Metal nanostructure-enhanced fluorescence and its biological applications

Over the last two decades, fluorescence-based detection has become one of the leading sensing technologies in biomedical, biological, and related sciences. Its sensitivity makes it possible to detect a single biomolecule through labelling with a suitable fluorophore. Two principal fluorophore properties, brightness and photostability, are fundamentally important to achieve a high level of sensitivity, and in many conventional fluorophores these often fall short of the requirements.. Among the methods used to improve the sensitivity of fluorescence detection, the metal-enhanced fluorescence (MEF) technique has been recently actively developed. The MEF phenomenon occurs when an excited fluorophore is located in close proximity to metals, and it is particularly pronounced near noble metal nanostructures.. Electrons in such metal nanostructures exhibit strong resonances often located in the visible part of the spectrum (also known as surface plasmon resonance). They can interact with proximal fluorophores, modifying their optical properties and producing increased quantum yield (fluorescence efficiency) and improved photostability. It has been experimentally demonstrated that the MEF technique can increase fluorescence intensity up to several hundreds times. This chapter provides an overview of MEF, covering its basic theory, synthesis of metal nanostructures, and biological applications based on MEF. We focus on silver nanostructures because their strong surface plasmon resonance in the visible matches the absorption and emission bands of most fluorophores. We discuss traditional planar MEF substrates as well as silver nanostructures deposited on micrometre-sized silica beads, generating a fluorescence enhancement of more than one order of magnitude. This achievement allowed us to demonstrate for the first time MEF immunoassays on silica beads by using high-throughput flow cytometry. Furthermore, we discovered that these silver nanostructure-coated silica beads are able to modify the luminescence decay lifetime of lanthanide fluorophores for time-gated luminescence bioimaging applications. These developments open up a broad range of opportunities for ultrasensitive and low-background fluorescence detection using lanthanide fluorophores.