Spectroscopic evidence and molecular simulation investigation of the pi-pi interaction between pyrene molecules and carbon nanotubes

The pi-pi interaction between pyrene molecules and single-walled carbon nanotubes (SWNTs) or multi-walled carbon nanotubes (MWNTs) was studied by fluorescence, FTIR, Raman spectroscopy and molecular simulation. The carbon nanotubes were incubated in pyrene solution and dried for characterization. A broadband fluorescence emission at 463 nm of the incubated samples was observed, which is similar to that of pyrene excimers but shifts to shorter wavelength. The typical FTIR bands of pyrene shift to lower wavenumbers in the incubated samples. D- and G-bands in Raman spectra of SWNTs also shift to low frequencies. All these spectroscopic evidences reveal the stronger pi-pi stacking interaction between the nanotubes and pyrene molecules over the pyrene dimers, which leads to the formation of pyrene-carbon nanotube complexes. The systems of SWNTs and pyrene molecules were also studied with molecular simulation. It was found from the binding energy calculation that a stronger interaction presents between the pyrene molecule and the nanotube. In addition, the simulation gives some structural information about the pyrene-nanotube complex, such as the staggered conformation of pyrene on nanotube. The effect of defects in carbon nanotube sidewall was also discussed.     

Label-free detection of DNA hybridization using pyrene-functionalized single-walled carbon nanotubes: effect of chemical structures of pyrene molecules on DNA sensing performance

We investigate the effect of functional groups of pyrene molecules on the electrical sensing performance of single-walled carbon nanotubes (SWNTs) based DNA biosensor, in which pyrenes with three different functional groups of carboxylic acid (Py-COOH), aldehyde (Py-CHO) and amine (Py-NH2) are used as linker molecules to immobilize DNA on the SWNT films. UV/Visible absorption spectra results show that all of the pyrene molecules are successfully immobilized on the SWNT surface via pi-pi stacking interaction. Based on fluorescence analysis, we show that the amide bonding of amine terminated DNA via pyrene containing carboxylic groups is the most efficient to immobilize DNA on the nanotube film. The electrical detection results show that the conductance of Py-COOH modified SWNT film is increased upon DNA immobilization, followed by further increase after hybridization of target DNAs. It indicates that the pyrene molecules with carboxylic acid groups play an important role to achieve highly efficient label-free detection by nondestructive and specific immobilization of DNAs.     

Carbon Nanotube Field‐Effect‐Transistor‐Based Biosensors

There is an explosive interest in 1D nanostructured materials for biological sensors. Among these nanometer‐scale materials, single‐walled carbon nanotubes (SWNTs) offer the advantages of possible biocompatibility, size compatibility, and sensitivity towards minute electrical perturbations. In particular, because of these inherent qualities, changes in SWNT conductivity have been explored in order to study the interaction of biomolecules with SWNTs. This Review discusses these interactions, with a focus on carbon nanotube field‐effect transistors (NTFETs). Recent examples of applications of NTFET devices for detection of proteins, antibody–antigen assays, DNA hybridization, and enzymatic reactions involving glucose are summarized. Examples of complementary techniques, such as microscopy and spectroscopy, are covered as well.     

A water-soluble hybrid material of single-walled carbon nanotubes with an amphiphilic poly(phenyleneethynylene): preparation, characterization, and photovoltaic properties

A novel rigid linear polymer poly(phenyleneethynylene) (PPE) was synthesized and the polymer exhibits good solubility in both water and common organic solvents. The interaction at both ground and excited state between this polymer and single-walled carbon nanotubes (SWNTs) was studied and a water-soluble nano-scale PPE/SWNTs hybrid was fabricated, where the water solubility of SWNTs was enhanced to 1.8 mg/ml. Steady state fluorescence spectra and fluorescence lifetime decay measurements showed that the emissions from PPEs in this hybrid at excited state were efficiently quenched by the attachment of SWNTs, where an efficient energy transfer happened from PPEs to SWNTs as the electron acceptor. Using this hybrid as the active layer we fabricated a photovoltaic cell with the bulk heterojunction configuration, and it showed a photoresponse with an open circuit voltage (Voc) of 105 mV and a short circuit current density (Isc) of 28.7 microA/cm2 under standard AM 1.5 illumination (100 mW/cm2).     

Detection of DNA hybridization using the near-infrared band-gap fluorescence of single-walled carbon nanotubes

We demonstrate the optical detection of DNA hybridization on the surface of solution suspended single-walled carbon nanotubes (SWNTs) through a SWNT band gap fluorescence modulation. Hybridization of a 24-mer oligonucleotide sequence with its complement produces a hypsochromic shift of 2 meV, with a detection sensitivity of 6 nM. The energy shift is modeled by correlating the surface coverage of DNA on SWNT to the exciton binding energy, yielding an estimated initial fractional coverage of 0.25 and a final coverage of 0.5. Hybridization on the nanotube surface is confirmed using Forster resonance energy transfer of fluorophore-labeled DNA oligonucleotides. This detection is enabled through a new technique to suspend SWNTs using adsorption of single-stranded DNA and subsequent removal of free DNA from solution. While the kinetics of free DNA hybridization are relatively fast (<10 min), the kinetics of the process on SWNTs are slower under comparable conditions, reaching steady state after 13 h at 25 degrees C. A second-order kinetic model yields a rate constant of k = 4.33 x 10(5) (M h)(-1). This optical, selective detection of specific DNA sequences may have applications in the life sciences and medicine as in vitro or in vivo detectors of oligonucleotides.     

Efficient fluorescence turn-on probe for zirconium via a target-triggered DNA molecular beacon strategy

It is well-known that Zr(4+) could selectively bind with two phosphate-functionalized molecules through a coordinate covalent interaction to form a sandwich-structured complex (-PO(3)(2-)-Zr(4+)-PO(3)(2-)-). In this paper, we for the first time converted such interaction into fluorescence sensing systems for Zr(4+) via a target-triggered DNA molecular beacon strategy. In the new designed sensing system, two phosphorylated and pyrene-labeled oligonucleotides were chosen as both recognition and reporter units, which will be linked by target Zr(4+) to form a hairpin structure and bring the two labeled pyrene molecules into close proximity, resulting in a "turn-on" excimer fluorescence signal. Moreover, γ-cyclodextrin was introduced to afford an amplified fluorescence signal and, therefore, provided an improved sensitivity for the target Zr(4+). This allows detection of Zr(4+) with high sensitivity (limit of detection, LOD = 200 nM) and excellent selectivity. The proposed sensing system has also been used for detection of Zr(4+) in river water samples with satisfactory result.     

Subcellular tracking of drug release from carbon nanotube vehicles in living cells

The direct observation of drug release from carbon nanotube vehicles in living cells is realized through a unique two-dye labeling approach. Single-walled carbon nanotubes (SWNTs) are firstly marked with fluorescein isothiocyanate (FITC) to track their location and movement inside the cell. Then a fluorescent anticancer drug doxorubicin (DOX) is attached by means of π-stacking onto SWNTs. Delivered by SWNTs into cells, DOX will detach from the vehicle in an acidic environment due to the pH-dependent π-π stacking interaction between DOX and SWNTs. From observation of the two different kinds of fluorescence (green and red) that respectively represent the carrier SWNTs and drug DOX, the process of drug release inside the living cell can be monitored under a confocal microscope. Results show that the drug DOX detaches from SWNTs inside the lysosomes to yield free molecules and escape into the cytoplasm and finally into the nucleus, while the vehicle SWNTs are trapped inside the lysosomes, without entering the nucleus. The current observations confirm previously proposed mechanisms for drug/DOX release inside cells. The experimental establishment of drug-release mechanisms in living cells here might provide important insights for future design of new drug-delivery and release systems.     

Nano-C(60) : a novel, effective, fluorescent sensing platform for biomolecular detection

Water-soluble nano-C(60) can serve as a novel, effective, fluorescent sensing platform for biomolecular detection with high sensitivity and selectivity. In this paper, fluorescent detection of DNA and thrombin via nano-C(60) is demonstrated for the first time. The principle of the assay lies in the fact that the adsorption of the fluorescently labeled single-stranded DNA (ssDNA) probe by nano-C(60) leads to substantial fluorescence quenching. In the presence of a target, the biomolecular mutual interaction suppresses this quenching, signaling the existence of the target. This sensing system rivals graphene oxide but is superior to other carbon-structure-based systems. The present method can also achieve multiplex DNA detection and withstand the interference from human blood serum.     

Diameter‐ and Metallicity‐Selective Enrichment of Single‐Walled Carbon Nanotubes Using Polymethacrylates with Pendant Aromatic Functional Groups

Current methods for the synthesis of single‐walled nanotubes (SWNTs) produce mixtures of semiconducting (sem‐) and metallic (met‐) nanotubes. Most approaches to the chemical separation of sem‐/met‐SWNTs are based on small neutral molecules or conjugated aromatic polymers, which characteristically have low separation/dispersion efficiencies or present difficulties in the postseparation removal of the polymer so that the resulting field‐effect transistors (FETs) have poor performance. In this Full Paper, the use of three polymethacrylates with different pendant aromatic functional groups to separate cobalt–molybdenum catalyst (CoMoCAT) SWNTs according to their metallicity and diameters is reported. UV/Vis/NIR spectroscopy indicates that poly(methyl‐methacrylate‐co‐fluorescein‐o‐acrylate) (PMMAFA) and poly(9‐anthracenylmethyl‐methacrylate) (PAMMA) preferentially disperse semiconducting SWNTs while poly(2‐naphthylmethacrylate) (PNMA) preferentially disperses metallic SWNTs, all in dimethylforamide (DMF). Photoluminescence excitation (PLE) spectroscopy indicates that all three polymers preferentially disperse smaller‐diameter SWNTs, particularly those of (6,5) chirality, in DMF. When chloroform is used instead of DMF, the larger‐diameter SWNTs (8,4) and (7,6) are instead selected by PNMA. The solvent effects suggest that diameter selectivity and change of polymer conformation is probably responsible. Change of the polymer fluorescence upon interaction with SWNTs indicates that metallicity selectivity presumably results from the photon‐induced dipole–dipole interaction between polymeric chromophore and SWNTs. Thin‐film FET devices using semiconductor‐enriched solution with PMMAFA have been successfully fabricated and the device performance confirms the sem‐SWNTs enrichment with a highly reproducible on/off ratio of about 10³.     

Chirality‐Selective Photoluminescence Enhancement of ssDNA‐Wrapped Single‐Walled Carbon Nanotubes Modified with Gold Nanoparticles

In this work, a convenient method to enhance the photoluminescence (PL) of single‐walled carbon nanotubes (SWNTs) in aqueous solutions is provided. Dispersing by single‐stranded DNA (ssDNA) and modifying with gold nanoparticles (AuNPs), about tenfold PL enhancement of the SWNTs is observed. More importantly, the selective PL enhancement is achieved for some particular chiralities of interest over all other chiralities, by using certain specific ssDNA sequences that are reported to recognize these particular chiralities. By forming AuNP–DNA–SWNT nanohybrids, ssDNA serves as superior molecular spacers that on one hand protect SWNT from direct contacting with AuNP and causing PL quench, and on the other hand attract the AuNP in close proximity to the SWNT to enhance its PL. This PL enhancement method can be utilized for the PL analysis of SWNTs in aqueous solutions, for biomedical imaging, and may serve as a prescreening method for the recognition and separation of single chirality SWNTs by ssDNA.     

Monitoring molecular beacon/DNA interactions using atomic force microscopy

The molecular beacon (MB) is a new fluorescence probe containing a single-stranded oligonucleotide with a probe sequence embedded in complementary sequences that form a hairpin stem. Due to the inherent fluorescent signal transduction mechanism, an MB functions as a sensitive probe with a high signal-to-background ratio for real-time monitoring and provides a variety of exciting opportunities in DNA, RNA, and protein studies. To better understand the properties of MBs, the specific interactions between MB and target DNA (complementary and one-base mismatch) have been directly investigated by atomic force microscopy. The interaction force between a linear DNA probe and the target DNA was also detected and compared to that between MB and target DNA. The results demonstrate the high specificity of the MB/target DNA compared to the linear DNA/target DNA interaction.     

Why semiconducting single-walled carbon nanotubes are separated from their metallic counterparts

The separation of single-walled carbon nanotubes (SWNTs) according to their electronic structure has attracted much recent attention. In many cases, metallic SWNTs are separated from semiconducting SWNTs and enriched in the supernatant due to stronger interaction between metallic SWNTs and adsorbates. However, the inverse separation of semiconducting from metallic SWNTs is often observed. In this computational study, the underlying mechanism is elucidated by density functional theory. We show that the shape of an aromatic molecule, the degree of hybridization between a molecule and a SWNT, and the oxidative state of SWNTs can affect the type of enriched SWNTs. In principle, one can control the type of enriched SWNTs by selecting a structurally compatible aromatic molecule or changing the hole concentration of the SWNTs.     

Subdiffraction far-field imaging of luminescent single-walled carbon nanotubes

Far-field near-infrared fluorescence microscopy of single-walled carbon nanotubes (SWNTs) has been hampered by the diffraction limit to resolution. A new analysis method is presented that allows subwavelength (相似文献   

Dispersion of single walled carbon nanotubes in amidine solvents

The excellent electronic and material properties of single walled carbon nanotubes (SWNTs) makes this nanomaterial very attractive for incorporation into flexible and stretchable electronics. However, the widespread application of SWNTs in electronic devices is still limited. To purify, process and place SWNTs, appropriate solvents for dispersion are needed. However, a fundamental understanding of the reasons why certain solvents are capable of dispersing SWNTs is still missing. Here we report on two new potential solvents containing amidine moieties, 1,8-diazabicycloundec-7-ene (DBU) and 1,5-diazabicyclo(4.3.0)non-5-ene (DBN). Even though these solvents' molecular structures differ by only two -CH(2)- groups, we found that DBU is capable of dispersing SWNTs, while DBN is not. We carried out density functional theory (DFT) calculations to investigate the interaction between DBU and DBN, and we elucidated the reasons for the different performances of the two solvents. DBU has a preferential edge-on interaction with the SWNT, thus allowing for a higher solvent coverage than DBN. In addition, the CH(2)-SWNT interaction present for DBU substantially increases the adsorption energy compared to DBN. Our results point to the important interplay between the interaction of pi electrons, nitrogen lone pairs and the -CH(2)- groups present in aprotic solvent molecules and the delocalized pi electrons in SWNTs.     

Substrate geometry-dependent nonlinear absorption of carbon nanotubes deposited onto D-shaped optical fibers

The effect of single-wall carbon nanotubes (SWNTs) on nonlinear optical absorption of D-shaped fibers with versatile the remaining length of the cladding region and the interaction length are investigated. The optical absorption based on SWNTs is induced by the energy bandgap in SWNTs. The bandgap energy depends on the tube diameter of SWNTs. After fabricating versatile D-shaped fiber, SWNTs are deposited on the polished surface of D-shaped fibers. The cladding region of single mode fibers is removed by a side-polishing technique and the D-shaped fiber is obtained. In the D-shaped fiber, the cladding region is thin enough to induce the evanescent field coupling of core mode to the other modes of the SWNT-overlay. The nonlinear absorption based on the SWNTs-overlay is changed by the remaining length of cladding region and the interaction length because the coupling strength of evanescent field strongly depends on the different remaining lengths of the cladding region and the interaction lengths as well.     

The evaluation of individual dispersion of single-walled carbon nanotubes using absorption and fluorescence spectroscopic techniques

We have investigated the degree of dispersion of single-walled carbon nanotubes (SWNTs) in solution using laser spectroscopic techniques. SWNTs were suspended in aqueous media using a sodium dodecyl sulfate (SDS) surfactant. SWNTs with different dispersion states were prepared by controlling the intensity and duration of sonication and centrifugation. The absorption and fluorescence spectroscopic techniques were employed to characterize the different dispersion state of the prepared samples. Nanotube suspensions with better dispersion showed higher fluorescence and sharper absorption peaks. The fluorescence data were characterized as a function of the nanotube chirality, and absorption peak shifts were analyzed depending on the first and second van Hove singularities (vHs) of semiconducting nanotubes.     

Hierarchical placement and associated optoelectronic impact of carbon nanotubes in polymer-fullerene solar cells

Since their discovery, carbon nanotubes (CNTs) have been considered to be promising candidates for polymer-based solar cells, but their functional incorporation and utilization in such devices have been limited due to processing bottlenecks. Here, we demonstrate the realization of controlled placement of a single-walled CNT (SWNT) monolayer network at four different positions in polymer-fullerene bulk-heterojunction (BHJ) solar cells. SWNTs were deposited by dip-coating from a hydrophilic suspension, and a very brief, largely nondestructive argon plasma treatment of the active layer was utilized for incorporation of a SWNT layer within or above it. We demonstrate that SWNTs on the hole-collection side of the active layer lead to an increase in power conversion efficiency (PCE) of the photovoltaic devices from 4 to 4.9% (under AM 1.5 G, 1.3 suns illumination). This is the highest reported PCE for polymer-based solar cells incorporating CNTs, upon consideration of expected scaling of device parameters for 1 sun illumination. We also observe that SWNTs deposited on the top of the active layer lead to major electro-optical changes in the device functionality, including an increased fluorescence lifetime of poly-3-hexylthiophene (P3HT).     

Separation of single-walled carbon nanotubes by use of ionic liquid-aided capillary electrophoresis

This paper presents a simple, highly efficient method for analyzing single-walled carbon nanotube (SWNT) bundles based on (1) ultrasound-assisted solubilization/dispersion of SWNTs in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, (2) encapsulation of the nanotubes in sodium dodecyl sulfate micelles, and (3) analysis by capillary electrophoresis. The process by which SWNTs disperse in the ionic liquid was studied by Raman spectroscopy. No degradation of SWNTs was observed under mild sonication conditions. The shape and position changes observed in the Raman spectral bands for the nanotubes are ascribed to debundling and interaction with the ionic liquid. Separation of solubilized SWNTs was accomplished by using a 50 mM formic acid solution at pH 2.0 as background electrolyte and a potential of -10 kV. Under these conditions, separation was completed within only 4 min. Eighteen peaks for SWNTs were identified in the analysis of commercial SWNT bundles. The two types of bundles studied exhibited distinct, highly characteristic electrophoretic profiles which could be used to control SWNTs purity.     

Cancer‐Cell Targeting and Photoacoustic Therapy Using Carbon Nanotubes as “Bomb” Agents

A unique approach using the large photoacoustic effect of single‐walled carbon nanotubes (SWNTs) for targeting and selective destruction of cancer cells is demonstrated. SWNTs exhibit a large photoacoustic effect in suspension under the irradiation of a 1064‐nm Q‐switched millisecond pulsed laser and trigger a firecracker‐like explosion at the nanoscale. By using such an explosion, a photoacoustic agent is developed by functionalizing the SWNTs with folate acid (FA) that can selectively bind to cancer cells overexpressing folate receptor on the surface of the cell membrane and kill them through SWNT explosion inside the cells under the excitation of millisecond pulsed laser. The uptake pathway of folate‐conjugated SWNTs into cancer cells is investigated via fluorescence imaging and it is found that the FA‐SWNTs can enter into cancer cells selectively with a high targeting capability of 17–28. Under the treatment of 1064‐nm millisecond pulsed laser, 85% of cancer cells with SWNT uptake die within 20 s, while 90% of the normal cells remain alive due to the lack of SWNTs inside cells. Temperature changes during laser treatment are monitored and no temperature increases of more than ± 3 °C are observed. With this approach, the laser power used for cancer killing is reduced 150–1500 times and the therapy efficiency is improved. The death mechanism of cancer cells caused by the photoacoustic explosion of SWNTs is also studied and discussed in detail. These discoveries provide a new way to use the photoacoustic properties of SWNTs for therapeutic applications.     

Fluorescent Polymer—Single‐Walled Carbon Nanotube Complexes with Charged and Noncharged Dendronized Perylene Bisimides for Bioimaging Studies

Fluorescent nanomaterials are expected to revolutionize medical diagnostic, imaging, and therapeutic tools due to their superior optical and structural properties. Their inefficient water solubility, cell permeability, biodistribution, and high toxicity, however, limit the full potential of their application. To overcome these obstacles, a water‐soluble, fluorescent, cytocompatible polymer—single‐walled carbon nanotube (SWNT) complex is introduced for bioimaging applications. The supramolecular complex consists of an alkylated polymer conjugated with neutral hydroxylated or charged sulfated dendronized perylene bisimides (PBIs) and SWNTs as a general immobilization platform. The polymer backbone solubilizes the SWNTs, decorates them with fluorescent PBIs, and strongly improves their cytocompatibility by wrapping around the SWNT scaffold. In photophysical measurements and biological in vitro studies, sulfated complexes exhibit superior optical properties, cellular uptake, and intracellular staining over their hydroxylated analogs. A toxicity assay confirms the highly improved cytocompatibility of the polymer‐wrapped SWNTs toward surfactant‐solubilized SWNTs. In microscopy studies the complexes allow for the direct imaging of the SWNTs' cellular uptake via the PBI and SWNT emission using the 1st and 2nd optical window for bioimaging. These findings render the polymer‐SWNT complexes with nanometer size, dual fluorescence, multiple charges, and high cytocompatibility as valuable systems for a broad range of fluorescence bioimaging studies.