Indentation of Highly Charged PSPM Brushes Measured by Force Spectroscopy: Application of a Compressible Fluid Model

Ion-specific nanoscale compaction of cysteine-modified poly(acrylic acid) brushes revealed by 3D scanning force microscopy with frequency modulation detection

Stimuli-responsive polyelectrolyte brushes adapt their physico-chemical properties according to pH and ion concentrations of the solution in contact. We synthesized a poly(acrylic acid) bearing cysteine residues at side chains and a lipid head group at the terminal, and incorporated them into a phospholipid monolayer deposited on a hydrophobic silane monolayer. The ion-specific, nanoscale response of polyelectrolyte brushes was detected by using three-dimensional scanning force microscopy (3D-SFM) combined with frequency modulation detection. The obtained topographic and mechanical landscapes indicated that the brushes were uniformly stretched, undergoing a gradual transition from the brush to the bulk electrolyte in the absence of divalent cations. When 1 mM calcium ions were added, the brushes were uniformly compacted, exhibiting a sharper brush-to-bulk transition. Remarkably, the addition of 1 mM cadmium ions made the brush surface significantly rough and the mechanical landscape highly heterogeneous. Currently, cadmium-specific nanoscale compaction of the brushes is attributed to the coordination of thiol and carboxyl side chains with cadmium ions, as suggested for naturally occurring, heavy metal binding proteins.

Polyelectrolyte multilayers under compression: concurrent osmotic stress and colloidal probe atomic force microscopy

Colloidal interactions have been characterised using both osmotic stress and surface forces. Here these methods are employed concurrently to measure the interaction forces of polyelectrolyte multilayers that when cross-linked form a dextran impermeable membrane. The force data, corrected for the thickness of the polyelectrolyte multilayer film, has been expressed as pressure versus separation enabling the interaction from osmotic stress measurements to be compared to the measured interaction from the colloid probe technique. The combined technique is valuable in evaluating the interaction forces associated with compression of polymer films at different rates and over a wide range of pressure and demonstrates features that are not revealed when just one technique is employed. The combination of the techniques allows both attractive forces and strongly repulsive forces to be measured and shows that the measured repulsion is greater in the force data than in the osmotic data. This is due to insufficient equilibration time in the AFM measurements, even at the slowest approach rates available, indicating that AFM force measurements between polyelectrolytes will always contain a dynamic component. That is we demonstrate that colloid probe measurements between polymer surfaces overestimate the equilibrium repulsive interaction due to the rate at which the measurement is performed.

Mapping nano-scale mechanical heterogeneity of primary plant cell walls

Nanoindentation experiments are performed using an atomic force microscope (AFM) to quantify the spatial distribution of mechanical properties of plant cell walls at nanometre length scales. At any specific location on the cell wall, a complex (non-linear) force-indentation response occurs that can be deconvoluted using a unique multiregime analysis (MRA). This allows an unambiguous evaluation of the local transverse elastic modulus of the wall. Nanomechanical measurements on suspension-cultured cells (SCCs), derived from Italian ryegrass (Lolium multiflorum) starchy endosperm, show three characteristic modes of deformation and a spatial distribution of elastic moduli across the surface. 'Soft' and 'hard' domains are found across length scales between 0.1 µm and 3 µm, which is well above a typical pore size of the polysaccharide mesh. The generality and wider applicability of this mechanical heterogeneity is verified through in planta characterization on leaf epidermal cells of Arabidopsis thaliana and L. multiflorum The outcomes of this research provide a basis for uncovering and quantifying the relationships between local wall composition, architecture, cell growth, and/or morphogenesis.

Quantitative mechanical analysis of thin compressible polymer monolayers on oxide surfaces

A clear understanding of the mechanical behavior of nanometer thick films on nanostructures, as well as developing versatile approaches to characterize their mechanical properties, are of great importance and may serve as the foundation for understanding and controlling molecular interactions at the interface of nanostructures. Here we report on the synthesis of thin, compressible polyethylene glycol (PEG) monolayers with a wet thickness of <20 nm on tin dioxide (SnO2) nanofibers through silane-based chemistries. Nanomechanical properties of such thin PEG films were extensively investigated using atomic force microscopy (AFM). In addition, tip-sample interactions were carefully studied, with different AFM tip modifications (i.e., hydrophilic and hydrophobic) and in different ionic solutions. We find that the steric forces dominate the tip-sample interactions when the polymer film is immersed in solution with salt concentrations similar to biological media (e.g., 1x phosphate buffer solution), while van der Waals and electrostatic forces have minimal contributions. A Dimitriadis thin film polymer compression model shows that the linear elastic regime is reproducible in the initial 50% indentation of these films which have tunable Young's moduli ranging from 5 MPa for the low molecular weight films to 700 kPa for the high molecular weight PEG films. Results are compared with the same PEG films deposited on silicon substrates which helped quantify the structural properties and understand the relationship between the structural and the mechanical properties of PEG films on the SnO2 fibers.

Measurement of the electrostatic interaction between polyelectrolyte brush surfaces by optical tweezers

We demonstrated an optical tweezers method to measure the electrostatic interaction between the strong polyelectrolyte brushes, poly(2-(methacryloyloxy)ethyltrimethylammonium chloride) (PMTAC), grafted on silica particles in aqueous media. The weak electrostatic interaction was successfully detected with a resolution of less than 0.1 μN m(-1). The apparent Debye length, including the charge distribution in the polymer brush and the surface potential, decreased as the salt concentration in the medium increased. The experimentally obtained surface charge density was much smaller than that estimated from the amount of polyelectrolyte on the surface. Furthermore, the dissociation of ionic groups was enhanced by decreasing the grafting density of the polyelectrolyte brush. The results suggest that the majority of chloride counterions was immobilized in the dense polyelectrolyte brush layer to neutralize the high charge density.

A novel approach for measuring the intrinsic nanoscale thickness of polymer brushes by means of atomic force microscopy: application of a compressible fluid model

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.