Reversible Thermoelectric Regulation of Electromagnetic and Chemical Enhancement for Rapid SERS Detection.

Actively controlling surface-enhanced Raman scattering (SERS) performance plays a vital role in highly sensitive detection or in situ monitoring. Nevertheless, it is still challenging to achieve further modulation of electromagnetic enhancement and chemical enhancement simultaneously in SERS detection. In this study, a silver nanocavity structure with graphene as a spacer layer is coupled with thermoelectric semiconductor P-type gallium nitride (GaN) to form an electric-field-induced SERS (E-SERS) for dual enhancement. After applying the electric field, the intensity of SERS signals is further enhanced by over 10 times. The thermoelectric field enables fast and reproducible doping of graphene, thereby modulating its Fermi level over a wide range. The thermoelectric field also regulates the position of the plasmon resonance peak of the silver nanocavity structure, rendering synchronous dual electromagnetic and chemical regulation. Additionally, the method enables the trace detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A detailed theoretical analysis is performed based on the experimental results and finite-element calculations, paving the way for the fabrication of high-efficient E-SERS substrates.

Thermoelectrically Driven Dual-Mechanism Regulation on SERS and Application Potential for Rapid Detection of SARS-CoV-2 Viruses and Microplastics.

Electric-induced surface-enhanced Raman scattering (E-SERS) has been widely studied for its flexible regulation of SERS after the substrate is prepared. However, the enhancement effect is not sufficiently high in the E-SERS technology reported thus far, and no suitable field of application exists. In this study, a highly sensitive thermoelectrically induced SERS substrate, Ag/graphene/ZnO (AGZ), was fabricated using ZnO nanoarrays (NRs), graphene, and Ag nanoparticles (NPs). Applying a temperature gradient to the ZnO NRs enhanced the SERS signals of the probe molecules by a factor of approximately 20. Theoretical and experimental results showed that the thermoelectric potential enables the simultaneous modulation of the Fermi energy level of graphene and the plasma resonance peak of Ag NPs, resulting in a double enhancement in terms of physical and chemical mechanisms. The AGZ substrate was then combined with a mask to create a wearable thermoelectrically enhanced SERS mask for collecting SARS-CoV-2 viruses and microplastics. Its SERS signal can be enhanced by the temperature gradient created between a body heat source (∼37 °C) and a cold environment. The suitability of this method for virus detection was also examined using a reverse transcription-polymerase chain reaction and SARS-CoV-2 virus antigen detection. This work offers new horizons for research of E-SERS, and its application potential for rapid detection of the SARS-CoV-2 virus and microplastics was also studied.

Zinc finger transcription factor MtZPT2-2 negatively regulates salt tolerance in Medicago truncatula.

Zinc finger proteins (ZFPs) are transcription factors involved in multiple cellular functions. We identified a C2H2 type ZFP (MtZPT2-2) in Medicago truncatula and demonstrated that it localizes to the nucleus and inhibits the transcription of 2 genes encoding high-affinity potassium transporters (MtHKT1;1 and MtHKT1;2). MtZPT2-2 transcripts were detected in stem, leaf, flower, seeds and roots, with the highest level in the xylem and phloem of roots and stems. MtZPT2-2 transcription in leaves was reduced after salt stress. Compared with the wild-type (WT), transgenic lines overexpressing MtZPT2-2 had decreased salt tolerance, while MtZPT2-2-knockout mutants showed increased salt tolerance. MtHKT1;1 and MtHKT1;2 transcripts and Na+ accumulation in shoots and roots, as well as in the xylem of all genotypes of plants, were increased after salt treatment, with higher levels of MtHKT1;1 and MtHKT1;2 transcripts and Na+ accumulation in MtZPT2-2-knockout mutants and lower levels in MtZPT2-2-overexpressing lines compared with the WT. K+ levels showed no significant difference among plant genotypes under salt stress. Moreover, MtZPT2-2 was demonstrated to bind with the promoter of MtHKT1;1 and MtHKT1;2 to inhibit their expression. Antioxidant enzyme activities and the gene transcript levels were accordingly upregulated in response to salt, with higher levels in MtZPT2-2-knockout mutants and lower levels in MtZPT2-2-overexpressing lines compared with WT. The results suggest that MtZPT2-2 regulates salt tolerance negatively through downregulating MtHKT1;1 and MtHKT1;2 expression directly to reduce Na+ unloading from the xylem and regulates antioxidant defense indirectly.

LSP-SPP Coupling Structure Based on Three-Dimensional Patterned Sapphire Substrate for Surface Enhanced Raman Scattering Sensing.

Although the fabrication of controllable three-dimensional (3D) microstructures on substrates has been proposed as an effective solution for SERS, there remains a gap in the detection and manufacturability of 3D substrates with high performance. In this study, photolithography is adopted to obtain a pyramid-like array on a patterned sapphire substrate (PSS), with Al2O3 as the dielectric layer. In addition, silver nanoparticles (AgNPs) are used to decorate Au films to obtain mass-producible 3D SRES substrates. In the case of low fluorescence, the substrate realizes the coupling of localized surface plasmon polaritons (LSPs) and surface plasmon polaritons (SPPs), which is consistent with the simulation results obtained using the finite element method. The performance of the SERS substrate is evaluated using rhodamine 6G (R6G) and toluidine blue (TB) as probe molecules with detection limits of 10-11 M and 10-9 M, respectively. The substrate exhibits high hydrophobicity and excellent light-capturing capability. Moreover, it shows self-cleaning ability and long-term stability in practical applications. Allowing for the consistency of the composite substrate in the preparation process and the high reproducibility of the test results, it is considered to be promising for mass production.

3D flexible compositing resonant cavity system for high-performance SERS sensing.

Arrayed resonant cavity with outstanding optical trapping ability have received increasing attention in surface-enhanced Raman spectroscopy (SERS). Here, a three-dimensional (3D) composite AgNPs-Al2O3/Au/inverted patterned sapphire substrate PMMA (IPSSPMMA) flexible resonant cavity system is theoretically and experimentally investigated as a flexible SERS sensor. With the help of an effective plasma coupling (localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs)), as shown by the Finite Element Method, a resonant cavity between IPSSPMMA and a particle-film nanostructure is created. Moreover, the proposed fabrication scheme can be easily used for large-scale fabrication. To measure the performance of IPSSPMMA, Rhodamine 6 G (R6G) and Crystalline violet (CV) were used as probe molecules with limit of detection (LOD) of 6.01 × 10-12 M and 5.36 × 10-10 M, respectively, and enhancement factors (EF) of R6G up to 8.6 × 109. Besides, in-situ detection of CV on the surface of aquatic products with a LOD of 3.96 × 10-5 M, enables highly sensitive in-situ detection of surface analytes. The Raman performance and in-situ detection results demonstrate that the proposed flexible compositing resonant cavity system has the advantages of ultra-sensitivity, stability, uniformity, and reproducibility, and has great potential for applications in the food safety field.

SERS Sensing Using Graphene-Covered Silver Nanoparticles and Metamaterials for the Detection of Thiram in Soil.

Multilayer hyperbolic metamaterial (HMM)-based SERS substrates have received special consideration because they accommodate various propagation modes such as surface plasmonic polaritons (SPP). However, the SPP modes are difficult to generate in HMM due to their weak electric field enhancement. In this article, we designed novel SERS substrates consisting of graphene-covered AgNPs and HMM. The graphene-covered AgNPs work as an external coupling structure for hyperbolic metamaterials due to this structure exhibiting significant plasmonic effects as well as unique optical features. The localized surface plasmonic resonance (LSPR) of the graphene-covered AgNPs excited the SPP and thus formed a strong hot spot zone in the nanogap area of the graphene. The Raman experiment was performed using rhodamine 6G (R6G) and crystal violet (CV), which showed high stability and a maximum enhancement factor of 2.12 × 108. The COMSOL simulation further clarified that enhanced SERS performance was due to the presence of monolayer graphene and provided an atomically flat surface for organic molecules in a more controllable manner. Interestingly, the proposed SERS structure carries out quantitative detection of thiram in soil and can satisfy the basic environmental need for pesticide residue in the soil.

Plasmonic optical fiber gratings based on few-layer Ta2C MXenes for refractive index sensing.

Tilted fiber Bragg grating (TFBG) is a widespread approach for developing refractive index (RI) sensors. The unique optoelectronic properties exhibited by MXene are expected to enhance the performance of TFBG-SPR sensors. In this study, a Ta2C coating appropriate for sensing was obtained by optimizing the photo-deposition time, which addressed the challenge of preparing large areas of MXene. The uniform coating of the few-layer Ta2C increases the wavelength sensitivity and FOM of the sensor to 229.5 nm/RIU and 2228.15 respectively. This significant enhancement was attributed to an ordered MXene phase of the grown Ta2C. The energy band theory verified the metallic nature of the Ta2C and the amplification effect on the RI response. Finite element analysis demonstrated that the stronger absorption band of Ta2C facilitated the generation of surface plasmon polariton. Based on the above benefits, the sensor detected melamine in milk with a detection limit of 7.9 × 10-9M. The TFBG/Au/Ta2C sensor is a promising approach for biochemical analysis and trace detection.

Evolution of optical harmonic generation near bound-states in the continuum in hybrid plasmonic-photonic structures.

We investigate the nonlinear optical harmonic generation behaviors near the bound-states in the continuum (BICs) in hybrid plasmonic-photonic structures. The hybrid structures are designed to consist of a plasmonic grating covered with a nonlinear dielectric waveguide layer, which support two distinct groups of BICs, i.e. the symmetry-protected BICs and Friedrich-Wintgen BICs. The evolution of second- and third-harmonic generation (SHG and THG) near the two groups of BICs was studied. The high dependence of nonlinear response on the local field distribution and tensor components of susceptibility of nonlinear materials was determined. Especially, there exists optimized angles of incidence for efficient SHG and THG response due to the interaction of photonic and plasmonic modes. The results are important to understand the nonlinear response behaviors in hybrid plasmonic-photonic structures and to design the nonlinear photonic devices.

Highly-sensitive sensor based on toroidal dipole governed by bound state in the continuum in dielectric non-coaxial core-shell cylinder.

The electromagnetic fields distributed on the surface region of the nanostructure is very important to improve the performance of the sensor. Here, we proposed a highly sensitive sensor based on toroidal dipole (TD) governed by bound state in the continuum (BIC) in all-dielectric metasurface consisting of single non-coaxial core-shell cylinder nanostructure array. The excitation of TD resonance in a single nanostructure is still challenging. The designed nanostructure not only supports TD resonance in a single nanostructure but also has very high Q-factor. More importantly, its electric field distributes at the surface of outer cylinder-shell, which is very suitable for biosensing. To evaluate the sensing performance of our proposed structure, we investigated the sensitivity and the figure of merit (FOM) of nanostructure with different structural parameters. Maximum sensitivity and FOM can reach up to 342 nm/RIU and 1295 when the asymmetric parameter d =10 nm. These results are of great significance to the research of TD resonance and the development of ultrasensitive sensor.

Composite substrate of graphene/Ag nanoparticles coupled with a multilayer film for surface-enhanced Raman scattering biosensing.

In this paper, we designed a surface-enhanced Raman scattering (SERS) substrate for graphene/Ag nanoparticles (Ag NPs) bonded multilayer film (MLF) using the hybrid nanostructures composed of graphene and plasmonic metal components with significant plasmonic electrical effects and unique optical characteristics. This paper achieved the advantages of efficient utilization of electromagnetic field and reduction of fluorescence background based on the electromagnetic enhancement activity of Ag NPs and unique physical/chemical properties of graphene with zero gap structures. Au/Al2O3 was stacked periodically to construct MLF. As indicated by the electric field intensity at the Au/Al2O3 interface of the respective layer, bulk plasmon polariton (BPP) in the MLF was excited and coupled with localized surface plasmon (LSP) in the Ag NPs, which enhanced the electromagnetic field on the top-layer of SERS substrate. To measure the performance of the SERS substrate, rhodamine 6G (R6G) and malachite green (MG) were used as the probe molecules, with the detection limits of 10-11 M and 10-8 M, respectively. The SERS substrate had high sensitivity and uniformity, which indicated that it has a broad application prospect in the field of molecular detection.

Graphene Oxide-Coated Metal-Insulator-Metal SERS Substrates for Trace Melamine Detection.

Surface-enhanced Raman spectroscopy (SERS) has long been an ultrasensitive technique for trace molecule detection. However, the development of a sensitive, stable, and reproducible SERS substrate is still a challenge for practical applications. Here, we demonstrate a cost-effective, centimeter-sized, and highly reproducible SERS substrate using the nanosphere lithography technique. It consists of a hexagonally packed Ag metasurface on a SiO2/Au/Si substrate. A seconds-lasting etching process of a self-assembled nanosphere mask manipulates the geometry of the deposited Ag metasurface on the SiO2/Au/Si substrate, which attains the wavelength matching between the optical absorbance of the Ag/SiO2/Au/Si substrate and the excitation laser wavelength as well as the enhancement of Raman signals. By spin-coating a thin layer of graphene oxide on the substrate, a SERS performance with 1.1 × 105 analytical enhancement factor and a limit of detection of 10-9 M for melamine is achieved. Experimental results reveal that our proposed strategy could provide a promising platform for SERS-based rapid trace detection in food safety control and environmental monitoring.

LSPR optical fiber sensor based on 3D gold nanoparticles with monolayer graphene as a spacer.

Localized surface plasmon resonance (LSPR) optical fiber biosensing is an advanced and powerful label-free technique which gets great attention for its high sensitivity to refractive index change in surroundings. However, the pursuit of a higher sensitivity is still challenging and should be further investigated. In this paper, based on a monolayer graphene/gold nanoparticles (Grm/Au NPs) three-dimensional (3D) hybrid structure, we fabricated a D-shaped plastic optical fiber (D-POF) LSPR sensor using a facile two-step method. The coupling enhancement of the resonance of this multilayer structure was extremely excited by the surface plasmon property of the stacked Au NPs/Grm layer. We found that the number of plasmonic structure layers was of high importance to the performance of the sensor. Moreover, the optimal electromagnetic field enhancement effect was found in three-layer plasmonic structure. Besides, the n*(Grm/Au NPs)/D-POF sensor exhibited outstanding performance in sensitivity (2160 nm/RIU), linearity (linear fitting coefficient R2 = 0.996) and reproducibility. Moreover, the sensor successfully detected the concentration of glucose, achieving a sensitivity of 1317.61 nm/RIU, which suggested a promising prospect for the application in medicine and biotechnology.

Indium Tin Oxide Nanowire Arrays as a Saturable Absorber for Mid-Infrared Er:Ca0.8Sr0.2F2 Laser.

We demonstrated a passively Q-switched Er:Ca0.8Sr0.2F2 laser with indium tin oxide nanowire arrays as an optical modulator in the mid-infrared region. In the Q-switched regime, the maximum output power of 58 mW with a slope efficiency of 18.3% was acquired. Meanwhile, the minimum pulse duration and highest repetition rate of the stable pulse trains were 490 ns and 17.09 kHz, corresponding to single pulse energy of 3.4 µJ and peak power of 6.93 W, respectively. To the best of our knowledge it was the first time that indium tin oxide nanowire arrays were employed as a saturable absorber to make pulse lasers carried out at 2.8 µm. The experimental data show that indium tin oxide nanowire arrays can be employed as a competitive candidate for saturable absorber in the field of mid-infrared solid-state lasers.

Broadband and Ultra-Low Threshold Optical Bistability in Guided-Mode Resonance Grating Nanostructures of Quasi-Bound States in the Continuum.

We model optical bistability in all-dielectric guide-mode resonance grating (GMR) nanostructures working at quasi-bound states in the continuum (BICs). The complementary metal-oxide-semiconductor (CMOS) compatible material silicon nitride (SiN) is used for the design of nanostructures and simulations. The ultra-low threshold of input intensity in the feasible nanostructure for nanofabrication is obtained at the level of ~100 W/cm2 driven by quasi-BICs. Additionally, the resonance wavelength in the GMR nanostructure can be widely tuned by incident angles with the slightly changed Q-factor that enables the optical bistable devices to work efficiently over a wide spectrum. The impact of the defects of grating that may be introduced in the fabrication process on the optical properties is discussed, and the tolerance of the defects to the optical performance of the device is confirmed. The results indicate that the GMR nanostructures of broadband and ultra-low threshold optical bistability driven by quasi-BICs are promising in the application of all-optical devices.

Low-Threshold Nanolaser Based on Hybrid Plasmonic Waveguide Mode Supported by Metallic Grating Waveguide Structure.

A high Q-factor of the nanocavity can effectively reduce the threshold of nanolasers. In this paper, a modified nanostructure composed of a silver grating on a low-index dielectric layer (LID) and a high-index dielectric layer (HID) was proposed to realize a nanolaser with a lower lasing threshold. The nanostructure supports a hybrid plasmonic waveguide mode with a very-narrow line-width that can be reduced to about 1.79 nm by adjusting the thickness of the LID/HID layer or the duty ratio of grating, and the Q-factor can reach up to about 348. We theoretically demonstrated the lasing behavior of the modified nanostructures using the model of the combination of the classical electrodynamics and the four-level two-electron model of the gain material. The results demonstrated that the nanolaser based on the hybrid plasmonic waveguide mode can really reduce the lasing threshold to 0.042 mJ/cm2, which is about three times lower than the nanolaser based on the surface plasmon. The lasing action can be modulated by the thickness of the LID layer, the thickness of the HID layer and the duty cycle of grating. Our findings could provide a useful guideline to design low-threshold and highly-efficient miniaturized lasers.

Noble metal modified ReS2 nanocavity for surface-enhanced Raman spectroscopy (SERS) analysis.

The rhenium disulphide (ReS2) nanocavity-based surface enhanced Raman scattering (SERS) substrates ware fabricated on the gold-modified silicon pyramid (PSi) by thermal evaporation technology and hydrothermal method. In this work, the ReS2 nanocavity was firstly combined with metal nanostructures in order to improve the SERS properties of ReS2 materials, and the SERS response of the composite structure exhibits excellent performance in sensitivity, uniformity and repeatability. Numerical simulation reveals the synergistic effect of the ReS2 nanocavity and the plasmon resonance generated by the metal nanostructures. And the charge transfer between the metal, ReS2 and the analytes was also verified and plays an non-ignorable role. Besides, the plasmon-driven reaction for p-nitrothiophenol (PNTP) to p,p'-dimercaptobenzene (DMAB) conversion was successfully in-situ monitored. Most importantly, it is found for the first time that the SERS properties of ReS2 nanocavity-based substrates are strongly temperature dependent, and the SERS effect achieves the best performance at 45 °C. In addition, the low concentration detection of malachite green (MG) and crystal violet (CV) molecules in lake water shows its development potential in practical application.

Surface-enhanced Raman scattering by the composite structure of Ag NP-multilayer Au films separated by Al2O3.

In the present study, a nanoparticle-multilayer metal film substrate was presented with silver nanoparticles (Ag NPs) assembled on a multilayer gold (Au) film by employing alumina (Al2O3) as a spacer. The SERS performance of the proposed structures was determined. It was suggested that the SERS effect was improved with the increase in the number of layers, which was saturated at four layers. The SERS performance of the structures resulted from the mutual coupling of multiple plasmon modes [localized surface plasmons (LSPs), surface plasmon polaritons (SPPs), as well as bulk plasmon polaritons (BPPs)] generated by the Ag NP-multilayer Au film structure. Furthermore, the electric field distribution of the hybrid system was studied with COMSOL Multiphysics software, which changed in almost consistency with the experimentally achieved results. For this substrate, the limit of detection (LOD) was down to 10-13 M for the rhodamine 6G (R6G), and the proposed SERS substrate was exhibited prominently quantitatively detected capability and high reproducibility. Moreover, a highly sensitive detection was conducted on toluidine blue (TB) molecules. As revealed from the present study, the Ag NP-multilayer Au film structure can act as a dependable SERS substrate for its sensitive molecular sensing applications in the medical field.

Composite Structure Based on Gold-Nanoparticle Layer and HMM for Surface-Enhanced Raman Spectroscopy Analysis.

Hyperbolic metamaterials (HMMs), supporting surface plasmon polaritons (SPPs), and highly confined bulk plasmon polaritons (BPPs) possess promising potential for application as surface-enhanced Raman scattering (SERS) substrates. In the present study, a composite SERS substrate based on a multilayer HMM and gold-nanoparticle (Au-NP) layer was fabricated. A strong electromagnetic field was generated at the nanogaps of the Au NPs under the coupling between localized surface plasmon resonance (LSPR) and a BPP. Additionally, a simulation of the composite structure was assessed using COMSOL; the results complied with those achieved through experiments: the SERS performance was enhanced, while the enhancing rate was downregulated, with the extension of the HMM periods. Furthermore, this structure exhibited high detection performance. During the experiments, rhodamine 6G (R6G) and malachite green (MG) acted as the probe molecules, and the limits of detection of the SERS substrate reached 10-10 and 10-8 M for R6G and MG, respectively. Moreover, the composite structure demonstrated prominent reproducibility and stability. The mentioned promising results reveal that the composite structure could have extensive applications, such as in biosensors and food safety inspection.

Aluminum nanoparticle films with an enhanced hot-spot intensity for high-efficiency SERS.

The weak plasmonic coupling intensity in an aluminum (Al) nanostructure has limited potential applications in excellent low-cost surface-enhanced Raman scattering (SERS) substrates and light harvesting. In this report, we aim to elevate the plasmonic coupling intensity by fabricating an Al nanoparticle (NP)-film system. In the system, the Al NP are fabricated directly on different Al film layers, and the nanoscale-thick alumina interlayer obtained between neighboring Al films acts as natural dielectric gaps. Interestingly, as the number of Al film layers increase, the plasmonic couplings generated between the Al NP and Al film increase as well. It is demonstrated that the confined gap plasmon modes stimulated in the nanoscale-thick alumina region between the adjacent Al films contribute significantly to elevating the plasmonic coupling intensity. The finite-difference time-domain (FDTD) method is used to carry out the simulations and verifies this result.

LSPR optical fiber biosensor based on a 3D composite structure of gold nanoparticles and multilayer graphene films.

In this paper, a localized surface-plasmon resonance (LSPR) biosensor, which uses a U-shaped multi-mode fiber (U-MMF), is introduced and investigated. It is modified with a complex of three-dimensional (3D) gold nanoparticles and multilayer graphene as spacer: n*(Au/G)@U-MMF, where n denotes the layer number of gold nanoparticles. The gold nanoparticles were synthesized by reducing chloroauric acid. Graphene films were formed using a liquid/chemical method. The number of gold-nanoparticle layers was found to be critical for the performance of the sensor. Moreover, using the finite-difference time domain, 3D nanostructures, with a wide range of gold-nanoparticle layers, were explored. The sensor showed the sensitivity of 1251.44 nm/RIU, as well as high stability and repeatability; for the measurement-process of time- and concentration-dependent DNA hybridization kinetics with detection concentrations, ranging from 0.1nM to 100 nM, the sensor displayed excellent performance, which points towards a vast potential in the field of medical diagnostics.