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Dielectric and Electrostatic Properties of the Silica Nanoparticle–Water Interface by EPR of pH-Sensitive Spin Probes
Interfacial electrostatic properties of monodisperse silica nanoparticles (SiNPs) in aqueous suspensions as a function of bulk pH were characterized by spin labeling EPR of two ionizable nitroxides: (1) IMTSL (S-(1-oxyl-2,2,3,5,5-pentamethylimidazolidin-4-yl)methylmethanesulfo-nothioate) and IKMTSL (S-4-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-1-oxyl-2,5-dihydro-1H-imidazol-2-yl). SiNPs of ca. 116 nm in diameter (by particle number) were synthesized using the Stöber method, and their surface was modified by silanization under harsh conditions to ensure robust attachment of the thiol-terminated ligands to the silica surface. These ligands were consequently modified with either IMTSL or IKMTSL to characterize the surface electrostatic potential of the nanoparticles from their EPR spectra. EPR titration data for these two pH-sensitive nitroxides allowed for differentiating the dielectric and electrostatic contributions to the interfacial properties of SiNPs. From such a titration at room temperature an effective local dielectric constant experienced by IMTSL at the silica nanoparticle–water interface was found to be εeff = 70.8 ± 5.0 whereas εeff ≈ 57 ± 4 was found for IKMTSL. Surface electrostatic potential calculated from EPR titration of IKMTSL demonstrated an approximately linear increase in the magnitude starting at about zero at pH ∼4.0 and reaching ∼−150 mV at pH ∼8.5. This is in agreement with the existing literature on the surface potential associated with the silanol deprotonation developing over a wide pH range. While the attachment linker employed for the two nitroxides has some flexibility, it still ensures the location of the pH-sensitive tags close to the surface. For these reasons the values of the electrostatic surface potential reported by these nitroxides are significantly higher than those reported by the zeta potential measurements. Overall, spin labeling methods developed here expand the applicability of spin-labeling EPR to measurements of interfacial electrostatic properties of metal oxide nanoparticles.