Anal. Chem. 2003, 75, 1578-1583
Selective Electroless and Electrolytic Deposition of Metal for Applications in Microfluidics: Fabrication of a Microthermocouple Peter B. Allen, Indalesio Rodriguez, Christopher L. Kuyper, Robert M. Lorenz, Paolo Spicar-Mihalic, Jason S. Kuo, and Daniel T. Chiu*
Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700
This paper describes a general strategy for the fabrication of a microthermocouple based on the spatially defined electroless deposition of metal, followed by annealing and electroplating. We present scanning electron microscopy and atomic force microscopy characterizations of the deposition and annealing process, as well as the performance of the microfabricated Ni-Ag thermocouple. The temperature-voltage curve for this Ni-Ag microthermocouple is linear over the range 0-50 °C with a slope of 61.9 °C mV-1. The sensitivity of our temperature measurement, which is limited by the uncertainty of our calibration curve, is ∼1 °C. The optimum figure of merit (Zopt) is 1.0 × 10-5 for this type of Ag-Ni thermocouple. We have fabricated microthermocouples ranging in size from 50 to 300 µm. The microthermocouple was integrated into microchannels and used to measure the inchannel temperature rise caused by the following: (1) a simple acid-base reaction, HCl + NaOH f H2O + NaCl, and (2) an enzyme-catalyzed biochemical reaction, H2O2 + catalase f H2O + 1/2O2. We have also profiled the temperature increase in the presence of electroosmotic flow for a 100-, 200-, and 300-µm channel.
separation techniques for analyzing complex mixtures,13-16 and integrated detection elements.17-22 Most of the optically17,18 and electrochemically19-22 based detection carried out in microfluidic devices are focused on detecting a response from the analytes present in the fluid rather than properties of the fluid itself, the most common form being optically based, most notably fluorescence because of its inherent sensitivity and compatibility with microscale assays. This paper describes the direct measurement of the thermal properties of the fluid using a microfabricated thermocouple. The ability to probe locally the thermal behavior of microfluidic systems is especially pertinent because of the superior heattransfer properties characteristic of these microdevices. Through the fabrication of the microthermocouple, this paper also details a general strategy - that is compatible both with microfluidics and with the procedures used in microfabricationsfor exploiting the spatially selective electroless and electrolytic deposition of metal for the creation of functional microelectrical elements. This approach can be applied to any solution-phase metal deposition procedures in which the plating solution is compatible with the polymer microchannels used to define the deposited metal patterns. This strategy is expected to be useful toward the integration of electrical elements into microfluidic devices both
Microfluidic systems are being developed rapidly into an integrated platform for microanalyses1-5 and, more recently, for chemical syntheses.6 These advances underlie the drive toward the integration of multiple functionalities on chip, including both active7-9 and passive10-12 methods for manipulating fluid flow,
(10) Beebe, D. J.; Moore, J. S.; Bauer, J. M.; Yu, Q.; Liu, R. H.; Devadoss, C.; Jo, B. H. Nature 2000, 404, 588-590. (11) Stroock, A. D.; Dertinger, S. K. W.; Ajdari, A.; Mezic, I.; Stone, H. A.; Whitesides, G. M. Science 2002, 295, 647-651. (12) Jeon, N. L.; Chiu, D. T.; Wargo, C. J.; Wu, H.; Choi, I. S.; Anderson, J. R.; Whitesides, G. M. Biomed. Microdevices 2002, 4, 117-121. (13) Paegel, B. M.; Emrich, C. A.; Weyemayer, G. J.; Scherer, J. R.; Mathies, R. A. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 574-579. (14) Chen, X.; Wu, H.; Mao, C.; Whitesides, G. M. Anal. Chem. 2002, 74, 17721778. (15) Rocklin, R. D.; Ramsey, R. S.; Ramsey, J. M. Anal. Chem. 2000, 72, 52445249. (16) Tang, T.; Badal, M. Y.; Ocvirk, G.; Lee, W. E.; Bader, D. E.; Bekkaoui, F.; Harrison, D. J. Anal. Chem. 2002, 74, 725-733. (17) Chabinyc, M. L.; Chiu, D. T.; McDonald, J. C.; Stroock, A. D.; Christian, J. F.; Karger, A. M.; Whitesides, G. M. Anal. Chem. 2001, 73, 4491-4498. (18) Lapos, J. A.; Manica, D. P.; Ewing, A. G. Anal. Chem. 2002, 74, 33483353. (19) Martin, R. S.; Ratzlaff, K. L.; Huynh, B. H.; Lunte, S. M. Anal. Chem. 2002, 74, 1136-1143. (20) Zeng, Y.; Chen, H.; Pang, D. W.; Wang, Z. L.; Cheng, J. K. Anal. Chem. 2002, 74, 2441-2445. (21) Hilmi, A.; Luong, J. H. T. Anal. Chem. 2000, 72, 4677-4682. (22) Baldwin, R. P.; Roussel, T. J., Crain, M. M.; Bathlagunda, V.; Jackson, D. J.; Gullapalli, J.; Conklin, J. A.; Pai, R.; Naber, J. F.; Walsh, K. M.; Keynton, R. S. Anal. Chem. 2002, 74, 3690-3697.
* To whom correspondence should be addressed. E-mail: chiu@chem. washington.edu. (1) Reyes, D. R.; Lossitidis, D.; Auroux, P.; Manz, A. Anal. Chem. 2002, 74, 2623-2636. (2) Auroux, P.; Lossitidis, D.; Reyes, D. R.; Manz, A. Anal. Chem. 2002, 74, 2637-2652. (3) Wang, J. Electrophoresis 2002, 23, 713-718. (4) Whitesides, G. M.; Stroock, A. D. Phys. Today 2001, 6, 42-48. (5) Simpson, P. C.; Roach, D.; Woolley, A. T.; Thorsen, T.; Johnston, R.; Sensabaugh, G. F.; Mathies, R. A. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 2256-2261. (6) Mitchell, M. C.; Spikmans, V.; Manz, A.; de Mello, A. J. J. Chem. Soc., Perkins Trans. 1 2001, 5, 514-518. (7) Unger, M. A.; Chou, H.; Thorsen, T.; Scherer, A.; Quake, S. R. Science 2000, 288, 113-116. (8) Terray, A.; Oakey, J.; Marr, D. W. M. Science 2002, 296, 1841-1844. (9) Jacobson, S. C.; Ermakov, S. V.; Ramsey, J. M. Anal. Chem. 1999, 71, 32733276.
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© 2003 American Chemical Society Published on Web 02/25/2003
for the manipulation of fluids and analytes and for the construction of electrically based detectors. Thermocouples are a common tool for measuring temperatures within a specified range very accurately.23-26 Thermocouples and thermopiles have been microfabricated or micromachined previously27 and have been demonstrated in applications such as thermal radiation sensors, thermal converters, and ac voltage meters. Here, we demonstrate the integration and applications of thermocouples in microfluidics. In addition, thermocouples are employed as power meters for measuring the output of light sources.28,29 This application of macroscale and thick-film thermocouples suggests the integration of microthermocouples with microfabricated lasers (e.g., Vertical Cavity Surface Emitting Lasers) for applications in microfluidics. The principle of operation for thermocouples relies on the Seebeck effect, which is the generation of a predictable voltage at the junction of two different metals that is related to the difference in temperature between the measuring junction and a reference junction. In comparison with other techniques for temperature measurements, such as resistive methods, thermocouples have stable temperature calibration curves, are noninvasive, and offer high spatial resolutions. It is also straightforward to apply a thin layer of protective coating on thermocouples to prevent their electrochemical degradation in harsh environments. These characteristics are especially pertinent to temperature-sensitive applications30-32 in microfluidics and microelectromechanical systems (MEMS), including their integration into microreactor platforms, microfluidic-based assays, and microfabricated sensors. EXPERIMENTAL SECTION Preparation of Glass. To prepare the surface of glass for electroless deposition, glass slides were sonicated in acetone for 10 min, followed by hydrolyzing the surface for 1 h in a boiling solution of water, ammonium hydroxide, and hydrogen peroxide in a 3:1:1 ratio. The glass slides were rinsed and briefly washed in soap and water, rinsed with ethanol, and dried in a stream of nitrogen gas. Electroless Nickel. The electroless deposition of Ni requires first sensitizing then activating the surface prior to electroless plating. The sensitizer, which contains 0.1 g/L stannous(II) chloride in 0.01% hydrochloric acid solution, must be prepared daily because the solution degrades over 24 h. The solution of activator, which contains 0.1 g/L palladium(II) chloride in 0.01% hydrochloric acid, also has a lifetime of ∼1 day. The dissolution of palladium chloride was facilitated by heating the solution to produce a yellow cloudy color, followed by sonication until the cloudiness disappears. The electroless Ni plating solution contains 20 g/L sodium hypophosphite, 40 g/L nickel(II) sulfate, 100 g/L sodium citrate, and 50 g/L ammonium chloride. (23) Sandfort, R. M.; Charlson, E. J. Solid-State Electron. 1968, 11, 635-637. (24) Beckman, P.; Roy, R. P.; Velidandla, V.; Capizzani, M. Rev. Sci. Instrum. 1995, 66, 4731-4733. (25) Fish, G.; Kokotov, B. S.; Lieberman, K.; Palanker, D.; Turovets, I.; Lewis, A. Rev. Sci. Instrum. 1995, 66, 3300-3306. (26) Forster, R.; Gmelin, E. Rev. Sci. Instrum. 1996, 67, 4246-4255. (27) Baltes, H.; Paul, O.; Oliver, B. Proc. IEEE 1998, 86, 1660-1678. (28) Offenberger, A. A. Appl. Opt. 1970, 9, 2594-2597. (29) Smetana, W.; Nicolics, J. Sens. Actuators, A 1993, 37-38, 565-570. (30) Ross, D.; Gaitan, M.; Locascio, L. E. Anal. Chem. 2001, 73, 4117-4123. (31) Mao, H. B.; Yang, T. L.; Cremer, P. S. J. Am. Chem. Soc. 2002, 124, 44324435. (32) Ross, D.; Locascio, L. E. Anal. Chem. 2002, 74, 2556-2564.
Annealing. The electrolessly deposited metal was annealed in a steel box with a copper inlet connected to a source of N2 gas and an outlet. The whole assembly was placed on top of a hot plate, with the temperature inside this box being monitored continuously with a thermocouple. The annealing was performed at 400 °C for 1 h under a positive N2 atmosphere created by introducing pressurized N2 to the inlet of this annealing box. Electrolytic Nickel. Electrolytic plating of the annealed Ni was carried out in a bath of 240 g/L nickel(II) sulfate, 45 g/L nickel(II) chloride, and 30 g/L boric acid until an even, mirrored surface was observed, which usually takes ∼45 s. The voltage and current applied were 2 V and ∼15 mA, respectively. Electroless Silver. Electroless deposition of silver was carried out using a commercially available silver-plating package (HC300 kit and No. 93 sensitizer) from Peacock Laboratories (Philadelphia, PA). The HC-300 kit consists of three parts: a silver solution, an activator solution, and a reducer solution. The concentrates from each of these solutions were diluted in deionized water in a 3.5:100 ratio. Immediately prior to electroless deposition, the diluted silver solution, activator solution, and reducer solution were mixed in equal proportions to form the electroless Ag-plating solution. Electrolytic Silver. The solution for electroplating silver consists of 36 g/L silver cyanide, 60 g/L potassium cyanide, and 45 g/L potassium carbonate. Plating is usually completed after an application of 2 V and ∼5 mA for ∼20 s. Preparation of Poly(dimethylsiloxane) (PDMS) Channels. The preparation of PDMS channels has been described in detail elsewhere.33 Briefly, a photomask transparency was designed using a vector drawing program (Freehand version 9, Macromedia, San Francisco, CA) and printed with a commercial image setter at 3500 dpi. Photoresist (SU-8 50, Microchem Corp., Newton, MA) was spun onto a clean silicon wafer at 3000 rpm for 30 s to yield a 50-µm-thick layer of resist, which was then baked and UV exposed under the transparency mask (contact photolithography). Postexposure, the resist was baked and then developed in propylene glycol methyl ether acetate (Aldrich, Milwaukee, WI). This bas-relief pattern of SU-8 on silicon wafer acts as a master for subsequent replication and molding of microchannels using PDMS. To facilitate removal of the PDMS mold, the surface of the wafer was treated by placing it in a desiccator containing an open vial of tridecafluouro-1,1,2,2-tetrahydrooctyl-1-trichlorsilane (United Chemical Technologies, Bristol, PA). To prepare PDMS for molding, the prepolymer was first mixed thoroughly with curing agent in a 10:1 mass ratio and then degassed in a desiccator for 30 min. The mixed and degassed PDMS was applied to the finished master and cured at 65 °C for ∼3 h. Once the appropriate mold has been prepared, reservoirs and tube inlets were created using a stopper punch or syringe needle, respectively. Materials and Chemicals. A vacuum pump, GASI model DOA-P104-AA from VWR (Irving, TX), with a vacuum range of 0-30 in. of mercury was used to apply negative pressure to microchannels. Silicon wafers were obtained from Montco Silicon Technologies, Inc (Spring City, PA) and PDMS (Sylgard 184) from Dow Corning Co. (Midland, MI). Catalase, sodium hypophosphite, (33) McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H.; Whiteside, G. M. Electrophoresis 2000, 21, 27-40.
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Figure 2. (A-C) SEM images of the electrolessly deposited silver without (A) and with (B) annealing as well as the electrolytic plating of silver after electroless deposition and annealing (C). (D-F) The corresponding AFM images of silver deposited electrolessly without annealing (D), with anneling (E), and electrolytically (F).
Figure 1. Schematic showing the procedure used for the microfabrication of a thermocouple, which consists of intersecting Ni and Ag metal lines.
and nickel sulfate were obtained from Sigma Chemical Co. (St. Louis, MO). SnCl2 and PdCl2 were from EM Science (Gibbstown, NY), Tris-HCl (0.5 M) buffer was purchased from Bio-Rad Laboratories (Hercules, CA), and phosphate-buffered saline (pH 7.4) from Fisher Scientific (Fair Lawn, NJ). RESULTS AND DISCUSSION Fabrication of Thermocouple with Selective Electroless and Electrolytic Deposition of Metals on Glass. A wide range of metals and metal alloys can be used for the fabrication of thermocouples. The selection of an appropriate type of thermocouple (i.e., the metals or alloys used in their construction) depends on the particular application, such as the temperature range to be measured, the solution or atmosphere into which the thermocouple is placed, and the accuracy required, as well as the ease of fabrication and integration. Our choice for fabricating a Ni-Ag thermocouple was based on the following: (1) linear output voltage curve within our temperature range of interest and (2) ease of microfabrication and its compatibility with most applications in microfluidics. Figure 1 shows the procedure we used for the patterned electroless plating of Ni and Ag. Prior to electroless Ni deposition, the surface to be platedswhich is defined by the pattern of microchannels in PDMS that is sealed conformally to the glass surfaceswas first primed with a stannous chloride solution by flowing the solution through the channels for ∼3 min and then rinsed by flowing deionized water through the channels for ∼30 s. The sensitized surface is subsequently activated by flowing a palladium chloride solution through the channels for another ∼3 min. This spatially selective priming and activation defines the 1580 Analytical Chemistry, Vol. 75, No. 7, April 1, 2003
final pattern of the plated Ni. After the surface was activated, the PDMS stamp was removed, and the slide was immersed into a 55-60 °C bath of electroless Ni plating solution. Ni was allowed to deposit until a dark and nearly opaque appearance was observed, after which the slide was removed and rinsed with deionized water. This electroless Ni was then annealed at ∼400 °C for 1 h under a N2 atmosphere. The annealed Ni was then electrolytically plated with a thin (
