Ultrasensitive detection of local acoustic vibrations at room temperature by plasmon-enhanced single-molecule fluorescence.

Sensitive detection of local acoustic vibrations at the nanometer scale has promising potential applications involving miniaturized devices in many areas, such as geological exploration, military reconnaissance, and ultrasound imaging. However, sensitive detection of weak acoustic signals with high spatial resolution at room temperature has become a major challenge. Here, we report a nanometer-scale system for acoustic detection with a single molecule as a probe based on minute variations of its distance to the surface of a plasmonic gold nanorod. This system can extract the frequency and amplitude of acoustic vibrations with experimental and theoretical sensitivities of 10 pm Hz-1/2 and 10 fm Hz-1/2, respectively. This approach provides a strategy for the optical detection of acoustic waves based on molecular spectroscopy without electromagnetic interference. Moreover, such a small nano-acoustic detector with 40-nm size can be employed to monitor acoustic vibrations or read out the quantum states of nanomechanical devices.

Controlled synthesis of gold nanorod dimers with end-to-end configurations.

End-to-end gold nanorod dimers provide unique plasmonic hotspots with extremely large near-field enhancements in the gaps. Thereby they are beneficial in a wide range of applications, such as enhancing the emissions from ultra-weak emitters. For practical purposes, synthesis of gold nanorod dimers with high yield, especially on the substrates, is essential. Here, we demonstrate two controllable strategies to synthesize gold nanorod dimers based on the self-assembly of gold nanorods, either in bulk solution or on the surface of a glass substrate directly. Both methods can give a high yield of gold nanorod dimers, yet, assembling them directly on the substrate provides more flexibility in controlling the shape and size of each nanorod within the dimer. We also show that these gold nanorod dimers can be used to enhance two-photon-excited fluorescence signals at the single-molecule level.

Two-Photon-Excited Single-Molecule Fluorescence Enhanced by Gold Nanorod Dimers.

We demonstrate two-photon-excited single-molecule fluorescence enhancement by single end-to-end self-assembled gold nanorod dimers. We employed biotinylated streptavidin as the molecular linker, which connected two gold nanorods in end-to-end fashion. The typical size of streptavidin of around 5 nm separates the gold nanorods with gaps suitable for the access of fresh dyes in aqueous solution, yet small enough to give very high two-photon fluorescence enhancement. Simulations show that enhancements of more than 7 orders of magnitude can be achieved for two-photon-excited fluorescence in the plasmonic hot spots. With such high enhancements, we successfully detect two-photon-excited fluorescence for a common organic dye (ATTO 610) at the single-molecule, single-nanoparticle level.

Long-term leachability of Sb in smelting residue stabilized by reactive magnesia under accelerated exposure to strong acid rain.

This study investigated the long-term leachability of antimony (Sb) in a smelting residue (39519 mg/kg) solidified/stabilized by reactive magnesia (MgO). Different dosages of MgO (0% as control, 2%, 5%, and 10% on a dry basis) were compared, and the long-term performance was evaluated by an accelerated exposure test consist of 20 consecutive leaching steps with simulated strong acid rain (SAR, HNO3: H2SO4 = 1:2, pH = 3.20) as the extractant. Notably, the MgO treatments efficiently reduced the Sb leachability. Compared to the original slag (8.3 mg/L), the leaching concentrations based on a Chinese standard HJ/T299-2007 were reduced by 58%, 79%, 85%, and 86% at MgO dosages of 0%, 2%, 5%, and 10%, respectively. Because the studied slag was rich in oxides like SiO2, CaO, and MgO, the hydration reactions probably happened during the aging processes with oxic water. It was inferred that the formed hydration products have a self-solidification/stabilization function to suppress the Sb leaching from the solid phase. The mineralogical characterization results proved that the hydrated Mg(OH)2 played an essential role in the decrease of Sb leachability. Besides, the MgO addition promoted the hydration of this smelting slag and formed new hydrate gels that immobilize Sb in this slag. Our results confirmed that MgO-amended slags were resistant to continuous SAR corrosion. Compared to the control, the dosage of 5% MgO could effectively reduce the cumulatively released Sb by 57%, with only 0.46% of total Sb could be leached. The decomposition of Mg(OH)2 and hydrate gels determined the re-release of Sb in a long term. Our work has demonstrated that reactive MgO amendment could be potentially selected as an effective strategy for the treatment of Sb-containing smelting residues in field conditions.

Immobilization of Sb in a smelting residue by micro-sized zero-valent iron: Long-term performance under accelerated exposure to strong acid rain.

This study investigated the long-term leachability of antimony (Sb) in a smelting residue immobilized by three commercial micro-sized zero-valent iron (ZVI) products. Effect of oxic incubation time (14 days and 120 days) on the immobilization efficiency of Sb were compared, and the long-term leaching risk was evaluated by an accelerated exposure test, in which the slag was consecutively extracted by simulated strong acid rain (SSAR, HNO3: H2SO4 = 1:2, pH = 3.20). Notably, all ZVI treatments efficiently immobilized the Sb in this slag in a short term (14 days); the one-step SSAR-leached Sb was reduced by 89%-91% compared to the original slag (5.9 mg/L) and was far below the environmental standard (0.6 mg/L) established by the US EPA. The sequential SSAR leaching results reflected that the 14-d incubated slags after ZVI treatments had strong H+ resistance, and the immobilized Sb was not easily activated by continuous SSAR corrosion. The binding of Sb with amorphous phase Fe oxyhydroxides (e.g. ferrihydrite) derived from ZVI corrosion played a dominant role in the Sb immobilization efficiency. However, the longer aging process (120 days) easily resulted in the reduction of Sb immobilization by ZVI treatments. The changes in crystallinity of Fe oxyhydroxides (transformation from poorly-crystalline to crystalline ones) and the pH elevation to alkaline range might explain the weakening of the immobilization of Sb in ZVI-amended slags with 120 days of incubation. In total, the effectiveness of Sb immobilization in smelting residue greatly depended on the type of ZVI and the aging process. Our work has demonstrated that the ZVI treatment was potentially feasible to mitigate the Sb leaching risk from smelting slags; however, the ZVI type needs to be carefully selected and its long-term performance should be adequately verified before practical application.

In situ monitoring of silver adsorption on assembled gold nanorods by surface-enhanced Raman scattering.

Self-assembly of metal nanocrystals is able to create a gap of sub-nanometer distance for concentrating incoming light by the strong coupling of surface plasmon resonance, known as a 'hot spot'. Although the plasmonic property of silver is better than other metals in the visible range, the superior Raman enhancement of silver compared to gold is still under debate. To provide direct evidence, in this work, we studied the silver adsorption on assembled gold nanorods (AuNRs) using in situ surface-enhanced Raman scattering (SERS) measurements. The self-assembled AuNR multimers were used as the SERS substrate, where the 4-mercaptophenol (MPh) molecules in our experiment played dual roles as both probe molecules for the Raman scattering and linking molecules for the AuNR assembly in a basic environment. Silver atoms were adsorbed on the surface of gold nanorod assemblies by reduction of Ag+ anions. The stability of the adsorbed silver was guaranteed by the basic environment. We monitored the SERS signal during the silver adsorption with a home-built in situ Raman spectroscope, which was synchronized by recording the UV-vis absorption spectra of the reaction solution to instantly quantify the plasmonic effect of the silver adsorption. Although a minor change was found in the plasmonic resonance wavelength or intensity, the measured SERS signal at specific modes faced a sudden increase by 2.1 folds during the silver adsorption. The finite element method (FEM) simulation confirmed that the silver adsorption corresponding to the plasmonic resonance variation gave little change to the electric field enhancement. We attributed the mode-specific enhancement mechanism of the adsorption of silver to the chemical enhancement from charge transfer (CT) for targeting molecules with a specific orientation. Our findings provided new insights to construct SERS substrates with higher enhancement factor (EF), which hopefully would encourage new applications in the field of surface-enhanced optical spectroscopies.

Single-molecule fluorescence enhancement of a near-infrared dye by gold nanorods using DNA transient binding.

Fluorescence enhancement by plasmonic nanostructures enables the optical detection of single molecules with weak fluorescence, extending the scope of molecular fluorescence imaging to new materials and systems. In this work, we study single-molecule fluorescence enhancement by individual gold nanorods exploiting a DNA-based transient binding technique. Single molecules are attached to short DNA oligomers that can reversibly hybridize to their complementary docking DNA strands immobilized on the surface of gold nanorods or the glass substrate next to gold nanorods. This method continuously refreshes the single molecule in the near field of the gold nanorod, and enables a study of fluorescence enhancement at a well-defined position, with long dwell time and without limitation by photobleaching. Docking strands attached to the glass substrate are found to be more photo-stable. We find over 3000-fold fluorescence enhancement of single molecules of IRDye800CW, a near-infrared dye with a low quantum yield of 7%. This strong enhancement, consistent with numerical simulations, arises from the combined effect of local field enhancement and the competition between radiative and nonradiative decay rate enhancements.

Plasmonic Enhancement of Two-Photon-Excited Luminescence of Single Quantum Dots by Individual Gold Nanorods.

Plasmonic enhancement of two-photon-excited fluorescence is not only of fundamental interest but also appealing for many bioimaging and photonic applications. The high peak intensity required for two-photon excitation may cause shape changes in plasmonic nanostructures, as well as transient plasmon broadening. Yet, in this work, we report on strong enhancement of the two-photon-excited photoluminescence of single colloidal quantum dots close to isolated chemically synthesized gold nanorods. Upon resonant excitation of the localized surface plasmon resonance, a gold nanorod can enhance the photoluminescence of a single quantum dot more than 10 000-fold. This strong enhancement arises from the combined effect of local field amplification and the competition between radiative and nonradiative decay rate enhancements, as is confirmed by time-resolved fluorescence measurements and numerical simulations.

Controllable Biosynthesis and Properties of Gold Nanoplates Using Yeast Extract.

ABSTRACT: Biosynthesis of gold nanostructures has drawn increasing concerns because of its green and sustainable synthetic process. However, biosynthesis of gold nanoplates is still a challenge because of the expensive source and difficulties of controllable formation of morphology and size. Herein, one-pot biosynthesis of gold nanoplates is proposed, in which cheap yeast was extracted as a green precursor. The morphologies and sizes of the gold nanostructures can be controlled via varying the pH value of the biomedium. In acid condition, gold nanoplates with side length from 1300 ± 200 to 300 ± 100 nm and height from 18 to 15 nm were obtained by increasing the pH value. Whereas, in neutral or basic condition, only gold nanoflowers and nanoparticles were obtained. It was determined that organic molecules, such as succinic acid, lactic acid, malic acid, and glutathione, which are generated in metabolism process, played important role in the reduction of gold ions. Besides, it was found that the gold nanoplates exhibited plasmonic property with prominent dipole infrared resonance in near-infrared region, indicating their potential in surface plasmon-enhanced applications, such as bioimaging and photothermal therapy.

Dispersive Plasmon Damping in Single Gold Nanorods by Platinum Adsorbates.

Surface modifications of plasmonic nanoparticles with metal adsorbates are essential in applications such as plasmonic sensing, plasmon-enhanced photocatalysis, etc., where spectral broadening is usually observed. A single particle study is presented on plasmon damping by adsorption of platinum (Pt) clusters. Single particle dark-field spectroscopy is employed to measure exactly the same gold nanorod before and after the Pt adsorption. The Pt-induced plasmon damping in terms of linewidth increase is found dependent on the resonance wavelength of the measured nanorod, which is dispersive in nature. The measured dispersion generally matches the theoretical prediction, and it basically exhibits a gradual increase with decreasing resonance energy. This increase can be attributed to the fact that the nanorod as a better resonator is more susceptible to the Pt adsorption than the spherical particles. Moreover, simulated results based on discrete dipole approximation method further indicate that the damping is mainly contributed from the adsorbates on the ends of the nanorod and independent on the type of the metal adsorbed. Knowledge and insights gained in this study can be very important for the design and fabrication of plasmonic heterostructures as functional nanomaterials.