An Effective Polymer Cross-Linking
Strategy To Obtain
Stable Dispersions of Upconverting NaYF<sub>4</sub> Nanoparticles
in Buffers and Biological Growth Media for Biolabeling Applications
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Abstract
Ligands on the nanoparticle surface provide steric stabilization,
resulting in good dispersion stability. However, because of their
highly dynamic nature, they can be replaced irreversibly in buffers
and biological medium, leading to poor colloidal stability. To overcome
this, we report a simple and effective cross-linking methodology to
transfer oleate-stabilized upconverting NaYF<sub>4</sub> core/shell
nanoparticles (UCNPs) from hydrophobic to aqueous phase, with long-term
dispersion stability in buffers and biological medium. Amphiphilic
poly(maleic anhydride-<i>alt</i>-1-octadecene) (PMAO) modified
with and without poly(ethylene glycol) (PEG) was used to intercalate
with the surface oleates, enabling the transfer of the UCNPs to water.
The PMAO units on the phase transferred UCNPs were then successfully
cross-linked using bis(hexamethylene)triamine (BHMT). The primary
advantage of cross-linking of PMAO by BHMT is that it improves the
stability of the UCNPs in water, physiological saline buffers, and
biological growth media and in a wide range of pH values when compared
to un-cross-linked PMAO. The cross-linked PMAO–BHMT coated
UCNPs were found to be stable in water for more than 2 months and
in physiological saline buffers for weeks, substantiating the effectiveness
of cross-linking in providing high dispersion stability. The PMAO–BHMT
cross-linked UCNPs were extensively characterized using various techniques
providing supporting evidence for the cross-linking process. These
UCNPs were found to be stable in serum supplemented growth medium
(37 °C) for more than 2 days. Utilizing this, we demonstrate
the uptake of cross-linked UCNPs by LNCaP cells (human prostate cancer
cell line), showing their utility as biolabels