Fig. 4
From: Dye-doped silica nanoparticles: synthesis, surface chemistry and bioapplications
(A: adapted from Zhu et al. (2012); B, C: adapted from Smith and Chen (2008))
Post-coating unmodified –OH bearing silica NPs is commonly done using siloxanes with reactive amines. A However, such coatings are not usually homogenous and individual silane molecules can interact through (a) hydrogen bonding, (b) electrostatic attraction, (c) covalent bonding with surface, (d) horizontal, and (e) vertical polymerisation with neighbouring silanes. (f) Oligomeric/polymeric silanes can also react/interact with functionalities present at the interface. B When the reactive amine is in the propyl position it can coordinate intra- or intermolecularly with silicon atoms and catalyse the cleavage of the siloxane bond via hydrolysis, thus eventually resulting in the loss of amine groups from the silica surface. C The longer chains of N-(6-aminohexyl)aminomethyltriethoxysilane are not capable of self-cleave and give stable amine layers on silica surfaces