Recent developments in the area of biological detection by optical sensing of molecular oxygen (O₂) are reviewed, with particular emphasis on the quenched-phosphorescence O₂ sensing technique. Following a brief introduction to the main principles, materials and formats of sensor technology, the main groups of applications targeted to biological detection using an O₂ transducer are described. These groups include: enzymatic assays; analysis of respiration of mammalian and microbial cells, small organisms and plants; food and microbial safety; monitoring of oxygenation in cell cultures, 3D models of live tissue, bioreactors and fluidic chips; ex vivo and in vivo O₂ measurements; trace O₂ analysis. For these systems, which enable a range of new bioanalytical tasks with different samples and models in a minimally invasive, contact-less manner, with high sensitivity, flexibility and imaging capabilities in 2D and 3D, relevant practical examples are presented and their merits and limitations discussed. An outlook of future scientific and technological developments in the field is also provided.

In recent years, significant progress has been achieved in the sensing and imaging of molecular oxygen (O₂) in biological samples containing live cells and tissue. We review recent developments in the measurement of O₂ in such samples by optical means, particularly using the phosphorescence quenching technique. The main types of soluble O₂ 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 O₂ 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 O₂ consumption rate, sample oxygenation, sensing of intracellular O₂, metabolic assessment of cells, and O₂ 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.

Mitochondrial biogenesis occurs in response to increased cellular ATP demand. The mitochondrial electron transport chain requires molecular oxygen to produce ATP. Thus, increased ATP generation after mitochondrial biogenesis results in increased oxygen demand that must be matched by a corresponding increase in oxygen supply. We found that overexpression of peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α), which increases mitochondrial biogenesis in primary skeletal muscle cells, leads to increased expression of a cohort of genes known to be regulated by the dimeric hypoxia-inducible factor (HIF), a master regulator of the adaptive response to hypoxia. PGC-1α-dependent induction of HIF target genes under physiologic oxygen concentrations is not through transcriptional coactivation of HIF or up-regulation of HIF-1α mRNA but through HIF-1α protein stabilization. It occurs because of intracellular hypoxia as a result of increased oxygen consumption after mitochondrial biogenesis. Thus, we propose that at physiologic oxygen concentrations, PGC-1α is coupled to HIF signaling through the regulation of intracellular oxygen availability, allowing cells and tissues to match increased oxygen demand after mitochondrial biogenesis with increased oxygen supply.