Rice University logo

Advanced Optical Detection of Single-Walled Carbon Nanotubes for Biomedical Applications and Photophysical Studies

Thesis Defense

Graduate and Postdoctoral Studies

By: Ching-Wei Lin
Doctoral Candidate
When: Tuesday, August 8, 2017
2:00 PM - 4:00 PM
Where: Dell Butcher Hall
Abstract: 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 singlet oxygen. 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 singlet oxygen 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.