Colorimetric sensing of various organic acids by using polydiacetylene/zinc oxide nanocomposites: Effects of polydiacetylene and acid structures

In this study, we utilize the polydiacetylene(PDA)/zinc oxide(ZnO) nanocomposites as colorimetric sensors of various organic acids. Series of organic acids with systematic variation of molecular structure and dissociation constant (K_a) are investigated. The PDA/ZnO nanocomposites are prepared by using three types of monomer with different alkyl chain length, 5,7-hexadecadiynoic acid, 10,12-tricosadiynoic acid and 10,12-pentacosadiynoic acid. Our results show that all PDA/ZnO nanocomposites exhibit color transition upon the addition of organic acids with concentration ranging from 0.01 to 100. mM. By varying PDA structures, detection of each acid at desired concentrations can be controlled. The color transition of the nanocomposites depends significantly on the dissociation ability and structure of acids. For linear alkanoic acids, PDA/ZnO nanocomposites exhibit higher sensitivity to the acids with higher K_a value. When the K_a values are comparable, the architecture of organic acids becomes a dominant factor. Acids with longer chain length induce the color transition of PDA/ZnO nanocomposites at lower concentration. However, acids with branch structure cause an opposite result. The amount of carboxylic group in acids also plays an important role on the color-transition behaviors. The colorimetric response of PDA/ZnO nanocomposites upon exposure to common organic acids used in industries including sorbic acid, oxalic acid, malic acid, citric acid, salicylic acid and acetylsalicylic acid are also investigated.