Direct measurement of singlet oxygen by using a photomultiplier tube-based detection system

• We tested new photomultiplier tube (PMT) based system.
• New PMT-based system is sensitive and reliable for measuring 1O2 concentrations.
• New PMT-based system can also measure 1O2 concentrations in vivo.
• This new 1O2 detection system can be used for dosimetry in photodynamic therapy.

The effective dosimetry for photodynamic therapy (PDT) can be specified by direct measurement of singlet oxygen (1O2) production. The purpose of this study was to investigate the feasibility of a newly developed photomultiplier tube (PMT)-based singlet oxygen detection (SOD) system. The lowest and highest 1O2 concentrations detectable by the PMT-SOD system were 15 nM and 10 μM, respectively. Dose-dependent quenching, by NaN3, of the fluorogenic reaction was observed, which was negatively correlated with the 1O2 level measured by the PMT-SOD system. The lifetime of 1O2, as measured by the PMT-SOD system, was found to be lengthened when H2O was replaced with deuterium oxide. 1O2 photon counts were significantly and dose-dependently correlated with intracellular fluorescence intensity after photosensitizer treatments. In vitro cell viability test and in vivo xenografted-tumor mass shrinkage showed a positive association between PDT-induced cytotoxicity and 1O2 production concomitantly measured by the PMT-SOD system. It was concluded that the PMT-SOD system is capable of measuring 1O2 production directly and accurately, demonstrating that this system can be useful in the determination of dosimetry for PDT.