Reactive oxygen species (ROS) such as superoxide anion radical (O2-·) play an essential role on normal cellular growth and homeostasis. However, excess ROS generated by perturbing O2-· homeostasis under various conditions of oxidative stress induce high radical toxicity, resulting in many diseases such as cancer, brain and myocardial infarction, and inflammation. Quantitative analysis of O2-· by a convenient method is a subject of intense research, since most of ROS are derived from O2-·. In situ real-time measurement of O2-· is very important to understand the relevance of ROS to many diseases. Recent progress in electrochemical sensors for the facile detection of O2-·, including biosensors utilizing a variety of metalloproteins as sensing elements for O2-· and very recently developed all-synthetic sensors with a high selectivity for O2-· detection, is reviewed. Emphasis is placed on the possibility of the all-synthetic sensor for convenient in vivo measurement of ROS.
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