Visual demonstration and prediction of the Hofmeister series based on a poly(ionic liquid) photonic array

The Hofmeister effect and associated Hofmeister series (HS) are ubiquitous in physicochemical phenomena and have demonstrated fundamental importance in a myriad of fields ranging from chemistry to biology. Visualization of the HS not only helps to straightforwardly understand the underpinning mechanism, but also enables the prediction of new ion positions in the HS and directs the applications of the Hofmeister effect. Owing to the difficulties of sensing and reporting complete multiple and subtle inter- and intramolecular interactions involved in the Hofmeister effect, facile and accurate visual demonstration and prediction of the HS remain highly challenging. Herein, a poly(ionic liquid) (PIL)-based photonic array containing 6 inverse opal microspheres was rationally constructed to efficiently sense and report the ion effects of the HS. The PILs can not only directly conjugate with HS ions due to their ion-exchange properties, but also provide sufficient noncovalent binding diversity with these ions. Meanwhile, subtle PIL-ion interactions can be sensitively amplified to optical signals owing to their photonic structures. Therefore, synergistic integration of PILs and photonic structures gives rise to accurate visualization of the ion effect of the HS, as demonstrated by correctly ranking 7 common anions. More importantly, assisted by principal component analysis (PCA), the developed PIL photonic array can serve as a general platform to facilely, accurately, and robustly predict the HS positions of an unprecedented amount of important and useful anions and cations. These findings indicate that the PIL photonic platform is very promising for addressing challenges in the visual demonstration and prediction of HS and promoting a molecular-level understanding of the Hoffmeister effect.