Functionalize AFM cantilevers and tips with particles to probe interactions with cells

Atomic force microscopy (AFM), developed in 1986, relies on the control of a force acting between a tip placed at the end of a flexible lever and a surface, while scanning the sample. This method is therefore a physical surface scanning method, which differentiates it from other types of microscopy such as electron or optical microscopy. As the tip scans the sample surface, the lever to which it is attached is deformed. This deflection is recorded by a laser reflected on a photodiode, enabling the signal to be collected. This technology is particularly well suited to the study of microorganisms in physiological conditions, as it can operate in a liquid medium, keeping the cells studied alive and exploring dynamic phenomena on their surface. It can be used in imaging mode, with nanometer-scale resolution, but its potential is in fact much greater, as the AFM is also a highly sensitive force machine, capable of recording forces as low as 20 pN. Indeed, in force spectroscopy mode, it is possible to record force-distance curves, where the force experienced by the tip is plotted as a function of the separation distance between the tip and the sample. These curves can then be interpreted using different physical models, giving access to the nanomechanical and nanoadhesive properties of cell surfaces. An important advantage of AFM is that these quantified forces can be simultaneously localized to the cell surface, enabling nanomechanics and molecular interactions to be correlated with cell surface ultrastructures. Finally, tips or levers used for AFM imaging or force spectroscopy can be functionalized with biomolecules or inert particles, opening up new avenues for probing specific interactions between these functionalized tips and cell surfaces. In this article, we describe how AFM levers or tips can be functionalized with inert particles and used to measure their interactions with living cells. We will illustrate the use of this technique through a specific application, where the measurement of interactions between levers functionalized with metallic zinc particles and microalgae cells has led to a better understanding of the uptake mechanisms of these heavy metals by the cells. This approach revealed the conditions favoring this process, and confirmed the potential of microalgae to clean up aquatic environments.