Energy Migration Upconversion in Manganese(II)‐Doped Nanoparticles

We report the synthesis and characterization of cubic NaGdF₄:Yb/Tm@NaGdF₄:Mn core–shell structures. By taking advantage of energy transfer through Yb→Tm→Gd→Mn in these core–shell nanoparticles, we have realized upconversion emission of Mn²⁺ at room temperature in lanthanide tetrafluoride based host lattices. The upconverted Mn²⁺emission, enabled by trapping the excitation energy through a Gd³⁺ lattice, was validated by the observation of a decreased lifetime from 941 to 532 μs in the emission of Gd³⁺ at 310 nm (⁶P_7/2→⁸S_7/2). This multiphoton upconversion process can be further enhanced under pulsed laser excitation at high power densities. Both experimental and theoretical studies provide evidence for Mn²⁺ doping in the lanthanide‐based host lattice arising from the formation of F^? vacancies around Mn²⁺ ions to maintain charge neutrality in the shell layer.     

Rare‐Earth‐Based Nanoparticles with Simultaneously Enhanced Near‐Infrared (NIR)‐Visible (Vis) and NIR‐NIR Dual‐Conversion Luminescence for Multimodal Imaging

Multifunctional NaGdF₄:Yb³⁺,Er³⁺,Nd³⁺@NaGdF₄:Nd³⁺ core–shell nanoparticles (called Gd:Yb³⁺,Er³⁺,Nd³⁺@Gd:Nd³⁺ NPs) with simultaneously enhanced near‐infrared (NIR)‐visible (Vis) and NIR‐NIR dual‐conversion (up and down) luminescence (UCL/DCL) properties were successfully synthesized. The resulting core–shell NPs simultaneously emitted enhanced UCL at 522, 540, and 660 nm and DCL at 980 and 1060 nm under the excitation of a 793 nm laser. The enhanced UCL and DCL can be explained by complex energy‐transfer processes, Nd³⁺→Yb³⁺→Er³⁺ and Nd³⁺→Yb³⁺, respectively. The effects of Nd³⁺ concentration and shell thickness on the UCL/DCL properties were systematically investigated. The UCL and DCL properties of NPs were observed under the optimal conditions: a shell Nd³⁺ content of 20 % and a shell thickness of approximately 5 nm. Moreover, the Gd:Yb³⁺,Er³⁺,Nd³⁺@Gd:20 % Nd³⁺ NPs exhibited remarkable magnetic resonance imaging (MRI) properties similar to that of a clinical agent, Omniscan. Thus, the core–shell NPs with excellent UCL/DCL/magnetic resonance imaging (MRI) properties have great potential for both in vitro and in vivo multimodal bioimaging.     

Unraveling Epitaxial Habits in the NaLnF4 System for Color Multiplexing at the Single‐Particle Level

We report an epitaxial growth technique for scalable production of hybrid sodium rare‐earth fluoride (NaLnF₄) microcrystals, including NaYF₄, NaYbF₄, and NaLuF₄ material systems. The single crystalline nature of the as‐synthesized products makes them strong upconversion emission. The freedom of combining a lanthanide activator (Er³⁺ or Tm³⁺) with a sensitizer (Yb³⁺) at various doping concentrations readily gives access to color multiplexing at the single‐particle level. Our kinetic and thermodynamic investigations on the epitaxial growth of core–shell microcrystals using NaLnF₄ particle seeds suggest that within a certain size regime it is plausible to exert precise control over shell thickness and growth orientation under hydrothermal conditions.     

Core‐Shell Structured Layered Lanthanide‐Organic Complexes with Stilbazolium‐type Dye Encapsulation for Multifunctional Performances

Host‐guest encapsulation of functional organic dye into a porous metal‐organic framework can give rise to the development of new functional materials. In this work, by intercalating the stilbazolium‐type dye (DEAST)I (4′‐diethylamino‐N‐methyl stilbazolium) into four lanthanide layered metal‐organic complexes (Ln‐LMOCs), i. e. {[Ln(BTB)(H₂O)₂]?3(DMF)?2(H₂O)}_n (Ln=La (1), Nd (2), Sm (3), Er (4)), four responsive (DEAST)I@Ln‐LMOC composites have been prepared, serving as multifunctional performance platform. The core–shell structures of (DEAST)I@Ln‐LMOC composites have been fully characterized by IR, UV/Vis, PXRD, SEM, TEM, TGA and ESR. Significantly, after intercalation of dyes, the (DEAST)I@Ln‐LMOC composites exhibit enhanced luminescent sensing properties in detecting Fe³⁺ with much higher water stabilities. The luminescent sensing behavior stems from the fluorescence resonance energy transfer (FRET) from the π‐electron‐rich BTB ligands to the Fe³⁺, and their higher water stabilities are induced by electrostatic interactions and lower porosity. Specially, the characteristic emissions of Sm³⁺ will not be affected after the encapsulation guest dyes, which provide a theoretical guide for the modulation of luminescence devices. Finally, better ion conductivities and diminished photocurrents can be achieved after the embedding of the functional organic dye. In all, the formation of (DEAST)I@Ln‐LMOC composites with core–shell structures can be utilized as a multifunctional platform with good stability.     

Three Lanthanide Metal‐Organic Frameworks Based on an Ether‐Decorated Polycarboxylic Acid Linker: Luminescence Modulation,CO2 Capture and Conversion Properties

Three new isostructural 3D lanthanide metal–organic frameworks (Ln‐MOFs), {H[LnL(H₂O)]?2 H₂O}_n ( 1‐Ln ) (Ln=Eu³⁺, Gd³⁺ and Tb³⁺), based on infinite lanthanide‐carboxylate chains were constructed by employing an ether‐separated 5,5′‐oxydiisophthalic acid (H₄L) ligand under solvothermal reaction. 1‐Eu and 1‐Tb exhibit strong red and green emission, respectively, through the antenna effect, as demonstrated through a combination of calculation and experimental results. Moreover, a series of dichromatic doped 1‐Eu_xTb_y MOFs were fabricated by introducing different concentrations of Eu³⁺ and Tb³⁺ ions, and they display an unusual variation of luminescent colors from green, yellow, orange to red. 1‐Eu with channels decorated by ether O atoms and the open metal sites displays good performance for CO₂ capture and conversion between CO₂ and epoxides into cyclic carbonates.     

Intrinsic Time‐Tunable Emissions in Core–Shell Upconverting Nanoparticle Systems

Color‐tunable luminescence has been extensively investigated in upconverting nanoparticles for diverse applications, each exploiting emissions in different spectral regions. Manipulation of the emission wavelength is accomplished by varying the composition of the luminescent material or the characteristics of the excitation source. Herein, we propose core–shell β‐NaGdF₄: Tm³⁺, Yb³⁺/β‐NaGdF₄: Tb³⁺ nanoparticles as intrinsic time‐tunable luminescent materials. The time dependency of the emission wavelength only depends on the different decay time of the two emitters, without additional variation of the dopant concentration or pumping source. The time‐tunable emission was recorded with a commercially available camera. The dynamics of the emissions is thoroughly investigated, and we established that the energy transfer from the ¹D₂ excited state of Tm³⁺ ions to the higher energy excited states of Tb³⁺ ions to be the principal mechanism to the population of the ⁵D₄ level for the Tb³⁺ ions.     

Efficient Luminescence from Fluorene‐ and Spirobifluorene‐Based Lanthanide Complexes upon Near‐Visible Irradiation

We describe herein the synthesis and photophysical characterization of new lanthanide complexes that consist of a (9,9‐dimethylfluoren‐2‐yl)‐2‐oxoethyl or a (9,9′‐spirobifluoren‐2‐yl)‐2‐oxoethyl unit as the antenna, covalently linked to a 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acid (DO3A) unit as the Ln³⁺ (Gd³⁺, Eu³⁺, Sm³⁺, Tb³⁺, Dy³⁺) coordination site. We were able to translate the spectroscopic properties of the innovative bipartite ligands into the formation of highly luminescent europium complexes that exhibit efficient emission (?_se>0.1) upon sensitization in the near‐visible region, that is, with an excitation wavelength above 350 nm. The luminescence of the Eu³⁺complexes is clearly detectable at concentrations as low as 10 pM . Furthermore, the structural organization of these bipartite ligands makes the complexes highly soluble in aqueous solutions and chemically stable over time.     

Confining Excitation Energy in Er3+‐Sensitized Upconversion Nanocrystals through Tm3+‐Mediated Transient Energy Trapping

A new class of lanthanide‐doped upconversion nanoparticles are presented that are without Yb³⁺ or Nd³⁺ sensitizers in the host lattice. In erbium‐enriched core–shell NaErF₄:Tm (0.5 mol %)@NaYF₄ nanoparticles, a high degree of energy migration between Er³⁺ ions occurs to suppress the effect of concentration quenching upon surface coating. Unlike the conventional Yb³⁺‐Er³⁺ system, the Er³⁺ ion can serve as both the sensitizer and activator to enable an effective upconversion process. Importantly, an appropriate doping of Tm³⁺ has been demonstrated to further enhance upconversion luminescence through energy trapping. This endows the resultant nanoparticles with bright red (about 700‐fold enhancement) and near‐infrared luminescence that is achievable under multiple excitation wavelengths. This is a fundamental new pathway to mitigate the concentration quenching effect, thus offering a convenient method for red‐emitting upconversion nanoprobes for biological applications.     

Crystal structure and pale‐yellow emitting properties of K3Gd1–xDyxB6O12 solid solutions

A new potassium dysprosium polyborate, K₃DyB₆O₁₂, has been prepared via the high‐temperature molten salt method and structurally characterized by single‐crystal X‐ray diffraction analysis. The structure can be described as a three‐dimensional framework composed of isolated bicyclic [B₅O₁₀]^5? groups and Dy³⁺ and K⁺ ions. The Fourier transform IR (FT–IR) and ultraviolet–visible (UV–Vis) spectra were investigated. A series of K₃Gd_1–xDy_xB₆O₁₂ phosphors was prepared and their photoluminescence properties were studied. The K₃Gd_1–xDy_xB₆O₁₂ phosphors exhibit a strong yellow emission band at 577 nm (the ⁴F_9/2→⁶H_13/2 transition of Dy³⁺) under UV excitation of 275 nm (the ⁸S_7/2→⁶I_J transition of Gd³⁺), suggesting the occurrence of the energy transfer Gd³⁺→Dy³⁺. The optimized doping concentration of the Dy³⁺ ion was 8 mol%. We may expect that K₃Gd_1–xDy_xB₆O₁₂ is a promising pale‐yellow emission phosphor for visual displays or solid‐state lighting.     

Aptamer‐Based Luminescence Energy Transfer from Near‐Infrared‐to‐Near‐Infrared Upconverting Nanoparticles to Gold Nanorods and Its Application for the Detection of Thrombin

A new luminescence energy transfer (LET) system has been designed for the detection of thrombin in the near‐infrared (NIR) region by utilizing NIR‐to‐NIR upconversion lanthanide nanophosphors (UCNPs) as the donor and gold nanorods (Au NRs) as the acceptor. The use of upconverting NaYF₄:Yb³⁺,Tm³⁺ nanoparticles with sharp NIR emission peaks upon NIR excitation by an inexpensive infrared continuous wave laser diode provided large spectral overlap between the donor and the acceptor. Both the Au NRs and carboxyl‐terminated NaYF₄:Yb³⁺,Tm³⁺ UCNPs were first modified with different thrombin aptamers. When thrombin was added, a LET system was then formed because of the specific recognition between the thrombin aptamers and thrombin. The LET system was used to monitor thrombin concentrations in aqueous buffer and human blood samples. The limits of detection for thrombin are as low as 0.118 nM in buffer solution and 0.129 nM in human serum. The method was also successfully applied to thrombin detection in blood samples.     

Synthesis and Infrared Multi‐band Absorption Properties of Core‐shell NaYF4:Yb3+, Er3+@SiO2 Nanoparticles

Due to the unique size effects, nanomaterials in infrared absorption have attracted much attention for their strong absorption in the infrared region. To achieve the infrared multi‐band absorption, we propose to synthesize a core‐shell structure nanomaterial consisting of NaYF₄:Yb³⁺, Er³⁺ core and a layer of SiO₂ as shell. A series of NaYF₄:Yb³⁺, Er³⁺ nanocrystals were synthesized through hydrothermal method by adjusting the ratio of citric acid(CA)‐to‐NaOH, and the effects of CA concentration, and NaOH concentration were studied in detail. NaYF₄:Yb³⁺, Er³⁺@SiO₂ nanoparticles were synthesized by sol‐gel method using TEOS as silica source. The results show that the core‐shell NaYF₄:Yb³⁺, Er³⁺@SiO₂ nanoparticles were successfully synthesized. Up‐conversion spectra of these nanoparticles were recorded with 980 nm laser excitation under room temperature. There are no changes of the emission centers of nanoparticles before or after silica coating, but the emission intensities of nanoparticles after silica coating are weakened. Furthermore, the property of infrared multi‐band absorption was tested through ultraviolet‐visible‐near infrared spectrophotometer and infrared absorption spectra. The results illustrate that the multi‐band infrared absorption nanomaterial was successfully synthesized.     

Encapsulation of LnIII Ions/Dyes within a Microporous Anionic MOF by Post‐synthetic Ionic Exchange Serving as a LnIII Ion Probe and Two‐Color Luminescent Sensors

A new anionic framework {[Me₂NH₂]_0.125[In_0.125(H₂L)_0.25] ? xDMF}_n ( 1 ) with one‐dimensional (1D) channels along the c axis of about 13.06×13.06 Ų, was solvothermally synthesized and well characterized. Post‐synthetic cation exchange of 1 with Eu³⁺, Tb³⁺, Dy³⁺, Sm³⁺ afforded lanthanide(III)‐loaded materials, Ln³⁺@ 1 , with different luminescent behavior, indicating that compound 1 could be used as a potential luminescent probe toward different lanthanide(III) ions. Additionally, compound 1 exhibits selective adsorption ability toward cationic dyes. Moreover, the RhB@ 1 realized the probing of different organic solvent molecules by tuning the energy transfer efficiency between two different emissions, especially for sensing DMF. This work highlights the practical application of luminescent guest@MOFs as sensors, and it paves the way toward other one/multi‐color luminescent host–guest systems by rational selection of MOF hosts and guest chromophores with suitable emissive colors and energy levels.     

Luminescence in NaLn(SO4)2H2O: A host lattice with high-energy vibrations

The luminescence of Ce³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, and Dy³⁺ in NaLn(SO₄)₂H₂O (Ln = lanthanide) is reported. Only Ce³⁺, Gd³⁺, and Tb³⁺ show efficient emission. This is explained in terms of an energy-gap law. Energy transfer is studied in several codoped compositions. The mutual transfer between Gd³⁺ ions is the only one encountered with high probability. The several transfers are discussed and where possible their rates are calculated.     

Luminescence Tuning and White‐Light Emission of Co‐doped Ln–Cd–Organic Frameworks

Four new three‐dimensional isostructural lanthanide–cadmium metal–organic frameworks (Ln–Cd MOFs), [LnCd₂(imdc)₂(Ac)(H₂O)₂]?H₂O (Ln=Pr ( 1 ), Eu ( 2 ), Gd ( 3 ), and Tb ( 4 ); H₃imdc=4,5‐imidazoledicarboxylic acid; Ac=acetate), have been synthesized under hydrothermal conditions and characterized by IR, elemental analyses, inductively coupled plasma (ICP) analysis, and X‐ray diffraction. Single‐crystal X‐ray diffraction shows that two Ln^III ions are surrounded by four Cd^II ions to form a heteronuclear building block. The blocks are further linked to form 3D Ln–Cd MOFs by the bridging imdc^3? ligand. Furthermore, the left‐ and right‐handed helices array alternatively in the lattice. Eu–Cd and Tb–Cd MOFs can emit characteristic red light with the Eu^III ion and green light with the Tb^III ion, respectively, while both Gd–Cd and Pr–Cd MOFs generate blue emission when they are excited. Different concentrations of Eu³⁺ and Tb³⁺ ions were co‐doped into Gd–Cd/Pr–Cd MOFs, and tunable luminescence from yellow to white was achieved. White‐light emission was obtained successfully by adjusting the excitation wavelength or the co‐doping ratio of the co‐doped Gd–Cd and Pr–Cd MOFs. These results show that the relative emission intensity of white light for Gd–Cd:Eu³⁺,Tb³⁺ MOFs is stronger than that of Pr–Cd:Eu³⁺,Tb³⁺ MOFs, which implies that the Gd complex is a better matrix than the Pr complex to obtain white‐light emission materials.     

The Series of Rare Earth Complexes [Ln2Cl6(μ‐4,4′‐bipy)(py)6], Ln=Y,Pr, Nd,Sm‐Yb: A Molecular Model System for Luminescence Properties in MOFs Based on LnCl3 and 4,4′‐Bipyridine

A series of 12 dinuclear complexes [Ln₂Cl₆(μ‐4,4′‐bipy)(py)₆], Ln=Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, ( 1 – 12 , respectively) was synthesized by an anhydrous solvothermal reaction in pyridine. The complexes contain a 4,4′‐bipyridine bridge and exhibit a coordination sphere closely related to luminescent lanthanide MOFs based on LnCl₃ and 4,4‐bipyridine. The dinuclear complexes therefore function as a molecular model system to provide a better understanding of the luminescence mechanisms in the Ln‐N‐MOFs ${\hbox{}{{\hfill 2\atop \hfill \infty }}}$ [Ln₂Cl₆(4,4′‐bipy)₃] ? 2(4,4′‐bipy). Accordingly, the luminescence properties of the complexes with Ln=Y, Sm, Eu, Gd, Tb, Dy, ( 1 , 4 – 8 ) were determined, showing an antenna effect through a ligand–metal energy transfer. The highest efficiency of luminescence is observed for the terbium‐based compound 7 displaying a high quantum yield (QY of 86 %). Excitation with UV light reveals typical emission colors of lanthanide‐dependent intra 4f–4f‐transition emissions in the visible range (Tb^III: green, Eu^III: red, Sm^III: salmon red, Dy^III: yellow). For the Gd^III‐ and Y^III‐containing compounds 6 and 1 , blue emission based on triplet phosphorescence is observed. Furthermore, ligand‐to‐metal charge‐transfer (LMCT) states, based on the interaction of Cl^? with Eu^III, were observed for the Eu^III compound 5 including energy‐transfer processes to the Eu^III ion. Altogether, the model complexes give further insights into the luminescence of the related MOFs, for example, rationalization of Ln‐independent quantum yields in the related MOFs.     

Synthesis of New LnxBi2–xSe3 (Ln: Sm3+, Eu3+, Gd3+, Tb3+) Nanomaterials and Investigation of Their Optical Properties

New Ln_xBi_2–xSe₃ (Ln: Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺) based nanomaterials were synthesized by a co‐reduction method. Powder XRD patterns indicate that the Ln_xBi_2–xSe₃ crystals (Ln = Sm³⁺, Eu³⁺, x = 0.00–0.44 and Ln = Gd³⁺, Tb³⁺, x = 0.00–0.50) are isostructural with Bi₂Se₃. The cell parameter c decreases for Ln = Eu³⁺, Gd³⁺, Tb³⁺ upon increasing the dopant content (x), while a slightly increases. Changes in lattice parameters could be related to the radii of cations. SEM images show that doping of the lanthanide ions in the lattice of Bi₂Se₃ generally results in nanoflowers. For the terbium compound two kinds of morphologies (nanoflowers and nanobelts) were observed. UV/Vis absorption and emission spectroscopy reveals mainly electronic transitions of the Ln³⁺ ions. Emission spectra show intense transitions from the excited to the ground state of Ln³⁺ and energy transfer from the Bi₂Se₃ lattice. Emission spectra of europium‐doped materials, in addition to the characteristic red emission peaks of Eu³⁺, show an intense blue emission band centered at 432 nm, originating from the 4f⁶5d¹ to 4f⁷ configuration in Eu²⁺. EPR measurements confirm the existence of Eu²⁺ in the materials. Interestingly, for all samples starting at low Ln³⁺ concentration, the emission intensity rises to a maximum at a Ln³⁺ concentration of x = 0.2 and falls again steadily to a minimum at x = 0.45.     

Infrared‐to‐Visible Light Conversion in Er3+–Yb3+:Lu3Ga5O12 Nanogarnets

Er³⁺–Yb³⁺ co‐doped Lu₃Ga₅O₁₂ nanogarnets were prepared and characterized; their structural and luminescence properties were determined as a function of the Yb³⁺ concentration. The morphology of the nanogarnets was studied by HRTEM. Under 488 nm excitation, the nanogarnets emit green, red, and near‐infrared light. The decay curves for the (⁴S_3/2, ²H_11/2) and ⁴F_9/2 levels of the Er³⁺ions exhibit a non‐exponential nature under resonant laser excitation and their effective lifetimes are found to decrease with an increase in the Yb³⁺ concentration from 1.0 to 10.0 mol %. The non‐exponential decay curves are well fitted to the Inokuti–Hirayama model for S=8, indicating that the mechanism of interaction for energy transfer between the optically active ions is of dipole–quadrupole type. Upon 976 nm laser excitation, an intense green upconverted emission is clearly observed by the naked eyes. A significant enhancement of the red‐to‐green intensity ratio of Er³⁺ ions was observed with an increase in Yb³⁺ concentration. The power dependence and the dynamics of the upconverted emission confirm the existence of two‐photon upconversion processes for the green and red emissions.     

Impact of High Ytterbium(III) Concentration in the Shell on Upconversion Luminescence of Core–Shell Nanocrystals

After coating 20 Yb/2 Er:NaGdF₄ core nanocrystals with a NaYbF₄ shell, upconversion emission of the rare earth ions weakens. So far, the exact reason for this phenomenon is still unclear due to lack of the direct evidence. In this report, a core@shell@shell sandwich‐like structure is designed and fabricated to investigate this phenomenon. We find that high Yb³⁺ concentration in the shell has mainly two adverse impacts: it promotes not only the deleterious back energy transfer from Er³⁺ in the core to Yb³⁺ in the shell but also the energy transfer from Yb³⁺ in the core to Yb³⁺ in the shell. To obtain nanocrystals with high upconversion efficency, appropriate Yb³⁺ concentration should be introduced into the shell or the transition layer.     

Controlling Lanthanide Exchange in Triple‐Stranded Helicates: A Way to Optimize Molecular Light‐Upconversion

The kinetic lability of hexadentate gallium‐based tripods is sufficient to ensure thermodynamic self‐assembly of luminescent heterodimetallic [GaLn( L3 )₃]⁶⁺ helicates on the hour time scale, where Ln is a trivalent 4f‐block cation. The inertness is, however, large enough for preserving the triple‐helical structure when [GaLn( L3 )₃]⁶⁺ is exposed to lanthanide exchange. The connection of a second gallium‐based tripod further slows down the exchange processes to such an extent that spectroscopically active [CrErCr( L4 )₃]⁹⁺ can be diluted into closed‐shell [GaYGa( L4 )₃]⁹⁺ matrices without metal scrambling. This feature is exploited for pushing molecular‐based energy‐transfer upconversion (ETU) at room temperature.     

Focal‐point anchoring: Large proton relaxivity observed for gadolinium‐cored dendrimer complexes in water

Water‐soluble Gd³⁺‐cored poly(ether amide) dendrimer complexes, bearing a diethylenetriaminepentaacetic acid Gd³⁺ complex (GdDTPA) core, were synthesized. The longitudinal proton relaxivity (R₁), governed mainly by the rotation correlation time of the paramagnetic Gd³⁺ center, was measured in water (37 °C, 20 MHz). An anchor effect at the focal point of the hydrophilic dendrimer was experimentally evaluated for the first time, through a comparison of the R₁ values with the ones reported for various Gd³⁺ complexes, including linear copolymers of GdDTPA linked by various lengths of aliphatic chains via amide bonds. Unusually large R₁ values for the first‐ and second‐generation Gd³⁺‐cored dendrimer complexes (6.1 and 9.3 mM^?1 s^?1, respectively), even with relatively low molecular weights (1355 and 3877, respectively), suggested a remarkable increase in the local rigidity around the GdDTPA core. The internal rigidity of the dendrimer was supposed to take advantage of the increased nominal molecular weight in water via multihydration to the dendron to increase the rotation correlation time, and this resulted in the observed R₁ gain. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2680–2689, 2004