| Metal-Organic Frameworks(MOFs)have attracted much attention due to their well-organized porosity,controllable pores and sizes and versatile chemical functionality,furthermore the MOFs have a porous structure and a large BET specific surface area.Benefit from these advantages,MOFs are of tremendous attractiveness and great value in many fields such as gas storage,chemical smart sensors and drug delivery.However,the excitation wavelengths of traditional rare earth MOFs are excitated by the ultraviolet light at room temperature generally,in which situation the MOFs we prepared are not conducive to the biological tissues due to a series of disadvantages of ultraviolet light such as the weak penetrability and the auto-fluorescence interference in the biological tissues and organs.As we know,the up-conversion materials which have excitation at near-infrared region and emission at visible region via a sequential absorption process of multiple photons.Besides,near-infrared light with a wavelength of more than 700 nm is located at the "optical window" in biological tissues,and has the characteristics of stronger light penetration ability,smaller light damage and minimized auto-fluorescent interference in the tissues,so it brings great research significances in the field of biological medicine and biological imaging.In this paper,we report the study on one-step strategy to prepare Ln-MOFs co-doped with Er3+/Tm3+ions at varied concentrations.These Ln-MOFs with adjustable and characteristic up-conversion emissions under the excitation at 980 nm were successfully synthesized using the corresponding lanthanide(?)nitrates(Ln=Er,Tm,Y)and N,N-dimethylformamide(DMF),1,3,5-trimesic acid(BTC)as a metal ion center and ligand respectively.We select two kinds of capping reagents which are sodium acetate and sodium citrate,as a kind of "auxiliary group",sodium acetate and sodium citrate can both provide a large number of carboxyl groups which can coordinate with trimesic acid ligands and rare earth ions,and inhibit the rapid growth of individual crystals and prevent the secondary nucleation process of crystals in the system in the early stage of crystal growth.However,the Ln-MOFs exhibit the different characteristic up-conversion emissions due to the introduction of sodium acetate and sodium citrate respectively,furthermore the optical properties and energy-transfer mechanism inside have been systematically analyzed and discussed.When we select sodium acetate as "auxiliary group",there are two different kinds of luminous mechanisms in the Ln-MOFs under the 980 nm excitation.At low Tm3+concentration level,the energy transfer from Tm3+to Er3+can be occurred,so it will promote the up-conversion emission of Er3+,and only exhibits the characteristic emissions of Er3+at 410,525,560 and 660 nm,respectively ascribed to the allowed electronic transition from 2H9/2 to 4I15/2,from 2H11/2 to 4I15/2,from 4S3/2 to 4I15/2,from 4F9/2 to 4I15/2 of Er energy level;when the concentration of Tm3+increases further,the energy transfer between Er3+and Tm3+become quite effective,and showing a significant up-conversion emissions of Tm3+at 360 and 460 nm,respectively ascribed to the allowed electronic transition from 1D2 to 3H6,1D2 to 3F4 of Tm energy level.In addition,the temperature sensing characteristics of Er3+were explored using the TCLs energy level of(2H11/2/4S3/2).The temperature-varying fluorescence spectrums of MOFs were measured at the temperature range from 303 k to 493k under the excitation of near-infrared light.According to the change of spectrums,the optimal temperature and the maximum sensitivity were found,the temperature was 433 k and the maximum sensitivity was 0.00532 K-1.When we use the sodium citrate as a substrate instead of sodium acetate.,we find that the introduction of sodium citrate can improve the energy transfer efficiency between Er3+ and Tm3+effectively under the 980 nm excitation,in which condition it also avoids the "self-fluorescence" interference effect caused by ultraviolet excitation in the biological detection.Besides,the Ln-MOFs co-doped with Er3+/Tm3+/Y3+ions in different proportions have exhibited the same characteristic up-conversion emissions at 294,350,363,455 and 478nm corresponding to Tm3+ions under 980 nm laser excitation at room temperature but no characteristic up-conversion emissions of Er3+ions at all times.It is worth noting that,the Ln-MOFs can still show strong blue emission without Yb3+doped.In addition,the temperature sensing characteristics of Tm3+were explored using the TCLs energy level.The temperature-varying fluorescence spectrums of MOFs were measured at the temperature range from 303k to 463k under the excitation of near-infrared light and the optimal temperature together with the maximum sensitivity were got,the temperature was 3133k and the maximum sensitivity was 0.00444 K-1.So,it can be applied to low-temperature fluorescence temperature sensing and provide a direction for the selection of temperature measuring ions or materials.In the preparation of Ln-MOFs,the g-C3N4 was directly introduced in the system and obtained the composite photocatalytic material of Ln-MOFs and g-C3N4 by using the one-step solvothermal method.The doping ratio of Ln-MOFs and g-C3N4 was tested by the method of liquid phase degradation of rhodamine B,and the optimal doping ratio was 1:1.The optimal doping concentration ratio of Er3+,Tm3+and Y3+ was determined as 6%,2%,and 92%by experiments in this paper,and the optimal molar doping ratio of total rare earth ions to organic ligands was 2:1.The experimental results also indicated that the up-conversion mechanism of Ln-MOFs:Er/Tm/Y materials can be attributed to the excited state absorption mechanism.The materials we fabricated could be co-doped with specific lanthanide ions and possess up-conversion luminescence under the excitation of near-infrared region.Furthermore,the up-conversion luminescence in Ln-MOFs with rare earth ions double-doped is less reported,especially without Yb3+doped,and the energy transfer mechanism between different kinds of rare earth ions has not been discussed detailed.In this paper we obtained the high intensity of up-conversion luminescence without Yb3+ions doped and explained the novel energy transfer mechanism in the Ln-MOFs,and filled this research area. |
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