Graphene-enhanced Raman Scattering Research Articles

Tuning the Fermi Level of Graphene by Two-Dimensional Metals for Raman Detection of Molecules.

Graphene-enhanced Raman scattering (GERS) offers great opportunities to achieve optical sensing with a high uniformity and superior molecular selectivity. The GERS mechanism relies on charge transfer between molecules and graphene, which is difficult to manipulate by varying the band alignment between graphene and the molecules. In this work, we synthesized a few atomic layers of metal termed two-dimensional (2D) metal to precisely and deterministically modify the graphene Fermi level. Using copper phthalocyanine (CuPc) as a representative molecule, we demonstrated that tuning the Fermi level can significantly improve the signal enhancement and molecular selectivity of GERS. Specifically, aligning the Fermi level of graphene closer to the highest occupied molecular orbital (HOMO) of CuPc results in a more pronounced Raman enhancement. Density functional theory (DFT) calculations of the charge density distribution reproduce the enhanced charge transfer between CuPc molecules and graphene with a modulated Fermi level. Extending our investigation to other molecules such as rhodamine 6G, rhodamine B, crystal violet, and F16CuPc, we showed that 2D metals enabled Fermi level tuning, thus improving GERS detection for molecules and contributing to an enhanced molecular selectivity. This underscores the potential of utilizing 2D metals for the precise control and optimization of GERS applications, which will benefit the development of highly sensitive, specific, and reliable sensors.

A light-mediated covalently patterned graphene substrate for graphene-enhanced Raman scattering (GERS).

We report covalently patterned graphene with acetic acid as a new potential candidate for graphene-enhanced Raman scattering (GERS). Rhodamine 6G molecules in direct contact with the covalently modified region show an enormous enhancement (∼25 times) compared to the pristine region at 532 nm excitation. The GERS enhancement with respect to the layer thickness of the probed molecule, excitation wavelength, and covalently attached groups is discussed.

Grain-Boundary-Induced Ultrasensitive Molecular Detection of Graphene Film.

Graphene has been considered a promising platform for molecular detection due to the graphene-enhanced Raman scattering (GERS) effect. However, the GERS performance of pristine graphene is limited by a low chemically active surface and insufficient density of states (DOS). Although diverse defects have been introduced, it remains a great challenge to improve the enhancement performance. Here, we show that graphene grain boundaries (GBs) possess stronger adsorption capacity and more abundant DOS. Thus, GERS performance increases with the atomic percentage of GBs, which makes nanocrystalline graphene (NG) film a superior GERS substrate. For R6G as a probe molecule, a low detection limit of 3 × 10-10 M was achieved. Utilizing the high chemical activity of GBs, we also fabricated NG film decorated with Au particles using a one-step quenching strategy, and this hybrid film exhibits an extremely low limit of detection down to 5 × 10-11 M, outperforming all the reported graphene-based systems.

(Invited) Multiplexed Label-Free Biosensing Using 2D-Heterostructures: Materials Stability and Signal Uniformity

Two-dimensional materials (2DM) has been already applied for bio- and chemical sensing[1],[2]. Even individual 2D materials often outperform bulk analogues, showing strong response and being compatible with flexible technologies, thus opening horizons for wearable and Point-of-Care applications[3]. Most recently, emergent need to achieve better, more precise and sensitive drug detection in medicine and health care has been addressed by developing new types of biosensors[4]. Extending properties of individual 2DMs by constructing their van der Waals heterostructures, either lateral or vertical, offers new response and/or transduction mechanisms and further improvement of device performance. Such heterostructures have potential for label-free biosensing and could be designed and/or integrated together to generate several signals in response to a single analyte. Such a capability enables a multimodal detection, which exceeds single-mode biosensing through its higher throughput, as well as by better ability to differentiate the analyte from background signals in a complex media, and potentially allows multiplexing (responding by several channels to a group of substances in parallel).Despite significant efforts were invested into discovery of newer and better 2D materials with biosensing capabilities, there is only limited knowledge of factors that could limit its performance. Indeed, atomically thin 2D materials have an ultimate surface-to-volume ratio which helps sensing, but may result in surface non-uniformities at the nanometer scale such as: atomic impurities, adsorbates, single atom and lattice defects, wrinkles and ruptures – to name just a few. Such defects may modulate their sensing properties. Here, a new multidimensional optical imaging technique will be presented which is capable to detect lattice mismatch and work function difference in the heterostructure material. Those result in strain and charge transfer and vary optical response at the nanometer scale, hard to detect and study by normal characterization tools.We present a vertical heterostructure comprised of monolayer graphene and single layer flakes of MoS2. An optical label-free detection of doxorubicin, a common cancer drug, is reported via three independent optical detection channels (photoluminescence shift, Raman shift and Graphene Enhanced Raman Scattering). Non-uniform broadening of components of multimodal signal correlates with the statistical distribution of local optical properties of the 2DM heterostructure. It will be shown how mapping of distribution for doping and strain (taken with sub-diffractional resolution) allows one to understand the role of those for modulation of electronic properties of 2D material.Acknowledgement: partial support is acknowledged from NSF CHE-2032582, CHE-2032601, DMR-1539916 and DMR-2011839 grants. [1] Oh, S.-H.; et.al, Nature Communications 2021, 12, 3824. [2] Bolotsky, A.; et.al, ACS Nano 2019, 13, 9781. [3] Pang, Y.; et.al, Small 2020, 16, 1901124. [4] Moradi, R; Small (2021). doi: 10.1002/smll.202104847

Comparative Evaluation of Graphene Nanostructures in GERS Platforms for Pesticide Detection.

Graphene-enhanced Raman scattering (GERS) produces enhancement of the Raman signal, which is based on chemical rather than electromagnetic mechanism such as in the surface-enhanced Raman scattering. Graphene oxide, amino- and guanidine-functionalized graphene oxide, exfoliated graphene, and commercial graphene nanoplatelets have been used to investigate the GERS response with the change of graphene properties. Different graphene nanostructures have been embedded into organic–inorganic microporous films to build a platform for the fast and sensitive detection of pesticides in water. The graphene nanostructures vary in the number of layers, lateral size, degree of oxidation, and surface functionalization. The GERS performances of the graphene nanostructures cast on silicon substrates and embedded in the nanocomposite films have been comparatively evaluated. After casting a few droplets of the pesticide aqueous solution on the graphene nanostructures, the Raman band enhancements of the analytes have been measured. In the nanocomposite films, the characteristic Raman bands originating from pesticides such as paraoxon, parathion, and glyphosate could be traced at concentrations below 10–7, 10–5, and 10–4 M, respectively. The results show that the surface functionalization reduces the GERS effect because it increases the ratio between the sp3 carbon and sp2 carbon. On the other hand, the comparison among different types of graphenes shows that the monolayers are more efficient than the few-layer nanostructures in enhancing the Raman signal.

Double‐Sided Graphene‐Enhanced Raman Scattering and Fluorescence Quenching in Hybrid Biological Structures

AbstractDue to their large contact and loading surfaces as well as high sensitivities to chemical changes, graphene‐based materials (GBMs) are increasingly being employed into novel nanomedicine technologies. Here biomolecule—monolayer graphene—kidney tissue hybrid structures are studied using mapping micro‐Raman and fluorescence spectroscopies. Because in this configuration graphene interacts with molecules on both sides, a double‐sided graphene‐enhanced Raman scattering (GERS) effect up to ≈10.1 is found for biomolecules adsorbed on graphene and amino acids in the kidney tissue below graphene. Moreover, graphene causes an efficient autofluorescence quenching (FLQ) up to ≈20% emitted by the kidney tissue. Despite the complexity of such layered materials, the intriguing simultaneous occurrence of double‐sided GERS (a new development of GERS) and FLQ phenomena can be well explained by suitable molecular structure and energy level alignment between molecules and graphene. These result in effective charge transfer mediated by non‐covalent interactions as indicated by correlative strain, doping, and defect analyses in graphene based on the Raman data and energy level calculations. Last, the advantages of using graphene over standard photobleaching are demonstrated. This work can be extended to other macromolecular entities toward integrating GBMs in versatile drug delivery, imaging, and sensing devices.

Graphenic substrates as modifiers of the emission and vibrational responses of interacting molecules: The case of BODIPY dyes.

Graphenic substrates (GS), such as reduced graphene oxide (rGO) and graphene oxide (GO), are 2D materials known for their unique physicochemical properties such as their ability to enhance vibrational spectroscopic signals and quench the fluorescence of adsorbed molecules. These properties provide an opportunity to develop nanostructured GS-based systems for detecting and identifying different analytes with high sensitivity and reliability through molecular spectroscopic techniques. This work evaluated the capacities of different GS to interact with a highly fluorescent compound, thereby changing its optical emission response (fluorescence quenching) and amplifying its vibrational signal, which is the base of graphene-enhanced Raman scattering (GERS). To test these properties, we used a derivative of highly fluorescent BODIPY (BP) compounds, which cover a wide range of applications from solar energy conversion to photodynamic cancer therapy. GS prepared by using the Langmuir-Blodgett (LB) technique allowed us to quench the fluorescence emission of BP and improve its Raman spectroscopy detection limit due to the GERS effect. These results were interpreted in light of the π-π interactions taking place between the Csp2 domains of GS and the aromatic core of the BP fluorophore.

Graphene-enhanced Raman scattering on single layer and bilayers of pristine and hydrogenated graphene

Graphene-enhanced Raman scattering (GERS) on isotopically labelled bilayer and a single layer of pristine and partially hydrogenated graphene has been studied. The hydrogenated graphene sample showed a change in relative intensities of Raman bands of Rhodamine 6 G (R6G) with different vibrational energies deposited on a single layer and bilayer graphene. The change corresponds qualitatively to different doping of graphene in both areas. Pristine graphene sample exhibited no difference in doping nor relative intensities of R6G Raman peaks in the single layer and bilayer areas. Therefore, it was concluded that strain and strain inhomogeneities do not affect the GERS. Because of analyzing relative intensities of selected peaks of the R6G probe molecules, it is possible to obtain these results without determining the enhancement factor and without assuming homogeneous coverage of the molecules. Furthermore, we tested the approach on copper phtalocyanine molecules.

Simultaneous Detection and Photocatalysis Performed on a 3D Graphene/ZnO Hybrid Platform.

The synergy between graphene foam (Gf) and ZnO nanoparticles (NPs) allows the detection of analytes at low conentrations, which can be subsequently photocatalyzed on the hybrid surface as well as in the liquid phase upon illumination with low-power UV-vis light-emitting diode (LED) lamps. Detection of methylene blue (MB) and bisphenol A (BPA) is monitored either by graphene-enhanced Raman scattering (GERS) or molecular doping/sensing upon analyte adsorption. Using GERS, we were able to detect concentrations as low as 0.3 ppm of MB, which remained adsorbed on the graphene surface after a photocatalytic conversion of 88% (total conversion). The photocatalysis performances of BPA and MB performed in the liquid phase were lower and corresponded to 73 and 33% as indicated by gas chromatography-mass spectrometry (GC/MS) and UV-vis, respectively. The kinetics of photocatalysis was fitted with a quasi-first-order reaction, and the apparent rate constant (kapp) was calculated according to the Langmuir-Hinshelwood model. The fastest kinetics was achieved with the hybrid platform named "Gf-ZnO400", which was thermally treated at high temperatures and with most of the Ni etched away. This is consistent with the excellent electronic interaction between ZnO and graphene foam as indicated by photoelectrochemistry analysis. We mainly employed Raman scattering and UV-vis spectroscopy analyses for detection and photocatalysis applications; however, we also used other complementary techniques such as focused ion-beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), diffuse reflectance, GC/MS, and photoelectrochemistry to explore the synergetic behavior of these two nanomaterials. This work brings about new insights into the detection of analyte molecules followed by photocatalysis performed in the solid and liquid states.

Perspectives of DCDR-GERS in the analysis of amino acids.

Graphene-enhanced Raman scattering (GERS) has attracted increasing attention from many scientists in recent years as a novel and potentially strong analytical technique since its discovery in 2010. GERS combines the ultra-high sensitivity and spectral analysis of the structural properties of many molecules found in surface-enhanced Raman scattering (SERS) and graphene simple sample processing and excellent uniformity. This facilitates a fast and very sensitive analysis of a wide range of analytes. Here, we present a new method for amino acid analysis based on the combination of drop coating deposition Raman (DCDR) and GERS. The potential of the method was evaluated in the analyses of Trp, Leu, Phe, and DOPA. Achievable limits of detection for all the studied amino acids are in tens of ng mL-1.

Adjusting Fermi Level of Graphene by Controlling the Linker Lengths of Dipolar Molecules.

Graphene modified by azo molecules has attracted much attention due to its promising graphene-enhanced Raman scattering (GERS) effect. Herein, we achieved the shortened distance between the dipole moment and graphene, by reducing the number of methylene groups and the chain length, and studied the Raman scattering in dipolar molecules/graphene hybrids. Whether for low or high concentrations, a more significant GERS signal can be observed, which can be attributed to the effective modulation of the dipole moment on interface energy level matching. Further, by fabricating FET devices, we demonstrate more obvious p-type and n-type doping in graphene that is affected by the chain length of the azobenzene molecule. On the basis of the dependence of the G band shift on the Fermi surface change, we further reveal the important role of dipole moment distance in dipolar molecules/graphene hybrids. These studies provide a new approach to optimize the interface matching between azobenzene and graphene and may help achieve more sensitive GERS detection.

Chemical and Bio Sensing Using Graphene-Enhanced Raman Spectroscopy.

Graphene is a two-dimensional (2D) material consisting of a single sheet of sp2 hybridized carbon atoms laced in a hexagonal lattice, with potentially wide usage as a Raman enhancement substrate, also termed graphene-enhanced Raman scattering (GERS), making it ideal for sensing applications. GERS improves upon traditional surface-enhanced Raman scattering (SERS), combining its single-molecule sensitivity and spectral fingerprinting of molecules, and graphene’s simple processing and superior uniformity. This enables fast and highly sensitive detection of a wide variety of analytes. Accordingly, GERS has been investigated for a wide variety of sensing applications, including chemical- and bio-sensing. As a derivative of GERS, the use of two-dimensional materials other than graphene for Raman enhancement has emerged, which possess remarkably interesting properties and potential wider applications in combination with GERS. In this review, we first introduce various types of 2D materials, including graphene, MoS2, doped graphene, their properties, and synthesis. Then, we describe the principles of GERS and comprehensively explain how the GERS enhancement factors are influenced by molecular and 2D material properties. In the last section, we discuss the application of GERS in chemical- and bio-sensing, and the prospects of such a novel sensing method.

Excitation Wavelength Dependence of Combined Surface- and Graphene-Enhanced Raman Scattering Experienced by Free-Base Phthalocyanine Localized on Single-Layer Graphene-Covered Ag Nanoparticle Arrays

Hybrid systems constituted by plasmonic nanostructures and single-layer graphene (SLG) as well as their employment as platforms for surface-enhanced Raman scattering (SERS) of the molecular species...

Graphene Grown on Ni Foam: Molecular Sensing, Graphene-Enhanced Raman Scattering, and Galvanic Exchange for Surface-Enhanced Raman Scattering Applications

The growth of graphene on an irregular three-dimensional (3D) Ni structure is demonstrated to be an interesting platform for molecular sensing, graphene-enhanced Raman scattering, and surface-enhanced Raman scattering (SERS) applications after galvanic exchange of Ag+ ions. Raman, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX), optical images, and diffuse reflectance demonstrate that graphene grows in a multilayer fashion with different stacking configurations. Statistics performed employing Raman show that the as-grown graphene can be classified into two main stacking configurations: AB (or Bernal stacking) and rotated graphene, which are separated by a full width at half-maximum (fwhm) threshold of ∼30 cm–1 corresponding to the 2D Raman band. The rotated stacking senses low concentrations of methylene blue (MB), whereas the AB stacking seems to be much less sensitive upon molecular adsorption. The galvanic exchange of Ag leads to agglomerates preferentially formed on top of gra...

Reversibility of Graphene‐Enhanced Raman Scattering with Fluorinated Graphene

The effect of the amount of sp3 defects in graphene on the graphene enhanced Raman scattering (GERS) is studied using gradual fluorination of chemical vapour deposition grown graphene covering the Rhodamine 6G molecules. While upon a mild fluorination the GERS effect is preserved, the GERS enhancement disappears in case of a strong fluorination. After partial removal of the fluorine from the strongly fluorinated graphene by water vapor treatment, the GERS effect is restored.

Raman excitation profiles of hybrid systems constituted by single‐layer graphene and free base phthalocyanine: Manifestations of two mechanisms of graphene‐enhanced Raman scattering

The ability of single‐layer graphene (SLG) to enhance Raman scattering of planar aromatic molecules denoted graphene‐enhanced Raman scattering (GERS) is currently the subject of focused interest. We report on manifestations of two mechanisms of GERS in Raman spectra of glass/SLG/free base phthalocyanine (H2Pc) monolayer (ML) hybrid systems: (i) photoinduced charge transfer from SLG Fermi level to LUMO of H2Pc excited at onset of the near IR region, and (ii) modification of resonance Raman scattering of H2Pc in the visible region by SLG–H2Pc interaction resulting into delocalization of the electronic transition over the benzene rings of H2Pc. Glass/SLG/H2Pc hybrid systems with either a bilayer or a monolayer of H2Pc molecules and a graphite/H2Pc (ML) reference system were prepared by a spectrally controlled adsorption–desorption of H2Pc from solution, followed by Raman mapping of samples at excitation wavelengths in the 532–830 nm range, construction of excitation profiles for H2Pc Raman bands of the glass/SLG/H2Pc samples and determination of GERS enhancement factors for the glass/SLG/H2Pc (ML) sample versus the graphite/H2Pc (ML) reference sample (3–24 at 633 nm and 3–19 at 647 nm excitations). Selectivity of the excitation profiles and of the GERS enhancement factors with respect to localization of the vibrational modes within the H2Pc molecule demonstrates involvement of a different resonant electronic transition in each of the two mechanisms of GERS. Copyright © 2017 John Wiley & Sons, Ltd.

Graphene-Based Enhanced Raman Scattering toward Analytical Applications

Due to the single-molecule sensitivity and the capability of chemical fingerprints recognition, surface-enhanced Raman scattering (SERS) has been an attractive analytical technique used in various fields. However, SERS sensing still suffers from several problems, including the heterogeneous adsorption of molecules on SERS substrates, the spectral fluctuation of molecules, the photo/chemical reactions of molecules in direct contact with metal, and the continuum spectral background originated from fluorescence or photocarbonization. Such problems greatly hinder its practical applications, in particular, in SERS quantification. Graphene, the star of the two-dimensional (2D) materials family, can be used for Raman enhancement, termed as graphene-based surface-enhanced Raman scattering (G-SERS). In this review, we will introduce the discovery of graphene-enhanced Raman scattering (GERS), the chemical enhancement, and its extension to other 2D materials beyond graphene. Then we will concentrate on graphene-base...

Ultrasensitive molecular sensor using N-doped graphene through enhanced Raman scattering.

As a novel and efficient surface analysis technique, graphene-enhanced Raman scattering (GERS) has attracted increasing research attention in recent years. In particular, chemically doped graphene exhibits improved GERS effects when compared with pristine graphene for certain dyes, and it can be used to efficiently detect trace amounts of molecules. However, the GERS mechanism remains an open question. We present a comprehensive study on the GERS effect of pristine graphene and nitrogen-doped graphene. By controlling nitrogen doping, the Fermi level (E F) of graphene shifts, and if this shift aligns with the lowest unoccupied molecular orbital (LUMO) of a molecule, charge transfer is enhanced, thus significantly amplifying the molecule's vibrational Raman modes. We confirmed these findings using different organic fluorescent molecules: rhodamine B, crystal violet, and methylene blue. The Raman signals from these dye molecules can be detected even for concentrations as low as 10(-11) M, thus providing outstanding molecular sensing capabilities. To explain our results, these nitrogen-doped graphene-molecule systems were modeled using dispersion-corrected density functional theory. Furthermore, we demonstrated that it is possible to determine the gaps between the highest occupied and the lowest unoccupied molecular orbitals (HOMO-LUMO) of different molecules when different laser excitations are used. Our simulated Raman spectra of the molecules also suggest that the measured Raman shifts come from the dyes that have an extra electron. This work demonstrates that nitrogen-doped graphene has enormous potential as a substrate when detecting low concentrations of molecules and could also allow for an effective identification of their HOMO-LUMO gaps.

Effect of Dipolar Molecule Structure on the Mechanism of Graphene-Enhanced Raman Scattering

Graphene-enhanced Raman scattering (GERS) is a promising characterization technique which uses a single layer of graphene. As the electronic coupling of adsorbates with graphene leads to enhancement in the Raman signal, it is of immense interest to explore the factors that affect the coupling of the adsorbates with graphene. To probe this effect we have designed and synthesized a series of dipolar molecules with the general structure, N-ethyl-N-(2-ethyl(1-pyrenebutyrate)-4-(4-R-phenylazo)aniline) where the R-groups are varied from methoxy (−OCH3), methyl (−CH3), hydrogen (−H), nitrile (−CN), nitro (−NO2) to tricyanofuran (TCF) groups. This systematically changes the dipole moments and electronic/optical band gap of the molecules. By noncovalently interfacing these molecules on graphene, the Raman signal is enhanced by a factor of 40–90 at the excitation wavelength of 532 nm. Measurements of the Raman enhancement factor and Raman cross section are complemented with DFT calculations to correlate the dipole ...

Graphene-Enhanced Raman Scattering from the Adenine Molecules.

An enhanced Raman scattering from a thin layer of adenine molecules deposited on graphene substrate was detected. The value of enhancement depends on the photon energy of the exciting light. The benzene ring in the structure of adenine molecule suggests π-stacking of adenine molecule on top of graphene. So, it is proposed that the enhancement in the adenine Raman signal is explained by the resonance electron transfer from the Fermi level of graphene to the lowest unoccupied molecular orbital (LUMO) level of adenine.