THE SPECTROSCOPIC INVESTIGATION OF THE UP-CONVERSION NANOPARTICLES FOR BIOMEDICAL APPLICATIONS

1. THE SPECTROSCOPIC INVESTIGATION OF THE UP-CONVERSION NANOPARTICLES FOR BIOMEDICAL APPLICATIONS D.V. Pominova, A.V. Ryabova, S.V. Kuznetsov, A.A. Luginina Laser Biospectroscopy Lab.,A.M. Prokhorov General Physics Institute,Russian Academy of Sciences,Moscow, Russia E-mail:pominovadv@gmail.comWeb:www.nsc.gpi.ru/lbs.html, www.biospec.ru

2. Introduction The problem of screening and application of luminescent agents currently remains one of the most vital in biology and medicine. There is great interest in the study of diagnostic and therapeutic properties of the phosphors as which can be used different nanoscale structures and particles. Some nanomaterials such as gold nanoparticles, quantum dots and polymers are already widely used. However, growing interest to the so-called up-conversion nanoparticles, which can absorb some NIR photons and then can emit luminescence in the visible region of the spectrum. Compared with organic phosphors and semiconductor nanocrystals, nanoparticles, with the possibility of the up-conversion mechanism luminescence excitation (up-NP) have a number of advantages, namely: providing a high photochemical stability, narrow emission band and large distances (up to 500 nm) between the individual luminescence peaks and excitation wavelength in the infrared range that allows them to be easily separated. Along with the increased penetration depth of light and lack of stray fluorescence from biomolecules under IR - excitation, such up-NP are perfect for use as fluorescent probes in biological studies and for fluorescence diagnostic.  Also, the photobleaching and phototoxicity are reduced under IR irradiation.

3. Purpose of investigation: • to research spectral-luminescence and up-conversion properties of nanoparticles with the host SrF2 doped with rare earth ions Yb3+- Er3+, Yb3+- Er3+- Tm3+, depending on the concentration and doping composition. Concentration Composition

4. Equipment • For the synthesis was used co-precipitation from water solutions method. • Hydrodynamic sizes of nanoparticles in liquids were estimated by multi-angle spectrometer of dynamic light scattering Photocor Complex (Russia). • Fluorescent studies were performed using the laser-induced fluorescence spectroscopy, including the optical fiber spectrum analyzer LESA-01-Biospec, modified with integrating sphere Avantes, laser 974 nm. • Kinetic characteristics of up-conversion luminescence in blue, green and red range were prepared using non descanned regime on microscope Carl Zeiss LSM-710-NLO with femtosecond laser excitation.

5. The experimental setup for up-conversion efficiency measurements PSample974_absorbed– absorbed by the sample exiting laser power; Psamplei_emitted– emitted up-conversion light (power integrated over the 380-780 nm or 420-870 nm ranges, depending on the used dopants); PReference974_scattered– scattered by the undopedsample power, which emulates the sample scattering; PSample974_scattered– the power scattered by doped sample.

6. Typical luminescence spectra of Yb3+ , Er3+ doped phosphors at room temperature under IR excitation and simplified energy level diagram of Er3+ and Yb3+ GSA – ground state absorbtion; ESA – exited state absorbtion; ETU – energy transfer up-conversion BET – back energy transfer

7. Up-conversion luminescence spectra for selected samples under 974 nm excitation

8. Kinetic characteristics of up-conversion luminescence In this paper was proposed and tested a new method for visualizing and studying the kinetics of up-conversion luminescence. Luminescence lifetime measurement was performed using a confocal microscope Carl Zeiss LSM 710 NLO. The luminescence was excited by a femtosecond pulsed tunable laser Chameleon Coherent (780-1080 nm) with a wavelength 974 nm. The measurements were prepared using non descanned regime. Laser scanned over the sample surface at a rate of about 1.3 microseconds per pixel. Detector fixed luminescence intensity corresponding to each pixel of the scan. Since the decay time of the samples is much larger than the scanning speed of one pixel, it were obtained glowing strips, the so-called fluorescent tracks . With further software processing and approximation were obtained the times of the luminescence decay.

9. The experimental values ​​of the up-conversion luminescence decay time The Er3+ (4S3/2,2H11/2) → 4I15/2transition has an instantaneous rise during the excitation pulse in addition to a delayed rise after the end of the excitation pulse, showing that both excited state absorption (ESA) and energy-transfer up-conversion (ETU) processes are involved in the Er3+ (4S3/2,2H11/2) green level population. The lifetimes associated with the green light emitting level become shorter with increasing concentration of dopantions.This confirms the predominance of back energy transfer in the depopulation mechanism of the up-conversion emitting levels. With increasing concentration of dopant ions, the distance between donor and acceptor is decreased and at high concentrations begins the concentration quenching.

10. Results • During the energy transfer from donor to acceptor the population of the level 4S3/2 is increases from which green emission occurs and thus the intensity of the luminescence of the acceptor increases . However, apart from the direct transfer of energy, there is a possibility of reverse energy transfer from donor to acceptor, at which the state 4S3/2relaxates to the state 4I13/2and the excess energy transfers back to the donor. The back energy transfer increases with the concentration of dopant ions, since the distance between the donor and acceptor ions reduced. At very high dopant ions concentrations, the excitation is also beginning to migrate by donors, which is a negative thing, because it leads to a decrease the acceptor ion pumping, a substantial decrease in the lifetime of the luminescence level and thus worsening the energy characteristics of up-conversion. • According to our experimental results, the dopant concentration 10%YbF3 1%ErF3 is optimal and the quantum yield for this sample is maximum. When the concentration of dopant is higher, this leads to decrease the distance between the donor and acceptor and reduction the lifetime and the intensity of the up-conversion luminescence. • Adding an Tm3+ ions can change the color of up-conversion emission, and obtain white luminescence. However, the investigated sample has low irradiation intensity and it requires more carefully optimization of the dopants concentrations ratio.

11. Conclusion • A system for the acquisition of up-conversion luminescence kinetic characteristics using non descanned regime of microscope Carl Zeiss LSM 710 NLO was developed. • We have established correlation between the dopant composition and up-conversion luminescence lifetime as well as up-conversion luminescence intensity from levels of Tm3+  1G4 → 3H6, ~470 nm (blue), Er3+ (2H11/2, 4S3/2) → 4I15/2 ~ 530 nm (green), and 4F9/2 → 4I15/2, ~ 650 nm (red). Based on these results, were made assumptions for the mechanism of radiative levels of Er3+, Tm3+ population. • The optimal concentration of up-converting doping ions for convert infrared radiation into a visible for ghost matrices SrF2 is 10%YbF3/1%ErF3. The quantum yield in visible range for this sample was 0.42%.

12. This work was supported by grants from the President of the Russian Federation (MK № 4408.2011.2) and MES (activity 1.2.2, research groups led by candidates: № 14.740.12.1343 from 10.03.2011)

13. Thank you for your attention!