Free oxygen radicals are a byproduct of any metabolism involving O₂. Cells usually neutralize radicals since elevated levels result in reactive species such as H₂O₂ – a state which is called oxidative stress. Oxidative stress can lead to inflammation and DNA damage, which are closely linked to the development of various diseases, e.g. cancer. Oxidative stress is also characteristic for ischemic reperfusion injury: tissues injured by an ischemic event (a restriction in blood supply) show a five-fold increase of H₂O₂. Unfortunately, there has been no method to detect pathophysiological levels of H₂O₂ in tissues.

Emilia Olson, University of California, San Diego, USA, and colleagues used intravenously injectable synthetic hollow enzyme-loaded nanospheres (SHELS) to detect sites of oxidative stress in vivo. SHELS are prepared from a nanoporous silica shell and catalase, an enzyme that converts H₂O₂ to water and O₂. The resulting catalase-SHELS (catSHELS) enclose the catalase, while still allowing the contact with H₂O₂ to produce O₂.

The catSHELS were tested in vivo in rats with an induced ischemic injury. After injection of the catSHELS, an increased ultrasound signal was detected due to O₂ microbubbles. These are the result of the catalase activity and the elevated H₂O₂ levels after the ischemic event. According to the researchers, their approach could allow using H₂O₂ elevation as a biomarker for early disease detection.

• Ultrasound Detection of Regional Oxidative Stress in Deep Tissues Using Novel Enzyme Loaded Nanoparticles,
  Emilia S. Olson, Inanc Ortac, Christopher Malone, Sadik Esener, Robert Mattrey,
  Adv. Healthcare Mater. 2017.
  DOI: 10.1002/adhm.201601163