Since the presence of sialic acid residues in SERS spectra was observable, we evaluated the capability of ML tool to quantify sialic acid levels in the samples containing both fetuin and asialofetuin. Fetuin and asialofetuin were mixed with various molar ratios in total of 1 mM, and SERS spectra of the mixed samples were collected on the nanopapers functionalized with different BAs. The sialic acid levels were quantified using Gaussian regression models. SI Figure illustrates the regression results using 4MBA, while SI Figure to SI Figure show the results with 1TBA, 3MBA, 4ABA, and PyriBA. Among the five BAs, 3MBA exhibit the best quantification performance, with R 2 of 0.9920 and NMSE of 0.007982. This is probably because more spectral differences between fetuin and asialofetuin were observed on 3MBA substrates, such as the peak intensity change around 785 cm -1 and 999 cm -1 . We also quantified the sialic acid levels using the collective spectra.