# Preparation and Characterization of Gold Nanorods Coated with Gellan Gum and Lipoic Acid
## 3 Results and Discussion
### Preparation and Characterization of Coated AuNRs
UV-VIS-NIR characterization of the newly produced nanorods (Figure 3) show that all three types of coatings are able to stabilize nanorods dispersions, but with some differences. In particular, GG-LA seemed able to prevent aggregation considerably, as it is possible to appreciate from the narrow longitudinal plasmon (\(\lambda_{\text{max}}\): 700 nm) of AuNRs/GG-LA. On the other hand, AuNRs/LA and AuNRs/LA,GG appeared properly stabilized by the coating agents used, although a slight widening of the longitudinal plasmons is registered. An interesting result lies in the differences between the maximum wavelengths of the three longitudinal plasmons. Indeed AuNRs/LA and AuNRs/LA,GG showed \(\lambda_{\text{max}}\) at about 740 and 730 nm, respectively, suggesting the possibility to achieve a more efficient photothermal effect with respect to AuNRs/GG-LA after laser exposition (810 nm). A study on the correct storage of the systems was carried out by performing lyophilization/redispersion tests in Milli-Q water and assessing their stability through UV-NIR analysis (Figure 4a). \begin{table}
\begin{tabular}{c c} \hline \hline
**Atom** & **\%** \\ \hline Au & 11.45 \\ S & 0.97 \\ C & 72.39 \\ O & 15.19 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Atomic abundance percentages of gold nanorods (AuNRs)/LA,GG measured by X-ray photoelectron spectroscopy (XPS) analysis. Figure 2: Scanning electron microscopy acquisitions of hexadecyltrimethylammonium bromide (CTAB)-gold nanorods (AuNRs) (**a**) and AuNRs/LA,GG (**b**) using magnifications 120,000\(\times\) (**a**) and 140,000\(\times\) (**b**). Scale bar: 500 nm. to note that both aqueous dispersions containing unmodified GG as well as derivatized GG, exhibit excellent stability after freeze-drying. These findings indicate that the presence of gellan gum can act as a cryoprotective agent of the proposed nanostructures. Consequently, it can be stated that AuNRs/LA,GG and AuNRs/GG-LA can be easily stored as lyophilize powder, a feature that increases their handling and ensures easy administration following immediate redispersion. The stability studies of the aqueous dispersion of AuNRs/LA, AuNRs/LA,GG, and AuNRs/GG-LA were carried out by UV analysis at 7 days intervals and for the total duration of 1 month. Figure 4b illustrates that AuNRs/LA and AuNRs/LA,GG were stable throughout the analysis time, not registering significant variations in terms of shifting or widening of the longitudinal plasmonic peak. In contrast, the AuNRs/GG-LA nanorods show signs of destabilization since the end of the first week of incubation. The aggregation of the sample appears more pronounced during the rest of the analysis, signaling an enlargement of the plasmonic band contextual to the decrease in the absorbance ratio between the longitudinal (about 700 nm) and the transversal (about 516 nm) plasmonic resonance peak, confirming the progressive destabilization of the system. This finding denotes that is not recommended to store AuNRs/GG-LA in aqueous dispersion. The nanorods produced were thus characterized in terms of hydrodynamic diameter and Zeta potential (Table 3). Hydrodynamic diameters obtained are in agreement with the forecasted values. It should be noted that the CTAB-AuNRs, AuNRs/LA, AuNRs/LA,GG, and AuNRs/GG-LA series possess increasing dimensions, with hydrodynamic diameters enlarging as a function of the progressive coating. From the comparison between the AuNRs/LA,GG and AuNRs/GG-LA systems, it has been seen that GG-LA-coated systems possess a higher hydrodynamic diameter. This result is probably attributable to the slight tendency towards self-aggregation of the GG-LA copolymer, which can lead to an increase in the size of the system. The Zeta potential data confirm the hypotheses formulated. In fact, the electrical surface potential decreases in accordance with the coating, registering strongly positive values for the system coated with the cationic surfactant and decreasing in modulus with the functionalization with LA.