Three-dimensional urchin-like K2Ti8O17 / Ag NPs composite as a SERS substrate for detecting folic acid and thiram.

Noble metal and semiconductor composite substrates possess high sensitivity, excellent stability, good biocompatibility, and selective enhancement, making them an important research direction in the field of surface-enhanced Raman scattering (SERS). Ta2O5, as a semiconductor material with high thermal stability, corrosion resistance, outstanding optical properties, and catalytic performance, has great potential in SERS research. This study aims to design and fabricate a composite SERS substrate based on Ta2O5 nanostructures, achieving optimal detection performance by combining the urchin-like structure of Ta2O5 with silver nanoparticles (Ag NPs). The urchin-like Ta2O5 nanostructures were prepared using a hydrothermal reaction method. The bandgap was modulated through structure design and the self-doping technique, the charge transfer efficiency and surface plasmon resonance effects were improved, thereby achieving better SERS performance. The composite substrate enables highly sensitive quantitative detection. This composite SERS substrate combines the electromagnetic enhancement mechanism (EM) and chemical enhancement mechanism (CM), achieving ultra-low detection limits of 10-13 M for R6G. Within the concentration range above 10-12 M, there is a good linear relationship between concentration and peak intensity, demonstrating excellent quantitative analysis capabilities. Furthermore, this composite SERS substrate is capable of precise detection of analytes such as crystal violet (CV) and methylene blue (MB), holding broad application prospects in areas such as food safety and environmental monitoring.

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.

Surface-enhanced Raman scattering (SERS), as one of the most powerful analytical methods, undertakes important inspection tasks in various fields. Generally, the performance of an SERS-active substrate relies heavily on its structure, which makes it difficult to integrate multiple-functional detectability on the same substrate. To address this problem, here we designed and constructed a film of graphene/Au nanoparticles (G/Au film) through a simple method, which can be conveniently transferred to different substrates to form various composite SERS substrates subsequently. By means of the combination of the electromagnetic enhancement mechanism (EM) and the chemical enhancement mechanism (CM) of this structure, the film realized good SERS performance experimentally, with the enhancement factor (EF) approaching ca. 1.40 × 105. In addition, the G/Au film had high mechanical strength and had large specific surface area and good biocompatibility that is beneficial for Raman detection. By further transferring the film to an Ag/Si composite substrate and PDMS flexible film, it showed enhanced sensitivity and in situ detectability, respectively, indicating high compatibility and promising prospect in Raman detection.

Noble metal nanoparticle (NMP)-based composite substrates have garnered significant attention as a highly promising technique for surface-enhanced Raman scattering (SERS) in diverse scientific disciplines because their remarkable ability to amplify and functionalize Raman signals has positioned them as valuable tools for molecular detection. However, optimizing the size and distribution of NMPs has not received sufficient emphasis because of challenges associated with the precise control of deposition and the modulation of reducing rates during growth. In this research, we achieved the optimized size and spatial patterns of AgNWs on electrospun poly(vinylidene fluoride) (PVDF) nanofibers by utilizing a polydopamine (PDA) layer as a mild and controllable reduction mediator, by which the size and density of the AgNWs could be relatively precisely manipulated, achieving a dense distribution of effective "hot spots". On the other hand, harnessing the inherent piezoelectric properties of the electrospun PVDF nanofibers further boosted the LSPR effect during the SERS test, forming a flexible dual-enhancing composite SERS substrate with excellent sensitivity. In addition to addressing structural aspects, exploiting synergistic systems capable of transferring external energy or forces to enhance the SERS performances presents a compelling avenue to broaden the practical applications of SERS. The dual-enhanced substrate achieved an exceptional enhancement factor (EF) of 1.05 × 108 and a low detection limit (LOD) of 10-10 M during the SERS test. This study focuses on integrating NMPs with electrospun piezoelectric polymer nanofibers to develop a dual-enhancing SERS substrate with excellent sensitivity and practicality. The findings provide valuable insights into controllably depositing NMPs on electrospun polymer fibers and hold significant implications for the development of highly sensitive and practical SERS substrates across various applications.

Graphene/noble metal substrates for surface enhanced RAMAN scattering (SERS) possess synergistically improved performance, due to the strong chemical enhancement mechanism accounted to graphene and the electromagnetic mechanism raised from the metal nanoparticles. However, only the effect of noble metal nanoparticles characteristics on the SERS performance was studied so far. In attempts to bring a light to the effect of quality of graphene, in this work, two different graphene oxides were selected, slightly oxidized GOS (20%) with low aspect ratio (1000) and highly oxidized (50%) GOG with high aspect ratio (14,000). GO and precursors for noble metal nanoparticles (NP) simultaneous were reduced, resulting in rGO decorated with AgNPs and AuNPs. The graphene characteristics affected the size, shape, and packing of nanoparticles. The oxygen functionalities actuated as nucleation sites for AgNPs, thus GOG was decorated with higher number and smaller size AgNPs than GOS. Oppositely, AuNPs preferred bare graphene surface, thus GOS was covered with smaller size, densely packed nanoparticles, resulting in the best SERS performance. Fluorescein in concentration of 10−7 M was detected with enhancement factor of 82 × 104. This work demonstrates that selection of graphene is additional tool toward powerful SERS substrates.

The composite substrate composed of precious metal, semiconductor and graphene has not only high sensitivity and uniform Raman signal but also stable chemical properties, which is one of the important topics in the field of surface-enhanced Raman scattering (SERS). In this paper, a sandwich SERS substrate based on tantalum oxide (Ta2O5) is designed and fabricated. The substrate has high sensitivity, stable performance and high quantification capability. The composite substrate can achieve a high sensitivity Raman detection of crystal violet (CV) with a detection limit of 10-11 M and an enhancement factor of 1.5 × 109. This is the result of the synergistic effect of electromagnetic enhancement and chemical enhancement, in which the chemical enhancement is the cooperative charge transfer in the system composed of probe molecules, silver nanoparticles (AgNPs) and Ta2O5, and the electromagnetic enhancement comes from the strong local surface plasmon resonance between the adjacent AgNPs. After exposing the composite substrate to the air for one month, the Raman signal did not weaken, indicating that the performance of the composite substrate is stable. In addition, there is an excellent linear relationship between the intensity of Raman characteristic peak and the concentration of probe molecules, which proves that the composite substrate has high quantification capability. In practical application, the composite SERS substrate can be used to detect harmful malachite green quickly and sensitively and has a broad application prospect in the field of food safety and chemical analysis.

Surface-enhanced Raman scattering (SERS) spectroscopy has been developed into a cross-disciplinary analytical technology through exploring various materials' Raman vibrational modes with ultra-high sensitivity and specificity. Although conventional noble-metal based SERS substrates have achieved great success, oxide-semiconductor-based SERS substrates are attracting researchers' intensive interest due to their merits of facile fabrication, high uniformity and tunable SERS characteristics. Among all the SERS active oxide semiconductors, molybdenum oxides (MoOx) possess exceptional advantages of high Raman enhancement factor, environmental stability, recyclable detection, etc. More interestingly, the SERS effect of the MoOx SERS substrates may involve both the electromagnetic enhancement mechanism and the chemical enhancement mechanism, which is determined by the stoichiometry and morphology of the material. Therefore, the focus of this review will be on two critical points: (1) synthesis and material engineering methods of the functional MoOx material and (2) MoOx SERS mechanism and performance evaluation. First, we review recent works on the MoOx preparation and material property tuning approaches. Second, the SERS mechanism and performance of various MoOx substrates are surveyed. In particular, the performance uniformity, enhancement factor and recyclability are evaluated. In the end, we discuss several challenges and open questions related to further promoting the MoOx as the SERS substrate for monitoring extremely low trace molecules and the theory for better understanding of the SERS enhancement mechanism.

This letter demonstrates a possibility to overcome the polarization-dependent problem in surface enhanced Raman scattering (SERS) performance by two-dimensional (2D) sinusoidal silver grating. A reproducible SERS substrate with a large area can be easily fabricated by maskless laser interference photolithography. The polarization-independent SERS performance and SERS enhancement factor (EF) of this substrate are deduced by finite difference time domain and demonstrated by R6G SERS detection experiments. SERS performance of 2D sinusoidal grating is polarization-independent over the whole 360° and EF can be 5 orders of magnitude as possible. Moreover, this long-range SERS substrate can realize label-free SERS detection of 2,4,6-Trinitrotoluene (TNT).

Tailored FTO/Ag/ZIF-8 structure as SERS substrate for ultrasensitive detection

Tailoring surface enhanced Raman scattering platform based on pulsed laser deposited MoS2 - Ag hybrid nanostructure

High sensitivity, good signal repeatability, and facile fabrication of flexible surface enhanced Raman scattering (SERS) substrates are common pursuits of researchers for the detection of probe molecules in a complex environment. However, fragile adhesion between the noble-metal nanoparticles and substrate material, low selectivity, and complex fabrication process on a large scale limit SERS technology for wide-ranging applications. Herein, we propose a scalable and cost-effective strategy to a fabricate sensitive and mechanically stable flexible Ti3C2Tx MXene@graphene oxide/Au nanoclusters (MG/AuNCs) fiber SERS substrate from wet spinning and subsequent in situ reduction processes. The use of MG fiber provides good flexibility (114 MPa) and charge transfer enhancement (chemical mechanism, CM) for a SERS sensor and allows further in situ growth of AuNCs on its surface to build highly sensitive hot spots (electromagnetic mechanism, EM), promoting the durability and SERS performance of the substrate in complex environments. Therefore, the formed flexible MG/AuNCs-1 fiber exhibits a low detection limit of 1 × 10-11 M with a 2.01 × 109 enhancement factor (EFexp), signal repeatability (RSD = 9.80%), and time retention (remains 75% after 90 days of storage) for R6G molecules. Furthermore, the l-cysteine-modified MG/AuNCs-1 fiber realized the trace and selective detection of trinitrotoluene (TNT) molecules (0.1 μM) via Meisenheimer complex formation, even by sampling the TNT molecules at a fingerprint or sample bag. These findings fill the gap in the large-scale fabrication of high-performance 2D materials/precious-metal particle composite SERS substrates, with the expectation of pushing flexible SERS sensors toward wider applications.

Functionalized UIO-66@Ag nanoparticles substrate for rapid and ultrasensitive SERS detection of di-(2-ethylhexyl) phthalate in plastics

Surface-enhanced Raman scattering (SERS) is extensively employed for detecting organics, where its sensitivity and selectivity are strongly influenced by the properties of the SERS substrates. In this work, a simple hydrothermal synthesis followed by a subsequent reduction was used to prepare Au-CeO2 composite nanocubes as a new SERS substrate, in which the side length of the CeO2 cubes was 20~30 nm and the diameter of the Au nanoparticles was 5~25 nm. Using methylene blue (MB) and crystal violet (CV) as probe molecules, the lowest detection limit (LDL) of methylene blue (MB) on the Au-CeO2 composite nanocubes substrate was 10−7 M, and the maximum SERS enhancement factor (EF) was 2.6 × 105. As a result, the lowest detection limit (LDL) of crystal violet (CV) was 10−7 M, and the maximum enhancement factor (EF) was 3.7 × 104. The above results proved that the Au-CeO2 composite nanocubes had a quite good Raman enhancement effect, which could be used as a SERS substrate. Finally, a Raman enhancement mechanism is proposed for the Au-CeO2 nanucubes.

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.

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