Defect Luminescence Based Persistent Phosphors—From Controlled Synthesis to Bioapplications

Persistent luminescence is an optical phenomenon where solid phosphors can store photoenergy in defects and release the energy by luminescence after stopping excitation.Due to the intriguing optical characteristics,the defect luminescence based persistent phosphors have attracted enormous attention in recent decades,especially in biomedical fields such as biosensing and bioimagin8.Persistent luminescence nanoparticles(PLNPs)can effectively avoid the autofluorescence interference from complex samples or tissues,leading to significantly improved sensitivity in biological analysis.In this review,we summarized the methods to control the optical performance of PLNPs from the perspectives of controlled synthesis and defect regulation,and emphasized the close relationship between their optical performance and applications.We further provided a summary about a series of PLNPs nanoprobes designed by our group for biosensing and bioimaging.Our efforts,summarized in this review,will not only open a window for manipulating luminescence in PLNPs,but also further promote the application of PLNPs in biomedicine.

Effect of excitation wavelength(blue vs near UV)and dopant concentrations on afterglow and fast decay of persistent phosphor SrAl_(2)O_(4):Eu^(2+),Dy^(3+)

The persistent phosphor SrAl_(2)O_(4):Eu^(2+),Dy^(3+)is the subject of numerous investigations.One often neglected aspect is that in this phosphor,as well as in Sr_(4)Al_(14)O_(25):Eu^(2+),Dy^(3+),there are two different Sr^(2)+sites which can be occupied by the dopant Eu^(2+)ions,We first introduce a general scheme of possible energy transfers in these persistent phosphor materials including explicitly both europium ions,This scheme is used as a generic starting point to study experimentally specific pathways.We illustrate this application with the study of the effect of excitation wavelength(444 and 382 nm)on the afterglow of differently doped SrAl_(2)O_(4):Eu^(2+),Dy^(3+)samples,as well as on the emission decay curves.With the same excitation intensity under 444 nm excitation,the resulting afterglow intensity is stronger than under near UV excitation.At 382 nm,Eu^(2+)ions on both Sr^(2)+s ites in SrAl_(2)O_(4)are excited,but at room temperature the blue emission is quenched,leading to a loss of photons.The observed effects can further be associated with the ratio of Eu^(2+)ions and trap states which are modulated by the concentrations of Eu^(2+)and Dy^(3+)in SrAl_(2)O_(4),as well as by temperature,Increasing the nominal Dy^(3+)content from 0.1 mol%to 0.5 mol%with respect to Sr results in the doubling of the integrated afterglow intensity and confirms thus that Dy^(3+)ions are indeed involved in the trapping process,The concentration of trap states is much lower than the concentration of Eu^(2+)ions,as even with low excitation densities,a plateau of integrated afterglow intensity(corresponding to the total number of accessible traps)is reached.We postulate that an important fraction of excited Eu^(2+)ions can potentially transfer their energy to trap states.Once that all traps are filled or in a dynamical filling-depletion process under illumination(with thermal and/or optical depletion processes),for the remaining Eu^(2+)a"normal"steady-state emission is observed.The luminescence decay curves at 520 nm measured at 77 K show a mono-exponential decay with a common lifetime of about 1140 ns for all 5 samples under 437 nm excitation,while under 375 nm excitation,a feed process originating from the energy transfer between Eu^(2+)ions is demonstrated.Under 375 nm excitation,the non-exponential decay observed at 440 nm can be quantitatively associated to a Forster energy transfer process with R_(0)=1.58(8)nm.For the overall understanding of the afterglow processes,it appears that one has to consider the individual contributions of all active ions on different lattice sites.

Effect of RE^3+electronegativities on luminescence property of Ba5Si8O21:Eu^2+persistent phosphor

A series of Ba5Si8O（21）：0.02Eu^2＋,0.09RE^3＋ persistent phosphors were synthesized by the solid-state reaction method.The measurement results of photoluminescence（PL）,phosphorescence and thermoluminescence（TL）were analysed and discussed.The XRD results showed that samples codoped with different RE^3＋ were Ba5Si8O（21） single pure phase.Under the excitation,all samples exhibited a broad Eu^2＋ characteristic emission,and the La^3＋ co-doped sample emitted the brightest photoluminescence even though its persistent luminescence property was the worst because of the weakest electronegativity.However,Nd^3＋ electronegativity was suitable,thus after activation,the Ba5Si8O（21）：Eu^2＋,Nd^3＋ sample had the best persistent luminescence performance with the highest phosphorescence intensity and the persistent luminescence decay time beyond 8 h.The Nd^3＋ co-doped sample also had the largest thermoluminescence integral area which proved effectively it had longer persistent luminescence time.The luminescence mechanism was also proposed to study the photoluminescence and persistent luminescence process.These results showed that RE^3＋ electronegativities were distinctly important for persistent phosphors and choosing suitable electronegativity codopant was conducive to enhancing the phosphorescent performance.

Bright yellow-emitting long persistent luminescence from Mn^(2+)-activated strontium aluminate phosphor

We developed bright yellow-emitting long persistent luminescence(LPL)materials Sr_(4)Al_(14)O_(25):Mn^(2+)and Sr_(4)Al_(14)O_(25):Mn^(2+),N(N=Zr^(4+),Ho^(3+),Er^(3+))by high temperature solid-state reaction.The addition of Zr^(4+),Ho^(3+)and Er^(3+)can regulate trap distributions and improve energy storage ability of the materials.The LPL perfo rmance of SAO:Mn^(2+),Ho^(3+)is optimal,considering LPL intensity and duration time.Bright LPL of SAO:Mn^(2+),Ho^(3+)can be observed for 3 h by naked eyes in dark after removing the excitation source.Profiles of LPL spectra are different from those of PL,because the two types of Mn^(2+)centers do not play equal parts in LPL and photoluminescence(PL).Trap depths of TL peaks centered at 354 K(peak 1)and 455 K(peak 2)are 0.60 and 0.72 eV,respectively.And peak 1 at 354 K is the effective TL peak responsible for LPL.In SAO:Mn^(2+),Ho^(3+),Mn^(2+)ions doped in Al^(3+)sites serve as emitting centers,and positively charged HoSr defects are the main effective trap centers.Finally,a feasible LPL mechanism of SAO:Mn^(2+),Ho^(3+)was proposed to clarify the generation process of LPL.