Oxygen-Doped Carbon Nanotubes Research Articles

(Invited) Enhancing Efficiency, Stability and Versatility in Dye-Sensitized Photoelectrochemical Cells Via Spatially Controlled Cascade Charge Transfer

In this talk, I will present my group's recent research on dye-sensitized photoelectrochemical cells (DSPECs) aimed at green energy generation. Initially, I will introduce a molecular-design strategy that integrates both strong and relatively weak electronic couplings within a single sensitizer. This approach aims to facilitate vectorial electron transfer, mirroring the efficiency seen in natural photosystems, thereby improving light harvesting efficiency.Next, to overcome the challenges of low photocurrent and limited stability in DSPECs, we develop a buried junction-type DSPEC. This utilizes a platinum-sputtered nickel foil to encapsulate both the dye-sensitized TiO2 layer and the redox mediator electrolyte. This design facilitates spatially controled cascade charge transfer, effective photoconversion, and water oxidation without the issues of current leakage. It also ensures high stability and versatility. By altering the photoredox electrolytes, we are able to reduce the onset potential required for water oxidation. Additionally, optimizing the kinetics of water oxidation is achieved by replacing the Ni catalyst with NiFe-LDH catalysts, which result in the photocurrent densities of 14.5 mA/cm² at 1.23 V versus the reversible hydrogen electrode.Moreover, by substituting the Pt catalyst with oxygen-doped carbon nanotubes, we broaden the application of our buried-junction DSPECs to include the bias-free production of hydrogen peroxide with a solar-to-fuel efficiency (STF) of approximately 3.76%, and a high photostability.

(Invited) Bio-Inspired Method for Degrading Carbon Nanotubes

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.[1] Y. Iizumi, M. Yudasaka, J. Kim, H. Sakakita, T. Takeuchi, T. Okazaki, Oxygen-doped carbon nanotubes for near-infrared fluorescent labels and imaging probes, Sci. Rep., 8, 6272 (2018).[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).

Oxygen-doped carbon nanotubes with dual active cites to enhance •OH formation through three electron oxygen reduction

The electro-Fenton (EF) process generates H2O2 through the 2e- oxygen reduction reaction (ORR), which is subsequently activated to •OH by iron-based catalysts. To alleviate the potential risk of external Fe-based catalysts, along with metal dissolution in acidic or neutral environments, in this study we employed oxygen-doped carbon nanotubes (OCNT) as a bifunctional, metal-free cathode to establish a metal-free EF process for organic pollutant degradation. The results demonstrate that the metal-free electrode has excellent H2O2 accumulation (12 mg L−1 cm−1) and degrades sulfathiazole (STZ) with 97.05 % efficiency in 180 min with an explanation kinetic of 0.0189 min−1. For the first time, this enhancement came from the dual active site centers in OCNT: Ⅰ) -COOH and defects active sites were responsible for H2O2 production, Ⅱ) then -CO triggered H2O2 into •OH, avoiding the introduction of metal-based catalysts. These findings suggest that the EF system with in situ oxygen-doped cathodes have great potential for treating antibiotic wastewater.

(Invited) Bio-Applications of Carbon Nanotubes for NIR Imaging Probes and Guided Bone Regeneration Membranes

Single-walled carbon nanotubes (CNTs) have been expected for biological and medical applications because of their unique optical and chemical properties. One of the promising applications is imaging probes because CNTs show fluorescence in the near-infrared (NIR) region, which is highly transparent for biomaterials. We have reported the synthesis method of the epoxide-type oxygen-doped CNTs (Ep-CNTs) and their application as a near-infrared fluorescent probe [1-3]. Treated (6, 5) nanotubes show photoluminescence at ~1280 nm, which corresponds to the most transparent regions for biomaterials [1]. Immunoassay and fluorescence vascular angiography of mice were demonstrated by using the produced Ep-CNTs as infrared fluorescent labels [1,3] and imaging agents [1,2], respectively. From a practical point of view, the biodistributions of the Ep-CNTs imaging probe after administration to mice were investigated in detail [2]. In addition, the biological responses of mice following administration, which are relevant to safety, were also introduced [2].More recently, we found that CNTs exhibit high compatibility with bone tissue [4]. The guided bone regeneration (GBR) technique is commonly applied to reconstruct alveolar bone and treat peri-implant bone defects. It was found that CNT-based membrane for GBR exhibits high strength to provide a space for bone formation and provide cellular shielding to induce osteogenesis.In this meeting, we will report above biological and medical applications of CNTs in detail.1. Y. Iizumi, M. Yudasaka, J. Kim, H. Sakakita, T. Takeuchi, T. Okazaki, Oxygen-doped carbon nanotubes for near-infrared fluorescent labels and imaging probes, Sci. Rep., 8, 6272 (2018).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. Y. Xu, E. Hirata, Y. Iizumi, N. Ushijima, K. Kubota, S. Kimura, Y. Maeda, T. Okazaki, A. Yokoyama, Single-walled Carbon Nanotube Membranes Accelerate Active Osteogenesis in Bone Defects: Potential of Guided Bone Regeneration Membranes, ACS Biomater. Sci. Eng., 8, 1667-1675 (2022).

Understanding the adsorption sites on nitrogen- and oxygen-doped carbon nanotubes for iodine uptake

The effective capture and storage of radioactive iodine is of great significance to the safe use of nuclear energy. In this study, nitrogen- and oxygen-doped carbon nanotubes acted as effective iodine capture agents in a low concentration of iodine solution of approximately 100 mg L−1. The iodine uptake capacity is proportionate to hydroxy-functional groups as oxygen species and graphitic nitrogen as nitrogen species, respectively, rather than defects and the specific surface area of carbon nanotubes. The essential adsorption activity of oxygen-doped carbon nanotubes is more than 2.5 times that of nitrogen-doped carbon nanotubes, indicating oxygen-doped carbon nanotubes as a potential carbon material in practical applications for the adsorption of iodine. Theoretical study further deepened the understanding of the iodine adsorption sites on carbon nanotubes. This work provided a new method for the design and synthesis of high-performance radioactive iodine solid adsorbents in the future.

Highly efficient electro-peroxone enhanced by oxygen-doped carbon nanotubes with triple role of in-situ H2O2 generation, activation and catalytic ozonation

The oxygen-doped carbon nanotubes coated on the carbon felt acted as cathode to enhance the electro-peroxone (OCNT-EP) process. Owing to the introduction of oxygen-containing functional groups and defect sites, the OCNT modified cathode could achieve in-situ H2O2 generation, activation, and catalytic ozonation in the EP process. The OCNT-EP showed a 2.3-folds apparent first-order rate constant (k) compared with the EP process using CNT cathode (CNT-EP) for atrazine (ATZ) degradation. According to the processes comparison, quenching experiments, electron paramagnetic resonance (EPR) analysis and determination of hydroxyl radical, it was proved that more.OH and 1O2 were generated by the in-situ H2O2 activation and catalytic ozonation in the OCNT-EP process. Especially, this process improved pollutants degradation under acidic condition for the in-situ H2O2 activation and catalytic ozonation, expanding the pH application range of the EP process with a lower energy consumption (0.10 kWh/g ATZ). What’ more, the OCNT modified cathode performed excellent stability with 20 cycle tests, and demonstrated all improved degradation rate (21.7%−78.0%) for various pollutants (carbamazepine, sulfamethazine, phenol and 2,4-dichlorophenol). Therefore, the OCNT-EP process could be considered as a promising and enhanced EP process for wastewater treatment.

Highly Efficient Electro-Peroxone Enhanced by Oxygen-Doped Carbon Nanotubes with Triple Role of In-Situ H2o2 Generation, Activation and Catalytic Ozonation

Highly Efficient Electro-Peroxone Enhanced by Oxygen-Doped Carbon Nanotubes with Triple Role of In-Situ H2o2 Generation, Activation and Catalytic Ozonation

Oxygen-Doped Carbon Nanotubes for Near-Infrared Imaging Probes

Optical imaging is one of the most important techniques in biomedical studies. Recently, usage of near-infrared light has been extensively tried because of the higher transparency in biomaterials than the visible light. Single-walled carbon nanotubes (SWCNTs) are nanocarbon materials that show optical absorption and fluorescence in the near infrared (NIR) wavelength region. Especially, due to the bright fluorescence in NIR, SWCNTs have been expected as imaging probes. In this talk, we show our recent results about the bio-application of oxygen-doped SWCNTs [1,2]. Interestingly, covalent doping of the nanotube surface with a low concentration of oxygen atoms can create a new optically allowed defect state [3]. Consequently, nanotube fluorescence is red-shifted and can be over 10 times brighter. Fluorescence vascular angiography and observation of the intestinal contractile activity of mice are demonstrated by using the produced oxygen-doped SWCNTs as infrared fluorescent labels and imaging agents [1]. Further, the biodistribution analysis after the administration of the oxygen-doped SWCNTs in mice is also discussed by the resonance Raman spectroscopy and NIR fluorescence microscopy [2]. [1] Y. Iizumi, M. Yudasaka, J. Kim, H. Sakakita, T. Takeuchi, T. Okazaki, Sci. Rep., 8, 6272 (2018). [2] T. Takeuchi, Y. Iizumi, M. Yudasaka, S. Kondoh, T. Okazaki, in preparation. [3] S. Ghosh, S. M. Bachilo, R. A. Simonette, K. M. Beckingham, R. B. Weisman, Science 330, 1656-1659 (2010).

Oxygen-doped carbon nanotubes for near-infrared fluorescent labels and imaging probes

Chemical modification of carbon nanotube surface can controllably modulate their optical properties. Here we report a simple and effective synthesis method of oxygen-doped single-walled carbon nanotubes (o-SWCNTs), in which a thin film of SWCNTs is just irradiated under the UV light for a few minutes in air. By using this method, the epoxide-type oxygen-adducts (ep-SWCNTs) were produced in addition to the ether-type oxygen-adducts (eth-SWCNTs). The Treated (6, 5) ep-SWCNTs show a red-shifted luminescence at ~1280 nm, which corresponds to the most transparent regions for bio-materials. Immunoassay, fluorescence vascular angiography and observation of the intestinal contractile activity of mice were demonstrated by using the produced o-SWCNTs as infrared fluorescent labels and imaging agents.

Effect of nitrogen and oxygen doped carbon nanotubes on flammability of epoxy nanocomposites

To overcome the flammability of epoxy resins, novel nitrogen (CNx) and oxygen (COx) doped CNT were synthesized via CVD method and epoxy nanocomposites at 2 wt.%. constant loading were prepared by three roll milling and cured. The structures of the nanotubes were confirmed by XRD, XPS, SEM and Raman and it was found a very high aspect ratio especially for COx.Thermal degradation as well as glass transition temperatures and elastic moduli were measured by TGA, DSC and DMTA. The flammability of epoxy nanocomposites was studied by microscale combustion calorimetry (MCC) and limiting oxygen index (LOI) determination. Results showed that the fire retardant properties of nanocomposites improved significantly specially for COx, which presented a very high LOI (35%) and a homogeneous and uniform surface after burning. This effect was tentatively attributed to the very high aspect ratio of COx tubes.

(Invited) Epoxide-Type Oxygen-Doped Carbon Nanotubes and Their Applications for NIR Imaging Probes

A novel procedure for effective synthesis of covalently oxygen-doped single-walled carbon nanotubes (SWCNTs) has been developed. The SWCNT thin film is directly irradiated by UV light and oxygen atoms substitute into the tube wall via the produced ozone molecules. The photoluminescence spectroscopy studies reveal that the obtained SWCNTs exhibit the characteristic spectroscopic features of epoxide-type oxygen-doped SWCNTs. Advantageously, the emission peak of the doped (6, 5) SWCNTs is centered at around 1300 nm, which is most transparent region of biomaterials. We demonstrate immunoassay, fluorescence vascular angiography and observation of the intestinal contractile activity of mice by using the produced oxygen-doped SWCNTs as infrared fluorescent labels and imaging agents.