Correlating Single Dye Molecule Fluorescence with Nanoparticle LSPR

It’s always tough when research doesn’t go the way you initially planned. Spending weeks on a certain project only to discover that the data doesn’t fully prove the hypothesis is beyond frustrating. Alas, that’s science.

My previous posts have focused on analyzing the fluorescence enhancement of Rhodamine B due to silica coated silver nanoparticles. But as I explained before, the enhancement ratios were not statistically different for various shell thicknesses, nor were they as high as they should have been. Up until now we were growing different thicknesses of silica shells on the silver nanoparticles to measure the distance dependence of plasmonic enhancement. This technique, which involves adding certain amounts of tetraethoxysilane (TEOS) that correspond to certain shell thicknesses, is commonplace according to numerous plasmonics articles. We usually spin-coat [RB] = 10-4 on top of the SCNPs and hope that there is an even dye coating. However, sometimes the dye is clumpy and more concentrated in certain areas, which is not ideal when trying to determine overall enhancements. Another problem with trying to grow the thinner silica shells is that they can have holes in them, and some nanoparticles are even left bare. This leads to fluorescence quenching, meaning a negative enhancement factor.

Last week we decided to take a step back and try a new approach to the project that hasn’t really been tried before. Instead of the higher concentration of dye, we chose to spin-coat[RB] = 10-9on a slide containing bare nanoparticles with no silica shell attached. In theory, this would result in a dispersion of dye molecules located at various distances from the nanoparticles. Thus, we will still be measuring the distance dependence of plasmonic enhancement, but the distances will no longer be fixed by a pre-determined shell thickness and the dye molecules won’t necessarily be on top of the particles. This could prove to be beneficial because we will have a wider range of dye-particle separations that could perhaps shed light on exactly what distance is associated with maximum enhancement, not to mention discovering where enhancement begins to significantly taper off at larger distances. This is a more “basic science” approach to the project because it has no real application besides proving the theory, yet it’s exciting because no one has tried this method before. The only complication with this procedure is correlating the single dye molecule fluorescence scans with the localized surface plasmon resonance (LSPR) images of the nanoparticles.