Hello. The surface plasmon resonance spectrum is dependent both on the size, and shape of gold nanoparticles. The peak absorbance wavelength increases with particle diameter. However, why is the peak absorbance of Au spherical nanoparticles near 550 nm compared to 800 nm for Au nanorods, please? Why dose Au sphere generate thermal energy exposed by NIR laser if there is no peak absorbance in NIR range?
If I understood well your questions, the first one is why the rods show absorption peak at 800 nm while the spheres show at 500 nm?
You answered this question yourself. The peak associated with the LSPR of the nanoparticles depends on the particle size, material, and geometry. For larger nanoparticles the LSPR generally shifts to the read and also broadens due to losses. This is what you see in the UV-vis spectra.
The second question is why the spheres absorb NIR light and convert it to thermal energy when there's no LSPR in that range? You need to tell more details about this. For example, you measure UV-vis for nanoparticles in a liquid dispersion, but when the nanoparticles are deposited on a substrate they can make aggregates. Due to the change of environment and the particle aggregation even in chain-like structures could result in plasmon modes in the NIR that you don't see in the UV-from the dispersed particles in liquid.
Can you please clarify how do you get to this conclusion?
"The slightly purple Au nanosphere in water converted thermal energy under NIR laser exposition"
@ Rodriguez. I think I've found the answers. The aggregation of Au nanosphere happened after polymer encapsulation, which induced the red shift. Thank you for your time again.