Surface-enhanced Raman scattering induced by the coupling of the guided mode with localized surface plasmon resonances

Grating-Integrated Gold Nanograsses Encapsulated with ZIF-8: A Quantitative and Ultrasensitive Surface-Enhanced Raman Scattering Substrate

We proposed and demonstrated highly sensitive hybrid surface-enhanced Raman scattering (SERS) substrates, which are grating-integrated gold nanograsses (GIGN) that are tip-selectively encapsulated by ZIF-8 nanospheres (GIGN/tip-ZIF). This unique structure is realized through the tip-selective modification of GIGN by polyvinylpyrrolidone (PVP), and then, the tips of the GIGN were encapsulated by ZIF-8 nanospheres. The ZIF-8 nanospheres can adsorb analyte molecules, resulting in the spatial overlap between the analyte molecules and the "hotspots" on the tips of GIGN. Such a unique GIGN/tip-ZIF hybrid SERS substrate exhibits high sensitivity and quantitative detection ability. The detection limits can reach as low as 10-11 M, and the relative standard deviation is 5.59% for 4-aminothiophenol (4-ATP). In a wide range of concentrations from 10-5 to 10-11 M, the SERS intensity and concentration relationship can be fitted as a sigmoidal curve with R2 = 0.988. These indicate that the GIGN/tip-ZIF hybrid SERS substrates have broad applications in detecting toxic and harmful substances in food safety, disease diagnosis, and environmental monitoring.

Plasma amplifiers: multiscale light-enhanced uniform SERS composite substrates for breaking through resonance limitations

A phenomenon known as plasmon resonance constitutes a unique optical effect that can induce an enhancement in localized electromagnetic fields, resulting in a substantial increase in the electromagnetic field intensity surrounding metallic nanostructures. In this work, the coupling effect of excitation of surface plasmon polaritons and local surface plasmons in nanoparticles is deeply studied under the background of nanoparticles/one-dimension grating composite structures through grating matching. By employing finite-difference time-domain simulations as our methodological approach, we discern gratings with a periodicity of 1.5 μm support surface plasmon bound states between the gratings. Furthermore, the modulation of SPs along the vertical sidewalls of the grating due to standing wave effects exhibits oscillatory behavior with varying grating heights. Experimental results obtained from the nanoparticle/grating composite SERS substrate validate theoretical predictions, demonstrating higher enhanced Raman signals at 633 nm compared to 532 nm. Remarkably, this structure exhibits good performance, with R6G detection sensitivity down to concentrations as low as 10-10 M and mapping achieving a relative standard deviation of 7.79%, underscoring its uniformity and capability of electromagnetic field enhancement.

Ag Nanoparticles@Au Nanograting Array as a 3D Flexible and Effective Surface-Enhanced Raman Scattering Substrate

Surface-enhanced Raman scattering (SERS) is a powerful analytical technique for chemical identification, but it remains a great challenge to realize the large-scale and well-controlled fabrication of sensitive and repeatable SERS substrates. Here, we report a facile strategy to fabricate centimeter-sized periodic Au nanograting (Au-NG) decorated with well-arranged Ag nanoparticles (Ag-NPs) (denoted as Ag-NPs@Au-NG) as a three-dimensional (3D) flexible hybrid SERS substrate with high sensitivity and good reproducibility. The Au-NG patterns with periodic ridges and grooves are fabricated through nanoimprint lithography by employing a low-cost digital versatile disc (DVD) as a master mold, and the Ag-NPs are assembled by a well-controlled interface self-assembly method without any coupling agents. Multiple coupling electromagnetic field effects are created at the nanogaps between the Ag-NPs and Au-NG patterns, leading to high-density and uniform hot spots throughout the substrate. As a result, the Ag-NPs@Au-NG arrays demonstrate an ultrahigh SERS sensitivity as low as 10-13 M for rhodamine 6G with a high average enhancement factor (EF) of 1.85 × 108 and good signal reproducibility. For practical applications, toxic organic pollutants including crystal violet, thiram, and melamine have been successfully detected with high sensitivity at a low detection limit, showing a good perspective in the rapid detection of toxic organic pollutants.

Free-standing plasmonic nanoarrays for leaky optical waveguiding and sensing

Flat optics nanogratings supported on thin free-standing membranes offer the opportunity to combine narrowband waveguided modes and Rayleigh anomalies for sensitive and tunable biosensing. At the surface of high-refractive index Si₃N₄ membranes we engineered lithographic nanogratings based on plasmonic nanostripes, demonstrating the excitation of sharp waveguided modes and lattice resonances. We achieved fine tuning of these optical modes over a broadband Visible and Near-Infrared spectrum, in full agreement with numerical calculations. This possibility allowed us to select sharp waveguided modes supporting strong near-field amplification, extending for hundreds of nanometres out of the grating and enabling versatile biosensing applications. We demonstrate the potential of this flat-optics platform by devising a proof-of-concept nanofluidic refractive index sensor exploiting the long-range waveguided mode operating at the sub-picoliter scale. This free-standing device configuration, that could be further engineered at the nanoscale, highlights the strong potential of flat-optics nanoarrays in optofluidics and nanofluidic biosensing.

Rational design of an allosteric G-quadruplex aptamer probe for ultra-sensitive detection of melamine in milk

Efficient and accurate detection of melamine in dairy products remains a crucial yet challenging task. Herein, an allosterically modulated G-quadruplex-integrated aptamer is rationally designed with thymine-rich recognition termini for melamine binding. The detection process is facile by simply introducing the analyte into the mixture consisting of G-quadruplex aptamer probes, exonuclease III, and thioflavin T (ThT). The detection feasibility is confirmed by the polyacrylamide gel electrophoresis and fluorescence measurement results. This exonuclease III-assisted signal amplifiable approach works well in a linear range from 0.1 nM to 0.1 μM. Moreover, a detection limit as low as 83 pM is easily achieved, which is almost five orders of magnitude smaller than the maximum allowable melamine levels (about 8 μM) defined by many countries all over the world. The whole assay time for each test is no longer than 1 h. Additionally, the scheme is highly specific and satisfactory recovery rates (from 91% to 104%) are readily obtained when challenged with melamine-spiked milk samples. Therefore, the label-free, turn-on, low-cost, and time-efficient method can be used for reliable detection of melamine in an easily manipulated and ultra-sensitive manner, which may find its utilization in the field of food safety, biomedical engineering, and clinical diagnosis.

Binary Plasmonic Assembly Films with Hotspot-Type-Dependent Surface-Enhanced Raman Scattering Properties

Tuning and controlling the plasmon coupling of noble metal nanoparticles are significant for enhancing their near-field and far-field responses. In this work, a novel heterogeneous plasmonic assembly with a controllable hot spot model was proposed by the conjugation of Au nanospheres (NSs) and Au@Ag core-shell nanocube (NC) films. Three hotspot configurations including point-to-point type, point-to-facet type, and facet-to-facet type were fabricated and transformed simply by adjusting the doping ratio of nanoparticles in the co-assembly film. Expectedly, the localized surface plasmon resonance (LSPR) property and surface-enhanced Raman scattering (SERS) performance of the binary assembly film exhibit distinct diversity due to the change in the hotspot conformation. Interestingly, the point-to-facet hotspot in hybrid assembly films can provide the most extraordinary enhancement for SERS behavior compared with single-component Au NS and Au@Ag NC plasmonic assemblies, which is further confirmed by the finite-different time-domain simulation results of dimer nanostructures. In addition, the two-dimensional binary assemblies of Au NS doping in Au@Ag NCs with excellent sensitivity and high reproducibility were successfully applied in the identification of ketamine. This work opens a new avenue toward the fabrication of plasmonic metal materials with collective LSPR properties and sensitive SERS behavior.

Silicon Microchannel-Driven Raman Scattering Enhancement to Improve Gold Nanorod Functions as a SERS Substrate toward Single-Molecule Detection

The investigation of enhanced Raman signal effects and the preparation of high-quality, reliable surface-enhanced Raman scattering (SERS) substrates is still a hot topic in the SERS field. Herein, we report an effect based on the shape-induced enhanced Raman scattering (SIERS) to improve the action of gold nanorods (AuNRs) as a SERS substrate. Scattered electric field simulations reveal that bare V-shaped Si substrates exhibit spatially distributed interference patterns from the incident radiation used in the Raman experiment, resulting in constructive interference for an enhanced Raman signal. Experimental data show a 4.29 increase in Raman signal intensity for bare V-shaped Si microchannels when compared with flat Si substrates. The combination of V-shaped microchannels and uniform aggregates of AuNRs is the key feature to achieve detections in ultra-low concentrations, enabling reproducible SERS substrates having high performance and sensitivity. Besides SIERS effects, the geometric design of V-shaped microchannels also enables a "trap" to the molecule confinement and builds up an excellent electromagnetic field distribution by AuNR aggregates. The statistical projection of SERS spectra combined with the SIERS effect displayed a silhouette coefficient of 0.83, indicating attomolar (10^-18 mol L^-1) detection with the V-shaped Si microchannel.

Coupling enhancement mechanisms, materials, and strategies for surface-enhanced Raman scattering devices

Surface-enhanced Raman scattering (SERS) has become one of the most sensitive analytical techniques for identifying the chemical components, molecular structures, molecular conformations, and the interactions between molecules. However, great challenges still need to be addressed until it can be widely accepted by the absolute quantification of analytes. Recently, many efforts have been devoted to addressing these issues via various electromagnetic (EM), chemical (CM), and EM-CM hybrid coupling enhancement strategies. In comparison with uncoupled SERS devices, they offer key advantages in terms of sensitivity, reproducibility, uniformity, stability, controllability and reliability. This review provides an in-depth analysis of coupled SERS devices, including coupling enhancement mechanisms, materials and approaches. Finally, we also discuss the remaining bottlenecks and possible strategies for the development of coupling-enhanced SERS devices in the future.

Design, Fabrication and Applications of Electrospun Nanofiber-Based Surface-Enhanced Raman Spectroscopy Substrate

Surface-enhanced Raman spectroscopy (SERS) is an advanced and powerful analysis tool. Due to the advantages of high sensitivity, high resolution, and nondestructive testing, it has been widely used in physics, chemistry, material science and other fields. In recent years, substantial progress has been made in developing flexible platforms for the design and fabrication of SERS substrates. One important kind of the flexible platforms is based on electrospun nanofibers. Electrospun nanofibers not only have unique advantages such as easy preparation, high porosity and large specific surface area, but also can increase the number of hotspots when combined with precious metal nanomaterials, thereby enhancing the SERS signal and expanding the application scope. In this review, we firstly focus on two strategies for the fabrication of metal nanostructure decorated in/on the electrospun nanofibers, namely in-situ and ex-situ. Then the applications of these SERS substrates in the fields of quantitative analysis, monitoring chemical reactions and recyclable detection are introduced in detail. Finally, the challenges as well as perspectives are presented to offer a guideline for the future exploration of these SERS substrates. We expect that it will provide new inspiration for the development of electrospun nanofiber-based SERS substrates.

Seed-Mediated Synthesis of Gold Nanorods at Low Concentrations of CTAB

Although gold nanorods capped with hexadecyltrimethylammonium bromide (CTAB) have been prepared through the seed-mediated method for their use in diagnostics and therapeutics, the toxicity of AuNRs@CTAB limits their practical applications in the biomedical field. In this work, the synthesis and tuning of gold nanorods at very low concentrations of CTAB (as low as 0.008 M) was successfully achieved by using the seed-mediated method. Furthermore, we managed to optimize the growth conditions by changing the amount of seeds, AgNO₃, and/or HCl. At low CTAB concentrations, gold nanorods with tunable size and aspect ratio, high monodispersity, and high purity were obtained and studied by UV-vis spectroscopy, transmission electron microscopy, and Mie-Gans theoretical calculations. This work revealed a method of nanoparticle growth that may be extended to synthesize other nanomaterials such as Ag, Cu, Pd, and Pt at such low CTAB concentrations.

Metal-Immobilized Micellar Aggregates of a Block Copolymer from a Mixed Solvent for a SERS-Active Sensing Substrate and Versatile Dip Catalysis

Here, we have reported micellar aggregations of an amphiphilic block copolymer in mixed solvent and their subsequent use as a template for the fabrication of a very dense, tunable metal nanoparticle-decorated surface for SERS and flexible dip catalysis applications. A silver nanoparticle-immobilized layer on silicon substrates shows excellent SERS (surface-enhanced Raman scattering)-based sensing performance for model analyte rhodamine B up to 10^-6 M concentration with a well-defined calibration curve. Furthermore, a facile approach to the preparation of metal NP-immobilized BCP membranes as efficient dip catalyst for two model reactions (the reduction of nitrophenol and the Suzuki-Miyaura reaction of iodobenzene or 2,7-diiodofluorene with phenyl boronic acid) is also demonstrated. The Ag NP-decorated film exhibits high efficiency and extensive reusability in a prototype reaction such as the reduction of nitrophenol by sodium borohydride with a very high turnover number, >126 (for a single use), whereas the Pd NP-immobilized film also has a high, ∼100%, reaction yield and extensive reusability and applicable for different aromatic systems. This work provides a new platform for the design and synthesis of a functionalizable, flexible, and highly mechanically stable dip catalyst which is highly demanded in the catalytic production of value-added chemicals and environmental applications such as wastewater treatment.

Recent advances in gold nanoparticles-based biosensors for food safety detection

Food safety issue remains a challenge worldwide. Common substances in food can pose a great threat to human health including but not limited to food borne-pathogens, heavy metals, mycotoxins, pesticides, herbicides, veterinary drugs, allergens and illegal additives. To develop rapid, low-cost, portable and on-site detection methods of those contaminants and allergens to ensure food safety, gold nanoparticles (AuNPs) of versatile shapes and morphologies such as nanorods, nanoclusters, nanoflowers, nanostars, nanocages, nanobipyramids and nanowires have been employed as probes because they possess extraordinary properties that can be used to design biosensors enabling detecting various contaminants and allergens. By means of surface modification, AuNPs can directly or indirectly sense specific targets based on different mechanisms, such as hydrogen bonds, nucleic acid hybridization, aptamer-target binding, antigen-antibody recognition, enzyme inhibition, and enzyme-mimicking activity. AuNPs can induce a distinct color change from red to blue when they transform from a monodispersed state to an aggregated state in liquid solution, which can be observed by naked eyes. If Raman molecules are functionalized on AuNPs, their aggregation will alter the interparticle distance and induce the surface-enhanced Raman scattering that can be employed for highly sensitive detection. Ultra-small AuNPs such as Au nanoclusters also feature in fluorescence that enable a fluorescent readout. The formats of AuNPs for food safety detection in real world range broadly including but not limited to films, fibers, liquid solutions, tapes, chips and lateral flow strips. In this review, recent applications of AuNPs-based biosensors for food safety detection will be discussed, mainly in the aspect of different contaminants and allergens encountered in food samples.

Integrated EC-SERS Chip with Uniform Nanostructured EC-SERS Active Working Electrode for Rapid Detection of Uric Acid

Toxemia of pregnancy is a very dangerous disease for pregnant women. The mortality rate of toxemia of pregnancy is close to 10% to 15%. Early detection of pregnancy toxemia is to monitoring uric acid concentration in urine. The current mainstream method for detecting uric acid requires an enzyme (urate oxidase), which needs to be stored in a low-temperature environment, and the method requires complex chemical steps, which takes a longer time and more samples. In this study, we propose an integrated miniature three-electrode electrochemical surface-enhanced Raman spectroscopy chip (EC-SERS chip) suitable for rapid EC-SERS detection applications. The integrated microfluidic reservoir on the chip makes it easy to use, which is very suitable for rapid detection applications. The SERS active working electrode for the proposed integrated EC-SERS chip is a nanocone array polycarbonate (PC) substrate decorated with an evenly distributed and tightly packed array of gold nanospheres. It showed good uniformity and can be easily reproduced. The integrated EC-SERS chip is very small compared to the traditional electrochemical cell, which reduces the sample volume required for the testing. In addition, the chip is for one-time use only. It eliminates the need to clean electrochemical cells for reuse, thereby reducing the possibility of contamination and inaccurate detection. Various low-concentration Rhodamine 6G (R6G) solutions were tested to verify the performance of the developed EC-SERS chip. Experimental results showed that the proposed EC-SERS chip has a strong enhancement factor of up to 8.5 × 10⁶ and a very good EC-SERS uniformity (the relative standard deviation of EC-SERS intensity is as low as 1.41%). The EC-SERS chip developed has been further tested for the detection of uric acid in synthetic urine. The results showed that the EC-SERS signal intensity has a highly linear relationship with the logarithm of the uric acid concentration in synthetic urine, which indicates that the developed EC-SERS chip is suitable for the quantitative detection of uric acid in synthetic urine. Therefore, the developed EC-SERS chip is very promising to be used in routine and early diagnosis of pregnancy toxemia and may be used in many other medical tests, food safety, and biotechnology applications.