Reporting SERS enhancement factor: molecular surface coverage standards by grazing-incidence X-ray fluorescence.

We used reference-free grazing incidence X-ray fluorescence to measure the density of four thiol Raman-active molecules assembled on gold surfaces. This value is needed to reliably quantify the number of molecules contributing to the SERS signal in a plasmonic substrate in the effort to standardize the enhancement factor calculation and such molecules constitute suitable probes for benchmarking SERS sensors.