A polydiacetylene multilayer film for naked eye detection of aromatic compounds

A blue-colored polyelectrolyte multilayer (PEM) film of poly(10,12-pentacosadiynoic acid), or poly(PCDA) vesicles, was successfully prepared by layer-by-layer deposition with polycationic chitosan, and its application as a colorimetric chemo-sensor for water soluble aromatic compounds was investigated in comparison to the same poly(PCDA) vesicles as a liquid sol. The color of the PEM film changes from blue to red within 5 min when immersed into 10 mM α-cyclodextrin (α-CD) solution giving the colorimetric response (CR) of ∼65%. The α-CD induced color transition of the PEM film was completely inhibited in the presence of 10 mM of either benzoic acid or 4-nitrophenol, which represents a 1:1 mole ratio of aromatic compound:α-CD, showing nearly zero percent CR and the film remained blue. In contrast, only partial inhibition was observed by eyes in the presence of 20 mM 4-methoxyphenol and indole as the film appeared purple with ∼15% CR. Phenol and nitrotoluene did not show inhibition detectable by naked eyes but the low level of inhibition, ∼35% CR remained, was observed spectroscopically at 20 mM. For nitrophenols, the degree of inhibition is varied by the isomeric structures in the following order: 4-nitrophenol > 3-nitrophenol > 2-nitrophenol. The competitive inclusion of the aromatic compound into the α-CD cavity is probably responsible for the observed inhibition of color transition. Compared with the liquid sol of poly(PCDA) vesicles, the PEM film, as a solid sol, offers less color interference from turbidity and intrinsic color of the samples being analyzed that the results of inhibition can be readily justified by naked eyes.