Singlet Oxygen luminescence detection with a fibre-coupled superconducting nanowire single-photon detector

Singlet oxygen (1O2), an excited state of the oxygen molecule, is a crucial intermediate in many biological processes. In the clinical domain, it is generated in most applications of photodynamic therapy (PDT) via photoactivation of a photosensitizing drug. Direct detection of the 1O2 luminescence at 1270 nm wavelength is extremely challenging, due to the high reactivity of 1O2 that results in very low emission probability (∼10−8) and short lifetime (<<1 μs) in biological media. About a decade ago, singlet oxygen luminescence dosimetry (SOLD) became technically feasible for the first time [1], using specialized PMTs with extended NIR sensitivity, but significant limitations remain. Superconducting nanowire single-photon detectors (SNSPDs) [2] offer high infrared single-photon sensitivity, combined with picosecond timing resolution, low dark count rates and free running operation. We report the first demonstration of SNSPDs for 1O2 luminescence detection [3]. We employed a fibre-coupled cavity-enhanced SNSPD with >20% practical detection efficiency at ∼1300 nm, representing a 20-fold improvement in efficiency over NIR PMTs used to date for SOLD. The singlet oxygen signature was isolated via time-resolved and spectrally-filtered measurements performed on Rose Bengal (RB) in solution as a model photosensitizer (fig. 1a). Quenching of the 1O2 luminescence was demonstrated using sodium azide (fig. 1b). Bovine serum albumen (BSA) was employed to simulate a proteinaceous biological environment; the evolution of the RB triplet and 1O2 singlet-state lifetimes was successfully observed as the BSA concentration was increased. Finally, and most compellingly, an optical fibre delivery and collection scheme is demonstrated as a breakthrough advance in SOLD for in vivo preclinical and clinical applications.