Direct 765 nm Optical Excitation of Molecular Oxygen in Solution and in Single Mammalian Cells

Singlet oxygen, O2(a(1)Δg), the first excited electronic state of molecular oxygen, is an important reactive oxygen species. Its chemistry plays a role in processes ranging from polymer degradation to cell death. Although O2(a(1)Δg) is routinely produced through natural events, including photosensitized processes mediated by organic chromophores, the controlled and selective laboratory production of O2(a(1)Δg) remains a challenge, particularly in biological systems. Here we exploit the fact that ground-state oxygen, O2(X(3)Σg(-)), absorbs 765 nm light to selectively produce O2(b(1)Σg(+)) which, in turn, decays to O2(a(1)Δg). We have quantified this process in different solvents using the time-resolved 1275 nm O2(a(1)Δg) phosphorescence as an optical probe. Most importantly, 765 nm falls in the so-called "biological window", where endogenous chromophores minimally absorb. We show that femtosecond-laser-based, spatially resolved 765 nm irradiation of human tumor cells induces O2(a(1)Δg)-mediated cell death. We thus provide an accessible tool for the controlled sensitizer-free production and study of O2(a(1)Δg) in complex biological systems.