3D composite SERS substrate constructed by Au-Ag core-satellite NPs and polystyrene sphere for ultrasensitive ratiometric Raman detection of cotinine.

Surface-enhanced Raman spectroscopy (SERS) is a powerful vibrational spectroscopy technique for highly sensitive structural detection of low concentration analyte. The SERS activities largely depend on the topography of the substrate. In this review, we summarize the recent progress in SERS substrate, especially focusing on the three-dimensional (3D) noble-metal substrate with hierarchical nanostructure. Firstly, we introduce the background and general mechanism of 3D hierarchical SERS nanostructures. Then, a systematic overview on the fabrication, growth mechanism, and SERS property of various noble-metal substrates with 3D hierarchical nanostructures is presented. Finally, the applications of 3D hierarchical nanostructures as SERS substrates in many fields are discussed.

Optimization of cotton SERS substrates based on different weave structures for surface trace detection

ConspectusSurface-enhanced Raman spectroscopy (SERS) fingerprinting is highly promising for identifying disease markers from complex mixtures of clinical sample, which has the capability to take medical diagnoses to the next level. Although vibrational frequency in Raman spectra is unique for each biomolecule, which can be used as fingerprint identification, it has not been considered to be used routinely for biosensing due to the fact that the Raman signal is very weak. Contemporary SERS has been demonstrated to be an excellent analytical tool for practical label-free sensing applications due its ability to enhance Raman signals by factors of up to 108–1014 orders of magnitude. Although SERS was discovered more than 40 years ago, its applications are still rare outside the spectroscopy community and it is mainly due to the fact that how to control, manipulate and amplify light on the “hot spots” near the metal surface is in the infancy stage.In this Account, we describe our contribution to develop nanoachitecture based highly reproducible and ultrasensitive detection capability SERS platform via low-cost synthetic routes. Using one-dimensional (1D) carbon nanotube (CNT), two-dimensional (2D) graphene oxide (GO), and zero-dimensional (0D) plasmonic nanoparticle, 0D to 3D SERS substrates have been designed, which represent highly powerful platform for biological diagnosis. We discuss the major design criteria we have used to develop robust SERS substrate to possess high density “hot spots” with very good reproducibility. SERS enhancement factor for 3D SERS substrate is about 5 orders of magnitude higher than only plasmonic nanoparticle and more than 9 orders of magnitude higher than 2D GO. Theoretical finite-difference time-domain (FDTD) stimulation data show that the electric field enhancement |E|2 can be more than 2 orders of magnitude in “hot spots”, which suggests that SERS enhancement factors can be greater than 104 due to the formation of high density “hot spots” in 3D substrate. Next, we discuss the utilization of nanoachitecture based SERS substrate for ultrasensitive and selective diagnosis of infectious disease organisms such as drug resistance bacteria and mosquito-borne flavi-viruses that cause significant health problems worldwide. SERS based “whole-organism fingerprints” has been used to identify infectious disease organisms even when they are so closely related that they are difficult to distinguish. The detection capability can be as low as 10 CFU/mL for methicillin-resistant Staphylococcus aureus (MRSA) and 10 PFU/mL for Dengue virus (DENV) and West Nile virus (WNV). After that, we introduce exciting research findings by our group on the applications of nanoachitecture based SERS substrate for the capture and fingerprint detection of rotavirus from water and Alzheimer’s disease biomarkers from whole blood sample. The SERS detection limit for β-amyloid (Aβ proteins) and tau protein using 3D SERS platform is several orders of magnitude higher than the currently used technology in clinics. Finally, we highlight the promises, major challenges and prospect of nanoachitecture based SERS in biomedical diagnosis field.

One-pot synthesis of silver nanoparticle-coated fiberglass as a reusable and flexible SERS substrate of malachite green

An in-situ method for SERS substrate preparation and optimization based on galvanic replacement reaction

With the development of flexible surface-enhanced Raman spectroscopy (SERS) substrates that can realize rapid in situ detection, the SERS technique accompanied by miniaturized Raman spectrometers holds great promise for point-of-care testing (POCT). For an in situ detection strategy, constructing high-performance flexible and transparent SERS substrates through a facile and cost-effective fabrication method is critically important. Herein, we present a simple method for fabricating a large-area flexible and transparent SERS substrate consisting of a silver-nanoparticle-grafted wrinkled polydimethylsiloxane (Ag NPs@W-PDMS) film, using a surface-wrinkling technique and magnetron sputtering technology. By characterizing rhodamine 6G as a probe molecule with a portable Raman spectrometer, the flexible SERS substrate shows a low detection limit (10-7 M), a high enhancement factor (6.11 × 106), and excellent spot-spot and batch-batch reproducibilities (9.0% and 4.2%, respectively). Moreover, the Ag NPs@W-PDMS substrate maintains high SERS activity under bending and twisting mechanical deformations of over 100 cycles, as well as storage in air for 30 days. To evaluate its practical feasibility, in situ detection of malachite green on apple and tomato peels is performed with a detection limit of 10-6 M. In addition, for point-of-care analysis, we develop a wireless transmission system to transmit the collected SERS spectral data to a computer in real time for signal processing and analysis. Therefore, the proposed Ag NPs@W-PDMS SERS substrate fabricated through a simple and mass-producible method, combined with the utilization of a portable Raman spectrometer and wireless communication, offers a promising opportunity to extend the SERS technique from the laboratory to POCT applications.

We describe a patterned surface-enhanced Raman spectroscopy (SERS) substrate with the ability to pre-concentrate target molecules. A surface-adsorbed nanosphere monolayer can serve two different functions. First, it can be made into a SERS platform when covered by silver. Alternatively, it can be fashioned into a superhydrophobic surface when coated with a hydrophobic molecular species such as decyltrimethoxy silane (DCTMS). Thus, if silver is patterned onto a latter type of substrate, a SERS spot surrounded by a superhydrophobic surface can be prepared. When an aqueous sample is placed on it and allowed to dry, target molecules in the sample become pre-concentrated. We demonstrate the utility of the patterned SERS substrate by evaluating the effects of inhibitors to acetylcholinesterase (AChE). AChE is a popular target for drugs and pesticides because it plays a critical role in nerve signal transduction. We monitored the enzymatic activity of AChE through the SERS spectrum of thiocholine (TC), the end product from acetylthiocholine (ATC). Inhibitory effects of paraoxon and carbaryl on AChE were evaluated from the TC peak intensity. We show that the patterned SERS substrate can reduce both the necessary volumes and concentrations of the enzyme and substrate by a few orders of magnitude in comparison to a non-patterned SERS substrate and the conventional colorimetric method.

A flowerlike mesoporous FeF3·0.33H2O with three-dimensional (3D) hierarchical nanostructure is successfully prepared for the first time through a facile PEG-assisted solvothermal route. By rationally regulating and controlling the amount of PEG-400 in solvent, a series of nanostructured FeF3·0.33H2O materials with various morphologies and sizes is obtained. The probable formation mechanism related to the role of the PEG is explored. Furthermore, the electrochemical performances of the as-prepared samples for Na-ion battery (NIB) are investigated and discussed. The flower-like mesoporous FeF3·0.33H2O can deliver the noticeable initial discharge capacity of 283 mAh g–1 and retain 190 mAh g–1 after 100 cycles within a potential range of 1.5–4.5 V at 0.1 C. Particularly, even at a high rate of 2.0 C, the material can still exhibit a high capacity of 155 mAh g–1. The excellent electrochemical performances can be attributed to the unique 3D hierarchical porous nanostructure and the small particle size, which ca...

Facile fabrication of silver nanoparticle decorated α-Fe2O3 nanoflakes as ultrasensitive surface-enhanced Raman spectroscopy substrates

A highly sensitive and uniform three-dimensional (3D) surface-enhanced Raman spectroscopy (SERS) substrate has been fabricated by in situ ultraviolet ozone cleaning and layer-by-layer self-assembly. The SERS properties and the structural changes of the substrates were systematically studied by adjusting the cleaning time upon the in situ and post cleaning strategy. Under the optimal cleaning condition, the cleaning technology could give rise to clean and optimal surfaces for SERS analysis, thus obtaining efficient plasmonic films populated with a large number of homogeneous ‘hot-spots’. Then with the optimal monolayer film, the SERS performance derived from plasmon coupling in multilayers of the Au@Ag nanocubes substrates was explored. It demonstrated that the plasmon coupling between layers (out-of-plane) contributed much to the SERS intensity, leading a more superior SERS enhancement from the 3D SERS substrates than that from the conventional two-dimensional SERS substrates. Also the in situ cleaning 3D SERS substrates displayed a nice uniformity and excellent time stability. With this method, the optimized substrates were further used to detect prohibited pigments in drink with an excellent linear relationship between characteristic peak intensity and analytes concentration over wide concentration ranges. Our experimental results clearly show that the in situ cleaning 3D SERS substrates provide an ideal candidate for SERS applications in food safety.

Rapid and flexible construction of inverted silicon architectures with nanogaps as high-performance SERS substrates

Silver nanoparticles as surface-enhanced Raman substrate for quantitative identification of label-free proteins

Discovery of new surface-enhanced Raman spectroscopy (SERS) substrates consisting of inexpensive and earth-abundant elements is an unmet need for the advancement of future analysis techniques for the ultrasensitive detection and quantification of chemical and biological analytes. Nanostructures (NSs) of noble metals such as Au, Ag, and Cu are the benchmarks for the preparation of highly efficient SERS substrates because of their unique localized surface plasmon resonance (LSPR) properties. Non-noble-metal SERS substrates, e.g., metal chalcogenide semiconductors and transition metal oxides, have been prepared to mitigate the cost; however, their low sensitivity restricts widespread applications. In this article, we report for the first time that the structure of oxygen-deficient, LSPR-active, nonstoichiometric tungsten oxide (i.e., WO3-x) NSs can control the SERS enhancement factor (EF). SERS substrates prepared with colloidally synthesized WO3-x nanowires, nanorods, and nanoplatelets display SERS EF of 2.5 × 106, 3.1 × 107, and 5.5 × 107, respectively, using rhodamine 6G (R6G) molecules as Raman probes. Our experimentally acquired SERS data and spectroscopically determined electronic band structure of LSPR-active WO3-x NSs, and time-domain density functional theory (TDDFT)-based calculations support a dual enhancement scheme involving a strong plasmonic effect controlled electromagnetic field and their oxygen vacancy-induced chemical enhancement mechanisms, respectively. To demonstrate the practical utility of our WO3-x NCs, we are able to detect aromatic nitro-explosives (tetryl, TNT, and DNT) with a very low limit of detection (LOD, 10-9 M). Importantly, machine learning-driven chemometric analysis for SERS-based detection shows excellent classification between these three explosives. Finally, three nonaromatic, nitro-explosives, HMX, RDX, and PETN are also successfully detected utilizing our LSPR-active, WO3-x-based SERS substrates. To the best of our knowledge, this is the first example where LSPR-active, non-noble-metal NSs are used for the detection of both aromatic and aliphatic nitro-explosives. Taken together, our work represents the advancement of the fabrication of non-noble-metal-based SERS substrates, which can be widely employed for the low-cost detection of analytes across forensic science, chemistry, and biomedical fields.

The performance of surface-enhanced Raman spectroscopy (SERS) substrates is typically evaluated by calculating an enhancement factor (EF). However, it is challenging to accurately calculate EF values since the calculation often requires the use of model analytes and requires assumptions about the number of analyte molecules within the laser excitation volume. Furthermore, the measured EF values are target analyte dependent and thus it is challenging to compare substrates with EF values obtained using different analytes. In this study, we propose an alternative evaluation parameter for SERS substrate performance that is based on the intensity of the surface plasmon enhanced Rayleigh band (IRayleigh) that originates from the amplified spontaneous emission (ASE) of the laser. Compared to the EF, IRayleigh reflects the enhancing capability of the substrate itself, is easy to measure without the use of any analytes, and is universally applicable for the comparison of SERS substrates. Six SERS substrates with different states (solid, suspended in liquid, and hydrogel), different plasmonic nanoparticle identities (silver and gold), as well as different nanoparticle sizes and shapes were used to support our hypothesis. The results show that there are excellent correlations between the measured SERS intensities and IRayleigh as well as between the SERS homogeneity and the variation of IRayleigh acquired with the six SERS substrates. These results suggest that IRayleigh can be used as an evaluation parameter for both SERS substrate efficiency and reproducibility.

In-situ galvanic replacement reaction assisted preparation of porous Cu-Au composites as highly sensitive SERS substrates.