pH-Controlled Assembly and Disassembly of Electrostatically Linked CdSe−SiO₂ and Au−SiO₂ Nanoparticle Clusters

Abstract

The effect of pH on the self-assembly and disassembly of nanoparticle aggregates (clusters) of amine modified silica spheres (50 nm) and carboxylic acid modified gold (7 nm) or cadmium selenide (6 nm) spheres was investigated using electron microscopy and infrared and electronic spectroscopy. 4-Mercaptobenzoic acid (MBA) ligated cadmium selenide nanoparticles were synthesized by ligand exchange on 6 nm trioctylphosphine oxide ligated CdSe nanoparticles in basic methanol solution at room temperature. Reaction of CdSe(MBA) and 11-mercaptoundecanoic acid (MUA) ligated gold nanoparticles with tetramethylammonium hydroxide in methanol or THF produced CdSe(MBA) - [N(CH 3 ) 4 + ] and Au(MUA) - [N(CH 3 ) 4 + ] as black solids. On the basis of signal integration in the 1 H NMR spectra, organic groups occupy areas of 0.44 nm 2 (MBA), 0.22 nm 2 (MUA), and 0.69 nm 2 (APS) on the nanoparticle surfaces. Combination of these modified colloids with solutions of 3-aminopropylsiloxane ligated colloidal silica in aqueous solution at various pHs yielded core−shell type clusters in which the smaller gold or cadmium selenide spheres are bound to the surfaces of the larger silicate spheres. In the pH interval from 6.8 to 11.1, the Au/CdSe nanocrystal content of the clusters was found to be inversely related to the pH. The addition of a 50 mM Na 3 PO 4 /H 3 PO 4 buffer solution attenuated the pH dependence of composition in the case of the SiO 2 @Au clusters, due to screening of the surface potential of the charged particles by ions of opposite charge. Whereas in neutral water the nanoparticle clusters are inert toward rearrangement and decomposition, at pH > 13, rapid disassembly of the clusters occurs as a consequence of deprotonation of the primary ammonium groups. The stability of the clusters is accurately described with a semiquantitative electrostatic binding model, in which the bond strength between nanoparticles is determined by their surface charge concentrations. These surface charge concentrations are calculated from the Henderson−Hasselbach equation using estimated and measured p K a values for 4-mercaptobenzoic acid (4.79), 3-aminopropylsilane (9.27), and MUA (5.7) monolayers.