Plasmonic enhancement of fluorescence for sensor applications

In this work we report on the so-called plasmonic enhancement effect, whereby the presence of metallic surfaces or particles in the vicinity of a fluorophore can dramatically alter the fluorescence emission and absorption properties of a fluorophore. The effect, which is associated with the surface plasmon resonance of the metallic surface, depends on parameters such as metal type, particle size, fluorophore type, and fluorophore-particle separation. This work focuses on the creation of metal nanoparticle arrays by a lithographic process and on optimisation strategies to maximise the fluorescence enhancement of dyes in the vicinity of the nanoparticles, for important applications such as fluorescence-based biochip platforms. Ordered arrays of metallic nano-islands were fabricated on glass substrates by a process of natural lithography using monodisperse polystyrene nanospheres. The metal particle dimensions were tailored in order to tune the plasmon resonance wavelength to match the spectral absorption of the fluorophore. The fluorophore, Cy5 dye, which is widely used in optical immunoassays and has a medium quantum efficiency (similar to 0.3), was used in this preliminary study of the plasmonic enhancement effect. The morphology of the metallic arrays was investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Absorption and emission spectroscopies were used to elucidate the enhancement effect and its dependence on metal island morphology. Results were correlated with existing theoretical models. The applicability of this important technique to sensor platforms, such as fluorescence-based biochips, is also discussed. (c) 2004 Elsevier B.V. All rights reserved.