Transition from Nanoparticles to Nanoclusters: Microscopic and Spectroscopic Investigation of Size-Dependent Physicochemical Properties of Polyamine-Functionalized Silver Nanoclusters

In this paper, an interesting process is described in the synthesis of silver nanoclusters capped by hyperbranched polyethyleneimine (PEI): a transition from nanoparticles to nanoclusters takes place spontaneously over time accompanied with the reappearance of size-dependent physicochemical properties of silver nanoclusters. The transmission electron microscopy images, X-ray powder diffraction patterns, and fluorescence and UV-visible spectra accurately record this process. As PEI-capped Ag particles were prepared by a heating process at 90 degrees C for 10 min and then a cooling process to ambient temperature, the average diameter changed from 5.3 nm after 24 h to 3.9 nm 168 A h later, and the percentage of the particles of 1.0-2.0 nm increased significantly over 168 h. Meanwhile, the color of Ag colloid solutions changed from deep reddish brown to bright yellow with an obvious enhancement of fluorescence intensity. The smart ligand shell of PEI plays a key role in the size transition. It could tailor the nanoparticles to just the right size of nanoclusters, resulting in the color change and fluorescence recovery. Therefore, this work clearly exhibits the reappearance of size-dependent physicochemical properties of silver nanoclusters with size reduction in the range where the transition from metallic to molecular behavior takes place.