Spatially and spectrally resolved plasmonic fluorescence enhancement

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Silver and gold particles with dimensions less than 100 nanometres (1/10000th of a millimetre), which is about 1/5th of the wave-length of green light, exhibit very different optical properties from larger pieces of these metals. In particular, these nanoparticles can absorb and scatter light very efficiently at specific wavelengths in the visible part of the electromagnetic spectrum, which depend on the size, shape and material of the nanoparticle. This occurs as a result of the resonant excitation of an oscillation of the electrons in the metal, called a localised surface plasmon, which results in very intense electromagnetic field around the nanoparticle. The ability in recent years to fabricate metal nanoparticles of well-defined shape and size has spawned the burgeoning area of plasmonics, which exploits the novel optical properties of these materials.In modern biology and biomedicine it has now become standard practice to use fluorescent tags (or 'fluorophores') which are used as labels for particular biological molecules, or to flag the occurrence of specific processes. These tags are molecules or nanoparticles which emit light of a well-defined colour when illuminated and are employed extensively in imaging, from the level of whole organisms down to individual cells and molecules, in DNA sequencing, in drug discovery and in biosensors. This project is concerned with a study of the ways in which the emission from fluorophores is modified significantly, when a fluorophore is in close proximity (less than 100 nm) to a silver or gold nanoparticle. The main effects observed are an enhancement of the fluorescence intensity and an enhancement in the quantum efficiency of a fluorophore (i.e. the ratio of the number of photons it emits, to the number it absorbs). Such effects have potential for high sensitivity fluorescence detection, enabling detection to much lower molecular concentrations. However, to inform exploitation for new technological applications, we need to first understand the basic physics of what happening! In particular, we will perform measurements over a wide range of excitation and emission wavelengths, to tie in changes in behaviour with the localised surface plasmon resonance spectrum of the metal nanoparticles. We will also investigate in detail the dependence of fluorescence emission on the nanometric separation of the metal nanoparticle and the fluorophore. To do this, we will combine advance optical microscopy techniques with scanning probe microscopy (in which a sharp needle maps out a surface) to provide full positioning information.