Gold nanoparticles AuNPs exhibit unique photochemical properties due to their size, shape, and surface plasmon resonance SPR . These properties have a significant impact on their optical and electronic properties, making them suitable for various applications in fields such as sensing, imaging, and catalysis.1. Surface Plasmon Resonance SPR : Gold nanoparticles exhibit a phenomenon called surface plasmon resonance, which is the collective oscillation of free electrons in the conduction band at the nanoparticle surface. This oscillation occurs when the nanoparticles are exposed to light, leading to a strong absorption and scattering of light at specific wavelengths. The SPR peak position depends on the size, shape, and surrounding medium of the nanoparticles.2. Size and Shape Dependency: The optical and electronic properties of gold nanoparticles are highly dependent on their size and shape. As the size of the nanoparticles decreases, the SPR peak shifts to shorter wavelengths blue shift . Similarly, the shape of the nanoparticles also influences the SPR peak position and intensity. For example, rod-shaped nanoparticles exhibit two SPR peaks, one for the longitudinal mode and another for the transverse mode.3. Quantum Confinement: In very small gold nanoparticles typically below 2 nm , the electronic properties are significantly affected by quantum confinement. This leads to discrete energy levels and a change in the optical properties, such as the disappearance of the SPR peak.4. Photothermal Effect: Gold nanoparticles can efficiently convert absorbed light into heat due to their strong SPR absorption. This photothermal effect can be used for applications such as photothermal therapy and drug delivery.5. Photochemical Reactivity: Gold nanoparticles can act as photocatalysts under specific conditions, promoting chemical reactions upon light irradiation. This property can be utilized in applications such as environmental remediation and solar energy conversion.In summary, the photochemical properties of gold nanoparticles, such as surface plasmon resonance, size and shape dependency, quantum confinement, photothermal effect, and photochemical reactivity, significantly affect their optical and electronic properties. These unique properties make gold nanoparticles suitable for a wide range of applications in various fields.