**3. Conclusion**

In recent decades functional EPR spectroscopy and imaging applications have moved closer to biomedical applicability. The bottleneck of *in vivo* EPR-based applications is the requirement of appropriate paramagnetic probes. NRs and triarylmethyl radicals, TAMs, represent two main classes of soluble paramagnetic materials used for EPR spectroscopy and imaging applications (Khramtsov & Zweier, 2010). TAMs have advantages over nitroxides in extraordinary stability toward tissue redox processes, longer relaxation time and narrower line width making them particularly attractive for imaging applications. Applications of TAM radicals as functional probes include EPR oximetry (Ardenkjaer-Larsen et al., 1998; Golman et al., 2000; Krishna et al., 2002) and recently reported sensitivity to the superoxide anion (Kutala et al., 2004; Kutala et al., 2008) and pH (Bobko et al., 2007a; Dhimitruka et al., 2008; Driesschaert et al., 2012). On the other hand NRs have tremendous advantage over TAMs in well-developed chemistry. As a consequence, the nitroxides provide a wide range of available structures that vary in solubility and tissue redistribution, spectral and functional sensitivity, ability to be targeted and lifetimes in living tissues. Specific nitroxides provide EPR-based spectroscopy and imaging with the capacity for functional temporally and spatially resolved mapping of physiologically relevant parameters of living tissues, such as oxygen, redox status, glutathione content and pH. The nitroxides were the first compounds applied to EPR oximetry and are still a useful tool for *in vivo* tissue oxygen mapping. The most well known reaction of the nitroxides, their reduction to EPR-silent hydroxylamines, provides information on redox state in living tissues. The disulfide biradical nitroxides were developed for *in vivo* assessment of intracellular glutathione concentration. The imidazoline nitroxide spin pH probes allowed for EPR spectroscopy and imaging of tissue pH. Nano*S*ized *P*articles with *I*ncorporated *N*itroxides, or nano*SPIN*s, may serve as a future platform for stabilized nitroxide-based biosensors (Woldman et al., 2009). In particular, encapsulation of NO-sensitive nitronyl nitroxides may protect the sensing probe from biological reductants while retaining sensitivity to a small analyte, molecule of nitric oxide. *In summary,* the area of nitroxide biomedical application as functional paramagnetic probes is truly emerging.

## **Author details**

Valery V. Khramtsov *Division of Pulmonary, Allergy, Critical Care & Sleep Medicine, Department of Internal Medicine, The Ohio State University, Columbus, OH, USA* 
