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This project has focused on the use of diacetylene-containing self-assembled monolayers (SAMs) to modify the surfaces of gold nanoparticles or on atomically flat gold as well as the assembly of gold nanoparticles into larger clusters. The diacetylenes undergo a photo-initiated polymerization reaction to form highly conjugated chains parallel to the gold surface. We have discovered that this reaction can be used to covalently attach diacetylenes from solutions, leading to a novel surface-derivitization methodology. Two manuscripts on this work are in preparation with plan to submit them late this year or early next year. The results obtained under the PRF award were critical for our MRI proposal to the NSF for a quartz crystal microbalance (QCM) and a dynamic light scattering (DLS) apparatus. These QCM will be key to the next phase of our investigation, where we quantify the amount of non-SAM diacetylene that is incorporated into the growing chain and the extent of polymerization as evidenced by the change in the viscoelastic behavior of the diacetylene SAM on photolysis. The DLS will allow us to further probe the cluster formation, particularly as we begin to control the directionality of the interparticle coupling with Janus particles (below).

As mentioned in last year’s report, the coupling of nanoparticles into larger structures has developed into a major new initiative within this project. Our goal is to design particles with anisotropic surface characteristics (Janus particles) to permit directional coupling of nanoparticles into well-defined clusters of varying shape and size. This year we branched out from our initial focus on gold nanoparticles to carbon-based materials. Using previously published methods, we have prepared nanocrystals of 1,6-bis(tosyl)-2,4-butadiyne. This material is known to undergo polymerization to nanocrystalline polydiacetylene either by heat or UV irradiation. We have been able to demonstrate that it will react with certain radical initiators to also form the polymeric crystal. This reaction is a radical addition process, meaning that we can selectively derivatize only certain faces of the particles. In addition, the use of radical initiators with functional groups able to undergo additional reactions means that we can add other molecules selectively to those faces.

In a complementary reaction, we have shown that we can effect a nucleophilic substitution of the tosylates from the polymerized nanoparticles. This derivatizes a different face of the crystal than the radical reaction, allowing for the same strategy of forming coupling centers, but with a different directionality. Specifically, the plate-like crystals are likely to be derivatized around the edges in the radical process and on the crystal faces for the substitution reaction. At this stage, we have demonstrated that the derivatization reactions occur, but we have not yet quantified the efficiency of the reaction, demonstrated the facial selectivity, or shown that controlled coupling of the particles is possible. With the aid of our new instruments, we plan to continue with these studies next year.

Another related initiative is the selective derivatization of buckyballs. It has been shown in the literature that five groups can be regioselectively added to each of the polar regions of the buckyball. We have a synthetic route to differentially derivatize these poles to that one side may be stereoselectively coupled to a complimentary buckyball, or used in a controlled synthesis of one-dimensional chains. Initial results are promising and this adds a unique new direction for the project that we will explore over the next couple of years.