Application of stimuli-responsive FRET behavior toward cyanide detection in a photo-switchable [2]pseudorotaxane polymer containing the BODIPY donor and the merocyanine acceptor

Abstract

We have developed a supramolecular (close form) [2]pseudorotaxane polymer containing the green-emissive (λ_em = 523 nm) BODIPY-based pillar[5]arene host and the non-emissive spiropyran (SP)-based cyano guest (close form), which can be converted to a red-emissive (λ_em = 630 nm) merocyanine (MC)-based guest (open form) upon UV exposure as well as turned into a bi-fluorophoric host–guest polymer with ratiometric PL emission of Förster resonance energy transfer (FRET) behavior in THF/water solution (60% H₂O, v/v). The equal host–guest molar ratio and the formation of the [2]pseudorotaxane polymer can be further verified using ¹H NMR titration, Job's plot analysis, DOSY and ITC experiments. Hence, the bi-fluorophoric host–guest (with the MC open form) polymer possessed the highest red-emission of the MC acceptor, which was also confirmed using TRPL measurements to possess the shortest lifetime of 0.37 ns and the best FRET efficiency of 83.5%. Since the MC unit could react with CN⁻ to induce a non-emissive quenching process, the mono-fluorophoric guest would show turn-off PL behavior and detect cyanide ions with a corresponding limit of detection (LOD) value of 0.94 μM. In comparison, the optimal LOD value of 0.48 μM toward CN⁻ anion could be achieved by our bi-fluorophoric host–guest polymer via the FRET-OFF process. Therefore, cyanide detection and cell viability tests using the open form of the [2]pseudorotaxane polymer suggest useful bio-imaging applications in living cells.