Destabilization of PVA-stabilized Ag NPs: color changes at low aqueous concentrations, induced by aggregation and coalescence

Aqueous concentrations of poly(vinyl alcohol) (PVA)-stabilized similar to 10 nm silver nanoparticles (Ag NPs), in the 1000 ppm concentration range, have been shown to be highly stable at elevated temperatures. However, lower concentrations of these NPs undergo color changes, without precipitation, when heated or when held for extended periods of time at room temperature. We have studied their optical and morphological changes at 80 degrees C, using UV-vis spectra and TEM, and found that their color, at a concentration of 10 ppm, changes from yellow to claret-red to black without precipitation. Further, the plasmon resonance peak at similar to 400 nm diminishes as a new peak develops at similar to 550 nm. These changes occur as the previously well-dispersed NPs (yellow color) agglomerate to chains (claret-red color) and, finally, coalesce (black color). We discuss the cause of the instability.