The immediate products of x-ray absorption in aqueous biological samples are free radicals including *OH, H2O2, *H\r\nand solvated electrons. Because their lifetimes and diffusion ranges are dependent on the local bio-molecular\r\nenvironment, imaging these free radicals in real-time while they are produced by a scanning x-ray nanobeam may\r\nprovide a biological microscopy method of high resolution. As a first step towards this goal, we investigated the\r\nfeasibility of imaging the initial free radical products of x-ray absorption in live cells using fluorescent free radical\r\nsensors. We selected six commercially available fluorescent sensors for screening tests of their sensitivities towards\r\nx-ray radiation in solution form. Two of the six dyes were found to have high sensitivities. One of the two was\r\nsuccessfully used for dynamic confocal fluorescence imaging of x-ray generated free radicals in the intracellular\r\nspace of mouse embryonic fibroblasts. Time series of fluorescence images before and during x-ray radiation were\r\nacquired. The rate of increase of cellular fluorescence showed both the initial production of free radicals by the\r\nphysical ionization events as well as stimulated biological production of reactive oxygen species later on. The\r\nimplications of the results for future development of microscopy techniques are discussed.
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