The size and shape of nanoparticles have a significant impact on their photochemical properties, including absorption and emission spectra. In the case of gold nanoparticles, these properties are primarily influenced by the localized surface plasmon resonance LSPR phenomenon. LSPR occurs when the conduction electrons in the nanoparticle oscillate collectively in response to the incident light, leading to enhanced absorption and scattering of light.1. Size effect: As the size of gold nanoparticles increases, the absorption and emission spectra shift towards longer wavelengths redshift . This is due to the increased interaction between the conduction electrons and the nanoparticle surface, which results in a decrease in the restoring force acting on the electrons. Consequently, the LSPR peak shifts to lower energies longer wavelengths . Additionally, larger nanoparticles exhibit broader LSPR peaks, which can be attributed to increased radiative damping and inhomogeneous broadening.2. Shape effect: The shape of gold nanoparticles also plays a crucial role in determining their photochemical properties. For instance, anisotropic shapes like nanorods, nanostars, and nanoprisms exhibit multiple LSPR peaks corresponding to different plasmon modes. These modes are associated with the oscillation of electrons along different axes or directions of the nanoparticle. The position and intensity of these peaks are highly dependent on the aspect ratio length/width and the sharpness of the nanoparticle's features.In summary, the absorption and emission spectra of gold nanoparticles are strongly influenced by their size and shape. Larger nanoparticles exhibit redshifted and broader LSPR peaks, while anisotropic shapes give rise to multiple peaks corresponding to different plasmon modes. By controlling the size and shape of gold nanoparticles, it is possible to tune their photochemical properties for various applications, such as sensing, imaging, and photothermal therapy.