Conjugated Oligoelectrolytes as Optical Probes

Optical probes are essential tools for interrogating biological and chemical systems invisible to the naked eye, providing insights into molecular interactions, protein activity, and cellular trafficking. Conjugated oligoelectrolytes (COEs), an emerging class of optical probes, are synthetic organic amphiphiles defined by a π-conjugated backbone and charged pendant groups. COEs with a linear conjugated structure and charged groups at the two termini can be designed to mimic the molecular dimensions and arrangements of hydrophobic and hydrophilic groups characteristic of lipid bilayers. This design drives their spontaneous intercalation into and prolonged residence within biological lipid bilayer membranes. By tailoring their molecular building blocks, their electronic and photophysical properties as well as their interactions with cells can be readily tuned, positioning COEs as a versatile platform for developing molecular probes for fundamental research and applied bioimaging across a range of biological systems. In this Account, we describe the design strategies elaborated by our group for developing COEs as optical probes, with a focus on their applications and uses in elucidation and tracking of cellular membrane properties. We show that COEs can be used to detect and visualize lipid membranes at multiple length scales, ranging from single microbial cells and exogenously isolated small extracellular vesicles and particles to subcellular organelles and whole cells in live animal models. COEs also function as effective nonlinear optical probes that are applicable in advanced imaging modalities such as two-photon microscopy and stimulated emission depletion microscopy to extract spatiotemporal information at high resolution. We also provide our insights into how COEs can be designed to be functional probes that exhibit predictable photophysical behavior in response to the local molecular and chemical environment. Using fluorescence lifetime imaging microscopy, the time-resolved emission of COEs can be leveraged to provide insight into dynamic processes such as rapid changes in membrane tension and long-term changes in membrane rigidity and composition. We additionally elaborate strategies for modulating interactions with biological membranes, designing membrane-specific probes that respond to specific cellular biophysical parameters, and offer perspectives and opportunities toward developing a new platform for disease detection and diagnosis.