Structural Characterization of Lecithin-Stabilized Tetracosane Lipid Nanoparticles. Part II: Suspensions

Mesoscopic Structure of Lipid Nanoparticle Formulations for mRNA Drug Delivery: Comirnaty and Drug-Free Dispersions

Lipid nanoparticles (LNPs) produced by antisolvent precipitation (ASP) are used in formulations for mRNA drug delivery. The mesoscopic structure of such complex multicomponent and polydisperse nanoparticulate systems is most relevant for their drug delivery properties, medical efficiency, shelf life, and possible side effects. However, the knowledge on the structural details of such formulations is very limited. Essentially no such information is publicly available for pharmaceutical dispersions approved by numerous medicine agencies for the use in humans and loaded with mRNA encoding a mimic of the spike protein of the severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) as, e.g., the Comirnaty formulation (BioNTech/Pfizer). Here, we present a simple preparation method to mimic the Comirnaty drug-free LNPs including a comparison of their structural properties with those of Comirnaty. Strong evidence for the liquid state of the LNPs in both systems is found in contrast to the designation of the LNPs as solid lipid nanoparticles by BioNTech. An exceptionally detailed and reliable structural model for the LNPs i.a. revealing their unexpected narrow size distribution will be presented based on a combined small-angle X-ray scattering and photon correlation spectroscopy (SAXS/PCS) evaluation method. The results from this experimental approach are supported by light microscopy, 1H NMR spectroscopy, Raman spectroscopy, cryogenic electron microscopy (cryoTEM), and simultaneous SAXS/SANS studies. The presented results do not provide direct insights on particle formation or dispersion stability but should contribute significantly to better understanding the LNP drug delivery process, enhancing their medical benefit, and reducing side effects.

Internal Structure of Nanometer-Sized Droplets Prepared by Antisolvent Precipitation

Antisolvent precipitation (AP) is a low-cost and less-invasive preparation alternative for organic nanoparticles compared to top-down methods such as high-pressure homogenization or milling. Here we report on particularly small organic nanoparticles (NPs) prepared by AP. It has been found for various materials that these NPs in their liquid state exhibit a significant degree of molecular order at their interface toward the dispersion medium including ubiquinones (coenzyme Q10), triglycerides (trimyristin, tripalmitin), and alkanes (tetracosane). This finding is independent of the use of a stabilizer in the formulation. While this is obviously a quite general interfacial structuring effect, the respective structural details of specific NPs systems might differ. Here, a detailed structural characterization of very small liquid coenzyme Q10 (Q10) NPs is presented as a particular example for this phenomenon. The Q10 NPs have been prepared by AP in the presence of two different stabilizers, sodium dodecyl sulfate (SDS) and pentaethylene glycol monododecyl ether (C₁₂E₅), respectively, and without any stabilizer. The NPs' size is initially analyzed by photon correlation spectroscopy (PCS). The SDS-stabilized Q10 NPs have been studied further by differential scanning calorimetry (DSC), small-angle X-ray and neutron scattering (SAXS, SANS), wide-angle X-ray scattering (WAXS), and cryogenic transmission electron microscopy (CryoTEM). A simultaneous analysis of SAXS and contrast variation SANS studies revealed the molecular arrangement within the interface between the NPs and the dispersion medium. The Q10 NPs stabilized by SDS and C₁₂E₅, respectively, are small (down to 19.9 nm) and stable (for at least 16 months) even when no stabilizer is used. The SDS-stabilized Q10 NPs reported here, are therewith, to the best of our knowledge, the smallest organic NPs which have been reported to be prepared by AP so far. In particular, these NPs exhibit a core-shell structure consisting of an amorphous Q10 core and a surrounding shell, which is mainly composed of oriented Q10 molecules and aligned SDS molecules. This structure suggests a significant amphiphilic behavior and a rather unexpected stabilizing role of Q10 molecules.

Concentration dependent morphology and composition of n-alcohol modified cetyltrimethylammonium bromide micelles

Cetyltrimethylammonium bromide (CTAB) is one of the most commonly used surfactants in nanoparticle synthesis and stabilization. Usually, CTAB is used in high concentrations besides co-surfactants leading to well defined products but the complex mesoscopic CTAB structures stay mostly unknown. N-alcohols for instance are widely used co-surfactants which modify the properties of native CTAB dispersions. In this paper we report about a detailed structure analysis of n-alcohol modified CTAB micelles. In particular, n-pentanol and n-hexanol exhibit a significantly different influence on the size, shape and composition of CTAB micelles. Using a combination of small-angle x-ray spectroscopy (SAXS) and neutron scattering spectroscopy (SANS), we applied a method for a complete structural characterization of such micelles. The incorporation of n-pentanol into CTAB micelles generally does not influence the morphology but enhances the number of micelles due to the volume of the added alcohol. N-hexanol, however, leads to an elongation of the micelles as a function of its concentration. It was found by extended contrast variation measurements that this difference is caused by a different distribution of the alcohols between the micellar core and shell. N-pentanol molecules are generally located at the core-shell interface of the CTAB micelles with not only the head group but also two additional methylene bridging groups located in the micellar shell. This leads to an increase of its effective head group volume. In comparison, in n-hexanol modified micelles the whole alkyl chain is located within the micellar core. The detailed structure for n-alcohol modified CTAB micelles is described herein for the first time. The knowledge of the structural details found is indispensable for an in-depth understanding of CTAB-n-alcohol-water interfaces in general which is relevant for the synthesis of many functional nanostructures like mesoporous silica and gold or silver nanoparticles.

Changes within the stabilizing layer of ZnO nanoparticles upon washing

ZnO nanoparticles (NPs) are highly relevant for various industrial applications, however, after synthesis of the NPs residual chemicals need to be removed from the colloidal raw product by washing, as they may influence the performance of the final device. In the present study we focus on the effect of washing by antisolvent flocculation with subsequent redispersion of the NPs on the stabilizing acetate shell. Purification of the ZnO nanoparticles is reported to be optimal with respect to zeta potential that has a maximum after one washing cycle. In this work, we will shed light on this observation using small angle X-ray and neutron scattering (SAXS, SANS) by demonstrating that after the first washing cycle the content of acetate in the ligand shell around the ZnO NPs increases. In detail, it was observed that the diffuse acetate shell shrinks to the size of a monolayer upon washing but the acetate content of this monolayer is higher than within the diffuse shell of the particles of the native dispersion. A second washing cycle reduces the acetate concentration within the stabilizing shell and the stability of the dispersion drops accordingly. After another (third) washing cycle strong agglomeration was observed for all investigated samples.