Stable Surface-enhanced Raman Scattering Substrate Research Articles

SERS Materials with Small‐Molecule Sensitivity for Biological Diagnosis

AbstractSmall molecule metabolites depict a biological system‘s status and help monitor disease conditions. Surface‐enhanced Raman scattering (SERS) holds considerable promise as an instant and non‐destructive detection tool for biological diagnosis. With single‐molecular sensitivity, molecular fingerprint and water compatible, SERS provides valuable information about chemical structures and analyte compositions, especially desirable in biological diagnosis. Design and fabrication of effective, stable SERS substrates can be one of the major research priorities to provide small‐molecule tracing with high sensitivity, selectivity, and reproducibility. Significant efforts have been directed towards creating high‐performance and multifunctional SERS substrates spanning from coinage metals to semiconductor materials. However, most SERS substrates are suffering from unsatisfied sensitivity for tracing small molecules such as volatile organic compounds (VOCs) and other biomarkers. This review summarized recent advances in SERS‐active substrates and their sensing for small biomarker molecules, especially VOCs in diagnosis.

SERS performance of silver nanoparticle/reduced graphene oxide-coated filter membranes

Polyvinylidene fluoride (PVDF), polyamide (Nylon) and anodic aluminum oxide (AAO) filter membranes were coated with in-situ synthesized silver nanoparticle/reduced graphene oxide (AgNP/rGO) nanocomposite by vacuum filtration, and surface-enhanced Raman scattering (SERS) performance of the prepared substrates was investigated using Rhodamine 6G (R6G) as a probe molecule. Analyte solutions were applied by drop casting and penetrated into the flexible PVDF and Nylon-based substrates within minutes, enabling very fast SERS measurements; however, this step took up to 2 hrs in the rigid AAO-based substrate. Scanning electron microscopy and Energy-dispersive X-ray spectroscopy mapping results showed that filter membrane surfaces were completely covered with AgNP/rGO nanocomposite at which AgNPs were uniformly distributed on rGO flakes; however, some large, partially reduced or unreduced GO flakes decorated with AgNP agglomerates (AgNP/GO) were also observed. Higher SERS signals were generally obtained from these AgNP/GO flakes compared to the AgNP/rGO coated regions, since they contain higher amount of oxygen-rich functional groups which enabled a higher chemical and electromagnetic enhancement in SERS signals. All the substrates could detect 10−7 M R6G even after being stored in a vacuum desiccator for 3 months, indicating that AgNP/rGO coated filter membranes are promising as stable SERS substrates thanks to their long shelf life.

Simultaneously improved SERS sensitivity and thermal stability on Ag dendrites via surface protection by atomic layer deposition

Silver has been considered as one of the most promising materials for surface-enhanced Raman scattering (SERS) thanks to its ultra-high sensitivity. Nevertheless, Ag based nanostructures suffer from both thermal and chemical instability, inhibiting their practical applications. Surface coating is effective for improving their stability, nevertheless, which sacrifices the SERS sensitivity. Here we proposed a novel strategy to fabricate highly sensitive and stable SERS substrates using atomic layer deposition (ALD) protected Ag nanogaps. The purpose of this work is to provide an ultrathin and conformal protective layer without destroying the hot spots in nanogaps. This idea was implemented in ALD protected Ag dendrites. Ultrathin ALD layer with precisely controlled thickness can be conformally coated on the Ag surface without blocking the nanogaps between adjacent branches. Experimental results revealed Ag dendrites@10-cycle ZnO exhibit the improved SERS sensitivity compared to the pristine Ag dendrites. COMSOL theoretical simulations also demonstrated that ultrathin ZnO coating can promote the plasmonic coupling in Ag nanogaps. More importantly, this SERS substrate also shows excellent thermal stability at high temperature of 200 °C. Therefore, both SERS sensitivity and thermal stability of Ag dendrites can be enhanced via ALD surface protection.

Ag Nanoparticles@Agar Gel as a 3D Flexible and Stable SERS Substrate with Ultrahigh Sensitivity.

Flexible surface-enhanced Raman scattering (SERS) substrates have become one of the research hot spots due to the facile sampling by swabbing or wrapping on rough surfaces and the sensitive and nondestructive detection of contaminants. In this work, we proposed a simple and fast in situ reduction method to prepare Ag nanoparticles (Ag NPs) composited agar hydrogel (Ag NPs@Agar) flexible SERS substrate. Owing to the three-dimensional (3D) structure, good hydrophilicity and adsorption of the agar hydrogel, Ag NPs were grown uniformly in the 3D cross-linked structure. The distribution density of Ag NPs was further increased by the volume shrinkage when the hydrogel was dried in air. This high density and uniformly distribution of Ag NPs produced a large number of highly active SERS regions. In addition, the sensitivity of Ag NPs@Agar was further improved with the assistance of hydrophilic agar gel, which can trap the probe molecules into highly active SERS areas. The SERS results showed that the substrate can be used to detect dye molecules (rhodamine 6G), the minimum detectable concentration was 10-15 M, the relative standard deviation tested at 18 different positions was only 7.58%, and the intensity of the characteristic peak at 611 cm-1 decreased only about 10% after 49 days of storage, demonstrating the superior stability. Moreover, the Ag NPs@Agar substrate also could successfully achieve the micro-trace detection of melamine and sodium penicillin G in Xinjiang specialty camel milk powder. The above available results show that the prepared flexible Ag NPs@Agar SERS substrates possess potentials for the illegal additives and antibiotics in food safety analysis.

Economically reproducible surface-enhanced Raman spectroscopy of different compounds in thin film

We report herein an economically cheap and functionally stable surfaceenhanced Raman scattering (SERS) protocol of two photoactive pigments Rhodamine 6G (R6G) and Kiton Red (KR), implemented in thin films of silver (Ag) and gold (Au) nanoparticles (AgNPs and AuNPs). Both commercially available and chemically synthesized nanoparticles were used. The suitability of the nanoparticles toward SERS activity was tested through UV-visible absorption spectroscopy and scanning electron microscopy (SEM). The AgNPs and AuNPs based SERS substrates in the form of films were fabricated onto square-sized aluminum(Al) plates by simple drop deposition of colloidal nanoparticles solution onto their polished surfaces. The prepared nanoparticle films were sufficiently dried and coated further with the probe (R6G and KR) molecules by employing the identical deposition technique. The enhanced Raman signals of R6G and KR in such composite film structures were then recorded through a custom-built dispersive Raman spectrometer with He-Ne laser excitation at 632.8 nm. Our AgNPsfilm-based SERS protocol could yield the magnitude of the Raman signal enhancement up to 104 times for both R6G and KR. Moreover, AuNPs-based film was found to be less efficient toward the Raman enhancement of both compounds. Our SERS substrates can be easily fabricated, and SERS spectra are reproducible and stable, allowing one to consistently get a reproducible result even after 6 months.
 J. Bangladesh Acad. Sci. 45(1); 1-11: June 2021

Hybridizing Carbon-Based Dot-Capped Manganese Dioxide Nanosheets and Gold Nanoparticles as a Highly Sensitive Surface-Enhanced Raman Scattering Substrate.

Surface-enhanced Raman Scattering (SERS) is a sensitive and nondestructive technique that provides fingerprint structural information of molecules. Designing and constructing sensitive and stable SERS substrates is of great significance for the application of the technique. In this study, single-layer carbon-based dots (CDs) are used as capping agents to synthesize gold nanoparticles (AuNPs/CDs) and manganese dioxide nanosheets (MnO2/CDs), which are then hybridized through a simple cocentrifugation method. After the hybridization, the monodispersive AuNPs/CDs aggregate obviously into some clusters exhibiting strong SERS activity due to the electromagnetic "hot spots" effect, and the MnO2/CDs also show outstanding SERS activity due to the charge-transfer resonance effect. The obtained nanohybrids (MnO2/CDs/AuNPs) with robust chemical stability combine well with the electromagnetic enhancement of AuNPs/CDs and chemical enhancement of MnO2/CDs, leading to an ultrahigh enhancement factor of 3.9 × 108. Based on the novel SERS substrate, a sensitive and rapid sensing system for the detection of malachite green is developed, with a low detection limit of 1 × 10-9 M. This work provides a valuable model for designing and fabricating high-performance SERS substrates.

Shaped femtosecond laser induced photoreduction for highly controllable Au nanoparticles based on localized field enhancement and their SERS applications

Abstract Gold nanoparticles (Au NPs) have a wide range of applications because of their localized surface plasmon resonance properties. Femtosecond laser is considered to be an effective method for preparing Au NPs because of its characteristics of ultrashort irradiation periods and ultrahigh intensities. In this study, a novel method is proposed to produce an Au NP-attached substrate using the spatially and temporally shaped femtosecond laser. Laser-induced periodic surface structures (LIPSS) are designed to obtain the localized optical field enhancement, which leads to the femtosecond laser spatially reshaping, enabling the deposition of Au NPs by photoreduction on silicon substrates. The Au NPs prepared by this method exhibit morphological controllability and chemical stability, especially excellent spatial selectivity and uniformity, resulting in the tunable and stable surface-enhanced Raman scattering (SERS) applications. Also, the temporally shaped femtosecond pulses are introduced to further increase the enhancement factors of the SERS. This method successfully achieves the controllable morphology synthesis and selective deposition of Au NPs on the substrate simultaneously, which provides a promising candidate for SERS substrates fabrication, and holds potential applications in optoelectronics, such as molecular detection and biosensors.

Few-layered vdW MoO3 for sensitive, uniform and stable SERS applications

Metal oxides have been considered as active substrates for surface-enhanced Raman scattering (SERS) through morphology-engineering and defect-introduction with the sacrifice of the uniformity and stability of the materials. In this work, large-size, few-layered (FL) wide band gap van der Waals (vdW) MoO3 nanosheets were prepared as SERS substrates. It’s shown that the decrease of layer number converted the intrinsic MoO3 from non-SERS active to sensitive SERS substrate, with limit of detection (LOD) of 2 × 10−8 M for R6G. The SERS of MoO3 showed high homogeneity with RSD as small as 2.95%, The Raman intensity of R6G on the SERS substrate remained higher than 90% after exposing in atmosphere for 160 days, demonstrating excellent air-stability. Experimental results demonstrated that the SERS effect of the plasmon-free MoO3 substrate comes from chemical enhancement mechanism (CM) when reducing its thickness. Furthermore, the CM effect can be further improved through non-destructive atomic intercalation into the vdW gap, resulting in more charges separation between the adsorbed molecules and substrates. Our result showed that non-destructive modulation strategy is effective for preparation of sensitive, uniform and stable SERS substrate.

Quasi-Metal for Highly Sensitive and Stable Surface-Enhanced Raman Scattering.

SummaryCompared with the noble-metal surface-enhanced Raman scattering (SERS) substrates activated by the surface plasmon resonance (SPR)-induced electromagnetic mechanism (EM), the relative low sensitivity and stability of the chemical mechanism (CM)-based substrates are the biggest obstacles to their applications. Herein, we report that quasi-metallic VO2 nanosheet arrays can be used as a sensitive and stable SERS substrate. The lowest detectable limit of analyte adsorbed on the VO2 nanosheets achieves 10−10 M and the maximum Raman enhancement factor (EF) reaches 6.7 × 107, which is comparable with that of the noble metals. The experimental and theoretical results demonstrate that the SERS performance of the VO2 nanosheets comes from the strong interfacial interactions based on charge transfer and the vigorous SPR effects. Our research results demonstrate that quasi-metals are very promising SERS detection platforms and reveal that CM, like EM, contributes significantly to the SERS activity of quasi-metals.

Hierarchical TiO2–Ag composite with three-dimensional hot spots for trace detection

The construction of sensitive materials for trace detection is significant for various applications, including structure analysis, environment monitoring, and biological analysis. A critical challenge is the construction of materials with a combination of high sensitivity, excellent reliability, and good reusability. Here, we describe a hierarchical Ag–TiO2 composite that can provide three-dimensional hot spots for local electric field enhancement. This substrate can offer stable surface-enhanced Raman scattering (SERS) signal with an absolute intensity that is ∼51 times higher than that of conventional Ag substrate. The SERS signal from the substrate is highly homogenous over a wide region, without any obvious peak fluctuations. Moreover, the composite can be regenerated through an in situ photocatalytic reaction of TiO2. This innovative, recyclable SERS substrate can provide an efficient platform for in situ structure analysis and trace detection.

Slanted Ag-Al alloy nanorods arrays for highly active and stable surface-enhanced Raman scattering substrates

Aluminum has been established as an earth-abundant and low-cost alternative to gold and silver for plasmonic applications. Particularly, aluminum largely tends to combines with oxygen compared with silver. Here, a simple glancing angle deposition technique is presented to prepare Ag-Al alloy nanorods (NRs) with a small amount of aluminum. The effect of aluminum is to combine oxygen or corroded substances under certain conditions, such as in the air and in etchants. Beside this, owing to the large diffusion coefficient of aluminum in a Si wafer, the aluminum diffuses easily into a Si wafer, so the bonding force between the Ag-Al alloy NRs and Si wafer can be improved accordingly. In this work, 3.5 at% Al alloy NRs are optimal to exhibit high surface-enhanced Raman scattering (SERS) sensitivity, long-time stability as well as strong bonding force with a Si wafer. Ag-Al alloy NRs make a metal-metal alloy a promising material platform to develop pretty sensitive as well as stable SERS substrates.

A facile method to directly deposit the large-scale Ag nanoparticles on a silicon substrate for sensitive, uniform, reproducible and stable SERS substrate

In this study, we report a facile replacement reduction method to directly deposit the large-scale Ag nanoparticles on a silicon substrate at the room temperature and short reaction time (3 min). The reaction process is a straightforward wet chemistry approach using silver nitrate and hydrogen fluoride as a silver source and reducing reagent, respectively. The Ag nanoparticles as a surface-enhanced Raman scattering (SERS) substrate present high sensitivity, excellent uniformity, and high stability for rhodamine 6G molecules. Also, Ag nanoparticles are also shown to provide a unique platform for the SERS detection of melamine, 5-fluorouracil, amoxicillin in different aqueous media. The fabrication of Ag nanoparticles is facile, low cost, strong enhancement, low detection limit, high reproducibility, and stability, which is advantageous for the applications of other SERS-based sensors.

A flexible and stable surface-enhanced Raman scattering (SERS) substrate based on Au nanoparticles/Graphene oxide/Cicada wing array

In this work, we presented an eco-friendly and low-cost method to fabricate a kind of flexible and stable Au nanoparticles/graphene oxide/cicada wing (AuNPs/GO/CW) substrate. By controlling the ratio of reactants, the optimum SERS substrate with average AuNPs size of 65 nm was obtained. The Raman enhancement factor for rhodamine 6G (R6G) was 1.08×106 and the limit of detection (LOD) was as low as 10−8 M. After calibrating the Raman peak intensities of R6G, it could be quantitatively detected. In order to better understand the experimental results, the 3D finite-different time-domain simulation was used to simulate the AuNPs/GO/CW-1 (the diameter of the AuNPs was 65 nm) to further investigate the SERS enhancement effect. More importantly, the AuNPs/GO/CW-1 substrates not only can provide strong enhancement factors but also can be stable and reproducible. This SERS substrates owned a good stability for the SERS intensity which was reduced only by 25% after the aging time of 60 days and the relative standard deviation was lower than 20%, revealing excellent uniformity and reproducibility. Our positive findings can pave a new way to optimize the application of SERS substrate as well as provide more SERS platforms for quantitative detection of organic contaminants vestige, which makes it very promising in the trace detection of biological molecules.

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.

Rapid qualitative and quantitative determination of food colorants by both Raman spectra and Surface-enhanced Raman Scattering (SERS)

Surface Enhanced Raman Scattering (SERS) spectroscopy technology is widely used in materials analysis, environmental monitoring, biomedical, food security and other fields. Flower-shaped silver nanoparticles have been successfully synthesized by a simple aqueous phase silver nitrate reduction by ascorbic acid in the presence of polyvinylpyrrolidone (PVP) surfactant. The nanoparticles diameters were adjusted from 450 to 1000nm with surface protrusions up to 10–25nm. The flower-shaped silver nanostructures obtained were used as stable SERS substrates with high SERS activity for detecting Rhodamine 6G (R6G), at a concentration of only 10−9mol/L, where the SERS signal is still clear. SERS spectroscopy of four different food colorants (e.g. food blue, tartrazine, sunset yellow, acid red) were analysed and the characteristic bands were identified. An improved principle component analysis (PCA) was used for four different food colorants detection, at concentrations down to about 10−8mol/L. Thus, the LOD of food blue, tartrazine, sunset yellow and acid red are 79.285μg/L, 5.3436μg/L, 45.238μg/L and 50.244μg/L, respectively.

A Low-cost, Highly-stable Surface Enhanced Raman Scattering Substrate by Si Nanowire Arrays Decorated with Au Nanoparticles and Au Backplate

We present a facile and cost-effective manner to fabricate a highly sensitive and stable surface enhanced Raman scattering (SERS) substrate. First, a silicon nanowire array (SiNWA) is tailored by metal-assisted chemical etching (MaCE) method as a scaffold of the desired SERS substrate. Next, with an oblique angle deposition (OAD) method, optimized gold nanoparticles (AuNPs) are successfully decorated on the surface of the SiNWA. These AuNPs enable a strong localized electric field, providing abundant hot spots to intensify the Raman signals from the targeting molecules. By applying a well-established methodology, Taguchi method, which is invented for designing experiments, the optimized combination of parameters is obtained efficiently. The experimental results are also confirmed by finite-difference time-domain (FDTD) simulation calculations. Besides, a gold metal backplate (AuMBP) is applied to further enhancing the Raman signal intensity. Based on this developed SERS substrate, we demonstrated an enhancement factor (EF) of 1.78 × 106 and a coefficient of variation (CV) of 4.2%. Both EF and CV indicate a highly stable property and the optimized SERS substrate substantially outperform the commercial product. In the end, we also demonstrate a quantitative measurement on practical application of detecting malachite green (MG) with concentration from 10 nM to 100 μM.

Reliable molecular trace-detection based on flexible SERS substrate of graphene/Ag-nanoflowers/PMMA

Flexible substrate consisted of PMMA-supported monolayer graphene with sandwiched Ag-nanoflowers (G/AgNFs/PMMA) for ultrasensitive, reproducible, and stable surface-enhanced Raman scattering (SERS) detection is reported. Graphene templated micro-current-assisted chemical reduction method was employed to support AgNFs growth, and uniform-distribution AgNFs with all directions nanotips can generate tremendous enhancement factor and intensive hotspots. The minimum detectable concentration (i.e., detection limit) for rhodamine 6G (R6G) in-situ detection by covering this as-synthesized G/AgNFs/PMMA flexible substrate can be as low as 10−14M. Moreover, graphene can effectively stabilize the SERS signals and protect AgNFs from oxidation, endowing this flexible substrate a long-term stability with maximum intensity deviation lower than 10%, for the quantitative measurements from spot-to-spot or substrate-to-substrate. In order to trial its practical applications with various real-world surfaces, the in-situ SERS detection of phenylalanine@apple, adenosine aqueous solution and methylene-blue@fish was performed by covering this G/AgNFs/PMMA flexible substrate. Clear Raman peaks can be obtained for all the selected samples with concentration of 10−10M and, importantly, good linear relationship between Raman intensity and molecular concentration indicates the potential application of the G/AgNFs/PMMA flexible substrate in quantitative determination. Thus, this high-efficiency and low-cost flexible SERS substrate may provide a new way for the molecular trace-detection in food security and environmental protection.

Fabrication of Silver Chevron Arrays as an Efficient and Stable SERS Substrate: Implications in Biological Sensing

Although glancing angle deposited silver substrates offer an excellent figures for surface enhanced Raman scattering (SERS) sensing, the chemical instability issues of silver substrates upon exposure to the aqueous solutions is still challenging and limits their application in SERS-based biosensors. In this context, we applied and compared two different strategies including sodium citrate as a self-assembled monolayer (SAM), and thermal deposition of gold at different thicknesses to stabilize the silver chevron arrays deposited by the glancing angle deposition method. Although optical characterization confirms good stabilization in both strategies, intact morphological features can only be detected after gold coating compared to sodium citrate coating which makes a homogenous monolayer on the surface. Furthermore, the SERS intensity of Rhodamine B increases after these coating processes, which might be due to the less oxidation and deformation of the nanostructures on the substrate or improved attachment of the analyte to the substrates. This research can extend our knowledge for the development of stable and reproducible SERS substrates in the field of biological sensing.

Site-Selective Controlled Dealloying Process of Gold-Silver Nanowire Array: a Simple Approach towards Long-Term Stability and Sensitivity Improvement of SERS Substrate.

Limitations of achieving highly sensitive and stable surface-enhanced Raman scattering (SERS) substrate greatly concern the suitable method for fabrication of large-area plasmonic nanostructures. Herein we report a simple approach using template-based synthesis to create a highly ordered two-dimensional array of gold-silver alloy nanowires, followed by the controlled dealloying process. This particular step of mild acid etching (15%v/v nitric acid for 5 min) allowed the formation of Raman hot spots on the nanowire tips while maintaining the integrity of highly active alloy composition and rigid nanowire array structure. Full consideration of SERS substrate performance was accomplished using 4-mercaptobenzoic acid (4-MBA) as a probe molecule. Exceedingly higher SERS signal (150-fold) can be achieved with respect to typical gold film substrate. Moreover, an excellent stability of SERS substrate was also determined for over 3 months storage time. In contrast to the previous studies which stability improvement was accomplished at a cost of sensitivity reduction, the simultaneous improvement of sensitivity and stability makes the controlled dealloying process an excellent choice of SERS substrate fabrication. In addition, uniformity and reproducibility studies indicated satisfactory results with the acceptable values of relative standard deviation.

Micro/nano-structured graphitic carbon nitride–Ag nanoparticle hybrids as surface-enhanced Raman scattering substrates with much improved long-term stability

Surface-enhanced Raman scattering (SERS) substrates with high SERS activity and stability are important for SERS sensors. A facile method was developed to fabricate efficient and stable SERS substrates by combining Ag nanoparticles (NPs) and micro-scale sheeted graphitic carbon nitride (g-C3N4). The g-C3N4/Ag NPs hybrid could provide a great number of hot spots and concentrated the analyte by the π–π stacking interaction between analyte molecules and g-C3N4, making a dramatic Raman enhancement. Moreover, the g-C3N4/Ag NPs hybrid uniformly immobilized Ag NPs on the surface and edges of g-C3N4 sheets by an interaction between Ag NPs and g-C3N4, leading to much improved long-term stability. This could be explained in terms of the electron–donor effect of g-C3N4, which was further confirmed by density functional theory calculations. The inherent Raman enhancing effect of g-C3N4 itself also contributed to the total SERS responses. Due to multiple enhancement contributions, the g-C3N4/Ag NPs hybrid exhibited a strong Raman enhancement effect for with an enhancement factor of 4.6×108 (evaluated by using crystal violet as a probe), and possessed wide adaptability from dyes, pesticides to bio-molecules.