Engineering plasmonic nanorattles for catalytic and photothermal therapy applications (PhD)

Abstract

The recent development in the design and synthesis of novel plasmonic nanostructures have drawn significant attention due to their variety of applications in sensing, catalysis, photonics and theranostics. Plasmonic nanorattles are the class of hollow, porous core-shell nanostructures which are composed of a solid core and a porous, thin shell. In this context, we have designed different plasmonic nanorattles of monometallic gold and bimetallic gold-palladium nanostructures. These, gold (Au NRT) and palladium nanorattles (Au-Pd NRTs) comprise of an octahedral solid gold core surrounded by a thin, porous gold or palladium shell respectively, and were synthesized following galvanic replacement reaction of Au@Ag nanocubes. Next, the catalytic activity of these two nanorattles, against the degradation of environmental pollutants such as p-nitrophenol and azo dyes (Congo red and methylene blue), was demonstrated. The kinetic and thermodynamic parameters revealed that Au-Pd NRT have higher catalytic efficiency with high-rate constant and lower activation energy in comparison to Au NRTs. Next, we exploited these Au-Pd NRTs for ORR applications. Compared with commercialized Pt/C, Au-Pd NRT displayed nearly comparable onset and halfwave potential values and excellent durability upon potential cycling. Engineering different plasmonic nanostructures with varying shapes and sizes, enables us to tune the localized surface plasmon resonance (LSPR) peak from visible to near infra-red (NIR) region of the electromagnetic spectrum which can further be exploited for plasmonic photothermal therapy (PPTT) against cancer. To achieve better PPTT, NIR active (extinction in the 700 nm to1300 nm region) plasmonic nanostructures are preferred because of the higher penetration depth of the NIR light. Thus, we examined the comparative photothermal efficiencies of the synthesized gold and bimetallic Au-Pd NRT. Both the nanorattles have wide range of absorbance– in the NIR I (700 – 900 nm) region. The photothermal conversion efficiency, in the aqueous medium for Au NRT and Pd NRT, was calculated to be 19.8 % and 35.4 % respectively. In vitro PTT result also suggested that bimetallic Au-Pd nanorattles have better PTT efficiency compared to gold nanorattles.

With an aim to further improve the PTT efficiency of the nanorattles and explore PTT in the second NIR window (NIR-II; 1000–1700 nm) which is much more promising due to its superiority in penetration depth (⁓ 2 cm) and maximum permissible exposure limit over NIR-I window, two capsular nanorattles of gold nanocapsules (Au Ncap) and palladium nanocapsules (Pd Ncap) were synthesized. The photothermal conversion efficiency of the synthesized Pd Ncap was calculated (49.2 %) and found to be higher than that of other synthesized nanorattles. In vitro studies demonstrated that bimetallic Pd Ncap have high PPTT efficiency against the breast cancer SK-BR- 3 cells, as compared to that of monometallic Au Ncap. Further investigation also revealed that both the nanocapsules caused apoptotic mode of cell death. Interestingly it was also observed that Pd Ncap exhibited ROS scavenging ability. Overall study suggested that Pd Ncap has better PTT efficiency in comparison to other synthesized nanorattles (Au NRT, Au-Pd NRT and Au Ncap) which leads to higher in vitro photothermal cell death at lower concentration, Further, the ROS scavenging ability of Pd Ncap offers added advantage in preventing oxidative damage of normal healthy cells during plasmonic photothermal therapy.