Design of dielectric-based platforms for Raman and fluorescence-based chemical detection

The detection of molecules at low concentrations using methods such as surface enhanced Raman spectroscopy (SERS) and surface enhanced fluorescence (SEF) is crucial in many fields, such as medical diagnostics, forensics, security, and environmental monitoring. Traditionally platforms for optical sensing are fabricated using expensive methods, requiring specialized equipment, such as lithography, and rely on the use of noble metals, which are limited by lack of biocompatibility and inertness as well as their large environmental impact. Dielectric materials, in particular semiconductors, have been recently proposed and researched as alternatives for sensing substrate designs, due to their unique properties, such as low cost, ease of manufacturing, recyclability as well as the large versatility of available materials and designs that can be utilized. Although many promising studies have been performed on dielectric-based SERS, the state-of-the-art designs are often very limited, due to the lower signal enhancements generated. The aim of this thesis is to study the efficiency of environmentally friendly dielectric-based platforms for optical sensing applications, fabricated using simple and cost-effective methods. In the thesis, the two main groups of dielectric materials: inorganics and organics are studied by investigating the performance of a representative subgroup. Both groups are studied as dielectric-only platforms, and as components of a metal/dielectric hybrid. The efficiency of the platforms is assessed in terms of signal enhancements of Raman and fluorescence signals and lowest detectable signals obtained, the effectiveness of signal boosting strategies employed, as well as the suitability of the materials for other applications, such as photocatalysis. First, we study metal oxides, which are inorganic semiconductors, largely beneficial in the SERS field due to their charge generation properties. Metal oxide nanowires with a focus on ZnO, TiO2, and WO3 are shown to be efficient SERS platforms, following decoration of the nanowires with silver nanoparticles, as well as without the use of metal. We show that heat treatment in an oxygen atmosphere can be used as an effective method for point defect introduction in the lattice of the semiconductor, which results in significant boosting of the SERS signal intensities generated, due to heat-induced wettability changes and optical gap shrinking that allows for more effective charge transfer. Additionally, for certain wide bandgap semiconductors, these templates can be used as an effective substrate for driving catalytic redox reactions based on a model reaction of oxidation of PATP to PNTP. Subsequently, we utilize cellulose, an abundant, naturally occurring biopolymer, for the design of cost-efficient, recyclable, and biocompatible and for Raman and fluorescence-based detection of molecules. We study the interactions of porphyrins, a representative group of biomolecules with crystalline surfaces of Iß cellulose, and demonstrate the Stranski-Krastanov growth of clusters for complex molecules. We show that randomly distributed cellulose nanofibers as well as other cellulose derivative materials can be used as efficient metal-free SERS platforms for porphyrin molecule detection and as platforms for fluorescence-based molecule detection of a range of biomolecules, including a model immunoassay. Additionally, we demonstrate that cellulose acetate can be shaped into uniform photonic crystals with regular features using a replica moulding method and then, following coating with a thin layer of silver, used as a sensing template characterized by flexibility and high signal reproducibility.