Reversible Aggregation Control of Polyvinylpyrrolidone Capped Gold Nanoparticles as a Function of pH

A simple procedure has been developed for aggregation of polyvinylpyrrolidone (PVP) capped gold nanoparticles by controlling the pH of the solution. PVP-capped gold nanoparticles were synthesized via wet chemical method. Transmission electron microscopy (TEM) and UV-vis spectroscopy techniques were employed for determining the morphological features and surface plasmon resonance (SPR) of the gold nanoparticles. When the pH of the solution containing PVP-capped gold nanoparticles was changed from 6.7 to 1.0, the color of the solution changed from deep red to blue due to SPR weakening by particle aggregate formation. The color of the solution returned to its original red when the pH of the solution was brought back to 6.7. Furthermore, citrate protection of the gold nanoparticles was found to inhibit the reverse action. This reversible/irreversible-selectable structural control of gold nanoparticles with drastic changes in their optical absorption characteristics may find various nanophotonic applications including contactless pH detection.