(Invited) Bio-Inspired Method for Degrading Carbon Nanotubes
Authors: Okazaki Toshiya , Zhang Minfang 
Published: December 2024

Carbon nanotubes (CNTs) have shown promise for biological and medical applications due to their biocompatibility, superior electrical conductivity, unique optical properties, and chemical modifiability. For instance, our research group has developed bright near-infrared imaging probe by using oxygen-doped CNTs, so far [1-3]. On the other hand, the adverse effects of CNTs on the environment and human health have recently become a concern. Since CNTs are made of stable graphite, their potential for accumulation in the environment and in living organisms has been discussed. Elucidation of the biodegradation properties of CNTs is thus an essential issue in discussing their long-term safety. In recent years, there have been several reports on the biodegradability of CNTs, but the results differ from report to report, and many details are still unknown. Under these circumstances, we used a technique to measure the amount of CNTs taken up into cells using the near-infrared absorption property of CNTs, and clarified the biodegradation of CNTs in immune cells [4]. The rate of CNT biodegradation was directly proportional to the amount of reactive oxygen species generated, indicating that the biodegradation of CNTs in immune cells was due to reactive oxygen species [5]. Furthermore, the biodegradation mechanism of CNTs is thought to involve oxidation by hypochlorous acid generated during the enzymatic reaction. When hypochlorous acid compounds were mixed with CNT dispersion, we found that the CNT dispersion easily became transparent [5]. This degradation method is applicable to single-walled carbon nanotubes, multi-walled carbon nanotubes, and even graphene [6,7]. We are currently working on international standardization activities for this method.

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[2] T. Takeuchi, Y. Iizumi, M. Yudasaka, S. K. Kondoh, T. Okazaki, Characterization and biodistribution analysis of oxygen-doped single-walled carbon nanotubes used as in vivo fluorescence imaging probes, Bioconjugate Chem., 30, 1323−1330 (2019). (Cover Article)

[3] K. Kojima, Y. Iizumi, M. Zhang, T. Okazaki, Streptavidin-conjugated oxygen-doped single-walled carbon nanotubes as near-infrared labels for immunoassays, Langmuir, 38, 1509-1513 (2022).

[4] M. Yang, M. Zhang, H. Nakajima, M. Yudasaka, S. Iijima, T. Okazaki, Time-Dependent Degradation of Carbon Nanotubes Correlates with Decreased Reactive Oxygen Species Generation In Macrophages, Int. J. Nanomedicine, 14, 2797-2807 (2019).

[5] M. Zhang, Y. Deng, M. Yang, H. Nakajima, M. Yudasaka, S. Iijima, T. Okazaki, A Simple Method for Removal of Carbon Nanomaterials from Wastewater Using Hypochlorite, Sci. Rep. 9, 1284 (2019).

[6] M. Zhang, M. Yang, H. Nakajima, M. Yudasaka, S. Iijima, T. Okazaki, Diameter-Dependent Degradation of 11 Types of Carbon Nanotubes: Safety Implications, ACS Appl. Nano Mater., 2, 4293-4301 (2019).

[7] M. Zhang, M. Yang, Y. Okigawa, T. Yamada, H. Nakajima, Y. Iizumi, T. Okazaki, Patterning of graphene using wet etching with hypochlorite and UV light, Sci. Rep., 12, 1541 (2022).