( Introduction ...)Singlet Oxygen measurement

Introduction
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.

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