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KTP nanoparticles

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Elodie Chaudan, Sandrine Perruchas, Thierry Gacoin
Ludovic Mayer (former PhD student) – Géraldine Dantelle

*Emmanuel Beaurepaire ’s group, Laboratorye for Optics and Biosciences (LOB), Ecole polytechnique
*Jean-François Roch’s group, Molecular Quantum Photonis Laboratory, ENS Cachan
*Michel Simonneau and Aude-Marie Bestel-Lepagnol, l’INSERM (French institute for health and medical research)

Single KTP nanocrystals as second-harmonic generation biolabels in cortical neurons, Nanoscale, 2013, 5, 8466, DOI: 10.1039/c3nr01251d
Dual light-emitting nanoparticles: second harmonic generation combined with rare-earth photoluminescence
J. Mat. Chemistry C, 2014, 2, 7681, DOI: 10.1039/c4tc01227e

KTiPO4 (KTP) is a material known for its non-linear optical properties. An illuminated KTP crystal - at the frequency f - can emit a light at the double frequency 2f. This property is called Second Harmonic Generation (SHG) and it is widely used in electro-optical modulators and lasers.
In the group we are interested in the KTP-nanoparticles synthesis. We developed a solid state synthesis at high temperature (700 à 1000 °C), in which KTP nano-crystals nucleation is limited by KCl crystals. At the end of the synthesis, we obtain stabilized nanoparticles in water - as the phosphate groups in surface of KTP nanoparticles force away other negatively charged groups. However, the size distribution of KTP nanoparticles is still quite large (see figure 1). Other steps are necessary to finally obtain stabilized particles with a narrow size distribution.

Figure 1 (a)KTP nanoparticules images, made by Scanning Electron Microscopy (SEM) (b) A single KTP nanocrystal image, made by Transmission Electron Microscopy.

Characterizations of KTP particles are allowed by coating nanocrystals on silicon (for SEM imaging) or on glass, for optical studies.
The SHG emission intensity depends on the excitation light power or on the particle’s size and volume that we can determine with AFM (Atomic Force Microscopy). The emission intensity is also related to the excitation light polarization. The experiments on radiation patterns and polarization have been done in collaborations with Jean-François Roch’group in ENS Cachan as they have great knowledge about single particle SHG radiation.
Other heterostructures with both a KTP core and a LaPO4: Eu shell have also been developed. As they combine SHG emission and photoluminescence they can be used as dual probe for imagery (see figure 2).

Figure 2 (a) AFM image of an area with several core-shell KTP@LaPO4:Eu particles (b) Photoluminescence (c) SHG and (d) radiations spectra.

SHG is a widely used property in the imaging field. Indeed, most biological tissues have a non-negligible transmission window in the infrared and SHG is an easy way to excite in the infrared.
In order to develop this potential application, cytoxicity studies and imaging of mice neuronal inserted nano-KTP have been accomplished with the collaboration of INSERM. As a result, KTP nanoparticles are not cytotoxic towards this kind of cells. Moreover, single nano-KTP can be imaged in the neuronal cells (see figure 3).

Figure 3: Single nano-KTP in a dendron of a mouse neuronal cell. On the optical image, the neuronal cell is in red, and in yellow, the SHG signal from a single KTP nano-crystal.

We also work with the group of Emmanuel Beaurepaire from the LOB (Laboratory for Optics and Biosciences of Ecole polytechnique). One of their main problematic is the zebra-fish growth. A way to investigate the different steps of their embryonic development is to inject nano-KTP in the blood vessels of these zebra fishes.