Modification of Porous Silicon with an Amphiphilic Quaternary Ammonium Hydrocarbon for Nanomedicine Applications

Octadecyldimethyl(3-trimethoxysilylpropyl)ammonium chloride (QAC₁₈-silane), an amphiphilic compound containing a quaternary ammonium group attached to an alkyl chain, has been widely used as a surface coating to impart antifouling and antimicrobial activity. Its combination of a cationic charge and a hydrophobic C₁₈-chain imparts unique amphiphilic properties relevant to biosensing and drug-delivery applications. This study evaluated QAC₁₈-silane for the modification of oxidized mesoporous silicon (Ox-pSi). Three aspects of the chemistry of QAC₁₈-silane are exploited here: first, its low tendency to cross-link with itself, avoiding buildup of multilayers and blocking of the pores in mesoporous materials─a problem commonly encountered in reactions of trialkoxysilanes; second, its ability to act as an effective host for hydrophobic molecules, to enable the loading of drugs of poor water solubility; and third, its ability to affiliate with other amphiphilic molecules to form a hybrid bilayer. The study involves two forms of Ox-pSi: thin films that are still attached to the silicon wafers from which they are prepared, and nanoparticles generated by removal of the electrochemically etched porous silicon material from the silicon wafer and ultrasonic fracture into ∼160 nm porous particles. The oxide layer in both sample types is prepared by treatment of the native mesoporous Si with hydrogen peroxide, generating a hydrophilic (water contact angle <10°) Si-SiO₂ core–shell structure with average pore diameters of 14 nm (thin films) and 20 nm (nanoparticles). It is found that QAC₁₈-silane has a low rate of intermolecular condensation in neutral solution, attributed to electrostatic repulsions between the positively charged quaternary ammonium species on the QAC₁₈-silane. The Ox-pSi surfaces react readily with QAC₁₈-silane in either water or a 90:10 (v/v) methanol/water mixed solvent, generating a hydrophobic surface coating that retains between 50 and 90% of the open porosity of the original nanostructure. QAC₁₈-silane reacts at the pore openings of Ox-pSi when water is the solvent, penetrating only ∼1 μm into the porous layer after 4 h of reaction. By contrast, QAC₁₈-silane in the methanol/water solvent system shows uniform penetration into the Ox-pSi pores, but it displays a lower overall degree of surface coverage. These results are attributed to competing effects of QAC₁₈-silane solvation, micelle formation, and charge screening of the quaternary ammonium species on QAC₁₈-silane. Modification of ∼160 nm Ox-pSi nanoparticles results in positively charged nanoparticles that, when coated with an amphiphilic polymer Pluronic F-127, retain their size distribution and do not aggregate in PBS buffer isotonic with blood plasma. As a proof of concept, rifampicin (RIF) is loaded, and the resulting QAC₁₈-modified, drug-loaded, and Pluronic F-127-coated nanoparticles are characterized.