Design and Preparation of a Nanoprobe for Imaging Inflammation Sites

To image inflammation sites, we developed a novel nanoparticle, hydroxylamine-containing nanoparticle (HANP), which emits an intense electron spin resonance (ESR)-signal triggered by enzymatic oxidation reaction and pH-sensitive self-disintegration. The nanoparticle was prepared from an amphiphilic block copolymer, poly (ethylene glycol)-b-poly[4-(2,2,6,6-tetramethylpiperidine-1-hydroxyl)aminomethylstyrene] (PEG-b-PMNT-H), which spontaneously forms a core-shell type polymeric micelle (particle diameter = ca. 50 nm) in aqueous media. Because the PMNT-H segment in the block copolymer possesses amino groups in each repeating unit, the particle can be disintegrated by protonation of the amino groups in an acidic pH environment such as inflammation sites, which is confined to the hydrophobic core of HANP. Mixing HANP with horseradish peroxidase (HRP)/H2O2 mixture resulted in enzymatic oxidization of the hydroxylamines in the PEG-b-PMNT-H and converted the hydroxylamine to the stable nitroxide radical form in PEG-b-poly[4-(2,2,6,6-tetramethylpiperidine-1-oxyl)aminomethylstyrene] (PEG-b-PMNT), which shows an intense ESR signal. It is interesting to note that the ESR signal increased at a greater rate under acidic conditions (pH 5.6) than that under neutral conditions (pH 7.4), although the enzymatic activity of HRP under neutral conditions is known to be much higher than that under acidic conditions. This indicates that enzymatic oxidation reaction was accelerated by synchronizing the disintegration of HANP under acidic conditions. On the basis of these results, HANP can be used as a high-performance ESR probe for imaging of inflammation sites.