A comprehensive review of theoretical approaches to simulate plasmonic-active metallic nano-arrangements is given. Further, various fabrication methods based on bottom-up, self-organization and top-down techniques are introduced. Here, analytical approaches are discussed to investigate the optical properties of isotropic and non-magnetic spherical or spheroidal particles. Furthermore, numerical methods are introduced to research complex shaped structures. A huge variety of fabrication methods are reviewed, e.g. bottom-up preparation strategies for plasmonic nanostructures to generate metal colloids and core-shell particles as well as complex-shaped structures, self-organization as well as template-based methods and finally, top-down processes, e.g. electron beam lithography and its variants as well as nanoimprinting. The review article is aimed at beginners in the field of surface enhanced spectroscopy (SES) techniques and readers who have a general interest in theoretical modelling of plasmonic substrates for SES applications as well as in the fabrication of the desired structures based on methods of the current state of the art.

Plasmonic nanostructures for surface enhanced spectroscopic methods

The synthesis of discrete nanostructures with a strong, persistent, stable plasmonic circular dichroism (PCD) signal is challenging. We report a seed-mediated growth approach to obtain discrete Au nanorods with high and stable chiroptical responses (c-Au NRs) in the visible to near-IR region. The morphology of the c-Au NRs was governed by the concentration of l- or d-cysteine used. The amino acids encapsulated within the discrete gold nanostructure enhance their PCD signal, attributed to coupling of dipoles of chiral molecules with the near-field induced optical activity at the hot spots inside the c-Au NRs. The stability of the PCD signal and biocompatibility of c-Au NRs was improved by coating with silica or protein corona. Discrete c-Au NR@SiO2 with Janus or core-shell configurations retained their PCD signal even in organic solvents. A side-by-side assembly of c-Au NRs induced by l-glutathione led to further PCD signal enhancement, with anisotropic g factors as high as 0.048.

Tuning the Morphology and Chiroptical Properties of Discrete Gold Nanorods with Amino Acids

https://onlinelibrary.wiley.com/doi/pdf/10.1002/smm2.1047

Plasmonic metal-organic frameworks

Plasmon-enhanced fluorescence (PEF) is considered to be a powerful signal amplification technology to overcome intrinsic shortcomings of photobleaching and brightness of the traditional fluorescent dyes. Nevertheless, exploitation of PEF-based probes for bioimaging application is still at a very early stage. In this work, a simple but powerful gold nanostar (Au NST)@SiO2-based PEF probe with 20 symmetric "hot spots" was developed for highly sensitive "lighting up" in situ imaging of intracellular microRNAs (miRNAs). By regulating the thickness of the silica shell, the distance between Au NSTs and fluorescent dyes was controlled, and the optimum fluorescence enhancement (21-fold) was obtained with the silica shell thickness of approximately 22 nm. Thanks to the 20 more powerful "hot spots" that can produce stronger localized electric fields, the Au NST-based PEF probe exhibits stronger PEF effects than the traditional plasmonic nanostructures such as gold nanorods (Au NRs), gold nanobipyramids (Au NBPs), and triangular gold nanoprisms (Au NPRs), resulting in high sensitivity and improved detection limit (LOD) of 0.21 pM for miRNA-21 analysis. Moreover, not only cancer cells (MCF-7 and Hela) and normal cells (L02) with distinct miRNA-21 expression levels can be discriminated but also tumor cells in co-cultured mixtures can be recognized, indicating its promising potential in clinical diagnosis.

More Symmetrical “Hot Spots” Ensure Stronger Plasmon-Enhanced Fluorescence: From Au Nanorods to Nanostars

Bimetallic nanoparticles (BNPs) have attracted greater attention compared to its monometallic counterpart because of their chemical/physical properties. The BNPs have a wide range of applications in the fields of health, energy, water, and environment. These properties could be tuned with a number of parameters such as compositions of the bimetallic systems, their preparation method, and morphology. Monodisperse and anisotropic BNPs have gained considerable interest and numerous efforts have been made for the controlled synthesis of bimetallic nanostructures (BNS) of different sizes and shapes. This review offers a brief summary of the various synthetic routes adopted for the synthesis of Palladium(Pd), Platinum(Pt), Nickel(Ni), Gold(Au), Silver(Ag), Iron(Fe), Cobalt(Co), Rhodium(Rh), and Copper(Cu) based transition metal bimetallic anisotropic nanostructures, growth mechanisms e.g., seed mediated co-reduction, hydrothermal, galvanic replacement reactions, and antigalvanic reaction, and their application in the field of catalysis. The effect of surfactant, reducing agent, metal precursors ratio, pH, and reaction temperature for the synthesis of anisotropic nanostructures has been explained with examples. This review further discusses how slight modifications in one of the parameters could alter the growth mechanism, resulting in different anisotropic nanostructures which highly influence the catalytic activity. The progress or modification implied in the synthesis techniques within recent years is focused on in this article. Furthermore, this article discussed the improved activity, stability, and catalytic performance of BNS compared to the monometallic performance. The synthetic strategies reported here established a deeper understanding of the mechanisms and development of sophisticated and controlled BNS for widespread application.

/pdf/modern-chemical-routes-for-the-controlled-synthesis-of-44g43562uy.pdf

Modern Chemical Routes for the Controlled Synthesis of Anisotropic Bimetallic Nanostructures and Their Application in Catalysis

The novel cellular-like gold nanofeet (CGNF) with movable gold core, which are derived from gold/silver core shell nanorods, have been generated by galvanic reaction protocol at room temperature. The optical property based on localized surface plasmon resonance (LSPR) has been evaluated in comparison with solid gold nanofeet, suggesting that obviously high LSPR sensitivity of CGNF contributes to enhancing optical effect for detection of analytes. In contrast with superquenching properties of nanogold for fluorescence detection of pollutants, highly sensitive detection of heavy metal contaminations, e.g., mercury ions, have been implemented via DNA functionalized silica-coated CGNF on the basis of surface enhanced fluorescence (SEF) approach.

Cellular-Like Gold Nanofeet: Synthesis, Functionalization,and Surface Enhanced Fluorescence Detection for Mercury Contaminations

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Cellular-Like Gold Nanofeet: Synthesis, Functionalization,and Surface Enhanced Fluorescence Detection for Mercury Contaminations