Overtone Vibrational Transition-Induced Lanthanide Excited-State

www.acsnano.org

Overtone Vibrational Transition-Induced Lanthanide Excited-State Quenching in Yb3+/ Er3+-Doped Upconversion Nanocrystals Bingru Huang,‡ Jan Bergstrand,§ Sai Duan,† Qiuqiang Zhan,*,‡ Jerker Widengren,§ Hans Ågren,† and Haichun Liu*,† Downloaded via KAOHSIUNG MEDICAL UNIV on November 27, 2018 at 10:39:38 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Theoretical Chemistry and Biology, KTH Royal Institute of Technology, Roslagstullsbacken 15, SE-106 91 Stockholm, Sweden ‡ Centre for Optical and Electromagnetic Research, Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, South China Academy of Advanced Optoelectronics, South China Normal University, 510006 Guangzhou, China § Experimental Biomolecular Physics, Department of Applied Physics, KTH Royal Institute of Technology, SE-106 91, Stockholm, Sweden

I

n a recent issue of ACS Nano, an article by Rabouw et al. was published,1 investigating the loss pathways competing with upconversion (UC) in lanthanide-doped UC nanocrystals (NCs). The authors studied the excited-state dynamics of UC material β-NaYF4 codoped with Yb3+ and Er3+ ions. For each of the energy levels involved in infrared-to-visible upconversion, they measured and modeled the competition between spontaneous emission, energy transfer between lanthanide ions, and other decay processes. The authors presented a microscopic quantitative model for the quenching dynamics in UC NCs, taking into account two important quenching pathways of the NCs, energy transfer from lanthanide excited states to fundamental vibration modes of the solvent and/or ligand molecules surrounding the UC NCs and cross-relaxation between lanthanide dopants at high doping concentrations. Their model was successful in reproducing the observed decay dynamics of the green (∼18459 cm−1) and red (∼15267 cm−1) Er3+ emissions and could determine the shell thickness of the core−shell NCs required for near-complete suppression of the solvent quenching. Significantly, they pointed out that the notably environment-induced UC luminescence quenching is a form of Förster resonance energy transfer (FRET) by dipole−dipole coupling of an electronic transition of a lanthanide dopant to vibrations of the solvent and ligand molecules, and that the transfer rate scales with the inverse sixth power of the separation between the energy donor (the luminescent center) and the energy acceptor (the solvent vibration). However, with this Comment, we like to point out that (1) their model is not complete and requires further theoretical considerations and that (2) their claim about the Yb3+ 2F5/2 state quenching is discussible. The fly in the ointment of their model is that it could not interpret the observed excited-state dynamics for the nearinfrared (NIR) emission originating from the NIR energy levels of Er3+ (4I11/2) and Yb3+ (2F5/2), despite that the quenching pathways of these levels are known to be fundamentally more influential to the UC emission,2,3 just as these authors analyzed and concluded in ref 1, “the major contribution to low UC efficiencies in NCs compared to bulk © 2018 American Chemical Society

material comes from losses in the NIR, not only because the NIR-emitting levels are quenched strongly at high doping concentrations but also because losses in the infrared affect the overall UC intensity more strongly than losses in the visible.” Their model considered only resonant solvent quenching between solvent fundamental vibration modes and electronic transitions of the Er3+ ion. The authors hold the view that solvent quenching by dipole−dipole coupling is negligible for the Yb3+ 2F5/2 excited state, originating from the fact that the gap of the two energy levels of Yb3+ (∼10000 cm−1) is over 3 times higher than the highest solvent vibrations (C−H stretch). In this context, they claimed that the Yb3+ 2F5/2 level was indirectly quenched via the Er3+ 4I11/2 level due to the rapid energy exchange between them by energy transfer and back-transfer, followed by 4I11/2 → 4I13/2 (Er3+) solventinduced quenching (Figure 1a). They proposed that undercoordinated Yb3+ centers at the NC surface could provide quenching pathways for the 2F5/2 state of inside Yb3+ ions, which, however, cannot be approved by their experimental results. Herein, we find that the authors of ref 1 may have ignored a key type of quenching mechanism for lanthanide excited states, i.e., the solvent quenching by the overtone vibrational transitions,2,4 that is, transitions between states separated by more than one vibrational quantum (Figure 1a).5 The overtone vibrational transitions have been extensively investigated experimentally and theoretically since the 1960s when researchers studied radiative and nonradiative transitions of rare earth ions in various form of materials.6−13 In 1971, Sveshnikova et al. proposed to consider the interaction of an electronically excited ion with surrounding molecular groups, causing the deactivation of the ion and excitation of vibrational states in molecular groups, as a nonradiative transition,14 under an inductive-resonant mechanism according to Förster15 and Dexter.16 In the general FRET mechanism, the nonradiative Received: July 5, 2018 Published: November 27, 2018 10572

DOI: 10.1021/acsnano.8b05095 ACS Nano 2018, 12, 10572−10575

Letter to the Editor

Cite This: ACS Nano 2018, 12, 10572−10575

ACS Nano


Figure 1. (a) Energy-level diagrams of Yb3+ and Er3+ and schematic representation of solvent quenching to lanthanide excited states. Transmission electron microscopy images of (b) NaYF4:18% Yb3+ and (c) NaYF4:20% Er3+ nanocrystals. Quenching effect of the increasing content of H2O on the infrared emission of (d) Yb3+ and (e) Er3+ upon 980 nm excitation in NaYF4:18% Yb3+ and NaYF4:20% Er3+ nanocrystals, respectively. The emission profile of NaYF4:18% Yb3+ was modulated by a 980 nm long-pass edge filter. (f) Laser intensity attenuation caused by water absorption and peak intensities of Yb3+ (2F5/2 state) and Er3+ (4I13/2 state) emission as a function of volume ratio of H2O (VH2O/Vall). (g) Decay curves of Yb3+ emission of NaYF4:18% Yb3+ nanocrystals in a H2O/D2O mixture with different H2O volume ratio, measured at 1005 nm upon 980 nm excitation. (h) Decay curves of Er3+ emission of NaYF4:20% Er3+ nanocrystals in H2O/D2O mixture with different H2O volume ratio, measured at 1520 nm upon 980 nm excitation. The rest of the solvent volume (excluding H2O) in (d−h) is D2O. The vibration modes depicted in (a) are related to O−H in our experiments but not limited to O−H in a general case.

vibrational modes, e.g., in Yb3+ complexes18 and in Yb3+-doped NCs.3 Actually, overtone transition-induced solvent quenching has emerged in ref 1, where Yb3+ could be significantly quenched directly by the solvent (Figure 4c in ref 1), as well as in our previous study.3 Particularly, in our previous study, we investigated the interactions of NaYF4:2% Er3+@NaYF4:20% Yb3+ nanostructures with the quenching environment by measuring their UC luminescence intensity in D2O/H2O mixtures.3 We found that with gradually increasing the amount of H2O, the UC luminescence intensity of the green Er3+ band dramatically decreased, disclosing that this sensitizer-rich shell is prone to quenching by H2O molecules. As the Er3+ ions were confined in the core, far from the quenching environment in our nanostructures, the reasonable explanation to the solvent quenching phenomena is that the Yb3+ excited state can be directly quenched by solvent vibrations, which is also consistent with other reports.2,4,19−21 More direct evidence for solvent quenching of the Yb3+ 2F5/2 state is provided by our recent observations on emission of Yb3+ in NaYF4 nanoparticles solely doped with Yb3+, as described below. NaYF4:18% Yb3+ nanoparticles were synthesized following a previously reported protocol (Figure 1b).22 The oleic acid capping of the UCNPs was removed using an acid-wash method reported previously,23 after which the UCNPs were redispersed in D2O/H2O solutions with the same nanoparticle concentration but varying the volume ratio of H2O. As shown in Figure 1d, with increasing H2O amounts, a significant

energy transfer rate from donor D to acceptor A may be expressed as11 k nr =

1 9000(ln 10)κ 2 τD 128π 5n 4NAR i6

∫0

∞

ID(∼ ν )ϵA (∼ ν )∼ ν −4 d∼ ν

(1)

where τD is the lifetime of the donor; n is the refractive index of the medium; NA is Avogadro’s number; Ri is the distance between donor and acceptor; κ2 is the orientational factor between the emission dipole moment of the donor and the excitation dipole moment of the acceptor; ID is the luminescence spectrum of the donor; ν̃ is the wavenumber in cm−1; and ϵA is the spectrum of the molar decimal vibrational absorption coefficient of the molecular group that overlaps with the donor luminescence spectrum. It is clearly shown that the decisive parameter for nonradiative FRET is the integral in eq 1. Note that ϵA contains contributions of not only fundamental vibrations but also overtone and combination vibrations.11 Quantum mechanically, overtone intensity originates from a combination of mechanical anharmonicity and electrical anharmonicity of the molecular potential, where the mechanical anharmonicity is more important than the electrical anharmonicity, especially at higher overtone levels.17 As the empirical rule of thumb, the first overtone is typically 100 times weaker than the fundamental, and each successive overtone is roughly a factor of 10 times weaker.17 Even so, the contribution of overtone transitions is prominent in situations where the energy gap is significantly larger than the highest 10573


ACS Nano


decrease of the emission originating from the Yb3+ 2F5/2 state is observed. Even if the mechanism proposed by the authors plays a role, i.e., that undercoordinated Yb3+ centers at the NC surface could provide quenching pathways for the Yb3+ 2F5/2 state, our experimental results, obtained with the same NCs (the same amount of undercoordinated Yb3+ centers at the NC surface) in different solvents, reveal that direct solvent quenching to the Yb3+ 2F5/2 state is significant. It should be noted that the variation of Yb3+ emission intensity cannot be explained by the difference in refractive indices of the solvent (D2O vs D2O/H2O mixture) because D2O and H2O have very similar refractive indices.24 Similarly, in ref 1, the solvent quenching of the Er3+ 4I13/2 state is ignored, due to the large energy gap (∼6572 cm−1) between this state to the ground state (4I15/2), which is significantly higher than the energy of the highest solvent vibrations. Note here that overtone vibrational transition-induced solvent quenching of the Er3+ 4 I13/2 state could also prevail, similar to the quenching of the Yb3+ 2F5/2 state. In order to verify this conjecture, NaYF4:20% Er3+ was synthesized and postprocessed to remove oleic acid capping (Figure 1c),22,23 and the emission originating from the Er3+ 4I13/2 state under excitation of 980 nm laser light was observed in different solvents. As shown in Figure 1e, with increasing amount of H2O, the Er3+ 4I13/2 emission gets dramatically quenched, even more significantly than the emission of the Yb3+ 2F5/2 state (Figure 1f). It is notable that laser light attenuation due to increased amounts of H2O, which has an absorption cross section at 980 nm that is larger than that of D2O,25 cannot account for such a remarkable decrease of the Yb3+ and Er3+ emission (Figure 1f). It is the difference between the vibration modes of O−H and O−D that makes the solvent quenching significant or negligible.23 In order to further elucidate the H2O quenching effect, we measured the luminescent decay curves of Yb3+ and Er3+ emissions of ligandfree NaYF4:18% Yb3+ and NaYF4:20% Er3+ nanocrystals dispersed in a H2O/D2O mixture with different H2O volume ratio, as shown in Figure 1g,h. The lifetimes of Yb3+ and Er3+ emissions were reduced with the increase of H2O content, indicating strong nonradiative energy transfer from Yb3+ and Er3+ ions to H2O molecules. It is worthwhile mentioning that the lifetime of Er3+ emission is more sensitive to water molecules than that of Yb3+ emission, which can be ascribed to a smaller energy gap between the Er3+ 4I13/2 and 4I15/2 states than that between the Yb3+ 2F5/2 and 2F7/2 states. This is also in accordance with the trend of luminescence intensity change shown in Figure 1d,e. Severe solvent quenching of the Er3+ 4 I13/2 state contributes to the relatively low brightness of Er3+ singly doped UC NCs. In ref 1, the authors investigated excited-state dynamics of UC NCs only in organic solvents, including hexane, octane, cyclohexane, chloroform, toluene, chlorobenzene, o-dichlorobenzene, but not in solvents containing O−H stretch. Even so, the authors did realize the importance of O−H stretch in quenching lanthanide excited states. They proposed that O−H vibrations of hydroxyl ions incorporated on F− sites in NaYF4 nanocrystal provide an additional quenching process to the NIR emitting Er3+ level (4I11/2) and that significant improvement in upconversion efficiency may be achieved by using alternative synthesis strategies that prevent incorporation of OH− in the NaYF4 lattice to suppress the quenching of the NIR emitting level. This suggestion has been recently proved experimentally.26 However, the authors did not point out the contribution of overtone transitions of O−H stretch to the

quenching of multiple lanthanide excited states, which we think should be emphasized, but considered only the fundamental transition-induced quenching of a single excited state, i.e., the Er3+ 4I11/2 state (4I11/2 → 4I13/2). In addition, in the organic solvents that the authors studied, we speculate that solvent quenching of Yb3+ 2F5/2 state and Er3+ 4I13/2 state due to overtone transitions of highest-energy vibrations, i.e., C−H stretch, should also be very significant, as the C−H stretch vibrational energy, ∼2930 cm−1 in aliphatic molecules and ∼3070 cm−1 in aromatic molecules, is close to that of O−H stretch (∼3500 cm−1). It is notable that overtone transitions typically have broader bands than the fundamental ones,27 as illustrated in Figure 1a, which makes the resonance requirement for overtone vibrational transition easily fulfilled. In all, we think that the authors of ref 1 correctly considered the dependence of the rate of energy transfer on the distance between ion and deactivating groups but not the energy gap law.11 They considered only resonant solvent quenching between solvent fundamental vibration modes and electronic transitions of the Er3+ ion but ignored the solvent quenching effect to the Yb3+ 2F5/2 state and the Er3+ 4I13/2 state, two important intermediary states for UC luminescence, via overtone vibrational transitions. Direct solvent quenching by overtone transitions to these key NIR states should be taken into account in order to better understand the quenching dynamics in UC NCs.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Qiuqiang Zhan: 0000-0002-5886-3795 Haichun Liu: 0000-0002-1780-7746 ACKNOWLEDGMENTS This work was supported by a Starting Grant (2016-03804) from the Swedish Research Council (Vetenskapsrådet) to H.L. Q.Z. acknowledges the grants from the National Natural Science Foundation of China (61675071, 61405062), the Natural Science Fund of Guangdong province (2018B030306015, 2014A030313445), the Pearl River Nova Program of Guangzhou (201710010010), and the Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (Grant No. 2017B030301007). REFERENCES (1) Rabouw, F. T.; Prins, P. T.; Villanueva-Delgado, P.; Castelijns, M.; Geitenbeek, R. G.; Meijerink, A. Quenching Pathways in NaYF4:Er3+,Yb3+ Upconversion Nanocrystals. ACS Nano 2018, 12, 4812−4823. (2) Arppe, R.; Hyppänen, I.; Perälä, N.; Peltomaa, R.; Kaiser, M.; Würth, C.; Christ, S.; Resch-Genger, U.; Schäferling, M.; Soukka, T. Quenching of the Upconversion Luminescence of NaYF4:Yb3+,Er3+ and NaYF4:Yb3+,Tm3+ Nanophosphors by Water: the Role of the Sensitizer Yb3+ in Non-Radiative Relaxation. Nanoscale 2015, 7, 11746−11757. (3) Huang, K.; Liu, H.; Kraft, M.; Shikha, S.; Zheng, X.; Ågren, H.; Würth, C.; Resch-Genger, U.; Zhang, Y. A Protected ExcitationEnergy Reservoir for Efficient Upconversion Luminescence. Nanoscale 2018, 10, 250−259. (4) Pilch, A.; Würth, C.; Kaiser, M.; Wawrzyńczyk, D.; Kurnatowska, M.; Arabasz, S.; Prorok, K.; Samoć, M.; Strek, W.; Resch-Genger, U.; Bednarkiewicz, A. Shaping Luminescent Properties of Yb3+ and Ho3+ 10574


ACS Nano


Co-Doped Upconverting Core−Shell β-NaYF4 Nanoparticles by Dopant Distribution and Spacing. Small 2017, 13, 1701635− 1701635. (5) Medvedev, E. S. Towards Understanding the Nature of the Intensities of Overtone Vibrational Transitions. J. Chem. Phys. 2012, 137, 174307−174307. (6) Haas, Y.; Stein, G. Radiative and Nonradiative Pathways in Solutions. Excited States of the Europium(III) Ion. J. Phys. Chem. 1972, 76, 1093−1104. (7) Haas, Y.; Stein, G.; Würzberg, E. Temperature Effects on Radiative and Radiationless Transitions of Gd3+ in Solution. J. Chem. Phys. 1973, 58, 2777−2780. (8) Haas, Y.; Stein, G.; Würzberg, E. Radiationless Transitions in Solutions: Isotope and Proximity Effects on Dy3+ by C-H and C-N bonds. J. Chem. Phys. 1974, 60, 258−263. (9) Stein, G.; Würzberg, E. Energy Gap Law in the Solvent Isotope Effect on Radiationless Transitions of Rare Earth Ions. J. Chem. Phys. 1975, 62, 208−213. (10) Bischof, C.; Wahsner, J.; Scholten, J.; Trosien, S.; Seitz, M. Quantification of C−H Quenching in Near-IR Luminescent Ytterbium and Neodymium Cryptates. J. Am. Chem. Soc. 2010, 132, 14334−14335. (11) Sveshnikova, E. B.; Ermolaev, V. L. Inductive-Resonant Theory of Nonradiative Transitions in Lanthanide and Transition Metal Ions (Review). Opt. Spectrosc. 2011, 111, 34−34. (12) Hsu, H.-L.; Leong, R. K.; Teng, I. J.; Halamicek, M.; Juang, J.Y.; Jian, S.-R.; Qian, L.; Kherani, P. N. Reduction of Photoluminescence Quenching by Deuteration of Ytterbium-Doped Amorphous Carbon-Based Photonic Materials. Materials 2014, 7, 5643−5663. (13) Siebrand, W. Radiationless Transitions in Polyatomic Molecules. I. Calculation of Franck-Condon Factors. J. Chem. Phys. 1967, 46, 440−447. (14) Sveshnikova, E. B.; Ermolaev, V. L. Opt. Spektrosk. 1971, 30, 379. (15) Fö rster, T. Zwischenmolekulare Energiewanderung und Fluoreszenz. Ann. Phys. 1948, 437, 55−75. (16) Dexter, D. L. A. Theory of Sensitized Luminescence in Solids. J. Chem. Phys. 1953, 21, 836−850. (17) Lehmann, K. K.; Smith, A. M. Where Does Overtone Intensity Come from? J. Chem. Phys. 1990, 93, 6140−6147. (18) Faulkner, S.; Pope, S. J. A.; Burton-Pye, B. P. Lanthanide Complexes for Luminescence Imaging Applications. Appl. Spectrosc. Rev. 2005, 40, 1−31. (19) Hossan, M. Y.; Hor, A.; Luu, Q.; Smith, S. J.; May, P. S.; Berry, M. T. Explaining the Nanoscale Effect in the Upconversion Dynamics of β-NaYF4:Yb3+, Er3+ Core and Core−Shell Nanocrystals. J. Phys. Chem. C 2017, 121, 16592−16606. (20) Guo, S.; Xie, X.; Huang, L.; Huang, W. Sensitive Water Probing through Nonlinear Photon Upconversion of Lanthanide-Doped Nanoparticles. ACS Appl. Mater. Interfaces 2016, 8, 847−853. (21) Würth, C.; Kaiser, M.; Wilhelm, S.; Grauel, B.; Hirsch, T.; Resch-Genger, U. Excitation Power Dependent Population Pathways and Absolute Quantum Yields of Upconversion Nanoparticles in Different Solvents. Nanoscale 2017, 9, 4283−4294. (22) Li, Z.; Zhang, Y. An Efficient and User-Friendly Method for the Synthesis of Hexagonal-Phase NaYF4:Yb,Er/Tm Nanocrystals with Controllable Shape and Upconversion Fluorescence. Nanotechnology 2008, 19, 345606−345606. (23) Bogdan, N.; Vetrone, F.; Ozin, G. A.; Capobianco, J. A. Synthesis of Ligand-Free Colloidally Stable Water Dispersible Brightly Luminescent Lanthanide-Doped Upconverting Nanoparticles. Nano Lett. 2011, 11, 835−840. (24) Kedenburg, S.; Vieweg, M.; Gissibl, T.; Giessen, H. Linear Refractive Index and Absorption Measurements of Nonlinear Optical Liquids in the Visible and Near-Infrared Spectral Region. Opt. Mater. Express 2012, 2, 1588−1611. (25) Zhan, Q.; Qian, J.; Liang, H.; Somesfalean, G.; Wang, D.; He, S.; Zhang, Z.; Andersson-Engels, S. Using 915 nm Laser Excited

Tm3+/Er3+/Ho3+-Doped NaYbF4 Upconversion Nanoparticles for In Vitro and Deeper In Vivo Bioimaging without Overheating Irradiation. ACS Nano 2011, 5, 3744−3757. (26) Homann, C.; Krukewitt, L.; Frenzel, F.; Grauel, B.; Würth, C.; Resch-Genger, U.; Haase, M. NaYF4:Yb,Er/NaYF4 Core/Shell Nanocrystals with High Upconversion Luminescence Quantum Yield. Angew. Chem., Int. Ed. 2018, 57, 8765−8769. (27) Burberry, M. S.; Morrell, J. A.; Albrecht, A. C.; Swofford, R. L. Local Mode Overtone Intensities of C−H Stretching Modes in Alkanes and Methyl Substituted Benzenes. J. Chem. Phys. 1979, 70, 5522−5526.

10575


Overtone Vibrational Transition-Induced Lanthanide Excited-State

Recommend Documents