Step-by-Step Design and Synthesis of Au@SiO₂@Phenyl-azathiacrown for SERS-Based Specific Quantification of Inorganic...

Direct SERS-based quantification of inorganic metal species has been a problem, because they have a small Raman cross-section or even no vibrational mode. Here, we report a new strategy for SERS-based quantification of such metal species, as exemplified by inorganic mercury (Hg^II) in waters. Step-by-step design and synthesis from azathioethers [3, 9-dithia-6-monoazaundecane (DMA) and 3,6,12,15-tetrathia-9-monoazaheptadecane (TTM)] to an azathiacrown [7-aza-1,4,10,13-tetrathiacyclohexadecane (NS4)] demonstrate an improved S-pulling effect and size-fit specificity towards Hg^II to form Hg-S bonds. Modification of NS4 on the surface of Au@SiO₂ by using a 4-(bromomethyl)benzoic linker enabled direct SERS-based specific quantification of Hg^II for the first time, in which the ultrathin layer (ca. 2 nm) that covered the Au core (55 nm) could be a barrier preventing the Au core from having direct interaction with the Hg^II, and with phenyl serving as an internal standard (IS). The ratio of the Hg-S SERS band intensity at 270 cm^-1 to that of IS [(γCC+γCCC) at 1046 cm^-1] was practically proportional to the concentration of Hg^II, eliminating the inevitable uncertainties encountered in SERS-based measurements. Such a methodology is expected to pave a new way for SERS-based quantification of inorganic metal species when specific complexing substrates and suitable ISs are designed.