Electrostatic interactions and covalent binding effects on the energy transfer between quantum dots and reaction centers of purple bacteria

The effect of electrostatic interaction and covalent binding on the energy transfer from negatively charged CdSe/CdS/ZnS-COО− and positively charged CdSe/CdS/ZnS-N H3+ quantum dots to reaction centers of Rb. sphaeroides bacteria (RC) in aqueous solutions is reported. The study was performed using optical absorption and steady-state and time resolved luminescence spectroscopies. The experiments were accompanied by theoretic modelling of charge distribution on the RC surface. More effective energy transfer from positively charged QD to RC, as compared with negatively charged QD, was observed. This effect was associated with the different localization of positively and negatively charged QD on the RC surface. The theoretic analysis has demonstrated that the periplasmic RC side is characterized by larger negative charge density as compared with the cytoplasmic side. Therefore, positively charged QD are localized mainly on periplasmic RC side while negatively charged QD are localized on the cytoplasmic RC side. In RC energy acceptors (porphyrins) are localized closer to the periplasmic side. Therefore, the energy transfer efficiency from positively charged QD is higher. The increase in ionic strength equalizes the charge distribution on both RC sides, thus levelling affinity of differently charged QD with RC. Equal energy transfer efficiency for both covalently bound and unbound QD is due to the fact of QD localization on the same RC side. This study demonstrates that for binding of charged QD with RC the electrostatic interaction is principal.