The particle size of gold nanoparticles AuNPs plays a significant role in determining their photochemical properties, including absorbance, emission, and quantum yield. These properties are influenced by the localized surface plasmon resonance LSPR phenomenon, which occurs when the conduction electrons in the metal nanoparticle oscillate in response to an incident electromagnetic field, such as light.1. Absorbance: The absorbance of AuNPs is highly dependent on their size. As the particle size increases, the LSPR peak shifts to longer wavelengths redshift . This is due to the increased interaction between the conduction electrons and the nanoparticle surface, leading to a stronger plasmonic response. Consequently, larger AuNPs will absorb light more efficiently at longer wavelengths compared to smaller ones. The absorbance also depends on the shape and the surrounding medium of the AuNPs.2. Emission: Gold nanoparticles typically exhibit weak photoluminescence due to their low radiative decay rates. However, the emission properties can be influenced by the particle size. Smaller AuNPs have a higher surface-to-volume ratio, which can lead to more efficient nonradiative decay processes and, therefore, weaker emission. On the other hand, larger AuNPs can exhibit enhanced emission due to the increased radiative decay rates associated with their larger size.3. Quantum yield: The quantum yield of AuNPs is the ratio of the number of emitted photons to the number of absorbed photons. It is generally low for gold nanoparticles due to their strong nonradiative decay processes. However, the quantum yield can be affected by the particle size. As mentioned earlier, smaller AuNPs have a higher surface-to-volume ratio, leading to more efficient nonradiative decay processes and, consequently, lower quantum yields. In contrast, larger AuNPs can have higher quantum yields due to the increased radiative decay rates.The photochemical properties of AuNPs under different light sources and intensities will also be influenced by the particle size. For instance, under low light intensities, smaller AuNPs may exhibit weaker absorbance and emission compared to larger ones. However, as the light intensity increases, the absorbance and emission properties of the AuNPs may change, and the differences between smaller and larger particles may become less pronounced.In summary, the particle size of gold nanoparticles significantly affects their photochemical properties, such as absorbance, emission, and quantum yield. These properties are influenced by the localized surface plasmon resonance phenomenon, which depends on the size, shape, and surrounding medium of the nanoparticles. Understanding these size-dependent properties is crucial for designing and optimizing AuNP-based applications in areas such as sensing, imaging, and photothermal therapy.