Volume 54 Number 72 16 September 2018 Pages 9999–10210 ChemComm Chemical Communications rsc.li/chemcomm ISSN 1359-7345 FEATURE ARTICLE K. Eszter Borbas et al. Lanthanide-based tools for the investigation of cellular environments ChemComm View Article Online FEATURE ARTICLE View Journal | View Issue Lanthanide-based tools for the investigation of cellular environments Cite this: Chem. Commun., 2018, 54, 10021 Emilie Mathieu, Agne`s Sipos, † Ellen Demeyere,‡ Dulcie Phipps,§ Dimitra Sakaveli¶ and K. Eszter Borbas * Biological probes constructed from lanthanides can provide a variety of readout signals, such as the luminescence of Eu(III), Tb(III), Yb(III), Sm(III)andDy(III), and the proton relaxation enhancement of Gd(III)and Eu(II). For numerous applications the intracellular delivery of the lanthanide probe is essential. Here, we review the methods for the intracellular delivery of non-targeted complexes (i.e. where the overall complex structure enhances cellular uptake), as well as complexes attached to a targeting unit (i.e. to a peptide or a small Received 1st July 2018, molecule) that facilitates delivery. The cellular applications of lanthanide-based supramolecules (dendrimers, Accepted 30th July 2018 metal organic frameworks) are covered briefly. Throughout, we emphasize the techniques that can confirm DOI: 10.1039/c8cc05271a the intracellular localization of the lanthanides and those that enable the determination of the fate of the Creative Commons Attribution-NonCommercial 3.0 Unported Licence. probes once inside the cell. Finally, we highlight methods that have not yet been applied in the context of rsc.li/chemcomm lanthanide-based probes, but have been successful in the intracellular delivery of other metal-based probes. Introduction Probes based on lanthanide(III) ions are ubiquitous in the life sciences. The luminescence of Eu (red) and Tb (green), and, Department of Chemistry, Ångstro¨m Laboratory, Uppsala University, Box 523, increasingly, Sm (orange), Dy (yellow), Yb and Nd (both near This article is licensed under a 75120, Uppsala, Sweden. E-mail: [email protected] infrared; NIR) makes them excellent tools for spectroscopy or † Exchange MSc student from De´partement de chimie, E´cole normale supe´rieure, microscopy. Highly paramagnetic Gd(III) with its 7 unpaired PSL University, 75005 Paris, France electrons and isoelectronic Eu(II) are used as MRI contrast ‡ Exchange BSc student from the Ghent University, Belgium 1–6 agents, although Eu(II) has interesting, and so far little Open Access Article. Published on 31 July 2018. Downloaded 11/29/2018 11:53:10 AM. § Erasmus student from the University of Glasgow, Scotland 7–10 ¶ Erasmus student from the University of Crete, Greece explored photophysics. Emilie Mathieu was born in Agne`s Sipos was born in Meudon Echirolles, France. She obtained (France) in 1996. She obtained her MSc in Chemistry in 2014 her BSc degree in 2017 from the (Sorbonne University and the Ecole Normale Supe´rieure in Ecole Normale Supe´rieure, Paris, Paris after an internship in France); she worked on the Christelle Hureau’s group in synthesis of gentiopicroside Toulouse, where she did research (Prof. Serge Thorimbert, Dr Luc on metal in a sperm peptide. She Dechoux). She then worked on the joined the Borbas group for an role of copper in Alzheimer’s internship during her 1st year of disease (Prof. Tim Storr, master’s. She is interested in Vancouver, Canada). During her inorganic chemistry and its Emilie Mathieu PhD, which she obtained in Agne`s Sipos applications to biology. She is September 2017, she studied preparing for the competitive manganese complex superoxide dismutase mimics and their teachers’ examination before cellular behavior under the supervision of Prof. Clotilde Policar finishing her master’s degree. and Dr Nicolas Delsuc. She is currently a postdoctoral fellow at Uppsala University, working on lanthanide coordination chemistry. This journal is © The Royal Society of Chemistry 2018 Chem. Commun., 2018, 54, 10021--10035 | 10021 View Article Online Feature Article ChemComm 13 Lanthanide luminescence is due to 4f–4f transitions. The lanthanide (kET c klum). The efficiency of the energy transfer f-electrons are buried and do not participate in bonding is dependent on the difference between the energy levels of the interactions with the ligand. Therefore, crystal field effects excited states of the ligand and the lanthanide. Quenching are small, and absorption and emission bands are sharp.11 mechanisms include back energy transfer (BET, most often The emission spectra of the lanthanide ions most often used in for Tb), photo-induced electron transfer (PeT, for the more 14,15 cellular imaging are shown in Fig. 1. The f–f transitions are reducible lanthanide(III) ions ), or coupling to X–H forbidden by the Laporte selection rule, which can be relaxed by oscillators (X = O, N, C).16,17 Lanthanide emitter design has vibronic coupling or mixing with other orbitals. As a consequence, been covered in several recent reviews.18,19 lanthanides have very weak molar extinction coefficients Lanthanide luminescence is characterized by long lifetimes (B1 L molÀ1 cmÀ1),12 which implies that direct excitation of (ms (Yb, Nd) to ms (Eu, Tb)) compared to that of organic the Ln3+ ion will lead to very weak emission. This limitation is fluorophores (ns).20 Large pseudo-Stokes shifts are observed, overcome by using a light-harvesting ‘antenna’, i.e. photo- because the excitation wavelength of the antenna is usually in sensitizer in the close vicinity of the metal ion, usually coupled the 320–400 nm range, while that of the lanthanide emission is to the ligand. Upon excitation, the antenna transfers its energy between 500 and 1500 nm. This makes lanthanide emitters less to the excited state of the Ln3+ ion, which can subsequently prone to auto-quenching. Furthermore, these complexes are emit light (Fig. 1). The rate constant of the energy transfer more photostable than organic fluorophores. between the antenna and the lanthanide ion is much higher Lanthanide luminescence is tailor-made for multiplex compared to the luminescence emission lifetime of the imaging. This includes not just the simultaneous detection of Ellen Demeyere was born in 1997 Dulcie Phipps was born in Creative Commons Attribution-NonCommercial 3.0 Unported Licence. in Ghent, Belgium. She earned Birmingham, England, but moved her BSc in chemistry in 2018 to Scotland when she was two from Ghent University after years old and has lived there all having completed an Erasmus her life. She is an undergraduate exchange at Uppsala University, studentattheUniversityof Sweden, where she worked in the Glasgow, with interest in nano- research group of Eszter Borbas science and in particular coordi- on the design and synthesis of nation chemistry. After graduating luminescent lanthanide complexes. from her final year in 2019, Dulcie This article is licensed under a She will start her MSc studies in plans to find a PhD outside of chemistry at Ghent University in the the UK. Ellen Demeyere fall of 2018 with focus on organic Dulcie Phipps Open Access Article. Published on 31 July 2018. Downloaded 11/29/2018 11:53:10 AM. chemistry. Dimitra Sakaveli was born in Eszter Borbas grew up in Hungary Heraklion of Crete, Greece. and in Yemen. After completing Dimitra completed her BSc in her undergraduate studies at 2017 at the Department of Eo¨tvo¨s University in Budapest Chemistry at the University of with research work in Jo´zsef Crete, after completing an under- Ra´bai’s group, she did her PhD graduate thesis in the Laboratory at the Open University (with of Bioinorganic Chemistry under James Bruce), and she was a the supervision of Professor A. postdoc at North Carolina State Coutsolelos. In 2018 she worked University (with Jonathan in the Borbas lab at Uppsala Lindsey), and a Marie Curie University as part of a three fellow at Stockholm University Dimitra Sakaveli month Erasmus+ internship. K. Eszter Borbas in Sweden. She started her She is interested in materials independent career in 2010 as a chemistry and medical applications. Next year she will start an Swedish Research Council assistant professor. She is currently an MSc in the Medical School at the University of Crete. associate professor at Uppsala University, where her research group works on lanthanide chemistry and photophysics, and on tetrapyrrole chemistry. 10022 | Chem. Commun., 2018, 54, 10021--10035 This journal is © The Royal Society of Chemistry 2018 View Article Online ChemComm Feature Article (3) The probes must be able to enter target cells, and must be able to detect their target analytes. Here, we will focus on the third aspect of probe design. We will present current strategies for delivering a lanthanide payload into cells, with an additional focus on the techniques that can confirm the internalization and the cellular localiza- tion of the metals. While we will focus on luminescent lanthanides, where appropriate, Gd(III) MRI contrast agents will be discussed. We will also briefly look at methods to target lanthanide complexes to the outside cell membranes. There are a number of relatively recent reviews on areas that partially overlap with the current topic: luminescent lanthanide labels in microscopy,22 lanthanide probes for detecting protein–protein interactions,23 responsive Eu-based probes,24 and the design of targeting extracellular receptors.25 Imaging techniques The luminescent properties of lanthanide complexes are highly valuable to study their mechanism(s) of internalization in living cells. Cell penetration of these complexes can be followed in real time in living cells and give insight into uptake pathways 26 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. and kinetics. Their long emission lifetimes limit signal contamination by endogenous chromophores (e.g. NAD(P)H, flavine, Trp, Tyr) that have nanosecond lifetimes.20 Imaging techniques that take advantage of the photo- physical properties of the lanthanides have been developed. Time-resolved fluorescence microscopy has been optimized to match the requirements of short and intense excitation pulses, tuneable delay times, and tuneable detection windows (Fig.