Mesoporous
silica materials are undergoing rapid development for
numerous environmental and biomedical applications. These materials
are commonly functionalized with small organic molecules through a
reaction between an organosilane and the surface silanols. Despite
widespread use and implementation of these materials, ligands on their
surfaces are challenging to characterize, particularly in aqueous
environments. Employing traditional physicochemical characterization
methods such as adsorption isotherms, X-ray diffraction, and electron
microscopy, as well as solution-phase 1H NMR methods including
one-dimensional NMR, diffusion ordered spectroscopy (DOSY) and two-dimensional
nuclear Overhauser effect spectroscopy (NOESY), the labile nature
of several different surface ligands on mesoporous silica nanoparticles
is revealed. The data presented indicate a dynamic model of organosilane
release from the surface, and adsorption of the released molecules
is ultimately dependent on the nature of the binding of the functional
group to the particle surface. A new paradigm for understanding chemical
changes that take place at the liquid–solid interface is described,
which incorporates a model of chemical dynamics in aqueous solution.
Covalently functionalized nanomaterials are widely used, and the characterization
of the ligands on their surfaces is of paramount importance, particularly
when they are implemented in biomedical and environmental applications.
The surface chemistry of zeolite nanoparticles functionalized with the organosilane aminopropyldimethylmethoxysilane (APDMMS) was selectively probed using solution (1)H NMR spectroscopy. The use of solution NMR spectroscopy results in high-resolution NMR spectra, and the technique is selective for protons on the surface organic functional groups due to their motional averaging in solution. In this study, (1)H solution NMR spectroscopy was used to investigate the interface of the organic functional groups of APDMMS-functionalized silicalite nanoparticles (∼35 nm) in D2O. The pKa for the amine group of APDMMS-functionalized silicalite nanoparticles in D2O was determined using an NMR-pH titration method based on the variation in the proton chemical shift for the alkyl group protons closest to the amine group with pH. The resulting NMR spectra demonstrate the sensitivity of solution NMR spectroscopy to the electronic environment and structure of the surface functional groups.
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