Determination of the interior pH of lipid nanoparticles using a pH-sensitive fluorescent dye-based DNA probe

Lipid nanoparticles (LNPs) containing ionizable cationic lipids are proven delivery systems for therapeutic nucleic acids, such as small interfering RNA (siRNA). It is important to understand the relationship between the interior pH of LNPs and the pH of the external environment to understand LNP formulation and function. Here, we developed a simple and rapid approach for determining the pH of the LNP core using a pH-sensitive fluorescent dye-based DNA probe. LNP siRNA systems containing pH-responsive DNA probes (LNP-siRNA&DNA) were generated by rapid mixing of lipids in ethanol and pH 4 aqueous buffer containing siRNA and DNA probes. We demonstrated that DNA probes were readily encapsulated in LNP systems and were sequestered into an environment at a high concentration as evidenced by an inter-probe FRET signal. It was shown that the pH of LNP encapsulated probes closely follows the pH increase or decrease of the external environment. This indicates that the clinically approved LNP RNA systems with similar lipid compositions (e.g., Onpattro and Comirnaty) are highly permeable to protons and that the pH of the interior environment closely mirrors the external environment. The pH-dependent response of the probe in LNPs was also confirmed under buffer conditions at various pHs. Furthermore, we showed that the pH-sensitive DNA probe can be incorporated into LNP systems at levels that allow the pH response to be monitored at a single LNP level using convex lens-induced confinement (CLiC) confocal microscopy. Direct visualization of the internal pH of single particles with the fluorescent DNA probe was achieved by CLiC for LNP-siRNA&DNA systems formulated under both high and normal ionic strength conditions.

Ultrafast Energy Transfer Process in Confined Gold Nanospheres Revealed by Femtosecond X-ray Imaging and Diffraction

Femtosecond laser pulses drive nonequilibrium phase transitions via reaction paths hidden in thermal equilibrium. This stimulates interest to understand photoinduced ultrafast melting processes, which remains incomplete due to challenges in resolving accompanied kinetics at the relevant space-time resolution. Here, by newly establishing a multiplexing femtosecond X-ray probe, we have successfully revealed ultrafast energy transfer processes in confined Au nanospheres. Real-time images of electron density distributions with the corresponding lattice structures elucidate that the energy transfer begins with subpicosecond melting at the specimen boundary earlier than the lattice thermalization, and proceeds by forming voids. Two temperature molecular dynamics simulations uncovered the presence of both heterogeneous melting with the melting front propagation from surface and grain boundaries and homogeneous melting with random melting seeds and nanoscale voids. Supported by experimental and theoretical results, we provide a comprehensive atomic-scale picture that accounts for the ultrafast laser-induced melting and evaporation kinetics.

Imaging adsorption of iodide on single Cu2O microparticles reveals the acid activation mechanism

Imaging an adsorption reaction taking place at the single-particle level is a promising avenue for fundamentally understanding the adsorption mechanism. Here, we employ a dark-field microscopy (DFM) method for in situ imaging the adsorption process of I^- on single Cu₂O microparticles to reveal the acid activation mechanism. Using the time-lapsed DMF imaging, we find that a relatively strong acid is indispensable to trigger the adsorption reaction of I^- on single Cu₂O microparticle. A hollow microparticle with the increase in size is obtained after the adsorption reaction, causing the enhancement of the scattering intensity. Correlating the change of the scattering light intensity or particle size with adsorption capacity of I^-, we quantitatively analyze the selective uptake, slightly heterogeneous adsorption behavior, pH/temperature-dependent adsorption capacity, and adsorption kinetics as well as isotherms of individual Cu₂O microparticles for I^-. Our observations demonstrate that the acid-initiated Kirkendall effect is responsible for the high-reaction activity of single Cu₂O microparticles for adsorption of I^- in the acidic environment, through breaking the unfavorable lattice energy between Cu₂O and CuI as well as generating high-active hollow intermediate microparticle.