Investigating Lysine Adsorption on Fumed Silica Nanoparticles

The adsorption of amino acids on silica surfaces has attracted considerable interest because it has a broad range of applications in various fields such as drug delivery, solid-phase peptide synthesis, and biocompatible materials synthesis. In this work, we systematically study lysine adsorption on fumed silica nanoparticles with thermal analysis and solid-state NMR. Thermogravimetric analysis results show that the adsorption behavior of lysine in low-concentration aqueous solutions is well-described by the Langmuir isotherm. With ultrafast magic-angle-spinning ¹H NMR and multinuclear and multidimensional ¹³C and ¹⁵N solid-state NMR, we successfully determine the protonation state of bulk lysine and find that lysine is adsorbed on silica nanoparticle surfaces through the side-chain amine groups. Density functional theory calculations carried out on lysine and lysine–silanol complex structures further confirm that the side-chain amine groups interact with the silica surface hydroxyl groups via strong hydrogen bonding. Furthermore, we find that lysine preferentially has monolayer coverage on silica surfaces in high salt concentration solutions because of the ionic complexes formed with surface bound lysine molecules.