Tianxing Ji2#, Xinqiang Xu1,2#, Xindong Wang1, Qiang Zhou2* and Guanying Chen1*
1School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, People’s Republic of China
2The Second Affiliated Hospital of Guangzhou Medical University, Guangzhou 510260, People’s Republic of China
#Co-first authors, these authors contributed equally to this work
Received: 28 December, 2016; Accepted: 16 January, 2017; Published: 20 January, 2017
Guanying Chen, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, People’s Republic of China, Tel: +86-0451-86483808; Fax: +86-0451-86483808; E-mail:
Ji T, Xu X, Wang X, Zhou Q, Chen G (2017) Monodisperse Water-Stable SiO2-Coated Fluoride Upconversion Nanoparticles with Tunable Shell Thickness. Int J Nanomater Nanotechnol Nanomed 3(1): 015-018. DOI: 10.17352/2455-3492.000015
Â© 2017 Ji T, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Upconversion nanoparticles; Silica; Reverse microemulsion
UCNPs: Upconversion Nanoparticles; TEM: Transmission Electron Microscopy; TEOS: Tetraethylorthosilicate
Monodisperse water-stable silica functionalization of upconversion nanoparticles are important for their applications in bio-imaging and bio-sensing. Here, we report on the uniform silica coating of NaYF4:Yb,Er,Tm upconversion nanoparticles (UCNPs) with a controlled thickness (3-16 nm) through a modified reverse microemulsion approach. We show that the molar ratio between tetraethylorthosilicate (TEOS) and UCNPs is important to tune the resulting thickness of silica, which can be controlled via simple increase of the concentration of UCNPs while maintaining the TEOS concentration constant. Our results show that those silica-coated upconversion nanoparticles are stable at pH = 7 sodium chloride-free solution, important for their bio-applications.
In recent years, with the deepened understanding of the luminescence mechanism of lanthanide-doped upconversion nanoparticles (UCNPs) and the maturity of their synthesis methods, those new luminescent nanomaterials have spurred wide interests on many applications ranging from biomarker detection to biomedical imaging [1-10]. UCNPs have many excellent physical and chemical properties, such as infrared light excitation, strong penetrability of tissue, little damage to biological tissue, zero background fluorescence, long fluorescence lifetime, narrow emission band, and wide anti-Stokes shift and so on. However, the as-prepared UCNPs are typically hydrophobic (water-insoluble) and unable to link with bio-targeting biomolecules, restricting their applications in biology. For example, the solvo (hydro) thermal method and thermal decomposition method are commonly used approaches for synthesizing monodisperse UCNPs with good reproducibility [11-14]. Yet, the long-chain oleic acid capped on the surface prevent their further uses in biological environment. To solve this problem, the surface chemistry of as-prepared UCNPs have to be designed to grant both a stable aqueous colloidal dispersion and the ability to conjugate biomolecules. Several approaches has been attempted toward this regard, including SiO2 coating, wrapping by small molecular surfactant, amphiphilic polymer, oxidizing oleic acid ligands with the Lemieuxvon Rudolf reagent, and ligand exchange [11,15-25]. In particular, SiO2-coating are of particular interest, due to its high biocompatibility, easy surface modification through silicon-enriched chemistry, and easy control of interparticle interactions [26-29]. However, there lacks a facile way to perform silica coating of UCNPs with a controlled thickness.
Herein, we used a high temperature co-precipitation method to synthesize hexagonal NaYF4: Yb, Er, Tm UCNPs capped by the oleic acid group. These UCNPs were then coated with a silica shell of different thickness via a modified reverse microemulsion. Moreover, we found that all these silica-coated UCNPs are stable at pH = 7 biological physiological saline (the sodium chloride NaCl solution).
Materials and Methods
Materials and characterization
Rare-earth oxides, including yttrium chloride (YCl3.6H2O, 99.99%), ytterbium chloride (YbCl3.6H2O, 99.99%), erbium chloride (ErCl3.6H2O, 99.99%), Thulium chloride (TmCl3.6H2O, 99.99%) Triton X-100, ammonia solution (25-28%) and tetraethyl orthosilicate (TEOS, 99.99%) were purchased from Aladdin (Shanghai, China). Other reagents (analytical grade) were also purchased from Aladdin (Shanghai, China). All reagents were used as received without further purification. Deionized water was used in all experiments. TEM images were performed on a JEM 2000FX (Jeol Ltd, Japan). Fluorescence spectra were recorded on Ocean optics spectrometers equipped with a near-infrared (NIR) laser with emission at 980 nm.
Synthesis of NaYF4: Yb,Er,Tm upconversion nanoparticles
The NaYF4: 20% Yb, 2% Er, 0.5% Tm nanocrystals were prepared using a high temperature co-precipitation method. First, the lanthanide chlorides containing 0.775 mmol YCl3.6H2O, 0.20 mmol YbCl3.6H2O, 0.02 mmol ErCl3.6H2O, 0.005 mmol TmCl3.6H2O were loaded to a 250 mL 3-necked flask, followed by adding 15 mL of octadecene and 9 mL of oleic acid. The flask was then heated to 150 under Ar for 30 min. Subsequently, the flask was cooled down to 50 and 10 mL methanol solution containing 0.1482 g NH4F and 0.1 g NaOH was added drop wise. The mixed solution was stirred at 50 for 2 h and heated slowly to 80 to remove methanol. The flask was heated up rapidly to 300 for 1 h, and then naturally cooled down to room temperature. The reaction solution was evenly divided into two 50 mL tubes, followed by adding 20 mL ethanol in each tube for centrifugation (6000 rpm, 5 min). After washing with ethanol for three more times, oleate-capped upconverting nanoparticles were stored in 10 mL cyclohexane for silica coating.
Synthesis of silica-coated NaYF4: Yb,Er,Tm upconversion nanoparticles with tunable shell thickness
The coating of upconversion nanoparticles with silica was achieved by a modified reverse micro-emulsion method. Cyclohexane-dispersed UCNPs (20 mg/mL) with various amount of volume (1, 0.5, or 0.2 mL) were diluted by adding pure cyclohexane solvent to 2 mL, and then got transferred to a 60 mL glass bottle. After that, 8 mL of cyclohexane and 0.5 mL of Triton X-100 were added into the bottle along with about 2 min ultrasonication. Subsequently, 0.5 mL of Triton X-100 and 0.12 mL of ammonium hydroxide (25-28 wt%) was added into the mixture under ultrasonication for about 20 min, yielding a transparent solution. Then, 40 L of TEOS were added and the mixture was ultrasonicated for 2 min, followed by magnetic stirring at room temperature for 48 h. The silica-coated NaYF4:Yb,Er, Tm UCNPs were collected by centrifugation, washed with ethanol three times, and finally dispersed in different NaCl concentration of aqueous solution at different pH values.
Results and Discussion
Characterization of NaYF4: Yb,Er,Tm and NaYF4: Yb,Er,Tm@SiO2 nanoparticles
UCNPs (Y:Yb:Er:Tm=77.5:20:2:0.5) were synthesized by using a previously reported method with modifications. The TEM image shows that the as-prepared nanoparticles have uniform size and morphology of hexagonal shape (Figure 1). The average diameter of these particles was about 40 nm, suitable for biological applications. The surface of these UCNPs was capped by oleic acid, making them hydrophobic and preventing them from biological applications. To address this problem, we coated these UCNPs with a shell layer of silica that makes the core-shell water soluble. The silica coating strategy had a number of advantages for bioapplications: (i) the multi-valency of an extensively polymerized polysilane ensures that these UCNPs stay soluble, providing a spatial isolation of UCNPs from the performance-degrading substances in the environment; (ii) the silica was known to be biocompatible, chemically inert and optically transparent at visible and near infrared wavelengths , (iii) the chemistry of silica was well investigated, which could incorporate different functional groups (-COOH, -NH2, and -SH) to control the interactions with biological environments. Moreover, the ability to control the shell thickness was of particular importance to probe the shell effect on the properties of UCNPs as well as the use of them as biolabels. To realize a tunable shell thickness of silica coating, we varied the ratio between the concentration of UCNPs and the concentration of tetraethyl orthosilicate (TEOS) while maintaining all other parameters unchanged. As one can see in Figure 2a-d, monodisperse UCNP@SiO2 core/shell nanoparticles with uniform shell thickness varying from 3-16 nm were synthesized successfully. An UCNP@SiO2-3 nm core/shell nanoparticle with 3 nm thick silica shell was obtained when 1 mL of cyclohexane-dispersed UCNPs were added to the reaction system, while 16 nm thick silica shell was obtained when 0.2 mL UCNPs were added. The larger the ratio of the concentration of UCNP versus the concentration of TEOS, the thinner the resulting silica shell thickness.3