Search

Herein, a novel pH‐responsive phosphorescent probe based on cyclometalated iridium(III) complex is reported. To prevent oxygen quenching of phosphorescence and to improve the probe biocompatibility, the complex is covalently conjugated with a water‐soluble block‐copolymer that also increases its pH sensitivity. The resulting polymeric nanoprobe demonstrates a strong response of the phosphorescence lifetime onto pH variations in physiological range. Cellular experiments with Chinese hamster ovary (CHO‐K1) cells show the predominant internalization of the probe in acidified cell compartments, endosomes and lysosomes. The analysis of phosphorescence lifetime imaging microscopy data confirms applicability of the sensor for monitoring of intra‐ and extracellular pH in cell cultures. [ABSTRACT FROM AUTHOR]

The encapsulation and/or surface modification can stabilize and protect the phosphorescence bio‐probes but impede their intravenous delivery across biological barriers. Here, a new class of biocompatible rhenium (ReI) diimine carbonyl complexes is developed, which can efficaciously permeate normal vessel walls and then functionalize the extravascular collagen matrixes as in situ oxygen sensor. Without protective agents, ReI‐diimine complex already exhibits excellent emission yield (34%, λem = 583 nm) and large two‐photon absorption cross‐sections (σ2 = 300 GM @ 800 nm) in water (pH 7.4). After extravasation, remarkably, the collagen‐bound probes further enhanced their excitation efficiency by increasing the deoxygenated lifetime from 4.0 to 7.5 µs, paving a way to visualize tumor hypoxia and tissue ischemia in vivo. The post‐extravasation functionalization of extracellular matrixes demonstrates a new methodology for biomaterial‐empowered phosphorescence sensing and imaging. [ABSTRACT FROM AUTHOR]

Luminescent metal complexes are increasingly being investigated as emissive probes and sensors for cell imaging using what is traditionally termed fluorescence microscopy. The nature of the emission in the case of second- and third-row metal complexes is phosphorescence rather than fluorescence, as it emanates from triplet rather than singlet excited states, but the usual terminology overlooks the distinction between the quantum mechanical origins of the processes. In steady-state imaging, such metal complexes may be alternatives to widely used fluorescent organic molecules, used in exactly the same way but offering advantages such as ease of synthesis and colour tuning. However, there is a striking difference compared to fluorescent organic molecules, namely the much longer lifetime of phosphorescence compared to fluorescence. Phosphorescence lifetimes of metal complexes are typically around a microsecond compared to the nanosecond values found for fluorescence of organic molecules. In this contribution, we will discuss how these long lifetimes can be put to practical use. Applications such as time-gated imaging allow discrimination from background fluorescence in cells and tissues, while increased sensitivity to quenchers provides a means of designing more responsive probes, for example, for oxygen. We also describe how the technique of fluorescence lifetime imaging microscopy (FLIM) - which provides images based on lifetimes at different points in the image - can be extended from the usual nanosecond range to microseconds. Key developments in instrumentation as well as the properties of complexes suitable for the purpose are discussed, including the use of two-photon excitation methods. A number of different research groups have made pioneering contributions to the instrumental set-ups, but the terminology and acronyms have not developed in a systematic way. We review the distinction between time-gating (to eliminate background emission) and true timeresolved imaging (whereby decay kinetics at each point in an image are monitored). For instance, terms such as PLIM (phosphorescence lifetime imaging microscopy) and TRLM (time-resolved luminescence microscopy) refer essentially to the same technique, whilst TREM (time-resolved emission imaging microscopy) embraces these long timescale methods as well as the more well-established technique of FLIM. [ABSTRACT FROM AUTHOR]