Plasmonic (Bio)sensors Based on Nanostructured Metallic Surfaces

The thesis describes the fabrication, functionalizing and (bio)analytical applications of nanostructured surfaces based on plasmonic effects. In Chapter 1 a review of the physical requirements of plasmons and as well as design and fabrications concepts are provided with emphasis on nanostructured platforms. Sensor applications of plasmonic nanostructured surfaces are introduced. The aim of this work is outlined in Chapter 2 to investigate nanostructured gold surfaces in their sensing capabilities in SPR and utilizing the hot spots of such structures in other optical transducing techniques. The fabrication of nanohole arrays with different hole sizes via a modified nanosphere lithography is reported in Chapter 3. Nanostructured surfaces were modified with reduced graphene oxide (rGO) as a thin receptor layer via spin-coating. In a comparison to a continuous film, nanohole arrays displayed a 250 - 350% higher shift in the SPR angle. The capabilities of nanohole arrays modified with rGO were investigated with the detection of the plasticizer diethyl phthalate in water. With the analysis of a small analyte binding to graphene via π-stacking on nanohole arrays with varying hole diameter, the effect of the plasmonic field on (bio)sensors was studied. For the nanohole array with a diameter-to-periodicity ratio of 0.43 roughly 12-fold enhancement of the maximum signal response was observed and a 10-times better LOD of ~20 nM was determined. The feasibility of the rGO-modified nanohole array as sensor platform was demonstrated with the analysis of environmental water samples without pre-treatment. Surprisingly the combination of the graphene with gold nanostructures resulted in further signal enhancement compared to continuous gold films. This effect can be assigned to an additional plasmonic enhancement by the 2D carbon nanomaterial itself. Chapter 4 deals with a more detailed investigation of the graphene receptor layer. The modification and sensing properties of differently prepared graphene, which differ in flake size, number of layers and numbers of defects types was studied on nanostructured surfaces. A wet transfer method for chemical vapor deposition (CVD) graphene was introduced. The more homogenous and reproducible graphene films on nanohole arrays were yielded for CVD graphene when compared to rGO with Raman spectroscopy. The higher signal response for CVD graphene is attributed to less defects and a large uniform layer. A 7-fold lower LOD 0.9 μM for a CVD-modified nanohole array for the detection of adenine with SPR revealed the enhanced sensing performance. In Chapter 5 the enhanced luminescence of NIR to UV upconversion at a nanostructured surface is described. An nanoengineered interface consisting of NaYF₄:Yb,Tm upconversion nanoparticles and a gold nanotriangle array was fabricated by a self-assembly method. Upconversion nanomaterials are known for low quantum efficiency which clearly hinders a wider impact of these materials in many applications. Hot spots of the nanostructured surface provided a local electromagnetic field enhancement, that improved the 4-photon upconversion process at 345 nm with an about six-times higher emission intensity. For UCNPs dispersed in solution, this emission is usually too weak to be used in any analytical application. Assembled as a nanoengineered interface a sensor for the detection of vitamin B12 in serum was constructed. A LOD of 0. 6 ± 0.2 nM was found. The strong enhancement in the UV emission at a very low laser density of 13 W·cm⁻² is outstanding and a very promising fact for the development of miniaturized point-of-care diagnostics. Challenges of nanostructured surfaces are addressed in Chapter 6. Preliminary results of a designed transmission setup for nanostructured surfaces are presented. Nanohole arrays are capable of extraordinary optical transmission and can significantly improve (bio)sensors based on transmission. Future directions and perspectives of nanostructured surfaces are highlighted.