# Gold Nanoparticles and Nanorods in Nuclear Medicine: A Mini Review
## 2 Gold Nanoparticle and Nanorods: Preparations and Features
### AuNPs  
AuNPs can be synthesized by means of several processes such as chemical, physical, and biological methods [49; 50; 51; 52; 53; 54; 55; 56]. A well-known bottom-up approach is a wet reduction in the presence of a capping agent [57; 59; 60; 61; 62; 64]. The obtained AuNPs can be easily investigated by microscope techniques, such as transmission electron microscope (TEM), scanning electron microscope (SEM), and atomic force microscope (AFM) [98; 99; 100; 101; 102]. Capped AuNPs are prepared by using a water solution of tetrachloroauric acid as the gold precursor, a ligand molecule, and a reducing agent \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Morphology**} & **Gold Surface** & \multirow{2}{*}{**Size (nm)**} & \multirow{2}{*}{**Radionuclides**} & **Nuclear Medicine** & \multirow{2}{*}{**Ref.**} \\  & **Functionality** & & & **Applications** & \\ \hline \multicolumn{5}{c}{**AuNPs**} \\ \hline PEI & 3 & \({}^{99}\)Tc & SPECT/TC & [80] \\ Tannic acid & 40 & \({}^{124}\)I & PET & [30] \\ PEG & 30 & \({}^{198}\)Au & SPECT & [81] \\ PEG & 15 & \({}^{211}\)At & cytotoxicity & [82] \\ PEG & 30 & \({}^{177}\)Lu & brachytherapy & [44] \\ Amino-PEG & 30 & \({}^{99}\)Tc & SPECT/CT & [83] \\ PEG & 5 & \({}^{199}\)Au & SPECT, biodistribution & [84] \\ PEI & 3 & \({}^{99}\)Tc & SPECT/TC & [85] \\ PEG & 30 & \({}^{177}\)Lu & treatment of breast & [86] \\  & & & cancer & [86] \\ thiol mannose & 20 & \({}^{99}\)Tc & sentinel lymph node & [87] \\  & & & detection & \\ allylamine & 5 & \({}^{89}\)Zr & PET, biodistribution & [88] \\ PEG * & 10 & \({}^{18}\)F & PET & [89] \\ \hline \multicolumn{5}{c}{**AuNRs**} \\ \hline \hline \multirow{2}{*}{**s**} & PEG & 20-60 & \({}^{169}\)Yb & brachytherapy & [90] \\ PEG & 25-90 & \({}^{131}\)I & biodistribution & [91] \\ PEG & 20-90 & \({}^{125}\)I & bioactivity & [92] \\ polydopamine & 15-60 & \({}^{125}\)I & SPECT/TC & [93] \\ PEG & 15-50:15-75 & \({}^{64}\)Cu & PET, biodistribution & [41] \\ HPMA ** & — & \({}^{90}\)Y & treatment of prostate & [94] \\  & & & tumors & [94] \\ \hline PEG & 10-40 & \({}^{198}\)Au & SPECT, biodistribution & [81] \\ PEG & 10–25 & \({}^{131}\)I & biodistribution & [95] \\ PEG & 10–50 & \({}^{125}\)I & biodistribution & [96] \\ PEG & — & \({}^{125}\)I & SPECT, PET & [97] \\ \hline \hline \end{tabular}  
* PEG = polyethylene glycol, ** HPMA = N-(2-hydroxyethyl)methacrylamide.  
\end{table} 
Table 1: Functionalized AuNPs and AuNRs reported with the conjugated radionuclide and the application in Nuclear Medicine. PET: positron emission tomography, SPECT: single-photon emission computed tomography. Functionalizations can be produced on the gold surface using different synthetic strategies: (i) the method of ligand exchange; (ii) the introduction of bifunctional thiols; (iii) the use of many different capping agents throughout the synthesis mixture; and (iv) the derivation reaction in a post-synthesis procedure [99; 103; 104; 105; 106]. The ligand exchange is an easy and cheap mode to functionalize the gold surface, but frequently leads to a partial coverage that is short lasting and stable in time. In fact, this approach is based on an equilibrium reaction associated with the Nernst distribution [99]. The use of bifunctional thiols (HS---X), which presents a free end group X, such as thiol, acid, amine, ester, and ether, allows the particle interactions with the external environment and enhance post-synthesis modifications. It is also possible to introduce different capping agents at the same time: in this way, many superficial functionalities can be introduced, but this approach needs strict control of the synthetic parameters (pH, concentrations, reducing agent, temperature) to assurance good reproducibility and dispersity [107; 108]. The monodispersity is a key feature for the noble metal nanoparticles, which can be obtained by fine tuning experimental parameters during synthesis or by separation after synthesis [108; 109; 110].