Controlled growth of Ag nanoparticles decorated onto the surface of SiO2 spheres: a nanohybrid system with combined SERS and catalytic properties

Abstract

A versatile water-based method for depositing silver nanoparticles (Ag NPs) with controllable and uniform metal size onto the surface of silica (SiO₂) spheres is reported. The hybrid particles exhibited a raspberry-like morphology, and their potential for SERS and catalytic applications has been demonstrated. SiO₂ spheres (∼120 nm) were synthesized first and modified with polyethyleneimine (PEI) to introduce amine surface functionalities. The amine groups were coordinated with silver ions (Ag⁺) and reduced to Ag seeds (∼4 nm), uniformly distributed onto the SiO₂ surface (SiO₂@Ag-seed). In order to improve the optical responses and catalytic activity of SiO₂@Ag-seed system, two subsequent silver growth steps were performed. The diameter of Ag seeds was increased to 12 and 19 nm, respectively, hereafter denoted as SiO₂@Ag-1 and SiO₂@Ag-2. The immobilization and controlled growth of Ag NPs was confirmed by UV-vis spectroscopy, and scanning and transmission electron microscopy (SEM and TEM, respectively). All specimens displayed satisfactory SERS activity increasing with the Ag NP size, showing clear Raman peaks of Rhodamine 6G (R6G) at very low concentration. The SiO₂@Ag particles were also tested and compared for their catalytic efficiency towards the reduction of 4-nitrophenol (4-Nip) by NaBH₄. The principal advantages of this study lie in the ability to tune the Ag NP size, the long-term colloidal stability of all fabricated SiO₂@Ag systems in aqueous media, and the limited use of hazardous chemicals and pollutant organic solvents during the synthetic process.