Pyridine-mediated B-B bond cleavage of tetrahydroxydiboron to synthesize n-doped SWCNTs with long-term air stability.

Neutral radicals, including carbon radicals, are highly useful chemical species for the functionalization of semiconducting materials to change their electrical and optical properties owing to their high reactivity. However, boron radicals have been limited to synthetic and reaction chemistry, with rare utilization in materials science. In this study, a mixture of tetrahydroxydiboron (B2(OH)4) and pyridine derivatives was found to act as an electron dopant for single-walled carbon nanotubes (SWCNTs) because of the electron transfer from pyridine-mediated boron radicals generated by B-B bond dissociation to neutral radicals. In particular, the radical formed from a mixture of B2(OH)4 and 4-phenylpyridine ((4-Phpy)B(OH)2·) efficiently doped electrons into the SWCNT films; thus, n-type SWCNTs with long-term air stability for more than 50 days at room temperature were prepared. Furthermore, the experimental and theoretical surface analyses revealed that the formation of stable cations from ((4-Phpy)B(OH)2·) and the efficient interaction with SWCNTs due to their high planarity served as the mechanism for their stable doping.

Predicting the anion conductivities and alkaline stabilities of anion conducting membrane polymeric materials: development of explainable machine learning models.

Anion exchange membranes (AEMs) are core components in fuel cells and water electrolyzers, which are crucial to realize a sustainable hydrogen society. The low anion conductivity and durability of AEMs have hindered the commercialization of AEM-based devices, and research and development (R&D) to improve AEM materials is often resource-intensive. Although machine learning (ML) is commonly used in many fields to accelerate R&D while reducing resource consumption, it is rarely used in the AEM field. Three problems hinder the adoption of ML models, namely, the low explainability of ML models; complication with expressing both homopolymers and copolymers in unity to train a single ML model; and difficulty in building a single ML model that comprehends various polymer types. This study presents the first ML models that solve all three problems. Our models predicted the anion conductivity for a diverse set of unseen AEM materials with high accuracy (root mean squared error = 0.014 S cm-1), regardless of their state (freshly synthesized or degraded). This enables virtual pre-synthesis screening of novel AEM materials, reducing resource consumption. Moreover, human-comprehensible prediction logic revealed new factors affecting the anion conductivity of AEM materials. Such capability to reveal new important variables for AEM materials design could shift the paradigm of AEM R&D. This proposed method is not limited to AEM materials, instead it presents a technology that is applicable to the diverse set of polymers currently available.

ortho-Substituted Aryldiazonium Design for the Defect Configuration-Controlled Photoluminescent Functionalization of Chiral Single-Walled Carbon Nanotubes.

Defect functionalization of single-walled carbon nanotubes (SWCNTs) by chemical modification is a promising strategy for near-infrared photoluminescence (NIR PL) generation at >1000 nm, which has advanced telecom and bio/medical applications. The covalent attachment of molecular reagents generates sp3-carbon defects in the sp2-carbon lattice of SWCNTs with bright red-shifted PL generation. Although the positional difference between proximal sp3-carbon defects, labeled as the defect binding configuration, can dominate NIR PL properties, the defect arrangement chemistry remains unexplored. Here, aryldiazonium reagents with π-conjugated ortho-substituents (phenyl and acetylene groups) were developed to introduce molecular interactions with nanotube sidewalls into the defect-formation chemical reaction. The functionalized chiral SWCNTs selectively emitted single defect PL in the wavelength range of ∼1230-1270 nm for (6,5) tubes, indicating the formation of an atypical binding configuration, different from those exhibited by typical aryl- or alkyl-functionalized chiral tubes emitting ∼1150 nm PL. Moreover, the acetylene-based substituent design enabled PL brightening and a subsequent molecular modification of the doped sites using click chemistry.

Azide modification forming luminescent sp2 defects on single-walled carbon nanotubes for near-infrared defect photoluminescence.

Azide functionalization produced luminescent sp2-type defects on single-walled carbon nanotubes, by which defect photoluminescence appeared in near infrared regions (1116 nm). Changes in exciton properties were induced by localization effects at the defect sites, creating exciton-engineered nanomaterials based on the defect structure design.

Protein-structure-dependent spectral shifts of near-infrared photoluminescence from locally functionalized single-walled carbon nanotubes based on avidin-biotin interactions.

Single-walled carbon nanotubes (SWCNTs) emit photoluminescence (PL) in the near-infrared (NIR) region (>900 nm). To enhance their PL properties, defect doping via local chemical functionalization has been developed. The locally functionalized SWCNTs (lf-SWCNTs) emit red-shifted and bright E11* PL originating from the excitons localized at the defect-doped sites. Here, we observe the E11* PL energy shifts induced by protein adsorption via the avidin-biotin interactions at the doped sites of lf-SWCNTs. We establish that the difference in the structures of the avidin derivatives notably influences the energy shifts. First, lf-SWCNT-tethering biotin groups (lf-SWCNTs-b) are synthesized based on diazonium chemistry, followed by post-modification. The responsiveness of the lf-SWCNTs-b to different microenvironments is investigated, and a correlation between the E11* PL energy shift and the induction-polarity parameters of surrounding solvents is established. The adsorption of neutravidin onto the lf-SWCNTs-b induces an increase in the induction-polarity parameters around the biotin-doped sites, resulting in the red-shift of the E11* PL peak. The E11* PL shift behaviors of the lf-SWCNTs-b change noticeably when avidin and streptavidin are introduced compared to the case with neutravidin. This is due to the different microenvironments formed at the biotin-doped sites, attributed to the difference in the structural features of the introduced avidin derivatives. Moreover, we successfully enhance the detection signals of lf-SWCNTs-b (>three fold) for streptavidin detection using a fabricated film device. Therefore, lf-SWCNTs exhibit significant promise for application in advanced protein detection/recognition devices based on NIR PL.

Polymer-coated carbon nanotube hybrids with functional peptides for gene delivery into plant mitochondria.

The delivery of genetic material into plants has been historically challenging due to the cell wall barrier, which blocks the passage of many biomolecules. Carbon nanotube-based delivery has emerged as a promising solution to this problem and has been shown to effectively deliver DNA and RNA into intact plants. Mitochondria are important targets due to their influence on agronomic traits, but delivery into this organelle has been limited to low efficiencies, restricting their potential in genetic engineering. This work describes the use of a carbon nanotube-polymer hybrid modified with functional peptides to deliver DNA into intact plant mitochondria with almost 30 times higher efficiency than existing methods. Genetic integration of a folate pathway gene in the mitochondria displays enhanced plant growth rates, suggesting its applications in metabolic engineering and the establishment of stable transformation in mitochondrial genomes. Furthermore, the flexibility of the polymer layer will also allow for the conjugation of other peptides and cargo targeting other organelles for broad applications in plant bioengineering.

Electron doping of single-walled carbon nanotubes using pyridine-boryl radicals.

Pyridine-boryl (py-boryl) radicals serve as efficient electron-doping reagents for single-walled carbon nanotubes (SWCNTs). The doping mechanism comprises electron transfer from the py-boryl radical to the SWCNT. The formation of a stable py-boryl cation is essential for efficient doping; the captodative effect of the py-boryl cation is important to this process.

Control of anisotropic conduction of carbon nanotube sheets and their use as planar-type thermoelectric conversion materials.

The large anisotropic thermal conduction of a carbon nanotube (CNT) sheet that originates from the in-plane orientation of one-dimensional CNTs is disadvantageous for thermoelectric conversion using the Seebeck effect since the temperature gradient is difficult to maintain in the current flow direction. To control the orientation of the CNTs, polymer particles are introduced as orientation aligners upon sheet formation by vacuum filtration. The thermal conductivities in the in-plane direction decrease as the number of polymer particles in the sheet increases, while that in the through-plane direction increases. Consequently, a greater temperature gradient is observed for the anisotropy-controlled CNT sheet as compared to that detected for the CNT sheet without anisotropy control when a part of the sheet is heated, which results in a higher power density for the planar-type thermoelectric device. These findings are quite useful for the development of flexible and wearable thermoelectric batteries using CNT sheets.

Antibody-Conjugated Gel-Coated Single-Walled Carbon Nanotubes as Photothermal Agents.

Photothermal therapy (PTT) using near-infrared (NIR) light is an attractive treatment modality for cancer, in which photothermal agents absorb energy from photons and convert it into thermal energy to lead to cancer cell death. Among the various organic and inorganic materials, single-walled carbon nanotubes (SWCNTs) are promising candidates for NIR photothermal agents due to their strong absorption in this region as well as their high photothermal conversion efficiency. In the development of the SWCNT-based PTT materials, modifications of SWCNTs to offer a stable dispersion for biocompatibility as well as to target the tumor of choice while maintaining their NIR absorption have been required. While modification of SWCNTs through noncovalent methods can be achieved, these modifications can be easily reversed in the body. Contrarily, modifications through covalent attachments, while more desirable, may compromise the NIR absorption characteristics of the SWCNTs. Previously, we reported the development of a synthetic strategy to coat SWCNTs with a cross-linked polymer (i.e., a gel) through a process called CNT Micelle Polymerization and successfully introduced maleimide groups that allowed for postmodification through the ene-thiol reaction without deteriorating the NIR absorption. In this report, we postmodify thiol-containing antibodies (anti-TRP-1, a melanoma specific protein) using maleimide chemistry and find that the SWCNTs conjugated with anti-TRP-1 maintain the characteristic NIR absorption as SWCNTs with dispersion stability. It is estimated that 50 maleimide groups are incorporated in one SWCNT (ca. 280 nm long) and they are modified with 32 TRP-1 fragments. Finally, we successfully use these targeted SWCNTs for the PTT of the melanoma cell line using NIR light (1064 nm; 2 W, 5 min). Our method can be extended to a vast array of specific antibodies as well as other targeting agents.

Thermoelectric Properties of Thin Films from Sorted Single-Walled Carbon Nanotubes.

Single-walled carbon nanotubes (SWCNTs) remain one of the most promising materials of our times. One of the goals is to implement semiconducting and metallic SWCNTs in photonics and microelectronics, respectively. In this work, we demonstrated how such materials could be obtained from the parent material by using the aqueous two-phase extraction method (ATPE) at a large scale. We also developed a dedicated process on how to harvest the SWCNTs from the polymer matrices used to form the biphasic system. The technique is beneficial as it isolates SWCNTs with high purity while simultaneously maintaining their surface intact. To validate the utility of the metallic and semiconducting SWCNTs obtained this way, we transformed them into thin free-standing films and characterized their thermoelectric properties.

Supramolecular Chemistry-Based One-Pot High-Efficiency Separation of Solubilizer-Free Pure Semiconducting Single-Walled Carbon Nanotubes: Molecular Strategy and Mechanism.

Single-walled carbon nanotubes (SWCNTs) have the potential to revolutionize nanoscale electronics and power sources; however, their low purity and high separation cost limit their use in practical applications. Here we present a supramolecular chemistry-based one-pot, less expensive, scalable, and highly efficient separation of a solubilizer/adsorbent-free pure semiconducting SWCNT (sc-SWCNT) using flavin/isoalloxazine analogues with different substituents. On the basis of both experimental and computational simulations (DFT study), we have revealed the molecular requirements of the solubilizers as well as provided a possible mechanism for such a highly efficient selective sc-SWCNT separation. The present sorting method is very simple (one-pot) and gives a promising sc-SWCNT separation methodology. Thus, the study provides insight for the molecular design of an sc-SWCNT solubilizer with a high (n,m)-chiral selectivity, which benefits many areas including semiconducting nanoelectronics, thermoelectric, bio and energy materials, and devices using solubilizer-free very pure sc-SWCNTs.

Alkaline Stability of Anion-Conductive Ionomer Coated on a Carbon Surface.

Anion-exchange membrane fuel cells (AEMFCs) are promising technologies that allow the use of nonprecious metals as catalysts because the oxidation reduction reaction at the cathode occurs readily at the high pH of AEMFCs. However, the insufficient chemical stability of the anion-conductive materials in AEMFCs currently limits their development. We studied the chemical stability of the electrolyte in the catalyst layer of AEMFCs containing cationic dimethyl polybenzimidazole (mPBI). Although degradation was observed in an mPBI membrane under alkaline conditions, mPBI coated on a carbon support showed excellent alkaline stability. Because no glass transition temperature was observed for mPBI after coating on the support, the increase of chemical stability was probably associated with the decrease of polymer flexibility.

Brighter near-IR emission of single-walled carbon nanotubes modified with a cross-linked polymer coating.

Photoluminescence (PL) in the near-infrared (NIR) region is an attractive feature of single-walled carbon nanotubes (SWNTs). In this study, we investigated the effect of the chemical structure of the cross-linked polymer coating of polymer-coated SWNTs on the NIR PL emission intensity. We found that brighter NIR emission can be achieved using a more hydrophobic polymer coating.

Thermoelectric properties of sorted semiconducting single-walled carbon nanotube sheets.

Single-walled carbon nanotubes (SWNTs), especially their semiconducting type, are promising thermoelectric (TE) materials due to their high Seebeck coefficient. In this study, the in-plane Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) of sorted semiconducting SWNT (s-SWNT) free-standing sheets with different s-SWNT purities are measured to determine the figure of merit ZT. We find that the ZT value of the sheets increases with increasing s-SWNT purity, mainly due to an increase in Seebeck coefficient while the thermal conductivity remaining constant, which experimentally proved the superiority of the high purity s-SWNT as TE materials for the first time. In addition, from the comparison between sorted and unsorted SWNT sheets, it is recognized that the difference of ZT between unsorted SWNT and high-purity s-SWNT sheet is not remarkable, which suggests the control of carrier density is necessary to further clarify the superiority of SWNT sorting for TE applications.

Solvatochromism of near infrared photoluminescence from doped sites of locally functionalized single-walled carbon nanotubes.

The doped sites of locally functionalized single-walled carbon nanotubes emit red-shifted and bright near-infrared photoluminescence compared to non-doped nanotubes. Here, we observe unique photoluminescent solvatochromism. Organic solvent environments induce photoluminescent energy shifts that linearly correlate with a solvent polarity function. A high responsiveness at the doped sites is found.

Single-step isolation of carbon nanotubes with narrow-band light emission characteristics.

Lack of necessary degree of control over carbon nanotube (CNT) structure has remained a major impediment factor for making significant advances using this material since it was discovered. Recently, a wide range of promising sorting methods emerged as an antidote to this problem, all of which unfortunately have a multistep nature. Here we report that desired type of CNTs can be targeted and isolated in a single step using modified aqueous two-phase extraction. We achieve this by introducing hydration modulating agents, which are able to tune the arrangement of surfactants on their surface, and hence make selected CNTs highly hydrophobic or hydrophilic. This allows for separation of minor chiral species from the CNT mixture with up to 99.7 ± 0.02% selectivity without the need to carry out any unnecessary iterations. Interestingly, our strategy is also able to enrich the optical emission from CNTs under selected conditions.

Multistep Wavelength Switching of Near-Infrared Photoluminescence Driven by Chemical Reactions at Local Doped Sites of Single-Walled Carbon Nanotubes.

Local chemical functionalization is used for defect doping of single-walled carbon nanotubes (SWNTs), to develop near-infrared photoluminescence (NIR PL) properties. We report the multistep wavelength shifting of the NIR PL of SWNTs through chemical reactions at local doped sites tethered to an arylaldehyde group. The PL wavelength of the doped SWNTs is modulated based on imine chemistry. This involves the imine formation of aldehyde groups with added arylamines, imine dissociation reaction, exchange reaction of bound arylamines in the imine, and the Kabachnik-Fields reaction of imine groups using diisopropyl phosphite. Using doped sites as a localized chemical reaction platform can exploit the versatile molecularly driven functionality of carbon nanotubes and related nanomaterials.

Synthesis of Single-Walled Carbon Nanotubes Coated with Thiol-Reactive Gel via Emulsion Polymerization.

Single-walled carbon nanotubes (SWNTs) have unique near-infrared absorption and photoemission properties that are attractive for in vivo biological applications such as photothermal cancer treatment and bioimaging. Therefore, a smart functionalization strategy for SWNTs to create biocompatible surfaces and introduce various ligands to target active cancer cells without losing the unique optical properties of the SWNTs is strongly desired. This paper reports the design and synthesis of a SWNT/gel hybrid containing maleimide groups, which react with various thiol compounds through Michael addition reactions. In this hybrid, the method called carbon nanotube micelle polymerization was used to noncovalently modify the surface of SWNTs with a cross-linked polymer gel layer. This method can form an extremely stable gel layer on SWNTs; such stability is essential for in vivo biological applications. The monomer used to form the gel layer contained a maleimide group, which was protected with furan in endo-form. The resulting hybrid was treated in water to induce deprotection via a retro-Diels-Alder reaction and then functionalized with thiol compounds through Michael addition. The functionalization of the hybrid was explored using a thiol-containing fluorescent dye as a model thiol, and the formation of the SWNT-dye conjugate was confirmed by energy transfer from the dye to SWNTs. Our strategy offers a promising SWNT-based platform for biological functionalization for cancer targeting, imaging, and treatment.