Au Nanobead Chains with Tunable Plasmon Resonance and Intense Optical Scattering: Scalable Green Synthesis, Monte Carlo Assembly Kinetics, Discrete Dipole Approximation Modeling, and Nano-Biophotonic Application

Abstract

The development of optically tunable and intensely scattering materials is of wide interest in biophotonics and medicine, provided they are biocompatible. Highly linear, tunable, narrow-gap, and intensely scattering gold nanobead chains were prepared by a green synthesis scheme, starting from ultrapure gold nanosphere monomers having a virgin surface. These Au nanochains exhibit a tunable longitudinal surface plasmon resonance (SPR) over the range from 590 to 710 nm. To determine the ensemble statistics of the synthesized nanobead chain lengths and their branching, transmission electron microscopy (TEM) images were acquired, in batch, correlated with the nanochain longitudinal SPR maxima (590 to 640 nm). Monte Carlo simulations quantify the selectivity of the chemical reaction: chain-end gold nanosphere units are about 4 times more reactive than chain-center units and assemble under a high-inverse-order-dependent diffusion. Discrete dipole approximation (DDA) computations predict the experimental extinction spectra and the narrow gap distance of the chains, as well as the elongated chain’s optical scattering enhancement, relative to Au nanosphere monomers (approximately 3× enhancement at equivalent mass). In vitro dark-field microscopy, cell studies, and biocompatibility tests demonstrate the nanochains’ (1) intense optical scattering cross sections, related to the plasmon coupling between the constituent nanospheres, (2) highly accessible gold surfaces, enabling facile conjugation with cell targeting ligands, and (3) absence of cell toxicity. The herein reported scalable, green synthesized, readily conjugatable gold nanobead chains are thus of great potential utility for serving as a wide range of biophotonic platforms, such as for enhanced in vitro and in vivo contrast imaging, diagnostics, and targeted nanotheranostics.

Au–Ag Alloy Nanocorals with Optimal Broadband Absorption for Sunlight-Driven Thermoplasmonic Applications

“…According to this set of experimental evidence, the anisotropic growth of NCs may be divided in two stages (Figure G), with the first one occurring just after photofragmentation generating the initial NCs, followed by a second slower one taking place over several days in which the NCs coalesce and increase their aspect ratio. This is in agreement with previous observations of unidirectional self-assembly of metallic nanoparticles, which occurs with the fast formation of oligomers followed by their slower assembly into larger structures. ,,,, …”

“…Unfortunately, the plasmon resonances of spherical or rod NPs are narrow and centered at specific wavelengths, which is not optimal for harvesting of solar energy, ,− even for the most effective plasmon heaters such as nanodoughnut . Symmetry reduction allows for tuning the number, position, and intensity of plasmons, which become broader and cover a wide spectral range when also the size of the nanostructure is increased over tens of nanometers. − ,,, Alternatively, new broad resonances from the visible to the NIR arise in large aggregates of NPs due to the mutual coupling of plasmon modes of the neighboring particles. ,,, In particular, several elongated or asymmetric networks of noble metal nanoparticles have been described for their multimodal plasmonic responses extending in the red and NIR. ,− However, the light-scattering component scales with the sixth power of object size and rapidly equals or overwhelms the absorption component in large objects, with a consequent loss of photothermal efficiency in big NPs or their aggregates. ,,, …”

Gold nanocrystals: optical properties, fine-tuning of the shape, and biomedical applications

“…Nevertheless, Chen et al provided a systematic introduction to the preparation methods of monopod, bipod, tripod, and tetrapod Au NCs. 71 To date, Au NCs with different shapes as spheres, [28][29][30] rods, [31][32][33]73 cubes, 29,74 triangle, 53 polyhedron 25,59 and others 34,75 have been synthesized successively. Such as those in the TEM or SEM images of Au NCs with different morphologies shown in Fig.…”

“…Shape control synthesis provides one of the most powerful means to tailor the properties of Au NCs. 34,35…”

“…Shape control synthesis provides one of the most powerful means to tailor the properties of Au NCs. 34,35 Due to extensive research and the importance of Au NCs, various reviews related to the preparation and applications of Au NCs have been published over the years. [36][37][38][39][40][41][42][43] Most of them provided a specic introduction to various aspects of Au NCs, such as photosynthesis, modication, functionalization, toxicity of Au NCs, as well as the application of Au NCs in therapeutic applications, targeted drug delivery, and energy or environment.…”