Facile preparation of hydrophilic sodium yttrium fluoride nanorods using hydrophobic nanospheres as precursor
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Yong Zhangb) Department of Bioengineering, National University of Singapore, Singapore 117576 (Received 23 February 2012; accepted 10 May 2012)
Synthesis of well-defined sodium yttrium fluoride (NaYF4) nanocrystals has been achieved in nonpolar solvents, but these nanocrystals possess a hydrophobic surface and need to be surfacemodified for various biological applications. Development of facile aqueous solution method to synthesize one-dimensional NaYF4 with a hydrophilic surface still remains challenging. Herein, we demonstrate a simple route to prepare hydrophilic NaYF4 nanorods by using hydrophobic NaYF4 nanospheres as precursor. It is interesting to find that hydrothermal treatment of oleic acid-capped NaYF4 nanocrystals can not only induce anisotropic growth of these nanocrystals but also change their surface properties. The hydrophilic NaYF4 nanorods synthesized in this work has been well characterized and possible formation mechanism has also been discussed.
I. INTRODUCTION
Recently, lanthanide upconversion nanocrystals (UCNs) have evoked considerable attention due to their unique optical properties and promising applications in biological fields.1–4 Upon the excitation of near-infrared (NIR) light, these kinds of nanocrystals can give various visible emissions through sequential upconvertion of two or three NIR photons into a visible photon. Since most biological species absorb minimally in the NIR window, application of these nanocrystals in fluorescent imaging will enable high degree of light penetration in tissues and minimize autofluorescence from biospecies and surroundings.4–8 Furthermore, lanthanide UCNs show superior photostability in various environments as the luminescent lanthanide ions are doped within the nanocrystal core and well protected by the host lattices.9,10 Moreover, since lanthanide ions give sharp emissions with a long lifetime, these UCNs can also serve as efficient electron donors and greatly improve the efficiency in fluorescence resonance energy transfer (FRET) detection, a sensitive and popular technology widely used in biological detection.11,12 In the last few years, lots of methods have been successfully developed to synthesize lanthanide UCNs with controllable size and shape. For example, Yan et al. developed a high-temperature decomposition route to prepare uniform NaYF4 nanocrystals using mixed trifluoroacetates as precursor13; Li et al. reported a liquid-solid-solution
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2012.177 J. Mater. Res., Vol. 27, No. 16, Aug 28, 2012
synthetic strategy to obtain regular NaYF4 nanocrystals14; we also presented a high-temperature recrystallization process to get NaYF4 nanocrystals with controllable shapes.15 Although high-quality and monodisperse NaYF4 nanocrystals could be prepared with above methods, the obtained nanocrystals possessed a hydrophobic surface, since some ligands such as oleic acid/oleylamine were required to control their growth kinetics. For utilizing these n
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