Multi-Stimuli-Responsive [2]Rotaxanes with Förster Resonance Energy Transfer Processes in Nanoscales for Cyanide Detection and Bioimaging

A [2]rotaxane-based mechanically interlocked molecule (MIM) containing a green-emissive naphthalimide (NP) donor stopper and a nonemissive spiropyran (SP) unit on the macrocycle was synthesized. Upon UV exposure, the SP isomer of MIM could be reversibly changed into a merocyanine (MC) probe with strong red emission in the semiaqueous solutions of ACN/H2O, higher water contents (fw = 70–95%) due to its excellent aggregation-induced emission (AIE) behaviors, and effective ratiometric fluorescent properties via Förster resonance energy transfer (FRET) processes between green emissions (λem = 545 nm) of the NP donor and red emissions (λem = 655 nm) of the MC acceptor. Furthermore, the red-emissive merocyanine probe of the [2]rotaxane-based supramolecule in the semiaqueous solution of ACN/H2O (5:95, v/v) could be reversibly shifted between two recognized sites on the axle, where various ratiometric PL properties occurred due to the shorter/longer distances of the NP donor and MC acceptor of stronger/weaker FRET-ON processes before/after shuttling by the acid/base controls, respectively. The highest energy transfer efficiencies of MIMs were obtained in neutral conditions (i.e., pH = 5–8) and at room temperature (25–40 °C) for the NP donor and MC acceptor with optimum FRET-ON processes, while [2]rotaxanes formed nanoaggregates at fw = 95% with average sizes of 110–189 nm. Hence, our designed [2]rotaxane (before shuttling) displayed the best detection results of FRET efficiency (EFRET = 76.5%) and a high sensitivity and selectivity toward cyanide ions with the best limit of detection LOD = 0.29 μM in ACN/H2O (fw = 95%) compared to those of [2]rotaxane (after shuttling) (EFRET = 62.3%, LOD = 1.61 μM), a mixture of macrocycle MC probe with axle (EFRET = 51.0%, LOD = 3.52 μM), and the MC probe alone (LOD = 6.18 μM). Thus, [2]rotaxane (before shuttling) was utilized for chemo- and biosensors along with bioimaging applications.