Charge Calibration Standard for Atomic Force Microscope Tips in Liquids

Abstract

An electric charge standard with nanoscale resolution is created using the known charge distribution of a single tobacco mosaic virus coat protein combined with the known packing of these proteins in the virus capsid. This advances the ability to measure charge on nanometric samples. Experimental atomic force microscope (AFM) force−distance curves are collected under aqueous conditions with controlled pH and ion concentration. A mathematical model that considers a polarizable dielectric tip immersed in an electrolyte is used to obtain charge density from the AFM measurements. Interactions between the tip and the sample are modeled using theory that includes monopolar electrostatic interactions, dipolar interactions, screening from both the dielectric nature of ambient water and solvated ions as described by the linear Poisson−Boltzmann equation, and hard-core repulsion. It is found that the tip charge density changes on a timescale of hours requiring recalibration of the tip for experiments lasting more than an hour. As an example of how a chargecalibrated tip may be used, the surface charge densities on 20 individual carboxylate-modified polystyrene (PS) beads are measured. The average of these AFM-measured bead charge densities is compared with the value obtained from conventional titration combined with electron microscopy. The two values are found to agree within 20%. While the comparison demonstrates similarity of the two charge measurements, hypotheses are put forward as to why the two techniques might be expected not to provide identical mean charge densities. The considerations used to build these hypotheses thus underscore the relevance of the method performed here if charge information is required on individual nanoparticles.

Force-Induced Changes of PilY1 Drive Surface Sensing by Pseudomonas aeruginosa

“…Gold cantilevers (PNP-TR probes [Pyrex nitride probe with triangular cantilevers] from NanoWorld) were treated for 16 h with a 1 mM 1-dodecanethiol solution in ethanol to render them hydrophobic and then rinsed with ethanol and kept in MilliQ water until the AFM experiments were ready to be performed. Prior any measurements, the cantilever’s spring constant was empirically determined by the thermal noise method ( 55 ). The AFM force volume mode was used to record the force-distance curve in a pixel-by-pixel manner (force mapping) on 6- by 6-μm 2 areas (32 by 32 pixels, i.e., 1,024 curves) with a bacterium at the center, previously localized by an optical microscope coupled to the AFM.…”

T cell stiffness is enhanced upon formation of immunological synapse

“…Cells were first settled on coverslips at 37°C with 5% CO 2 for 15 min (Berquand et al, 2010). Prior to each experiment, the AFM cantilever (MLCT cantilever B, Bruker) was calibrated using the thermal tune method (Li, Steinmetz et al, 2020). The spring constant given by manufacturer is 0.02 N/m (min 0.005, max 0.04 N/m), and the calibrated spring constant is 0.06 -0.1 N/m, which varies slightly in different cantilevers.…”

T cell stiffness is enhanced upon formation of immunological synapse

“…For the inhibitor treatment, T cells were pre-treated with the indicated inhibitor or vehicle control (DMSO) at room temperature for 30 min. Prior to each experiment, the AFM cantilever (MLCT cantilever B, Bruker) was calibrated using the thermal tune method (Li, Steinmetz et al, 2020). Elasticity mapping was carried out with the following parameters: line scan rate: 0.25 Hz, feedback gain: 0.5, peak force amplitude: 100 nm, peak force threshold: 700 pN and a resolution of ~60 nm.…”