The size and shape of gold nanoparticles play a crucial role in determining their surface plasmon resonance SPR properties. Surface plasmon resonance is a phenomenon where the conduction electrons in the nanoparticle oscillate collectively in response to incident light, leading to strong absorption and scattering of light at specific wavelengths. This unique property of gold nanoparticles has led to their widespread use in various applications, such as sensing, imaging, and drug delivery.The size and shape of gold nanoparticles influence their SPR properties in the following ways:1. Size: As the size of gold nanoparticles increases, the SPR peak shifts towards longer wavelengths redshift . This is because larger nanoparticles have a higher number of conduction electrons, which leads to stronger interaction with the incident light and a redshift in the SPR peak. Additionally, larger nanoparticles exhibit broader SPR peaks due to increased damping and scattering effects.2. Shape: The shape of gold nanoparticles has a significant impact on their SPR properties. Different shapes, such as spheres, rods, cubes, and stars, exhibit distinct SPR peaks due to the variation in their electron distribution and oscillation modes. For example: a. Spherical nanoparticles: These nanoparticles exhibit a single SPR peak, which is mainly due to the symmetric oscillation of electrons along all three axes. b. Rod-shaped nanoparticles nanorods : Gold nanorods have two SPR peaks one corresponding to the transverse mode oscillation of electrons along the short axis and the other corresponding to the longitudinal mode oscillation of electrons along the long axis . The longitudinal mode is highly sensitive to the aspect ratio length/width of the nanorods, and its peak position can be tuned from the visible to the near-infrared region by varying the aspect ratio. c. Other shapes: Gold nanoparticles with more complex shapes, such as cubes, stars, and shells, exhibit multiple SPR peaks due to the presence of various oscillation modes and hotspots. These shapes can provide enhanced sensitivity and tunability in sensing and imaging applications.In summary, the size and shape of gold nanoparticles significantly affect their surface plasmon resonance properties, which in turn influence their optical properties and potential applications. By controlling the size and shape of gold nanoparticles, it is possible to tune their SPR properties for specific applications in areas such as sensing, imaging, and drug delivery.