# Gold Nanoparticles for Drug Delivery and Cancer Therapy
## 5 Gold Nanoparticle-Based Therapy  
Photothermal therapy (PTT) is a form of cancer therapy where NPs are embedded within the tumor and generate heat in response to exogenously applied laser light. Research shows that PTT is highly effective at cancer treatment. In addition, Au NPs are mostly used for PTT due to their superior biocompatibility, simple Au thiol bioconjugation chemistry for the attachment of desire molecules, small diameter that enable tumor penetration, efficient light to heat conversion, and the ability to be tuned to absorb near infrared light, which penetrates tissue more deeply than other light wavelengths. PPT is being implemented with other therapies as a combination, such as with chemotherapy and gene regulation [96]. Au NPs are promising in cancer theranostics due to their superior synthesis, modification and biocompatibility. The localized surface plasmon resonance and multivalent coordination effects on the NP surface are ideal for photothermal imaging, therapy-controlled drug release and targeted drug delivery [97]. NIR light has photons of longer wavelengths and less energy. NIR light penetrates deeper into living tissue but cannot carry out large amounts of photothermal reaction without external energy. However, Au NPs can convert NIR light into heat, which can then be applied to a specific site for hyperthermia. Hence, utilizing the Au NPs as photothermal agent can release heat which can damage or ablate tumor cells directly [98]. In chemo-photothermal synergetic cancer therapy, the co-delivery of multi-agents is being studied, where NPs can be used for drug delivery and photothermal effect. It shows promising results in breast cancer mouse models [99]. In cancer therapy, cell target imaging and efficient therapy are vital for tumor diagnosis and treatment. Multifunctional plasmonic NPs with high performance surface-enhanced Raman scattering (SERS)-imaging and NIR light-triggered plasmonic photothermal therapy of cancer cells in both the NIR-I and NIR-II were studied. The results show that Au nano-stars possess a broad NIR absorption band that covers both the NIR-I and NIR-II windows with good NIR SERS activity and photothermal effects. These nano-stars were designed to be multifunctional nano-agents by labeling Raman and conjugating arginine-glycine-aspartic acid, which serves as a cell targeted SERS-imaging tag. In vitro studies on A549 human lung adenocarcinoma cells show that these multifunctional nano-stars have high potential in SERS mapping and a wide spectrum of light mediated applications [100]. A customized hybrid and NIR light-triggered thermal responsive drug delivery system was introduced, where bupivacaine was used as a model drug, and Au hollow NPs were used as carriers. Their temperature responsiveness was studied and can be useful in PTT [101]. NIR-absorbing NPs, for use as photothermal cancer therapeutic agents, have been promising. A study was carried out to observe the properties of Au-Au sulfide NIR-absorbing NPs that provide higher absorption as well as better tumor penetration. They have the ability to ablate tumor cells in vitro and are effective for photothermal cancer therapy. The results show a significant increase in long-term tumor free survival [102]. Au NPs can also be used as a dose enhancer in cancer radiotherapy. When Au NPs are added to the patient, dose enhancement at tumor can increase the cancer cell kill while sparing the critical tissues surrounding the tumor. Figure 3 shows the dependences of dose enhancement factor (DER) vs. Au NP concentration for different prostate sizes using the 6 MV FFF (Figure 3A) and FF (Figure 3B) photon beams. It can be seen that the DER value increased with a reduction in prostate size. Moreover, higher DER values could be obtained by using the FFF (Figure 3A) photon beams compared to the FF (Figure 3B) [1]. For skin cancer radiotherapy, Au NPs are added to the skin lesion with different thicknesses irradiated by different energies of kV photon beams.