The size and shape of gold nanoparticles have a significant impact on their photochemical properties, which include absorption, scattering, and localized surface plasmon resonance LSPR . These properties are crucial in various applications such as sensing, imaging, catalysis, and drug delivery. Here, we will discuss how the size and shape of gold nanoparticles affect their photochemical properties:1. Absorption and Scattering: The size and shape of gold nanoparticles determine their absorption and scattering cross-sections. Smaller nanoparticles have a higher absorption cross-section, while larger nanoparticles exhibit a higher scattering cross-section. This means that smaller gold nanoparticles are more efficient at absorbing light, while larger ones are more efficient at scattering light. The shape of the nanoparticles also influences the absorption and scattering properties. For example, rod-shaped nanoparticles have higher absorption and scattering cross-sections compared to spherical nanoparticles of the same volume.2. Localized Surface Plasmon Resonance LSPR : LSPR is a collective oscillation of free electrons in gold nanoparticles when they interact with light. The size and shape of gold nanoparticles play a crucial role in determining the LSPR frequency and its sensitivity to the surrounding environment. As the size of the nanoparticles increases, the LSPR peak redshifts shifts to longer wavelengths , and the peak broadens. The shape of the nanoparticles also affects the LSPR properties. For example, gold nanorods exhibit two LSPR peaks, one corresponding to the transverse mode short axis and the other to the longitudinal mode long axis . The longitudinal LSPR peak is highly sensitive to the aspect ratio length/width of the nanorods and can be tuned across a wide range of wavelengths.3. Photothermal Conversion: Gold nanoparticles can convert absorbed light into heat, a property known as photothermal conversion. The size and shape of the nanoparticles affect the efficiency of this conversion process. Larger nanoparticles and those with anisotropic shapes e.g., nanorods, nanostars have higher photothermal conversion efficiencies due to their enhanced absorption and scattering properties.4. Surface-Enhanced Raman Scattering SERS : The size and shape of gold nanoparticles can also influence their SERS properties. SERS is a phenomenon where the Raman scattering signal of molecules adsorbed on the surface of gold nanoparticles is significantly enhanced. Nanoparticles with sharp edges or corners, such as nanostars or nanocubes, can generate strong electromagnetic "hot spots" that lead to a higher SERS enhancement factor.In summary, the size and shape of gold nanoparticles play a critical role in determining their photochemical properties, including absorption, scattering, LSPR, photothermal conversion, and SERS. By controlling the size and shape of gold nanoparticles, their photochemical properties can be tuned for specific applications in areas such as sensing, imaging, catalysis, and drug delivery.