A "turn-off" luminescence resonance energy transfer aptamer sensor based on near-infrared upconverting NaYF4:Yb3+,Tm3+ nanoparticles as donors and gold nanorods as acceptors

Near-infrared upconverting NaYF₄:Yb^3*,Tm^3* nanophosphors modified with poly（acrylic acid） were prepared and characterized by transmission electron microscopy and luminescence spectroscopy.Based on the observed overlap between the emission spectrum of the NaYF₄:Yb^3*,Tm^3* nanophosphors and the absorption spectrum of the gold nanorods,we believe that a new "turn-off luminescence resonance energy transfer aptamer sensor was constructed for sensing thrombin in near-infrared region.     

Synthesis of Upconverting Hydrogel Nanocomposites Using Thiol‐Ene Click Chemistry: Template for the Formation of Dendrimer‐Like Gold Nanoparticle Assemblies

The synthesis of upconverting hydrogel nanocomposites by base‐catalyzed thiol‐ene click reaction between 10‐undecenoic acid capped Yb³⁺/Er³⁺‐doped NaYF₄ nanoparticles and pentaerythritol tetrakis(3‐mercaptopropionate) (PETMP) as tetrathiol monomer is reported. This synthetic strategy for nanocomposite gels is quite different from works where usually the preformed gels are mixed with the nanoparticles. Developing nanocomposites by surface modification of capping ligands would allow tuning and controlling of the separation of the nanoparticles inside the gel network. The hydrogel nanocomposites prepared by thiol‐ene click reaction show strong enhancement in luminescence intensity compared to 10‐undecenoic acid‐capped Yb³⁺/Er³⁺‐doped NaYF₄ nanoparticles through the upconversion process (under 980 nm laser excitation). The hydrogel nanocomposites display strong swelling characteristics in water resulting in porous structures. Interestingly, the resulting nanocomposite gels act as templates for the synthesis of dendrimer‐like Au nanostructures when HAuCl₄ is reduced in the presence of the nanocomposite gels.     

Synthesis and characterization of nanoporous NaYF4:Yb3+, Tm3+@SiO2 nanocomposites

Novel upconversion nanocomposites with nanoporous structure were presented in this paper. Silica-coated cubic NaYF₄:Yb³⁺, Tm³⁺ nanoparticles were first prepared. After annealing, monodisperse cubic/hexagonal mixed phases NaYF₄:Yb³⁺, Tm³⁺@SiO₂ nanoparticles were obtained, and the NaYF₄:Yb³⁺, Tm³⁺ cores became nanoporous. To the best of our knowledge, the nanoporous structure in NaYF₄:Yb³⁺, Tm³⁺@SiO₂ nanocomposites was observed for the first time. They demonstrate increased upconversion emission compared with unannealed dense NaYF₄:Yb³⁺, Tm³⁺ nanoparticles due to the appearance of the hexagonal NaYF₄:Yb³⁺, Tm³⁺. The silica shell not only makes the nanocomposites possess bio-affinity but also protects the NaYF₄:Yb³⁺, Tm³⁺ cores from aggregating and growing up. Thus the upconversion, nanoporous and bio-affinity properties were combined into one single nanoparticle. The nanocomposites have been characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), small angle X-ray diffraction (SAXRD) and emission spectroscopy. These multifunctional nanocomposites are expected to find applications in biological fields, such as biolabels, drug storage and delivery.     

Synthesis and surface modification of ultrasmall monodisperse NaYF4:Yb3+/Tm3+ upconversion nanoparticles

The emerging upconversion nanoparticles (UCNP) have gained substantial consideration in the field of bioanalytical as well as diagnostic applications. Therefore, great progress has been made in the synthesis and surface modification of luminescent UCNPs over the last two decades. In this paper, we have reported monodispersed and high luminescent upconversion nanoparticles NaYF₄: 20%Yb³⁺, 2%Tm³⁺ have been synthesized using a solvothermal method, followed by a coating of the NaYF₄ shell with a thin layer of SiO₂ on the surface to afford the core-shell NaYF₄:Yb³⁺, Tm³⁺@SiO₂ nanoparticles (NP@SiO₂). The prepared nanoparticles were of strong upconversion fluorescent emission intensity, hexagonal phase, and with an average size of about 8 ± 1 nm, which have been characterized by luminescence spectroscopy, powder X-ray diffraction (P-XRD), Dynamic light scattering (DLS), and Transmission electron microscopy (TEM). The results indicate that the NP@SiO₂ can be used for the conjugation of fluorescent probes for various biomolecules and can find applications in cancer cell imaging and disease diagnosis.     

Shape,Size, and Phase‐Controlled Rare‐Earth Fluoride Nanocrystals with Optical Up‐Conversion Properties

High‐quality rare‐earth fluorides, α‐NaMF₄ (M=Dy, Ho, Er, Tm, Y, Yb, and Lu) nanocrystals and β‐NaMF₄ (M=Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Y, Yb, and Lu) nanoarrays, have been synthesized by using oleic acid as a stabilizing agent through a facile hydrothermal method at 130–230 °C. The phase, shape, and size of the products are varied by careful control of synthetic conditions, including hydrothermal temperature and time, and the amounts of reactants and solvents. Tuning the hydrothermal temperature, time, and the amount of NaOH can cause the transformation from the cubic α‐NaMF₄ to hexagonal phase β‐NaMF₄. Upon adjustment of the amount of NaOH, NaF, M³⁺, and ethanol, the morphologies for the β‐NaMF₄ nanoarrays can range from tube, rod, wire, and zigzagged rod, to flower‐patterned disk. Simultaneously, the size of the rare‐earth fluoride crystals is variable from 5 nm to several micrometers. A combination of “diffusion‐controlled growth” and the “organic–inorganic interface effect” is proposed to understand the formation of the nanocrystals. An ideal “1D growth” of rare‐earth fluorides is preferred at high temperatures and high ethanol contents, from which the tube‐ and rodlike nanoarrays with high aspect ratio are obtained. In contrast, the disklike β‐NaMF₄ nanoarrays with low aspect ratios are produced by decreasing the ethanol content or prolonging the reaction time, an effect probably caused by “1D/2D ripening”. Multicolor up‐conversion fluorescence is also successfully realized in the Yb³⁺/Er³⁺ (green, red) and Yb³⁺/Tm³⁺ (blue) co‐doped α‐NaYF₄ nanocrystals and β‐NaYF₄ nanoarrays by excitation in the NIR region (980 nm).     

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.     

Comparative investigation on structure and luminescence properties of fluoride phosphors codoped with Er/Yb

This paper reports on comparative investigation of structure and luminescence properties of tetragonal LiYF₄ and BaYF₅, and hexagonal NaYF₄ phosphors codoped with Er³⁺/Yb³⁺ by a facile hydrothermal synthesis. The products were characterized by X-ray diffractometer, scanning electron microscope, and photoluminescence spectroscopy. Intense visible emissions centered at around 525, 550 and 650 nm, originated from the transitions of ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 of Er³⁺, respectively, have been observed in all the samples upon excitation with a 980 nm laser diode, and the involved mechanisms have been explained. Based on the green up-conversion emission performance, the Yb³⁺ concentrations of Er³⁺/Yb³⁺-codoped LiYF₄, BaYF₅, and NaYF₄ phosphors have been optimized to be 10, 20, and 20 mol.%, respectively. The quadratic dependence of fluorescence on excitation laser power has confirmed that two-photon contribute to up-conversion of the green–red emissions.     

Phase‐ and Size‐Controllable Synthesis of Hexagonal Upconversion Rare‐Earth Fluoride Nanocrystals through an Oleic Acid/Ionic Liquid Two‐Phase System

Herein, we introduce a facile, user‐ and environmentally friendly (n‐octanol‐induced) oleic acid (OA)/ionic liquid (IL) two‐phase system for the phase‐ and size‐controllable synthesis of water‐soluble hexagonal rare earth (RE=La, Gd, and Y) fluoride nanocrystals with uniform morphologies (mainly spheres and elongated particles) and small sizes (<50 nm). The unique role of the IL 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BmimPF₆) and n‐octanol in modulating the phase structure and particle size are discussed in detail. More importantly, the mechanism of the (n‐octanol‐induced) OA/IL two‐phase system, the formation of the RE fluoride nanocrystals, and the distinctive size‐ and morphology‐controlling capacity of the system are presented. BmimPF₆ is versatile in term of crystal‐phase manipulation, size and shape maintenance, and providing water solubility in a one‐step reaction. The luminescent properties of Er³⁺‐, Ho³⁺‐, and Tm³⁺‐doped LaF₃, NaGdF₄, and NaYF₄ nanocrystals were also studied. It is worth noting that the as‐prepared products can be directly dispersed in water due to the hydrophilic property of Bmim⁺ (cationic part of the IL) as a capping agent. This advantageous feature has made the IL‐capped products favorable in facile surface modifications, such as the classic Stober method. Finally, the cytotoxicity evaluation of NaYF₄:Yb,Er nanocrystals before and after silica coating was conducted for further biological applications.     

Hydrophilic,Upconverting, Multicolor,Lanthanide‐Doped NaGdF4 Nanocrystals as Potential Multifunctional Bioprobes

Monodisperse water‐soluble hexagonal phase Ln³⁺‐doped NaGdF₄ upconverting nanocrystals (UCNCs) have been successfully fabricated by means of a fast, facile, and environmentally friendly microwave‐assisted route with polyethylenimine as the surfactant. Fine‐tuning of the UC emission from visible to near‐IR and finally to white light has been achieved. Furthermore, studies of the magnetic resonance imaging as well as the magnetization (magnetization–magnetic field curves) and the targeted recognition properties of FA‐coupled amine‐functionalized NaGdF₄@SiO₂ UCNCs indicate that the obtained NaGdF₄ UCNCs can be potential candidates for dual‐mode optical/magnetic bioapplications.     

Direct formation of mesoporous upconverting core-shell nanoparticles for bioimaging of living cells

We have developed a one-step method for the synthesis of mesoporous upconverting nanoparticles (MUCNs) of the type NaYF₄:Yb,Er@mSiO₂ in ammoniacal ethanol/water solution. The mesoporous silica is directly encapsulating the hydrophobic upconversion nanoparticles (UCNs) due to the presence of the template CTAB. Intense green emission (between 520 and 560 nm) and weaker red emission (between 630 and 670 nm) is observed upon 980-nm laser excitation. The MUCNs display low cytotoxicity (as revealed by an MTT test) and were successfully applied to label and image human nasopharyngeal epidermal carcinoma (KB) cells.

Synthesis of improved upconversion nanoparticles as ultrasensitive fluorescence probe for mycotoxins

Rare earth-doped upconversion nanoparticles (UCNPs) have promising potentials in biodetection due to their unique frequency upconverting capability and high detection sensitivity. This paper reports an improved UCNPs-based fluorescence probe for dual-sensing of Aflatoxin B1 (AFB1) and Deoxynivalenol (DON) using a magnetism-induced separation and the specific formation of antibody-targets complex. Herein, the improved UCNPs, which were namely NaYF₄:Yb/Ho/Gd and NaYF₄:Yb/Tm/Gd, were systematically studied based on the optimization of reaction time, temperature and the concentration of dopant ions with simultaneous phase and size controlled NaYF₄ nanoparticles; and the targets were detected using the pattern of competitive combination assay. Under an optimized condition, the advanced fluorescent probes revealed stronger fluorescent properties, broader biological applications and better storage stabilities compared to traditional UCNPs-based ones; and ultrasensitive determinations of AFB1 and DON were achieved under a wide sensing range of 0.001–0.1 ng ml⁻¹ with the limit of detection (LOD) of 0.001 ng ml⁻¹. Additionally, the applicability of the improved nanosensor for the detection of mycotoxins was also confirmed in adulterated oil samples.     

Designing Upconversion Nanocrystals Capable of 745 nm Sensitization and 803 nm Emission for Deep‐Tissue Imaging

A crystal design strategy is described that generates hexagonal‐phased NaYF₄:Nd/Yb@NaYF₄:Yb/Tm luminescent nanocrystals with the ability to emit light at 803 nm when illuminated at 745 nm. This is accomplished by taking advantage of the large absorption cross‐section of Nd³⁺ between 720 and 760 nm plus efficient spatial energy transfer and migration through Nd³⁺→Yb³⁺→Yb³⁺→Tm³⁺. Mechanistic investigations suggest that a cascaded two‐photon energy transfer upconversion process underlies the emission mechanism. This protocol enables deep‐tissue imaging to be achieved while mitigating the attenuation effect associated with the visible emission and the overheating constraint imposed by conventional 980 nm excitation.     

Controllable Multicolor Upconversion Luminescence by Tuning the NaF Dosage

Multicolor upconversion (UC) luminescence of NaYF₄:Yb³⁺/Er³⁺ nanoparticles (NPs) was successfully tuned by simply controlling the NaF dosage. Unlike UC nanocrystals previously reported in the literature with multicolor emission obtained by varying the rare‐earth dopants, the current work developed a new approach to tune the UC emission color by controlling the NaF concentration without changing the ratio and dosage of rare‐earth ions. TEM and powder XRD were used to characterize the shape, size, and composition of the UC luminescent nanocrystals. The luminescence images, emission spectra, and multicolor emission mechanism of the NPs have also been demonstrated. As a result of the excellent ability of this new method to manipulate color emission, this will open up new avenues in the areas of bioprobes, light‐emitting devices, color displays, lasers, and so forth. To demonstrate their biological applications, the water‐stable, biocompatible, and bioconjugatable NaYF₄:Yb³⁺/Er³⁺@poly(acrylic acid) NPs were synthesized by this developed strategy and applied in targeted‐cell UC luminescence imaging.     

Direct Synthesis of Hexagonal NaGdF4 Nanocrystals from a Single‐Source Precursor: Upconverting NaGdF4:Yb3+,Tm3+ and Its Composites with TiO2 for Near‐IR‐Driven Photocatalysis

A novel single‐source precursor NaGd(TFA)₄(diglyme) (TFA=trifluoroacetate) was synthesized, characterized thoroughly, and used to obtain the hexagonal phase of NaGdF₄ nanoparticles as an efficient matrix for lanthanide‐doped upconverting nanocrystals (NCs) that convert near‐infrared radiation into shorter‐wavelength UV/visible light. These NCs were then used to prepare well‐characterized TiO₂@NaGdF₄:Yb³⁺,Tm³⁺ nanocomposites to extend the absorption range of the TiO₂ photocatalyst from the UV to the IR region. While the visible/near IR part of the photoluminescent spectra remains almost unaffected by the presence of TiO₂, the UV part is strongly quenched due to the absorption of TiO₂ above its gap at approximately 380 nm by energy transfer or FRET. Preliminary results on the photocatalytic activity of the above obtained nanocomposites are presented.     

Tunable photoluminescence of NaYF4:Eu nanocrystals by Sr codoping

NaYF₄:Eu/Sr nanocrystals were synthesized by a hydrothermal method. Tunable photoluminescence of the NaYF₄:Eu nanocrystals was successfully achieved by codoping with Sr²⁺ ions. With increasing Sr²⁺ concentration, not only the X-ray diffraction peaks of the nanocrystals become broader, but also the positions of them shift toward larger lattice parameters. Eu³⁺ and Eu²⁺ have been found to coexist in an NaYF₄:Eu/Sr. The Eu³⁺/Eu²⁺ emission intensity ratio changed with the Sr²⁺ concentration and excitation wavelength. More interestingly, the spectral configurations of Eu²⁺ and Eu³⁺ also varied with the excitation wavelength, indicating that the nanocrystals have multiple luminescence centers or emitting states.     

Orthogonal Functionalisation of Upconverting NaYF4 Nanocrystals

A simple and straightforward method for the orthogonal functionalisation of upconverting NaYF₄ nanocrystals (UCNCs)—doped withYb³⁺ and Er³⁺—based on N‐(3‐dimethylaminopropyl)‐N′‐ethylcarbodiimide/N‐hydroxysuccinimide (EDC/NHS) selective reactions between two dyes and two different reactive groups present at the periphery of the upconverting nanocrystals is reported. Organic‐soluble UCNCs of 10 and 50 nm in size are encapsulated efficiently in a 1:1 mixture of two commercial 3000 Da poly(ethylene glycol) derivatives with two different reactive groups (amino and carboxylic groups). The water‐dispersible UCNCs are non‐cytotoxic, stable in the physiological environment, and present free amine and carboxylic reactive groups on their periphery, allowing rapid, selective, and modular covalent conjugation to payloads through EDC/NHS reactions. PEG‐encapsulated UCNCs with and without covalent conjugation to payloads are characterised in vitro through spectroscopic, dynamic light scattering, and electron microscopy measurements. Living cell analyses coupled with TEM measurements confirm the uptake and low cytotoxicity of the coated UCNCs. They are linked covalently to two different dyes, internalised by living cells, and analysed by confocal microscopy. The related colocalisation measurements prove the reactivity of both amines and carboxylic acids on the periphery of the nanocrystals. This approach demonstrates that it is possible to produce water‐dispersible and cyto‐compatible dual‐functional UCNCs.     

Synthesis,properties and mechanism of photodegradation of core–shell structured upconversion luminescent NaYF4:Yb3+,Er3+@BiOCl

Microspherical bismuth oxychloride (BiOCl) can only utilize ultraviolet (UV) light to promote photocatalytic reactions. To overcome this limitation, a uniform and thin BiOCl nanosheet was synthesized with a particle size of about 200 nm. As results of UV–visible diffuse reflectance spectroscopy showed, the band gap of this nanostructure was reduced to 2.78 eV, indicating that the BiOCl nanosheet could absorb and utilize visible light. Furthermore, the upconversion material NaYF₄ doped with rare earth ions Yb³⁺ and Er³⁺ emitted visible light at 410 nm following excitation with near‐infrared (NIR) light (980 nm), which could be utilized by BiOCl to produce a photocatalytic reaction. To produce a high‐efficiency photocatalyst (NaYF₄:Yb³⁺,Er³⁺@BiOCl), BiOCl‐loaded NaYF₄:Yb³⁺,Er³⁺ was successfully synthesized via a simple two‐step hydrothermal method. The as‐synthesized material was confirmed using X‐ray diffraction, scanning electron microscopy, X‐ray photoelectron spectroscopy as well as other characterizations. The removal ratio of methylene blue by NaYF₄:Yb³⁺,Er³⁺@BiOCl was much higher than that of BiOCl alone. Recycling experiments verified the stability of NaYF₄:Yb³⁺,Er³⁺@BiOCl, which demonstrated excellent adsorption, strong visible‐light absorption and high electron–hole separation efficiency. Such properties are expected to be useful in practical applications, and a further understanding of the NIR‐light‐responsive photocatalytic mechanism of this new catalytic material would be conducive to improving its structural design and function.     

Apoptosis induced by NaYF<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> nanoparticles in liver cells via mitochondria damage dependent pathway

As lanthanide-doped sodium yttrium flouride (NaYF₄) nanoparticles have great potential in biomedical applications, their biosafety is important and has attracted significant attention. In the present work, three different sized NaYF₄:Eu³⁺ nanoparticles have been prepared. Liver BRL 3A cell was used as a cell model to evaluate their biological effects. Cell viability and apoptosis assays were used to confirm the cytotoxicity induced by NaYF₄:Eu³⁺ NPs. Apart from the elevated malondialdehyde (MDA), the decrease of superoxide dismutase (SOD), glutathione peroxidase (GSH-PX) and catalase (CAT) activity indicated reactive oxygen species (ROS) generation, which were associated with oxidative damage. The decrease of mitochondrial membrane potential (MMP) value demonstrated the occurrence of mitochondria damage. Then, release of cytochrome c from mitochondria and activation of caspase-3 confirmed that NaYF₄:Eu³⁺ NPs induced apoptosis was mitochondria damage-dependent.     

Quenching of the luminescence of upconverting luminescent nanoparticles by heavy metal ions

We report that the luminescence of upconverting luminescent nanoparticles (UCLNPs) is quenched by heavy metal ions and halide ions in aqueous solution. The UCLNPs consist of hexagonal NaYF₄ nanocrystals doped with trivalent rare earth ions and were synthesized by both the oleic acid (solvothermal) method and the ethylenediaminetetraacetic acid (co‐precipitation) method. Quenching was studied for the Cu^II, Hg^II, Pb^II, Cd^II, Co^II, Ag^I, Fe^III, Zn^II, bromide, and iodide ions and is found to be particularly strong for Hg^II. Stern–Volmer plots are virtually linear up to quencher concentrations of 10–25 mM , but deviate from linearity at higher quencher concentrations, because static quenching causes an additional effect. The UCLNPs display two main emission bands (blue, green, red or near‐infrared), and the quenching efficiencies for these are found to be different. The effect seems to be generally associated with UCLNPs because it was observed for all UCLNPs doped with trivalent lanthanide ions including Yb^III, Er^III, Ho^III, and Tm^III. The results are discussed in terms of quenching mechanisms and with respect to potential applications such as optical sensing.     

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.