Critical shell thickness and emission enhancement of NaYF 4 :Yb,Er/NaYF 4 /silica core/shell/shell nanoparticles
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Amorphous silica shells, used for functionalization of inorganic nanoparticles in bioapplications, were coated on chemically synthesized NaYF4:Yb,Er upconversion fluorescent nanoparticles via a reverse microemulsion method by using dual surfactants of polyoxyethylene (5) nonylphenylether and 1-hexanol, and tetraethyl orthosilicate as precursor. NaYF4:Yb,Er nanoparticles were equiaxed with a particle size of 11.1 1.3 nm. The thickness of silica shell was 8 nm. NaYF4:Yb,Er/silica core/shell nanoparticles were well dispersed in solvents such as ethanol and deionized water. The emission intensities of NaYF4:Yb,Er/silica core/shell nanoparticles remained the same as that of uncoated nanoparticles after surface functionalization with an amine group using (3-aminopropyl)trimethoxysilan. Silica, although providing a good barrier to the nonradiative relaxation between the upconversion nanoparticles and the environments, did not enhance the emission intensity of upconversion nanoparticles. To increase the emission intensity of NaYF4:Yb,Er/silica core/shell nanoparticles, an undoped NaYF4 shell (3-nm thick) was deposited on the upconversion nanoparticles before the silica coating. The total emission intensity of NaYF4:Yb,Er/NaYF4/silica core/shell/shell nanoparticles increased by 15 times compared to that without the intermediate NaYF4 shell. The critical shell thickness of NaYF4 was 3 nm, beyond which no further emission intensity enhancement was observed.
I. INTRODUCTION
Near infrared (NIR)-to-visible lanthanide (Ln)-doped upconversion (UC) fluorescent nanoparticles have attracted significant interests due to potential applications as sensitive bioprobes.1–3 Unlike the ultraviolet (UV) excitation that causes a background signal from the imaged biomolecules, the optical transparency of NIR results in improved signal-to-noise ratio in imaging.4,5 In vivo bioimaging is potentially feasible due to the high penetration ability of NIR and low NIR absorption coefficient for water and hemoglobin (main absorption from 650 to 900 nm).6 Photobleaching may be significantly reduced because of the photochemical stability of these inorganic particles.7 For bioimaging, the size of targeted molecules range from several to tens of nanometers, ideal bioprobes should therefore have a size 10 nm with a narrow size distribution,8 high fluorescence efficiency, and watera)
Present address: nanoBright Technologies Pte. Ltd., 8 Boon Lay Way, #05-03, 8@TradeHub21, Singapore 609964, Republic of Singapore. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0432 J. Mater. Res., Vol. 24, No. 12, Dec 2009
dispersibility.9,10 Surface functionalization of these nanoscale probes is often required to enable attachment of targeted molecules. Among all the reported upconversion phosphors, NaYF4:Yb,Er is one of the most efficient NIR-to-visible upconversion materials for Ln-doped phosphors.11 NaYF4 host has high chemical stability and low phonon energy (420 cm 1).12 Yb (as sensitizer) and Er (as activator) cod
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