Electric field-induced assembly of Au-Ag alloy nanoparticles into nano-reticulation for ultrasensitive SERS.

This paper discusses a method for assembling Au-Ag alloy nanoparticles (NPs) using direct current (DC) electric field to fabricate highly active SERS substrates. Different nanostructures could be obtained by regulating the intensity and action time of DC electric field. Under the condition of 5mA*10 min, we obtained Au-Ag alloy nano-reticulation (ANR) substrate with excellent SERS activity (Enhancement factor on order of magnitude of 106). ANR substrate has excellent SERS performance due to the resonance matching between its LSPR mode and excitation wavelength. The uniformity of the Raman signal on ANR is greatly improved than bare ITO glass. ANR substrate also has the ability to detect multiple molecules: ANR substrate can respectively detect Rh6G and CV molecules with a concentration as low as 10-10 M and 10-9 M and the Raman spectral intensity of the probe molecules on the surface of the ANR substrate has good linear correlation with the molecular concentration (R2 > 0.95). In addition, ANR substrate can detect both thiram and aspartame (APM) molecules far below (thiram for 0.0024 ppm and APM for 0.0625 g/L) the safety standard, which demonstrate its practical application potential.

Capillary-force-assisted self-assembly of gold nanoparticles into highly ordered plasmonic thin films for ultrasensitive SERS.

In this study, a capillary device based on the surface plasmon-enhanced Raman scattering effect was prepared by a simple and easy method. First, the capillary was treated with APTES solution. Due to the electrostatic effect, gold nanoparticles could be easily and tightly assembled in the capillary inner wall. On this basis, the effects of changing the concentration of APTES, the concentration of colloids and the soaking time of the capillary in the colloids on the assembly of gold nanoparticles on the inner wall of the capillary were studied, and the SERS enhancement effect under different conditions was analyzed, and the optimal solution was successfully found. At the same time, the reason why the capillary substrate shows better SERS performance than the traditional planar substrate is deeply discussed. Since the nanoparticles can be attached to the upper and lower surfaces of the inner wall of the capillary, the utilization rate of nanoparticles and laser is improved, thereby achieving higher enhancement. For the detection of the probe molecule rhodamine 6G, it was proved that the substrate has good uniformity and the lowest detection limit can reach 10-10 M. Finally, the real-life pesticide thiram and the food additive aspartame were tested, and the detection limits could reach 10-6 M and 0.25 g L-1. It is confirmed that the prepared capillary shows excellent SERS performance and can be used for rapid detection in various fields.

Low-cost and flexible paper-based plasmonic nanostructure for a highly sensitive SERS substrate.

The application of a noble-metal-based plasmon-enhanced substrate to detect low-concentration analytes has attracted extensive attention. Most of the substrates used in recently reported researches are based on two-dimensional structures. Hence, we prepared a higher efficiency Raman activity substrate with a filter paper structure, which not only provides more plasmonic "hot spots," but also facilitates analyte extraction and detection due to the flexibility of the paper. The preparation of the plasmonic paper substrate adopted centrifugation to deposit the alloy nanoparticles onto the paper base. The optimal particle deposition condition was found by adjusting the centrifugal force and centrifugation time. Then, the surface-enhanced Raman spectroscopy (SERS) performance of the substrate was enhanced by altering the plasmon resonance peak on the surface of the nanoparticles. The enhancement factor of this paper-based substrate was 1.55×107, with high detection uniformity (10-6 M, rhodamine 6G) and a low detection limit (10-11 M, rhodamine 6G). Then, we applied the SERS substrate to pesticide detection; the detection limit of the thiram reached 10-6 M. As a result, the simple and cost-effective paper-based SERS substrate obtained in this way has high detection performance for pesticides and can be used for rapid detection in the field, which is beneficial to food safety and environmental safety.

Electrically controllable self-assembly of gold nanorods into a plasmonic nanostructure for highly efficiency SERS.

In this Letter, a method for the rapid and efficient preparation of ultrasensitive detection substrates by assembling gold nanorod suspensions with the application of an alternating current (AC) field is proposed, and it is found that frequency and voltage are the effective means of regulation. A sandwich structure (parallel SiO2 plate) not only effectively slows down the evaporation rate, but also visually reveals the changes in the assembly process. Under the optimal assembly conditions, the sensitivity and uniformity of the substrate to different probe molecules are tested. The Raman detection results experimentally show that the detection limits of Rhodamine 6G (Rh6G), crystal violet (CV), and Aspartame (APM) molecular solutions are 10-14 M, 10-10 M, and 62.5 mg/L, respectively, and the mixed dye molecular solutions can also be effectively distinguished. Furthermore, Rh6G and CV characteristic peaks at 1647 cm-1 and 1619 cm-1 were measured at randomly selected positions, and their relative standard deviations (RSDs) were 5.63% and 8.45%, respectively, indicating that the substrate has good uniformity. The effective regulation of the self-assembly results of nanoparticles will further enhance the practical application effect of surface-enhanced Raman technology and expand the application prospects of this technology.

Nanoscale engineering of ring-mounted nanostructure around AAO nanopores for highly sensitive and reliable SERS substrates.

Surface-enhanced Raman scattering (SERS) is recognized as one of the most favored techniques for enhancing Raman signals. The morphology of the SERS substrate profoundly affects molecular Raman spectra. This study aimed to construct a ring-mounted nanostructured substrate via liquid-liquid two-phase self-assembly incorporated with anodic aluminum oxide (AAO) membrane transfer techniques. High-density nanoparticles (NPs) assembled on AAO membranes were ascribed to reduce the diameters of the nanopores, with Au-Ag alloy NPs to regulate the dielectric constant so as to reveal the local surface plasmon resonance tunability. SERS engineered in this way allowed for the fabrication of a ring-mounted nanostructured substrate where the distribution density of NPs and dielectric constant could be independently fine-tuned. High SERS activity of the substrate was revealed by detecting the enhanced factor of crystal violet and rhodamine 6G molecules, which was up to 1.56 × 106. Moreover, SERS of thiram target molecules confirmed the supersensitivity and repeatability of the substrate as a practical application. The results of this study manifested a low-cost but high-efficiency ring-mounted nanostructured SERS substrate that might be suitable in many fields, including biosensing, medical research, environmental monitoring, and optoelectronics.

Manipulating upconversion luminescence intensity in a single crystal particle with a waveguide structure.

Lanthanide (Ln)-doped upconversion luminescence (UCL) materials have attracted worldwide attention due to their unique photophysical characteristics. However, how to effectively improve their UCL efficiency has always been an important scientific issue. Here, we design and fabricate ß-NaYF4 microtubes (MTs) with a natural hexagonal shape in the cross section and wedge shape on both top vertexes, which can be regarded as an optical waveguide. The UCL property of a single ß-NaYF4:Yb3+,Er3+(or Tm3+) MT is systematically investigated based on waveguide-excitation modes. It is found that the excitation light can be efficiently coupled in the ß-NaYF4:Yb3+,Er3+(or Tm3+) MT by modulating the angle between the wedge-shape end plane of MT and the microscope slide. In addition, it is clearly observed that the excitation light can be confined and propagate in the MT by introducing a 633 nm laser, which is mainly due to the natural waveguide structure with a stronger confinement and propagation effect of light, thereby enhancing light-to-MT interactions. The current work provides a powerful solution to build high-efficiency Ln-doped UCL materials, which may have potential applications in the optical communication and biomedical fields.

Induced circular dichroism of achiral dielectric elliptical hole arrays with a monolayer borophene film.

Induced circular dichroism (ICD) is widely used in miniature polarizers, molecular detection, and negative refractive index media. However, enhancing and the dynamic regulation of ICD signals of achiral nanostructures in the visible and near-infrared range remain the current challenges. Here, monolayer borophene (MB) with anisotropic conductance was incorporated into achiral nanostructures, which consisted of achiral dielectric elliptical hole arrays (DENAs) placed on a silver substrate. Two narrowband ICD signals for DENAs/MB were achieved in the near-infrared range under different circularly polarized lights. The distributions of the magnetic field of DENAs/MB could explain the two narrowband ICD signals originating from the coupling of surface plasmon polariton resonances along the x- and y directions. Not only could the ICD signals be tuned by the structural parameters of DENAs/MB, but they could also be actively tuned by the incident angles of the excitation light and the carrier concentration of MB. In addition, the sensitivity and the figure of merit of DENAs/MB could reach 302/RIU and 61.0. These results provide a concise method for the design of dynamically adjustable chiral devices based on borophene and promote its application in molecular recognition and chiral catalysis.

Dynamically adjustable-induced THz circular dichroism and biosensing application of symmetric silicon-graphene-metal composite nanostructures.

Induced circular dichroism (ICD) has been used to detect biomolecular conformations through the coupling between chiral molecules and achiral metal nanostructures with the localized surface plasmon (LSP). However, this ICD is always weak and cannot be dynamically adjusted. Here, we put dielectric and graphene nanostructures on a metal-substrate for restricting more light energies and obtaining dynamic adjustable performance. A composite nanostructure array composed of achiral silicon-nanorods on a metal-substrate and graphene-ribbons (ASMG) is theoretically investigated. Two strong ICD signals appear in the THz region. Near-field magnetic distributions of ASMG reveal that the two strong ICD signals are mainly due to the surface plasmon resonances (SPPs) on the metal-substrate and LSP in the graphene nanostructures, respectively. The ICD signals strongly depend on the geometric parameters of ASMG and are dynamically adjusted by just changing the Fermi levels of graphene-ribbons. In addition, left-handed ASMG and right-handed ASMG can be used to identify the chiral molecular solutions with different chiralities. The maximum enhancement factor of the chiral molecular solutions could reach up to 3500 times in the THz region. These results can help to design dynamically adjustable THz chiral sensors and promote their application in biological monitoring and asymmetric catalysis.

Plasmonic alloy nanochains assembled via dielectrophoresis for ultrasensitive SERS.

It is great challenge and interesting for researchers to fabricate substrates for enhanced Raman and sensor, and assemble some easy-to-synthesize metallic nanomaterials into controllable nanostructures with special morphologies and arrangements, via alternating current (AC) electric field. The Au-Ag alloy nanoparticles (Au-Ag alloy NPs) colloidal suspension with excellent dispersibility synthesized by wet chemical method, and the morphology of the assembly can be well controlled by regulating the frequency of the AC electric field. Au-Ag alloy nanochains array (Au-Ag ANCs) with dense plasmonic "hot spots" is formed when the AC electric field of 4Vpp-30kHz is applied, which is supported by the result of finite element method (FEM) numerical simulation. Experimental results demonstrate that Au-Ag ANCs show excellent SERS activity: Au-Ag ANCs can detect both Rhodamine 6G (Rh6G) and crystal violet (CV) in the magnitude order of 10-10 M, and the Raman peaks intensity and analyte concentration has a strong linear correlation (R2 is 0.99339 and 0.95916, respectively). Besides, the introduction of Au-Ag ANCs makes the Raman spectra intensity of thiram (a pesticide) with a concentration of 30 ppm on the surface of the blank ITO glass significantly enhanced, and it can detect thiram with a concentration as low as 0.03 ppm. In addition, Au-Ag ANCs substrate exhibits great uniformity and stability, so they have considerable application potential in the field of quantitative detection of trace substances.

Nanoscale flexible Ag grating/AuNPs self-assembly hybrid for ultra-sensitive sensors.

In this paper, Au nanoparticles (AuNPs) are prepared using wet chemical reduction transfer of dense AuNPs film by self-assembly to the surface of Ag grating, which is inverted from the inner DVD after evaporation. The Ag grating/AuNPs self-assembly hybrid substrate commonly used in surface-enhanced Raman scattering (SERS) research is produced. The coupling effect between AuNP-AuNP and AuNPs-Ag slugs can evidently enhance the local electric field. Experimental results show that the hybrid SERS substrate can detect 10-9 M Rh6G, and the enhancement factor reaches 4.4 × 105. This small, cheap hybrid substrate has enormous potential in the field of SERS sensing.

Circular dichroism enhancement and dynamically adjustment in planar metal chiral split rings with graphene sheets arrays.

Chiral plasmonic nanostructures have become a promising platform for polarization converters and molecular analysis. However, the circular dichroism (CD) of planar chiral plasmonic nanostructures is always weak and difficult for dynamic adjustment. In this work, graphene sheets (GSs) are introduced in planar metal chiral split rings (MCSRs) to enhance and dynamically adjust their CD effect. The chiral split rings consist of rotated big and small split rings. Simulation results show that the plasmonic coupling between MCSRs and GSs can enhance the absorption and CD spectra of MCSRs at two resonant wavelengths. The surface current distributions reveal that the CD signals are due to the localized surface plasmon resonances on the big and small split rings, respectively. The loss distributions illustrate that the increased loss mainly locates in GSs. In addition, the CD spectra of MCSRs/GSs can be dynamically adjusted and influenced by the Fermi energy of GSs, the geometric parameters of MCSRs and, the mediums in the environment. It can be used to detect the environmental temperature and concentration. The results help to design dynamically adjustable chiral nanostructures and promote their applications in environment detection.

Plasmon-exciton coupling for nanophotonic sensing on chip.

The monolayer graphene-noble metallic nanostructure hybrid system with excellent optical characteristic, which is deserved pay attentions in the study of surface-enhanced Raman scattering spectroscopy. In this work, a hybrid sandwich structure is designed to transfer single-layer graphene to the surface of discs substrate covered by silver film and assembly of the dense Au nanoparticles (AuNPs). Blu-ray disc has a cycle density of approximately 5.7 times that of DVD-R due to the different storage capacities of these optical discs. In the research, enhancement effects have been explored for two different periodic grating structures. Compared to spectra of Si/G structure, Graphene Raman spectra from Blu-grating/AuNPs/G structure and Blu-grating/G/AuNPs enhancement multiples at the 2D peak position possesses different Raman responses of 1.09 and 2.51 times, respectively. The sandwich hybrid structure of Ag grating/graphene/AuNPs obtains a Raman enhancement factor (EF) of 6.2×108 for Rhodamine 6G and surface-enhanced Raman Scattering(SERS) detection limit of 0.1 nM. These findings can be attributed to the electric field enhancement of the hybrid structure and the chemical enhancement of graphene. This study provides a new approach for SERS detection and offers a new technique for designing SERS sensors with grapheme-plasmon hybrid structures.

Strong circular dichroism enhancement by plasmonic coupling between graphene and h-shaped chiral nanostructure.

Circular dichroism (CD) is useful in polarization conversion, negative refraction chemical analysis, and bio-sensing. To achieve strong CD signals, researchers constantly break the symmetry of nanostructures. However, how to further enhance the CD based on a new mechanism has become a new challenge in this field. In this work, a hybrid plasmonic chiral system composed of an array of graphene ribbons (GRs) over h-shaped sliver chiral nanostructures (HSCNs) is theoretically investigated. Results demonstrate that the plasmonic coupling between HSCNs and GRs results in different enhanced absorptions for different circularly polarized lights. The absorbance of right circularly polarized light is enhanced to perfect absorption; the absorption of left circularly polarized light is enhanced weakly. It leads to the CD effect of HSCNs@GRs approaching 88%. The loss distributions of HSCNs and HSCNs@GRs reveal that the absorption is enhanced and transferred from HSCNs to GRs. Moreover, the current distributions of HSCNs@GRs are simplified to equivalent LC resonant circuits, which can qualitatively explain the change of CD signals by tuning geometrical parameters of HSCNs@GRs. The findings of this work provide a new method of enhancing chirality and benefit the design of graphene-based chiral optoelectronic devices.

Multiband circular dichroism from bilayer rotational F4 nanostructure arrays.

A chiral nanostructure array is designed, which is composed of a bilayer rotational F4-shaped nanoarray configuration. The surface plasmon resonance and circular dichroism are studied by changing the parameters of the structure. The results show that the structure has strong multiband circular dichroism, which is attributed to the coupling of the layers. In theory, based on the Born-Kuhn model, the upper and lower nanostructures are equivalent to electric dipoles. By analyzing the coupling mode of electric dipoles in the upper and lower layer, the mechanism of circular dichroism and the shift of the circular dichroism resonance are revealed. Besides, there are several specific modes that are fault tolerant of fabrication issues. This feature unveils the bright prospect of spectral anti-interference. So, the suggested chiral nanostructure can be used in biologically targeted molecular detection and spectral sensing.

Preparation of SiO2@Ag Composite Nanoparticles and Their Antimicrobial Activity.

At normal atmospheric temperature, the modified sol­gel method was employed to synthesize SiO2 nanospheres (SiO2 NSs) whose average size was about 352 nm. Silver nanoparticles (Ag NPs) were uniformly distributed on the surface of SiO2 nanospheres (SiO2 NSs) by applying chemical reduction method at 95 °C and the size of silver nanoparticles (Ag NPs) could be controlled by simply tuning the reaction time and the concentration of sodium citrate. Besides, the size, morphology, structure and optical absorption properties of SiO2@Ag composite nanoparticles were measured and characterized by laser particle size analyzer (LPSA), transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD) and ultraviolet visible absorption spectrometer (UV-Vis), respectively. Furthermore, antimicrobial effect experiments that against gram-negative bacteria (E. coli) and gram-positive bacteria (S. aureus) were carried out to characterize the antibacterial activity of synthesized SiO2@Ag composite nanoparticles. The results show that the prepared SiO2@Ag composite nanoparticles have strong antimicrobial activity, which is associated with the size of silver nanoparticles.

Metal-enhanced fluorescence of single shell-isolated alloy metal nanoparticle.

A single silica-shell isolated Au-Ag alloy nanoparticle is used for investigating a metal-enhanced fluorescence effect. Well-dispersed alloy nanoparticles are prepared by the facile chemical method, and the property of local surface plasmon resonance is controlled by adjusting the metal component of the alloy and shell thickness. The distance dependence of fluorescence enhancement for a single Au-Ag alloy nanoparticle is studied systematically with different silica shell thickness ranging from 2 to 35 nm. The isolation shell not only adjusts the distance between metal surface and fluorophore emitters but also improves the chemical stability of the metal particle.

Facile fabrication and upconversion luminescence enhancement of LaF3:Yb3+/Ln3+@SiO2 (Ln = Er, Tm) nanostructures decorated with Ag nanoparticles.

A novel hybrid nanostructure, that is a Ag nanoparticle decorated LaF(3):Yb(3+)/Ln(3+)@SiO(2) nanosphere (Ln=Er, Tm), was constructed by a facile strategy, and characterized by XRD, TEM, FTIR, XPS and UV-vis-NIR absorption. Obvious spectral broadening and red-shift on the surface plasmon resonance were obtained by adjusting the size and configuration of Ag nanoparticles. Effective upconversion luminescence enhancements for Er(3+) and Tm(3+) containing samples were obtained. It is suggested that the luminescence enhancement results from both the excitation and emission processes, and the configuration of the studied hybrid nanostructure is an efficient system to enhance the luminescence emission of rare earth doped nanomaterials. It is believed that the enhancement from the hybrid nanostructure will find great potential in the development of photovoltaic solar cells.

Fabrication of complexed nanostructure using AAO template for ultrasensitive SERS detection.

In the study of surface-enhanced Raman scattering (SERS) processes, a simple and fast approach is needed to ensure the large-scale preparation of SERS substrates. This article uses anodic aluminum oxide (AAO) as a template to assemble gold nanoparticles (Au NPs) into an ordered array. By changing the pore size of AAO and silanizing the pores, the number and density of Au NPs entering the pores through liquid-liquid two-phase self-assembly (LLSA) can be effectively regulated. Using Rh6G (Rhodamine 6G) and CV (Crystal Violet) molecules as probe molecules, substrate sensitivity was evaluated with an enhancement factor of up to 6.34 × 107. In addition, the uniformity of the substrate is good, with a relative standard deviation (RSD) of 9.94%, and the logarithmic concentration and the Raman signal presented significant linear correlations R2 was 0.997 and 0.985, respectively. The detection limit of the substrate for APM (aspartame) as a solvent is as low as 0.0078 g/L. Finally, the substrate was subjected to high sensitivity testing on two types of beverages containing APM sold, proving the practicality of the substrate. It is expected to achieve simple and rapid detection in food additive trace detection in the future.

Enhancement and sensing applications of ultra-narrow band circular dichroism of the chiral nanopore films based on Bragg reflector.

Narrow-band circular dichroism (CD) has attracted considerable attention in the high-sensitivity detection of chiral molecules and chiral catalysis. However, achieving dynamic adjustment of narrow-band CD signals is challenging. In this study, we introduce a disruption layer (DL) and molybdenum disulfide (MoS2) into an L-shaped chiral nanohole array based on a distributed Bragg reflector (DBR), forming L-shaped chiral nanoholes (LCNAs/DL-DBR/MoS2), and investigate the mechanism of CD signal generation. Simulation results show that LCNAs/DL-DBR/MoS2 generate three narrow-band CD signals in the visible region. Analysis of the near-field electric field maps reveals that the three CD peaks of LCNAs/DL-DBR/MoS2 are caused by three Tamm resonances in the DBR layer. The producing and adjusting mechanisms of the CD signals are achieved by changing the structural parameters and the number of MoS2 layers. Dynamic adjustment of the CD signals of LCNAs/DL-DBR/MoS2 can be achieved by changing the environmental temperature. Furthermore, by altering the refractive index of the environment and the DBR layer, it is demonstrated that LCNAs/DL-DBR/MoS2 has a high-quality factor. Our theoretical simulations aid in the design of UNB chiral devices, opening up new avenues for environmental monitoring and the detection of chiral molecules with exceptional sensitivity.

Enhancement and Sensing Application of Ultra-Narrowband Circular Dichroism in the Chiral Nanostructures Based on Monolayer MoS2 and a Distributed Bragg Reflector.

Narrowband circular dichroism (CD) has aroused wide concerns in high-sensitivity detections of chiral molecular and chiral catalysis. Nevertheless, the dynamical adjustment of ultra-narrowband (UNB) CD signals is hard to achieve. In this work, single-layer molybdenum disulfide (MoS2), vanadium dioxide (VO2), and a distributed Bragg reflector (DBR) are introduced into X-shaped chiral nanostructures (XCNs) for overcoming the above challenge. The simulation results show that XCNs can generate four strong UNB CD signals in the near-infrared band, and XCNs/MoS2 can further enhance the UNB CD signals. The full width at half-minimum of UNB CD signals can reach 0.14 nm. The electric field distributions of XCNs/MoS2 show that the four CD signals originate from the coupling between the guided mode resonances along the x and y axes in the VO2 layer and the Tamm plasmon polaritons along the x and y axes in the DBR layer. Four UNB CD peaks can be actively tuned by varying the structural parameters, the number of MoS2 layers, and the environmental temperature. The FOM of XCNs/MoS2 can reach 1487 by changing the refractive index of the DRB layers. These findings contribute to the design of UNB chiral devices and provide new possibilities for environmental monitoring and ultrasensitive detection of chiral molecules.