A Frequency Domain Existence Proof of Single-Molecule Surface-Enhanced Raman Spectroscopy

Wafer-scale metasurface for total power absorption, local field enhancement and single molecule Raman spectroscopy

The ability to detect molecules at low concentrations is highly desired for applications that range from basic science to healthcare. Considerable interest also exists for ultrathin materials with high optical absorption, e.g. for microbolometers and thermal emitters. Metal nanostructures present opportunities to achieve both purposes. Metal nanoparticles can generate gigantic field enhancements, sufficient for the Raman spectroscopy of single molecules. Thin layers containing metal nanostructures ("metasurfaces") can achieve near-total power absorption at visible and near-infrared wavelengths. Thus far, however, both aims (i.e. single molecule Raman and total power absorption) have only been achieved using metal nanostructures produced by techniques (high resolution lithography or colloidal synthesis) that are complex and/or difficult to implement over large areas. Here, we demonstrate a metasurface that achieves the near-perfect absorption of visible-wavelength light and enables the Raman spectroscopy of single molecules. Our metasurface is fabricated using thin film depositions, and is of unprecedented (wafer-scale) extent.

Superlocalization surface-enhanced Raman scattering microscopy: comparing point spread function models in the ensemble and single-molecule limits

In this report, we compare the effectiveness of various dipole and Gaussian point spread function (PSF) models for fitting diffraction-limited surface-enhanced Raman scattering (SERS) emission images from rhodamine 6G-labeled nanoparticle dimers at both the high-concentration and single-molecule limit. Of all models tested, a 3-axis dipole PSF gives the best approximation to the experimental PSF, although none of the models utilized in the study were without systematic error when fitting the experimental data. In the high-concentration regime, all models localize the SERS emission to a stationary centroid position, with the dipole models providing additional orientation parameters that closely match the geometry of the dimer, indicating that the molecules are coupled to all resonant plasmon modes of the nanostructure. In the single-molecule case, the different models show a mobile SERS centroid, consistent with single-molecule motion on the surface, but the behavior of the centroid is model-dependent. Despite the centroid mobility in the single-molecule regime, the dipole PSF models still give accurate orientation information on the underlying dimer structure, although with less precision than the ensemble-averaged samples.

Nanogap structures: combining enhanced Raman spectroscopy and electronic transport

Surface-enhanced Raman spectroscopy (SERS) is an experimental tool for accessing vibrational and chemical information, down to the single molecule level. SERS typically relies on plasmon excitations in metal nanostructures to concentrate the incident radiation and to provide an enhanced photon density of states to couple emitted radiation to the far field. Many common SERS platforms involve metal nanoparticles to generate the required electromagnetic enhancements. Here we concentrate on an alternative approach, in which the relevant plasmon excitations are supported at a truly nanoscale gap between extended electrodes, rather than discrete subwavelength nanoparticles. The ability to fabricate precise gaps on demand, and in some cases to tune the gap size in situ, combined with the additional capability of simultaneous electronic transport measurements of the nanogap, provides access to information not previously available in standard SERS. We summarize the rich plasmonic physics at work in these extended systems and highlight the recent state of the art including tip-enhanced Raman spectroscopy (TERS) and the application of mechanical break junctions and electromigrated junctions. We describe in detail how we have performed in situ gap-enhanced Raman measurements of molecular-scale junctions while simultaneously subjecting these structures to electronic transport. These extended electrode structures allow us to study the pumping of vibrational modes by the flow of tunneling electrons, as well as the shifting of vibrational energies due to the applied bias. These experiments extend SERS into a tool for examining fundamental processes of dissipation, and provide insight into the mechanisms behind SERS spectral diffusion. We conclude with a brief discussion of future directions.

Recent developments and future directions in SERS for bioanalysis

The ability to develop new and sensitive methods of biomolecule detection is crucial to the advancement of pre-clinical disease diagnosis and effective patient specific treatment. Surface enhanced Raman scattering (SERS) is an optical spectroscopy amenable to this goal, as it is capable of extremely sensitive biomolecule detection and multiplexed analysis. This perspective highlights where SERS has been successfully used to detect target biomolecules, specifically DNA and proteins, and where in vivo analysis has been successfully utilised. The future of SERS development is discussed and emphasis is placed on the steps required to transport this novel technique from the research laboratory to a clinical setting for medical diagnostics.

Silicon nanowire based single-molecule SERS sensor

One-dimensional nanowire (NW) optical sensors have attracted great attention as promising nanoscale tools for applications such as probing inside living cells. However, achieving single molecule detection on NW sensors remains an interesting and unsolved problem. In the present paper, we investigate single-molecule detection (SMD) on a single SiNW based surface-enhanced Raman scattering (SERS) sensor, fabricated by controllably depositing silver nanoparticles on a SiNW (AgNP-SiNW). Both Raman spectral blinking and bi-analyte approaches are performed in aqueous solution to investigate SMD on individual SiNW SERS sensors. The results extend the functions of the SiNW sensor to SMD and provide insight into the molecule level illustration on the sensing mechanism of the nanowire sensor.

Directional Raman scattering from single molecules in the feed gaps of optical antennas

Controlling light from single emitters is an overarching theme of nano-optics. Antennas are routinely used to modify the angular emission patterns of radio wave sources. "Optical antennas" translate these principles to visible and infrared wavelengths and have been recently used to modify fluorescence from single quantum dots and single molecules. Understanding the properties of single molecules, however, would be advanced were one able to observe their vibrational spectra through Raman scattering in a very reproducible manner but it is a hugely challenging task, as Raman scattering cross sections are very weak. Here we measure for the first time the highly directional emission patterns of Raman scattering from single molecules in the feed gaps of optical antennas fabricated on a chip. More than a thousand single molecule events are observed, revealing that an unprecedented near-unity fraction of optical antennas have single molecule sensitivity.

Understanding Plasmonic Properties in Metallic Nanostructures by Correlating Photonic and Electronic Excitations

A large number of optical phenomena rely on the excitation of localized surface plasmon resonances (LSPR) in metallic nanostructures. Electron-energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) has emerged as a technique capable of mapping plasmonic properties on length scales 100 times smaller than optical wavelengths. While this technique is promising, the connection between electron-driven plasmons, encountered in EELS, and photon-driven plasmons, encountered in plasmonic devices, is not well understood. This Perspective highlights some of the contributions that have been made in correlating optical scattering and STEM/EELS from the exact same nanostructures. The experimental observations are further elucidated by comparison with theoretical calculations obtained from the electron-driven discrete dipole approximation, which provides a method to calculate EEL spectra for nanoparticles of arbitrary shape. Applications of plasmon mapping to the electromagnetic hot-spots encountered in single-molecule surface-enhanced Raman scattering and electron beam induced damage in silver nanocubes are discussed. It is anticipated that the complementarity of both techniques will address issues in fundamental and applied plasmonics going forward.

Single-Molecule Surface-Enhanced Raman Scattering Sensitivity of Ag-Core Au-Shell Nanoparticles: Revealed by Bi-Analyte Method

Single-molecule surface-enhanced Raman scattering (SM-SERS) is an important application of localized surface plasmons in metallic nanostructures. Conventionally, Ag nanoparticles are used in solution-based SM-SERS experiments, but their usage is limited due to toxicity and oxidation issues. Au nanoparticle solutions are relatively biocompatible and SERS-active, but they do not facilitate large-scale SERS enhancement factors, which is an important prerequisite for SM-SERS. Under such constraints, silver-core gold-shell nanoparticles can be an excellent alternative for SM-SERS. Motivated by this, herein we report on the experimental evidence of SM-SERS sensitivity of Ag-core Au-shell nanoparticles by employing bianalyte method. Additionally, by detecting resonant molecules at femtomolar concentrations, we show that Ag-core Au-shell nanoparticle can be harnessed for ultrasensitive detection of molecules. The provided evidence will further motivate usage of such gold-shell-based bimetallic nanostructures for SM-SERS in biological environments.

Maximal Raman optical activity in hybrid single molecule-plasmonic nanostructures with multiple dipolar resonances

We show that a hybrid system built of a plasmonic nanoparticle cluster and a single molecule can attain maximal Raman optical activity (ROA), converting linearly polarized light into purely circularly polarized light at the Raman-scattered frequency. In contrast to standard molecular ROA, the effect described here does not involve magnetic modes and is attributed to off-resonance excitation of electric-dipole plasmon modes of the nanoparticle cluster. A model based on a combination of harmonic oscillators excited at the frequency of the Raman-scattered light is shown to successfully capture the physics of the effect.

Investigating electron-transfer processes using a biomimetic hybrid bilayer membrane system

Here we report a protocol to investigate the electron-transfer processes of redox-active biomolecules in biological membranes by electrochemistry using biomimetic hybrid bilayer membranes (HBMs) assembled on gold electrodes. Redox-active head groups, such as the ubiquinone moiety, are embedded in HBMs that contain target molecules, e.g., nicotinamide adenine dinucleotide (NADH). By using this approach, the electron-transfer processes between redox molecules and target biomolecules are mediated by mimicking the redox cycling processes in a natural membrane. Also included is a procedure for in situ surface-enhanced Raman scattering (SERS) to confirm the electrochemically induced conformational changes of the target biomolecules in the HBMs. In addition, each step in constructing the HBMs is characterized by electrochemical impedance spectroscopy (EIS), high-resolution X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The time required for the entire protocol is ∼12 h, whereas the electrochemical measurement of electron-transfer processes takes less than 1 h to complete.

Probing the plasmonic near-field by one- and two-photon excited surface enhanced Raman scattering

Strongly enhanced and spatially confined near-fields in the vicinity of plasmonic nanostructures open up exciting new capabilities for photon-driven processes and particularly also for optical spectroscopy. Surface enhanced Raman signatures of single molecules can provide us with important information about the optical near-field. We discuss one- and two-photon excited surface enhanced Raman scattering at the level of single molecules as a tool for probing the plasmonic near-field of silver nanoaggregates. The experiments reveal enhancement factors of local fields in the hottest hot spots of the near-field and their dependence on the photon energy. Also, the number of the hottest spots and their approximate geometrical size are found. Near-field amplitudes in the hottest spots can be enhanced by three orders of magnitudes. Nanoaggregates of 100 nm dimensions provide one hot spot on this highest enhancement level where the enhancement is confined within less than 1nm dimension. The near-field enhancement in the hottest spots increases with decreasing photon energy.

Plasmon based biosensor for distinguishing different peptides mutation states

Periodic and reproducible gold nanocuboids with various matrix dimensions and with different inter-particle gaps were fabricated by means of top-down technique. Rhodamine 6G was used as a probe molecule to optimize the design and the fabrication of the cuboid nanostructures. The electric field distribution for the nanocuboids with varying matrix dimensions/inter-particle gap was also investigated. These SERS devices were employed as biosensors through the investigation of both myoglobin and wild/mutated peptides. The results demonstrate the probing and the screening of wild/mutated BRCA1 peptides, thus opening a path for the fabrication of simple and cheap SERS device capable of early detection of several diseases.

Surface-enhanced Raman spectroscopy at single-molecule scale and its implications in biology

Single-molecule (SM) spectroscopy has been an exciting area of research offering significant promise and hope in the field of sensor development to detect targets at ultra-low levels down to SM resolution. To the experts and developers in the field of surface-enhanced Raman spectroscopy (SERS), this has often been a challenge and a significant opportunity for exploration. Needless to say, the opportunities and excitement of this multidisciplinary area impacts span the fields of physics, chemistry and engineering, along with a significant thrust in applications constituting areas in medicine, biology, environment and agriculture among others. In this review, we will attempt to provide a quick snapshot of the basics of SM-SERS, nanostructures and devices that can enable SM Raman measurement. We will conclude with a discussion on SERS implications in biomedical sciences.

Dispersible surface-enhanced Raman scattering nanosheets

Ultrathin and flexible silica nanosheets, synthesized with gold nanorod dimers embedded uniformly throughout, can be dispersed in solution and deposited onto arbitrary surfaces. These novel materials conform and maintain the as-synthesized density of dimers, allowing them to be used reliably in labeling and detection applications.

Tuning and maximizing the single-molecule surface-enhanced Raman scattering from DNA-tethered nanodumbbells

We extensively study the relationships between single-molecule surface-enhanced Raman scattering (SMSERS) intensity, enhancement factor (EF) distribution over many particles, interparticle distance, particle size/shape/composition and excitation laser wavelength using the single-particle AFM-correlated Raman measurement method and theoretical calculations. Two different single-DNA-tethered Au-Ag core-shell nanodumbbell (GSND) designs with an engineerable nanogap were used in this study: the GSND-I with various interparticle nanogaps from ∼4.8 nm to <1 nm or with no gap and the GSND-II with the fixed interparticle gap size and varying particle size from a 23-30 nm pair to a 50-60 nm pair. From the GSND-I, we learned that synthesizing a <1 nm gap is a key to obtain strong SMSERS signals with a narrow EF value distribution. Importantly, in the case of the GSND-I with <1 nm interparticle gap, an EF value of as high as 5.9 × 10(13) (average value = 1.8 × 10(13)) was obtained and the EF values of analyzed particles were narrowly distributed between 1.9 × 10(12) and 5.9 × 10(13). In the case of the GSND-II probes, a combination of >50 nm Au cores and 514.5 nm laser wavelength that matches well with Ag shell generated stronger SMSERS signals with a more narrow EF distribution than <50 nm Au cores with 514.5 nm laser or the GSND-II structures with 632.8 nm laser. Our results show the usefulness and flexibility of these GSND structures in studying and obtaining SMSERS structures with a narrow distribution of high EF values and that the GSNDs with < 1 nm are promising SERS probes with highly sensitive and quantitative detection capability when optimally designed.

Near-infrared surface-enhanced Raman spectroscopy (NIR-SERS) for the identification of eosin Y: theoretical calculations and evaluation of two different nanoplasmonic substrates

This work demonstrates the development of near-infrared surface-enhanced Raman spectroscopy (NIR-SERS) for the identification of eosin Y, an important historical dye. NIR-SERS benefits from the absence of some common sources of SERS signal loss including photobleaching and plasmonic heating, as well as an advantageous reduction in fluorescence, which is beneficial for art applications. This work also represents the first rigorous comparison of the enhancement factors and the relative merits of two plasmonic substrates utilized in art applications; namely, citrate-reduced silver colloids and metal film over nanosphere (FON) substrates. Experimental spectra are correlated in detail with theoretical absorption and Raman spectra calculated using time-dependent density functional theory (TDDFT) in order to elucidate molecular structural information and avoid relying on pigment spectral libraries for dye identification.

pH-Response Mechanism of p-Aminobenzenethiol on Ag Nanoparticles Revealed By Two-Dimensional Correlation Surface-Enhanced Raman Scattering Spectroscopy

The existence of pH-dependent surface-enhanced Raman scattering (SERS) of p-aminobenzenethiol (PATP) on Ag nanoparticles has been confirmed by numerous studies, but its mechanism still remains to be clarified. Discussion of the mechanism is at a standstill because of the lack of a systematic investigation of the process behind the pH-induced variation of the PATP behavior. Two-dimensional correlation spectroscopy is one of the most powerful and versatile spectral analysis methods for investigating perturbation-induced variations in dynamic data. Herein, we have analyzed the pH-dependent behavior of PATP using a static buffer solution with pH ranging from 3.0 to 2.0. The order of the variations in the different vibrational intensities was carefully investigated based on 2D correlation SERS spectroscopy. These results have demonstrated that the very first step of the pH-response process involves protonation of the amine group. The pH-response mechanism revealed is an important new component to our understanding of the origin of the b2-type bands of PATP.

Ordered arrays of Raman nanosensors for ultrasensitive and location predictable biochemical detection

Surface enhanced Raman scattering (SERS) is sensitive enough for single-molecule biochemical detection, but it is extremely difficult to obtain a large number of SERS hotspots for sensitive and reproducible detection. It is even more challenging to assemble the hotspots at designated positions for location predictable sensing. Here, we report an original strategy for the synthesis, manipulation, and assembling of plasmonic nanocapsule SERS sensors for high-sensitivity biochemical detection at predictable locations.

Super-resolution SERS imaging beyond the single-molecule limit: an isotope-edited approach

Super-resolution imaging of single-molecule surface-enhanced Raman scattering (SM-SERS) reveals a spatial relationship between the SERS emission centroid and the corresponding intensity. Here, an isotope-edited bianalyte approach is used to confirm that shifts in the SERS emission centroid are directly linked to the changing position of the molecule on the nanoparticle surface. By working above the single-molecule limit and exploiting SERS intensity fluctuations, the SERS centroid positions of individual molecules are found to be spatially distinct.

A chemical route to increase hot spots on silver nanowires for surface-enhanced Raman spectroscopy application

The effective number of surface-enhanced Raman spectroscopy (SERS) active hot spots on plasmonic nanostructures is the most crucial factor in ensuring high sensitivity in SERS sensing platform. Here we demonstrate a chemical etching method to increase the surface roughness of one-dimensional Ag nanowires, targeted at creating more SERS active hot spots along Ag nanowire's longitudinal axis for increased SERS detection sensitivity. Silver nanowires were first synthesized by the conventional polyol method and then subjected to chemical etching by NH(4)OH and H(2)O(2) mixture. The surfaces of silver nanowires were anisotropically etched off to create miniature "beads on a string" features with increased surface roughness while their crystallinity was preserved. Mapping of single-nanowire SERS measurements showed that the chemical etching method has overcome the limitation of conventional one-dimensional Ag nanowires with limited SERS active area at the tips to produce etched Ag nanowires with an increase in Raman hot spots and polarization-independent SERS signals across tens of micrometers length scale.

Electrospun nanofibrous membranes surface-decorated with silver nanoparticles as flexible and active/sensitive substrates for surface-enhanced Raman scattering

The development of novel nanomaterials with well-controlled morphologies/structures to achieve excellent activities/sensitivities in surface-enhanced Raman scattering (SERS) is crucial in advancing the high-performance SERS detections of chemical and biological species. In this study, amidoxime surface-functionalized polyacrylonitrile (ASFPAN) nanofibrous membranes surface-decorated with silver nanoparticles (Ag NPs) were prepared via the technique of electrospinning followed by the method of seed-mediated electroless plating. High SERS activities/sensitivities were observed from the ASFPAN-Ag NPs nanofibrous membranes, while the density and size of Ag NPs had an important impact on the SERS activity/sensitivity. The results confirmed that the enhancement of Raman signals is due to the presence of hot spots between/among Ag NPs on the nanofiber surfaces. Electrospun nanofibrous membranes surface-decorated with Ag NPs were mechanical flexible/resilient and could be used as highly active/sensitive SERS substrates for a broad range of applications.

Creating, characterizing, and controlling chemistry with SERS hot spots

In this perspective we discuss the roles of hot spots in surface-enhanced Raman spectroscopy (SERS). After giving background and defining the hot spot, we evaluate a variety of SERS substrates which often contain hot spots. We compare and discuss the differentiating properties of each substrate. We then provide a thorough analysis of the hot spot contribution to the observed SERS signal both in ensemble-averaged and single-molecule conditions. We also enumerate rules for determining the SERS enhancement factor (EF) to clarify the use of this common metric. Finally, we present a forward-looking overview of applications and uses of hot spots for controlling chemistry on the nanoscale. Although not exhaustive, this perspective is a review of some of the most interesting and promising methodologies for creating, controlling, and using hot spots for electromagnetic amplification.

Importance of Correctly Describing Charge-Transfer Excitations for Understanding the Chemical Effect in SERS

The enhancement mechanism due to the molecule-surface chemical coupling in surface-enhanced Raman scattering (SERS) is governed to a large extent by the energy difference between the highest occupied molecular orbital (HOMO) of the metal and the lowest unoccupied molecular orbital (LUMO) of the molecule. Here, we investigate the importance of correctly describing charge-transfer excitations, using time-dependent density functional theory (TDDFT), when calculating the chemical coupling in SERS. It is well-known that TDDFT, using traditional functionals, underestimates the position of charge-transfer excitations. Here, we show that this leads to a significant overestimation of the chemical coupling mechanism in SERS. Significantly smaller enhancements are found using long-range corrected (LC) functionals as compared with a traditional generalized gradient approximation (GGA) and hybrid functionals. Enhancement factors are found to be smaller than 530 and typically less than 50. Our results show that it is essential to correctly describe charge-transfer excitations for predicting the chemical enhancement in SERS.

Single-Molecule Surface-Enhanced Raman Scattering: Can STEM/EELS Image Electromagnetic Hot Spots?

Since the observation of single-molecule surface-enhanced Raman scattering (SMSERS) in 1997, questions regarding the nature of the electromagnetic hot spots responsible for such observations still persist. For the first time, we employ electron-energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) to obtain maps of the localized surface plasmon modes of SMSERS-active nanostructures, which are resolved in both space and energy. Single-molecule character is confirmed by the bianalyte approach using two isotopologues of Rhodamine 6G. Surprisingly, the STEM/EELS plasmon maps do not show any direct signature of an electromagnetic hot spot in the gaps between the nanoparticles. The origins of this observation are explored using a fully three-dimensional electrodynamics simulation of both the electron-energy-loss probability and the near-electric field enhancements. The calculations suggest that electron beam excitation of the hot spot is possible, but only when the electron beam is located outside of the junction region.

Mapping the energy distribution of SERRS hot spots from anti-Stokes to Stokes intensity ratios

The anomalies in the anti-Stokes to Stokes intensity ratios in single-molecule surface-enhanced resonance Raman scattering were investigated. Brilliant green and crystal violet dyes were the molecular probes, and the experiments were carried out on an electrochemically activated Ag surface. The results allowed new insights into the origin of these anomalies and led to a new method to confirm the single-molecule regime in surface-enhanced Raman scattering. Moreover, a methodology to estimate the distribution of resonance energies that contributed to the imbalance in the anti-Stokes to Stokes intensity ratios at the electromagnetic hot spots was proposed. This method allowed the local plasmonic resonance energies on the metallic surface to be spatially mapped.

Nanoparticle assembly for sensitive DNA detection using SERRS

SERRS (surface-enhanced resonance Raman scattering) is a vibrational technique, whereby a relatively weak Raman scattering effect is enhanced through the use of a visible chromophore and a roughened metal surface. The direct analysis of DNA by SERRS requires the modification of a nucleic acid sequence to incorporate a chromophore, and adsorption of the modified sequence on to a roughened metal surface. Aggregated metallic nanoparticles are commonly used in the analysis of dye-labelled DNA by SERRS, allowing for detection levels that rival those gained from standard fluorescence-based techniques. In the present paper, we report on how SERRS can be exploited for the analysis of clinically relevant DNA samples. We also report on the ability of nanoparticles to aggregate as the result of a biologically significant event, as opposed to the use of an external charge-modifying agent. The self-assembly of metallic nanoparticles is shown to be a promising new technique in the move towards extremely sensitive methods of DNA analysis by SERRS.

Inhibition assay of yeast cell walls by plasmon resonance Rayleigh scattering and surface-enhanced Raman scattering imaging

We report on plasmon resonance Rayleigh scattering (PRRS) and surface enhanced Raman scattering (SERS) imaging for inhibition assay of yeast cell walls. This assay reveals that the proteins having alkali sensitive linkage bound to β1,3 glucan frameworks in cell walls are involved in SERS activity. The result is further confirmed by comparison of genetically modified cells and wild type cells. Finally, we find that PRRS and SERS spots do not appear on cell walls when daughter cells are enough smaller than parent ones, but appear when size of daughter cells are comparable to parent cells. This finding indicates the relationship between expression of the proteins that generate SERS spots and cell division. These results demonstrate that PRRS and SERS imaging can be a convenient and sensitive method for analysis of cell walls.

Shedding Light on Surface-Enhanced Raman Scattering Hot Spots through Single-Molecule Super-Resolution Imaging

Super-resolution imaging has recently been utilized to develop a better understanding of the properties of surface-enhanced Raman scattering (SERS) hot spots. SERS hot spots are much smaller than the diffraction limit of light, and therefore, obtaining a clear picture of the enhanced electromagnetic (EM) fields comprising these hot spots is a challenging task. In this Perspective, we discuss recent work applying super-resolution imaging to single-molecule SERS (SM-SERS) of rhodamine 6G (R6G) adsorbed to randomly assembled silver colloidal aggregates, allowing the shape, size, and local enhancement of the hot spots to be imaged with <5 nm resolution. The results are compared with studies applying super-resolution imaging to surface-enhanced fluorescence (SEF) of analytes diffusing into silver nanoparticle hot spots. Both studies show a strong correlation between emission intensity and position, allowing the EM field enhancements of SERS hot spots to be mapped with sub-5 nm resolution.

The need for speed

One of the key enabling features of coherent Raman scattering (CRS) techniques is the dramatically improved imaging speed over conventional vibrational imaging methods. It is this enhanced imaging acquisition rate that has guided the field of vibrational microscopy into the territory of real-time imaging of live tissues. In this feature article, we review several aspects of fast vibrational imaging and discuss new applications made possible by the improved CRS imaging capabilities. In addition, we reflect on the current limitations of CRS microscopy and look ahead at several new developments towards real-time, hyperspectral vibrational imaging of biological tissues. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

Graphene-based high-efficiency surface-enhanced Raman scattering-active platform for sensitive and multiplex DNA detection

We have developed a surface-enhanced Raman scattering (SERS)-active substrate based on gold nanoparticle-decorated chemical vapor deposition (CVD)-growth graphene and used it for multiplexing detection of DNA. Due to the combination of gold nanoparticles and graphene, the Raman signals of dye were dramatically enhanced by this novel substrate. With the gold nanoparticles, DNA capture probes could be easily assembled on the surface of graphene films which have a drawback to directly immobilize DNA. This platform exhibits extraordinarily high sensitivity and excellent specificity for DNA detection. A detection limit as low as 10 pM is obtained. Importantly, two different DNA targets could be detected simultaneously on the same substrate just using one light source.

Super-resolution imaging reveals a difference between SERS and luminescence centroids

Super-resolution optical imaging of Rhodamine 6G surface-enhanced Raman scattering (SERS) and silver luminescence from colloidal silver aggregates are measured with sub-5 nm resolution and found to originate from distinct spatial locations on the nanoparticle surface. Using correlated scanning electron microscopy, the spatial origins of the two signals are mapped onto the nanoparticle structure, revealing that, while both types of emission are plasmon-mediated, SERS is a highly local effect, probing only a single junction in a nanoparticle aggregate, whereas luminescence probes all collective plasmon modes within the nanostructure. Calculations using the discrete-dipole approximation to calculate the weighted centroid position of both the |E|(2)/|E(inc)|(2) and |E|(4)/|E(inc)|(4) electromagnetic fields were compared to the super-resolution centroid positions of the SERS and luminescence data and found to agree with the proposed plasmon dependence of the two emission signals. These results are significant to the field of SERS because they allow us to assign the exact nanoparticle junction responsible for single-molecule SERS emission in higher order aggregates and also provide insight into how SERS is coupled into the plasmon modes of the underlying nanostructure, which is important for developing new theoretical models to describe SERS emission.

Anchoring molecular chromophores to colloidal gold nanocrystals: surface-enhanced Raman evidence for strong electronic coupling and irreversible structural locking

High-affinity anchoring groups such as isothiocyanate (ITC, -N═C═S) are often used to attach organic chromophores (reporter molecules) to colloidal gold nanocrystals for surface-enhanced Raman scattering (SERS), to atomically smooth gold surfaces for tip-enhanced Raman scattering, and to scanning tunneling microscopy probes (nanosized electrodes) for single-molecule conductance measurements. However, it is still unclear how the attached molecules interact electronically with the underlying surface, and how the anchoring group might affect the electronic and optical properties of such nanoscale systems. Here we report systematic surface-enhanced Raman studies of two organic chromophores, malachite green (MG) and its ITC derivative (MGITC), that have very different functional groups for surface binding but nearly identical spectroscopic properties. A surprise finding is that, under the same experimental conditions, the SERS signal intensities for MGITC are nearly 500-fold higher than those of MG. Correcting for the intrinsic difference in scattering cross sections of these two dyes, we estimate that the MGITC enhancement factors are ~200-fold higher than for MG. Furthermore, pH-dependent studies reveal that the surface structure of MGITC is irreversibly stabilized or "locked" in its π-conjugated form and is no longer responsive to pH changes. In contrast, the electronic structure of adsorbed MG is still sensitive to pH and can be switched between its localized and delocalized electronic forms. These results indicate that ITC is indeed an unusual anchoring group that enables strong electronic coupling between gold and the adsorbed dye, leading to more efficient chemical enhancement and higher overall enhancement factors.

Single-molecule surface-enhanced Raman spectroscopy

A general overview of the field of single-molecule (SM) surface-enhanced Raman spectroscopy (SERS) as it stands today is provided. After years of debates on the basic aspects of SM-SERS, the technique is emerging as a well-established subfield of spectroscopy and SERS. SM-SERS is allowing the observation of subtle spectroscopic phenomena that were not hitherto accessible. Examples of the latter are natural isotopic substitutions in single molecules, observation of the true homogeneous broadening of Raman peaks, Raman excitation profiles of individual molecules, and SM electrochemistry. With background examples of the contributions produced by our group, properly interleaved with results by other practitioners in the field, we present some of the latest developments and promising new leads in this new field of spectroscopy.

Surface-enhanced Raman spectroscopy (SERS): progress and trends

Surface-enhanced Raman spectroscopy (SERS) combines molecular fingerprint specificity with potential single-molecule sensitivity. Therefore, the SERS technique is an attractive tool for sensing molecules in trace amounts within the field of chemical and biochemical analytics. Since SERS is an ongoing topic, which can be illustrated by the increased annual number of publications within the last few years, this review reflects the progress and trends in SERS research in approximately the last three years. The main reason why the SERS technique has not been established as a routine analytic technique, despite its high specificity and sensitivity, is due to the low reproducibility of the SERS signal. Thus, this review is dominated by the discussion of the various concepts for generating powerful, reproducible, SERS-active surfaces. Furthermore, the limit of sensitivity in SERS is introduced in the context of single-molecule spectroscopy and the calculation of the 'real' enhancement factor. In order to shed more light onto the underlying molecular processes of SERS, the theoretical description of SERS spectra is also a growing research field and will be summarized here. In addition, the recording of SERS spectra is affected by a number of parameters, such as laser power, integration time, and analyte concentration. To benefit from synergies, SERS is combined with other methods, such as scanning probe microscopy and microfluidics, which illustrates the broad applications of this powerful technique.