(65a) Insulator-Based Dielectrophoresis of Protein Particles Using Direct Current Electric Fields

This paper presents protein manipulation and immobilization employing insulator-based dielectrophoresis (iDEP) and DC electric fields. The manipulation of proteins employing dielectrophoresis has been reported by several research groups [1,2]. However, these studies have used electrode-based dielectrophoresis and AC electric fields. Our work differs from the mentioned studies since we utilized insulating structures instead of arrays of electrodes, and DC instead of AC electric fields.

Dielectrophoresis (DEP), the motion of particles in nonuniform electric fields, is an electrokinetic transport mechanism with a great potential for the manipulation of bioparticles [3,4]. Employing microelectrodes to generate nonuniform electric fields has some disadvantages, including elevated cost of microelectrode fabrication [3,5]. An alternative is to employ arrays of insulating structures, instead of electrodes, which provides with cheaper and more durable devices. The present study investigated the potential of iDEP for protein manipulation. A glass microdevice containing eight microchannels was employed. Microchannels were 10-mm deep, 10.16-mm long and 2-mm wide. Each microchannel contained an array of insulating cylindrical posts; diameter is 440 mm, height 10 mm and spaced 520 mm center-to-center (Figure 1). Electric fields were applied across the post array, creating regions of higher and lower electric field intensity. Bovine serum albumin (BSA) was employed, which was dyed using fluorescein-isothiocyanate. A sample of dyed protein solution was injected into the microchannel the electric field was applied. The dielectrophoretic response was recorded in the form of videos and pictures. Experiments were conducted varying the electric field, pH and conductivity of the suspending medium, in order to study the effect of these parameters on the dielectrophoretic response of protein BSA.

Figure 1: Schematic representation of experimental set-up, showing expected results of dielectrophoretic trapping of protein particles across the insulating post array.

Figure 2a shows the results obtained when 700 V/cm are applied with conductivity of 25 μS/cm and pH=8; no dielectrophoretic trapping is observed. Under these conditions, the dielectrophoretic force does not overcome the electrokinetic force. Figures 2b and 2c show results with the same medium and applying 900 and 1200 V/cm, respectively. Increasing the applied field leads to higher dielectrophoretic force, which overcomes the electrokinetic force, producing dielectrophoretic trapping. Dielectrophoretic trapping is negative, since protein particles are less conductive than the medium [1]. Figure 2d shows results with 700 V/cm, pH=8 and conductivity of 100 μS/cm. Comparing Figures 2b and 2d, shows that increasing conductivity increases particle immobilization at the same field. This is due to an increase in the magnitude of the Claussius-Mosoti factor, which increases the DEP force. Figure 2e shows protein immobilization employing a conductivity of 100 μS/cm, pH=9 and E= 900 V/cm. Comparing Figures 2d and 2e, it is demonstrated that increasing the pH leads to higher electric field required to achieve dielectrophoretic immobilization, since 900 V/cm were required to achieve trapping at pH=9. This is due to an increase of the electroosmotic flow. Figure 3 shows the effects of pH and conductivity on the minimum field required to achieve DEP trapping. These results demonstrate that it is possible to trap proteins employing iDEP and DC electric fields, and that optimal operating conditions are highest conductivity and lowest pH possible.

Figure 2. Dielectrophoretic response of protein particles Flow direction is from left to right, post diameter is 440 μm. Protein particles are shown green-yellow. Pictures (a) to (c), conductivity 25 μS/cm and pH=8. Picture (d), conductivity 100 μS/cm and pH=8. Picture (e), conductivity 100 μS/cm and pH=9. (a) E = 700 V/cm, (b) E = 900 V/cm, (c) E = 1200 V/cm (d) E= 700 V/cm and (e) E=900 V/cm.

Figure 3. Minimum applied electric field required to achieve dielectrophoretic trapping of protein particles. Results obtained by varying the conductivity and pH of the suspending medium.

REFERENCES

[1]        L.F. Zheng, J.P. Brody, P.J. Burke, Biosens. Bioelectron. 20 (2004) 606.

[2]        M. Washizu, O. Kurosawa, I. Arai, S. Suzuki, N. Shimamoto, IEEE Trans. Ind. Appl. 31 (1995) 447.

[3]        E.B. Cummings, A.K. Singh, Anal. Chem. 75 (2003) 4724.

[4]        H.A. Pohl, Dielectrophoresis, Cambridge University Press, Cambridge, 1978.

[5]        B.H. Lapizco-Encinas, B.A. Simmons, E.B. Cummings, Y. Fintschenko, Anal. Chem. 76 (2004) 1571.