In-fiber light-fluorophore interactions for spectroscopy and photochemistry

• Main Author: Yong, Derrick Zhi Yong
• Other Authors: Chan Chi Chiu, Julian
• Format: Theses and Dissertations
• Language: English
• Published: 2016
• Subjects: DRNTU::Engineering::Bioengineering; DRNTU::Science::Physics::Optics and light; DRNTU::Science::Chemistry::Biochemistry::Spectroscopy; DRNTU::Science::Chemistry::Physical chemistry::Photochemistry
• Online Access: https://hdl.handle.net/10356/65957
• Institution: Nanyang Technological University

Optical fibers have enabled the miniaturization of a host of optical devices and systems. The further development of photonic crystal fibers (PCFs) has broadened this miniaturization capability to fluidics. Exploiting the optofluidics offered by PCFs, in-fiber light-fluorophore interactions have been studied in this thesis for applications in fluorescence spectroscopy and the generation of photochemical reactions. A compact optofluidic platform was first devised to facilitate these in-fiber studies. The simple application of a bend in the length of PCF created wavelength-specific core-cladding coupling that were observed as dips in the output spectrum. Variations in the refractive index of the PCF’s air hole array were hence reflected as dip shifts. A performance study of the platform was next performed using this bending-induced sensitivity to refractive index changes. Fluorospectroscopy was subsequently studied within bent lengths of PCFs,which were observed to possess heightened sensitivities in contrast to their nonbent counterparts. In order to account for light-fluorophore interactions within PCFs, a ray-tracing model was developed. Numerical results demonstrated good concurrence with experiments and were able to attribute the higher sensitivities to higher number of reflections in bent PCFs. Additionally, a ratiometric method of spectral analysis mitigated the effects of intensity fluctuations and enabled spectroscopy under a filterless regime. Light-fluorophore interactions were further utilized in the generation of photochemical reactions. This enabled the novel demonstration of in-fiber surface functionalization through photochemical means. This method was simple and involved non-hazardous chemicals. The functionalization primarily entailed the photo-induced radicalization of a biotin-conjugated fluorophore. Intermediary products of the process further led to the radicalization of an amino acid reside in proteins adsorbed on the in-fiber surfaces. Consequently, radical-radical reactions would lead to bond formation and thus the immobilization of biotin. The subsequent introduction tagged streptavidin (a biotin-binding biomolecule) enabled quantification of these biomolecules’ coverage density via means of in-fiber fluorospectroscopy. The feasibility of creating bio-integrated photonic devices was next explored using the biomolecule-functionalized in-fiber surfaces. Liposomes (biomolecular constructs) loaded with fluorescent dyes were chosen for tethering within PCFs due to their likeness to dye-doped nanoparticles and ease of characterization by fluorospectroscopy. Investigation of these dye-loaded liposomes external of PCFs revealed an intriguing counterintuitive phenomenon. This was the observation of increasing fluorescence emission intensities upon photobleaching. A devised mathematical model accounted for this unique phenomenon, attributing it to a reduction in quencher concentration upon photobleaching. The original demonstration of in-fiber tethered liposomes was further substantiated by the observation of this phenomenon. Since the phenomenon only presented when fluorescent dye was present at high concentrations within liposomes, it indicated the maintenance of liposomal structural integrity post-surface attachment. In sum, in-fiber light-fluorophore interactions were studied and exploited. This was facilitated by the developed optofluidic platform that provided infiltrationevacuation efficiency and repeatability. Ratiometry was demonstrated to enable fluorescence measurement capabilities comparable to other PCF-based techniques, but with an all-fiber configuration and just an LED source. The further devised ray-tracing model was generalized for all fluorospectroscopy using index-guiding PCFs. Next, the first biomolecular photo-immobilization and subsequent first successful tethering and probing of liposomes within PCFs were demonstrated. The first report of photobleaching-induced dequenching was also made, with its observation substantiating the intactness of the in-fiber tethered liposomes. Future work would explore the application of the developed bio-integrated photonic device. This would comprise utilization of the current established understanding of light-fluorophore interactions for implanted waveguides as well as further investigate the potential of liposomes for use in photonics.