Lanthanide-based luminescent hybrid materials.

Lanthanide ions exhibit unique luminescent properties, including the ability to convert near infrared long-wavelength excitation radiation into shorter visible wavelengths through a process known as photon upconversion. In recent years lanthanide-doped upconversion nanocrystals have been developed as a new class of luminescent optical labels that have become promising alternatives to organic fluorophores and quantum dots for applications in biological assays and medical imaging. These techniques offer low autofluorescence background, large anti-Stokes shifts, sharp emission bandwidths, high resistance to photobleaching, and high penetration depth and temporal resolution. Such techniques also show potential for improving the selectivity and sensitivity of conventional methods. They also pave the way for high throughput screening and miniaturization. This tutorial review focuses on the recent development of various synthetic approaches and possibilities for chemical tuning of upconversion properties, as well as giving an overview of biological applications of these luminescent nanocrystals.

Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals

Uniform, spherical-shaped TiO2:Eu nanoparticles with different doping concentrations have been synthesized through controlled hydrolysis of titanium tetrabutoxide under appropriate pH and temperature in the presence of EuCl3·6H2O. Through air annealing at 500°C for 2 h, the amorphous, as-grown nanoparticles could be converted to a pure anatase phase. The morphology, structural, and optical properties of the annealed nanostructures were studied using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy [EDS], and UV-Visible diffuse reflectance spectroscopy techniques. Optoelectronic behaviors of the nanostructures were studied using micro-Raman and photoluminescence [PL] spectroscopies at room temperature. EDS results confirmed a systematic increase of Eu content in the as-prepared samples with the increase of nominal europium content in the reaction solution. With the increasing dopant concentration, crystallinity and crystallite size of the titania particles decreased gradually. Incorporation of europium in the titania particles induced a structural deformation and a blueshift of their absorption edge. While the room-temperature PL emission of the as-grown samples is dominated by the 5D0 - 7Fj transition of Eu+3 ions, the emission intensity reduced drastically after thermal annealing due to outwards segregation of dopant ions.

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Effects of crystallization and dopant concentration on the emission behavior of TiO2:Eu nanophosphors

Nanomaterials, such as metal or semiconductor nanoparticles and nanorods, exhibit similar dimensions to those of biomolecules, such as proteins (enzymes, antigens, antibodies) or DNA. The integration of nanoparticles, which exhibit unique electronic, photonic, and catalytic properties, with biomaterials, which display unique recognition, catalytic, and inhibition properties, yields novel hybrid nanobiomaterials of synergetic properties and functions. This review describes recent advances in the synthesis of biomolecule-nanoparticle/nanorod hybrid systems and the application of such assemblies in the generation of 2D and 3D ordered structures in solutions and on surfaces. Particular emphasis is directed to the use of biomolecule-nanoparticle (metallic or semiconductive) assemblies for bioanalytical applications and for the fabrication of bioelectronic devices.

Integrated Nanoparticle–Biomolecule Hybrid Systems: Synthesis, Properties, and Applications

Magnetic Nanoparticles in MR Imaging and Drug Delivery

Fluorescence resonant energy transfer biosensor based on upconversion-luminescent nanoparticles

To demonstrate their applications in biological and medical fields such as in immunoassays, magnetic separation of cells or proteins, drug or gene delivery, and magnetic resonance imaging, the template-free syntheses of water-soluble and surface functionalized magnetic nanomaterials have become essential and are challenging. Herein, we developed a facile one-pot template-free method for the preparation of amine-functionalized magnetite nanoparticles and hollow nanospheres by using FeCl(3)6 H(2)O as single iron source. These magnetic nanomaterials were characterized by TEM, SEM, XRD, and FTIR technologies. Their magnetic properties were also studied by using a superconducting quantum interference device (SQUID) magnetometer at room temperature. Then the amine-functionalized magnetite nanoparticles were applied to immunoassays and magnetic resonance imaging in live mice.

One-pot synthesis and bioapplication of amine-functionalized magnetite nanoparticles and hollow nanospheres

A facile wet chemical synthesis method was used to prepare a range of single-crystal Na(Y1.5Na0.5)F6 nanorods with controllable aspect ratios. Their novel multicolor upconversion (UC) fluorescence has been successfully realized by doping Yb3+/Er3+ (green) and Yb3+/Tm3+ (blue) ion pairs. When doped with Eu3+ and Tb3+ ions, the strong red and green downconversion (DC) fluorescence has also been observed, respectively. Being covered with oleic acids, these luminescent nanorods have been transparently dispersed in nonpolar solvent. For their unique luminescence and controllable morphology and surface properties, these nanorods may find great applications in the fields of color displays, biolabels, light-emitting diodes (LEDs), optical storage, optoelectronics, anticounterfeiting, and solid-state lasers.

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Na(Y1.5Na0.5)F6 Single-Crystal Nanorods as Multicolor Luminescent Materials

The rapidly increasing health care is now challenging the development of fluorescent biolabel materials used in diagnostics and bioanalysis such as immunoassay, fluorescence imaging and DNA or toxin detection. Due to their multicolor emission, high brightness and long lifetime, lanthanide ions based luminescent nanomaterials have tremendous promise as indicators and photon sources for numerous applications such as biolabels, light-emitting devices, sensor technology, and low-threshold lasers. However, when they are used in polymer-based luminescence devices, their strong luminescence is often quenched by the organic compounds. Therefore, doping the luminescent rare-earth ions into inorganic host materials can efficiently maintain their novel luminescence. But if their surface is not coated with organic compounds, these lanthanide-doped inorganic materials can not be dispersed in organic environment, which will further limits their application in polymer-based devices. So, it is very important to successfully prepare the rare-earth doped inorganic nanocrystals with good dispersibility in organic solvents. Up to now, lots of facile strategies have been developed for the preparation and application of the one-dimensional nanomaterials such as carbon nanotubes, semiconductor and metal nanocrystals, however, it is still a challenge for the synthesis of highly luminescent LaF3 and NaLaF4 nanorods with uniform shapes and sizes, high crystallinity, and especially controllable surface properties. Herein, we developed a facile one-pot strategy for the preparation of rareearth doped lanthanum fluoride (LaF3) and sodium lanthanum fluoride (NaLaF4) nanorods with controllable sizes and shapes. By doping different rare-earth ions, these nanorods can emit strong downconversion and upconversion fluorescence. For the coating surfactants on the surfaces, these asprepared nanocrystals are redispersible in nonpolar organic solvents such as cyclohexane to form a transparent colloidal solution, and can be easily aggregated by adding polar solvents such as ethanol, which facilitates their applications in polymer-based devices. Meanwhile, for their novel luminescence, they will also find great application in clinical and biological fields In brief, in water-ethanol-oleic acid mixing system, with a facile solvothermal technology, we successfully prepared the LaF3 and NaLaF4 nanorods by tuning sodium fluoride content and reaction time. By doping Ce-Tb and YbEr ion-pairs, novel downand up-conversion luminescence have been obtained, respectively. The crystal structure, shape and size of the resulting nanocrystals were quite sensitive to the reaction time, dopant concentration and NaF content. The results have been demonstrated by transmission electron microscopy (TEM) in Figure 1. In the case of low NaF content (2.0 mL, 1.0 mol L) and 6 h, only the short nanorods were obtained (Fig. 1a and b). High NaF content (2.7 mL, 1.0 mol L) and long reaction time (10 h) are beneficial for the formation of long nanorods (Fig. 1c and d). But with the elongation of the nanorods, their crystal phase has been already changed from LaF3 (Fig. 1a and b) to NaLaF4 C O M M U N IC A TI O N

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Down-and up-conversion luminescent nanorods

Complex rare earth fluoride (NaRF4, R = Ce,Y,Gd) nanocrystals of 30?50?nm were synthesized by hydrothermal and solvothermal techniques. The size, morphology and phase transition of the complex rare earth fluorides are discussed as the effects of different solvents, reaction time and temperature. Hexagonal phase NaRF4 nanocrystals with good dispersibility can be prepared in the presence of EDTA using ethanol as the solvent. After doping with Yb3+, Er3+; Yb3+, Tm3+, and Eu3+, some typical up-conversion and down-conversion photoluminescence was characterized and discussed.

https://iopscience.iop.org/article/10.1088/0957-4484/17/14/032/pdf

Leyu Wang

Papers

Fluorescence resonant energy transfer biosensor based on upconversion-luminescent nanoparticles

One-pot synthesis and bioapplication of amine-functionalized magnetite nanoparticles and hollow nanospheres

Na(Y1.5Na0.5)F6 Single-Crystal Nanorods as Multicolor Luminescent Materials

Down-and up-conversion luminescent nanorods

Synthesis of complex rare earth fluoride nanocrystal phosphors