The interfacing structural effect of Ag/graphene oxide nanohybrid films on surface enhanced Raman scattering

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.

Lateral Size Effect of Graphene Oxide on Its Surface-Enhanced Raman Scattering Property

The lateral size effect of graphene oxide (GO) on surfaced-enhanced Raman scattering (SERS) property is systematically investigated by using size-fractionalized GO. For the size fractionalization without changes of chemical structure, large-sized GO (LGO) and small-sized GO (SGO) are separated from the as-synthesized GO (AGO) by centrifugation and membrane filtration, respectively. The size-fractionalized GO sheets are immobilized on a solid substrate for the parallel comparison of their SERS property. As a result, we find that LGO shows considerably higher SERS property than SGO for typical Raman probes such as rhodamine 6G and crystal violet. Furthermore, the lateral size effect of GO derivatives is consistently observed when they are hybridized with plasmonic silver nanoparticles. These results indicate that LGO is superior to AGO and SGO as a SERS platform, and it is also quantitatively confirmed by calculating their enhancement factor.

Quantitative Surface-Enhanced Raman Spectroscopy for Field Detections Based on Structurally Homogeneous Silver-Coated Silicon Nanocone Arrays

Practical application of surface-enhanced Raman spectroscopy (SERS) is greatly limited by the inaccurate quantitative analyses due to the measuring parameter's fluctuations induced by different operators, different Raman spectrometers, and different test sites and moments, especially during the field tests. Herein, we develop a strategy of quantitative SERS for field detection via designing structurally homogeneous and ordered Ag-coated Si nanocone arrays. Such an array is fabricated as SERS chips by depositing Ag on the template etching-induced Si nanocone array. Taking 4-aminothiophenol as the typical analyte, the influences of fluctuations in measuring parameters (such as defocusing depth and laser powers) on Raman signals are systematically studied, which significantly change SERS measurements. It has been shown that the silicon underneath the Ag coating in the chip can respond to the measuring parameters' fluctuations synchronously with and similar to the analyte adsorbed on the chip surface, and the normalization with Si Raman signals can well eliminate the big fluctuations (up to 1 or 2 orders of magnitude) in measurements, achieving highly reproducible measurements (mostly, <5% in signal fluctuations) and accurate quantitative SERS analyses. Finally, the simulated field tests demonstrate that the developed strategy enables quantitatively analyzing the highly scattered SERS measurements well with 1 order of magnitude in signal fluctuation, exhibiting good practicability. This study provides a new practical chip and reliable quantitative SERS for the field detection of real samples.

Plasmonic tunable Ag-coated gold nanorod arrays as reusable SERS substrates for multiplexed antibiotics detection

Antibiotic contaminants in aqueous media pose a serious threat to human and ecological environments. Therefore, it is necessary to develop robust strategies to detect antibiotic residues. For this purpose, a self-assembly and in situ electrochemical reduction method is utilized to tailor silver nanoparticles (AgNPs)-coated GNRs (AgNPs/GNRs) large-scale vertical arrays. These AgNPs/GNRs arrays exhibit outstanding surface-enhanced Raman scattering (SERS) activities because of abundant Raman hot-spots among the adjacent AgNPs and GNRs, but also excellent stability and reproducibility due to the close-packed arrayed nanostructure. These remarkable features validate this arrayed substrate for high-sensitivity 4-aminothiophenol analysis with a detection limit of 0.35 pM and self-cleaning via electrochemical stripping of the adsorbed analytes and AgNPs from the GNRs arrays, therefore realizing renewable SERS applications. Moreover, the distinct SERS performance of AgNPs/GNRs arrays is verified via the analysis of multiplexed antibiotics at tens of picomolar level and no apparent changes of SERS activities are observed when recyclability is explored. The result demonstrates that the proposed AgNPs/GNRs arrays provide a novel strategy for avoiding conventional, disposable SERS substrates, as well as expanding SERS applications for simultaneous sensing and stripping of environmental contaminants.

Resolving the stacking fault structure of silver nanoplates

The stacking fault structure (SFT) is the key to understanding the symmetry breaking of fcc nanocrystals and the origin of two-dimensional (2D) anisotropic growth of nanoplates. After resolving the SFT in Ag nanoplates under aberration-corrected transmission electron microscope (TEM) observations, it is found that there are three basic stacking faults, namely, twinned stacking fault (SF-t), a layer missed stacking fault (SF-m) and a layer inserted stacking fault (SF-i). The SFT is composed of one or a combination of two or all of the three kinds of stacking faults with a total number varying from 4 to 9. It has been demonstrated that the SFT could generate concave faces, step faces and (100) faces in the lateral directions, which provides sites for adding-atoms with a higher coordination number than on the top and bottom flat (111) faces, and results in the anisotropic growth along the 2D direction. Additionally, Ag nanoplates fall into either center symmetry or mirror symmetry when the corresponding number is even or odd. The center symmetry and mirror symmetry with different side face arrangements in turn manipulate the shape evolution to cubes and bipyramids, respectively. Our study provides a comprehensive understanding of the formation and growth of 2D metal nanomaterials.

Effect of Ag/rGO on the Optical Properties of Plasmon-Modified SnO2 Composite and Its Application in Self-Powered UV Photodetector

A facile hydrothermal method was employed to synthesize silver–reduced graphene oxide (Ag/rGO) plasmon-modified SnO2 composite, by incorporating Ag–reduced graphene oxide (Ag/rGO) into SnO2 nanorods as a photoanode for assembling a self-powered ultraviolet photodetector (UVPD). The as-synthesized samples were investigated in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and UV visible spectrophotometer. The as-prepared Ag/rGO films show enhanced light absorption attributed to the localized surface plasmon resonance (LSPR). The optimized 1.0 wt.% Ag/rGO incorporated into SnO2-based UVPD exhibits a significant photocurrent response due to the enhanced absorption light and effective suppression of charge recombination. This UVPD demonstrates a high performance, with photocurrent density reaching 0.29 mAcm−2 compared to the SnO2-based device with 0.16 mAcm−2. This device also exhibits a high on:off ratio of 195 and fast response time, which are superior to that of the free-modified one. In addition, the UVPD based on plasmon-modified SnO2 photoanode treated with TiCl4-aqueous solution has attained a higher photocurrent with a maximum value reaching 5.4 mAcm−2, making this device favorable in ultraviolet detection.

Graphene Oxide as an Optical Biosensing Platform: A Progress Report

A few years ago, crucial graphene oxide (GO) features such as the carbon/oxygen ratio, number of layers, and lateral size were scarcely investigated and, thus, their impact on the overall optical biosensing performance was almost unknown. Nowadays valuable insights about these features are well documented in the literature, whereas others remain controversial. Moreover, most of the biosensing systems based on GO were amenable to operating as colloidal suspensions. Currently, the literature reports conceptually new approaches obviating the need of GO colloidal suspensions, enabling the integration of GO onto a solid phase and leading to their application in new biosensing devices. Furthermore, most GO-based biosensing devices exploit photoluminescent signals. However, further progress is also achieved in powerful label-free optical techniques exploiting GO in biosensing, particularly using optical fibers, surface plasmon resonance, and surface enhanced Raman scattering. Herein, a critical overview on these topics is offered, highlighting the key role of the physicochemical properties of GO. New challenges and opportunities in this exciting field are also highlighted.

A Rapid Surface-Enhanced Raman Scattering (SERS) Method for Pb2+ Detection Using L-Cysteine-Modified Ag-Coated Au Nanoparticles with Core–Shell Nanostructure

A rapid surface-enhanced Raman scattering (SERS) method for Pb2+ detection has been developed based on l-cysteine-modified Ag-coated Au nanoparticles with core-shell nanostructure. Specifically, l-cysteine-functionalized Au@Ag core-shell probes bearing Raman-labeling molecules (4-ATP) are used to detect Pb2+ upon the formation of nanoparticle aggregates. The proposed SERS-based method shows a linear range between 5 pM and 10 nM, with an unprecedented limit of detection (LOD) of 1 pM for Pb2+; this LOD shows the method to be a few orders of magnitude more sensitive than the typical colorimetric approach that is based on the aggregation of noble metal nanoparticles. Real water samples diluted with pure water have been successfully analyzed. This SERS-based assay may provide a general and simple approach for the detection of other metal ions of interest, and so could have wide-ranging applications in many areas.

Flexible Asymmetric Supercapacitor Based on Functionalized Reduced Graphene Oxide Aerogels with Wide Working Potential Window

Flexible energy storage devices are in great demand since the advent of flexible electronics. Until now, flexible supercapacitors based on graphene analogues usually have had low operating potential windows. To this end, two dissimilar electrode materials with complementary potential ranges are employed to obtain an optimum cell voltage of 1.8 V. A low-temperature organic sol-gel method is used to prepare two different types of functionalized reduced graphene oxide aerogels (rGOA) where Ag nanorod functionalized rGOA acts as a negative electrode while polyaniline nanotube functionalized rGOA acts as a positive electrode. Both materials comprehensively exploit their unique properties to produce a device that has high energy and power densities. An assembled all-solid-state asymmetric supercapacitor gives a high energy density of 52.85 W h kg^-1 and power density of 31.5 kW kg^-1 with excellent cycling and temperature stability. The device also performs extraordinarily well under different bending conditions, suggesting its potential to meet the requirements for flexible electronics.

Silicon nanohybrid-based SERS chips armed with an internal standard for broad-range, sensitive and reproducible simultaneous quantification of lead(ii) and mercury(ii) in real systems

Lead ions (Pb²⁺) and mercury ions (Hg²⁺), the two commonly coexisting heavy metal ions, pose severe risks to environment and human health. To date, no surface-enhanced Raman scattering (SERS) sensor has been reported for the simultaneous quantification of Pb²⁺ and Hg²⁺ in real systems. Herein, the first demonstration of SERS chips for simultaneous quantification of Pb²⁺ and Hg²⁺ in real systems is presented based on the combination of reproducible silicon nanohybrid substrates and a corrective internal standard (IS) sensing strategy. This chip was made of a silver nanoparticle-decorated silicon wafer via modification of the IS, i.e. 4-aminothiophenol, molecules. The as-prepared chip was further functionalized with Pb²⁺- and Hg²⁺- specific DNA strands capable of simultaneously detecting Pb²⁺ and Hg²⁺. Quantitatively, upon correction by the IS Raman signals, the broad dynamic ranges from 100 pM to 10 μM for Pb²⁺ and from 1 nM to 10 μM for Hg²⁺ were achieved, with the detection limit down to 19.8 ppt for Pb²⁺ and 168 ppt for Hg²⁺. For real applications, we further demonstrated that Pb²⁺ and Hg²⁺ spiked into industrial wastewater could be readily distinguished via the presented chip, and the relative standard deviation (RSD) value was less than ∼15%. More significantly, the resulting SERS chip can be well coupled with a hand-held Raman instrument and can then be used for the qualitative analysis of both Pb²⁺ and Hg²⁺ in real systems in a portable manner. Our results suggest that this high-quality SERS chip is a powerful tool for on-site detection of various heavy metal ions in real samples in the field of food safety and environment protection.