Over the past 15 years, fluorescence has become the dominant detection/sensing technology in medical diagnostics and biotechnology. Although fluorescence is a highly sensitive technique, where single molecules can readily be detected, there is still a drive for reduced detection limits. The detection of a fluorophore is usually limited by its quantum yield, autofluorescence of the samples and/or the photostability of the fluorophores; however, there has been a recent explosion in the use of metallic nanostructures to favorably modify the spectral properties of fluorophores and to alleviate some of these fluorophore photophysical constraints. The use of fluorophore-metal interactions has been termed radiative decay engineering, metal-enhanced fluorescence or surface-enhanced fluorescence.

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The use of fluorescent nanoparticles as indicators in biological applications such as imaging and sensing has dramatically increased since the 1990s. 1 These applications require that the fluorescent nanoparticles are monodisperse, bright, photostable, and amenable to further surface modification for the conjugation of biomolecules and/or fluorophores. Among the many types of fluorescent nanoparticles available today, nanoparticles with core-shell architecture fulfill all these requirements, combining diverse functionalities into a single hybrid nanocomposite. 2 In this work, we have developed core-shell (silver core-silica shell) nanoparticles with various shell thicknesses featuring a variety of fluorophores, to show the versatility of the core-shell architecture, and have demonstrated their applicability for two platform technologies, metal-enhanced fluorescence (MEF) and single nanoparticle sensing. We demonstrate the broad potential applications of our technology by employing near-infrared emitting probes (Rh800) for potential applications in cellular imaging and the use of highly photostable long lifetime (μS) lanthanide probes, probes suitable for off-gating biological autofluorescence. The use of Alexa 647 serves to demonstrate that fluorophores can be readily covalently linked to the core-shell particles also, for metal-enhanced benefits.MEF is an established technology, 3a-d where the interactions of fluorophores with metallic nanoparticles results in fluorescence enhancement, increased photostability, decreased lifetime owing to increased rates of system radiative decay, reduced blinking in single molecule fluorescence spectroscopy, 3b and increased transfer distances for fluorescence resonance energy transfer. 3c Single-molecule fluorescence spectroscopy is the prime tool in single nanoparticle sensing, and it provides several advantages over ensemble measurements, such as, the elimination of averaging of the spectral properties over all members of the ensemble, which can reveal fundamental features otherwise masked in

The effects of thermally annealed silver island films have been studied with regard to their potential applicability in applications of metal-enhanced fluorescence, an emerging tool in nano-biotechnology. Silver island films were thermally annealed between 75 and 250 • C for several hours. As a function of both time and annealing temperature, the surface plasmon band at ≈420 nm both diminished and was blue shifted. These changes in plasmon resonance have been characterized using both absorption measurements, as well as topographically using Atomic Force Microscopy. Subsequently, the net changes in plasmon absorption are interpreted as the silver island films becoming spherical and growing in height, as well as an increased spacing between the particles. Interestingly, when the annealed surfaces are coated with a fluorescein-labeled protein, significant enhancements in fluorescence are osbserved, scaling with annealing temperature and time. These observations strongly support our recent hypothesis that the extent of metal-enhanced fluorescence is due to the ability of surface plasmons to radiate coupled fluorophore fluorescence. Given that the extinction spectrum of the silvered films is comprised of both an absorption and scattering component, and that these components are proportional to the diameter cubed and to the sixth power, respectively, then larger structures are expected to have a greater scattering contribution to their extinction spectrum and, therefore, more efficiently radiate coupled fluorophore emission. Subsequently, we have been able to correlate our increases in fluorescence emission with an increased particle size, providing strong experiment evidence for our recently reported metal-enhanced fluorescence, facilitated by radiating plasmons hypothesis.KEY WORDS: Metal-enhanced fluorescence; radiative decay rate; increased excitation rate; radiative decay engineering; surface-enhanced fluorescence; silver island films; fluorescence spectroscopy.

In the last century the production and application of halides assumed an ever greater importance. In the fields of medicine, dentistry, plastics, pesticides, food, photography etc many new halogen containing compounds have come into everyday use. In an analogous manner many techniques for the detection and determination of halogen compounds and ions have been developed with scientific journals reporting ever more sensitive methods. The 19th century saw the discovery of what is now thought of as a classical method for halide determination, namely the quenching of fluorescence. However, little analytically was done until over 100 years after its discovery, when the first halide sensors based on the quenching of fluorescence started to emerge. Due to the typical complexity of fluorescence quenching kinetics of optical halide sensors and their lack of selectivity, they have found little if any place commercially, despite their sensitivity, where other techniques such as ion-selective electrodes, x-ray fluorescence spectroscopy and colorimetry have dominated the analytical market. In this review article the author summarizes the relevant theory and work to date for halide sensing using fluorescence quenching methods and outlines the future potential that fluorescence quenching based optical sensors have to offer in halide determination.

We report recent achievements in metal-enhanced fluorescence from our laboratory. Several fluorophore systems have been studied on metal particle-coated surfaces and in colloid suspensions. In particular, we describe a distance dependent enhancement on silver island films (SIFs), release of self-quenching of fluorescence near silver particles, and the applications of fluorescence enhancement near metalized surfaces to bioassays. We discuss a number of methods for various shaped silver particle deposition on surfaces.

We examined the effects of metallic silver colloids on the fluorescence spectral properties of indocyanine green (ICG), which is a dye widely used for in vivo medical testing. Silver colloids from a suspension bind spontaneously to amine-coated surfaces. These colloid-coated surfaces were found to cause a 30-fold increase in the intensity of ICG, which was held close to the metal surface by adsorbed albumin. The increased intensities of ICG were also associated with decreased lifetimes and increased photostability, which are indicative of modifying the fluorophores radiative decay rate. These results suggest the use of metal colloid-enhanced ICG for applications to retinal angiography and vascular imaging and as a contrast agent for optical tomography.

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