Photosynthetic reaction centers embedded in polyelectrolyte multilayer as a tool in the determination of PSII herbicides

The photosynthetic bacterial reaction center (RC) has been adsorbed onto a glass surface by alternating deposition with the cationic polimer poly(dimethyldiallylammonium chloride) (PDDA) obtaining as an end result an ordinate polyelectrolyte multilayer (PEM) where the protein retains its integrity and photoactivity over a period of several months. This polymer/RC assembly revealed to be extremely sensitive to the presence of herbicides belonging to the so called "PSII family". The RC-herbicides interaction can be easily monitored since it alters the protein photoactivity, such variation resulting in an absorbance decrement at 870 lira. To this purpose, an instrumental setup has been developed based on a monochromatic light source and on a solid state detector connected to a DAQ acquisition board for the measurement storage onto a PC. The system PEM/instrument has been tested on atrazine based herbicides. The bound herbicide can be easily removed from PEM, thus allowing the reuse of the same sample. Moreover, the system has been tested for sensitivity, reproducibility and reliability of the measured data. The results of these tests were so promising to encourage us to propose the PEM as an optical biosensor for herbicides.