# Gold Nanoparticles for Drug Delivery and Cancer Therapy
## 3 Cellular Uptake of Gold Nanoparticles and Cytotoxicity  
Nanoparticles are very small and comparable in size to many biomolecules. The properties of nanostructures are simple to modify to better suit their companion in a biological system. Gold particles that are modified with nuclear localization signal from simian virus (SV40) have been widely used for drug carrier to nucleus [43]. In many cases, the nano-peptide complex is injected directly into the cytoplasm near the nucleus. Tat peptide can be used to translocate Au NPs into the nucleus; recently, Au NP coated with mycobacterium, and a cell penetrating peptide were used to penetrate a membrane bound compartment inside the HeLa cells, where Au NPs are attached with peptides via linking agent [44]. Study shows that cellular uptake of Au NPs is dependent on surface functionalization. Key factors that determine the cellular uptake are the nature of the ligand, molecular weight, and grafting density. The report also states that cellular uptake is not significantly dependent on the size of Au NPs as seen in the laser scanning confocal microscopy and flow cytometry. However, in multicellular tumor spheroids, the smaller Au NPs had higher cellular uptake and advantages [45]. The cellular uptake of Au NPs also depends on the NP shape and the type of cell. A study was conducted to examine the effect of Au NP shape on RAW264.7 cells. The experimental results showed that the Au NPs with triangle shapes are more efficient than the rod shape in RAW264.7 cellular uptake, while the star-shaped Au NPs had the lowest cellular uptake [46]. Figure 2A shows the cellular uptake of the Au NPs, and Figure 2B-D are transmission electron microscopy images indicating the localizations of Au NPs. It is seen from Figure 2 that the NPs were internalized as single particles after incubation and were uptaken in vacuoles. However, the particles were not found in the cell nucleus [46]. Another study compares the post-transcriptional gene silencing, internalization, loading and release efficiencies with hollow Au nano-cages, nanorods and plasmons with siRNA. The result shows that nano-shells and nano-cages have the highest downregulation of green florescent protein after treatment. Hollow Au nano-shells load more than three times the amount of siRNA compared to nano-cages. The shape of the particle strongly affects the internalization of the Au NPs [47]. The Au  
Figure 2: Cellular uptake of P-GNPs (**A**) and transmission electron microscopy images of RAW 264.7 after 24 h of incubation with P-GNSs (**B**), P-GNRs (**C**), P-GNTs (**D**). Data represent mean \(\pm\) SEM (\(n=3\)). Statistical significance is represented by \({}^{*}p<0.05\), \({}^{**}p<0.01\), \({}^{***}p<0.001\)[46]. Reproduced from reference [46] with Springer Nature under the Creative Commons Attribution 4.0 International License ([[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/))). NPs with sizes ranging from 5 to 50 nm were investigated for genotoxicity in vitro and in vivo assays. In the comet assay, 20 and 50 nm Au NPs were found to not induce significant DNA damage in the HepG2 cells. However, the 5 nm Au NPs gave rise to a dose-related enhancement in the DNA damage. The 5 nm Au NP caused the cell cycle arresting in the G1 phase to produce the DNA damage. In a chromosomal aberration test using the Chinese hamster lung cells, it did not show any significant aberration. For the in vivo micronucleus test, no significant increase in micronucleus formation was found in mice after 4 days of exposure. For the two weeks of exposure, however, the 5 nm Au NPs were found to have significant clastogenic damage with an increase in micronuclei frequencies due to the dose [48]. Cytotoxicity of most Au NPs depends on various parameters such as size, cell type, tissue distribution, tissue absorption, and penetration capacity.