Surface enhanced Raman imaging: periodic arrays and individual metal nanoparticles

Dimensional Design for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique that enables specific identification of target analytes with sensitivity down to the single-molecule level by harnessing metal nanoparticles and nanostructures. Excitation of localized surface plasmon resonance of a nanostructured surface and the associated huge local electric field enhancement lie at the heart of SERS, and things will become better if strong chemical enhancement is also available simultaneously. Thus, the precise control of surface characteristics of enhancing substrates plays a key role in broadening the scope of SERS for scientific purposes and developing SERS into a routine analytical tool. In this review, the development of SERS substrates is outlined with some milestones in the nearly half-century history of SERS. In particular, these substrates are classified into zero-dimensional, one-dimensional, two-dimensional, and three-dimensional substrates according to their geometric dimension. We show that, in each category of SERS substrates, design upon the geometric and composite configuration can be made to achieve an optimized enhancement factor for the Raman signal. We also show that the temporal dimension can be incorporated into SERS by applying femtosecond pulse laser technology, so that the SERS technique can be used not only to identify the chemical structure of molecules but also to uncover the ultrafast dynamics of molecular structural changes. By adopting SERS substrates with the power of four-dimensional spatiotemporal control and design, the ultimate goal of probing the single-molecule chemical structural changes in the femtosecond time scale, watching the chemical reactions in four dimensions, and visualizing the elementary reaction steps in chemistry might be realized in the near future.

On the Large Near-Field Enhancement on Nanocolumnar Gold Substrates

One of the most important and distinctive features of plasmonic nanostructures is their ability to confine large electromagnetic fields on nanometric volumes; i.e., the so-called hot spots. The generation, control and characterization of the hot spots are fundamental for several applications, like surface-enhanced spectroscopies. In this work, we characterize the near-field distribution and enhancement of nanostructured gold thin films fabricated by glancing angle deposition magnetron sputtering. These films are composed of columnar nanostructures with high roughness and high density of inter-columnar gaps, where the electromagnetic radiation can be confined, generating hot spots. As expected, the hot spots are localized in the gaps between adjacent nanocolumns and we use scattering-type scanning near-field optical microscopy to image their distribution over the surface of the samples. The experimental results are compared with finite-difference time-domain simulations, finding an excellent agreement between them. The spectral dependence of the field-enhancement is also studied with the simulations, together with surface-enhanced Raman spectroscopy at different excitation wavelengths in the visible-NIR range, proving a broad-band response of the substrates. These findings may result in interesting applications in the field of surface-enhanced optical spectroscopies or sensing.

Raman Open Database: first interconnected Raman-X-ray diffraction open-access resource for material identification

Detailed crystallographic information provided by X-ray diffraction (XRD) is complementary to molecular information provided by Raman spectroscopy. Accordingly, the combined use of these techniques allows the identification of an unknown compound without ambiguity. However, a full combination of Raman and XRD results requires an appropriate and reliable reference database with complete information. This is already available for XRD. The main objective of this paper is to introduce and describe the recently developed Raman Open Database (ROD, http://solsa.crystallography.net/rod). It comprises a collection of high-quality uncorrected Raman spectra. The novelty of this database is its interconnectedness with other open databases like the Crystallography Open Database (http://www.crystallography.net/cod and Theoretical Crystallography Open Database (http://www.crystallography.net/tcod/). The syntax adopted to format entries in the ROD is based on the worldwide recognized and used CIF format, which offers a simple way for data exchange, writing and description. ROD also uses JCAMP-DX files as an alternative format for submitted spectra. JCAMP-DX files are compatible to varying degrees with most commercial Raman software and can be read and edited using standard text editors.

Femtosecond Laser-Induced, Nanoparticle-Embedded Periodic Surface Structures on Crystalline Silicon for Reproducible and Multi-utility SERS Platforms

Fabrication of reproducible and versatile surface-enhanced Raman scattering (SERS) substrates is crucial for real-time applications such as explosive detection for human safety and biological imaging for cancer diagnosis. However, it still remains a challenging task, even after several methodologies were developed by various research groups, primarily due to (a) a lack of consistency in detection of a variety of molecules (b) cost-effectiveness of the SERS substrates prepared, and (c) byzantine preparation procedures, etc. Herein, we establish a procedure for preparing reproducible SERS-active substrates comprised of laser-induced nanoparticle-embedded periodic surface structures (LINEPSS) and metallization of silicon (Si) LINEPSS. LINEPSS were fabricated using the technique of femtosecond laser ablation of Si in acetone. The versatile SERS-active substrates were then achieved by two ways, including the drop casting of silver (Ag)/gold (Au) nanoparticles (NPs) on Si LINEPSS and Ag plating on the Si LINEPSS structures. By controlling the LINEPSS grating periodicity, the effect of plasmonic nanoparticles/plasmonic plating on the Si NPs embedded periodic surface structures enormously improved the SPR strength, resulting in the consistent and superior Raman enhancements. The reproducible SERS signals were achieved by detecting the molecules of Methylene Blue (MB), 2,4-dinitrotoluene (DNT), and 5-amino-3-nitro-l,2,4-triazole (ANTA). The SERS signal strength is determined by the grating periodicity, which, in turn, is determined by the input laser fluence. The SERS-active platform with grating periodicity of 130 ± 10 nm and 150 ± 5 nm exhibited strong Raman enhancements of ∼10⁸ for MB and ∼10⁷ for ANTA molecules, respectively, and these platforms are demonstrated to be capable, even for multiple usages.

Treating cancer stem cells and cancer metastasis using glucose-coated gold nanoparticles

Cancer ranks among the leading causes of human mortality. Cancer becomes intractable when it spreads from the primary tumor site to various organs (such as bone, lung, liver, and then brain). Unlike solid tumor cells, cancer stem cells and metastatic cancer cells grow in a non-attached (suspension) form when moving from their source to other locations in the body. Due to the non-attached growth nature, metastasis is often first detected in the circulatory systems, for instance in a lymph node near the primary tumor. Cancer research over the past several decades has primarily focused on treating solid tumors, but targeted therapy to treat cancer stem cells and cancer metastasis has yet to be developed. Because cancers undergo faster metabolism and consume more glucose than normal cells, glucose was chosen in this study as a reagent to target cancer cells. In particular, by covalently binding gold nanoparticles (GNPs) with thio-PEG (polyethylene glycol) and thio-glucose, the resulting functionalized GNPs (Glu-GNPs) were created for targeted treatment of cancer metastasis and cancer stem cells. Suspension cancer cell THP-1 (human monocytic cell line derived from acute monocytic leukemia patients) was selected because it has properties similar to cancer stem cells and has been used as a metastatic cancer cell model for in vitro studies. To take advantage of cancer cells' elevated glucose consumption over normal cells, different starvation periods were screened in order to achieve optimal treatment effects. Cancer cells were then fed using Glu-GNPs followed by X-ray irradiation treatment. For comparison, solid tumor MCF-7 cells (breast cancer cell line) were studied as well. Our irradiation experimental results show that Glu-GNPs are better irradiation sensitizers to treat THP-1 cells than MCF-7 cells, or Glu-GNPs enhance the cancer killing of THP-1 cells 20% more than X-ray irradiation alone and GNP treatment alone. This finding can help oncologists to design therapeutic strategies to target cancer stem cells and cancer metastasis.

Use of surface-enhanced Raman scattering as a prognostic indicator of acute kidney transplant rejection

We report an early, noninvasive and rapid prognostic method of predicting potential acute kidney dysfunction using surface-enhanced Raman scattering (SERS). Our analysis was performed on urine samples collected prospectively from 58 kidney transplant patients using a He-Ne laser (632.8 nm) as the excitation source. All abnormal kidney function episodes (three acute rejections and two acute kidney failures that were eventually diagnosed independently by clinical biopsy) consistently exhibited unique SERS spectral features in just one day following the transplant surgery. These results suggested that SERS analysis provides an early and more specific indication to kidney function than the clinically used biomarker, serum creatinine (sCr).

Sub-wavelength confinement of the orbital angular momentum of light probed by plasmonic nanorods resonances

We discuss how the topological charge of an OAM-carrying plasmon (Plasmonic Vortex) can be probed by monitoring the near-field response of plasmonic nanostructures suitably arranged inside a Plasmonic Vortex Lens. The turning "on" or "off" of four gold nanorods, detected by a Scanning Near field Optical Microscope (SNOM), acts as a fingerprint of the OAM state of the PV at the nanoscale. Different configurations are studied numerically, the integrated structure is fabricated and near field characterization is performed for a particularly meaningful case.

Polarization-dependent SERS effects of laser-generated sub-100 nm antenna structures

Sub-100 nm antenna arrays consisting of a star-like ridge or dome-like structures with needles in their centers are prepared in thin gold films on glass substrates using femtosecond laser pulses. The needles can be bent mechanically to be horizontally aligned to the substrate surface. Controlled variation of the pulse energy allows one to obtain nanostructures of different defined morphologies. These arrays of nanostructures are covered with a thin homogeneous layer of rhodamine molecules. Raman spectra using linearly polarized laser light of 632.8 nm are taken with the laser spot centered on individual nanostructures and at positions on the unstructured film. The average Raman enhancement within the laser spot focused onto a nanostructure is two orders of magnitude higher than on the unstructured film. The nanostructures with bent needles exhibit a polarization dependence of the SERS effect, i.e., typically the enhancement is larger by about a factor of two for excitation light polarized parallel to the needle direction than for the perpendicular case. The enhancement factor of the star-like ridge structures with needles is analyzed by the finite-element method, which agrees with the experiment. We show that the variation of the SERS activity of almost similar structures arises from the inherent randomness of the hot spots created in the fabrication process. Nevertheless, these antenna structures may be useful as elements in novel SERS devices as they can be accurately positioned on a device using a cheap fabrication process compatible with microfabrication technology.

Polarization-resolved two-photon luminescence microscopy of V-groove arrays

Using two-photon luminescence (TPL) microscopy and local reflection spectroscopy we investigate electromagnetic field enhancement effects from a µm-sized composition of 450-nm-deep V-grooves milled by focused ion beam in a thick gold film and assembled to feature, within the same structure, individual V-grooves as well as one- and two-dimensional 300-nm-period arrays of, respectively, parallel and crossed V-grooves. We analyze TPL signal levels obtained at different spatial locations and with different combinations of excitation and detection polarizations, discovering that the TPL emitted from the V-grooves is polarized in the direction perpendicular to that of the V-grooves. This feature implies that the TPL occurs solely in the form of (p-polarized) surface plasmon modes and originates therefore from the very bottom of V-grooves, where no photonic modes exist. Implications of the results obtained to evaluation of local field enhancements using TPL microscopy, especially when investigating extended structures exhibiting different radiation channels, are discussed.

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.

Complex bound and leaky modes in chains of plasmonic nanospheres

Bound and leaky modes with complex wavenumber in chains (linear arrays) of plasmonic nanospheres are characterized for both longitudinal and transverse polarization states (with respect to the array axis). The proposed method allows for the description of each mode evolution when varying frequency. As a consequence, full characterization of the guided modes with complex wavenumber is provided in terms of propagation direction, guidance or radiance, proper or improper, and physical or nonphysical conditions. Each nanosphere is modeled according to the single dipole approximation, and the metal permittivity is described by the Drude model. Modal wavenumbers are obtained by computing the complex zeroes of the homogeneous equation characterizing the field in the one dimensional periodic array. The required periodic Green's function is analytically continued into the complex wavenumber space by using the Ewald method. Furthermore, a parametric analysis of the mode wavenumbers is performed with respect to the geometrical parameters of the array.

On-chip approach for traceable quantification of biomarkers based on isotope-dilution surface-enhanced Raman scattering (IDSERS)

We present an on-chip approach for the quantification of biomarkers based on isotope-dilution surface-enhanced Raman scattering (IDSERS). The full procedure was realized on a few square millimetres of a SERS-active substrate, covered with either lithographically engineered gold nanotriangles or silver nanospheres generated by galvanic displacement deposition. The use of certified reference materials combined with the ID principle ensures traceability of the quantitation to SI units. A series of substance spots was deposited onto the SERS active area and measured one by one in fast sequence. The SERS spectra were used to generate and validate a PLS model and also to predict the creatinine concentration of an unknown serum sample.

Lithographical gap-size engineered nanoarrays for surface-enhanced Raman probing of biomarkers

Engineered gold nanostructured arrays with feature separation in the sub-10-nm range were fabricated and used for highly sensitive surface enhanced Raman scattering (SERS) detection of biomarkers. Nanostructuring is achieved by combining electron beam lithography with the so-called shadow evaporation technique which involves a two-step Au vapor deposition through a suspended Ge mask while the substrate is tilted in opposite directions. This results in a regular triangular surface pattern with extremely small gap distances that allow positive enhancement of the local electric fields by enabling improvements in the electromagnetic coupling between adjacent nanoparticles. The resulting SERS active surfaces are suitable for the realization of reference procedures for quantifying marker molecules like urea or creatinine at physiologically relevant concentrations.

Nanoparticles as biochemical sensors

There is little doubt that nanoparticles offer real and new opportunities in many fields, such as biomedicine and materials science. Such particles are small enough to enter almost all areas of the body, including cells and organelles, potentially leading to new approaches in nanomedicine. Sensors for small molecules of biochemical interest are of critical importance. This review is an attempt to trace the use of nanomaterials in biochemical sensor design. The possibility of using nanoparticles functionalized with antibodies as markers for proteins will be elucidated. Moreover, capabilities and applications for nanoparticles based on gold, silver, magnetic, and semiconductor materials (quantum dots), used in optical (absorbance, luminescence, surface enhanced Raman spectroscopy, surface plasmon resonance), electrochemical, and mass-sensitive sensors will be highlighted. The unique ability of nanosensors to improve the analysis of biochemical fluids is discussed either through considering the use of nanoparticles for in vitro molecular diagnosis, or in the biological/biochemical analysis for in vivo interaction with the human body.

Investigation on the second part of the electromagnetic SERS enhancement and resulting fabrication strategies of anisotropic plasmonic arrays

In general, the electromagnetic mechanism is understood as the strongest contribution to the overall surface-enhanced Raman spectroscopy (SERS) enhancement. Due to the excitation of surface plasmons, a strong electromagnetic field is induced at the interfaces of a metallic nanoparticle leading to a drastic enhancement of the Raman scattering cross-section. Furthermore, the Raman scattered light expierences an emission enhancement due to the plasmon resonances of the nanoantennas. Herein, this second part of the electromagnetic enhancement phenomenon is investigated for different Raman bands of crystal violet by utilizing the anisotropic plasmonic character of gold nanorhomb SERS arrays. We aim at evaluating the effects of localized and propagating surface plasmon polariton modes as well as their combination on the scattered SERS intensity. From that point of view, design and fabrication strategies towards the fabrication of SERS arrays for excitation wavelengths in the visible and near-infrared (NIR) spectral region can be given, also using a double-resonant electromagnetic enhancement.