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45426-B10
Topotactic Polymerization of Diacetylene-Containing SAMs as a Method for Surface-Derivatization of Nanoparticles
Progress in the first year of this award has been slowed by the construction of a new wing on the laboratory building as well as renovation of the old facility. However, some significant findings have been obtained.
The focus of this work is to construct multifunctional nanoscale objects. A key concept is the self-assembly of long-chain alkanes containing a conjugated butadiyene (diacetylene, DA) moiety in the chain. If these can be organized properly, they will undergo a topotactic polymerization initiated by UV irradiation. We are currently collecting evidence that this polymerization can take place on the surface of nanoparticles in the 7-9 nm size regime. When irradiated for a few minutes with a high intensity UV light in a good solvent, we observe a shift of the gold plasmon band to shorter wavelength. In marginal solvents, this shift is accompanied by dramatic broadening of the absorbance and a decrease in intensity. Long irradiation times result in the precipitation of a gold-containing solid. Control experiments on nanoparticles of similar size, but coated with simple long alkane chain thiols show no change in the intensity or frequency of the plasmon band. We interpret our results as evidence for the polymerization reaction. In the case of highly solvated nanoparticles, we believe the DAs on a single particle undergo polymerization. In weaker solvents, we believe the particles are aggregating, with the DA-containing chains interpenetrating. Upon photolysis, the particles cross-polymerize to form insoluble materials.
The above argument would be strengthened if we were able to obtain spectroscopic evidence that the polymerization had indeed occurred. The best way to do this would be using Raman spectroscopy, since the DA polymers (pDAs) display an intense absorption between 2100 and 2200 cm-1 due to the polymer backbone. This is in a region devoid of absorbances by competing vibrational modes. However, the nanoparticles are intensely colored and prone to fluorescence. Despite examination of them with spectrometers using excitation wavelengths from 532 to 1064 nm, we have not been able to obtain spectral data. We are currently exploring the use of iodine to remove the gold from the particle in order to collect the Raman data.
Another aspect of this project is to use the polymerization reaction to alter the surface of the nanoparticles. In these experiments, we treat THF solutions of the nanoparticle with a long-chain alkane containing a DA and terminated by a functionality of interest – usually a carboxylic acid or an ionic fluorophore. Water is added to the solution, followed by sonication. We believe that this drives the hydrophobic alkane chain to interpenetrate, leaving a hydrophilic surface. Evidence that this is the case comes from the fact that the nanoparticles become fluorescent when treated with the fluorophore-DA and modestly soluble in dilute aqueous base when treated with carboxylic acid-DA.
Finally, we have also applied this technology to magnetic (iron oxide) nanoparticles. While these are far less soluble than their gold counterparts. Our initial results suggest that they too are amenable to surface modification by this method.
