Layer-by-Layer Self-Assembly of Oppositely Charged Ag Nanoparticles on Silica Microspheres for Trace Analysis of Aqueous Solutions Using Surface-Enhanced Raman Scattering

A layer-by-layer (LbL) self-assembly strategy involving oppositely charged Ag nanoparticles was used to deposit a nanoshell of Ag nanoparticles on silica microspheres for trace chemical measurements in aqueous solutions by means of surface-enhanced Raman scattering (SERS). Positively charged Ag nanoparticles were produced by reduction of Ag nitrate in a solution mixture of branched polyethyleneimine (BPEI) and N-(2-hydroxyethyl)piperazine-N′-2-ethanesulfonic acid (HEPES) under UV irradiation whereas negatively charged Ag nanoparticles were synthesized by the conventional citrate reduction method. The density of Ag nanoparticles in the nanoshell exhibits a strong correlation with the layer number and the nanoparticle type. Thiocyanate (SCN⁻) and crystal violet and were used as model positively and negatively charged analytes respectively to assess the robustness of the resultant core–shell nanostructures for SERS measurements. High sensitivity, at ppt for crystal violet and ppb for SCN⁻, was obtained when the surface charge of the terminating Ag layer in the LbL self-assembled nanoshell was opposite to the ionic nature of the analyte of interest due to enhanced adsorption of the analyte to the Ag nanoparticles facilitated by strong electrostatic attraction. The microsphere-nanoshell structures were all individually SERS-active, making them excellent candidate platform for integration with microfluidic systems for in situ SERS-based sensing and measurements.