Uniaxially Stretched Flexible Surface Plasmon Resonance Film for Versatile Surface Enhanced Raman Scattering Diagnostics

Plasmonic Nanostructures Grown from Reacting Droplet-In-Microwell Array on Flexible Films for Quantitative Surface-Enhanced Raman Spectroscopy in Plant Wearable In Situ Detection

Plant wearable detection has garnered significant interest in advancing agricultural intelligence and promoting sustainable food production amidst the challenges of climate change. Accurately monitoring plant health and agrochemical residue levels necessitates qualities such as precision, affordability, simplicity, and noninvasiveness. Here, a novel attachable plasmonic film is introduced and designed for on-site detection of agrochemical residues utilizing surface-enhanced Raman spectroscopy (SERS). By functionalizing a thin polydimethylsiloxane film with silver nanoparticles via controlled droplet reactions in micro-well arrays, a plasmonic film is achieved that not only maintains optical transparency for precise analyte localization but also conforms closely to the plant surface, facilitating highly sensitive SERS measurements. The reliability of this film enables accurate identification and quantification of individual compounds and their mixtures, boasting an ultra-low detection limit ranging from 10-16 to 10-13 m, with mini mal relative standard deviation. To showcase its potential, on-field detection of pesticide residues on fruit surfaces is conducted using a handheld Raman spectrometer. This advancement in fabricating plasmonic nanostructures on flexible films holds promise for expanding SERS applications beyond plant monitoring, including personalized health monitoring, point-of-care diagnosis, wearable devices for human-machine interface, and on-site monitoring of environmental pollutants.

Capping layer enabled controlled fragmentation of two-dimensional materials by cold drawing

Cold drawing, a well-established processing technique in the polymer industry, was recently revisited and discovered as an efficient material structuring method to create ordered patterns in composites consisting of both cold-drawable polymers and brittle target materials. Such a high-yield and low-cost manufacturing technique enables the large-scale fabrication of micro-ribbon structures for a wide range of functional materials, including two-dimensional (2D) layered materials. Compared to the abundant phenomenological results from experiments, however, the underlying mechanisms of this technique are not fully explored. Here, supported by experimental investigation, finite element calculations, and theoretical modeling, we systematically study the effect of a capping layer on the controlled fragmentation of 2D materials deposited on polymer substrates during the cold drawing. The capping layer is found to prevent the premature fracture of the 2D thin films during elastic deformation of the substrate, when a specific requirement proposed by the theoretical model is satisfied. Controlled fragmentation is enabled in the necking stage due to the protective effect of the capping layer, which also influences the size of the resulting fragments. Flexible and stretchable electrodes based on 2D material ribbons are fabricated to demonstrate the effectiveness of the proposed roadmap. This study gives an accurate understanding of interactions between 2D materials, polymer substrates, and capping layers during cold drawing, and offers guidance for potential applications such as flexible electronics.

Recent Development and Applications of Stretchable SERS Substrates

Surface-enhanced Raman scattering (SERS) is a cutting-edge technique for highly sensitive analysis of chemicals and molecules. Traditional SERS-active nanostructures are constructed on rigid substrates where the nanogaps providing hot-spots of Raman signals are fixed, and sample loading is unsatisfactory due to the unconformable attachment of substrates on irregular sample surfaces. A flexible SERS substrate enables conformable sample loading and, thus, highly sensitive Raman detection but still with limited detection capabilities. Stretchable SERS substrates with flexible sample loading structures and controllable hot-spot size provide a new strategy for improving the sample loading efficiency and SERS detection sensitivity. This review summarizes and discusses recent development and applications of the newly conceptual stretchable SERS substrates. A roadmap of the development of SERS substrates is reviewed, and fabrication techniques of stretchable SERS substrates are summarized, followed by an exhibition of the applications of these stretchable SERS substrates. Finally, challenges and perspectives of the stretchable SERS substrates are presented. This review provides an overview of the development of SERS substrates and sheds light on the design, fabrication, and application of stretchable SERS systems.

Stretchable and Flexible Micro-Nano Substrates for SERS Detection of Organic Dyes

Surface-enhanced Raman spectroscopy (SERS) is a precise and noninvasive analytical technique to identify vibrational fingerprints of trace analytes with sensitivity down to the single-molecule level. However, substrates can influence this capability, and current SERS techniques lack uniform, reproducible, and stable substrates to control plasma hot spots over a wide spectral range. Herein, we demonstrate a flexible SERS substrate via longitudinal stretching of a polydimethylsiloxane (PDMS) film. This substrate, after stretching and shrinking, exhibits an irregular wrinkled structure with abundant gaps and grooves that function as hot spots, thereby improving the hydrophobic properties of the material. To investigate the enhancement effect of Raman signals, silver nanoparticles (AgNPs) were mixed with Rhodamine 6G (R6G) solution, and the obtained blend was dropped onto the PDMS film to form a coffee ring pattern. According to the results, the hydrophobicity of the substrate increases with the degree of PDMS stretching, achieving the optimal level at 150% stretching. Moreover, the increase in hydrophobicity makes the measured molecules more aggregated, which enhances the Raman signal. The stretching and shrinkage of the PDMS film lead to a much higher density of nanogaps among nanoparticles and nanogrooves, which serve as multiple hot spots. Being highly localized regions of intense local fields, these hot spots make a significant contribution to SERS performance, improving the sensitivity and reproducibility of the method. In particular, the relative standard deviation (RSD) was found to be 2.5544%, and the detection limit was 1 × 10^-7 M. Therefore, SERS using stretchable and flexible micro-nano substrates is a promising way for detecting dyes in wastewater.

An adhesive SERS substrate based on a stretched silver nanowire-tape for the in situ multicomponent analysis of pesticide residues

Two essential factors in powerful surface-enhanced Raman spectroscopy analysis of trace pesticide residues are viz., high sensitivity and efficient sampling. Herein, owing to elastic properties, a stretched Ag nanowire (Ag NW)-tape under the strain of 15% formed a wrinkled structure with periodic microridges and microgrooves, where abundant nanogaps were generated by the aggregated Ag NWs. Compared with the unstretched Ag NW-tape substrate, an appreciable signal enhancement of the modified 4-mercaptobenzoic acid (4-MBA) molecules with a ratio of 2.6 was discerned from the sophisticated SERS substrate due to the electromagnetic enhancement induced by the relatively high density of "hot spots" around the Ag NW aggregates. The as-fabricated Ag NW-tape substrate performed admirably in detecting 4-MBA and demonstrated an enhancement factor of 1.16 × 10⁶. Moreover, for the in situ detection of tetramethylthiuram disulfide, thiabendazole, and their mixture, the relatively high recovery rates of over 88% were favorably realized by the Ag NW-tape substrate with superior sensitivity, distinct flexibility, and adhesiveness. This fascinating SERS substrate, dependent on the flexible and adhesive Ag NW-tape, is promising for application in SERS analysis of trace residues on various practical surfaces.

Label-Free Detection of DNA via Surface-Enhanced Raman Spectroscopy Using Au@Ag Nanoparticles

DNA is a building block of life; surface-enhanced Raman spectroscopy (SERS) has been broadly applied in the detection of biomolecules but there are challenges in obtaining high-quality DNA SERS signals under non-destructive conditions. Here, we developed a novel label-free approach for DNA detection based on SERS, in which the Au@AgNPs core-shell structure was selected as the enhancement substrate, which not only solved the problem of the weak enhancement effect of gold nanoparticles but also overcame the disadvantage of the inhomogeneous shapes of silver nanoparticles, thereby improving the sensitivity and reproducibility of the SERS signals of DNA molecules. The method obtained SERS signals for four DNA bases (A, C, G, and T) without destroying the structure, then further detected and qualified different specific structures of DNA molecules. These results promote the application of SERS technology in the field of biomolecular detection.

Investigation of a Highly Sensitive Surface-Enhanced Raman Scattering Substrate Formed by a Three-Dimensional/Two-Dimensional Graphene/Germanium Heterostructure

Three-dimensional graphene (3D-graphene) is used in surface-enhanced Raman spectroscopy (SERS) because of its plasmonic nanoresonator structure and good ability to interact with light. However, a thin (3-5 nm) layer of amorphous carbon (AC) inevitably appears as a template layer between the 3D-graphene and object substrate when the 3D-graphene layer is synthesized, weakening the enhancement factor. Herein, two-dimensional graphene (2D-graphene) is employed as a template layer to directly synthesize 3D-graphene on a germanium (Ge) substrate via plasma-assisted chemical vapor deposition, bypassing the formation of an AC layer. The interaction and photoinduced charge transfer ability of the 3D-graphene/Ge heterojunction with incident light are improved. Moreover, the high density of electronic states close to the Fermi level of the heterojunction induces the adsorbed probe molecules to efficiently couple to the 3D-graphene-based SERS substrate. Our experimental results imply that the lowest concentrations of rhodamine 6G and rhodamine B that can be detected on the 3D/2D-graphene/Ge SERS substrate correspond to 10^-10 M; for methylene blue, it is 10^-8 M. The detection limits of the 3D/2D-graphene/Ge SERS substrate with respect to 3-hydroxytyramine hydrochloride and melamine (in milk) are both less than 1 ppm. This work may provide a viable and convenient alternative method for preparing 3D-graphene SERS substrates. It also constitutes a new approach to developing SERS substrates with remarkable performance levels.

Flexible Plasmonic Biosensors for Healthcare Monitoring: Progress and Prospects

The noble metal nanoparticle has been widely utilized as a plasmonic unit to enhance biosensors, by leveraging its electric and/or optical properties. Integrated with the "flexible" feature, it further enables opportunities in developing healthcare products in a conformal and adaptive fashion, such as wrist pulse tracers, body temperature trackers, blood glucose monitors, etc. In this work, we present a holistic review of the recent advance of flexible plasmonic biosensors for the healthcare sector. The technical spectrum broadly covers the design and selection of a flexible substrate, the process to integrate flexible and plasmonic units, the exploration of different types of flexible plasmonic biosensors to monitor human temperature, blood glucose, ions, gas, and motion indicators, as well as their applications for surface-enhanced Raman scattering (SERS) and colorimetric detections. Their fundamental working principles and structural innovations are scoped and summarized. The challenges and prospects are articulated regarding the critical importance for continued progress of flexible plasmonic biosensors to improve living quality.

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.

Silk fibroin fibers decorated with urchin-like Au/Ag nanoalloys: a flexible hygroscopic SERS sensor for monitoring of folic acid in human sweat

Surface-enhanced Raman scattering (SERS) spectroscopy has become a powerful and sensitive analytical tool for the detection and assessment of chemical/biological molecules in special scenarios. Herein we propose a flexible hygroscopic SERS biocompatible sensor based on the silk fibroin fibers (SFF) decorated with urchin-like Au/Ag nanoalloys (NAs). The hybrid SFF-Au/Ag NAs with a stronger absorbance capacity (500∼1100 nm) and excellent hygroscopicity provide a remarkable higher near-infrared (NIR)-SERS activity than that of bare urchin-like Au/Ag NAs. The interesting NIR-SERS sensor enables the limit of detection (LOD) of folic acid (FA) to be achieved at nanomolar (nM, 10^-9 M) level, facilitating the ultrasensitive monitoring of FA in human sweat and offering reliable real-time personal health management in the near future.

Enhancing SERS detection on a biocompatible metallic substrate for diabetes diagnosing

A method to realize surface-enhanced Raman spectroscopy (SERS) at a titanium alloy substrate for glucose detection has been experimentally demonstrated. A silver-coated laser-induced periodic surface structure (LIPSS) was prepared via femtosecond laser micro-processing. The low detection limit of glucose is 10^-7mol/L and, a good linear relationship between the glucose concentration and Raman intensity is found in the range between 1×10^-7 and 1×10^-3mol/L. Moreover, we investigate SERS detection for glucose sensing in human urine samples, while the results are in good agreement with clinical results. The Letter provides a facile method for producing a structure-controlled SERS substrate to realize glucose detection, which is promising for long-term in vivo diagnostics.

One-step fabrication of metal nanoparticles on polymer film by femtosecond LIPAA method for SERS detection

Flexible transparent SERS substrates have aroused great interest as a rapid and in situ detection method for trace chemicals. We demonstrate a one-step and environmentally friendly method to fabricate flexible fluorinated ethylene propylene (FEP) surface plasmon resonance film by femtosecond laser induced plasma assisted ablation (LIPAA) for in situ SERS detection. By tuning laser fluence, the distributions and sizes of silver and gold nanoparticles generated by femtosecond LIPAA are studied. Using a Rhodamine 6G (R6G) Raman probe with a 532 nm laser excitation, the proposed Ag NPs/FEP and Au NPs/FEP substrates show enhancement factors of 5.6 × 10⁷ and 2.4 × 10⁶, respectively, as compared to a bare FEP film without the metallic nanoparticles. The Raman signals show good uniformity and a linear relationship with the concentration of R6G solution. In addition, the detection limit of thiram on an apple for in situ measurement is 0.1 mg/Kg, corresponding to 7.96 ng/cm². The proposed SERS detection approach has great potential to pave a new way in food safety applications, such as detecting pesticides in harvested fruits.

Femtosecond laser patterned superhydrophobic/hydrophobic SERS sensors for rapid positioning ultratrace detection

Ultratrace molecular detections are vital for precancer diagnosis, forensic analysis, and food safety. Superhydrophobic (SH) surface-enhanced Raman scattering (SERS) sensors are regarded as an ideal approach to improve detection performance by concentrating analyte molecules within a small volume. However, due to the low adhesion of SH surfaces, the analyte droplet is prone to rolling, making it hard to deposit molecules on a predetermined position. Furthermore, the sediment with a very small area on the SH-SERS surface is difficult to be captured even with a Raman microscope. In this study, femtosecond laser fabricated hybrid SH/hydrophobic (SH/HB) surfaces are successfully applied to realize a rapid and highly sensitive SERS detection. By modulating dual surface structures and wetting behaviors, the analyte molecules can be enriched at the edge of HB pattern. This improves the convenience and speed of Raman test. On a hybrid SH/HB SERS substrate with a circular HB pattern at 300-µm-diameter, a femtomolar level (10^-14 M) of rhodamine 6G can be detected by using analyte volumes of just 5 µL. The SERS enhancement factor can reach 5.7×10⁸ and a good uniformity with a relative standard deviation of 6.98% is achieved. Our results indicate that the laser fabrication of hybrid SERS sensor offers an efficient and cost-effective approach for ultratrace molecular detection.

Tunable rainbow light trapping in ultrathin resonator arrays

Rainbow light trapping in plasmonic devices allows for field enhancement of multiple wavelengths within a single device. However, many of these devices lack precise control over spatial and spectral enhancement profiles and cannot provide extremely high localised field strengths. Here we present a versatile, analytical design paradigm for rainbow trapping in nanogroove arrays by utilising both the groove-width and groove-length as tuning parameters. We couple this design technique with fabrication through multilayer thin-film deposition and focused ion beam milling, which enables the realisation of unprecedented feature sizes down to 5 nm and corresponding extreme normalised local field enhancements up to 10³. We demonstrate rainbow trapping within the devices through hyperspectral microscopy and show agreement between the experimental results and simulation. The combination of expeditious design and precise fabrication underpins the implementation of these nanogroove arrays for manifold applications in sensing and nanoscale optics.

Bioinspired Disordered Flexible Metasurfaces for Human Tear Analysis Using Broadband Surface-Enhanced Raman Scattering

Flexible surface-enhanced Raman scattering (SERS) has received attention as a means to move SERS-based broadband biosensing from bench to bedside. However, traditional flexible periodic nano-arrangements with sharp plasmonic resonances or their random counterparts with spatially varying uncontrollable enhancements are not reliable for practical broadband biosensing. Here, we report bioinspired quasi-(dis)ordered nanostructures presenting a broadband yet tunable application-specific SERS enhancement profile. Using simple, scalable biomimetic fabrication, we create a flexible metasurface (flex-MS) of quasi-(dis)ordered metal-insulator-metal (MIM) nanostructures with spectrally variable, yet spatially controlled electromagnetic hotspots. The MIM is designed to simultaneously localize the electromagnetic signal and block background Raman signals from the underlying polymeric substrate-an inherent problem of flexible SERS. We elucidate the effect of quasi-(dis)ordering on broadband tunable SERS enhancement and employ the flex-MS in a practical broadband SERS demonstration to detect human tear uric acid within its physiological concentration range (25-150 μM). The performance of the flex-MS toward noninvasively detecting whole human tear uric acid levels ex vivo is in good agreement with a commercial enzyme-based assay.

The simultaneous detection of the squamous cell carcinoma antigen and cancer antigen 125 in the cervical cancer serum using nano-Ag polydopamine nanospheres in an SERS-based lateral flow immunoassay

The accurate analysis of tumor related biomarkers is extremely critical in the diagnosis of the early stage cervical cancer. Herein, we designed a novel and inexpensive surface-enhanced Raman scattering-based lateral flow assay (SERS-based LFA) strip with a single test line, which was applied for the rapid and sensitive quantitative simultaneous analysis of SCCA and CA125 in serum samples from patients with cervical cancer. In the presence of target antigens, the monoclonal antibody-coupled and Raman reporter-labeled nano-Ag polydopamine nanospheres (PDA@Ag-NPs) aggregated on the test line modified by the polyclonal antibody to form a double-antibody sandwich structure. The finite difference time domain simulation demonstrated that large number of “hot spots” was generated among the nanogaps of aggregated PDA@AgNPs, which resulted in a huge enhancement of the signal of the Raman reporters. Accordingly, the limit of detection was determined to be 7.156 pg mL⁻¹ for SCCA and 7.182 pg mL⁻¹ for CA125 in phosphate buffer and 8.093 pg mL⁻¹ for SCCA and 7.370 pg mL⁻¹ for CA125 in human serum, revealing high sensitivity of this SERS-based LFA strip. Significantly, the detection of SCCA and CA125 using the SERS-based LFA was observed to have high specificity and reproducibility, and the whole detection was completed within 20 min. Furthermore, the SERS-based LFA and enzyme-linked immunosorbent assay were also employed in serum samples obtained from patients with cervical cancer, cervical intraepithelial neoplasia and healthy subjects, and perfect agreement existed between both the methods. Thus, clinically, the developed SERS-based LFA strip has strong potential for the simultaneous detection of multiple cancer biomarkers in serum.

Based on SERS-based lateral flow immunoassay, nano-Ag polydopamine nanospheres was used for detecting squamous cell carcinoma antigen and cancer antigen 125 simultaneously in cervical cancer serum.

Enhanced second harmonic emission with simultaneous polarization state tuning by aluminum metal-insulator-metal cross nanostructures

Aluminum (Al) plasmonic nanostructures have recently demonstrated remarkable optical nonlinear phenomena, such as enhanced second harmonic (SH) generation. However, the relatively weak field enhancement resulted from large optical losses associated with aluminum nanostructures in combination with the difficulties in controlling the emission polarization pose as a challenge for SH enhancement and tuning. In this paper, we show that the SH emission of aluminum nanostructures can be efficiently enhanced with the polarization properties simultaneously tunable by using metal-insulator-metal (MIM) nanostructures, constituting of Al cross nanoantenna arrays on top of Al mirrors with a SiO₂ spacing layer. Specifically, femtosecond laser beam with a linear polarization parallel to one arm illuminates on the structure while the orthogonal arms were physically modified by the laser-induced photothermal reshaping technique to control the SH radiation by the plasmonic resonances. Under the resonance at the SH wavelength, we observed one order of magnitude larger emission enhancement compared to that at the off-resonant condition. Interestingly, the polarization states can be well manipulated simultaneously by controlling the resonances of the orthogonal arms. The enhanced SH conversion and tunable polarization states pave the way for the development of nonlinear optical sources and advanced functional metasurfaces.

Effective plasmon coupling in conical cavities for sensitive surface enhanced Raman scattering with quantitative analysis ability

Conical silver nanocavity arrays are fabricated by directly depositing Ag on porous alumina templates with V-shaped nanopores. By controlling the thickness of deposited Ag, complete and cracked cavity arrays are constructed respectively. The cracked cavity arrays with the cavity wall consisting of Ag nanoparticles are demonstrated to exhibit higher surface enhanced Raman scattering (SERS) activity than the complete one. Numerical simulation reveals that an effective coupling of the cavity modes with the surface plasmons of Ag nanoparticles (NPs) generates a significantly enhanced local electric field on the cavity wall responsible for the high SERS activity. The optimized cavity array presents an enhancement factor (EF) of ∼7.4 × 106 and an excellent uniformity with a relative standard deviation (RSD) as small as ∼5% for rhodamine 6G (R6G) molecules. Moreover, a good linear correlation between the logarithmic Raman intensity and the molecular concentration endows the array with quantitative analysis ability. These cavity arrays therefore are of great potential for qualitative and quantitative chemical and biomedical analysis with high sensitivity and reproducibility.

Scratch on Polymer Materials Using AFM Tip-Based Approach: A Review

As a brand new nanomachining method, the tip-based nanomachining/nanoscratching (TBN) method has exhibited a powerful ability at machining on polymer materials and various structures have been achieved using this approach, ranging from the nanodot, nanogroove/channel, bundle to 2D/3D (three-dimensional) nanostructures. The TBN method is widely used due to its high precision, ease of use and low environmental requirements. First, the theoretical models of machining on polymer materials with a given tip using the TBN method are presented. Second, advances of nanostructures achieved by this method are given, including nanodots/nanodot arrays, a nanogroove/channel, 2D/3D nanostructures and bundles. In particular, a useful approach called the ultrasonic vibration-assisted method introduced to integrate with TBN method to reduce the wear of the tip is also reviewed, respectively. Third, the typical applications of the TBN method and the nanostructures achieved by it are summarized in detail. Finally, the existing shortcomings and future prospects of the TBN method are given. It is confirmed that this review will be helpful in learning about this method and push the technology toward industrialization.

Flexible PET/ITO/Ag SERS Platform for Label-Free Detection of Pesticides

We show a new type of elastic surface-enhanced Raman spectroscopy (SERS) platform made of poly(ethylene terephthalate) (PET) covered with a layer of indium tin oxide (ITO). This composite is subjected to dielectric barrier discharge (DBD) that develops the active surface of the PET/ITO foil. To enhance the Raman signal, a modified composite was covered with a thin layer of silver using the physical vapor deposition (PVD) technique. The SERS platform was used for measurements of para-mercaptobenzoic acid (p-MBA) and popular pesticides, i.e., Thiram and Carbaryl. The detection and identification of pesticides on the surface of fruits and vegetables is a crucial issue due to extensive use of those chemical substances for plant fungicide and insecticide protection. Therefore, the developed PET/ITO/Ag SERS platform was dedicated to quantitative analysis of selected pesticides, i.e., Thiram and Carbaryl from fruits. The presented SERS platform exhibits excellent enhancement and reproducibility of the Raman signal, which enables the trace analysis of these pesticides in the range up to their maximum residues limit. Based on the constructed calibration curves, the pesticide concentrations from the skin of apples was estimated as 2.5 µg/mL and 0.012 µg/mL for Thiram and Carbaryl, respectively. Additionally, the PET/ITO/Ag SERS platform satisfies other spectroscopic properties required for trace pesticide analysis e.g., ease, cost-effective method of preparation, and specially designed physical properties, especially flexibility and transparency, that broaden the sampling versatility to irregular surfaces.

Plasmonic-based platforms for diagnosis of infectious diseases at the point-of-care

Infectious diseases such as HIV-1/AIDS, tuberculosis (TB), hepatitis B (HBV), and malaria still exert a tremendous health burden on the developing world, requiring rapid, simple and inexpensive diagnostics for on-site diagnosis and treatment monitoring. However, traditional diagnostic methods such as nucleic acid tests (NATs) and enzyme linked immunosorbent assays (ELISA) cannot be readily implemented in point-of-care (POC) settings. Recently, plasmonic-based biosensors have emerged, offering an attractive solution to manage infectious diseases in the developing world since they can achieve rapid, real-time and label-free detection of various pathogenic biomarkers. Via the principle of plasmonic-based optical detection, a variety of biosensing technologies such as surface plasmon resonance (SPR), localized surface plasmon resonance (LSPR), colorimetric plasmonic assays, and surface enhanced Raman spectroscopy (SERS) have emerged for early diagnosis of HIV-1, TB, HBV and malaria. Similarly, plasmonic-based colorimetric assays have also been developed with the capability of multiplexing and cellphone integration, which is well suited for POC testing in the developing world. Herein, we present a comprehensive review on recent advances in surface chemistry, substrate fabrication, and microfluidic integration for the development of plasmonic-based biosensors, aiming at rapid management of infectious diseases at the POC, and thus improving global health.

Fabrication of highly sensitive and reproducible 3D surface-enhanced Raman spectroscopy substrates through in situ cleaning and layer-by-layer assembly of Au@Ag nanocube monolayer film

A highly sensitive and uniform three-dimensional (3D) surface-enhanced Raman spectroscopy (SERS) substrate has been fabricated by in situ ultraviolet ozone cleaning and layer-by-layer self-assembly. The SERS properties and the structural changes of the substrates were systematically studied by adjusting the cleaning time upon the in situ and post cleaning strategy. Under the optimal cleaning condition, the cleaning technology could give rise to clean and optimal surfaces for SERS analysis, thus obtaining efficient plasmonic films populated with a large number of homogeneous 'hot-spots'. Then with the optimal monolayer film, the SERS performance derived from plasmon coupling in multilayers of the Au@Ag nanocubes substrates was explored. It demonstrated that the plasmon coupling between layers (out-of-plane) contributed much to the SERS intensity, leading a more superior SERS enhancement from the 3D SERS substrates than that from the conventional two-dimensional SERS substrates. Also the in situ cleaning 3D SERS substrates displayed a nice uniformity and excellent time stability. With this method, the optimized substrates were further used to detect prohibited pigments in drink with an excellent linear relationship between characteristic peak intensity and analytes concentration over wide concentration ranges. Our experimental results clearly show that the in situ cleaning 3D SERS substrates provide an ideal candidate for SERS applications in food safety.

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.

Large-Scale Fabrication of Nanostructure on Bio-Metallic Substrate for Surface Enhanced Raman and Fluorescence Scattering

The integration of surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) has attracted increasing interest and is highly probable to improve the sensitivity and reproducibility of spectroscopic investigations in biomedical fields. In this work, dual-mode SERS and SEF hierarchical structures have been developed on a single bio-metallic substrate. The hierarchical structure was composed of micro-grooves, nano-particles, and nano-ripples. The crystal violet was selected as reporter molecule and both the intensity of Raman and fluorescence signals were enhanced because of the dual-mode SERS-SEF phenomena with enhancement factors (EFs) of 7.85 × 105 and 14.32, respectively. The Raman and fluorescence signals also exhibited good uniformity with the relative standard deviation value of 2.46% and 5.15%, respectively. Moreover, the substrate exhibited high sensitivity with the limits of detection (LOD) as low as 1 × 10-11 mol/L using Raman spectroscopy and 1 × 10-10 mol/L by fluorescence spectroscopy. The combined effect of surface plasmon resonance and "hot spots" induced by the hierarchical laser induced periodical surface structures (LIPSS) was mainly contributed to the enhancement of Raman and fluorescence signal. We propose that the integration of SERS and SEF in a single bio-metallic substrate is promising to improve the sensitivity and reproducibility of detection in biomedical investigations.

A porous Au-Ag hybrid nanoparticle array with broadband absorption and high-density hotspots for stable SERS analysis

Constructing high-density hotspots is of crucial importance in surface enhanced Raman scattering (SERS). In this paper, we present a large-area and broadband porous Au-Ag hybrid nanoparticle array which was fabricated by an ultra-thin alumina mask (UTAM) technique incorporated with annealing and galvanic replacement techniques. Experimental results and numerical simulations demonstrated that the porous Au-Ag hybrid nanoparticle array possessed enormous hotspots for high sensitivity, uniformity, and stability in SERS analysis. A large Raman enhancement factor of 2.2 × 107 was achieved with a relative standard deviation (RSD) of 7.7%, leading to excellent reliability for Raman detection. Furthermore, this novel substrate exhibited a long shelf time in an ambient environment and promising practical applications in many SERS-based quantitative analytical and biomedical sensing techniques.

Preparation of a Novel SERS Platform Based on Mantis Wing with High-Density and Multi-Level "Hot Spots"

The recent development of SERS substrates based on irregular nanostructures for directly molecule recognition has aroused increasing attention. By combining the irregular flake-like nanostructures of mantis wings, high SERS performance of Ag nanofilms, and the chemical stability of Au nanoparticles (NPs), an ultra-sensitive and flexible SERS substrate based on Au NPs functionalized Ag nanofilms-mantis wings (Au-Ag-M.w.) hybrid system is successfully fabricated. When 4-aminothiophenol is selected as the probe molecule, the limit of detection (LOD) is as low as 10-13 M and the relative standard deviation (RSD) is lower than 7.15%. This novel SERS platform exhibits high SERS performance in terms of sensitivity, reproducibility and practicability mainly because there are high-density and multi-level "hot spots" in the appropriate nanogaps. Meanwhile, it also systematically compares the differences of the SERS performance of Cu and Ag decorated M.w. hybrids and how these differences can alter their response. Moreover, the proposed substrate is employed to rapidly detect the pesticide residues on apple peels and the LOD for cypermethrin is estimated at 10-10 mg/mL. Therefore, this novel SERS substrate has great potential in rapid sampling of pesticide residues on real samples and expands the investigation to other natural materials for fabricating various SERS platforms.

Extraordinary optical fields in nanostructures: from sub-diffraction-limited optics to sensing and energy conversion

Along with the rapid development of micro/nanofabrication technology, the past few decades have seen the flourishing emergence of subwavelength-structured materials and interfaces for optical field engineering at the nanoscale. Three remarkable properties associated with these subwavelength-structured materials are the squeezed optical fields beyond the diffraction limit, gradient optical fields in the subwavelength scale, and enhanced optical fields that are orders of magnitude greater than the incident field. These engineered optical fields have inspired fundamental and practical advances in both engineering optics and modern chemistry. The first property is the basis of sub-diffraction-limited imaging, lithography, and dense data storage. The second property has led to the emergence of a couple of thin and planar functional optical devices with a reduced footprint. The third one causes enhanced radiation (e.g., fluorescence), scattering (e.g., Raman scattering), and absorption (e.g., infrared absorption and circular dichroism), offering a unique platform for single-molecule-level biochemical sensing, and high-efficiency chemical reaction and energy conversion. In this review, we summarize recent advances in subwavelength-structured materials that bear extraordinary squeezed, gradient, and enhanced optical fields, with a particular emphasis on their optical and chemical applications. Finally, challenges and outlooks in this promising field are discussed.

Fabrication of a Flexible Gold Nanorod Polymer Metafilm via a Phase Transfer Method as a SERS Substrate for Detecting Food Contaminants

Surface enhanced Raman scattering (SERS) has been widely used in detection of food safety due to the nondestructive examination property. Here, we reported a flexible SERS film based on a polymer-immobilized gold nanorod polymer metafilm. Polystyrene-polyisoprene-polystyrene (SIS), a transparent and flexible, along with having excellent elasticity, polymer, was chosen as the main support of gold nanorods. A simple phase transfer progress was adopted to mix the gold nanorods with the polymer, which can further be used in most water-insoluble polymers. The SERS film performed satisfactorily while being tested in a series of standard Raman probes, like crystal violet (CV) and malachite green (MG). Moreover, the excellent reproducibility and elastic properties make the film a promising substrate in practical detection. Hence, the MG detection on the fish surface and trace thiram detection on orange pericarp were inspected with detection results of 1 × 10^-10 and 1 × 10^-6 M, which were below the demand of the National standard of China, exactly matching the realistic application requirements.

Inverse Stellation of CuAu-ZnO Multimetallic-Semiconductor Nanostartube for Plasmon-Enhanced Photocatalysis

One-dimensional (1D) metallic nanocrystals constitute an important class of plasmonic materials for localization of light into subwavelength dimensions. Coupled with their intrinsic conductive properties and extended optical paths for light absorption, metallic nanowires are prevalent in light-harnessing applications. However, the transverse surface plasmon resonance (SPR) mode of traditional multiply twinned nanowires often suffers from weaker electric field enhancement due to its low degree of morphological curvature in comparison to other complex anisotropic nanocrystals. Herein, simultaneous anisotropic stellation and excavation of multiply twinned nanowires are demonstrated through a site-selective galvanic reaction for a pronounced manipulation of light-matter interaction. The introduction of longitudinal extrusions and cavitation along the nanowires leads to a significant enhancement in plasmon field with reduced quenching of localized surface plasmon resonance (LSPR). The as-synthesized multimetallic nanostartubes serve as a panchromatic plasmonic framework for incorporation of photocatalytic materials for plasmon-assisted solar fuel production.

Cathodoluminescence as a probe of the optical properties of resonant apertures in a metallic film

Here we present the results of an investigation of resonances of azimuthal trimer arrangements of rectangular slots in a gold film on a glass substrate using cathodoluminescence (CL) as a probe. The variation in the CL signal collected from specific locations on the sample as a function of wavelength and the spatial dependence of emission into different wavelength bands provides considerable insight into the resonant modes, particularly sub-radiant modes, of these apertures. By comparing our experimental results with electromagnetic simulations we are able to identify a Fabry-Pérot mode of these cavities as well as resonances associated with the excitation of surface plasmon polaritons on the air-gold boundary. We obtain evidence for the excitation of dark (also known as sub-radiant) modes of apertures and aperture ensembles.

Assembled growth of 3D Fe3O4@Au nanoparticles for efficient photothermal ablation and SERS detection of microorganisms

A leukocyte-like Fe₃O₄@Au-polydopamine nanohybrid performed a magnetism-based enhanced SERS signal and efficient NIR-irradiated photothermal antibacterial agent.

Transparent and Flexible Surface-Enhanced Raman Scattering (SERS) Sensors Based on Gold Nanostar Arrays Embedded in Silicon Rubber Film

Integration of surface-enhanced Raman scattering (SERS) sensors onto transparent and flexible substrates enables lightweight and deformable SERS sensors which can be wrapped or swabbed on various nonplanar surfaces for the efficient collection and detection of analytes on various surfaces. However, the development of transparent and flexible SERS substrates with high sensitivity is still challenging. Here, we demonstrate a transparent and flexible SERS substrate with high sensitivity based on a polydimethylsiloxane (PDMS) film embedded with gold nanostar (GNS) assemblies. The flexible SERS substrates enable conformal coverage on arbitrary surfaces, and the optical transparency allows light interaction with the underlying contact surface, thereby providing highly sensitive detection of analytes adsorbed on arbitrary metallic and dielectric surfaces which otherwise do not provide any noticeable Raman signals of analytes. In particular, when the flexible SERS substrates are covered onto metallic surfaces, the SERS enhancement is greatly improved because of the additional plasmon couplings between GNS and metal film. We achieve the detection capability of a trace amount of benzenethiol (10^-8 M) and enormous SERS enhancement factor (∼1.9 × 10⁸) for flexible SERS substrates on Ag film. In addition, because of the embedded structure of GNS monolayers within the PDMS film, SERS sensors maintain the high sensitivity even after mechanical deformations of stretching, bending, and torsion for 100 cycles. The transparent and flexible SERS substrates introduced in this study are applicable to various SERS sensing applications on nonplanar surfaces, which are not achievable for hard SERS substrates.