Strong light-matter interactions in sub-nanometer gaps defined by monolayer graphene: toward highly sensitive SERS substrates

In-situ surface enhanced Raman spectroscopy revealing the role of metal-organic frameworks on photocatalytic reaction selectivity on highly sensitive and durable Cu-CuBr substrate

Photocatalytic reactions using copper-based nanomaterials have emerged as a new paradigm in green technology. Selective photocatalysis is very important for improving energy utilization efficiency, and in order to directional improve catalytic selectivity, it is necessary to understand the mechanism of interfacial reactions at the molecular level. Therefore, a unique bifunctional Cu-CuBr substrate is first fabricated via an electrochemical method, which overcomes the instability of traditional copper-based materials and endows high surface-enhanced Raman spectroscopy (SERS) sensitivity and photocatalytic performance and can be stored stably for more than a year. Further modification of the surface with Metal-Organic Frameworks (MOFs) containing carboxyl functional groups can significantly tune the surface properties of the substrate. This increases the adsorption of cationic dyes to improve the SERS effect, and 10-10 M methylene blue can easily be detected with this substrate. Surprisingly, in-situ SERS monitoring of the interfacial photocatalytic dehalogenation reaction of aromatic halides through its intrinsic SERS effect reveal two competing selective reaction pathways, self-coupling and hydrogenation. Typically, the SERS spectra reveal that the latter's selectivity was greatly enhanced after MOFs modification, and the yield rate of the hydrogenated product increased from 27.6 % to 46.9 % (selectivity increased from 32.7 % to 51.5 %). This proves that the surface properties of catalysts, especially the affinity for reaction intermediates, can effectively regulate catalytic selectivity.

Flexible nano-cloth-like Ag cluster@rGO with ultrahigh SERS sensitivity for capture-optimization-detection due to effective molecule-substrate interactions

Surface-enhanced Raman scattering (SERS) is a rapid and promising detection technique for trace molecules. A central goal of research in this area is to achieve the highly sensitive detection of molecules built on a systematic understanding of enhancement mechanisms. Herein, we develop a Ag cluster@rGO composite nanostructure, which utilizes strong molecular adsorption to achieve ultrahigh SERS sensitivity. Ag clusters are prepared without additional reducing agents, leaving a low carbon footprint in the fabrication process. Finite-difference time-domain (FDTD) simulations show strong electromagnetic field enhancements generated at the edges and interstices of Ag clusters due to the specificity of their structure. Density Functional Theory (DFT) calculations show that the HOMO-LUMO energy gap value is significantly reduced when Ag cluster@rGO forms a composite system with the target molecule, which enables efficient charge transfer between the substrate and molecules, resulting in charge transfer enhancement. A detection limit of 10^-14 M using our substrate can be achieved for the environmental pollutant dye rhodamine 6G (Rh6G). The detection limits of bisphenol A (BPA) and its derivatives reach nanomolar levels with good signal stability. More importantly, we demonstrate the ability to rapidly screen BPA migration in Chinese Baijiu. Our SERS platform can be further developed for environmental pollution control and food safety.

An improved surface enhanced Raman spectroscopic method using a paper-based grape skin-gold nanoparticles/graphene oxide substrate for detection of rhodamine 6G in water and food

Organic toxins are persistent chemicals of global concern capable of accumulating in environment and food. Surface enhanced Raman spectroscopy (SERS) is a promising technique that facilitates onsite detection of organic toxins. However, the fabrication of a SERS substrate is complicated and difficult to provide flexibility, fastness and cost-effectiveness. This study aims to develop a paper-based SERS method using grape skin-gold nanoparticles/graphene oxide (GE-AuNPs/GO) as SERS substrate and evaluate its efficiency with rhodamine 6G (Rh6G) as a model organic toxin and a real water and food contaminant. GE-AuNPs synthesized by green method using grape skin waste extract and GE-AuNPs/GO showed a surface plasmon resonance at 536 and 539 nm, particle size 18.6 and 19.5 nm, and zeta potential -44.6 and -59.7 mV, respectively. Paper-based SERS substrates were prepared by coating a hydrophobic thin-film of 30% polydimethylsiloxane solution in hexane on Whatman no. 1 filter paper, followed by drop-casting GE-AuNPs or GE-AuNPs/GO and drying. The SERS signals of Rh6G showed an enhancement factor of 5.8 × 10⁴ for GE-AuNPs and 1.92 × 10⁹ for GE-AuNPs/GO, implying that a combination of electromagnetic surface plasmon, charge transfer and molecular resonances may be responsible for a higher enhancement of signal by the latter. A low detection limit of 7.33 × 10^-11 M in the linear range of 10^-11-10^-5 M was obtained for GE-AuNPs/GO, while the relative standard deviation of repeatability and reproducibility was 9.6 and 12.6%, respectively. Paper-based GE-AuNPs/GO SERS substrate was highly stable as <20% loss in efficiency was shown over a 60-day storage period. Application to real samples showed a high recovery of Rh6G from tap water (93.9-100.8%) as well as food samples such as red chilli powder (91.0-95.4%), red glutinous rice ball (96.6-98.3%) and tomato ketchup (98.9-102.3%) after QuEChERS extraction. Collectively, the developed paper-based GE-AuNPs/GO can be a potential substrate for sensitive onsite detection of rhodamine 6G by SERS method.

Plasmonic Metal Nanoparticles Hybridized with 2D Nanomaterials for SERS Detection: A Review

In SERS analysis, the specificity of molecular fingerprints is combined with potential single-molecule sensitivity so that is an attractive tool to detect molecules in trace amounts. Although several substrates have been widely used from early on, there are still some problems such as the difficulties to bind some molecules to the substrate. With the development of nanotechnology, an increasing interest has been focused on plasmonic metal nanoparticles hybridized with (2D) nanomaterials due to their unique properties. More frequently, the excellent properties of the hybrids compounds have been used to improve the drawbacks of the SERS platforms in order to create a system with outstanding properties. In this review, the physics and working principles of SERS will be provided along with the properties of differently shaped metal nanoparticles. After that, an overview on how the hybrid compounds can be engineered to obtain the SERS platform with unique properties will be given.

Investigating the thermal transport in gold decorated graphene by opto-thermal Raman technique

We report a systematic study on the thermal transport properties of gold nanoparticles (Au NPs) decorated single-layer graphene on a SiO₂/Si substrate by the opto-thermal Raman technique. Our results, with moderate Au NPs coverage (<10%), demonstrate an enhancement in the thermal conductivity of graphene by ∼55% from its pristine value and a decrement in the interface conductance by a factor of 1.5. A detailed analysis of our results shows the importance of the photo-thermal conversion efficiency of Au NPs, plasmon-phonon coupling and lattice modifications in the graphene developed after gold nanoparticles deposition in enhancing the thermal conductivity and reducing the interface thermal conductance of the system. Our study paves way for a better understanding of the thermal management in such hybrid systems, which are envisioned as excellent candidates for optoelectronics and photonics applications.

Fabrication of a surface-enhanced Raman spectroscopy-based analytical method consisting of multifunctional DNA three-way junction-conjugated porous gold nanoparticles and Au-Te nanoworm for C-reactive protein detection

C-Reactive protein (CRP) is a biomarker of inflammatory responses and an index for assessing the risk of cardiovascular disease and estimating prognosis. In this study, we constructed a surface-enhanced Raman spectroscopy (SERS) biosensor composed of a multifunctional DNA three-way junction (DNA 3WJ), porous gold nanoplates (pAuNPs), and an Au-Te nanoworm structure for detection of CRP. The pAuNP and Au-Te nanostructures were synthesized by galvanic replacement reactions, and the morphology was confirmed by transmission electron microscopy, scanning electron microscopy, and dynamic light scattering (DLS). To generate the SERS signal, the Au-Te nanostructure was immobilized on an indium-tin oxide substrate, and the thiol-modified CRP aptamer was then self-assembled onto the modified substrate for CRP recognition. To amplify the SERS signal and identify the Raman tag, the multifunctional DNA 3WJ was conjugated with the pAuNPs, and each fragment of 3WJ was functionalized to biotin (pAuNP conjugation), methylene blue (Raman reporter), and CRP aptamer (target binding). The results were confirmed by gel electrophoresis. For conjugation between pAuNPs and DNA 3WJ, avidin was encapsulated in pAuNPs, and the conjugation structure was confirmed by DLS. The fabricated SERS biosensor showed detection limits of 2.23 pM in phosphate-buffered saline and 3.11 pM in diluted human serum. Overall, the proposed biosensor may have potential applications as a SERS biosensor platform.

Sensitive, Reusable, Surface-Enhanced Raman Scattering Sensors Constructed with a 3D Graphene/Si Hybrid

Surface-enhanced Raman scattering (SERS) substrates based on graphene and its derivatives have recently attracted attention among those interested in the detection of trace molecules; however, these substrates generally show poor uniformity, an unsatisfactory enhancement factor, and require a complex fabrication process. Herein, we design and fabricate three-dimensional (3D) graphene/silicon (3D-Gr/Si) heterojunction SERS substrates to detect various types of molecules. Notably, the detection limit of 3D-Gr/Si can reach 10^-10 M for rhodamine 6G (R6G) and rhodamine B (RB), 10^-7 M for crystal violet (CRV), copper(II) phthalocyanine (CuPc), and methylene blue (MB), 10^-8 M for dopamine (DA), 10^-6 M for bovine serum albumin (BSA), and 10^-5 M for melamine (Mel), which is superior to most reported graphene-based SERS substrates. Besides, the proposed 3D-Gr/Si heterojunction SERS substrates can achieve a high uniformity with relative standard deviations (RSDs) of less than 5%. Moreover, the 3D-Gr/Si SERS substrates are reusable after washing with ethyl alcohol to remove the adsorbed molecules. These excellent SERS performances are attributed to the novel 3D structure and abundantly exposed atomically thin edges, which facilitate charge transfer between 3D-Gr and probe molecules. We believe that the 3D-Gr/Si heterojunction SERS substrates offer potential for practical applications in biochemical molecule detection and provide insight into the design of high-performance SERS substrates.

Three-dimensional SERS sensor based on the sandwiched G@AgNPs@G/PDMS film

Three-dimensional (3D) SERS substrate with the denser "hotspots" is synthesized by the constriction of PDMS film decorated with sandwiched graphene@AgNPs@graphene (G@AgNPs@G) nanostructure. Graphene layers above and below the AgNPs are used to absorb molecules onto the "hotspots", and prevent the oxidation of AgNPs in our design. PDMS films can be easily shrunk for 3D structures, causing advantages in enhancement ability and light-matter interaction. Benefiting from the above advantages, a detection limit of 10^-14 M (CV) and enhancement factor (EF) of 3.9 × 10⁹ were obtained in our experiment. Theoretical analyses (FDTD) were also used to study the enhancement mechanism. For practical purposes, in-situ detection of MG molecules on the fish surface and the label-free detection of DNA base of adenine (A) and cytosine (C) were also studied. The high enhancement factor, great sensitivity, reliability, and stability of substrate reasonably proved that it can be used as an excellent SERS substrate for biomolecular detection.

Silver nanoparticles decorated g-C3N4: An efficient SERS substrate for monitoring catalytic reduction and selective Hg2+ions detection

Graphitic carbon nitride supported Ag NPs(AgNPs@g-C₃N₄) were synthesized by an in-situ chemical reduction using a green reducing agent, tannic acid. They were characterized by UV-Vis, FTIR, XPS, XRD, FESEM, EDAX and HRTEM. They were very much SERS sensitive, and capable of detecting methylene blue and 4-aminothiophenol at 1 × 10^-12 M and 1 × 10^-10 M, respectively with the corresponding SERS enhancement factor of 1.4 × 10⁸ and 4.7 × 10⁷. Apart from its high SERS sensitivity, it exhibited high catalytic activity for the reduction of MB with NaBH₄. So, their SERS activity and catalytic activity were combined successfully to monitor catalytic reduction of MB by SERS technique. Further, the SERS activity towards MB was also employed for the detection/quantification of free Hg²⁺ ions in aqueous solution. The SERS intensity of MB drastically decreased in the presence of Hg²⁺ ions, and hence it provides novel route to detect and quantify the latter. Presence of Ca²⁺, Mg²⁺, Cu²⁺ and Cd²⁺ions showed zero interference for it. So, this study proves that Ag NPs@g-C₃N₄ as a unique substrate for multiple SERS applications.

Ultra-sensitive SERS detection, rapid selective adsorption and degradation of cationic dyes on multifunctional magnetic metal-organic framework-based composite

In-situ and real-time ultra-sensitive monitoring for the degradation process of environmental pollutants is always an important issue of concern to many people. Herein, a multifunctional magnetic metal-organic framework (MOF)-based composite has been successfully constructed and applied in monitoring the disposal of cationic dyes. Owing to its particular MOFs shell and internal gold particles, the composite can be used as an efficient SERS substrate to ultra-sensitively detect the cationic dyes. Furthermore, the prepared MOF-based composite is also a peroxidase-like nanozyme, which can catalytically degrade the adsorbed cationic dyes. Additionally, the magnetic core in the MOF-based composite offers a good magnetic separation capacity, which makes a facile and rapid separation of the catalyst from the reacted solution for recyclability. This work has provided a new way to monitor the catalytic degradation process by SERS technique in the co-existence of catalyst and dye molecules in the reaction system, which can effectively eliminate the absorption of the catalyst compared with the UV-vis technique, showing promising applications in in-situ and real-time pollution disposal monitoring.

Fabrication of Bioprobe Self-Assembled on Au-Te Nanoworm Structure for SERS Biosensor

In the present study, we propose a novel biosensor platform using a gold-tellurium (Au-Te) nanoworm structure through surface-enhanced Raman spectroscopy (SERS). Au-Tenanoworm was synthesized by spontaneous galvanic replacement of sacrificial Te nanorods templated with Au (III) cations under ambient conditions. The fabricated Au-Te nanoworm exhibited an interconnected structure of small spherical nanoparticles and was found to be effective at enhancing Raman scattering. The Au-Te nanoworm-immobilized substrate exhibited the ability to detect thyroxine using an aptamer-tagged DNA three-way junction (3WJ) and glycoprotein 120 (GP120) human immunodeficiency virus (HIV) using an antibody. The modified substrates were investigated by scanning electron microscopy and atomic force microscopy (AFM). The optimal Au-Te nanoworm concentration and immobilization time for the thyroxine biosensor platform were further determined by SERS experimentation. Thus, the present study showed that the Au-Te nanoworm structure could be applied to various biosensor platforms.

Magnetic assembled 3D SERS substrate for sensitive detection of pesticide residue in soil

Three-dimensional (3D) surface enhanced Raman scattering (SERS) substrates were produced by magnetic force assisting self-assembled nanoparticles in arrayed holes. Compared to '2D' plasmonic structures used in conventional SERS substrates, the 'hot spots' existed on whole depth of the 3D SERS substrates, which greatly enhanced the sensitivity. The prepared 3D SERS substrate was able to detect 4-aminothiophenol with a concentration down to 1 pM. Furthermore, the substrate was applied to detect hexachlorobenzene residue in soil, indicating its great potential for rapid and sensitive detection of extreme low concentrated molecules, especially pollutants residues in foods and environments.

Graphene Plasmonic Fractal Metamaterials for Broadband Photodetectors

Metamaterials have recently established a new paradigm for enhanced light absorption in state-of-the-art photodetectors. Here, we demonstrate broadband, highly efficient, polarization-insensitive, and gate-tunable photodetection at room temperature in a novel metadevice based on gold/graphene Sierpinski carpet plasmonic fractals. We observed an unprecedented internal quantum efficiency up to 100% from the near-infrared to the visible range with an upper bound of optical detectivity of 10¹¹ Jones and a gain up to 10⁶, which is a fingerprint of multiple hot carriers photogenerated in graphene. Also, we show a 100-fold enhanced photodetection due to highly focused (up to a record factor of |E/E₀| ≈ 20 for graphene) electromagnetic fields induced by electrically tunable multimodal plasmons, spatially localized in self-similar fashion on the metasurface. Our findings give direct insight into the physical processes governing graphene plasmonic fractal metamaterials. The proposed structure represents a promising route for the realization of a broadband, compact, and active platform for future optoelectronic devices including multiband bio/chemical and light sensors.

Graphene oxide-highly anisotropic noble metal hybrid systems for intensified surface enhanced Raman scattering and direct capture and sensitive discrimination in PCBs monitoring

Graphene oxide (GO)-anisotropic noble metal hybrid systems were developed as highly sensitive and reproducible surface enhanced Raman scattering (SERS) platform, in which ultrathin GO was embedded between two metallic layers of flower-like Ag nanoparticles (AgNFs) and gold nanostars (AuNSts). Due to multi-dimensional plasmonic coupling effect, the well-designed AgNFs-GO-AuNSts sandwich structures possessed ultrahigh sensitivity with the detection limit of R6G as low as 1.0 × 10^-13 M and high enhancement factor of 2.59 × 10⁷. Additionally, the GO interlayer could function as protective shell to suppress the oxidation of bottom silver layer and efficiently position the target analytes within hot spots. These features endow the substrate with high stability and excellent reproducibility (Signal variations < 7%). Particularly, the GO sandwiched substrate can be explored for the direct capture and sensitive detection of polychlorinated biphenyls (PCBs) without any organic modifier as molecule harvester. This minimum detected concentration was estimated as low as 3.4 × 10^-6 M. The detection method based on GO mediated sandwich substrate avoids complicated surface modification manipulations and improves the substrate cleanness. Moreover, the resultant sandwich substrates can be used to recognize fingerprint peaks of different PCBs in their complex mixture, revealing great potential applications in SERS-based simultaneous detection of multiple pollutants with low affinity.

Two-dimensional MgX2Se4 (X = Al, Ga) monolayers with tunable electronic properties for optoelectronic and photocatalytic applications

Two new two-dimensional (2D) layered materials, namely, MgX₂Se₄ (X = Al, Ga) monolayers, are predicted to possess novel electronic properties. Ab initio electronic structure calculations show that both MgAl₂Se₄ and MgGa₂Se₄ monolayers are direct-gap semiconductors with bandgaps of 3.14 eV and 2.34 eV, respectively. The bandgap of both 2D materials is very sensitive to the in-plane biaxial strain, while the strain induced bandgap changes allow the tuning of optical absorption from the violet to green-light region. Also importantly, the in-plane electron mobility of both 2D materials is predicted to be as high as ∼0.7-1.0 × 10³ cm² V^-1 s^-1, notably higher than that of the MoS₂ sheet (∼200 cm² V^-1 s^-1), while it is comparable to that of black phosphorene (∼1000 cm² V^-1 s^-1), suggesting their potential application in n-type field-effect transistors. Moreover, suitable bandgap and band-edge alignment make the monolayer MgX₂Se₄ a potential photocatalyst for water splitting. Lastly, we show that MgX₂Se₄ possesses a lower monolayer cleavage energy than that of graphite, indicating easy exfoliation of MgX₂Se₄ layers from their bulk.

Toward the highly sensitive SERS detection of bio-molecules: the formation of a 3D self-assembled structure with a uniform GO mesh between Ag nanoparticles and Au nanoparticles

We report a structure to form a hybrid system in which a mesh is sandwiched between Au nanoparticles (AuNPs) and Ag nanoparticles (AgNPs). This self-assembly method uses smaller and denser AgNPs "hot spots" that are spin-coated on a AuNPs@GO mesh nanostructure formed by the reaction of GO@MoS₂ and HAuCl₄ to form AuNPs@GO mesh@AgNPs SERS substrates. Sub-40-nm mesh and 10-nm gaps ensure the landing sites and spacing of the AgNPs. Consequently, the design integrates the strong plasmonic effects of AgNPs and AuNPs with the biological compatibility of the GO mesh. Crystal violet (CV) as low as 10^-15 M can be detected, which confirms the ultrahigh sensitivity of AuNPs@GO mesh@AgNPs. Furthermore, the reproducibility, stability, and finite-difference time-domain (FDTD) simulations confirm the value of this SERS substrate. This material can be used for label-free DNA detection, and the AuNPs@GO mesh@AgNPs substrate facilitated single-molecule DNA detection limits.

Ultrasensitive and Stable Plasmonic Surface-Enhanced Raman Scattering Substrates Covered with Atomically Thin Monolayers: Effect of the Insulating Property

We demonstrated the effects of monolayer graphene and hexagonal boron nitride (h-BN) on the stability and detection performance of two types of substrates in surface-enhanced Raman scattering (SERS): a two-dimensional (2D) monolayer/Ag nanoparticle (NP) substrate and a Au NP/2D monolayer/Ag NP substrate. Graphene and h-BN, which have different electrical and chemical properties, were introduced in close contact with the metal NPs and had distinctly different effects on the plasmonic near-field interactions between metal NPs in the subnanometer-scale gap and on the electron transport behavior. A quantitative comparison was possible due to reproducible SERS signals across the entire substrates prepared by simple and inexpensive fabrication methods. The hybrid platform, an insulating h-BN monolayer covering the Ag NP substrate, ensured the long-term oxidative stability for over 80 days, which was superior to the stability achieved using conducting graphene. Additionally, a sandwich structure using an h-BN monolayer exhibited excellent SERS sensitivity with a detection limit for rhodamine 6G as low as 10^-12 M; to the best of our knowledge, this is the best SERS detection limit achieved using monolayer h-BN as a gap-control material. In this study, we suggest an efficient strategy for hybridizing the desired 2D layers with metal nanostructures for SERS applications, where the substrate stability and electromagnetic field enhancement are particularly crucial for the various applications that utilize metal/2D hybrid structures.

Nanomaterial-based SERS sensing technology for biomedical application

Over the past few years, nanomaterial-based surface-enhanced Raman scattering (SERS) detection has emerged as a new exciting field in which theoretical and experimental studies of the structure and function of nanomaterials have become a focus.

Dual-plasmonic Au/graphene/Au-enhanced ultrafast, broadband, self-driven silicon Schottky photodetector

High-performance photodetectors are desirable for various applications, including multi-wavelength image sensing, communication, and safety monitoring. In this study, we report the construction of a dual-surface plasmon-enhanced silicon Schottky photodetector using Au nanoparticles (NPs)/graphene/Au NPs hybrid structure as the electrode. It was found that the as-assembled device exhibited broad sensitivity, ranging from ultraviolet to near-infrared light (360-1330 nm) at room temperature, with a high response speed of 360 ns and a 3 dB bandwidth of 780 kHz at zero bias. Further theoretical simulation based on the finite-element method revealed that good device performance is associated with the contribution of the Au NPs/graphene/Au NPs electrode: intense dual-plasmonic resonance coupling is induced in a hybrid structure of two layers of metallic NPs separated by a uniform monolayer graphene. It not only can enhance light trapping and the localized electric field at the resonant and off-resonant wavelength regions, but is also beneficial for the tunneling of hot electrons. This work demonstrated the great potential of dual-plasmonic resonance coupling in optoelectronic devices and will lead to the development of advanced plasmonic devices.

Tunable broadband terahertz absorber based on multilayer graphene-sandwiched plasmonic structure

We numerically demonstrate a tunable broadband terahertz absorber with near-unity absorption by using multilayer graphene ribbons sandwiched in a plasmonic integrated structure. By stacking slightly different widths of graphene ribbons in a sandwiched configuration, the absorption bandwidth can be increased because of the different resonant modes closely positioned together. The absorption spectrum's center frequency can be manipulated by varying the graphene's chemical potential, which provides a flexible way to design and optimize absorption property after fabrication. Furthermore, the structure can tolerate a wide range of incident angles, while the improved structure with graphene nanoparticles also shows polarization-independent feature. In this routine, stacking more graphene ribbons or particles with well-designed dimensions can further increase the bandwidth, as long as the metamaterial dimension satisfies the sub-wavelength condition. Therefore, our research provides an important theoretical guide for designing various graphene-based tunable broadband absorbers at terahertz, infrared, and microwave frequencies. This may have promising applications in imaging, sensing, and novel optoelectronic devices.

Nanocavity-in-Multiple Nanogap Plasmonic Coupling Effects from Vertical Sandwich-Like Au@Al2O3@Au Arrays for Surface-Enhanced Raman Scattering

The development of ideal three-dimensional (3D) tailorable surface-enhanced Raman scattering (SERS) substrates with the properties of timesaving, large area, high throughput, single or few molecules detection, reproducibility, reusable ability, and high density of "hot spots" has been the mainstream challenge and the robust task. Here, we construct perpendicular sandwich-like Au@Al₂O₃@Au hybrid nanosheets (PSHNs) on the Al foil as a 3D flexible substrate for SERS. The design of 3D PSHNs incorporates several advantageous aspects for SERS to enhance the performance of plasmonic diamers via bifunctions of vertical Al₂O₃ nanosheets (NSs) including the nanoscaffold and nanobaffle plate effects. As a nanoscaffold, it increases the space utilization of Au-Au diamers, whereas as a nanobaffle, it forms densely homogeneous Au@Al₂O₃@Au nanojunctions by sub-4 nm thickness of Al₂O₃ NSs as the dielectric isolated layer for the double-sided exposure of slitlike surface plasmon resonance. The optimized PSHN substrate exhibits a fascinating SERS sensitivity with an experimental enhancement factor of 10¹² and is able to detect rhodamine B at an extremely low concentration up to the limit of single or few molecules (10^-18 M), as well as can be recycled without the loss of SERS enhancement via the simple impregnation process. These advantages will greatly facilitate the wider use of SERS in many fields.

Recent progress on graphene-based substrates for surface-enhanced Raman scattering applications

Graphene-based SERS substrates are classified and introduced, and their applications in biosensing-related fields are reviewed.

Improved Thermal Stability of Graphene-Veiled Noble Metal Nanoarrays as Recyclable SERS Substrates

The ability to enhance the heat resistance of noble metals is vital to many industrial and academic applications. Because of its exceptional thermal properties, graphene was used to enhance the thermal stability of noble metals. Monolayer graphene-covered noble metal triangular nanoarrays (TNAs) showed excellent heat resistance, which could maintain their original triangular nanoarrays at high temperatures, whereas bare noble metal TNAs all agglomerate into spherical nanoparticles. On the basis of this mechanism, we obtained a universal recyclable surface-enhanced Raman scattering (SERS) substrate; after 16 cycles, the SERS substrate still worked well. The improvement of the heat resistance of noble metals by graphene has a great significance to the working reliability and service life of electronic devices and the single-use problem of traditional SERS substrates.

Scalable fabrication of the graphitic substrates for graphene-enhanced Raman spectroscopy

We propose direct synthesis of ultra-thin graphitic films on a dielectric substrate using sacrificial Ni catalyst layer, which significantly increases the crystallinity of the photoresist pyrolyzed at the temperature of 800 °C and above. A considerable amount of multilayer graphene in the photoresist film pyrolyzed in the presence of the Ni catalyst gives rise to an enhancement of the Raman signal of dye Sudan III molecules deposited on the substrate. We demonstrate comparable enhancement of the Raman signal from Sudan III molecules deposited on the fabricated graphitic substrate and those deposited on graphene, which was conventionally transferred to the silica substrate.

Toward highly sensitive surface-enhanced Raman scattering: the design of a 3D hybrid system with monolayer graphene sandwiched between silver nanohole arrays and gold nanoparticles

We report a novel graphene-metal hybrid system by introducing monolayer graphene between gold nanoparticles (Au NPs) and silver nanohole (Ag NH) arrays. The design incorporates three key advantages to promote the surface-enhanced Raman scattering (SERS) sensing capacity: (i) making full use of the single-atomic feature of graphene for generating uniform sub-nanometer spaces; (ii) maintaining the bottom layer of Ag nanoarrays with an ordered manner for facilitating the transfer of graphene films and assembly of the top layer of Au NPs; (iii) integrating the advantages of the strong plasmonic effect of Ag, the chemical stability of Au, as well as the mechanical flexibility and biological compatibility of graphene. In this configuration, the plasmonic properties can be fine-tuned by separately optimizing the horizontal or vertical gaps between the metal NPs. Exactly, sub-20 nm spaces between the horizontally patterned Ag tips constructed by adjacent Ag NHs, and sub-nanometer scale graphene gaps between the vertically distributed Au NP-Ag NH have been achieved. Finite element numerical simulations demonstrate that the multi-dimensional plasmonic couplings (including the Au NP-Au NP, Au NP-Ag NH and Ag NH-Ag NH couplings) promote for the hybrid platform an electric field enhancement up to 137 times. Impressively, the as-prepared 3D Au NP-graphene-Ag NH array hybrid structure manifests ultrahigh SERS sensitivity with a detection limit of 10^-13 M for R6G molecules, as well as good reproducibility and stability. This work represents a step towards high-performance SERS substrate fabrication, and opens up a new route for graphene-plasmonic hybrids in SERS applications.

SERS-enhanced piezoplasmonic graphene composite for biological and structural strain mapping

Thin-film optical strain sensors have the ability to map small deformations with spatial and temporal resolution and do not require electrical interrogation. This paper describes the use of graphene decorated with metallic nanoislands for sensing of tensile deformations of less than 0.04% with a resolution of less than 0.002%. The nanoisland-graphene composite films contain gaps between the nanoislands, which when functionalized with benzenethiolate behave as hot spots for surface-enhanced Raman scattering (SERS). Mechanical strain increases the sizes of the gaps; this increase attenuates the electric field, and thus attenuates the SERS signal. This compounded, SERS-enhanced "piezoplasmonic" effect can be quantified using a plasmonic gauge factor, and is among the most sensitive mechanical sensors of any type. Since the graphene-nanoisland films are both conductive and optically active, they permit simultaneous electrical stimulation of myoblast cells and optical detection of the strains produced by the cellular contractions.

Sandwiched gold/PNIPAm/gold microstructures for smart plasmonics application: towards the high detection limit and Raman quantitative measurements

A smart plasmonic SERS platform, comprising a layer of a stimuli-responsive polymer sandwiched between two gold layers, is reported.

A three-dimensional Au nanoparticle–monolayer graphene–Ag hexagon nanoarray structure for high-performance surface-enhanced Raman scattering

The three-dimensional Au nanoparticle–monolayer graphene–Ag hexagon nanoarray structure exhibits 3107-fold enhancement of the Raman response of graphene and high-performance SERS sensing.

Graphene sandwiched platform for surface-enhanced Raman scattering

The gold nanoparticle–monolayer graphene–silver nanoarray sandwich structure exhibits excellent SERS activity of high sensitivity and good reproducibility.

Morphological modulation of graphene-mediated hybridization in plasmonic systems

We investigated the plasmonic response of a 2-dimensional ordered array of closely spaced (few-nm apart) Au nanoparticles covered by a large-area single-layer graphene sheet. The array consisted of coherently aligned nanoparticle chains, endowed with a characteristic uniaxial anisotropy. The joint effect of such a morphology and of the very small particle size and spacing led to a corresponding uniaxial wrinkling of graphene in the absence of detectable strain. The deposition of graphene redshifted the Au plasmon-resonance, strongly increased the optical absorption of the array and, most importantly, induced a marked optical anisotropy in the plasmonic response, absent in the pristine nanoparticle array. The experimental observations are accounted for by invoking a graphene-mediated resistive coupling between the Au nanoparticles, where the optical anisotropy arises from the wrinkling-induced anisotropic electron mobility in graphene at optical frequencies.

Optical responses of a metal with sub-nm gaps

If the size of a metallic structure is reduced to be comparable to or even smaller than the typical quantum-mechanical lengths such as the Fermi wavelength or Thomas-Fermi wavelength, the electronic structure and optical responses are modulated by quantum effects. Here, we calculate the optical responses of a metal with sub-nm gaps using the eigenstates obtained from an effective-mass quantum theory. According to our simulation, the dielectric responses can be significantly modified by tuning the inter-gap distances. Remarkably, sub-nm gaps occupying a 0.3% volumetric fraction can elongate the penetration depth by an order of magnitude in the terahertz regime. We find that the detailed dependences of electron-photon interaction matrix elements on the involved electronic wavefunctions play an important role in the optical responses. The results draw our attention to these recently fabricated systems.

Controlled Veiling of Silver Nanocubes with Graphene Oxide for Improved Surface-Enhanced Raman Scattering Detection

Hybrid graphene oxide (GO)/metal nanocomposites have been recently proposed as novel surface-enhanced Raman scattering (SERS) substrates. Despite an increasing interest in these systems, standardization in their fabrication process is still lacking but urgently required to support their use for real-life applications. In this work we investigate how the assembly of GO should be conducted to control adsorption geometry and optical properties at the interface with plasmonic nanostructures as monolayer assemblies of silver nanocubes, by tuning main experimental parameters including GO concentration and self-assembly time. We finally identified the experimental conditions for building up a close-fitting soft dressing of the plasmonic surface, which shows optimal characteristics for flexible and reliable SERS detection.

Mangifera indica, Ficus religiosa and Polyalthia longifolia leaf extract-assisted green synthesis of graphene for transparent highly conductive film

A green approach to synthesize transparent conducting films of graphene nanosheets by reduction of graphene oxide is presented.

Enhanced spatial near-infrared modulation of graphene-loaded perfect absorbers using plasmonic nanoslits

Modulating spatial near-infrared light for ultra-compact electro-optic devices is a critical issue in optical communication and imaging applications. To date, spatial near-infrared modulators based on graphene have been reported, but they showed limited modulation effects due to the relatively weak light-graphene interaction. In combination with graphene and metallic nanoslits, we design a kind of ultrathin near-infrared perfect absorber with enhanced spatial modulation effects and independence on a wide range of incident angles. The modulated spectral shift of central wavelength is up to 258.2 nm in the near-infrared range, which is more promising in applications than state-of-the-art devices. The modulation enhancement is attributed to the plasmonic nanoslit mode, in which the optical electric field is highly concentrated in the deep subwavelength scale and the light-graphene interaction is significantly strengthened. The physical insight is deeply revealed by a combination of equivalent circuit and electromagnetic field analysis. The design principles are not only crucial for spatial near-infrared modulators, but also provide a key guide for developing active near-infrared patch nanoantennas based on graphene.

Simulation Evidence of Hexagonal-to-Tetragonal ZnSe Structure Transition: A Monolayer Material with a Wide-Range Tunable Direct Bandgap

2D material with tunable direct bandgap in the intermediate region (i.e., ≈2-3 eV) is key to the achievement of high efficiency in visible-light optical devices. Herein, a simulation evidence of structure transition of monolayer ZnSe from the experimental pseudohexagonal structure to the tetragonal structure (t-ZnSe) under lateral pressure is shown, suggesting a possible fabrication route to achieve the t-ZnSe monolayer. The as-produced t-ZnSe monolayer exhibits highly tunable bandgap under the biaxial strains, allowing strain engineering of t-ZnSe's bandgap over a wide range of 2-3 eV. Importantly, even under the biaxial strain up to 7%, the t-ZnSe monolayer still keeps its direct-gap property in the desirable range of 2.40-3.17 eV (corresponding to wavelength of green light to ultraviolet). The wide-range tunability of direct bandgap appears to be a unique property of the t-ZnSe monolayer, suggesting its potential application as a light-emitting 2D material in red-green-blue light emission diodes or as complementary light-absorption material in the blue-yellow region for multijunction solar cells. The straddling of the band edge of the t-ZnSe monolayer over the redox potential of water splitting reaction also points to its plausible application for visible-light-driven water splitting.

Surface-enhanced Raman scattering from AgNP-graphene-AgNP sandwiched nanostructures

We developed a facile approach toward hybrid AgNP-graphene-AgNP sandwiched structures using self-organized monolayered AgNPs from wet chemical synthesis for the optimized enhancement of the Raman response of monolayer graphene. We demonstrate that the Raman scattering of graphene can be enhanced 530 fold in the hybrid structure. The Raman enhancement is sensitively dependent on the hybrid structure, incident angle, and excitation wavelength. A systematic simulation is performed, which well explains the enhancement mechanism. Our study indicates that the enhancement resulted from the plasmonic coupling between the AgNPs on the opposite sides of graphene. Our approach towards ideal substrates offers great potential to produce a "hot surface" for enhancing the Raman response of two-dimensional materials.

Nitrogen-doped graphene network supported copper nanoparticles encapsulated with graphene shells for surface-enhanced Raman scattering

In this study, we demonstrated nitrogen-doped graphene network supported few-layered graphene shell encapsulated Cu nanoparticles (NPs) (Cu@G-NGNs) as a sensing platform, which were constructed by a simple and scalable in situ chemical vapor deposition (CVD) technique with the assistance of a self-assembled three-dimensional (3D) NaCl template. Compared with pure Cu NPs and graphene decorated Cu NPs, the graphene shells can strengthen the plasmonic coupling between graphene and Cu, thereby contributing to an obvious improvement in the local electromagnetic field that was validated by finite element numerical simulations, while the 3D nitrogen-doped graphene walls with a large surface area facilitated molecule adsorption and the doped nitrogen atoms embedded in the graphene lattice can reduce the surface energy of the system. With these merits, a good surface enhanced Raman spectroscopy (SERS) activity of the 3D Cu@G-NGN painting film on glass was demonstrated using rhodamine 6G and crystal violet as model analytes, exhibiting a satisfactory sensitivity, reproducibility and stability. As far as we know, this is the first report on the in situ synthesis of nitrogen-doped graphene/copper nanocomposites and this facile and low-cost Cu-based strategy tends to be a good supplement to Ag and Au based substrates for SERS applications.

Graphene-based hybrid films for plasmonic sensing

Graphene, a one-atomic-layer-thick planar sheet of sp(2)-bonded carbon configured in a two-dimensional hexagonal lattice, has attracted considerable research interest with regard to sensing-related applications owing to its extraordinary electronic, optical, chemical, and mechanical properties. Graphene plasmonics may be excited in the mid-infrared-to-terahertz regions with high spatial confinement, low loss, and excellent tunability. Meanwhile, graphene can be utilized to tune the plasmonic properties of conventional metallic nanostructures in the visible and near-infrared regions, allowing it to act as a versatile component in various plasmonic applications. This article reviews the recent progress in graphene-based hybrid films used for plasmonic sensing and detection. We particularly emphasize on the unique roles and advantages of graphene in surface-enhanced Raman scattering (SERS) for bare graphene or graphene-metal hybrid films, and plasmonic refractive index (RI) sensing for graphene-metal or graphene-insulator hybrids, among other plasmonic sensing applications. The preparation of graphene-based hybrid films, their functionalization and signal detection techniques are also reviewed. Finally, the perspectives and current challenges in the use of graphene-based hybrid films for plasmonic sensing are outlined.

Electromagnetic Saturation of Angstrom-Sized Quantum Barriers at Terahertz Frequencies

Metal-graphene-metal hybrid structures allow angstrom-scale van der Waals gaps, across which electron tunneling occurs. We squeeze terahertz electromagnetic waves through these λ/10 000 000 gaps, accompanied by giant field enhancements. Unprecedented transmission reduction of 97% is achieved with the transient voltage across the gap saturating at 5 V. Electron tunneling facilitated by the transient electric field strongly modifies the gap index, starting a self-limiting process related to the barrier height. Our work enables greater interplay between classical optics and quantum tunneling, and provides optical indices to the van der Waals gaps.

Lighting up the Raman signal of molecules in the vicinity of graphene related materials

Surface enhanced Raman scattering (SERS) is a popular technique to detect the molecules with high selectivity and sensitivity. It has been developed for 40 years, and many reviews have been published to summarize the progress in SERS. Nevertheless, how to make the SERS signals repeatable and quantitative and how to have deeper understanding of the chemical enhancement mechanism are two big challenges. A strategy to target these issues is to develop a Raman enhancement substrate that is flat and nonmetal to replace the conventional rough and metal SERS substrate. At the same time, the newly developed substrate should have a strong interaction with the adsorbate molecules to guarantee strong chemical enhancement. The flatness of the surface allows better control of the molecular distribution and configuration, while the nonmetal surface avoids disturbance of the electromagnetic mechanism. Recently, graphene and other two-dimensional (2D) materials, which have an ideal flat surface and strong chemical interaction with plenty of organic molecules, were developed to be used as Raman enhancement substrates, which can light up the Raman signals of the molecules, and these substrates were demonstrated to be a promising for microspecies or trace species detection. This effect was named "graphene enhanced Raman scattering (GERS)". The GERS technique offers significant advantages for studying molecular vibrations due to the ultraflat and chemically inert 2D surfaces, which are newly available, especially in developing a quantitative and repeatable signal enhancement technique, complementary to SERS. Moreover, GERS is a chemical mechanism dominated effect, which offers a valuable model to study the details of the chemical mechanism. In this Account, we summarize the systematic studies exploring the character of GERS. In addition, as a practical technique, the combination of GERS with a metal substrate incorporates the advantages from both conventional SERS and GERS. The introduction of graphene to the Raman enhancement substrate extended SERS applications in a more controllable and quantitative way. Looking to the future, we expect the combination of the SERS concept with the GERS technology to lead to the solution of some important issues in chemical dynamics and in biological processes monitoring.

An all-copper plasmonic sandwich system obtained through directly depositing copper NPs on a CVD grown graphene/copper film and its application in SERS

A simple, low-cost, all-copper sandwich system has been obtained through directly depositing Cu nanoparticles (NPs) onto a graphene sheet, which has already been grown on a Cu foil (Cu-NGF). The new design inherits two key advantages: (1) the materials of the NGF coupling system are composed of only cheaper Cu instead of Au and Ag, (2) direct fabrication of the system without transferring graphene will greatly lower the fabrication cost. More importantly, the Cu-NFG system shows a high sensitivity in surface-enhanced Raman scattering (SERS) with the highest enhancement factor (EF, over 1.89 × 10(7)) reported to date in Cu plasmonic systems. Experimental and theoretical results reveal that the strong EF is mainly because of the strong near-field coupling between Cu NPs and Cu films at the optimal angle of incidence, opening up a new route for Cu materials in SERS applications.

Hybrid nanostructures for SERS: materials development and chemical detection

This review focuses on recent developments in hybrid and nanostructured substrates for SERS (surface-enhanced Raman scattering) studies. Thus substrates composed of at least two distinct types of materials, in which one is a SERS active metal, are considered here aiming at their use as platforms for chemical detection in a variety of contexts. Fundamental aspects related to the SERS effect and plasmonic behaviour of nanometals are briefly introduced. The materials described include polymer nanocomposites containing metal nanoparticles and coupled inorganic nanophases. Chemical approaches to tailor the morphological features of these substrates in order to get high SERS activity are reviewed. Finally, some perspectives for practical applications in the context of chemical detection of analytes using such hybrid platforms are presented.

Explosive and chemical threat detection by surface-enhanced Raman scattering: a review

Acts of terror and warfare threats are challenging tasks for defense agencies around the world and of growing importance to security conscious policy makers and the general public. Explosives and chemical warfare agents are two of the major concerns in this context, as illustrated by the recent Boston Marathon bombing and nerve gas attacks on civilians in the Middle East. To prevent such tragic disasters, security personnel must be able to find, identify and deactivate the threats at multiple locations and levels. This involves major technical and practical challenges, such as detection of ultra-low quantities of hazardous compounds at remote locations for anti-terror purposes and monitoring of environmental sanitation of dumped or left behind toxic substances and explosives. Surface-enhanced Raman scattering (SERS) is one of todays most interesting and rapidly developing methods for label-free ultrasensitive vibrational "fingerprinting" of a variety of molecular compounds. Performance highlights include attomolar detection of TNT and DNT explosives, a sensitivity that few, if any, other technique can compete with. Moreover, instrumentation needed for SERS analysis are becoming progressively better, smaller and cheaper, and can today be acquired for a retail price close to 10,000 US$. This contribution aims to give a comprehensive overview of SERS as a technique for detection of explosives and chemical threats. We discuss the prospects of SERS becoming a major tool for convenient in-situ threat identification and we summarize existing SERS detection methods and substrates with particular focus on ultra-sensitive real-time detection. General concepts, detection capabilities and perspectives are discussed in order to guide potential users of the technique for homeland security and anti-warfare purposes.

Adjustable plasmonic optical properties of hollow gold nanospheres monolayers and LSPR-dependent surface-enhanced Raman scattering of hollow gold nanosphere/graphene oxide hybrids

Colloidal hollow gold nanospheres with adjustable localized surface plasmon resonance (LSPR) properties were synthesized and self-assembled into HGNs monolayers for investigation of LSPR-dependent surface enhanced Raman scattering (SERS) behavior.

A graphene-interlayered magnetic composite as a multifunctional SERS substrate

Fe₃O₄@Ag-rGO-Au composites for efficient SERS detection of different analytes.

Highly sensitive surface-enhanced Raman scattering based on multi-dimensional plasmonic coupling in Au-graphene-Ag hybrids

We report an efficient surface-enhanced Raman scattering (SERS) substrate by utilizing the multi-dimensional plasmonic coupling in Au nanoparticle (NP)-graphene-Ag NP hybrid structures. An ultrasensitive SERS detection with a limit of down to 10(-13) M has been achieved when the sandwiched hybrid film is fabricated on an Ag substrate.