Optical Properties and Persistent Spectral Hole Burning of Near Infrared-Absorbing Hollow Gold Nanospheres

We report on the characteristics and mechanism of persistent spectral hole burning of hollow gold nanospheres (HGNs) with near IR (NIR) surface plasmon resonance (SPR) absorption induced by femtosecond (fs) laser pulses. The HGNs were exposed to pulsed laser light at 810 nm, on resonance with their absorption band, at various laser powers while the total photon flux incident on the samples was kept constant. Depletion of HGNs responsible for 830 nm absorption was observed while simultaneous growth of bluer-absorbing HGNs, solid gold nanoparticles (AuNPs), or a mixture thereof was observed. This occurred only with fs pulsed laser irradiation and did not occur with continuous wave (CW) laser irradiation. Based on combined UV-vis spectroscopy and transmission electron microscopy data, the mechanism behind the persistent hole burning with fs laser irradiation is proposed to involve two possible processes: (i) conversion of NIR-absorbing HGNs to bluer-absorbing HGNs, and (ii) breakdown of NIR-absorbing HGNs into solid AuNPs directly. The branching ratio between these two processes depends on the peak power of the fs laser pulses with higher peak power favoring the second process. This hole-burning study is useful for understanding and exploring the potential use of HGNs in drug delivery with a laser as a trigger for release. In conjunction, theoretical calculations have been carried out to gain further insight into the optical absorption of HGNs, especially in terms of how the extinction coefficient depends on the structure of the HGNs in comparison to solid AuNPs. One interesting finding is that HGNs with thicker shells not only absorb at bluer wavelengths, but also have lower absorption as well as extinction coefficient. An HGN with the same outer diameter as a solid AuNP has a substantially larger extinction coefficient. These theoretical findings are supported by experimental results. The enhanced extinction coefficient is advantageous for applications such as photothermal ablation therapy (PTA) for cancer treatment. The dependence of the absorption coefficients on the HGN structure is also important for interpreting the hole-burning results that rely on changes in spectral position as well as intensity