*2.1. Infrared Spectroscopies (μ-FTIR and r-FTIR)*

Micro-FTIR (μ-FTIR) analysis was carried out in transmission mode using a Nicolet iS50-FTIR coupled with a Thermo Nicolet Continuum infrared microscope equipped with an MCT detector. Spectra were collected in the 4000–600 cm−<sup>1</sup> range with a 4 cm−<sup>1</sup> resolution and 128 scans and using the Thermo Scientific OMNIC 9.0 software package.

Reflectance FTIR (r-FTIR) was carried out using a portable Bruker ALPHA spectrometer equipped with an external reflection module and a DTGS detector. Spectra were collected in the 7000–400 cm−<sup>1</sup> range and a 4 cm−<sup>1</sup> resolution over 128 scans. Measurements were made in non-contact mode over a spot size of approximately 4 mm in diameter and a 1 to 1.5 cm distance from the object (the focal distance from the surface was adjusted using the instrument's built-in camera). The background was periodically acquired using a built-in flat gold mirror. The spectra were converted to pseudo-absorbance.

Spectra were examined using the Spectral Search and Multicomponent Search tools available in the Thermo Scientific OMNIC Specta 2.0 software, as well as the IRUG FTIR spectral database [17]. Both original and Kramers–Kronig (KK) transformed r-FTIR spectra were evaluated.

#### *2.2. Raman Spectroscopies (Confocal Raman, p-Raman, SERS)*

Confocal Raman spectra (Raman) were collected using a Renishaw In-via Raman system equipped with a 785 nm diode laser at powers between 0.3 to 3 mW, a 1200 lines/mm grating, and a Leica confocal microscope with a 100x objective. Final spectra represent an average of five acquisitions of 10 s. AgNPs for SERS were synthesized using the Lee-Meisel method [18] and measurements were obtained before and after using acid pretreatments for hydrolyzing colorants into the colloidal solution [19] (see supplementary information (SI) for a complete description). SERS spectra were collected using a 532 nm diode laser at powers between 0.5 to 2.5 mW, 1800 lines/mm grating, and a Leica confocal microscope with a ×100 objective. Final spectra represent an average of five acquisitions of 10 s. Portable Raman (p-Raman) spectra were acquired using a Bruker BRAVO Handheld Raman spectrometer equipped with 785 nm and 853 nm diode lasers (DuoLASERTM), a resolution of 10–12 cm−<sup>1</sup> and 400 ms exposure time over 30 acquisitions. The output laser is ≈ 50 mW and spot size diameter 1 mm) measurements were carried in non-contact mode at 1–2 mm to prevent thermal degradation or damage to the object. All Raman spectra were evaluated using Omnic Specta.

Reagents for surface-enhanced Raman (SERS) spectroscopy: silver nitrate (AgNO3) (99.9999% trace metals basis) and sodium citrate (≥99%, FG) were used for silver nanoparticle (AgNPs) synthesis. Potassium nitrate (KNO3) (BioXtra, ≥99.5%) was used for AgNP aggregation. Nitric acid (HNO3) (ACS Reagent, 70%) and hydrofluoric acids (HF) (ACS Reagent 48%) were used for colorant hydrolysis into AgNP colloid. All materials were purchased from Millipore-Sigma.
