Two-photon-induced Förster resonance energy transfer (FRET) from fluorescent CdSe/ZnS quantum dots (QDs) to the photosensitive protein bacteriorhodopsin (bR) in purple membranes (PMs) from bacteria Halobacterium salinarum was studied. The experimental dependences of the QD fluorescence intensity and average fluorescence lifetime (AFL) on the bR-to-QD molar ratio upon one-photon and two-photon excitations were obtained. An increase in the protein concentration in QD–bR complexes in aqueous solution provoked pronounced decrease in the fluorescence intensity and AFL for both one-photon and two-photon excitations. For detecting the possible FRET from QDs to bR in the hybrid material, the fluorescence intensities and AFL were measured and compared with those for complexes of QDs with so-called white membranes (WMs), PMs from which their only photosensitive chromophore retinal (the FRET acceptor) was carefully extracted. The data show that, at the same QD-to-bR molar ratio, WM quenched the QD fluorescence much weaker (up to 4-fold) than PMs did. This fact clearly confirms that FRET occurred in QD–PM complexes upon two-photon and one-photon QD excitations. The two-photon absorption cross section (TPACS) of QDs at 790 nm was found to be about 20,000 GM, two orders of magnitude larger than that of bR. This means that two-photon selective excitation of QDs occurred in the QD–PM hybrid material and FRET from the QDs (excited in the two-photon mode) to bR was observed.
In conclusion, our study has demonstrated the possibility of initiating bR photoconversion through excitation of QDs in the infrared spectral region in a two-photon mode, followed by FRET from the QDs to bR. Our finding may significantly improve the photovoltaic and photochromic properties of bR in the QD–bR hybrid system, thus opening new prospects for industrial applications of this promising material.
This work within theGovernment Decree №220 ofApril, 2010.