Highly Efficient Multicolour Upconversion Emission in Transparent Colloids of Lanthanide‐Doped NaYF₄ Nanocrystals

Abstract

Excitation of luminescing biolabels via two-photon absorption processes allows the use of near-infrared (NIR) light, which is only weakly absorbed by biological tissue.[1] Excitation in the NIR induces only a very weak autofluorescence background and avoids photodegradation in biotagging applications, thus simplifying the detection of the labeled target molecules and increasing the sensitivity of the method. Organic dyes as well as semiconductor nanoparticles can be employed as emitters. [1±3] Due to the nature of the two-photon absorption (TPA) process which involves a non-stationary (ªvirtualº) quantum mechanical state, however, its efficiency is very low, requiring high excitation densities. To avoid thermal decomposition processes in the sample, expensive pulsed lasers are frequently employed as light sources with pulse durations in the pico-or even femtosecond range. Photon upconversion is an alternative process for the generation of visible radiation by NIR excitation. It is based on sequential absorption and energy transfer steps involving real metastable excited states of the chromophore.[4] Therefore its efficiency can be much higher than for TPA processes, and continuous wave (CW) laser or lamp excitation is possible. Typical excitation densities fall in the range of 1±10 3 W cm ±2for upconversion, compared to 10 6 ±10 9 W cm ±2 for twophoton absorption. [2,4,5] Applications range from display devices [6] and lasers [7] to commercially used reporters for nucleic acid microarrays. [8,9] [10] Crystalline materials doped with these ion couples, however, normally consist of sub-micrometer [8,9] to micrometer-sized grains [10] which do not form transparent colloids and are much too large to substitute for molecular dyes in biological tagging applications. Therefore it was a challenging task to synthesize nanocrystals of these materials which can be transparently dispersed in solution. Recently we have been able to dope lanthanide phosphate nanocrystals with these ion couples and have demonstrated upconversion emission in transparent colloidal solution for the first time.[11] Due to competing radiationless processes, however, the efficiency of the upconversion luminescence was still rather poor. In the present paper we report on the successful synthesis and very intense multicolor upcon- nanocrystals transparently dispersed in solution. The upconversion efficiency of such solutions is about eight orders of magnitude higher than for the previously reported colloids of lanthanide-doped phosphate nanocrystals. We believe that this enormous improvement of the upconversion efficiency of these materials opens the door for interesting future applications in the field of biolabeling. The characterization of NaYF 4 :20 %Yb,2 %Er is summarized in Figure 1. The transmission electron microscopy (TEM) images (Fig. 1A) show crystalline particles of roughly spherical shape. A histogram of the particle size distribution, deduced from several overview TEM images, is given in Figure 1B and shows a relatively broad distributi...

Large Enhancement of Upconversion Luminescence of NaYF4:Yb3+/Er3+ Nanocrystal by 3D Plasmonic Nano‐Antennas

“…Today, one of the most effi cient lanthanide-ion-based UCL materials is the Er 3 + doped hexagonal phase NaYF 4 crystal sensitized by Yb 3 + ions (i.e., the Yb 3 + ions collect the pumping light and trasfer the energy to Er 3 + ions, which then upconvert photon enery via the lone-lived ladder-form energy levels). [18][19][20][21][22] In this energytransfer (ET) process, the UCL effi ciency depends on the light collection effi ciency of the sensitizers (i.e., Yb 3 + ions); it is therefore possible to improve the UCL effi ciency by using metal nanostructures, which can further enhance the optical absorption cross-section of the sensitizers.…”

Versatile Photosensitizers for Photodynamic Therapy at Infrared Excitation

“…[11] They are synthesized following the procedures reported in the literature [12] with some modifications. When excited by an infrared (974 nm) source, strong visible bands appear around 537 nm and 635 nm.…”

Morphologically controlled synthesis of colloidal upconversion nanophosphors and their shape-directed self-assembly

“…These NCs often possess "peculiar" optical properties [e.g., quantum cutting (16) and photon upconversion (17)], allowing the management of photons that could benefit a variety of areas including biomedical imaging (18,19) and therapy (20), photovoltaics (16,21), solid state lighting (22), and display technologies (23). Colloidal upconversion nanophosphors (UCNPs) are capable of converting long-wavelength near-infrared excitation into short-wavelength visible emission through the long-lived, metastable excited states of the lanthanide dopants (24). In contrast to the Stokes-shifted emissions from semiconductor NCs or organic fluorophores and the multiphoton process employing fluorescent dyes, UCNPs offer several advantages including narrow emission bands tunable through the choice of dopants (25).…”