During the past several years we have studied the interactions of fluorophores with metallic particles or surfaces. We will refer to conducting metallic structures as metals. We have observed a number of important spectral changes, including increases in intensity and photostability, decreased lifetimes due to increased rates of radiative decay, and increased distances for FRET. We have also shown that fluorophores can create surface plasmons in metals which in turn create light. We believe the potential of these phenomena is far greater than we imagined several years ago. Our results suggest that by using fluorophore-metal interactions it will be possible to control the migration of electromagnetic energy across and through metal surfaces, and to control when and where the energy is converted back into light. We refer to these phenomenon as plasmon-controlled fluorescence (PCF). We believe that fluorescence is now poised for a paradigm shift which will change how we think about fluorescence and will expand its capabilities in research, medical diagnostics and imaging.
Photophysics Near Metallic Particles and Surfaces
The spectral properties of fluorophores can be well described by Maxwell's equations for an oscillating dipole radiating energy into free space. From this perspective a fluorophore is like an antenna which oscillates at high frequency and radiates at UV to NIR wavelengths. In standard fluorescence experiments the effects of surfaces or particles are not usually seen because of the small size of fluorophores relative to the sample holder. However, in the near-field around the fluorophore the nearby conducting metal surfaces can respond to the oscillating dipole, and modify the absorption, modify the rate of emission, and modify the spatial distribution of the radiated energy.
Nanofabrication of Plasmonic Structures
Quasi-two-dimensional, collective electron oscillations known as localized surface plasmons (LSP) can be excited in metal nanoparticles (typically gold or silver) along a dielectric surface resulting in strong amplification of the local electromagnetic field and appearance of surface plasmon absorption bands. These enhanced fields are evanescent i.e. they are confined to a distance of within 300 nm from the particles and decay significantly beyond it. State-of-the-art lithographic techniques provide tools for tailoring the interaction of nanosize structures with light; they provide precise control of size and spacing for the fabrication of a wide variety of complex shapes. These nanoparticles can be used to control, transmit, scatter, amplify, radiate and modify electromagnetic field of the incident light for applications in subwavelength optics, data storage, biophotonics, detection and sensing.
Single Molecule Spectroscopy Near Plasmonic Structures
The use of fluorophore-metal interactions has the potential to dramatically increase the detectability ofsingle fluorophores for both SMD and FCS experiments. Our ensemble measurements have shown increased quantum yields, decreased lifetimes and increased photostabilities. Decreased lifetimes will result in higher emission rates prior to saturation. This is possible because the fluorophores can cycle faster between the ground and excited states. Decreased lifetimes should result in higher photostability because there is less time for chemical reactions to occur in the excited state. Decreased lifetimes should also result in decrease blinking because there will be less time for the fluorophores to go to the triplet state. These effects will provide longer observation times prior to photobleaching. We will use these effects to create the next generation of probes, which could be fluorophores in nanoshells, fluorophores bound to colloids, or fluorophores trapped in small volumes. These effects may also be used for increased detectability of single molecules bound to surfaces which contain metallic structures, for either biophysical studies or high sensitivity assays.
Surface Plasmon-coupled Emission
Surface plasmon-coupled emission (SPCE) is a phenomenon which occurs with excited fluorophores near continuous metallic surfaces covered with thin 50 nm metal films. These films are visually almost completely opaque. Excited fluorophores within about 100 nm of the surface result in strongly directional emission through the metal film and into the substrate. A large fraction of the total light energy is coupled into the substrate. This remarkable phenomenon is the result of near-field interactions of the excited fluorophores with thin metal film, and is not a reflective or transmissive phenomenon.
Calculations on Metallic Nanostructures
An understanding of MEF, SPCE and PCF will ultimately be based on an understanding of the interactions of incident light with plasmonic structures and the interactions of excited state fluorophores with electron oscillations in these structures. A complete emphasis on such calculations would be diversion from our efforts on nanofabrication and experimentation. Most of these experiments measured light extinction or transmission, not fluorescence. Many of the recently discovered properties of plasmonic structures are based on experimentation rather than theory. Our goal is to use commercially available software and selective calculations to allow comparison of our experimental results with electrodynamics theory for a more complete understanding of the underlying phenomena.