Date of Award

2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Chemical Engineering

Department

Chemical Engineering

First Advisor

Daniel E. Roxbury

Abstract

Single-walled carbon nanotubes (SWCNTs) functionalized with synthetic or natural polymers are a class of advanced optical nanomaterials that have recently found applications in a wide range of molecular diagnostics platforms as biosensors and bioimaging agents. Despite their widespread applications, there is an urgent need to develop standard characterization assays to evaluate their optical, physical, and interfacial properties in vitro and in complex biological environments. The results from such assays are essential for defining general design principles for nanotube-based optical probes and biosensing devices. Moreover, to fully exploit the nanotube's unique properties for health monitoring, it is essential to employ micro- and nano-fabrication processes to produce bulk or thin-film optical materials capable of real-time biomarker monitoring for wearable and implantable biosensing applications.

In this dissertation, we developed an entirely experimental platform based on time-resolved near-infrared fluorescence spectroscopy, hyperspectral fluorescence microscopy, and confocal Raman imaging, among other experimental approaches, to investigate the optical and hybrid stabilities of DNA-wrapped SWCNTs (DNA-SWCNTs) in solution phase and in live cells. We systematically quantified the time constant of the DNA displacement as a measure for the DNA-nanotube hybrid stability through a surfactant exchange mechanism and measuring the resultant fluorescence response. The stability was demonstrated to directly correlate with the DNA sequence length, although no statistical correlation was observed between the stability and optical parameters, indicating that the stability is not a result of the surface coverage afforded by DNA. To translate these findings into biological environments, utilizing hyperspectral fluorescence and confocal Raman imaging, we showed that a longer DNA length leads to a greater relative cellular uptake, intracellular optical stability, and retention of DNA-SWCNTs in mammalian cells. Additionally, by labeling the DNA with a fluorophore that dequenches upon removal from the SWCNT surface, we found that shorter DNA strands are displaced from the SWCNT within the cell, altering the physical identity and biocompatibility, thus changing the fate of the internalized nanomaterial.

Next, we employed atomic force microscopy (AFM) and thermogravimetric analysis (TGA) to investigate the physical and material characteristics of the DNA-nanotubes, namely their length distribution, DNA coverage density, and thermal stability, as a function of the DNA sequence. We discovered that the DNA coverage densities and conformations on the nanotubes significantly differ by varying the DNA sequence length. Consequently, this introduces an overlooked artifact to the electrostatic repulsions between the hybrids and the imaging substrate. Thus, we developed a modified standardized protocol for AFM-based size quantification by suppressing the wrapping and free diffusion effect so that the deposited hybrids represent their parent solutions. Moreover, the DNA conformations revealed in this study confirmed the instabilities of the short DNA sequences in the biological environment. Utilizing TGA, for the first time, we also discovered that the thermal stability of the purified nanotubes can be substantially enhanced upon hybridization with DNA, and their thermal decomposition behavior can be manipulated by modulating the bases in the DNA sequence.

Finally, we introduced the first generation of the nanotube-based optical wearable textiles by encapsulating the DNA-nanotube hybrids inside of core-shell microfibers without altering their unique nanoscale optical properties. We optimized a robust fabrication process that can be applied to a wide range of water-soluble polymer-wrapped nanotube biosensors. Utilizing probe fluorescence spectroscopy and confocal Raman microscopy, we demonstrated that the microfibers maintain their optical stability and structural integrity for up to 21 days without releasing the nanotube biosensors into their surrounding environment. We ultimately designed and calibrated a smart wound dressing for continuous wireless monitoring of oxidative stress in the wound environment.

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