The SALSAC approach: comparing the reactivity of solvent-dispersed nanoparticles with nanoparticulate surfaces

We demonstrate that the ‘surface-as-ligand, surface-as-complex’ (SALSAC) approach that we have

established for annealed nanoparticulate TiO2 surfaces can be successfully applied to nanoparticles (NPs)

dispersed in solution. Commercial TiO2 NPs have been activated by initial treatment with aqueous HNO3

followed by dispersion in water and heating under microwave conditions. We have functionalized the

activated NPs with anchoring ligands 1–4; 1–3 contain one or two phosphonic acid anchoring groups

and 4 has two carboxylic acid anchors; ligands 1, 2 and 4 contain 6,60-dimethyl-2,20-bipyridine (Me2bpy)

metal binding domains and 3 contains a 2,20:60,200-terpyridine (tpy) unit. Ligand functionalization of the

activated NPs has been validated using infrared and 1H NMR spectroscopies, and thermogravimetric

analysis. NPs functionalized with 1, 2 and 4 react with [Cu(MeCN)4][PF6] and those with 3 react with

FeCl2$4H2O; metal binding has been investigated using solid-state absorption spectroscopy and

scanning electron microscopy (SEM). Competitive binding of ligands 1–4 to TiO2 NPs has been

investigated and shows preferential binding of phosphonic acid over carboxylic acid anchors. For the

phosphonic acids, the binding orders are 3 > 1 > 2 which is rationalized in terms of relative pKa values

(phosphonic acid and [HMe2bpy]+ or [Htpy]+) and the number of anchoring groups in the ligands. Ligand

exchange between ligand-functionalized NPs and homoleptic metal complexes gives NPs functionalized

with heteroleptic copper(I) or iron(II) complexes