Breaking the optical resolution limit with novel FRET-based nanocrystal sensors – NanoFRET

405-nm evanescent-field excitation with a laser diode of luminescence of a hydrophilic nanocrystal (NC) donor permits the resonant energy transfer to organic acceptor dye molecules bound to its surface. This NanoFRET sensor borrows its sensitivity from the use of an ion concentration indicator as acceptor and is addressed to live cells via antibody targeting. The FRET efficiency steeply varies with donor/acceptor separation, providing contrast on a 1-10nm length scale well below optical resolution (~l/2). Hence, NanoFRET sensors only generate acceptor fluorescence when the donor is excited and the acceptor absorbs maximally following ion binding. This coincidence requirement makes NanoFRET sensors highly selective reporters of localized ion concentration gradients, e.g., for [Ca2+] or [Zn2+] nanodomains, until now drowned in the resolutionlimited signal. We aim at developing a family of NanoFRET-based sensors by i) donor calibration (fine-tuning of donor emission), ii) organic synthesis of a perfect acceptor match, with adapted KD of the ion sensor, and iii) minimizing the donor/acceptor distance. We are a team of experts in nanomaterials, physicochemistry, dye synthesis, imaging, modelling and neurobiology. We will apply NanoFRET biosensors to study (i) the modulation of postsynaptic neurotransmitter receptors by extracellular Zn2+, co-released from secretory vesicles during neurotransmission. (ii) The visualization of intracellular near-membrane Ca2+ nanodomains that are a prerequisite for the compartmentalization and specific action of the ubiquitous second messenger Ca2+ in a multitude of cellular signaling cascades.