Photoacoustic Detection of Superoxide Using Oxoporphyrinogen and Porphyrin

The superoxide (O2•-) ion is a highly reactive oxygen species involved in many diseases; hence, its noninvasive detection is desirable to identify the onset of pathological processes. Here, we employed photoacoustic (PA) spectroscopy, which enables imaging at ultrasound resolution with the sensitivity of optical modality, for the first time to detect O2•-, using stimuli-responsive contrast agents. meso-(3,5-Di-tert-butyl 4-hydroxyphenyl) porphyrins and oxoporphyrinogens were used as PA contrast agents, which trap the O2•- and enable its detection. The trapped O2•- increased the PA signal amplitude of chromophores up to 9.6-fold, and induced a red-shift in the PA signal maxima of up to 225 nm. Therefore, these trigger-responsive probes may be highly valuable as smart diagnostic PA probes to investigate pathological events stimulated by O2•- species.