Diameter-Dependent Competitive Adsorption of Sodium Dodecyl Sulfate and Single-Stranded DNA on Carbon Nanotubes

The equilibrium compositions of coatings on single-wall carbon nanotubes were spectroscopically deduced for samples dispersed in dilute sodium dodecyl sulfate (SDS) and then exposed to low concentrations of ssDNA oligomers. With all studied oligomers, displacement of the SDS tended to occur at lower ssDNA concentrations for smaller diameter nanotubes than for larger diameter ones. However, the behavior varied significantly with oligomer. For example, the diameter dependence was steeper for (TAT)4 than for (ATT)4, suggesting that interstrand head-to-tail hydrogen bonding interactions play a role in SWCNT wrapping. Concentrations of ssDNA in the range of several μg/mL displace SDS from nanotubes dispersed in 1500 μg/mL SDS solutions. This effect allows the use of coating exchange to prepare ssDNA dispersions with minimal oligomer costs. Another demonstrated use exploits the structure-dependent relative coating affinities in a simple filtration method for the diameter enrichment of SWCNT mixtures.

Kinetics of Single-Wall Carbon Nanotube Coating Displacement by Single-Stranded DNA Depends on Nanotube Structure

Time-resolved fluorescence spectroscopy has been used to study the displacement of adsorbed sodium dodecyl sulfate (SDS) from the surface of single-wall carbon nanotubes (SWCNTs) by short strands of single-stranded DNA. Intensity changes in near-infrared emission peaks of various SWCNT structures were analyzed following the addition of six different (GT)n oligomers (n from 3 to 20) to SDS-coated nanotube samples. There is a strong kinetic dependence on the oligomer length, with (GT)3 giving an initial rate more than 300 times greater than that of (GT)20. For shorter oligos in the (GT)n series, we observe an inverse dependence of the displacement rate on the SWCNT diameter, with SDS displaced from (6,5) more than twice as fast as from (8,7). However, this diameter dependence is reversed for oligos with more than six (GT) units. There is also a systematic dependence of the displacement rate on the nanotube chiral angle that is strongest for (GT)5, leading to a factor of ∼3 initial rate difference between (9,1) and (6,5) despite their identical diameters. To account for these findings, we propose a simple two-step kinetic model in which disruption of the original SDS coating is followed by conformational relaxation of ssDNA on the nanotube surface. The relaxation is relatively fast for ssDNA oligos shorter than 12 bp, making the first step rate-determining. Conversely, relaxation of the longer oligomers is slow enough that the second step becomes rate-determining.

Structure-Resolved Monitoring of Single-Wall Carbon Nanotube Functionalization from Raman Intermediate Frequency Modes

Single-wall carbon nanotubes (SWCNTs) can be covalently modified to generate useful changes in their spectroscopic and photophysical properties. We report here a new method to monitor the extent of such functionalization reactions for different nanotube structures. Raman spectra are analyzed to find the intensities of structure-specific intermediate frequency mode (IFM) features in the range of ca. 350 to 650 cm-1, which are induced by introduction of sp3 defects. The IFM frequencies are found to depend on both the nanotube diameter and Raman excitation wavelength. The growth of IFM features is accompanied by a decrease in RBM intensities, so the IFM to RBM intensity ratio can provide a sensitive, structure-specific measure of nanotube functionalization.

Photostable Small-Molecule NIR-II Fluorescent Scaffolds that Cross the Blood-Brain Barrier for Noninvasive Brain Imaging

The second near-infrared (NIR-II, 1000-1700 nm) fluorescent probes have significant advantages over visible or NIR-I (600-900 nm) imaging for both depth of penetration and level of resolution. Since the blood-brain barrier (BBB) prevents most molecules from entering the central nervous system, NIR-II dyes with large molecular frameworks have limited applications for brain imaging. In this work, we developed a series of boron difluoride (BF₂) formazanate NIR-II dyes, which had tunable photophysical properties, ultrahigh photostability, excellent biological stability, and strong brightness. Modulation of the aniline moiety of BF₂ formazanate dyes significantly enhances their abilities to cross the BBB for noninvasive brain imaging. Furthermore, the intact mouse brain imaging and dynamic dye diffusion across the BBB were monitored using these BF₂ formazanate dyes in the NIR-II region. In murine glioblastoma models, these dyes can differentiate tumors from normal brain tissues. We anticipate that this new type of small molecule will find potential applications in creating probes and drugs relevant to theranostic for brain pathologies.

Compositional Analysis of ssDNA-Coated Single-Wall Carbon Nanotubes through UV Absorption Spectroscopy

Aqueous suspensions of single-wall carbon nanotubes (SWCNTs) coated by ssDNA are analyzed using UV absorption and total carbon measurements. The results give absolute average concentrations of both components in samples without free ssDNA. From those values, the average mid-UV SWCNT absorptivity is deduced for three different batches of relatively small diameter nanotubes: two HiPco and one CoMoCAT. The absorptivity values enable the use of simple spectrophotometry to measure absolute concentrations of similar SWCNT samples in aqueous SDS. The results also quantify the mass ratio of ssDNA to SWCNT, defining the average number of nanotube carbon atoms suspended by one ssDNA strand of T₁₅GT₁₅ or T₃₀G. Comparing this experimental parameter with results from replica exchange molecular dynamics simulations of one ssDNA strand freely adsorbed on a (6,5) segment shows close agreement between the computed number of SWCNT atoms covered per strand and the measured number of SWCNT atoms suspended per strand.

Next-generation 2D optical strain mapping with strain-sensing smart skin compared to digital image correlation

This study reports next generation optical strain measurement with "strain-sensing smart skin" (S⁴) and a comparison of its performance against the established digital image correlation (DIC) method. S⁴ measures strain-induced shifts in the emission wavelengths of single-wall carbon nanotubes embedded in a thin film on the specimen. The new S⁴ film improves spectral uniformity of the nanotube sensors, avoids the need for annealing at elevated temperatures, and allows for parallel DIC measurements. Noncontact strain maps measured with the S⁴ films and point-wise scanning were directly compared to those from DIC on acrylic, concrete, and aluminum test specimens, including one with subsurface damage. Strain features were more clearly revealed with S⁴ than with DIC. Finite element method simulations also showed closer agreement with S⁴ than with DIC results. These findings highlight the potential of S⁴ strain measurement technology as a promising alternative or complement to existing technologies, especially when accumulated strains must be detected in structures that are not under constant observation.

Guanine-Specific Chemical Reaction Reveals ssDNA Interactions on Carbon Nanotube Surfaces

Understanding the conformations of physisorbed single-stranded DNA (ssDNA) oligos on single-wall carbon nanotube (SWCNT) surfaces is important for advancing basic nanoscience and for developing applications in biomedicine and quantum information processing. Here we report evidence that the ssDNA strands are partly desorbed from the nanotube surface under common conditions. SWCNT suspensions were prepared in eight ssDNA oligos, each containing 1 guanine and 30 thymine bases but differing in the position of the guanine within the strand. Singlet oxygen exposure then covalently functionalized the guanine to the SWCNT surface, red-shifting the nanotube fluorescence by an amount reflecting the guanine spatial density at the surface. Spectral shifts were greatest for central guanine positions and smallest for end positions. In conjunction with steered molecular dynamics simulations, the results suggest that steric interference between neighboring ssDNA strands on an individual nanotube causes significant dislocation or desorption of the strand ends while central regions remain better wrapped around the nanotube. This effect decreases with decreasing concentrations of free ssDNA.

Oxime as a general photocage for the design of visible light photo-activatable fluorophores

Photoactivatable fluorophores have been widely used for tracking molecular and cellular dynamics with subdiffraction resolution. In this work, we have prepared a series of photoactivatable probes using the oxime moiety as a new class of photolabile caging group in which the photoactivation process is mediated by a highly efficient photodeoximation reaction. Incorporation of the oxime caging group into fluorophores results in loss of fluorescence. Upon light irradiation in the presence of air, the oxime-caged fluorophores are oxidized to their carbonyl derivatives, restoring strong fluorophore fluorescence. To demonstrate the utility of these oxime-caged fluorophores, we have created probes that target different organelles for live-cell confocal imaging. We also carried out photoactivated localization microscopy (PALM) imaging under physiological conditions using low-power light activation in the absence of cytotoxic additives. Our studies show that oximes represent a new class of visible-light photocages that can be widely used for cellular imaging, sensing, and photo-controlled molecular release.

Quantum Light Emission from Coupled Defect States in DNA-Functionalized Carbon Nanotubes

Solid-state single-photon sources are essential building blocks for quantum photonics and quantum information technologies. This study demonstrates promising single-photon emission from quantum defects generated in single-wall carbon nanotubes (SWCNTs) by covalent reaction with guanine nucleotides in their single-stranded DNA coatings. Low-temperature photoluminescence spectroscopy and photon-correlation measurements on individual guanine-functionalized SWCNTs (GF-SWCNTs) indicate that multiple, closely spaced guanine defect sites within a single ssDNA strand collectively form an exciton trapping potential that supports a localized quantum state capable of room-temperature single-photon emission. In addition, exciton traps from adjacent ssDNA strands are weakly coupled to give cross-correlations between their separate photon emissions. Theoretical modeling identifies coupling mechanism as a capture of band-edge excitons. Because the spatial pattern of nanotube functionalization sites can be readily controlled by selecting ssDNA base sequences, GF-SWCNTs should become a versatile family of quantum light emitters with engineered properties.

Delayed Fluorescence from Carbon Nanotubes through Singlet Oxygen-Sensitized Triplet Excitons

Single-wall carbon nanotubes (SWCNTs) in liquid suspension have been observed to emit delayed, microsecond-scale fluorescence arising from upconverted triplet excitons that are directly created through energy transfer from singlet oxygen molecules (¹O₂). The singlet oxygen is produced through quenching of an optically excited organic sensitizer. The mechanism of this delayed fluorescence has been deduced from measurements of time-resolved emission kinetics, delayed emission spectra, and polarization-resolved excitation-emission spectra. The observed strong dependence of ¹O₂ sensitization efficiency on SWCNT structure suggests that (7,6) triplet excitons have an energy near 970 meV. The yields for E₁₁^T → E₁₁^S upconversion are found to be in the range of several percent. These yields increase with increasing temperature and decrease with increasing excitation intensities, reflecting thermal activation and triplet-triplet exciton annihilation processes.

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.

Tailoring the Properties of Single-Wall Carbon Nanotube Samples through Structure-Selective Near-Infrared Photochemistry

As-produced samples of single-wall carbon nanotubes (SWCNTs) contain many structural forms with characteristic properties. In particular, semiconducting SWCNTs display distinct near-IR optical absorption and emission peaks. We show that the selective irradiation of these absorption features can induce structure-specific functionalization in unsorted SWCNT samples. This approach is demonstrated with an ambient temperature photoreaction involving dissolved O₂ and irradiation at 955, 985, and 1130 nm, causing preferential covalent reactions of (8,3), (6,5), and (7,6) SWCNTs, respectively. Treated samples showed permanent fluorescence quenching and absorption bleaching near the irradiation wavelength and an increase in the Raman D/G intensity ratio, indicating the formation of covalent defects. The reaction has a very low photochemical quantum yield and was observed for samples suspended in single-stranded DNA and in conventional surfactants that gave incomplete coverage of the nanotube surface. The approach of exploiting sharp nanotube near-IR transitions for structure-selective photochemistry provides a path to tailor SWCNT optical properties for several potential applications without the need for physical sorting.

Photoexcited Aromatic Reactants Give Multicolor Carbon Nanotube Fluorescence from Quantum Defects

Covalent functionalization of single-wall carbon nanotubes (SWCNTs) can be valuable for modifying their electronic properties and creating fluorescent quantum defects. We report here a previously unreported category of such reactions involving interactions of photoexcited aromatic compounds with SWCNT sidewalls. When aqueous suspensions of SWCNTs are exposed to organic aromatic compounds and then irradiated by UV light, fluorescent defects are formed in the nanotubes at rates that depend on the aromatic ring substituents. In reactions with aniline or iodoaniline, strong spectral sidebands appear within 1 min. Total SWCNT photoluminescence can be enhanced by a factor as large as ∼5. Notably, emission spectra of reacted SWCNTs depend on the presence or absence of dissolved oxygen during the reaction. For (6,5) SWCNTs, treatment when oxygen is present gives an additional emission band red-shifted by 160 meV from the pristine position, whereas treatment without oxygen leads to two additional emission bands red-shifted by 140 and 270 meV. Variance spectroscopy shows the presence of individual "multicolor" nanotubes with three distinct emission bands (pristine plus two shifted). The facile generation of dual fluorescent quantum defects in SWCNTs provides emission closer to standard telecom wavelengths, advancing the prospects for applications as single-photon sources in quantum information processing.

Synchro-Excited Free-Running Single Photon Counting: A Novel Method for Measuring Short-Wave Infrared Emission Kinetics

Time-resolved measurements of short-wave infrared (SWIR) photoluminescence on the submicrosecond to millisecond scale are needed for physical and chemical studies involving singlet oxygen, single-walled carbon nanotubes, and other samples with weak, slow emission. We present here an alternative to the common method of time-correlated single photon counting (TCSPC) that is well suited to indium gallium arsenide avalanche photodiode (APD) detectors operated in Geiger mode. In the new method, termed synchro-excited free-running single photon counting (SEFR-SPC), excitation pulses from inexpensive laser diodes (providing a variety of wavelengths) are synchronized to detection events from a free-running detector covering the 900 to 1700 nm range. In contrast to traditional TCSPC, data from this method can be rigorously corrected for pile-up distortions, allowing operation with high excitation powers and low repetition rates. A technique is described to extend the system's dynamic range to approximately 10⁸. We also show that SEFR-SPC provides state-of-the-art sensitivity in the SWIR spectral region and that spectrally filtered kinetic data can offer additional insights. A six-step correction protocol has been developed and implemented as a LabVIEW program for very accurate acquisition of kinetic shapes. The SEFR-SPC method will be a valuable tool for studies of weak, long-lived emission sources.

Controlled Patterning of Carbon Nanotube Energy Levels by Covalent DNA Functionalization

Each structural form of single-wall carbon nanotube (SWCNT) has specific electronic and optical properties, but it has not been possible to achieve spatial or energetic modulation of those properties in controllable ways. We present here a simple method for using chemical reactions with single-stranded DNA (ssDNA) to accomplish such modulation. When aqueous suspensions of SWCNTs coated with ssDNA are exposed to singlet oxygen under ambient conditions, the nanotubes selectively form covalent bonds to the guanine nucleotides. This locally modulates semiconducting SWCNT energy levels and red-shifts their emission wavelengths by up to 10%. Both the magnitude and spatial pattern of these shifts can be controlled by selecting the nucleotide sequence used to coat the nanotubes. Biomedical, optoelectronic, and single-photon emission applications are foreseen.

Creating fluorescent quantum defects in carbon nanotubes using hypochlorite and light

Covalent doping of single-walled carbon nanotubes (SWCNTs) can modify their optical properties, enabling applications as single-photon emitters and bio-imaging agents. We report here a simple, quick, and controllable method for preparing oxygen-doped SWCNTs with desirable emission spectra. Aqueous nanotube dispersions are treated at room temperature with NaClO (bleach) and then UV-irradiated for less than one minute to achieve optimized O-doping. The doping efficiency is controlled by varying surfactant concentration and type, NaClO concentration, and irradiation dose. Photochemical action spectra indicate that doping involves reaction of SWCNT sidewalls with oxygen atoms formed by photolysis of ClO^- ions. Variance spectroscopy of products reveals that most individual nanotubes in optimally treated samples show both pristine and doped emission. A continuous flow reactor is described that allows efficient preparation of milligram quantities of O-doped SWCNTs. Finally, we demonstrate a bio-imaging application that gives high contrast short-wavelength infrared fluorescence images of vasculature and lymphatic structures in mice injected with only ~100 ng of the doped nanotubes.

Enantiomers of Single-Wall Carbon Nanotubes Show Distinct Coating Displacement Kinetics

It is known that specific oligomers of single-stranded DNA (ssDNA) can show remarkable selectivity when coating different structural species of single-wall carbon nanotubes (SWCNTs). We report that (ATT)₄ ssDNA coatings strongly distinguish between the two optical isomers of (7,5) SWCNTs. This causes resolvable shifts in their fluorescence spectra and differences of 2 orders of magnitude in the room temperature rates of coating displacement, as monitored through changes in nanotube fluorescence wavelength and intensity on exposure to sodium deoxycholate. During coating displacement, the enantiomer with high affinity for the ssDNA oligomer is deduced to form an intermediate hybrid that is not observed for the low affinity enantiomer. These results reveal that enantiomeric differences in SWCNTs complexed with ssDNA are more diverse and dramatic than previously recognized.

In Vivo Optical Detection and Spectral Triangulation of Carbon Nanotubes

In the first in vivo demonstration of spectral triangulation, biocompatible composites of single-walled carbon nanotubes in Matrigel have been surgically implanted into mouse ovaries and then noninvasively detected and located. This optical method deduces the three-dimensional position of a short-wave IR emission source from the wavelength-dependent attenuation of fluorescence in tissues. Measurements were performed with a second-generation optical scanner that uses a light-emitting diode matrix emitting at 736 nm for diffuse specimen excitation. The intrinsic short-wave IR fluorescence of the nanotubes was collected at various positions on the specimen surface, spectrally filtered, and detected by a photon-counting InGaAs avalanche photodiode. Sensitivity studies showed a detection limit of ∼120 pg of nanotubes located beneath ∼3 mm of tissue. In addition, the mass and location of implanted nanotubes could be deduced through spectral triangulation with sub-millimeter accuracy, as validated with the aid of magnetic resonance imaging (MRI) data. Dual-modality imaging combining spectral triangulation with computed tomography or MRI will allow accurate registration of emission centers with anatomical features. These results are a step toward the future use of probes with targeting agents such as antibodies linked to nanotube tags for the noninvasive detection and imaging of tumors in preclinical research on small animals. Translation to the clinic could aid in early detection of ovarian cancer and identification of metastases for resection during primary surgery.

Skewness Analysis in Variance Spectroscopy Measures Nanoparticle Individualization

An important enabling step in nanoparticle studies is the sorting of heterogeneous mixtures to prepare structurally homogeneous samples. It is also necessary to detect and monitor aggregation of the individual nanoparticles. Although variance spectroscopy provides a simple optical method for finding low concentrations of heteroaggregates in samples such as single-walled carbon nanotube dispersions, it cannot detect the homoaggregates that are relevant for well-sorted samples. Here we demonstrate that variance spectral data can be further analyzed to find third moments of intensity distributions (skewness), which reveal the presence of emissive homoaggregates. Using experimental measurements on variously processed nanotube dispersions, we deduce a simple numerical standard for recognizing aggregation in the highly sorted samples that are increasingly available to nanoscience researchers.

Quenching of Single-Walled Carbon Nanotube Fluorescence by Dissolved Oxygen Reveals Selective Single-Stranded DNA Affinities

The selective interactions between short oligomers of single-stranded DNA (ssDNA) and specific structures of single-walled carbon nanotubes have been exploited in powerful methods for nanotube sorting. We report here that nanotubes coated with ssDNA also display selective interactions through the selective quenching of nanotube fluorescence by dissolved oxygen. In aqueous solutions equilibrated under 1 atm of O₂, emission intensity from semiconducting nanotubes is reduced by between 9 and 40%, varying with the combination of ssDNA sequence and nanotube structure. This quenching reverses promptly and completely on the removal of dissolved O₂ and may be due to physisorption on nanotube surfaces. Fluorescence quenching offers a simple, nondestructive approach for studying the structure-selective interactions of ssDNA with single-walled carbon nanotubes and identifying recognition sequences.

(n,m)-Specific Absorption Cross Sections of Single-Walled Carbon Nanotubes Measured by Variance Spectroscopy

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.

Spectral triangulation: a 3D method for locating single-walled carbon nanotubes in vivo

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.

Resonance Raman Optical Activity Spectra of Single-Walled Carbon Nanotube Enantiomers

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 E22(S) 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.

Variance Spectroscopy

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.

Structure-Dependent Thermal Defunctionalization of Single-Walled Carbon Nanotubes

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.

Enabling in vivo measurements of nanoparticle concentrations with three-dimensional optoacoustic tomography

In this report, we demonstrate the feasibility of using optoacoustic tomography (OAT) to evaluate biodistributions of nanoparticles in animal models. The redistribution of single-walled carbon nanotubes (SWCNTs) was visualized in living mice. Nanoparticle concentrations in harvested organs were measured spectroscopically using the intrinsic optical absorption and fluorescence of SWCNTs. Observed increases in optoacoustic signal brightness in tissues were compared with increases in optical absorption coefficients caused by SWCNT accumulation. The methodology presented in this report can further be extended to calibrate the sensitivity of an optoacoustic imaging system for a range of changes in optical absorption coefficient values at specific locations or organs in a mouse body to enable noninvasive measurements of nanoparticle concentrations in vivo. Additionally, qualitative information provided by OAT and quantitative information obtained ex vivo may provide valuable feedback for advancing methods of quantitative analysis with OAT.

Directly measured optical absorption cross sections for structure-selected single-walled carbon nanotubes

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.

Chromatic aberration short-wave infrared spectroscopy: nanoparticle spectra without a spectrometer

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.

Measuring single-walled carbon nanotube length distributions from diffusional trajectories

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.

Strain paint: noncontact strain measurement using single-walled carbon nanotube composite coatings

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.

Films of bare single-walled carbon nanotubes from superacids with tailored electronic and photoluminescence properties

The use of single-walled carbon nanotubes (SWCNTs) in fabricating macroscopic devices requires addressing the challenges of nanotube individualization and organization in the desired functional architectures. Previous success in depositing bare SWCNTs from chlorosulfonic acid onto silicon oxide microporous and mesoporous nanoparticles has motivated this study of their deposition onto fused silica substrates. A facile dip-coating method is reported that produces thin homogeneous films in which the carbon nanotubes are not covered by surfactants or shortened by sonication. Photophysical, electrical, chemical, and morphological properties of these SWCNT films have been characterized. When prepared at low densities, the films exhibit near-IR photoluminescence from individualized SWCNTs, whereas when prepared at high densities the films behave as transparent conductors. Sheet resistance of 471 ohm/sq has been achieved with film transmittance of ∼ 86%.

Length- and defect-dependent fluorescence efficiencies of individual single-walled carbon nanotubes

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.

Evidence for long-lived, optically generated quenchers of excitons in single-walled carbon nanotubes

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/cm(2), 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.

Efficient spectrofluorimetric analysis of single-walled carbon nanotube samples

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.

Analyzing absorption backgrounds in single-walled carbon nanotube spectra

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.

Advanced sorting of single-walled carbon nanotubes by nonlinear density-gradient ultracentrifugation

Existing methods for growing single-walled carbon nanotubes produce samples with a range of structures and electronic properties, but many potential applications require pure nanotube samples. Density-gradient ultracentrifugation has recently emerged as a technique for sorting as-grown mixtures of single-walled nanotubes into their distinct (n,m) structural forms, but to date this approach has been limited to samples containing only a small number of nanotube structures, and has often required repeated density-gradient ultracentrifugation processing. Here, we report that the use of tailored nonlinear density gradients can significantly improve density-gradient ultracentrifugation separations. We show that highly polydisperse samples of single-walled nanotubes grown by the HiPco method are readily sorted in a single step to give fractions enriched in any of ten different (n,m) species. Furthermore, minor variants of the method allow separation of the mirror-image isomers (enantiomers) of seven (n,m) species. Optimization of this approach was aided by the development of instrumentation that spectroscopically maps nanotube contents inside undisturbed centrifuge tubes.

Surfactant-dependent exciton mobility in single-walled carbon nanotubes studied by single-molecule reactions

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 approximately 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.

Enrichment of armchair carbon nanotubes via density gradient ultracentrifugation: Raman spectroscopy evidence

We have used resonant Raman scattering spectroscopy to fully analyze the relative abundances of different (n,m) species in single-walled carbon nanotube samples that are metallically enriched by density gradient ultracentrifugation. Strikingly, the data clearly show that our density gradient ultracentrifugation process enriches the metallic fractions in armchair and near-armchair species. We observe that armchair carbon nanotubes constitute more than 50% of each (2n + m) family.

Multidomain peptides as single-walled carbon nanotube surfactants in cell culture

We present a series of short, multidomain peptides as biocompatible solubilizing agents of single-walled carbon nanotubes (SWCNTs). These peptides are organized into an ABA block motif, where the A block is composed of charged amino acids, such as glutamic acid, and the B block is composed of alternating hydrophilic and hydrophobic residues. The hydrophobic amino acid residues interact with SWCNT sidewalls, while the hydrophilic residues interact primarily with water in an aqueous solution. When many peptides assemble along the length of the nanotube, it becomes effectively encapsulated within a peptide nanofiber. This noncovalent interaction between the peptide and the nanotube solubilizes SWCNTs while keeping the electronic structure of the nanotube intact, thereby preserving the optical and electrical properties that make SWCNTs promising for use in biological applications. To assess the toxicity of these peptide coatings, they were added to cultures of NIH 3T3 mouse fibroblasts and the effect on cell viability was measured. Toxicity was found to be far lower than for ionic surfactants typically used for SWCNT suspension and similar to Pluronics. The near-IR fluorescence intensity of SWCNTs in peptide suspensions was comparable to that in Pluronics. Five surfactants were tested for their effect on the proliferation of NIH 3T3 cells with and without SWCNTs. Although some differences were observed among surfactants, in no case did the presence of SWCNTs make a statistically significant difference. Based on their ability to solubilize SWCNTs, the fluorescence of the suspended tubes, their minimal impact on cell viability, and their potential for easy chemical modification, multidomain peptides have been found to have excellent potential as a biocompatible surfactant for suspension of SWCNTs.

Do inner shells of double-walled carbon nanotubes fluoresce?

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.

Quantifying the semiconducting fraction in single-walled carbon nanotube samples through comparative atomic force and photoluminescence microscopies

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.

Diameter-dependent bending dynamics of single-walled carbon nanotubes in liquids

By relating nanotechnology to soft condensed matter, understanding the mechanics and dynamics of single-walled carbon nanotubes (SWCNTs) in fluids is crucial for both fundamental and applied science. Here, we study the Brownian bending dynamics of individual chirality-assigned SWCNTs in water by fluorescence microscopy. The bending stiffness scales as the cube of the nanotube diameter and the shape relaxation times agree with the semiflexible chain model. This suggests that SWCNTs may be the archetypal semiflexible filaments, highly suited to act as nanoprobes in complex fluids or biological systems.

In Vivo Therapeutic Silencing of Hypoxia-Inducible Factor 1 Alpha (HIF-1α) Using Single-Walled Carbon Nanotubes Noncovalently Coated with siRNA

A new approach is described for delivering small interfering RNA (siRNA) into cancer cells by noncovalently complexing unmodified siRNA with pristine single-walled carbon nanotubes (SWCNTs). The complexes were prepared by simple sonication of pristine SWCNTs in a solution of siRNA, which then served both as the cargo and as the suspending agent for the SWCNTs. When complexes containing siRNA targeted to hypoxia-inducible factor 1 alpha (HIF-1α) were added to cells growing in serum containing culture media, there was strong specific inhibition of cellular HIF-1α activity. The ability to obtain a biological response to SWCNT/siRNA complexes was seen in a wide variety of cancer cell types. Moreover, intratumoral administration of SWCNT-HIF-1α siRNA complexes in mice bearing MiaPaCa-2/HRE tumors significantly inhibited the activity of tumor HIF-1α. As elevated levels of HIF-1α are found in many human cancers and are associated with resistance to therapy and decreased patient survival, these results imply that SWCNT/siRNA complexes may have value as therapeutic agents.

Self-assembling peptide coatings designed for highly luminescent suspension of single-walled carbon nanotubes

A series of self-assembling multidomain peptides have been designed, synthesized, and tested for their ability to individually suspend single-walled carbon nanotubes (SWCNTs) in water while preserving strong near-IR nanotube luminescence. Photometric and spectral measurements on individual SWCNTs revealed that emission in the common biocompatible coating agents Pluronic F127, ss-DNA, and BSA is approximately an order of magnitude weaker than in the bioincompatible ionic surfactant SDBS. By contrast, one of the engineered peptides gave SWCNT emission approximately 40% as intense as in SDBS. A strong inverse correlation was also found between the spectral line widths of coated SWCNTs and the efficiency of their emission. Peptides with rationally designed self-assembly properties appear to be promising coatings that may enable SWCNT optical sensing applications in biological environments.

Translational and rotational dynamics of individual single-walled carbon nanotubes in aqueous suspension

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 microm(2)/s. The nanotube lengths deduced from these values ranged between approximately 130 nm and 6 microm. 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.

Chirality-resolved length analysis of single-walled carbon nanotube samples through shear-aligned photoluminescence anisotropy

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.