Quantum Yield Measurements of Fluorophores in Lipid Bilayers Using a Plasmonic Nanocavity

Metal-Induced Energy Transfer (MIET) for Live-Cell Imaging with Fluorescent Proteins

Metal-induced energy transfer (MIET) imaging is an easy-to-implement super-resolution modality that achieves nanometer resolution along the optical axis of a microscope. Although its capability in numerous biological and biophysical studies has been demonstrated, its implementation for live-cell imaging with fluorescent proteins is still lacking. Here, we present its applicability and capabilities for live-cell imaging with fluorescent proteins in diverse cell types (adult human stem cells, human osteo-sarcoma cells, and Dictyostelium discoideum cells), and with various fluorescent proteins (GFP, mScarlet, RFP, YPet). We show that MIET imaging achieves nanometer axial mapping of living cellular and subcellular components across multiple time scales, from a few milliseconds to hours, with negligible phototoxic effects.

Excitation Intensity-Dependent Quantum Yield of Semiconductor Nanocrystals

One of the key phenomena that determine the fluorescence of nanocrystals is the nonradiative Auger-Meitner recombination of excitons. This nonradiative rate affects the nanocrystals' fluorescence intensity, excited state lifetime, and quantum yield. Whereas most of the above properties can be directly measured, the quantum yield is the most difficult to assess. Here we place semiconductor nanocrystals inside a tunable plasmonic nanocavity with subwavelength spacing and modulate their radiative de-excitation rate by changing the cavity size. This allows us to determine absolute values of their fluorescence quantum yield under specific excitation conditions. Moreover, as expected considering the enhanced Auger-Meitner rate for higher multiple excited states, increasing the excitation rate reduces the quantum yield of the nanocrystals.

Graphene- and metal-induced energy transfer for single-molecule imaging and live-cell nanoscopy with (sub)-nanometer axial resolution

Super-resolution fluorescence imaging that surpasses the classical optical resolution limit is widely utilized for resolving the spatial organization of biological structures at molecular length scales. In one example, single-molecule localization microscopy, the lateral positions of single molecules can be determined more precisely than the diffraction limit if the camera collects individual photons separately. Using several schemes that introduce engineered optical aberrations in the imaging optics, super-resolution along the optical axis (perpendicular to the sample plane) has been achieved, and single-molecule localization microscopy has been successfully applied for the study of 3D biological structures. Nonetheless, the achievable axial localization accuracy is typically three to five times worse than the lateral localization accuracy. Only a few exceptional methods based on interferometry exist that reach nanometer 3D super-resolution, but they involve enormous technical complexity and restricted sample preparations that inhibit their widespread application. We developed metal-induced energy transfer imaging for localizing fluorophores along the axial direction with nanometer accuracy, using only a conventional fluorescence lifetime imaging microscope. In metal-induced energy transfer, experimentally measured fluorescence lifetime values increase linearly with axial distance in the range of 0-100 nm, making it possible to calculate their axial position using a theoretical model. If graphene is used instead of the metal (graphene-induced energy transfer), the same range of lifetime values occurs over a shorter axial distance (~25 nm), meaning that it is possible to get very accurate axial information at the scale of a membrane bilayer or a molecular complex in a membrane. Here, we provide a step-by-step protocol for metal- and graphene-induced energy transfer imaging in single molecules, supported lipid bilayer and live-cell membranes. Depending on the sample preparation time, the complete duration of the protocol is 1-3 d.

Absolute quantum yield measurements of fluorescent proteins using a plasmonic nanocavity

One of the key photophysical properties of fluorescent proteins that is most difficult to measure is the quantum yield. It describes how efficiently a fluorophore converts absorbed light into fluorescence. Its measurement using conventional methods become particularly problematic when it is unknown how many of the proposedly fluorescent molecules of a sample are indeed fluorescent (for example due to incomplete maturation, or the presence of photophysical dark states). Here, we use a plasmonic nanocavity-based method to measure absolute quantum yield values of commonly used fluorescent proteins. The method is calibration-free, does not require knowledge about maturation or potential dark states, and works on minute amounts of sample. The insensitivity of the nanocavity-based method to the presence of non-luminescent species allowed us to measure precisely the quantum yield of photo-switchable proteins in their on-state and to analyze the origin of the residual fluorescence of protein ensembles switched to the dark state.

Direct observation of alpha-lactalbumin, adsorption and incorporation into lipid membrane and formation of lipid/protein hybrid structures

The interaction between proteins and membranes is of great interest in biomedical and biotechnological research for its implication in many functional and dysfunctional processes. We present an experimental study on the interaction between model membranes and alpha-lactalbumin (α-La). α-La is widely studied for both its biological function and its anti-tumoral properties. We use advanced fluorescence microscopy and spectroscopy techniques to characterize α-La-membrane mechanisms of interaction and α-La-induced modifications of membranes when insertion of partially disordered regions of protein chains in the lipid bilayer is favored. Moreover, using fluorescence lifetime imaging, we are able to distinguish between protein adsorption and insertion in the membranes. Our results indicate that, upon addition of α-La to giant vesicles samples, protein is inserted into the lipid bilayer with rates that are concentration-dependent. The formation of heterogeneous hybrid protein-lipid co-aggregates, paralleled with protein conformational and structural changes, alters the membrane structure and morphology, leading to an increase in membrane fluidity.

Probe-location dependent resonance energy transfer at lipid/water interfaces: comparison between the gel- and fluid-phase of lipid bilayer

Effect of dielectric environment and lipid fluidity/rigidity in multi-chromophoric FRET from a series of donors to acceptors at lipid/water interfaces are monitored by tailored donor–acceptor pairs.