# Preparation and Characterization of Gold Nanorods Coated with Gellan Gum and Lipoic Acid
## 3 Results and Discussion
### Preparation and Characterization of Coated AuNRs  
Although the cationic surfactant CTAB is a powerful stabilizing agent of lyophobic colloids, its toxicity avoids the use of CTAB-coated AuNRs in biomedical applications. For these reasons, gold nanoparticles need to be coated with molecules and/or macromolecules capable of physically stabilizing them as well as improving their cytocompatibility, allowing their use as drug delivery systems. The first coating agent used in this study is lipoic acid (LA), a natural molecule whose structure is characterized by the presence of an intracycle disulfide bond and a carboxylic functional group. The preparation procedure of the AuNRs/LA systems involved the incubation of the freshly synthesized AuNRs with lipoic acid in the presence of a reducing environment for the addition of glucose. The thiol/disulfide redox pair was thus exploited to obtain a stable interaction with the gold surface. According to data reported in the literature [34], gellan gum gels prepared in a basic environment have a reduced number of junction areas. In virtue of this, gel meshes are looser with increasing pH, an effect due to the greater electrostatic repulsive force exerted by the GG chains. Hence, the possibility of coating the nanorods in a basic environment has been investigated, with the aim to allow higher mobility of the polysaccharide chains and minimize aggregation phenomena. The preparation was therefore carried out at pH 7, to maintain an unfolded conformation of the GG during the coating procedure as well as to minimize the destabilization of the systems occurring at excessively high pH. To calculate the amount of lipoic acid interacted with the gold nanorods surface, an AuNRs/LA preparation was carried out using LA labeled with the Texas Red Hydrazide (TRH) fluorescent probe. LA-TRH was synthesized via carbodiimide reaction between the carboxylic group of LA and the hydrazide group of TRH and used for the preparation of AuNRs/LA-TRH. The system produced was used to quantify LA, comparing the fluorescence intensity values recorded with a calibration curve obtained with TRH standards. The calculated lipoic acid content was 20% w/w with respect to the gold weight. AuNRs/LA were thus coated with gellan gum, exploiting the abundance of hydroxyl groups to establish physical interactions with LA and the gold surface. The functionalization of AuNRs/LA with gellan gum was carried out incubating AuNRs/LA with an aqueous polymer dispersion at pH 7, leading to the formation of AuNRs/LA,GG. The rationale of gellan gum coating was the achievement of a second protective layer that further stabilizes the system, improving its cytocompatibility and allowing the future loading of bioactive molecules. This time, considering the heterogeneous sample, analysis of sulfur content of AuNRs/LA,GG was performed by X-ray photoelectron spectroscopy (XPS). The analysis showed a sulfur percentage of 0.97%, corresponding to 4.43% of LA with respect to the gold weight (Table 2). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Mw (Da)** & **Mn (Da)** & **PDI** \\ \hline GG sodium salt & 43,600 & 31,500 & 1.38 \\ GG-LA & 70,600 & 54,412 & 1.30 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Gellan gum (GG) sodium salt and GG-LA derivative weight average molecular weight, number average molecular weight, and polydispersity index. Finally, AuNRs/GG-LA were prepared by direct incubation of the nanorods pellet obtained after removal of the CTAB, with a polymeric dispersion of GG-LA in bidistilled water at pH 7 and in the presence of glucose. Scanning electron microscopy (SEM) was carried out on CTAB-AuNRs and AuNRs/LA,GG to characterize Au nanorods in terms of morphology and size (Figure 2). SEM acquisitions confirmed the rod-like shape with an aspect ratio (length/width) of 3 and average dimensions of 54 nm \(\times\) 18 nm (Figure 2a). Microscopies demonstrated that the coating was successfully performed (Figure 2b) and that no morphological changes occurred.