Highly Efficient siRNA Delivery from Core-Shell Mesoporous Silica Nanoparticles with Multifunctional Polymer Caps

A new general route for siRNA delivery is presented combining porous core-shell silica nanocarriers with a modularly designed multifunctional block copolymer. Specifically, the internal storage and release of siRNA from mesoporous silica nanoparticles (MSN) with orthogonal core-shell surface chemistry was investigated as a function of pore-size, pore morphology, surface properties and pH. Very high siRNA loading capacities of up to 380 microg/mg MSN were obtained with charge-matched amino-functionalized mesoporous cores, and release profiles show up to 80% siRNA elution after 24 h. We demonstrate that adsorption and desorption of siRNA is mainly driven by electrostatic interactions, which allow for high loading capacities even in medium-sized mesopores with pore diameters down to 4 nm in a stellate pore morphology. The negatively charged MSN shell enabled the association with a block copolymer containing positively charged artificial amino acids and oleic acid blocks, which acts simultaneously as capping function and endosomal release agent. The potential of this multifunctional delivery platform is demonstrated by highly effective cell transfection and siRNA delivery into KB-cells. A luciferase reporter gene knock-down of up to 90% was possible using extremely low cell exposures with only 2.5 microg MSN containing 32 pM siRNA per 100 microL well.

💡 Research Summary

In this work the authors present a novel siRNA delivery platform that combines core‑shell mesoporous silica nanoparticles (MSNs) with a multifunctional block‑copolymer cap, achieving exceptionally high loading capacities and potent gene‑silencing efficacy at remarkably low dosages. The MSNs are synthesized via a co‑condensation route that yields particles of approximately 150 nm with a positively charged amino‑functionalized core and a negatively charged mercapto‑functionalized shell. The core‑shell architecture provides orthogonal surface chemistries: the interior is enriched with primary amine groups (APTES) while the exterior is terminated with thiol groups (MPTES). This design directs negatively charged siRNA molecules into the internal pore network rather than adsorbing on the particle surface.

Pore characterization shows a stellate, conically widening morphology with average pore diameters of 4–5 nm, which is sufficient for the diffusion of siRNA (≈2 × 8 nm). By varying the APTES content from 1 mol % to 9 mol % the isoelectric point (IEP) of the particles can be tuned from pH 4 to pH 10, resulting in a positive ζ‑potential of +30 to +40 mV at physiological pH. Under these conditions, siRNA loading reaches up to 380 µg · mg⁻¹ of MSN—an order of magnitude higher than previously reported values for mesoporous silica carriers. Release studies performed in PBS (pH 7.4) demonstrate a gradual release profile, with about 5 % of the cargo liberated within the first hour and up to 80 % after 24 h, indicating that the electrostatic interaction is strong yet reversible under biologically relevant conditions.

To prevent premature release and to promote endosomal escape, the authors synthesize a modular block copolymer comprising positively charged artificial amino acids, succinoyl‑tetraethylene‑penta‑mide (stp) segments that bind to the negatively charged silica shell, and hydrophobic oleic‑acid blocks that act as fusogenic agents. The polymer caps the MSN surface through electrostatic attraction, forming a stable “cap” that also serves as a pH‑responsive membrane‑disrupting element. Upon endosomal acidification, the oleic‑acid domains insert into the endosomal membrane, facilitating cytosolic delivery of the siRNA payload.

Cellular experiments using KB cancer cells and a luciferase reporter system confirm the functional superiority of the platform. With a mere 2.5 µg of MSN per 100 µL well (containing 32 pM siRNA), the authors achieve up to 90 % knock‑down of luciferase expression after 24 h. This performance surpasses that of conventional polyethylenimine (PEI)‑based carriers, which typically require substantially higher siRNA concentrations and are associated with notable cytotoxicity. Gel electrophoresis and zeta‑potential measurements corroborate the strong yet reversible binding of siRNA within the pores and the successful capping by the polymer.

The study highlights three pivotal insights: (1) medium‑sized (≈4 nm) mesopores, when combined with a positively charged interior, can accommodate large oligonucleotides efficiently, challenging the prevailing notion that only large‑pore (>10 nm) MSNs are suitable for nucleic‑acid delivery; (2) orthogonal surface functionalization enables selective internal loading while minimizing nonspecific external adsorption; and (3) a rationally designed block copolymer cap can simultaneously inhibit premature release and trigger endosomal escape without the toxicity associated with high‑molecular‑weight polycations.

Overall, the work provides a robust, scalable, and versatile platform for nucleic‑acid therapeutics. Future directions may involve grafting targeting ligands (antibodies, peptides) or stealth polymers (PEG) to the shell for in‑vivo applications, extending the approach to other nucleic‑acid modalities such as miRNA, mRNA, or CRISPR‑Cas components, and exploring combination therapies with chemotherapeutics loaded in the same carrier. The integration of precise pore engineering with smart polymer capping represents a significant advance toward safe and effective gene‑silencing nanomedicines.