Sensing membrane stress with near IR-emissive porphyrins

Probes embedded within a structure can enable prediction of material behavior or failure. Carefully assembled composites that respond intelligently to physical changes within a material could be useful as intrinsic sensors. Molecular rotors are one such tool that can respond optically to physical environmental changes. Here, we propose to use molecular rotors within a polymersome membrane to report membrane stress. Using supermolecular porphyrin-based fluorophores as rotors, we characterize changes in the optical emission of these near-infrared (NIR) emissive probes embedded within the hydrophobic core of the polymersome membrane. The configuration of entrapped fluorophore depends on the available space within the membrane; in response to increased volume, emission is blue shifted. We used this feature to study how shifts in fluorescence correlate to membrane integrity, imparted by membrane stress. We monitored changes in emission of these porphyrin-based fluorophores resulting from membrane stress produced through a range of physical and chemical perturbations, including surfactant-induced lysis, hydrolytic lysis, thermal degradation, and applied stress by micropipette aspiration. This paper comprehensively illustrates the potential for supermolecular porphyrin-based fluorophores to detect intrinsic physical changes in a wide variety of environments, and suggests how molecular rotors may be used in soft materials science and biology as sensors.