Anion recognition properties of acridine-urea conjugate in bulk and silica nanopore systems.

This study investigated the anion recognition properties of acridine-urea conjugate (AcU) in bulk DMSO and silica nanopore. The free AcU in bulk DMSO was in tautomeric equilibrium between fluorescent amino form (a-AcU) and non-fluorescent imino form (i-AcU). Owing to this tautomerization of AcU, the free AcU worked as fluorescence enhancement sensing by hydrogen bonds mediated complexation between urea unit and anion. The estimated dissociation constants were 1.4 ± 0.2 mM for CHCOO and 3.1 ± 1.0 mM for HPO, whereas those for Cl, ClO, and HSOwere quite large. The hydrogen bond between urea unit and anion was also available for the anion recognition by AcU immobilized at the pore surface of mesoporous silica when the anion concentration is above 0.2 mM. In addition, we found that fluorescence of protonated a-AcU (a-AcHU) could also be utilized for the recognition of weak acid anions over strong acid anions when the anion concentration is below 0.1 mM. The AcU@MPS with two recognition system has potential application for the anion recognition.