(167d) Post-Synthetically Amine Grafted Silica Nanoparticles As Model Materials for Separations and Catalysis

Peter Miller¹ and Daniel F. Shantz¹

¹Department of Chemical and Biomolecular Engineering, Tulane University, 6823 St. Charles Avenue, New Orleans, Louisiana 70118, United States

Contact Email: pmiller5@tulane.edu

Abstract:

Silica nanoparticles with grafted organic functional groups have been extensively studied due to applications such as separations, catalysis, pharmaceuticals, and thin films. However, the majority of the functionalized particles have a significant size increase compared with pristine particles. In a typical synthesis of uniform spheres, tetraethylorthosilicate is reacted with ammonium hydroxide forming nanoparticles that immediately aggregate and are very difficult to subsequently redisperse. To avoid this, amino acid capping groups, such as L-lysine, have instead been used as a base catalyst during synthesis which results in a transparent solution with monodisperse silica spheres around 10 nm in diameter. Applications of these particles could significantly expand by adding covalently bound organic functionalities while maintaining monodispersity, small diameter, and colloidal stability in a wide range of solvents.

In this work, amine functional groups were post-synthetically grafted onto monodispersed silica nanoparticles initially synthesized with L-lysine. The grafted particles could then be resuspended in water up to a concentration of 20 mg/mL while maintaining monodispersity. Particle grafting was completed by precipitating the aqueous suspended particles then transferring to acetonitrile. The particles could then be resuspended with the addition of trifluoroacetic acid and sonication. Amine grafting was then completed with 3-aminopropyldimethylethoxysilane and the resulting particles could be transferred to an aqueous acidic solution. The average diameter was found to be 25 nm by dynamic light scattering, transmission electron microscopy, and small-angle x-ray scattering. The amine grafting was confirmed and quantified by ninhydrin assay and copper chloride binding with loading calculated to be about 0.25 mmol amine/g silica.

The amine grafted nanoparticles were then used to analyze organic acid-amine binding with a suite of solution NMR techniques. It was found that the organic acid hydrophobicity and alkyl chain length was the main factor in binding quantity and strength, which could be determined through analysis of molecular relaxation times and exchange rates. This technique could also be used to analyze many molecular binding/reactions that occur at a functionalized particle interface.

Additionally, it was shown that a wide variety of organic functionalities could be reacted onto the bound amine to control the surface properties. This was done by first reacting cyanuric chloride with the amine, then using that as a base ligand for a variety of dendritic structures terminated with groups including aromatic amines, benzene derivatives, and alkyl chains. Structures were then confirmed by thermogravimetric analysis, infrared spectroscopy, and mass spectroscopy. The base catalyzed Henry reaction was then used as a probe reaction to show increased reaction rates for the grafted nanoparticles when compared to mesoporous silica due to decreased diffusion resistance.