Conventional Surface-enhanced Raman Scattering Research Articles

Preface to the special issue dedicated to Professor Richard P. Van Duyne (1945–2019)

Preface to the special issue dedicated to Professor Richard P. Van Duyne (1945–2019)

Core-shell magnetic Ag-molecularly imprinted composite for surface enhanced Raman scattering detection of carbaryl

Surface-enhanced Raman scattering (SERS) is a promising technique for rapid detection of pesticide residues. However, conventional SERS substrates require extraction processes which are time consuming and they also lack selectivity, stability and reproducibility. Herein, we present a multifunctional stable zero-valent iron based core-shell substrate. It combines magnetic separation, selective adsorption by molecular imprinting technique and sensitive detection of carbaryl by SERS. The core-shell substrate was successfully prepared by immobilizing silver on the surface of zero-valent iron microspheres. Subsequent molecular imprinting on the bimetallic magnetic silver microspheres ensured selective removal and detection. The substrate exhibited magnetization saturation of 8.89 emu/g providing efficient analyte separation. It showed high sensitivity and selectivity toward carbaryl detection to nanomolar concentration level. Linear regression models for peaks at Raman shift 1599 cm−1 and 2233 cm−1 demonstrated a good linear fit with R2=0.9738 and R2=0.8952 respectively. The composite was successfully applied on spiked water samples resulting in average recovery rate of 89%. The findings of this study demonstrate great substrate potential for application in separation and detection of trace quantities of chemical contaminants for environment safety and protection.

Point-of-care testing in the early diagnosis of acute pesticide intoxication: The example of paraquat.

Acute pesticide intoxication is a common method of suicide globally. This article reviews current diagnostic methods and makes suggestions for future development. In the case of paraquat intoxication, it is characterized by multi-organ failure, causing substantial mortality and morbidity. Early diagnosis may save the life of a paraquat intoxication patient. Conventional paraquat intoxication diagnostic methods, such as symptom review and urine sodium dithionite assay, are time-consuming and impractical in resource-scarce areas where most intoxication cases occur. Several experimental and clinical studies have shown the potential of portable Surface Enhanced Raman Scattering (SERS), paper-based devices, and machine learning for paraquat intoxication diagnosis. Portable SERS and new SERS substrates maintain the sensitivity of SERS while being less costly and more convenient than conventional SERS. Paper-based devices provide the advantages of price and portability. Machine learning algorithms can be implemented as a mobile phone application and facilitate diagnosis in resource-limited areas. Although these methods have not yet met all features of an ideal diagnostic method, the combination and development of these methods offer much promise.

Tip-Enhanced Raman Scattering on Bulk MoS2Substrate

Surface-enhanced Raman scattering (SERS) has many applications in nanotechnology, biophotonics, and sensing. Conventional SERS is based on surface plasmon resonances of noble metal nanostructures that enhance molecular Raman signals via electromagnetic mechanism and on molecule-substrate interaction via chemical mechanism. Recent studies used 2-D materials as atomically flat SERS substrates based on the chemical mechanism. Here, we use tip-enhanced Raman scattering (TERS) to enhance Raman signals from copper phthalocyanine molecules deposited on a bulk MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> substrate by a nanosized gold tip via electromagnetic enhancement. We present a comparative study of MoS2, gold, and SiO2 substrates. MoS2 changes relative intensities of molecular vibrations and provides additional gap-mode enhancement of TERS via interaction with the gold tip. Our results can be used to improve sensitivity and resolution in single molecule detection and imaging on flat substrates.

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.

Waveguide-coupled directional Raman radiation for surface analysis.

Kretschmann-type waveguide structures, including Plasmon Waveguide (PW) and Resonant Mirror (RM), have been applied in interfacial Raman spectroscopy due to the following unique features: (1) unlike the classic surface enhanced Raman scattering (SERS) substrates made of either gold or silver, both PW and RM can be prepared using a large variety of inexpensive materials; (2) the field enhancement factors using these structures can be theoretically predicted and experimentally controlled, which enables us to manipulate the surface Raman sensitivity with high repeatability; (3) the use of transverse electric (TE) and transverse magnetic (TM) modes for Raman excitation allows us to evaluate the orientation of target molecules immobilized on the waveguide surface; (4) the unwanted impact of noble metals on the Raman fingerprints of target molecules, which is often observed for conventional SERS substrates, can be avoided upon the use of dielectric waveguides. In this paper, guided-mode-coupled directional Raman emission, which is an additional important feature of the waveguide Raman technique, was theoretically investigated based on the optical reciprocity theorem combined with the Fresnel equations. The simulation results indicate that the directional Raman emission from a dipole located within the field confinement and penetration depth of a guided mode depends on both the orientation of the dipole and its distance from the waveguide surface. Raman light from the TE-oriented dipoles is launched into the prism coupler at the TE-mode resonance angle and that from the non-TE-oriented dipoles propagates at the TM-mode resonance angle. The intensity of the guided-mode-excited Raman signal propagating at the mode resonance angle is proportional to the fourth power of the mode field (E(4)) at the depth of the dipole from the waveguide surface. This means that the guided-mode-excited and guided-mode-coupled directional Raman spectroscopy has a detection depth that is as small as a quarter of the evanescent-field penetration depth, indicating the excellent surface selectivity of this technique. The directional Raman emission also facilitates high-efficiency signal collection compared with conventional SERS. It is worth noting that Raman light from the dipoles confined in the core layer of a single-mode waveguide can be simultaneously coupled into both the guided mode and the substrate mode, especially the surface plasmon resonance (SPR) mode for PW.

Horizontal slot waveguide channel for enhanced Raman scattering

Herein we characterize and experimentally demonstrate a new type of a horizontal slot waveguide structure for remarkably enhanced Raman scattering detection in nanometer-scale void channels. As the measurement sensitivity is one of the key limiting factors in nanofluidic detection, it is essential to search advanced solutions for such detection. Combining an all dielectric resonance waveguide grating and a surface enhanced Raman scattering (SERS) substrate in a close proximity it is possible to create high electromagnetic field energy hot zones within an adjustable slot region. This results in a strong enhancement in Raman scattering. We show the theoretical principles and demonstrate, with rhodamine 6G molecules, an approximately 20-fold enhancement compared to a conventional SERS substrate within the corresponding slot arrangement. We foresee potential applications for the proposed approach in the fields of medical, biological and chemical sensing, where the high detection sensitivity is essential due to integration with nanofluidic devices.