# Gold Nanoparticles and Nanorods in Nuclear Medicine: A Mini Review
## 2 Gold Nanoparticle and Nanorods: Preparations and Features  
The peculiar chemical-physical features presented by gold-based nanomaterials make them an excellent platform in nanomedicine, especially in the case of AuNPs and AuNRs [71; 72]. As it is well-known, these materials show a large surface to volume ratio that increases the interaction with the surrounding biochemical medium. They are easy synthesized and functionalized by chemical routes. The Turkevich process [73] produces highly stable AuNPs with dimensions in the range of 20 to 50 nm, wherein the capping agent is citrate, giving a net negative superficial charge that prevents particle aggregation. The Schiffrin-Brust method [74] is the most used synthetic route, which is always based on the reduction in two phases of Au\({}^{3+}\) using different capping agents and reduction agents. Alongside these well-known approaches, the seed-mediated growth and use of bacteria are the other usual preparation methods to obtain gold-based nanomaterials, which are inert and stable [75; 76].  
Further, gold-based nanomaterials possess local surface plasmonic resonance (LSPR), which can be modulated by variations in shape, dimension, and aggregate formation [59; 77; 78]. The size and shape of particles modify the local electron confinement, which produces a variation in the LSPR absorption maxima. These properties endure evident changes when an anisotropic system is proposed, such as in the synthesis of nanorods instead of nanoparticles. AuNRs present strong plasmon resonance absorption, with both transversal and longitudinal LSPR peaks in two different spectrum ranges: the first in the visible range, and the second in the infrared (IR) region of electromagnetic radiation.  
The use of AuNPs and AuNRs often requires specific functionalization wherein they are conjugated with a target-specific ligand. The ligand molecule was anchored with a gold surface but preserved a free ending functional group, which was able to form successive bond. Essentially, there are three ways for the introduction of targeted ligands on a gold surface. (i) First, there is physical adsorption, which occurs by means of electrostatic or hydrophobic interaction. It is a rapid process and avoids the need for a complex synthesis process. Nevertheless, any modification in experimental parameters (pH, ionic strength etc.) produces a detachment of the ligand molecule. (ii) Second, there is a covalent bond that in general is based on the strong bonding interaction between the gold surface and S or N atoms (alkanethiolates, alkaneamines glutathione, cysteine, xanthates, anthocyanins, disulfide, thioethers, aminoethers, dithiocarbamate). (iii) Lastly, there is an explicit tag with specific ligand molecules to obtain the desired target.  
AuNPs and AuNRs are easily conjugated with radionuclides for diagnostic or therapeutic purposes. In Table 1, examples of AuNPs and AuNRs are reported together with: (i) the dimension and  
Figure 1: Key aspects of gold nanoparticles (AuNPs) and gold nanorods (AuNRs) in nuclear medicine.  
(ii) surface functionalization of gold-based nanomaterials before the radionuclides conjugation, (iii) the conjugated radionuclide, (iv) the application in nuclear medicine, and (v) the reference to recent literature. All of this is presented with the aim of providing readers with a useful guide for orientation in this sector.  
In several of these cases, the gold surface functionalization was realized by polyethylene glycol (PEG) coating and showed increased therapeutic efficacy of the formulation for radiosensitization. In fact, PEG covering reduces the AuNPs recognition by RES and products the circulation half-life. The RES involves cells descending from the monocytes, which are able to perform the phagocytosis of extraneous materials and particles [79]. The following paragraphs will illustrate the possibilities and advantages of using AuNPs and AuNRs in biotechnological applications, focusing on the field of nuclear medicine.