an electrowetting-based microfluidic platform for magnetic bioassays

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ABSTRACT. Here we present our recent work on a droplet-based microfluidic device for manipulating microliter-sized droplets. By replacing the formerly used ...
AN ELECTROWETTING-BASED MICROFLUIDIC PLATFORM FOR MAGNETIC BIOASSAYS S. Chang1, 2, V. Schaller1, B. Raeissi1, A. Kalabukhov1, J. F. Schneiderman1, F. Öisjöen1, A. Jesorka1, A. Prieto Astalan3, C. Johansson3, P. Enoksson1, D. Winkler1 and A. Sanz-Velasco1 1

Chalmers University of Technology, SWEDEN, 2East China University of Science and Technology, CHINA, and 3The Imego Institute, SWEDEN

ABSTRACT Here we present our recent work on a droplet-based microfluidic device for manipulating microliter-sized droplets. By replacing the formerly used common dielectric SiO2 with Si3N4 and applying a 33 nm thick Teflon top layer to create a hydrophobic surface, we successfully lowered the actuation voltage from 450 V to 50 Vdc/ 40 Vac. Sputtered HfO2 with high dielectric constant was also investigated as an insulator, which could reproducibly yield thin defect-free insulation layers and lower the actuation voltage to less than 40 V. KEYWORDS: Digital microfluidics, ElectroWetting-On-Dielectric (EWOD), High-k dielectric, Silicon nitride (Si3N4), Hafnium dioxide (HfO2), Superconducting Quantum Interference Device (SQUID). INTRODUCTION Miniaturization and integration of microfluidic technology has demonstrated the increase in sensitivity and speed of biological analyses while drastically reducing their cost [1]. Previously, we demonstrate ElectroWetting-On-Dielectric (EWOD) transport and Superconducting Quantum Interference Device (SQUID) gradiometer detection of magnetic nanoparticles (MNPs) suspended in a 2 μl de-ionized water droplet (Figure 1). We observed that the signal amplitude per unit volume is 2.5 times higher for a 2 μl sample droplet compared to a 30 μl sample volume [2]. This proof-of-concept methodology constitutes the first development step towards a highly sensitive magnetic bioassay platform with SQUID readout and droplet-based sample handling. Compared to established bioassay detection principles such as fluorescence emission, SQUID magnetometric detection provides the highest achievable sensitivity, accompanied by simple assay chemistry and low time demand. This detection has the potential to reach fM analyte concentrations, comparable with or better than PolyHRP ELISA methods [3]. Real-time monitoring of controlled on-chip biochemical reactions is within reach.

EWOD chip with gold electrodes, oxide dielectric and hydrophobic coating.

Figure 1: Set-up for AC susceptibility measurements: (a) A Helmholtz pair, surrounding the SQUID and the sample holder, is used to excite the MNPs with a magnetic field H of 50 PT. (b) The MNPs (1) in suspension in a 2 Pl water droplet (2) are transported on the multilayered EWOD chip (3.) The SQUID gradiometer chip (4) sits on a sapphire rod (5) in a non-magnetic cryostat (6). A 250 Pm thick sapphire window (7) separates the cooled SQUID from the sample stage at room temperature. EXPERIMENTAL The hydrophobic layer, in particular thickness and surface roughness, were investigated first. Teflon® AF 1600 solution, diluted in Fluorinert® FC-40 to different concentrations ranging from 0.0075% - 6%, was deposited by spin coating onto silicon wafers at 6000 rpm for 60 s. The thickness of the Teflon layer was measured by a Wollam Elipsometer, while the detailed surface properties were checked by the optical Profilometer “Wyko” and SPM (Scanning Probe Microscopy) for higher resolution. In one set of experiments, a Teflon® AF 1600 solution was spin coated onto a 240 nm thick PECVD deposited silicon nitride (Si3N4) layer, which replaced the 500 nm thick dielectric layer of silicon dioxide previously used [2]. Alternatively, a 175 nm thick hafnium dioxide (HfO2) layer was deposited onto the electrodes by reactive RF sputtering, followed by 10 min annealing at 500°C in a N2 atmosphere and by spin-coating of the Teflon AF layer. We used

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14th International Conference on Miniaturized Systems for Chemistry and Life Sciences 3 - 7 October 2010, Groningen, The Netherlands

capacitance measurements to characterize the dielectric properties of both Si3N4 and HfO2, as well as contact angle measurements to characterize the effectiveness of the coating for droplet actuation. RESULTS AND DISCUSSION Further optimization of the chip layout has now led to greatly enhanced performance. In particular, lower actuation voltages were achieved by improving both the hydrophobic and dielectric insulation layers. ® The surface characteristics depending on Teflon AF content is shown in Figure 2. The spin coating of the Teflon solutions resulted in different layer thicknesses with varying surface roughness, depending on polymer content. A content of 1.2%, resulting in an average thickness of about 33 nm was found to give the most suitable coating for our application.

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(b) Figure 2: Hydrophobic layer thickness versus Teflon concentration. The inset shows an AFM scan of 10x10 µm2 of the sample with a Teflon concentration of 1.2%. The z-axis resolution is 12nm/div and the measured surface roughness of Teflon: Ra = 0.335 nm; RMS = 0.457 nm;

Figure 3: C-V measurements of Si3N4 (a) and HfO2 (b) dielectric layers.

To evaluate the dielectric properties of the insulation layers, we measured the capacitance versus voltage data (C-V) at room temperature for  = 2 × 1 kHz (Figure 3). Si3N4 has a relatively high dielectric constant (6.04), while it also displays hysteresis behaviour (Figure 3a). The hysteresis indicates a noticeable degree of power loss, and indeed, the droplets did not preserve the initial contact angle over the course of the experiment. This indicates defects in the Si3N4 layers, which could have their origin in the deposition method. Further experiments are required to evaluate and optimize the deposition conditions. Compared to Si3N4, HfO2 has a dielectric constant as high as 20.8 and shows no hysteresis (Figure 3b). Therefore it is a superior material for electrode insulation.

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Figure 4: Contact angle measurements of Si3N4. (black curves). (a) With DC voltage applied. (b) With AC voltage applied. Theoretical values (red curves) were calculated as in Ref. [4]. The coplanar electrode layout is shown as inset. 1332

The contact angle change of a 5 Pl de-ionized water droplet was measured using the coplanar electrode design [4] illustrated in Figs. 4a & 5a (inset). The initial contact angle was 115°. The new Si3N4 layout could tolerate actuation voltages of +300 Vdc, and -180 Vdc, respectively, without loss of integrity. The maximum contact angle change before saturation was 25° for DC and 33° for AC (Figure 4). We confirmed that AC voltage is more effective in actuating droplets. At Vac = 40 VRMS and f = 20 Hz, the contact angle change was about 19°, resulting in droplet actuation. No breakdown of the dielectric layer was observed even when a voltage of 150 VRMS was applied.

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Figure 5: Contact angle measurements of HfO2 (black curves). (a) With DC voltage applied. (b) With AC voltage applied. Theoretical values (red curves) were calculated as in Ref. [4]. The coplanar electrode layout is shown as inset. For HfO2, the insulation breakdown appeared at 40 VRMS. The actual voltage required for droplet actuation on the HfO2 layout remains to be measured but the value is expected to be lower than for the Si3N4 layout since the contact angle change at V = 40 VRMS and f = 20 Hz was about 22° for HfO2 (Figure 5b) compared to 19° for Si3N4. CONCLUSIONS We were able to achieve a performance optimization of our microfluidic platform for magnetic assays by a new surface coating methodology. Due to a thinner Teflon layer of 33 nm and deposition of silicon nitride as insulating layer on top of the patterned electrodes, the required actuation voltages noticeably decreased from ±450 Vdc to about 40 VRMS, well below the breakdown voltage for the new dielectric layer. Moreover, the high dielectric constant material HfO2 was investigated, and preliminary results show that a further decrease of actuation voltage can be achieved with this material. ACKNOWLEDGMENTS We thank Chemicell for supplying the FluidMAG-D CF nanoparticles and Crystec GmbH for supplying sapphire windows. Financial support from the EU Sixth Framework Program, Biodiagnostics (contract no. NMP4-CT-2005017002) is gratefully acknowledged. REFERENCES [1] S. Haeberle and R. Zengerle, Lab on a chip, pp. 1094-1100, 7 (2007). [2] V. Schaller, A. Sanz-Velasco, A. Kalabukhov, J.F. Schneiderman, F. Oisjöen, A. Jesorka, A.P. Astalan, A. Krozer, C. Rusu, P. Enoksson, D. Winkler, Lab on a chip, pp. 3433-3436, 9 (2009). [3] Y.R. Chemla, H.L. Grossman, Y. Poon, R. McDermott, R. Stevens, M.D. Alper, J. Clarke, PNAS, pp. 1426814272, 97 (2000). [4] U. C. Yi and C. J. Kim, Journal of Micromechanics and Microengineering, pp. 2053-2059, 16 (2006). CONTACT: Anke Sanz-Velasco, Tel: +46 31 772 18 76; [email protected]

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