45426-B10
Topotactic Polymerization of Diacetylene-Containing SAMs as a Method for Surface-Derivatization of Nanoparticles
After two and a half years of construction and renovation, our new science facility is essentially completed and the PIs have moved into their new research laboratories. Despite the challenges of working through the construction and moving the labs, we have made substantial progress this year.
In our last report, we noted that nanoparticles coated with long chain thiols containing a diacetylene moiety along the chain underwent a dramatic change in the gold plasmon resonance upon irradiation with UV light. The absorption band broadened and moved to longer wavelength. With continued irradiation, the particles precipitated out of solution. Control nanoparticles coated with simple alkane thiols showed no change in the plasmon band under the same conditions. We proposed that the diacetylenes were undergoing a photopolymerization, similar to that seen in diacetylene single crystals. Further, we argued that the shift in the plasmon band was due to cross-linking of the chains on different nanoparticles, forming clusters that eventually precipitated. This interpretation was supported by thermogravimetric analysis, but we have been unable to collect Raman data on the samples due to very high absorption cross section that stretches into the infrared (the samples decompose or even burst into flame). Recently, we have been able to use transmission electron microscopy to examine our materials. The TEM data has shown that both the alkane-coated and diacetylene-coated gold nanoparticles have an average diameter of approximately 4 nm, somewhat smaller that we had anticipated. The data also show that the irradiated diacetylene-containing samples clearly form large aggregates upon irradiation. These structures are completely unlike anything observed with either the control particles or the diacetylene-containing particles prior to irradiation.
We now believe that unlike diacetylene thiol self-assembled monolayers on flat gold surfaces, the curvature of these nanoparticles prevents significant polymerization of the diacetylene group on an individual particle. In solution, there is some degree of aggregation of the nanoparticles, with the alkyl chains on adjacent particles interpenetrating to some extent. In the diacetylene-containing systems, this leads to a geometry and monomer density suitable for photopolymerization, leading to aggregation. In a related experiment, we added a large excess of a long-chain, carboxylic acid-terminated diacetylenes to a solution of the diacetylene-containing nanoparticles. These were then exposed to UV radiation. We observed a shift in the plasmon band, though smaller than that observed when the excess diacetylene was not present. The IR of the resulting products indicated the presence of the carboxylic acid group in the nanoparticles that could not be removed by thoroughly washing the particles. These particles also showed resistance to aggregation. Thus, we have demonstrated a method for covalently binding gold nanoparticles into covalently linked aggregates as well as a method for modifying the surface of the nanoparticles using a photo-induced coupling. We are currently writing up our initial report on this work for publication. We are also finishing up experiments on the surface-modification work by attempting to demonstrate the technique with a fluorophore. This work will be reported in a second publication.
Current work and our plans for year three include expanding the diacetylene-coupling technique to atomically flat gold platelets. We are attempting, with some encouraging initial results, to photopattern these surfaces. We have also developed a strategy to prepare Janus gold nanoparticles. The term "Janus particle" refers to a particle with zones of differing substitution on the surface. In our case, this will mean areas of diacetylene-containing thiols and areas of non-polymerizable thiols. The goal in this work is to essentially create giant atoms that could permit the construction aggregates with controlled structure.
