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From the journal: Journal of Materials Chemistry B. At the beginning of the reaction, the precipitated nuclei of the product are first formed. When the size of the particles approaches the droplet size, the film formed by the surfactant molecules adheres to the surface of the particles and. The products prepared by the microemulsion method have poor monodispersity, and the relative yield of the prepared nanoparticles is relatively small, which is suitable for laboratory research and is difficult to be applied to large-scale production.
Usually, metal nitrate is mixed with organic fuel in aqueous solution and water is evaporated by heating to cause explosive reaction, and the generated large amount of heat promotes the formation of the target product. The size of the product can be controlled by varying the ratio of fuel to oxidant. Combustion method is a very meaningful and energy-efficient synthesis method in the synthesis of luminescent materials. The luminescent materials synthesized by it have corresponding adaptability and the burning gas can protect the rare earth ions from being oxidized, thus eliminating the need for reducing protective atmosphere.
However, the purity and luminescence properties of the products produced are not very good. Among these synthetic routes, thermal decomposition, hydrothermal solvent heat and precipitation methods are the most popular and most effective methods for preparing high quality upconverting nanoparticles, the UCNPs prepared by these methods have good monodispersity, uniform size and controllable morphology.
Combustion method and microemulsion method are relatively less applied because of some inevitable disadvantages, such as severe agglomeration, poor light performance and difficult size control. The monodispersed UCNPs with controlled morphology and uniform size can be obtained by a variety of methods. However, the surface of UCNPs prepared by these methods usually contains hydrophobic organic ligands such as oleic acid, oleylamine and carbon octadecene, etc.
Hydrophobic UCNPs have limited their use in the biomedical applications. Therefore, the hydrophobic UCNPs must be changed into hydrophilic for biocompatible.
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Surface modification methods mainly contain ligand exchange method, polymer coating method and silica coating method etc. UCNPs have inappreciable cytotoxicity and negligible organ toxicity through various modifications on the surface, showing overall safety. UCNPs prepared by thermal decomposition method, are covered with a layer of oleic acid molecules.
Chen [ 40 ] and Wang [ 41 ] changed hydrophobic UCNPs into water-soluble by using the ligand interaction between polyacrylic acid PAA macromolecule and surface oleic acid molecules. Hydrophilic PAA molecules can not only convert UCNPs into water-soluble, but also the surface carboxyl groups can be further coupled with biomolecules. Ai [ 43 ] synthesized a highly soluble core-shell lanthanide-doped upconversion nanocrystals by using PAA for surface modification. And the carboxyl group on the PAA surface can be further coupled to biomolecules for further cellular applications. Beyerrell [ 44 ] prepared hydrophilic function UCNPs by poly ethylene glycol -phosphate ligands through the solvent heat pathways.
Wang [ 45 ] prepared water-soluble UCNPs by surface modification using citrates. Research shows that ligand exchange method is a simple, convenient and effective method. UCNPs can be encapsulated by amphiphilic polymers and polyelectrolyte polymers through the layer-by-layer self-assembly method LBL.
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The amphiphilic polymer coating is mainly carried on the surface of the nanoparticle by the Van der Waals force between the hydrophobic chain of polymer and the long alkyl chain of the hydrophobic UCNPs surface, finally the entire nanoparticle is water-soluble and biocompatible. Positively charged polyallylamine hydrochloride PAH and negatively charged sodium polystyrene sulfonate PSS are connected to the surface of nanocrystals by LBL, and a large number of amino groups on peripheral linear polymer PAH can be further functionalized.
As a result, the ligand is shortened and becomes more hydrophilic, also the dispersion of the particles is remarkably improved. Kamimura et al.
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The surface coating of UCNP can also be achieved in hydrothermal method using protective block poly ethylene glycol copolymer layer [ 55 ]. Respectively [47, 61—63].
UCNPs can be coated by silicon oxidition by covalent bonding of silane hydrolysis. Zhang [ 21 ] reported UC-NPs compounds combined with dyes and quantum dots wrapped with silica by using the silica-coated method, as shown Figure 6. The functional compounds can be issued in different colors of light and widely used in biological probes. The silica coating method not only can use reverse microemulsion to encapsulate hydrophobic oil-soluble nanoparticles, but also can use microemulsion to encapsulate hydrophilic water-soluble.
The encapsulated nanoparticles with good water solubility and biocompatibility are widely used in biomedical fields [ 64 ] by silica coating method. Qian [ 65 ] prepared mesoporous silica-coated UCNP by microemulsion method. Then, the photosensitizer zinc phthalocyanine is incorporated into mesoporous layer. Upconversion emission activates the photosensitizer to release singlet oxygen which may kill the cancer cells.
Wang et al. In silica coating method, neutral polymer can be used to stabilize the silica shell and control its thickness. Recently Wang et al. MOFs are a class of crystalline nanoporous materials with well-defined pore structures [ 69 ]. Their unique properties such as high surface area and structural flexibility have endowed them a wide bio-application, ranging from sensors, drug delivery to bioimaging [ 70 , 71 ]. Due to ZIF-8 excellent performance, such as high surface area, open metal sites, excellent water stability and biocompatibility, ZIF-8 has become a research hotspot in biomedical materials.
Due to more special advantages over organic dyes and quantum dots, UCNPs have raised to be the most interesting candidate in biological field [40, 46—48, 51, 73, 74]. In recent years, with the development of nanotechnology and biotechnology, upconversion luminescent nanomaterials have been widely used in biomedicine, mainly in the following aspects:.
Because of few background fluorescence interference, UC-NPs have high imaging sensitivity. There are clear biological imaging by nm excitation light irradiation which is shown in Figure 9. The element Gd as a protective layer enhanced UCNPs fluorescence intensity, and also as an contrast agent was used for magnetic resonance imaging. UCNPs can be used to integrate a variety of imaging or therapeutic functions through surface modification, making them promising multifunctional nanoplatforms for diagnosis and treatment.
PDA not only shows a good photothermal effect, but also can be used as a drug carrier for chemotherapy. Liu and colleagues [ 76 ] developed a binary contrast agent based on PAA modified BaYbF 5 :Tm nanoparticles for direct visualization of the gastrointestinal GI tract, displaying low cytotoxicity and negligible hemolysis.
Unlike clinically used barium meal, low concentrations of PAA-BaYbF 5 :Tm can be performed for X-ray imaging of the digestive tract and exhibiting admirable solubility and monodispersity, which greatly reduces the artifacts in the imaging process greatly improve the imaging effect. Moreover, blood biochemistry assay unambiguously reveal their overall safety and great potentials in biomedicine. The tumor site was marked by red circle [ 75 ].
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Mn-doped silica shell is sensitive to intratumoral acidity and reducibility, causing biodegradation of the shell and further accelerating Si-O-Si bond rupture. Biologists study intracellular dynamic behavior or nanoscale proteins through a lossless, real-time imaging optical microscope.
Due to the existence of optical diffraction limits, conventional far-field optical microscopy cannot observe these life activities at the nm scale. In recent years, super-resolution imaging technology that overcomes the optical diffraction limit has developed rapidly. Currently, UCNPs have been successfully applied to super-resolution microscopy due to significant light penetration depth, ultra-low autofluorescence background and minimal phototoxicity. They use this property to design a low-power super-resolution stimulated emission depletion STED microscope and realize nanoscale optics.
In addition to the above findings, many research teams are now working on UCNPs bioimaging. FRET means when the fluorescence spectrum of one fluorescent molecule donor overlaps with the excitation spectrum of another fluorescent molecule acceptor , donor fluorescent molecule can induce the fluorescence of acceptor, the fluorescence intensity of the acceptor increases with the donor fluorescent molecule itself decaying.
The emission wave-length of UCNPs can be controlled by adjusting the elements species and doping ratio. When the groups are coupled, the FRET between them can be achieved and a high sensitive chemical sensor can be designed for bimoleculars detection. With the miR concentration increasing, the signal at nm weakens, and the peak at nm strengthens.
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