Multi-Color Upconversion Luminescence Probes For Biological Detection

Inorganic up-conversion（UC）luminescence materials have great advantages as biological fluorescent probes,for example:near infrared light excitation source with low energy can reduce the damage to the target detector;Inorganic host has good photostability,and the detection sensitivity can be greatly improved due to the non background fluorescence of UC luminescence;Biological tissues cannot absorb infrared light,which can avoid auto-fluorescence of organisms.The red UC luminescence,witch have both emission and excitation wavelengths located within the“tissue transparency window”,present greater penetration depth in biological tissue.However,due to lack of an effective way to populate the red emission energy level,it is very difficult to obtain red UC fluorescent probes at present.UC luminescent materials with multi-color luminescence have important applications in the fields of multi-mode biological imaging,display,and multiple anti-counterfeiting.However,due to the limited types of sensitizer and cross relaxation caused by multi-ion doping,it is even more difficult to achieve multi-color luminescence in single sample.In order to resolve the above problem,we synthesized a series of red UC fluorescent probes,and on this basis,tuning the UC luminescence color from single red light to double-color to multi-color luminescence,and carried out preliminary research on these probes in biological imaging application.The specific research content is as follows:（1）Er³⁺single doped,Yb³⁺,Er³⁺co-doped,and Er³⁺,Tm³⁺co-doped Y₂Ti₂O₇UC luminescence phosphor are prepared by the high temperature solid state reaction method.High doping concentration of rare earth ions can be achieved relying on the specific layered distribution of Y³⁺ions in the pyrochlore Y₂Ti₂O₇host,which solves the problem of low doping concentration of activator in traditional host.The doping concentration of Er³⁺ions in Y₂Ti₂O₇is up to 28 mol%,luminescence brightness of Er³⁺heavy doped（28 mol%）sample are 4 and 7 times stronger than that of low concentration（5 mol%）Er³⁺doped sample under980 and 1550 nm excitation.Red emission efficiency can be further improved by co-doping with Yb³⁺ions,red to green emission ration are 6.97 and 23.26 while the concentration of Yb³⁺ion increase to 30 mol%excited by 980 and 1550 nm.Moreover,taking the advantage of large absorption cross section of Er³⁺ions at 1550 nm,single band red emission can be achieved by introducing Tm³⁺ions as energy capture centers under 1550 nm excitation,red-green ratio of the sample is>10³.Compared with 980 nm excitation,the red UC luminescence excited by 1550 nm present better color purity and stability.（2）Er³⁺,Tm³⁺co-doped Na YF₄nanocrystals are prepared by the co-precipitation method,which solved the limitation of oversize powder material in biological detection.Based on the research results of Y₂Ti₂O₇powders,single red emission is obtained in the currently recognized most efficient Na YF₄host.Luminescence efficiency of the sample is further improved by coating an inert shell,total intensity of the samples is increased by 4.69 and16.19 times under 980 and 1510 nm excitation,respectively.In addition,Yb³⁺doped active shell with higher absorption efficiency at 980 nm are coated on the surfaces of Na YF₄:Er³⁺,Tm³⁺nanoparticles,which solves the problem of low luminescence efficiency of Er³⁺,Tm³⁺co-doped systems excited by 980 nm.Luminescence intensity of the nanoparticles with active epitaxial shell is 9 times that of Na YF₄:Er³⁺,Tm³⁺core nanoparticles,and a thick high-quality active shell（>3 nm）could be used to further improve red UC luminescence efficiency.These nanoparticles are used as fluoresce probes for the imaging of onion epidermal cells and Marine fish.（3）Er³⁺doped Na Yb F₄nanoparticles are prepared by the co-precipitating method.The nanoparticles present red and green dual-color luminescence under 980 and 1550 nm excitation,respectively,which avoid the complex structure and synthesis processes for realizing multi-color luminescence in a single sample.The platymona were incubated with these UC nanoparticles,long-term culture and life activity monitoring are carried out to verify the feasibility of UC fluorescent probes for fluorescent imaging monitoring of organisms in harsh seawater environments.On the basis of the dual-color particles,five layers core-shell nanoparticles are prepared,which show blue,green and red primary colors luminescence excited by 808,980 and 1550 nm,respectively.Nd³⁺,Yb³⁺and Er³⁺ions are used as sensitizes,UC luminescence in the whole spectrum can be achieved by simply adjusting the relative intensity of the three excitation sources.It is the first study to achieve steady full-color luminescence in a single sample at present.