Oxygen detection lights up
Highly selective chemiluminescent probes for singlet oxygen separate the trapping and detection processes by forming stable dioxetane derivatives.
Singlet oxygen (1O2) is the lowest excited state of the dioxygen molecule, with a lifetime of about 4?sec in water. It is classified as a highly reactive oxygen species (ROS), acting as a cytotoxin in living systems and reacting with organic pollutants in aqueous environments.
Several types of molecular probes exist for trapping 1O2, but the absorbance-based type suffer from relatively poor sensitivity while chemiluminescent probes emit light spontaneously on reaction with 1O2, or are not selective as they also react with other ROS. These limitations have been overcome by researchers at the University of Minnesota, US, who have developed a trap-and-trigger method that permits 1O2 molecules to be trapped for later analysis.
A spiroadamantyl-substituted vinyl ether (2-[1-[(3-tert-butyl-dimethylsilyloxy)phenyl]-1-methoxymethylene]adamantane) (TPMA) and its tetraglycol analogue were employed as probes. On reaction with 1O2, a thermally stable dioxetane derivative was formed in both cases, that from TPMA being stable in solution for up to one day at room temperature and for several months at 0°C. On reaction with tetrabutylammonium fluoride, desilylation of the dioxetane occurred in a chemiluminescent reaction.
The reactivity of TPMA was low compared with typical absorbance-based probes like furfuryl alcohol but could be improved by replacing the methyl group of the vinyl ether with a stronger electron-donating group. However, the poor reactivity was countered by strong luminescence activity and the high selectivity for 1O2, even in the presence of the superoxide anion and hydrogen peroxide.
The system was demonstrated by confirming the formation of 1O2 from the reaction of dibenzoyl peroxide with the superoxide anion and illustrating that the reaction followed second-order kinetics.
Steady-state concentrations of 1O2 down to 5 × 10-13M, particularly suitable for environmental systems, can be measured accurately with an exposure time of 10 minutes. 'The work has been extended to surface water samples from Lake Superior,' says group leader Kris McNeill, where the probes appear to associate with the natural organic matter to give enhanced signals, a phenomenon under further investigation.
L A MacManus-Spencer et al, Anal. Chem., 2005 (DOI: 10.1021/ac048293s)