Gold-nanoparticle-embedded polydimethylsiloxane elastomers for highly sensitive Raman detection

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.

Strong synergy between gold nanoparticles and cobalt porphyrin induces highly efficient photocatalytic hydrogen evolution

The reaction efficiency of reactants near plasmonic nanostructures can be enhanced significantly because of plasmonic effects. Herein, we propose that the catalytic activity of molecular catalysts near plasmonic nanostructures may also be enhanced dramatically. Based on this proposal, we develop a highly efficient and stable photocatalytic system for the hydrogen evolution reaction (HER) by compositing a molecular catalyst of cobalt porphyrin together with plasmonic gold nanoparticles, around which plasmonic effects of localized electromagnetic field, local heating, and enhanced hot carrier excitation exist. After optimization, the HER rate and turn-over frequency (TOF) reach 3.21 mol g-1 h-1 and 4650 h-1, respectively. In addition, the catalytic system remains stable after 45-hour catalytic cycles, and the system is catalytically stable after being illuminated for two weeks. The enhanced reaction efficiency is attributed to the excitation of localized surface plasmon resonance, particularly plasmon-generated hot carriers. These findings may pave a new and convenient way for developing plasmon-based photocatalysts with high efficiency and stability.

High-Performance Organic Electrochemical Transistor Based on Photo-annealed Plasmonic Gold Nanoparticle-Doped PEDOT:PSS

Organic electrochemical transistors (OECTs), particularly the ones based on PEDOT:PSS, are excellent candidates for chemical and biological sensing because of their unique advantages. Improving the sensitivity and stability of OECTs is crucially important for practical applications. Herein, the transconductance of OECT is improved by 8-fold to 14.9 mS by doping the PEDOT:PSS channel with plasmonic gold nanoparticles (AuNPs) using a solution-based process followed by photo annealing. In addition, the OECT also possesses high flexibility and cyclic stability. It is revealed that the doping of AuNPs increases the conductivity of PEDOT:PSS and the photo annealing improves the crystallinity of the PEDOT:PSS channel and the interaction between AuNPs and PEDOT:PSS. These changes lead to the increase in transconductance and cyclic stability. The prepared OECTs are also demonstrated to be effective in sensitive detection of glucose within a wide concentration range of 10 nM-1 mM. Our OECTs based on photo-annealed plasmonic AuNP-doped PEDOT:PSS may find great applications in chemical and biological sensing, and this strategy may be extended to prepare many other high-performance OECT-based devices.

Plasmonic/magnetic nanoarchitectures: From controllable design to biosensing and bioelectronic interfaces

Controllable design of the nanocrystal-assembled plasmonic/magnetic nanoarchitectures (P/MNAs) inspires abundant methodologies to enhance light-matter interactions and control magnetic-induced effects by means of fine-tuning the morphology and ordered packing of noble metallic or magnetic building blocks. The burgeoning development of multifunctional nanoarchitectures has opened up broad range of interdisciplinary applications including biosensing, in vitro diagnostic devices, point-of-care (POC) platforms, and soft bioelectronics. By taking advantage of their customizability and efficient conjugation with capping biomolecules, various nanoarchitectures have been integrated into high-performance biosensors with remarkable sensitivity and versatility, enabling key features that combined multiplexed detection, ease-of-use and miniaturization. In this review, we provide an overview of the representative developments of nanoarchitectures that being built by plasmonic and magnetic nanoparticles over recent decades. The design principles and key mechanisms for signal amplification and quantitative sensitivity have been explored. We highlight the structure-function programmability and prospects of addressing the main limitations for conventional biosensing strategies in terms of accurate selectivity, sensitivity, throughput, and optoelectronic integration. State-of-the-art strategies to achieve affordable and field-deployable POC devices for early multiplexed detection of infectious diseases such as COVID-19 has been covered in this review. Finally, we discuss the urgent yet challenging issues in nanoarchitectures design and related biosensing application, such as large-scale fabrication and integration with portable devices, and provide perspectives and suggestions on developing smart biosensors that connecting the materials science and biomedical engineering for personal health monitoring.

In Situ Surface-Enhanced Raman Scattering Detection of a SARS-CoV-2 Biomarker Using Flexible and Transparent Polydimethylsiloxane Films with Embedded Au Nanoplates

Coronavirus disease 2019 (COVID-19) remains an ongoing issue worldwide and continues to disrupt daily life. Transmission of infection primarily occurs through secretions when in contact with infected individuals, but more recent evidence has shown that fomites are also a source of virus transmission, especially in cold-chain logistics. Traditional nucleic acid testing for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) contamination in cold-chain logistics is time-consuming and inaccurate because of the multiplex sampling sites. Surface-enhanced Raman spectroscopy (SERS) provides a rapid, sensitive, and label-free detection route for various molecules, including viruses, through the identification of the characteristic peaks of their outer membrane proteins. In this study, we embedded arbitrarily orientated gold nanoplates (Au NPLs) in polydimethylsiloxane (PDMS) elastomer and used it as biosensor for the ultrasensitive detection of the SARS-CoV-2 spike protein in cold-chain logistics. This transparent and flexible substrate can be wrapped onto arbitrary surfaces and permits light penetration into the underlying contact surface, enabling in situ and point-of-care SERS diagnostics. The developed assay displayed high reproducibility (8.7%) and a low detection limit of 6.8 × 10-9 g mL-1, indicating its potential to serve as a promising approach with increased accuracy and sensitivity for the detection of the S protein.

Molecular Cocatalyst of p-Mercaptophenylboronic Acid Boosts the Plasmon-Mediated Reduction of p-Nitrothiophenol

Localized surface plasmon resonance (LSPR) has been demonstrated to be highly effective in the initialization or acceleration of chemical reactions because of its unique optical properties. However, because of the ultrashort lifetime (fs to ps) of plasmon-generated hot carriers, the potential of LSPR in photochemical reactions has not been fully exploited. Herein, we demonstrate an acceleration of the plasmon-mediated reduction of p-nitrothiophenol (PNTP) molecules on the surface of silver nanoparticles (AgNPs) with in situ Raman spectroscopy. p-Mercaptophenylboronic acid (PMPBA) molecules coadsorbed on AgNP surfaces act as a molecular cocatalyst in the plasmon-mediated reaction, resulting in a boosting of the PNTP reduction. This boosting is attributed to the improved transfer and separation of the plasmon-generated hot carriers at the interface of the AgNPs and coadsorbed PMPBA molecules. Our finding provides a highly simple, cost-effective, and highly effective strategy to promote plasmonic photochemistry by introducing a molecular cocatalyst, and this strategy can be extended to promote various plasmon-mediated reactions.

Stimuli-responsive microgels with fluorescent and SERS activities for water and temperature sensing

Design and application of stimulus-responsive microgels is still in its infancy but is an exhilarating topic in controllable sensing device. Here, we have fabricated a dual-responsive platform capable of both sensitive on-spot fluorescence analysis and reliable surface-enhanced Raman scattering (SERS) quantification of water and temperature by in-situ encapsulating 4,4'-dimercaptoazobenzene (DMAB), meso-formyl-1,3,5,7-tetramethyl pyrromethene fluoroborate (FPF) probe and Ag nanoparticles (AgNPs) into polyvinyl alcohol (PVA) microgels. The smart microgels exhibit ultra-sensitive (detection limit 10^-4% v/v) and reversible response towards water due to the liner relationship between network volume and SERS performance of the microgels. Furthermore, the presence of water triggers the conversion of FPF to aldehyde hydrate, facilitating visual assay of trace water in matrix samples through the enhanced fluorescence signals. Interestingly, the SERS signals can be precisely tuned by the thermo-sensitive microgels substrate, thus achieving the temperature monitoring from 32 to 50 °C. The microgels-based sensor has fast-response (2 min), excellent stability, and enables accurate and reliable response of water in organic solvent and pharmaceutical products. As a smart and flexible sensor, the hybrid microgels will facilitate the field of POC analysis, as well as molecular recognition in the future.

Electrospun polymeric nanofiber decorated with sea urchin-like gold nanoparticles as an efficient and stable SERS platform

Surface enhanced Raman scattering (SERS)-based nanoprobes have been used as well-established analytical tools enabling single-molecule detection. In this work, we report a facile method to decorate sea urchin-like gold nanoparticles (SUGNPs) on the surface of PMMA/P4VP nanofibers. Firstly, PMMA/P4VP nanofibers within the submicrometer size range were prepared by applying the electrospinning technique. Then, the incorporation of SUGNPs on the surface of PMMA/P4VP nanofiber was achieved by immersing PMMA/P4VP nanofiber into freshly prepared SUGNP aqueous solution through the specific Au-N interactions. The as-fabricated SUGNP-coated PMMA/P4VP nanofibers exhibited good sensitivity and reproducibility in SERS measurements with the relative standard deviation down to 6.6%, by employing 4-mercaptobenzoic acid as a probe molecule with 30 min of soaking time. Hence, we envisage that the SUGNP-coated PMMA/P4VP nanofibers can act as efficient and stable SERS substrates for potential applications in molecular detection as well as chemical and biological analysis.

Recent developments on gold nanostructures for surface enhanced Raman spectroscopy: Particle shape, substrates and analytical applications. A review

Surface enhanced Raman spectroscopy (SERS) is a powerful technique for sensitive analysis which is attracting great attention in the last decades. In this review, different gold nanostructures that have been exploited for SERS analysis are described, ranging from gold nanospheres to anisotropic and complex-shaped gold nanostructures, in which the presence of high aspect ratio features leads to an increment of the electromagnetic field at the surface of the nanomaterial, resulting in enhanced SERS response. In addition to the shape of the nanostructure, the interparticle nanogaps play a prominent role in the SERS efficiency. In this sense, different approaches such as nanoaggregation and formation of assemblies and ordered structures lead to the creation of the so-called hot spots. SERS measurements may be performed in solution, while usually the nanostructures are deposited building a SERS substrate, which can be created via attachment of chemically prepared gold nanostructures, as well as via top-down physical methods. Among the classical supports for creating the SERS substrates, in the last years there is a trend towards the development of flexible supports based on polymers as well as paper. Finally, some recent applications of gold nanostructures-based SERS substrates within the analytical field are discussed to spotlight the potential of this technique in real-world analytical scenarios.

Contrastive Study of In Situ Sensing and Swabbing Detection Based on SERS-Active Gold Nanobush-PDMS Hybrid Film

Surface-enhanced Raman scattering (SERS) with fast and intuitive property has been extensively utilized in the field of food safety. Here, we demonstrated a novel noble metal-polymer hybrid film as a SERS substrate for food fungicide analysis. Benefiting from its transparency and flexibility, poly(dimethylsiloxane) (PDMS) film was chosen as a versatile supporting matrix to grow gold nanobushes (Au NBs) through a seed-mediated process. The as-prepared AuNB-PDMS hybrid film performed satisfactorily in testing 4-nitrothiophenol (4NTP) and exhibited an enhancement factor (EF) of 2.56 × 10⁶. Moreover, the high sensitivity and elastic properties make the hybrid film a promising substrate in practical detection. Hence, the in situ sensing of TBZ, carbaryl, and their mixture was finally realized using the developed hybrid film, which exhibited higher sensitivity than that obtained by the swabbing method. This high-performance SERS substrate based on the flexible and transparent AuNB-PDMS hybrid film has great potential applications in the fast in situ monitoring of biochemical molecules.

Enhancing Nonfouling and Sensitivity of Surface-Enhanced Raman Scattering Substrates for Potent Drug Analysis in Blood Plasma via Fabrication of a Flexible Plasmonic Patch

Surface-enhanced Raman scattering (SERS) is an ultrasensitive analytical technique, which is capable of providing high specificity; thus, it can be used for toxicological drug assay (detection and quantification). However, SERS-based drug analysis directly in human biofluids requires mitigation of fouling and nonspecificity effects that commonly appeared from unwanted adsorption of endogenous biomolecules present in biofluids (e.g., blood plasma and serum) onto the SERS substrate. Here, we report a bottom-up fabrication strategy to prepare ultrasensitive SERS substrates, first, by functionalizing chemically synthesized gold triangular nanoprisms (Au TNPs) with poly(ethylene glycol)-thiolate in the solid state to avoid protein fouling and second, by generating flexible plasmonic patches to enhance SERS sensitivity via the formation of high-intensity electromagnetic hot spots. Poly(ethylene glycol)-thiolate-functionalized Au TNPs in the form of flexible plasmonic patches show a twofold-improved signal-to-noise ratio in comparison to triethylamine (TEA)-passivated Au TNPs. Furthermore, the plasmonic patch displays a SERS enhancement factor of 4.5 ×10⁷. Utilizing the Langmuir adsorption model, we determine the adsorption constant of drugs for two different surface ligands and observe that the drug molecules display stronger affinity for poly(ethylene glycol) ligands than TEA. Our density functional theory calculations unequivocally support the interaction between drug molecules and poly(ethylene glycol) moieties. Furthermore, the universality of the plasmonic patch for SERS-based drug detection is demonstrated for cocaine, JWH-018, and opioids (fentanyl, despropionyl fentanyl, and heroin) and binary mixture (trace amount of fentanyl in heroin) analyses. We demonstrate the applicability of flexible plasmonic patches for the selective assay of fentanyl at picogram/milliliter concentration levels from drug-of-abuse patients' blood plasma. The fentanyl concentration calculated in the patients' blood plasma from SERS analysis is in excellent agreement with the values determined using the paper spray ionization mass spectrometry technique. We believe that the flexible plasmonic patch fabrication strategy would be widely applicable to any plasmonic nanostructure for SERS-based chemical sensing for clinical toxicology and therapeutic drug monitoring.

Plasmonic SERS Biosensor Based on Multibranched Gold Nanoparticles Embedded in Polydimethylsiloxane for Quantification of Hematin in Human Erythrocytes

This work reports a plasmonic surface-enhanced Raman scattering (SERS) biosensor that allows for quantitative analysis of hematin in erythrocytes without the need of separating it from hemoglobin (Hb). The biosensor exploits the tunable localized surface plasmon resonance (LSPR) characteristics of multibranched gold nanoparticles (M-AuNPs) and the strong plasmon coupling between an Au thin film and a flexible substrate consisting of M-AuNPs embedded in polydimethylsiloxane (PDMS) (i.e., M-AuNP-embedded PDMS substrate). In the assay, the hematin (or hematin-containing erythrocyte hemolysate) was deposited on Au film surface and covered with M-AuNP-embedded PDMS. Strong SERS signals were generated under excitation at 785 nm; the signals were sensitive to hematin concentration but not to several common coexisting biological substances. The intensities of the SERS signal (at 1623 cm^-1) displayed a wide linear range using hematin concentrations in a range of at least ∼1.5 nM-1.1 μM; the limit of detection (LOD) was ∼0.03 ± 0.01 nM at a signal/noise (S/N) of 3. This assay is simple and sensitive without tedious separation procedures, thereby saving time and enhancing efficiency. This biosensor can be used to determine hematin concentration in human erythrocyte cytosols giving concentrations of ∼18.5 ± 4.5 (by averaging eight samples) and 51.5 ± 6.2 μM (by averaging three samples) for healthy and sickle erythrocytes, respectively, making it a potential application in clinical detection.

Macroscopic two-dimensional monolayer films of gold nanoparticles: fabrication strategies, surface engineering and functional applications

In the last few decades, two-dimensional monolayer films of gold nanoparticles (2D MFGS) have attracted increasing attention in various fields, due to their superior attributes of macroscopic size and accessible fabrication, controllable electromagnetic enhancement, distinctive optical harvesting and electron transport capabilities. This review will focus on the recent progress of 2D monolayer films of gold nanoparticles in construction approaches, surface engineering strategies and functional applications in the optical and electric fields. The research challenges and prospective directions of 2D MFGS are also discussed. This review would promote a better understanding of 2D MFGS and establish a necessary bridge among the multidisciplinary research fields.

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.

SERS based protocol using flow glass-hemostix for detection of neuron-specific enolase in blood plasma

An inexpensive and disposable lateral flow glass-hemostix (FGH) has been developed as an immunoassay, in which surface-enhanced Raman scattering (SERS) is utilized for sensing signal transduction.

A critical review of flexible and porous SERS sensors for analytical chemistry at the point-of-sample

For decades surface enhanced Raman spectroscopy (SERS) has been intensely investigated as a possible solution for performing analytical chemistry at the point of sample origin. Unfortunately, due to cost and usability constraints, conventional rigid SERS sensors and microfluidic SERS sensors have yet to make a significant impact outside of the realm of academics. However, the recently introduced flexible and porous paper-based SERS sensors are proving to be widely adaptable to realistic usage cases in the field. In contrast to rigid and microfluidic SERS sensors, paper SERS sensors feature (i) the potential for roll-to-roll manufacturing methods that enable low sensor cost, (ii) simple sample collection directly onto the sensor via swabbing or dipping, and (iii) equipment-free separations for sample cleanup. In this review we argue that movement to paper-based SERS sensors will finally enable point-of-sample analytical chemistry applications. In addition, we present and compare the numerous fabrication techniques for paper SERS sensors and we describe various sample collection and sample clean-up capabilities of paper SERS sensors, with a focus on how these features enable practical applications in the field. Finally, we present our expectations for the future, including emerging ideas inspired by paper SERS.

A flexible SERS-active film for studying the effect of non-metallic nanostructures on Raman enhancement

Since the discovery of surface enhanced Raman scattering (SERS), the choice of SERS-active materials has been limited mainly to metals, especially gold and silver in the visible spectrum. Although non-metals can also be SERS-active by forming nanostructures or composite structures with SERS-active materials, the mechanism behind it is still unclear and there is no perfect technique to study it. In this work, by constructing a SERS structure on a flexible polydimethylsiloxane film, we provide a way to study the effect of non-metallic nanostructures on Raman enhancement by attaching the above film onto flat and nanostructured surfaces. It was found that a nanoporous silicon surface contributes to an additional, up to five times, Raman enhancement. The pore depth and pore size also influence the observed Raman enhancement. These findings will help us not only to understand the mechanism of SERS involving non-metallic nanostructures, but also to design more efficient SERS structures for various applications.

SERS substrate based on the flexible hybrid of polydimethylsiloxane and silver colloid decorated with silver nanoparticles

Various flexible SERS sensors have attracted widespread concern in performing the direct identification of the analytes adsorbed on arbitrary surfaces. Here, a sample method was proposed to integrate plasmonic nanoparticles into polydimethylsiloxane (PDMS) to fabricate flexible substrate for the decoration of silver nanoparticles (AgNPs). The flexible SERS sensor based on AgNPs/AgNPs-PDMS offers highly sensitive Raman detection with enhancement factor up to 8.3 × 10⁹, which can be attributed to the integrative effects from both the increase of the light absorption of the embedded AgNPs in PDMS substrate and the EM enhancement from the adjacent top-top, bottom-bottom and top-bottom AgNPs. After undergoing the cyclic mechanical deformation, the SERS substrate still maintains high mechanical stability and stable SERS signals. However, upon stretching the flexible substrate, there was an amusing phenomenon that SERS signals can be highly increased, which results from that the reduction of lateral nanogaps between top and bottom of the PDMS boundary strengthens the trigger of the plasmon coupling as demonstrated by the simulated result. This result reveals that the tuning and the coupling of the electromagnetic fields can be effectively controlled by the macroscopic mechanical solicitation. That will have an important significance for practical applications in strain-dependent sensors and detectors.

Flexible, transparent and highly sensitive SERS substrates with cross-nanoporous structures for fast on-site detection

A flexible and transparent film assembled from the cross-nanoporous structures of Au on PET (CNS of Au@PET) is developed as a versatile and effective SERS substrate for rapid, on-site trace analysis with high sensitivity. The fabrication of the CNS of Au can be achieved on a large scale at low cost by employing an etching process with super-aligned carbon nanotubes as a mask, followed by metal deposition. A strongly enhanced Raman signal with good uniformity can be obtained, which is attributed to the excitation of "hot spots" around the metal nanogaps and sharp edges. Using the CNS of Au@PET film as a SERS platform, real-time and on-site SERS detection of the food contaminant crystal violet (CV) is achieved, with a detection limit of CV solution on a tomato skin of 10-7 M. Owing to its ability to efficiently extract trace analytes, the resulting substrate also achieves detection of 4-ATP contaminants and thiram pesticides by swabbing the skin of an apple. A SERS detection signal for 4-ATP has a relative standard deviation of less than 10%, revealing the excellent reproducibility of the substrate. The flexible, transparent and highly sensitive substrates fabricated using this simple and cost-effective strategy are promising for practical application in rapid, on-site SERS-based detection.

Two-Dimensional Metal Nanomaterials: Synthesis, Properties, and Applications

As one unique group of two-dimensional (2D) nanomaterials, 2D metal nanomaterials have drawn increasing attention owing to their intriguing physiochemical properties and broad range of promising applications. In this Review, we briefly introduce the general synthetic strategies applied to 2D metal nanomaterials, followed by describing in detail the various synthetic methods classified in two categories, i.e. bottom-up methods and top-down methods. After introducing the unique physical and chemical properties of 2D metal nanomaterials, the potential applications of 2D metal nanomaterials in catalysis, surface enhanced Raman scattering, sensing, bioimaging, solar cells, and photothermal therapy are discussed in detail. Finally, the challenges and opportunities in this promising research area are proposed.

A silver/graphene oxide nanocomposite film as a flexible SERS substrate for reliable quantitative analysis using high-speed spiral scanning spectrometry

Excellent uniformity (∼1.5% RSD) in SERS signals was obtained from an Ag/GO decorated adhesive tape on a simple in-house cylindrical scanning system. The calibration curve for the quantitative analysis of CV shows reliable linearity ranging from 75 nM to 50 μM. This novel method is promising to be an adept tool for universal quantitative analysis and be used complementarily with the conventional Raman mapping method for a more time efficient and reliable analysis.

Viable stretchable plasmonics based on unidirectional nanoprisms

Well-defined ordered arrays of plasmonic nanostructures were fabricated on stretchable substrates and tunable plasmon-coupling-based sensing properties were comprehensively demonstrated upon extension and contraction. Regular nanoprism patterns consisting of Ag, Au and Ag/Au bilayers were constructed on the stretchable polydimethylsiloxane substrate. The nanoprisms had the same orientation over the entire substrate (3 × 3 cm²) via metal deposition on a single-crystal microparticle monolayer assembly. The plasmonic sensor based on the Ag/Au bilayer showed a 6-fold enhanced surface enhanced Raman scattering signal under 20% uniaxial extension, whereas a 3-fold increase was observed upon 6% contraction, compared with the Au nanoprism arrays. The sensory behaviors were corroborated by finite-difference time-domain simulation, demonstrating the tunable electromagnetic field enhancement effect via the localized surface plasmon resonance coupling. The advanced flexible plasmonic-coupling-based devices with tunable and quantifiable performance herein suggested are expected to unlock promising potential in practical bio-sensing, biotechnological applications and optical devices.

Effect of nanostructured silicon on surface enhanced Raman scattering

Non-metallic materials are often employed in SERS systems by forming composite structures with SERS-active metal materials. However, the role of the non-metallic structures in these composites and the effect of them on the SERS enhancement are still unclear. Herein, we studied the effect of silicon morphology on SERS enhancement on silver nanoparticles-coated different structured silicon surfaces. Our finding will help to further understand the SERS mechanism and pave the way for making more efficient SERS systems.

The surface morphology of non-metallic silicon has a big effect on the SERS enhancement of silver nanoparticle-coated silicon surfaces.

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.

Au nanostructure fabrication by pulsed laser deposition in open air: Influence of the deposition geometry

We present a fast and flexible method for the fabrication of Au nanocolumns. Au nanostructures were produced by pulsed laser deposition in air at atmospheric pressure. No impurities or Au compounds were detected in the resulting samples. The nanoparticles and nanoaggregates produced in the ablated plasma at atmospheric pressure led to the formation of chain-like nanostructures on the substrate. The dependence of the surface morphology of the samples on the deposition geometry used in the experimental set up was studied. Nanocolumns of different size and density were produced by varying the angle between the plasma plume and the substrate. The electrical, optical, and hydrophobic properties of the samples were studied and discussed in relation to their morphology. All of the nanostructures were conductive, with conductivity increasing with the accumulation of ablated material on the substrate. The modification of the electrical properties of the nanostructures was demonstrated by irradiation by infrared light. The Au nanostructures fabricated by the proposed technology are difficult to prepare by other methods, which makes the simple implementation and realization in ambient conditions presented in this work more ideal for industrial applications.

In situ synthesis of Au-shelled Ag nanoparticles on PDMS for flexible, long-life, and broad spectrum-sensitive SERS substrates

A simple, fast, one-step fabrication of silver nanoparticles with atomically thin gold coatings on polydimethylsiloxane affords oxidation-resistant and highly sensitive surface enhanced Raman scattering (SERS) substrates.

Ag Nanoparticle-Grafted PAN-Nanohump Array Films with 3D High-Density Hot Spots as Flexible and Reliable SERS Substrates

A facile fabrication approach of large-scale flexible films is reported, with one surface side consisting of Ag-nanoparticle (Ag-NP) decorated polyacrylonitrile (PAN) nanohump (denoted as Ag-NPs@PAN-nanohump) arrays. This is achieved via molding PAN films with ordered nanohump arrays on one side and then sputtering much smaller Ag-NPs onto each of the PAN-nanohumps. Surface-enhanced Raman scattering (SERS) activity of the Ag-NPs@PAN-nanohump array films can be improved by curving the flexible PAN film with ordered nanohump arrays during the Ag-sputtering process to increase the density of the Ag-NPs on the sidewalls of the PAN-nanohumps. More 3D hot spots are thus achieved on a large-scale. The Ag-NPs@PAN-nanohump array films show high SERS activity with good Raman signal reproducibility for Rhodamine 6G probe molecules. To trial their practical application, the Ag-NPs@PAN-nanohump array films are employed as SERS substrates for trace detection of trinitrotoluene and a congener of polychlorinated biphenyls. A lower detection limit of 10(-12) m and 10(-5) m can be achieved, respectively. Furthermore, the flexible Ag-NPs@PAN-nanohump array films can also be utilized as swabs to probe traces of methyl parathion on the surface of fruits such as apples. The as-fabricated SERS substrates therefore have promising potential for applications in rapid safety inspection and environmental protection.

Surface enhanced Raman scattering by graphene-nanosheet-gapped plasmonic nanoparticle arrays for multiplexed DNA detection

We have developed a new type of surface enhanced Raman scattering (SERS) substrate with thiolated graphene oxide (tGO) nanosheets sandwiched between two layers of closely packed plasmonic nanoparticles. The trilayered substrate is built up through alternative loading of interfacially assembled plasmonic nanoparticle arrays and tGO nanosheets, followed by coating the nanoparticle surfaces with poly(ethylene glycol) (PEG). Here tGO plays multifunctional roles as a 2D scaffold to immobilized interfacially assembled plasmonic nanoparticles, a nanospacer to create SERS-active nanogaps between two layers of nanoparticle arrays, and a molecule harvester to enrich molecules of interest viaπ-π interaction. In particular, the molecule harvesting capability of the tGO nanospacer and the stealth properties of PEG coating on the plasmonic nanoparticles collectively lead to preferential positioning of selective targets such as aromatic molecules and single-stranded DNA at the SERS-active nanogap hotspots. We have demonstrated that an SERS assay based on the PEGylated trilayered substrate, in combination with magnetic separation, allows for sensitive, multiplexed "signal-off" detection of DNA sequences of bacterial pathogens.

A facile method for fabrication of buckled PDMS silver nanorod arrays as active 3D SERS cages for bacterial sensing

We have successfully demonstrated a simple and facile method to increase the SERS signal of bacteria due to the formation of high density hotspots among the AgNRs and the increase in the area for better interaction of bacteria with the metal surface.

A SERS protocol as a potential tool to access 6-mercaptopurine release accelerated by glutathione-S-transferase

The developed method for monitoring GST, an important drug metabolic enzyme, could greatly facilitate researches on relative biological fields.

Thin metal nanostructures: synthesis, properties and applications

Two-dimensional nanomaterials, especially graphene and single- or few-layer transition metal dichalcogenide nanosheets, have attracted great research interest in recent years due to their distinctive physical, chemical and electronic properties as well as their great potentials for a broad range of applications. Recently, great efforts have also been devoted to the controlled synthesis of thin nanostructures of metals, one of the most studied traditional materials, for various applications. In this minireview, we review the recent progress in the synthesis and applications of thin metal nanostructures with a focus on metal nanoplates and nanosheets. First of all, various methods for the synthesis of metal nanoplates and nanosheets are summarized. After a brief introduction of their properties, some applications of metal nanoplates and nanosheets, such as catalysis, surface enhanced Raman scattering (SERS), sensing and near-infrared photothermal therapy are described.

Synthesis of novel gold mesoflowers as SERS tags for immunoassay with improved sensitivity

A new class of flowerlike gold mesostructure in high yield is successfully synthesized through a facile one-step route using ascorbic acid as a reducing agent of gold salt with cetyltrimethylammonium chloride (CTAC) as surfactant. The as-prepared Au particles have spherical profiles with an averaged diameter of 770 ± 50 nm, but showing a highly rough surface consisting of many irregular and randomly arranged protrusions. The Au mesoflowers exhibit strong surface-enhanced effects and near-infrared absorption which were utilized in the design of efficient surface-enhanced Raman scattering (SERS) tags as immunosensors for immunoassay with improved sensitivity. The experimental results indicate that a good linear relationship is found between the peak intensity at 1071 cm(-1) and the logarithm of H-IgG concentration in the range between 1 ng/mL and 1 fg/mL, and the limit of detection (LOD) is 1 fg/mL.

Solution processed polydimethylsiloxane/gold nanostar flexible substrates for plasmonic sensing

Gold nanostars can display tunable optical properties in the visible and near IR, which lead to strong electromagnetic field enhancement at their tips. We report generalized application of gold nanostars for ultrasensitive identification of molecules, based on both localized surface plasmon resonance (LSPR) and surface enhanced Raman scattering (SERS). We address the requirements of plasmonic sensors, related to sufficiently large areas where nanoparticles are uniformly immobilized with high density, as well as mechanical flexibility, which offers additional advantages for real-world applications. Gold nanostar monolayers were thus immobilized on transparent, flexible polydimethylsiloxane substrates, and their refractive index sensitivity and SERS performance were studied. The application of such substrates for LSPR based molecular sensing is demonstrated via detection of a model analyte, mercaptoundecanoic acid. We further demonstrate SERS-based pesticide detection on fruit skin, by simply covering the fruit surface with the flexible plasmonic substrate, at the area where the target molecule is to be detected. The transparency of the substrate allows SERS detection through backside excitation, thereby facilitating practical implementation.

A rational biomimetic approach to structure defect generation in colloidal nanocrystals

Controlling the morphology of nanocrystals (NCs) is of paramount importance for both fundamental studies and practical applications. The morphology of NCs is determined by the seed structure and the following facet growth. While means for directing facet formation in NC growth have been extensively studied, rational strategies for the production of NCs bearing structure defects in seeds have been much less explored. Here, we report mechanistic investigations of high density twin formation induced by specific peptides in platinum (Pt) NC growth, on the basis of which we derive principles that can serve as guidelines for the rational design of molecular surfactants to introduce high yield twinning in noble metal NC syntheses. Two synergistic factors are identified in producing twinned Pt NCs with the peptide: (1) the altered reduction kinetics and crystal growth pathway as a result of the complex formation between the histidine residue on the peptide and Pt ions, and (2) the preferential stabilization of {111} planes upon the formation of twinned seeds. We further apply the discovered principles to the design of small organic molecules bearing similar binding motifs as ligands/surfactants to create single and multiple twinned Pd and Rh NCs. Our studies demonstrate the rich information derived from biomimetic synthesis and the broad applicability of biomimetic principles to NC synthesis for diverse property tailoring.

Transparent free-standing metamaterials and their applications in surface-enhanced Raman scattering

Integration of metamaterials onto a flexible substrate can provide many advantages such as transparency, deformability, light weight and biocompatibility. Here we demonstrate a simple and convenient nickel sacrificial layer-assisted transfer method to fabricate visible-near infrared (IR) metamaterials embedded into a thin polydimethylsiloxane (PDMS) film. Both the structures and the optical properties are maintained after transferring into the PDMS film from a rigid substrate. This PDMS-based metamaterial can behave as a high performance surface enhanced Raman scattering (SERS) device with tunable plasmonic bands, which decouple the preparation of SERS structure and the linkage of targeted molecules to the plasmonic structures. By simply covering the PDMS metamaterials device on the surface with molecules of interest, we demonstrate the application of 2-naphthalenethiol molecules self-assembled on a Au film, highlighting the considerable potential of these PDMS metamaterials as a SERS stamp onto any other substrate. What's more, the PDMS-based nanostructures offer a representative model to investigate the interaction between the plasmonic nanostructure and the substrate consisting of different materials by placing PDMS on the surface of the substrate.

A silver nanoparticle embedded hydrogel as a substrate for surface contamination analysis by surface-enhanced Raman scattering

A surface enhanced Raman scattering (SERS) hydrogel substrate, capable of extracting small amounts of organic species from surfaces of different types of materials with variable roughness, has been fabricated.

Gold-modified silver nanorod arrays for SERS-based immunoassays with improved sensitivity

Silver nanorod arrays and Au-modified AgNR arrays are fabricated for SERS immunoassays with improved sensitivity.

Towards low-cost flexible substrates for nanoplasmonic sensing

Plasmonic nanostructures have played a significant role in the field of nanotechnology due to their unprecedented ability to concentrate light at the nanometre scale, which renders them precious for various sensing applications. The adsorption of plasmonic nanoparticles and nanostructures onto solid substrates in a controlled manner is a crucial process for the fabrication of nanoplasmonic devices, in which the nanoparticles amplify the electromagnetic fields for enhanced device performance. In this perspective article we summarize recent developments in the fabrication of flexible nanoplasmonic devices for sensing applications based on surface enhanced Raman scattering (SERS) and localized surface plasmon resonance (LSPR) shifts. We introduce different types of flexible substrates such as filter paper, free-standing nanofibres, elastomers, plastics, carbon nanotubes and graphene, for the fabrication of low-cost flexible nanoplasmonic devices. Various techniques are described that allow impregnation of such flexible substrates with plasmonic nanoparticles, including solution processes, physical vapour deposition and lithographic techniques. From the discussion in this Perspective, it is clear that highly sensitive and reproducible flexible plasmonic devices can currently be fabricated on a large scale at relatively low-cost, toward real-world applications in diagnostics and detection.