*2.4. Characterization*

For transmission electron microscopy (TEM), samples were prepared by drying a diluted ethanolic suspension of MSN on a carbon-coated copper grid at room temperature for several hours. The measurements were performed on a Tecnai G2 20 S-Twin instrument operated at 200 kV with a TVIPS TemCam-F216 camera.

Dynamic light scattering (DLS) and zeta potential measurements were performed with a Malvern Zetasizer Nano instrument equipped with a 4 mW He-Ne-Laser (633 nm). For DLS data, a diluted colloidal suspension of the particles was measured in PMMA cuvettes at 25 ◦C. For zeta potential measurements, the additional Zetasizer titration system (MPT-2) was used based on diluted NaOH and HCl as titrants. For this purpose, a colloidal suspension of MSNs in water at a concentration of 0.1 mg mL−<sup>1</sup> was prepared.

Raman spectroscopy was performed on a Bruker Equinox 55 with FRA-106 Raman attachment with a ND:YAG laser (1064 nm) and a laser power of 100 mW. Infrared spectra of dried sample powder were recorded on a ThermoScientific Nicolet iN10 IRmicroscope in reflection–absorption mode with a liquid-N2 cooled MCT-A detector.

A Quantachrome Instrument NOVA 4000e was used for nitrogen sorption analysis at −196 ◦C. Sample outgassing was performed for 12 h at 120 ◦C and at a vacuum of 13.3 × 10−<sup>3</sup> mbar. A quenched solid density functional theory (QSDFT) equilibrium model of N2 on silica at a relative pressure *p*/*p*0 = 0.8 was used to calculate the pore size and pore volume, based on the adsorption and desorption branch of the isotherm. A BET model in the range *p*/*p*0 = 0.05–0.2 was used to determine the specific surface area.

Thermogravimetric analysis (TGA) of the powder samples (10 mg) was performed on a Netzsch STA 440 C TG/DSC using a heating rate of 10 ◦C/min up to 900 ◦C with a stream of synthetic air of about 25 mL·min−1. siRNA concentrations were determined by UV measurements performed with the Nanodrop 2000c spectrometer (Thermo Scientific).

#### *2.5. Loading of Ctrl and miR200c for Gene Silencing*

In all experiments, 100 μL Ctrl or miR200c solution (c = 50 ng μL−1) in MES buffer (pH = 5) was added to 100 μg MSN-NH2in-SHout samples, resulting in a final RNA concentration of 50 μg mg<sup>−</sup><sup>1</sup> of MSN carrier. Samples were vortexed and shaken at 37 ◦C for 30 min to complete adsorption. Particles were washed via centrifugation and were resuspended in 100 μL HEPES buffer. The supernatant was collected to verify complete adsorption of RNA.

## *2.6. Loading of Fluorescent Dye for Cellular Internalization Studies*

A total of 1 mL MSN-NH2in-SHout samples (pH = 5; 1 mg mL−<sup>1</sup> MES buffer) was mixed with 2 μL Atto-633 carboxy (2 mg mL−<sup>1</sup> in anhydrous DMSO), 10 μL EDC and a catalytic amount of sulfo-NHS. The samples were vortexed and shaken for 4 h, were washed multiple times afterwards with MES and HEPES buffer (1 mL, respectively) (centrifugation steps: 10 min, 16,900 rcf) and were redispersed in 1 mL MES buffer.
