Bare Nanoparticle Quenching and Future Work

Towards the end of this summer I was able to reproduce fluorescence quenching on bare nanoparticles. Upon photoexcitation by the laser (or sunlight), there is a locally enhanced electromagnetic field produced around the nanoparticle. As mentioned in previous posts, a fluorophore oriented closer to the surface of a plasmonic particle will experience a greater EM field. However, fluorophores located too close or touching the surface of a particle will undergo quenching of fluorescence because of the significantly stronger EM field. To save some time, I will not go into a lengthy discussion on the exact science behind quenching, mainly because I don’t understand most of it. In this context all we are concerned with is the fact that fluorophores located on the surface of bare nanoparticles are showing a decreased fluorescence enhancement. As opposed to positive ratios, the quenched particles show up as dark spots on a bright background, meaning these particles are emitting less photons than the background dye. A typical correlated fluorescence scan and LSPR image of bare NPs can be seen below. After analyzing 25 particles, the average enhancement was 0.84 ± 0.07. 25 particles isn’t necessarily a substantial amount, but the quenching measurements thus far prove the background theory.

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By utilizing the current spin-coating procedure, sometimes the dye is highly concentrated and clumpy in one section and light in other areas. This ultimately makes compiling fluorescence enhancement ratios difficult. With this method it is impossible to control the concentration of Rhodamine B on each nanoparticle and there’s no way of knowing the exact orientation of the dye when it coats the silica shell. Future work will involve covalent attachment of dyes to the silica shell in order to control both the concentration of dyes around each nanoparticle and the orientation of the dyes. This will entail amino-functionalizing the surface using (3-aminopropyl)-trimethoxysilane (APTMS) to create a link for the dye as opposed to just randomly spin-coating Rhodamine B onto the nanoparticles. In theory, this should result in more accurate and even fluorescence backgrounds, yet it is unclear how it will affect average enhancement ratios.

We also hope to better analyze single nanoparticles using the TEM machine. Although most of the particles in solution are single NPs encapsulated in silica, some aggregation of particles within a single silica shell occurs. There is reason to believe that, over time, significant particle aggregation has occurred and caused our fluorescence enhancements to be lower than normal. So once the TEM is working again, we would like to re-analyze our old colloid solutions to see what they look like now. Additionally, there are small variations in shell thickness and shape within each batch. Therefore, future studies will involve a more detailed structural characterization of individual particle size, the exact silica shell thickness, and the multiplicity of nanoparticles within each shell using TEM.

Working in the Wustholz lab this summer has been one of the best jobs that I’ve ever had. Not only did I learn an incredible amount about my project that would not have been possible during the semester, but I also formed great friendships with my labmates. Spending 40 hours a week in a room with no windows can really bring people together… I recommend that everyone interested in research to get involved immediately because it is the best way to get hands-on experience and it teaches you valuable lessons about working independently. Before joining this group I had no idea what physical chemists did, and now I’m an expert (in my mind at least).