This proposed project is to develop efficient sensing materials and devices that can detect trace amounts of reactive oxygen species (ROS), in particular a type of activated oxygen, namely singlet oxygen. Detection of singlet oxygen is an important but very challenging task in biological and pharmaceutical applications. The main difficulty in detecting singlet oxygen is due to its very low stability and trace amounts. Certain molecules can capture singlet oxygen through rapid chemical reactions, which result in emission of fluorescence light. They are therefore useful fluorescent molecular probes for rapid and sensitive detection of singlet oxygen. In this work, we will synthesize a range of low-cost but high-performance molecular probes to achieve highly sensitive detection of singlet oxygen as well as other ROS. Prototype devices will be fabricated using these molecular probes as the key components, so that real technological applications can be found in various fields.