The size and shape of gold nanoparticles play a crucial role in determining their photochemical properties, especially in their ability to generate reactive oxygen species ROS for potential applications in photodynamic therapy PDT . Gold nanoparticles AuNPs have unique optical properties due to their localized surface plasmon resonance LSPR , which can be tuned by altering their size and shape. This LSPR effect can enhance the generation of ROS, which are essential for inducing cell death in PDT.1. Size: As the size of gold nanoparticles increases, their LSPR peak shifts to longer wavelengths redshift . This shift affects the absorption and scattering properties of the nanoparticles, which in turn influences their ability to generate ROS. Smaller AuNPs have a higher surface-to-volume ratio, which can lead to more efficient ROS generation. However, extremely small AuNPs may have reduced stability and increased toxicity, which could limit their applicability in PDT.2. Shape: The shape of gold nanoparticles also significantly affects their LSPR and, consequently, their photochemical properties. Spherical AuNPs are the most common and well-studied shape, but other shapes such as rods, stars, and cages have been investigated for their potential in PDT. For example, gold nanorods have a higher aspect ratio, which leads to a stronger LSPR effect and enhanced ROS generation compared to spherical AuNPs. Gold nanostars, with their multiple sharp branches, can also generate strong local electromagnetic fields, leading to efficient ROS production.Several factors should be considered when optimizing the size and shape of gold nanoparticles for photodynamic therapy:1. Absorption and scattering: The LSPR effect should be tuned to match the wavelength of the light source used in PDT to maximize the absorption and scattering of light, leading to efficient ROS generation.2. Cellular uptake and biodistribution: The size and shape of AuNPs can affect their cellular uptake and biodistribution in the body. Smaller and non-spherical nanoparticles may have better cellular uptake and tumor penetration, which is essential for effective PDT.3. Biocompatibility and toxicity: The size and shape of AuNPs should be optimized to minimize toxicity and ensure biocompatibility for safe use in PDT.In conclusion, the size and shape of gold nanoparticles significantly affect their photochemical properties, particularly their ability to generate reactive oxygen species for potential applications in photodynamic therapy. By carefully tuning the size and shape of AuNPs, researchers can optimize their properties for efficient ROS generation, cellular uptake, and biocompatibility, making them promising candidates for PDT applications.