# Internalization of Phospholipid-Coated Gold Nanoparticles
## 4 Results
### Endocytosis Occurs in C2C12 and A549 Cells and is Cell Type Specific
Moreover, in the first four hours there is little change in the amount of PCAuNPs in the cells. However, after 24 h, the intensity decreased to less than half, consistent with the images shown in Figure 5 and suggesting that the PCAuNPs are being expelled or degraded (or at least the fluorescent lipids disappear). The cluster density (i.e., the number of compartments containing PCAuNPs per square micron) remains low for the first four hours, but increases significantly after 24 h, consistent with a dispersal of the PCAuNPs within the cells. Significantly, the cluster density is much lower in the A549 cells, suggesting a more aggregated state for the same number of PCAuNPs. Correspondingly, the degree of aggregation (i.e., the number of PCAuNPs per compartment) decreases rapidly, consistent again with a dispersal of the gold nanoparticles (or the fluorescent lipids) with time. Again, the degree of aggregation is much greater throughout in the A549 cells. \begin{table}
\begin{tabular}{l c c} \hline  & **C2C12** & **A549** \\ \hline \(\langle F\left(\mathit{Rab}11\ |PCAuNPs\ \mathit{Rab}5\right)\rangle\) & \(0.23\pm 0.07\)\({}^{\ddagger}\) & \(0.04\pm 0.01\) \\ \hline \(\langle F\left(\mathit{Rab}5\ |PCAuNPs\ \mathit{Rab}11\ \right)\rangle\) & \(0.31\pm 0.12\) & \(0.03\pm 0.01\) \\ \hline \(\langle F\left(\mathit{Rab}5\ |PCAuNPs\ \mathit{Rab}7\right)\rangle\) & \(0.13\pm 0.03\) & \(0.02\pm 0.00\) \\ \hline \(\langle F\left(\mathit{Rab}5\ |PCAuNPs\ \mathit{cAMP}-1\ \right)\rangle\) & \(0.14\pm 0.03\) & \(0.05\pm 0.01\) \\ \hline \(\langle F\left(\mathit{Rab}7\ |PCAuNPs\ \mathit{Rab}5\right)\rangle\) & \(0.20\pm 0.07\) & \(0.05\pm 0.01\) \\ \hline \(\langle F\left(\mathit{Rab}7\ |PCAuNPs\ \mathit{cAMP}-1\ \right)\rangle\) & \(0.65\pm 0.17\) & \(0.01\pm 0.00\) \\ \hline \(\langle F\left(\mathit{LAMP}-1\ |PCAuNPs\ \mathit{Rab}5\right)\rangle\) & \(0.12\pm 0.01\) & \(0.02\pm 0.00\) \\ \hline \(\langle F\left(\mathit{LAMP}-1\ |PCAuNPs\ \mathit{Rab}7\right)\rangle\) & \(0.45\pm 0.23\) & \(0.06\pm 0.01\) \\ \hline \end{tabular} \({}^{\ddagger}\) Standard error.  
\end{table} 
Table 4: Fractions of marker distribution that co-localize with compartments containing PCAuNPs and one other marker. Figure 5: Images of phospholipid-coated gold nanoparticles after fixed exposure with variable uptake in A549 Cells. Confocal fluorescence microscopy images of A549 cells exposed to phospholipid-coated gold nanoparticles for 2 hours, washed, and re-incubated for 1, 2, 4 and 24 h respectively, then fixed. In summary, it appears that both cells types take up comparable numbers of gold nanoparticles, but that these are distributed in fewer and larger clusters (compartments) in the A549 cells. This is consistent with the observation from the cross-correlation experiments, that there are fewer compartments in the A549 cells that contain PCAuNPs. Phospholipid-Coated Gold Nanoparticles Aggregate Significantly with Continuous Exposure in A549 Cells  
In the previous section, we observed that the gold nanoparticles taken up initially will eventually disperse into more, but smaller clusters. We now examine what happens with continuous exposure of the cells to the PCAuNPs over a similar time period. The images in Figure 7 correspond to one confocal fluorescence microscopy image of one cell from each of the four samples prepared in the experiment, in which A549 cells were exposed to phospholipid-coated gold nanoparticles for 1, 2, 4 and 24 h, prior to fixation. The images show that there is a gradual increase in intensity and size of PCAuNP compartments over the 24 h.  
Table 5 shows the averages of the auto correlation amplitude, average fitted laser beam width, and average intensity from a data set of 20-30 images of A549 cells exposed to phospholipid-coated gold nanoparticles for 1, 2, 4, and 24 h.  
Figure 6: Fixed exposure, variable processing of phospholipid-coated gold nanoparticles in C2C12 cells and A549 cells.