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.

The deposition and hydrolysis reaction of SO2 + H2O in small clusters of sulfuric acid and water are studied by theoretical calculations of the molecular clusters SO2-(H2SO4)n-(H2O)m (m = 1,2; n = 1,2). Sulfuric acid exhibits a dramatic catalytic effect on the hydrolysis reaction of SO2 as it lowers the energy barrier by over 20 kcal/mol. The reaction with monohydrated sulfuric acid (SO2 + H2O + H2SO4 - H2O) has the lowest energy barrier of 3.83 kcal/mol, in which the cluster H2SO4-(H2O)2 forms initially at the entrance channel. The energy barriers for the three hydrolysis reactions are in the order SO2 + (H2SO4)-H2O > SO2 + (H2SO4)2-H2O > SO2 + H2SO4-H2O. Furthermore, sulfurous acid is more strongly bonded to the hydrated sulfuric acid (or dimer) clusters than the corresponding reactant (monohydrated SO2). Consequently, sulfuric acid promotes the hydrolysis of SO2 both kinetically and thermodynamically. Kinetics simulations have been performed to study the importance of these reactions in the reduction of atmospheric SO2. The results will give a new insight on how the pre-existing aerosols catalyze the hydrolysis of SO2, leading to the formation and growth of new particles.

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As a powerful non-destructive and label-free detection technology, surface-enhanced Raman scattering (SERS) has been widely used in environmental-pollutant detection, biological-tissue sensing, molecular fingerprint analysis and so on. Different from the traditional SERS substrates represented by noble metals and semiconductors, herein, we report a new highly sensitive SERS substrate material with high stability, biocompatibility, and low cost, namely nucleusfree two-dimensional electron gas (2DEG) Ti 3 C 2 monolayer nanosheets. The highly crystalline monolayer Ti 3 C 2 nanosheets with clean surface are synthesized by an improved chemical exfoliation and microwave heating method. The unique structure of nucleus-free-2DEG in Ti 3 C 2 monolayer provides an ideal transport channel without nuclear scattering, which makes the highly crystalline monolayer Ti 3 C 2 nanosheets achieve a Raman enhanced factor of 3.82×10 8 and a 10 −11 level detection limit for typical environmental pollutants such as azo dyes, trichlorophenol, and bisphenol A. Singlemolecule imaging is also realized on the surface of the Ti 3 C 2 monolayers, which may be the first time that approximate single-molecule imaging has been achieved on a non-noblemetal SERS substrates. Preliminary toxicological experiments show that the cytotoxicity of this material is very low. Considering the facile synthesis, high biocompatibility, low cost and high chemical stability of carbide nanosheets, these Ti 3 C 2 monolayer nanosheets show significant promise for the design and fabrication of flexible SERS substrates for the sensing of trace substances with ultrahigh sensitivity.