Lipid Bilayer-Confined J-Aggregation Transduces Cell Membrane Mechanics into Photoacoustic Signals

Abstract

Membrane tension, defined as the in-plane mechanical stress of the lipid bilayer, encodes membrane stretch and lipid packing density. Despite its central role in regulating cellular behavior, measuring membrane tension in a direct and noninvasive way in deeper tissue (beyond microscopy) remains a major challenge. Here, we report a supramolecular strategy that transduces membrane mechanics into a spectrally resolved photoacoustic signal through lipid bilayer-confined J-aggregation. We employ two complementary probes: a fluorescence-active system to report aggregation behavior in the lipid bilayer of LUVs and an optimized analogue to enable cellular photoacoustic readout. We found that amphipathic BODIPY-based probes are inserted into lipid membranes and exist as either monomers or slip-stacked J-aggregates with aggregate populations dictated by lipid packing density, which is in turn influenced by membrane tension or lipid composition. Only ordered J-aggregates formed under membrane-confined conditions enable efficient conversion of absorbed light into PA805 signals. Kinetically driven large aggregates spectrally observable via absorption experiments in protein-rich environments are inactive in photoacoustic mode. This decoupling between optical absorption and photoacoustic output reveals aggregate geometry as a critical determinant of photothermal efficiency. Furthermore, the bilayer-confined PA805 on/off behavior minimizes background signals from uninserted probes, enabling wash-free photoacoustic imaging. In model membranes and live cells, PA805 inversely correlates with lipid packing density, enabling noninvasive readout of membrane tension-associated changes.