Triphase Enzyme Electrode Based on ZIF-8 with Enhanced Oxidase Catalytic Kinetics and Bioassay Performance.

Oxidase enzyme-based electrochemical bioassays have garnered considerable interest due to their specificity and high efficiency. However, in traditional solid-liquid diphase enzyme electrode systems, the low solubility of oxygen and its slow mass transfer rate limit the oxidase catalytic reaction kinetics, thereby affecting the bioassay performance, including the detection accuracy, sensitivity, and linear dynamic range. ZIF-8 nanoparticles (NPs) possess hydrophobic and high-porosity characteristics, enabling them to serve as oxygen nanocarriers. In this work, we constructed a solid-liquid-air triphase enzyme electrode by encapsulating ZIF-8 NPs within an oxidase network. Hydrophobic ZIF-8 NPs can provide a rapid and sufficient supply of oxygen for the oxidase-catalyzed reactions, which enhances and stabilizes the kinetics of oxidase-catalyzed reactions. This approach eliminates the issue of "oxygen deficiency" at the traditional solid-liquid diphase interface. Consequently, the triphase enzyme electrode exhibits a 12-fold higher linear detection range than the diphase system and possesses good detection accuracy in electrolytes even with fluctuating oxygen levels. This work proposes a novel approach to construct triphase reaction systems for addressing the gas deficiency problem in heterogeneous catalysis.

Circular Differential Photocurrent Mapping of Hot Electron Response from Individual Plasmonic Nanohelicoids.

Chiral plasmonic nanocrystals have recently attracted increasing attention in circular polarization-dependent photocatalysis driven by hot carriers. While being concealed in traditional ensemble measurements, the individual chiral photocatalytic activity of nanocrystals can exclusively be revealed by directly correlating the circular differential photocurrent response to helical morphologies using single-particle techniques. Herein, we develop a method named circular differential photocurrent mapping (CDPM) and demonstrate that CDPM can be used to characterize the circular differential hot electron (CDHE) response from individual Au nanohelicoids (AuNHs) on a TiO2 photoanode in a photoelectrochemical cell. The single-particle circular differential scattering and CDHE measurements were interpreted with calculations performed on a model in direct correlation to the helical morphologies of the nanocrystal. While CDHE response was found inactive at a dipolar resonance of 750 nm, helicity-convoluted sites of HE generation were identified on the AuNH at a specific higher-order mode of 550 nm, resulting in a significant response of CDHE in association with the handedness of the AuNH. Details of circular differential contributions were further resolved by examining the efficiencies of individual AuNHs in terms of g-factors. Our study provides a powerful microscopic method at the single-particle level for the photocatalytic characterization of chiral nanocrystals, gaining fundamental insights into the photocatalysis of chirality, especially toward plasmon-induced asymmetrical photochemistry or photoelectrochemistry.

Construction of Gold/Rhodium Freestanding Superstructures as Antenna-Reactor Photocatalysts for Plasmon-Driven Nitrogen Fixation.

Precisely controlling the architecture and spatial arrangement of plasmonic heterostructures offers unique opportunities to tailor the catalytic property, whereas the lack of a wet-chemistry synthetic approach to fabricating nanostructures with high-index facets limits their practical applications. Herein, we describe a universal synthetic strategy to construct Au/Rh freestanding superstructures (SSs) through the selective growth of ordered Rh nanoarrays on high-index-faceted Au nanobipyramids (NBPs). This synthetic strategy works on various metal nanocrystal substrates and can yield diverse Au/Rh and Pd/Rh SSs. Especially, the obtained Au NBP/Rh SSs exhibit high photocatalytic activity toward N2 fixation as a result of the spatially separated architecture, local electric field enhancement, and the antenna-reactor mechanism. Both theoretical and experimental results reveal that the Au NBPs can function as nanoantennas for light-harvesting to generate hot charge carriers for driving N2 fixation, while the Rh nanoarrays can serve as the active sites for N2 adsorption and activation to synergistically promote the overall catalytic activity in the Au NBP/Rh SSs. This work offers new avenues to rationally designing and constructing spatially separated plasmonic photocatalysts for high-efficiency catalytic applications.

Trace detection of chiral J-aggregated molecules adsorbed on single Au nanorods.

Trace detection of chiral molecules, which is of great significance in chemical, biological, medical and pharmaceutical sciences, requires microscopic techniques at the single-particle or single-molecule level. Although ensemble experiments show that the circular dichroism of chiral molecules can be amplified by plasmonic nanocrystals, trace detection of small chiral molecules remains challenging due to weak signals that are far below the detection limit. Herein, we demonstrate trace detection of chiral J-aggregated molecules adsorbed on individual Au nanorods (NRs) using single-particle circular differential scattering (CDS) spectroscopy. Through measuring the single-particle CDS spectra, we identified dip-peak bisignatures and further determined the chirality by matching them with calculations modelled with chiral media. We therefore find that plasmonic nanocrystals can dramatically amplify the circular dichroism of strongly coupled molecules to a detectable level so that the detection limit is as low as 3.9 × 103 molecules on an individual plasmonic nanoparticle, whereas 2.5 × 1012 molecules free in solution are barely detectable using a commercial circular dichroism instrument, suggesting a significant amplification factor of 108. Our method provides a promising strategy with a high amplification factor, shedding light on the trace detection of chiral molecules using optical microscopic methods.

Engineering a Hollow Carbon Sphere-Based Triphase Microenvironment for Enhanced Enzymatic Reaction Kinetics and Bioassay Performance.

Electrochemical bioassays based on oxidase reactions are frequently used in biological sciences and medical industries. However, the enzymatic reaction kinetics are severely restricted by the poor solubility and slow diffusion rate of oxygen in conventional solid-liquid diphase reaction systems, which inevitably compromises the detection accuracy, linearity, and reliability of the oxidase-based bioassay. Herein, an effective solid-liquid-air triphase bioassay system is provided that uses hydrophobic hollow carbon spheres (HCSs) as oxygen nanocarriers. The oxygen stored in the cavity of HCS can rapidly diffuse to the oxidase active sites through the mesoporous carbon shell, providing sufficient oxygen for oxidase-based enzymatic reactions. As a result, the triphase system can significantly improve the enzymatic reaction kinetics and obtain a 20-fold higher linear detection range than the normal diphase system. Other biomolecules can also be determined using this triphase technique, and the triphase design strategy offers a new route to address the gas deficiency problem in catalytic reactions that involve gas consumption.

Observation of Enantiomeric Switching of Individual Plasmonic Metamolecules.

Active plasmonic metamolecules under microscopic observation are promising for optical reporters in single molecule sensing applications. While self-assembled reconfigurable chiral plasmonic metamolecules can be conveniently engineered with sensing functionalities, their observation is usually based on ensemble measurements, where the chiroptical response of enantiomers tend to cancel each other in ensemble circular dichroism. Herein, we demonstrate microscopic observation of enantiomeric switching of individual active DNA origami-assembled plasmonic metamolecules. The metamolecules are immobilized on a glass substrate in a microfluidic chamber, in which the plasmonic metamolecule can maintain their activities upon certain local stimuli as in solution. In circular differential scattering, two enantiomeric states controlled by the strand-displacement reaction display opposite spectral signals to each other, representing successful enantiomeric switching of the chirality. Moreover, in a close-to-racemic mixture of chiral metamolecules controlled by pH-sensitive strands, the coexistence of enantiomeric individuals, which is concealed in ensemble measurements, is clearly identified.

DNA-Assembled Chiral Satellite-Core Nanoparticle Superstructures: Two-State Chiral Interactions from Dynamic and Static Conformations.

A significant challenge exists in obtaining chiral nanostructures that are amenable to both solution-phase self-assembly and solid-phase preservation, which enable the observation of unveiled optical responses impacted by the dynamic or static conformation and the incident excitations. Here, to meet this demand, we employed DNA origami technology to create quasi-planar chiral satellite-core nanoparticle superstructures with an intermediate geometry between the monolayer and the double layer. We disentangled the complex chiral mechanisms, which include planar chirality, 3D chirality, and induced chirality transfer, through combined theoretical studies and thorough experimental measurements of both solution- and solid-phase samples. Two distinct states of optical responses were demonstrated by the dynamic and static conformations, involving a split or nonsplit circular dichroism (CD) line shape. More importantly, our study on chiral nanoparticle superstructures on a substrate featuring both a dominant 2D geometry and a defined 3D represents a great leap toward the realization of colloidal chiral metasurfaces.

Single-particle spectroscopic investigation on the scattering spectrum of Au@MoS2 core-shell nanosphere heterostructure.

Owing to the uniform shape of the nanospheres, the Au@MoS2 core-shell nanosphere heterostructure enables us to design nano-optoelectronic devices and nanosensors with highly tunable and reproducible optical properties. However, until now, at the single-particle level, there is still uncertainty as to how much the scattering characteristics depend on the particle size and the local environment. In this letter, we performed an in situ single-particle study of the scattering spectrum of the Au@MoS2 core-shell nanosphere heterostructure before and after coating with the MoS2 layer. Single-particle characterization confirms that the classic quasi-static approximation (QSA) theory can be used to predict the scattering spectra of Au@MoS2 core-shell nanoparticles. Moreover, we have found that the A and B-exciton absorption peaks do not rely on the local refractive index change, while the position of the particle plasmon resonances does. Such features can be used as an internal reference for sensing applications against measurement errors, such as defocusing the imaging. Our results show that Au@MoS2 core-shell nanoparticles have the potential to become one of the promising nanosensors in the field of single-particle sensing.

4.9% Au stabilizes Ag in an atomically homogenous bimetallic alloy for anisotropic nanocrystals with enhanced stability under light irradiation.

Instability problems encountered by Ag nanocrystals largely limit their use in practical applications. In AuAg bimetallic alloys, the stability of Ag can be greatly enhanced, whereas doping a high fraction of Au to the alloy usually leads to the loss of the superior properties of Ag and undesirable degradation of the quality factor of the plasmonic resonance. Herein, we provide experimental evidence that the atomically homogenous AuAg alloy nanocrystals with Au fraction as low as 4.9% (at%) possess comparable stability to pure Au, while the superior plasmonic properties of Ag are largely reserved. The study is based on the synthetic strategy developed for the overgrowth on the Au nanorods of atomically homogenous AuAg alloy shells with a tunable Au/Ag ratio but constant size and anisotropic shape. The stability of over 800 individual alloy nanocrystals in the absence of surfactants is simultaneously characterized at the single-particle level for over 10 h under light irradiation. The stability transition is explained in correlation with the charge redistribution of Ag occurring at the same critical Au fraction. We note that such bimetallic alloy nanocrystals with a low Au fraction possessing both high stability and high quality of resonance are preferred in fundamental researches and practical applications.

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.

Plasmonic thermochromism based on a reversible redox reaction of Ag+/Ag on Au nanorods.

Reversible redox reaction-based thermochromism using plasmonic nanocrystals is challenging due to the requirements set based on the complexity of the reaction system where the oxidizing and reducing agents must not interfere with each other, and both should possess temperature sensitivity. Herein, we demonstrate plasmonic thermochromism based on a reversible redox reaction of Ag+/Ag on Au nanorods (AuNRs) by incorporating temperature-sensitive reducing and oxidizing agents into the same system. The competition between reduction and oxidation is solely dependent on temperature. When the temperature is above (below) the transition temperature, the reduction of Ag+ (oxidation of Ag) dominates on the surface of AuNRs, and the thermochromic nanostructure solution appears green (red). An experimental study on the mechanism reveals that HOCl produced at low concentrations by H2O2 is the source of the observed temperature dependence of the Ag oxidation. Rationally tuning the transition temperature in a range from 27 to 40 °C can be realized by changing the concentration of some key chemical compounds in the solution. The thermochromic solution can be standalone-functional within multiple cycles of heating and cooling and long-term storage without any additional reagents. Our study provides new insight into plasmonic thermochromism and may pave the way for fabricating smart thermochromic materials.

Mapping Hot Electron Response of Individual Gold Nanocrystals on a TiO2 Photoanode.

Incorporating metal nanocrystals with semiconductor photoanodes significantly enhances the efficiency of the energy conversion in the visible range during water splitting due to the excitation of hot electrons. While extensively studied on ensemble samples, hot electron response of metal nanocrystals in a photoelectrochemical cell remains unexploited at the single-particle level. Herein, we systematically investigate hot electron response of individual single-crystalline gold nanocrystals (AuNCs) on a TiO2 photoanode during water splitting. We directly correlate the morphology of the AuNC and its plasmonic property to the efficiencies involving hot electrons with the help of single-particle dark-field microscopy and photocurrent mapping. Our results show that the efficiencies of individual AuNCs are dependent on a variety of factors including interface condition, applied bias, excitation power, incident angle, and AuNC size. Our research may shed light on optimizing the light-harvesting capability of metal/semiconductor photoanodes by providing insights into the photocatalytic processes.

Fano-like chiroptical response in plasmonic heterodimer nanostructures.

Plasmonic chirality has attracted more and more attention recently due to the enhanced chiroptical response and its potential applications in biosensing. Plasmonic Fano resonance arises from the interference between a dark narrow resonance and a bright broad resonance, and it provides a new paradigm to control the plasmon mode interactions. Even though a strong circular dichroism (CD) effect has been predicted in chiral nanostructures with a Fano resonance, there are few experimental studies, and the correlation between the two effects is unclear. In this research, we investigate these two effects in plasmonic heterodimer nanorods in the same spectral range. We find that the heterodimer nanostructure exhibits a Fano-like resonance and Fano-like chiroptical response, both of which are correlated with the coupling between a super-radiant electric dipole and a sub-radiant magnetic dipole mode. Due to the interference nature of the Fano resonance, the Fano-like chiroptical response exhibits distinctively sharp features in a narrow spectral range. This Fano-like chiroptical response can be explained by a modified chiral molecule theory and a simplified coupled electric-magnetic dipole model. This research may provide new insight into the physics picture of plasmonic chirality and paves the way for the development of sensitive plasmonic sensors.

Reconfigurable Plasmonic Diastereomers Assembled by DNA Origami.

Herein, we report self-assembled reconfigurable plasmonic diastereomers based on DNA nanotechnology. Up to three plasmonic chiral centers were organized by dynamic DNA origami platforms. Meanwhile, each chiral center could be individually controlled to switch between left-handed and right-handed states. Thus, complex and reconfigurable chiral plasmonic diastereomers with eight plasmonic stereoisomers were achieved, driven by programmed DNA reactions. With these plasmonic diastereomers, we demonstrated the existence of strong cross-talk near-field coupling among chiral centers, and the coupling of chiral centers could substantially contribute to the overall CD signals. Our work provides an important bottom-up approach for building complex and dynamic chiral plasmonics and for probing the interactions of plasmonic chiral centers.

Tailoring optical cross sections of gold nanorods at a target plasmonic resonance wavelength using bromosalicylic acid.

In many applications, the optical cross sections of gold nanorods (AuNRs) are required to be tailored at a fixed target longitudinal surface plasmon resonance (LSPR) wavelength depending on the excitation source and the photodetector. In this work, we demonstrate the fine tailoring of optical cross sections of AuNRs at a fixed target resonance wavelength, on the basis of AuNR overgrowth using a binary surfactant mixture consisting of 5-bromosalicylic acid (BSA) and cetyltrimethylammonium bromide (CTAB). A systematic study was performed on the sum effects of the BSA concentration and the volume of the growth solution, which gives a formula for quantitative instructions. Based on the formula, we gave examples for the successful synthesis of AuNRs with different optical cross sections at target LSPR wavelengths. From simulation, a nonlinear relationship was further derived to understand the relationship between the aspect ratio and the width of the AuNRs at a target LSPR wavelength for the dimension design of AuNRs. The ratio of optical against physical cross sections was calculated and plotted as a function of the width. The results clearly indicate that AuNRs with a width of 30 nm possess the highest efficiency in terms of optical per physical cross section. Our study provides reliable methods for the synthesis, as well as guidelines for the dimension design of AuNRs, for use in a variety of applications.

Quantum phase transitions of a generalized compass chain with staggered Dzyaloshinskii-Moriya interaction.

We consider a class of one-dimensional compass models with staggered Dzyaloshinskii-Moriya exchange interactions in an external transverse magnetic field. Based on the exact solution derived from Jordan-Wigner approach, we study the excitation gap, energy spectra, spin correlations and critical properties at phase transitions. We explore mutual effects of the staggered Dzyaloshinskii-Moriya interaction and the magnetic field on the energy spectra and the ground-state phase diagram. Thermodynamic quantities including the entropy and the specific heat are discussed, and their universal scalings at low temperature are demonstrated.

Gold nanorod@iron oxide core-shell heterostructures: synthesis, characterization, and photocatalytic performance.

Iron oxides are directly coated on the surface of cetyl-trimethylammonium bromide (CTAB)-capped gold nanorods (AuNRs) in aqueous solutions at room temperature, which results in AuNR@Fe2O3, AuNR@Fe3O4, and AuNR@Fe2O3@Fe3O4 core-shell heterostructures. The iron oxide shells are uniform, smooth, with characteristic porous structure, and their thickness can be readily tuned. The shell formation is highly dependent on the reaction parameters including pH and CTAB concentration. The Fe2O3 shell is amorphous and exhibits nearly zero remanence and coercivity, while the Fe3O4 shell is ferromagnetic with a low saturation magnetization of about 0.5 emu g-1 due to its low crystallinity and the porous structure. At elevated temperatures achieved by plasmonic heating of the Au core, the Fe2O3 shell transforms from amorphous to γ-Fe2O3 and α-Fe2O3 phases, while the Fe3O4 phase disappears because of the oxidation of Fe2+. A 1.4-fold increase of photocatalytic performance is observed due to the plasmonic resonance provided by the Au core. The photocatalytic efficiency of Fe3O4 is about 1.7-fold higher than Fe2O3 as more surface defects are present on the Fe3O4 shell, promoting the adsorption and activation of reagents on the surface during the catalytic reactions. This approach can be readily extended to other nanostructures including Au spherical nanoparticles and nanostars. These highly uniform and multifunctional core-shell heterostructures can be of great potential in a variety of energy, magnetic, and environment applications.

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.

Massively Screening the Temporal Spectra of Single Nanoparticles to Uncover the Mechanism of Nanosynthesis.

Nanosynthesis is the basis of nanotechnology and its applications. It is necessary to understand the growth mechanism of nanoparticles and the functions of growth factors. An effective way to study the synthesis is at the single nanoparticle level. This study reports a single nanoparticle spectrometer, which is based on a commercial dark-field microscopy and a group of narrowband filters. This spectrometer has many advantages, such as high light transparency (35%-75%), low cost (<$1500), massive screening (≈200 nanoplates at a time), and a high time resolution (<5 s). By using this spectrometer, the galvanic replacement reaction (GRR) is studied on single Ag nanoplates in situ and in real time. The research reveals that GRR on single Ag nanoplates has three different types according to the change of peak wavelength during reaction. Such diverse reaction types can be attributed to the different relative reaction rates of GRR on the faces and edges of Ag nanoplate with different facets. Further research shows that the relative reaction rates of different facets vary a lot under different concentrations of tri-sodium citrate. This research successfully demonstrates that the new single nanoparticle spectrometer can study the growth of single nanoparticles and the effect of growth factors.

"Hot spots" growth on single nanowire controlled by electric charge.

"Hot spots" - a kind of highly active site, which are usually composed of some unique units, such as defects, interfaces, catalyst particles or special structures - can determine the performance of nanomaterials. In this paper, we study a model system, i.e. "hot spots" on a single Ag nanowire in the galvanic replacement reaction (GRR), by dark-field microscopy. The research reveals that electric charge can be released by the formation reaction of AgCl, and consequently the electrochemical potential on Ag nanowire drops. The electric charge could induce the reduction of Ag(+) to form the "hot spots" on the nanowire during the GRR. The appearance probability of "hot spots" is almost even along the Ag nanowire, while it is slightly lower near the two ends. The spatial distance between adjacent "hot spots" is also controlled by the charge, and obeys a model based on Boltzmann distribution. In addition, the distance distribution here has an advantage in electron transfer and energy saving. Therefore, it's necessary to consider the functions of electric charge during the synthesis or application of nanomaterials.