Chemistry Reference
In-Depth Information
Another method for achieving high luminescence efficiency is to reduce the amount of surface ligands in order to minimise
the quenching effect. Besides the annealing method to remove surface ligands [37], Li and co-workers have reported the
synthesis of small-sized NaLuF
4
-based UCNPs [4]. The efficiency is much higher than the NaYF
4
-based analogues, because
of fewer surface-capping ligands, probably resulting from the different coordination abilities of oA molecules to Y
3+
and
Lu
3+
ions.
A recently developed route to enhance the UCL is the metal surface-enhanced luminescence method. Although such
metal-enhanced fluorescence effects were found early in 1980, the use of this method for Ln-UCNPs was only reported a
few years ago. Because the surface plasmon resonance of noble metals have the ability to change the spatial distribution of
the excitation energy field, the emitters located at the hot spot will gain more excitation energy, thus generating intense
upconversion luminescence. Yan and co-workers reported the surface-enhanced upconversion luminescence of UCNPs on
silver nanowires [38]. Duan and co-workers fabricated a NaYF
4
:Yb,Tm@Au heterostructure and observed a more than
150% enhancement of the blue upconversion emission [39]. Recently, Kennedy [40] and Qin [41] reported the similar sur-
face-enhanced upconversion luminescence phenomenon, employing Au nanoparticles to the surface of UCNPs, respectively.
This kind of enhancement was further confirmed by a single particle experiment carried out on a combined optical and
atomic force microscope setup by Schietinger and coworkers [42].
Relatively speaking, the colour tuning of Ln-UCNPs is easier. The common method is to dope multiple kinds of activator ions
within the lattice [43]. Controlling the kinds and amounts of activators will lead to different upconversion emission colours [44,
45]. For example, Er
3+
usually gives green and red emissions, Tm
3+
usually gives blue emission, while Ho
3+
usually gives green
emissions. The mixture of these three kinds of activators can produce a series of upconversion emission colours [46-48].
Another method of colour tuning is to employ a quencher to selectively quench one emission in order to change the
emission ratios of Ln-UCNPs. For example, gold nanoparticles have absorption in the visible range. In a heterostructure
composed of Au NPs and Ln-UCNPs, the emission of Ln-UCNP will be quenched by the Au NPs [49]. varying the ratio of
these two NPs can adjust the emission ratio in the visible range.
In all, tuning the efficiency and colour of Ln-UCNPs can be achieved by controlling the structure and composition of the
nanoparticles. However, how to obtain small Ln-UCNPs with enough efficiency and tunable upconversion emissions is still
the main challenge for their bioimaging application.
13.3
SurFace modIFIcatIon oF Ln-ucnPs
Even though many groups have obtained Ln-UCNPs with the desired size and upconversion properties, the materials are not
ready for bioimaging use. The important requirements for ideal biolabels include biocompatibility and some functional
groups, which are convenient for linking to biological molecules in order to achieve selective targeting. Therefore, it is
necessary to modify the surface of the Ln-UCNPs in order to endow them with these functions.
As mentioned in section 2.2, most of the successful synthetic methods use organic species as capping ligands to obtain
uniform Ln-UCNPs. However, these organic species make the surface of Ln-UCNPs hydrophobic. Therefore, surface modifi-
cation protocols need to be developed to transform the surface to a hydrophilic nature to suit the biological surroundings. Until
now, several strategies have been developed to successfully modify the surface properties of Ln-UCNPs (Scheme 13.2) [5].
The first surface modification strategy is to directly alter the surface ligands. Ligand exchange is the most commonly used
protocol in this strategy. Because these ligands are usually small molecules, the exchange process does not affect the crys-
tallinity and morphology of Ln-UCNPs. The detailed exchange process is driven by the higher affinity between rare-earth
ions and the additional ligands than those for the original ligands or by just the high concentration of the additional ligands.
In particular, oA-coated Ln-UCNPs usually have strong interactions between rare-earth ions and oA ligands because of the
high affinity of rare-earth ions to the oxygen terminated species. Thus the exchange of oA molecules usually requires excess
additional ligands or the application of a chelating agent. Up to now, poly(ethyleneglycol) (PEg)-phosphate, polyacrylic
acid (PAA), polyethylenimine (PEI), polyvinylpyrrolidone, hexanedioic acid, 3-mercaptopropionic acid, dimercaptosuc-
cinic acid, mercaptosuccinic acid, citrate, 1,10-decanedicarboxylic acid, 11-mercaptoundecanoic acid, and poly(amidoamine)
have been used to replace surface oA ligands to achieve hydrophilic properties [5].
Another way to modify the surface ligand is to perform some chemical reactions. Li and co-workers have developed a versa-
tile protocol to oxidise surface oA molecules with Lemieux − von Rudloff reagent [50]. The oxidation agents break the double
bond in the oA molecule to form azelaic acid products. The terminal carboxylic groups in the products endow the Ln-UCNPs
with hydrophilic properties. Yan and co-workers have used ozone as an oxidation agent, in which the terminal group can be con-
trolled as -oH, -CHo, or -CooH units by employing specific hydrolysis conditions [51]. The hydrophilic terminal functional
groups can also be reacted directly to the biofunctional species, which provides a direct route for the biofunctionalisation.
Another strategy is to introduce some amphiphilic species outside the hydrophobic Ln-UCNPs. The hydrophobic groups
of amphiphilic molecules will interact with the hydrophobic chains of the surface ligands in the Ln-UCNPs. The hydrophilic
