Selective detection of metal ions based on nanocrystalline ionochromic polydiacetylene

Metal ion detection using commercially available ionochromic polydiacetylenes has not been selective. In this work, we report selective detection of metal ions in small organic compound (SOC)-stabilized poly(10,12-pentacosadiynoic acid) (PDA) nanocrystals suspended in an aqueous solution. These PDA nanocrystals were topochemically polymerized using a 254-nm UV light from the corresponding SOC-stabilized DA nanocrystals prepared by a solvent/nonsolvent micellization method. In this detection, the stabilizing SOC could selectively regulate the intercalation of specific metal ions into the galleries of PDA nanocrystals. Whenever the specific metal ions were intercalated, a fast (<1 min) ionochromic transition was observed. In addition, different metal ion selectivity could be achieved by utilization of different SOCs. For example, selective detections of Pb2+ and Ag+ with different detection sensitivities were achieved for a PDA/glycine (Gly) nanocrystal suspension and selective detection of Zn2+ was found for a PDA/boric acid nanocrystal suspension. X-ray diffraction and Fourier transform infrared studies revealed that Pb2+ and Ag+ intercalated into the galleries of PDA nanocrystals and deprotonated most –COOH groups in the side chains of PDA, forming a strong ionic interaction. This strong ionic interaction effectively reduced the conjugation length in the PDA main chain, and therefore selectively induced a blue-to-red ionochromic transition.