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.

Active Control and Sensing Application of Ultra-Narrowband Circular Dichroism in Multilayer Chiral Nanorod Arrays.

Narrowband circular dichroism (CD) has attracted wide attention for its high sensitivity in detecting chiral molecules and catalysis. However, designing a chiral metasurface with excellent sensing performance that can be dynamically tuned still poses challenges. This paper introduces lithium niobate, an electrically tunable material, and a distributed Bragg reflector into chiral nanorod structures to form multilayer chiral nanorod arrays (MCNAs). Simulation results show that MCNAs can generate four strong ultra-narrowband (UNB) CD signals in the visible light spectrum. The UNB CD signal intensity was up to 0.86, and the minimum full width at half-maximum (FWHM) was up to 0.21 nm. The surface electric field and current distribution of MCNAs indicate that the four UNB CD signals mainly originate from the x and y direction Tamm resonances in the chiral nanorod layer. The refractive index of lithium niobate can be tuned by changing the electric field, allowing the active tuning of UNB CD signals. In addition, the sensing performance of MCNAs in the SARS-CoV-2 solution was analyzed, and the figure of merit (FOM) can reach an astonishing 2092. These findings not only assist with the design of UNB chiral devices but also offer new possibilities for the environmental monitoring and ultrasensitive detection of chiral molecules.

Preparation of a Three-Dimensional Composite Structure Based on a Periodic Au@Ag Core-Shell Nanocube with Ultrasensitive Surface-Enhanced Raman Scattering for Rapid Detection.

The absorption and scattering frequencies of surface plasmon resonance can be selectively adjusted by changing the morphology, size, structure, arrangement, and gap between noble metal nanoparticles so that the local electromagnetic field on the substrate surface can be further enhanced. This change will promote and popularize surface-enhanced Raman spectroscopy. This paper reports the research results and improvement scheme of surface enhanced Raman scattering (SERS) activity of silver-coated gold nanocubed/organism (Au@Ag/CW NCs) prepared by three-phase self-assembly. In the experiment, the uppermost oil phase in the three-phase self-assembly process was optimized as ethanol and n-hexane solution containing a specific concentration of a probe molecule rhodamine 6G or aspartame. The probe molecules were directly self-assembled on the surface of the composite substrate to avoid the possible loss and pollution during immersion and preservation and achieve the purpose of rapid detection. The results show that the Au@Ag/CW NC array substrate is a periodic cubic ring structure. The sensitivity, uniformity, reproducibility, and stability of composite Au@Ag/CW NC array substrates are verified by comparing the Raman activities of various substrates. The feasibility of using the substrate to realize rapid SERS detection, compared with the advantages and disadvantages of the traditional soaking method, proved that the prepared substrate and improvement direction have excellent potential for application and development prospects in the field of rapid food additive detection.

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.

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.

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.

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.

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.

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.

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.

Surface-Plasmon-Assisted Growth, Reshaping and Transformation of Nanomaterials.

Excitation of surface plasmon resonance of metal nanostructures is a promising way to break the limit of optical diffraction and to achieve a great enhancement of the local electromagnetic field by the confinement of optical field at the nanoscale. Meanwhile, the relaxation of collective oscillation of electrons will promote the generation of hot carrier and localized thermal effects. The enhanced electromagnetic field, hot carriers and localized thermal effects play an important role in spectral enhancement, biomedicine and catalysis of chemical reactions. In this review, we focus on surface-plasmon-assisted nanomaterial reshaping, growth and transformation. Firstly, the mechanisms of surface-plasmon-modulated chemical reactions are discussed. This is followed by a discussion of recent advances on plasmon-assisted self-reshaping, growth and etching of plasmonic nanostructures. Then, we discuss plasmon-assisted growth/deposition of non-plasmonic nanostructures and transformation of luminescent nanocrystal. Finally, we present our views on the current status and perspectives on the future of the field. We believe that this review will promote the development of surface plasmon in the regulation of nanomaterials.

Two-Dimensional Self-Assembly of Au@Ag Core-Shell Nanocubes with Different Permutations for Ultrasensitive SERS Measurements.

Different self-assembly methods not only directly change the arrangement of noble metal particles on the substrate but also indirectly affect the local electromagnetic field distribution and intensity of the substrate under specific optical excitation conditions, which leads to distinguished different enhancement effects of the structure on molecular Raman signals. In this paper, first, the gold species growth method was used to prepare the silver-coated gold nanocubes (Au@Ag NCs) with regular morphology and uniform size, and then the two-phase and three-phase liquid-liquid self-assembly and evaporation-induced self-assembly methods were used to obtain the substrate structure with different NC arrangement patterns. The optimal arrangement of NCs was found by transverse comparison of Raman signal detection of probe molecules with the same concentration. Subsequently, surface-enhanced Raman scattering (SERS) measurements of Rhodamine (Rh6G) and aspartame (APM) were carried out. Furthermore, the finite element method (FEM) was employed to calculate the local electromagnetic fields of the substrates with different Au@Ag NC arrangements, and the calculated results were in agreement with the experimental results. The experimental results show that the SERS-active substrate was largely associated with the different arrangements of Au@Ag NCs, and the island membrane Au@Ag NCs array substrate obtained by evaporation-induced self-assembly can generate a strong local electromagnetic field due to the edge and corner bonding gap between the tightly arranged NCs; this endows the substrate with benign sensitivity and reproducibility and has great potential in molecular detection, biosensing, and food safety monitoring.

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.

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.

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.

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.

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.

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.

Electrochemical synthesis of tin plasmonic dendritic nanostructures with SEF capability through in situ replacement.

Dendrite nanostructures with noble metals, such as Au and silver, act as plasmonic substrates with excellent potential in enhanced fluorescence technology. However, tin dendritic nanostructures are poorly investigated. In this study, we proposed a method of in situ electrochemical synthesis replacement to fabricate highly branched tin dendritic nanostructures on aluminum substrates. The surface enhanced fluorescence performance of the tin dendrites was tested for the detection of rhodamine 6G as probe molecules, and the result showed that the enhancement factors can reach to 36.5-fold that of an aluminum substrate. The fabricated tin dendrites have numerous nanogaps between the stratified and adjacent ones, thereby creating many plasmon-active "hotspots" dedicated to enhanced fluorescence. Electrical field simulation results for the tin dendritic nanostructures proved that its nanogaps can enhance the nearby local electromagnetic field. As a result, tin dendritic nanostructures exhibit outstanding surface enhanced fluorescence and promising application in biomolecule detection and sensor devices.