We have prepared cyclic peptide nucleic acids (PNAs). These compounds do not bind complementary nucleic acids. One carboxylic ester group was introduced in the backbone of the cyclic PNAs. This group is cleaved in the presence of Cu2+ or coordinatively unsaturated Cu2+ complexes. The cleavage products are linear PNAs. In contrast to the cyclic PNAs, they are efficient nucleic acid binders. The rate of formation of the linear PNAs is proportional to the concentration of the cleaving agents. Therefore, one may apply highly sensitive methods of detection of linear PNAs for determination of Cu2+ concentration. In particular, we have demonstrated that both fluorescent spectroscopy in combination with molecular beacons and MALDI-TOF mass spectrometry are suitable for the detection of Cu2+. A range of related divalent metal ions and Eu3+, Ln(3+), Pr3+, Ce3+, and Zr4+ do not interfere with Cu2+ detection.