DETECTION OF REDOX PROPERTIES OF (6,5)-ENRICHED SINGLE-WALLED CARBON NANOTUBES USING POTASSIUM PERMANGANATE (KMNO4)

C Journal of
 Carbon Research

Article
Detection of Redox Properties of (6,5)-Enriched
Single-Walled Carbon Nanotubes Using Potassium
Permanganate (KMnO4)
Yuji Matsukawa * and Kazuo Umemura
 Department of Physics, Graduate School of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku,
 Tokyo 1628601, Japan; meicun2006@163.com
 * Correspondence: 2214101@alumni.tus.ac.jp; Tel.: +81-3-5228-8228
 



 Received: 2 April 2020; Accepted: 7 May 2020; Published: 11 May 2020 


 Abstract: It has been reported that even if single-walled carbon nanotubes (SWNTs) are coated
 with the same polymer, the redox characteristics change of each chirality may differ. Particularly,
 the addition of hydrogen peroxide (H2 O2 ) minimally affects the near-infrared (NIR) absorption spectra
 of the dsDNA-(6,5)-enriched SWNT complex (DNA-SWNT complex). Detecting the redox properties
 of (6,5) chirality using NIR absorption spectra has been one of the issues to be solved. We hypothesized
 that an oxidizing agent with high oxidizing power is required to detect the absorption spectra of
 (6,5) chirality. In this study, we used KMnO4 , which contains atoms with a high oxidation number.
 A dispersion was prepared by mixing 0.5 mg of (6,5)-enriched SWNT powder with 1 mg/mL of DNA
 solution. After adding H2 O2 or KMnO4 to this dispersion and oxidizing it, catechin solutions were
 added to reduce the dispersion. The absorption peak of the DNA-SWNT complex decreased by 23.9%
 following the addition of KMnO4 (final concentration: 0.5 µM) and recovered 30.7% following the
 addition of the catechin solution. We revealed that the changes in the absorption spectra change of
 (6,5) chirality, which could not be detected by H2 O2 , can be detected by using KMnO4 . We also varied
 the concentration of KMnO4 and verified whether the adsorption of KMnO4 can be modeled as a
 Langmuir adsorption isotherm.

 Keywords: carbon nanotube; DNA; potassium permanganate; near-infrared; redox

1. Introduction
 Single-walled nanotubes (SWNTs) can be synthesized by using different methods. Common
methods include the chemical vapor deposition (CVD) method, in which a carbon-based gas is blown
into a heating furnace with a carrier gas to synthesize SWNTs [1–4]; the arc discharge method, in which
DC or AC is applied to graphite electrodes to grow SWNTs [5,6]; and the laser evaporation method,
in which graphite is used as a carbon source to facilitate heating and evaporation by laser irradiation
and obtain SWNTs [7]. In a recent study, Haque et al. reported that CVD-synthesized multi-walled
carbon nanotubes were converted to diamonds, without the addition of a catalyst, following application
of a nanosecond pulsed laser melting process at ambient temperature and air pressure [8]. Additionally,
Narayan et al. realized the epitaxial growth of diamond nanofibers by applying a nanosecond laser
melting technique to supercooled carbon nanofibers and nanotubes, followed by rapid cooling [9].
 The SWNTs produced as a result of employing these methods have a cylindrical one-dimensional
structure rolled with graphene sheets, and have excellent electrical characteristics. SWNTs have
optical characteristics that are strongly dependent on how the graphene sheet is wound, which can be
represented by the chiral vector (n, m) [10–14]. It is very important to have a deep understanding of
the electrical properties of each chirality from the fundamental and applied perspectives of carbon

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C 2020, 6, 30 2 of 9

nanotubes. To date, many studies on redox characteristics focusing on the optical response of carbon
nanotubes have been reported [15–19]. However, because the optical response of (6,5) chirality to
H2 O2 is weak, detecting the redox properties in the near-infrared (NIR) absorption spectra is one of the
issues to be solved.
 Polo et al. investigated the effects of reducing and oxidizing small molecules on the NIR
fluorescence of polymer-wrapped SWNTs. They reported that the (6,5) chirality of SWNTs decreased
by 81% from the initial state as a result of adding the oxidizing agent riboflavin; it also increased by
141% from the initial state as a result of adding the reducing agent ascorbic acid. They found that the
fluorescence of reductive molecules of compounds such as ascorbic acid, epinephrine, and Trolox only
significantly increases when SWNTs are suspended in a negatively charged polymer such as DNA
or polyacrylic acid [20,21]. However, their study focused on the photoluminescence (PL) intensity of
SWNTs, and did not include an investigation into the characteristics of the absorption spectra.
 Zheng et al. found that electron transfer occurs readily between small molecule redox reagents
and semiconducting carbon nanotubes. Furthermore, a direct correlation between the bandgap of
semiconductor nanotubes and their reduction potential was identified. In their study, they showed
that (6,5)-enriched SWNTs can be easily oxidized by adding potassium (IV) chloroiridate (K2 IrCl6 ) as
an oxidant; they found that changes in concentration affected the absorption spectra [22]. However,
they did not report on the KMnO4 -induced redox action of SWNTs.
 Our group has previously investigated the (9,4) and (8,7) chirality of SWNTs; we consequently
determined the redox characteristics of SWNTs by applying NIR and PL in order to reveal the optical
response of SWNTs to H2 O2 and catechin [23,24]. In these papers, we reported that it was difficult to
determine the extent of oxidation because the optical response was weak, even when H2 O2 was added
to the DNA-(6,5)-enriched SWNT complex coated with DNA.
 In this study, we focused on the oxidation numbers of the atoms contained in the oxidizing agent
and compared the oxidation numbers of the atoms contained in H2 O2 and KMnO4 . The oxidation
number of the oxygen (O2) in H2 O2 is −1, and the oxidation number of the manganese (Mn) in
KMnO4 is +7. The difference is clear; therefore, we used KMnO4 , which contains atoms with a high
oxidation number. KMnO4 (oxidant) and catechin (reductant) solutions were sequentially added to the
DNA-SWNT complex, and the NIR and PL spectra were measured. The results revealed a clear optical
response that can be attributed to redox of the (6,5) chirality. Additionally, we show that the oxidation
state of the (6,5) chirality can be determined by using KMnO4 and analyzing the absorption spectra.

2. Materials and Methods
 For this study, (6,5)-enriched SWNT powder (No. 773735-250G), which was manufactured by
using CoMoCAT synthesis method, and dsDNA (deoxyribonucleic acid sodium salt from salmon
testes, D1626) were purchased from Sigma-Aldrich Co., LLC (St. Louis, MO, USA). Hydrogen peroxide
(abt. 30%, 084-07441) and catechin (553-74471) were purchased from Wako Pure Chemical Industries,
Ltd. (Osaka City, Osaka, Japan). Potassium permanganate solution (No. 42000375) was obtained from
Hayashi Pure Chemical Ind., Ltd. (Osaka City, Osaka, Japan).
 A 1 mg/mL dsDNA solution was prepared with a 10 mM tris(hydroxymethyl)aminomethane-
HCl (Tris-HCL) buffer (pH 7.9). To untangle the dsDNA molecules, the solution was sonicated, on ice,
in a bath-type ultrasonicator (80 W) for 90 min. Finally, the dsDNA solution was gently shaken for 3 h.
To prepare the dsDNA-(6,5)-enriched SWNT complex, 0.5 mg of SWNT powder and 1 mL of the dsDNA
stock solution were mixed and sonicated on ice for 1.5 h using a probe-type sonicator (3W, VCX130,
Sonic & Materials, Inc., Newtown, CT, USA). The supernatant of the prepared dsDNA-(6,5)-enriched
SWNT dispersion was stored after undergoing 17,360 g centrifugation at a temperature of 8 ◦ C for a
period of 3 h [25–27].
 A UV–vis spectrophotometer (V-630, JASCO Corp., Hachioji City, Tokyo, Japan) was employed
for NIR measurements. NIR measurement entailed mixing the DNA-SWNT complex and 440 µL of
Tris-HCL buffer in a cuvette and recording the initial absorbance spectra. Subsequently, an oxidant

CC 2020,
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 6, x30FOR PEER REVIEW 33of
 of99

(final concentration: 9.8 mM) diluted with sterilized water was added to the sample, and the
(H2 O2 or KMnO
absorbance 4 ) was
 spectra added
 were separately
 measured to the sample.
 following 30 min In
 of the case of Hat
 incubation 2 O24
 2 addition, 5 µL
 °C. In the of H
 case of2 O 2 (final
 KMnO 4
 concentration: 9.8 mM) diluted with sterilized water was added to the sample, and the
addition, 5 µL of KMnO4 (final concentration: 0.5 µM) diluted with sterilized water was added to the absorbance
 spectra were measured following 30 were
 min ofmeasured
 incubation at 24 ◦
sample, and the absorbance spectra after 10 C.
 minInof
 the case of KMnO
 incubation addition,
 at 244 °C. Finally,5 µL
 5 µLof
 KMnO (final
of catechin
 4 concentration: 0.5 µM) diluted with sterilized water was added to the sample,
 solution (final concentration: 6.2 mM) was added to the sample and the absorbance spectra and the
 absorbance
were spectra were measured after 10 min of incubation at 24 ◦ C. Finally, 5 µL of catechin solution
 measured.
(final concentration: 6.2 mM) was added to the sample and the absorbance spectra were measured.
3. Results
 3. Results
 Figure
 Figure 11 shows
 shows aa conceptual
 conceptual diagram
 diagram of
 of the
 the experiment.
 experiment. The
 The DNA-SWNT
 DNA-SWNT complexes
 complexes were
 were
oxidized with H
oxidized with H2 O2 or KMnO4 , and catechin was added to detect changes in the NIR-ABSspectra.
 2 O2 or KMnO 4 , and catechin was added to detect changes in the NIR-ABS spectra.

 1.AAconceptual
 Figure 1.
 Figure conceptualdiagram
 diagram of
 of the
 the experiment.
 experiment. The
 The final
 final concentrations
 concentrations of
 of H O22 and
 H22O and KMnO
 KMnO44 were
 were
 9.8 mM and 0.50 µM, respectively, for NIR-ABS. The final catechin concentration was 6.2 mM.
 9.8 mM and 0.50 µM, respectively, for NIR-ABS. The final catechin concentration was 6.2 mM. SWNT: SWNT:
 single-walled carbon
 single-walled carbon nanotube;
 nanotube; NIR:
 NIR: near-infrared;
 near-infrared; ABS:
 ABS: Absorbance.
 Absorbance.

 Figure 2 shows the absorption spectra that were obtained following H2 O2 addition to the
 Figure 2 shows the absorption spectra that were obtained following H2O2 addition to the DNA-
 DNA-SWNT complex and subsequent addition of an aqueous catechin solution. The absorbance in the
SWNT complex and subsequent addition of an aqueous catechin solution. The absorbance in the
 initial state was 0.466, and the absorption peak appeared at 992 nm. Thirty minutes after the addition
initial state was 0.466, and the absorption peak appeared at 992 nm. Thirty minutes after the addition
 of H O , the absorbance was 0.465, which corresponds to a decrease of only 0.13% from the initial
of H22O22, the absorbance was 0.465, which corresponds to a decrease of only 0.13% from the initial
 state. Additionally, 10 min after the addition of the catechin solution, the absorbance was 0.462, which
state. Additionally, 10 min after the addition of the catechin solution, the absorbance was 0.462, which
 corresponds to a decrease of 0.73%. This was 99.1% of the initial state. The absorption peaks at the
corresponds to a decrease of 0.73%. This was 99.1% of the initial state. The absorption peaks at the
 times of H O addition and catechin addition were both 992 nm, and no peak shift was observed.
times of H22O22addition and catechin addition were both 992 nm, and no peak shift was observed.
These results are similar to those reported in previous studies [24].
These results are similar to those reported in previous studies [24].
 Because an H2 O2 concentration of 9.8 mM did not significantly change the absorbance at oxidation,
 the concentration of H2 O2 was increased by 10 times (final concentration: 98 mM) to determine whether
 changes in the absorption spectra could be observed. The results are available as Supplemental Materials
 Figure S1. There was no significant change in the absorbance, even though the H2 O2 concentration
was increased by 10 times. To confirm that H2 O2 was effectively acting on the SWNTs, the absorption

C 2020, 6, 30 4 of 9

spectra of the (8,7) chirality of the dsDNA-SWNT (HiPco: High-Pressure carbon monoxide) complex
were measured using the same H2 O2 solution. The (8,7) chirality of the dsDNA-SWNT (HiPco)
complex was observed to have SWNT-specific redox characteristics (Supplemental Materials Figure
S2). Specifically, the absorbance decreased by 15.0 % as a result of H2 O2 addition, and then recovered
by 21.7%
 C 2020, 6, xfollowing the addition of catechin.
 FOR PEER REVIEW 4 of 9

 Figure 2. Absorption spectra of the DNA-SWNT complex following the addition of H2 O2 and catechin.
 Figure 2. Absorption spectra of the DNA-SWNT complex following the addition of H2O2 and catechin.
 H2 O2 did not significantly change the spectra.
 H2O2 did not significantly change the spectra.

 Figure 3 shows the absorption spectra of the DNA-SWNT complex with and without KMnO4
 Because an H2O2 concentration of 9.8 mM did not significantly change the absorbance at
(final concentration: 0.5 µM), and with catechin. The absorbance, which was negligibly affected by the
 oxidation, the concentration of H2O2 was increased by 10 times (final concentration: 98 mM) to
addition of H2 O2 , significantly decreased in response to KMnO4 addition. The peak wavelength was
 determine whether changes in the absorption spectra could be observed. The results are available as
992 nm for the initial state. Ten minutes after the addition of KMnO4 , the peak shifted to 985.5 nm.
 Supplemental Materials S1. There was no significant change in the absorbance, even though the H2O2
However, 10 min
 concentration wasafter the addition
 increased of catechin,
 by 10 times. To confirm thethat
 peak H2wavelength recovered
 O2 was effectively actingtoon992
 thenm.
 SWNTs,This
finding confirms that
 the absorption spectra of theKMnO 4 can effectively oxidize the (6,5) chirality. Additionally,
 (8,7) chirality of the dsDNA-SWNT (HiPco: High-Pressure H O
 2 2 requires
 carbonan
incubation
 monoxide)time of 30 were
 complex min. measured
 To determineusingthethe
 incubation
 same H2O time required under the condition of KMnO
 2 solution. The (8,7) chirality of the dsDNA-4
addition,
 SWNT (HiPco) complex was observed to have SWNT-specificstate
 the time required for the complex to reach a stable redoxfollowing KMnO(Supplemental
 characteristics 4 addition was
measured; the incubation
 Materials S2). Specifically,time the was determined
 absorbance to be by
 decreased 10 min
 15.0 (Supplemental
 % as a result of Materials Figure
 H2O2 addition, andS3).
 then
 Table 1 summarizes the effects of
 recovered by 21.7% following the addition of catechin.KMnO 4 concentration on the peak absorbance and peak
wavelength results for the DNA-SWNT complex. In this experiment,
 Figure 3 shows the absorption spectra of the DNA-SWNT complex with and without KMnO we detected changes in the
 4
absorption spectrum 0.5
 (final concentration: in µM),
 the KMnO
 and with4 concentration
 catechin. Therange of 0.05which
 absorbance, to 1.0was
 µM. Increasing
 negligibly the final
 affected by
concentration
 the addition of ofHKMnO 4 was found
 2O2, significantly to correspond
 decreased to a negative
 in response to KMnOpeak wavelength
 4 addition. The peakshift. The peak
 wavelength
wavelength was not affected by the addition of 0.05 µM of KMnO ; however,
 was 992 nm for the initial state. Ten minutes after the addition of4 KMnO4, the peak shifted a KMnO 4 concentration
 to 985.5
ofnm.1.0However,
 µM corresponded
 10 min after tothe
 a total decrease
 addition of 6.5 the
 of catechin, nm.peak Then, when the
 wavelength complex
 recovered wasnm.
 to 992 reduced
 This
with catechin, the initial peak wavelength was fully recovered. Supplemental
 finding confirms that KMnO4 can effectively oxidize the (6,5) chirality. Additionally, H2O2 requires Materials Figure
S4anshows the three-experiment-average
 incubation time of 30 min. To determine absorption spectra, astime
 the incubation measured
 requiredbyunder
 increasing the KMnO
 the condition of4
concentration
 KMnO4 addition, fromthe 0.05time
 to 1.0 µM. It can
 required for be
 theseen that the
 complex absorbance
 to reach gradually
 a stable decreased
 state following KMnO as the KMnO4
 4 addition

concentration
 was measured; increased.
 the incubation time was determined to be 10 min (Supplemental Materials S3).

C 2020, 6, 30 5 of 9
 C 2020, 6, x FOR PEER REVIEW 5 of 9

 Figure3.3. Absorption
 Figure Absorption spectra
 spectra of
 of DNA-SWNT
 DNA-SWNT complex
 complex with KMnO44 and
 and catechin.
 catechin. The
 Theabsorption
 absorptionpeak
 peak
 decreased by 23.9% following the addition of KMnO , and recovered by 30.7% as a result of adding
 decreased by 23.9% following the addition of KMnO4 , and recovered by 30.7% as a result of adding the
 4

 the catechin
 catechin solution.
 solution.

 Table1. 1Effects
 Table of KMnO4the
 summarizes concentration
 effects of on the absorption
 KMnO spectra and shift in the peak position. These
 4 concentration on the peak absorbance and peak
 data are presented as the average of three independent experiments.
 wavelength results for the DNA-SWNT complex. In this experiment, The final
 we catechin
 detectedconcentration
 changes in the
 was 6.2 mM. There was a correlation between KMnO4 concentration and the extent of the redox reaction.
 absorption spectrum in the KMnO4 concentration range of 0.05 to 1.0 µM. Increasing the final
 concentration
 KMnO4 Final
 of KMnO4 was found to correspond to aAbsorbance
 Peak negative peak wavelength shift. The peak
 wavelength was
 Concentration not affected by the addition of 0.05 µM of KMnO4; however, a KMnO4 concentration
 KMnO4 Change Rate Catechin Change Rate from
 of 1.0(µM) Initial State
 µM corresponded to a totalAddition
 decrease of 6.5 nm. Then, when the complex was reduced with
 from Initial State Addition KMnO 4 Addition
 catechin, the initial peak wavelength was fully recovered. Supplemental Materials S4 shows the
 1.00 0.465 ± 0.003 0.321 ± 0.009 −31.1 ± 1.22% 0.462 ± 0.003 44.2 ± 2.46%
 three-experiment-average absorption spectra, as measured by increasing the KMnO4 concentration
 0.50 0.466 ± 0.004 0.354 ± 0.004 −23.9 ± 0.18% 0.463 ± 0.004 30.7 ± 0.43%
 from 0.05 to 1.0 µM. It can be seen that the absorbance gradually decreased as the KMnO4
 0.25
 concentration 0.469 ± 0.004
 increased. 0.406 ± 0.005 −13.5 ± 0.31% 0.467 ± 0.004 14.9 ± 0.38%
 0.17 0.466 ± 0.005 0.425 ± 0.004 −8.72 ± 0.11% 0.463 ± 0.005 8.72 ± 0.11%
 Table
 0.10 1. Effects of KMnO 4 concentration on the absorption spectra and shift in the peak position.
 0.467 ± 0.003 0.449 ± 0.001 −3.97 ± 0.43% 0.465 ± 0.003 3.53 ± 0.51%
 These data are presented as the average of three independent experiments. The final catechin
 0.05 0.467 ± 0.003 0.459 ± 0.003 −1.61 ± 0.01% 0.464 ± 0.003 0.915 ± 0.03%
 concentration was 6.2 mM. There was a correlation between KMnO4 concentration and the extent of
 KMnO Finalreaction.
 the 4redox Peak Wavelength (nm)
 Concentration KMnO4 Shift from Initial Catechin Shift from Initial
 (µM)4 Final Initial State Peak Absorbance
 KMnO Addition State Addition State
 Change Rate
 Concentration KMnO4 Change Rate from
 1.00 992.0 ± 0.000
 Initial State 985.5 ± 1.886 −6.5
 from ± 1.540 Catechin
 Initial 992.0 ± 0.000
 Addition 0.0 ± 0.000
 (µM) Addition KMnO4 Addition
 State
 0.50 992.0 ± 0.000 988.0 ± 0.816 −4.0 ± 0.667 992.0 ± 0.000 0.0 ± 0.000
 1.00 0.465 ± 0.003 0.321 ± 0.009 −31.1 ± 1.22% 0.462 ± 0.003 44.2 ± 2.46%
 0.25
 0.50 992.0 ± 0.000
 0.466 0.354±± 0.000
 ± 0.004 990.0 0.004 −2.0
 −23.9 ± 0.000
 ± 0.18% 992.0
 0.463 ± 0.000
 ± 0.004 0.0 ±±0.43%
 30.7 0.000
 0.25
 0.17 0.469 ± 0.004
 992.0 ± 0.000 0.406 ± 0.005
 991.0 ± 0.000 −13.5 ± 0.31%
 −1.0 ± 0.000 0.467 ± 0.004
 992.0 ± 0.000 14.9 ± 0.38%
 0.0 ± 0.000
 0.17 0.466 ± 0.005 0.425 ± 0.004 −8.72 ± 0.11% 0.463 ± 0.005 8.72 ± 0.11%
 0.10
 0.10 992.0 ± 0.000
 0.467 0.449±± 0.471
 ± 0.003 991.0 0.001 −1.0
 −3.97 ± 0.000
 ± 0.43% 992.0
 0.465 ± 0.000
 ± 0.003 0.0 ±±0.51%
 3.53 0.000
 0.05
 0.05 0.467
 992.0 ± 0.003 992.0
 ± 0.000 0.459±± 0.000
 0.003 −1.61
 0.0± 0.01%
 ± 0.000 0.464 ± 0.003
 992.0 ± 0.000 0.915
 0.0 ± ± 0.03%
 0.000
 KMnO4 Final Peak Wavelength (nm)
 Concentration KMnO4 Shift from Shift from Initial
4. Discussion Initial State Catechin Addition
 (µM) Addition Initial State State
 1.00 992.0 ± 0.000 985.5 ± 1.886 −6.5 ± 1.540 992.0 ± 0.000 0.0 ± 0.000
 Regarding
 0.50
 the mechanism
 992.0 ± 0.000
 of 988.0
 oxidation,
 ± 0.816
 Tu et al. reported that
 −4.0 ± 0.667
 the spectral intensity
 992.0 ± 0.000
 of the first
 0.0 ± 0.000
interband0.25
 transitions for the
 992.0 (10,5)/(8,7)
 ± 0.000 990.0chirality
 ± 0.000 of −2.0
 semiconducting
 ± 0.000 nanotubes
 992.0 ± 0.000 decreased0.0as± 0.000
 the SWNTs
 0.17 992.0 ± 0.000 991.0 ± 0.000 −1.0 ± 0.000 992.0 ± 0.000 0.0 ± 0.000
 0.10 992.0 ± 0.000 991.0 ± 0.471 −1.0 ± 0.000 992.0 ± 0.000 0.0 ± 0.000

0.05 992.0 ± 0.000 992.0 ± 0.000 0.0 ± 0.000 992.0 ± 0.000 0.0 ± 0.000

4. Discussion
 Regarding the mechanism of oxidation, Tu et al. reported that the spectral intensity of the first
C 2020, 6, 30 6 of 9
interband transitions for the (10,5)/(8,7) chirality of semiconducting nanotubes decreased as the
SWNTs reacted with H2O2, and that the transition intensity was reduced. This phenomenon occurs
reactedHwith
when 2O2 forms
 H2 O2a, and
 charge-transfer complexintensity
 that the transition with thewassidewall of SWNTs,
 reduced. which removes
 This phenomenon electrons
 occurs when
from
H O
 2 2
 the valence
 forms a band and
 charge-transfer increases
 complex the bandgap
 with the [17].
 sidewall of SWNTs, which removes electrons from the
 We assumed that the shift in
valence band and increases the bandgap [17]. the peak position was caused by redox reactions. At the time of
oxidation, KMnO4that
 We assumed removes
 the shiftelectrons fromposition
 in the peak the valence band and
 was caused increases
 by redox the bandgap.
 reactions. This of
 At the time is
believed to
oxidation, KMnOincrease
 4
 energy
 removes and
 electrons decrease
 from the the peak
 valence wavelength.
 band and Conversely,
 increases the bandgap.during
 This reduction,
 is believed
catechin
to increasepromotes
 energy and the decrease
 removal the of oxygen, suppressesConversely,
 peak wavelength. the transferduring
 of electrons fromcatechin
 reduction, the valence band,
 promotes
andremoval
the reduces the bandgapsuppresses
 of oxygen, energy [28]. the This process
 transfer of is assumed
 electrons to reduce
 from energyband,
 the valence and increase the peak
 and reduces the
wavelength.
bandgap energy [28]. This process is assumed to reduce energy and increase the peak wavelength.
 Here, we
 Here, we focused
 focused on on the
 themaximum
 maximumadsorption
 adsorptionamount
 amountofofKMnO KMnO 4 and examined applicability of
 4 and examined applicability
thethe
of Langmuir
 Langmuir adsorption
 adsorption isotherm.
 isotherm. For
 For this
 thispurpose,
 purpose,2 2toto1010 µM
 µM concentrations
 concentrations of KMnO44 were
 of KMnO were
prepared, and the effects of KMnO
prepared, and the effects of KMnO4 concentration on the absorbance were investigated. Figure 4 shows4
 4 concentration on the absorbance were investigated. Figure
shows
the the results.
 results. It can beIt can
 seenbethat
 seenthere
 that was
 thereminimal
 was minimal
 change change in absorbance
 in absorbance betweenbetween
 5 µM5and
 µM 10 andµM,
 10
µM, suggesting
suggesting thatsystem
 that the the system
 in thisinexperiment
 this experiment was saturated
 was saturated at 5 µM.
 at 5 µM.

 Figure 4. Absorbance as a function of KMnO4 concentration (0.05 to 10 µM).
 Figure 4. Absorbance as a function of KMnO4 concentration (0.05 to 10 µM).
 The absorbance peak at 5 µM was 0.253. Because the density of KMnO4 is 2.7 (g/cm33 = kg/L)
 The absorbance peak at 5 µM was 0.253. Because the density of KMnO4 is 2.7 (g/cm = kg/L)
(PubChem CID 516875), the expected adsorption amount was (5.00 µmol/L ÷ 2.7 kg/L) = 1.852 µmol/kg.
(PubChem CID 516875), the expected adsorption amount was (5.00 µmol/L ÷ 2.7 kg/L) = 1.852
Table S1 in Supplemental Materials shows the expected amount of adsorption at each concentration,
µmol/kg. Table S1 in Supplemental Materials shows the expected amount of adsorption at each
as determined based on the absorbance. More specifically, this is the value when the expected
concentration, as determined based on the absorbance. More specifically, this is the value when the
adsorption amount at 5 µM is assumed to be the adsorption amount corresponding to saturation
expected adsorption amount at 5 µM is assumed to be the adsorption amount corresponding to
(100%). The Langmuir adsorption isotherm is given as follows:
saturation (100%). The Langmuir adsorption isotherm is given as follows:
 qm KC
 q=
 1 + KC

where q is the expected adsorption amount (µmol/kg), qm is the maximum adsorption amount (µmol/kg),
where
C q is the expected
 is equilibrium adsorption
 concentration amount
 (i.e., KMnO (µmol/kg),
 4 molar qm is the
 concentration) maximum
 (µmol/L), and adsorption amount
 K is the adsorption
(µmol/kg), coefficient.
equilibrium is equilibrium concentration
 To determine (i.e., KMnO
 whether 4 molar concentration)
 the measured (µmol/L), and
 data fits the Langmuir’s K is the
 equation,
adsorption
the measured equilibrium
 values werecoefficient.
 applied to To
 the determine whether
 linear equation theC measured
 below. data
 and C/q were fits the
 plotted Langmuir’s
 to confirm that
they have a linear relationship. The adsorption constants were derived from the slope and intercept of
the linear line by solving the following equation.
 ! !
 C 1 1
 = + C
 q qm K qm

C 2020, 6, x FOR PEER REVIEW 7 of 9

equation, the measured values were applied to the linear equation below. C and C/q were plotted to
confirm that they have a linear relationship. The adsorption constants were derived from the slope
and intercept of the linear line by solving the following equation.
C 2020, 6, 30 7 of 9
 1 1
 = + C
 
 Table
 Table S2
 S2 in
 in Supplemental
 SupplementalMaterials
 Materialsdescribes
 describesthetherelationship betweenCCand
 relationshipbetween andC/q, and
 C/q, Figure
 and S5 S5
 Figure in
Supplemental Materials shows C/q as a function of C. A linear line with
in Supplemental Materials shows C/q as a function of C. A linear line with slope slope 1/q and intercept 1/q
 m 1/qm and interceptmK
was
1/ obtained. The slope
 was obtained. andslope
 The intercept of the linear
 and intercept line linear
 of the in the line
 graphinisthe
 0.523 andis0.0958,
 graph 0.523 respectively.
 and 0.0958,
From these results, the maximum adsorption amount q m = 1.90 µmol/L and equilibrium
respectively. From these results, the maximum adsorption amount qm = 1.90 µmol/L and equilibrium adsorption
coefficient K = 5.48 kg/L were obtained. Figure 5 shows the results of using the above
adsorption coefficient K = 5.48 kg/L were obtained. Figure 5 shows the results of using the above equation to
calculate and
equation to plot theand
 calculate expected adsorption
 plot the amount. The
 expected adsorption resultsThe
 amount. suggest that
 results the system
 suggest meets
 that the the
 system
requirements for a Langmuir adsorption isotherm.
meets the requirements for a Langmuir adsorption isotherm.

 5. Expected
 Figure 5. Expected amount of adsorption at
 at various
 various concentrations.
 concentrations. The adsorption amount was
 values in
 calculated by implementing measured values in the
 the Langmuir
 Langmuir adsorption
 adsorption formula.
 formula.

5. Conclusions
5. Conclusions
 We proved
 We proved that
 that the
 the oxidation
 oxidation properties
 properties of
 of (6,5)
 (6,5) SWNT,
 SWNT,which
 whichcould
 couldnot
 notbebedetected
 detectedwhen
 whenH H22O
 O22
was
was used
 usedasasanan
 oxidant, cancan
 oxidant, be detected by using
 be detected KMnOKMnO
 by using 4 , and that thethat
 4, and redoxthe
 behavior
 redox can be elucidated
 behavior can be
when KMnO
elucidated 4 is used
 when KMnO in4 combination with catechin
 is used in combination withsolution. We foundWe
 catechin solution. that by increasing
 found KMnO4
 that by increasing
concentration, the peak wavelength decreased and the absorbance gradually
KMnO4 concentration, the peak wavelength decreased and the absorbance gradually decreased. decreased. Analysis of
the measured
Analysis absorbance
 of the measuredresults revealed
 absorbance that the
 results magnitude
 revealed of the
 that the absorbance
 magnitude wasabsorbance
 of the correlated with
 was
the concentration of KMnO
correlated with the concentration
 4 ; furthermore, we demonstrated that Langmuir
 of KMnO4; furthermore, we demonstrated thatisothermal adsorption
 Langmuircan
be used to represent
isothermal adsorption KMnO
 can be 4 adsorption on SWNTs.
 used to represent KMnO4 adsorption on SWNTs.
Supplementary Materials: The following are available online at http://www.mdpi.com/2311-5629/6/2/30/s1:
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1: Figure S1: Absorption
Figure S1: Absorption spectra of dsDNA– (6,5)-Enriched SWNT Complex; Figure S2: Absorption spectra of
spectra of dsDNA–
dsDNA-SWNT (6, 5)-Enriched
 (HiPco) SWNT
 complex; Figure S3: Complex; Figure
 Measurement S2: Absorption
 of incubation spectra
 time for KMnOof dsDNA-SWNT (HiPco)
 4 ; Figure S4: Absorption
complex;
spectra forFigure S3: Measurement
 each concentration of incubation
 of KMnO time
 4 ; Table S1: for KMnO
 Expected 4; Figure
 amount S4: Absorption
 of adsorption spectra for Table
 at each concentration; each
S2: Relationship of
concentration between
 KMnO C 4and C/q; Figure
 ; Table S5: Relationship
 S1: Expected amount between C and C/q.
 of adsorption at each concentration; Table S2:
Author Contributions:
Relationship and C/q; Figure S5:Y.M.;
 between C Conceptualization: methodology:
 Relationship between C and
 Y.M.; C/q.
 validation: Y.M.; formal analysis: Y.M.;
investigation: Y.M.; data curation: Y.M.; writing—original draft preparation: Y.M.; writing—review and editing:
Author
Y.M.; Contributions:
 supervision: Conceptualization:
 K.U.; project Y.M.;
 administration: methodology:
 Y.M. and K.U. All Y.M.; validation:
 authors have readY.M.; formaltoanalysis:
 and agreed Y.M.;
 the published
version of the Y.M.;
investigation: manuscript.
 data curation: Y.M.; writing—original draft preparation: Y.M.; writing—review and editing:
Acknowledgments: We would like to thank Miyashiro of ESTEC CORP.
Conflicts of Interest: The authors declare that there are no conflicts of interest.

C 2020, 6, 30 8 of 9

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