## 3 Results and Discussion
This is true in the case of rhodamine 6G, where a higher enhancement was observed with the green laser, but the opposite occurs for riboflavin. This may be explained by the fact that the adsorption process of riboflavin is different as compared with R6G, which results in a greater plasmon wavelength of the gold substrate. As a result, the LSPR peak will be closer to the wavelength of the red laser and a greater enhancement can be achieved with this excitation source. Figure 6a shows the normalized absorbance of the gold nanoisland substrate without analyte and with R6G and riboflavin adsorbed on the surface. It can be seen how the plasmon wavelength of the gold substrate is redshifted by the different analytes. Initially, the LSPR wavelength of the clean substrate is at 532 nm, which is shifted to 551 nm with rhodamine 6G, and to 585 nm with riboflavin. The latter is closer to the wavelength of the 633 nm excitation source than to 532 nm. Furthermore, Figure 6b shows the spectra of the normalized absorbance of the two analyte solutions. In the 400-750 nm region, the absorption peak of riboflavin is located below 510 nm, while R6G has strong band in the 425-600 nm region. Comparing this with the excitation wavelengths, it can be concluded that a resonant Raman scattering could occur mainly for the R6G and for that only with 532 nm excitation. As a consequence, the SERS spectrum of this analyte can also be of resonant with 532 nm excitation. Figure 6: _Cont._  
Table 3 summarizes the plasmon wavelengths of the gold nanoisland substrates after the riboflavin and R6G solution was adsorbed on their surface. Furthermore, the table contains average EF values calculated for both riboflavin and rhodamine 6G based on the intensities of the peaks described above as a function of the two excitation sources (green and red). The standard deviation of these values was 3-5%. The initial plasmon wavelength of the gold nanostructures was in the range of 510-550 nm, which was already shown earlier [12; 36]. The cleaned substrates were verified, and there was no difference in the plasmon wavelength as compared with the as-deposited substrates. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **Plasmon Wavelength of** & \multicolumn{2}{c}{**SERS Enhancement Factor for**} & \multicolumn{2}{c}{**Plasmon Wavelength of**} & \multicolumn{2}{c}{**SERS Enhancement Factor for**} \\ **No.** & **Gold Nanoisland** & \multicolumn{2}{c}{**Different Excitations**} & \multicolumn{2}{c}{**Gold Nanoisland Substrate**} & \multicolumn{2}{c}{**Different Excitations**} \\ \cline{3-6}  & **Substrate with R6G, nm** & **532 nm** & **633 nm** & **with Riboflavin, nm** & **532 nm** & **633 nm** \\ \hline  
\hline \hline \end{tabular}
\end{table} 
Table 3: The plasmon wavelength of gold nanoislands SERS with different analyte solutions, and their associated EF values. Figure 6: Normalized absorbance of the gold nanoisland SERS substrate: (**a**) clean substrate, and with riboflavin and R6G solutions; (**b**) normal absorbance of analyte solutions. The data in the table demonstrate that, for the rhodamine 6G and riboflavin, the higher EF could be achieved with different laser excitations, which could also be connected with the adsorption of the analyte on the SERS substrate. The polarity and the polar surface area (PSA) can be used to quantify the adsorption. The PSA is defined as the surface area of a molecule that arises from oxygen or nitrogen atoms, plus hydrogen atoms attached to nitrogen or oxygen atoms [32]. Roughly, PSA is the ability of a molecule to form hydrogen bonds due to the presence of nitrogen and oxygen atoms [37]. For the riboflavin, it is 155 A\({}^{2}\), while for the R6G is 63.5 A\({}^{2}\)[38; 39]. As the PSA value is in a good connection with the contact angle [40], the latter was measured on glass and gold nanoisland substrates.