An energy transfer relationship between core-shell CdSe/ZnS quantum dots (QDs) and the optical protein bacteriorhodopsin (bR) is shown, demonstrating a distance-dependent energy transfer with 88.2% and 51.1% of the QD energy being transferred to the bR monomer at separation distances of 3.5？nm and 8.5？nm, respectively. Fluorescence lifetime measurements isolate nonradiative energy transfer, other than optical absorptive mechanisms, with the effective QD excited state lifetime reducing from 18.0？ns to 13.3？ns with bR integration, demonstrating the F？rster resonance energy transfer contributes to 26.1% of the transferred QD energy at the 3.5？nm separation distance. The established direct energy transfer mechanism holds the potential to enhance the bR spectral range and sensitivity of energies that the protein can utilize, increasing its subsequent photocurrent generation, a significant potential expansion of the applicability of bR in solar cell, biosensing, biocomputing, optoelectronic, and imaging technologies. 1. Introduction Integrated nano biosystems are expected to offer applications in multiple technologies, such as biodetection and sensing [1, 2], biomedical diagnostics [3], single molecule dynamics [4], and photovoltaics [5]. In this work, the fundamental properties of such multifunctional hybrid nano biosystems involving core-shell quantum dots (QDs) and the optical protein bacteriorhodopsin (bR) are presented. Bacteriorhodopsin has been a subject of intense study over the past four decades due to its photoconducting properties and exceptionally high long-term stability against thermal, chemical, and photochemical degradation [6–8]. As a retinal protein found in the cell membrane of the extremophile Halobacterium salinarum, it is utilized to generate a proton motive force that energizes ATP synthase to drive the conversion of ADP and Pi to ATP and H2O, thereby providing the energy to drive the cell’s internal machinery [9]. The proton motive force is achieved when bR’s attached retinal chromophore absorbs photons in the 570？nm region, resulting in a cis-trans isomerization of the retinal. This structural alteration initiates proton transport from the retinal region to the extracellular side of the membrane creating a proton gradient within the membrane, with subsequent reprotonation from the cytoplasm [10]. This proton gradient across the cell membrane, which facilitates ATP synthesis in living systems, can be utilized to produce a measurable electrical response in engineered applications. Applications of bR require it to be extracted from the
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