The chemistry, spectroscopy, and reactivity of singlet oxygen, 1O2, is of considerable interest in environmental applications, novel organic syntheses and mechanisms of photo-oxygenation.1 Recently, research has been directed towards understanding the biological effects of 1O2, such as the photodynamic destruction of cells in the presence of a dye, oxygen, and light.
An understanding of the mechanisms that produce 1O2 is required for the synthesis of new anti-cancer agents used in photodynamic therapy. The spectroscopic properties of the dye and 1O2 may be characterized using fluorescence spectroscopy. The emission of 1O2 is in the near-IR, around 1280 nm. The SPEX FLUOROLOGsystem easily detects this signal, with the appropriate emission gratings and a suitable detector. Most multi-alkali photomultiplier tubes, such as the R928P, respond to wavelengths up to 800 nm. InGaAs detectors extend this range to nearly 1700 nm.
The SPEX DSS-IGA020L InGaAs detector is an excellent choice for 1O2 measurements. It operates at liquid-nitrogen temperature to minimize thermal noise. Usable from 8001700 nm, the InGaAs detector provides a smooth overlap with the standard R928P photomultiplier tube.
Experiment and Results
As an example of 1O2 detection, generated using a transition-metal complex, a solution of tris2-2-bipyridine ruthenium(II) chloride, [Ru(bpy)3]Cl2, in D2O, was used as a generator of 1O2.2 The optical density of the solution was 0.3 A above the D2O baseline at 450 nm. To enhance the yield of 1O2, the solution was saturated with O2 by bubbling the solution with pure oxygen for 5 min before measurement. The excitation maximum of the [Ru(bpy)3]2+ complex is in the visible region of the spectrum, near 450 nm.
A FLUOROLOG system, equipped with a 450-W xenon lamp, single-grating excitation and emission monochromators, and near-IR accessories (DSS-IGA020L InGaAs detector and emission grating at 600 grooves mm1, blazed at 1000 nm) was used to measure the 1O2 emission spectrum. A high-pass filter (λ = 780 nm) was used as an order-sorter, necessary when such large wavelength ranges are spanned. Figure 1 shows the emission spectrum obtained using an integration time of 0.1 s and a bandpass of 12 nm. The excellent signal-to-noise ratio is seen in the graph.
1 Wasserman, H.H.; Murray, R.W., in Singlet Oxygen; Academic Press: New York, 1979.
2 Mulazzani, et. al. J. Phys. Chem. 1994, 98, 1145.