*2.3. Raman and SERS Analysis*

Conventional Raman and SERS experiments were performed with a Horiba Jobin-Yvon HR-Evolution spectrometer equipped with a microscope and a 632 nm laser. A motorized mapping stage was used for inspecting the sample and collecting the Raman signal from specific locations on the sample.

For conventional Raman measurements, a preliminary collection of spectra was carried out. Set conditions for the spectra acquisition were: 100× objective magnification and the laser intensity varied between 0.15 and 15 mW in order to maximize the Raman signal and to observe clear features of the investigated compounds. The Ag-reduced colloid was prepared according to the protocol developed by Leopold and Lendl [41]. Briefly, a solution of AgNO3 <sup>1</sup> × <sup>10</sup>−<sup>3</sup> M in MilliQ Water was prepared. Separately, the same volumes of a solution of NH2OH·HCl 6 × <sup>10</sup>−<sup>2</sup> M and a solution of NaOH 1 × <sup>10</sup>−<sup>1</sup> M were mixed together. Ten mL of NH2OH·HCl solution was added to 100 mL of AgNO3 solution under stirring, with direct formation of a colloid. The colloid was left under stirring for 20 min, and it was used after its production. Aggregation of the colloid was induced by dropping 20 μL of 0.01 M MgSO4 solution: 200 μL of Ag colloid was inserted in an Eppendorf tube and 20 μL of 0.01 M MgSO4 solution was added and stirred to induce nanoparticle aggregation. The aggregated colloid was poured on some colored fibers sampled from the stubbons, and it was left to dry. SERS spectra were acquired in correspondence of Ag nanoclusters close to the stubbon fibers or on them. In order to evaluate eventual spectral interferences deriving from the colloid, spectra of the blank (200 μL of Ag colloid and 20 μL of MgSO4) were also collected in the dried form. Set conditions for the SERS spectra acquisition were: 50× objective magnification, laser intensity varied between 0.15 and 0.38 mW according to the sample, maximum accumulation time of 5 s per scan, 60 scans maximum. Generally, six spectra for every typology of analyzed sample were acquired. For the spectral band assignment and compound identification, experimental spectra were compared to databases and literature [42–54].

#### **3. Results**
