Multidentate-protected colloidal gold nanocrystals: pH control of cooperative precipitation and surface layer shedding.

Colloidal gold nanocrystals (AuNCs) with broad size tunability and unusual pH-sensitive properties have been synthesized using multidentate polymer ligands. Because they contain both carboxylic functional groups and sterically hindered aliphatic chains, the multidentate ligands can not only reduce gold precursors but also stabilize gold nanoclusters during nucleation and growth. The "as-synthesized" AuNCs are protected by an inner coordinating layer and an outer polymer layer and are soluble in water and polar solvents. When the solution pH is lowered by just 0.6 units (from 4.85 to 4.25), the particles undergo a dramatic cooperative transition from being soluble to insoluble, allowing rapid isolation, purification, and redispersion of the multidentate-protected AuNCs. A surprising finding is that when a portion of the surface carboxylate groups are neutralized by protonation, the particles irreversibly shed their outer polymer layer and become soluble in nonpolar organic solvents. Furthermore, the multidentate polymer coatings are permeable to small organic molecules, in contrast to the tightly packed self-assembled monolayers of alkanethiols on gold. These insights are important in regard to the design of "smart" imaging and therapeutic nanoparticles that are activated by small pH changes in the tumor interstitial space or endocytic organelles.