Ferrocenyl-functionalized silica nanoparticles: preparation, characterization, and molecular recognition at interfaces.

Ferrocenyl-functionalized silica nanoparticles (Fc-SiO(2), 6a-6c) of about 60 nm with supramolecular "guest" properties were prepared. Nanoparticles 6a-6c differed by the addition of different molar ratios of starting compounds during the functionalization step, i.e., 1:0, 1:10, and 1:90 of 2-ferrocenyl amidoethoxyethanol and diethylene glycol for 6a,6b, and 6c, respectively. X-ray photoelectron spectroscopy (XPS) proved the presence of ferrocenyl groups on the surfaces of 6a-6c, whereas the elemental analysis revealed an iron content of particles 6a-6c of 0.10-0.16%. Dynamic light scattering (DLS) results showed that, compared with 6a, 6b dispersed well in aqueous media, possibly due to the presence of diethylene glycol at the surfaces of 6b that significantly increases its overall hydrophilicity. Cyclic voltammetry of 6b indicated a totally irreversible system and a "mixed" diffusion-adsorption behavior, which is attributed to sluggish electron transfer. The shifted |I(p,C)/I(p,A*)| ratio showed that the ferrocenyl groups are robustly attached to the nanoparticle surface within the experimental potential range. The supramolecular recognition of Fc-SiO(2) nanoparticles at interfaces was verified by their adsorption on beta-cyclodextrin (beta-CD) self-assembled monolayers, as monitored by surface plasmon resonance (SPR) spectroscopy. The ability of the Fc-SiO(2) nanoparticles to form host-guest interactions was also demonstrated by the attachment of beta-CD-functionalized Au nanoparticles (2.8 nm) on the Fc-SiO(2) surfaces, when mixed in solution.