# Preparation and Characterization of Gold Nanorods Coated with Gellan Gum and Lipoic Acid
## 1 Introduction  
Nanomaterials science constantly provides new approaches to overcome the limitations that lie in disparate fields of current technology. Biomedical research represents among the top areas that take advantage of nanomaterials' progress. As a consequence, there has been recently registered an expanding production of new nanometric devices that cover a wide panorama of biomedical applications such as targeted drug delivery, biosensing, and bioimaging [1; 2; 3]. In this scenario, a clear breakthrough has been achieved exploiting inorganic nanostructures, which endow to synergistically combine the advantageous dimensional characteristics of nanomaterials with their unique features including hyperthermia, conductivity, magnetism, and contrast properties [4; 5]. For these reasons, diverse nanometric inorganic materials, such as superparamagnetic iron oxide nanoparticles (SPIONs) [6; 7], graphene oxide [8; 9], silica nanoparticles [10; 11], quantum dots [12; 13], colloidal gold [14; 15], have been already successfully employed for drug delivery or theranostic purposes. Gold nanoparticles (AuNPs) are highly versatile inorganic nanoparticles that have paved the way for a wide range of new biomedical applications, with a special focus on cancer treatment. By modulating the synthesis procedures and parameters, AuNPs can be easily produced in a large variety of shapes and sizes, displaying different properties and usage [16; 17; 18]. As a noble metal, gold in nanometric size exhibits localized surface plasmon resonance (LSPR), a peculiar feature responsible for its optical behavior. When gold nanoparticles are irradiated with electromagnetic radiation with the wavelength corresponding to the LSPR band, free electrons oscillate harmoniously and in a localized manner around the particle lattice. Because of this phenomenon, diffuse electromagnetic waves can be used for the acquisition of diagnostic images using imaging techniques such as computed tomography (CT) or confocal microscopy. In addition, the energy of the absorbed electromagnetic radiation can be converted into heat, and for this reason, gold colloids are useful agents in photothermal therapy (PTT) of cancer. LSPR bands of AuNPs lay in the UV-Visible-Near InfraRed region (UV-VIS-NIR) although their definite position and number are dependent on the size, shape, and state of aggregation of the nanoparticles. It follows that the modification of the synthesis parameters allows to obtain AuNPs with tailored optical properties [19; 20]. In this context, cylindrical gold nanoparticles (nanorods-AuNRs) have gained increasing interest due to the facile synthesis and the position of their longitudinal LSPR located in the NIR region (wavelength: 650-900 nm), which overlaps the biological window where the absorption of water and hemoglobin (and hence of body tissues) is minimized. Therefore, the use of AuNRs allows to combine the possibility to obtain a targeted delivery (via Enhanced Permeability and Retention effect and/or active targeting), as well as the ability to exert cancer PTT with superior penetration and efficacy. Although colloidal gold stands out for its bioinertia, the use of capping agents is fundamental for biomedical applications [21]. The coating operation is manifold because it can stabilize AuNRs creating a protective hydrophilic film that prevents aggregation in the aqueous physiological environment, further replacing cytotoxic stabilizing agents used during the synthesis procedure. Besides, the choice of appropriate coating agents allows to increase cytocompatibility, as well as to load therapeutic molecules and active targeting functionalities, obtaining a targeted drug delivery. Lipoic acid (LA) is a small molecule naturally found in the human body that possesses a metal chelating activity and proven ROS scavenging ability [22; 23; 24].