Single-molecule imaging of the biophysics of molecular interactions with precision and control, in cell-like conditions, and without tethers

High-throughput fluorescence lifetime imaging flow cytometry

Flow cytometry is a vital tool in biomedical research and laboratory medicine. However, its accuracy is often compromised by undesired fluctuations in fluorescence intensity. While fluorescence lifetime imaging microscopy (FLIM) bypasses this challenge as fluorescence lifetime remains unaffected by such fluctuations, the full integration of FLIM into flow cytometry has yet to be demonstrated due to speed limitations. Here we overcome the speed limitations in FLIM, thereby enabling high-throughput FLIM flow cytometry at a high rate of over 10,000 cells per second. This is made possible by using dual intensity-modulated continuous-wave beam arrays with complementary modulation frequency pairs for fluorophore excitation and acquiring fluorescence lifetime images of rapidly flowing cells. Moreover, our FLIM system distinguishes subpopulations in male rat glioma and captures dynamic changes in the cell nucleus induced by an anti-cancer drug. FLIM flow cytometry significantly enhances cellular analysis capabilities, providing detailed insights into cellular functions, interactions, and environments.

Quantitative Visualization of Lipid Nanoparticle Fusion as a Function of Formulation and Process Parameters

Lipid nanoparticles (LNPs) have proven to be promising delivery vehicles for RNA-based vaccines and therapeutics, particularly in LNP formulations containing ionizable cationic lipids that undergo protonation/deprotonation in response to buffer pH changes. These nanoparticles are typically formulated using a rapid mixing technique at low pH, followed by a return to physiological pH that triggers LNP-LNP fusion. A detailed understanding of these dynamic processes is crucial to optimize the overall performance and efficiency of LNPs. However, knowledge gaps persist regarding how particle formation mechanisms impact drug loading and delivery functions. In this work, we employ single-molecule Convex Lens-induced Confinement (CLiC) microscopy in combination with Förster resonance energy transfer (FRET) measurements to study LNP fusion dynamics in relation to various formulation parameters, including lipid concentration, buffer conditions, drug loading ratio, PEG-lipid concentrations, and ionizable lipid selection. Our results reveal a strong correlation between the measured fusion dynamics and the formulation parameters used; these findings are consistent with DLS and Cryo-TEM-based assays. These measurements offer a cost-effective method for characterizing and screening potential drug candidates and can provide additional insights into their design, with opportunities for optimization.

Polymer Brushes on Silica Nanostructures Prepared by Aminopropylsilatrane Click Chemistry: Superior Antifouling and Biofunctionality

In nanobiotechnology, the importance of controlling interactions between biological molecules and surfaces is paramount. In recent years, many devices based on nanostructured silicon materials have been presented, such as nanopores and nanochannels. However, there is still a clear lack of simple, reliable, and efficient protocols for preventing and controlling biomolecule adsorption in such structures. In this work, we show a simple method for passivation or selective biofunctionalization of silica, without the need for polymerization reactions or vapor-phase deposition. The surface is simply exposed stepwise to three different chemicals over the course of ∼1 h. First, the use of aminopropylsilatrane is used to create a monolayer of amines, yielding more uniform layers than conventional silanization protocols. Second, a cross-linker layer and click chemistry are used to make the surface reactive toward thiols. In the third step, thick and dense poly(ethylene glycol) brushes are prepared by a grafting-to approach. The modified surfaces are shown to be superior to existing options for silica modification, exhibiting ultralow fouling (a few ng/cm2) after exposure to crude serum. In addition, by including a fraction of biotinylated polymer end groups, the surface can be functionalized further. We show that avidin can be detected label-free from a serum solution with a selectivity (compared to nonspecific binding) of more than 98% without the need for a reference channel. Furthermore, we show that our method can passivate the interior of 150 nm × 100 nm nanochannels in silica, showing complete elimination of adsorption of a sticky fluorescent protein. Additionally, our method is shown to be compatible with modifications of solid-state nanopores in 20 nm thin silicon nitride membranes and reduces the noise in the ion current. We consider these findings highly important for the broad field of nanobiotechnology, and we believe that our method will be very useful for a great variety of surface-based sensors and analytical devices.

Diffusive dynamics of charged nanoparticles in convex lens-induced confinement

Transport through heterogeneous confined geometries is encountered in many processes and applications such as filtration, drug delivery, and enhanced oil recovery. We have used differential dynamic microscopy (DDM) and particle tracking to investigate dynamics of 36 nm negatively-charged polystyrene particles in convex lens-induced confinement (CLiC). The confinement gap height was controlled from 0.085 μm to 3.6 mm by sandwiching the aqueous particle solution between a glass coverslip and a convex lens using a homemade sample holder. With an inverted fluorescence microscope, sequences of micrographs were taken at various radial positions and gap heights for five particle concentrations (i.e. φ = 0.5 × 10^-5, 1 × 10^-5, 5 × 10^-5, 10 × 10^-5, 50 × 10^-5) and ionic strengths ranging from 10^-3 to 150 mM. The resulting image structure functions were fitted with a simple exponential model to extract the ensemble-averaged diffusive dynamics. It was found that particle diffusion was more hindered as a function of increased confinement. In addition, the ensemble-averaged diffusion coefficient was found to depend on the bulk concentration, and the concentration dependence increased as a function of confinement. Increasing particle and salt concentration led to confinement-dependent adsorption onto the geometry surface. Overall, we show that CLiC devices are simple and effective and can be used to study dynamics in continuous confinement from sub 100 nm to 100's of μm. These findings could lead to better understanding of separations and interactions in confining devices.