Nanocrystals-bacteriorhodopsin hybrid materials as electric field controllable light emitters with photo-voltaic properties. – NANO-BAC

The NANO-BAC project will develop a nano-bio hybrid material and investigate, on the nano-scale, the interactions of fluorescent semiconductor nanocrystals quantum dots (QDs) and a biological photochromic material, wild-type and mutated membrane protein bacteriorhodopsin (bR), incorporated within its natural purple membrane (PM) of bacteria Halobacterium salinarum. Upon illumination by light, bR acts as light-driven proton pump transporting protons across the PM. The resulting proton gradient initiates a cyclic sequence of photo-intermediate states (photocycle) providing the change of bR absorptions in the blue-to-red region of the spectrum. The high quantum efficiency of the initial bR state guarantees photoisomerization of the retinal, a protein-linked bR chromophore, located near to the centre of the PM, 25 Å from each of the PM surfaces. Thus, the fluorescence emission from the QDs immobilized on the PM surface may be modulated due to the fluorescence resonance energy transfer (FRET) effect between the emitting QDs of selected emission colours (donors) and strongly absorbing bR (acceptor), depending on the actual stage of its photocycle. Additionally, the intra- and extracellular surfaces of PM are charged oppositely which makes a fabrication of oriented PM films a routine task. This allows for the design of a unique nanoscale optically switchable light emitter operating in a FRET regime. An electric field applied to the oriented PM films containing the wild-type or mutated bR, induces bR spectral changes. When the electric field is removed, the original bR photo-state is regenerated and this process is completely reversible and extremely robust. Thus, the QDs-bR oriented hybrid material operating in the FRET-regime may be electrically controlled. In this case, varying the electric field will modulate FRET between the QDs and bR. This variation will change both intensity and wavelength of the light-emitting hybrid material. It's worth to be mentioned that the electric field-controlled light emitting materials should have numerous optoelectronic applications. PM-immobilized QDs will also serve as a built-in light harvesting antenna converting photons of light wavelengths from the near UV to the blue region of spectrum to photons of wavelengths that can be absorbed by bR. This provides a nano-unit of a novel photovoltaic material, in view of more effective voltage production. The routine PM purification produces homogeneous membrane fractions composed of lipids and a single protein, bRh. The long-term stability of PM against drastic thermo-chemical alterations together with its desirable photochromic properties, has made bR one of the most promising biological candidates for device applications. It can be dehydrated, fabricated into multilayers, or embedded into a polymer matrix. The substantial number of patents filed on bR applications is a strong indicator that commercial use of this protein in technical applications is feasible. Otherwise, the unique optical properties of QDs coupled with PM for development of optoelectronic materials are not yet explored. Our strategy to combine the quantum-size properties of QD emitters with optically switchable properties of nanoscale PM fragments will make feasible a new method to design efficient energy transfer nano-materials. The proposed approach to develop hybrid nanoscale QDs/bR system is attractive since the coupled photonic element (QDs) can record and transduce the irradiation-switchable biological functions. The NANO-BAC project will: 1. Engineer the bR-QD hybrid nanomaterials as an example of advanced material engineering where biotechnology is used as a tool for protein production and nanotechnology for design of the nanostructures which will improve or adapt selected bR biological functions to specific device applications. 2. Develop a model of resonance energy transfer mechanism on a nanoscale within a bR-QDs hybrid material and apply this model to engineering of hybrids with desired optical properties. 3. Investigate the photovoltaic properties of bR-QDs material in which the light harvesting QDs-based built-in antenna converts the photons of wavelengths from the near UV to the blue region of the visible spectrum to photons of wavelengths that can be absorbed by bR and used for more effective voltage production. 4. Engineer the prototypes of all-optical and opto-electrical switches based on bR-QD hybrids operating in the FRET-regime. NANO-BAC is a highly innovative proposal employing the ideas and approaches lying far away from the well-established research itineraries and we feel that it entirely corresponds to the scope of a program 'Blanc' both in terms of its significance and scientific merit.