To understand better the structure and function of biological systems, cell biologists and biochemists would like to have methods that minimally perturb living systems. The development of emissive optical probes is essential for improving our observation of intracellular signaling and recognition processes. Following excitation of the probe, photons emitted from the probe may be observed by spectroscopy or microscopy and encode information about their environments in their energy, lifetime, and polarization. Such optical probes may be based on organic fluorophores, quantum dots, recombinant proteins, or emissive metal complexes. In this Account, we trace the emergence of lanthanide coordination complexes as emissive optical probes. These probes benefit from sharp emission bands and long lifetimes. We can design these complexes to report on the concentration of key biochemical variables by modulation of spectral form, lifetime, or circular polarization. These properties allow us to apply ratiometric methods of analysis in spectroscopy or microscopy to report on local pH, pM (M = Ca, Zn), or the concentration of certain anionic metabolites, such as citrate, lactate, bicarbonate, or urate. For optical microscopy studies in living cells, these probes must be cell-permeable and, ideally, should localize in a given cell organelle. We undertook systematic studies of more than 60 emissive complexes, examining the time dependence of cellular uptake and compartmentalization, cellular toxicity, protein affinity, and quenching sensitivity. These results and their relationship to probe structure have allowed us to identify certain structure-activity relationships. The nature and linkage mode of the integral sensitizing group-introduced to harvest incident light efficiently-is of primary importance in determining protein affinity and cellular uptake and trafficking. In many cases, uptake may occur via macropinocytosis. We have defined three main classes of behavior: complexes exhibit predominant localization profiles in protein-rich regions (nucleoli/ribosomes), in cellular mitochondria, or in endosomes/lysosomes. Therefore, these systems offer considerable promise as intracellular optical probes, amenable to single- or two-photon excitation, that may report on the local ionic composition of living cells subjected to differing environmental stresses.

Beyond the more common chemical delivery strategies, several physical techniques are used to open the lipid bilayers of cellular membranes. These include using electric and magnetic fields, temperature, ultrasound or light to introduce compounds into cells, to release molecular species from cells or to selectively induce programmed cell death (apoptosis) or uncontrolled cell death (necrosis). More recently, molecular motors and switches that can change their conformation in a controlled manner in response to external stimuli have been used to produce mechanical actions on tissue for biomedical applications. Here we show that molecular machines can drill through cellular bilayers using their molecular-scale actuation, specifically nanomechanical action. Upon physical adsorption of the molecular motors onto lipid bilayers and subsequent activation of the motors using ultraviolet light, holes are drilled in the cell membranes. We designed molecular motors and complementary experimental protocols that use nanomechanical action to induce the diffusion of chemical species out of synthetic vesicles, to enhance the diffusion of traceable molecular machines into and within live cells, to induce necrosis and to introduce chemical species into live cells. We also show that, by using molecular machines that bear short peptide addends, nanomechanical action can selectively target specific cell-surface recognition sites. Beyond the in vitro applications demonstrated here, we expect that molecular machines could also be used in vivo, especially as their design progresses to allow two-photon, near-infrared and radio-frequency activation.

Authenticating products and documents with security inks is vital to global commerce, security and health. Lanthanide complexes are widely used in luminescent security inks due to their unique and robust photophysical properties. Lanthanide complexes can also be engineered to undergo circularly polarised luminescence (CPL), which encodes chiral molecular fingerprints in luminescence spectra that cannot be decoded by conventional optical measurements. However, chiral CPL signals have not yet been exploited as an extra security layer in advanced security inks. This Review introduces CPL and related concepts that are necessary to appreciate the challenges and potential of lanthanide-based CPL-active security inks. We describe recent advances in CPL analysis and read-out technologies that have expedited CPL-active security ink applications. Further, we provide a systematic meta-analysis of strongly CPL-active Eu III , Tb III , Sm III , Yb III , Cm III , Dy III and Cr III complexes, discussing the suitability of their photophysical properties and highlighting promising candidates. We conclude by providing key recommendations for the development and advancement of the field.

The synthesis, structure and photophysical properties of a series of highly emissive europium complexes is reported. Certain complexes enter mammalian cells by

Nine very bright europium(III) complexes with different macrocyclic ligands have been prepared that exhibit excellent cell uptake behaviour and distinctive sub-cellular localisation profiles, allowing the use of fluorescence microscopy and time-gated spectral imaging to track their fate in cellulo. Their use as cellular imaging stains is described for the selective illumination of mitochondria, lysosomes or the endoplasmic reticulum of various mammalian cell types.