Complex Deformation Characteristics of Liposomes during Atomic Force Microscopy Force Curves

We use Atomic Force Microscopy (AFM) topography imaging and force curve analysis to quantify the shape and effective Young's modulus of liposomal doxorubicin nanoparticles (NPs). In ambient air conditions, the NPs collapse to a height of 3 ± 1 nm, but when submerged in phosphate-buffered saline (PBS), they are 27 ± 6 nm tall. The Johnson-Kendall-Roberts (JKR) model is used to fit the effective Young's modulus values from AFM force curves, with Monte Carlo uncertainty analysis to account for systematic uncertainties. In PBS, the mean Young's modulus at the NPs' center is 25 MPa with a 68% coverage interval of [1.5-46] MPa. Values at the edge are lower, likely due to contact geometry and tip slip, and higher in air due to the collapse of NPs. This study highlights the impact of AFM measurement conditions on soft NP characterization. Additionally, we show that large modulus uncertainty bounds can arise from systematic uncertainties in the calibration parameters. These uncertainties should be considered carefully when planning AFM experiments.