Sensitive Surface-enhanced Raman Spectroscopy Research Articles

Ag Nanoparticles Modified Sea Urchin-like W18O49 Microstructures for Sensitive Surface Enhanced Raman Spectroscopy Detection

Surface-enhanced Raman scattering (SERS) has gained significant attention in various fields, including biomedical diagnostics, environmental monitoring, and food safety. This technique offers many advantages such as label-free detection and the ability to detect analytes at low concentrations. In this study, we present a variety of tungsten oxide (W18O49) microstructures as solid SERS substrates using various solvents realized by a simple hydrothermal approach. Among them, the well-defined sea urchin-like W18O49 microstructures support greater light scattering and confinement to enhance the SERS signal when compared to other W18O49 structures. The surface of well-defined sea urchin-like W18O49 microstructures was then modified with uniformly distributed silver nanoparticles (Ag NPs). The combination of the unique morphology of the Ag and sea urchin-like W18O49 microstructures (W18O49@Ag) led to improved detection capabilities for analytes such as rhodamine 6G (R6G) and thiram molecules. The SERS enhancement factor (EF) of sea urchin-like W18O49@Ag microstructures was estimated to be as high as 1.53×106 with the lowest detectable concentrations of 1nM and 1μM for R6G and thiram, respectively. Thus, the well-defined sea urchin-like W18O49@Ag microstructures are excellent choice to replace the pure noble metal SERS substrates in the fields of rapid detection and trace analysis due to their high sensitivity, repeatability, and long-term stability.

Determination of Trace Levels of Fenbendazole in Milk, Yogurt and Curd Using Surface Enhanced Raman Spectroscopy (SERS)

A simple, rapid and sensitive surface enhanced Raman spectroscopy (SERS) method is described for the determination of fenbendazole (FBZ) residues in milk and fermented dairy products using yogurt and curd as examples. An extraction method was used to purify the samples from interfering components of the matrix. 5-Nitrothiabendazole, which has similar physicochemical properties, was used as an internal standard. The detection limits for milk were 1.8 nM, 8.8 nM for yogurt, and 13.5 nM for curd. Using partial least squares (PLS), a calibration model was built for the determination of fenbendazole from 100 to 1000 nM. The root-mean square error of calibration (RMSEC) and the root-mean square error of prediction (RMSEP) were 39.4 and 46.5 nM, respectively. The recovery rate was from 97% to 120% and the relative standard deviation was less than 5.0%. The difference between this technique from the widely used liquid chromatography–mass spectrometry (LC–MS) method is in the reduction in time for preliminary sample preparation and significantly lower costs for reagents and consumables. The article examines the influence of order when mixing reagents in a SERS experiment, which is an important tool for optimizing the sensitivity and reproducibility of the method. It was shown for the first time that when various alkaline earth metal chlorides were used for agglomeration of silver nanoparticles (AgNPs), a change in the ratio of peak intensities of the two analytes in the solution was observed. This phenomenon may be used to increase the selectivity of SERS analysis of multicomponent samples.

Simultaneous photocatalytic degradation and SERS detection of tetracycline with self-sustainable and recyclable ternary PI/TiO2/Ag flexible microfibers

Facile and efficient photocatalysts using sunlight, as well as fast and sensitive surface-enhanced Raman spectroscopy (SERS) substrates, are urgently needed for practical degradation of tetracycline (TC). To meet these requirements, a new paradigm for PI/TiO2/Ag organic‒inorganic ternary flexible microfibers based on semiconducting titanium dioxide (TiO2), the noble metal silver (Ag) and the conjugated polymer polyimide (PI) was developed by engineering a simple method. Under sunlight, the photocatalytic characteristics of the PI/TiO2/Ag flexible microfibers containing varying amounts of Ag quantum dots (QDs) were evaluated with photocatalytic degradation of TC in aqueous solution. The results demonstrated that the amount of Ag affected the photocatalytic activity. Among the tested samples, PI/TiO2/Ag-0.07 (93.1%) exhibited a higher photocatalytic degradation rate than PI/TiO2 (25.7%), PI/TiO2/Ag-0.05 (77.7%), and PI/TiO2/Ag-0.09 (63.3%). This observation and evaluation conducted in the present work strongly indicated a charge transfer mechanism. Moreover, the PI/TiO2/Ag-0.07 flexible microfibers exhibited highly sensitive SERS detection, as demonstrated by the observation of the Raman peaks for TC even at an extremely low concentration of 10–10 moles per liter. The excellent photocatalytic performance and SERS detection capability of the PI/TiO2/Ag flexible microfibers arose from the Schottky barrier formed between Ag and TiO2 and also from the outstanding plasmonic resonance and visible light absorptivity of Ag, along with immobilization by the PI. The successful synthesis of PI/TiO2/Ag flexible microfibers holds significant promise for sensitive detection and efficient photocatalytic degradation of antibiotics.

Highly Sensitive Microarray Immunoassay for Multiple Mycotoxins on Engineered 3D Porous Silicon SERS Substrate with Silver Nanoparticle Magnetron Sputtering.

A high-throughput, rapid, and highly sensitive surface-enhanced Raman spectroscopy (SERS) microarray for screening multiple mycotoxins has been developed on a three-dimensional silver nanoparticle porous silicon (3D AgNP-Psi) SERS substrate, which was easy to be engineered by electrochemical etching and magnetron sputtering technology. The etching current density, etching waveform, and target material for magnetron sputtering have been investigated to obtain an optimal 3D SERS substrate. The optimized 3D AgNP-Psi SERS substrate showed an enhancement factor of 2.3 × 107 at 400 mA/cm2 constant current density etching for 20 s and Ag target magnetron sputtering for 200 nm thickness on the surface of Psi. The simulation electric field distribution showed the near-field enhancement can reach 3× higher than that of AuNPs. A protein microarray has been designed to screen multiple mycotoxins by AuNP Raman tags and a competitive immunoassay protocol on the surface of the 3D SERS substrate. The SERS protein microarray displayed wide linear detection ranges of 0.001-100 ng/mL for ochratoxin A, 0.01-100 ng/mL for aflatoxin B1, 0.001-10 ng/mL for deoxynivalenol, along with pg/mL low limit of detection, good recovery rates, repeatability, and reproducibility. The 3D SERS protein microarray is easily engineered and has a great potential application in medicine, environment, and food industry fields.

High sensitive SERS film of Ag-coated aggregated nanowire structure and benzene gas detection

A highly sensitive surface-enhanced Raman spectroscopy (SERS) film was fabricated based on anodic aluminum oxide template and Ag coating techniques. The array of alumina nanowires of ∼5 μm length and 26 nm thickness was fabricated and then made collapse to build a micro-scale funnel array, where each cell consists of hundreds of nanowires. Ag film was deposited on the nanowire structure by thermal evaporation. Due to numerous tapered nanogaps and efficient illumination of incident light in the micro-funnel structures, Raman signal enhancement factor (E.F.) as high as ∼1.8 × 107 could be obtained at 785 nm. The SERS film functionalized by 1-propanethiol was used for benzene gas detection, where the sensitivity of <0.3 ppm (3-σ) was realized within 120 s. This nanostructure is expected to be widely used for highly sensitive chemical and bio-sensors and have higher E.F. when the morphology is optimized further.

Sensitive Determination of Carbendazim in Tobacco Leaves by Surface Enhanced Raman Spectroscopy (SERS) Using a Silica – Silver Nanoparticle Substrate and QuEChERs Pretreatment

A sensitive surface enhanced Raman spectroscopy (SERS) substrate based on SiO2@Ag core-shell with a QuEChERs pretreatment method were constructed for the determination of carbendazim residue in tobacco leaves. The SERS substrate was formed by self-assembly of monolayer homogeneous SiO2 nanospheres onto silicon wafer following by vacuum thermal evaporation of silver film. The QuEChERs pretreatment method was used to eliminate the interfering effects of pigments and organic acids in tobacco leaves. The results for carbendazim in tobacco leaves show the limit of detection (LOD) of the SERS substrate was 0.1 mg/kg and the recovery rate was from 82.56% to 95.93%. The proposed low-cost sensitive SERS substrates provide potential application for tobacco safety monitoring.

Plasmon mediated SERS and photocatalysis enhancement in Au nanoparticle decorated 2D-TiSe2

The combination of 2D materials and noble metallic nanostructure is becoming an attractive research domain for label-free, highly sensitive surface-enhanced Raman spectroscopy (SERS) applications. This study demonstrated photocatalysis degradation and SERS enhancements of organic fluorophore (Rhodamine 6G) on two-dimensional (2D) TiSe2 using Raman spectroscopy. The Au nanoparticles (NPs) were decorated on TiSe2 thin films by thermal annealing at variable temperatures. The selective deposition of Au NPs on the hexagonal TiSe2 nanocrystals increases surface roughness, creating a larger surface area for molecule adsorption. It has been observed that the Au decoration at 250 °C on TiSe2 exhibits efficient detection capabilities for R6G with the Raman intensity enhancement factors of the order of ≈105 along with the significantly improved visible light-induced photodegradation efficiency. The optimized Au NP size creates large electromagnetic hot spots produced by strong plasmon coupling that assists in the charge transfer mechanism among TiSe2, Au NPs, and R6G for enhanced SERS and photocatalysis activities. It has been observed that the intensity of Raman scattering decreases as the Au NP size increases on the TiSe2 material. A possible charge transfer mechanism is proposed with an energy band diagram. The simultaneous measurement of SERS and photocatalytic dye degradation in Au decorated TiSe2 can be used as a sensitive technique for water pollution treatment and biodegradable organic contaminants for the environmental ecosystem.

Sensitive detection of dextromethorphan hydrobromide based on portable Raman spectrometer and CuO@AgNPs nano composite SERS substrate

A highly sensitive surface-enhanced Raman spectroscopy approach was established for the detection of dextromethorphan hydrobromide (DXM) utilizing nano CuO modified silver nanoparticles (CuO@AgNPs) as substrate. Ultraviolet visible spectra (UV-vis), X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterized the synthesized CuO@AgNPs. UV-vis and fourier transform infrared (FT-IR) were adopted to investigate the interaction between DXM and CuO@AgNPs. The optimal experimental conditions (the dosages of CuO@AgNPs and NaCl as well as mixing time) were explored. The enhancement factor (EF) was 1.71 × 106. The linear relationship between SERS intensity and the concentration of DXM in the range of 67 – 1000 nmol L−1 was obtained as ISERS = 25.81 c + 40398.77, and the limit of detection (LOD) was 2.12 nmol L−1 (S/N = 3). The interference of K+, Mg2+, Zn2+, Ca2+, Cu2+, Fe3+, glucose, HSA, L-tryptophan, soluble starch and ibuprofen were investigated. The method was successfully applied to test DXM in serum samples. The recovery was 99.06% – 101.51% with the relative standard deviation (RSD) of 0.74% – 3.87%.

Ultrahighly Sensitive Surface-Enhanced Raman Spectroscopy Film of Silver Nanoparticles Dispersed in Three Dimensions on a Thin Alumina Nanowire Framework.

To develop highly sensitive surface-enhanced Raman spectroscopy (SERS) films, various types of aggregated Ag nanowire (NW) and nanoparticle (NP) complex structures were fabricated using anodic aluminum oxide (AAO) templates and thermal evaporation. Aggregated AgNW structures with numerous tapered nanogaps were fabricated via Ag deposition on aggregated thin alumina nanowires of different lengths. AgNP complex structures were obtained by collapsing vertically aligned thin alumina nanowires 1 μm in length and depositing AgNPs on their tops and sides using surface tension during ethanol drying after functionalization. The Raman signal enhancement factors (EFs) of the samples were evaluated by comparing the SERS signal of the thiophenol (TP) self-assembled monolayer (SAM) on the nanostructures with the Raman signal of neat TP. EFs as high as ~2.3 × 107 were obtained for the optimized aggregated AgNW structure (NW length of 1 μm) and ~3.5 × 107 for the optimized AgNP complex structure. The large EF of the AgNP complex film is attributed mainly to the AgNPs dispersed in three dimensions on the sides of the thin alumina nanowires, strongly implying some important, relevant physics yet to be discovered and also a very promising nanostructure scheme for developing ultrahighly sensitive SERS films with EF > 108.

Plasmonic cellulose microfilament assisted SERS detection in microfluidics

Limited by the narrow enhanced area of nanoscale on the metal surface, the sensitivity of surface-enhanced Raman spectroscopy (SERS) detection in solution is usually much lower than the detection in a solid substrate, which is dramatic in microfluidics for online detection. In this work, a cellulose microfilament embraced by Ag nanoparticles, called plasmonic cellulose microfilament, is located in a microchannel for SERS detection in microfluidics. Benefiting from the congestion caused by the plasmonic cellulose microfilament in a microchannel, the trace molecule in the solution is much easier to gather in Ag nanoparticles for Raman enhancement. To obtain high sensitivity, the structure of plasmonic cellulose microfilament is optimized. The SERS spectra collected in this novel microfluidics demonstrate that the plasmonic cellulose microfilament presents a high sensitivity at 10−13 M and good reproducibility in SERS detection. In addition, automatic identification of urea presence or absence was achieved based on deep learning (DL) here. The results show excellent diagnostic accuracy (99 %), which suggests that a fast, label-free urea screening tool can be developed. These results point out this SERS microfluidics with plasmonic cellulose microfilament has a great application prospective in online SERS detection with high sensitivity.

Ultra-sensitive SERS detection of Aβ 1–42 for Alzheimer's disease using graphene oxide/gold nanohybrids

Alzheimer's disease (AD) is an irreversible neurodegenerative brain disorder and the most common cause of dementia with significant social and economic impact. Despite the severity and prevalence of AD, early diagnosis of AD remains quite challenging. One of the important markers of neuropathology in AD patients is the aggregation of Aβ1–42 monomers into oligomers, which then become fibrils and eventually form beta-amyloid plaques. Herein, a highly sensitive surface-enhanced Raman spectroscopy (SERS) method based on graphene oxide/gold nanoparticles (GO/Au NPs) for the detection of Aβ 1–42 is proposed. Specifically, Au NPs were deposited on the GO surface by in situ reduction to form high-density hot spots for SERS. The limits of detection are 0.0232 ng mL-1 and 0.0192 ng mL-1 for Aβ1–42 monomer and fibrils. In addition, support vector machine (SVM) and one-dimensional convolutional neural network (1DCNN) algorithm were used to identify samples with different fibrils degrees. SERS is expected to be used for label-free diagnosis and early detection of AD, which has exciting potential for biomedical detection.

Designing of a functional paper-tip substrate for sensitive surface-enhanced Raman spectroscopy (SERS) detection

A simple and flexible fabrication method of paper SERS substrate was developed by nanoparticles (NPs) droplet self-assembly at the paper tip with a temperature gradient (PTTG). We turned the drawback of the coffee ring effect into an effective way of preparing paper SERS substrate. When the NPs droplets were continuously dripped onto the PTTG, NPs were densely and uniformly distributed at the paper-tip front based on the combination of gravity and the coffee ring effect, which could achieve 91.2-fold improvement of SERS performance compared to a flat filter paper. Meanwhile, the analytes could also be enriched at the paper-tip front, which could achieve 9.3-fold signal enhancement compared to the paper-tip tail. Thus, the PTTG realized an excellent signal amplification for SERS detection. The paper-tip SERS substrate combined with a portable Raman spectrometer yielded an excellent analytical enhancement factor of 1.15 × 105 with the detection limit of 10 nM Rhodamine 6G (R6G). The whole fabrication procedure was completed within 2 h, and the paper-tip substrate showed a satisfactory substrate-to-substrate reproducibility with a relative standard deviation (RSD) of 5.13% (n = 10). It was successfully applied for quantitatively detecting real samples of oxytetracycline and malachite green with recoveries of 83.84–105.25% (n = 3). Meanwhile, we further evaluated the SERS performance of the PTTG using a laboratory-based Raman spectrometer, and it could realize the detection as low as 10 pM R6G. The proposed paper-tip substrate would offer a promising potential application for the on-site SERS analysis of food safety and environmental health.

Localized surface plasmon resonance enhanced charge transfer effect in MoO2/ZnSe nanocomposites enabling efficient SERS detection and visible light photocatalytic degradation

The detection of pharmaceutical residues in the environment based on surface enhanced Raman spectroscopy (SERS) technology has become a popular tool due to its superior molecular specificity and high sensitivity. However, the practical applicability of semiconductor-based SERS sensors for trace pharmaceutical contaminants is limited due to their weak and unstable SERS performance. Herein, a donor-bridge-acceptor (D-B-A) system of MoO2 and ZnSe was fabricated to enhance the separation efficiency of electron-hole pairs in ZnSe and the charge transfer between MoO2 and ZnSe, which causes the MoO2/ZnSe nanocomposites to exhibit excellent SERS sensitivity and photocatalytic activity for the detection of multi-component contaminants and the antibiotic. The LOD of ciprofloxacin hydrochloride (CIP HCl) on MoO2/ZnSe nanocomposites is 8.30 × 10−8 M, and the degradation rate of CIP HCl is 80 % after visible light irradiation for 180 min. Our research effectively improves the SERS sensitivities and expands the practical SERS application of plasma-nonmetal/semiconductor composites, and avoids secondary contamination in the detection process. The enhancement mechanism of SERS and the interpretation of photocatalytic phenomena provide promising guidance for providing a low-cost and stable composite SERS platform in the detection and treatment of emerging contaminants.

Atomic layer deposition assisted fabrication of insertable silver dendrites-based SERS substrates with high adhesion

Considerable efforts have been dedicated to the development of low-cost and highly sensitive surface-enhanced Raman spectroscopy (SERS) substrates, which have found applications in numerous fields. However, the critical factors of adhesion and stability have been insufficiently addressed. Such as Ag dendrites (Ag D) have been recognized for their cost-effectiveness and superior efficiency as SERS substrates, nonetheless, the issue of weak adhesion to the substrate has remained a challenge. Herein, a novel strategy to fabricate a Planar Ag dendrites-based SERS substrate with high adhesion is reported, that employs a combination of atomic layer deposition, roll milling, and chemical etching. The resulting Planar Ag dendrites show excellent adhesion, with a surface stripping force 18 times greater than that of pristine Ag dendrites, allowing for prolonged ultrasonic cleaning. Moreover, the substrate can be used as an insertable SERS sensor to obtain spectral information inside apples due to its excellent adhesion. The proposed design achieves high SERS sensitivity, with a limit of detection of 10-9 M and an analytical enhancement factor of 1.5 × 108 for R6G. Overall, this work provides a valuable reference for improving the adhesion of noble metal nanostructures on supporting substrates, offering a reliable SERS substrate for practical applications.

Au Nanoparticles on Two-Dimensional MoWS2 as Highly Sensitive Surface-Enhanced Raman Spectroscopy Substrates for Adenine Detection

Au Nanoparticles on Two-Dimensional MoWS₂ as Highly Sensitive Surface-Enhanced Raman Spectroscopy Substrates for Adenine Detection

Monolithic 3D structural-substrate SERS sensing platform for ultrasensitive and highly-specific analysis of trace bisphenol A

Constructing advanced substrates with excellent features is promising for sensitive surface-enhanced Raman spectroscopy (SERS) detection. Here a novel capillary monolithic 3D structural-substrate SERS platform with Au@cDNA@Ag@Cyanine 3-aptamer nanoparticles (Au@cDNA@Ag@Cy3-Apt NPs) was fabricated for rapid, highly specific profiling of ultra-trace Bisphenol A (BPA). The proposed SERS platform combined both in-capillary SERS and aptamer-affinity recognition strategies, in which the superior SERS properties of Au–Ag NPs, aptamer selectivity, and the advantages of capillary monolith were integrated. A 3D hierarchically porous network was constructed in the monolithic column, which was endowed with rich hotspots for SERS, rapid sample permeation, and better analysis efficiency than most plane-shaped SERS modes. By varying the amount of Ag+ precursor, the Ag-shell thickness on SERS was finely tuned to guarantee Cy3 label in proximity to the plasmonic surface. Based on the biorecognition of aptamer, the selective identification of BPA occurred and exhibited a significant change in SERS intensity without obvious interference. As a result, the monolithic SERS platform featured facile operation, excellent specificity, and rapid analysis (10 min, much less than the solution-based or planar substrate SERS modes). Ultra-high sensitivity and robust reproducibility for BPA analysis was achieved with a low limit of detection (LOD) at 9.12 × 10-4 ng/L. The feasibility of this SERS platform for monitoring BPA in water and milk samples was also validated. This work lights a new access to capillary monolithic SERS-sensing platform for ultrasensitive and specific analysis of BPA.

Development of a Sensitive SERS Method for Label-Free Detection of Hexavalent Chromium in Tea Using Carbimazole Redox Reaction.

Tea plants absorb chromium-contaminated soil and water and accumulate in tea leaves. Hexavalent chromium (Cr6+) is a very toxic heavy metal; excessive intake of tea containing Cr6+ can cause serious harm to human health. A reliable and sensitive surface-enhanced Raman spectroscopy (SERS) method was developed using Au@Ag nanoparticles as an enhanced substrate for the determination of Cr6+ in tea. The Au@AgNPs coated with carbimazole showed a highly selective reaction to Cr6+ in tea samples through a redox reaction between Cr6+ and carbimazole. The Cr6+ in the contaminated tea sample reacted with methimazole-the hydrolysate of carbimazole-to form disulfide, which led to the decrease in the Raman intensity of the peak at 595 cm-1. The logarithm of the concentration of Cr6+ has a linear relationship with the Raman intensity at the characteristic peak and showed a limit of detection of 0.945 mg/kg for the tea sample. The carbimazole functionalized Au@AgNPs showed high selectivity in analyzing Cr6+ in tea samples, even in the presence of other metal ions. The SERS detection technique established in this study also showed comparable results with the standard ICP-MS method, indicating the applicability of the established technique in practical applications.

A sensitive surface-enhanced Raman spectroscopy detection for gentamicin and tobramycin using γ-Al2O3-modified silver nanoparticles coated with bovine serum albumin as substrate

Two aminoglycoside antibiotics (AGs), gentamicin (GEN) and tobramycin (TOB), have good antibacterial activity against most pseudomonas aeruginosa and staphylococcus. The molecular structure of these drugs lack chromogenic groups, which brings challenges to their detection. In this project, the detecting methods for GEN and TOB utilizing surface-enhanced Raman spectroscopy (SERS) based on γ-Al2O3-modified silver nanoparticles (AgNPs) coated with bovine serum albumin (BSA) were established. The enhancement factors (EFs) of GEN and TOB were 2.44 × 105 and 2.67 × 106, respectively. The transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and spectrophotometric techniques were used to characterize the substrate and the combination of the substance with drugs. The pH, the addition amounts for the substrate and coagulant, as well as the mixing time were optimized. On the basis of single factor experiments, a more scientific response surface model was established. The concentrations of GEN and TOB showed good linear relationships with their Raman signals in the ranges of 6.67 × 10−8 – 2.00 × 10−6 and 6.67 × 10−9 – 3.00 × 10−7 mol L−1 respectively. The limits of detection (LODs) were 11.88 and 1.26 nmol L−1 for GEN and TOB, respectively. The methods were used successfully for the samples determination of the two AGs in commercial drugs and meat products.

A surface-enhanced Raman spectroscopy based smart Petri dish for sensitive and rapid detection of bacterial contamination in shrimp

Indole level in seafood induced by food-borne bacteria is a vital parameter to determine whether the seafood has bacterial contamination. Therefore, to detect pathogens and ensure food safety, rapid, easy to use, and highly sensitive analytical tools are needed for quantifying indole in seafood. In this study, we developed a highly sensitive surface-enhanced Raman spectroscopy (SERS) based tool to detect indole in shrimp. The tool relies on a smart sample container made of a gold nanosphere (AuNS) coated glass Petri dish, AuNS@Dish. The AuNS@Dish was proven to be an efficient plasmonic SERS substrate having an analytical enhancement factor (AEF) of 106. When the indole-containing shrimp extracts were incubated in the plasmonic AuNS@Dish, the SERS signal of indole was significantly enhanced due to the Raman enhancement effects of the AuNS@Dish plasmonic substrate. The change in SERS intensity as a function of indole concentration was used to construct a standard calibration curve leading to a limit of detection (LOD) of 4.97 nM for indole. The standard calibration curve allowed determination of indole level in shrimp samples as low as 0.009 μg/100 g of shrimp which is way below the FDA regulated level, i.e., 25 μg indole/100 g of shrimp. Apart from the high sensitivity, the detection technique was rapid (<10 min), easy to handle, and fulfilled requirements for determination of bacterial contamination in shrimp. Overall, this is a novel, facile, and sensitive analytical method to detect indole in shrimp with possible future applications in other seafood.

The Decisive Role of Li2O2 Desorption for Oxygen Reduction Reaction in Li–O2 Batteries

Fundamental issues relevant to the oxygen reduction reaction (ORR) mechanism and reaction interface are ambiguous in Li–O2 batteries. Herein, we utilized highly sensitive surface-enhanced Raman spectroscopy (SERS) to reveal the spontaneous desorption behavior of insoluble products of lithium peroxide (Li2O2) from the electrode surface. Furthermore, the electrochemical ORR mechanism is elucidated at the electrode/Li2O2 interface based on a dynamic equilibrium between the generation and desorption of Li2O2. The desorption of adsorbed Li2O2 species (Li2O2*) is crucial to releasing surface sites and maintaining the electrochemical ORR process at the electrode substrate surface instead of the Li2O2/electrolyte interface. The proceeding of Li2O2* desorption can guarantee the stability of Li2O2* concentration and the discharge plateau in the galvanostatic ORR process. The suppression of Li2O2* desorption is proved to induce the termination of Li2O2* generation, which is accompanied by the growth of adsorbed lithium superoxide (LiO2*), leading to rapid potential attenuation.