Browsing by Author "Lin, Ching-Wei"

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    Advanced Optical Detection of Single-Walled Carbon Nanotubes for Biomedical Applications and Photophysical Studies
    (2017-08-09) Lin, Ching-Wei; Weisman, R. Bruce
    Fluorescence of single-walled carbon nanotubes (SWCNTs) is of great interest for biomedical applications and optoelectronic devices because of their unusual emission wavelengths in the short-wave infrared (SWIR; 900-1600 nm). Many applications require developing novel experimental techniques with special capabilities. In this thesis, I demonstrate the use of a SWIR avalanche photodiode (APD) detector to expand biomedical applications and photophysical studies. Taking advantage of the detector’s high sensitivity, we developed a new approach for the noninvasive imaging of nanotubes in biological specimens and observed new photophysical phenomena of SWCNTs. A custom-built system was designed and constructed to detect and locate small amounts of SWCNTs inside living animals. This instrument uses diffuse LED illumination to excite nanotubes, a scanning optical probe to capture emission at specific locations, and a spectrally filtered APD to sensitively detect the nanotube fluorescence. This system also implements a new method called spectral triangulation, which determines the 3D locations of SWCNT emission sources in vivo. The unique feature of spectral triangulation is taking advantage of the differential water absorption in the SWIR region to gauge the path length in tissue between emission source and the probe. The SWCNT fluorescence signals attenuate differently in two selected spectral channels, so that the distance between source and probe can be deduced from the ratio of channel intensities. By probing at more than three positions on the specimen surface, we gauge the depth of the source and triangulate its 3D position with high precision. The SWIR-scanner system and spectral triangulation method were demonstrated first in tissue phantoms and then in living mice. Results show that the depth of a SWCNT source and the attenuation coefficients of nearby tissues can be obtained simultaneously. Future prospects for advancing the detection of SWCNTs in vivo are also quantitatively discussed. The high time resolution provided by APD detection allows exploration of novel SWCNT photophysics. I have developed a novel kinetic apparatus to monitor SWCNT luminescence changes on the sub-microsecond to millisecond time scale, with particular value for detecting weak delayed emission in the SWIR. For the first time, the “pile-up” distortions that commonly hamper traditional time-correlated single photon counting measurements have been overcome by mathematical analysis. This allows data acquisition to be conducted using a low repetition-rate, high power excitation source. This novel system is a powerful tool for studying SWCNT delayed fluorescence or photophysics involving singlet oxygen. Electronic excitation of SWCNTs has almost always been achieved through light absorption, electron injection, or molecular energy transfer, creating singlet excitons in the nanotubes. Very little is therefore known about SWCNT triplet excitons. I describe here the first production of SWCNT triplet excitons through singlet oxygen sensitization. A specialized apparatus was built to perform SWIR delayed luminescence spectrometry (SWIR-DLS) and selectively measure delayed emission spectra at intensities more than 20 times lower than normal SWCNT fluorescence. In these experiments, an optically excited organic sensitizer is quenched by dissolved oxygen to generate excited 1O2. This species then quenched in energy transfer encounters with nanotubes that produce triplet SWCNT excitons. Thermal activation of the SWCNT triplet state to its emissive singlet rate results in the detected delayed fluorescence emission. The delayed spectrum shows strong (n,m) selectivity that reflects the relative energy levels of SWCNT triplet excitons and singlet oxygen. Evidence is also seen for an alternate process that excites (n,m) species with triplet energies higher than 1O2 through triplet exciton ̶ singlet oxygen annihilation (TESOA). These studies demonstrate the use of advanced experimental probes for expanding basic scientific knowledge about carbon nanotubes and developing applications that make use of their remarkable properties.
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    Creating fluorescent quantum defects in carbon nanotubes using hypochlorite and light
    (Springer Nature, 2019) Lin, Ching-Wei; Bachilo, Sergei M.; Zheng, Yu; Tsedev, Uyanga; Huang, Shengnan; Weisman, R. Bruce; Belcher, Angela M.; Smalley-Curl Institute
    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.
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    (n,m)-Specific Absorption Cross Sections of Single-Walled Carbon Nanotubes Measured by Variance Spectroscopy
    (American Chemical Society, 2016) Sanchez, Stephen R.; Bachilo, Sergei M.; Kadria-Vili, Yara; Lin, Ching-Wei; Weisman, R. Bruce; Smalley-Curl Institute
    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 ofﾠmod 1ﾠtype than forﾠmod 2. Directly measured and extrapolated values are now available to support quantitative analysis of SWCNT samples through absorption spectroscopy.
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    Spectral triangulation: a 3D method for locating single-walled carbon nanotubes in vivo
    (Royal Society of Chemistry, 2016) Lin, Ching-Wei; Bachilo, Sergei M.; Vu, Michael; Beckingham, Kathleen M.; Weisman, R.Bruce; Smalley-Curl Institute
    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.
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    Variance Spectroscopy
    (American Chemical Society, 2015) Streit, Jason K.; Bachilo, Sergei M.; Sanchez, Stephen R.; Lin, Ching-Wei; Weisman, R. Bruce; Smalley-Curl Institute
    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.