Investigation into the Regulation of Ag NPs/ZnO NRs/GaN Heterostructure SERS Substrate via Pyroelectric Effects.

Ag/ZnO/Au 3D hybrid structured reusable SERS substrate as highly sensitive platform for DNA detection

We fabricated surface-enhanced Raman scattering (SERS) substrates using gold nanoparticle (AuNP)-decorated zinc oxide (ZnO) nanorods (NRs). Prior to decoration with AuNPs, ZnO NRs on the glass substrate fabricated using the sol–gel method could enhance the SERS signal for detecting 10−5 M rhodamine 6G (R6G). Microscopic analysis revealed that the thermal-annealing process for fabricating the seed layers of ZnO facilitated the growth of ZnO NRs with the highly preferred c-axis (002) orientation. A decrease in the diameter of ZnO NRs occurred because of the use of annealed seek layers further increased the surface-to-volume ratio of ZnO NRs, resulting in an increase in the SERS signal for R6G of 10−5 M. To combine the localized surface plasmon resonance (LSPR) mode with the charge transfer (CT) mode, ZnO NRs were decorated with AuNPs through pulsed-laser-induced photolysis (PLIP). However, the preferred vertical (002) orientation of ZnO NRs was prone to the aggregation of AuNPs, which hindered the SERS signal. The experimental results revealed that ZnO NRs with the crystalline structure of horizontal (100) and (101) orientations facilitated the growth of homogeneous, independent and isolated AuNPs which serves as “hot spots” for SERS signal of detecting R6G at a low concentration of 10−9 M. Comparing to previous fabrication of SERS substrate, our method has advantage to fabricate AuNP-decorated ZnO NR in a short time. Moreover, the optimization of the SERS behaviors for different fabrication conditions of AuNPs using the PLIP method was investigated in detail.

The discovery of surface-enhanced Raman spectroscopy (SERS) presents an opportunity for the development of ultrasensitive biosensors. SERS allows for fingerprint detection of biomolecules at low concentrations and can perform single-molecule detection. The most used SERS substrates are Plasmonic metal nanostructures such as Au and Ag mainly because of their superior electrical conductivity. The Plasmonic metal nanoparticles enhance the Raman signal of the target biomolecules through the electromagnetic field mechanism. Despite the successful use of metal nanostructures, alternative materials have been sought which can exhibit similar or better properties than their metal counterparts. Materials such as graphene which has excellent electrical properties and high surface area are good candidates for SERS applications. Graphene is a 2D nanomaterial consisting of sp2 hybridized carbon atoms. The use of graphene in SERS is termed graphene-enhanced Raman spectroscopy (GERS) and is reported to use a charge transfer mechanism. Both graphene and metal nanostructures can be used together as SERS substrate and the synergistic effect of their mechanism can improve the enhancement factor and the sensitivity of the biosensor, however, the use of graphene as a SERS substrate is still at an early stage. GERS can be applied in disease diagnosis, detection of food additives, and biological species biosensors. The use of GERS in biosensors such as glucose biosensors presents an opportunity for ultrasensitive biosensors which can greatly improve the lives of individuals living with diabetes. In this review insights will be given on how to successfully circumvent the challenges associated with small Raman cross-section of glucose detection for use in medical application.

Surface enhanced Raman scattering (SERS) is a highly sensitive spectroscopy technique, which is widely used in chemical reaction detecting, medical diagnostics, and food analysis. The construction of the substrate structure has a very important influence on enhancing the SERS signal of the probe molecule. In this paper, a three-dimensional (3D) pyramid stereo composite SERS substrate is prepared by using polymethyl methacrylate (PMMA) to encapsulate silver nanoparticles, which achieves high sensitivity detection of Rhodamine 6G (R6G) molecules. By adjusting the dispersion density of silver nanoparticles in the PMMA acetone solution, the effective oscillation of light in the pyramid valley is realized, which not only ensures the high-density "hot spot" effect of the 3D structure, but also avoids deforming the adsorption probe molecules caused by the metal-molecule interaction. It also effectively prevents the silver nanoparticles from being oxidized and provides a larger range of electromagnetic enhancement for probe molecules, resulting in a stable output of the enhanced Raman signal. This research result provides an effective strategy for designing a high performance and reusable SERS substrate, meanwhile, it has important guiding significance for further designing an SERS substrate with improved 3D structure in the future study.

Metal organic frameworks (MOFs), crystalline solids consisting of organic ligands and metal ions, have attracted increasing interest in various areas, including catalysis and biology. Functionalizable pore interiors and ultrahigh surface-to-volume ratios of MOFs make them excellent materials, especially for surface-enhanced Raman scattering (SERS) by the photoinduced charge transfer (PICT) between the MOFs and adsorbed molecules for SERS signal amplification. In our previous work, we demonstrated a p-n junction-assisted MOF substrate for enhancing the SERS signal through additional charge transfer, while the notable structural characteristics of MOFs benefit the SERS selectivity. However, due to this characteristic, a single MOF can only detect analytes at specific energy levels, thereby reducing the efficiency and limiting the detection range of SERS. To address this challenge, in this article, we attempted to synthesize multiple MOFs on the same substrate to achieve SERS with multiple detection functionality. In this study, we demonstrated a ZIF-8/Zn(OH)2 n-n junction SERS substrate, which can achieve a 4.44 nM limit of detection using methyl orange (MO) as a model analyte, by trapping the additional electrons from Zn(OH)2 to ZIF-8 to provide stronger electromagnetic enhancement. Then, we developed a multiple MOFs-based SERS analytical platform, incorporating both in situ ZIF-8 and ZIF-67, and utilized them together as SERS substrates. These two systems operated independently under different incident light wavelengths and successfully detected MO and Rhodamine 6G, respectively. Furthermore, this approach broadens the detection range of analytes while maintaining the tailorability and selectivity of MOF substrates simultaneously. This work offers a cutting-edge method for creating high-performance SERS substrates by demonstrating the ability to manipulate hot electrons for remarkable Raman amplification.

In this paper, we present bridged-bowtie nanohole arrays and cross bridged-bowtie nanohole arrays in a gold thin film as surface enhanced Raman scattering (SERS) substrates. These SERS substrates not only exhibit large electromagnetic enhancement of SERS but also have the SERS enhancement spread over a much larger area than what could be present in SERS substrates consisting of nanopillar arrays or nanopillar plasmonic nanoantennas. Numerical simulations based on the finite difference time domain (FDTD) method are employed to determine electric field enhancement factors (EFs) and therefore the electromagnetic SERS enhancement factor for these SERS substrates. It was observed that bridged-bowtie nanohole arrays and cross bridge-bowtie nanohole arrays exhibit a highest electromagnetic SERS enhancement factor (EF) of ~109, which is orders of magnitude higher than what has been previously reported for nanohole arrays as SERS substrates. This electromagnetic SERS EF (of ~109) is spread over a hotspot region of ~100 nm2 (in each periodic unit of the array), which is larger than the case of nanopillar arrays. In addition, it was observed that an electromagnetic SERS enhancement factor of at least 108 is spread over a large area (500 nm2 in each periodic unit of the array), thus increasing the average enhancement factor. It was observed that the bridged-bowtie nanohole arrays and the cross bridged-bowtie nanohole arrays can be employed as effective SERS substrates in both the transmission mode and the reflection mode. The resonance wavelength of these arrays of nanoholes can be tuned by altering the size of the nanoholes. The effects of varying the gold film thickness and the diameter of the bridged-bowtie nanoholes forming the arrays were also analyzed. The bridged-bowtie nanohole arrays and cross bridged-bowtie nanohole arrays exhibit very high electric field enhancement factors (EFs) at more than one wavelength, and can therefore be used to obtain a multi-wavelength SERS response. Moreover, the cross bridged-bowtie nanohole array allows the tunability of the position of the hotspot with the rotation of the direction of the polarization of incident field.

Surface Enhanced Raman Scattering (SERS) is a technique that is increasingly being used to detect trace amounts of various types of molecules, especially organic and biological molecules. The SERS effect is available mainly due to the SERS substrate - a noble metal surface that is rough at the nano level or a set of noble metal nanoparticles in a certain arrangement. Such a SERS substrate acts as an analyte Raman signal amplifier and can provide amplification up to millions of times and even more. The amplification coefficient of the SERS substrate is determined mainly by the number of ‘hot spots’ it contains as well as the ‘hotness’ of these spots. In turn, a ‘hot spot’ is a certain space around the tips or a nanogap between particles, where the local electromagnetic field is intensely enhanced, while the ‘hotness’ is determined by the sharpness of the tips (the sharper the hotter) and tightness of the gaps (the narrower the hotter). This report presents an overview of the research results of fabricating a type of SERS substrate with a high enhancement factor, which is the SERS substrate made from silver nanostructures coated on the silicon surface. With the aim of increasing the number of ‘hot spots’ and their quality, as well as ensuring uniformity and reproducibility of the SERS substrate, silver nanostructures have been fabricated in various forms, such as nanoparticles, nanodendrites and nanoflowers. In addition, the report also mentions the use of the above silver nanostructures as SERS substrates to detect trace amounts of some pesticides and other toxic agents such as paraquat, pyridaben, thiram, cyanide...

The small Raman scattering cross section of hemoglobin (Hb) molecules limits its application using a Raman spectroscopy based optical biosensor. Label-free surface enhanced Raman scattering (SERS) detection and degradation of Hb have been achieved using 3D reusable superhydrophobic SERS substrates based on a Ag/ZnO/Ag hybrid structure. The fabrication process follows the decoration of thermally evaporated non-spherical like Ag nanoparticles on hydrothermally grown ZnO nanorods on a catalytic ultra-thin Ag film. From SERS point of view, these 3D SERS substrates exhibit four important characteristics such as a higher surface to volume ratio, surface plasmon resonance in the broad wavelength region of the visible spectrum, a strong electric field at the Ag–ZnO interface due to the formation of a Schottky barrier, and the superhydrophobic surface. The SERS substrates not only performed an outstanding Raman enhancement effect due to the above factors but also displayed multiple recyclabilities owing to their excellent self-cleaning property via a UV light assisted photocatalytic degradation process. The quantitative SERS analysis has been performed by a linear regression method and resulted in 10−13.42M and 10−7.24M limit of detection for Rhodamine 6G (Rh6G) and Hb molecules, respectively, with an enhancement factor of 6 × 1011. The effect of the 3D hybrid structure toward higher SERS activity has been compared with that of 2D SERS substrates, and the SERS mapping of Rh6G molecules proves good homogeneity of the 3D SERS substrates. These ultra-sensitive 3D SERS substrates with reusable capability open the possibility of their use toward biosensors for the early detection of diseases.

As a noninvasive and label-free optical technique, Raman spectroscopy offers significant advantages in studying the structure and properties of biomacromolecules, as well as real-time changes in cellular molecular structure. However, its practical applications are hindered by weak scattering responses, low signal intensity, and poor spectral uniformity, which affect the subsequent accuracy of spectral analysis. To address these issues, we report a novel surface-enhanced Raman scattering (SERS) substrate based on a pyramidal pitted silicon (PPSi) array structure adhered with Au-shell Ag-core nanospheres (Au@Ag NSs). By preparing a highly uniform PPSi array substrate with controllable size and arrangement, and constructing SERS-active Au@Ag NSs on this substrate, a three-dimensional (3D) composite SERS substrate is realized. The enhancement performance and spectral uniformity of 3D composite SERS substrate were examined using crystal violet (CV) and Rhodamine 6G (R6G) molecules, achieving a minimum detectable concentration of R6G at 10-9 M and the analytical enhancement factor (AEF) of 4.2 × 108. Moreover, SERS detection of biological samples with varying concentrations of Staphylococcus aureus demonstrated excellent biocompatibility of the SERS substrate and enabled quantitative analysis of bacterial concentration (R 2 = 99.7 %). Theoretical simulations using finite-difference time-domain (FDTD) analysis were conducted to examine the electromagnetic field distribution of the three-dimensional SERS composite substrate, confirming its local electric field enhancement effect. These experimental and theoretical results indicate that the Au@Ag NSs/PPSi substrate with a regulable pyramidal pitted array is a promising candidate for sensitive, label-free SERS detection in medical and biotechnological applications.

Indirect surface-enhanced Raman scattering (SERS)-based methods are highly efficient in detecting and quantitatively analyzing trace antibiotics in complex samples. However, the poor reproducibility of indirect SERS assays caused by the diffusion and orientation changes of the probing molecules on SERS substrates still presents a significant challenge. To address this issue, this study reports the construction of a novel SERS sensing platform using tetrahedral framework nucleic acid (tFNA) as SERS probes in conjunction with a long-range SERS (LR-SERS) substrate. The tFNA was modified with sulfhydryl groups at three vertices and appended with a probing DNA at the remaining vertex, anchored on the substrate surface with a well-ordered orientation and stable coverage density, resulting in highly reproducible SERS signals. Owing to the weak SERS signal of tFNA inherited from its size being larger than the effective range of the enhancing electric field (E-field) of conventional SERS substrates, we utilized an LR-SERS substrate to enhance the signal of tFNA probes by capitalizing on its extended E-field. Correspondingly, the LR-SERS substrate demonstrated a 54-fold increase in the intensity of tFNA probes compared to the conventional substrate. Using this novel platform, we achieved a highly reliable detection of the antibiotic ampicillin with a wide linear range (10 fM to 1 nM), low detection limit (3.1 fM), small relative standard deviation (3.12%), and yielded quantitative recoveries of 97-102% for ampicillin in water, milk, and human serum samples. These findings, therefore, effectively demonstrate the achievement of highly reliable SERS detection of antibiotics using framework nucleic acids and an LR-SERS substrate.

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

Surface-enhanced Raman scattering technology plays a prominent role in spectroscopy. By introducing plasmonic metals and photonic crystals as a substrate, SERS signals can achieve further enhancement. However, the conventional doping preparation methods of these SERS substrates are insufficient in terms of metal-loading capacity and the coupling strength between plasmonic metals and photonic crystals, both of which reduce the SERS activity and reproducibility of SERS substrates. In this work, we report an approach combining spin-coating, surface modification, and in situ reduction methods. Using this approach, a photonic crystal array of SiO2@Au core-shell structure nanoparticles was prepared as a SERS substrate (SiO2@Au NP array). To study the SERS properties of these substrates, Rhodamine 6G was employed as the probe molecule. Compared with a Au-SiO2 NP array prepared using doping methods, the SiO2@Au NP array presented better SERS properties, and it reproduced the SERS spectra after one month. The detection limit of the Rhodamine 6G on SiO2@Au NP array reached 1 × 10-8 mol/L; furthermore, the relative standard deviation (9.82%) of reproducibility and the enhancement factor (1.51 × 106) were evaluated. Our approach provides a new potential option for the preparation of SERS substrates and offers a potential advantage in trace contaminant detection, and nondestructive testing.

Graphene(G)-noble metal-ZnO hybrid systems were developed as highly sensitive and recyclable surface enhanced Raman scattering (SERS) platforms, in which ultrathin graphene of varying thickness was embedded between two metallic layers on top of a ZnO layer. Due to the multi-dimensional plasmonic coupling effect, the Au/G/Ag@ZnO multilayer structure possessed ultrahigh sensitivity with the detection limit of Rhodamine 6 G (R6G) as low as 1.0×10-13 mol/L and a high enhancement factor of 5.68×107. Both experimental and simulation results showed that graphene films could significantly regulate the interlayer plasmon resonance coupling strength, and single-layer graphene had the best interlayer regulation effect. Additionally, the SERS substrate structure prepared through physical methods exhibited high uniformity, the graphene component of the substrate possessed excellent molecular enrichment ability and silver oxidation inhibition characteristics, resulting in a substrate with high stability and exceptional reproducibility. The signal change was less than 15%. Simultaneously, due to the excellent photocatalytic performance of the low-cost and wide-band-gap semiconductor material ZnO, the SERS substrate exhibited exceptional reusability. Even after five cycles of adsorption-desorption, the SERS performance remained stable and maintained a reliable detection limit. The study introduced a novel approach to creating multilayer composite SERS substrates that exhibited exceptional performance, offering a new analytical tool with high sensitivity, stability, and reusability.

Surface enhanced Raman scattering (SERS) is a widely used analytical technique for detecting trace-level molecules based on an indispensable SERS substrate. SERS substrates with high tailorability are assumed to be attractive and desirable for SERS detection, because the substrates match the need for the selective detection of different species. Nevertheless, the rational design of such SERS substrates is rather challenging for both noble-metal and semiconductor substrates. Herein, expanding beyond conventional SERS substrates, we demonstrate that metal-organic framework (MOF) materials can serve as a type of SERS substrate with molecular selectivity, which are rarely realized for SERS detection without any special pretreatment. A salient structural characteristic of MOF-based SERS substrates benefiting the SERS selectivity is their high tailorability. By controlling the metal centers, organic ligands, and framework topologies of our MOF-based SERS substrates, we show that the electronic band structures of MOF-based SERS substrate can be purposively manipulated to match those of the target analytes, thus resulting in different detectable species. Going further, the SERS enhancement factors (EFs) of the MOF-based SERS substrates can be greatly enhanced to as high as 106 with a low detection limit of 10-8 M by pore-structure optimization and surface modification, which is comparable to the EFs of noble metals without "hot spots" and recently reported semiconductors. This selective enhancement is interpreted as being due to the controllable combination of several resonances, such as the charge-transfer, interband and molecule resonances, together with the ground-state charge-transfer interactions. Our study opens a new venue for the development of SERS substrates with high-design flexibility, which is especially important for selective SERS detection toward specific analytes.

Surface enhanced Raman scattering (SERS) has been proved to be a highly sensitive method to detect organic molecules at very low concentrations. In recent years, many researchers have reported that 1-dimension semiconductor nanomaterials assembled noble metal nanoparticles can get a strong SERS effect. In this paper, we succeeded to synthesize TiO2 nanorod thin films on fluorine-doped tin oxide (FTO) glass with hydrothermal synthesis which were able to be used as SERS substrates. Gold nanoparticles were assembled to TiO2 nanorod thin films using the physical sputtering method and the citrate reduction method, respectively. The field emission scanning electron microscope (FESEM) images show that the later method could achieve the more desirable and uniform distribution of gold nanoparticles. Rhodamine 6G (R6G) was chosen as the probe molecule to study the SERS performance of our novel SERS substrates. Raman scattering measurement proved that the substrates were able to enhance Raman signals by several orders of magnitude and could be applied to biochemical detection. The whole fabrication process was facile and cost-effective, and the SERS activity and reproducibility of the substrates were pretty good.