Surface-enhanced Raman scattering (SERS) spectroscopy provides a noninvasive and highly sensitive route for fingerprint and label-free detection of a wide range of molecules. Recently, flexible SERS has attracted increasingly tremendous research interest due to its unique advantages compared to rigid substrate-based SERS. Here, the latest advances in flexible substrate-based SERS diagnostic devices are investigated in-depth. First, the intriguing prospect of point-of-care diagnostics is briefly described, followed by an introduction to the cutting-edge SERS technique. Then, the focus is moved from conventional rigid substrate-based SERS to the emerging flexible SERS technique. The main part of this report highlights the recent three categories of flexible SERS substrates, including actively tunable SERS, swab-sampling strategy, and the in situ SERS detection approach. Furthermore, other promising means of flexible SERS are also introduced. The flexible SERS substrates with low-cost, batch-fabrication, and easy-to-operate characteristics can be integrated into portable Raman spectroscopes for point-of-care diagnostics, which are conceivable to penetrate global markets and households as next-generation wearable sensors in the near future.

https://www.onlinelibrary.wiley.com/doi/epdf/10.1002/advs.201900925

Toward Flexible Surface-Enhanced Raman Scattering (SERS) Sensors for Point-of-Care Diagnostics

Semiconductor-based surface-enhanced Raman spectroscopy (SERS) substrates represent a new frontier in the field of SERS. However, the application of semiconductor materials as SERS substrates is still seriously impeded by their low SERS enhancement and inferior detection sensitivity, especially for non-metal-oxide semiconductor materials. Herein, we demonstrate a general oxygen incorporation-assisted strategy to magnify the semiconductor substrate–analyte molecule interaction, leading to significant increase in SERS enhancement for non-metal-oxide semiconductor materials. Oxygen incorporation in MoS2 even with trace concentrations can not only increase enhancement factors by up to 100,000-fold compared with oxygen-unincorporated samples but also endow MoS2 with low limit of detection below 10−7 M. Intriguingly, combined with the findings in previous studies, our present results indicate that both oxygen incorporation and extraction processes can result in SERS enhancement, probably due to the enhanced charge-transfer resonance as well as exciton resonance arising from the judicious control of oxygen admission in semiconductor substrate. The application of non-metal-oxide semiconductor materials as surface-enhanced Raman spectroscopy (SERS) substrates is impeded by their low SERS enhancement and detection sensitivity. Here, the authors develop a general oxygen incorporation strategy to magnify these parameters.

/pdf/semiconductor-sers-enhancement-enabled-by-oxygen-3gavybzo72.pdf

Semiconductor SERS enhancement enabled by oxygen incorporation

The quantitative determination and identification of bio-/chemical molecules at ultra-low concentrations is a hot topic in several fields including medical diagnostics, environmental science, and homeland security. Molecular detection techniques are conventionally based on optical, electrochemical, electronic, or gravimetric methodologies. Among these methods, surface-enhanced Raman spectroscopy (SERS) is considered as one of the most reliable, sensitive and selective techniques for non-destructive molecular analysis through the amplification of electromagnetic fields and/or creation of charge-transfer states between the chemisorbed analyte molecule and SERS active platform. Unfortunately, the applicability of SERS is rather limited, which is mainly due to the lack of highly sensitive SERS platforms with good stability and reproducibility. In line with this, metal nanoparticles (e.g., Au, Ag, and Cu) have been extensively exploited as SERS active platforms. Although the utilization of metallic nanoparticles in SERS is simple and cost-effective, the poor controllability of the structures and limited formation of hot spots in the detection zone leads to discrepancy in the resulting SERS signals. For these reasons, in the past few years, researchers have focused on fabricating 3-dimensional (3D) SERS platforms, which increase the adsorption of analyte molecules and facilitate hot spot formation in all three dimensions. However, the fabrication of 3D SERS platforms is mostly expensive and technologically demanding. Therefore, the discovery of non-metal alternative approaches is of great interest not only to widen SERS applications but to further elucidate fundamental questions. Considering recent developments on the fabrication and application of SERS active platforms, this review is structured in 3 main directions; (1) implementation of the plasmonic nanoparticles having different shapes into SERS-active platforms, (2) highlighting recent developments in the fabrication and application of 3D SERS-active platforms, and (3) examination of recent novel inorganic and organic semiconductor based platforms for SERS applications. At the end, we conclude with the promises and challenges for the future evolution of SERS.

Surface-enhanced Raman spectroscopy (SERS): an adventure from plasmonic metals to organic semiconductors as SERS platforms

Current plasmonic photocatalysts are mainly based on noble metal nanoparticles and rarely work in the infrared (IR) light range. Herein, cost-effective Bi2 O3-x with oxygen vacancies was formed in situ on commercial bismuth powder by calcination at 453.15 K in atmosphere. Interestingly, defects introduced into Bi2 O3-x simultaneously induced a localized surface plasmon resonance (LSPR) in the wavelength range of 600-1400 nm and enhanced the adsorption for CO2 molecules, which enabled efficient photocatalysis of CO2 -to-CO (ca. 100 % selectivity) even under low-intensity near-IR light irradiation. Significantly, the apparent quantum yield for CO evolution at 940 nm reached 0.113 %, which is approximately 4 times that found at 450 nm. We also showed that the unique LSPR allows for the realization of a nearly linear dependence of photocatalytic CO production rate on light intensity and operating temperature. Finally, based on an IR spectroscopy study, an oxygen-vacancy induced Mars-van Krevlen mechanism was proposed to understand the CO2 reduction reactions.

Plasmonic Hot Electrons from Oxygen Vacancies for Infrared Light-Driven Catalytic CO 2 Reduction on Bi 2 O 3-x

A novel 3D glass surface‐enhanced Raman spectroscopy (SERS) microfluidic chip is fabricated by an all‐femtosecond‐laser‐processing technique. 2D periodic Cu‐Ag nanodots are generated for SERS probes through femtosecond‐laser‐induced orthorhombic surface structures. This technique shows significant promise for the fabrication of a high‐performance microfluidic SERS platform for the real‐time online sensing of toxic substances with ultrahigh sensitivity.

3D Microfluidic Surface-Enhanced Raman Spectroscopy (SERS) Chips Fabricated by All-Femtosecond-Laser-Processing for Real-Time Sensing of Toxic Substances

Compared with noble metals, semiconductors with surface plasmon resonance effect are another type of SERS substrate materials. The main obstacles so far are that the semiconducting materials are often unstable and easy to be further oxidized or decomposed by laser irradiating or contacting with corrosive substances. Here, we report that metallic MoO2 can be used as a SERS substrate to detect trace amounts of highly risk chemicals including bisphenol A (BPA), dichloropheno (DCP), pentachlorophenol (PCP) and so on. The minimum detectable concentration was 10−7 M and the maximum enhancement factor is up to 3.75 × 106. To the best of our knowledge, it may be the best among the metal oxides and even reaches or approaches to Au/Ag. The MoO2 shows an unexpected high oxidation resistance, which can even withstand 300 °C in air without further oxidation. The MoO2 material also can resist long etching of strong acid and alkali. Semiconducting materials are potential SERS substrates as alternatives to noble metals, but often suffer from poor stabilities and sensitivities. Here, the authors use molybdenum dioxide as a SERS material, showing high enhancement factors and stability to oxidation even at high temperatures.

A metallic molybdenum dioxide with high stability for surface enhanced Raman spectroscopy.

Transition metal nitrides have been widely studied due to their high electrical conductivity and excellent chemical stability. However, their preparation traditionally requires harsh conditions because of the ultrahigh activation energy barrier they need to cross in nucleation. Herein, we report three-dimensional porous VN, MoN, WN, and TiN with high surface area and porosity that are prepared by a general and mild molten-salt route. Trace water is found to be a key factor for the formation of these porous transition metal nitrides. The porous transition metal nitrides show hydrophobic surface and can adsorb a series of organic compounds with high capacity. Among them, the porous VN shows strong surface plasmon resonance, high conductivity, and a remarkable photothermal conversion efficiency. As a new type of corrosion- and radiation-resistant surface-enhanced Raman scattering substrate, the porous VN exhibits an ultrasensitive detection limit of 10−11 M for polychlorophenol. Fabrication of transition metal nitrides generally requires harsh conditions due to the high activation energy barrier for nucleation. Here, the authors report a mild molten-salt route for the synthesis of three-dimensional porous metal nitrides, VN, MoN, WN, and TiN.

/pdf/general-molten-salt-route-to-three-dimensional-porous-2kxz9i80oc.pdf

Jing Han

Papers

A metallic molybdenum dioxide with high stability for surface enhanced Raman spectroscopy.

General molten-salt route to three-dimensional porous transition metal nitrides as sensitive and stable Raman substrates

Enhanced pyrolysis of Huadian oil shale at high temperature in the presence of water and air atmosphere