Nanotechnology. 2017 May 5;28(18):185101. doi: 10.1088/1361-6528/aa6834.

Ultrasmall, water dispersible, TWEEN80 modified Yb:Er:NaGd(WO4)2 nanoparticles with record upconversion ratiometric thermal sensitivity and their internalization by mesenchymal stem cells.

Cascales C1, Paíno CL, Bazán E, Zaldo C.

Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, c/Sor Juana Inés de la Cruz 3, E-28049 Madrid, Spain.





Upconversion is a luminescence process observed in some trivalent lanthanides in which light with low energy photons is transformed into light with higher energy photons, i.e. infrared light can be transformed into visible light, see Figure 1. In addition to upconversion, there are several other non linear optical processes (non related with lanthanides) able to make similar optical transformations. Second harmonic generation and multiphoton absorption are examples of these non linear optical processes, but the latter processes require of high light intensity to be effective. The singularity of upconversion is the presence of intermediate electronic levels in lanthanides able to store excited electrons for a long (μs-ms) time, thus low infrared light intensities can be used for excitation of high energy levels through a two (or more) sequential excitation steps, i.e. a first infrared photon populates the intermediate level and a second photon excites electrons in intermediate levels to higher energy leves giving rise to fluorescence photons with energy higher than those of excitation. The ladder-like energy level structure of Er3+ is very suitable for this aim, but it should be incorporated to a transparent solid host which modifies to some extend its fluorescent properties, see Figure 1b and 1c. For selected Er-host combinations bright green light emission can be observed under infrared light excitation, see Figure 1d.

Thanks to these properties, upconversion nanoparticles are recognized non-contact optical probes for biomedical applications. In fact, irradiation in the so-called first (λ= 700-980 nm) and second (λ= 1000-1400 nm) optical windows of tissular specimens allows deep scanning of optical probes incorporated to them.  At difference, conventional downconversion fluorescence processes require ultraviolet light for excitation what limits the light penetration in tissues, excites tissue autofluorescence (reducing testing resolution), and have harmful consequences after extended irradiation.

In addition to the imaging capabilities of fluorescence techniques, upconversion has also the capability of accurate (≤1ºC) determination of the temperature of the used probe, assumed in thermodynamic equilibrium with its environment. This is related to the evolution with temperature of the relative intensity of two green emissions of Er3+ ions, while Yb3+ is used to enhance the excitation light absorption (Yb3+ transfers its energy to Er3+). This property has promoted the use of nanosized particles doped with lanthanides to monitor and control different biomedical protocols, although in most cases these proposals are still in a primary stage and a lot of work is still required to reach clinical trials.

The hyperthermia therapy of cancerous cells is one of the subjects that has received more attention in upconversion literature. So far b-NaYF4:Er:Yb nanoparticles, with sizes in the 10 to 50 nm range, are widely proposed for this task. The most relevant parameter for these applications is the thermal sensitivity of the used nanoprobe, i.e. how much the measured magnitude change with a given change in temperature. Despite fluoride nanoparticles show the most efficient upconversion yield in terms of infrared to green light intensity conversion, their relatively low thermal sensitivity is a limit for their widespread application in hyperthermia control. Further, thermal nanoprobes must travel and accumulate selectively at the cancerous cells, which so far is being pursuit by chemical modifications of the nanoprobe surface, thus increasing largely the nanoprobe size.

Our work in references 1 and 2 show that NaT(XO4)2:Er:Yb nanoparticles (being T= Y, La, Gd or Lu, and X= Mo or W)  exhibit five times higher thermal sensitivity than classic b-NaYF4:Er:Yb upconversion probes while still having comparable fluorescence efficiency. Further, we propose the use of human mesenchymal stem cells (hMSCs) as vehicles to carry the nanoprobes to cancerous cells profiting  of the MSCs capability to engraft and grow around cancerous tissue. As a first step, we have shown the ability  of the hMSCs to engulf water soluble TWEEN80 modified diamond-shaped NaT(XO4)2:Er:Yb nanoprobes with diagonal size as small as 5´10 nm, and the conditions of cell survival after particle intake. Furthermore, thanks to the high upconversion efficiency of NaT(XO4)2:Yb:Er particles, subcutaneous depths (in excess of 2 mm) have been tested with NaLu(MoO4)2:Er:Yb by monitoring the attenuation of the emitted green light in ex-vivo chicken breast tissues, with equal results than with b-NaYF4:Er,Yb. Additionally, the upconversion signal of these novel nanoprobes perfused in a mouse has been unequivocally identified by using fluorescence lifetime imaging microscopy, and further compared with the SHG or autofluorescence images of the surrounding tissues, showing that these nanoprobes are distributed through the circulatory system to all mouse organs, including heart, lung, kidney, liver, spleen, eye and brain.



Figure 1. Physical basis of near infrared excited (λ≈ 980 nm) upconversion nanoprobes. (a) Excitation, energy transfer and emission processes in a Yb:Er coupled system. (b) Yb and Er ions incorporated in diamond-shaped nanoparticle. (c) Transmission electron microscopy image of a Yb:Er upconversion nanoprobe with a TWEEN80 shell. (d) Image of the upconversion fluorescence of NaY(WO4)2:Yb:Er nanoprobes dispersed in water.



Figure 2. Scheme of the proposed upconversion control of tumor hyperthermia. Upconverting nanoparticles are incorporated to hMSC during culture. After hMSCs are perfused in the patient they migrate and engraft the tumor releasing the nanoparticles at tumoral sites. Infrared light irradiates the nanoparticles and the intensity ratio of the Er fluorescence at 525 and 555 nm provides the tissue temperature. This measurement allows the control and modulation of the heat applied to the tumor in order to avoid overheating of surrounding healthy tissues.  The new developed NaY(WO2)4:Yb:Er nanoparticles show five times larger thermal sensitivity than classic b-NaYF4:Er:Yb nanoparticles, which would allow a more precise control of the temperature and thus heat regulation.



  1. “Efficient Up-conversion in Yb:Er:NaT(XO4)2 Thermal Nanoprobes. Imaging of their Distribution in a Perfused Mouse” C. Zaldo, M. D. Serrano, X. Han, C. Cascales, M. Cantero, L. Montoliu, E. Arza, V. R. Caiolfa, M. Zamai, PLOS one 12(5) e0177596, 2017 (
  2. “Ultrasmall, water dispersible,  TWEEN80 modified Yb:Er:NaGd(WO4)2  nanoparticles with record upconversion ratiometric thermal sensitivity and their internalization by mesenchymal stem cells” C. Cascales, C. Paino, E. Bazán, C. Zaldo. Nanotechnology 28, 185101, 2017 (