Synthesis and Evaluation of Heat-Resistant Silver-Platinum Alloy Nanoprisms for Application in Cancer Therapy and Imaging

Abstract

Nano-sized metal particles have been received great attention in the field of bioimaging and therapy, i.e. cancer treatment, because of their unique optical properties resulting from localized surface plasmon resonance (LSPR). Multifunctional nanoparticles have been developed for the precisive diagnosis and noninvasive cancer therapy in place of traditional surgery and chemotherapy, which can be restricted by region, physical strength, and so on. While various metal nanoparticles have been explored, the use of those consisting of Ag has been limited because of their physical instability. In particular, Ag nanoprisms (AgPRs) can be easily degraded by thermal energy generated along with the LSPR excitation because of their high surface energy. However, use of the AgPRs will provide a sufficient possibility for precisive cancer diagnosis due to the generation of significantly-enhanced electromagnetic fields. In this study, we developed AgPRs-based Pt-Ag alloyed nanomaterials (AgPtPRs), which can act as multifunctional therapeutic/diagnostic agents with improved thermal stability and bioimaging capability. The Ag nanoprisms (AgPRs) exhibiting a peak within near-infrared (NIR) wavelengths were synthesized according to the method reported by Mirkin.1) The formation of the AgPRs with an average edge length of 40 nm and thickness of 8nm were confirmed by TEM images. The AgPtPRs were synthesized by galvanic replacement of the as-prepared AgPRs with hexachloroplatinic (IV) acid. After the reaction, while the size as well as the morphology of them were almost maintained, the plasmon extinction peak slightly red-shifted. This result implies that thin Pt layers formed over the AgPRs surfaces. Thermal-durability evaluated by the irradiation of a continuous laser (2 W, 808 nm) for 1 h and heating in waterbath at 95 °C for 1 h. The morphology of the non-alloyed AgPRs were easily degraded by both of the laser irradiation and the exterior heating because of their high vertices energy. The optical properties of the AgPRs were destroyed by the deformation. In contrast, thermal-durability of the AgPtPRs against both of them was appreciably improved by the alloying reaction with Pt atoms. The photothermal conversion efficiency (PCE) obtained by the irradiation of NIR laser (808 nm, 2 W) were calculated to be ca. 65% even after the alloying reaction. The value was quite higher than those of the previous-reported major photothermal nanoparticles, e.g. Au nanorods (22 %) and Au nanoshells (13 %).2) In order to search the source of the higher PCE, the scattering and absorption components consisting of the extinction were calculated using a simulation of boundary element method (BEM). The absorption of the AgPRs modeled as the triangular structures with edge length of 40 nm and the thickness of 8 nm was dominant in the extinction peak. The optical tendency was almost maintained even after covering the AgPRs with Pt thin shells with 0.4 nm in thickness. The factor of large PCE of AgPtPRs and AgPRs was considered to be due to the high absorption coefficient. Next, we evaluated the effect of the photothermal phenomenon of AgPtPRs on the cancer cells. After the Hela cells were incubated with various concentrations of AgPtPRs modified with folic acid-conjugated polyethyleneimine, the cells were washed with phosphate buffered saline (PBS) several times. While the irradiation of the laser with 808 nm (2W) to the cells incubated with the AgPtPRs led to the efficient death of cells, non-incubated cells was not affected by the irradiation. The cell viability with and without the laser irradiation was ca. 12.9 % and 87.3 %, respectively. It was found from these results that the AgPtPRs have sufficient ability as a photothermal therapy nanomaterial with a high biocompatibility. In order to evaluate the imaging ability, surface-enhanced Raman scattering (SERS) properties of 4-mercaptobenzonic acid (4-MBA) immobilized on the nanoprisms were measured with a NIR laser of 785 nm. Those on the AgPRs and AgPtPRs showed Raman peaks characteristic of 4-MBA at 1590 cm-1 (C-C stretching) and 1080 cm-1(ring breathing). These results demonstrated that the strong local fields generated even after covering with the Pt shell led to the enhancement of the Raman signals for the SERS-imaging technique. 1). G. S. Métraux et al., Adv. Mater., 2005, 17, 412. 2). C. M. Hessel et al., Nano Lett., 2011, 11, 2560. Figure 1