Excitable Oil Droplets ‐ FRET Across a Liquid‐Liquid Phase Boundary

Abstract

FRET forms the basis for energy transfer in biological systems and organisms and it has become an investigative tool in the analysis of protein-protein interactions and in the study of semiconductors (SC). Until now, FRET has been restricted to the simultaneous presence of both components in the same phase. Here, we report on the first successful prototype demonstrating interfacial FRET. This innovative FRET between inorganic SCnanoparticles and illuminating protein chimeras takes place across an oil/water interface. As a 'proof of concept' oil droplets were stabilized by hydrophobin-derivatives in aqueous solution. These proteins possess the ability to attach fused functional domains close to an interface. Moreover, an optically active nanostructure directly docks to the hydrophobin at the oil/ water interface. Due to its modular design, this signal amplification array has the potential to be exploited in numerous fields ranging from biosensors, biotechnology to medical applications.Fö rster Resonance Energy Transfer (FRET) is a widely used quantum phenomenon for detection and sensor application on the nanometer scale in one phase systems. Excitation energy is transferred between two fluorescent species, from a donor to an acceptor, through a dipole-dipole interaction i. e. without the emission and reabsorption of a photon. Subsequently, the emission of the donor fluorophore is quenched and the acceptor fluorophore becomes excited. Such systems are extremely sensitive to changes in distance between donor and acceptor and the effect decreases as the sixth power of the distance separating the two. In common FRET cascades the donor and acceptor molecules are located in one phase so that the distance over which FRET occurs is of the dimensions of the Fö rster radius. [1,2] When FRET occurs, the fluorescence intensity of the acceptor increases while a decrease in donor fluorescence intensity and fluorescence lifetime takes place. [1] In nature FRET is a prevalent concept to guide energy between biological molecules. [3] These well known systems are also used in molecular biology for example to detect protein-protein-interactions. [4] FRET is not only possible between biological proteins or molecules, it is also possible to use quantum dots (QDs) and transfer energy between different kinds of nanoparticles. [5][6][7] QDs are well suited for this task because they have a broad absorption spectrum and narrow emission. Furthermore, QDs show a high photostability and can be synthesized such that they possess a high quantum yield. [8] The combination of biological molecules and QDs has already been demonstrated and hold many advantages such as size tunable absorption and emission. [9,10] Also their inherently high photostability and brightness result in an increase in the efficiency of the FRET process. Despite their many benefits, QDs are normally synthesized in organic solvents and thus have to be transferred to the aqueous phase in order to interact with biological samples. [11,12] The hydrophobic ligands on the surface...