Nanostructured Photonic Chips for High Throughput Biomolecule Sensing

Abstract

Optical biosensors have emerged as primary candidates to realize integrated, self-contained analytical systems capable of replacing or out-performing traditional benchtop set-ups for applications requiring high-throughput or ultra-sensitive biomolecular detection. As such, this work investigated the use of photonic waveguide-based sensing devices for the detection of chemical and biological compounds via fluorescence and Raman scattering modalities. Aluminum nitride (AlN) optical waveguides were fabricated and used to excite Raman scattering of benzene derivative mixtures, from which characteristic peaks and concentrations could be resolved. Low-index optical waveguides were combined with metallic nanoparticle-conjugated molecular reporters for waveguide-assisted, surface-enhanced Raman detection of an immobilized cardiac biomarker assay. Further, the use of a nano-slot silicon nitride (Si3N4) fluidic waveguide to detect a tagged oligonucleotide with a viral DNA sequence was also demonstrated, showing sensitivity enhancement of 9 x over a strip waveguide. A resin mate-rial platform was used for direct-write fabrication of waveguides and micro-ring resonators, where the low index contrast and reduced coupling loss allowed for increased excitation of a fluorescence-labeled biomolecule, resulting in an overall 12 x signal over a titanium dioxide (TiO2) waveguide. The work also examined methods for further improving the performance of photonics-based biosensors. A surface functionalization scheme to immobilize probe molecules for affinity-based sensing with high selectivity was applied, utilizing a self-assembled silane layer and linker molecule to covalently bond biomolecules to the device surface. Nanoporous anodic aluminum oxide (AAO) membranes were employed for further improvement of detection sensitivity, owing to their extensive surface area. Surface-functionalized AAO membranes were demonstrated for detection of fluorophore-labeled DNA and RNA sequence targets, yielding an increase in signal nearly 100 x compared to a non-porous substrate. Moreover, direct fabrication of AAO from thin films to create AAO-clad optical waveguides was presented and used to facilitate antibody and unlabeled DNA detection assays. Further improvement to AAO-waveguide devices through fabrication and a functionalization strategy for orientation-specific protein immobilization is proposed.