R6G on graphene: high Raman detection sensitivity, yet decreased Raman cross-section

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.

Highly sensitive fluorescent explosives detection via SERS: based on fluorescence quenching of graphene oxide@Ag composite aerogels

High fluorescence background poses a substantial challenge to surface-enhanced Raman scattering (SERS), thereby limiting its broader applicability across diverse domains. In this work, silver nanoparticle (Ag NP)-loaded graphene oxide aerogel nanomaterials (GO-Ag ANM) were prepared for sensitive SERS detection of fluorescent explosive 2,4,8,10-tetranitrobenzo-1,3a,6,6a-tetraazapentaenopyridine (BPTAP) by a fluorescence quenching strategy. By harnessing the fluorescence quenching properties of graphene and the localized surface plasmon resonance of silver nanoparticles, the synthesized aerogels exhibited effective fluorescence quenching and Raman enhancement capabilities when employed for BPTAP analysis with 532 nm laser excitation. Significantly, precise control over the loading quantity of silver nanoparticles (Ag NPs) resulted in the remarkable sensitivity of the surface-enhanced Raman scattering (SERS) effect. This method allowed for the detection of fluorescent explosive BPTAP at an extraordinarily low concentration of 1 × 10-7 M. Furthermore, the approach also demonstrated excellent detection capabilities for the dyes R6G, CV, and RhB. This study offers valuable insights for the sensitive detection of fluorescent molecules.

Bifunctional Mo2N Nanoparticles with Nanozyme and SERS Activity: A Versatile Platform for Sensitive Detection of Biomarkers in Serum Samples

The combined application of nanozymes and surface-enhanced Raman scattering (SERS) provides a promising approach to obtain label-free detection. However, developing nanomaterials with both highly efficient enzyme-like activity and excellent SERS sensitivity remains a huge challenge. Herein, we proposed one-step synthesis of Mo2N nanoparticles (NPs) as a "two-in-one" substrate, which exhibits both excellent peroxidase (POD)-like activity and high SERS activity. Its mimetic POD activity can catalyze the 3,3',5,5'-tetramethylbenzidine (TMB) molecule to SERS-active oxidized TMB (ox-TMB) with high efficiency. Furthermore, combining experimental profiling with theory, the mechanism of POD-like activity and SERS enhancement of Mo2N NPs was explored in depth. Benefiting from the outstanding properties of Mo2N NPs, a versatile platform for indirect SERS detection of biomarkers was developed based on the Mo2N NPs-catalyzed product ox-TMB, which acts as the SERS signal readout. The feasibility of this platform was validated using glutathione (GSH) and target antigens alpha-fetoprotein antigen (AFP) and carcinoembryonic antigen (CEA) as representatives of small molecules with a hydroxyl radical (·OH) scavenging effect and proteins with a low Raman scattering cross-section, respectively. The limits of detection of GSH, AFP, and CEA were as low as 0.1 μmol/L, 89.1, and 74.6 pg/mL, respectively. Significantly, it also showed application in human serum samples with recoveries ranging from 96.0 to 101%. The acquired values based on this platform were compared with the standard electrochemiluminescence method, and the relative error was less than ±7.3. This work not only provides a strategy for developing highly active bifunctional nanomaterials but also manifests their widespread application for multiple biomarkers analysis.

DNA induced CTAB-caped gold bipyramidal nanoparticles self-assembly using for Raman detection of DNA molecules

DNA is an indispensable part of metabolism, which affects many important processes in the body, including gene expression, protein synthesis, and drug delivery. Surface-enhanced Raman spectroscopy (SERS) is one of the most important methods used to study the structure and function of DNA and can obtain rich DNA molecular fingerprints. However, it is still a great challenge to use SERS to directly analyze the characteristic Raman signals of the DNA molecule and achieve rapid and simple detection. Hence, a detection platform based on gold bipyramidal nanoparticles (AuNBs) self-assembly that can be directly used for the detection of DNA molecules without the need for additional aggregators and cleaning agents was designed in this study. The original hexadecyltrimethylammonium bromide (CTAB) of AuNBs can be used as the internal standard for DNA quantification without an additional standard. This is the first time that the Raman signals of the analyte molecule can be obtained directly without labels by using the interaction between the molecule and the enhanced substrate. We used this method to capture the original DNA molecules in methylated DNA, serum, and cell metabolites and obtained spectral data processing results using linear discriminant analysis (LDA). This provides new ideas for the digitization of disease treatment and the study of the metabolic processes of life.

Arrays of Ag-nanoparticles decorated TiO2 nanotubes as reusable three-dimensional surface-enhanced Raman scattering substrates for molecule detection

Three-dimensional surface-enhanced Raman scattering (SERS) substrates usually provide more hot spots in the excitation light beam and higher sensitivity when compared with the two-dimensional counterpart. Here a simple approach is presented for the fabrication of arrays of Ag-nanoparticles decorated TiO2 nanotubes. Arrays of ZnO nanorods were fabricated in advance by a hydrothermal method. Then TiO2 nanotube arrays were achieved by immersing the arrays of ZnO nanorods in an aqueous solution of (NH4)2TiF6 for 1.5 h. Vertically aligned TiO2 nanotube arrays were modified with dense Ag nanoparticles by Ag mirror reaction. High density of Ag nanoparticles decorated on the fabricated TiO2 nanotubes provide plenty of hotspots for Raman enhancement. In addition, the fabricated array of Ag nanoparticles modified TiO2 nanotubes can serve as a reusable SERS substrate because of the photocatalytic activity of the TiO2 nanotubes. The SERS substrate adsorbed with analyte molecules can realize self-cleaning in deionized water after UV irradiation for 2.5 h. The sensitivity of the fabricated SERS substrate was investigated by the detection of organic dye molecules. The detectable concentration limits of rhodamine 6G (R6G), malachite green (MG) and methylene blue (MB) were found to be 10−12 M, 10−9 M and 10−8 M, respectively. The enhancement factor (EF) of the three-dimensional SERS substrate was estimated to be as high as ∼1.4×108. Therefore, the prepared Ag nanoparticles modified TiO2 nanotube arrays have promising potentials to be applied to rapid and trace SERS detection of organic chemicals.

AgNPs and MIL-101(Fe) self-assembled nanometer materials improved the SERS detection sensitivity and reproducibility

Recently, in the research of Surface-enhanced Raman scattering (SERS) technology, it is found that the preparation of enhanced substrate is particularly important. In this work, the most commonly used methods were used to synthesize AgNPs and MIL-101(Fe), and AgNPs/MIL-101(Fe) nanocomposite was obtained through self-assembly of the two substances. Four different probe molecules were detected with the self-assembled substrate and compared with the results of same probe molecules with AgNPs and MIL-101(Fe) as SERS substrate separately, it was found that AgNPs/ MIL-101 (Fe) nanocomposites had a strong enhancing effect as SERS substrate. The Enhancement Factor (EF) value of 10^-6 mol/L Rhodamine 6G (R6G) was calculated as 2.09 × 10⁹, and the Raman intensities of the peak relative standard deviation (RSD) of R6G Raman attribution was calculated as 7.55%. The time stability of the material was studied and it was found that the reduced Raman signal and poor reproducibility were due to the AgNPs placement time. AgNPs/ MIL-101 (Fe) nanocomposites were used as SERS substrate to detect Paraquat with a minimum concentration of 10^-12 mol/L. The signal values of Paraquat Raman detected at 10^-6 mol/L in different pH environments were relatively stable.

Dipole-Induced Raman Enhancement Using Noncovalent Azobenzene-Functionalized Self-Assembled Monolayers on Graphene Terraces

Graphene is a promising material in the field of interface science, especially for noncovalent functionalization, sensing, and for applications in catalysis and nanoelectronics. The noncovalent self-assembly of aromatic molecules on graphene promotes electronic coupling through π-π interactions that allows for quenching of the fluorescence of adsorbent molecules and the enhancement of their Raman spectra via graphene-enhanced Raman spectroscopy (GERS). Although recent work has explored the Raman enhancement on mono- and bilayer graphene, the layer dependence of both electronic phenomena (i.e., fluorescence quenching and Raman enhancement) has largely remained underexplored. Similarly, the effect of near-surface molecular dipoles on GERS has sparsely been examined. In this work, we employ self-assembled monolayers of azobenzene-decorated triazatriangulene molecules (AzoTATA) on graphene terraces to examine the effect of switchable molecular dipoles on the GERS effect, which occurs as a function of azobenzene photoisomerization. Furthermore, using empirical and computational methods, we present a systematic study for deriving the mechanism of GERS enhancement and fluorescence quenching on graphene terraces.

Pressure-induced SERS enhancement in a MoS2/Au/R6G system by a two-step charge transfer process

Pressure-induced surface-enhanced Raman spectroscopy (PI-SERS) represents a new frontier in the research field of SERS. However, relatively few studies have focused on PI-SERS due to many difficulties, such as easy aggregation of nanoparticles, and difficulty in understanding the interaction mechanisms between probe molecules and the SERS substrate at high pressure. Here we developed an efficient semiconductor-metal SERS substrate (MoS₂/Au) to study PI-SERS. Different from the previously reported monotonous decrease in Raman intensities upon compression, an anomalous Raman enhancement of R6G molecules adsorbed on the MoS₂/Au substrate was observed up to 2.39 GPa, at which the degree of charge transfer (ρ_CT) between the R6G molecules and the MoS₂/Au substrate reaches a maximum. By comparison, it is proposed that the decoration of Au on the SERS system could bring about a two-step charge transfer (CT) process, introduce localized surface plasmon resonance (LSPR), and thus favor the PI-SERS enhancement. Moreover, this charge transfer also causes obvious changes in the optical behaviors of R6G molecules upon compression. This brings new insights into the SERS study and also offers new ideas for the development of SERS application in high pressure studies.

Dual-Enhanced Raman Scattering-Based Characterization of Stem Cell Differentiation Using Graphene-Plasmonic Hybrid Nanoarray

Surface-enhanced Raman scattering (SERS) has demonstrated great potential to analyze a variety of bio/chemical molecular interactions within cells in a highly sensitive and selective manner. Despite significant advancements, it remains a critical challenge to ensure high sensitivity and selectivity, while achieving uniform signal enhancement and high reproducibility for quantitative detection of targeted biomarkers within a complex stem cell microenvironment. Herein, we demonstrate an innovative sensing platform, using graphene-coated homogeneous plasmonic metal (Au) nanoarrays, which synergize both electromagnetic mechanism (EM)- and chemical mechanism (CM)-based enhancement. Through the homogeneous plasmonic nanostructures, generated by laser interference lithography (LIL), highly reproducible enhancement of Raman signals could be obtained via a strong and uniform EM. Additionally, the graphene-functionalized surface simultaneously amplifies the Raman signals by an optimized CM, which aligns the energy level of the graphene oxide with the target molecule by tuning its oxidation levels, consequently increasing the sensitivity and accuracy of our sensing system. Using the dual-enhanced Raman scattering from both EM from the homogeneous plasmonic Au nanoarray and CM from the graphene surface, our graphene-Au hybrid nanoarray was successfully utilized to detect as well as quantify a specific biomarker (TuJ1) gene expression levels to characterize neuronal differentiation of human neural stem cells (hNSCs). Collectively, we believe our unique graphene-plasmonic hybrid nanoarray can be extended to a wide range of applications in the development of simple, rapid, and accurate sensing platforms for screening various bio/chemical molecules.

Photogating in the Graphene-Dye-Graphene Sandwich Heterostructure

In this work, we developed an atomically thin (∼2.5 nm) heterostructure consisting of a monolayer rhodamine 6G (R6G) film as a photoactive layer that was sandwiched between graphene films functioning as channels (graphene-R6G-graphene, G-R-G). Through a comparison of results of both photocurrent measurements and chemically enhanced Raman scattering (CERS) experiments, we found that our G-R-G heterostructure exhibited ∼7 and ∼30 times better performance than R6G-attached single-graphene (R6G-graphene, R-G) and MoS₂ devices, respectively; here, the CERS enhancement factor was highly correlated with the relative photoinduced Dirac voltage change. Furthermore, the photocurrent of the G-R-G device was found to be ∼40 times better than that of the R-G photodetector. The top graphene was highly operative in the monolayer, of which the performance is significantly deteriorated by fluorescence and tailored charge transfer efficiency with the increment of R6G film thickness. Overall, the responsivity of the G-R-G photodetector was ∼40 times higher than that of the R-G photodetector because of the more efficient carrier transfer between the organic dye and graphene induced by weaker π-π interactions between the top and bottom graphene channels in the former device. This atomically thin (∼2.5 nm) and highly photosensitive photodetector can be employed for post-Si-photodiode (PD) image sensors, single-photon detection devices, and optical communications.

Graphene-Activated Optoplasmonic Nanomembrane Cavities for Photodegradation Detection

Graphene, with its excellent chemical stability, biocompatibility, and capability of electric field enhancement, has a great potential in optical and optoelectronic applications with superior performances by integrating with conventional optical and plasmonic devices. Here, we design and demonstrate graphene-activated optoplasmonic cavities based on rolled-up nanomembranes, which are employed for in situ monitoring the photodegradation dynamics of organic dye molecules on the molecular level in real time. The presence of the graphene layer significantly enhances the electric field of hybrid optoplasmonic modes at the cavity surface, enabling a highly sensitive surface detection. The degradation of rhodamine 6G molecules on the graphene-activated sensor surface is triggered by localized laser irradiation and monitored by measuring the optical resonance shift. Our demonstration paves the way for real-time, high-precision analysis of photodegradation by resonance-based optical sensors, which promises the comprehensive understanding of degradation mechanism and exploration of effective photocatalysts.

Raman intensity enhancement of molecules adsorbed onto HfS2 flakes up to 200 layers

Newly emerging two-dimensional transition metal dichalcogenide HfS2 has received considerable attention recently due to its ultrahigh photoresponsivity, well-balanced carrier mobility and an appropriate band gap which offer potential in electronic and optoelectronic devices. In this work, HfS2 flakes up to 200 layers with varying color contrasts are fabricated and transferred on a SiO2/Si substrate. The Raman intensities of HfS2 flakes and Raman intensities of molecules adsorbed on HfS2 flakes are quantitatively studied both theoretically and experimentally by considering an optical interference effect. The effects of the main experimental factors: thickness of SiO2 and excitation wavelength on Raman intensities are also theoretically investigated. Due to the low absorption of HfS2, many strong high-order interference-induced enhancement peaks are observed which are different from high absorption materials like graphene and MoS2, in which only 2-4 interference-induced enhancement peaks exist. Due to the environmental instability of single layer HfS2 under ambient conditions, multi-layer HfS2 is a better choice than single layer HfS2 as a Raman scattering substrate which has a stronger Raman enhancement and a better environmental stability. The discovery here will expand the application of HfS2 flakes in molecular detection.

Graphene-Enhanced Raman Spectroscopy Reveals the Controlled Photoreduction of Nitroaromatic Compound on Oxidized Graphene Surface

Although graphene-enhanced Raman spectroscopy has been investigated for several years, there have been no studies that have applied it to real-time observations of chemical catalytic reactions. Here, we report that UV/ozone-treated oxidized graphene was used to both control and monitor the photoreduction of an adsorbed nitroaromatic dye compound. Graphene-enhanced Raman spectroscopy studies show that more oxidized graphene surface leads to faster photoreduction. This is due to the lowering of the Fermi level in the oxidized graphene, which is in agreement with the highest occupied molecular orbital level of the adsorbed dye molecule, leading to a rapid electron transfer from graphene to the dye. Our findings will be useful in understanding and exploiting the photocatalytic properties of oxidized graphene on adsorbed molecular species.

In situ formation of catalytically active graphene in ethylene photo-epoxidation

Ethylene epoxidation is used to produce 2 × 10⁷ ton per year of ethylene oxide, a major feedstock for commodity chemicals and plastics. While high pressures and temperatures are required for the reaction, plasmonic photoexcitation of the Ag catalyst enables epoxidation at near-ambient conditions. Here, we use surface-enhanced Raman scattering to monitor the plasmon excitation-assisted reaction on individual sites of a Ag nanoparticle catalyst. We uncover an unconventional mechanism, wherein the primary step is the photosynthesis of graphene on the Ag surface. Epoxidation of ethylene is then promoted by this photogenerated graphene. Density functional theory simulations point to edge defects on the graphene as the sites for epoxidation. Guided by this insight, we synthesize a composite graphene/Ag/α-Al₂O₃ catalyst, which accomplishes ethylene photo-epoxidation under ambient conditions at which the conventional Ag/α-Al₂O₃ catalyst shows negligible activity. Our finding of in situ photogeneration of catalytically active graphene may apply to other photocatalytic hydrocarbon transformations.

Photosensitive Graphene P-N Junction Transistors and Ternary Inverters

We investigate the electric transport in a graphene-organic dye hybrid and the formation of p-n junctions. In the conventional approach, graphene p-n junctions are produced by using multiple electrostatic gates or local chemical doping, which produce different types of carriers in graphene. Instead of using multiple gates or typical chemical doping, a different approach to fabricate p-n junctions is proposed. The approach is based on optical gating of photosensitive dye molecules; this method can produce a well-defined sharp junction. The potential difference in the proposed p-n junction can be controlled by varying the optical power of incident light. A theoretical calculation based on the effective medium theory is performed to thoroughly explain the experimental data. The characteristic transport behavior of the photosensitive graphene p-n junction opens new possibilities for graphene-based devices, and we use the results to fabricate ternary inverters. Our strategy of building a simple hybrid p-n junction can further offer many opportunities in the near future of tuning it for other optoelectronic functionalities.

Raman enhancement on ultra-clean graphene quantum dots produced by quasi-equilibrium plasma-enhanced chemical vapor deposition

Graphene is regarded as a potential surface-enhanced Raman spectroscopy (SERS) substrate. However, the application of graphene quantum dots (GQDs) has had limited success due to material quality. Here, we develop a quasi-equilibrium plasma-enhanced chemical vapor deposition method to produce high-quality ultra-clean GQDs with sizes down to 2 nm directly on SiO₂/Si, which are used as SERS substrates. The enhancement factor, which depends on the GQD size, is higher than conventional graphene sheets with sensitivity down to 1 × 10⁻⁹ mol L⁻¹ rhodamine. This is attributed to the high-quality GQDs with atomically clean surfaces and large number of edges, as well as the enhanced charge transfer between molecules and GQDs with appropriate diameters due to the existence of Van Hove singularities in the electronic density of states. This work demonstrates a sensitive SERS substrate, and is valuable for applications of GQDs in graphene-based photonics and optoelectronics.

Surface-enhanced Raman spectroscopy (SERS) is a promising technology for sensitive optical sensors, generally using rough metal films. Here, Liu et al. synthesize high-quality graphene quantum dot films which offer a large SERS enhancement due to a strong light-matter interaction with Van Hove singularities.

Graphene Oxide Nanoprisms for Sensitive Detection of Environmentally Important Aromatic Compounds with SERS

Recent advances in graphene-based sensors have shown that heavily oxidized (GO) and reduced graphene oxide (rGO) are attractive materials for environmental sensing due to their unique chemical and physical properties. We describe here the fabrication of nanostructured GO assemblies with Ag nanoprisms for improved detection with surface enhanced Raman scattering (SERS). Specifically, 100-μm-sized, periodic-nanoprism-array domains were fabricated on top of the GO layers by GO-assisted lithography (GOAL). The atomically thin GO underlayers are shown to attract cyclic aromatic molecules to the surface, likely via π-π stacking interactions. The close proximity of the analyte to GO and nanoprism (NP) tips effectively suppresses fluorescent background and affords a plausible tertiary enhancement of photon emissions via an electron charge transfer (CT) process. The adsorption of analyte to rGO-NP leads to the appearance and/or shift of several Raman bands, which provided a means to gain molecular insights into the graphene-enhanced scattering process. The analytical merits were characterized with model compound Rhodamine 6G, where the detection limit could reach subnanomolar concentrations. The nanoprism GO substrates also prove effective for SERS multiplex measurement of several legacy aromatic pollutants. Three tetrachlorobiphenyl isomers could be identified from a mixture using their autonomous nonoverlapping molecular fingerprints, and the substrate offers remarkable trace detection of 2,2',3,3'-tetrachlorobiphenyl (PCB-77).

Controlled preparation of Ag nanoparticles on graphene with different amount of defects for surface-enhanced Raman scattering

Graphene with different amounts of defects was prepared by chemical vapor deposition by controlling the flow rate of hydrogen, on which Ag nanoparticles (NPs) were deposited by magnetron sputtering.

Nanoarchitecture Based SERS for Biomolecular Fingerprinting and Label-Free Disease Markers Diagnosis

Surface-enhanced Raman spectroscopy (SERS) fingerprinting is highly promising for identifying disease markers from complex mixtures of clinical sample, which has the capability to take medical diagnoses to the next level. Although vibrational frequency in Raman spectra is unique for each biomolecule, which can be used as fingerprint identification, it has not been considered to be used routinely for biosensing due to the fact that the Raman signal is very weak. Contemporary SERS has been demonstrated to be an excellent analytical tool for practical label-free sensing applications due its ability to enhance Raman signals by factors of up to 10⁸–10¹⁴ orders of magnitude. Although SERS was discovered more than 40 years ago, its applications are still rare outside the spectroscopy community and it is mainly due to the fact that how to control, manipulate and amplify light on the “hot spots” near the metal surface is in the infancy stage.

In this Account, we describe our contribution to develop nanoachitecture based highly reproducible and ultrasensitive detection capability SERS platform via low-cost synthetic routes. Using one-dimensional (1D) carbon nanotube (CNT), two-dimensional (2D) graphene oxide (GO), and zero-dimensional (0D) plasmonic nanoparticle, 0D to 3D SERS substrates have been designed, which represent highly powerful platform for biological diagnosis. We discuss the major design criteria we have used to develop robust SERS substrate to possess high density “hot spots” with very good reproducibility. SERS enhancement factor for 3D SERS substrate is about 5 orders of magnitude higher than only plasmonic nanoparticle and more than 9 orders of magnitude higher than 2D GO. Theoretical finite-difference time-domain (FDTD) stimulation data show that the electric field enhancement |E|² can be more than 2 orders of magnitude in “hot spots”, which suggests that SERS enhancement factors can be greater than 10⁴ due to the formation of high density “hot spots” in 3D substrate. Next, we discuss the utilization of nanoachitecture based SERS substrate for ultrasensitive and selective diagnosis of infectious disease organisms such as drug resistance bacteria and mosquito-borne flavi-viruses that cause significant health problems worldwide. SERS based “whole-organism fingerprints” has been used to identify infectious disease organisms even when they are so closely related that they are difficult to distinguish. The detection capability can be as low as 10 CFU/mL for methicillin-resistant Staphylococcus aureus (MRSA) and 10 PFU/mL for Dengue virus (DENV) and West Nile virus (WNV). After that, we introduce exciting research findings by our group on the applications of nanoachitecture based SERS substrate for the capture and fingerprint detection of rotavirus from water and Alzheimer’s disease biomarkers from whole blood sample. The SERS detection limit for β-amyloid (Aβ proteins) and tau protein using 3D SERS platform is several orders of magnitude higher than the currently used technology in clinics. Finally, we highlight the promises, major challenges and prospect of nanoachitecture based SERS in biomedical diagnosis field.

Improving the Selective Efficiency of Graphene-Mediated Enhanced Raman Scattering through Molecular Imprinting

Enhancement of Raman scattering signal through graphene is an important property that could be exploited for producing innovative sensing devices with advanced properties. Because the enhancement of Raman scattering is due to only a chemical mechanism, the amplification of the signal is lower than that one produced by excitation of localized surface plasmons. The combination of a highly selective technique, which is molecular imprinting, with graphene-mediated enhanced Raman scattering, represents a new synergistic approach that we have developed in the present work. The careful material design has allowed obtaining a porous composite embedding exfoliated graphene and molecular cavities specifically designed for recognizing Rhodamine 6G. The molecularly imprinted porous samples have shown a signal enhancement that increases as a function of the number of molecular cavities, which are also accountable for the molecular recognition properties. Environmental ellipsometric porosimetry has shown no substantial difference between molecularly imprinted and not-imprinted films confirming that the signal enhancement of the imprinted samples is due to the molecular cavities. Interestingly, the most efficient sample has shown a Raman enhancement per cavity that exceeds the value of 1 × 10¹² and a remarkable molecular selectivity allowing for a Rhodamine 6G signal amplification 4.5 higher than structural analogues such as Rhodamine B and methylene blue. The robust and flexible matrix ensures a good recyclability of the samples without lack of linear response. These results prove the great potential of molecular imprinting as a general strategy to provide selectivity to GERS-active substrates for a new generation of sensing devices.

Three-dimensional (3D) plasmonic hot spots for label-free sensing and effective photothermal killing of multiple drug resistant superbugs

Drug resistant superbug infection is one of the foremost threats to human health. Plasmonic nanoparticles can be used for ultrasensitive bio-imaging and photothermal killing by amplification of electromagnetic fields at nanoscale "hot spots". One of the main challenges to plasmonic imaging and photothermal killing is design of a plasmonic substrate with a large number of "hot spots". Driven by this need, this article reports design of a three-dimensional (3D) plasmonic "hot spot"-based substrate using gold nanoparticle attached hybrid graphene oxide (GO), free from the traditional 2D limitations. Experimental results show that the 3D substrate has capability for highly sensitive label-free sensing and generates high photothermal heat. Reported data using p-aminothiophenol conjugated 3D substrate show that the surface enhanced Raman spectroscopy (SERS) enhancement factor for the 3D "hot spot"-based substrate is more than two orders of magnitude greater than that for the two-dimensional (2D) substrate and five orders of magnitude greater than that for the zero-dimensional (0D) p-aminothiophenol conjugated gold nanoparticle. 3D-Finite-Difference Time-Domain (3D-FDTD) simulation calculations indicate that the SERS enhancement factor can be greater than 10⁴ because of the bent assembly structure in the 3D substrate. Results demonstrate that the 3D-substrate-based SERS can be used for fingerprint identification of several multi-drug resistant superbugs with detection limits of 5 colony forming units per mL. Experimental data show that 785 nm near infrared (NIR) light generates around two times more photothermal heat for the 3D substrate with respect to the 2D substrate, and allows rapid and effective killing of 100% of the multi-drug resistant superbugs within 5 minutes.

Modulating the Morphology of Gold Graphitic Nanocapsules for Plasmon Resonance-Enhanced Multimodal Imaging

With their unique optical properties and distinct Raman signatures, graphitic nanomaterials can serve as substrates for surface-enhanced Raman spectroscopy (SERS) or provide signal amplification for bioanalysis and detection. However, a relatively weak Raman signal has limited further biomedical applications. This has been addressed by encapsulating gold nanorods (AuNRs) in a thin graphitic shell to form gold graphitic nanocapsules. This step improves plasmon resonance, which enhances Raman intensity, and has the potential for integrating two-photon luminescence (TPL) imaging capability. However, changing the morphology of gold graphitic nanocapsules such that high quality and stability are achieved remains a challenge. To address this task, we herein report a confinement chemical vapor deposition (CVD) method to prepare the construction of AuNR-encapsulated graphitic nanocapsules with these properties. Specifically, through morphological modulation, we (1) achieved higher plasmon resonance with near-IR incident light, thus achieving greater Raman intensity, and (2) successfully integrated two-photon luminescence dual-modal (Raman/TPL) bioimaging capabilities. Cancer-cell-specific aptamers were further modified on the AuNR@G graphitic surface through simple, but strong, π-π interactions to achieve imaging selectivity through differential cancer cell recognition.

R6G molecule induced modulation of the optical properties of reduced graphene oxide nanosheets for use in ultrasensitive SPR sensing

A proper understanding of the role that molecular doping plays is essential to research on the modulation of the optical and electronic properties of graphene. The adsorption of R6G molecules onto defect-rich reduced graphene oxide nanosheets results in a shift of the Fermi energy and, consequently, a variation in the optical constants. This optical variation in the graphene nanosheets is used to develop an ultrasensitive surface plasmon resonance biosensor with a detection limit of 10(-17) M (0.01 fM) at the molecular level. A density functional theory calculation shows that covalent bonds were formed between the R6G molecules and the defect sites on the graphene nanosheets. Our study reveals the important role that defects play in tailoring the properties and sensor device applications of graphene materials.

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.

Electronically coupled hybrid structures by graphene oxide directed self-assembly of Cu(2-x)S nanocrystals

Here, we describe an electronically coupled hybrid material consisting of graphene oxide (GO) flakes and inorganic Cu(2-x)S nanocrystals (NCs) formed via a self-assembly route. As a result of the amphiphilic nature of the water-dispersible GO flakes, the hydrophobic Cu(2-x)S NCs self-assemble in between the GO flakes, resulting in a large-interface hybrid structure with ordered close-packed NCs. We demonstrate that the optical properties of the hybrid GO/Cu(2-x)S structures are governed by the injection of electrons from the GO flakes to the valence band of the vacancy-doped plasmonic Cu(2-x)S NCs. This leads to a suppression of the plasmon band of the Cu(2-x)S NCs and to a softening of the Raman G-band of the GO flakes. Our results indicate that graphene derivatives can act not only as a self-assembly directing template, but also as a tool to affect the optical properties of self-assembled NCs in a chemical process, enhanced by the high interface area of the composite.

A case study: effect of defects in CVD-grown graphene on graphene enhanced Raman spectroscopy

PMMA-transferred graphene provides much larger GERS signal enhancement than TRT-transferred graphene.

Graphene thickness-controlled photocatalysis and surface enhanced Raman scattering

Exceptional photocatalytic enhancement of graphene-semiconductor composites has been widely reported, but our understanding of the role that graphene plays in this enhancement remains limited, which arises from the difficulty of precisely controlling graphene hybridization. Here we present a general platform of a graphene-semiconductor hybrid panel (GHP) system wherein a precise number of layers of graphene are hybridized with photoactive semiconductors (e.g. TiO2, ZnO) to study systematically how graphene affects the photocatalysis. The results show that the graphene enhancement of the photocatalysis depends on the number of graphene layers, with the maximum performance observed at 3 layers. Photodeposited indicators of gold particles further reveal that graphene thickness governs the density of photocatalytic sites and charge transfer efficiency at the graphene-semiconductor interfaces. We suggest that quantized energy levels caused by different numbers of stacked graphene sheets along the vector normal to the graphene basal plane affect the charge transfer routes and lead to the graphene thickness-controlled photocatalysis. GHP substrates deposited with gold particles are promising, uniform substrates for surface enhanced Raman scattering (SERS) applications with the enhancement factor as high as ∼10(8) on 3-layer graphene.

Silicon nanoparticles as Raman scattering enhancers

In this communication we demonstrate the large amplification values of the Raman signal of organic molecules attached to silicon nanoparticles (SiNPs). Light induced Mie resonances of high refractive index particles generate strong evanescent electromagnetic (EM) fields, thus boosting the Raman signal of species attached to the nanoparticles. The interest of this process is justified by the wide range of experimental configurations that can be implemented including photonic crystals, the sharp spectral resonances easily tuneable with the particle size, the biocompatibility and biodegradability of silicon, and the possibility of direct analysis of molecules that do not contain functional groups with high affinity for gold and silver. Additionally, silicon nanoparticles present stronger field enhancement due to Mie resonances at larger sizes than gold.

Identification of metalloporphyrins with high sensitivity using graphene-enhanced resonance Raman scattering

Graphene-enhanced resonance Raman scattering (GERRS) was performed for the detection of three different metallo-octaethylporphyrins (M-OEPs; M = 2H, FeCl, and Pt) homogeneously thermal vapor deposited on a graphene surface. GERRS of M-OEPs were measured using three different excitation wavelengths, λ(ex) = 405, 532, and 633 nm, and characterized detail vibrational bands for the identification of M-OEPs. The GERRS spectra of Pt-OEP at λ(ex) = 532 nm showed ~29 and ~162 times signal enhancement ratio on graphene and on graphene with Ag nanoclusters, respectively, compared to the spectra from bare SiO2 substrate. This enhancement ratio, however, was varied with M-OEPs and excitation wavelengths. The characteristic peaks and band shapes of GERRS for each M-OEP were measured with high sensitivity (100 pmol of thermal vapor deposited Pt-OEP), and these facilitate the selectively recognition of molecules. Also, the peaks shift and broadening provide the evidence of the interaction between graphene and M-OEPs through the charge transfer and π-orbital interaction. The increase of graphene layer induced the decrease of signal intensity and GERRS effect was almost not observed on the thick graphite flakes. Further experiments with various substrates demonstrated that the interaction of single layer of graphene with molecule is the origin of the Raman signal enhancement of M-OEPs. In this experiment, we proved the graphene is a good alternative substrate of Raman spectroscopy for the selective detection of various metalloporphyrins with high sensitivity.

Multifunctional hybrid graphene oxide for label-free detection of malignant melanoma from infected blood

The development of multifunctional graphene oxide for label-free detection of malignant melanoma from infected blood is reported.

Is chemically synthesized graphene 'really' a unique substrate for SERS and fluorescence quenching?

We demonstrate observation of Raman signals of different analytes adsorbed on carbonaceous materials, such as, chemically reduced graphene, graphene oxide (GO), multi-walled carbon nanotube (MWCNT), graphite and activated carbon. The analytes selected for the study were Rhodamine 6G (R6G) (in resonant conditions), Rhodamine B (RB), Nile blue (NBA), Crystal Violet (CV) and acetaminophen (paracetamol). All the analytes except paracetamol absorb and fluoresce in the visible region. In this article we provide experimental evidence of the fact that observation of Raman signals of analytes on such carbonaceous materials are more due to resonance effect, suppression of fluorescence and efficient adsorption and that this property in not unique to graphene or nanotubes but prevalent for various type of carbon materials.

Functional hybrid systems based on large-area high-quality graphene

The properties of sp² carbon allotropes can be tuned and enriched by their interaction with other materials. The large interface to the outside world in these forms of carbon is ideally suited for combining in an optimal manner several functionalities thanks to this interaction. A wide range of novel materials holding strong promise in energy, optoelectronics, microelectronics, mechanics, or medical applications have been designed accordingly. Graphene, the last representative of this family of sp² carbon materials, has already yielded a wealth of hybrid systems. A new class of these hybrids is emerging, which allows researchers to exploit the properties of truly single-layer graphene. These systems rely on high-quality graphene. In this Account, we describe our recent efforts to develop hybrid systems through various approaches and with various scopes. Depending on the interaction between graphene and molecules, metal clusters, layers, and substrates, either graphene may essentially preserve the electronic properties that make it a unique platform for electronic transport, or new organization and properties in the materials may arise due to the graphene contact at the expense of deep modification of graphene's properties. We prepare our graphene samples by both mechanical exfoliation of graphite and chemical vapor deposition on metals. We use this to study graphene in contact with various species, which either decorate graphene or are intercalated between it and its substrate. We first address the electronic and magnetic properties in systems where graphene is in epitaxy with a metal and discuss the potential to manipulate the properties of both materials, highlighting graphene's role as a protective capping layer in magnetic functional systems. We then present graphene/metal dot hybrids, which can utilize the two-dimensional gas properties of Dirac fermions in graphene. These hybrids allow one to tune the coupling between clusters hosting electronically ordered states such as superconductivity and explore quantum phase transitions controlled by electrostatic back gates. We finally discuss the optical properties of hybrids in which graphene is decorated with optically active molecules. Depending on how close these molecules are to the graphene's electromechanical systems, the interaction of the system with light can be changed. Fields such as spintronics and catalysis could benefit from high-quality graphene based hybrid systems, which have not been fully explored.

Exploring graphene nanocolloids as potential substrates for the enhancement of Raman scattering

Graphene, especially few-layer graphene solid film, has been found to strongly suppress fluorescence and enhance Raman signals of probe molecules. In this paper, we attempt to explore the possibility of using graphene nanocolloids as potential substrates for the enhancement of Raman scattering. Graphene nanocolloids chemically produced from the reduction of graphene oxide by sodium citrate are nearly all monolayers in solution and are also found to exhibit certain surface-enhanced Raman scattering (SERS) activity to common aromatic probe molecules. Interestingly, largely different from few-layer graphene solid film, graphene nanocolloids show maximal SERS activity only when the probe molecules are at resonant laser excitation. According to our analysis, this phenomenon should arise from a combined effect of fluorescence quenching of graphene and a photoinduced charge transfer mechanism, in which the strong charge transfer accounts for the main contribution from close coupling between graphenes and probe molecules photoinduced by resonant excitation as well as the desolvation of graphene sheets and probe molecules.

Graphene: a platform for surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) imparts Raman spectroscopy with the capability of detecting analytes at the single-molecule level, but the costs are also manifold, such as a loss of signal reproducibility. Despite remarkable steps having been taken, presently SERS still seems too young to shoulder analytical missions in various practical situations. By the virtue of its unique molecular structure and physical/chemical properties, the rise of graphene opens up a unique platform for SERS studies. In this review, the multi-role of graphene played in SERS is overviewed, including as a Raman probe, as a substrate, as an additive, and as a building block for a flat surface for SERS. Apart from versatile improvements of SERS performance towards applications, graphene-involved SERS studies are also expected to shed light on the fundamental mechanism of the SERS effect.

Tuning surface-enhanced Raman scattering from graphene substrates using the electric field effect and chemical doping

Graphene recently has been demonstrated to support surface-enhanced Raman scattering. Here, we show that the enhancement of the Raman signal of methylene blue on graphene can be tuned by using either the electric field effect or chemical doping. Both doping experiments show that hole-doped graphene yields a larger enhancement than one which is electron-doped; however, chemical doping leads to a significantly larger modulation of the enhancements. The observed enhancement correlates with the changes in the Fermi level of graphene, indicating that the enhancement is chemical in nature, as electromagnetic enhancement is ruled out by hybrid electrodynamical and quantum mechanical simulations.

Probing the Structure and Chemistry of Perylenetetracarboxylic Dianhydride on Graphene Before and After Atomic Layer Deposition of Alumina

The superlative electronic properties of graphene suggest its use as the foundation of next generation integrated circuits. However, this application requires precise control of the interface between graphene and other materials, especially the metal oxides that are commonly used as gate dielectrics. Towards that end, organic seeding layers have been empirically shown to seed ultrathin dielectric growth on graphene via atomic layer deposition (ALD), although the underlying chemical mechanisms and structural details of the molecule/dielectric interface remain unknown. Here, confocal resonance Raman spectroscopy is employed to quantify the structure and chemistry of monolayers of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on graphene before and after deposition of alumina with the ALD precursors trimethyl aluminum (TMA) and water. Photoluminescence measurements provide further insight into the details of the growth mechanism, including the transition between layer-by-layer growth and island formation. Overall, these results reveal that PTCDA is not consumed during ALD, thereby preserving a well-defined and passivating organic interface between graphene and deposited dielectric thin films.