Synergistic roles of vapor- and liquid-phase epitaxy in the seed-mediated synthesis of substrate-based noble metal nanostructures

Colloidal growth modes reliant on the replication of the crystalline character of a preexisting seed through homoepitaxial or heteroepitaxial depositions have enriched both the architectural diversity and functionality of noble metal nanostructures. Equivalent syntheses, when practiced on seeds formed on a crystalline substrate, must reconcile with the fact that the substrate enters the syntheses as a chemically distinct bulk-scale component that has the potential to impose its own epitaxial influences. Herein, we provide an understanding of the formation of epitaxial interfaces within the context of a hybrid growth mode that sees substrate-based seeds fabricated at high temperatures in the vapor phase on single-crystal oxide substrates and then exposed to a low-temperature liquid-phase synthesis yielding highly faceted nanostructures with a single-crystal character. Using two representative syntheses in which gold nanoplates and silver-platinum core-shell structures are formed, it is shown that the hybrid system behaves unconventionally in terms of epitaxy in that the substrate imposes an epitaxial relationship on the seed but remains relatively inactive as the metal seed imposes an epitaxial relationship on the growing nanostructure. With epitaxy transduced from substrate to seed to nanostructure through what is, in essence, a relay system, all of the nanostructures formed in a given synthesis end up with the same crystallographic orientation relative to the underlying substrate. This work advances the use of substrate-induced epitaxy as a synthetic control in the fabrication of on-chip devices reliant on the collective response of identically aligned nanostructures.

Ag@TiO2 Nanoprisms with Highly Efficient Near-Infrared Photothermal Conversion for Melanoma Therapy

Melanoma is a primary reason of death from skin cancer and associated with high lethality. Photothermal therapy (PTT) has been developed into a powerful cancer treatment technique in recent years. Here, we created a low-cost and high-performance PTT agent, Ag@TiO₂ NPs, which possesses a high photothermal conversion efficiency of ≈65 % and strong near-infrared (NIR) absorption about 808 nm. Ag NPs were synthesized using a two-step method and coated with TiO₂ to obtain Ag@TiO₂ NPs by a facile sol-gel method. Because of the oxide, Ag@TiO₂ NPs exhibit remarkable high photothermal conversion efficiencies and biocompatibility in vivo and in vitro. Cytotoxicity and therapeutic efficiency of photothermal cytotoxicity of Ag@TiO₂ NPs were tested in B16-F10 cells and C57BL/6J mice. Under light irradiation, the elevated temperature causes cell death in Ag NPs-treated (100 μg mL^-1 ) cells in vitro (both p<0.01). In the case of subcutaneous melanoma tumor model, Ag@TiO₂ NPs (100 μg mL^-1 ) were injected into the tumor and irradiated with a 808 nm laser of 2 W cm^-2 for 1 minute. As a consequence, the tumor volume gradually decreased by NIR laser irradiation with only a single treatment. The results demonstrate that Ag@TiO₂ NPs are biocompatible and an attractive photothermal agent for cutaneous melanoma by local delivery.

Dynamically Switchable Multicolor Electrochromic Films

The dynamic optical switch of plasmonic nanostructures is highly desirable due to its promising applications in many smart optical devices. To address the challenges in the reversibility and transmittance contrast of the plasmonic electrochromic devices, here, a strategy is reported to fabricate color switchable electrochromic films through electro-responsive dissolution and deposition of Ag on predefined hollow shells of Au/Ag alloy. Using the hollow Au/Ag alloy nanostructures as stable seeds for site-specific deposition of Ag, elimination of the random self-nucleation events is enabled and optimal reversibility in color switching is allowed. The hollow structure further enables excellent transmittance contrast between the bleached and colored states. With its additional advantages such as the convenience for preparation, high sensitivity, and field-tunable optical property, it is believed that this new electrochromic film represents a unique platform for designing novel smart optical devices.

Structural Change of a Single Ag Nanoparticle Observed by Dark-field Microspectroscopy

Silver nanoparticles (AgNPs) have been widely used as photocatalysts and nanosensors. Observation of the spectroscopy of a single AgNP greatly helps us understand the catalytic characteristics and morphology change of the AgNP during reactions. In the present study, AgNPs physically adsorbed on indium tin oxide (ITO) conductive glass were electrochemically reduced and oxidized, and the plasmonic resonance Rayleigh scattering (PRRS) spectrum of an individual AgNP was observed under a dark-field microscopy (DFM) equipped with a spectrometer. The electrochemical oxidization of the AgNP under constant potential caused a redshift of the PRRS peak for 30±5 nm. However, electrochemical reduction of the AgNP could not make the PRRS peak completely shift back to the initial position. In situ AFM and SEM characterization confirmed that very small Ag fragments (<10 nm) formed around the AgNP core during electrochemical oxidization. Results showed that dark-field microspectroscopy could be used as a sensitive tool for estimating the morphology/structural changes of nanoparticles that can hardly be observed through the cyclic voltammograms of multiple AgNPs.

Seed-Mediated Growth of Colloidal Metal Nanocrystals

Seed-mediated growth is a powerful and versatile approach for the synthesis of colloidal metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, composition, and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also determine their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a number of existing applications and opening the door to new developments. This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and reduction kinetics. New capabilities and future directions are also highlighted.

Designed self-assembled hybrid Au@CdS core–shell nanoparticles with negative charge and their application as highly selective biosensors

Schematic illustration of the reaction mechanism of Au@CdS core–shell structure with DA in the presence of UA and AA.

Silver/gold core-shell nanoprism-based plasmonic nanoprobes for highly sensitive and selective detection of hydrogen sulfide

A simple and highly sensitive and selective hydrogen sulfide assay utilizing plasmonic nanoprobes is presented in this report. The assay employs the etching of silver in the Ag/Au core-shell nanoprisms, accompanied by surface plasmon resonance (SPR) signal depression and shift. Briefly, thin layers of gold are first coated onto silver nanoprisms. The thin gold layer not only guarantees the high stability of the plasmonic nanoprobes but also ensures the high selectivity toward hydrogen sulfide. Once hydrogen sulfide is introduced, the silver core is converted to Ag2S mainly from its lateral walls. Moreover, the SPR peak is located in the NIR region that makes these plasmonic nanoprobes more appealing for the detection of hydrogen sulfide in real-world samples and in in vivo applications.

Ultrastrong plasmon-exciton coupling in metal nanoprisms with J-aggregates

In this Letter, ultrastrong plasmon-exciton coupling in an Ag nanoprism-J-aggregate hybrid nanostructure is reported. A localized surface plasmon wavelength of Ag nanoprisms is tunable starting from 400 to 1100 nm. Because of the large electric field localization at the corners of the nanoprisms, the observed Rabi splitting energy is higher than the previously reported Rabi splitting energies using metal nanoparticles. A giant Rabi splitting energy of more than 400 meV corresponding to ~19% of the j-band energy has been observed, thus indicating the ultrastrong coupling regime. The hybrid nanostructure of nanoprism-J-aggregate is easy to prepare in large quantities and it can be uniformly assembled on solid substrates.