Lanthanide
ion doped upconversion nanoparticles (UCNPs) hold great
promise as multimodal contrast agents for a range of medical imaging
techniques, including optical bioimaging (OBI), magnetic resonance
imaging (MRI), and computed tomography (CT). However, it is challenging
to obtain UCNPs with both maximal contrast enhancement effects for
both OBI and MRI simultaneously owing to the dilemma in the size of
UCNPs. UCNPs in large dimensions contain more photonic Ln ions and
less surface defects, which is favored for high luminescent emissions,
while small UCNPs with high specific surface areas allow a higher
proportion of paramagnetic Ln ions to be more accessible to water
molecules, which offers enhanced contrast in MRI. In this work, we
report the novel design of core−porous shell UCNPs with both
high luminescent emissions and magnetic relaxivities as potential
dual-modal contrast agents. The core–porous shell UCNPs were
fabricated via the selective etching of the inert shell of NaYF4: 30%Gd at the active core of NaYF4: 20%Yb, 1%Er.
Their morphology and composition were carefully characterized using
transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy,
X-ray diffraction, and high resolution TEM. Their photoluminescent
and magnetic resonance properties were experimentally determined and
compared for the core, core–dense shell, and core–porous
shell UCNPs. Core–porous shell UCNPs were found to display
bright luminescence and superior MRI contrast enhancement, thus showing
great potential as bimodal OBI and MRI contrast agents.