Nanoemulsion-based silver ion-selective optode based on colorimetrically silver ion-responsive ionic liquid-based dye.

In this study, we describe the fast-responsive nanoemulsion (NE)-based silver ion (Ag+)-selective optode based on colorimetrically silver ion-responsive ionic liquid-based dye (ILD). The ILD comprises purely functional sensing molecules, a protonated cationic merocyanine dye (KD-M13-H+) and an anionic Ag+ ionophore (BDM-SO3-), and thus, it can be used for highly sensitive silver ion (Ag+) sensing due to the extremely high content of dye in the organic phase (ionic-liquid phase). However, during the Ag+ sensing, the cationic merocyanine dye is converted into electrically neutral form by deprotonation of the dye, which leads to the conversion of liquified dye into solid form in the organic phase, which makes the response time slower when ILD is used for poly(vinyl chloride) (PVC) membrane-based ion-selective optode, especially for sensing of high Ag+ concentration. To solve this problem, we focused on the use of the nano-emulsification technique. The response time of the ILD-based nanoemulsion (NE) was considerably shorter (1 s) compared to that of the ILD-based PVC membrane (a few minutes) owing to the large surface area and excellent diffusivity of the emulsion. The ILD-based NE contained a very high dye concentration (833 mmol kg-1) and exhibited approximately 12 times higher sensitivity than that of the plasticizer-based conventional NE. In the cation measurements, the ILD-based NE responded to Ag+ via a cation-exchange mechanism and demonstrated a highly selective response to Ag+ (log [Formula: see text] = - 3.0). ILD-NE was successfully applied to the detection of spiked Ag+ in a tap water sample with recoveries of 98 - 103% with a relative standard deviation (RSD) of less than 5%. In comparison with NE based on non-ionic ionophores without charge, NE based on BDM-SO3- responded to lower Ag+ concentrations owing to the effect of negative charge on the binding property. The novel ILD-based NE was capable of highly sensitive, rapid, and selective Ag+ sensing, providing potential for analytical devices applicable to high-performance on-site analysis.