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

Abstract

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.

Advances in Atomic Force Microscopy for Probing Polymer Structure and Properties

“…AFM has subangstrom height resolution and, therefore, can measure the layer thickness fast and accurately by cross-section analysis of the polymer covered and uncovered regions . Recently, AFM force–distance curve measurements were used to determine the layer thickness. − The applied loading force should must be considered when using AFM to measure the layer thickness, especially with compliant polymer layers, since the deformation of the polymers will cause errors in the measured height values. , For polymer brush modified surfaces, average molecular weights of the brush were determined by measuring the heights of the brushes in a good solvent in comparison to the known monomer length …”

Nature of the Specific Anion Response of a Hydrophobic Weak Polyelectrolyte Brush Revealed by AFM Force Measurements

“…In situ force measurements of polyelectrolyte brush-coated surfaces can provide insight into brush conformation, thickness, and physicochemical properties like interfacial adhesion and lubricity, all as a function of environmental conditions like pH and ionic strength, with surface force apparatus, − surface force balance, , optical tweezers, − and most commonly sharp tip − and colloidal probe − ,− atomic force microscopy (AFM) measurements reported. When two identical polyelectrolyte brush surfaces are brought close together they repel each other, and the range of these repulsive predominantly steric but also electrostatic forces reveals information about brush thickness, charge, and conformation at varying environmental conditions. − , For potential applications like particle stabilization, molecular binding, and biosensing, understanding the interaction forces between polyelectrolyte brushes and dissimilar colloidal bodies is of great importance.…”

“…When two identical polyelectrolyte brush surfaces are brought close together they repel each other, and the range of these repulsive predominantly steric but also electrostatic forces reveals information about brush thickness, charge, and conformation at varying environmental conditions. − , For potential applications like particle stabilization, molecular binding, and biosensing, understanding the interaction forces between polyelectrolyte brushes and dissimilar colloidal bodies is of great importance. Recently, the interaction of unmodified silica microparticles with both strong and weak polyelectrolyte brushes has revealed that both attractive and repulsive forces exist between the two, dependent on the pH and ionic strength of the surrounding medium. − ,, …”

“…Recently, the interaction of unmodified silica microparticles with both strong and weak polyelectrolyte brushes has revealed that both attractive and repulsive forces exist between the two, dependent on the pH and ionic strength of the surrounding medium. [11][12][13]53,56 Herein we present in situ AFM force measurements between a silica−colloid probe and a hydrophobic, weakly basic PDPA brush at pH 4.5 as a function of a range of potassium acetate, nitrate, and thiocyanate electrolyte solutions with overall ionic strength values between 0.04 and 100 mM (4 × 10 −5 and 0.1 M). These measurements were performed at ∼1.5 units below the apparent pK a of the PDPA brush where the brush is significantly swollen at intermediate salt concentrations.…”

Biosensing, Characterization of Biosensors, and Improved Drug Delivery Approaches Using Atomic Force Microscopy: A Review

“…Atomic force microscopy (AFM) (Lal and John 1994;Parot et al, 2007), with its unique capability to perform high-resolution imaging and characterize organic and inorganic samples in ambient air, liquid, and vacuum, is advantageous over other scanning probe microscopy (SPM) techniques to investigate all the scenarios mentioned above. Other than topographical imaging, AFM (Cuellar et al, 2013;Senapati et al, 2013;Vahabi et al, 2013) offers the opportunity to measure nanomechanical properties like elastic modulus, adhesion, deformation, and dissipation of any substate, and AFM cantilever tip acts a nanoindenter in that case. In addition, AFM probe might be transformed into a specific biosensor to measure piconewton (pN) range interaction forces with particular cellular receptor on live cells with high specificity and high signal-to-noise ratio.…”