Reversible, Band-Gap-Selective Protonation of Single-Walled Carbon Nanotubes in Solution

Solubilization of Carbon Nanobelts in Aqueous Solutions: Optical and Colloidal Properties

Carbon nanobelts (CNBs) correspond to carbon nanotube (CNT) segments and are insoluble in most common aqueous solutions, posing challenges across diverse applications. In this study, [12] CNB, which corresponds to a (6,6) CNT segment, was solubilized by aliphatic surfactant micelles through host-guest complexation, which was confirmed by comprehensive analyses involving spectrophotometry, mass spectrometry, and molecular dynamics simulations. Through this solubilization, zero-Stokes shift emission of the CNB could occur, which could be ascribed to the symmetry-allowed transition. In contrast, CNB was insoluble in non-aliphatic surfactant solutions. The mechanism by which CNB is solubilized using aliphatic surfactants is completely distinct from that of the CNT dispersion mechanism. The present finding provides knowledge of the effectiveness of aliphatic compounds in solubilizing CNBs and their derivatives (carbon nanohoops), which show significant potential for various applications in aqueous systems, including biological applications.

Densification effect on field emission characteristics of CNT film emitters for electron emission devices

Field electron emission characteristics of the carbon nanotube (CNT) film emitters were investigated according to densification conditions such as nitric acid, acetic acid, and salicylic acid. The emission performance of the CNT film emitters was strongly affected by the densification conditions. Salicylic acid exhibits the best field electron emission properties of the CNT film emitters, followed by nitric acid and acetic acid. The efficient densification of the CNT film emitter by salicylic acid is caused by the role of polarity and p orbitals, nitric acid by hydrogen ions, and acetic acid by weak polarity. After the densification with salicylic acid, the turn-on field of the CNT film emitter decreases from 1.94 Vμm-1to 1.86 Vμm-1, the threshold field decreases from 3.41 Vμm-1to 2.95 Vμm-1, the emission current significantly increases from 20.92 mA to 43.98 mA, and the degradation rate from the long-term emission stability decreases from 49.9% to 21%. The improved emission characteristics are attributed to the increased emission sites at the CNT film and the increased electrical conductivity of the CNT film. The densification is a useful way to enhance the field electron emission properties of CNT film emitters.

Novel Green Fluorescent Probe Stem From Carbon Quantum Dots for Specific Recognition of Tyrosinase in Serum and Living Cells

Tyrosinase (TYR), an important biomarker for melanoma, offered significant information early detection of melanoma and may decrease the likelihood of mortality. Therefore, this article constructed a highly sensitive and selective green fluorescent functionalized carbon quantum dots (TYR-CQDs) for tyrosinase (TYR) activity detection by one-step hydrothermal protocol utilizing catechol, citric acid and urea as precursors. The prepared TYR-CQDs illustrated excellent linear relationship and broad linear range with a low detection limit, which exhibited high accuracy and recovery in quantitative determination of TYR in human serum samples. Furthermore, the TYR-CQDs had successfully realized intracellular TYR detection owing to excellent biocompatibility, high anti-interference ability and good cellular imaging capability, suggesting the potential biomedical applications in early diagnosis of melanoma and other tyrosinase-related diseases.

Coenzyme corona formation on carbon nanotubes leads to disruption of the redox balance in metabolic reactions

Carbon nanotubes (CNTs) have adverse impacts on metabolism in biological systems. The impacts should be associated with interactions of the CNTs with coenzymes, such as nicotinamide adenine dinucleotide (NAD), because most metabolic processes are governed by coenzyme-dependent reactions. This study demonstrates that NAD molecules adsorb onto the CNT surface, leading to the formation of interfacial NAD layers-in other words, a coenzyme corona (coenzyme-based biomolecular corona). Coenzyme corona formation is accompanied by the oxidation of NAD at biological concentrations through electron transfer. Similar phenomena are observed for NAD derivatives. Molecular dynamics simulations indicate that the adsorption of NAD onto CNTs is driven by interactions between the aromaphilic groups of NAD and the CNT surfaces, leading to coenzyme corona formation. Generally, in living biological systems, the balance of NAD redox (NADH/NAD⁺ redox) is maintained to sustain metabolism. The present results suggest that CNTs affect coenzyme-dependent metabolic reactions by disrupting the redox balance through coenzyme corona formation and subsequent coenzyme oxidation. The proposed molecular mechanism not only advances the fundamental understanding of the biological impact of CNTs in terms of metabolism but also contributes to biological CNT applications.

Hybrid, dual visible and near-infrared fluorescence emission of (6,5) single-walled carbon nanotubes modified with fluorescein through aryl diazonium salt chemistry

The aryl diazonium salt chemistry offers enhancement of near-infrared (NIR) emission of single-walled carbon nanotubes (SWCNTs), although, the attachment of functional molecules which could bring hybrid properties through the process is underdeveloped. In this work, we utilize aryl diazonium salt of fluorescein to createsp³defects on (6,5) SWCNTs. We study the influence of pH on the grafting process identifying that pH 5-6 is necessary for a successful reaction. The fluorescein-modified (6,5) SWCNTs (F-(6,5) SWCNTs) exhibit red-shiftedE₁₁* emission in the NIR region attributed to luminescentsp³defects, but also visible (Vis) fluorescence at 515 nm from surface-attached fluorescein molecules. The fluorescence in both Vis and NIR regions of F-(6,5) SWCNTs exhibit strong pH-dependency associated with the dissociation of fluorescein molecules with an indication of photoinduced-electron transfer quenching the Vis emission of fluorescein dianion. The F-(6,5) SWCNTs could potentially be used for dual-channel medical imaging as indicated by our preliminary experiments. We hope that our research will encourage new, bold modifications of SWCNTs with functional molecules introducing new, unique hybrid properties.

Immobilization of Polyoxometalates on Carbon Nanotubes: Tuning Catalyst Activity, Selectivity and Stability in H2O2-Based Oxidations

In recent years, carbon nanotubes (CNTs), including N-doped ones (N-CNTs), have received significant attention as supports for the construction of heterogeneous catalysts. In this work, we summarize our progress in the application of (N)-CNTs for immobilization of anionic metal-oxygen clusters or polyoxometalates (POMs) and use of (N)-CNTs-supported POM as catalysts for liquid-phase selective oxidation of organic compounds with the green oxidant–aqueous hydrogen peroxide. We discuss here the main factors, which favor adsorption of POMs on (N)-CNTs and ensure a quasi-molecular dispersion of POM on the surface and their strong attachment to the support. The effects of the POM nature, N-doping of CNTs, acid additives, and other factors on the POM immobilization process and catalytic activity/selectivity of the (N)-CNTs-immobilized POMs are analyzed. Particular attention is drawn to the critical issue of the catalyst stability and reusability. The scope and limitations of the POM/(N)-CNTs catalysts in H2O2-based selective oxidations are discussed.

Gel Chromatography for Separation of Single-Walled Carbon Nanotubes

Carbon nanotubes (CNTs), having either metallic or semiconducting properties depending on their chirality, are advanced materials that can be used for different devices and materials (e.g., fuel cells, transistors, solar cells, reinforced materials, and medical materials) due to their excellent electrical conductivity, mechanical strength, and thermal conductivity. Single-walled CNTs (SWNTs) have received special attention due to their outstanding electrical and optical properties; however, the inability to selectively synthesize specific types of CNTs has been a major obstacle for their commercialization. Therefore, researchers have studied different methods for the separation of SWNTs based on their electrical and optical properties. Gel chromatography methods enable the large-scale separation of metallic/semiconducting (m/s) SWNTs and single-chirality SWNTs with specific bandgaps. The core principle of gel chromatography-based SWNT separation is the interaction between the SWNTs and gels, which depends on the unique electrical properties of the former. Controlled pore glass, silica gel, agarose-based gel, and allyl dextran-based gel have been exploited as mediums for gel chromatography. In this paper, the interaction between SWNTs and gels and the different gel chromatography-based SWNT separation technologies are introduced. This paper can serve as a reference for researchers who plan to separate SWNTs with gel chromatography.

Optical Response Characteristics of Single-Walled Carbon Nanotube Chirality Exposed to Oxidants with Different Oxidizing Power

Semiconductor single-walled carbon nanotubes (SWNTs) have unique characteristics owing to differences in the three-dimensional structure (chirality) expressed by the chiral index (n,m), and many studies on the redox characteristics of chirality have been reported. In this study, we investigated the relationship between the chirality of SWNTs and the oxidizing power of oxidants by measuring the near-infrared (NIR) absorption spectra of two double-stranded DNA-SWNT complexes with the addition of three oxidants with different oxidizing powers. A dispersion was prepared by mixing 0.5 mg of SWNT powder with 1 mg/mL of DNA solution. Different concentrations of hydrogen peroxide (H₂O₂), potassium hexachloroidylate (IV) (K₂IrCl₆), or potassium permanganate (KMnO₄) were added to the dispersion to induce oxidation. Thereafter, a catechin solution was added to observe if the absorbance of the oxidized dispersion was restored by the reducing action of the catechin. We found that the difference in the oxidizing power had a significant effect on the detection sensitivity of the chiralities of the SWNTs. Furthermore, we revealed a detectable range of oxidants with different oxidizing powers for each chirality.

Dye Quenching of Carbon Nanotube Fluorescence Reveals Structure-Selective Coating Coverage

Many properties and applications of single-wall carbon nanotubes (SWCNTs) depend strongly on the coatings that allow their suspension in aqueous media. We report that SWCNT fluorescence is quenched by reversible physisorption of dye molecules such as methylene blue, and that measurements of that quenching can be used to infer structure-specific exposures of the nanotube surface to the surrounding solution. SWCNTs suspended in single-stranded DNA oligomers show quenching dependent on the combination of nanotube structure and ssDNA base sequence. Several sequences are found to give notably high or low surface coverages for specific SWCNT species. These effects seem correlated with the selective recognitions used for DNA-based structural sorting of nanotubes. One notable example is that dye quenching of fluorescence from SWCNTs coated with the (ATT)₄ base sequence is far stronger for one (7,5) enantiomer than for the other, showing that coating coverage is associated with the coating affinity difference reported previously for this system. Equilibrium modeling of quenching data has been used to extract parameters for comparative complexation constants and accessible surface areas. Further insights are obtained from molecular dynamics simulations, which give estimated contact areas between ssDNA and SWCNTs that correlate with experimentally inferred surface exposures and account for the enantiomeric discrimination of (ATT)₄.

Quantitative analysis of the effect of reabsorption on the Raman spectroscopy of distinct (n, m) carbon nanotubes

We quantitatively analyze the effect of reabsorption on the Raman spectroscopy of (10, 3) and (8, 3) single-chirality single-wall carbon nanotube (SWCNT) solutions by varying the detection depth in confocal micro-Raman measurements and SWCNT concentration the in sample solution. The increase of the detection depth and concentration of SWCNTs enhances the reabsorption effect and decreases the intensities of the Raman features. More importantly, reabsorption exhibits different effects on different Raman features such as the radial breathing mode (RBM) and G+ band, strongly depending on the resonance degree of the scattered light energy and the interband transition of SWCNTs. When (10, 3) SWCNTs are excited with a 633 nm laser, the scattered light from RBM has stronger resonance with the interband transition of the SWCNTs than that from the G+ band, leading to a faster reduction in the RBM intensity and a lower intensity ratio of RBM to the G+ band. In contrast, when (8, 3) SWCNTs are excited with a 633 nm laser, reabsorption has the same effect on the RBM and G+ band intensities and thus maintains a constant intensity ratio of RBM to the G+ band. Furthermore, we precisely establish a quantitative relationship of the intensities of the Raman features such as RBM, the G+ band and their intensity ratio as a function of the focal depth and SWCNT concentration by theoretical calculations and numerical simulation, which reproduces the experimental results well. These results are very useful in the precise analysis of the Raman spectroscopy of SWCNTs and thus their applications in molecular detection and imaging.

Aggregation and stability of nanoscale plastics in aquatic environment

The widespread use and release of plastics in nature have raised global concerns about their impact on public health and the environment. While much research has been conducted on macro- and micro-sized plastics, the fate of nanoscale plastics remains unexplored. In this study, the aggregation kinetics and stability of polyethylene and polystyrene nanoscale plastics were investigated over a wide range of aquatic chemistries (pH, salt types (NaCl, CaCl_2, MgCl₂), ionic strength) relevant to the natural environment. Results showed that salt types and ionic strength had significant effects on the stability of both polyethylene and polystyrene nanoscale plastics, while pH had none. Aggregation and stability of both polyethylene and polystyrene nanoscale plastics in the aquatic environment followed colloidal theory (DLVO theory and Schulze-Hardy rule), similar to other colloidal particles. The critical coagulation concentration (CCC) values of polyethylene nanoscale plastics were lower for CaCl₂ (0.1 mM) compared to NaCl (80 mM) and MgCl₂ (3 mM). Similarly, CCC values of polystyrene nanospheres were 10 mM for CaCl₂, 800 mM for NaCl and 25 mM for MgCl₂. It implies that CaCl₂ destabilized both polyethylene and polystyrene nanoscale plastics more aggressively than NaCl and MgCl₂. Moreover, polystyrene nanospheres are more stable in the aquatic environment than polyethylene nanoscale plastics. However, natural organic matter improved the stability of polyethylene nanoscale plastics in water primarily due to steric repulsion, increasing CCC values to 0.4 mM, 120 mM and 8 mM for CaCl_2, NaCl and MgCl₂ respectively. Stability studies with various water conditions demonstrated that polyethylene nanoscale plastics will be fairly stable in the natural surface waters. Conversely, synthetic surface water, wastewater, seawater and groundwater rapidly destabilized polyethylene nanoscale plastics. Overall, our findings indicate that significant aqueous transport of nanoscale plastics will be possible in natural surface waters.

Carbon dots with pH-responsive fluorescence: a review on synthesis and cell biological applications

This review summarizes state of the art synthesis and applications of carbon dots (CDs) with pH-responsive fluorescence. Following an introduction, the first section covers methods for the preparation of pH-responsive CDs, with subsections on general methods for preparing CDs (by hydrothermal, solvothermal, electrochemical, microwave, laser ablation, pyrolysis or chemical oxidation polymerization methods), and on precursors for synthesis. This is followed by a section on the mechanisms of pH-responsivity (by creating new functional groups, change of energy levels, protonation and deprotonation, aggregation, or by introduction shells). Several Tables are presented that give an overview of the wealth of methods and materials. A final section covers applications of carbon dots (CDs) with pH-responsive fluorescence for sensing, drug delivery, and imaging. The conclusion summarizes the current status, addresses challenges, and gives an outlook on potential future trends. Graphical abstract The synthesis and biological applications of carbon dots(CDs) with pH-responsive fluorescence are summarized. Precursors and methods for preparation of pH-responsive CDs, mechanisms of pH-responsivity, and biological applications of CDs with pH-responsive fluorescence for sensing, drug delivery, and imaging are discussed.

Packaging vertically aligned carbon nanotubes into a heat-shrink tubing for efficient removal of phenolic pollutants

Owing to their extremely high surface-to-volume ratio, carbon nanotubes (CNTs) are excellent adsorbents for the removal of organic pollutants. However, retrieval or collection of the CNTs after adsorption in existing approaches, which utilize CNTs dispersed in a solution of pollutants, is often more challenging than the removal of pollutants. In this study, we address this challenge by packaging vertically aligned CNTs into a PTFE heat-shrink tubing. Insertion of CNTs into the tubing and subsequent thermal shrinkage densified the CNTs radially by 35% and also reduced wrinkles in the nanotubes. The CNT-based adsorption tube with a circular cross-section enabled both easy functionalization of CNTs and facile connection to a source of polluted water, which we demonstrated for the removal of phenolic compounds. We purified and carboxylated CNTs, by flowing a solution of nitric acid through the tubing, and obtained adsorption capacities of 115, 124, and 81.2 mg g⁻¹ for 0.5 g L⁻¹ of phenol, m-cresol, 2-chlorophenol, respectively. We attribute the high adsorption capacity of our platform to efficient adsorbate-CNT interaction within the narrow interstitial channels between the aligned nanotubes. The CNT-based adsorption tubes are highly promising for the simple and efficient removal of phenolic and other types of organic pollutants.

An adsorption tube prepared by heat-shrinkage of vertically aligned carbon nanotubes provides high adsorption capacity for phenolic compounds.

DNA-SWCNT Biosensors Allow Real-Time Monitoring of Therapeutic Responses in Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a highly desmoplastic cancer with limited treatment options. There is an urgent need for tools that monitor therapeutic responses in real time. Drugs such as gemcitabine and irinotecan elicit their therapeutic effect in cancer cells by producing hydrogen peroxide (H₂O₂). In this study, specific DNA-wrapped single-walled carbon nanotubes (SWCNT), which precisely monitor H₂O₂, were used to determine the therapeutic response of PDAC cells in vitro and tumors in vivo. Drug therapeutic efficacy was evaluated in vitro by monitoring H₂O₂ differences in situ using reversible alteration of Raman G-bands from the nanotubes. Implantation of the DNA-SWCNT probe inside the PDAC tumor resulted in approximately 50% reduction of Raman G-band intensity when treated with gemcitabine versus the pretreated tumor; the Raman G-band intensity reversed to its pretreatment level upon treatment withdrawal. In summary, using highly specific and sensitive DNA-SWCNT nanosensors, which can determine dynamic alteration of hydrogen peroxide in tumor, can evaluate the effectiveness of chemotherapeutics. SIGNIFICANCE: A novel biosensor is used to detect intratumoral hydrogen peroxide, allowing real-time monitoring of responses to chemotherapeutic drugs.

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.

Separation of Small-Diameter Single-Walled Carbon Nanotubes in One to Three Steps with Aqueous Two-Phase Extraction

An aqueous two-phase extraction (ATPE) technique capable of separating small-diameter single-walled carbon nanotubes in one, two, or at the most three steps is presented. Separation is performed in the well-studied two-phase system containing polyethylene glycol and dextran, but it is achieved without changing the global concentration or ratio of cosurfactants. Instead, the technique is reliant upon the different surfactant shell around each nanotube diameter at a fixed surfactant concentration. The methodology to obtain a single set of surfactant conditions is provided, and strategies to optimize these for other diameter regimes are discussed. In total, 11 different chiralities in the diameter range 0.69-0.91 nm are separated. These include semiconducting and both armchair and nonarmchair metallic nanotube species. Titration of cosurfactant suspensions reveal separation to be driven by the pH of the suspension with each ( n, m) species partitioning at a fixed pH. This allows for an ( n, m) separation approach to be presented that is as simple as pipetting known volumes of acid into the ATPE system.

Oxidative Stress of Carbon Nanotubes on Proteins Is Mediated by Metals Originating from the Catalyst Remains

Nanomaterials introduced into biological systems are immediately coated by proteins in vivo. They induce oxidative stress on adsorbed proteins and hence alter the protein structures, which determines the fate pathways and biological impacts of nanomaterials. Carbon nanotubes (CNTs) have been suggested to cause protein oxidation. In this work, we discovered that CNTs induce oxidative stress on proteins in cooperation with coexisting metals originating from catalyst remains. Protein sulfhydryl groups were readily oxidized by the coexistence of CNTs and metals. Numerical simulations of the reaction demonstrated that the metals effectively mediate electron transfer between the CNTs and protein sulfhydryl groups. Thus, the coexistence of CNTs and metals, even in low concentrations, generates oxidative stress on proteins with high reaction rates. Metal catalysts used for CNT growth, in turn, catalyze the oxidation reaction of proteins. The proposed protein oxidation mechanism will advance the fundamental understanding of the biological safety and toxicity of nanomaterials synthesized using metal catalysts.

Chemometric Approaches for Developing Infrared Nanosensors To Image Anthracyclines

Generation, identification, and validation of optical probes to image molecular targets in a biological milieu remain a challenge. Synthetic molecular recognition approaches leveraging the intrinsic near-infrared fluorescence of single-walled carbon nanotubes are promising for long-term biochemical imaging in tissues. However, generation of nanosensors for selective imaging of molecular targets requires a heuristic approach. Here, we present a chemometric platform for rapidly screening libraries of candidate single-walled carbon nanotube nanosensors against biochemical analytes to quantify the fluorescence response to small molecules, including vitamins, neurotransmitters, and chemotherapeutics. We further show this method can be applied to identify biochemical analytes that selectively modulate the intrinsic near-infrared fluorescence of candidate nanosensors. Chemometric analysis thus enables identification of nanosensor-analyte "hits" and also nanosensor fluorescence signaling modalities such as wavelength shifts that are optimal for translation to biological imaging. Through this approach, we identify and characterize a nanosensor for the chemotherapeutic anthracycline doxorubicin (DOX), which provides a ≤17 nm fluorescence red-shift and exhibits an 8 μM limit of detection, compatible with peak circulatory concentrations of doxorubicin common in therapeutic administration. We demonstrate the selectivity of this nanosensor over dacarbazine, a chemotherapeutic commonly co-injected with doxorubicin. Lastly, we establish nanosensor tissue compatibility for imaging of doxorubicin in muscle tissue by incorporating nanosensors into the mouse hindlimb and measuring the nanosensor response to exogenous DOX administration. Our results motivate chemometric approaches to nanosensor discovery for chronic imaging of drug partitioning into tissues and toward real-time monitoring of drug accumulation.

Dopant-Modulated Conjugated Polymer Enrichment of Semiconducting SWCNTs

Conjugated polymer extraction (CPE) is a low-cost, scalable process that can enrich single-walled carbon nanotube (SWCNT) materials in organic media. For other separation methods in aqueous phases, redox chemistry and/or pH control dramatically affect the sorting process of the SWCNTs. We have previously determined that the CPE process can be fine-tuned by adjusting the pH on the tube surface. Here, we systematically studied the effect of redox chemistry on the CPE process by adding organic p-/n-dopants. At a very strong p-/n-doping level, static repulsions dominated the interactions between the tubes and the CPE lost selectivity. When the doping level changed from a medium p-doping to a neutral state, the yield of CPE increased and the selectivity was compromised. We also observed chiral selectivity when a weak p-dopant was used. A photoluminescence excitation mapping under different titration conditions provided more insight into the doping level of the tubes relative to their diameters, chiralities, and redox potentials. We proposed a mechanism for the CPE process. The semiconducting and metallic tubes are separated because of their different solubilities, which are determined by the bundling energy between the tubes and are related to their doping level in polymer solutions.

Carbon Nanotube Formic Acid Sensors Using a Nickel Bis( ortho-diiminosemiquinonate) Selector

Formic acid is corrosive, and a sensitive and selective sensor could be useful in industrial, medical, and environmental settings. We present a chemiresistor for detection of formic acid composed of single-walled carbon nanotubes (CNTs) and nickel bis( ortho-diiminosemiquinonate) (1), a planar metal complex that can act as a ditopic hydrogen-bonding selector. Formic acid is detected in concentrations as low as 83 ppb. The resistance of the material decreases on exposure to formic acid, but slightly increases on exposure to acetic acid. We propose that 1 assists in partial protonation of the CNT by formic acid, but the response toward acetic acid is dominated by inter-CNT swelling. This technology establishes CNT-based chemiresistive discrimination between formic and acetic acid vapors.

Ratiometric photoluminescence sensing based on Ti3C2 MXene quantum dots as an intracellular pH sensor

Intracellular pH sensing is of importance and can be used as an indicator for monitoring the evolution of various diseases and the health of cells. Here, we developed a new class of surface-functionalized MXene quantum dots (QDs), Ti₃C₂, by the sonication cutting and hydrothermal approach and further explored their intracellular pH sensing. The functionalized Ti₃C₂ QDs exhibit bright excitation-dependent blue photoluminescence (PL) originating from the size effect and surface defects. Meanwhile, Ti₃C₂ QDs demonstrate a high PL response induced by the deprotonation of the surface defects. Furthermore, combining the highly pH sensitive Ti₃C₂ QDs with the pH insensitive [Ru(dpp)₃]Cl₂, we developed a ratiometric pH sensor to quantitatively monitor the intracellular pH values. These novel MXene quantum dots can serve as a promising platform for developing practical fluorescent nanosensors.

Multicolour nitrogen-doped carbon dots: tunable photoluminescence and sandwich fluorescent glass-based light-emitting diodes

The first use of the combination of ammonium citrate (AC) and ethylenediamine tetraacetic acid (EDTA) as coordinating precursors for the synthesis of highly fluorescent (quantum yield = 67%) multicolour nitrogen-doped carbon dots (CDs) is reported. Under UV light, these CDs emitted outstanding luminescence in colours from dark blue to red. Interestingly, a single component white-light CD point with high fluorescence efficiency was obtained by surface control. Alterations of the photoluminescence (PL) emission of these full-colour CDs were tentatively proposed to benefit from surface functional groups, such as C[double bond, length as m-dash]O and C[double bond, length as m-dash]N. An energy-level model was proposed to explain the continuously adjustable full-colour emission. The white light may be attributed to the overlap of diverse light emission induced by electron transitions between the energy levels. Subsequently, to avoid aggregation-induced solid-state fluorescence quenching, multicolour CD-based sandwich glasses with various colour emission was fabricated, which is anticipated to be compatible with the all-optical light-emitting diodes (LEDs). The facile preparation and outstanding optical features are believed to provide an alternative synthesis route and inspire more research into applications and CD-based materials of multicolour CDs.

Ionic Strength-Mediated Phase Transitions of Surface-Adsorbed DNA on Single-Walled Carbon Nanotubes

Single-stranded DNA oligonucleotides have unique, and in some cases sequence-specific molecular interactions with the surface of carbon nanotubes that remain the subject of fundamental study. In this work, we observe and analyze a generic, ionic strength-mediated phase transition exhibited by over 25 distinct oligonucleotides adsorbed to single-walled carbon nanotubes (SWCNTs) in colloidal suspension. The phase transition occurs as monovalent salts are used to modify the ionic strength from 500 mM to 1 mM, causing a reversible reduction in the fluorescence quantum yield by as much as 90%. The phase transition is only observable by fluorescence quenching within a window of pH and in the presence of dissolved O₂, but occurs independently of this optical quenching. The negatively charged phosphate backbone increases (decreases) the DNA surface coverage on an areal basis at high (low) ionic strength, and is well described by a two-state equilibrium model. The resulting quantitative model is able to describe and link, for the first time, the observed changes in optical properties of DNA-wrapped SWCNTs with ionic strength, pH, adsorbed O₂, and ascorbic acid. Cytosine nucleobases are shown to alter the adhesion of the DNA to SWCNTs through direct protonation from solution, decreasing the driving force for this phase transition. We show that the phase transition also changes the observed SWCNT corona phase, modulating the recognition of riboflavin. These results provide insight into the unique molecular interactions between DNA and the SWCNT surface, and have implications for molecular sensing, assembly, and nanoparticle separations.

Mediatorless, Reversible Optical Nanosensor Enabled through Enzymatic Pocket Doping

Single-walled carbon nanotubes (SWCNTs) exhibit intrinsic near-infrared fluorescence that benefits from indefinite photostability and tissue transparency, offering a promising basis for in vivo biosensing. Existing SWCNT optical sensors that rely on charge transfer for signal transduction often require exogenous mediators that compromise the stability and biocompatibility of the sensors. This study presents a reversible, mediatorless, near-infrared glucose sensor based on glucose oxidase-wrapped SWCNTs (GOx-SWCNTs). GOx-SWCNTs undergo a selective fluorescence increase in the presence of aldohexoses, with the strongest response toward glucose. When incorporated into a custom-built membrane device, the sensor demonstrates a monotonic increase in initial response rates with increasing glucose concentrations between 3 × 10^-3 and 30 × 10^-3 m and an apparent Michaelis-Menten constant of K_M (app) ≈ 13.9 × 10^-3 m. A combination of fluorescence, absorption, and Raman spectroscopy measurements suggests a fluorescence enhancement mechanism based on localized enzymatic doping of SWCNT defect sites that does not rely on added mediators. Removal of glucose reverses the doping effects, resulting in full recovery of the fluorescence intensity. The cyclic addition and removal of glucose is shown to successively enhance and recover fluorescence, demonstrating reversibility that serves as a prerequisite for continuous glucose monitoring.

Carbon Nanotubes Facilitate Oxidation of Cysteine Residues of Proteins

The adsorption of proteins onto nanoparticles such as carbon nanotubes (CNTs) governs the early stages of nanoparticle uptake into biological systems. Previous studies regarding these adsorption processes have primarily focused on the physical interactions between proteins and nanoparticles. In this study, using reduced lysozyme and intact human serum albumin in aqueous solutions, we demonstrated that CNTs interact chemically with proteins. The CNTs induce the oxidation of cysteine residues of the proteins, which is accounted for by charge transfer from the sulfhydryl groups of the cysteine residues to the CNTs. The redox reaction simultaneously suppresses the intermolecular association of proteins via disulfide bonds. These results suggest that CNTs can affect the folding and oxidation degree of proteins in biological systems such as blood and cytosol.

Chirality-dependent growth of single-wall carbon nanotubes as revealed inside nano-test tubes

Growth dynamics of single-wall carbon nanotubes (SWCNTs) have been studied with nickelocene as a precursor encapsulated in the interior of template SWCNTs. By means of multi-laser Raman spectroscopy, growth curves of nine different SWCNTs, (8,8), (12,3), (13,1), (9,6), (10,4), (11,2), (11,1), (9,3) and (9,2), have been determined upon in situ annealing at various temperatures. The data reveal that the nanotubes grow through fast and slow reaction pathways with high and low activation energies, respectively. While the activation energy of the slow growth is independent of the nanotube's chiral vector, that of the fast growth exhibits a monotonic increase as the tube diameter reduces from ∼1.1 down to 0.8 nm and no dependency on the chiral angle, which can be attributed to the size-dependent properties of catalyst clusters. The chirality dependent catalytic growth properties exploited in this study provide the basis for a large-scale synthesis of single-chiral vector SWCNTs.

Disulfide bond formation of thiols by using carbon nanotubes

Clarification of the interactions between carbon nanotubes (CNTs) and proteinogenic amino acids is a key approach to understanding CNT-protein interactions. Previous studies have addressed the mechanism of the physical adsorption of amino acids onto CNTs. However, little is known about their chemical reactions in aqueous solutions. Here, we established dispersant-free systems to clarify intrinsic CNT-thiol interactions. We demonstrated that the redox reaction of CNTs with cysteine, containing a thiol group, leads to disulfide bond formation between cysteine molecules, even under acidic conditions. The generality of the redox reaction is validated using other thiols such as dithiothreitol and glutathione. The present results suggest that structures of proteins and peptides containing free thiol groups are chemically modified and misfolded on CNT surfaces by this disulfide bond formation in biological systems.

Wide dynamic range enrichment method of semiconducting single-walled carbon nanotubes with weak field centrifugation

The potential of single-walled carbon nanotubes (SWCNTs) to outperform silicon in electronic application was finally enabled through selective separation of semiconducting nanotubes from the as-synthesized statistical mix with polymeric dispersants. Such separation methods provide typically high semiconducting purity samples with narrow diameter distribution, i.e. almost single chiralities. But for a wide range of applications high purity mixtures of small and large diameters are sufficient or even required. Here we proof that weak field centrifugation is a diameter independent method for enrichment of semiconducting nanotubes. We show that the non-selective and strong adsorption of polyarylether dispersants on nanostructured carbon surfaces enables simple separation of diverse raw materials with different SWCNT diameter. In addition and for the first time, we demonstrate that increased temperature enables higher purity separation. Furthermore we show that the mode of action behind this electronic enrichment is strongly connected to both colloidal stability and protonation. By giving simple access to electronically sorted SWCNTs of any diameter, the wide dynamic range of weak field centrifugation can provide economical relevance to SWCNTs.

Sorting Carbon Nanotubes

Sorting of single-wall carbon nanotubes by their electronic and atomic structures in liquid phases is reviewed in this chapter. We first introduce the sorting problem, and then provide an overview of several sorting methodologies, following roughly the chronological order of their development over the past 15 years or so. Major methods discussed include ion-exchange chromatography, density-gradient ultracentrifugation, selective extraction in organic solvents, gel chromatography, and aqueous two-phase extraction. A main focus of the review is on the common mechanisms underlining all sorting processes. We propose that differences in solvation among different nanotube species are the ultimate driving force of sorting, and we corroborate this proposal by presenting analysis on how the differences are realized in electronic-structure-based sorting and atomic-structure-based sorting. In the end, we offer some suggestions on future directions that may grow out of carbon nanotube sorting. In particular, the prospect of expanding the function of DNA/carbon nanotube hybrid to control inter-particle interactions both inside and outside the nanotube is discussed.

Real Time Hyperspectroscopy for Dynamical Study of Carbon Nanotubes

Photoluminescence excitation mapping used routinely to characterize semiconducting single-walled carbon nanotubes (sc-SWCNTs) is extended to provide time-dependent data. A hyperspectral method based on a supercontinuum light source and an imaging detector is demonstrated with excitation and emission wavelengths spanning 600-1000 and 1050-1650 nm, respectively, at time scales <100 ms. An example of a titration experiment is given to highlight the potential offered by the technique. Specifically, the dependence of luminescence yield on acceptor concentration is tracked for 18 different chirality assigned sc-SWCNT species. Dopant-induced quenching presents a simple dependence on nanotube diameter, with larger diameter being most affected, while chirality does not appear to be a dominant factor. In a second example, the successive addition of acceptor and donor molecules leads to a modulation of luminescence intensity upon hole doping and dedoping.

π-Plasmon absorption of carbon nanotubes for the selective and sensitive detection of Fe3+ ions

Inspired by the remarkable electronic and optical properties of single walled carbon nanotubes (SWNTs), various molecular sensing devices with sensitivity down to the single molecule level have been developed. However, most sensing approaches such as field effect transistors or near infrared (NIR) fluorescence require the rigorous debundling and separation of metallic tubes and semiconducting tubes in order to reach the desired high sensitivity. Interestingly, all carbon nanomaterials including carbon nanotubes, graphite, graphene, and even amorphous carbon exhibit extremely strong π-plasmon absorption in the ultraviolet region. This strong absorption has been studied as an undesired optical background for applications based on visible and NIR absorptions. For the first time, we found that the strong π-plasmon absorption of SWNTs in the ultraviolet region is extremely sensitive to ion binding. It is even much more sensitive than the absorption in the visible and NIR regions. Herein, we present our first exploration into using the extremely strong plasmon absorption of SWNTs to develop a new sensing platform for the detection of metallic ions. The detection selectivity is realized by modifying the surface of SWNTs with molecular ligands that have a high specificity for metal ions. As a demonstration, the new method is applied to selectively detect iron ions (Fe3+) by modifying the surface of the SWNTs with deferoxamine (DFO), a natural bacterial siderophore, which has a high specificity and affinity for Fe3+. The selective detection of Fe3+ in both aqueous solution and complex rain water is achieved with a pM level of sensitivity and detection limit. In situ resonant Raman spectroscopy demonstrated that the sensitive detection possibly involves electron transfer between the formed Fe-DFO complexes and the SWNTs. We envisage that it can be used to detect other metal ions when a specific binding chelator is attached to the carbon nanotube surface.

Highly Efficient and Scalable Separation of Semiconducting Carbon Nanotubes via Weak Field Centrifugation

The identification of scalable processes that transfer random mixtures of single-walled carbon nanotubes (SWCNTs) into fractions featuring a high content of semiconducting species is crucial for future application of SWCNTs in high-performance electronics. Herein we demonstrate a highly efficient and simple separation method that relies on selective interactions between tailor-made amphiphilic polymers and semiconducting SWCNTs in the presence of low viscosity separation media. High purity individualized semiconducting SWCNTs or even self-organized semiconducting sheets are separated from an as-produced SWCNT dispersion via a single weak field centrifugation run. Absorption and Raman spectroscopy are applied to verify the high purity of the obtained SWCNTs. Furthermore SWCNT - network field-effect transistors were fabricated, which exhibit high ON/OFF ratios (10⁵) and field-effect mobilities (17 cm²/Vs). In addition to demonstrating the feasibility of high purity separation by a novel low complexity process, our method can be readily transferred to large scale production.

Selective Surface Charge Sign Reversal on Metallic Carbon Nanotubes for Facile Ultrahigh Purity Nanotube Sorting

Semiconducting (semi-) single-walled carbon nanotubes (SWNTs) must be purified of their metallic (met-) counterparts for most applications including nanoelectronics, solar cells, chemical sensors, and artificial skins. Previous bulk sorting techniques are based on subtle contrasts between properties of different nanotube/dispersing agent complexes. We report here a method which directly exploits the nanotube band structure differences. For the heterogeneous redox reaction of SWNTs with oxygen/water couple, the aqueous pH can be tuned so that the redox kinetics is determined by the availability of nanotube electrons only at/near the Fermi level, as predicted quantitatively by the Marcus-Gerischer (MG) theory. Consequently, met-SWNTs oxidize much faster than semi-SWNTs and only met-SWNTs selectively reverse the sign of their measured surface zeta potential from negative to positive at the optimized acidic pH when suspended with nonionic surfactants. By passing the redox-reacted nanotubes through anionic hydrogel beads, we isolate semi-SWNTs to record high electrically verified purity above 99.94% ± 0.04%. This facile charge sign reversal (CSR)-based sorting technique is robust and can sort SWNTs with a broad diameter range.

Synthesis of 1D-glyconanomaterials by a hybrid noncovalent-covalent functionalization of single wall carbon nanotubes: a study of their selective interactions with lectins and with live cells

To take full advantage of the remarkable applications of carbon nanotubes in different fields, there is a need to develop effective methods to improve their water dispersion and biocompatibility while maintaining their physical properties. In this sense, current approaches suffer from serious drawbacks such as loss of electronic structure together with low surface coverage in the case of covalent functionalizations, or instability of the dynamic hybrids obtained by non-covalent functionalizations. In the present work, we examined the molecular basis of an original strategy that combines the advantages of both functionalizations without their main drawbacks. The hierarchical self-assembly of diacetylenic-based neoglycolipids into highly organized and compacted rings around the nanotubes, followed by photopolymerization leads to the formation of nanotubes covered with glyconanorings with a shish kebab-type topology exposing the carbohydrate ligands to the water phase in a multivalent fashion. The glyconanotubes obtained are fully functional, and able to establish specific interactions with their cognate receptors. In fact, by taking advantage of this selective binding, an easy method to sense lectins as a working model of toxin detection was developed based on a simple analysis of TEM images. Remarkably, different experimental settings to assess cell membrane integrity, cell growth kinetics and cell cycle demonstrated the cellular biocompatibility of the sugar-coated carbon nanotubes compared to pristine single-walled carbon nanotubes.

Inhibition of Cancer Cell Migration by Multiwalled Carbon Nanotubes

Inhibiting cancer cell migration and infiltration to other tissues makes the difference between life and death. Multiwalled carbon nanotubes (MWCNTs) display intrinsic biomimetic properties with microtubules, severely interfering with the function of these protein filaments during cell proliferation, triggering cell death. Here it is shown MWCNTs disrupt the centrosomal microtubule cytoskeletal organization triggering potent antimigratory effects in different cancer cells.

Multicolor fluorescent graphene quantum dots colorimetrically responsive to all-pH and a wide temperature range

Smart functional nanomaterials colorimetrically responsive to all-pH and a wide temperature range are urgently needed due to their widespread applications in biotechnology, drug delivery, diagnosis and optical sensing. Although graphene quantum dots possess remarkable advantages in biological applications, they are only stable in neutral or weak acidic solutions, and strong acidic or alkaline conditions invariably suppress or diminish the fluorescence intensity. Herein, we report a new type of water-soluble, multicolor fluorescent graphene quantum dot which is responsive to all-pH from 1 to 14 with the naked eye. The synthesis was accomplished by electrolysis of the graphite rod, followed by refluxing in a concentrated nitric and sulfuric acid mixed solution. We demonstrate the novel red fluorescence of quinone structures transformed from the lactone structures under strong alkaline conditions. The fluorescence of the resulting graphene quantum dots was also found to be responsive to the temperature changes, demonstrating their great potential as a dual probe of pH and temperature in complicated environments such as biological media.

Regeneration of Light-Harvesting Complexes via Dynamic Replacement of Photodegraded Chromophores

All-synthetic molecular donor-acceptor complexes are designed, which are capable of counteracting the effect of photoinduced degradation of donor chromophores. Anionic gallium protoporphyrin IX (GaPP) and semiconducting carbon nanotube (CNT) are used as a model donor-acceptor complex, which is assembled using DNA oligonucleotides. The GaPP-DNA-CNT complex produces an anodic photocurrent in a photoelectrochemical cell, which steadily decays due to photo-oxidation. By modulating the chemical environment, we showed that the photodegraded chromophores may be dissociated from the complex, whereas the DNA-coated carbon nanotube acceptors are kept intact. Reassociation with fresh porphyrins leads to the full recovery of GaPP absorption and photocurrents. This strategy could form a basis for improving the light-harvesting performance of molecular donor-acceptor complexes and extending their operation lifetime.

Redox sorting of carbon nanotubes

This work expands the redox chemistry of single-wall carbon nanotubes (SWCNTs) by investigating its role in a number of SWCNT sorting processes. Using a polyethylene glycol (PEG)/dextran (DX) aqueous two-phase system, we show that electron-transfer between redox molecules and SWCNTs triggers reorganization of the surfactant coating layer, leading to strong modulation of nanotube partition in the two phases. While the DX phase is thermodynamically more favored by an oxidized SWCNT mixture, the mildly reducing PEG phase is able to recover SWCNTs from oxidation and extract them successively from the DX phase. Remarkably, the extraction order follows SWCNT bandgap: semiconducting nanotubes of larger bandgap first, followed by semiconducting nanotubes of smaller bandgap, then nonarmchair metallic tubes of small but nonvanishing bandgap, and finally armchair metallic nanotubes of zero bandgap. Furthermore, we show that redox-induced surfactant reorganization is a common phenomenon, affecting nanotube buoyancy in a density gradient field, affinity to polymer matrices, and solubility in organic solvents. These findings establish redox modulation of surfactant coating structures as a general mechanism for tuning a diverse range of SWCNT sorting processes and demonstrate for the first time that armchair and nonarmchair metallic SWCNTs can be separated by their differential response to redox.

Pyrene-based mechanically interlocked SWNTs

Pyrene serves as a recognition motif to template the synthesis of mechanically interlocked derivatives of SWNTs.