While fluorescent ROS-sensing dyes respond to their target molecules without further spectroscopic signal structure, which impedes the selectivity of the otherwise highly sensitive fluorescence technique, the detection of the electron paramagnetic resonance (EPR) with spin traps enables a more selective technique for ROS monitoring. Since some ROS species are radicals, the application of spin traps appears sound to focus on spin-carrying ROS. Therefore, spin traps are widely applied to EPR- detectable ROS species like superoxide and hydroxyl radicals (Hideg et al., 1994, 1998, 2000; Zulfugarov et al., 2011). Fluorescent spin traps for ROS detection like DanePy which is quenched in presence of :Ag O2 are suitable for an optical measurement of the interaction between ROS and spin trap molecules (Hideg et al. 1998, 2000, 2001).
The decisive advantage of spin traps is their specific response to the external magnetic field. Even one spin trap that can bind different ROS typically exhibits specific EPR resonance for different bound ROS which makes the applications of spin traps to the most specific approach for selective ROS sensing (see Figure 64).
Figure 64. Principal scheme for the function of a spin trap and generation of the corresponding specific EPR signal.
Table 4 gives an overview on spin traps used for ROS imaging in plants. Disadvantages of spin traps are the rather large spectroscopic effort for EPR measurements and the impossibility of microscopic applications. Therefore spin traps are highly specific in their target molecules but do not easily carry information about the localization of ROS.
The detection of ROS in cyanobacteria faces additional difficulties because their accessibility to EPR and fluorescent spin traps is limited. An alternative technique is chemical trapping by ROS scavengers like histidine. Recently, it was shown that chemical trapping by histidine is suitable to monitor singlet oxygen generation in Synechocystis sp. PCC 6803 (Rehman et al., 2013).