Recent startling interest for lanthanide luminescence is stimulated by the continuously expanding need for luminescent materials meeting the stringent requirements of telecommunication, lighting, electroluminescent devices, (bio-)analytical sensors and bio-imaging set-ups. This critical review describes the latest developments in (i) the sensitization of near-infrared luminescence, (ii) “soft” luminescent materials (liquid crystals, ionic liquids, ionogels), (iii) electroluminescent materials for organic light emitting diodes, with emphasis on white light generation, and (iv) applications in luminescent bio-sensing and bio-imaging based on time-resolved detection and multiphoton excitation (500 references).

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Lanthanide luminescence for functional materials and bio-sciences

Doping is a widely applied technological process in materials science that involves incorporating atoms or ions of appropriate elements into host lattices to yield hybrid materials with desirable properties and functions. For nanocrystalline materials, doping is of fundamental importance in stabilizing a specific crystallographic phase, modifying electronic properties, modulating magnetism as well as tuning emission properties. Here we describe a material system in which doping influences the growth process to give simultaneous control over the crystallographic phase, size and optical emission properties of the resulting nanocrystals. We show that NaYF(4) nanocrystals can be rationally tuned in size (down to ten nanometres), phase (cubic or hexagonal) and upconversion emission colour (green to blue) through use of trivalent lanthanide dopant ions introduced at precisely defined concentrations. We use first-principles calculations to confirm that the influence of lanthanide doping on crystal phase and size arises from a strong dependence on the size and dipole polarizability of the substitutional dopant ion. Our results suggest that the doping-induced structural and size transition, demonstrated here in NaYF(4) upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.

Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping

Lanthanide ions exhibit unique luminescent properties, including the ability to convert near infrared long-wavelength excitation radiation into shorter visible wavelengths through a process known as photon upconversion. In recent years lanthanide-doped upconversion nanocrystals have been developed as a new class of luminescent optical labels that have become promising alternatives to organic fluorophores and quantum dots for applications in biological assays and medical imaging. These techniques offer low autofluorescence background, large anti-Stokes shifts, sharp emission bandwidths, high resistance to photobleaching, and high penetration depth and temporal resolution. Such techniques also show potential for improving the selectivity and sensitivity of conventional methods. They also pave the way for high throughput screening and miniaturization. This tutorial review focuses on the recent development of various synthetic approaches and possibilities for chemical tuning of upconversion properties, as well as giving an overview of biological applications of these luminescent nanocrystals.

Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals

Keywords: Time-Resolved Fluorescence ; Resonance Energy-Transfer ; Near-Infrared Luminescence ; Double-Stranded Dna ; Prostate-Specific Antigen ; Photoinduced Electron-Transfer ; Europium-Tetracycline Complex ; Sybr-Green-I ; Terbium Complexes ; Optical Probes Reference EPFL-ARTICLE-149396doi:10.1021/cr900362eView record in Web of Science Record created on 2010-06-17, modified on 2017-05-12

Lanthanide Luminescence for Biomedical Analyses and Imaging

Sasidharan Swarnalatha Lucky,†,§ Khee Chee Soo,‡ and Yong Zhang*,†,§,∥ †NUS Graduate School for Integrative Sciences & Engineering (NGS), National University of Singapore, Singapore, Singapore 117456 ‡Division of Medical Sciences, National Cancer Centre Singapore, Singapore, Singapore 169610 Department of Biomedical Engineering, Faculty of Engineering, National University of Singapore, Singapore, Singapore 117576 College of Chemistry and Life Sciences, Zhejiang Normal University, Zhejiang, P. R. China 321004

Nanoparticles in photodynamic therapy.

Novel materials doped with trivalent lanthanides and transition metal ions showing near-infrared to visible photon upconversion

The expected excitation power dependencies for any upconversion emission band of an acceptor ion is investigated theoretically when the excitation takes place on a sensitizer ion and subsequent energy transfer upconversion from the sensitizer to the acceptor ion is exclusively responsible for the excitation of the acceptor ion. Under these limitations it is shown that emission from a state that requires $k$ energy transfer upconversion steps will have a slope of $k$ in the low-power regime when the luminescence intensity is plotted in a double-logarithmic representation versus absorbed pump intensity. In the high-power regime, any emission band will show a slope of 1, irrespective of the number of energy transfer steps from the sensitizer to the acceptor ions that are involved. The theoretical results are verified experimentally by data on three different inorganic systems with different types of sensitizer and acceptor ions: rare earth (RE) ions as well as transition metal (TM) ions. The active ions in the systems that are studied experimentally are RE/RE, RE/TM, and TM/TM, where the first dopant indicates the sensitizer ion and the second dopant indicates the upconverting ion. These different classes of sensitizer and upconverter ions all agree with the theoretical predictions put forward by the model. Thus providing confidence in the applicability (within the boundary conditions put forward here) of the model described.

Anomalous power dependence of sensitized upconversion luminescence

The efficiency with which the surroundings of trivalent lanthanide ions sensitize their luminescence (ηsens) is a key parameter in the design of highly emitting molecular edifices and materials. Evaluation of ηsens requires the measurement of the overall and intrinsic quantum yields obtained upon ligand and metal excitation, respectively. We describe a modified integration sphere enabling absolute determination of these quantities on small amounts of solid samples or solutions (60 μL). The sphere is tested for linear response of emitted versus absorbed light intensities with increasing concentration of Cs3[Ln(dpa)3] solutions (Ln = Eu, Tb). The overall (QEuL = 29 ± 2%) and intrinsic (QEuEu = 41 ± 2%) quantum yields obtained for Eu allow the direct calculation of ηsens (71 ± 6%) while the radiative lifetime (τrad = 4.1 ± 0.3 ms) is calculated from QEuEu and the observed lifetime. The intrinsic quantum yield matches the value extracted from emission parameters using the simplified equation proposed by Werts et al. but, on the other hand, the theoretical estimate using spontaneous transition probabilities calculated from Judd–Ofelt (JO) parameters is off by −25% (3.15 ms). In the case of Cs3[Tb(dpa)3], the molar absorption coefficient of the 5D4←7F6 transition is too small to measure QTbTb for the solution but this quantity could be determined for the microcrystalline sample (72 ± 5%, τrad = 1.9 ± 0.1 ms). In this case, the JO theoretical estimate leads to a much too short τrad value. The large difference in ηsens for microcrystalline samples of Eu (85%) and Tb (42%) tris(dipicolinates) is attributed to back energy transfer in the latter compound consecutive to a sizeable overlap between the 5D4→7F6 emission and the absorption spectrum of the dipicolinate triplet, this overlap being smaller in the case of the solution. The overall quantum yield of Na3[Yb(dpa)3] in aqueous solution is very low (0.015 ± 0.002%) due to both poor sensitization efficiency (8%) and small intrinsic quantum yield (QYbYb = 0.178 ± 0.003%; τrad = 1.31 ± 0.02 ms). For evaluating intrinsic quantum yields of Yb in aqueous solutions of coordination compounds from lifetimes, a value of 1.2–1.3 ms is recommended.

Intrinsic quantum yields and radiative lifetimes of lanthanide tris(dipicolinates)

Structural and Spectroscopic Characterization of Active Sites in a Family of Light-Emitting Sodium Lanthanide Tetrafluorides

Giving the green light: The connection of two CrIII sensitizers around a central ErIII acceptor in a self-assembled cation provides high local metal concentrations that favor efficient nonlinear energy transfer upconversion luminescence (see picture). Upon selective low-energy near-infrared irradiation of CrIII-centered transitions, 1 displays an unprecedented molecular two-photon upconverted green ErIII-centered emission. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Annina Aebischer

Papers

Novel materials doped with trivalent lanthanides and transition metal ions showing near-infrared to visible photon upconversion

Anomalous power dependence of sensitized upconversion luminescence

Intrinsic quantum yields and radiative lifetimes of lanthanide tris(dipicolinates)

Structural and Spectroscopic Characterization of Active Sites in a Family of Light-Emitting Sodium Lanthanide Tetrafluorides

Near‐Infrared→Visible Light Upconversion in a Molecular Trinuclear d–f–d Complex