1 Introduction
UCNPs are being decorated with chromophores with interesting photophysical
properties, such as high extinction coefficients, high fluorescence or triplet quantum
yields or capability to generate singlet oxygen. According to the chromophore or dye
features, the prepared nanohybrids would be applied to different purposes, namely,
sensing, imaging, photodynamic therapy (PDT) and so on [1, 2].
This chapter focuses on the requirements needed to succeed when designing a
photoactive functional nanohybrid based on the combination of Ln-UCNPs
and dyes.
For a detailed description of the preparation methods of this kind of nanohybrids,
the reader is directed to recent published reviews and the references cited therein
[3, 4]. Briefly, several synthetic protocols have been outlined to load a high concentration of chromophores on the UCNP surface or its periphery by means of,
e.g. interdigitation of amphiphilic molecules, direct anchoring to the NP surface,
ionic or covalent linkage to the NP ligand or ligand oxidation [3–5].
Along this chapter, a few representative examples have been selected to illustrate
the advances in the application of the luminescence of UCNPs to transfer energy to
conventional dyes and the applications emerging from these photophysical events.
Accordingly, we have set out this chapter in sections dealing, first, with a
description of the UCNPs and, next, with each type of applications. Thus, two
main types of applications are introduced in the last two sections: sensing and
photodynamic therapy. Finally, a brief section about theragnosis is also presented.
1.1 UCNP Properties
UCNPs are usually made of a nanometre-sized inorganic photochemically inert
matrix, such as NaYF 4 , doped with at least two photoactive trivalent lanthanide
cations. One of them serves as sensitizer, absorbs NIR light and transfers it to the
other one, the activator, which emits at certain characteristic wavelengths [6, 7].
The unique intrinsic properties of UCNPs such as NIR excitation and multiple
long-lifetime emission in the UV-NIR region have attracted the attention as
nanomaterials for being implemented in (bio)imaging, sensing, optoelectronics and
theragnosis with incredible potential in bioassays.
1.1.1 NIR Excitation
Commonly used organic bioprobes and nanoparticles – such as quantum dots (QDs)
or carbon dots – are excited in the UV-Vis region. Consequently, there is an
important limitation for in vivo or in vitro applications due to the limited penetration
depth of UV-Vis light. UV-Vis radiation cannot penetrate effectively in the
Functional Nanohybrids Based on Dyes and Upconversion Nanoparticles
373
UCNPs are being decorated with chromophores with interesting photophysical
properties, such as high extinction coefficients, high fluorescence or triplet quantum
yields or capability to generate singlet oxygen. According to the chromophore or dye
features, the prepared nanohybrids would be applied to different purposes, namely,
sensing, imaging, photodynamic therapy (PDT) and so on [1, 2].
This chapter focuses on the requirements needed to succeed when designing a
photoactive functional nanohybrid based on the combination of Ln-UCNPs
and dyes.
For a detailed description of the preparation methods of this kind of nanohybrids,
the reader is directed to recent published reviews and the references cited therein
[3, 4]. Briefly, several synthetic protocols have been outlined to load a high concentration of chromophores on the UCNP surface or its periphery by means of,
e.g. interdigitation of amphiphilic molecules, direct anchoring to the NP surface,
ionic or covalent linkage to the NP ligand or ligand oxidation [3–5].
Along this chapter, a few representative examples have been selected to illustrate
the advances in the application of the luminescence of UCNPs to transfer energy to
conventional dyes and the applications emerging from these photophysical events.
Accordingly, we have set out this chapter in sections dealing, first, with a
description of the UCNPs and, next, with each type of applications. Thus, two
main types of applications are introduced in the last two sections: sensing and
photodynamic therapy. Finally, a brief section about theragnosis is also presented.
1.1 UCNP Properties
UCNPs are usually made of a nanometre-sized inorganic photochemically inert
matrix, such as NaYF 4 , doped with at least two photoactive trivalent lanthanide
cations. One of them serves as sensitizer, absorbs NIR light and transfers it to the
other one, the activator, which emits at certain characteristic wavelengths [6, 7].
The unique intrinsic properties of UCNPs such as NIR excitation and multiple
long-lifetime emission in the UV-NIR region have attracted the attention as
nanomaterials for being implemented in (bio)imaging, sensing, optoelectronics and
theragnosis with incredible potential in bioassays.
1.1.1 NIR Excitation
Commonly used organic bioprobes and nanoparticles – such as quantum dots (QDs)
or carbon dots – are excited in the UV-Vis region. Consequently, there is an
important limitation for in vivo or in vitro applications due to the limited penetration
depth of UV-Vis light. UV-Vis radiation cannot penetrate effectively in the
Functional Nanohybrids Based on Dyes and Upconversion Nanoparticles
373