Sunday, April 8, 2007

Photovoltaic devices from viruses

Viral capsids are attractive scaffolds for the preparation of nanomaterials; they are highly robust in nature, monodisperse, easy to assemble, and small in size. In particular, the hollow tube-like capsid of the tobacco mosaic virus (TMV) provides an intriguing template for the development of organic nanowires. When fully assembled, each TMV particle is 300nm in length and is made up of over 2000 identical protein subunits that can be assembled into other aggregate structures; depending on pH and ionic strength conditions during assembly.

Thus, the idea to create light-harvesting systems out of assembled TMV capsids is not a surprising one. In fact, in the literature numerous methods have been developed to modify both the interior and exterior of the viral capsid with inorganic substrates,[1,2,3] but little success has been as been achieved with organic ones. So you can imagine that I was especially excited when I saw the title "Self-Assembling Light-Harvesting Systems from Synthetically Modified Tobacco Mosaic Virus Coat Proteins" in JACS quite r
ecently. Ever since Prof. Matthew Francis gave a seminar here, I've had his group website bookmarked and have been watching for new developments.

In nature, sunlight is converted into chemical bonds with a high efficiency, mostly due to the fact that photosynthetic systems incorporate several types of chromophores (covering a large spectral bandwidth) spaced precisely to optimize energy transfer. In an attempt to mimic the ingenuity of nature, Miller and coworkers utilized a mutant TMV monomer bearing a reactive cysteine residue. At pH 7 in a phosphate buffer, this reactive cysteine was coupled with maleimide functionalized Oregon Green (primary donor), tetrame
thylrhodamine (secondary acceptor), or Alexa Fluor 594 (acceptor). In order for this to work, FRET must occur between the selected donors and acceptor, and these dyes were chosen for their high degree of overlap in the solar spectrum as well as their high extinction coefficients and stability. By mixing various ratios of donor and acceptor monomers together and then adjusting ionic strength and pH, both disk and long rod aggregate structures were formed; the attached chromophores apparently had no effect on the systems ability to self assemble. A ratio of 33:1 (donor Oregon green) to acceptor (Alexa Fluor 594) produced an overall efficiency of 47%, while the 3-chromophore system containing 8:4:1 Oregon green: tetramethylrhodamine: Alexa Fluor 594 resulted in a stunning 90% efficiency!

Though it is a simple concept, the combination of self-assembling biological scaffolds and synthetic organic chromophores seems to have great potential for the development of new solar cells.


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