Imaging in Biologically-Relevant Environments with AFM Using Stiff qPlus Sensors

Area of Science:

• Biophysics
• Materials Science
• Nanotechnology

Background:

• High-resolution atomic force microscopy (AFM) of soft biological samples is difficult due to the need for low imaging forces to prevent sample deformation.
• Traditional AFM methods often employ soft cantilevers and optical detection, which are limited in non-transparent or optically changing liquid environments.

Purpose of the Study:

• To develop and demonstrate a novel AFM setup capable of high-resolution imaging of biological samples in various liquid media.
• To overcome the limitations of optical detection in complex biological solutions using an alternative detection method.

Main Methods:

• Utilized a stiff qPlus sensor (stiffness ≥ 1 kN/m) with electrical detection, enabling imaging in frequency-modulation mode with small amplitudes.
• Implemented a liquid cell setup where samples are immersed, with only the sensor tip apex submerged, using long tips.
• Analyzed noise terms and compared the minimal detectable signal with that of soft cantilevers.

Main Results:

• Achieved atomic resolution on muscovite mica in multiple liquids, including water, Tris buffer, and cell culture medium.
• Successfully imaged lipid membranes, resolving individual head groups, demonstrating the system's capability with complex biological samples.
• The qPlus sensor's high stiffness facilitated high Q factors even in liquid environments.

Conclusions:

• The developed qPlus-based AFM system with electrical detection provides a robust solution for high-resolution imaging of soft biological matter in challenging liquid environments.
• This approach significantly expands the applicability of AFM for studying biological systems in physiologically relevant conditions, overcoming optical limitations.