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.

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.

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 E11T → E11S 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.

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.

(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.

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.

High precision fractionator for use with density gradient ultracentrifugation.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Near-infrared photoluminescence of Portland cement.

Portland cement emits bright near-infrared photoluminescence that can be excited by light wavelengths ranging from at least 500-1000 nm. The emission has a peak wavelength near 1140 nm and a width of approximately 30 nm. Its source is suggested to be small particles of silicon associated with calcium silicate phases. The luminescence peak wavelength appears independent of the cement hydration state, aggregates, and mechanical strain but increases weakly with increasing temperature. It varies slightly with the type of cement, suggesting a new non-contact method for identifying cement formulations. After a thin opaque coating is applied to a cement or concrete surface, subsequent formation of microcracks exposes the substrate's near-infrared emission, revealing the fracture locations, pattern, and progression. This damage would escape detection in normal imaging inspections. Near-infrared luminescence imaging may therefore provide a new tool for non-destructive testing of cement-based structures.

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.

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.

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.