DNA aptamers were combined with platinum complexes to form DNA-Pt complexes, which exhibited peroxidase enzymatic activity while retaining the specific binding ability of aptamers. The DNA-Pt complexes were fractioned by size through ultrafiltration membranes having several molecular weight cut-offs, and the peroxidase enzymatic activities of these fractions were compared. The enzymatic activity was highest in the filtrate of DNA-Pt complex solutions prepared with cisplatin and K-2[PtCl4] after ultrafiltration through membranes having MWCO = 300,000. These showed 1.2-fold and 1.6-fold higher activity, respectively, than the corresponding unfractioned complexes. Sandwich-type DNAzyme-linked aptamer assays (DLAAs) using unfractioned or fractioned DNA-Pt complexes successfully detected the target protein thrombin. DLAAs incorporating a DNA-Pt (K-2[PtCl4]) complex fractioned through ultrafiltration membranes having MWCO = 300,000 showed the highest sensitivity among DLAAs made using fractioned DNA-Pt complexes, and had a 13-fold higher sensitivity than those made with unfractioned DNA-Pt complexes. (c) 2008 Elsevier B.V. All rights reserved.