Gold@silver bimetal nanoparticles/pyramidal silicon 3D substrate with high reproducibility for high-performance SERS

Preparation and SERS applications of Ta2O5 composite nanostructures

Noble metal and semiconductor composite substrates possess high sensitivity, excellent stability, good biocompatibility, and selective enhancement, making them an important research direction in the field of surface-enhanced Raman scattering (SERS). Ta2O5, as a semiconductor material with high thermal stability, corrosion resistance, outstanding optical properties, and catalytic performance, has great potential in SERS research. This study aims to design and fabricate a composite SERS substrate based on Ta2O5 nanostructures, achieving optimal detection performance by combining the urchin-like structure of Ta2O5 with silver nanoparticles (Ag NPs). The urchin-like Ta2O5 nanostructures were prepared using a hydrothermal reaction method. The bandgap was modulated through structure design and the self-doping technique, the charge transfer efficiency and surface plasmon resonance effects were improved, thereby achieving better SERS performance. The composite substrate enables highly sensitive quantitative detection. This composite SERS substrate combines the electromagnetic enhancement mechanism (EM) and chemical enhancement mechanism (CM), achieving ultra-low detection limits of 10-13 M for R6G. Within the concentration range above 10-12 M, there is a good linear relationship between concentration and peak intensity, demonstrating excellent quantitative analysis capabilities. Furthermore, this composite SERS substrate is capable of precise detection of analytes such as crystal violet (CV) and methylene blue (MB), holding broad application prospects in areas such as food safety and environmental monitoring.

Plasmonic nanomaterials-based flexible strips for the SERS detection of gouty arthritis

Flexible biochips that enable sensitive detection and simultaneous quantification of biomarkers are of great importance in the field of point-of-care testing. Recently, surface-enhanced Raman scattering (SERS)-based flexible biochips have attracted a great deal of research attention for disease detection due to their rapid, sensitive, and noninvasive sensing abilities. Phenomenal progress in the synthesis of structure-controlled plasmonic nanomaterials has made SERS a powerful sensing platform for disease diagnosis and trace detection. Here, we demonstrate flexible plasmonic biochips for the SERS-based detection of uric acid (UA). Flexible strips exhibited excellent sensing performance with a detection limit of around 10 μM of UA, which is lower than the average level of UA in tears. This rapid and sensitive detection method enables the noninvasive diagnosis of gouty arthritis.

Recent advances of Au@Ag core-shell SERS-based biosensors

The methodological advancements in surface-enhanced Raman scattering (SERS) technique with nanoscale materials based on noble metals, Au, Ag, and their bimetallic alloy Au-Ag, has enabled the highly efficient sensing of chemical and biological molecules at very low concentration values. By employing the innovative various type of Au, Ag nanoparticles and especially, high efficiency Au@Ag alloy nanomaterials as substrate in SERS based biosensors have revolutionized the detection of biological components including; proteins, antigens antibodies complex, circulating tumor cells, DNA, and RNA (miRNA), etc. This review is about SERS-based Au/Ag bimetallic biosensors and their Raman enhanced activity by focusing on different factors related to them. The emphasis of this research is to describe the recent developments in this field and conceptual advancements behind them. Furthermore, in this article we apex the understanding of impact by variation in basic features like effects of size, shape varying lengths, thickness of core-shell and their influence of large-scale magnitude and morphology. Moreover, the detailed information about recent biological applications based on these core-shell noble metals, importantly detection of receptor binding domain (RBD) protein of COVID-19 is provided.

Wafer-scale Silver Nanodendrites with Homogeneous Distribution of Gold Nanoparticles for Biomolecules Detection

We report the fabrication and demonstrate the superior performance of robust, cost-effective, and biocompatible hierarchical Au nanoparticles (AuNPs) decorated Ag nanodendrites (AgNDs) on a Silicon platform for the trace-level detection of antibiotics (penicillin, kanamycin, and ampicillin) and DNA bases (adenine, cytosine). The hot-spot density dependence studies were explored by varying the AuNPs deposition time. These substrates’ potential and versatility were explored further through the detection of crystal violet, ammonium nitrate, and thiram. The calculated limits of detection for CV, adenine, cytosine, penicillin G, kanamycin, ampicillin, AN, and thiram were 348 pM, 2, 28, 2, 56, 4, 5, and 2 nM, respectively. The analytical enhancement factors were estimated to be ∼10⁷ for CV, ∼10⁶ for the biomolecules, ∼10⁶ for the explosive molecule, and ∼10⁶ for thiram. Furthermore, the stability of these substrates at different time intervals is being reported here with surface-enhanced Raman spectroscopy/scattering (SERS) data obtained over 120 days.

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Wafer-scale surface-enhanced Raman spectroscopy/scattering (SERS) substrate of Ag nanodendrites decorated with Au nanoparticles prepared

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Trace level detection of antibiotics achieved

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Versatility of these substrates demonstrated by detecting explosive, dye molecules

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Typical enhancement factors achieved were 10⁵–10⁷

Heterogeneous Nanoplasmonic Amplifiers for Photocatalysis’s Application: A Theoretical Study

The higher cost of Ag and Au and their resonance frequency shift limitation opened the way to find an alternative solution by developing new nanohybrid antenna based on silicon and silicon dioxide coated with metallic nanoparticles. The latter has been recently solicited as a promising configuration for more large-scale plasmonic utilisation. This work reports a multitude of fascinating new phenomenon on LSPR on silicon antenna wires coated with core-shell nanospheres and the studying of the nanoplasmonics amplifiers to control optical and electromagnetic properties of materials. The LSPR modes and their interaction with the silicon nanowires are studied using numerical methods. The suggested configuration offers resonance covering the UV-visible and NIR regions, making them an adaptable addition to the nanoplasmonics toolbox.

Tuning the topographical parameters of Si pyramids for a better surface enhanced Raman response

Development of facile routes for the fabrication of surface enhanced Raman substrates (SERS) along with optimal conditions for a high enhancement factor are significant from an application perspective of SERS. Despite steady efforts to establish high SERS signals, cost effectiveness without compromising the enhanced and robust Raman signal remains a major challenge. To address this aspect, herein, we try to tune the topographical aspects of Si pyramidal textures in pursuit of efficient SERS substrates. These pyramidal surfaces are deployed as a pre-template for adopting a SERS substrate using a cost-effective wet chemical etching method. By controlling the etching time, various topographical parameters namely base size, height, pyramidal number density and uniformity of pyramidal textures are modulated. To make all the surfaces SERS active, a Au (50%)-Ag (50%) alloy nanolayer is post-deposited over them. Furthermore, SERS behavior of all the surfaces is investigated by using Rh6G dye as an analyte molecule. In addition to the high density of hot spots in terms of pyramidal number density, base size and uniformity shows a strong correlation in deciding the substantial SERS response. Furthermore, we find a high enhancement factor (∼1.42 × 10⁸) for the substrate consisting of dense, small and uniformly sized pyramids. Finite Difference Time Domain (FDTD) simulations done on similar structures corroborate our results. Additionally, universal applicability of the proposed substrate is also verified by detecting methylene blue and methyl parathion analyte molecules. These substrates are much cheaper (∼5 USD for 1 × 1 cm²) in comparison with commercially available Klarite SERS substrates (∼100 USD for 2 × 2 mm²). We believe this work provides a critical insight into the design of potential SERS substrates using a significantly cost-effective wet chemical etching process.

Synergistic SERS Enhancement in GaN-Ag Hybrid System toward Label-Free and Multiplexed Detection of Antibiotics in Aqueous Solutions

Noble metal-based surface-enhanced Raman spectroscopy (SERS) has enabled the simple and efficient detection of trace-amount molecules via significant electromagnetic enhancements at hot spots. However, the small Raman cross-section of various analytes forces the use of a Raman reporter for specific surface functionalization, which is time-consuming and limited to low-molecular-weight analytes. To tackle these issues, a hybrid SERS substrate utilizing Ag as plasmonic structures and GaN as charge transfer enhancement centers is presented. By the conformal printing of Ag nanowires onto GaN nanopillars, a highly sensitive SERS substrate with excellent uniformity can be fabricated. As a result, remarkable SERS performance with a substrate enhancement factor of 1.4 × 1011 at 10 fM for rhodamine 6G molecules with minimal spot variations can be realized. Furthermore, quantification and multiplexing capabilities without surface treatments are demonstrated by detecting harmful antibiotics in aqueous solutions. This work paves the way for the development of a highly sensitive SERS substrate by constructing complex metal-semiconductor architectures.

Elucidating Raman Image-Guided Differential Recognition of Clinically Confirmed Grades of Cervical Exfoliated Cells by Dual Biomarker-Appended SERS-Tag

Ultrasensitive detection of cancer biomarkers via single-cell analysis through Raman imaging is an impending approach that modulates the possibility of early diagnosis. Cervical cancer is one such type that can be monitored for a sufficiently long period toward invasive cancer phenotype. Herein, we report a surface-enhanced Raman scattering (SERS) nanotag (SERS-tag) for the simultaneous detection of p16/K-i67, a dual biomarker persisting in the progression of squamous cell carcinoma of human cervix. A nanoflower-shaped SERS-tag, constituted of hybrid gold nanostar with silver tips to achieve maximum fingerprint enhancement from the incorporated reporter molecule, was further functionalized with the cocktail monoclonal antibodies against p16/K-i67. The recognition by the SERS-tag was first validated in cervical squamous cell carcinoma cell line SiHa as a foot-step study and subsequently implemented to different grades of clinically confirmed exfoliated cells including normal cell (NC), high-grade intra-epithelial lesion (HC), and squamous cell carcinoma (CC) samples of the cervix. Precise Raman mapped images were constituted based on the average intensity gradient of the signature Raman peaks arising from different grades of exfoliated cells. We observed a distinct intensity hike of around 10-fold in the single dysplastic HC and CC samples in comparison to NC specimen, which clearly justify the prevalence of p16/Ki-67. The synthesized probe is able to map the abnormal cells within 20 min with high reproducibility and stability for 1 mm × 1 mm mapping area with good contrast. Amidst the challenges in Raman image-guided modality, the technique was further complemented with the gold standard immunocytochemistry (ICC) dual staining analysis. Even though both are time-consuming techniques, tedious steps can be avoided and real-time readout can be achieved using the SERS mapping unlike immunocytochemistry technique. Therefore, the newly developed Raman image-guided SERS imaging emphasizes the approach of uplifting of SERS in practical utility with further improvement for clinical applications for cervical cancer detection in future.

Tunable LSPR of silver/gold bimetallic nanoframes and their SERS activity for methyl red detection

Ag/Au bimetallic nanostructures have received much attention in surface-enhanced Raman scattering (SERS). However, the synthesis of this nanostructure type still remains a challenge. In the present research, Ag/Au nanoframes were synthesized via a simple room temperature solution phase chemical reduction method using pre-synthesized triangular Ag nanoplates as templates in the presence of appropriate amounts of HAuCl4. Controlling experimental parameters was applied for understanding of the growth mechanism. The galvanic exchange reaction resulted in a uniform deposition of the Au shell on the Ag nanoplates and the Ag core was removed which generated triangular hollow nanoframes. It is found that the amount of HAuCl4 added to the growth solution played a key role in controlling the Ag/Au nanoframes. The resultant silver/gold nanoframes with average size of 50 nm were applied in detecting methyl red (MR) in the solution-phase using an excitation wavelength laser of 532 nm. The SERS signal was greatly enhanced owing to the tunable plasmonic peaks in the visible region (400-650 nm). The limit of detection (LOD) of MR in diluted solution was 10-6 M. The enhancement factor (EF) was about 8 × 104 toward 10-5 M of MR. Interestingly, the linear dependence between the logarithm of the SERS signal intensity (log I) and the logarithm of the MR concentration (log C) occurred in the range from 10-6 to 10-4 M. Our work promises the application of Ag/Au nanoframes as a chemical sensor in detecting MR molecules at low concentration with high performance.

Polyelemental Nanoparticles as Catalysts for a Li-O2 Battery

The development of highly efficient catalysts in the cathodes of rechargeable Li-O₂ batteries is a considerable challenge. Polyelemental catalysts consisting of two or more kinds of hybridized catalysts are particularly interesting because the combination of the electrochemical properties of each catalyst component can significantly facilitate oxygen evolution and oxygen reduction reactions. Despite the recent advances that have been made in this field, the number of elements in the catalysts has been largely limited to two metals. In this study, we demonstrate the electrochemical behavior of Li-O₂ batteries containing a wide range of catalytic element combinations. Fourteen different combinations with single, binary, ternary, and quaternary combinations of Pt, Pd, Au, and Ru were prepared on carbon nanofibers (CNFs) via a joule heating route. Importantly, the Li-O₂ battery performance could be significantly improved when using a polyelemental catalyst with four elements. The cathode containing quaternary nanoparticles (Pt-Pd-Au-Ru) exhibited a reduced overpotential (0.45 V) and a high discharge capacity based on total cathode weight at 9130 mAh g^-1, which was ∼3 times higher than that of the pristine CNF electrode. This superior electrochemical performance is be attributed to an increased catalytic activity associated with an enhanced O₂ adsorbability by the quaternary nanoparticles.

Structure-Adjustable Gold Nanoingots with Strong Plasmon Coupling and Magnetic Resonance for Improved Photocatalytic Activity and SERS

Au nanoingots, on which an Au nanosphere is accurately placed in an open Au shell, are synthesized through a controllable hydrothermal method. The prepared Au nanoingots exhibit an adjustable cavity structure, strong plasmon coupling, tunable magnetic plasmon resonance, and prominent photocatalytic and SERS performances. Au nanoingots exhibit two resonance peaks in the extinction spectrum, one (around 550 nm) is ascribed to electric dipole resonance coming from the central Au, and the other one (650-800 nm) is ascribed to the magnetic dipole resonance originating from the open Au shell. Numerical simulations verify that the intense electric and magnetic fields locate in the bowl-shaped nanogap between the Au nanosphere and shell, and they can be further optimized by changing the size of the outer Au shell. Au nanoingots with the largest shell have the strongest electric field because of large-area plasmon coupling, while Au nanoingots with the largest shell opening size have the strongest magnetic field. As a result, the structure-adjustable Au nanoingots show a high tunability and enhancement of catalytic reduction of p-nitrophenol and SERS detection of Rhodamine B. Specially, Au nanoingots with the largest shell size exhibit the highest catalytic activity and Raman signals at 532 nm excitation. However, Au nanoingots with the largest shell opening size have the highest photocatalytic activity with light irradiation (λ > 420 nm) and exhibit the best SERS performance at 785 nm excitation.

Instantaneous trace detection of nitro-explosives and mixtures with nanotextured silicon decorated with Ag-Au alloy nanoparticles using the SERS technique

The development of recyclable surface enhanced Raman scattering (SERS) based sensors has been in huge demand for trace level explosives detection. A simple, hybrid Silicon (Si) nanotextured target-based SERS platform is fabricated through patterning micro square arrays (MSA) on Si using femtosecond (fs) laser ablation technique at different fluences. Using the hybrid target Si MSA substrate loaded/decorated with Ag-Au alloy NPs (obtained using femtosecond ablation in liquids) we demonstrate the trace level detection of organic nitro-explosives [picric acid (PA), 2,4-dinitrotoluene (DNT), and 1, 3, 5-trinitroperhydro-1, 3, 5-triazine (RDX)] and their mixtures. The microstructures/nanostructures of MSA fabricated at an input fluence of 9.55 J/cm², and decorated with Ag-Au alloy NPs, exhibited exceptional SERS enhancement factors (EFs) up to ∼10¹⁰ for MB, ∼10⁶ for PA, and ∼10⁴ for RDX with the detection limits obtained being ∼5 pM, ∼36 nM, and ∼400 nM for MB, PA and RDX respectively. Furthermore, we demonstrate these SERS substrates possess good reproducibility (RSD values < 15%) and a superior performance compared to a commercial Ag substrate (SERSitive, Poland). Three binary mixtures, i.e. MB-PA, MB-DNT, PA-DNT at different concentrations, were also investigated using the same SERS substrate to test the efficacy. Further, the SERS spectra of dyes, explosives, and complex mixtures were utilized for discrimination/classification using principal component analysis.

Interaction of ZnO nanorods with plasmonic metal nanoparticles and semiconductor quantum dots

We model the enhancement of near band edge emission from ZnO nanorods using plasmonic metal nanoparticles and compare it with emission enhancement from ZnO with semiconducting quantum dots. Selected CdSe quantum dots with absorption energies close to those of Ag and Au nanoparticles are chosen to construct model systems with ZnO to comprehend the role of ZnO's intrinsic defects and plasmonic excitation in realizing the spectrally selective luminescence enhancement. Excitation wavelength dependent photoluminescence spectra along with theoretical models quantifying the related transitions and plasmonic absorption reveal that a complex mechanism of charge transfer between the ZnO nanorods and metal nanoparticles or quantum dots is essential along with an optimal energy band alignment for realizing emission enhancement. The theoretical model presented also provides a direct method of quantifying the relative transition rate constants associated with various electronic transitions in ZnO and their change upon the incorporation of plasmonic nanoparticles. The results indicate that, while the presence of deep level defect states may facilitate the essential charge transfer process between ZnO and the plasmonic nanoparticles, their presence alone does not guarantee UV emission enhancement and strong plasmonic coupling between the two systems. The results offer clues to designing novel multicomponent systems with coupled plasmonic and charge transfer effects for applications in charge localization, energy harvesting, and luminescence enhancement, especially in electrically triggered nanophotonic applications.

Large-Area and Patternable Nano-Dot Array from Electrolysis of ITO Film for Surface-Enhanced Raman Spectroscopy

Fabrication of large-area devices with patternable nanostructures is important for practical applications in optical or electrical devices. In this work, we describe an easy and environment-friendly method for preparing large-area nano-dot (ND) arrays via the electrolytic reaction of a metal oxide film. NDs with various size and morphology can be obtained by adjusting the applied voltage, electrolysis time, and the film thickness of the indium tin oxide (ITO) layer. High-density NDs with size of 50-60 nm can be obtained by electrolysis of a 25-nm-thick ITO film at 150 V for 1.5 min under a water droplet medium, which have been applied for surface-enhanced Raman spectroscopy (SERS) after depositing a thin layer of silver. The SERS substrate with optimized ND structure exhibits sensitive detection of Rhodamine 6G (R6G) with detection limit down to 5 × 10^-12 M. The enhancement factors (EFs) of 1.12 × 10⁶ and 6.79 × 10⁵ have been achieved for characterization of 4-methylbenzenethiol (4-MBT) and R6G, respectively. With an additional photolithographic step, multiple areas of ND arrays can be created on one substrate, enabling simultaneous detection of various samples containing different molecules at once experiment. Such a method is quick, easy, patternable, and environment-friendly, being suitable for on-site quick and synchronous determination of various molecules for applications in point-of-care, environmental monitoring, and airport security fields.

Robust and cost-effective silver dendritic nanostructures for SERS-based trace detection of RDX and ammonium nitrate

We report the fabrication and performance evaluation of cost-effective, reproducible silver nanodendrite (AgND) substrates, possessing high-density trunks and branches, achieved by a simple electroless etching process and subsequently utilized them for the trace detection of 1,3,5-trinitroperhydro-1,3,5-triazine (Research Development Explosive, RDX) and Ammonium Nitrate (AN). The intricate structural features in AgNDs offer high-density hotspots for effective molecular detection based on the surface enhanced Raman scattering (SERS) technique. The active SERS-substrate was initially tested with standard Rhodamine 6G (R6G) molecules at 1 nM concentration, which established an effective enhancement factor (EF) of ∼10⁸. The AgNDs were subsequently utilized in the detection of the explosives RDX and AN, down to concentrations of 1 μM. The typical EF achieved in the case of RDX and AN was ∼10⁴. The sensitivity of 1 μM R6G was further enhanced by two-fold through the deposition of Au nanoparticles on the AgNDs. The reproducibility of the low-cost substrate was also demonstrated, with a ∼9% RSD value in the measurements.

We report the fabrication and performance evaluation of cost-effective, reproducible silver nanodendrite (AgND) substrates, possessing high-density trunks and branches, achieved by a simple electroless etching and used for the trace detection of RDX and Ammonium Nitrate.

Label-free highly sensitive probe detection with novel hierarchical SERS substrates fabricated by nanoindentation and chemical reaction methods

Nanostructures have been widely employed in surface-enhanced Raman scattering (SERS) substrates. Recently, in order to obtain a higher enhancement factor at a lower detection limit, hierarchical structures, including nanostructures and nanoparticles, appear to be viable SERS substrate candidates. Here we describe a novel method integrating the nanoindentation process and chemical redox reaction to machine a hierarchical SERS substrate. The micro/nanostructures are first formed on a Cu(110) plane and then Ag nanoparticles are generated on the structured copper surface. The effect of the indentation process parameters and the corrosion time in the AgNO₃ solution on the Raman intensities of the SERS substrate with hierarchical structures are experimentally studied. The intensity and distribution of the electric field of single and multiple Ag nanoparticles on the surface of a plane and with multiple micro/nanostructures are studied with COMSOL software. The feasibility of the hierarchical SERS substrate is verified using R6G molecules. Finally, the enhancement factor using malachite green molecules was found to reach 5.089 × 10⁹, which demonstrates that the production method is a simple, reproducible and low-cost method for machining a highly sensitive, hierarchical SERS substrate.

Quantitative SERS by hot spot normalization - surface enhanced Rayleigh band intensity as an alternative evaluation parameter for SERS substrate performance

The performance of surface-enhanced Raman spectroscopy (SERS) substrates is typically evaluated by calculating an enhancement factor (EF). However, it is challenging to accurately calculate EF values since the calculation often requires the use of model analytes and requires assumptions about the number of analyte molecules within the laser excitation volume. Furthermore, the measured EF values are target analyte dependent and thus it is challenging to compare substrates with EF values obtained using different analytes. In this study, we propose an alternative evaluation parameter for SERS substrate performance that is based on the intensity of the surface plasmon enhanced Rayleigh band (I_Rayleigh) that originates from the amplified spontaneous emission (ASE) of the laser. Compared to the EF, I_Rayleigh reflects the enhancing capability of the substrate itself, is easy to measure without the use of any analytes, and is universally applicable for the comparison of SERS substrates. Six SERS substrates with different states (solid, suspended in liquid, and hydrogel), different plasmonic nanoparticle identities (silver and gold), as well as different nanoparticle sizes and shapes were used to support our hypothesis. The results show that there are excellent correlations between the measured SERS intensities and I_Rayleigh as well as between the SERS homogeneity and the variation of I_Rayleigh acquired with the six SERS substrates. These results suggest that I_Rayleigh can be used as an evaluation parameter for both SERS substrate efficiency and reproducibility.

Fabrication, Characterization, and Application of Large-Scale Uniformly Hybrid Nanoparticle-Enhanced Raman Spectroscopy Substrates

Surface-enhanced Raman spectroscopy (SERS) substrates with high sensitivity and reproducibility are highly desirable for high precision and even molecular-level detection applications. Here, large-scale uniformly hybrid nanoparticle-enhanced Raman spectroscopy (NERS) substrates with high reproducibility and controllability were developed. Using oxygen plasma treatment, large-area and uniformly rough polystyrene sphere (URPS) arrays in conjunction with 20 nm Au films (AuURPS) were fabricated for SERS substrates. Au nanoparticles and clusters covered the surface of the URPS arrays, and this increased the Raman signal. In the detection of malachite green (MG), the fabricated NERS substrates have high reproducibility and sensitivity. The enhancement factor (EF) of Au nanoparticles and clusters was simulated by finite-difference time-domain (FDTD) simulations and the EF was more than 10⁴. The measured EF of our developed substrate was more than 10⁸ with a relative standard deviation as low as 6.64%–13.84% over 15 points on the substrate. The minimum limit for the MG molecules reached 50 ng/mL. Moreover, the Raman signal had a good linear relationship with the logarithmic concentration of MG, as it ranged from 50 ng/mL to 5 μg/mL. The NERS substrates proposed in this work may serve as a promising detection scheme in chemical and biological fields.

Graphene-Ag nanoparticles-cicada wings hybrid system for obvious SERS performance and DNA molecular detection

In recent years, biomaterials have increasingly attracted attention on surface-enhanced Raman spectroscopy (SERS) due to their well Raman performance while metal particles are combined with biological substrates. Therefore, we propose an environmentally friendly substrate based on silver-plated cicada wings with seamless graphene layer (Gr-AgNPs-C.w.), which can be prepared with a simple and inexpensive method. Compared with AgNPs-C.w., Gr-AgNPs-C.w. hybrids show better SERS performance with high sensitivity, good uniformity and good stability with R6G detection. The minimum detected concentration can reach 10^-15 M, and the value of R² can reach 0.996, respectively. Theoretical simulation demonstrates the situation of electromagnetic field through COMSOL software. In addition, due to the affinity of graphene for biomolecules, we can successfully detect the DNA biomolecules through a simple process. Therefore, this cheap and efficient natural SERS substrate has great potential for a considerable number of biochemical SERS applications and can broaden the way in which multiple SERS platforms derived from other natural materials are prepared.

Heterogeneous and cross-distributed metal structure hybridized with MoS2 as high-performance flexible SERS substrate

The heterogeneous metal nanostructures have attracted great interest in various applications due to the synergistic effects between two noble metals, especially in surface enhanced Raman scattering (SERS) region. Herein, we prepared a 3D SERS active substrate based on heterogeneous and cross-distributed metal structure hybridized with MoS₂by in situ synthesizing gold nanoparticles (AuNPs) on MoS₂ membrane. The AuNPs-AgNPs/MoS₂/P-Si hybrid SERS substrate were characterized by a scanning electron microscope (SEM), a transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) to investigate the character and the content of elements. In virtue of the heterogeneous and cross-distributed structure and ultra-narrow interparticle gap generating strong electric fields enhancement, the ultra-low concentration of probe molecule were detected (the LOD of 10^-12 M for R6G and CV, 10^-11 M for MG), serving the optimal SERS performance. The excellent uniformity and reproducibility were achieved by the proposed substrate. Moreover, the flexible MoS₂/AuNPs-AgNPs/PMMA pyramidal SERS substrate was applied to detect melamine molecule in liquid milk (the LOD reached 10^-9 M), which revealed great potential to be an outstanding SERS substrate for biological and chemical detection.

3D SERS substrate based on Au-Ag bi-metal nanoparticles/MoS2 hybrid with pyramid structure

It is very vital to construct the dense hot spots for the strong surface-enhanced Raman scattering (SERS) signals. We take full advantage of the MoS₂ edge-active sites induced from annealing the Ag film on the surface of the MoS₂. Furthermore, the composite structure of Au-Ag bi-metal nanoparticles (NPs)/MoS₂ hybrid with pyramid structure is obtained by the in situ grown AuNPs around AgNPs, which serves the optimal SERS performance (enhancement factor is ~9.67 × 10⁹) in experiment. Due to the introduction of AuNPs with the simple method, the denser hot spots contribute greatly to the stronger local electric field, which is also confirmed by the finite-different time-domain (FDTD) simulation. Therefore, the ultralow limit of detection (the LOD of 10^-13 and 10^-12 M respectively for the resonant R6G and non-resonant CV), quantitative detection and excellent reproducibility are achieved by the proposed SERS substrate. For practical application, the melamine molecule is detected with the LOD of 10^-10 M using the proposed SERS substrate that has the potential to be a food security sensor.

Grating-like SERS substrate with tunable gaps based on nanorough Ag nanoislands/moth wing scale arrays for quantitative detection of cypermethrin

Considering the complexity and high-consumption of the existing approaches to fabricate three-dimensional (3D) regular substrate templates, the scales of the moth wings with evenly-distributed nanoarrays were discovered to provide an ideal bioscaffold for metal silver (Ag) to decorate on to fabricate a flexible, highly-ordered, low-cost and large-scale Ag nanoislands/moth wing (Ag/MW) SERS-active substrate. The grating-like substrate with the optimal morphology of rough and hierarchical Ag nanoislands exhibited high enhancement factor (EF, ~4.16 × 10⁵), low detection limit (10^-10 M) to 4-aminothiophenol (4-ATP), outstanding signal uniformity (the relative standard deviations were less than 15%) and superior identification performance in the quantitative detection of pesticide cypermethrin. The three-dimensional finite-difference time-domain (3D-FDTD) method simulated the spatial distribution of the electric field intensity in the substrates with different morphologies, showing a potential strong enhancement of Raman signals in sub-10 nm gaps between two adjacent Ag nanoislands of different layers. These prominent SERS properties of novel Ag/MW SERS-active substrates suggest their potential value in rapid on-side biological and chemical sensing. Meanwhile, the highly-ordered nanoarrays of moth wings provide a new idea for the preparation of regular biomimetic nanomaterials.

Controlling the 3D Electromagnetic Coupling in Co-Sputtered Ag⁻SiO₂ Nanomace Arrays by Lateral Sizes

Ag–SiO₂ nanomace arrays were prepared on a two-dimensional ordered colloidal (2D) polystyrene sphere template by co-sputtering Ag and SiO₂ in a magnetron sputtering system. The lateral size of the nanomaces and the distance between the neighbor nanomaces were controlled by adjusting the etching time of the 2D template. The nanomaces were composed of SiO₂-isolated Ag nanoparticles, which produced surface-enhanced Raman scattering (SERS) enhancement, and 3D hot spots were created between the neighbor nanomaces. When the distance between the nanomaces was sufficiently large, triangle-shaped nanostructures on silicon substrate were observed, which also contributed to the enhancement of the SERS signals. The finite-difference time-domain (FDTD) method was used to calculate the electromagnetic field distributions in the Ag–SiO₂ nanomace arrays, which generated physical reasons for the change of the SERS signals.

Bioderived Three-Dimensional Hierarchical Nanostructures as Efficient Surface-Enhanced Raman Scattering Substrates for Cell Membrane Probing

In this work, we propose the use of complex, bioderived nanostructures as efficient surface-enhanced Raman scattering (SERS) substrates for chemical analysis of cellular membranes. These structures were directly obtained from a suitable gold metalization of the Pseudonitzchia multistriata diatom silica shell (the so called frustule), whose grating-like geometry provides large light coupling with external radiation, whereas its extruded, subwavelength lateral edge provides an excellent interaction with cells without steric hindrance. We carried out numerical simulations and experimental characterizations of the supported plasmonic resonances and optical near-field amplification. We thoroughly evaluated the SERS substrate enhancement factor as a function of the metalization parameters and finally applied the nanostrucures for discriminating cell membrane Raman signals. In particular, we considered two cases where the membrane composition plays a fundamental role in the assessment of several pathologies, that is, red blood cells and B-leukemia REH cells.

Controlling surface morphology and sensitivity of granular and porous silver films for surface-enhanced Raman scattering, SERS

The design of efficient substrates for surface-enhanced Raman spectroscopy (SERS) for large-scale fabrication at low cost is an important issue in further enhancing the use of SERS for routine chemical analysis. Here, we systematically investigate the effect of different radio frequency (rf) plasmas (argon, hydrogen, nitrogen, air and oxygen plasma) as well as combinations of these plasmas on the surface morphology of thin silver films. It was found that different surface structures and different degrees of surface roughness could be obtained by a systematic variation of the plasma type and condition as well as plasma power and treatment time. The differently roughened silver surfaces act as efficient SERS substrates showing greater enhancement factors compared to as prepared, sputtered, but untreated silver films when using rhodamine B as Raman probe molecule. The obtained roughened silver films were fully characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron (XPS and Auger) and ultraviolet-visible spectroscopy (UV-vis) as well as contact angle measurements. It was found that different morphologies of the roughened Ag films could be obtained under controlled conditions. These silver films show a broad range of tunable SERS enhancement factors ranging from 1.93 × 10² to 2.35 × 10⁵ using rhodamine B as probe molecule. The main factors that control the enhancement are the plasma gas used and the plasma conditions, i.e., pressure, power and treatment time. Altogether this work shows for the first time the effectiveness of a plasma treatment for surface roughening of silver thin films and its profound influence on the interface-controlled SERS enhancement effect. The method can be used for low-cost, large-scale production of SERS substrates.

Electrospun Polymer Nanofibers Decorated with Noble Metal Nanoparticles for Chemical Sensing

The integration of different noble metal nanostructures, which exhibit desirable plasmonic and/or electrocatalytic properties, with electrospun polymer nanofibers, which display unique mechanical and thermodynamic properties, yields novel hybrid nanoscale systems of synergistic properties and functions. This review summarizes recent advances on how to incorporate noble metal nanoparticles into electrospun polymer nanofibers and illustrates how such integration paves the way towards chemical sensing applications with improved sensitivity, stability, flexibility, compatibility, and selectivity. It is expected that further development of this field will eventually make a wide impact on many areas of research.

Annealing Induced Morphology of Silver Nanoparticles on Pyramidal Silicon Surface and Their Application to Surface-Enhanced Raman Scattering

This paper reports on a simple and cost-effective process of developing a stable surface-enhanced Raman scattering (SERS) substrate based on silver (Ag) nanoparticles deposited on silicon (Si) surface. Durability is an important issue for preparing SERS active substrate as silver nanostructures are prone to rapid surface oxidation when exposed to ambient conditions, which may result in the loss of the enhancement capabilities in a short period of time. Here, we employ the galvanic displacement method to produce Ag nanoparticles on Si(100) substrate prepatterned with arrays of micropyramids by chemical etching, and subsequently, separate pieces of such substrates were annealed in oxygen and nitrogen environments at 550 °C. Interestingly, while nitrogen-annealed Si substrates were featured by spherical-shaped Ag particles, the oxygen annealed Si substrates were dominated by the formation of triangular shape particles attached with the spherical one. Remarkably, the oxygen-annealed substrate thus produced shows very high SERS enhancement compared to the either unannealed or nitrogen annealed substrate. The hitherto unobserved coexistence of triangular morphology with the spherical one and the gap between the two (source of efficient hot-spots) are the origin of enhanced SERS activity for the oxygen-annealed Ag particle-covered Si substrate as probed by the combined finite-difference time domain (FDTD) simulation and cathodoluminesensce (CL) experiment. As the substrate has already been annealed in an oxygen environment, further probability of oxidation is reduced in the present synthesis protocol that paves the way for making a novel long-lived thermally stable SERS substrate.

Seed-mediated synthesis and SERS performance of graphene oxide-wrapped Ag nanomushroom

A facile seed-mediated method was developed to modify core-shell Ag nanosphere@PSPAA with another Ag layer for achieving an enhancement of their surface-enhanced Raman scattering (SERS) activity. Interestingly, an Ag bridge in the polymer shell connected the inner and outer Ag layers, resulting in a mushroom-like nanostructure. The outer Ag grew around the polymer shell to form the cap of the nanomushrooms (NMs) with the extension of the reaction time. The epitaxial growth mechanism of this novel nanostructure was investigated by tuning the type of seed from nanosphere to nanocube and nanorod. With the growth of the outer Ag cap, the SERS intensity of these Ag NMs increased significantly together with the red-shifting and broadening of their typical localized surface plasmon resonance band. Such a phenomenon can be attributed to the formation of SERS hotspots between the inner and outer Ag layers. The Ag NMs were then wrapped with a graphene oxide (GO) shell via static interactions. The GO-wrapped Ag NMs exhibited a further better SERS performance in terms of sensitivity, homogeneity and stability compared with non-wrapped ones, indicating that the heterostructure could be potentially useful for SERS-based immunoassay.

Theoretical design of a surface plasmon resonance sensor with high sensitivity and high resolution based on graphene–WS2 hybrid nanostructures and Au–Ag bimetallic film

We proposed a high sensitivity and a high resolution surface plasmon resonance sensor composed of graphene–WS₂ hybrid nanostructure and Au–Ag bimetallic-layers film.

A primary SERS-active interconnected Si-nanocore network for biomolecule detection with plasmonic nanosatellites as a secondary boosting mechanism

We report in this study, the development of a polymorphic biosensitive Si nanocore superstructure as a SERS biosensing platform.

Tailoring gold and silver colloidal bimetallic nanoalloys towards SERS detection of rhodamine 6G

Nanoparticles of gold, silver and their bimetallic alloys were tailored for SERS sensing towards trace amounts of rhodamine 6G.

High-performance SERS substrate based on hybrid structure of graphene oxide/AgNPs/Cu film@pyramid Si

We present a novel surface-enhanced Raman scattering (SERS) substrate based on graphene oxide/silver nanoparticles/copper film covered silicon pyramid arrays (GO/AgNPs/PCu@Si) by a low-cost and simple method. The GO/AgNPs/PCu@Si substrate presents high sensitivity, good homogeneity and well stability with R6G molecules as a probe. The detected concentration of Rhodamine 6 G (R6G) is as low as 10⁻¹⁵ M. These sensitive SERS behaviors are also confirmed in theory via a commercial COMSOL software, the electric field enhancement is not only formed between the AgNPs, but also formed between the AgNPs and Cu film. And the GO/AgNPs/PCu@Si substrates also present good property on practical application for the detection of methylene blue (MB) and crystal violet (CV). This work may offer a novel and practical method to facilitate the SERS applications in areas of medicine, food safety and biotechnology.