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In this study, sub-10 nm BaLuF 5 nanocrystals with cubic phase structure were synthesized by a solvothermal method using oleic acid as the stabilizing agent. The as-prepared BaLuF 5 nanocrystals were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and analyzed by the upconversion (UC) spectra. The TEM results reveal that these samples present high uniformity. Compared with Gd-free samples, the size of BaLuF 5 :Yb/Er doped with 10% Gd 3+ decreased to 5.6 nm. In addition, BaLuF 5 :Yb/Tm/Gd upconversion nanoparticles (UCNPs) presented efficient near-infrared (NIR)-NIR UC luminescence. Therefore, it is expected that these ultra-small BaLuF 5 nanocrystals with well-controlled shape, size, and UC emission have potential applications in biomedical imaging fields.

In this paper, the monodispersed NaErF 4 :Yb, Mn nanoparticles (NPs) were successfully synthesized by a simple and mild solvothermal method. The as-prepared NPs were characterized by X-ray diffraction, transmission electron microscopy, and spectrophotometer. The crystal phase, size, morphology, and upconversion (UC) properties of these NPs can be readily tuned by doping Mn 2+ . With increasing Mn 2+ contents, the crystal phase of NaErF 4 :Yb NPs was transferred from hexagonal to cubic, and then a new orthorhombic phase NaMn 3 F 10 appeared. The morphology of NaErF 4 :Yb was tuned from nanodisks to nanocubes and then to large nanoflakes with increasing Mn 2+ dopant contents. Moreover, the UC luminescence (UCL) intensity can be enhanced by increasing Mn 2+ content. Interestingly, single-band red emission can be achieved when the Mn 2+ content reaches 20%. This is mainly ascribed to the efficient energy transfer between Er 3+ and Mn 2+ . The enhanced UCL intensity and single-band red UCL make these NaErF 4 :Yb/Mn NPs ideal probes for optical bioimaging owing to the low tissue absorption and deep tissue penetration of red light.

A strategy is demonstrated for simultaneous phase/size manipulation, multicolor tuning, and remarkably enhanced upconversion luminescence (UCL), particularly in red emission bands in fixed formulae of general lanthanide-doped upconverting nanoparticles (UCNPs), namely NaLnF4:Yb/Er (Ln: Lu, Gd, Yb), simply through transition metal Mn2+-doping. The addition of different Mn2+ dopant contents in NaLnF4:Yb/Er system favors the crystal structure changing from hexagonal (β) phase to cubic (α) phase, and the crystal size of UCNPs can be effectively controlled. Moreover, the UCL can be tuned from green through yellow and to dominant red emissions under the excitation of 980 nm laser. Interestingly, a large enhancement in overall UCL spectra of Mn2+ doped UCNPs (∼59.1 times for NaLuF4 host, ∼39.3 times for NaYbF4 host compared to the UCNPs without Mn2+ doping) is observed, mainly due to remarkably enhanced luminescence in the red band. The obtained result greatly benefits in vitro and in vivo upconversion bioimaging with highly sensitive and deeper tissue penetration. To prove the application, a select sample of nanocrystal is used as an optical probe for in vitro cell and in vivo bioimaging to verify the merits of high contrast, deeper tissue penetration, and the absence of autofluorescence. Furthermore, the blood vessel of lung of a nude mouse with the injection of Mn2+-doped NaLuF4: Yb/Er UCNPs can be readily visualized using X-ray imaging. Therefore, the Mn2+ doping method provides a new strategy for phase/size control, multicolor tuning, and remarkable enhancement of UCL dominated by red emission, which will impact on the field of bioimaging based on UCNP nanoprobes.

Visualization of blood vessel of lung can improve the detection of the lung and pulmonary vascular diseases. However, research on visualization of blood vessel of lung using the new generation upconversion nanoprobes is still scarce. Herein, high quality hexagonal phase NaLuF4:Gd/Yb/Er nanorods were synthesized by a simple hydrothermal method through doping Gd(3+). Doping Gd can not only promote the phase transformation from cubic to hexagonal and the shape evolution from microtube to rod-like, but also provide an additional magnetic properties for biomedical application. The as-prepared nanorods were further converted to water solubility by treating with HCl for eliminating the capped oleic acid. The ligand-free nanorods were successfully used for high-contrast upconversion fluorescent bioimaging of HeLa cells. Moreover, the in vivo synergistic upconversion fluorescent and X-ray imaging of nude mice were demonstrated by subcutaneously and intravenously administrated the ligand-free nanorods. The X-ray signals were matched well with the upconversion signal, indicating the successfully synergistic bioimaging. The ex-vivo X-ray and upconversion fluorescent imaging of various organs revealed that the nanorods were mainly accumulated in liver and lung. More importantly, the blood vessel of the lung can be readily visualized when these ligand-free nanorods are intravenously injected. Apart from the synergistic X-ray and upconversion bioimaging, the ligand-free nanorods can also possess excellent paramagnetic property for potential magnetic resonance imaging contrast agent. Our results have demonstrated the enhanced visualization of blood vessel of lung performed by dual-modal bioimaging of X-ray and upconversion fluorescence, revealing the great promise of these nanoprobes in angiography imaging. Such a new technique enables the integration of the two bioimaging techniques by combining their collective strengths and minimizing their shortcomings.

In this paper, the lanthanide (Ln) doped fluorescent/magnetic NaYbF 4 microtubes with hexagonal phase were synthesized via a hydrothermal method using oleic acid as a capping ligand and surface modifier. The as-prepared samples were characterized by X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), upconversion luminescence (UCL) spectra, and vibrating sample magnetometer (VSM). The TEM and FE-SEM observations revealed that the NaYbF 4 microtubes present high quality hexagonal prism shape. Under the excitation of a 980 nm laser diode (LD) with a power density of 0.5 W cm −2 , the UCL colors can be tuned from purplish blue to blue, greenish white, and further to yellowish green by adjusting the Tm 3+ and Ho 3+ content in NaYbF 4 system. Moreover, the brighter greenish white light can be readily achieved via increasing excitation power. The UCL mechanisms for the white emission were proposed based on the spectral and pumping power dependence analyses. Moreover, these as-prepared NaYbF 4 microtubes exhibit excellent paramagnetic property at room temperature. The magnetic mass susceptibility and magnetization are measured to 4.17 × 10 −5 emu/gOe and ∼0.84 emu g −1 at 20 kOe, respectively. Therefore, these NaYbF 4 microtubes with hexagonal prism shape, tunable multicolor UCL and excellent paramagnetic property imply that NaYbF 4 is an excellent host material and may find applications in flat-panel displays, lasers photonics and dual-modal bioprobes.

Polyethylene glycol (PEG) modified BaLuF5:Gd/Yb/Er upconversion nanoparticles (UCNPs) were synthesized by a facile one-pot hydrothermal method for simultaneous synthesis and surface functionalization. The novel, excellently biocompatible and water-soluble bioprobes were used for simultaneous upconversion (UC) luminescence and X-ray bioimaging for the first time. The as-prepared BaLuF5:Gd/Yb/Er UCNPs possess a face-centered cubic structure with an average size of 23.7 ± 2.7 nm. Under 980 nm laser excitation, these UCNPs emitted intense UC luminescence via a two-photon process. In vitro bioimaging and localized luminescence spectra detected from HeLa cells and the background reveal that these UCNPs are ideal candidates for optical bioimaging in the absence of autofluorescence. Furthermore, the synergistic in vivo UC luminescence and X-ray bioimaging reveal that these PEG-modified BaLuF5:Gd/Yb/Er UCNPs can be successfully used as ideal dual-modal bioprobes. These results demonstrate that these PEG modified UCNPs are ideal multi-modal nanoprobes for bioimaging.

Small-sized BaLaF5:Mn/Yb/Er upconversion nanoparticles (UCNPs) were successfully synthesized for dual-modal X-ray and upconversion (UC) luminescence bioimaging by a simple solvothermal method. The size, shape, and UC luminescence intensity of the as-prepared UCNPs can be readily modified by changing the contents of Mn2+. The size of BaLaF5 UCNPs doped with Mn2+ decreased largely compared with Mn-free UCNPs. When increasing the content of Mn2+ from 5% to 20%, the size of UCNPs was gradually increased from 6.5 nm to 9.7 nm. The as-prepared BaLaF5 UCNPs doped with 20% Mn2+ present intense UC luminescence. The in vitro UC luminescence imaging of HeLa cells and localized spectra detected from HeLa cells and the background based on these BaLaF5:Mn/Yb/Er (20/20/2%) UCNPs indicate that this sample can serve as an ideal bioprobe with the absence of autofluorescence under the excitation of 980 nm laser. Moreover, an obvious UC signal was observed in in vivo UC bioimaging, demonstrating that these BaLaF5:Mn/Yb/Er (20/20/2%) UCNPs can also be used as bioprobes for whole body optical bioimaging. In addition, owing to the high X-ray mass absorption coefficients of Ba2+, La3+ and the doped Yb3+, the simultaneous X-ray and UC in vivo bioimaging of a nude mouse further demonstrate that the as-prepared UCNPs can be successfully used as dual-modal bioprobes. Ex vivo UC bioimaging revealed that these UCNPs gathered at the lung of a mouse at the initial time, demonstrating that this sample was suitable for the detection of the lung diseases. In addition, the cytotoxicity test showed that the UCNPs possessed little toxicity. Therefore, the small-sized BaLaF5:Yb/Er/Mn UCNPs are ideal nanoprobes for dual-modal UC luminescence/X-ray bioimaging with non-autofluorescence, and enhanced detection of the lung diseases.

Simultaneous in vivo luminescence and X-ray bioimaging in a tissue or animal integrates the advantages of each single-modal imaging technology, and will find widespread application in biological and clinical fields. However, synergistic dual-modal bioimaging that utilizes a new generation of upconversion nanoprobes is still limited. In addition, investigations concentrated on in vivo biodistribution of these nanoprobes may contribute to diagnosis and treatment, but long-term in vivo tracking based on these nanoprobes is rarely reported. In this work, water-soluble NaLuF4: Yb/Er nanophosphors were prepared through modified one-pot simultaneous synthesis and surface modification method. Owing to the outstanding upconverting emissions and large X-ray absorption coefficient/K-edge value of Lu and doped Yb ions, the obtained nanoprobes were successfully used as luminescent nanoprobes and X-ray contrast agents for in vivo synergistic upconversion luminescence and X-ray bioimaging. The in vivo biodistribution of these nanoprobes were observed, and the results based on long-term tracking reveal that the as-prepared nanoprobes first aggregated in the lung of the mouse, transferred to the liver, and finally moved to the spleen.

In this work, the amine-functionalized NaYbF4:Er nanoparticles were developed as dual-modal nanoprobes for synergistic upconversion (UC) luminescence and X-ray imaging in a single system by a simple one-step method of simultaneous synthesis and surface modification. The water-soluble NaYbF4:Er nanoparticles present excellent green and dominant red UC emissions. The in vitro cell imaging shows that the high-contrast green and intense red UC emissions can be observed from HeLa cells treated with these nanoparticles, indicating the successful labeling of HeLa cells. Moreover, the localized spectra measured from HeLa cells and background presented significant green and dominant red UC emissions with the absence of any autofluorescence, further verifying that these nanoparticles can be successfully used as ideal probes for optical UC bioimaging with high contrast and non-autofluorescence. In addition, the amine-functionalized NaYbF4:Er nanoparticles maintained low cell toxicity in HeLa cells evaluated by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. More importantly, these amine-functionalized NaYbF4:Er nanoparticles can also be used as X-ray imaging, owing to the large X-ray absorption efficiency of the Yb ion. The synergistic in vivo UC and X-ray imaging present significant UC luminescence and X-ray signals in the same region of a nude mouse, and the two signals are matched very well, which provides direct evidence for simultaneous UC luminescence and X-ray imaging in a single compound of lanthanide-doped material. Moreover, ex vivo UC imaging shows that these nanoparticles are first accumulated in the lung and gradually translocated from the lung into the liver. These results demonstrate that the amine-functionalized NaYbF4:Er nanoparticles presented here are very attractive nanoprobes for dual-modal UC luminescence and X-ray imaging with low cytotoxicity, autofluorescence free, and synergistic combination of the advantages of the two imaging modalities.

In this paper, multi-functional hexagonal phase NaErF4:Yb nanorods were synthesized by a facile hydrothermal method. The upconversion luminescence (UCL) intensity and red to green ratio of the multi-functional NaErF4 nanorods can be improved by Yb(3+) doping. More importantly, owing to the decreased distance of Er and Yb, the significant enhancement of red UCL can be obtained, which is different to the usual green UCL of Yb/Er doped NaYF4 host. In addition, the intensity of UCL is strongest when the Yb(3+)-doped concentration reached 30%. The in vitro cell imaging and localized UCL spectra taken from HeLa cells revealed that these NaErF4: 30% Yb(3+) nanorods are ideal nanoprobes with absence of autofluorescence for optical bioimaging. Moreover, these nanorods possess large X-ray absorption ions (Er(3+) and doped Yb(3+)), and were successfully used as contrast agents for in vivo X-ray bioimaging for the first time. In addition to the excellent UCL and X-ray absorption properties, these nanorods present significant paramagnetic properties and can be used as T2-weighted magnetic resonance imaging (MRI) agents. Therefore, these enhanced red UCL NaErF4 nanocrystals with excellent paramagnetic properties and X-ray absorption properties can be used as promising multi-modal nanoprobes for optical bioimaging, MRI, computed X-ray tomography (CT), and may have potential applications in bioseparation.

In this study, monodispersed and high-quality hexagonal phase LaF3 nanocrystals with different shapes and sizes were synthesized by a solvothermal method using oleic acid as the stabilizing agent. The as-prepared LaF3 nanocrystals were characterized by transmission electron microscopy (TEM), x-ray diffraction (XRD), and analysis of the upconversion spectra. The TEM results reveal that the samples present high uniformity and monodispersity and are self-assembled into a two-dimensional ordered array. Moreover, the shape, size and structure of the nanocrystals can be readily tuned by adjusting the NaF content. With increasing content of NaF, the shape of the LaF3 nanocrystals changed from particle to rod and the size gradually increased. More importantly, high NaF content favors the formation of one-dimensional nanorods. High Y b(3+) and Er(3+) content is beneficial to synthesizing the hexagonal phase of NaLaF4 nanocrystals. Furthermore, the TEM results show that the shape and size of the LaF3 nanocrystals can also be tuned by doping lanthanide ions, which provides a new route for size and shape control of nanocrystals. In addition, LaF3 nanocrystals co-doped with Y b(3+)/Tm(3+) present efficient near-infrared (NIR)-NIR upconversion luminescence. More importantly, the upconversion luminescent colors can be readily tuned from blue-white to blue by adjusting the excitation power. Therefore, it is expected that these LaF3 nanocrystals with well-controlled shape, size and NIR-NIR upconversion emission have potential applications in biomedical imaging fields.

Yb/Er codoped Gd(OH)3was synthesized firstly via a simple hydrothermal treatment of the corresponding nitrate in the presence of alkali by tuning the pH values. The Gd2O3:Yb/Er nanocrystals were obtained via sintering the corresponding hydroxides precursors. The as-prepared samples were characterized by the typical X-ray diffraction, energy-dispersive X-ray spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, and spectrophotometer. The results revealed that two shapes of the as-prepared Gd2O3:Yb/Er nanocrystals can be readily tuned from lemon-like particle to rod-like structure via tuning pH values from 7 to 14. Moreover, compared with the samples prepared at pH 7, the Gd2O3:Yb/Er nanocrystals prepared at pH 14 exhibit enhanced red upconversion emission and higher upconversion luminescence intensity under the excitation of 980 nm laser.

Highly crystalline body-centered cubic structure Y2O3with lanthanide (Ln) codopants (Ln = Yb3+/Er3+and Yb3+/Ho3+) has been synthesized via a moderate hydrothermal method in combination with a subsequent calcination. The structure and morphology of Y(OH)3precursors and Y2O3nanoparticles were characterized by X-ray diffraction and transmission electron microscopy. The results reveal that the Y2O3nanoparticles possess cubic phase and form the quasispherical structure. The upconversion luminescence properties of Y2O3nanoparticles doped with different Ln3+(Yb3+/ Er3+and Yb3+/ Ho3+) ions were well investigated under the 980 nm excitation. The results show that the Yb3+/Er3+and Yb3+/Ho3+codoped Y2O3nanoparticles exhibit strong red and light yellow upconversion emissions, respectively. It is expected that these Y2O3nanoparticles with tunable multicolor output and intense red upconversion emission may have potential application in color displays and biolabels.

In this work, we developed a novel and biocompatible dual-modal nanoprobe based on single-phase amine-functionalized BaYF5:Yb/Er nanoparticles (NPs) for upconversion (UC) fluorescence and in vivo computed X-ray tomography (CT) bioimaging for the first time. High-quality water-soluble amine-functionalized BaYF5:Yb/Er NPs with an average size of 24 nm were synthesized by a facile environmentally friendly hydrothermal method for simultaneous synthesis and surface functionalization. Structure investigation based on the Rietveld refinement method revealed that the as-synthesized BaYF5:Yb/Er NPs present a cubic phase structure, which differs from the previously reported tetragonal structure. Under 980 nm excitation, high-contrast green and red UC emissions were observed from HeLa cells incubated with these amine-functionalized NPs. The UC spectra measured from the NPs incubated with HeLa cells presented only green and red UC emissions without any autofluorescence, further revealing that these NPs are ideal candidates for fluorescent bioimaging. In addition, the cell cytotoxicity test showed low cell toxicity of these NPs. These amine-functionalized NPs were also successfully used as CT agents for in vivo CT imaging because of the efficient X-ray absorption efficiency of Ba and doped Yb ions. A prolonged (2 h) signal enhancement of the spleen in a mouse was observed in CT imaging, which can improve the detection of splenic diseases. More importantly, the simultaneous X-ray and UC in vivo bioimaging was demonstrated in a nude mouse for the first time, indicating the as-prepared UCNPs can be successfully used as dual-modal bioprobes. These results demonstrate that BaYF5:Yb/Er NPs are ideal nanoprobes for dual-modal fluorescent/CT bioimaging with low cytotoxicity, non-autofluorescence, and enhanced detection of the spleen.

In this paper, optical/magnetic multi-functional NaErF4 nanocrystals with high quality, different phases and shapes were synthesized by a simple hydrothermal method using oleic acid as the capping agent. The structure, upconversion luminescence and magnetic properties were characterized by various techniques. The transmission electron microscopy results reveal that the nanocrystals are of high quality and can be self-assembled into a two-dimensional ordered structure. Moreover, the crystal phase and shape can be readily controlled by adjusting the reaction temperature and F− content. The results reveal that high temperature can favor the formation of a hexagonal phase structure and promote the phase transformation from the cubic to hexagonal phase. The phase transformation mechanism based on the free energy theory was discussed in detail. In addition, the F− content plays a critical role in determining the morphology of the final products. A high F− content is beneficial for the formation of the one-dimensional rod-like shape. Interestingly, phase-induced upconversion luminescence color tuning from red to green was observed. All of the as-prepared NaErF4 nanocrystals possess paramagnetic properties at room temperature and the magnetizations of the nanocrystals with spherical-like cubic, cubic, and rod-like shapes were measured as 1.69 emu g−1, 2.53 emu g−1and 2.29 emu g−1 at 20 kOe, respectively, which are larger than most previously reported Gd-based nanocrystals. In addition, T2-weighted magnetic resonance imaging based on these NaErF4 nanocrystals was demonstrated for the first time. Therefore, these tunable upconversion fluorescent NaErF4 nanocrystals with excellent paramagnetic properties can be used as promising dual-modal nanoprobes for optical bioimaging and magnetic resonance imaging, and may have potential applications in bioseparation.