Catalytic nanocrystalline coordination polymers as an efficient peroxidase mimic for labeling and optical immunoassays

We report that nanocrystalline Prussian blue of the type Fe-4[Fe(CN)(6)](3) is a powerful peroxidase mimic for use in labeling of biomolecules. The cubic nanocrystals typically have a diameter of 15 nm and are capable of catalyzing the oxidation of colorless 3,3',5,5'-tetramethylbenzidine in the presence of H2O2 to form an intensively colored product with an absorption maximum at 662 nm. The determined pseudo turnover number is similar to 20,000 s(-1) which is the highest value reported for nanoparticles of a size comparable to common proteins. We also present a method for the biotinylation of the surface of these nanocrystals, and show their use in competitive bioaffinity based assays of biotin and human serum albumin. The limits of detection are 0.35 and 0.27 mu g mL(-1), respectively. The results prove the applicability of coordination polymers for signal amplification and also their compatibility with the format of enzyme linked immunosorbent assays.