Sensitive Surface-Enhanced Raman Scattering Detection Using On-Demand Postassembled Particle-on-Film Structure

High-performance, large-area flexible SERS substrates prepared by reactive ion etching for molecular detection

In the realm of molecular detection, the surface-enhanced Raman scattering (SERS) technique has garnered increasing attention due to its rapid detection, high sensitivity, and non-destructive characteristics. However, conventional rigid SERS substrates are either costly to fabricate and challenging to prepare over a large area, or they exhibit poor uniformity and repeatability, making them unsuitable for inspecting curved object surfaces. In this work, we present a flexible SERS substrate with high sensitivity as well as good uniformity and repeatability. First, the flexible polydimethylsiloxane (PDMS) substrate is manually formulated and cured. SiO2/Ag layer on the substrate can be obtained in a single process by using ion beam sputtering. Then, reactive ion etching is used to etch the upper SiO2layer of the film, which directly leads to the desired densely packed nanostructure. Finally, a layer of precious metal is deposited on the densely packed nanostructure by thermal evaporation. In our proposed system, the densely packed nanostructure obtained by etching the SiO2layer directly determines the SERS ability of the substrate. The bottom layer of silver mirror can reflect the penetrative incident light, the spacer layer of SiO2and the top layer of silver thin film can further localize the light in the system, which can realize the excellent absorption of Raman laser light, thus enhancing SERS ability. In the tests, the prepared substrates show excellent SERS performance in detecting crystalline violet with a detection limit of 10-11M. The development of this SERS substrate is anticipated to offer a highly effective and convenient method for molecular substance detection.

Wafer-Level Highly Dense Metallic Nanopillar-Enabled High-Performance SERS Substrates for Molecular Detection

Seeking sensitive, large-scale, and low-cost substrates is highly important for practical applications of surface-enhanced Raman scattering (SERS) technology. Noble metallic plasmonic nanostructures with dense hot spots are considered an effective construction to enable sensitive, uniform, and stable SERS performance and thus have attracted wide attention in recent years. In this work, we reported a simple fabrication method to achieve wafer-scale ultradense tilted and staggered plasmonic metallic nanopillars filled with numerous nanogaps (hot spots). By adjusting the etching time of the PMMA (polymethyl methacrylate) layer, the optimal SERS substrate with the densest metallic nanopillars was obtained, which possessed a detection limit down to 10^-13 M by using crystal violet as the detected molecules and exhibited excellent reproducibility and long-term stability. Furthermore, the proposed fabrication approach was further used to prepare flexible substrates; for example, a SERS flexible substrate was proven to be an ideal platform for analyzing low-concentration pesticide residues on curved fruit surfaces with significantly enhanced sensitivity. This type of SERS substrate possesses potential in real-life applications as low-cost and high-performance sensors.

Ag NPs@PDMS nanoripple array films as SERS substrates for rapid in situ detection of pesticide residues

The large-area fabrication of flexible and transparent surface-enhanced Raman scattering (SERS) substrates with high performance by a facile and efficient method is still challenging. Here, we demonstrated a large-scale, flexible and transparent SERS substrate composed of PDMS nanoripple array film decorated with silver nanoparticles (Ag NPs@PDMS-NR array film) prepared by a combination of plasma treatment and magnetron sputtering. The performances of SERS substrates were characterized by rhodamine 6G (R6G) using a handheld Raman spectrometer. The optimal Ag NPs@PDMS-NR array film exhibited high SERS sensitivity, with a detection limitation of R6G reaching 8.20 × 10^-8 M as well as excellent uniformity (RSD = 6.8%) and batch-to-batch reproducibility (RSD = 2.3%). In addition, the substrate showed outstanding mechanical stability and good SERS enhancement by backside illumination, thus it was suitable for in situ SERS detection on curved surfaces. The detection limit of malachite green on apple and tomato peels was 1.19 × 10^-7 and 1.16 × 10^-7 M, respectively, and quantitative analysis of pesticide residues could be realized. These results demonstrate that the Ag NPs@PDMS-NR array film has great practical potential in rapid in situ detection of pollutants.

ZnO nanorods decorated with Ag nanoflowers as a recyclable SERS substrate for rapid detection of pesticide residue in multiple-scenes

Pesticide residues threaten the ecological environment and human health. Therefore, developing high performance SERS substrate to achieve highly sensitive detection of pesticide residues is meaningful. In this study, based on the strategy of combining "hot spots" engineering and material hybridization, we construct a novel hybrid SERS substrate by depositing Ag nanoflowers (NFs) on ZnO nanorods (NRs). Benefiting from the synergistic effect of electromagnetic enhancement and charge transfer effect, the Ag NFs@ZnO NRs substrate exhibits a low detection limit (10^-13 M) for crystal violet molecules. This SERS substrate has good uniformity with a relative standard deviation of 7.463 %. Besides, owning to the photocatalytic property of ZnO NRs, the hybrid substrate can degrade probe molecules after SERS detection and realize recyclability. As a demonstration, we employed our SERS substrate for the trace detection of pesticide residues on apple surface and in river water. This study provides a new idea for improving the SERS performance of hybrid substrates.

Self-assembly of Au nanocrystals into large-area 3-D ordered flexible superlattice nanostructures arrays for ultrasensitive trace multi-hazard detection

Plasmonic nanoparticles that self-assemble into highly ordered superlattice nanostructures hold substantial promise for facilitating ultra-trace surface-enhanced Raman scattering (SERS) detection. Herein, we propose a boiling-point evaporation method to synthesize ordered monocrystal-like superlattice Au nanostructures (OML-Au NTs) with a polyhedral morphology. Combined with thermal nanoimprint technology, OML-Au NTs were directly transferred to impact-resistant polystyrene (IPS) flexible SERS substrates, the obtained flexible substrates (donated as OML-Au NTs/IPS) detection limit for R6G molecules as low as 10^-13 M. These results were confirmed by simulating the electromagnetic field distribution of ordered/unordered two-dimensional single-layer and three-dimensional aggregated gold nanostructures. The OML-Au NTs/IPS substrates were successfully used to detect and quantify three commonly-used agricultural pesticides, achieving detection limits as low as 10^-11 M and 10^-12 M, and in situ real-time detection limit reached 0.24 pg/cm² for thiram on apple peels, which was 3 orders of magnitude lower than the current detection limit. In addition, the Raman intensity from multiple locations showed a relative standard deviation lower than 7 %, exhibiting the reliability necessary for practical applications. As a result, this research demonstrates a highly reproducible method to enable the development of plasmonic nanomaterials with flexible superstructures.

One-pot fabrication of Mo1-xWxS2 alloy nanosheets as SERS substrates with highly Raman enhancement effect and long-term stability

A new Mo_1-xW_xS₂ two-dimensional nanosheets were prepared by the one-pot method. After certain Mo atoms in MoS₂ were replaced by W ones in a hydrothermal reduction procedure, Mo_1-xW_xS₂ was formed on the Mo foil. Well enhanced Mo_1-xW_xS₂ nanosheets were prepared when the sodium tungstate concentration got under control. Various characterizations were carried out, which indicate that Mo_1-xW_xS₂ nanosheets with good crystallinity. Compared with MoS₂, the Raman intensity of Rhodamine 6G (10^-6 M) was amplified by 1.7 times with Mo_1-xW_xS₂ nanosheets as the substrate. The characteristic Raman peaks could still be clearly distinguished until the concentration of Rhodamine 6G (R6G), Methylene blue (MB) and Crystal violet (CV) down to 10^-8, 10^-8 and 10^-7 M, respectively. With abundant edge active sites that facilitate charge transfer, Mo_1-xW_xS₂ nanosheets could better enhance SERS signals of target detection molecules and get a good linear relationship exists within the concentration and Raman peak strength. In addition, R6G SERS detection also shows excellent reproducibility and long-term stability of this TMDs SERS substrate.

Rapid Immobilization of Silver Nanoparticles via Amino-quinone Coatings Enables Surface-Enhanced Raman Scattering Detection

Immobilization of metal nanoparticles (NPs) on flexible substrates for surface-enhanced Raman scattering (SERS) has received great attention. Anchoring NPs on substrates generally involves the process of surface modification, thanks to its simple, universal, and nondestructive features. 2-Hydroxy-1,4-naphthoquinone (HNQ), a plant-derived compound used to dye hairs and nails, may interact with polyamine or metal ions to form a surface coating. Here, we report the formation of amino-quinone coatings via the co-deposition of HNQ and polyethyleneimine, which provides a functionalized platform to rapidly immobilize Ag NPs on substrates such as a poly(dimethylsiloxane) (PDMS) film to fabricate Ag-PDMS substrates for SERS detection. The detection concentrations are down to 10^-8 M for rhodamine 6G. This work expands the system of surface co-deposition and further provides a facile route to prepare a highly efficient SERS substrate.

Flexible surface-enhanced Raman scatting substrates: recent advances in their principles, design strategies, diversified material selections and applications

Surface-enhanced Raman scattering (SERS) is widely used as a powerful analytical technology in cutting-edge areas such as food safety, biology, chemistry, and medical diagnosis, providing ultra-fast, ultra-sensitive, nondestructive characterization and achieving ultra-high detection sensitivity even down to the single-molecule level. Development of Raman spectroscopy is strongly dependent on high-performance SERS substrates, which have long evolved from the early days of rough metal electrodes to periodic nanopatterned arrays building on solid supporting substrates. For rigid SERS substrates, however, their applications are restricted by sophisticated pretreatments for detecting solid samples with non-planar surfaces. It is therefore essential to reassert the principles in constructing flexible SERS substrates. Herein, we comprehensively review the state-of-the-art in understanding, preparing and using flexible SERS. The basic mechanisms behind the flexible SERS are briefly outlined, typical design strategies are highlighted and diversified selection of materials in preparing flexible SERS substrates are reviewed. Then the recent achievements of various interdisciplinary applications based on flexible SERS substrates are summarized. Finally, the challenges and perspectives for future evolution of flexible SERS and their applications are demonstrated. We propose new research directions focused on stimulating the real potential of SERS as an advanced analytical technique for commercialization.

Hybridizing Silver Nanoparticles in Hydrogel for High-Performance Flexible SERS Chips

An ideal surface-enhanced Raman scattering (SERS) substrate should have high sensitivity, long-term stability, excellent repeatability, and strong anti-interference. In the present work, single-layer carbon-based dot (CD)-capped Ag nanoparticle aggregates (a-AgNPs/CDs) with high SERS activity are synthesized and hybridized with a hydrogel to prepare novel hydrogel SERS chips. Benefiting from the unique properties of a-AgNPs/CDs and the hydrogel, the constructed hydrogel SERS chips show excellent performances. Taking crystal violet detection as an example, the hydrogel SERS chips show a detection limit of around 1 × 10^-16 mol/L (high sensitivity), maintain above 96.40% of SERS activity even after 14 weeks of storage (long-term stability), and display point-to-point relative standard deviation (RSD) in one chip as low as 1.43% (outstanding repeatability) and RSD in different chips as low as 2.75% (excellent reproducibility). Furthermore, the self-extraction effect of the hydrogel enables the flexible hydrogel SERS chips to be used for analyzing various real samples including soybean milk, juices, and fruits without any complex pretreatment. For instance, the hydrogel SERS chips are able to detect trace thiram and 2-(4-thiazolyl)benzimidazole with the detection limits of 1 and 5 ppb in liquid samples, respectively, and of 1 and 2.5 ng/cm² on the peel of fruits, respectively. The self-extraction functional flexible SERS chips offer a reliable and convenient platform for the quick detection and on-site monitoring of chemical contaminants.

Controllable Nanoparticle Aggregation through a Superhydrophobic Laser-Induced Graphene Dynamic System for Surface-Enhanced Raman Scattering Detection

Surface-enhanced Raman scattering (SERS) is widely used for low-concentration molecular detection; however, challenges related to detection uniformity and repeatability are bottlenecks for practical application, especially as regards ultrasensitive detection. Here, through the coupling of bionics and fluid mechanics, a lotus-leaf effect and rose-petal effect (LLE-RPE)-integrated superhydrophobic chip is facilely developed using laser-induced graphene (LIG) fabricated on a polyimide film. Dense and uniform aggregation of gold nanoparticles (AuNPs) in droplets is realized through a constant contact angle (CCA) evaporation mode in the dynamic enrichment process, facilitating reliable ultrasensitive detection. The detection chip consists of two components: an LLE zone containing an ethanol-treated LIG superhydrophobic surface with a low-adhesive property, which functions as an AuNP-controllable aggregation zone, and an RPE zone containing an as-fabricated LIG superhydrophobic surface with water-solution pinning ability, which functions as a droplet solvent evaporation and a AuNP blending zone. AuNPs realize uniform aggregation during rolling on the LLE zone, and then get immobilized on the RPE zone to complete evaporation of the solvent, followed by Raman detection. Here, based on dense and uniform AuNP aggregation, the detection system achieves high-efficiency (242 s/18 μL) and ultralow-concentration (10^-17 M) detection of a target analyte (rhodamine 6G). The proposed system constitutes a simple approach toward high-performance detection for chemical analysis, environmental monitoring, biological analysis, and medical diagnosis.

SERS substrate fabrication for biochemical sensing: towards point-of-care diagnostics

Rapid technology development and economic growth have brought attention to public health issues, such as food safety and environmental pollution, which creates an ever-increasing demand for fast and portable sensing technologies. Portable surface-enhanced Raman spectroscopy (SERS) capable of various analyte detection with low concentration in a convenient manner shows advantages in sensing technology including enhanced diagnostic precision, improved diagnostic efficiency, reduced diagnostic cost, and alleviation of patient pain, which emerges as a promising candidate for point-of-care testing (POCT). SERS detection technology based on different nanostructures made of noble metal-based nanomaterials can increase the sensitivity of Raman scattering by 6-8 orders of magnitude, making Raman based trace detection possible, and greatly promote the application scenarios of portable Raman spectrometers. In this perspective, we provide an overview of fundamental knowledge about the SERS mechanism including chemical and electromagnetic field enhancement mechanisms, the design and fabrication of SERS substrates based on materials, progress of using SERS for POCT in biochemical sensing and its clinical applications. Furthermore, we present the prospective of developing new nanomaterials with different functionalities for advanced SERS substrates, as well as the future advancement of biomedical sensing and clinical potential of SERS technology.

Towards a point-of-care SERS sensor for biomedical and agri-food analysis applications: a review of recent advancements

The growing demand for reliable and robust methodology in bio-chemical sensing calls for the continuous advancement of sensor technologies. Over the last two decades, surface-enhanced Raman spectroscopy (SERS) has emerged as one of the most promising analytical techniques for sensitive and trace analysis or detection in biomedical and agri-food applications. SERS overcomes the inherent sensitivity limitation associated with Raman spectroscopy, which provides vibrational "fingerprint" spectra of molecules that makes it unique and versatile among other spectroscopy techniques. This paper comprehensively reviews the recent advancements of SERS for biomedical, food and agricultural applications over the last 6 years, and we envision that, in the near future, some of these platforms have the potential to be translated as a point-of-care and rapid sensor for real-life end-user applications. The merits and limitations of various SERS sensor designs are analysed and discussed based on critical features such as sensitivity, specificity, usability, repeatability and reproducibility. We conclude by highlighting the opportunities and challenges in the field while stressing the technological gaps to be addressed in realizing commercially viable point-of-care SERS sensors for practical biomedical and agri-food technological applications.

DNA-assisted synthesis of Ortho-NanoDimer with sub-nanoscale controllable gap for SERS application

Nanostructure with precisely controllable narrow gap width remains a great challenge, especially at the sub-nanoscale level. Here, a versatile strategy named as DNA-assisted synthesis of ortho-nanodimer (DaSON) is proposed to fabricate Ag (Au) nanodimers with a uniform gap width from nanometers to angstroms. In such a strategy, two nanoparticles are constrained by the equilibrium state of the DNA hybridization and electrostatic repulsion to form zipper-like ortho-nanostructures with an extremely uniform gap whose width can be finely adjusted at nanoscale or sub-nanoscale by changing the DNA sequence and the surface charge of nanoparticles. The inherent strong electromagnetic coupling in the uniform sub-nanometer gap can generates an unparalleled SERS enhancement together with an extraordinary reproducibility. Compared with conventional DNA-based nano-gap fabrication strategy, the DaSON strategy enhances the SERS intensity for more than two orders of magnitude with a detection limit of 100 aM for DNA, and significantly improves the reproducibility in both labeled and label-free SERS sensing applications. Moreover, the DaSON strategy holds wide applicability for arbitrary kinds of DNA-modifiable nanoparticles. Therefore, we believe that the DaSON strategy provides an innovative method for the synthesis of nanostructures with controllable nanogaps and has a promising future in multiple fields including nanotechnology, catalysis and photonics.

Ultrasensitive SERS detection of antitumor drug methotrexate based on modified Ag substrate

Methotrexate (MTX) is a drug with broad-spectrum antitumor activity that is of great importance in therapeutic drug monitoring applications. In this essay, the two-step modified concentrated Ag colloid with the assistance of KF and MgSO₄ was used as the SERS active substrate for the ultrasensitive detection of MTX and its commercial formulations (tablets). It can be found that the two-step modification of the samples is a crucial procedure to remove the by-products in the synthesis of Ag colloid and further concentrate the Ag colloid. Under the optimal detection conditions, the minimum detection concentration of MTX is 1 × 10^-16 mol/L. And, there is a good linear relationship over a wide concentration range of 1 × 10^-16-1 × 10^-6 mol/L. The labelled amounts of the two manufacturers of MTX commercial tablets are in the range of 96.4-104.3% with the RSDs between 1.8% and 3.5% by this method, which are in accordance with the methodological requirements. These results prove that the proposed SERS method exhibits a good reproducibility and ultra-high sensitivity for the detection of the antitumor drug MTX.

Huge field enhancement and high transmittance enabled by terahertz bow-tie aperture arrays: a simulation study

Sub-wavelength aperture arrays featuring small gaps have an extraordinary significance in enhancing the interactions of terahertz (THz) waves with matters. But it is difficult to obtain large light-substance interaction enhancement and high optical response signal detection capabilities at the same time. Here, we propose a simple terahertz bow-tie aperture arrays structure with a large electric field enhancement factor and high transmittance at the same time. The field enhancement factor can reach a high value of 1.9×10⁴ and the transmission coefficient of around 0.8 (the corresponding normalized-to-area transmittance is about 14.3) at 0.04 µm feature gap simultaneously. The systematic simulation results show that the designed structure can enhance the intensity of electromagnetic hotspot by continuously reducing the feature gap size without affecting the intensity of the transmittance. We also visually displayed the significant advantages of extremely strong electromagnetic hot spots in local terahertz refractive index detection, which provides a potential platform and simple strategy for enhanced THz spectral detection.

Toward Flexible Surface-Enhanced Raman Scattering (SERS) Sensors for Point-of-Care Diagnostics

Surface-enhanced Raman scattering (SERS) spectroscopy provides a noninvasive and highly sensitive route for fingerprint and label-free detection of a wide range of molecules. Recently, flexible SERS has attracted increasingly tremendous research interest due to its unique advantages compared to rigid substrate-based SERS. Here, the latest advances in flexible substrate-based SERS diagnostic devices are investigated in-depth. First, the intriguing prospect of point-of-care diagnostics is briefly described, followed by an introduction to the cutting-edge SERS technique. Then, the focus is moved from conventional rigid substrate-based SERS to the emerging flexible SERS technique. The main part of this report highlights the recent three categories of flexible SERS substrates, including actively tunable SERS, swab-sampling strategy, and the in situ SERS detection approach. Furthermore, other promising means of flexible SERS are also introduced. The flexible SERS substrates with low-cost, batch-fabrication, and easy-to-operate characteristics can be integrated into portable Raman spectroscopes for point-of-care diagnostics, which are conceivable to penetrate global markets and households as next-generation wearable sensors in the near future.

Direct electron-beam patterning of transferrable plasmonic gold nanoparticles using a HAuCl4/PVP composite resist

Reliable fabrication of gold nanoparticles with desirable size, geometry and spatial arrangement is essential for plasmonic applications. A common fabrication flow usually involves electron-beam lithography and a vacuum-evaporation-based lift-off process or etching. In this work, we evaluate an alternative approach to directly fabricate a plasmonic gold nanoparticle array without involving the vacuum evaporation process by using a chloroauric acid/poly(vinyl pyrrolidone) (HAuCl4/PVP) hybrid as a functional electron-beam resist. Systematic experiments were conducted to investigate the patterning behaviors in the fabrication process. With the optimized fabrication parameters, we show that the HAuCl4/PVP composite resist has a high patterning resolution and pure gold nanoparticles with tens of nanometers can be obtained after an annealing-based pyrolysis process. More particularly, compared to the patterned plasmonic gold nanoparticles obtained by conventional methods, the gold nanoparticles fabricated by our method can be transferred to soft substrates due to the absence of an adhesion layer, enabling various potential applications in flexible and stretchable optics. As an example, we demonstrated that the transferred gold nanoparticle array can be conformably assembled onto a flat gold surface to form a particle-on-film structure for surface-enhanced Raman scattering (SERS) applications.

High-Throughput Fabrication of Ultradense Annular Nanogap Arrays for Plasmon-Enhanced Spectroscopy

The confinement of light into nanometer-sized metallic nanogaps can lead to an extremely high field enhancement, resulting in dramatically enhanced absorption, emission, and surface-enhanced Raman scattering (SERS) of molecules embedded in nanogaps. However, low-cost, high-throughput, and reliable fabrication of ultra-high-dense nanogap arrays with precise control of the gap size still remains a challenge. Here, by combining colloidal lithography and atomic layer deposition technique, a reproducible method for fabricating ultra-high-dense arrays of hexagonal close-packed annular nanogaps over large areas is demonstrated. The annular nanogap arrays with a minimum diameter smaller than 100 nm and sub-1 nm gap width have been produced, showing excellent SERS performance with a typical enhancement factor up to 3.1 × 10⁶ and a detection limit of 10^-11 M. Moreover, it can also work as a high-quality field enhancement substrate for studying two-dimensional materials, such as MoSe₂. Our method provides an attractive approach to produce controllable nanogaps for enhanced light-matter interaction at the nanoscale.