All fluorophores experience some form of quenching effects from oxygen,21 but certain dyes exhibit a greater sensitivity to oxygen. In particular, polycyclic aromatic hydrocarbons, metalloporphyrin and transition metal polypyridyls such as, pyrene, platinum, palladium, and ruthenium have been used extensively. Pyrenebyutic acid has previously been used for intracellular measurements of oxygen concentration in rat liver cells.22 However, there are number of factors that make these derivative unsuitable as an oxygen sensitive dye. For example, emission overlap with autofluorescence, poor chemical- and photostability,23 limited sensitivity in the physiological range24 and quenching caused by reactive oxygen species.25 Platinum and palladium based probes have phosphorescent decays allowing encapsulation which protects it from the applied environment. However, only single point measurements are collated due to the restriction in acquisition speed derived from the long lifetime decay.26,27 Ruthenium(II) complexes have been used for the detection and quantification of oxygen in-vitro28 and in-vivo27,29 and have lifetimes in the order of 600 ns making them suitable for TCSPC detection. The other advantages of ruthenium(II) complexes in homogeneous solution are the linear response to oxygen concentration,30 large Stokes shift which provides high photo-stability,31 and a lifetime longer than endogenous fluorophores and mono-exponential decay. Tris(2,2’-bipyridyl)ruthenium(II) chloride hexahydrate, , is the least likely to bind to protein and DNA,32 is hydrophilic, and inexpensive.