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The 23rd PPC Symposium on Petroleum, Petrochemicals, and Polymers and The 8th Research Symposium on
Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 1
Electrospinning Carbonized Hybrid Metal Nano-fiber Composite for Electrochemical
Applications in Biosensor Teeraseth Ariyathanakul
a, Pongpol Ekabutr
a, Sudkate Chaiyo
b, Orawon Chailapakul
b, Pitt Supaphol
a
a The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand
b Electrochemistry and Optical Spectroscopy Research Unit, Department of Chemistry, Faculty of Science,
Chulalongkorn University, Bangkok, Thailand
ABSTRACT
Dopamine (DA) is an essential neurotransmitter that controls the mammalian central
nervous system, therefore irregular DA levels can cause such various diseases as Parkinson’s
disease, Alzheimer’s disease, and mental illness. Screen-printed carbon electrode (SPCE), a type
of electrochemical technique, has been used to detect DA because possesses high sensitivity, low
cost, and rapid detection rate. Modification surface of SPCE at working electrode is necessary to
enhance charge transfer properties for extending limit of detection by using conductive material.
In this study, the conductive material, that is used to modify electrode surface, is carbon
nanofibers and its composites incorporating with metal nanoparticles. Poly(vinyl pyrrolidone),
PVP, is electro-spun and used as a precursor to turn into carbon nanofibers and metal
nanoparticles that are used to enhance electrochemical properties of carbon nanofibers are Co.
Additionally, ruthenium (Ru) and zinc (Zn) are incorporated to study electrochemical behavior
as well. The electrochemical behavior was examined by Cyclic Voltammetry (CV) and
Differential Pulse Voltammetry (DPV). Scanning electron microscopy (SEM), Transmission
electron microscopy (TEM), and X-ray diffraction (XRD) were used to characterize the
morphology and physical properties of carbonized metal nanofiber composites.
Keywords: Poly(vinyl pyrrolidone), Cobalt, Ruthenium, Zinc, Carbon Nanofibers, Electrochemical
Sensors, Dopamine Corresponding author e-mail pitt.s@chula.ac.th
INTRODUCTION
Nanotechnology is interesting and tremendously studied in nowadays, which everything
can turn into tiny thing but enormous capability. Nanofibers, as the productivity of
nanotechnology, are useful and developed in many applications, in addition they can encapsulate
small particles in themselves to enhance ability in specific applications. One of the most
interesting techniques which can fabricate nanofibers is electrospinning technique because of
easy fabricating with various polymers and their composites, providing alignment and
controllable diameter of fibers, and low cost. The electrospinning came from electro-static
spinning that means utilizing electrostatic force to produce ultra-fine fibers from polymer
solution or molten polymer, therefore produced fibers have a tiny diameter (nanometer to
micrometer) and a large surface area comparing with conventional spinning processes
(Bhardwaj, et al., 2010).
Carbon nanofibers (CNFs) are widely used in many applications such as reinforcement
materials, rechargeable batteries, templates for nanotubes, high-temperature filters, supports for
high-temperature catalysis, nanoelectronics, and supercapacitors because they have distinctive
properties including high aspect ratio, large specific surface area, high-temperature resistance
and good electrical/thermal conductivities. A simple and inexpensive electrospinning process has
been developed and reported by Cooley in 1902 as the optimum process for the preparation of
The 23rd PPC Symposium on Petroleum, Petrochemicals, and Polymers and The 8th Research Symposium on
Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 2
continuous and uniform carbon nanofibers (Liu, et al., 2009). Preparation of CNFs can achieved
by transformation of precursor polymers through involving two main steps; stabilization and
carbonization (Liu, et al., 2009). Poly(vinyl pyrrolidone), PVP, as a thermoplastic polymer
which can soluble in various solvents including environmentally friendly solvents as water and
ethanol, is able to form nanofibers by electrospinning and retain fibrous structure through three-
step heat treatment process consisting of stabilization, pre-oxidization, and carbonization (Wang,
et al., 2012). Among metallic materials, transition metals are noticeable due to their high
electrical conductivity and rapid redox reactions. Cobalt (Co) is an interesting non-noble metal
and used as electrocatalyst in water splitting process which involves electrochemical oxidation of
O2-
to produce oxygen and proton reduction to generate hydrogen, but it is not much studied in
the field of electrochemical sensors, including ruthenium (Ru) and zinc (Zn) as well (Wu, et al.,
2015).
Screen-printed carbon electrode (SPCE) is attractive and widely used in recent years
because they possess good electrochemical performance (broad potential window, decreased
residual current, and better reproducibility of currents), corrosion resistance in various
electrolytes, suitability for various sensing and detection applications, low cost, and as a
disposable electrode (Fanjul-Bolado, et al., 2008). A number of researches have been developed
SPCE to enhance charge transfer properties, extent limiting of detection, and specify to that
analytes by modified surface of SPCE at working electrode.
In this work, a screen-printed carbon electrode was used as electrochemical sensor and
modified on working electrode by various types of carbonized metal nanofiber composites. The
carbonized metal nanofiber composites was prepared from poly(vinyl pyrrolidone) (PVP) and
various types of metal chloride solution by electrospinning and carbonization steps. The aim was
to study morphology and physical properties of various types of carbonized metal nanofiber
composites and their electrochemical behavior when they were used to modify on working
electrode surface of screen-printed carbon electrode.
EXPERIMENTAL
Materials
All chemicals were analytical grade and used without further purification. Poly(vinyl
pyrrolidone) (PVP; average MW 360,000), cobalt(II) chloride hexahydrate, ruthenium(III)
chloride hydrate, zinc chloride, ethanol, potassium ferricyanide (K3[Fe(CN)6]), potassium
ferrocyanide (K4[Fe(CN)6]), polyphosphate-buffered saline (PBS), dopamine hydrochloride, L-
ascorbic acid, and uric acid were purchased from Sigma-Aldrich. The electrospinning apparatus
was a Gamma High Voltage Research Model D-ES30PN/M692 equipped with a DC power
source. The electrochemical measurements were determined by using a potentiostat from
PalmSens at ambient temperature. CV and DPV were operated by using a three electrode system,
which consisted of a working electrode (WE), with a working area of 0.126 cm2; an Ag/AgCl
electrode as the reference electrode; and a carbon plate as the counter electrode.
Preparation of Carbonized Nanofiber Composites The PVP concentration used for electropinning were prepared by dissolving PVP
concentration of 8 %w/v (PVP8) in ethanol 10 mL. Subsequently, cobalt(II) chloride
concentration of 10 %w relative to PVP (0.336 mmol/10 mL) was mixed with PVP solution,
followed by vigorously stirring at ambient temperature for 12 hr. Ruthenium(III) chloride and
zinc chloride were mixed with PVP solution at the same components as well.
The 23rd PPC Symposium on Petroleum, Petrochemicals, and Polymers and The 8th Research Symposium on
Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 3
The electrospinning apparatus was set to fabricate nanofibers. A 5 mL syringe with a
needle tip (D=0.5 mm) was placed and clamped near an anode connected to a high-voltage
power supply. The cathode was connected to aluminum foil, which was controlled to an applied
voltage of 14 kV relative to the anode. The distance between the electrode and needle tip was 20
cm. After preparation of electro-spun nanofibers under ambient condition, the electro-spun
samples were stabilized in oven under air atmosphere at 150 oC for 24 hr, and then stabilized
samples were pre-oxidized in a tube furnace under air atmosphere at 360 oC for 4 hr, and finally
carbonized in a nitrogen atmosphere at a ramp rate 5 oC/min up to 800
oC, held for 4 hr.
Preparation of Modified Electrodes
SPCE modified with carbon nanofiber composites were prepared by drop casting 1 µL of
the carbon nanofiber composites at various types of metal on the carbon layer of the WE. After
drying in an oven at 80 oC for 30 min, the modified electrodes were rinsed with ultra-pure water.
Unmodified electrodes were labeled as unmod, and the modified electrodes treated with bare
carbon nanofibers were labeled as PVP8 followed by the various types of metal for instance
PVP8Co0.336, PVP8Ru0.336, and PVP8Zn0.336. All modified electrodes were stored in a
desiccator at room temperature prior to use.
Physical Properties of Carbonized Nanofibers
The surface morphologies of samples before and after carbonization were observed using
a Hitachi S-4800 field-emission scanning electron microscope (FE-SEM) operated at 15 kV.
SemAphore 4.0 Software was used for image processing. The fiber also observed under Philips,
TECNAI 20 transmission electron microscope (TEM) operated at 120 kV. X-ray diffraction
(XRD) patterns were collected using an X-ray diffractometer (Rigaku Smartlab, Japan) based on
Cu-Kα radiation. The 2Ɵ angle of the diffractometer was stepped from 10o to 100
o and the
crystallite sizes were determined using MDI JADE 6 software, relating to the Scherrer formula,
with a residual error of less than 10%.
CV of Modified Electrodes
Electrochemical responses of both modified and unmodified electrodes were
characterized using single-loop CV experiments, comprising a forward sweep in the anodic
direction from -0.5 V vs. Ag/AgCl up to 0.5 V, followed by a sweep back down to -0.5 V.
Voltammograms were measured for 0.05 M PBS at a pH of 7.4 a number of scan rates to
establish background currents. To evaluate electrochemistry, the current response of 1 mM
[Fe(CN)6]3-/4-
in 0.05 M PBS at a pH of 7.4 was evaluated at a number of scan rates, and
corrected by subtracting out the corresponding background currents.
Differential Pulse Voltammetry (DPV) of the Modified Electrodes to dopamine detection
The optimum parameters for the DPV measurements were determined in preliminary
studies (data not shown) to be a pulse amplitude of 150 mV, step potential of 5 mV, and a pulse
period of 200 ms for scanning the potential between -0.5 and 0.5 V vs. Ag/AgCl. This optimized
potential control scheme was used in further studies with 0.05 M PBS at a pH of 7.4.
RESULTS AND DISCUSSION
Changing of electro-spun nanofibers to carbonaceous nanofibers
The as-spun PVP8 nanofibers were transformed into carbon nanofibers through three
steps including stabilization, pre-oxidization, and carbonization process (Wang, et al., 2012).
The morphology and changing in chemical structure of each step were traced by SEM and FT-
IR, respectively. Figure 1 revealed fibrous morphology of each step and these fibers were
The 23rd PPC Symposium on Petroleum, Petrochemicals, and Polymers and The 8th Research Symposium on
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measured and informed in Figure 2, therefore carbon nanofibers could be achieved by these
steps. The fiber diameter was reduced after carbonization step from about 274 nm to 183 nm.
FIGURE 1 SEM images of (a) as-spun PVP8, (b) stabilization, (c) pre-oxidization, and (d)
carbonization and TEM image of (e) carbonized PVP8.
(a) (b)
(c) (d)
(e)
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FIGURE 2 Comparison of fiber diameter before and after carbonization steps, according to
SEM images.
In Figure 3 showed FT-IR spectra of each step comparing with as-spun PVP8 and found
that PVP8 did not change in chemical structure at stabilization step, but changing in molecular
structure arose at pre-oxidization step, where it was confirmed by absence of ‒CH2 stretching
(2950 cm-1
) and C‒N stretching (1290 cm-1
), resulting from the destroyed side chain (Wang, et
al., 2012). Additionally, the remaining band at 1720 cm-1
corresponds to the C=O stretching,
which main chain was oxidized during pre-oxidization step (Wang, et al., 2012). Eventually,
chemical structure dramatically changed in carbonization step, observed by disappearing of
characteristic peak of PVP.
Figure 3 FT-IR spectra of as-spun PVP8 and each step of carbonization.
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Incorporating with metal to form carbonized metal nanofiber composites
The PVP8 nanofibers incorporating with various types of metal at 0.336 mmol/10 mL
polymer solution to form metal nanofiber composites were further studied. As the SEM results,
Figure 4(a-1), 4(b-1), and 4(c-1) revealed fibrous morphology of various types of metal
nanofiber composites before carbonization process, therefore electrospinning technique could be
exploited to prepare these nanofiber composites successfully. Then fiber diameter was measured,
in Figure 5 showed that uncarbonized fiber diameter of each type of incorporated metal was
bigger than bare PVP8 because viscosity of polymer solution was increased when metal chloride
was added, since viscosity provides the charged jet to totally withstand the coulombic stretching
force resulting in the observed larger diameter of the electro-spun fibers (Sutasinpromprae, et al.,
2006).
After that, carbonization steps, in following study of PVP8 carbon nanofibers, were used
to produce carbonized metal nanofiber composites and the average diameter of PVP8Co0.336,
PVP8Ru0.336, and PVP8Zn0.336, reduced after through carbonization steps, was about 203 nm,
127 nm, and 454 nm, respectively, as shown in Figure 5.
The TEM images revealed surface morphology of carbonized nanofibers (PVP8) and
various types of carbonized metal nanofiber composites (PVP8Co0.336, PVP8Ru0.336, and
PVP8Zn0.336). Figure 1(e) and 4(c-3) showed smooth surface and did not find any black spots
on carbonized PVP8 nanofibers and PVP8Zn0.336, respectively, but it was found on surface of
PVP8Co0.336, as shown in Figure 4(a-3). Additionally, TEM image of PVP8Ru0.336 (Figure
4(b-3)) showed that many black spots were jointed each other to form something like a coral.
The XRD could characterize and reveal various black spots what they were.
(a-1) (b-1) (c-1)
(a-2) (b-2) (c-2)
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FIGURE 4 SEM images of uncarbonized (a-1) PVP8Co0.336; (b-1) PVP8Ru0.336; (c-1)
PVP8Zn0.336 and carbonized (a-2) PVP8Co0.336; (b-2) PVP8Ru0.336; (c-2) PVP8Zn0.336,
and TEM images of carbonized (a-3) PVP8Co0.336; (b-3) PVP8Ru0.336; (c-3) PVP8Zn0.336.
FIGURE 5 Comparison of fiber diameter before and after carbonization of PVP8 nanofibers and
their composites, according to SEM images.
According to Figure 6, all XRD patterns showed broad diffraction peak at 2Ɵ values of
20-30o, associating with the disorders carbon phase, so PVP8 nanofibers and their composites
could be turned into carbon nanofibers successfully (Kim, et al., 2014). Figure 6(b) showed
diffraction peaks of carbonized PVP8Co0.336 at 2Ɵ values of 44.18o, 51.48
o, 75.82
o, and 92.16
o
corresponding to (111), (200), (220), and (311) crystal planes that indicated formation of face
centered cubic crystalline cobalt metal (JCPDS card no 15–0806). XRD patterns of carbonized
PVP8Ru0.336 showed diffraction peak at 2Ɵ values of 38.32o, 42.11
o, 43.95
o, 58.27
o, 69.35
o,
78.32o, 82.14
o, 84.62
o, 85.88
o, 92.03
o, and 97.02
o corresponding to (100), (002), (101) (102),
(110), (103), (200), (112), (201), (004), and (202) crystal planes that indicated formation of
hexagonal crystalline ruthenium metal (JCPDS card no. 65–7646), whereas XRD patterns of
carbonized PVP8Zn0.336 (Figure 6(d)) did not show any diffraction peaks of zinc metal or zinc
oxide. Therefore, arising of black spots on surface of PVP8Co0.336 and black spots segment of
PVP8Ru0.336 were cobalt metal nanoparticles and ruthenium metal nanoparticles, respectively
that were affirmed by XRD. Furthermore, the MDI JADE software was used to calculate the
crystallite size of cobalt and ruthenium nanoparticles, according to the Scherrer formula. The
crystallite size of cobalt and ruthenium nanoparticles were listed in Table 1.
(a-3) (b-3) (c-3)
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The Scherrer formula was used to measure the crystallite size of each metal nanoparticle;
Where d is the average crystallite size, λ is the wavelength of the X-ray, β is the full width at half
maximum intensity of Bragg diffraction peak at diffraction angle θ (in radians)
(Pithakratanayothin, et al., 2017).
FIGURE 6 XRD patterns of carbonized (a) PVP8, (b) PVP8Co0.336, (c) PVP8Ru0.336, and (d)
PVP8Zn0.336.
(a)
(c) (d)
(b)
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TABLE 1 Fiber diameter, crystallite size, and electrochemical data.
* CV with a scan rate of 100 mV/s in 0.05 M PBS at pH 7.4 (n=3).
** DPV in 0.05 M PBS at pH 7.4 (n=3).
a Data were obtained from SEM images.
b Data were obtained from XRD by using MDI JADE 6 software with residual error is less than 10 %.
Electrochemical Behavior of Modified Electrodes
After preparation of carbonized PVP8 nanofibers and PVP8Co0.336, PVP8Ru0.336, and
PVP8Zn0.336 as the carbonized metal nanofiber composites successfully, they were used as
modifier on surface of working electrode of screen-printed carbon electrodes (labeled as PVP8,
PVP8Co0.336, PVP8Ru0.336, and PVP8Zn0.336, respectively), and their electrochemical
behavior was monitored by CV using 1 mM [Fe(CN)6]3-/4-
as a model redox couple (Ekabutr, et
al., 2014). Figure 7 showed the oxidation potential of each electrode that was modified and
unmodified (labeled as unmod) and found that incorporating of various types of metal caused
shifting of the oxidation potential to lower values and increasing of anodic current response.
According to Table 1, the measurement of anodic current (Ipa), oxidation potential (Epa), Ipa/Ipc,
and peak potential separation (∆Ep=Epa-Epc) calculated from the CV curves. The unmod showed
slow electron transfer kinetics for the redox reaction of [Fe(CN)6]3-/4-
due to a low anodic current
density and broad peak potential separation, whereas electrodes that were modified by
PVP8Co0.336, PVP8Ru0.336, and PCP8Zn0.336 exhibited higher current density, narrower
peak potential separations, and Ipa/Ipc ~ 1, as the results of the enhancement of electron transfer
kinetics and excellent electrochemical sensitivity of these modified electrodes surface (Ekabutr,
et al., 2014).
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FIGURE 7 CV images of unmodified/modified electrodes with a scan rate of 100 mV/s in 0.05
M PBS at a pH of 7.4 in the presence of 1 mM [Fe(CN)6]3-/4-
.
Determination of Dopamine at Various Modified Electrodes
The differential pulse voltammogram (DPV) profiles of dopamine detection on surface of
various modified electrodes were shown in Figure 8 and the measured anodic current density and
oxidation potential values were listed in Table 1. In this study, dopamine was detected in PBS
solution at a pH of 7.4. As the results, the incorporating of various types of metal affected on
increasing of anodic current density with the lowest oxidation potential comparing with PVP8
and unmod electrodes. The higher anodic current density and negatively shifted oxidation
potential indicated that these modified electrodes could enhance the oxidation of dopamine
effectively. In addition, Figure 9(a), 9(b), 9(c), and 9(d) showed typical DPV trace profiles for
the individual curve of dopamine, uric acid, ascorbic acid, and mixtures of these compounds at
unmod, PVP8Co0.336, PVP8Ru0.336, and PVP8Zn0.336, respectively, in PBS at a pH of 7.4
(simulated human bodily fluid). In human serum, the typical levels of ascorbic acid and uric acid
are 80 and 400 μM, respectively (Chauhan, et al., 2011). The unmod electrode in this study
(Figure 9(a)) were unable to distinguish mixtures of these compounds obviously. Interestingly,
PVP8Ru0.336 and PVP8Co0.336 electrodes were able to distinguish the dopamine peak from
uric acid peak evidently, and ascorbic acid peak disappeared. Therefore, existence of metal
nanoparticles at the surface of carbonized metal nanofiber composites attributed to enhance
electrocatalytic activity toward the oxidation of dopamine (Hsu, et al., 2010).
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FIGURE 8 DPV profiles of unmodified and modified electrodes in 0.05 M PBS at a pH of 7.4 in
the presence of 40 μM dopamine.
FIGURE 9 DPV profiles of (a) unmodified electrode and modified electrodes by (b)
PVP8Co0.336, (c) PVP8Ru0.336, and (d) PVP8Zn0.336 in 0.05 M PBS at a pH of 7.4 in the
presence of 40 μM dopamine, 80 μM ascorbic acid, and 400 μM uric acid.
(a) (b)
(c) (d)
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CONCLUSIONS
A PVP solution in ethanol at concentration of 8 %w/v was fabricated to nanofibers by
using electrospinning technique, after that they were turned into carbon nanofibers successfully
by following three steps carbonization. Preparation of carbonized metal nanofiber composites
were achieved through electrospinning and carbonization steps as well. TEM images and XRD
patterns revealed existence of cobalt nanoparticles and ruthenium nanoparticles and crystallite
size was also measured, whereas zinc metal was absent. In the part of electrochemical behavior,
the working electrode surface of screen-printed carbon electrode was modified by drop casting of
PVP8Co0.336, PVP8Ru0.336, and PVP8Zn0.336 and results showed high current density, low
ΔEp, and Ipa/Ipc ~ 1 that indicated the faster electron transfer kinetics and excellent
electrochemical sensitivity of these modified electrodes. Eventually, modified electrodes by
cobalt and ruthenium metal nanofiber composites exhibited clearly distinguish dopamine peak
from uric acid peak due to existence of metal nanoparticles at the surface of carbonized metal
nanofiber composites. Therefore, carbonized metal nanofiber composites can act as a conductive
material that is easy to produce and use, but powerfulness in electrochemical sensor applications.
ACKNOWLEDGEMENTS
This work was supported in part by (1) Rachadapisek Sompote Fund for Postdoctoral
Fellowship, Chulalongkorn University, and (2) Research Pyramid, Ratchadaphiseksomphot
Endowment Fund (GCURP_58_02_63_01) of Chulalongkorn University.
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