## 3 Results
### FT-IR Spectroscopy  
#### 3.5.1 Characterisation Laboratory Sample  
Compositional information on the banknote and the applied colourants was obtained through FTIR spectroscopy. Analyses conducted on the unpainted white areas of the banknote confirmed the cellulosic nature of the paper support, with characteristic peaks at 3335 and 3294 cm\({}^{-1}\) (O-H stretching), 2920 and 2852 cm\({}^{-1}\) (CH\({}_{2}\)/CH\({}_{3}\) stretching), 1643 cm\({}^{-1}\) (O-H bending), the region between 1363 and 1000 cm\({}^{-1}\) (pyranose ring and glycosidic bond vibrations), and between 1000 and 700 cm\({}^{-1}\) (glucosidic ring deforma \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Coating** & **Non-Treated** & \multicolumn{2}{c}{**Non-Aged**} & \multicolumn{2}{c}{**Aged**} \\ \cline{3-5}  & **VIS** & **UV** & **VIS** & **UV** \\ \hline CS & & & & \\ CS/CNC3 & & & & \\ CS/CNC6 & & & & \\ CS/CNC1.5 & & & & \\ \hline CS/CNC1.5 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Optical microscopy of coated painted areas (red, blue, yellow, green)—VIS/UV, at several magnifications, before and after ageing (scale bar: 0.2 mm, located in the bottom right corner). tions) [71; 72; 73]. A weak absorption band at 1724 cm\({}^{-1}\), associated with C=O stretching, suggests the presence of hemicellulose or minor lignin residues (Figure 3) [72]. Figure 3: FTIR spectra collected on black ballpoint pen applied to both non-aged and aged areas. Applications are compared with both white and purple substrates, as they were applied to regions with distinct characteristics (white for the non-aged side and purple for the aged side). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \(\Delta\)**E Before Coating:** & \(\Delta\)**E Before vs. After** & \(\Delta\)**E Coated:** \\
**Marker** & **Non-Aged vs.** & **Coating** & **Non-Aged vs.** \\  & **Aged** & **Application** & **Aged** \\ \hline Black & 14.86 & 2.46 & 1.99 \\ Red & 6.67 & 1.24 & 0.85 \\ Light blue & 5.42 & 1.94 & 2.15 \\ Blue & 18.20 & 4.27 & 6.90 \\ White & 1.37 & 0.58 & 0.51 \\ Yellow & 4.56 & 1.53 & 2.03 \\ Green & 1.10 & 2.03 & 1.89 \\ Pink & 3.57 & 1.41 & 1.67 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Colour differences (\(\Delta\)E) measured on each treated area, comparing the condition before and after coating application and before and after artificial ageing (0.2 < SD < 0.8). The purple area of the banknote showed a similar profile but with increased intensity at 1727 cm\({}^{-1}\) (C=O stretching) and 1427 cm\({}^{-1}\) (asymmetric stretching of CO\({}_{3}\)\({}^{2-}\)), supported by signals at 1792, 867, and 704 cm\({}^{-1}\), indicating the possible presence of carbonate compounds [74; 75]. The additional peak at 1584 cm\({}^{-1}\) suggests the presence of aromatic structures, likely from dye components [76; 77]. Spectra collected on the black marker, both aged and unaged, reflected contributions from both the substrate and the marker dye. New bands at 1585, 1530, and 1255 cm\({}^{-1}\) were attributed to C=C aromatic stretching, C=N vibrations, and C-N/C-O-C modes, compatible with triarylmethane-based dyes [78; 79]. The UniPosca acrylic markers showed spectra distinct from the substrate, confirming the presence of a synthetic acrylic medium. Peaks in the 2969-2858 cm\({}^{-1}\) region corresponded to aliphatic stretching, while the strong band at 1737 cm\({}^{-1}\) reflected C=O stretching typical of acrylic esters. Additional signals between 1443 and 1200 cm\({}^{-1}\) were attributed to C-O and C-C vibrations within the polymer matrix [69; 70; 80; 81; 82]. The red marker presented vibrational features coherent with a naphthol-based azo pigment, with peaks between 1670 and 813 cm\({}^{-1}\) associated with N=N and C=C stretching, and C-N/C-O modes (Figure S1) [83; 84; 85]. The yellow marker displayed a distinct profile, consistent with diarylide pigments: the diagnostic bands between 1592 and 860 cm\({}^{-1}\) corresponded to N=N bonds, aromatic ring modes, and substituted ring deformations (Figure S2) [85].