A new method based on variance spectroscopy has enabled the determination of absolute absorption cross sections for the first electronic transition of 12 (n,m) structural species of semiconducting single-walled carbon nanotubes (SWCNTs). Spectrally resolved measurements of fluorescence variance in dilute bulk samples provided particle number concentrations of specific SWCNT species. These values were converted to carbon concentrations and correlated with resonant components in the absorbance spectrum to deduce (n,m)-specific absorption cross sections (absorptivities) for nanotubes ranging in diameter from 0.69 to 1.03 nm. The measured cross sections per atom tend to vary inversely with nanotube diameter and are slightly greater for structures of mod 1 type than for mod 2. Directly measured and extrapolated values are now available to support quantitative analysis of SWCNT samples through absorption spectroscopy.Keywords: absorption cross section; fluctuation spectroscopy; molar absorptivity; quantitative analysis; SWCNT; SWIR spectroscopy;

Nanomaterials with luminescence in the short-wave infrared (SWIR) region are of special interest for biological research and medical diagnostics because of favorable tissue transparency and low autofluorescence backgrounds in that region. Single-walled carbon nanotubes (SWCNTs) show well-known sharp SWIR spectral signatures and therefore have potential for noninvasive detection and imaging of cancer tumours, when linked to selective targeting agents such as antibodies. However, such applications face the challenge of sensitively detecting and localizing the source of SWIR emission from inside tissues. A new method, called spectral triangulation, is presented for three dimensional (3D) localization using sparse optical measurements made at the specimen surface. Structurally unsorted SWCNT samples emitting over a range of wavelengths are excited inside tissue phantoms by an LED matrix. The resulting SWIR emission is sampled at points on the surface by a scanning fibre optic probe leading to an InGaAs spectrometer or a spectrally filtered InGaAs avalanche photodiode detector. Because of water absorption, attenuation of the SWCNT fluorescence in tissues is strongly wavelength-dependent. We therefore gauge the SWCNT–probe distance by analysing differential changes in the measured SWCNT emission spectra. SWCNT fluorescence can be clearly detected through at least 20 mm of tissue phantom, and the 3D locations of embedded SWCNT test samples are found with sub-millimeter accuracy at depths up to 10 mm. Our method can also distinguish and locate two embedded SWCNT sources at distinct positions.

Photoluminescence spectra of single-walled carbon nanotubes (SWCNTs) have been recorded and analyzed for selected individual nanotubes and structurally sorted bulk samples to clarify the nature of secondary emission features. Room temperature spectra show, in addition to the main peak arising from the E11 bright exciton, three features at lower frequency, which are identified here (in descending order of energy difference from E11 emission) as G1, X1, and Y1. The weakest (G1) is interpreted as a vibrational satellite of E11 involving excitation of the ∼1600 cm–1 G mode. The X1 feature, although more intense than G1, has a peak amplitude only ∼3% of E11. X1 emission was found to be polarized parallel to E11 and to be separated from that peak by 1068 cm–1 in SWCNTs with natural isotopic abundance. The separation remained unchanged for several (n,m) species, different nanotube environments, and various levels of induced axial strain. In 13C SWCNTs, the spectral separation decreased to 1023 cm–1. The measured isotopic shift points to a phonon-assisted transition that excites the D vibration. This supports prior interpretations of the X1 band as emission from the dark K-momentum exciton, whose energy we find to be ∼230 cm–1 above E11. The remaining sideband, Y1, is red-shifted ∼300 cm–1 from E11 and varies in relative intensity among and within individual SWCNTs. We assign it as defect-induced emission, either from an extrinsic state or from a brightened triplet state. In contrast to single-nanotube spectra, bulk samples show asymmetric zero-phonon E11 peaks, with widths inversely related to SWCNT diameter. An empirical expression for this dependence is presented to aid the simulation of overlapped emission spectra during quantitative fluorimetric analysis of bulk SWCNT samples.

We present experimental Raman optical activity (ROA) spectra of enantio-enriched single-walled carbon nanotubes (SWCNTs). Enantiomeric samples of (6,5) SWCNTs were prepared using nonlinear density gradient ultracentrifugation (DGU). Upon excitation at 2.33 eV, remarkably strong G-band signals are obtained due to strong resonance enhancement with the E22S transition of (6,5) SWCNTs. Enhancement allows measuring the vibrational optical activity (VOA) at unusually low concentrations. The obtained results are in good agreement with the single-excited-state theory (SES). To our knowledge, these are the first experimental VOA spectra of SWCNTs.

Covalent sidewall functionalization of single-walled carbon nanotubes (SWCNTs) is an important tool for tailoring their properties for research purposes and applications. In this study, SWCNT samples were first functionalized by reductive alkylation using metallic lithium and 1-iodododecane in liquid ammonia. Samples of the alkyl-functionalized SWCNTs were then pyrolyzed under an inert atmosphere at selected temperatures between 100 and 500 °C to remove the addends. The extent of defunctionalization was assessed using a combination of thermogravimetric analysis, Raman measurements of the D, G, and radial breathing bands, absorption spectroscopy of the first- and second-order van Hove peaks, and near-IR fluorescence spectroscopy of (n,m)-specific emission bands. These measurements all indicate a substantial dependence of defunctionalization rate on nanotube diameter, with larger diameter nanotubes showing more facile loss of addends. The effective activation energy for defunctionalization is estimated to be a factor of ∼1.44 greater for 0.76 nm diameter nanotubes as compared to those with 1.24 nm diameter. The experimental findings also reveal the quantitative variation with functionalization density of the Raman D/G intensity ratio and the relative near-IR fluorescence intensity. Pyrolyzed samples show spectroscopic properties that are equivalent to those of SWCNTs prior to functionalization. The strong structure dependence of the defunctionalization rate suggests an approach for scalable diameter sorting of mixed SWCNT samples.Keywords: covalent functionalization; D/G ratio; dodecylation; nanotube fluorescence restoration; pyrolysis;

Spectroscopic analysis and study of nanoparticle samples is often hampered by structural diversity that presents a complex superposition of spectral signatures. By probing the spectra of small volumes within dilute samples, we can expose statistical variations in composition to obtain information unavailable from bulk spectroscopy. This new approach is demonstrated using fluorescence spectra of unsorted single-walled carbon nanotube samples to deduce structure-specific abundances and emissive efficiencies. Furthermore, correlations between intensity variations at different wavelengths provide two-dimensional covariance maps that isolate the spectra of homogeneous subpopulations. Covariance analysis is also a sensitive probe of particle aggregation. It shows that well-dispersed nanotube samples can spontaneously form loose aggregates of a type not previously recognized. Variance spectroscopy is a simple and practical technique that should find application in many nanoparticle studies.

We have measured peak and spectrally integrated absolute absorption cross sections for the first (E11) and second (E22) optical transitions of seven semiconducting single-walled carbon nanotube (SWCNT) species in bulk suspensions. Species-specific concentrations were determined using short-wave IR fluorescence microscopy to directly count SWCNTs in a known sample volume. Measured cross sections per atom are inversely related to nanotube diameter. E11 cross sections are larger for mod 1 species than for mod 2; the opposite is found for E22.

The recent application of density gradient ultracentrifugation (DGU) for structural sorting of single-walled carbon nanotube samples has created a need for highly selective extraction of closely spaced layers formed in the centrifuged tube. We describe a novel computer-controlled device designed for this purpose. Through the use of fine needles, systematic needle motions, and slow flow rates, multiple sample layers can be aspirated under program control with minimal cross contamination between layers. The fractionator’s performance is illustrated with DGU-sorted samples of single-walled carbon nanotubes.

Purification of raw single-walled carbon nanotube (SWCNT) material remains an important challenge in nanotube research and applications. We describe here a simple but effective purification method that uses permanent magnets to remove many nanotube aggregates, as well as residual metallic catalyst, from aqueous suspensions of surfactant-coated SWCNTs. Samples have been characterized by optical absorption, fluorescence, and Raman spectroscopies; atomic force microscopy and near-infrared fluorescence microscopy; and thermogravimetric analysis. It is found that magnetic purification reduces absorption backgrounds and increases average fluorescence efficiencies to levels comparable to those in ultracentrifuged samples. The ratio of individualized SWCNTs to aggregates in magnetically processed HiPco samples is estimated to be approximately 4:1. As compared to ultracentrifugation, magnetic processing promises major advantages in cost, simplicity, energy consumption, and scalability.

A new method is described for measuring the short-wave infrared (SWIR) emission wavelengths of numerous individual nanoparticles without using a dedicated spectrometer. Microscope objectives designed for use at visible wavelengths often show severe axial chromatic aberration in the SWIR. This makes coplanar objects emitting at different SWIR wavelengths appear to focus at different depths. After this aberration has been calibrated for a particular objective lens, the depth at which an emissive nanoparticle appears brightest and best focused can be used to deduce its peak emission wavelength. The method is demonstrated using a dilute, structurally polydisperse sample of single-walled carbon nanotubes deposited onto a microscope slide. Discrete emission centers in this sample have different peak wavelengths corresponding to specific nanotube structural species. A set of images was recorded at stepped focus settings and analyzed to find the sharpest focus depth of each nanotube. The chromatic aberration calibration curve converted these depths into peak emission wavelengths with a spectral resolution better than 3 nm, allowing identification of each nanotube’s structure. Chromatic aberration spectroscopy is a practical tool for using existing microscopic equipment to extract significant spectral information on coplanar nanoparticle samples that emit or scatter light.

Composite coatings have been developed that reveal strains in underlying structural elements through noncontact optical measurement. Dilute individualized single-walled carbon nanotubes are embedded in a polymeric host and applied to form a thin coating. Strain in the substrate is transmitted through the polymer to the nanotubes, causing systematic and predictable spectral shifts of the nanotube near-infrared fluorescence peaks. This new method allows quick and precise strain measurements at any position and along any direction of the substrate.

The nonlinear dependence of near-infrared photoluminescence (PL) emission on excitation intensity has been measured for individual nanotubes representing six different (n,m) species. Significant deviations from linearity are observed for intensities as low as ∼100 W/cm2, and an approximate inverse correlation is found between nonlinearity and PL action cross section (brightness). A model in which all PL nonlinearity arises from exciton–exciton annihilation is insufficient to account for the experimental data using realistic parameters. It is proposed that additional nonlinear quenching arises from photoinduced quenching states or species with longer lifetimes than emissive excitons. Evidence is also found for metastable photogenerated PL quenchers with lifetimes up to 20 s.

Using near-infrared fluorescence videomicroscopy with spectrally selective excitation and imaging, more than 400 individual (10,2) single-walled carbon nanotubes (SWCNTs) have been studied in unsorted liquid dispersions. For each nanotube, the spatially integrated emission intensity was measured under controlled excitation conditions while its length was found either from direct imaging or from the diffusion coefficient computed by analyzing its Brownian motion trajectory. The studied nanotubes ranged in length from 170 to 5300 nm. For any length, a wide variation in emission intensities was observed. These variations are attributed to differing densities of nanotube imperfections that cause fluorescence quenching. The brightest nanotubes at each length (presumed near-pristine) show total emission nearly proportional to length. This implies a nearly constant fluorescence quantum yield and a constant absorption cross section per carbon atom, validating conventional Beer–Lambert analysis for finding concentrations of SWCNT species. Ensemble-averaged emission is also proportional to length, but at only ca. 40% of the near-pristine values. Further research is needed to investigate the extrinsic effects causing wide variation in quantum yields and assess their implications for SWCNT fluorimetry.Keywords: carbon nanotubes; extrinsic quenching; fluorescence efficiency; fluorescence microscopy; length dependence; photoluminescence; SWCNT

A new method is demonstrated for measuring the length distributions of dispersed single-walled carbon nanotube (SWCNT) samples by analyzing diffusional motions of many individual nanotubes in parallel. In this method, termed “length analysis by nanotube diffusion” (LAND), video sequences of near-IR fluorescence microscope images showing many semiconducting SWCNTs are recorded and processed by custom image analysis software. This processing locates the individual nanotubes, tracks their translational trajectories, computes the corresponding diffusion coefficients, and converts those values to nanotube lengths. The deduced length values are then compiled into a histogram of lengths present in the sample. By using specific excitation wavelengths and emission filters, this analysis is performed on selected (n,m) structural species. The new LAND method has been found to give distributions in very good agreement with those obtained by conventional AFM analysis of the same samples. Because it is fluorescence-based, LAND monitors only semiconducting, relatively pristine SWCNTs. However, it is less sensitive to artifacts from impurities and bundled nanotubes than AFM or light scattering methods. In addition, samples can be analyzed with less time and operator attention than by AFM. LAND is a promising alternative method for characterizing length distributions of SWCNTs in liquid suspension.Keywords: diffusion; fluorescence; length; single-particle tracking; single-walled carbon nanotubes; SWCNT

A new method and instrumentation are described for rapid compositional analysis of single-walled carbon nanotube (SWCNT) samples. The customized optical system uses multiple fixed-wavelength lasers to excite NIR fluorescence from SWCNTs individualized in aqueous suspensions. The emission spectra are efficiently captured by a NIR spectrometer with InGaAs multichannel detector and then analyzed by a computer program that consults a database of SWCNT spectral parameters. The identities and relative abundances of semiconducting SWCNTs species are quickly deduced and displayed in graphs and tables. Results are found to be consistent with those based on manual interpretation of full excitation–emission scans from a conventional spectrofluorometer. The new instrument also measures absorption spectra using a broadband lamp and multichannel spectrometers, allowing samples to be automatically characterized by their emission efficiencies. The system provides rapid data acquisition and is sensitive enough to detect the fluorescence of a few picograms of SWCNTs in ∼50 μL sample volumes.

The sources of broad backgrounds in visible−near-IR absorption spectra of single-walled carbon nanotube (SWCNT) dispersions are studied through a series of controlled experiments. Chemical functionalization of nanotube sidewalls generates background absorption while broadening and red-shifting the resonant transitions. Extensive ultrasonic agitation induces a similar background component that may reflect unintended chemical changes to the SWCNTs. No major differences are found between spectral backgrounds in sample fractions with average lengths between 120 and 650 nm. Broad background absorption from amorphous carbon is observed and quantified. Overlapping resonant absorption bands lead to elevated backgrounds from spectral congestion in samples containing many SWCNT structural species. A spectral modeling method is described for separating the background contributions from spectral congestion and other sources. Nanotube aggregation increases congestion backgrounds by broadening the resonant peaks. Essentially no background is seen in sorted pristine samples enriched in a single semiconducting (n,m) species. By contrast, samples enriched in mixed metallic SWCNTs show broad intrinsic absorption backgrounds far from the resonant transitions. The shape of this metallic background component and its absorptivity coefficient are quantitatively assessed. The results obtained here suggest procedures for preparing SWCNT dispersions with minimal extrinsic background absorptions and for quantifying the remaining intrinsic components. These findings should allow improved characterization of SWCNT samples by absorption spectroscopy.Keywords: absorption; functionalization; single-walled carbon nanotubes; spectral background; SWCNT; ultrasonication

Measurements of stepwise photoluminescence quenching in individual, (n,m)-selected single-walled carbon nanotubes (SWCNTs) undergoing chemical reaction have been analyzed to deduce mobilities of optically generated excitons. For (7,5) nanotubes, the mean exciton range varies between ∼140 and 240 nm for different surfactant coatings and correlates weakly with nanotube PL intensity. The results are consistent with a model of localized SWCNT excitons having substantial diffusional mobility along the nanotube axis.

Single-walled carbon nanotubes (SWCNTs) are a family of structurally related artificial nanomaterials with unusual properties and many potential applications. Most SWCNTs can emit spectrally narrow near-IR fluorescence at wavelengths that are characteristic of their precise diameter and chiral angle. Near-IR fluorimetry therefore offers a powerful approach for identifying the structural species present in SWCNT samples. Such characterization is increasingly important for nanotube production, study, separation, and applications. General-purpose and specialized instruments suitable for SWCNT fluorimetric analysis are described, and methods for interpreting fluorimetric data to deduce the presence and relative abundances of different SWCNT species are presented. Fluorescence methods are highly effective for detecting SWCNTs in challenging samples such as complex environmental or biological specimens because of the methods’ high sensitivity and selectivity and the near absence of interfering background emission at near-IR wavelengths. Current limitations and future prospects for fluorimetric characterization of SWCNTs are discussed.

Optically generated excitons in semiconducting single-walled carbon nanotubes (SWCNTs) display substantial diffusional mobility. This property allows excitons to encounter ∼104 carbon atoms during their lifetime and accounts for their efficient deactivation by sparse quenching sites. We report here experimental determinations of the mobilities of optically generated excitons in 10 different (n,m) species of semiconducting SWCNTs. Exciton diffusional ranges were deduced from measurements of stepwise photoluminescence quenching in selected individual SWCNTs coated with sodium deoxycholate surfactant and immobilized in agarose gel. A refined data analysis method deduced mean exciton ranges from 190 to 370 nm. The results suggest that exciton range increases weakly with nanotube diameter over the 0.7−1.2 nm diameter range and that species with near-armchair roll-up angles have the smallest exciton ranges. No significant correlation was found between the exciton range and measured photoluminescence action cross section, which represents fluorimetric brightness. These findings highlight the importance of complementary photophysical studies to elucidate factors controlling SWCNT exciton mobility.Keywords (keywords): exciton diffusion; photoluminescence action cross sections; single-particle fluorescence; stepwise photoluminescence quenching;

The reported fluorescence from inner shells of double-walled carbon nanotubes (DWCNTs) is an intriguing and potentially useful property. A combination of bulk and single-molecule methods was used to study the spectroscopy, chemical quenching, mechanical rigidity, abundance, density, and TEM images of the near-IR emitters in DWCNT samples. DWCNT inner shell fluorescence is found to be weaker than SWCNT fluorescence by a factor of at least 10 000. Observable near-IR emission from DWCNT samples is attributed to SWCNT impurities.

A new method was used to measure the fraction of semiconducting nanotubes in various as-grown or processed single-walled carbon nanotube (SWCNT) samples. SWCNT number densities were compared in images from near-IR photoluminescence (semiconducting species) and AFM (all species) to compute the semiconducting fraction. The results show large variations among growth methods and effective sorting by density gradient ultracentrifugation. This counting-based method provides important information about SWCNT sample compositions that can guide controlled growth methods and help calibrate bulk characterization techniques.

The fluorescence spectra of individual semiconducting single-walled carbon nanotubes embedded in polymer films were measured during the application of controlled stretching and compressive strains. Nanotube band gaps were found to shift in systematic patterns that depend on the (n,m) structural type and are in excellent agreement with the predictions of theoretical models. Loss of nanotube−host adhesion was revealed by abrupt irregularities in plots of spectral shift vs strain.

The effect of external electric fields on the photoluminescence intensity of single-walled carbon nanotubes was investigated for individual nanotubes and bulk samples in polymeric films. Fields of up to 107 V/m caused dramatic, reversible decreases in emission intensity. Quenching efficiency varied as the cosine of the angle between the field and nanotube axis and decreased with increasing optical band gap. Photoluminescence intensity was found to follow a reciprocal hyperbolic cosine dependence on electric field.

Energy transfer from photoexcited porphyrin molecules to single-walled carbon nanotubes (SWNTs) has been experimentally detected for samples in aqueous Triton X-100 micellar suspensions. Addition of SWNTs to micelle-suspended porphyrin results in strong quenching of porphyrin fluorescence. Measurements of concentration-dependent quenching and spectra suggest that this process arises from formation of ground state non-covalent complexes between porphyrins and SWNTs. Optical excitation of the porphyrin generates characteristic near-IR emission from the SWNTs, indicating efficient energy transfer within the complexes. This energy transfer is deduced to occur through a Dexter-type electron exchange mechanism. Complexation of SWNTs with organic photosensitizers provides a novel way of uniformly exciting a wide range of nanotube structural species in polydisperse samples using only a single excitation wavelength.

An efficient new method is demonstrated for measuring length distributions of semiconducting single-walled carbon nanotubes (SWCNTs) through analysis of their highly polarized photoluminescence when aligned by shear flows. Instrumentation and procedures are developed to characterize nanotube lengths in bulk suspensions with rapid data acquisition and interpretation. Applying the method with spectrally resolved SWCNT emission provides the first measurements of (n,m)-specific length distributions. A positive correlation is found between average length and nanotube diameter, although this correlation is weaker following extensive sample centrifugation. Intense sonication shortened all nanotube species and had the strongest effect on those with small diameters. The new method should provide a useful alternative to atomic force microscopy for characterizing SWCNT lengths.Keywords: length distribution; near-IR fluorescence; photoluminescence anisotropy; shear alignment; single-walled carbon nanotubes

Near-infrared fluorescence videomicroscopy has been used to study simultaneously the translational and rotational diffusion of individual semiconducting single-walled carbon nanotubes (SWCNTs) in aqueous suspension. Analysis of translational trajectories revealed diffusion coefficient values from approximately 0.3 to 6 μm2/s. The nanotube lengths deduced from these values ranged between ∼130 nm and 6 μm. From the minor bending motions observed in individual nanotubes several micrometers in length, we confirmed that the shorter SWCNTs of primary interest here can be considered to be rigid rods under normal conditions. Because the nanotubes act as highly rigid, photostable, steady, and anisotropic fluorophores, it was possible to monitor their rotational reorientations through fluctuations in emission intensity under linearly polarized excitation. The magnitudes of observed orientational fluctuations varied substantially among individual nanotubes. These magnitudes correlated strongly with translational diffusion coefficient, reflecting the length dependence of both types of motions. Combined translational and rotational measurements also revealed the influence of local environment on nanotube mobility.Keywords: Brownian motion; diffusion; fluorescence microscopy; rotation; single-nanotube imaging; single-walled carbon nanotubes; SWCNT

Abstract

Single-molecule chemical reactions with individual single-walled carbon nanotubes were observed through near-infrared photoluminescence microscopy. The emission intensity within distinct submicrometer segments of single nanotubes changed in discrete steps after exposure to acid, base, or diazonium reactants. The steps were uncorrelated in space and time and reflected the quenching of mobile excitons at localized sites of reversible or irreversible chemical attack. Analysis of step amplitudes revealed an exciton diffusional range of about 90 nanometers, independent of nanotube structure. Each exciton visited about 10,000 atomic sites during its lifetime, providing highly efficient sensing of local chemical and physical perturbations.

Individualized, chemically pristine single-walled carbon nanotubes have been intravenously administered to rabbits and monitored through their characteristic near-infrared fluorescence. Spectra indicated that blood proteins displaced the nanotube coating of synthetic surfactant molecules within seconds. The nanotube concentration in the blood serum decreased exponentially with a half-life of 1.0 ± 0.1 h. No adverse effects from low-level nanotube exposure could be detected from behavior or pathological examination. At 24 h after i.v. administration, significant concentrations of nanotubes were found only in the liver. These results demonstrate that debundled single-walled carbon nanotubes are high-contrast near-infrared fluorophores that can be sensitively and selectively tracked in mammalian tissues using optical methods. In addition, the absence of acute toxicity and promising circulation persistence suggest the potential of carbon nanotubes in future pharmaceutical applications.

Energy transfer from photoexcited porphyrin molecules to single-walled carbon nanotubes (SWNTs) has been experimentally detected for samples in aqueous Triton X-100 micellar suspensions. Addition of SWNTs to micelle-suspended porphyrin results in strong quenching of porphyrin fluorescence. Measurements of concentration-dependent quenching and spectra suggest that this process arises from formation of ground state non-covalent complexes between porphyrins and SWNTs. Optical excitation of the porphyrin generates characteristic near-IR emission from the SWNTs, indicating efficient energy transfer within the complexes. This energy transfer is deduced to occur through a Dexter-type electron exchange mechanism. Complexation of SWNTs with organic photosensitizers provides a novel way of uniformly exciting a wide range of nanotube structural species in polydisperse samples using only a single excitation wavelength.