Design And Preparation Of Tm³⁺ Based NIR-Ⅱ Luminescent Nanoparticles For Bioimaging

Compared with visible light（400-700 nm）and NIR-I（700-1000 nm）bioimaging,NIR-Ⅱ（1000-3000 nm）fluorescence imaging has the advantages of deeper penetration,higher resolution,and higher sensitivity.And due to the longer wavelength of NIR-Ⅱb（1500-1700 nm）fluorescence imaging,less light scattering and absorption,and weaker tissue autofluorescence,it shows a more excellent imaging effect in the detection range of indium gallium arsenic（In Ga As）detectors（900-1700 nm）.In addition,compared with NIR-Ⅱb bioimaging,which is widely studied now,NIR-Ⅱc（1700-2000 nm）bioimaging has a longer wavelength,less light scattering and absorption,resulting in weaker tissue autofluorescence,and its imaging penetration depth is deeper,thus improving the resolution and sensitivity of imaging.Therefore,the application of NIR-Ⅱc in vivo bioimaging is more promising.Rare-earth nanoparticles have the advantages of abundant energy levels（rich emission spectra）,large Stokes shifts,long luminescence lifetime,no photobleaching,and low biotoxicity,which have received increasing attention in the field of bioimaging.However,current studies have mainly focused on fluorescent bioprobes activated by Er³⁺（1530 nm）,Nd³⁺（800 nm）and Yb³⁺（980 nm）ions,whose luminescence is also mainly located in the NIR-Ⅱa/b window,while fluorescent bioprobes whose luminescence is located in the NIR-Ⅱc window have rarely been reported.In the NIR-Ⅱc window,the emission of Tm³⁺ions at 1800 nm offers the possibility to realize NIR-Ⅱc bioimaging.In this paper,we focus on the controlled synthesis and luminescence properties of Na Tm F₄nanoparticles with cubic-phase（α-phase）core-shell structure（denoted asα-Tm NPs）,and based on the study of the crystalline phase,we realize the efficient downshifting luminescence ofα-Tm NPs under laser excitation at 808 nm by rational design and optimization of the structure,and use its emission at 1800 nm for NIR-Ⅱc window in vivo bioimaging.The specific research of this paper is as follows two aspects.（1）Design and preparation of NIR-Ⅱc bioprobes:First,monodisperseα-Na YF₄:x Tm core nanoparticles with controlled morphological particle size were prepared by high-temperature thermal decomposition,and the optimal doping concentration of Tm³⁺ions was obtained by varying the doping concentration of Tm³⁺ions for the strongest downshifting luminescence.Then,theα-Na YF₄:x%Tm nanoparticles with core-shell structure were synthesized by epitaxial growth to improve the downshifting luminescence efficiency ofα-Tm NPs under laser excitation at 808 nm.Theα-Tm NPs showed the strongest downshifting luminescence at 1800nm when the Tm³⁺ion concentration reached 100%.The core-shell structure ofα-Na Tm F₄nanoparticles with efficient luminescence in the 1600-2100 nm band was obtained by changing the shell matrix and shell thickness ofα-Na Tm F₄.It was found that theα-Tm NPs emitted the strongest downshifting luminescence at 1800 nm by epitaxially growing a 5 nm thick Na YF₄shell on the surface ofα-Na Tm F₄with a size of about 9 nm.Meanwhile,it was found that the cubic-phase Na Tm F₄@Na YF₄with a size of about 19 nm downshifting luminescence at 1800 nm is about 2.53 times higher than that of the hexagonal phase with the same size when the crystalline phase was studied.This work shows that the core-shellα-Tm NPs at 100%Tm³⁺ion doping concentration with efficient downshifting luminescence at 1800 nm under laser excitation at 808 nm provides the basis for NIR-Ⅱc bioimaging,andα-Tm NPs can be an efficient contrast agent for NIR-Ⅱc fluorescence imaging.（2）Evaluation of NIR-Ⅱc bioimaging quality:First,the surface ofα-Tm NPs was modified with polyethylene glycol(DSPE-PEG₂₀₀₀)to make them water-soluble as well as biocompatible to obtain fluorescent bioprobes that can be used for NIR-Ⅱc bioimaging.Then,the widely studied Er-based NIR-Ⅱb bioprobeα-Na Er F₄@Na YF₄（denoted asα-Er NPs）was prepared and its NIR-Ⅱb bioimaging effect was compared with the NIR-Ⅱc bioimaging effect ofα-Tm NPs.Simulations of in vitro imaging were performed in intralipid at a concentration of 1%to compare the penetration depth,signal-to-background ratio（SBR）,and full width half maximum（FWHM）of both imaging.It was shown that the penetration depth ofα-Tm NPs was 6 mm under 808nm laser excitation,which was better than that ofα-Er NPs（5 mm）,and the SBR and FWHM ofα-Tm NPs was better than that ofα-Er NPs.α-Tm NPs showed better NIR-Ⅱc bioimaging thanα-Er NPs for NIR-Ⅱb bioimaging.Finally,vascular imaging and gastrointestinal imaging of mice were performed usingα-Tm NPs,and the vascular and gastrointestinal contours of mice could be clearly seen.Among them,the SBR of mouse vascular imaging was up to 2.72 with a resolution of 648.18μm;the peristaltic direction of the intestine could also be observed in gastrointestinal imaging.Due to the excellent performance ofα-Na Tm F₄@Na YF₄nanoparticles in NIR-Ⅱc bioimaging,this probe design will be expected to be combined with therapy to develop an integrated imaging and therapy nanoplatform.