The Fig. is giving the SERS spectrum from Sherman et al., 2020. The researchers "conducted preliminary studies to optimize the metabolite SERS response while maintaining a method that can be used in future high-throughput experiments" . L-cysteine was one of the substances used to evaluate some standard SERS substrates. The researchers opted for Ag colloids prepared with sodium citrate. The reason is in a superior signal enhancement; moreover, the presence of some functional groups in biological molecules, such as thiols and amines, is relevant for their "strong affinity to silver" (Sherman et al., 2020, and references therein). Also, "the colloid substrate prepared with sodium citrate produced a smaller background signal" compared to other substances. In the Fig. of Brolo and coworkers we can find the SERS spectra of L-Cys, which has been adsorbed on the surface of an electrochemically roughened silver electrode, from measurements at different applied potentials. "The band at ca. 665 cm -1 was predominant for potentials more positive than -600 mV (Figure -c [of ). This band can be assigned to the C-S stretching mode. The frequency of this mode is red-shifted from around 690 cm -1 for L-Cys in solution". "This shift indicates that the sulfur atom of L-Cys should be directly bonded to the surface" . When the potential becomes "more negative than -600 mV", we can see new SERS bands. "The SERS intensity for the C-S stretching mode does not change significantly as the potential becomes negative, indicating that L-Cys does not leave the Ag surface in the potential range studied. … Moreover, all spectral features observed at potentials more negative than -600 mV (Figure and 4e [by Brolo and coworkers]) can be assigned to L-Cys vibrations" , see references therein). We can find also assignments of the bands. "The shoulder at 630 cm -1 and the peaks at 799 and 905 cm -1 can be related to the COO-wagging, the COO-bending, and the C-COO-stretching, respectively. All of the new SERS bands involved vibrational modes related to the carboxylate group" .