Luminescent Metal Complexes in Bioimaging

Transition metal complex luminophores have emerged over the past decade as versatile probes for intracellular imaging and sensing. While complexes such as Ru(II) and Re(I) polypyridyl complexes have long been studied with biomaterials such as DNA, their application in cells or tissues has been limited by their poor membrane permeability. Fueled by the dramatic expansion in diversity in cell imaging technologies advanced over recent decades, in confocal fluorescence imaging, lifetime imaging, super-resolution, and vibrational imaging methods, there has been growing interest in metal complex luminophores as contrast agents. This is due to their attractive tunable, optical, and photophysical properties that can make them particularly amenable to challenging imaging methods. In this chapter, focusing on (d-block) transition metal luminophores, we examine the properties that make metal complex luminophores potentially valuable imaging and sensing probes in the context of modern optical microscopy methods and discuss the methods that have been applied to improve their permeability and targeting within the cellular environment and in tissue. Recent examples of their application across imaging methodologies are discussed as well as emerging applications in sensing and theranostic methods using microscopy. We examine the key probe demands in advanced microscopy methods such as stimulation depletion microscopy, two-photon microscopy, and resonance Raman imaging and examine how inorganic probes address these. Finally, their potential therapeutic capability, where they can fulfill the dual role of contrast agent and phototherapeutic in theranostic applications, is discussed, along with future probe developments that can be made to improve their performance.