Abdul-Hadi, Jalal
(2018).
Enhanced fluorescence evanescent wave spectroscopy with
integrated polymer optical waveguides.
Thèse.
Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 231 p.
Résumé
Research in optical biosensors has gained great interest in the last two decades. Although
many optical technologies were developed, integrated optics technologies revealed to be
popular due to their potential for quick detection, highly sensitive measurements and
particularly for miniaturization. Indeed, miniaturization of an optical sensing device offer
the possibility to integrate it in a lab-on-a-chip (LOC) system. Which consists, in the best
scenario, of a device that takes care of the delivery of the bio or chemical samples to the
sensing area with microfluidic channels and treatment of the detection signal in a single
chip with micro dimensions. Optical sensing structures with nano to micro dimensions can
then be easily integrated with microfluidics, hence ideal for LOC devices. Also, they
surpass electrochemical sensors for this task since they are immune to electromagnetic
interferences which could alter the detection results.
Optical sensing devices could be in many forms, but for the purpose of miniaturization
channel waveguides are the best candidates. Indeed, the fabrication processes for
channel waveguides benefited immensely from microelectronics circuits manufacturing
technologies which have been refined for decades.
In this work, polymer channel waveguides were used as the sensing platforms for
evanescent wave fluorescence spectroscopy among other optical bio sensing methods
because of its high sensitivity and specificity.
Firstly, a novel fabrication process was developed for SU-8 polymer optical waveguides
on a quartz substrate which revealed different characteristics than SU-8 polymer
waveguides on a silica substrate fabricated with the standard protocol. They differed from
each other mainly in their lower optical propagation losses and lower asymmetry.
The SU-8 on quartz waveguides were then compared with the SU-8 on silica waveguides
for evanescent wave fluorescence spectroscopy to investigate the impact of the different
parameters from the novel and standard fabrication protocols. Finite difference time
domain (FDTD) simulations were performed to evaluate the impact of the different
asymmetries of both SU-8/quartz and SU-8/silica waveguide on the fluorescence collection efficiency for simulated emitting fluorophores. Also, an evanescent wave
fluorescence spectroscopy experiment was performed with Alexa 647 labeled Bovine
serum albumin (BSA) molecules to evaluate the sensitivity of both waveguides for
fluorescence detection. In brief, the FDTD simulations showed an increase in the
fluorescence collection efficiency for the SU-8/quartz waveguide compared to the SU-
8/silica waveguide and the experimental results revealed that the SU-8/quartz waveguide
offered a greater sensitivity than the SU-8/silica waveguide.
Moreover, to evaluate the impact of the variations of the different parameters
(dimensions, core & substrate refractive indices) of both the SU-8/quartz and SU-8/silica
waveguides on the fluorescence collection efficiency, a new calculation method was
developed. This calculation method combined a modal propagation approach to find the
fluorescence collection efficiency for planar waveguides with the effective index method.
Finally, new geometries were designed with the purpose of increasing the area of
immobilization of the labeled targets for increased sensitivity. Namely, the cascaded,
doubled and tripled waveguides. The geometries of these new designs were first
optimized for the excitation light propagation with numerical simulations (beam
propagation method) and then by experimental characterization. Evanescent wave
fluorescence spectroscopy experiments with Alexa 647 labeled biotin were performed to
compare their performance with a single straight waveguide. The experimental results
revealed increased sensitivity for the new designs compared to the straight waveguide.
Concentrations of Alexa 647-biotin as low as 15, 9, 10 and 7 pg·mL-1 were detected for
the single, cascaded, doubled and tripled waveguides respectively. Experimental results
were expected to be higher, taking into account the increased area of immobilization. To
understand this discrepancy, additional numerical simulations with the beam propagation
method were performed to analyze the optical losses of the evanescently coupled
fluorescence guided throughout the sensors.
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