Attonewton Force Resolution Measurements with Silicon Nanospheres at the Thermal Noise Limit in Ambient-Temperature Liquids

Precise force measurements are crucial for understanding fundamental physics or nanoscale interactions, such as those of molecular machines in biology. Optical tweezers are versatile force transducers for such measurements, enabling meticulous manipulation of small particles. However, achieving high-resolution, subfemtonewton force measurements under physiological conditions remains challenging due to thermal fluctuations and instrument noise. Here, we employed an ultrastable optical tweezers setup in an isolated environment with precise temperature control, which minimized instrumental noise and enabled prolonged, low-force measurements. We utilized water-suspended, high-refractive index silicon nanospheres for improved resolution and trapping stability. Our system achieved a force resolution of ≈60 aN, with a sensitivity of 2.7 fN Hz^-0.5, allowing us to measure forces as low as 0.30 ± 0.06 fN. Our drag force measurements demonstrate the importance of optimized experimental conditions for low-force measurements, providing a robust framework for scientific investigations that require high-precision force characterization.