# Gold Nanoparticles for Drug Delivery and Cancer Therapy
## 2 Synthesis and Properties
effect on the size, morphology and polydispersity of the Au NPs. Size specifically can be changed by varying the pH of solution or solvent type [35]. In addition, pH was used to control the particle size in glutathione-capped Au NPs, where the size ranges from 5.5 to 8.0 nm. This method is based on polymeric NPs precursors, Au (I) glutathione polymers, that changes size and density based on the pH A lower pH favors larger and denser precursors, which leads to larger Au NPs, and higher pH favors smaller and denser precursors resulting in smaller Au NPs [36]. In the seed-mediated synthesis of Au NPs, the chemical components determine the shape and rate of the NP formation. Halide is used to adjust the reduction potential of the Au ions and species in the solution and passivate the particle surface. Therefore, Halides can be used to control the reaction kinetics and enable the selective synthesis of NPs with different shapes [37]. Silver ions are also used to access various particle shapes by controlling the growth of Au NPs through surface passivation. The density of the Ag coverage can be adjusted by the concentration and type of the halide in the solution [38]. Gold silica nano-shells consist of silica cores within a tiny overly of Au, where the first Au particle is easily tunable to the near infrared (NIR) [39]. Seed-mediated growth is used to fabricate solid silica nano-shell, where the seeds of Au colloid are linked to the silica cores. They can induce cancer cell kill by photothermal effect. In vitro silica-based Au nano-shells were tested as targeted therapy probes for prostrate, breast, brain and liver cancer. In scatter-based imaging, the large size of the nano-shell provides clear advantage. Gold nanorods are smaller than Au nano-shells and they can easily be adjusted to NIR region by simply modifying their aspect ratio. Their small size allows them to have high absorption coefficients and narrow spectral bandwidths. The Au nanorods have two absorption peaks for the longitudinal and transverse resonances. Longitudinal resonance can range from the visible to the NIR wavelength, where transverse resonance occurs at 520 nm. Gold nanorods have superior cross-section vs. Au silica and Au-Au sulfide NPs, when normalizing for the particle size differences. Their heating per gram of Au is at least 6 times faster than Au silica nano-shell, but Au silica has significant larger photothermal transduction cross-section per particle basis. Gold-gold sulfide NP has small size. Some examples of Au-Au sulfide are hollow Au nano-shells and Au nano-cages, where both have delivery advantage for photothermal ablation over larger Au NPs. Hollow Au NPs consist of a thin Au shell with a hollow center. Gold hollow shells can be fabricated using Co or Ag NPs, then oxidizing this material with chloroauric acid. Using this process, a thin Au shell can be created. Gold nano-cages are synthesized by creating a template of Ag NPs, then replacing the Ag with Au. By tuning the nano-cage wall thickness, the absorption peak can be changed from 400 to 1200 nm with an edge length in the range of 30-200 nm. Gold colloidal nanospheres are widely being studied in hyperemia applications and image probe. Their small size and simple synthesis make them ideal for converting light into heat in the photothermal therapy (PTT) [40]. Au NPs can be used as a photothermal agent to generate tumor heating and can subsequently be depleted using laser exposure. Two Au nanorods are compared with single core and multicore. The generated heat was measured in water dispersion and in cancer cells using lasers at the wavelengths of 680, 808 and 1064 nm. It is found that the Au nanospheres provide no significant heating in water while Au nanorods are efficient in the first biological window (NIR-I), and only nano-raspberries heat in the second biological window (NIR-II) [41]. The plasmonic superstructure of Au NPs can have enhanced photothermal effects and properties.