Opt. Express 21, 5005–5013 (2013)

It is desirable to monitor the level of singlet oxygen 1O2, the excited state of molecular oxygen, owing to its importance in many biological systems. However, luminescence from 1O2 occurs only with a very low emission probability (10−8) and a short lifetime (<<1 μs), thus making optical detection difficult. Now, scientists in the UK, Canada and the Netherlands have developed a detection system based on a superconducting nanowire single-photon detector. In a proof-of-concept demonstration, they delivered a pulsed laser beam with a wavelength of 532 nm to the sample and collected the photoluminescence using a fibre-coupled NbTiN superconducting nanowire single-photon detector. Instead of a series of lenses and a dichroic beamsplitter, they used two separate delivery and collection fibres. The time-correlated signal through a bandpass filter centred at 1,270 nm showed a weak but definitive signal from 1O2 luminescence. They confirmed the results by adding a singlet oxygen quencher and a protein to a sample. The researchers believe that the system is a significant step towards realizing dose monitoring in clinical treatments such as photodynamic therapy.