Reports claiming that certain biomaterials have the potential to act as artificial solar energy converters are flawed, suggest Canadian scientists.    

In nature, photosynthetic systems, such as plants, capture the energy from sunlight using molecular units called chromophores, which are attached to proteins.  Researchers have tried to mimic this process using thin layers of chromophore-functionalised peptides on gold electrode surfaces.  When lasers shine on these electrodes, a current is generated.    

Heinz-Bernhard Kraatz, of the University of Saskatchewan, Saskatoon, says that although reports claim that the current is caused by the chromophores capturing the laser light and converting it to electricity,^1-3 this might not be the case.

Kraatz and colleagues performed similar experiments using peptides without chromophores.  They found that current was still generated, indicating that the chromophores are not responsible its production.⁴ Current was even generated when they used a bare gold electrode.  

Kraatz explained: 'A significant consequence of the laser irradiation has been overlooked.  The so-called photocurrent phenomenon is probably largely a result of the heating of the electrode.'  
 
Shunsaku Kimura, a biomaterials chemist from Kyoto University, Japan, has published a number of reports on photocurrent generation.^1-3 'The findings are not so special,' said Kimura.  'They used an intensive laser that could cause a substantial temperature change.  We did not use a laser but a xenon lamp.  We do not think that this low intensity light caused a significant temperature change.'
 
Kraatz hopes to separate and quantify the chromophore-based photocurrent, if any, from the temperature-induced current.  This should allow him to learn more about the electron transfer in biomolecular films.  However, he does not expect the work to be straightforward.

'The increased temperature of the electrode may also change the structure of the peptides, which may change their electron transfer mechanism.  It will be a challenge to isolate the responses due to the individual contributions,' said Kraatz.   

Joanne Thomson