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8LS-I-6
NIR Fluorescence Concentration Self-Quenching and Quenching by OH- Acceptors in Aqueous Colloids of Nd3+ Doped Fluoride Nanocrystals
Yu.V. Orlovskii 12, A.V. Popov \ E.O. Orlovskaya \ A.S. Vanetsev 12
1-Prokhorov General Physics Institute RAS, 38 Vavilov street, Moscow, Russia 2- Institute of Physics, University of Tartu, 1 Ostwald street, Tartu, Estonia
orlovski@lst.gpi.ru
Aqueous colloidal solutions of Nd3+: LaF3 nanocrystals (NCs) synthesized using "green" hydrothermal -microwave treatment (HTMW) have established themselves as a promising fluorescent agent for near IR imaging in the first biological window (750 - 950 nm) [1,2], due to their higher temporal stability and lower fluorescence quenching in comparison with NCs synthesized by the traditional co-precipitation (CO) method [3]. Two processes control the fluorescence quenching in NCs synthesized by water-based methods: 1) Nd*-Nd self-quenching and 2) quenching caused by OH- molecular groups located in their volume.
The main issue considered in the report is what are the main regularities and features of these processes in NCs, and why can we control the fluorescence quenching using a specific synthesis method and a specific crystal matrix for doping with Nd3+?
At first, when choosing a synthesis method, we connected fluorescence-quenching processes with defects in the crystal structure of nanoparticles. These defects are
1) Inhomogeneity of dopant distribution over the La3+ sites in the volume of the synthesized NCs, leading to the formation of closely spaced Nd3+ ions (pairs), which exhibit strong fluorescence self-quenching;
2) The arrangement of OH- molecular groups in the volume of the NPs, which enhances the quenching of Nd3+ fluorescence and manifests itself in a decrease in the relative quantum yield and fluorescence brightness.
As a result, we found that a higher temperature of the reaction mixture for the HTMW synthesis method as compared to CO leads to a more uniform distribution of the Nd3+ dopant over La3+ sites and a lower concentration of OH- acceptors in the NC volume, which leads to a higher brightness of NCs fluorescence in an aqueous colloid.
The effect of oven modification for HTMW treatment on fluorescence quenching will also be discussed.
In addition, we have established simple criteria for choosing a crystal matrix for doping with Nd3+ ions in the synthesis of aqueous colloidal solutions of fluoride nanocrystals using them as luminescent probes for obtaining images in the first biological window, which is most convenient for recording luminescence [4]. This is a large ratio of intensity parameters Q4IQ6, used in the Judd-Ofelt theory to calculate the probability of radiative transitions, which increases the luminescence branching coefficient / at the 4F3i2 ^ %I2 transition in the first biological window. A low value of the intensity parameter Qe weakens the Nd-Nd luminescence self-quenching and the Nd-OH- quenching due to the weaker dipole-dipole ion-ion interaction. The hypothesis was tested on concentration series of aqueous colloidal solutions of Nd3+: LaF3 and Nd3+: KY3F10 NCs, synthesized by HTMW treatment with PVP as a biocompatible surfactant. We found that due to the higher Q4IQ6 ratio and the lower value of Qe in the Nd3+: LaF3 compared to Nd3+: KY3F10 NCs, the fluorescence branching ratio at the 4F3i2 ^ %I2 transition significantly increases for the former, while the fluorescence brightness increases four times. The latter is associated with a weaker donor-acceptor interaction, which depends on the specific crystal structure.
[1] U. Rocha, J. Hu, E. M. Rodríguez, A.S. Vanetsev, M. Rahn, V. Sammelselg, Yu.V. Orlovskii, J. García Solé, D. Jaque, D.H. Ortgies, Small, 12, pp. 5394 - 5400 (2016).
[2] D.H. Ortgies, F.J. Teran, U. Rocha, L. de la Cueva, G. Salas, D. Cabrera, A.S. Vanetsev, M. Rahn, V. Sammelselg, Yu.V. Orlovskii and D. Jaque, Advanced Functional Materials, 28(11) p. 1704434 (2018).
[3] A. Vanetsev, K. Kaldvee, L. Puust, K. Keevend, A. Nefedova, S. Fedorenko, A. Baranchikov, I. Sildos, M. Rahn, V. Sammelselg, Yu. Orlovskii, ChemistrySelect, 2, pp. 4874 - 4881 (2017).
[4] Yu.V. Orlovskii, A.V. Popov, E.O. Orlovskaya, A.S. Vanetsev, E.A. Vagapova, M. Rahn, V. Sammelselg, I. Sildos, A.E. Baranchikov, P.V. Grachev, V.B. Loschenov, A.V. Ryabova, Journal of Alloys and Compounds, 756, pp. 182 - 192 (2018).
