Transition metal luminophores show significant potential as multifunctional probes with applications in many fields extending from medicinal chemistry to bioimaging and biosensing. Conjugation of Ru(II)/ Os(II) luminophores to signal peptides is a useful strategy to reliably deliver the probes intracellularly and to specific organelles such as the nucleus and mitochondria. While conventional microscopy methods are used for cellular-imaging, biosensing techniques, can range from fluorescence lifetime imaging microscopy (FLIM) to simple fluorescence intensity-based measurements. This thesis explores the design and photophysical properties of biocompatibleprobes for imaging and sensing within the cellular environment. Chapter 1 reviews the literature and background to the thesis. Chapter 2 describes an achiral Os(II) complex, [Os(tpybenzCOOH)2] 2+ conjugated to the mitochondrial targeting peptide, MPP, at two conjugation sites. The novel bis-MPP conjugate showed NIR emission coincident with the biological window and an interesting approach to theranostics was proposed whereby switch in cell death mechanism (dark toxicity) is reflected in specific probe localisation observed via confocal microscopy. Chapter 3 describes the preparation of two polyarginine-based Os(II) conjugates and their uptake in live cells. While confocal imaging revealed probe- membrane interactions, Os-bisR8 was not taken up by cells, attributed to the large arginine-chains resulting in a significantly high cationic charge. In order to balance lipophilicity and overall cationic charge, Os-bisR4 was prepared and shown to be taken up by A549 lung carcinoma cells. Chapter 4 presents the preparation and characterization of novel ratiometric Ru(II)/ BODIPY core- shell nanoparticles. Single excitation at 480 nm resulted in dual emission corresponding to the BODIPY and Ru(II) particle component. The poly-L-lysine coated particles were studied in two cancerous cell lines as oxygen sensors. The changes in oxygen levels under hypoxic conditions were monitored using xy-lamda (xyλ) scanning microscopy. Chapter 5 investigated a series of novel Ru(biq) 2 (trzbenzCOOH)-conjugates (NLS,R8,MPP,PEG) in CHO and HeLa cell lines. All conjugates exhibited dark cytotoxicity and the origin of toxicity was studied in detail using the mitochondrial depolarization and caspase activity assays. This chapterhighlighted the importance of balancing lipophilicity and targeting ability in order to reduce overall probe toxicity.