Synthesis Of Ag(Ⅰ)-linked Polymolybdates As Crystalline Materials And Their SERS Sensing For Reductive Small Molecules

Surface enhanced Raman scattering spectroscopy(SERS)is a highly efficient and multifunctional leading analytical technique that can be used for the qualitative and quantitative detection of trace molecules.However,for the analysis of SERS detection,designing efficient SERS substrates and stable internal standards are crucial issues in achieving reliable SERS detection.In this thesis,silver ions are used to connect polyoxometalates(POMs)with characteristic Raman signals to form two-dimensional(2D)or three-dimensional(3D)structural substrates,where POMs can simultaneously serve as internal standards or signal molecules for the qualitative and quantitative analysis of small molecules with reducibility.The main research results obtained are as follows.1.In order to investigate the SERS response of POMs as signal molecules to the reductive hydrazine(N2H4),a novel 2D crystalline material of Ag2[Ag(CH3CN)2]2[βMo8O26](Ag4Mo8)has been designed and synthesized.The structural analysis shows that Ag4Mo8 is composed of the classical Lindqvist-type polyoxoanions,Ag(I)cations and acetonitrile molecules.Among them,polyoxoanion and Ag(I)cations are covalently bonded to form a 2D layer.Due to the strong reducibility of N2H4,when Ag4Mo8 is exposed to the vapor atmosphere of N2H4,part Ag+on the surface of Ag4Mo8 can be reduced to form silver nanoparticles(AgNPs).At this time,the surrounding plasma of AgNPs is excited by Raman laser to generate a strong electromagnetic field.Therefore,the Mo-O bond closer to Ag0 will generate the SERS signals,and the intensity values exhibit a good linear relationship with the logarithm of N2H4 concentrations.The minimum detected N2H4 concentration is 10-10 mg/L.These results indicate that using the coordination action of silver ions with oxygen atoms,it is easy to connect POMs into stable crystalline materials.Especially,the formation of single crystal structures is more conducive to obtain stable SERS signals,thereby achieving sensitive and accurate detection of molecules under detection.2.A novel 3D porous silver linked POM compound Ag6[MnMo9O32]·4H2O(Ag6MnMo9)has been designed and synthesized.According to the structural analysis,Ag6MnMo9 is composed of Waugh-type[MnMo9O32]6-polyoxoanions,Ag(I)cations,and water molecules.The Ag(I)cation covalently forms Ag-O bond with the oxygen terminal atoms of the[MnMo9O32]6-polyoxoanion,forming a 2D interconnected 3D framework structure.When Ag6MnMo9 is exposed to N2H4 vapor,small sized N2H4 molecules enter the pores.At this time,the reductive N2H4 reduces a portion of the Ag+in the pores to form AgNPs.Under the excitation of Raman lasers,the surrounding plasma generates a strong electromagnetic field.At the same time,the[MnO6]group in[MnMo9O32]6-is wrapped by the[Mo3O13]group,so the generated strong electromagnetic field has no effect on the vibration of the Mn-O bond,but it has a significant impact on the vibration of the Mo-O bond near AgNPs,thereby increasing the Raman signal of the Mo-O bond.Based on this,a linear relationship between the ratio SERS signal of IMo-O/IMn-o and the logarithm of N2H4 concentrations can be obtained in the detection of N2H4(10-10～10-3 mg/L)for Ag6MnMo9.The minimum detection limit for N2H4 is 40 pg/L(1.8×10-5 ppb).Finally,the SERS signal of Ag6MnMo9 is more sensitive and stable than that of similar 3D composites with completely filled pore channels,further demonstrating that its porous structure promotes the interaction of silver with targeted molecules.The 3D porous material Ag6MnMo9 has good specificity and sensitivity for N2H4 without any sample pretreatment,providing a new idea for the detection of trace N2H4.3.A novel 3D compound Ag2[Ag(trz)2][H6CrMo6O24]·2H2O(Ag3CrMo6,trz=1,2,4-triazole)has been constructed by selecting Anderson type heteropolymolybdates and Ag(I)cations.Structural analysis shows that Ag3CrMo6 is composed of Anderson type[H6CrMo6O24]4-polyoxoanion,Ag(Ⅰ)cations,and trz molecules.Ag(Ⅰ)cations connect to terminal bridge oxygen atoms in the adjacent[H6CrMo6O24]4-polyoxoanions through Ag-O bonds to form a 2D layer,and then the 2D layers connect to the nitrogen atoms in the trz molecule through Ag-N bonds to form a 3D framework structure.When Ag3CrMo6 is exposed to a formaldehyde(FA)atmosphere,the reductive FA reduces a portion of the surface Ag+to form AgNPs.Under the excitation of Raman lasers,the Raman signals ascribed to the polyoxoanions are significantly enhanced,thereby achieving a sensitive response to the concentration of FA gas.At the same time,the Cr-O bond in polyoxoanion farther from AgNPs has little change in its Raman signal after contaction with formaldehyde,so it can be used as an internal standard group.To sum up,in this thesis,Ag(I)-POM crystalline materials with different dimensions are confirmed to be superior SERS substrates to detect reductive small molecules.In the SERS detection process,POM can be used as both an internal standard and a signal molecule,not only opening up an innovative way for the rational design and synthesis of SERS substrates based on Ag(I)-POM,but also promoting the in-depth development of SERS quantitative detection technology.