Determining the oxygen concentration is important in various fields of chemical, clinical analysis and environmental monitoring. Traditionally, oxygen concentration measurements are based on the Clark electrode and the Winkler titration approach which suffer from drawbacks such as oxygen consumption during sensing process, long response times and the tendency of electrodes to be poisoned by sample constituents (e.g., H2S, proteins, and certain anesthetics). In addition, both techniques necessitate the use of sophisticated instrumentations and require complicated pretreatment procedures, not suitable for on-line or in-field monitoring. Therefore, in the last decades, the focus of research has shifted to the construction of new oxygen sensing systems. Oxygen is a powerful quencher of the emission intensity and excited state lifetime of some luminescent complexes. Optical oxygen sensors based on photoluminescence quenching have gained increasing attention as a superior method for continuous monitoring of oxygen. Among them, coordination complexes have been successfully applied in optical oxygen sensing by virtue of their excellent performance.
Quenchmetric O2 sensors are based on the bimolecular quenching of excited states. The basic processes are:
D + hν → D* excitation
D* → D + hν or ∆
D* + O2 → D + *1O2
luminescence or deactivation
(where "*" denotes an excited state and D denotes luminescent coordination complexes)
For O2, the primary quenching path is energy transfer to form singlet O2. Any bimolecular processes with the analyte that deactivate the excited state will reduce emission intensity and τ.
Oxygen quenching is diffusion-limited and can be described by the Stern-Volmer relationship. It is valid for both phosphorescence and fluorescence lifetimes and intensities:
I0/I = τ0/τ = 1 + KSV[O2] = 1 + κτ0 [O2]
where I and τ are the luminescence intensity and excited state lifetime of the luminophore, respectively, the subscript 0 denotes the absence of oxygen, KSV is the Stern-Volmer constant, κ is the bimolecular quenching rate constant, and [O2] is the oxygen concentration. Because of their long τ's, large shifts between excitation and emission, and efficient luminescences, many coordination complexes play an important role in optical oxygen sensing.
Various luminescent coordination complexes have been synthesized and used as indicators for optical oxygen sensing. Among these indicators, metals including Fe (II), Pt (II), Pd (II), Ru (II), and Os (II) have been utilized. New coordination complexes for optical oxygen sensing are still being discovered. For example, a cyclometallated Ir (III) complex tethered with coumarin-343(C343) fluorophore has been investigated as a ratiometric sensor for monitoring the O2 levels in living cells and tissues . In their design, the C343 fluorophore was linked to the phosphorescent Ir (III) complex using a tetraproline linker to avoid direct contact between the two luminophores. Excitation of the C343 moiety at 405 nm results in partial energy transfer to the Ir (III) complex by a single electron transfer mechanism, producing both blue fluorescence from the C343 moiety and red phosphorescence from the Ir (III) complex. However, upon the addition of O2, the red phosphorescence emission was completely quenched, while the blue fluorescence remained unaffected. The phosphorescence quenching was oxygen-concentration dependent in solution.
Figure 1. Ratiometric coumarin–Ir (III) complex for O2 sensing in living cells
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- Tobita, S.; et al. Ratiometric molecular sensor for monitoring oxygen levels in living cells. Angew. Chem. Int. Ed. 2012, 51: 4148.