Due to resonant electrons in metals surfaces, nanoparticles of noble metals are able to absorb a large proportion of incoming light. For gold and silver nanoparticles the resonant frequency, and therefore the frequency where the absorption takes place, is located squarely in the visible spectrum making them the target or various research projects. The other main effect of metal nanoparticles is the increased electric field strength near the particles surface. For photoluminescent dyes, these electric fields have been shown to either severely mitigate or greatly enhance the dyes effects.
As a general rules the quenching occurs when the nanoparticles are at a separation of 5nm from the dye particles and the enhancement occurs at about 10nm. Beyond this the interaction quickly falls off an no effect is observed. One of the suggestions is that the enhancement is caused by a combination of the greater dye excitation due to the stronger electric field but also the fact that the nanoparticles may be able to transfer part of their own excitation energy to the dye in a very efficient way. The whole effect is highly dependent on distance based on both the mitigating to magnifying boundary already described but also because the the electric field around the nanoparticles degrades rapidly when moving away from the particle’s surface.
To control the separation between particles of metal and dye, to optimise enhancement, a spacer material is often used such as silica; although more advanced versions based on DNA and technical polymers have also been investigated. The materials have to be very specific as if they fail, even slightly, to hold the components apart the metal will switch to quenching the dye instead. Silica seems to be still the best as it offers rigidity, chemical resistance and an ease of surface adaptability. Despite the quite clearly drawn lines for the regular enhancement to quenching boundary, in silica there have been inconsistent results which recent research aims to explain. Through a series of experiments on gold nanoparticles in a suitable silica medium it was shown that the likely cause of recorded discrepancies was the inner filter effect. This is where absorbed and reemitted light is reabsorbed once again and so the recorded fluorescence at certain distances was always undercounted as these effects were not accounted for in previous studies.