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Sensors and Materials, Vol. 35, No. 9 (2023) 3201–3209
MYU Tokyo
S & M 3385

Amino-functionalized Cup-stacked Carbon Nanofibers
as Electrode Materials for Suppressing Protein Fouling
Yuka Ashitani,1 Kazutake Takada,2 and Kikuo Komori1,3*
1Graduate School of System Engineering, Kindai University,
Takaya-Umenobe, Higashi-Hiroshima, Hiroshima 739-2116, Japan
2Graduate School of Engineering, Nagoya Institute of Technology,
Gokiso, Showa, Nagoya 466-8555, Japan
3Department of Biotechnology and Chemistry, Kindai University,
Takaya-Umenobe, Higashi-Hiroshima, Hiroshima 739-2116, Japan(Received July 13, 2023; accepted September 22, 2023)

Keywords: carbon nanomaterials, graphene edges, electrochemical sensing, electron transfer rate
constant

Amino-functionalized cup-stacked carbon nanofibers (CSCNFs), the surface of which
provides highly ordered graphene edges and amino groups, were investigated as electrodematerials by using typical redox species in electrochemistry, namely, [Fe(CN)6]3–/4– and Fe2+/3+.
The electron transfer rate of [Fe(CN)6]3–/4– was accelerated and that of Fe2+/3+ was decelerated
by amino groups, mainly owing to the electrostatic interaction and repulsion, respectively. The
hydrophilic amino-functionalized CSCNF surface also allowed the suppression of protein
adsorption. Thus, the amino-functionalized CSCNFs would be useful electrode materials forbiosensing and other electrochemical devices.

1.

Introduction

Carbon nanomaterials including single-walled carbon nanotubes, graphene flakes, andgraphite nanofibers are one of the outstanding and promising electrode materials for batteries,
fuel cells, and electrochemical sensors because of their large specific surface area and highelectrical conductivity. In particular, graphene edges are known to often exhibit uniqueelectronic, chemical, and magnetic properties.(1) In the electrochemistry field, the edgeorientated pyrolytic graphite surface is widely known to be more electroactive than the basal
plane pyrolytic graphite surface.(2) This phenomenon is also considered to be akin to stacked
graphene nanofibers, such as platelet carbon nanofibers, herringbone carbon nanofibers, andcup-stacked carbon nanofibers (CSCNFs), which provide a high density of exposed and reactive
graphene edges at fiber surfaces.(3,4) Hence, the stacked graphene nanofibers are suitable forconstructing highly electroactive three-dimensional electrode networks.
Recently, we have reported that the CSCNFs worked as an electric nanowire and an enzyme
support for electrochemical biosensors.(5,6) In addition, oxygenated CSCNFs, at the surface of*Corresponding author: e-mail: komori@hiro.kindai.ac.jp

https://doi.org/10.18494/SAM4577

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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which negatively charged oxygen-containing functional groups are introduced by oxygenation,
allowed the tuning of sensitivities and selectivities to redox species.(7,8) If positively charged
functional groups, such as amino groups, are introduced to the CSCNF surface, the sensitivities
and selectivities to redox species should also be tunable and different from those for theoxygenated CSCNF electrode. However, the electrochemical properties of amino-functionalized
CSCNFs (NH2-CSCNFs) have not yet been well investigated to the best of our knowledge.
To functionalize the surface of the carbon electrode, plasma treatment methods areoften employed, but special apparatuses are used for the synthesis of nitrogen-dopedcarbon.(9,10) We previously treated the surfaces of graphite nanofibers using a general air
plasma apparatus. Although air plasma is associated with oxygen and nitrogen gases,
oxygen atoms were almost doped to the graphite nanofiber surface (11) probably owing to
the reaction with oxygen or ambient moisture. On the other hand, amino-functional
molecules, such as amino silane coupling agents, have been widely used for the chemical
introduction of the functional group on the surfaces of oxygenated carbon electrodes
including diamond electrodes without any special apparatuses.(12,13) Since the CSCNF
surface is known to be easily oxidized,(5,7) we herein report the electrochemical properties
of CSCNFs chemically functionalized with amino groups using 3-aminopropyltriethoxysilane(APTES). We investigated electrochemical responses for the obtained NH2-CSCNF electrode to
two redox species, potassium ferricyanide (K3[Fe(CN)6]) and iron(II) sulfate (FeSO4), whichhave been widely used for the characterization of the carbon electrode in the electrochemistry
field. We also examined the prevention of protein adsorption at the NH2-CSCNF surface for its
application to bio-electroanalytical chemistry.

2.

Experimental Procedure

2.1

Preparation of NH2-CSCNF electrodes

A fluorine-doped tin-oxide (FTO, 10 Ω/□)-coated glass plate (Nippon Sheet Glass Co., Ltd.,ø ~ 0.4 cm) was treated with a 1.0 M sodium hydroxide aqueous solution for 4 h to provide anegatively charged surface, followed by further treatment with a 2.5% acetic acid aqueous
solution containing 2.5% APTES (Tokyo Chem. Ind. Co., Inc., Japan) for 4 h. After drying in anelectric oven at 80 ℃ for 1 h, an aliquot of N,N-dimethylformamide (DMF) dispersion of
CSCNF (Vision Development Co., Ltd., Japan) at 1.0 mg mL –1 was cast onto the APTESmodified FTO glass plate (ca. 40 µg cm–2). Afterwards, the electrode was dried at room
temperature and then stored in the electric oven at 80 ℃ for 15 min to completely remove DMF.
The surface of the obtained CSCNF electrode was further treated for 1 min with a vacuum
oxygen plasma apparatus YHS-G200 (SAKIGAKE-Semiconductor Co., Ltd., Japan) at theoxygen flow ratio of 70 NL min–1 to introduce oxygen-containing functional groups. To obtainthe NH2-CSCNF electrode, the oxygen plasma-treated CSCNF (O2-CSCNF) electrode was thenimmersed in the 2.5% acetic acid aqueous solution containing 2.5% APTES for 12 h, followedby drying in the electric oven at 80 ℃ for 1 h.

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2.2

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Evaluation of NH2-CSCNF electrodes

The physical properties of bare CSCNFs, O2-CSCNFs, and NH2-CSCNFs at the FTO surface
were evaluated by scanning electron microscopy (SEM, S-4800, Hitachi High-Tech Co., Japan),
X-ray photoelectron spectroscopy (XPS, PHI5000, ULVAC-PHI, Inc.), and Raman spectroscopy(NRS-3300, JASCO Corp., Japan). The water contact angles of those CSCNF electrodes were
measured using an optical microscope equipped for contact angle measurements.
Electrochemical measurements were performed using a potentiostat SP150 (Bio-Logic
Science Instruments Ltd.). A three-electrode system was employed in the present work. Preparedelectrodes were working electrodes. A Ag|AgCl|KCl (sat., RE-1CP, BAS, Japan) and a coiled
platinum wire were the reference and counter electrodes, respectively. The electrochemical
properties of the obtained electrodes were evaluated in a Briton Robinson buffer (BRB)
containing 1 mM K3[Fe(CN)6] in the presence and absence of 100 mg mL –1 bovine serum
albumin (BSA, Fujifilm Wako Pure Chem. Ind., Corp., Japan) and 0.1 M sulfuric acid (H2SO4)
solution containing 1 mM FeSO4 by cyclic voltammetry. The BRB was prepared by mixing
phosphoric acid (40 mM), acetic acid (40 mM), and boric acid (40 mM) with sodium hydroxide
to reach the desired pH 7.4.

3.

Results and Discussion

3.1

Characterization of NH2-CSCNF electrodes

We first examined the physical properties of bare CSCNF, O2-CSCNF, and NH2-CSCNF
electrodes using SEM. Figure 1(a) shows the SEM images of the bare CSCNFs, O2-CSCNFs, andNH2-CSCNFs at the FTO glass plate. Those three types of CSCNF seemed to be similar without
any significant destruction and conformational changes even after the irradiation of oxygen
plasma and the modification of APTES. Additionally, the diameter of those CSCNFs wasdetermined to be about 50 nm. However, the water contact angle observed at the NH2-CSCNF
surface (ca. 23.4°) was clearly smaller than that at the bare CSCNF surface (ca. 82.2°) andslightly larger than that at the O2-CSCNF surface (ca. 12.0°), as shown in Fig. 1(b). The oxygen
plasma treatment has been known to produce carbonyl groups mainly on both basal and edge
sites of graphite.(14) Therefore, hydrophilicity at the CSCNF surface after oxygen plasma
treatment increased. Note that the water contact angle was almost the same even though the
CSCNF surface was treated with oxygen plasma for 2 min. On the other hand, the pKa value forthe amino group in APTES is known to be 9.6(15) and smaller than that for the carbonyl group(ca. 24–29),(16) namely, protonation might easily take place at the O2-CSCNF surface incomparison with the NH2-CSCNF surface. Thus, the pKa value likely contributed to thedifference in water contact angle between NH2-CSCNFs and O2-CSCNFs. In addition, as
distilled water generally shows a near-neutral pH, the NH2-CSCNF surface should positively becharged. To verify this, the atomic oxygen-to-carbon (O/C) and nitrogen-to-carbon (N/C) ratios
for the surfaces of the CSCNF samples were examined by XPS. The O/C ratio increased from
7% for bare CSCNF to 17% for O2-CSCNF after the oxygen plasma treatment and then

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Sensors and Materials, Vol. 35, No. 9 (2023)

(a)

(b)

(c)

Fig. 1. (Color online) (a) SEM images of bare CSCNF, O2-CSCNF, and NH2-CSCNF electrodes. (b) Micrographsof 3 µL of water droplets on bare CSCNF, O2-CSCNF, and NH2-CSCNF electrode surfaces. (c) Raman spectra ofbare CSCNF and O2-CSCNF.

decreased to 14% for NH2-CSCNF probably owing to the modification of APTES. This result
may be reasonable, but almost no difference was observed for the N/C ratio among the threesamples (~3%). The reason for this is unclear. Only a small amount of amino groups might beintroduced to the CSCNF surface.
We also verified the structure of CSCNFs before and after the irradiation of oxygen plasma
by Raman spectroscopy. As shown in Fig. 1(c), two strong Raman bands were observed at ~1350
and ~1580 cm–1 typically called the D- and G-bands, respectively. It has been known that the
D-band corresponds to carbon atoms adjacent to a defect or a graphene edge, whereas the
G-band is associated with a well-ordered graphitic framework.(17) A shoulder peak called D’band, which arises from the edge plane of graphite,(18) also appeared at ~1615 cm–1. This result
certainly indicates that CSCNFs possess edge sites of graphene. According to the previous
report, the relative intensity ratio of the D- and G-band peaks (ID/IG ratio) is often employed forthe characterization of sp2 carbon nanomaterials to estimate defects in their graphite structure.(19)On the basis of Fig. 1(c), the ID/IG ratio for O2-CSCNF was determined to be about 1.14, which
was slightly smaller than that for bare CSCNF (ca. 1.20). This result might suggest that peroxides
or linkages forming between graphene sheets cause the carbon ends to become capped with thegraphene sheets.(11) Thus, no significant severe damage was observed, even if the CSCNF
surface was treated with oxygen plasma in this work. The ID/IG ratio for NH2-CSCNF was also
determined to be about 1.32, which was slightly larger than those for bare CSCNF (ca. 1.20) andO2-CSCNF (ca. 1.14) owing to the presence of APTES. This result agreed well with the previous
report.(20)Sensors and Materials, Vol. 35, No. 9 (2023)

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Electrochemical behavior of redox species

We next examined the electrochemical behavior of the two selected redox species. One is[Fe(CN)6]3–/4–, the redox behavior of which has been extensively studied in the electrochemistry
field and widely used for the characterization of electrodes.(21) The other is Fe2+/3+, the electrontransfer kinetics of which has been known to be strongly sensitive to functional groups at the
carbon surface.(22)Figures 2(a) and 2(b) show cyclic voltammograms (CVs) in an air-saturated BRB containing 1
mM [Fe(CN)6]3–/4– at the scan rate of 750 mV s–1 and an air-saturated 0.1 M H2SO4 aqueous
solution containing 1 mM FeSO4 at the scan rate of 300 mV s–1. For [Fe(CN)6]3–/4–, there seemeda small difference in peak-to-peak potential separation (∆EP) among the bare CSCNF (ca. 86.3mV), O2-CSCNF (ca. 92.2 mV), and NH2-CSCNF electrodes (ca. 86.2 mV). In addition, bothanodic and cathodic peak currents for the three electrodes increased linearly with the square rootof the scan rate below at least 1.0 V s–1, indicating that the redox reaction is the diffusioncontrolled process [Fig. 2(c)]. In this context, the apparent electroactive surface area in contact
with the electrolyte solution was roughly estimated to be about 0.11, 0.12, and 0.12 cm2 for thebare CSCNF, O2-CSCNF, and NH2-CSCNF on the FTO surface with a diameter of 0.4 cm,
respectively, on the basis of the following Randles–Sevicik equation:

(a)

(b)

(c)

Fig. 2. (Color online) CVs of (a) 1 mM [Fe(CN)6]3–/4– in the BRB and (b) 1 mM Fe2+/3+ in the 0.1 M H2SO4
aqueous solution on bare CSCNF, O2-CSCNF, and NH2-CSCNF electrodes. Scan rates are (a) 750 and (b) 300
mV s –1. (c) Dependences of anodic and cathodic peak currents on the square root of the scan rate at bare CSCNF, O2CSCNF, and NH2-CSCNF electrodes in BRB containing 1 mM [Fe(CN)6]3–/4–.

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Sensors and Materials, Vol. 35, No. 9 (2023)=
I P 2.69 × 105 AD1/2 n3/2ν 1/2C ,(1)

where IP is the peak current for cyclic voltammetry, A is the electroactive surface area, D is the
diffusion coefficient of [Fe(CN)6]3–/4– as the redox probe to be 7.4 × 10 –6 cm2 s–1,(23) n is thenumber of electrons participating in the redox reaction (1 in the present case), ν is the scan rate,
and C corresponds to the concentration of the redox probe. As mentioned above, the obtained
apparent electroactive surface areas of the three electrodes were almost the same. The fast
kinetics obtained here may therefore be strongly attributed to the edge plane sites, which may
facilitate the electron transfer, but not the functional groups. In contrast, for Fe2+/3+, the ∆EPvalue for NH2-CSCNF (ca. 373 mV) was clearly larger than those for bare CSCNF (ca. 235 mV)
and O2-CSCNF (ca. 208 mV). This reflects that the electron transfer kinetics of Fe2+/3+ isstrongly sensitive to the carbon surface, as described above.
On the basis of the cyclic voltammetry measurements, we calculated an apparent
heterogenous electron transfer rate constant k0app at the projected surface area. For this, weemployed the method developed by Nicholson [Eq. (2)],(24) which is widely used for thedetermination of k 0app based on a quasi-reversible electrochemical reaction controlled bydiffusion.
 πDnν F  RT 

0ψ = kapp


−1/2

(2)

Here, ψ is the kinetic parameter, F is the Faraday constant, R is the gas constant, and T is thetemperature. The ψ value is easily calculated using Eq. (3) proposed by Lavagnini et al.,(25)where the ∆EP value obtained at each scan rate is assigned.
=ψ( 0.6288 + 0.0021∆EP ) / (1 − 0.017∆EP )(3)

A plot of the ψ values against (πDnνF/ RT)−1/2 should therefore give a straight line of slope
k0app. The k0app value was calculated to be about 3.3 × 10 –2, 2.8 × 10 –2, and 3.8 × 10 –2 cm s–1 forthe bare CSCNF, O2-CSCNF, and NH2-CSCNF electrodes, respectively. Compared with the k0app
value for bare CSCNF, that for NH2-CSCNF was slightly larger, whereas that for O2-CSCNF
was slightly smaller. This result suggests that cationic amino groups positively and anionic
oxygen-containing functional groups negatively contribute to the electron transfer rate of[Fe(CN)6]3–/4–, which is the anionic molecule, owing to the electrostatic interaction andrepulsion, respectively. This point may be reasonable because the electron transfer of[Fe(CN)6]3–/4– is known to be decelerated at oxygenated edge-oriented pyrolytic graphite,(26)oxygen-terminated diamond,(27) and graphene oxide electrodes,(28) whereas it is accelerated at
the APTES-modified multiwalled carbon nanotube(12) and APTES-modified diamond.(13)Similarly, the electron transfer of [Fe(CN)6]3–/4– is also known to be accelerated at nitrogendoped carbon film electrodes, which consist of sp2- and sp3-bonded carbons with a low amountSensors and Materials, Vol. 35, No. 9 (2023)

3207

of nitrogen-containing graphite-like and pyridine-like bonding structures.(29,30) Therefore,amino groups at the carbon surface contribute to the electron transfer kinetics for redox species.
However, from the results obtained here, the electron transfer kinetics for [Fe(CN)6]3–/4– in thepresent work is controlled by mainly graphene edges and partially functional groups at the
CSCNF surface, such as carbonyl and amino groups.
In the case of Fe2+/3+, when the diffusion coefficient D = 5.1 × 10 –6 cm2 s–1,(31) the k0app value
was determined to be about 1.3 × 10 –3 for bare CSCNF, 2.0 × 10 –3 for O2-CSCNF, and
6.0 × 10 –4 cm s–1 for NH2-CSCNF. As expected, the opposite property to [Fe(CN)6]3–/4– wasobserved here, namely, the largest k0app value was O2-CSCNF, followed in order by bare CSCNF
and NH2-CSCNF. Therefore, cationic amino groups negatively and anionic oxygen-containing
functional groups positively contribute to the electron transfer rate of Fe2+/3+, which is known tousually undergo inner-sphere redox reactions.(21,22) Thus, the oxygen plasma-treated and aminofunctionalized CSCNF surface would allow the selectivity of electrochemical reactions to be
tuned.
3.3

Electrochemical behavior of [Fe(CN)6]3–/4– in the presence of protein

Considering the application to bioanalytical sensors, anti-protein fouling at the electrodesurface is one of the crucial issues to maintain the performance of electrodes, such as sensitivity.
Niwa and his co-workers have reported that the adsorption of a protein such as BSA is suppressedat electron cyclotron resonance (ECR)-sputtered nanocarbon film electrodes, the surface of
which is treated with water vapor or ammonia gas plasma.(32) The decrease in current ratio for[Fe(CN)6]3–/4– in the presence of 100 mg mL–1 BSA was within 30% in comparison with that in
the absence of BSA.(10) We therefore examined the electrochemical behavior of [Fe(CN)6]3–/4– at
the O2-CSCNF and NH2-CSCNF electrodes in the BRB containing 100 mg mL –1 BSA by cyclic
voltammetry. At the NH 2-CSCNF electrode, the anodic and cathodic peak currents of[Fe(CN)6]3–/4– in the presence of 100 mg mL –1 BSA decreased about 22%, compared with thosein the absence of BSA at the scan rate of 100 mV s–1. In addition, the ∆EP value for the former
was about 70.9 mV, which was slightly larger than that for the latter (ca. 65.4 mV). Unfortunately,
the NH2-CSCNF electrode surface might be unable to completely eliminate the effect of protein
fouling. However, this value was similar to the previous report.(10) We also found that the ∆EPvalue gradually increased as the BSA concentration increased probably owing to the effect of
BSA in the electrolyte solution [Fig. 3(a)]. Additionally, ∆EP increased with the scan rate. On thebasis of the CVs, similarly, the k0app value for [Fe(CN)6]3–/4– in the presence of 100 mg mL –1
BSA (k0app,BSA) was determined to be about 1.5 × 10 –2 for bare CSCNF, 1.8 × 10 –2 for O2-CSCNF,
and 2.8 × 10 –2 cm s–1 for NH2-CSCNF. Note that although the viscosity of the electrolyte likely
increased owing to the presence of BSA, such an effect was ignored in the calculation here. The
obtained values were slightly smaller than those in the absence of BSA. Since k0app,BSA depends
on ∆EP, as described above, such a decrease in k0app,BSA would be based on the BSA adsorption
partially.
We also evaluated the apparent heterogenous electron transfer rate constant ratio, k 0app,BSA/k0app
As shown in Fig. 3(b), the k0app,BSA/k0app value for the bare glassy carbon (GC) electrode, which is3208

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(a)

(b)

Fig. 3. (Color online) (a) ∆EP values of 1 mM [Fe(CN)6]3–/4– at the NH2-CSCNF electrode in BRB at the scan rateof 500 mV s –1. (b) Comparison of k 0app,BSA/k 0app ratio for [Fe(CN)6]3–/4– in the presence and absence of 100 mg mL –1
BSA at GC, bare CSCNF, O2-CSCNF, and NH2-CSCNF electrodes.

widely used in the electrochemistry field, was about 0.16 owing to significant BSA adsorption.
Such a blocking effect agreed well with the previous report.(33) The present O2-CSCNF and
NH2-CSCNF electrodes cannot eliminate the BSA adsorption completely, but the k0app,BSA/k0app
values for those electrodes were much larger than that for the GC electrode. This result isreasonable because a highly hydrophilic surface at the electrode surface has been known tosuppress protein fouling.(33) Incidentally, k0app,BSA/k0app was slightly larger for NH2-CSCNF thanfor O2-CSCNF. As the isoelectric point for BSA is known to be 4.9, BSA should be negatively
charged in the BRB (pH 7.4). If this holds, the BSA adsorption should easily take place at thepositively charged NH2-CSCNF surface owing to the electrostatic interaction rather than thenegatively charged O2-CSCNF surface. The factors for significant anti-BSA fouling might be
not only the hydrophilicity and surface charge but also the surface roughness and morphology.
This point is still unclear and one of the research topics in the future.

4.

Conclusions

The electrochemical properties of NH2-CSCNFs were investigated to explore theirapplicability as electrode materials. The kinetics of [Fe(CN)6]3–/4– and Fe2+/3+ is tunablewith the NH2-CSCNF surface. In addition, protein adsorption was suppressed owing to thehydrophilic and amino-functionalized surface. These factors are therefore subject tooptimization in different applications, such as biosensing and other electrochemical devices.

Acknowledgments
We are very grateful to Prof. Kohei Shiraishi and Mr. Seiryu Hirao (Kindai University) for
their help with the water contact angle measurement. This work was supported by Furukawa
Technology Foundation.

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