Diameter-Dependent Degradation of 11 Types of Carbon

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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Diameter-Dependent Degradation of 11 Types of Carbon Nanotubes: Safety Implications Minfang Zhang,*,† Mei Yang,† Hideaki Nakajima,† Masako Yudasaka,‡,§ Sumio Iijima,†,§ and Toshiya Okazaki†

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†

CNT-Application Research Center, National Institute of Advanced Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Research Institute of Nanomaterials, National Institute of Advanced Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan § Faculty of Science and Technology, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan S Supporting Information *

ABSTRACT: Understanding the biodegradation profiles of various types of carbon nanotubes (CNTs) is important for their safe use according to their structural characteristics. To investigate the relationship between the biodegradation and the physicochemical properties of CNTs, sodium hypochlorite was used to degrade CNTs because it is believed to be a key oxidizer of CNTs inside macrophages and neutrophils. The complete degradation of CNTs in response to sodium hypochlorite was confirmed, and the half-lives of 11 different types of commercially available single-wall CNTs (SWNTs), carbon nanohorns (CNHs), and multiwall CNTs (MWNTs) in sodium hypochlorite were determined. The degradation of CNTs generally correlated with diameter, as follows: SWNTs ≥ CNHs > thinner MWNTs > thicker MWNTs. In addition, defects and the degree of dispersion also affected the degradation rate, but these were not the primary factors determining CNT degradation. KEYWORDS: carbon nanotubes, oxidation, sodium hypochlorite, biodegradation, diameter dependence acute toxicity in vitro and in vivo,7−12 the long-term safety of CNTs has yet to be clarified. Inhalation exposure to one kind of MWNT induced malignant mesothelioma in animal studies, suggesting that some types of CNTs may pose hazards similarly to asbestos.13−15 To use CNTs safely and in an environmentally friendly way, we need to understand the degradation profiles of various types of CNTs with different physicochemical properties and then use them in specific applications accordingly. Recently, studies have shown that CNTs can be degraded by peroxidases16−21 such as horseradish peroxidase,16 eosinophil peroxidase,17 and myeloperoxidase (MPO).18−20 The partial degradation of CNTs was also found to occur in macrophages

1. INTRODUCTION Carbon nanotubes (CNTs)1 are hollow tubes consisting of carbon atoms that are formed by rolling one or several layers of graphene into cylinders called single-walled carbon nanotubes (SWNTs)2 or multiwalled carbon nanotubes (MWNTs),1 respectively. Due to their remarkable properties, CNTs have an incredible range of applications in electronics, materials science, energy management, chemical processing, and many other fields, such as nanomedicine.3−6 CNTs are mechanically strong and resistant to degradation due to their unique graphite structure and have been used in advanced composites, fabrics, and fibers, and even in the preparation of cables for the space elevator. However, these characteristics have induced long-term latent risk. Studies on the biodistribution of CNTs after administration to mice indicate that some CNTs persist in organs or tissues for long periods of time.6−12 Although many studies have shown that most types of CNTs have low © XXXX American Chemical Society

Received: April 23, 2019 Accepted: June 10, 2019 Published: June 10, 2019 A

DOI: 10.1021/acsanm.9b00757 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials Table 1. Characteristics of 11 Types of CNTs (As Purchased)

in vitro and in vivo.18,21−27 Results on the stability of different types of CNTs were not consistent across studies. Although the degradation of CNTs by enzymes may be related to their specific properties, including their length, degree of oxidation, and surface functionalization,28−31 the main factors influencing the biodegradation of CNTs are still unclear. The complicated process of enzymatic degradation has limited comparison studies using different types of CNTs. It is known that the biodegradation of CNTs by neutrophils or macrophages is mediated by the oxidation of the CNTs by hypochlorite (ClO−), which is generated as a byproduct of various enzymatic reactions.32−35 It has been reported that 2 × 106 activated neutrophils produced approximately 100 nmol of HClO within 2 h.36 Therefore, using sodium hypochlorite (NaClO) directly as an oxidant is a relevant system to evaluate the degradation characteristics of CNTs. Recent results showed that CNTs can be completely degraded with NaClO.37 SWNTs were reported to degrade faster than MWNTs,37 and two-dimensional (2D) graphene oxide sheets degraded faster than one-dimensional (1D) oxidized CNTs.38 The results also suggest that the wall thickness of CNTs might not be a crucial factor for their degradation by hypochlorite, and that other factors such as impurities might also affect the degradation rate.39 However, the above studies were limited to a comparison of two or three types of carbon nanomaterials, and the general profile of CNT biodegradation is therefore still not clear. In this study, we carried out comparative studies of 11 types of commercially available CNTs, including SWNTs, carbon nanohorns (CNHs),40 and MWNTs produced by different manufacturers, to clarify the major factors that influence the degradation rates of CNTs. Our results could potentially be used to predict the biodegradation rates of CNTs based on their physicochemical properties, which would be useful for selecting the appropriate type of CNTs for specific applications based on their structural characteristics.

based on more than 10 images, which were acquired by transmission electron microscopy (TEM; 120 keV, 36 μA; 002B HRTEM; Topcon Corporation). To investigate the impurities and combustion temperatures of the CNTs, each sample was assessed using a thermogravimetric analyzer (TGA Q500, TA Instruments) from room temperature to 1000 °C at a heating rate of 10 °C/min. 2.2. Preparation of CNT Dispersions. Each CNT was dispersed in an aqueous solution of bovine serum albumin (BSA; fatty acid-free; Nacalai Tesque) following a standard process (ISO/TS 19337), as reported previously.41,42 Briefly, CNTs (about 50 mg) were dispersed in 50 mL of an aqueous solution of 10 mg/mL BSA by ultrasonic homogenization (VC-750, Sonics and Materials) for 5 h with cooling using an ice-water bath. The resulting dispersions were homogeneous, with a concentration of approximately 1 mg/mL. The morphologies of CNTs in dispersion were observed by TEM (120 keV, 36 μA; 002B HRTEM; Topcon Corporation). One drop of CNT dispersion was placed on a copper grid attached to an ultrathin carbon film and then dried at room temperature. For Raman spectrum measurement, the CNT dispersion was washed several times with deionized water by filtration (membrane pore: 0.2 μm). Thereafter, CNTs were redispersed in water, and one drop was placed on a glass plate and dried at room temperature. The Raman spectra were measured at an excitation wavelength of 532 nm with a Raman spectrometer in combination with a confocal microscope (inVia, Renishaw). 2.3. Treatment of CNT Dispersions with Sodium Hypochlorite. Sodium hypochlorite solution (1.1%) was prepared by dissolving sodium hypochlorite pentahydrate (NaClO·5H2O; Tokyo Chemical Industry Co.) in deionized water. About 1 mL of each CNT dispersion (1 mg/mL) was added to 50 mL of sodium hypochlorite solution and kept at 37 °C for 0−720 h. The concentration of each CNT in NaClO solution was about 0.02 mg/mL. The relative change in CNT concentration after treatment with sodium hypochlorite was estimated by measuring the optical absorbance at 700 nm at each time point over a period of 7 days using an ultraviolet/visible light/nearinfrared (UV/vis/NIR) spectrometer (Lambda 1050, PerkinElmer). To analyze changes in the structure of CNTs after treatment, a sample of CNT dispersion (about 5 mL) at each time point was filtrated and rinsed with deionized water using a centrifugal filter unit (Amicon Ultra; membrane, 3 kDa). The obtained samples were dried at room temperature for measurement of the Raman spectrum (inVia, Renishaw) and observation by TEM (120 keV, 36 μA; 002B HRTEM; Topcon Corporation).

2. EXPERIMENTAL METHODS 2.1. Characterization of CNTs. Eleven different types of CNTs were purchased from different manufacturers (Table 1). The diameters and layer numbers of each type of CNT were estimated B


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ACS Applied Nano Materials

Figure 1. Photos of 11 types of CNTs dispersed in water with BSA after treatment with sodium hypochlorite solution for 0−336 h.

Figure 2. Estimation of the quantity of CNTs in aqueous dispersions following treatment with sodium hypochlorite solution. Black lines indicate the absorbance at 700 nm for each dispersion of CNTs after treatment for 0−500 h at 37 °C.

3. RESULTS

All types of CNTs were dispersed in BSA solution to a concentration of approximately 1 mg/mL using the same dispersion process. SWNT dispersions consisted of individual tubes and bundles of two to five tubes, and MWNT dispersions showed individual tubes (Figure S-3), consistent with a previous study.42 The Raman spectra of CNTs dispersed in BSA showed G-band peaks at about 1592 cm−1 and D-band peaks at about 1350 cm−1 for all types of CNTs.43 The G-band is a characteristic feature of the graphite layers and corresponds to the tangential vibration of carbon atoms. The D-band is characteristic of defective graphite structures. The ratio of these two peaks for each CNT sample, which is a measure of the quality or crystallinity of CNTs, is shown in Table 1. 3.2. Rate of CNT Degradation with Hypochlorite. When CNT dispersions were mixed with sodium hypochlorite

3.1. Characteristics of 11 Types of CNTs. The characteristics of the 11 types of CNTs used in this study and their abbreviated names are shown in Table 1. They included four types of SWNTs (SWNT1, SWNT2, SWNT3, and SWNT4) with mean tube diameters of 0.7, 1.0, 1.4, and 2.5 nm, respectively. CNHs had single tube diameters of 1−5 nm and aggregate diameters of 100 nm. The five types of MWNTs had diameters of 10−150 nm and different layer numbers based on estimation of the average values using TEM (Figure S-1, Table 1). The combustion temperatures of each CNT were estimated by thermogravimetric analysis (TGA) performed in oxygen gas (Figure S-2), which showed that the burning temperatures of SWNTs, CNHs, and MWNTs were between 400 and 660 °C (Table 1). C


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D

MWNT5 10 42 2.8 456 MVV.NI4 0.9 16 0.2−0.6 41 MWNT3 0.9 (0.7−1.0) 12 0.8−1.2 27.5 MWNT2 0.8 10 0.6−1.0 22 MWNT1 0.91.0 7 0.2−0.6 4.5 1.02

CNHs

CNHs 1.2 1−5

FloTubeSOOO

NIKKISO (Japan) MWCNT CNano (China)

CoMoCat (USA) SMW200 NEC (Japan)

SWNT2 8.7 0.8−1.2 0.2−0.4 0.55 SWNTl >20 0.7−0.9 0.5−0.6 0.22 abbreviation in this study intensity ratio of G/D Raman bands mean diameter (urn) length of CNT- bundles (pm) degradation half-life (1.1% NaCIO, 37 °C)(h)

product name

SWNT3 52.7 1.5 0.1−0.3 1.52

ZEON (Japan) HT (SGCNT) SWNT4 1.9−5.7 3 0.1−0.4 1.75 Meijo (Japan) EC (eDIPs) HiPco (USA) HiPco CoMoCat (USA) Signis SG65i manufacturer

single-wall carbon nanotubes (SWNTs)

Table 2. Characteristics of 11 Types of CNTs Dispersed in BSA

nauoboms (C:NHs)

JEIO (Korea) JC140

Nanocyl (Belgian) NC7000

multiwall carbon nanotubes (MWNTs)

Showadenko (Japan) VGCF

solution (1.1%, w/w) and kept at 37 °C for 0.5−336 h, the black color of the dispersions faded gradually with increasing treatment time (Figure 1). The SWNT1, SWNT2, SWNT3, SWNT4, and CNH dispersions became transparent within 48 h, whereas the MWNTs took a longer time (72 or 336 h) to become transparent. The MWNT4 and MWNT5 dispersions did not become transparent during the test period of 500 h. The changes in concentration of the CNT dispersions after treatment with sodium hypochlorite were monitored by measurement of the optical absorbance at 700 nm based on our previous study42 and no absorption of NaClO and BSA in the region of 600−900 nm (Figure S-4). The relative concentration of the CNTs decreased with time and reached zero for the SWNTs and CNHs and MWNT1, MWNT2, MWNT3, and MWNT4 (Figure 2). The half-lives for the complete degradation of the SWNTs, CNHs, and MWNTs were estimated to be 0.22−1.75, 1.02, and 4.5−456 h, respectively, as noted in Figure 2 and Table 2. 3.3. Relationship between CNT Degradation and Their Major Physicochemical Properties. The relationship between the degradation of each CNT sample and its diameter (Table 1), layer number, combustion temperature, and crystallinity (G/D; the ratio of the Raman G-band and Dband) is shown in Figure 3a−d. The half-life of the CNTs increased with increasing diameter (Figure 3a) and layer number (Figure 3b). In particular, the half-life of MWNTs increased linearly with their layer number. The half-life of the SWNTs and MWNTs also increased with increasing combustion temperature (Figure 3c), which was estimated by TGA performed in oxygen at a heating rate of 10 °C/min (Figure S-2). However, there was no obvious correlation between the half-life of CNT degradation and the Raman band intensity ratio (Figure 3d). Furthermore, the half-lives of the SWNTs were clearly shorter than those of CNHs and the MWNTs (Figure 3a). In addition, we have found that SWNT1 was longer than SWNT2 and SWNT4 but had a faster degradation rate; MWNT1 and MWNT4 had almost the same lengths but different half-lives of degradation (Table 2). This result indicated that the length was not a factor to influence degradation of CNTs by NaClO solution. 3.4. Structural Changes in CNTs during Degradation with Hypochlorite. The structures of CNTs before and after treatment with NaClO for 1−48 h were determined by TEM and using Raman spectra. The tubular structures of SWNTs were destroyed by NaClO treatment for 1 h. At that time, there were only small pieces of graphene sheets remaining in SWNT1 (Figure 4a) and SWNT2 (not shown). For SWNT3 and SWNT4 (Figure 4b), the SWNTs were also difficult to identify after NaClO treatment, but a few double-wall CNTs or multiwall CNTs could be found at 1 h, while after 5 h, only a few multiwall CNTs remained, consisting of pieces or sheets of graphene or other disordered carbons. The double-wall and multiwall CNTs inside these samples are believed to be impurities in SWNT samples. Extensive structural damage was also seen in MWNT1−MWNT4 (Figure 4c and d) after NaClO treatment for 24 h, while MWNT5 (Figure 4e) and MWNT6 (not shown) only had slight damage to the CNT walls. The typical Raman spectra of SWNTs (SWNT4) and MWNTs (MWNT4) after treatment with NaClO are shown in Figure 5a and b. The G-bands of both samples decreased relative to their D-bands, and a D′-band at about 1620 cm−1 corresponding to small or disordered CNTs42 appeared. No

MW.NI6 6.6 150 2.01 >456



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Figure 3. Correlations between the half-lives of 11 types of CNTs upon incubation in sodium hypochlorite with the diameters of CNTs (a), the layer numbers of MWNTs (b), the combustion temperatures (c), and the Raman G/D ratios (d), which were average values obtained from more than 10 measurements. The inset in b shows an enlarged view of the boxed area.

(H2O2),45 oxygen,46,47 and nitric acid.47,48 Studies on the oxidation of MWNTs in oxygen indicated that their oxidative stability was diameter-dependent.49,50 The pattern of oxidation of CNTs with hypochlorite described here is consistent with results obtained with other oxidizers such as hydrogen peroxidase and oxygen. On the other hand, the recent study by Newman et al. demonstrated39 that the wall thickness of MWNTs is not a crucial factor for their degradation by NaClO, which is in contrast with our results. By comparing the morphological changes occurring in two different types of NaClO-treated MWNTs by TEM, they found that thicker MWNTs became degraded on the outside wall and inner sidewall, while thinner MWNTs became degraded only at the inner sidewall. The authors suggested that the biodegradation of CNTs likely depends not only on the wall thickness but also on defects in the inner wall.39 Our studies were performed at a higher temperature, and therefore the reaction rates were much faster than the rates reported in that study.39 We did not find that oxidation of MWNTs with NaClO occurred only at the surface or at both the surface and the inner wall. The complete degradation of CNTs enabled us to estimate their half-lives, which provided strong evidence that the degradation of CNTs, whether SWNTs or MWNTs, was diameter-dependent and layer-number-dependent overall. 4.2. The Effects of Defects on Degradation. It is known that the defects in CNTs can influence their oxidation, as suggested in the study described above.38 Defects and derivatization moieties in CNT walls can lower the thermal stability. Oxidized CNTs have been shown to be more easily degraded than pristine CNTs by enzymatic oxidation.18,28 To further investigate the effects of defects on degradation of

Raman peaks could be found once the CNT dispersions became transparent after treatment with NaClO.

4. DISCUSSION The results obtained here confirmed those reported in our previous study,37 indicating that CNTs are completely degraded by NaClO solution. The chemical reaction formula for complete degradation of CNTs was estimated to be CCNT + 2NaClO → CO2 + 2NaCl, where CO2 is partially or fully changed to [CO32−] or [HCO3−] in aqueous solutions containing NaClO (pH 10).37 On the basis of the complete degradation of CNTs by NaClO reported in this study, we performed a systematic study to measure the biodegradation rates of 11 types of CNTs. By studying the relationship between the half-life of degradation of a given CNT sample and its chemical and physical properties, we were able to determine that the main factor influencing the biodegradation of a CNT sample was its diameter. 4.1. The Effect of CNT Diameter on Biodegradation. Our results (Figure 3) showed clearly that the degradation rates of CNTs correlated with their diameters and layer numbers. Both SWNTs and MWNTs with smaller diameters had faster degradation rates. The degradation of CNTs with hypochlorite followed the order of SWNTs ≥ CNHs > thin MWNTs > thick MWNTs. The larger surface areas of CNTs with small diameters increased their interaction with NaClO, which may explain their faster degradation. Another potential reason for this finding is that CNTs with smaller diameters are believed to have greater curvature (i.e., strain), which could decrease their stability, leading to higher oxidation rates.44 Many previous studies have shown that SWNTs with smaller diameters are more readily oxidized with hydrogen peroxidase E


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Figure 4. Representative observations of the structural changes in CNTs exposed to NaClO (1.1 wt %) as detected by TEM. SWNT1 before and after treatment for 1 h (a); SWNT4 before and after treatment for 1 and 5 h (b); and MWNT2 (c), MWNT4 (d), and MWNT5 (e) before and after treatment for 24 h.

CNTs by NaClO, we induced different degrees of defects in SWNT4 by sonication for 0.5−5 h. The Raman G/D ratio decreased with increasing sonication time, indicating an increase in defects (Figure S-5). The degradation half-life of SWNT4 in NaClO decreased slightly with increasing sonication time. This suggested that these defects increased the degradation of CNTs by hypochlorite. However, comparing the 11 types of CNTs used in this study, there was no correlation between the Raman G/D ratio and the degradation half-life. As shown in Figure 3c, CNTs with lower Raman G/D ratios, which indicate more defects, did not have faster degradation rates than CNTs with higher Raman G/D ratios (Figure 3c). For example, SWNT3 and SWNT4 had very different Raman G/D ratios (50 and 1.9− 5.7) but similar degradation half-lives (1.52 and 1.72 h). MWNT3 and MWNT4 had similar Raman G/D ratios but different degradation rates. Therefore, we could not determine

a general relationship between defects and susceptibility to degradation by NaClO for the different types of CNTs. 4.3. The Effects of Impurities and Dispersion Degree on Degradation. The CNTs used in this study were welldispersed in BSA solution by sonication before NaClO treatment. The SWNTs were dispersed as individual tubes or in small bundles, whereas the MWNTs were almost all dispersed as individual tubes (Figure S-3). Therefore, the results of this study reflect the degradation of individual CNTs but not CNT aggregates. The degradation of aggregated CNTs in dispersion or in powder would be different from the degradation of individual CNTs due to differences in their interaction with NaClO. We also evaluated the degradation of CNTs in powder with NaClO and found that it was much slower than the degradation of CNTs in dispersion. It was difficult to establish a correlation between the degradation rates of different types of CNTs in powder, indicating that the dispersion state influences the degradation of CNTs. We also F


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linear relationship between the layer numbers and half-lives of MWNTs (Figure 3b) also suggests that degradation of MWNTs occurs via a layer-by-layer mechanism as reported previously.50 In addition, our recent result confirmed that the final products of CNTs after complete degradation with NaClO were carbon dioxide or carbonate, and no organic perox compounds were found.37 Therefore, the process of CNT degradation after exposure to NaClO would proceed from etching of the CNT walls, to the destruction of the CNT walls, to the formation of oxidized graphene sheets, and finally to the production of the carbon dioxide or carbonate.

5. CONCLUSION We measured the degradation rates of 11 types of carbon nanotubes using sodium hypochlorite as an oxidizer. By analyzing the half-life of CNT degradation, the CNT diameter, the Raman G/D ratio, and the combustion temperature by TGA, we determined that a general feature of CNT degradation is that it is diameter-dependent. While defects and dispersion states influenced the degradation rate, these were not the major factors determining the degradation rates of the different types of CNTs. The degradation rates of CNTs were as follows: SWNTs ≥ CNHs > thinner MWNTs > thicker MWNTs. In addition, we found that graphene oxide was an intermediate product formed during the degradation of SWNTs, suggesting that the degradation of CNTs might involve an unzipping process. Although the biodegradation of CNTs in living systems is more complicated, and is influenced by interactions of CNTs with enzymes and macrophage-derived reactive oxygen species, we believe that the results of this study will help us to understand the lack of consistency of studies about CNT stability and to compare different studies on enzymatic degradation using different types of CNTs. The results also could enhance our understanding of the features of CNT degradation based on their physical and chemical properties. This information will help researchers to choose the appropriate type of CNT for a specific application.

Figure 5. Raman spectra of SWNT4 after treatment with sodium hypochlorite solution for 0, 1, 3, 5, and 24 h (a) and of MWNT4 after treatment with sodium hypochlorite solution for 0, 5, 24, 48, and 96 h (b).

evaluated the degradation of oxidized SWNT4 (oxSWNT4), which was obtained by oxidation of SWNT4 with H2SO4/ HNO3 at 70 °C for 40 min.37 oxSWNT4 dispersed in water with BSA (oxSWNT4/BSA), in water with polyethylene glycol [DSPE-PEG(5000) amine] (oxSWNT4/PEG), or in water alone (oxSWNT4/H2O) was then treated with NaClO. The results (Figure S-6) demonstrated that all three samples could be almost completely degraded by NaClO treatment within 24 h. However, the half-lives of the three samples were different (oxSWNT4-BSA < ox-SWNT4/H2O ≤ ox-SWNT4-PEG). These results suggested that the dispersers might influence the degradation rates of the CNTs. It is also known that impurities such as metal catalysts affect the oxidation of CNTs in oxygen. A higher concentration of a metal catalyst lowered the combustion temperature of CNTs by TGA, indicating that metal impurities might catalyze the oxidation of CNTs. However, for the oxidation of CNTs with NaClO, no obvious effect of metals on CNT degradation was found. For example, MWNT4 had a higher amount of metal impurities (Fe/Mo, 8%) than MWNT3 (Co/Mo, 2.2%; Table 1), but a lower degradation rate. Although we did not conduct a detailed investigation of the effect of metals on the degradation of CNTs by NaClO, impurities in metals did not appear to have a strong effect. In addition, BSA, which was used to disperse CNTs, would also be oxidized and removed by NaClO, which might consume a small amount of NaClO.51−53 However, this effect was disregarded because an excess of NaClO was used in this study. The rapid reaction of BSA with NaClO (a few minutes or seconds)51−53 also meant that the influence of BSA on degradation of CNTs was negligible. 4.4. The Mechanism of CNT Degradation by NaClO. On the basis of the color change observed in CNT samples (from black to yellow to transparent), we infer that the degradation of CNTs occurred as a stepwise process, with CNTs first degrading into graphene sheets before completely degrading. TEM confirmed that there were graphene-like pieces in SWNT samples after exposure to NaClO for 1 h (Figure 4a and b). These results indicate that CNT degradation might proceed via an unzipping process. The unzipping process of CNTs or CNHs into graphene sheets by oxidation has also been observed in previous studies.54−57 The

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00757. Figure S-1, TEM images of 11 types of CNTs (as purchased); Figure S-2, TGA measurement results; Figure S-3, TEM images of 11 types of CNTs after dispersal in BSA solution; Figure S-4, UV−vis−NIR absorption spectra of BSA and NaClO solutions and CNTs dispersions; Figure S-5, investigation of the defects in degradation of SWNT4; and Figure S-6, investigation of the effect of dispersers on degradation of SWNT4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 0081-298616758. E-mail: [email protected]. ORCID

Minfang Zhang: 0000-0002-3983-3355 Toshiya Okazaki: 0000-0002-5958-0148 G


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M.Z. designed the study. M.Z., M.Y., and H.N. performed the experiments. M.Z., M.Y., I.S., and T.O. analyzed the data. M.Z. wrote the manuscript. All authors discussed the results and contributed to the final manuscript. Funding

M.Z. received funding from the Japan Society for the Promotion of Science (Grant-in-Aid for Scientific Research B, 17H02742); M.Z. and T.O. received financial support from ZEON Corporation. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a KAKENHI grant (Grant-in-Aid for Scientific Research B, 17H02742) from the Japan Society for the Promotion of Science (JSPS) and by ZEON Corporation.

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