electrospinning carbonized hybrid metal nano-fiber ... · pdf fileapplications in biosensor...

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

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 [email protected]

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



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 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



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)



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.



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)



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)



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)



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).



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).



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)



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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