# Gold Nanoparticles and Nanorods in Nuclear Medicine: A Mini Review
## 2 Gold Nanoparticle and Nanorods: Preparations and Features
### AuNRs  
The chemical synthesis is a strong tool for the bottom-up approach in gold-based materials preparations [49; 63]. In particular for AuNRs, by the strict control of chemical procedures, it can be possible to control dimensions and monodispersity, as reported in Figure 3. [PERSON] et al. performed detailed studies on the growth mechanism of AuNRs and the role of seed growth [113]. They proposed changing in classic seed-mediated synthesis via the time-based control of seed and reactant concentration increasing the production of AuNRs more than 100 times with respect to the conventional concentration, while keeping autonomous control and narrow distribution of the nanoparticle dimensions, aspect ratio, and volume. In this way, they are confident that they cost-effectively meet the increased demand for large quantities of AuNRs in emergent applications. Moreover, AuNRs present amazing optical properties, such as two characteristic transverse and longitudinal surface plasmonic bands: the first in the visible (\(\sim\)520 nm) range, and the second in the near-infrared (\(\sim\)900 nm) region. This optical property is perfect for in vivo imaging and therapy, because the near-infrared region allows the maximum penetration of light into tissues. Additionally, the AuNRs surface chemistry allows multiple functionalization by various pharmaceutical agents. Capping molecules, such as cetyltrimethylammonium bromide (CTAB), can be replaced or conjugated  
Figure 3: (**a**) Different AuNRs volume in function of seed concentration; (**b**) AuNRs TEM images, scale bars 100 nm; (**c**) Comparing the conventional seed-mediated protocol and the two steps approach. Adapted from [113] with permission from the American Chemical Society (2017). with many molecules. PEG-modified AuNRs show a nearly neutral surface and have little in vivo cytotoxicity. The application of AuNRs in nuclear medicine has attracted great interest in the last few years [91; 95; 114]. PEG-modified AuNRs were radiolabeled with \({}^{131}\)I, and their in vivo biodistribution was investigated by intravenous injection into the rats [95]. Moreover, the in vivo stability and biological metabolism of the radiolabeled AuNRs in the rat's body were studied using a \(\gamma\)-camera. The obtained results showed the prominent uptake of \({}^{131}\)I-PEG-AuNRs in the blood after intravenous administration, which showed that the \({}^{131}\)I-PEG-AuNRs as whole particles are hardly sequestered on the gland-type organs. One of the reasons for the high blood retention of \({}^{131}\)I-PEG-AuNRs was AuNRs surface modification with PEG, which postponed the uptake of radio conjugates by the RES and influenced the biodistribution profile and the clearance kinetics. The PEG-coated AuNRs persisted in the blood for long time, but the accumulation in liver and spleen decreased drastically. This protocol revealed that \({}^{131}\)I-PEG-AuNRs are extraordinary stable in vivo condition. In a recent work, cyclic arginine-glycine-aspartate acid (RGD)-conjugated gold nanoparticles were used to follow integrins in high resolution by means of cryo-electron tomography: it is possible to localize AuNPs in cells and identify the precise interactors within cells [115; 116]. This development is a clear example of the AuNPs use of structural cell biology. [PERSON] et al. conjugated \({}^{131}\)I-labeled AuNRs with cyclic arginine-glycine-aspartic acid (RGD) and investigated its biological characteristics for the targeted imaging of integrin \(\alpha_{\mathrm{V}}\beta_{3}\)-expressing tumors [91]. The \({}^{131}\)I-labeled AuNRs-PEG-cRGD probe targeted selectively and could be taken up by tumor cells mainly via integrin \(\alpha\)-receptor-mediated endocytosis. In vivo imaging, biodistribution, and autoradiography studies showed tumor uptake in integrin \(\alpha_{\mathrm{V}}\beta_{3}\)-expressing tumors. For better investigation of inflammation, [PERSON] et al.