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

Abstract

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 °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 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.

The pH-switchable agglomeration and dispersion behavior of fluorescent Ag nanoclusters and its applications in urea and glucose biosensing

“…The fast Fourier transform patterns of Figure 2g and h show that the Ag nanocrystals in the etching process maintained a good lattice, corresponding to metallic silver, whereas the small nanoclusters failed to possess a metallic crystal lattice because of the quantum size effect (insets in Figure 2d and f). 3 The loss of the lattice was accompanied by the appearance of the size-dependent physicochemical properties of Figure 3a and b describes the changes in the fluorescence and UV-vis absorption spectra during the etching process, showing that the fluorescence intensity at 428 nm and absorbance at 349 nm increased synchronously, and an obvious blue shift was observed at the UV-vis absorption peak within 12 h after the addition of GSH, which showed that the nanoparticles in the solution changed from larger nanocrystals to small nanoclusters, and the increasing absorbance showed the increase in the concentration of Ag NCs as the etching process progressed. 35,36 The color of the Ag colloid solution changed from the original dark brown (before adding GSH) to reddish-brown (72 h after the addition of GSH) and then to colorless after 168 h (Figure 3c), and the blue emission of the diluted Ag NCs solutions brightened correspondingly over time (Figure 3d).…”

“…To investigate the mechanism of the pH-dependent signal switching of the fluorescent Ag NCs, the HRTEM technique was used to obtain HRTEM images of the Ag NCs at different stages in one cycle of this reversible behavior. The original Ag NCs (pH 5) dispersed well in the solution (Figure 4e), and the Ag NCs gathered (Figure 4f) when the pH of the solution was adjusted to 7 by adding 0.1 M NaOH and incubated for 40 min at room temperature, whereas the Ag NCs would redisperse well in the solution (Figure 4g) when the pH was first adjusted to 7 and then to 5 again with 0.1 M NaOH and HNO 3 . The HRTEM results showed that under the experimental conditions, the Ag NCs possess the capability of reversible agglomeration and dispersion in aqueous solution with the change in pH.…”

“…Fluorescent metal nanoclusters composed of a small number of atoms have attracted research interest because of their unique properties and molecule-like behavior. 1,2 In recent decades, many fluorescent metal nanoclusters, especially nanoclusters of Au and Ag, among noble metals, have been reported, and the ligands that stabilize these small nanoparticles have a key role in their fundamental properties: (i) ligands can influence optical properties by transferring their charge to the metal cores or by directly donating delocalized electrons contained on them to the metal cores; 3,4 (ii) surface ligands can determine the physical properties of nanoclusters (for example, hydrophily and hydrophoby), and amphiphilic metal nanoclusters can be produced by skillfully controlling the type and amount of ligands; 5 (iii) the functional groups of the surface ligands determine the chemical properties of the metal nanoclusters; thus, fluorescent nanoclusters can be more widely used as probes for detecting various targets. 6,7 Moreover, published studies show that the optical properties of Au and Ag nanoclusters (Ag NCs) protected by thiols are more likely to be dependent on the ligands and functional groups on the nanocluster surface, 8 and because of good biocompatibility and active chelation to metal atoms, glutathione (GSH) has been widely used as a stabilizer to obtain fluorescent Au and Ag NCs for use in the biomedicine and biosensor fields.…”

“…In addition, to validate the practicability of the interesting pH-dependent R-AIQ phenomenon of the Ag NCs in the sensor field, we established urea and glucose sensors as an extended application study of the Ag NCs. According to reports that urea and glucose can change the pH of a solution by producing NH 3 and gluconic acid in the presence of urase and glucose oxidase (GOD), 30,31 we used the prepared Ag NCs as an optical probe to establish a regenerated sensing platform for detecting urea and glucose (Glu) in one sample, and the results were satisfactory. The fluorescence and ultraviolet-visible (UV − vis) absorption spectra measurements were conducted using an F-2700 fluorescence spectrophotometer (Hitachi, Japan) and a UV-2450 spectrophotometer (Shimadzu, Japan), respectively.…”

A facile synthesis of highly stable and luminescent Ag clusters: a steady-state and time-resolved spectroscopy study

“…Among various methods, template-based synthesis techniques are the most recognized ones. Different kinds of templates such as polymers, 19,24,25 proteins, 26,27 DNA, 21,28 oligonucleotides, 29,30 polyelectrolytes, 31 dendrimers, 32,33 and polymer microgels 34 have been used so far. Small molecules, especially those containing carboxylic or thiol groups, 35,36 were also utilized as stabilizers.…”