Surface-enhanced Raman Scattering Platform Research Articles

Electrostatic Self-Assembly of Ti3C2Tx MXene and Gold Nanorods as an Efficient Surface-Enhanced Raman Scattering Platform for Reliable and High-Sensitivity Determination of Organic Pollutants.

A reliable surface-enhanced Raman scattering (SERS) substrate composed of two-dimensional (2D) MXene (Ti3C2Tx) nanosheets and gold nanorods (AuNRs) is designed and fabricated for sensitive detection of organic pollutants. The AuNRs are uniformly distributed on the surface of the 2D MXene nanosheets because of the strong electrostatic interactions, forming abundant SERS hot spots. The MXene/AuNR SERS substrate exhibits high sensitivity and excellent reproducibility in the determination of common organic dyes such as rhodamine 6G, crystal violet, and malachite green. The detection limits are 1 × 10-12, 1 × 10-12, and 1 × 10-10 M, and relative standard deviations determined from 13 areas on each sample are 18.1, 10.1, and 15.6%, respectively. In the determination of more complex organic pesticides and pollutants, the substrate also shows excellent sensitivity and quantitative detection, and the detection limits for thiram and diquat of 1 × 10-10 and 1 × 10-8 M, respectively, are much lower than the contaminant levels stipulated by the US Environmental Protection Agency. The MXene/AuNR composite constitutes an efficient SERS platform for reliable and high-sensitivity environmental analysis and food safety monitoring.

Utilization of doped GQDs for ultrasensitive detection of catastrophic melamine: A new SERS platform.

The detection and filtration of melamine in food products has become an emergence due to its harmful effect on humans. In present work, we have investigated the binding mechanism of melamine over carboxyl group edge-functionalized graphene quantum dots doped with oxygen and sulphur atoms (O-GQD and S-GQD). In order to monitor melamine, surface enhanced Raman scattering (SERS) is adopted which is an effective vibrational spectroscopic approach. Electronic and vibrational properties were analysed by means of well adapted density functional theory (DFT). The calculated adsorption energy of melamine over O-GQD and S-GQD is-1.18 and-0.15 eV respectively. The characteristic peak of melamine at 688 cm-1 is in good agreement with previously reported experimental work and enhances by 348.4% in SERS spectra of Mel-O-GQD and 48% in SERS spectra of Mel-S-GQD. We have calculated the chemical enhancement factor (EF) for melamine over O-GQD and S-GQD and found the enhancement of 4.51 and 1.48 which is greater than melamine‑silver complexes. Our theoretical studies on SERS of melamine over O-GQD and S-GQD suggest that oxygen is a better candidate for SERS. Our work demonstrates that the graphene quantum dots are remarkable platforms for the detection of melamine.

Hierarchic Interfacial Nanocube Assembly for Sensitive, Selective, and Quantitative DNA Detection with Surface-Enhanced Raman Scattering.

Surface-enhanced Raman scattering (SERS)-based sensing is promising in that it has potential to allow for highly sensitive, selective, and multiplexed detection and imaging. However, the controlled assembly and gap formation between plasmonic particles for generating strong SERS signals in a quantitative manner is highly challenging, especially on biodetection platforms, and particle-to-particle variation in the signal enhancement can vary by several orders of magnitude in a single batch, largely limiting the reliable use of SERS for practical sensing applications. Here, a hierarchic-nanocube-assembly based SERS (H-Cube-SERS) bioassay to controllably amplify the electromagnetic field between gold nanocubes (AuNCs) is developed. Based on this strategy, H-Cube-SERS assay allows for detecting target DNA with a wide dynamic range from 100 aM to 10 pM concentrations in a stable and reproducible manner. It is also found that the uniformly formed AuNCs with flat surfaces are much more suitable for highly sensitive, reliable, and quantitative biodetection assays due to faster DNA binding kinetics, sharper DNA melting transition, wider hot spot regions, and less dependence on light polarization direction than spherical Au nanoparticles with curved interfaces. This work paves the pathways to the quantitative and sensitive biodetection on a SERS platform and can be extended to other particle assembly systems.

3D Hierarchical Gallium Oxynitride Nanostructures Decorated with Ag Nanoparticles Applied as Recyclable Substrates for Ultrasensitive SERS Sensing

AbstractDeveloping plasmon‐assisted semiconductor nanocomposites as multifunctional surface‐enhanced Raman scattering (SERS) substrate for detection as well as photodegradation of toxic organic pollutants appears to be a crucial step in upgrading the SERS technique to a quantitative analytical level. However, conventional photocatalytic semiconductor scaffolds are confined in ultraviolet‐light‐active metal oxide nanostructures (TiO2, ZnO, etc.). Herein, the scope of semiconductor scaffold is expanded to visible‐light‐active oxynitrides by developing a robust and ultrasensitive recyclable SERS substrate based on 3D hierarchical gallium oxynitrides (GaON) nanosheets loading with Ag nanoparticles (Ag NPs). Significantly, the bandgap of GaON nanosheets shrinks to 2.9 eV to enable visible‐light adsorption. The as‐fabricated SERS substrate shows a high sensitivity with enhancement factor of 5.2 × 107 in R6G detection, and yields a wide detection range of 1.0 × 10−6 –1.0 × 10−12 mol L−1. Assisted with the enhanced charge transfer by generating the Schottky contact at the metal/semiconductor interface and the electron traps played by the residue oxygen, the Ag/GaON nanocomposites show attractive recyclability at visible‐light irradiation with well‐maintained SERS activity after four cycles. This work demonstrates the superiority of taking advantage of plasmon‐assisted oxynitride semiconductors as a high‐performance SERS platform and provides new opportunities for developing robust visible‐light‐driven recyclable SERS substrates.

Possible application of gold thin films formed from surfactant free gold nano-particle dispersed in aqueous solutions to surface-enhanced Raman scattering spectroscopy

Detailed procedure to form gold (Au) thin films from surfactant free Au nano-particle dispersed aqueous solution prepared by solution plasma processing was investigated for surface-enhanced Raman scattering (SERS) spectroscopy measurement. The averaged diameter, thickness, and the weight of each Au thin film in our experimental conditions from 40 mL of 1% Au dispersed aqueous solution, were evaluated to be around 2.4 mm, 4.6 μm and 0.4 mg, respectively. SERS spectra of dye molecule (crystal violet: CV) adsorbed on Au thin film and silver (Ag) substrate were successfully measured upon excitation at 532 nm of solid type laser and 632.8 nm of He-Ne laser, respectively. These SERS spectra of CV adsorbed both on Au and Ag surfaces were coincided with each other, and the intensity from Au surface was unexpectedly much stronger as compared to those from Ag surface. The possible application of Au thin film for SERS platform on solid/liquid interfaces was demonstrated by observation of SERS spectra from p-nitrothiophenol (PNTP) adsorbed on Au thin film/aqueous solution interface.

Dynamic Liquid Surface Enhanced Raman Scattering Platform Based on Soft Tubular Microfluidics for Label-Free Cell Detection.

Cell detection is of great significance for biomedical research. Surface enhanced Raman scattering (SERS) has been widely applied to the detection of cells. However, there is still a lack of a general, low-cost, rapid, and sensitive SERS method for cell detection. Herein, a dynamic liquid SERS platform, which combines label-free SERS technique with soft tubular microfluidics for cell detection, is proposed. Compared with common static solid and static liquid measurement, the dynamic liquid SERS platform can present dynamical mixing, precise control of the mixing time, and continuous spectra collection. By characterizing the model molecules, the proposed dynamic liquid SERS platform has successfully demonstrated good stability and repeatability with 1.90% and 4.98% relative standard deviation (RSD), respectively. Three cell lines including one normal breast cell line (MCF-10A) and two breast cancer cell lines (MCF-7 and MDA-MB-231) were investigated in this platform. 270 cell spectra were selected as the training set for the classification of the models based on the K-Nearest Neighbor (K-NN) algorithm. In three independent experiments, three types of cells were identified by a test set containing 180 cell spectra with sensitivities above 83.3% and specificities above 91.6%. The accuracy was 94.1 ± 1.14% among three independent cell identifications. The dynamic liquid SERS platform has shown higher signal intensity, better repeatability, less pretreatment, and obtainment of more spectra with less time consumption. It will be a powerful detection tool in the area of cell research, clinical diagnosis, and food safety.

Three-Dimensional Surface-Enhanced Raman Scattering Platforms: Large-Scale Plasmonic Hotspots for New Applications in Sensing, Microreaction, and Data Storage.

Surface-enhanced Raman scattering (SERS) is a molecular-specific spectroscopic technique that provides up to 1010-fold enhancement of signature Raman fingerprints using nanometer-scale 0D to 2D platforms. Over the past decades, 3D SERS platforms with additional plasmonic materials in the z-axis have been fabricated at sub-micrometer to centimeter scale, achieving higher hotspot density in all x, y, and z spatial directions and higher tolerance to laser misalignment. Moreover, the flexibility to construct platforms in arbitrary sizes and 3D shapes creates attractive applications besides traditional SERS sensing. In this Account, we introduce our library of substrate-based and substrate-less 3D plasmonic platforms, with an emphasis on their non-sensing applications as microlaboratories and data storage labels. We aim to provide a scientific synopsis on these high-potential yet currently overlooked applications of SERS and ignite new scientific discoveries and technology development in 3D SERS platforms to tackle real-world issues. One highlight of our substrate-based SERS platforms is multilayered platforms built from micrometer-thick assemblies of plasmonic particles, which can achieve up to 1011 enhancement factor. As an alternative, constructing 3D hotspots on non-plasmonic supports significantly reduces waste of plasmonic materials while allowing high flexibility in structural design. We then introduce our emerging substrate-less plasmonic capsules including liquid marbles and colloidosomes, which we further incorporate the latter within an aerosol to form centimeter-scale SERS-active plasmonic cloud, the world's largest 3D SERS platform to date. We then discuss the various emerging applications arising only from these 3D platforms, in the fields of sensing, microreactions, and data storage. An important novel sensing application is the stand-off detection of airborne analytes that are several meters away, made feasible with aerosolized plasmonic clouds. We also describe plasmonic capsules as excellent miniature lab-in-droplets that can simultaneously provide in situ monitoring at the molecular level during reaction, owing to their ultrasensitive 3D plasmonic shells. We highlight the emergence of 3D SERS-based data storage platforms with 10-100-fold higher storage density than 2D platforms, featuring a new approach in the development of level 3 security (L3S) anti-counterfeiting labels. Ultimately, we recognize that 3D SERS research can only be developed further when its sensing capabilities are concurrently strengthened. With this vision, we foresee the creation of highly applicable 3D SERS platforms that excel in both sensing and non-sensing areas, providing modernsolutions in the ongoing Fourth Industrial Revolution.

Surface‐enhanced Raman scattering studies of gold‐coated ripple‐like nanostructures on iron substrate achieved by femtosecond laser irradiation in water

AbstractDuring the past several years, numerous efforts have been accomplished on designing efficient surface‐enhanced Raman scattering (SERS) substrates with high sensitivity, good reproducibility, and recyclable nature. However, it still remains a significant challenge to realize all these qualities in a single substrate. We have fabricated ripple‐like nanostructures (NSs) on Iron (Fe) substrate using femtosecond (fs) laser irradiation of Fe target in distilled water. These ripple‐like structures had periodicities in the range of ~90–190 nm, which was confirmed from the analysis of field emission scanning electron microscope (FESEM) data. We subsequently deposited a thin layer of gold on these ripple‐like Fe NSs using thermal evaporation. Later, these coated NSs were effectively utilized as SERS substrates for methylene blue (MB) detection and were found to be sensitive evident from the data obtained at a very low concentration of 500pM. Furthermore, we observed that the SERS signal variations on the substrate were <11.2% indicating the reproducible nature of these substrates. Additionally, we demonstrate that these substrates can be reused (we establish this for three times consecutively) by detecting malachite green (MG) and an explosive molecule (picric acid, PA) followed by the mixture compound (Rhodamine 6G+MB) through employment of simple cleaning procedures. The detected molecular concentrations in terms of masses were found to be 3.2 pg, 36.5 pg, and 23 ng for MB (for a concentration of 500pM), MG (for a concentration of 5nM), and PA (for a concentration of 5μM), respectively. Furthermore, these Fe NSs exhibited superior batch‐to‐batch reproducibility with a relative standard deviation (RSD) value of ~l4%. The proposed method of fabricating ripple structures as SERS platforms are highly feasible, reliable, and have great potential for on‐site detection of several analyte molecules with an easily transportable Raman spectrometer.

Surface-Enhanced Raman Spectroscopy on Self-Assembled Au Nanoparticles Arrays for Pesticides Residues Multiplex Detection under Complex Environment.

The high reproducibility of trace detection in complex systems is very hard but crucial to analytical technology and science. Here, we present a surface-enhanced Raman scattering (SERS) platform made by large-scale self-assembly of Au nanoparticle (NP) arrays at the cyclohexane/water interface and its use for pesticides residues trace detection. The analyte molecules spontaneously localize into the Au NPs’ nanogaps during the self-assembly process, yielding excellent Raman signal enhancement by surface effects, and possibly both by the concentration of the analytes into the array and by plasmonic hot-spot formation. Transmission electron microscopy (TEM) images demonstrate a good uniformity of interparticle distances (2–3 nm) in the Au NP arrays. SERS experiments on crystal violet (CV) molecules demonstrated that the relative standard deviations (RSD) of the band intensities at 1173, 1376, and 1618 cm−1 were 6.3%, 6.4%, and 6.9%, respectively, indicating high reproducibility of the substrate. Furthermore, we demonstrate that two pesticides dissolved in organic and aqueous phases could be simultaneously detected, suggesting an excellent selectivity and universality of this method for multiplex detection. Our SERS platform opens vast possibilities for repeatability and sensitivity detection of targets in various complex fields.

Surface‐enhanced Raman scattering platform operating over wide pH range with minimal chemical enhancement effects: Test case of tyrosine

AbstractDecoupling the contributions of electromagnetic and chemical enhancements in surface‐enhanced Raman scattering (SERS) would lead to greater reliability and applicability in biological systems that involve large, localized pH gradients. However, suppressing short‐range, chemical enhancement effects in SERS has proved challenging due to unavoidable charge‐transfer interactions between analyte and the SERS platforms. This extends to both lithographically fabricated top‐down and self‐assembled bottom‐up SERS platforms. Here, we employ a Soret effect‐driven monodisperse nanoparticle assembly (Soret colloids [SCs]) with negligible surface charge (ξ < 16 mV) that provides exceptional SERS enhancement (analytical enhancement factor ~106) contributed predominantly by excitation of localized surface plasmon resonance (electromagnetic enhancement) with minimal electrostatic or charge‐transfer–based short‐range interactions. Significantly, the low ξ of SCs is invariant over a wide pH range (pH 4–7) indicating minimal electrostatic surface charge. The interaction of such SCs with ionic analytes such as tyrosine leads to uniform SERS dictated by electromagnetic enhancement mechanism. The absence of contribution from chemical enhancement is clearly established by the vibrational fingerprints of tyrosine that are devoid of any spectral shifts originating from charge transfer between tyrosine and SCs. Accordingly, all features of the tyrosine that are independent of pH such as C―H stretch (2,700–3,000 cm−1) and C―C stretch (1,440 cm−1) and C―C―C bending (1,010 cm−1) are completely preserved without any spectral shifts. Simultaneously, pH‐dependent vibrational features of tyrosine such as N―H stretch (1,545 cm−1), CO stretch (1,700 cm−1), COO− stretch (1,640 cm−1), C―O stretch (1,240 cm−1), and amide III bands (1,246 and 1,260 cm−1) are clearly observed. The intensities of such bands correspond exactly to the ionic state of tyrosine as dictated by the pH of the medium. This is reinforced by the crossover in the intensity of the pH‐dependent vibrational modes (C―O vs. COO‐ stretch, COO− vs. NH3+ stretch) that coincides with the isoelectric pH of tyrosine. Finally, reliable quantification of the charge and concentration of tyrosine using SCs are established, enabling its use for studying intrinsic cellular pathways using SERS.

Rapid and fingerprinted monitoring of pesticide methyl parathion on the surface of fruits/leaves as well as in surface water enabled by gold nanorods based casting-and-sensing SERS platform

An effective, simple and portable gold nanorod (Au NR) based casting-and-sensing surface enhanced Raman scattering (SERS) platform was developed for rapid and fingerprinted detection of pesticide methyl parathion. Monodispersed Au NRs with an average length of 60 nm and an aspect ratio of ca. 3 were synthesized through a seed mediated method and then systematically characterized. After a proof-of-concept detection for methyl parathion in DI water and on solid surface, the sensing platform was further applied to real samples (lake water, orange, apple and plant leave) contaminated with methyl parathion. The results show that the sensitivity of the SERS sensor for methyl parathion was satisfactory for real application, with detection limits of 1 μM in spiked lake water and 110–440 ng/cm2 on the surface of various fruits and plant leaves. This study indicates that the developed casting-and-sensing SERS sensor shows great promise to secure agricultural, food and environmental safety.

Silver nanocubes/graphene oxide hybrid film on a hydrophobic surface for effective molecule concentration and sensitive SERS detection

Surface-enhanced Raman scattering (SERS) is a promising analytical technique with molecular “fingerprint” characteristic and high sensitivity. To achieve high sensitivity, analytes should be located in or around the so-called SERS “hot spots” that are normally formed in sub-10 nm gaps between the neighboring plasmonic nanostructures. Ordinary SERS substrates generally show low sensitivity to the molecules that have weak affinity to noble-metal surfaces. Here, we fabricate an effective SERS system that can concentrate and adsorb organic pollutants in water for sensitive SERS detection. Such a SERS platform is composed of a thin film of Ag nanocubes/graphene oxide hybrid covered on a small Cu foil, and the Cu foil is placed on a hydrophobic teflon surface. The Ag nanocubes/graphene oxide hybrid film is synthesized by growth of graphene oxide on a Cu foil using electrophoresis and subsequent assembly of Ag nanocubes on the graphene oxide in ethanol via van der Waals interactions. Organic pollutant molecules can be concentrated on the hybrid film by adding a droplet of aqueous analyte solution on the hybrid film and evaporating. Due to the hydrophobic nature of teflon surface, the droplet will shrink onto the hybrid film and realize the concentration of analyte molecules on a SERS platform. There exists large delocalized π-electron system in the graphene oxide, so the Ag nanocubes/graphene oxide hybrid can effectively adsorb a large number of analyte molecules. As there are plenty of nano-sized gaps between the Ag nanocubes that are densely attached on the graphene oxide sheets, high density SERS hot spots can be formed in the gaps. As a result, the Ag nanocubes/graphene oxide hybrid has shown high SERS activity with an enhancement factor of 3.9 × 108 and good signal uniformity (relative standard deviation ∼12%). It is further demonstrated that the hybrid SERS substrates have exhibited high SERS sensitivity to organic pollutants such as 4-chlorobiphenyl and methyl parathion, showing promising applicability in many fields as diverse as chemical analysis, environmental monitoring, and food safety inspection.

In Situ Synthesis of Silver Nanospheres, Nanocubes, and Nanowires over Boron-Doped Graphene Sheets for Surface-Enhanced Raman Scattering Application and Enzyme-Free Detection of Hydrogen Peroxide.

An effective in situ synthesis strategy is demonstrated for the preparation of silver nanostructures (nanospheres (NSs), nanocubes (NCs), and nanowires (NWs)) on the surface of boron-doped graphene (BG). Further, these functional nanomaterials are employed for the surface-enhanced Raman scattering (SERS) and non-enzymatic electrochemical detection of H2O2. The results confirm the superior performance of BG-Ag nanostructures as SERS platform. Among various geometries of silver nanoparticles studied in this work, we find that the AgNCs over BG (BG-AgNC) present outstanding SERS performance for detecting 4-mercaptobenzoic acid, with a limit of detection of 1.0 × 10-13 M. Furthermore, BG-AgNC exhibits excellent capability to detect melamine as low as 1.0 × 10-9 M. Electrochemical results confirm that the BG-AgNW-based platform exhibits a superior biosensing performance toward H2O2 detection. The enhanced performance is due to the presence of graphene, which improves the conductivity and provides more active sites. The synthesis of doped graphene with metallic nanoparticles described in this work is expected to be a key strategy for the development of an efficient SERS and electrochemical sensor that offers simplicity, cost-effectiveness, long-term stability, and better reproducibility.

Room Temperature, Multiphasic Detection of Explosives, and Volatile Organic Compounds Using Thermodiffusion Driven Soret Colloids

Achieving a label-free spectroscopic platform for multiphasic analytical detection in real-time, ambient conditions is extremely challenging due to the fundamental dichotomy between ultrahigh sensitivity and reliability of detection. Addressing these challenges, we demonstrate a versatile surface-enhanced Raman scattering (SERS) platform capable of multiphasic, reliable detection with ultrahigh sensitivity. The SERS platform is extremely sensitive and relies on vapor molecules present at the solid–vapor or liquid–vapor interface for reliable detection in ambient conditions (298 K, 1 atm). We observe that metal nanoparticles, subjected to a temperature gradient, migrate and self-assemble into precise nanoparticle assemblies, in a nanoscale analogue of Soret effect. The formation and monodispersity of these nanoparticles assemblies, termed Soret colloids (SCs), is kinetically controllable using the thermal gradient. Soret colloids exhibit excellent size uniformity (monodispersity index, MDI ∼ 0.8) and stron...

Tip-Selective Growth of Silver on Gold Nanostars for Surface-Enhanced Raman Scattering

Nanogaps as "hot spots" with highly localized surface plasmon can generate ultrastrong electromagnetic fields. Superior to the exterior nanogaps obtained via aggregation and self-assembly, interior nanogaps within Au and Ag nanostructures give stable and reproducible surface-enhanced Raman scattering (SERS) signals. However, the synthesis of nanostructures with interior hot spots is still challenging because of the lack of high-yield strategies and clear design principles. Herein, gold-silver nanoclusters (Au-Ag NCs) with multiple interior hot spots were fabricated as SERS platforms via selective growth of Ag nanoparticles on the tips of Au nanostars (Au NSs). Furthermore, the interior gap sizes of Au-Ag NCs can be facilely tuned by changing the amount of AgNO3 used. Upon 785 nm excitation, single Au-Ag NC350 exhibits 43-fold larger SERS enhancement factor and the optimal signal reproducibility relative to single Au NS. The SERS enhancement factors and signal reproducibility of Au-Ag NCs increase with the decrease of gap sizes. Collectively, the Au-Ag NCs could serve as a flexible, reproducible, and active platform for SERS investigation.

Plasmonically Enhanced Galactoxyloglucan Endowed Gold Nanoparticles Exposed Tumor Targeting Biodistribution Envisaged in a Surface-Enhanced Raman Scattering Platform.

Biopolymer-capped gold nanoparticles (AuNPs) were perceived for tracing biodistribution in a solid tumor mice through surface-enhanced Raman scattering (SERS) fingerprinting. In this strategy, a robust and ecofriendly green chemistry approach was adopted to construct galactoxyloglucan (PST001) endowed AuNPs (PST-GNPs) with cancer-cell-selective toxic nature and excellent biocompatibility. Plasmonically enhanced light-scattering properties facilitated PST-GNPs to be a superior SERS substrate with high Raman signal enhancement. In this context, PST-GNPs were scrutinized for the noninvasive label-free SERS live-cell spectral imaging to evaluate the fingerprint molecular details of cellular processes. Consequently, the inherent SERS feature of PST-GNPs enabled us to investigate the dynamic and complex nature with NP biodistrubution in tumor-bearing mice on a SERS platform that illustrated the tumor targeting nature. Henceforth, the present findings emphasized a futuristic clinically relevant scenario for tracing the in vivo NP dissemination in a label-free fashion for providing vital biochemical details on a molecular level.

Three-Dimensional Au-Coated Electrosprayed Nanostructured BODIPY Films on Aluminum Foil as Surface-Enhanced Raman Scattering Platforms and Their Catalytic Applications.

The design and development of three-dimensional (3D) nanostructures with high surface-enhanced Raman scattering (SERS) performances have attracted considerable attention in the fields of chemistry, biology, and materials science. Nevertheless, electrospraying of organic small molecules on low-cost flexible substrates has never been studied to realize large-scale SERS-active platforms. Here, we report the facile, efficient, and low-cost fabrication of stable and reproducible Au-coated electrosprayed organic semiconductor films (Au@BDY-4T-BDY) on flexible regular aluminum foil at a large scale (5 cm × 5 cm) for practical SERS and catalytic applications. To this end, a well-designed acceptor-donor-acceptor-type solution-processable molecular semiconductor, BDY-4T-BDY, developed by our group, is used because of its advantageous structural and electrical properties. The morphology of the electrosprayed organic film changes by solution concentration, and two different 3D morphologies with out-of-plane features are obtained. Highly uniform dendritic nanoribbons with sharp needle-like tips and vertically oriented nanoplates (∼50 nm thickness) are achieved when electrospraying solution concentrations of 240 and 253% w/v (mg/mL) are, respectively, used. When these electrosprayed organic films are coated with a nanoscopic thin (30 nm) Au layer, the resulting Au@BDY-4T-BDY platforms demonstrate remarkable SERS enhancement factors up to 1.7 × 106 with excellent Raman signal reproducibility (relative standard deviation ≤ 0.13) for methylene blue over the entire film. Finally, Au@BDY-4T-BDY films showed good catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol with rate constants of 1.3 × 10-2 and 9.2 × 10-3 min-1. Our results suggest that electrospraying of rationally designed organic semiconductor molecules on flexible substrates holds great promise to enable low-cost, solution-processed, SERS-active platforms.

SERS detection of hepatitis B virus DNA in a temperature‐responsive sandwich‐hybridization assay

In this study, we report the design of a DNA sensor for the detection of a DNA sequence representative of the hepatitis B virus (HBV) based on a sandwich assay and surface‐enhanced Raman scattering (SERS) measurement. The temperature‐responsive hybrid silicon substrate was first prepared via immobilization of capture DNA strand at the surface of gold nanoparticles on the hybrid silicon substrate. A sandwich strategy was then applied for the detection of target DNA that brings a reporter DNA labeled with indocyanine green to the proximity of the surface, leading to high SERS signals. The temperature‐responsive hybrid silicon substrate‐based SERS platform can detect the remarkably lowest HBV DNA concentrations at ~0.44 fm at 25 °C and ~0.14 fm at 37 °C, respectively, which are comparable with the lowest HBV DNA concentration ever via other techniques. We expect this highly sensitive and robust hybrid silicon substrate‐based SERS platform can be extended to detect other biomolecules and chemical species such as viruses, proteins, and small molecules without any labeling. Copyright © 2017 John Wiley & Sons, Ltd.

Two-Dimensional Heterostructure as a Platform for Surface-Enhanced Raman Scattering

Raman enhancement on a flat nonmetallic surface has attracted increasing attention, ever since the discovery of graphene enhanced Raman scattering. Recently, diverse two-dimensional layered materials have been applied as a flat surface for the Raman enhancement, attributed to different mechanisms. Looking beyond these isolated materials, atomic layers can be reassembled to design a heterostructure stacked layer by layer with an arbitrary chosen sequence, which allows the flow of charge carriers between neighboring layers and offers novel functionalities. Here, we demonstrate the heterostructure as a novel Raman enhancement platform. The WSe2 (W) monolayer and graphene (G) were stacked together to form a heterostructure with an area of 10 mm × 10 mm. Heterostructures with different stacked structuress are used as platforms for the enhanced Raman scattering, including G/W, W/G, G/W/G/W, and W/G/G/W. On the surface of the heterostructure, the intensity of the Raman scattering is much stronger compared with isolated layers, using the copper phthalocyanine (CuPc) molecule as a probe. It is found that the Raman enhancement effect on heterostructures depends on stacked methods. Phonon modes of CuPc have the strongest enhancement on G/W. W/G and W/G/G/W have a stronger enhancement than that on the isolated WSe2 monolayer, while lower than the graphene monolayer. The G/W/G/W/substrate demonstrated a comparable Raman enhancement effect than the G/W/substrate. These differences are due to the different interlayer couplings in heterostructures related to electron transition probability rates, which are further proved by first-principle calculations and probe-pump measurements.

Flexible, Transparent, and Free-Standing Silicon Nanowire SERS Platform for in Situ Food Inspection.

We demonstrated a flexible transparent and free-standing Si nanowire paper (SiNWP) as a surface enhanced Raman scattering (SERS) platform for in situ chemical sensing on warping surfaces with high sensitivity. The SERS activity has originated from the three-dimension interconnected nanowire network structure and electromagnetic coupling between closely separated nanowires in the SiNWP. In addition, the SERS activity can be highly improved by functionalizing the SiNWP with plasmonic Au nanoparticles. The hybrid substrate not only showed excellent reproducibility and stability of the SERS signal, but also maintained the flexibility and transparency of the pristine SiNWP. To demonstrate its potential application in food inspection, the Au nanoparticles-modified SiNWP was directly wrapped onto the lemon surface for in situ identification and detection of the pesticide residues. The results showed that the excellent SERS activity and transparency of the hybrid substrate enabled the detection of the pesticides down to 72 ng/cm2, which was much lower than the permitted residue dose in food safety.