Controllable Preparation Of Eu³⁺ Doped Calcium Phosphate Fluorescent Nanodots And Its Application For Bioimaging

Fluorescence imaging technology shows a promising application for the early diagnosis of tumor due to its non-hazardous optical radiation,high sensitivety and low cost.Thereinto,the small size and highly fluorescent bioimaging agent plays a crucial role.Currently,a lot of bioimaging agents have been reaseached and achieved a series of research progress.But the good biocompatibility is a prerequisite for bioimaging agent.Calcium phosphate(CaP)has a good biocompatibility and biological activity,including hydroxyapatite(HAP),amorphous calcium phosphate(ACP),carbonate apatite(CHA)and so on.Additionally,CaP can be used as optical host material with good adaptation for the doping of rare earth ions(REs).The previous studies mainly focus on the fluorescence improvement by the diffusion of lanthanide to the different crystallographic Ca2+site and energy transfer between REs for REs:CaP nanoparticles.However,the size increase and agglomeration of REs:CaP inevitably occur.This has been a bottleneck for developing the bioimaging application of REs:CaP.We develop a poly(acrylic acid)(PAA)complexation-precipitation method for the controllable synthesis of monodisperse and sub-10 nm Eu:CaP nanodots.The results show that the Eu:CaP nanodots can be successfully prepared as the following parameters:5.6<pHcom<7.0,2.0<R<5.0,7.0<pHpre<9.3,25??Tpre?120?.The micro-structure and phase of Eu:CaP product can be regularly changed with increased the R or pHpre.Particularly,0<R<0.5,Eu:CaP product(HAP)with partial crystallization was prepared;0.5<R<1.0,Eu:CaP product(ACP)with inner short-range ordered regions was prepared;1.0<R<2.0,Eu:CaP product(ACP)with inner high electron density regions was prepared;2.0<R<5.0,the monodisperse and sub-10 nm Eu:CaP nanodots(CHA)were prepared.The combination mode between PAA and Ca2+(Eu3+)ions determines the formation of Eu:CaP nanodots.Particularly,0<R<2.0,the large size PAA-Ca(Eu)complexes(>100 nm)were firstly formed due to the hydrophobic effect plays a dominant role and then which as a template to synthesize the Eu:CaP particles(>100 nm);2.0<R<5.0,the ultrasmall PAA-Ca(Eu)complexes(2-3 nm)were firstly formed due to electrostatic repulsion plays a dominant role and then which as a template to synthesize the Eu:CaP nanodots.Eu:CaP nanodots display an excellent fluorescence property.Eu:CaP nanodots(<10 nm)show about 280%and 70%enhancement of fluorescence intensity at 617 nm(excited at 394 nm)compared to Eu:CaP nanoparticles(114 nm)and PAA-Ca(Eu)complexes(2-3 nm)respectively,corresponding to about 340%and 190%enhancement of quantum yields,and about 450%and 200%extension of fluorescence lifetimes.Meanwhile,Eu:CaP nanodots display a stable fluorescence property as the following condition:25??Tpre<120?,7.0<pHpre<9.3.Such Eu:CaP nanodots lead to strong surface effect(SE),and achieve the significant enhancement of fluorescence based on the SE-CTB(Eu3+-02" charge transfer band)energy transfer synergy(excited at 254 nm)and the SE-CFE(crystal field effect)synergy(excited at 394 nm),respectively.Eu:CaP nanodots(<10 nm)showed no toxicity to L02 cells and showed no obvious morphological damage to liver and kidney of nude mice.Compared to Eu:CaP nanoparticles(114 nm),Eu:CaP nanodots exhibit stronger fluorescent signals in vitro and in vivo.The results of tissue distribution indicated that the Eu:CaP fluorescent nanodots were mainly renal clearance rather than reticuloendothelial system(RES)clearance,and their accumulation in tumors was 5.1 times higher than that of Eu:CaP nanoparticles,indicating that the passive targeting ability of tumors was enhanced.Moreover,the Eu:CaP fluorescent nanodots showed blood clearance rapidly within the first 3h and can be cleared about 74%in the blood at 8 h.