## 3 Results and Discussion  
Raman and SERS spectra of a molecule can be very different depending on the nature of the interaction of a molecule with the substrate surface. In order to give an idea to the reader, we provide a comparison of average intensity-normalized and baseline-corrected Raman and SERS spectra of PMT in Figure 3. In an interesting study, AuNPs-PMT conjugates were synthesized and characterized with the aim of utilizing AuNPs as drug carriers [35]. In that study, both Raman and SERS spectra of PMT were reported. Although the experimental conditions for both Raman and SERS spectral acquisitions are different, it is important to mention this study as spectral reference point. A bulk Raman spectrum of PMT is also provided in Figure S1. As expected, many new spectral bands on SERS spectra appears and there is almost no spectral overlap between two spectra indicating a high affinity of PMT to the AgNPs surface. Table S1 shows the tentative SERS band assignments for PMT and as it degrades under several experimental conditions based on the previous studies [20, 24, 36, 37]. The forced degradation studies indicated that the PMT substance degrades in solution via two main degradation pathways, hydrolysis of the amide linkage at low pH, and oxidation of the five-member ring of the pyrrolopyrimidine moiety [15, 19, 23]. Figure 4a shows the two principal reactive sites for the PMT molecule, where chemistry can take place. Figure 4b shows the main potential DPs of PMT, oxidative dimers, ring-opened keto-amine, lactams, des-glutamate and \(\alpha\)-hydroxy lactams. This information is important to be able interpret the spectral changes under stress conditions. The other aim is to investigate how stress conditions affect the SERS activity of AgNPs after exposing them with PMT and degradants following the forced degradation conditions. Figure 3: Comparison of Raman and SERS spectra of PMT. Figure 5 shows the SERS spectra of PMT exposed to NaOH at increasing times. The drug and the NaOH concentrations remain the same but the exposure times increase. An aliquot from the solution during incubation was taken and mixed with the AgNPs colloidal suspension. As seen, the spectral pattern remains almost the same with increasing NaOH exposure time but several changes in relative band intensities are apparent indicating the concentration changes of the molecular structures originating from their parent compound. In Table S1, the assignment of each band observed on the SERS spectra is provided. It was reported that the DPs in basic conditions are the oxidative dimer, ring-opened keto-amine, and \(\alpha\)-hydroxy lactams [18]. According to the study conducted by [PERSON] et al., 4.2% of PMT exposed 24 h to NaOH was degraded. In our previous study employing mesoporous silica-coated AgNPs as SERS substrates, we also reached the conclusion that NaOH does not cause a destructive effect on AgNPs [28]. From the consistent spectral patterns at increasing exposure times, it is clear that NaOH is compatible with AgNPs and they do not lose their SERS effect and the observed spectral changes is the result of possible mild degradation of PMT. Figure 6 shows the SERS spectra of PMT exposed to HCl at increasing times. The DPs in acidic medium are reported to be an oxidative dimer, ring-opened ketoamine, desglutamate, and lactams [15,18,20]. When PMT was exposed to HCl for 3 h, 15.32% of PMT degraded [20]. As seen in Figure 6, at shorter exposure times, SERS spectra are quite noisy and do not provide much molecular information. After 30 h exposure, new bands start to appear at 640, 1051, 1100, 1253, 1437, 1615 cm\({}^{-1}\) that can be assigned HCH bending, CH in plane bending, NH\({}_{2}\) rocking, CH\({}_{2}\) symmetric deformation and scissoring of NH\({}_{2}\), respectively [28,35].