Surface Enhanced Raman Scattering Substrates Based On Semiconductor And Noble Metal Nanocomposites And Its Analytical Application

The illegal addition of prohibited substances in food,and the deterioration of long-term food will pose a threat to human health.At present,the methods for the detection of prohibited substances in food mainly include fluorescence,electrochemical and liquid chromatography.However,these methods are complex and time-consuming to preprocess.Therefore,the development of simple,fast and sensitive methods for analysis and detection is the key to evaluate food safety.As a powerful technology,surface enhanced Raman scattering（SERS）can provide fast,highly sensitive and non-mark detection of trace chemicals.At present,the SERS enhancement mechanism mainly includes electromagnetic and chemical mechanisms.The electromagnetic enhancement is caused by local surface plasmon resonance（LSPR）and SERS hot spots.The physical or chemical adsorption of analyte molecules on the surface may lead to charge transfer,which is called chemical enhancement.EM enhancement is considered to play a leading role in SERS enhancement mechanism.The challenge of SERS in practical application is how to prepare SERS substrates with high sensitivity and good stability.Noble metals have unique local surface plasmon resonance and plasmon resonance frequency in the common excitation wavelength range of Raman spectroscopy.Therefore,noble metals are the attractive materials for advanced SERS substrates.However,their disadvantages like easy agglomeration,high cost,limited wavelength range and unfavorable biocompatibility are the main challenge for its real application.In order to improve the stability of the substrate,semiconductor and noble metal are combined for SERS substrates.Semiconductor（ZnO）not only has certain SERS activity,but also has the advantages of good biocompatibility,pH resistance,optical stability and high repeatability.The shape,size and composition of noble metal nanoparticles and semiconductors can adjust their absorption wavelength and sensitivity.SERS substrates composed of noble metal and semiconductors are considered as excellent dual function system due to the unique combination of LSPR and photocatalytic activity in the composition.The coupling of noble metal and semiconductors can provide high sensitivity and recyclable SERS substrate respectively.This paper aims to expand the application of semiconductors in the field of SERS.Herein,we innovatively propose to combine the high stability,high reproducibility and selectivity of semiconductors with the high sensitivity of noble metals to achieve strong combination and complementary advantages for obtaining advanced SERS substrate.A series of SERS matrix composites based on semiconductors and noble metals were prepared,and their characterization and SERS properties were also studied.Finally,they were conducted to detect harmful substances in food.The specific research contents as follows:1.A two-step method is used to prepare a regenerative three-dimension（3D）ZnO/Ag@Au substrate for developing superior sensitive surface enhanced Raman scattering（SERS）method for detecting antibiotics.A great electromagnetic enhancement is observed from as-prepared composite substrate,which is triggered by tuning electron distribution of metals and semiconductor metal oxide.The strong interaction between target sample and the huge surface area of ZnO/Ag@Au composite promotes the charge transfer to produce promising chemical enhancement.The synergistically physical and chemical enhancement mechanism is validated by density functional theory（DFT）and finite difference time domain（FDTD）simulation.Additionally,the presence of light“echo effect”in 3D structure of ZnO support could also amplify the efficiency of light excitation for Raman scattering.The above-stated merits benefit to boost the Raman scattering detection sensitivity for real samples.The ZnO/Ag@Au-based SERS substrate could detect Rhodamine 6G（R6G）molecules with an enhancement factor up to 1.48×10⁹and the lowest detectable concentration of 10^-10mol/L.As a real application,antibiotics sulfapyridine in milk is determined by using proposed SERS protocol and the limit of detection（LOD）at1×10^-9mol/L could be reached.As a prospective,the ZnO/Ag@Au-based SERS method would be extended for food safety and biomedicine analysis.2.In the second work,the energy level of ZnO carrier is adjusted by doping cobalt（Co）,so as to adjust the metal carrier interaction of the final composite Co-ZnO/Au.The“doping modification”effect of Co in certain ion ratio is beneficial to increase of separation efficiency of electron-hole-pair of ZnO under visible laser and shift of LSPR peak to visible region.Such metal-like property of the Co-ZnO semiconductor with the abundant free electrons improves surface enhancement Raman scattering（SERS）performance of Au nanoparticles.By using Co-ZnO/Au the detection performance is effectively promoted and R6G could be detectable as low as1×10^-9mol/L.An enhancement mechanism of Co-ZnO/Au is deeply discussed by observation of electron microscope,UV-visible,X-ray diffraction,photoluminescence and X-ray photoelectron spectroscopy.As a real application,tyramine in beer is analyzed by Co-ZnO/Au-based Raman method and detection limit of 1×10^-8mol/L is achieved.It paves a way to develop a controllable metal-semiconductor interaction strategy for preparing high performance SERS substrates.3.In the third work,the contribution of semiconductor ZnO in the composites was further developed.Based on the absorption of needle-like ZnO in the visible region,the properties of the composites ZnO/Au were adjusted for better SERS performance.Needle-like ZnO with high surface area and abundant surface tips,and then loaded dense gold nanoparticles on its surface.On the one hand,the needle like structure effectively prevented the aggregation of noble metal nanoparticles;on the other hand,the synthesis method was simple and easy to repeat,which provided convenience for on-site detection or rapid screening of a large number of samples.ZnO/Au-based-SERS method could be applied for rapid detection of nicotine with a detection limit of 1×10^-10mol/L.