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

Abstract

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.

Photochemistry and Spectroscopy of Singlet Oxygen in Solvents. Recent Advances which Support the Old Theory

“…Hurst et al [25] were the first experimentalists who have noticed that k r a-X in a number of aromatic solvents grows up with increases of solvent polarizability. This trend is supported by numerous experiments for various solvents [32,36,40,46,47] however, the reason of the phenomenon is not simple [41].…”

“…In experiment with direct oxygen excitation Krasnovsky have found recently a tiny effect of solvent polarity on the extinction coefficient of the 760 nm band [35]; the X→b absorption band seams to be slightly more intense in polar solvents in agreement with our estimations. All recent experiments [35][36][37][38] on direct oxygen excitation are crucial to verify the old theory predictions [23]. Comparison of the absorption intensity of the two bands at 1270 and 760 nm in solvents clearly demonstrates that the former band enhancement (1.27 μm) is much more efficient [35,38].…”

“…The low radiative rate constant (k r ) for 1270 nm a→X emission at low pressure in gases [21] and its essential enhancement in solvents [15,18] have been explained in a number of old papers [10,22,23]. Nowadays the total theoretical backgrounds of this theory are supported by numerous experimental studies [26][27][28][29][30][31][32] including recent experiments [33][34][35][36][37][38] on direct oxygen excitation. Thus, it can be interesting to present a short review of such backgrounds accounting a great importance of singlet oxygen photochemistry in solvents for modern medical diagnostics and therapy [39,40] as well as for some braches of chemical technology [5,32].…”

“…(10) various ideas were considered in terms of vibration mechanics during last three decades [16][17][18][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. Unfortunately, the spin-prohibition of non-radiative C -maximum likelihood fitting coefficient [32].…”

“…They consider the potential energy curves (PECs) for both states, O 2 ( ) species, presented thirty years ago [22,23,43], can be used to explain the results of recent experiments [35][36][37][38].…”