Chemistry Reference
In-Depth Information
13
LanthanIde-baSed uPconverSIon
nanoPhoSPhorS For bIoImagIng
Fuyou Li, Wei Feng, Jing Zhou and Yun Sun
Department of Chemistry, Fudan University, Shanghai, China
13.1
IntroductIon
Lanthanide-based upconversion nanomaterials have attracted much attention in recent years because of their unique ability
to generate visible or near-infrared (NIR) upconversion emission with excitation at near-infrared wavelengths [1]. For
bioimaging applications, this unique upconversion luminescent property promises low background signals and deep pene-
tration [2-4], which are key factors in determining the imaging quality in luminescence imaging techniques. Together with
the high photostability and low phototoxicity originating from the inorganic nature of the lattice, lanthanide-based upconver-
sion nanomaterials are considered candidates for a new generation of biolabels for luminescent imaging [5-8].
The upconversion process is different from the conventional luminescent process, as described in Scheme 13.1 [1]. After
being excited to state
1
from ground state
0
by absorbing one excitation photon, the conventional luminescent process
includes a non-radiative relaxation process to state
2
, followed by the radiative transition back to the ground state and the
generation of an emission photon. Due to the law of conservation of energy, the energy of the emission photon is less than
that of the absorbed excitation photon, which brings about a Stokes shift. However, in the upconversion process, the excited
state
1
can absorb energy from another excitation photon to generate excited state
2
, followed by the radiative transition to
generate an anti-Stokes shift emission photon.
The upconversion mechanism can be divided into three main classes: excited state absorption, energy transfer upconver-
sion (ETU), and photon avalanche. Because all of these mechanisms require relatively high stability of the intermediate
excited state (such as state
1
in Scheme 13.1b), lanthanide ions are the most commonly used luminescent centres for upcon-
version luminescence as a result of their abundant f-electron configurations and stable excitation state. Detailed discussion
about these mechanisms can be found in the review articles and references therein [1, 9].
In light of the rapid development of the field concerning lanthanide-based upconversion nanophosphors (Ln-UCNPs),
numerous works have described their synthesis, surface modification, and bioimaging applications [10, 11]; several review
articles summarise recent progress [5-8]. Herein we will introduce the main methods and techniques developed in recent
years that promote the bioimaging application of Ln-UCNPs. Some typical examples will be included to describe the
methods or techniques in detail.
13.2
FabrIcatIon oF Ln-ucnPs SuItabLe For bIoImagIng
An ideal luminescent biolabel must have the following unique properties: high luminescent efficiency to improve the
sensitivity; uniform size, shape, and luminescent properties; suitable surface properties, which enable the dispersion and
functionalisation of biomolecules in biological surroundings; and low cytotoxicity in living systems [5].
