DNA sequence motifs for structure-specific recognition and separation of carbon nanotubes.

Single-walled carbon nanotubes (SWNTs) are a family of molecules that have the same cylindrical shape but different chiralities. Many fundamental studies and technological applications of SWNTs require a population of tubes with identical chirality that current syntheses cannot provide. The SWNT sorting problem-that is, separation of a synthetic mixture of tubes into individual single-chirality components-has attracted considerable attention in recent years. Intense efforts so far have focused largely on, and resulted in solutions for, a weaker version of the sorting problem: metal/semiconductor separation. A systematic and general method to purify each and every single-chirality species of the same electronic type from the synthetic mixture of SWNTs is highly desirable, but the task has proven to be insurmountable to date. Here we report such a method, which allows purification of all 12 major single-chirality semiconducting species from a synthetic mixture, with sufficient yield for both fundamental studies and application development. We have designed an effective search of a DNA library of approximately 10(60) in size, and have identified more than 20 short DNA sequences, each of which recognizes and enables chromatographic purification of a particular nanotube species from the synthetic mixture. Recognition sequences exhibit a periodic purine-pyrimidines pattern, which can undergo hydrogen-bonding to form a two-dimensional sheet, and fold selectively on nanotubes into a well-ordered three-dimensional barrel. We propose that the ordered two-dimensional sheet and three-dimensional barrel provide the structural basis for the observed DNA recognition of SWNTs.

Rod hydrodynamics and length distributions of single-wall carbon nanotubes using analytical ultracentrifugation.

Because of their repetitive chemical structure, extreme rigidity, and the separability of populations with varying aspect ratio, SWCNTs are excellent candidates for use as model rodlike colloids. In this contribution, the sedimentation velocities of length and density sorted single-wall carbon nanotubes (SWCNTs) are compared to predictions from rod hydrodynamic theories of increasing complexity over a range of aspect ratios from <50 to >400. Independently measuring all contributions to the sedimentation velocity besides the shape factor, excellent agreement is found between the experimental findings and theoretical predictions for numerically calculated hydrodynamic radius values and for multiterm analytical expansion approximations; values for the hydrodynamic radii in these cases are additionally found to be consistent with the apparent hydrated particle radius determined independently by buoyancy measurements. Lastly, we utilize this equivalency to calculate the apparent distribution of nanotube lengths in each population from their sedimentation coefficient distribution without adjustable parameters, achieving excellent agreement with distributions from atomic force microscopy. The method developed herein provides an alternative for the ensemble measurement of SWCNT length distributions and others rodlike particles.

Chirality-dependent vapor-phase epitaxial growth and termination of single-wall carbon nanotubes.

Structurally uniform and chirality-pure single-wall carbon nanotubes are highly desired for both fundamental study and many of their technological applications, such as electronics, optoelectronics, and biomedical imaging. Considerable efforts have been invested in the synthesis of nanotubes with defined chiralities by tuning the growth recipes but the approach has only limited success. Recently, we have shown that chirality-pure short nanotubes can be used as seeds for vapor-phase epitaxial cloning growth, opening up a new route toward chirality-controlled carbon nanotube synthesis. Nevertheless, the yield of vapor-phase epitaxial growth is rather limited at the present stage, due in large part to the lack of mechanistic understanding of the process. Here we report chirality-dependent growth kinetics and termination mechanism for the vapor-phase epitaxial growth of seven single-chirality nanotubes of (9, 1), (6, 5), (8, 3), (7, 6), (10, 2), (6, 6), and (7, 7), covering near zigzag, medium chiral angle, and near armchair semiconductors, as well as armchair metallic nanotubes. Our results reveal that the growth rates of nanotubes increase with their chiral angles while the active lifetimes of the growth hold opposite trend. Consequently, the chirality distribution of a nanotube ensemble is jointly determined by both growth rates and lifetimes. These results correlate nanotube structures and properties with their growth behaviors and deepen our understanding of chirality-controlled growth of nanotubes.

High-resolution length fractionation of surfactant-dispersed carbon nanotubes.

Length fractionation of colloidal single-wall carbon nanotube (SWCNT) dispersions is required for many studies. Size-exclusion chromatography (SEC) has been developed as a reliable method for high-resolution length fractionation of DNA-dispersed SWCNTs but has not been applied to surfactant-dispersed SWCNTs due to their lower dispersion stability and tendency to adsorb onto SEC stationary phases. Here, we report that SEC length fractionation can be achieved for bile salt dispersed SWCNTs by using porous silica-based beads as the stationary phase and bile salt solution as the mobile phase. We demonstrate that the SEC length sorting method can be combined with existing ultracentrifugation SWCNT sorting methods to produce "orthogonally sorted" samples, including length sorted semiconducting SWCNTs, which are important for electronics applications as well as length sorted empty-core SWCNTs. Importantly, we show that unlike simple length fractionation by SEC or any other method, orthogonal sorting produces samples of consistent quality for different length fractions, with similar UV-vis-nearIR absorption and Raman spectral features.

Protective roles of single-wall carbon nanotubes in ultrasonication-induced DNA base damage.

The overall level of ultrasonication-induced DNA damage is reduced in the presence of single-wall carbon nanotubes (SWCNTs), particularly for DNA lesions formed by one-electron reduction of intermediate radicals. The protective role of SWCNTs observed in this work suggests a contrary view to the general idea that carbon nanotubes have damaging effects on biomolecules.

Concentration measurement of length-fractionated colloidal single-wall carbon nanotubes.

The determination of the carbon concentration of single-wall carbon nanotubes (SWCNTs) in a given dispersion is a basic requirement for many studies. The commonly used optical absorption-based concentration measurement is complicated by the spectral change due to variations in nanotube chirality and length. In particular, the origin of the observed length-dependent spectral change and its effect on concentration determination has been the subject of considerable debate. Here, we use length-fractionated DNA-wrapped SWCNTs to establish the relationship between SWCNT carbon concentration and optical absorption spectra by directly quantifying the amount of wrapping DNA and, independently, the DNA/carbon nanotube mass ratio. We find that SWCNT carbon concentrations derived from either the E(11) peak or spectral baseline deviate significantly from the SWCNT carbon concentrations derived from the DNA measurement method. Instead, SWCNT carbon concentrations derived from the spectral integration of the E(11) optical transition region match most closely with the DNA-derived SWCNT carbon concentrations. We also observe that shorter SWCNT fractions contain more curved carbon nanotubes, and propose that these defective nanotubes are largely responsible for the observed spectral variation with nanotube length.

Quantum interference between the third and fourth exciton states in semiconducting carbon nanotubes using resonance Raman spectroscopy.

We exploit an energy level crossover effect [Haroz et al., Phys. Rev. B 77, 125405 (2008)] to probe quantum interference in the resonance Raman response from carbon nanotube samples highly enriched in the single semiconducting chiralities of (8,6), (9,4), and (10,5). UV Raman excitation profiles of G-band spectra reveal unambiguous signatures of interference between the third and fourth excitonic states (E(33) and E(44)). Both constructive and destructive responses are observed and lead to anomalous intensity ratios in the LO and TO modes. Especially large anomalies for the (10,5) structure result from nearly identical energies found for the two E(ii) transitions. The interference patterns demonstrate that the sign of the exciton-phonon coupling matrix elements changes for the LO mode between the two electronic states, and remains the same for the TO mode. Significant non-Condon contributions to the Raman response are also found.

Bright fluorescence from individual single-walled carbon nanotubes.

Single-walled carbon nanotubes (SWNTs) have unique photophysical properties but low fluorescence efficiency. We have found significant increases in the fluorescence efficiency of individual DNA-wrapped SWNTs upon addition of reducing agents, including dithiothreitol, Trolox, and ß-mercaptoethanol. Brightening was reversible upon removal of the reducing molecules, suggesting that a transient reduction of defect sites on the SWNT sidewall causes the effect. These results imply that SWNTs are intrinsically bright emitters and that their poor emission arises from defective nanotubes.

[Advances in risk factors and genetic risk of head and neck squamous cell carcinoma].

With the incidence of head and neck squamous cell carcinoma increasing year by year and the epidemiological trend changes, the research on risk factors of head and neck squamous cell carcinoma and its primary prevention has attracted more and more attention. At present, many risk factors of head and neck squamous cell carcinoma have been studied, which can be divided into genetic factors and non-genetic factors. Among the non-genetic factors, viral infection （HPV virus, Epstein-Barr virus）, lifestyle （smoking, drinking, diet, chewing betel nut）, social behavior factors, immunosuppression, occupational exposure, and other factors are closely related to HNSCC incidence. In this paper, the research progress of HNSCC related risk factors is reviewed to provide evidence for the identification of the risk of head and neck squamous cell carcinoma and reduce the risk behaviors to reduce the incidence of head and neck squamous cell carcinoma.

The Association between Self-Rated Health and Health Self-Management Ability of Healthcare Undergraduates: The Chain Mediating Roles of eHealth Literacy and Resistance to Peer Influence.

Health self-management is important for healthcare undergraduates who are at the late adolescent or early adulthood stage, and will play an important part in health promotion for the general population. Previous research has shown that perceived health status affects health self-management. However, few studies have uncovered the mechanism between self-rated health and health self-management among healthcare undergraduates. Based on social ecology theory and Pender's health promotion model, this study aimed to explore the associations between health self-management ability, self-rated health, eHealth literacy and resistance to peer influence of healthcare undergraduates, with a focus on identifying the mediating effects of eHealth literacy and resistance to peer influence. A cross-sectional study was conducted among 515 healthcare undergraduates in Eastern China between July and September 2021. Serial mediation analysis was performed using Haye's Model 6 PROCESS macro to examine the mediating effects. The study found that health self-management ability was significantly and positively correlated with eHealth literacy, resistance to peer influence and self-rated health. Self-rated health had a direct and positive predictive effect on health self-management, with a direct effect value of 0.654. eHealth literacy and resistance to peer influence played both an independent mediating and a chain-mediating role in the mechanism of self-rated health affecting health self-management among healthcare undergraduates, with indirect effect values of 0.085, 0.101, and 0.013, respectively. The results suggest that eHealth literacy and resistance to peer influence could be intervention targets in programs for improving these students' health self-management ability.

Evolution of DNA sequences toward recognition of metallic armchair carbon nanotubes.

The armchair carbon nanotube is an ideal system to study fundamental physics in one-dimensional metals and potentially a superb material for applications such as electrical power transmission. Synthesis and purification efforts to date have failed to produce a homogeneous population of such a material. Here we report evolutionary strategies to find DNA sequences for the recognition and subsequent purification of (6,6) and (7,7) armchair species from synthetic mixtures. The new sequences were derived by single-point scanning mutation and sequence motif variation of previously identified ones for semiconducting tubes. Optical absorption spectroscopy of the purified armchair tubes revealed well-resolved first- and second-order electronic transitions accompanied by prominent sideband features that have neither been predicted nor observed previously. Resonance Raman spectroscopy showed a single Lorentzian peak for the in-plane carbon-carbon stretching mode (G band) of the armchair tubes, repudiating the common practice of using such a line shape to infer the absence of metallic species. Our work demonstrates the exquisite sensitivity of DNA to nanotube metallicity and makes the long-anticipated pure armchair tubes available as seeds for their mass amplification.

Recognition ability of DNA for carbon nanotubes correlates with their binding affinity.

The ability to sort mixtures of carbon nanotubes (CNTs) based on chirality has recently been demonstrated using special short DNA sequences that recognize certain matching CNTs of specific chirality. In this work, we report on a study of the relationship between recognition sequences and the strength of their binding to the recognized CNT. We have chosen the (6,5) CNT and its corresponding DNA recognition sequences for investigation in this study. Binding strength is quantified by studying the kinetics of DNA replacement by a surfactant, which is monitored by following shifts in the absorption spectrum. We find that recognition ability correlates strongly with binding strength thus measured; addition or subtraction of just one base from the recognition sequence can enhance the kinetics of DNA displacement some 20-fold. The surfactant displaces DNA in two steps: a rapid first stage lasting less than a few seconds, followed by progressive removal lasting tens of minutes. The kinetics of the second stage is analyzed to extract activation energies. Fluorescence studies support the finding that the DNA sequence that recognizes the (6,5)-CNT forms a more stable hybrid than its close relatives.

Multiple exciton generation in single-walled carbon nanotubes.

Upon absorption of single photons, multiple excitons were generated and detected in semiconducting single-walled carbon nanotubes (SWNTs) using transient absorption spectroscopy. For (6,5) SWNTs, absorption of single photons with energies corresponding to three times the SWNT energy gap results in an exciton generation efficiency of 130% per photon. Our results suggest that the multiple exciton generation threshold in SWNTs can be close to the limit defined by energy conservation.

Prognostic value of prognostic nutritional index in nasopharyngeal carcinoma: A meta-analysis containing 4511 patients.

OBJECTIVES: The prognostic nutritional index (PNI) is an index reflecting the nutritional and inflammatory status of patients and is explored for prognosis in nasopharyngeal carcinoma (NPC). However, the data are conflicting. In the current study, a meta-analysis was performed to comprehensively clarify the association between PNI and prognosis of NPC. MATERIALS AND METHODS: PubMed, Web of Science, the Cochrane library, China National Knowledge Infrastructure (CNKI), and Wanfang database were searched up to July 25, 2020. Hazard ratio (HR) and with 95% confidence interval (CI) were calculated to assess the impact of PNI on the survival outcomes of patients with NPC. RESULTS: A total of 10 studies containing with 4511 patients were identified. The pooled results showed that NPC patients with a low PNI would have a worse overall survival (OS) (HR = 1.89, 95%CI = 1.59-2.25, p < 0.001), distant metastasis-free survival (DMFS) (HR = 2.01. 95%CI = 1.66-2.43, p < 0.001), progression-free survival (PFS) (HR = 1.59, 95%CI = 1.32-1.91, p < 0.001), and locoregional recurrence-free survival (LRRFS) (HR = 1.51, 95%CI = 1.04-2.21, p = 0.032). Subgroup analysis showed that the low PNI was still a significant prognostic factor for OS and DMFS. CONCLUSIONS: Our meta-analysis demonstrated that a low PNI was significantly correlated to poor OS, DMFS, PFS, and LRRFS in NPC. Therefore, we suggest PNI applied as an indicator for prediction of the short- and long- term survival outcomes in patients with NPC.

Optical characterizations and electronic devices of nearly pure (10,5) single-walled carbon nanotubes.

It remains an elusive goal to achieve high-performance single-walled carbon nanotube (SWNT) field-effect transistors (FETs) composed of only single-chirality SWNTs. Many separation mechanisms have been devised and various degrees of separation demonstrated, yet it is still difficult to reach the goal of total fractionation of a given nanotube mixture into its single-chirality components. Chromatography has been reported to separate small SWNTs (diameter < or = 0.9 nm) according to their diameter, chirality, and length. The separation efficiency decreased with increasing tube diameter when the ssDNA sequence d(GT)(n) (n = 10-45) was used. Here we report our results on the separation of single-chirality (10,5) SWNTs (diameter = 1.03 nm) from HiPco tubes using ion-exchange chromatography. The separation efficiency was improved by using the new DNA sequence (TTTA)(3)T, which can recognize SWNTs with the specific chirality (10,5). The chirality of the separated tubes was examined by optical absorption, Raman, photoluminescence excitation/emission, and electrical transport measurements. All of the spectroscopic methods gave a single peak of (10,5) tubes. The purity was 99% according to the electrical measurement. The FETs composed of separated SWNTs in parallel gave an I(on)/I(off) ratio up to 10(6) due to the single-chirality-enriched (10,5) tubes. This is the first time that SWNT FETs with single-chirality SWNTs have been achieved. The chromatography method has the potential to separate even larger diameter semiconducting SWNTs from other starting materials to further improve the performance of the SWNT FETs.

Assessment of chemically separated carbon nanotubes for nanoelectronics.

It remains an elusive goal to obtain high performance single-walled carbon-nanotube (SWNT) electronics such as field effect transistors (FETs) composed of single- or few-chirality SWNTs, due to broad distributions in as-grown materials. Much progress has been made by various separation approaches to obtain materials enriched in metal or semiconducting nanotubes or even in single chiralties. However, research in validating SWNT separations by electrical transport measurements and building functional electronic devices has been scarce. Here, we performed length, diameter, and chirality separation of DNA functionalized HiPco SWNTs by chromatography methods, and we characterized the chiralities by photoluminescence excitation spectroscopy, optical absorption spectroscopy, and electrical transport measurements. The use of these combined methods provided deeper insight to the degree of separation than either technique alone. Separation of SWNTs by chirality and diameter occurred at varying degrees that decreased with increasing tube diameter. This calls for new separation methods capable of metallicity or chirality separation of large diameter SWNTs (in the approximately 1.5 nm range) needed for high performance nanoelectronics. With most of the separated fractions enriched in semiconducting SWNTs, nanotubes placed in parallel in short-channel (approximately 200 nm) electrical devices fail to produce FETs with high on/off switching, indicating incomplete elimination of metallic species. In rare cases with a certain separated SWNT fraction, we were able to fabricate FET devices composed of small-diameter, chemically separated SWNTs in parallel, with high on-/off-current (I(on)/I(off)) ratios up to 105 owing to semiconducting SWNTs with only a few (n,m) chiralities in the fraction. This was the first time that chemically separated SWNTs were used for short channel, all-semiconducting SWNT electronics dominant by just a few (n,m)'s. Nevertheless, the results suggest that much improved chemical separation methods are needed to produce nanotube electronics at a large scale.

Selective synthesis combined with chemical separation of single-walled carbon nanotubes for chirality selection.

Single-walled carbon nanotubes (SWNTs) are potential materials for future nanoelectronics. Since the electronic and optical properties of SWNTs strongly depend on tube diameter and chirality, obtaining SWNTs with narrow (n,m) chirality distribution by selective growth or chemical separation has been an active area of research. Here, we demonstrate that a new, bimetallic FeRu catalyst affords SWNT growth with narrow diameter and chirality distribution in methane CVD. At 600 degrees C, methane CVD on FeRu catalyst produced predominantly (6,5) SWNTs according to UV-vis-NIR absorption and photoluminescence excitation/emission (PLE) spectroscopic characterization. At 850 degrees C, the dominant semiconducting species produced are (8,4), (7,6), and (7,5) SWNTs, with much narrower distributions in diameter and chirality than materials grown by other catalysts. Further, we show that narrow diameter/chirality growth combined with chemical separation by ion exchange chromatography (IEC) greatly facilitates achieving single (m,n) SWNT samples, as demonstrated by obtaining highly enriched (8,4) SWNTs with near elimination of metallic SWNTs existing in the as-grown material.

Bright Fraction of Single-Walled Carbon Nanotubes through Correlated Fluorescence and Topography Measurements.

Correlated measurements of fluorescence and topography were performed for individual single-walled carbon nanotubes (SWNTs) on quartz using epifluorescence confocal microscopy and atomic force microscopy (AFM). Surprisingly, only ~11% of all SWNTs in DNA-wrapped samples were found to be highly emissive on quartz, suggesting that the ensemble fluorescence quantum yield is low because only a small population of SWNTs fluoresces strongly. Qualitatively similar conclusions were obtained from control studies using a sodium cholate surfactant system. To accommodate AFM measurements, excess surfactant was removed from the substrate. Though individual SWNTs on nonrinsed and rinsed surfaces displayed differences in fluorescence intensities and line widths, arising from the influence of the local environment on individual SWNT optical measurements, photoluminescence data from both samples displayed consistent trends.

Fundamental optical processes in armchair carbon nanotubes.

Single-wall carbon nanotubes provide ideal model one-dimensional (1-D) condensed matter systems in which to address fundamental questions in many-body physics, while, at the same time, they are leading candidates for building blocks in nanoscale optoelectronic circuits. Much attention has been recently paid to their optical properties, arising from 1-D excitons and phonons, which have been revealed via photoluminescence, Raman scattering, and ultrafast optical spectroscopy of semiconducting carbon nanotubes. On the other hand, dynamical properties of metallic nanotubes have been poorly explored, although they are expected to provide a novel setting for the study of electron-hole pairs in the presence of degenerate 1-D electrons. In particular, (n,n)-chirality, or armchair, metallic nanotubes are truly gapless with massless carriers, ideally suited for dynamical studies of Tomonaga-Luttinger liquids. Unfortunately, progress towards such studies has been slowed by the inherent problem of nanotube synthesis whereby both semiconducting and metallic nanotubes are produced. Here, we use post-synthesis separation methods based on density gradient ultracentrifugation and DNA-based ion-exchange chromatography to produce aqueous suspensions strongly enriched in armchair nanotubes. Through resonant Raman spectroscopy of the radial breathing mode phonons, we provide macroscopic and unambiguous evidence that density gradient ultracentrifugation can enrich ensemble samples in armchair nanotubes. Furthermore, using conventional, optical absorption spectroscopy in the near-infrared and visible range, we show that interband absorption in armchair nanotubes is strongly excitonic. Lastly, by examining the G-band mode in Raman spectra, we determine that observation of the broad, lower frequency (G(-)) feature is a result of resonance with non-armchair "metallic" nanotubes. These findings regarding the fundamental optical absorption and scattering processes in metallic carbon nanotubes lay the foundation for further spectroscopic studies to probe many-body physical phenomena in one dimension.

Chirality-controlled synthesis of single-wall carbon nanotubes using vapour-phase epitaxy.

Chirality-controlled synthesis of single-wall carbon nanotubes with predefined chiralities has been an important but elusive goal for almost two decades. Here we demonstrate a general strategy for producing carbon nanotubes with predefined chiralities by using purified single-chirality nanotubes as seeds for subsequent metal catalyst free growth, resembling vapour-phase epitaxy commonly used for semiconductor films. In particular, we have successfully synthesized (7, 6), (6, 5) and (7, 7) nanotubes, and used Raman spectroscopy to show unambiguously that the original chiralities of the nanotube seeds are preserved. Furthermore, we have performed electrical measurements on synthesized individual (7, 6) and (6, 5) nanotubes, confirming their semiconducting nature. The vapour-phase epitaxy approach is found to be highly robust and should enable a wide range of fundamental studies and technological developments.