What is Singlet Oxygen?

Singlet oxygen is produced from a photosensitizer molecule reacting with ground state oxygen. Molecular triplet states are chemically reactive due to their long decay times and the presence of unpaired valence electrons. Reactivity with ground state oxygen (³O₂) will yield singlet oxygen (¹O₂). Singlet oxygen has an emission spectrum peak around 1270 nm, which results in photons emitted from a triplet state (phosphorescence).

The lifetime of singlet oxygen is very long, but quenched when singlet oxygen reacts or comes in contact with different species. Instruments used to detect singlet oxygen typically need near-infrared detectors such as InGaAs detectors (PMT, Analog, or Arrays) and emission gratings that are blazed for efficiency in the NIR wavelength region.

The production of singlet oxygen involves the irradiation of a photosensitizer molecule in the presence of oxygen and subsequent reaction of the excited triplet state photosensitizer with the ground state oxygen resulting in the creation of excited singlet state oxygen. These include molecules such as Rose Bengal, transition metal complexes (as shown below), porphyrins, fluorescein, and others. (DeRosa, 2002) Exciting these molecules at their peak absorbance wavelengths can lead to the reactivity of these molecules in the excited state with ground state oxygen, producing singlet oxygen.

Mechanisms that produce singlet oxygen are important to photodynamic therapeutics, anti-cancer agents and other skin treatments.

The reactivity of singlet oxygen itself can be damaging to organic molecules, including those in biological systems, but the reactivity, if controlled is also a potential method of cancer therapy and photodynamic medicine.

In photodynamic therapy, a patient with malignant cancer has a fiber optic light either inserted into, or placed just outside their body. This light emits visible wavelengths. It reacts with photosensitizer molecules (photodynamic drugs), provides energy to oxygen in the microenvironment, which, in turn, generates non-toxic singlet oxygen species to shrink or kill the tumor.

Singlet oxygen species are chemically reactive chemical varieties containing oxygen.

Also known as light therapy, photodynamic therapy is a treatment for cancers that are near the surface of a body’s tissue, where the light can act on the chemical substances.

It is not like radiation therapy, which uses radicals and a toxic light source. Nor does it cause systematic side effects like chemotherapy. The light and molecules photodynamic therapy uses are non-toxic and benign.