Silica and silica-coated nanoparticles (NPs) are among the most versatile nanomaterials with applications spanning chemical, materials, and biomedical fields. Interfacial physical and surface chemistry properties of NPs play crucial roles in their interactions with other molecules and cellular components that could lead to the formation of protein coronas and determine intracellular uptake and biocatalytic activity. Typically, untreated silica NPs are negatively charged at neutral and alkaline pH and the resulting repulsive electrostatic forces provide for an extended colloidal stability. These considerations make understanding and probing these charge-based interactions at the bionano interface an important task. Here we describe development and applications of spin-labeling EPR methods based pn pH-sensitive molecular tags to measure interfacial electrostatic properties of silica NPs in aqueous suspensions. Monodisperse NPs of ca. 116 nm in diameter were synthesized by the Stöber method and then surface-modified by a silanization under harsh conditions to attach thiol-terminated ligands. The ligands were consequently modified with EPR-active pH-reporting molecular tags - either IMTSL IMTSL (S-(1-oxyl-2,2,3,5,5-pentamethylimidazolidin-4-yl)methylmethanesulfonothioate) or IKMTSL (S-4-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-1-oxyl-2,5-dihydro-1H-imidazol-2-yl). From EPR titration data for these two ionizable nitroxides at room temperature effective local dielectric constant experienced by IMTSL at the silica nanoparticle−water interface was determined to be 70.8 ± 5.0 whereas 57 ± 4 was found for IKMTSL. Surface electrostatic potential demonstrated an approximately linear increase in the magnitude starting at about zero at pH ∼4.0 and reaching ∼−150 mV at pH ∼8.5. While the tether linking the two nitroxides to silica surface allowed for some flexibility, it still ensured the location of the pH-sensitive tags close to the surface. For this reason, the values of the electrostatic surface potential reported by these nitroxides are significantly higher than determined from conventional zeta potential measurements. We the extended these measurements to aqueous solvents with an increased ionic strength to evaluate effects of counterions on screening the surface charge and changes in the local dielectric constant. Overall, developed spin labeling EPR methods are uniquely suited for studying interfacial electrostatic properties of colloid systems.

Dielectric and electrostatic properties of silica nanoparticle−water Interface

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.