Surface-Enhanced Raman Scattering Using 2D Materials

The use of surface-enhanced Raman scattering (SERS) as a technique for detecting small amounts of (bio)chemical analytes has become increasingly popular in various fields. While gold and silver nanostructures have been extensively studied as SERS substrates, the availability of other types of substrates is currently expanding the applications of this spectroscopic method. Recently, researchers have begun exploring two-dimensional (2D) materials (e. g., graphene-like nanostructures) as substrates for SERS analysis. These materials offer unique optical properties, a well-defined structure, and the ability to modify their surface chemistry. As a contribution to advance this field, this concept article highlights the significance of understanding the chemical mechanism that underlies the experimental Raman spectra of chemisorbed molecules onto 2D materials' surfaces. Therefore, the article discusses recent advancements in fabricating substrates using 2D layered materials and the synergic effects of using their metallic composites for SERS applications. Additionally, it provides a new perspective on using Raman imaging in developing 2D materials as analytical platforms for Raman spectroscopy, an exciting emerging research area with significant potential.

Surface-Enhanced Raman Spectroscopy Using a Silver Nanostar Substrate for Neonicotinoid Pesticides Detection

Surface-enhanced Raman spectroscopy (SERS) has been introduced to detect pesticides at low concentrations and in complex matrices to help developing countries monitor pesticides to keep their concentrations at safe levels in food and the environment. SERS is a surface-sensitive technique that enhances the Raman signal of molecules absorbed on metal nanostructure surfaces and provides vibrational information for sample identification and quantitation. In this work, we report the use of silver nanostars (AgNs) as SERS-active elements to detect four neonicotinoid pesticides (thiacloprid, imidacloprid, thiamethoxam and nitenpyram). The SERS substrates were prepared with multiple depositions of the nanostars using a self-assembly approach to give a dense coverage of the AgNs on a glass surface, which ultimately increased the availability of the spikes needed for SERS activity. The SERS substrates developed in this work show very high sensitivity and excellent reproducibility. Our research opens an avenue for the development of portable, field-based pesticide sensors, which will be critical for the effective monitoring of these important but potentially dangerous chemicals.

Engineered Two-Dimensional Nanostructures as SERS Substrates for Biomolecule Sensing: A Review

Two-dimensional nanostructures (2DNS) attract tremendous interest and have emerged as potential materials for a variety of applications, including biomolecule sensing, due to their high surface-to-volume ratio, tuneable optical and electronic properties. Advancements in the engineering of 2DNS and associated technologies have opened up new opportunities. Surface-enhanced Raman scattering (SERS) is a rapid, highly sensitive, non-destructive analytical technique with exceptional signal amplification potential. Several structurally and chemically engineered 2DNS with added advantages (e.g., π-π* interaction), over plasmonic SERS substrates, have been developed specifically towards biomolecule sensing in a complex matrix, such as biological fluids. This review focuses on the recent developments of 2DNS-SERS substrates for biomolecule sensor applications. The recent advancements in engineered 2DNS, particularly for SERS substrates, have been systematically surveyed. In SERS substrates, 2DNS are used as either a standalone signal enhancer or as support for the dispersion of plasmonic nanostructures. The current challenges and future opportunities in this synergetic combination have also been discussed. Given the prospects in the design and preparation of newer 2DNS, this review can give a critical view on the current status, challenges and opportunities to extrapolate their applications in biomolecule detection.

Plasmonic Core-Shell Nanoparticle Enhanced Spectroscopies for Surface Analysis

Probing the properties and components of reactive surfaces is crucial for illustrating reaction mechanisms. However, common surface analysis techniques are restricted to in situ acquisition of surface information at the molecular scale in the human environment and industrial catalysis processes. Plasmonic spectroscopies are promising tools to solve this problem. This Feature is intended to introduce the plasmonic core-shell nanoparticle enhanced spectroscopies for qualitatively and quantitatively analyzing surface trace species. Four different working modalities are designed for meeting varied needs, involving in situ surface species detection, catalytic process monitoring, labeled sensing, and dual mode analysis. These newly developed plasmonic spectroscopies show great potential not only in fundamental research but also in practical applications.

Large Area Few-Layer Hexagonal Boron Nitride as a Raman Enhancement Material

Increasingly, two-dimensional (2D) materials are being investigated for their potential use as surface-enhanced Raman spectroscopy (SERS) active substrates. Hexagonal Boron Nitride (hBN), a layered 2D material analogous to graphene, is mostly used as a passivation layer/dielectric substrate for nanoelectronics application. We have investigated the SERS activity of few-layer hBN film synthesized on copper foil using atmospheric pressure chemical vapor deposition. We have drop casted the probe molecules onto the hBN substrate and measured the enhancement effect due to the substrate using a 532 nm excitation laser. We observed an enhancement of ≈10³ for malachite green and ≈10⁴ for methylene blue and rhodamine 6G dyes, respectively. The observed enhancement factors are consistent with the theoretically calculated interaction energies of MB > R6G > MG with a single layer of hBN. We also observed that the enhancement is independent of the film thickness and surface morphology. We demonstrate that the hBN films are highly stable, and even for older hBN films prepared 7 months earlier, we were able to achieve similar enhancements when compared to freshly prepared films. Our detailed results and analyses demonstrate the versatility and durability of hBN films for SERS applications.

Half-metallic two-dimensional polyaniline with 3d-transition-metal decoration

Possible half-metallic behavior was explored in 3d-transition-metal (Fe, Co, and Ni) decorated two-dimensional polyaniline (C₃N) on the basis of density-functional theory. 3d-transition-metal atoms would prefer to adsorb on top of the carbon hexagonal ring. The calculated electronic structures suggest the Fe and Co decorated polyanilines ([Formula: see text]Fe and [Formula: see text]Co) are magnetic half-metals, while the Ni-decorated polyaniline ([Formula: see text]Ni) is a nonmagnetic semiconductor with an enlarged band gap. In [Formula: see text]Fe, the half-metallic energy window can be as large as 0.7 eV. Interestingly, there are two half-metallic energy windows with opposite spins near Fermi level in [Formula: see text]Co. The energy windows and band gaps can be modulated by the distance between 3d-transition-metal atoms and C₃N. Due to the large half-metallic energy window and the appropriate band gap, 3d-transition-metal decorated C₃N may be used in nanoscale spintronic devices.

Achieving extremely high optical contrast of atomically-thin MoS2

Extraordinarily high optical contrast is instrumental to research and applications of two-dimensional materials, such as, for rapid identification of thickness, characterisation of optical properties, and quality assessment. With optimal designs of substrate structures and light illumination conditions, unprecedented optical contrast of MoS₂ on Au surfaces exceeding 430% for monolayer and over 2600% for bilayer is achieved. This is realised on custom-designed substrates of near-zero reflectance near the normal incidence. In particular, by using an aperture stop to restrict the angle of incidence, high-magnification objectives can be made to achieve extraordinarily high optical contrast in a similar way as the low-magnification objectives, but still retaining the high spatial resolution capability. The technique will allow small flakes of micrometre size to be located easily and identified with great accuracy, which will have significant implications in many applications.

Controlled nanofabrication of metal-free SERS substrate on few layered black phosphorus by low power focused laser irradiation

Black Phosphorous (BP) has intrinsic in-plane ferroelectric properties that may have the inherent capability of SERS response and can be considered as a replacement of metal nanoparticle-based SERS substrates. A simple one-step process has been demonstrated for the controlled nano-structuring and rapid prototyping on a BP flake to develop a metal-free SERS substrate by low power focused laser irradiation. The effect of focused laser irradiation on the surface morphology of the pristine BP flakes has been thoroughly investigated by real time Raman spectroscopy measurements and corresponding AFM height profiling, which confirms that the proposed laser irradiation technique has more advantages over the conventional lithography and is free from undesired contamination. For a 532 nm laser line, the minimum laser power needed to create a nano-void on the BP flake is 25 mW (Power density = ∼15.62 × 10⁵ W cm^-2) with 5 s exposure time, where the etching rate is controlled by the laser power and exposure time. By analyzing the geometrical shape of the nano-void created due to laser irradiation, it is possible to identify the armchair and zigzag directions of the BP flake. The experimental results revealed that by controlling the exposure time and laser power, it was possible to perform layer by layer thinning of BP flakes. The proposed thinning process of the BP flake did not alter the pristine quality and no signature of oxidation was found in the Raman spectra, which signified the reliability of this low power laser irradiation technique towards the future nano-fabrication of BP-based devices. The controlled formation of the nano-void array on a few layered BP flake enhanced the local electric field (hot spots) in the vicinity of the nano-voids, resulting in ∼30% Raman intensity enhancement. Such nano-void induced hotspots on the BP flake open up a new species of metal-free SERS substrate, demonstrating pronounced enhancement in the Raman signal of Rhodamine B as high as ∼10⁶ and a limit of detection (LOD) up to ∼10 nM.

Recent Advances in 2D Inorganic Nanomaterials for SERS Sensing

Surface-enhanced Raman spectroscopy is a powerful and sensitive analytical tool that has found application in chemical and biomolecule analysis and environmental monitoring. Since its discovery in the early 1970s, a variety of materials ranging from noble metals to nanostructured materials have been employed as surface enhanced Raman scattering (SERS) substrates. In recent years, 2D inorganic materials have found wide use in the development of SERS-based chemical sensors owing to their unique thickness dependent physico-chemical properties with enhanced chemical-based charge-transfer processes. Here, recent advances in the application of various 2D inorganic nanomaterials, including graphene, boron nitride, semiconducting metal oxides, and transition metal chalcogenides, in chemical detection via SERS are presented. The background of the SERS concept, including its basic theory and sensing mechanism, along with the salient features of different nanomaterials used as substrates in SERS, extending from monometallic nanoparticles to nanometal oxides, is comprehensively discussed. The importance of 2D inorganic nanomaterials in SERS enhancement, along with their application toward chemical detection, is explained in detail with suitable examples and illustrations. In conclusion, some guidelines are presented for the development of this promising field in the future.

Plasmonic silvered nanostructures on macroporous silicon decorated with graphene oxide for SERS-spectroscopy

A method for fabricating surface-enhanced Raman scattering (SERS)-active substrates by immersion deposition of silver on a macroporous silicon (macro-PS) template with pore diameters and depth ranging from 500-1000 nm is developed. The procedure for the formation of nanostructured silver films in the layers of macro-PS was optimized. Silver particles of dimensions in the nano- and submicron-scale were formed on the external surface of the macro-PS immersed in the water-ethanol solution of AgNO_3, while the inner pore walls were covered by smaller, 10-30 nm diameter, silver nanoparticles. Upon introducing the hydrofluoric acid to the reaction mixture, the size of nanoparticles grown on the pore walls increased up to 100-150 nm. Such nanostructures were found to yield SERS-signal intensities from CuTMpyP4 analyte molecules of the same order to those obtained from silvered mesoporous silicon reported previously. The tested storage stability for the silvered macro-PS-based samples reached up to 8 months. However, degradation of the SERS intensity under illumination by the laser beam during spectral measurements was observed. To improve the stability of the SERS-signal a hybrid structure involving graphene oxide deposited on the top of analyte molecules adsorbed on the Ag/macro-PS was formed. A systematic observation of the time evolution of the characteristic peak at 1365 cm^-1 showed that the addition of the oxidized graphene layer over the analyte results in ∼2 times slower decay of the Raman intensity, indicating that the graphene coating can be used to enhance the stability of the SERS-signal from the CuTMpyP4 molecules.