Constructing Background-free Luminescent Nanoprobes And Exploring Their Applications

Fluorescence analysis based on luminescent materials is one of the most commonly used methods in analytical science.Because of its high-sensitivity and high-resolution,fluorescence analysis has been widely used in fields including disease diagnosis and treatment,environmental analysis,and criminal investigation.Thus,exploring luminescent materials is crucial for the development of fluorescence analysis.Up to now,many kinds of luminescent materials have been developed,such as organic dyes,fluorescent proteins,metal complexes,semiconductor quantum dots,and so on.The traditional fluorescent materials still face sort of challenges in fluorescence analysis.One of the commonly encountered challenge is the background fluorescence interference.Traditional fluorescent materials are usually excited by ultraviolet(UV)or visible light.However,the background chromophores in sample matrix can also be excited by UV or visible light.Thus,the background fluorescence will blur the signal of traditional fluorescent materials and significantly reduce detection sensitivity and imaging resolution.In some severe cases,the signal of fluorescent probes may even be completely buried by background fluorescence,resulting in detection and imaging failure.Due to the presence of background fluorescence,false positive signals may also be generated and incorrect detection and imaging results would be given.Therefore,exploiting fluorescent materials that can eliminate background fluorescence interference is of great significance for the advance of fluorescence analysis.In the past two decades,lots of analysis methods based on persistent luminescence nanoparticles(PLNPs)and upconversion nanoparticles(UCNPs)have been developed,leading to great advance of fluorescence analysis.PLNPs can remain luminescent after the ceases of excitation,and the decay time of persistent luminescence can be up to several hundred hours.Whereas,the fluorescence lifetimes of background fluorescent substances are usually tens of nanoseconds.Thus,the background fluorescence interference can be completely eliminated by collecting persistent luminescence signal after the background fluorescence completely decays,leading to effectively improved detection sensitivity and imaging resolution.Lanthanides have abundant 4f-5d energy levels,which can produce plentiful absorption and emission transitions.The near-infrared(NIR)light absorption capability of Yb³⁺and Nd³⁺enables lanthanide doped-upconversion nanoparticles with multiple emissions under NIR excitation.Since the background chromophores cannot be excited by NIR light irradiation,UCNPs can also effectively avoid the generation of background fluorescence in fluorescence analysis.In addition,the unique electronic structure of lanthanide endows upconversion nanoparticles with long luminescence lifetime,usually in the microsecond to millisecond range.Therefore,background fluorescence interference can be effectively eliminated through time-resolved fluorescence analysis.In this thesis,based on the unique optical properties of PLNPs and UCNPs,we constructed persistent luminescence nanoprobes and upconversion nanoprobes.Furthermore,we explored their applications in fluorescence analysis and information science.The main content of this thesis is as follows:1.The Ge⁴⁺ions doped ZnGa₂O₄:Ge,Cr PLNPs were synthesized through hetero-valence ion doping strategy.Compared with ZnGa₂O₄:Cr,the ZnGa₂O₄:Ge,Cr displayed stronger persistent luminescence and longer luminescence decay time.Rietveld refinements and electron paramagnetic resonance measurement demonstrated that the content of charged defects in ZnGa₂O₄:Ge,Cr gradually increases with the increase of the doping amount of Ge⁴⁺,resulting in enhanced persistent luminescence properties of PLNPs.Based on the unique luminescence properties of ZnGa₂O₄:Ge,Cr,the aptamer-modified ZnGa₂O₄:Ge,Cr probe was further constructed and successfully used for background-free in vivo imaging.2.The Zn₂GeO₄:Ga,Mn PLNPs were synthesized by hydrothermal method and were further used for background-free latent fingerprint imaging.The Zn₂GeO₄:Ga,Mn showed uniform size,good dispersibility,strong luminescence intensity and long decay time.The Zn₂GeO₄:Ga,Mn-COOH nanoprobes were constructed by surface-modified carboxyl group,which can combine with fingerprint ridge through EDC/NHS process by forming an amido bond with the amino groups in fingerprint.Thus,background-free latent fingerprint imaging can be realized by collecting the persistent luminescence signal of the probe after excitation ceases.Furthermore,Concanavalin A modified Zn₂GeO₄:Ga,Mn probe was prepared and background-free glycosylated proteins imaging in latent fingerprint was realized through specifically identifying the glycosylated proteins by Concanavalin A.3.The Zn₂GeO₄:Mn PLNPs were synthesized by hydrothermal method and used for background-free information storage and encryption.The obtained Zn₂GeO₄:Mn possessed uniform size,good dispersibility,and bright green persistent luminescence.PLNPs ink with good stability and bright persistent luminescence was further prepared based on the Zn₂GeO₄:Mn,and background-free text/pattern encryption on paper substrates was successfully performed by inkjet printing technology.Based on the long decay time of PLNPs inks,an information encryption strategy was further designed.That is,one can only obtain the complete and correct encrypted information by collecting the persistent luminescence signals of the PLNPs ink after the stoppage of excitation.4.By making use of upconversion nanoparticles with different excitation wavelengths,a high-throughput and easy-to-perform background-free information encryption strategy was developed.The UCNPs with different excitation wavelengths were prepared through thermal decomposition method.The obtained UCNPs possessed uniform size,high colloidal stability,and bright upconversion luminescence.The upconversion inks with good dispersibility were loaded into inkjet printers and background-free information encryption on paper substrates was achieved by collecting the upconversion luminescence signal under NIR laser excitation.Furthermore,a new information encryption strategy was designed based on upconversion inks with different excitation wavelengths.The information was encrypted with two types of upconversion inks separately,and one can only read the complete and correct encrypted information under 980 nm laser excitation.