Norovirus-like particles were imaged using atomic force microscopy. The mechanical stability of the virus-like particles (VLPs) was probed by nanoindentation at pH values ranging from 2 to 10. This range includes pH values of the natural environment during the life cycle of noroviruses. The resistance of VLPs to indentation was constant at acidic and neutral pH. The Young's modulus was of the order of 30 MPa. At basic pH the compliance of the capsid increased along with an increase in diameter. This specific pH-dependent mechanical response of the capsid may be related to mechanisms controlling uptake and release of the RNA during infection. Consecutive indentations with pressures ≤ 300 bar demonstrated the ability of the capsids to fully recover from deformations comparable with the size of the capsid. The capsids can be viewed as nanocontainers with an inbuilt self-repair mechanism. At pH 10 the capsids lost their stability and were irreversibly destroyed after one single indentation.

The thickness of a poly(sulfo propyl methacrylate) (PSPM) brush is determined by Atomic Force Microscopy (AFM) imaging as a function of the loading force at different ionic strengths, ranging from Milli-Q water to 1 M NaCl. Imaging is performed both with a sharp tip and a colloidal probe. The brush thickness strongly depends both on the applied load and on the ionic strength. A brush thickness of 150 nm is measured in Millipore water when applying the minimal loading force. Imaging with an 8 μm silica particle as a colloidal probe results in a thickness of 30 nm larger than that measured with the tip. Increasing the ionic strength causes the well known reduction of the thickness of the brush. The apparent thickness of the brush decreases with increasing loading forces. An empirical model analogous to that of a compressible fluid is applied to describe the dependence of the apparent thickness of the brush with loading force. The model comprises three ionic strength dependent parameters for the brush: thickness at infinite compression, energy, and cohesive force. The meaning and significance of these parameters are discussed. A particular advantage of the model is that it allows for determination of the brush thickness at zero loading force.

Highly charged dense poly(sulfopropyl methacrylate) polyelectrolyte brushes were indented with an atomic force microscopy (AFM) tip as well as with an 8 μm silica colloidal probe at different ionic strengths ranging from Millipore water to 1 M NaCl. The force response during indentation was fitted to a phenomenological equation analogous to the equation of state of a compressible fluid. In this way, internal energy and brush thickness were obtained as a function of ionic strength. Long-range forces decayed exponentially with distance. The characteristic decay lengths were much larger than the Debye screening lengths at the respective ionic strengths. It was therefore concluded that long-range repulsion was due to compression of a loose corona of polymers in front of the dense part of the brush. The size of the indentor determines which region of the brush can be explored by AFM. The tip probes the denser parts of the brush, while with the colloidal probe the corona of the brush can be investigated. The obtained fits of the experimentally measured force distance curves were used as regularization tools for obtaining the brush swelling pressure or "force per unit area" as a function of brush compression. The swelling pressure as a function of brush thickness, h, followed over a wide range a power law close to ∼h −2 . This approach allowed deriving fundamental brush parameters on a thermodynamical basis like the compressibility as a function of thickness.

In this manuscript we review work of our group on the assembly of lipid layers on top of polyelectrolyte multilayers (PEMs). The assembly of lipid layers with zwitterionic and charged lipids on PEMs is studied as a function of lipid and polyelectrolyte composition by the Quartz Crystal Microbalance. Polyelectrolyte lipid interactions are studied by means of Atomic Force Spectroscopy. We also show the coating of lipid layers for engineering different nanomaterials, i.e., carbon nanotubes and poly(lactic-co-glycolic) nanoparticles and how these can be used to decrease in vitro toxicity and to direct the intracellular localization of nanomaterials.

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