The marginal increase of stripping current for the Cd 2+ ion is likely to be caused in a similar way but
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to a lesser extent. In contrast, the affinity of the EGAMPANI to the Hg 2+ ion is lower than that of the neat CPE. Overall, the results show that EGAMPANI has a strong influence on the sensitivity of metal ion sensing by means of CPEs and that it is most appropriate for Pb2+
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ED
analysis.
CPE EGAMPANI
-4
Anodic stripping current (A)
1.6x10
-4
1.2x10
-5
8.0x10
-5
4.0x10
0.0 Pb
Hg
Cd
Metal ions
Fig. 6: Differential pulse anodic stripping currents of EGAMPANI modified CPE and neat CPE in individual metal ion solutions of 1×10-5 M concentration.
15
ACCEPTED MANUSCRIPT
The assessment of the EGAMPANI modified CPE as a multi-elemental metal ion sensor has been carried out in mixed equimolar metal ion solutions in order to facilitate selectivity
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studies. The voltammograms recorded with the EGAMPANI modified CPE and the neat CPE when using these multi-elemental solutions are compiled in Fig. 7a. The voltammogram of the
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EGAMPANI modified CPE exhibits three characteristic oxidation peaks, corresponding to Cd2+, Pb2+ and Hg2+ at -0.73, -0.51 and +0.34 V respectively. In contrast, the voltammogram of
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the neat CPE displays only two peaks, corresponding to Pb 2+ and Hg2+ at -0.58 and +0.27 V respectively. The observed slight shifts in the reduction potentials of Pb 2+ and Hg2+ are caused by differences in the interaction between the respective ions and electrode surfaces.
The
disappearance of the Cd2+ signal on the neat CPE indicates the virtual absence of such an
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interaction in the multi-elemental solution, which is in agreement with the observations obtained for the neat CPE using the individual solutions (Fig. 6).
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It is furthermore evident from Fig. 7a that the stripping currents for Pb 2+ and Cd2+ ions in the multi-elemental solution are significantly improved for the EGAMPANI modified CPE
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compared to those of the neat CPE in the same solution.
This is primarily because the
EGAMPANI facilitates the accumulation of lead and cadmium onto the electrode surface
sensing.
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through the functional groups on its surface, which is a requirement for multi-elemental In contrast, the deposition of pure mercury onto the surface of the EGAMPANI
modified CPE is dramatically reduced with regard to that of the neat CPE in the same solution. These effects are in agreement with the observations made for the individual metal ion solutions (Fig. 6).
Together, Fig. 7a and Fig. 6 reveal the influence of other metal ions on the stripping current of the Pb2+ ion. For the EGAMPANI modified CPE, the Pb2+ stripping current in the multi-elemental solution (Fig. 7a) shows a moderate decrease compared to that in the individual metal ion solution (Fig. 6). This indicates an interference of other metal ions on the Pb 2+ sensing with an EGAMPANI based electrode.
16
ACCEPTED MANUSCRIPT -4
1.6x10
A
CPE EGAMPANI
-4
-5
4.0x10
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Pb
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Hg
Cd
-5
8.0x10
Pb
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Current (A)
1.2x10
Hg
0.0 -1.5
-1.0
-0.5
0.0
0.5
1.0
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Potential (V)
-4
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1.2x10 1.0x10
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a
a
-5
6.0x10
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Current (A)
8.0x10
Pb
Cd
-5
Current (A)
B
-4
Hg Pb Cd
1E-5
1E-6
a
-5
4.0x10
Hg
1E-5 1E-6 1E-7 Concentration (M) a) 0.5E-4 b) 1.0E-5 c) 0.5E-5
-5
d) 1.0E-6
2.0x10
0.0
e
-1.0
f
e) 0.5E-6
c
-0.5
0.0
f) 1.0E-7
0.5
1.0
1.5
Potential (V)
Fig. 7: DPASV of A) EGAMPANI modified CPE and neat CPE, recorded using mixed equimolar solutions of Pb2+, Cd2+ and Hg2+ ions with concentrations of 1×10-5 M for each ion, and B) EGAMPANI modified CPE recorded using mixed equimolar solutions of these metal ions with concentrations ranging from 5.0×10-5 to 1.0×10-7 M for each ion. The inset graph shows the cross-sensitivity of each metal ion in equimolar solution.
17
ACCEPTED MANUSCRIPT Fig. 7b compiles the variations of the anodic stripping currents obtained with EGAMPANI modified CPEs for mixed equimolar metal ion solutions with concentrations ranging from 5×10-5 to 1×10-7 M. It is evident that the electrode sensitivity of the EGAMPANI modified CPE is a function of the metal ion concentration and that the relative intensity of each 6
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peak is characteristic of the metal ion. The limit of detection is 2.2×10 -6 M for Hg2+ ion, 1.2×10M for Cd2+ ion, and 9.8×10-7 M for Pb2+ ion. The inset graph shows the cross-sensitivity for
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each metal ion. It is observed that Pb2+ ion sensing becomes less affected by cross-sensitivity
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when the concentration of the multi-elemental solution decreases. This improvement in crosssensitivity at lower concentrations of other metal ions holds promise for the utilization of the EGAMPANI modified CPE as a selective and sensitive Pb 2+ ion sensor in environments with
Conclusions
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4.
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small concentrations of interfering metal ions.
An amine functionalized polyaniline derivative (AMPANI) has been grafted onto the
denoted as EGAMPANI.
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surface of exfoliated graphite oxide (EGO) to synthesize a polymer/graphite hybrid material The EGAMPANI was characterized by FT-IR and FT-Raman
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spectroscopy, CHNO analysis, XPS, TGA, XRD and SEM to assess its structure, composition, thermal stability, crystallinity and morphology. In this study, the polyaniline/EGO hybrid material has been for the first time applied for metal ion sensor tests in aqueous media. In these, the response of an EGAMPANI modified carbon paste electrode has been assessed for Pb 2+, Cd2+ and Hg2+ metal ion sensing in an acetate buffer by stripping voltammetric techniques. Limits of detection of respectively 2.2×10-6 M, 1.2×10-6 M, and 9.8×10-7 M have been achieved under appropriate conditions for Hg 2+, Cd2+ and Pb2+. This feasibility study has thus indicated that the functionalized electrode has the potential to be employed as a multi-elemental sensor and that it may be of particular use for the sensing of Pb2+ ions in aqueous media with very low metal ion concentration. It is clear that supplementary studies will be required to gain a better understanding of the cross-sensitivities observed and to optimize the application of any sensor incorporating this material.
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ACCEPTED MANUSCRIPT Acknowledgements The authors express their gratitude to the Indo-UK UKIERI program for the award of a research project to carry out this investigation (SA08-005). The NMR Research Centre, Indian
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Institute of Science, Bangalore, is gratefully acknowledged for the permission to record the NMR spectra. One of the authors (AJ) thanks the Council of Scientific and Industrial Research,
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MA
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opportunity to visit the University of Cambridge, UK.
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New Delhi, for the grant of a senior research fellowship and the UKIERI program for the
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ACCEPTED MANUSCRIPT References
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[27]
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245611.
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ACCEPTED MANUSCRIPT
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Highlights
An amine functionalized polyaniline has been grafted to exfoliated graphite oxide.
Synthesized EGAMPANI is characterised using FTIR, TGA, XRD, Raman and elemental
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analysis.
A DPASV based multi-elemental sensor is prepared using EGAMPANI modified CPE.
Analysis was carried out to study the electrochemical detection of Pb2+, Cd2+ and Hg2+.
The LoD observed is 2.2×10-6 M for Hg2+ ion, 1.2×10-6 M for Cd2+ ion, and 9.8×10-7 M for
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Pb2+ ion.
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S1572-6657(15)30111-9 doi: 10.1016/j.jelechem.2015.09.015 JEAC 2278
To appear in:
Journal of Electroanalytical Chemistry
Received date: Revised date: Accepted date:
6 February 2015 9 September 2015 11 September 2015
Please cite this article as: Alex Joseph, Sankaran Subramanian, Praveen C. Ramamurthy, Srinivasan Sampath, R. Vasant Kumar, Carsten Schwandt, Functionalized polyaniline grafted to exfoliated graphite oxide: Synthesis, characterization and multi-element sensor studies, Journal of Electroanalytical Chemistry (2015), doi: 10.1016/j.jelechem.2015.09.015
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ACCEPTED MANUSCRIPT Functionalized polyaniline grafted to exfoliated graphite oxide: Synthesis, characterization and multi-element sensor studies [a]
, Sankaran Subramanian
[b]
[e]
[c]
, Srinivasan Sampath [d],
and Carsten Schwandt
[f]
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R. Vasant Kumar
, Praveen C. Ramamurthy
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Alex Joseph
[a] Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India & Department of Chemistry, Newman College, Thodupuzha, Kerala, 685585, India. Email: [email protected]
Email: [email protected]
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[b] Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India.
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[c] Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. Email: [email protected]
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[d] Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore
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560012, India. Email: [email protected] [e] Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK. Email: [email protected] [f] Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK. Email: [email protected]
Correspondence author: S. Subramanian Professor Materials Engineering Department, Indian Institute of Science, Bangalore-560012, India Phone +91-80-22932261 Fax +91-80-23600472 Email [email protected]
1
ACCEPTED MANUSCRIPT Abstract: A functionalized polyaniline derivative (AMPANI) has been grafted onto exfoliated graphite oxide (EGO). The synthesis involved the in-situ chemical oxidative polymerization of
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functionalized aniline monomer in the presence of EGO, with diaminobenzene acting as a bridging ligand, to yield EGAMPANI. The synthesized compound was characterized by FT-IR and
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FT-Raman spectroscopy as well as thermogravimetric and X-ray diffraction analysis.
The
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EGAMPANI was then used to modify a carbon paste electrode (CPE), which was applied for multi-elemental sensing of Pb2+, Cd2+ and Hg2+ ions using differential pulse anodic stripping voltammetry. The limits of detection achieved using the EGAMPANI modified CPE were 2.2×10-6 M for Hg2+ ion, 1.2×10-6 M for Cd2+ ion, and 9.8×10-7 M for Pb2+ ion.
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Keywords:
Exfoliated graphite oxide, Surface functionalization, Graft polymerization, Differential pulse
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anodic stripping voltammetry, Multi-elemental sensor.
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Acronyms:
EG - Exfoliated graphite, EGO - Exfoliated graphite oxide, DAB - 1,4-diaminobenzene, EGO-
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DAB - exfoliated graphite oxide modified with 1,4-diaminobenzene, AMPANI - Amine functionalized polyaniline, EGAMPANI - Amine functionalized polyaniline grafted onto exfoliated graphite oxide, DCC - N,N’-dicyclohexylcarbodiimide, DMAP - dimethylaminopyridine, APS - Ammonium persulfate.
1.
Introduction Exfoliated graphite oxide (EGO) is generally regarded as a key material for the
preparation of functional materials for diverse applications.
EGO carries various oxygen
containing functional groups, such as the carboxyl, hydroxyl and epoxy groups, on its surface [1]
, which are distributed on the basal planes and edges. These groups facilitate the chemical
functionalization of the EGO with both simple compounds polymers
[3,4,5]
.
[2]
and high molecular weight
Another important property of EGO is its ability to adsorb simple and
macromolecules through non-covalent electronic interactions. Depending upon the nature of 2
ACCEPTED MANUSCRIPT functionalization, modified EGO exhibits different physical and chemical properties which in turn determine its applications for electronic devices and electrochemical sensors modified electrodes
[10]
[6,7,8,9]
, surface
, and others. Studies have also been reported on the functionalization of
EGO intended to make it more lyophilic in nature
[11,12]
. Chemical functionalization has the
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advantage of very good polymer layer adhesion and long term performance
compared to
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physical deposition by non-covalent interaction.
[13]
EGO in its native form has already been reported for use in metal ion sensors [14]. EGO
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which is surface modified with an amine is expected to show sensor properties that are different from those of pure EGO. A convenient way of achieving amine carrying EGO surfaces is to graft amine containing polymers, such as functionalized polyaniline, onto them. It has been reported in an earlier investigation that functionalized polyaniline possesses an improved metal [15]
. It is therefore likely that this property can be
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uptake capacity compared to its native form
effectively used in metal ion sensors consisting of an EGO substrate and surface grafted
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polyaniline. A distinct advantage of using EGO as the substrate for metal ion sensors is its surface functional groups which will provide extra metal chelation in addition to the polymer.
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In this investigation, the synthesis and characterization of EGO modified with amine functionalized polyaniline (EGAMPANI) have been performed, and its application in a multi-
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elemental electrochemical sensor for selected metal ions has been assessed. Initially, the EGO has been synthesized by following a modified procedure Hummers and Offeman
[17]
[16]
of that originally reported by
, and then the EGO surface has been enriched with diaminobenzene
(DAB) to obtain EGO-DAB. Subsequently, the amine functionalized polyaniline (AMPANI) has been grafted onto the EGO-DAB surface by in-situ chemical oxidative polymerization, yielding EGAMPANI as the reaction product. In each step of the synthesis, the structure of the intermediary product formed has been confirmed by using NMR or FT-IR and FT-Raman techniques.
The synthesized EGAMPANI has then been characterized with regard to its
compositional, thermal, structural and topographic properties by means of CHNO analysis and TGA, XRD, SEM and microanalytical techniques.
The differential pulse anodic stripping
voltammetry (DPASV) technique has been adopted for Pb2+, Cd2+ and Hg2+ metal ion sensing with an EGAMPANI modified carbon paste electrode (CPE).
3
ACCEPTED MANUSCRIPT 2.
Experimental
2.1.
Chemical syntheses and analytical methods The chemical synthesis work involved several parts, (i) the preparation of the amine
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functionalized aniline monomer starting from 2-hydroxyaniline, (ii) the preparation of exfoliated
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graphite oxide (EGO) from crushed commercial graphite, and (iii) the formation of the desired EGAMPANI by in-situ chemical oxidative graft polymerization of the AMPANI monomer onto the
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EGO surface with 1,4-diaminobenzene (p-phenylenediamine) (DAB) as the bridging ligand. Details of the individual synthesis protocols, the results from the NMR analysis of the intermediary products, and the suppliers of the chemicals used are provided in the Supporting Information.
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NMR spectra were recorded using a Bruker 400 MHz spectrometer. Molecular mass of compounds was determined with a Waters Q-TOF micro mass spectrometer.
For Fourier
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transform infrared (FT-IR) studies, samples were pressed into pellets using KBr and scanned in the range of 4000 to 400 cm-1 in a Perkin Elmer FT-IR spectrometer. FT-Raman spectra were
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acquired in a LabRAM HR spectrometer with Ar ion laser of wavelength 514 nm as source at magnification 100x and using a CCD camera ranging from 0 to 3000 cm -1 as detector. CHNO
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contents were determined using an EA Flash 1112 series instrument. Thermograms of the samples were recorded using a Netzsch STA 409 instrument at heating rate 2 °C min-1. X-ray diffraction (XRD) patterns of powdered samples were recorded with a PANalytical X-ray diffractometer ranging from 6 to 70° with Cu anode K-α radiation of wavelength 1.54 Å as source. For the morphological studies, an FEI Quanta 200 scanning electron microscope was used. Xray photoelectron studies (XPS) were carried out using a SPECS GmbH Phoibos 100 MCD Energy Analyzer with AlK radiation of energy 1486.6 eV. The electrochemical studies were carried out using a CHI 660D electrochemical workstation. 2.2.
Preparation of EGAMPANI modified carbon paste electrode (CPE) and differential pulse anodic stripping voltammetry (DPASV) test procedure A carbon past electrode (CPE) modified with EGAMPANI was prepared by mixing 5
wt% of EGAMPANI to a blend of 4:1 ratio of carbon black and paraffin oil. This paste was then filled into a glass tube of inner diameter 4 mm, into which a copper wire was inserted for electrical contact. All stripping voltammetric analyses were conducted in a three terminal cell, 4
ACCEPTED MANUSCRIPT which contained a CPE as the working electrode, a calomel electrode as the reference electrode, and a platinum wire of 1 mm diameter as the counter electrode. Individual solutions of Pb 2+, Cd2+ and Hg2+ ions as well as mixed equimolar solutions of these metal ions were used for analysis. All solutions were prepared from the respective nitrate salts, using an acetate buffer to
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maintain a constant pH of 5. Volumes of 10 ml were used in each experiment, and the solutions were purged with argon to remove dissolved oxygen prior to investigation. The deposition of
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metal onto the CPE surface was performed at a constant potential of -1.3 V for about 10 min,
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and the stripping of the deposit from the electrode surface was achieved by sweeping from -1.1 to +1.0 V with a scan rate of 25 mV s-1. The stripping analysis was carried out with step potential of 0.004 V, amplitude of 0.05 V, pulse period of 0.5 s, pulse width of 0.2 s and sampling width of 0.0167 s. A neat CPE was prepared and used under the same conditions in
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order to distinguish the response of the EGAMPANI modified CPE from that of the neat one. All electrochemical studies were carried out under nitrogen atmosphere. The limit of detection (LoD) has been determined according to its definition given in the literature
[18]
, using the
ED
relation LoD = LoB + 1.645 SD (low concentration sample), where LoB = Mean (blank) + 1.645 SD (blank) and
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SD is the standard deviation of three determinations.
Results and discussion
3.1.
Synthesis of EGAMPANI
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3.
The synthesis of the aniline monomer is depicted in Scheme 1. The synthesis was initiated from 2-hydroxyaniline, in which the amine functional group was protected by reacting it with acetic anhydride to form the corresponding acetanilide (1). This was followed by the reaction with epichlorohydrin in the presence of a weak base to produce compound (2). The epoxide ring introduced in step 2 was then reacted with dimethylamine so that compound (3) was formed.
Finally, the acetanilide moiety in compound (3) was deprotected to obtain the
monomer (4). This was later used for chemical oxidative grafting. The further reaction pathway to EGAMPANI is illustrated in Scheme 2. EGO prepared according to Hummers’ method was used as the starting material. As it is well known, EGO is a good adsorbing agent for organic molecules and macromolecules by π-π stack interactions
[19]
and, beyond this, the oxygen containing functional groups on the EGO surface enable grafting 5
ACCEPTED MANUSCRIPT polymers onto it through the formation of covalent bonds
[1,20,21]
. Taking advantage of both
these properties of EGO, the EGO surface has been enriched with DAB. Chemical linking was achieved by amide bond formation in the presence of DCC as the coupling agent and DMAP as a base. Grafting of AMPANI was then accomplished through reaction of EGO-DAB with
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monomer (4) by in-situ chemical oxidative polymerization using APS as an oxidizing agent. This produced the desired EGAMPANI. One of the advantages of using AMPANI in the
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synthesis of EGAMPANI is that it is highly water soluble at all pH, which facilitates washing
NH2 OH
NHCOCH3
(CH3CO) 2O
OH
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off any ungrafted excess AMPANI that may have formed during the in-situ polymerization.
Cl
O
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70 C, 3 h
(1)
ED
CH3
CH3
N
O
CH3
Reflux, 12 h
O
Reflux, 24 h
NHCOCH3
CH3
O
K2CO3, Dry THF
o
HN
NHCOCH3
(2) +
-
NH3 Cl
1:2 Con HCl/EtOH
O
Reflux 24 h
OH
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HO
(3)
+
NH Cl H3C CH3
-
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2-oxy (1-dimethylamine-2-hydroxy propyl) aniline hydrochloride Monomer (4)
Scheme 1: Synthesis of the monomeric unit 2-oxy (1-dimethylamine-2-hydroxypropyl) aniline hydrochloride (4) starting from 2-hydroxyaniline.
6
ACCEPTED MANUSCRIPT H2N O
HOOC
OH
H N
H2N
OH
C
O
NH2
O
OH
OH
DCC/DMAP O
H2N
O
NH
reflux H2 N
COOH
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2O 8
4) 2
CH3
N
+
HO
-
NH4 Cl
(N H
H3C
O
H3C
CH3
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OH O
N
CH3
NH
N
CH3
O
+
HO
H3C
NH O
NH2 H3C
N
CH3
HO
HO
N
NH Cl CH3
-
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HO
O
EGO-DAB
S
EGO
NH2
C
H3C
N
CH3
O
O H3C
N
H3C
NH
O
NH
O
HO
CH3 H3C
H N
H2N C
N
NH
OH
OH
O
O
NH
O NH2
HO
H N
O
HN O N NH
N
OH
CH3
N
EGAMPANI
CH3
ED
H3C
CH3
N H3C
H3C
O
HO
OH
OH
O N
CH3
H2N
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HO
H3C
N
CH3
O
H3C
O
N
HN
Scheme 2: Synthesis of EGO-DAB and in-situ chemical oxidative polymerization grafting
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of monomer (4) onto EGO-DAB to form EGAMPANI. (The tetramer shown is a
3.2.
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representative of the type of polymer formed.)
Characterization of EGAMPANI To study the type of bonding of the AMPANI to the EGO surface, the synthesized
EGAMPANI and its intermediate compounds were analyzed using FT-IR spectroscopy. The spectra recorded for EGO, EGO-DAB and EGAMPANI are compiled in Fig. 1.
7
ACCEPTED MANUSCRIPT A 1234
B
3456
2857
1632 1503 1709 1654
1064
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2926 3453
C
832
2994
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Transmittance
1457
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1733 1622
1710
3444 3003
4000
3500
3000
1509 1578
2500
2000
1500
1000
500
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-1 Wavenumber (cm )
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Fig. 1: FT-IR spectra of a) EGO b) EGO-DAB and c) EGAMPANI.
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The spectrum of EGO (Fig. 1a) shows major bands at 1064, 1234, 1457, 1622 and 1733 cm-1, which correspond respectively to the vibrational modes of C–O stretching, C–OH
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stretching of phenolic group, C=C phenyl ring stretching, and carbonyl group stretching at the edge of layer planes or in conjugate functional groups. The intense band observed at 3456 cm -1 is due to O–H stretching vibration of graphite oxide. These results confirm that EGO has a large number of oxygen containing functional groups on its surface. At both 2926 and 2857 cm-1 the C–H stretching vibrations of substituted aromatic compounds of EGO are observed, which is in line with earlier findings
[3]
. After the reaction of EGO with DAB to EGO-DAB,
the carboxylic stretching frequency previously observed at 1733 cm -1 for EGO is shifted to 1709 cm-1 (Fig. 1b), indicating the formation of an amide bond. The amide bond is further confirmed by the stretching vibrations observed at 1654 and 1632 cm-1 and the bending vibration at 1503 cm-1. The presence of the phenylene moiety is confirmed by the new intense band observed at 2994 cm-1, which is due to the C–H stretching vibration of phenylic group from the DAB. The formation of EGAMPANI by AMPANI grafting onto the EGO-DAB surface is confirmed by the formation of new characteristic intense bands corresponding to the polyaniline backbone (Fig. 1c). The stretching vibrations observed at 1578, 1509 and 832 cm -1 are respectively due to the quininoid, benzenoid and 1,4-disubstituted phenylic stretching 8
[22]
of
ACCEPTED MANUSCRIPT the polyaniline backbone present in the AMPANI. Furthermore, the increased intensity of the C–H stretching frequency at 3003 cm-1 is due to the phenylic group present at the AMPANI backbone, and thus attests to the AMPANI grafting onto the EGO-DAB.
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Further evidence for the AMPANI grafting onto the EGO-DAB is obtained from the CHNO elemental analysis. The elemental compositions of EGO, EGO-DAB and EGAMPANI,
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expressed in atomic percentages, are as shown in Table 1.
Table 1: Elemental compositions of EGO, EGO-DAB and EGAMPANI. C (%)
EGO
41.6
EGO-DAB
44.7
EGAMPANI
52.0
H (%)
N (%)
O (%)**
C/N
2.6
1.0*
54.8
41.6
2.8
4.6
47.9
9.72
3.8
5.5
38.7
9.45
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Compound
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* The small content of nitrogen is either from the concentrated acid used for exfoliation or from impurities. ** The oxygen content is calculated by subtracting the other elemental percentages from 100%.
The significant increase in the content of nitrogen in EGO-DAB compared to EGO is due to DAB absorption on the EGO surface by π-π stack interaction as well as amide bond formation. A further additional increase in the content of nitrogen in EGAMPANI suggests that the AMPANI polymer has been successfully grafted onto the EGO-DAB surface. As has been shown in a recent study
[23]
, XPS analysis is a suitable technique for the analysis of a
polyaniline backbone on an EGO substrate. However, in the present study, attempts to obtain further information on the structure of the EGAMPANI by means of XPS analysis were not successful as only a faint peak for nitrogen was obtained. This observation can be explained by combining the CHNO results of EGO-DAB and EGAMPANI with the following assumptions. Firstly, the adsorption of AMPANI onto the EGO-DAB surface is unfavorable as both AMPANI and EGO-DAB carry free amino groups and hence acquire a positive charge in acidic 9
ACCEPTED MANUSCRIPT media and tend to repel each other electrostatically. This will favor the formation of chemically grafted AMPANI rather than physically adsorbed AMPANI.
Secondly, the high water
solubility of AMPANI facilitates the complete removal of any ungrafted AMPANI that may still be present immediately after the chemical polymerization step. This will leave behind only
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the chemically grafted AMPANI. Finally, the AMPANI formed will only have a low degree of polymerization due to its bulky ortho substituent. Altogether, the above will lead to a rather
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low amount of additional nitrogen on the EGAMPANI surface compared to that on the EGO-
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DAB.
FT-Raman spectra can provide additional evidence for the structural modification of the EGO as a result of its interaction with the AMPANI grafted onto it. It is presumed that this interaction will introduce changes in the vibrational properties of the EGO carbon skeleton.
D
G
Intensity
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Fig. 2 presents the Raman spectra obtained for EGO, EGO-DAB and EGAMPANI.
EGO
EGO-DAB
EGAMPANI
1000
1200
1400
1600
1800
-1 Raman shift (cm )
Fig. 2: FT-Raman spectra of EGO, EGO-DAB and EGAMPANI.
The Raman spectrum of EGO exhibits two distinct peaks at 1590 and 1350 cm -1, which are attributed to the tangential mode (G-band) and disorder mode (D-band) respectively. These
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ACCEPTED MANUSCRIPT two peaks are correlated with the ordered and disordered skeletal structures of the graphite layers present in the EGO. The ratios of the G-band to D-band intensities (IG/ID) measured for EGO, EGO-DAB and EGAMPANI are shown in Table 2. It is evident that the intensity ratio decreases successively as the EGO changes to EGO-DAB and then further to EGAMPANI.
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This is indicative of the formation of new chemical bonds impacting on the EGO carbon skeletal vibration frequencies. These interactions provide evidence in support of the formation
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of an amide bond as well as the grafting of AMPANI onto the EGO surface.
Table 2: G-band to D-band intensity ratio (IG/ID) of EGO, EGO-DAB and EGAMPANI
Material
IG/ID
EGO
0.996
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calculated from Raman spectra.
0.920
EGAMPANI
0.894
The
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EGO-DAB
thermal
properties
of
the
compounds
synthesized
were
assessed
using
thermogravimetric analysis carried out under inert atmosphere. Fig. 3 displays the thermograms of EGO, EGO-DAB and EGAMPANI.
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100
EGO-DAB
70
EGAMPANI
50
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60
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Mass (%)
80
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90
EGO
40 200
400
600
800
1000
Temperature ( C)
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Fig. 3: Thermograms of EGO, EGO-DAB and EGAMPANI recorded under inert atmosphere.
All materials were analyzed in the heating range from 30 to 1000 °C by increasing the
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temperature at 2 °C min-1, and the onset temperatures of decomposition were determined from the corresponding derivative curves. The EGO exhibits a weight loss of 26% up to around 100
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°C which is caused by the release of physically absorbed water. An additional weight loss of 19% is observed at around 190 °C which is mainly due to the pyrolysis of the labile oxygen [24]
containing functional groups, such as –OH and –COOH, yielding CO, CO2 and H2O
.
Thereafter the EGO residue formed remains stable up to around 870 °C. Both the EGO-DAB and the EGAMPANI only exhibit a comparatively small weight loss up to around 120 °C caused by the liberation of physically bound water. Beyond that, both the EGO-DAB and the EGAMPANI undergo a slower and more gradual weight loss than the EGO, which points to their improved thermal stability due to the presence of the relatively stable amide bond
[25]
. In
case of the EGO-DAB this gradual weight loss extends up to around 700 °C, whereafter it accelerates because of the breakage of the amide bond and the loss of the DAB entity
[25]
. In
case of the EGAMPANI, there is a similar gradual weight loss up to around 840 °C, whereafter the curve shows a downward deflection due to the decomposition of the AMPANI side chains. Overall, the EGO-DAB displays a larger weight loss up to 1000 °C than the EGAMPANI, because the decomposition of the DAB entity leads to some volatile entities while the surface
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ACCEPTED MANUSCRIPT grafted polyaniline leaves behind a mainly nonvolatile carbonaceous residue
[26]
. The trend of
decomposition for the AMPANI in the EGAMPANI is similar to that of polyaniline reported earlier
[27]
. It should also be noted that the comparatively low plateau for the EGO at high
temperatures is simply a consequence of its high initial water content and is not an indication of
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lower thermal stability.
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X-ray powder diffraction was used to investigate the crystallinity of the pristine graphite and the compounds synthesized from it. Fig. 4 displays the XRD patterns of pristine graphite,
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Graphite
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Intensity
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EGO and EGAMPANI.
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EGO
EGAMPANI 20
30
40
50
60
70
2 Theta
Fig. 4: X-Ray diffraction patterns of graphite, EGO and EGAMPANI.
For pristine graphite the characteristic peak corresponding to the (002) plane appears at 2θ = 26.4°
[5]
along with other less intense peaks at higher angles. In the case of EGO the
characteristic peak is observed at 2θ = 11.5°. This indicates that the graphite oxide layers stack with a larger interlayer separation due to steric hindrance
[3]
, which may readily be attributed to
the presence of oxygen containing functional groups within the material. The interlayer spacing of the EGO is calculated to be around 0.8 nm, which is significantly larger than that of the
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ACCEPTED MANUSCRIPT pristine graphite of 0.338 nm. The grafting of AMPANI onto the EGO surfaces results in the disappearance of the sharp peak observed at 11.5° for EGO, which demonstrates that the grafting reaction was successful and led to an AMPANI assisted expansion of the EGO layers [5]
. The AMPANI intervening between the EGO layers will bring out a steric effect which
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explains the lack of the hitherto orderly arrangement of the graphite oxide layers in the stacking direction. A scanning electron microscopic image of an EGAMPANI film prepared from a
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dispersion of EGAMPANI in deionized water is shown in Fig. 5. This puts weightage to the
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same observation as there is a clearly visible disorder in the morphological arrangements.
Fig. 5: Scanning electron micrograph of EGAMPANI film.
3.3.
Differential pulse anodic stripping voltammetric (DPASV) studies of EGAMPANI modified CPE: sensitivity and cross-sensitivity for metal ions The application of EGAMPANI as an active material for metal ion sensing was carried
out using differential pulse anodic stripping voltammetric (DPASV) analysis. A CPE modified with 5 wt% of EGAMPANI has been used to investigate the sensor ability for Pb 2+, Cd2+ and Hg2+, and the results were compared to those of a neat CPE.
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ACCEPTED MANUSCRIPT Fig. 6 shows the anodic stripping currents recorded for individual metal ion solutions using both the EGAMPANI modified CPE and the neat CPE. It is seen that the anodic stripping current for Pb2+ ions on the EGAMPANI modified CPE is significantly increased in comparison to that of the neat CPE. The improvement in stripping current for Cd 2+ ions is rather marginal
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and the observed current remains significantly below that of Pb 2+. The stripping current for Hg2+ ions using the EGAMPANI modified CPE is found to be lower than that of the neat CPE.
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These results imply that the EGAMPANI modified CPE shows a higher sensitivity for Pb2+
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compared to the other two metals. The large stripping current for the Pb 2+ ion can be attributed to the high binding affinity of the EGAMPANI to the Pb 2+ ion, which enhances its surface concentration on the electrode during the deposition step prior to the stripping process. The marginal increase of stripping current for the Cd 2+ ion is likely to be caused in a similar way but
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to a lesser extent. In contrast, the affinity of the EGAMPANI to the Hg 2+ ion is lower than that of the neat CPE. Overall, the results show that EGAMPANI has a strong influence on the sensitivity of metal ion sensing by means of CPEs and that it is most appropriate for Pb2+
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analysis.
CPE EGAMPANI
-4
Anodic stripping current (A)
1.6x10
-4
1.2x10
-5
8.0x10
-5
4.0x10
0.0 Pb
Hg
Cd
Metal ions
Fig. 6: Differential pulse anodic stripping currents of EGAMPANI modified CPE and neat CPE in individual metal ion solutions of 1×10-5 M concentration.
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The assessment of the EGAMPANI modified CPE as a multi-elemental metal ion sensor has been carried out in mixed equimolar metal ion solutions in order to facilitate selectivity
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studies. The voltammograms recorded with the EGAMPANI modified CPE and the neat CPE when using these multi-elemental solutions are compiled in Fig. 7a. The voltammogram of the
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EGAMPANI modified CPE exhibits three characteristic oxidation peaks, corresponding to Cd2+, Pb2+ and Hg2+ at -0.73, -0.51 and +0.34 V respectively. In contrast, the voltammogram of
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the neat CPE displays only two peaks, corresponding to Pb 2+ and Hg2+ at -0.58 and +0.27 V respectively. The observed slight shifts in the reduction potentials of Pb 2+ and Hg2+ are caused by differences in the interaction between the respective ions and electrode surfaces.
The
disappearance of the Cd2+ signal on the neat CPE indicates the virtual absence of such an
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interaction in the multi-elemental solution, which is in agreement with the observations obtained for the neat CPE using the individual solutions (Fig. 6).
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It is furthermore evident from Fig. 7a that the stripping currents for Pb 2+ and Cd2+ ions in the multi-elemental solution are significantly improved for the EGAMPANI modified CPE
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compared to those of the neat CPE in the same solution.
This is primarily because the
EGAMPANI facilitates the accumulation of lead and cadmium onto the electrode surface
sensing.
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through the functional groups on its surface, which is a requirement for multi-elemental In contrast, the deposition of pure mercury onto the surface of the EGAMPANI
modified CPE is dramatically reduced with regard to that of the neat CPE in the same solution. These effects are in agreement with the observations made for the individual metal ion solutions (Fig. 6).
Together, Fig. 7a and Fig. 6 reveal the influence of other metal ions on the stripping current of the Pb2+ ion. For the EGAMPANI modified CPE, the Pb2+ stripping current in the multi-elemental solution (Fig. 7a) shows a moderate decrease compared to that in the individual metal ion solution (Fig. 6). This indicates an interference of other metal ions on the Pb 2+ sensing with an EGAMPANI based electrode.
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ACCEPTED MANUSCRIPT -4
1.6x10
A
CPE EGAMPANI
-4
-5
4.0x10
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Pb
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Hg
Cd
-5
8.0x10
Pb
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Current (A)
1.2x10
Hg
0.0 -1.5
-1.0
-0.5
0.0
0.5
1.0
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Potential (V)
-4
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1.2x10 1.0x10
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a
a
-5
6.0x10
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Current (A)
8.0x10
Pb
Cd
-5
Current (A)
B
-4
Hg Pb Cd
1E-5
1E-6
a
-5
4.0x10
Hg
1E-5 1E-6 1E-7 Concentration (M) a) 0.5E-4 b) 1.0E-5 c) 0.5E-5
-5
d) 1.0E-6
2.0x10
0.0
e
-1.0
f
e) 0.5E-6
c
-0.5
0.0
f) 1.0E-7
0.5
1.0
1.5
Potential (V)
Fig. 7: DPASV of A) EGAMPANI modified CPE and neat CPE, recorded using mixed equimolar solutions of Pb2+, Cd2+ and Hg2+ ions with concentrations of 1×10-5 M for each ion, and B) EGAMPANI modified CPE recorded using mixed equimolar solutions of these metal ions with concentrations ranging from 5.0×10-5 to 1.0×10-7 M for each ion. The inset graph shows the cross-sensitivity of each metal ion in equimolar solution.
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ACCEPTED MANUSCRIPT Fig. 7b compiles the variations of the anodic stripping currents obtained with EGAMPANI modified CPEs for mixed equimolar metal ion solutions with concentrations ranging from 5×10-5 to 1×10-7 M. It is evident that the electrode sensitivity of the EGAMPANI modified CPE is a function of the metal ion concentration and that the relative intensity of each 6
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peak is characteristic of the metal ion. The limit of detection is 2.2×10 -6 M for Hg2+ ion, 1.2×10M for Cd2+ ion, and 9.8×10-7 M for Pb2+ ion. The inset graph shows the cross-sensitivity for
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each metal ion. It is observed that Pb2+ ion sensing becomes less affected by cross-sensitivity
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when the concentration of the multi-elemental solution decreases. This improvement in crosssensitivity at lower concentrations of other metal ions holds promise for the utilization of the EGAMPANI modified CPE as a selective and sensitive Pb 2+ ion sensor in environments with
Conclusions
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4.
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small concentrations of interfering metal ions.
An amine functionalized polyaniline derivative (AMPANI) has been grafted onto the
denoted as EGAMPANI.
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surface of exfoliated graphite oxide (EGO) to synthesize a polymer/graphite hybrid material The EGAMPANI was characterized by FT-IR and FT-Raman
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spectroscopy, CHNO analysis, XPS, TGA, XRD and SEM to assess its structure, composition, thermal stability, crystallinity and morphology. In this study, the polyaniline/EGO hybrid material has been for the first time applied for metal ion sensor tests in aqueous media. In these, the response of an EGAMPANI modified carbon paste electrode has been assessed for Pb 2+, Cd2+ and Hg2+ metal ion sensing in an acetate buffer by stripping voltammetric techniques. Limits of detection of respectively 2.2×10-6 M, 1.2×10-6 M, and 9.8×10-7 M have been achieved under appropriate conditions for Hg 2+, Cd2+ and Pb2+. This feasibility study has thus indicated that the functionalized electrode has the potential to be employed as a multi-elemental sensor and that it may be of particular use for the sensing of Pb2+ ions in aqueous media with very low metal ion concentration. It is clear that supplementary studies will be required to gain a better understanding of the cross-sensitivities observed and to optimize the application of any sensor incorporating this material.
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ACCEPTED MANUSCRIPT Acknowledgements The authors express their gratitude to the Indo-UK UKIERI program for the award of a research project to carry out this investigation (SA08-005). The NMR Research Centre, Indian
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Institute of Science, Bangalore, is gratefully acknowledged for the permission to record the NMR spectra. One of the authors (AJ) thanks the Council of Scientific and Industrial Research,
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opportunity to visit the University of Cambridge, UK.
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New Delhi, for the grant of a senior research fellowship and the UKIERI program for the
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Highlights
An amine functionalized polyaniline has been grafted to exfoliated graphite oxide.
Synthesized EGAMPANI is characterised using FTIR, TGA, XRD, Raman and elemental
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analysis.
A DPASV based multi-elemental sensor is prepared using EGAMPANI modified CPE.
Analysis was carried out to study the electrochemical detection of Pb2+, Cd2+ and Hg2+.
The LoD observed is 2.2×10-6 M for Hg2+ ion, 1.2×10-6 M for Cd2+ ion, and 9.8×10-7 M for
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Pb2+ ion.
22