Nanofabrication of plasmonic structures for photothermal applications

Abstract

Plasmonic nanostructures, with novel light-to-heat features, exhibit tremendous potential in bio- and energy-related areas. Typically, under the exposure of external coherent irradiation, a collective oscillation of charge carriers occurs on plasmonic nanostructures with a subsequently thermalized carrier cloud. The energetic carriers are further relaxed through non-radiative processes by electron-phonon (e-p) and phonon-phonon (p-p) scattering, which transfer the thermal energy to the surrounding environment. This intriguing light-to-heat feature has drawn much attention in many applications, due to the capability of manipulation of the localized heat with remote controls. However, the limited heat conversion efficiency (HCE) of photothermal agents remains the major obstacle in the current stage of photothermal applications. In recent years, the copper chalcogenide nanocrystal (Cu2-xS NC, 0≤x≤1) was proposed as a promising candidate for gold-based plasmonic nanostructures, due to its chemical abundance, tunable optical properties, robust thermal stability, and limited cytotoxicity. As self-doped semiconductor nanocrystals, copper-deficient Cu2-xS NCs can provide more charge carriers than copper-rich alternatives. Hence, with bigger value of x, Cu2-xS NCs can afford stronger carrier oscillation and heat generation under the irradiation of external light. In the past decade, different synthetic routes and post-treatment methods were developed in order to tune the value of x. However, even with the biggest value of x, copper-poor CuS NCs may not be powerful enough to generate sufficiently high heat to overcome the bottleneck in conventional photothermal applications. To address this challenge, rattle type nanostructures, composed of a gold nanorod (AuNR) core and a Cu2-xS shell are proposed in this work. On the one hand, the inner plasmonic metal cores are capable to concentrate the incident light, which is beneficial for producing local electromagnetic field enhancement to induce a resonant energy transfer (RET) from plasmonic metal to semiconductor, resulting in enhanced electron-hole pair generation in semiconductors. On the other hand, the AuNRs core can also provide additional heat when responding to the external irradiation. In this research, the effects of the overall size, aspect ratio of inner core, shell thickness of Cu2-xS, and gap distance between core and shell on the optical and photothermal responses were investigated. The results suggest that the gap distance plays a dominant role over the other parameters on HCE, with maximum value of ~52%. Plasmonic nanostructures of Cu2-xS NCs and AuNR@Cu2-xS NRs prepared in this work are further applied in vitro cancer therapy and solar vapor generation. By integrating the photothermal effect of plasmonic nanostructures with gene therapy, enhanced cell apoptosis was achieved. Moreover, with quercetin to reduce the heat shock response level in cancer cells, photothermal therapy (PTT) exhibits stronger effects in treating cancer. These results demonstrate an alternative way to improve the overall performance in bio-applications by strengthening the relationship with the biological response. Furthermore, CuS NRs were embedded in PVA gel to perform solar vapor generation. Owing to the hierarchical inner structure of PVA and promising light-to-heat feature of CuS NRs, a maximum water evaporation efficiency of ~87% was obtained.