Light from Within: Sensing Weak Strains and FemtoNewton Forces in Single Molecules

Weak mechanical forces acting on molecules are in control of a wide variety of (bio)chemical and physical processes. The spatially inhomogeneous nature of these forces has a profound effect on the structure and mechanics of soft and biological materials. Yet, the lack of methods for probing sub-picoNewton forces at high resolution leaves our understanding of these effects incomplete. Here, we solve this challenge by engineering semiconducting polymers to act as ultraweak force sensors. Combining simulations, chemical synthesis, and single-molecule fluorescence spectroscopy, we demonstrate force sensing in single molecules. We achieve grayscale force detection, at a resolution as low as 300 fN, down to the molecular scale. Our approach opens the way to illuminating and quantifying molecular mechanics with unprecedented resolution.