Probing electrical forces with optical trapping

Session

Poster Session

Authors

William Hardiman (1), Tania Mendonca (1), Amanda J. Wright (1), Sidahmed Abayzeed (1)

Affiliations

1. Optics and Photonics Group, University of Nottingham

Keywords

Optical Trapping

Electrophoretic Force

Force Sensing

Bioelectrostatics

Abstract text

            Electrostatic interactions play a crucial role in cellular interactions with surfaces and the immune response, yet relatively little is known about bio-electrostatics. This is, in part, driven by a lack of techniques for probing electric fields at the microscale. Electric force microscopy, a variant of atomic force microscopy, can be applied to measure electric potential in the vertical direction but not lateral[1]. We have developed optical trapping methodologies for measuring and mapping electric forces in a manner which can be applied to three dimensional force and surface charge measurements.

Figure 1: Measuring electrical forces on optical trapped microsphere. a) Lateral force measurement using sinusoidal voltage waveform applied to gold electrodes. Position autocorrelation function (ACF) shown for experimental data and Brownian dynamics simulation show oscillations at the frequency of the applied voltage (500 Hz). Inset: Force calculated by fitting to ACF. b) Vertical force measurement scanning across edge of poly-lysine surface coating which carries a positive charge. Force is calculated by shift in the equilibrium position of the bead within the trap and normalised to the mean force when above the poly-lysine

            Optical tweezers are a well-established tool for micromanipulation and force measurement[2]. Here we trap sulfate-modified polystyrene microspheres (2 micron diameter, PolySciences, USA), which carry a negative surface charge, using an infrared laser inside a custom sample chamber which includes surface-integrated gold electrodes. Once trapped, the microsphere can be tracked in three-dimensions using well-established particle tracking techniques. The electrodes allow application of a known electric field to the microsphere. By applying a sinusoidal voltage waveform of varying amplitude, the effective charge of the microsphere can be measured[3]. Once the effective charge is known, the microsphere can be used as an electric field probe as shown in Figure 1a). Optical trapping allows the microsphere to be manipulated in three dimensions, making it possible to approach a surface of interest and measure the interaction force by the displacement of the microsphere relative to the trap centre. Poly-l-lysine coated glass coverslips are prepared following established protocols and the interaction force, shown in Figure 1b) is measured by considering the microsphere’s displacement from the trap centre. Brownian dynamics simulations are used to support interpretation of experimental data and to quantify the sensitivity of the technique.

            Here we present proof-of-principle measurements to demonstrate that we are sensitive to static and dynamic electric forces upon the trapped microsphere in three dimensions.

References

[1]: Ruiz-Cabello, F., et al. Physical Review E 90.1 (2014): 012301.

[2]: Neuman, K.C., and Block, S.M. " Review of scientific instruments 75.9 (2004): 2787-2809.

[3]: Pesce, G., et al. Optics Express 23.7 (2015): 9363-9368.