World Journal of Nano Science and Engineering, 2012, 2, 41-46
http://dx.doi.org/10.4236/wjnse.2012.22007 Published Online June 2012 (http://www.SciRP.org/journal/wjnse)
A Method to Improve the Up-Conversion Fluorescence of
Polymer Modified NaYF4:Yb,Er(Tm) Nanocomposites
Weina Cui1, Siyu Ni1, Shunan Shan2, Xingping Zhou1*
1College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, China
2College of Life and Environment, Shanghai Normal University, Shanghai, China
Email: *xpzhou@dhu.edu.cn
Received October 3, 2011; revised November 11, 2011; accepted December 15, 2011
ABSTRACT
The modification of NaYF4:Yb,Er(Tm) nanoparticles synthesized in the presence of an ionic surfactant is critical to
their application in biological fields for better solubility and biocompatibility. In this work, NaYF4:Yb,Er(Tm) was
transformed from insoluble, inactive to hydrophilic, biocompatible via ligand exchange modification with polyacrylic
acid (PAA). Ligand exchange was carried out at room temperature when a colloidal solution of NaYF4:Yb,Er(Tm) in
tetrahydrofuran (THF) was treated with excess PAA. The PAA modified NaYF4:Yb,Er(Tm) nanoparticles got better
surface properties but with declined inner up-conversion fluorescence. Generally, coating an analogous layer of material
outside the core nanoparticles can improve the optical properties of the core. Accordingly, NaYF4:Yb,Er(Tm)/NaYF4
nanoparticles were synthesized before PAA modification to avoid the optical intensity decaying. The result of fluores-
cence test proved that the water soluble NaYF4:Yb,Er(Tm)/NaYF4/PAA nanocomposites had a sound up-conversion
property compared with that of NaYF4:Yb,Er(Tm)/PAA. Furthermore, the up-conversion fluorescence property of the
nanocomposite varied with the doping ratio of Er(Tm) to Yb and the possible mechanism for this change was also dis-
cussed.
Keywords: Doping Ratio; Fluorescence; Ligand Exchange; Polyacrylic Acid; Sodium Yttrium Fluoride;
Surface Modification
1. Introduction
Up-conversion fluorescence materials are attracting much
attention, owing to their unique optical properties and
potential applications. Among all the up-conversion fluo-
rescence materials, hexagonal-phase NaYF4:Yb,Er(Tm)
is one of the most efficient 980 nm near-infrared (NIR)
to visible (green and blue) up-conversion phosphors [1,2].
In bulk, the low phonon energy of the host strongly sup-
presses multiphonon relaxation process in the emission
centers, leading to up-conversion emission. Yb3+ work-
ing as a activator with Y3+ sensitizes Er3+(Tm3+) to emit
green (blue) and red lights. In the recent decade, the re-
search of up-conversion materials was mainly focused on
the biological and medical fields such as low-intensity IR
imaging and sensitive bioprobes [3,4].
Biological application of the up-conversion fluores-
cence materials presents new challenges of the develop-
ment of nano-sciecnce and nano-technology, especially
the surface modification study of nanoparticles. As well
known, ligand exchange is a versatile way to modify the
surfaces of nanoparticles for better solubility and bio-
compatibility. Dykstra et al. [5] prepared the CdSe/ZnS
nanoparticles by an organometallic route in the presence
of trioctylphosphine oxide (TOPO) and then used (PEG)-
phosphine oxide copolymer to remove the TOPO for
obtaining hydrophilic nanoparticles. Zhang et al. [6] chose
polyacrylic acid and poly-allylamine to replace the ori-
ginnal hydrophobic ligand on magnetic nanoparticles at
an elevated temperature in a glycol solvent and rendered
it high water solubility. Lin et al. [7] successfully synthe-
sized at 150˚C the OA-capped PbS QDs with emission in
the NIR and modified the PbS by ligand exhange with
polyelectrolytes for the application of deep-tissue in vivo
imaging.
Ligand exchange is also suitable for the modification of
NaYF4. In our work, PAA was chosen as the ligand polymer
because of its excellent properties including hydrophilicity,
abundant reactant groups (carboxyl) and non-toxicity. In
order to eliminate the quenching effect from polymer, the
nanoparticles of NaYF4:Yb,Er(Tm)/NaYF4 core-shell were
firstly synthesized before PAA modification [8]. The middle
NaYF4 layer protected the inner core and its crystal structure,
thus enhancing the fluorescence property. The final resultant
of NaYF4:Yb,Er(Tm)/NaYF4/PAA nanocomposites prom-
*Corresponding author.
C
opyright © 2012 SciRes. WJNSE
W. N. CUI ET AL.
42
ised to have better comprehensive properties, which made
it possible to be applied in biological fields.
To the best of our knowledge, fluorescence properties
of the NaYF4 with a size of several nanometers after
modification are barely discussed, although the research
about the up-converting fluorescence of NaYF4 core ma-
terial has been widely carried out. However, in our work,
fluorescence properties of the NaYF4:Yb,Er(Tm)/PAA
core-shell nanoparticles and the NaYF4:Yb,Er(Tm)/ NaYF4/
PAA core-shell-shell nanocomposites were mainly ex-
plored. It is more significant to study the fluorescence of
polymer modified NaYF4, as the existence of polymer
often causes the quenching effect which is one of the
most important influence factors for a bio-labeling mate-
rial. Besides that, the color changing rules of NaYF4:Yb,
Er(Tm)/NaYF4/PAA nanocomposites were also revealed
in this article. It is the first time to explore the relation-
ship in details between lanthanide doping ratio and fluo-
rescence property of the nanoparticles after modificatioin
with PAA.
2. Experimental
2.1. Synthesis of the Host Material: Hexagonal
NaYF4 Nanoparticles
The synthesis basicdth:84.510000px;height:11.826000px;background-color:rgba(255,255,255,0.000001);">