Urea-Functionalized Poly(ionic liquid) Photonic Spheres for Visual Identification of Explosives with a Smartphone.

Current effort merging rational design of colorimetric sensor array with portable and easy-to-use hand-held readers delivers an effective and convenient method for on-site detection and discrimination of explosives. However, on the one hand, there are rare relevant reports; on the other hand, some limitations regarding direct sensing, color retention, and array extendibility still remain. Herein, urea-functionalized poly(ionic liquid) photonic spheres were employed to construct a brand-new colorimetric sensor array for directly identifying five nitroaromatic explosives with a smartphone. It is found that the strong hydrogen bonding between the urea motifs and the nitro groups offers the spheres high affinity for binding the targets, whereas the existence of other abundant intermolecular interactions in poly(ionic liquid) units renders one single sphere eligible for prominent cross-responses to a broad range of analytes. Besides, in our case, opal-like photonic crystal structures other than chemical dyes are used to fabricate a new style of colorimetric array. Such structural colors can be vivid and unchanged over a long period even in hazard environments. Importantly, through simply altering the preparation conditions of our PIL spheres, a pool of sensing elements could be added to the developed array for discrimination of extended target systems such as more explosives and even their mixtures in real-world context.