Trapping Single Molecules by Dielectrophoresis

We have trapped single protein molecules of $R$-phycoerythrin in an aqueous solution by an alternating electric field. A radio frequency voltage is applied to sharp nanoelectrodes and hence produces a strong electric field gradient. The resulting dielectrophoretic forces attract freely diffusing protein molecules. Trapping takes place at the electrode tips. Switching off the field immediately releases the molecules. The electric field distribution is computed, and from this the dielectrophoretic response of the molecules is calculated using a standard polarization model. The resulting forces are compared to the impact of Brownian motion. Finally, we discuss the experimental observations on the basis of the model calculations.

Figure 1

Electron micrograph of gold electrodes on a silicon substrate. Scale bar $5\mu \mathrm{m}$. The largest electrode pair with 500 nm gap width has been used.

Figure 2

Dielectrophoretic trapping of single $R$-phycoerythrin molecules between triangular electrodes. Fluorescence micrographs are in false color representation. (a) Protein solution (0.6 nM) and electrodes before field application. (b) After 10 s field application at 1 MHz and 10 V. (c) Difference of images (b),(c). Scale bar $5\mu \mathrm{m}$.

Figure 3

Fluorescence intensity histogram of $R$-phycoerythrin adsorbed onto a glass surface.

Figure 4

(a) Fluorescence autocorrelation function $G(t)$ of the $R$-phycoerythrin solution. (b) Difference between data and numerical simulation assuming a single fluorescent species.

Figure 5

Temporal course of fluorescence intensity at the electrode tips at 0.7 MHz and 1.6 V. Bars emphasize “on” periods.

Figure 6

Calculation of the electric field gradient $\mathrm{grad}|E_{\mathrm{rms}}{|}^2$. The color coded projection shows the field gradient in the electrode plane (note the logarithmic scale). The gray shaded surface in the upper part encloses the space where the electric gradient force exceeds molecular diffusion ($F>3\times {10}^{-16}\mathrm{N}$).