Seeing Is Believing: How Does the Surface of Silver Nanocubes from a Polyol Synthesis Change during Sample Collection, Washing, and Redispersion

Abstract

While the synthesis of Ag nanocubes has been extensively studied, sample preparation (including collection, washing, and redispersion) after the synthesis has received far less attention. Herein, we leverage the unique capability of surface-enhanced Raman scattering to investigate how the solvent used for sample preparation affects the surface chemistry of Ag nanocubes. Our findings reveal that the use of an appropriate solvent for sample preparation plays a vital role in preserving the cubic shape. Crushing the reaction mixture with acetone before centrifugation greatly improves collection efficiency by inducing reversible aggregation among the particles. It also promotes the coadsorption of the carbonyl group from acetone and Cl^- ions on the Ag surface to suppress oxidative etching and thereby help preserve the cubic shape. Subsequent washing of the collected nanocubes with water or ethanol enables effective redispersion while facilitating the desorption of Cl^- ions and the adsorption of the carbonyl group from poly(vinylpyrrolidone). Collectively, these results underscore the importance of processing conditions after a colloidal synthesis in preserving the desired properties of Ag nanocubes for an array of applications.

1

SERS spectra of the Ag nanocubes suspended in the original reaction solution; after dilution by a factor of 4 in volume with acetone; after centrifugation and then redispersion in water; after centrifugation and redispersion in water; after centrifugation, washing with water once, and then redispersion in water; and after centrifugation, washing with water twice, and then redispersion in water. The concentrations of Ag nanocubes in all aqueous samples were kept roughly the same as that of the original reaction solution.

2

(a) Digital photograph, (b) UV–vis spectra, and (c) SERS spectra of Ag nanocubes dispersed in the original reaction solution or in EG–acetone binary mixtures with different ratios. The samples for UV–vis and SERS measurements were prepared from the original reaction solution by dilution with EG or acetone at a factor of 4.

3

SERS spectra recorded from the Ag nanocubes collected by centrifugation after being crashed with acetone while removing (a) 95% and (b) >99% of the supernatant, respectively, followed by redispersion in water, with the sample washed with water once.

4

A model detailing the surface of two Ag nanocubes suspended in the original reaction solution (top panel), after dilution with acetone (middle), and after collection by centrifugation and then redispersion in water (bottom).

5

SERS spectra of Ag nanocubes suspended in the original reaction solution or diluted by a factor of 4 in volume with EG, water, acetone, and ethanol, respectively. The sample was allowed to sit for 40 min prior to SERS measurement.