A mediatorless photoelectrochemical cell based on LIFT-immobilized Reaction Centers for the amperometric detection of herbicides

Herbicide detection by means of photosynthetic proteins is the most straightforward strategy since the inhibition of the photosynthetic process is the natural target of this class of chemicals. The bacterial pigment-protein complex called Reaction Center (RC) promotes the transfer of electrons from the cytochrome c2 to the ubiquinone (UQ10) capturing the free energy of visible photons, thus allowing the first conversion of light energy into chemical energy (Fig 1). A number of photo-electrochemical cells employing the RC as photoactive component have been recently described 1. Here we report on the immobilization of RC onto gold electrodes by Laser Induced Forward Transfer (LIFT) technique. This laser printing method enhances the physical adsorption of biomolecules due to the high impact pressure of the transferred droplets at the receiver substrate, improving the electrochemical communication with the electrode 2. At the potential of -100 mV vs quasi-reference Ag/AgCl the efficient reduction of the photo-oxidized dimer occurrs by direct electron transfer between gold electrode and surface-confined RC, in absence of any mediator at the donor side. The photocurrents increase upon addition of the substrate UQ0, a water-soluble analogue of ubiquinone, in agreement with its better ability to withdraw electrons from the QB pocket. Nevertheless, in agreement with the competitive inhibition mechanism, the attenuation of the photocurrents by the herbicide terbutryn is higher for the UQ0-free system, resulting in a lower limit of detection. These findings demonstrate that the LIFT technology allows the direct electronic wiring between RC and metal surface, enhancing the biosensor sensitivity towards herbicides by means of a mediatorless approach.