A highly selective method for synthesizing monoadduct derivatives of triaminopentanoic acid anhydride (DTPD) was developed by the sequential heating and cooling of DTPD and an amine catalyst, followed by the addition of polymer material. After chelation with Eu3+, the concentration of the polymers thus obtained was detected by time-resolved fluorescence even when these polymers were incubated with mouse serum, indicative of the applicability of this method for the pharmacokinetic analyses of polymer-based nanoparticles.