Fluorescence Temperature Sensing Properties Of Rare Earth Doped Fluoride Core-Shell Nanocrystals Excited By Near-Infrared

Temperature plays a very important role in human body system,industrial manufacturing,agricultural production and other fields.How to measure the temperature quickly and accurately has become a very important problem.In recent decades,because of the fluorescence intensity ratio（FIR）non-contact optical temperature measurement method has self-reference characteristics,so it has aroused the interest of the majority of scientific researchers.In addition,lanthanide activators have great potential in the field of optical temperature measurement due to their abundant radiation energy levels and diverse thermal-dependent electronic transition possibility,which lead to their emission range from ultraviolet to infrared.However,the current temperature measurement method has many shortcomings,such as:the narrow temperature measurement range which can not meet the use of special places;low temperature sensitivity which can not accurately and quickly determine the temperature.Based on these problems,we carried out the following research and achieved some results:（1）Employ a novel strategy to achieve temperature probes with ultrahigh relative sensitivity through integrating both positive and negative thermal quenching effect into a hydrogel.Specifically,Er³⁺ions show evidently a positive thermal quenching effect in Na YF₄:Yb/Er@Na YF₄nanocrystals while Nd³⁺and Tm³⁺ions in a Yb₂W₃O₁₂bulk exhibit prominently a negative thermal quenching effect.With elevating temperature from 313 to 553 K,the fluorescence intensity ratio of Er（540nm）to Nd（799 nm）and Tm（796 nm）to Er（540 nm）is significantly decreased about 1654 times and increased about 14,422 times,respectively.The maximum relative sensitivity of 15.3%/K at 553 K and 23.84%/K at 380 K are achieved.The strategy developed in this work sheds light on highly sensitive probes using lanthanide ion-doped materials.（2）The concept of indirectly thermally coupled levels is introduced and employed to actualize high performance temperature sensing.By means of the temperature-dependent phonon-assisted non-radiative relaxation,the ⁴I_13/2excited state（with infrared emission）of Er³⁺can be indirectly thermally coupled with the⁴S_3/2excited state（with visible emission）under 808 nm or 980 nm excitation.This is experimentally realized in specially designed Na Er F₄:10Yb@Na YF₄nanocrystals,and the corresponding ratiometric nanothermometer shows excellent luminescence thermal sensing performance with a maximum relative sensitivity value up to3.76%/K at 295 K.（3）The positive and negative thermal quenching effect were integrated to realize NIR nanothermometer with high sensitivity.For the Na Nd F₄:Yb@Na YF₄and Na Nd F₄:Yb@Na YF₄nanoparticles involved transparent film,the Yb³⁺emission intensity at～975 nm decreases while that of Er³⁺ions at～1532 nm increases with the rising of temperature upon 808 nm laser excitation.These opposite temperature-dependent emission intensity variations give out a maximum absolute temperature sensitivity of 0.18 K^-1and a relative sensitivity of 3.1%/K.Moreover,the calculated minimum S_aand S_rare up to 0.04 K^-1and 1.55%/K,respectively.