Detection Of Rare Earth Doped NaGdF₄ Upconversion Nanocrystals In Food Synthetic Pigment Amaranth

In recent years,food safety problems caused by the abuse of edible synthetic pigments have emerged in an endless stream.In order to better supervise the rational use of food synthetic pigments in the food field and ensure food safety,it is critically important to obtain the colorants concentration accurately and quickly.Therefore it is of great significance to design and build a sensitive,simple and low-cost food pigment detection platform.Upconversion nanocrystals,as a new optical probe,have been widely used in the detection of heavy metal ions and other fields.In addition,high-sensitivity detection requires the above-mentioned nanocrystals with high luminous intensity as probes.In this paper,to achieve amaranth concentration detection with high sensitivity as the guidance,in-situ observation of nucleation and crystallization process of NaGdF₄ nanocrystals is studied by transmission electron microscope(TEM).On this basis,the upconversion nanocrystals with excellent optical properties were prepared.In addition,the surface defects during the growth process of nanocrystals were passivated by coating the inert-shell layer to further optimize the optical properties of nanocrystals.Finally,the optimized Na G d F₄:Yb³⁺,Er³⁺@Na YF₄upconversion particles were used as probes to detect the concentration of edible synthetic colorant amaranth.Specific research content is as follows:Firstly,NaGdF₄ nanocrystals were synthesized by a co-precipitation method,the in-si t u nucleation and crystallization process of nanocrystals were observed by TEM to explore the relationship between precursor concentration and nucleation and crystallization of nanocrystals.Studies have shown that under the irradiation of TEM electron beams,there are two situations for the nucleati on of NaGdF₄ precursors.On one hand,when the precursor concentration is high,the nucleation undergo three steps:(1)the separation of the precursors into a rich and poor-NaGdF₄ phase,(2)amorphous NaGdF₄ nanoclusters arise from the rich-NaGdF₄ and(3)the amorphous nanoclusters crystallize.On the other hand,when the precursor concentration is low,small crystal nuclei are directly formed under TEM electron beam irradiation,and then free atoms are attached to complete the nucleation and crystallizati on process.In addition,the organic ligands attached to the surface of the particles and the incompact arrangement of atoms on the surface during the gr owth process will weaken the up conversion luminescence intensity in the form of surface defect states.Secondly,optical properties optimization of Na G d F₄:Yb³⁺,Er³⁺upconversion nanocrystals.Based on understanding the above mentioned studies,we obtained homogeneous NaGdF₄:Yb³⁺,Er³⁺with excellent optical properties by adjusting the concentration of the precursors and doped ions,and the optical properties of the upconversion nanoparticles are further optimized by passivating the surface defect states of the upconversion nanocrystals by coating the inert shell Na YF ₄.The results show that the shell coating significantly improves the upconversion intensity without changing the luminescence mechanism of activator ion.Finally,we explore the detection of edible synthetic pigment amaranth via NaGdF₄:Yb³⁺,Er³⁺@Na YF₄ nanocrystals.The nanocrystals encapsulated with epoxy resin forming a solid-state sensor enables the recycled use in the dye detection.By characterizing the optical performance of the nanocrystal s before and after encapsulation,it is found that the impact of the encapsulation on the optical performance of the nanocrystal s can be ignored.In the study,we found that the amaranth absorbs the energy of the nanocrystals selectively,which reduces the upconversion luminescence intensity of the nanocrystals in a specific wavelength range.Therefore,the fluorescence intensity ratio(FIR)technique is achieved,establishing relationship between the fluorescence intensity variation and the amaranth concentration.The detection sensitivity of this sensor reach es 0.01mg m L^-1with characteristics of high selectivity and stability.Meanwhile,the reliability and accuracy of this sensor is demonstrated for the colorants detection in commercial soft drinks.