Tris(2,2'-bipyridine)ruthenium(II)-silver nanoparticle electrostatic nanoaggregates (AgNPs@[Ru(bpy)] ENAs) as novel SERS nanotags for rapid, sensitive and selective immunosensing.

Tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy)]), as a versatile molecule, has been widely applied in various fields, such as photocatalysis, electrochemiluminescence and fluorescence probes, solar cell and LED due to its excellent optical and electrical properties, good water solubility, high chemical stability. In this work, we prepared electrostatic nanoaggregates from [Ru(bpy)] and silver nanoparticles (AgNPs@[Ru(bpy)] ENAs) as a new type of SERS nanotags. Each [Ru(bpy)] ion carries two positive charges with strong affinity to negative surfaces, which enables a strong electrostatic interaction between [Ru(bpy)] and negatively charged silver nanoparticles (AgNPs) and fast (within 10 min) formation of AgNPs@[Ru(bpy)] ENAs. The prepared AgNPs@[Ru(bpy)] ENAs had a very strong and stable SERS activity due to abundant bipyridine molecules in [Ru(bpy)] and the location of many [Ru(bpy)] SERS reporters at the electromagnetic "hot spots" (i.e. the junction of two adjacent AgNPs), and thus could act as novel and excellent SERS nanotags. Further conjugated with antibodies, AgNPs@[Ru(bpy)] nanotags were used to develop new SERS-based immunochromatography test strips (SERS-ICTSs), showing excellent sensing performances. The AgNPs@[Ru(bpy)] ENAs based SERS-ICTSs not only inherit the merit of fast and visualize quantitative analysis from traditional ICTSs, but also realize much more sensitive biosensing (with detection limit of 25 pg/mL HCG) using the SERS technology.