Altering Surface Charge of Silica Nanoparticles through Co-condensation of Choline Chloride and Tetraethyl Orthosilicate (TEOS)

Abstract

We demonstrate an alternative route to synthesize functionalized silica nanoparticles through incorporation of alcohol compounds in the Stöber process. The Stöber process has been widely utilized for the synthesis of silica nanoparticles due to its simplicity and reliability. Silane based compounds have been incorporated in this process in order to tailor surface properties of the silica nanoparticles. These compounds do, however, have limitations in their utility due to side reactions with water and intermolecular polymerization. In this article, we report the incorporation of alcohol based reagents in the Stöber process as an alternative means of synthesis and functionalization of silica nanoparticles. In particular, choline chloride was chosen as an exemplary alcohol to be incorporated in the process for tuning overall surface charge of the silica nanoparticles. These silica nanoparticles with incorporated choline chloride were characterized by atomic force microscopy (AFM), zeta potential measurements, and X-ray photoelectron spectroscopy (XPS) in comparison with silica nanoparticles synthesized from the traditional Stöber process. While the size and shape of the nanoparticles exhibited little difference between the two synthetic routes, the zeta potential of the choline chloride incorporated nanoparticle was ∼10 mV higher than that of the traditional silica nanoparticles. Composition of the choline chloride containing silica nanoparticles was verified by XPS with the observation of strong N1s and C1s signals. The methods introduced in this article could be expanded to incorporate a range of alcohol containing compounds including choline chloride for the synthesis of silica nanoparticles with a tuned surface chemistry.

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“…An alternative route to synthesize functionalized silica particles incorporating alcohol compounds through the Stöber process has been described by Lee et al [ 72 ]. The Stöber method is one of the most common methods to synthesize nanoparticles due to its simplicity and reliability.…”

“…This method involves the hydrolysis and condensation of silicon alkoxide precursors, such as tetraethyl orthosilicate (TEOS) in an ethanol solution, using water and a base (e.g., ammonium hydroxide) as catalysts. In particular, the authors [ 72 ] chosen cholinium chloride (ChCl) as a functional alcohol to create quaternary ammonium functionalized SILs. Usually, the formation of quaternary ammonium functionality on the silica surface comprises a multi-step process involving silanes and other compounds [ 73 ].…”

“…Usually, the formation of quaternary ammonium functionality on the silica surface comprises a multi-step process involving silanes and other compounds [ 73 ]. However, Lee et al [ 72 ] have reported this distinct one-step process that can be easily accomplished by incorporating cholinium chloride into the Stöber synthesis. In detail, the hydroxyl group of cholinium chloride and silanol groups created during hydrolysis of tetraethyl orthosilicate (TEOS) can react by condensation, forming silyl ether (Si-O-C) bonds and exposing a positively charged quaternary ammonium group at the surface of the particles [ 72 ].…”

“…However, Lee et al [ 72 ] have reported this distinct one-step process that can be easily accomplished by incorporating cholinium chloride into the Stöber synthesis. In detail, the hydroxyl group of cholinium chloride and silanol groups created during hydrolysis of tetraethyl orthosilicate (TEOS) can react by condensation, forming silyl ether (Si-O-C) bonds and exposing a positively charged quaternary ammonium group at the surface of the particles [ 72 ]. The results showed an increase of the zeta potential (from 3 to 10) of silica particles incorporating cholinium chloride.…”