External-Stimuli-Assisted Control over Assemblies of Plasmonic Metals

Design of Multifunctional Janus Metasurface Based on Subwavelength Grating

In this paper, a Janus metasurface is designed by breaking the structural symmetry based on the polarization selection property of subwavelength grating. The structure comprises three layers: a top layer having a metallic nanostructure, a dielectric spacer, and a bottom layer having subwavelength grating. For a forward incidence, the metal-insulator-metal (MIM) structure operates as a gap plasmonic cavity if the linearly polarized (LP) component is parallel to the grating wires. It also acts as a high-efficiency dual-layer grating polarizer for the orthogonal LP component. For the backward incidence, the high reflectance of the grating blocks the function of the gap plasmonic cavity, leading to its pure functioning as a polarizer. A bifunctional Janus metasurface for 45 degrees beam deflector and polarizer, with a transmission of 0.87 and extinction ratio of 3840, is designed at 1.55 μm and is investigated to prove the validity of the proposed strategy. Moreover, the proposed metasurface can be cascaded to achieve more flexible functions since these functions are independent in terms of operational mechanism and structural parameters. A trifunctional Janus metasurface that acts as a focusing lens, as a reflector, and as a polarizer is designed based on this strategy. The proposed metasurface and the design strategy provide convenience and flexibility in the design of multifunctional, miniaturized, and integrated optical components for polarization-related analysis and for detection systems.

Stimuli-Responsive Plasmonic Assemblies and Their Biomedical Applications

Among the diverse development of stimuli-responsive assemblies, plasmonic nanoparticle (NP) assemblies functionalized with responsive molecules are of a major interest. In this review, we outline a comprehensive and up-to-date overview of recently reported studies on in vitro and in vivo assembly/disassembly and biomedical applications of plasmonic NPs, wherein stimuli such as enzymes, light, pH, redox potential, temperature, metal ions, magnetic or electric field, and/or multi-stimuli were involved. Stimuli-responsive assemblies have been applied in various biomedical fields including biosensors, surfaced-enhanced Raman scattering (SERS), photoacoustic (PA) imaging, multimodal imaging, photo-activated therapy, enhanced X-ray therapy, drug release, stimuli-responsive aggregation-induced cancer therapy, and so on. The perspectives on the use of stimuli-responsive plasmonic assemblies are discussed by addressing future scientific challenges involving assembly/disassembly strategies and applications.

Use of Photoacids and Photobases To Control Dynamic Self-Assembly of Gold Nanoparticles in Aqueous and Nonaqueous Solutions

Dynamic self-assembly of nanoparticles (NPs) for the formation of aggregates takes place out of thermodynamic equilibrium and is sustained by external energy supply. Herein, we present light energy driven dynamic self-assembly process of AuNPs, decorated with pH sensitive ligands. The process is being controlled by the use of photoacids and photobases that undergo excited state proton or hydroxide transfer, respectively, due to their large p K_a change between their ground and excited electronic states. The unique design is underlined by record subsecond conversion rates between the assembled and disassembled AuNPs states, and the ability to control the process using only light of different wavelengths. Measurements in both aqueous and nonaqueous solutions resulted in different self-assembly mechanisms, hence showing the wide versatility of photoacids and photobases for dynamic processes.