Chemistry Reference
In-Depth Information
13.2.3
colour and efficiency tuning
In addition to the synthesis of uniform Ln-UCNPs, there is also the need to find an efficient way to optimise the upconversion
properties of the as-prepared Ln-UCNPs. There are two main aims in the optimisation of Ln-UCNPs: one is the upconver-
sion efficiency and the other is multicolour emissions for multiplexed labelling.
The factors that influence the luminescence efficiency of phosphors have been investigated for the conventional lumines-
cent materials. These principles can also be applied for Ln-UCNPs. The most important factor restricting the luminescence
efficiency of Ln-UCNPs is the abundant quenching route for nanoparticles. The small size of the nanoparticles results in
poor crystallinity and a large surface-to-volume ratio, which generates more quenching centres. In this aspect, Ln-UCNPs
with larger size will have better luminescence efficiency [33].
To achieve high luminescence efficiency in small-sized Ln-UCNPs, one effective way is to construct a core-shell struc-
ture to block the quenching route to the surface-quenching centres. Pioneering reports include the simple growth of a pure
host layer around the Ln-UCNPs [34, 35]. The identical crystal lattice minimises the lattice strains to enable the growth of a
core-shell structure. These as-prepared core-shell structured Ln-UCNPs are proven to have higher luminescence efficiency
than the simple Ln-UCNPs. Recently, Yan and co-workers reported a core-shell structured Ln-UCNP employed CaF
2
as the
shell (FigureĀ 13.3) [36]. Benefitting from a lattice similar to that of the cubic-phased NaREF
4
, CaF
2
shells can easily grow
around NaREF
4
-based UCNPs to obtain core-shell structures with high luminescent efficiency. meanwhile, the CaF
2
shell
can minimise the leakage of rare earth ions from Ln-UCNPs in the bio-surroundings, releasing Ca
2+
instead, which has low
biotoxicity.
(a)
(b)
NaYF
4
: Yb, Er
@
CaF
2
50 nm
(c)
[Ca] / [RE]=0
[Ca] / [RE]=1
[Ca] / [RE]=2
[Ca] / [RE]=4
350
400
450
500
550
Ī» (nm)
600
650
700
750
FIgure 13.3
Schematic illustration of Ln-UCNPs@CaF
2
core-shell structures (a), HAADF-STEm images of the nanoparticles and
the corresponding upconversion emission spectra of Ln-UCNPs@CaF
2
nanoparticles with different thicknesses of shell layer. Reprinted
with permission from Ref. [36]. Copyright 2012 Wiley-vCH verlag gmbH&Co. KgaA, Weinheim.
