Optical probes and techniques for O ₂ measurement in live cells and tissue

In recent years, significant progress has been achieved in the sensing and imaging of molecular oxygen (O ₂) in biological samples containing live cells and tissue. We review recent developments in the measurement of O ₂ in such samples by optical means, particularly using the phosphorescence quenching technique. The main types of soluble O ₂ sensors are assessed, including small molecule, supramolecular and particle-based structures used as extracellular or intracellular probes in conjunction with different detection modalities and measurement formats. For the different O ₂ sensing systems, particular attention is paid to their merits and limitations, analytical performance, general convenience and applicability in specific biological applications. The latter include measurement of O ₂ consumption rate, sample oxygenation, sensing of intracellular O ₂, metabolic assessment of cells, and O ₂ imaging of tissue, vasculature and individual cells. Altogether, this gives the potential user a comprehensive guide for the proper selection of the appropriate optical probe(s) and detection platform to suit their particular biological applications and measurement requirements.