Nanophotonic materials for biomedical sensing

Abstract

Sensitive and rapid detection of chemical and biological species has become a central component in the rapidly emerging fields of information-aided personalized medicine. Currently available technologies are dominantly based on amplification strategies for the analysis of target analytes such as polymerase chain reaction or other enzymatic reactions. These approaches offer sufficient sensitivity but typically rely on multiple assay steps and purification of the sample that prolong the analysis time and that can only be conducted in a specialized laboratory environment. Alternatively, faster direct detection assays are pursued in order to expand the capability of sensing technology and potentially perform tests closer to the patients. Fluorescence represents a common and established optical readout method in both enzymatic and enzyme-free optical sensors. Nanophotonic surfaces have been developed to boost the sensitivity of fluorescence assays by implementing advanced light management, increasing the fluorescence emission strength, improving optical collection efficiency and reducing the background. However, these efforts rarely found practical applications as nanophotonic structures are more commonly prepared only for research purposes by complex manufacturing processes. This work presents nanostructured optical materials that can be prepared by scaled-up fabrication means, that support novel multimodal biosensor concepts and increases the sensitivity on optical readout. In particular, it focuses on tailoring surface plasmonic resonances on metallic nanostructures for optical spectroscopy amplification in conjunction with the development of dedicated optical readers. Plasmonic nanostructure architectures and an optical instrument are developed that provide large field of view required for multiplexing and parallelized kinetics measurements. The combination offers improved collection efficiency of fluorescence light that is emitted by the selectively excited molecules on the sensor surface. The work is organized in three parts, starting with the exploration of nanophotonic 2D materials by numerical simulation, their lab scale preparation and characterization. The optical properties of architectures composed of periodic arrays of nanoparticles, nanomeshes, holographic gratings and combinations of such elements are studied. Furthermore, thermo-responsive hydrogel elements are introduced to the architectures, serving as binding matrix for affinity reactions and as an actively tunable optical component. In the second part, new instruments are developed to take advantage of the optical properties of these surfaces. An epifluorescence reader is presented for the simultaneous observation of affinity binding events by fluorescence spectroscopy and monitoring of associated refractive index changes. The label-free surface plasmon resonance modality is in a novel instrument integrated with an electrolyte-gated field effect transistor and used for simultaneous and time resolved observation of surface mass deposition and reorganization of polymer assembly at the common sensor surface. The final section deals with the utilization of light management and surface plasmon-enhanced fluorescence to push forward the sensitivity of fluorescence heterogenous assays. A sensor for the continuous monitoring of low molecular weight chemical analytes based on a hairpin aptamer and fluorophore quenching at metallic surface is presented. Finally, there is developed a multi-resonant plasmonic substrate that provides fluorescence enhancement by a factor of 300 while maintaining compatibility with scaled-up industrial manufacturing processes. Together with a newly developed imaging reader, this fluorescence enhancement is achieved by utilizing plasmonically enhanced excitation and surface plasmon-mediated emission of fluorescence. The system is employed for the highly multiplexed observation of immunoassay binding kinetics on a large surface area with low femtomolar limit of detection (LOD). Concluding, nanophotonic 2D materials have been implemented in novel biosensor concepts that push forward the sensitivity and open door for combined sensor modalities providing additional insights into surface processes, beyond state-of-the-art methods.