"Elastic" property of mesoporous silica shell: for dynamic surface enhanced Raman scattering ability monitoring of growing noble metal nanostructures via a simplified spatially confined growth method.

Abstract

The Raman enhancing ability of noble metal nanoparticles (NPs) is an important factor for surface enhanced Raman scattering (SERS) substrate screening, which is generally evaluated by simply mixing as-prepared NPs with Raman reporters for Raman signal measurements. This method usually leads to incredible results because of the NP surface coverage nonuniformity and reporter-induced NP aggregation. Moreover, it cannot realize in situ, continuous SERS characterization. Herein, we proposed a dynamic SERS monitoring strategy for NPs with precisely tuned structures based on a simplified spatially confined NP growth method. Gold nanorod (AuNR) seed NPs were coated with a mesoporous silica (mSiO2) shell. The permeability of mSiO2 for both reactive species and Raman reporters rendered the silver overcoating reaction and SERS indication of NP growth. Additionally, the mSiO2 coating ensured monodisperse NP growth in a Raman reporter-rich reaction system. Moreover, "elastic" features of mSiO2 were observed for the first time, which is crucial for holding the growing NP without breakage. This feature makes the mSiO2 coating adhere to metal NPs throughout the growing process, providing a stable Raman reporter distribution microenvironment near the NPs and ensuring that the substrate's SERS ability comparison is accurate. Three types of NPs, i.e., core-shell Au@AgNR@mSiO2, Au@AuNR@mSiO2, and yolk-shell Au@void@AuNR@mSiO2 NPs, were synthesized via core-shell overgrowth and galvanic replacement methods, showing the versatility of the approach. The living cell SERS labeling ability of Au@AgNR@mSiO2-based tags was also demonstrated. This strategy addresses the problems of multiple batch NP preparation, aggregation, and surface adsorption differentiation, which is a breakthrough for the dynamic comparison of SERS ability of metal NPs with precisely tuned structures and optical properties.