Anal. Chem. 2007, 79, 4720-4723
Adaptive Nanowires for Switchable Microchip Devices Evandro Piccin,†,‡ Rawiwan Laocharoensuk,† Jared Burdick,† Emanuel Carrilho,‡ and Joseph Wang*,†
Departments of Chemical Engineering and Chemistry and Biochemistry, Biodesign Institute, Arizona State University, Tempe, Arizona 85287-5801, and Departamento de Quı´mica e Fı´sica Molecular, Instituto de Quı´mica de Sa˜o Carlos, Universidade de Sa˜o Paulo, Sa˜o Carlos, SP, Brazil
This paper demonstrates for the first time the use of adaptive functional nickel nanowires for switching ondemand operation of microfluidic devices. Controlled reversible magnetic positioning and orientation of these nanowires at the microchannel outlet offers modulation of the detection and separation processes, respectively. The former facilitates switching between active and passive detection states to allow the microchip to be periodically activated to perform a measurement and reset it to the passive (“off”) state between measurements. Fine magnetic tuning of the separation process (postchannel broadening of the analyte zone) is achieved by reversibly modulating the nanowire orientation (i.e., detector alignment) at the channel outlet. The concept can be extended to other microchip functions and stimuli-responsive materials and holds great promise for regulating the operation of microfluidic devices in reaction to specific needs or unforeseen scenarios. Microfluidic (lab-on-a-chip) devices, involving the handling and manipulation of ultrasmall liquid volumes, provide a bridge between the molecular world that governs the processes of life and the digital world of computing and communication.1,2 Such devices offer great promise for microscale bioanalysis, automated drug discovery, or security surveillance. Yet, despite extensive activity for over a decade, little effort has been devoted to the development of “smart” (adaptive) microchips able to change their operation to meet specific needs or opportunities.3 In this paper, we describe a novel nanowire-based strategy for controlling on demand the separation and detection processes in microfluidic devices. “Adaptive materials”, whose function can be controlled via external (photonic or magnetic) stimuli, offer great promise for changing the operation of sensors or microchips in response to a specific need (event, opportunity, etc.).1 For example, magneto-switchable processes have been useful for ondemand magnetic control of biosensing events.4 Similarly, stimuli* Corresponding author. E-mail:
[email protected]. † Arizona State University. ‡ Universidade de Sa˜o Paulo. (1) Byrne, R.; Diamond, D. Nat. Mater. 2006, 5, 421-424. (2) Thorsen, T.; Maerkl, S. J.; Quake, S. R. Science 2002, 298, 580-584. (3) Wang, J.; Pumera, M.; Chatrathi, M. P.; Escarpa, A.; Musameh, M.; Collins, G.; Mulchandani, A.; Lin, Y.; Olsen, K. Anal. Chem. 2002, 74, 1187-1191. (4) Katz, E.; Sheeney-Haj-Ichia, L.; Buckmann, A. F.; Willner, I. Angew. Chem., Int. Ed. 2002, 41, 1343-1346.
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responsive nanowires have been employed for reversible magnetoswitchable control of electrochemical processes.5 As will be illustrated below, adaptive nanowires can add a unique dimension to the operation of microfluidic devices, including control of the separation and detection processes. The present use of adaptive nanowires for magneto-switchable microchip operation relies on placing, reorienting, and removing on-demand catalytic/magnetic nickel nanowires at the exit of the separation channel (Figure 1). This allows magnetic switching between active and passive detection states as well as fine-tuning the microchip separation efficiency (peak resolution). The movie shown in the Supporting Information (SI) illustrates the magnetic positioning of the nickel nanowires at the channel exit and the reversible switching of the nanowires orientation. The magnetic properties of nickel and its electrocatalytic activity toward aliphatic alcohols, carbohydrates, and amino acids6 make nickel nanowires ideally suited for such adaptively controlled operation of microchip detection. Magnetic manipulations are particularly suitable for such adaptive microchip operation as they offer unique possibilities for controlling externally matters inside and outside a microchannel.7 Such capabilities were illustrated below in connection with placement of magnetic particle bioreactors8,9 or magnetics for valving/mixing,10,11 but not for generating “temporary” detectors such as those described below for adaptive microchip detection and separation. EXPERIMENTAL SECTION Chemicals. All solutions were prepared from double-distilled water. Sodium hydroxide, glucose, arginine, histidine, glycine, NiCl2‚6H2O, Ni(H2NSO3)2‚4H2O, and H3BO3 were purchased from Sigma. Anodisc alumina membranes with a pore size of 200 nm and thickness of 60 µm were purchased from Whatman (Catalog No. 6809-6022; Maidstone, U.K.). Stock solutions of the various (5) Wang, J.; Scampicchio, M.; Laocharoensuk, R.; Valentini, F.; GonzalezGarcia, O.; Burdick, J. J. Am. Chem. Soc. 2006, 128, 4562-4563. (6) Cassela, I. G.; Cataldi, T. R.; Salvi, A.; Desimoni, E. Anal. Chem. 1993, 65, 3143-3150. (7) Pamme, N. Lab Chip 2006, 6, 24-38. (8) Fan, Z. H.; Mangru, S.; Granzow, R.; Heaney, P.; Ho, W.; Dong, Q. P.; Kumar, R. Anal. Chem. 1999, 71, 4851. (9) Choi, J. W.; Oh, K. W.; Thomas, J. H.; Heineman, W. R.; Halsall, H. B.; Nevin, J. H.; Helmicki, A. J.; Henderson, H. T.; Ahn, C. H. Lab Chip 2002, 2, 27. (10) Bau, H. H.; Zhu, J. Z.; Qian, S. Z.; Xiang, Y. Sens. Actuators, B 2003, 88, 207. (11) Ryu, K. S.; Shaikh, K.; Goluch, E.; Fan, Z. F.; Liu, C. Lab Chip 2004, 4, 608. 10.1021/ac0705519 CCC: $37.00
© 2007 American Chemical Society Published on Web 05/11/2007
Figure 1. Schematic representation of the magneto-switchable microchip detector based on adaptive nanowires. Magnetic control is used to position and reorient the nickel nanowires on the gold layer sputtered onto the outlet of the separation microchannel. Also shown (top) are corresponding electropherograms of a mixture without (A) and with the nanowires aligned in vertical (B) and horizontal (C) positions at the microchannel exit area.
analytes were prepared daily in the correspondent running buffer solution. Apparatus. The design of the Plexiglas holder, housing the separation chip and the detector, has been described previously.1 The glass microchannel chips were purchased from Microlyne Inc. (model MC-BF4-001, Edmonton, Canada). The detection reservoir of the chip was cut off to facilitate the end-channel electrochemical detection. The chip consisted of a 68 × 17 mm glass plate with a simple cross-channel layout. The separation channel (between the buffer and detection reservoirs) was 65 mm in length, while the injection channel (between the sample and sample-waste reservoirs) was 10 mm long. The intersection of the separation and injection channels was located 5 mm from the buffer reservoir, yielding a separation channel with an effective length of 60 mm. The channels had a maximum depth of 20 µm and a width of 50 µm. Short pipet tips were placed in the holes of the various reservoirs. Platinum wires, inserted into the individual reservoirs on the holder, served as contacts to the high-voltage power supply. The homemade high-voltage power supply had an adjustable voltage range between 0 and +4000 V. Electrode Construction. Figure 1 displays the nanowires electrode assembly at the end of the separation channel. A thin gold film, sputtered around the outlet of the separation channel, served as an electrical contact for the catalytic nickel nanowires during amperometric detection. The chip surface surrounding the outlet was cleaned first by dipping into a “piranha” solution (4:1 concentrated H2SO4/30% H2O2) for 5 min. It was then rinsed thoroughly with deinonized water and allowed to air-dry. Gold sputtering on the chip outlet was performed with a Denton Vacuum Desk-III sputtering machine (Denton Vaccum, Inc., Moorestown, NJ), using an argon pressure of 50 mTorr and a current of 40 mA for 360 s; this resulted in a patterned film thickness of ∼200 nm. Patterning the sputtered gold (usually 0.5
× 5 mm) was accomplished by protecting the resulting uncoated area with Scotch tape. Electrical contact was achieved by connecting a copper wire to the gold layer using a conductive epoxy (SPI Supplies, West Chester, PA). Finally, an insulator ink (Ercon ink R-488C1) was used for covering part of the gold layer, exposing an electrode area (around the channel outlet) of ∼1.0 mm × 1.0 mm. Nickel Nanowire Synthesis. Nickel nanowires were prepared by electrochemical deposition into the 200-nm-diameter nanopores of a 60-µm-thick alumina membrane template (Anodisc, Whatman, Inc.).2 A gold film was sputtered on one side of the template to provide an electrical contact. Nickel was deposited using a potential of -1.0 V (vs Ag/AgCl), from a solution of 20 g L-1 NiCl2‚ 6H2O, 515 g L-1 Ni(H2NSO3)2‚4H2O, and 20 g L-1 H3BO3 (pH 3.4). Nickel nanowires (∼15-µm length) were grown by controlling the electrodeposition charge to 50 C. Following the nickel electrodeposition, the sputtered gold film was removed by polishing with 3-µm alumina powder using a grinder/polisher (model 900, South Bay Technology Inc., San Clemente, CA). The nanowires were released by dissolving the template in a 3 M NaOH solution for 10 min under stirring. The resulting nickel nanowires were repeatedly washed with water to remove residual base and salts. After the washing step, the nanowires were collected by placing a small magnet on the side of the flask and were suspended in the 35 mM NaOH electrophoretic medium for storage. The nanowires were activated electrochemically daily by positioning them vertically on a 0.8 cm × 0.8 cm screen printed electrode and scanning the potential between -1.0 and +1.0 V at 0.1 V s-1 for 50 scans (in a 1 M NaOH). Electrophoresis Procedure. Before use, the channels were treated by rinsing with deionized water, 0.1 M NaOH, and deionized water solutions for 10, 20, and 5 min, respectively. Each of the reservoirs in the chip holder and the corresponding pipet Analytical Chemistry, Vol. 79, No. 12, June 15, 2007
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Figure 2. Current-time recording for injections of 1 mM glucose obtained for reversible switching of the microchip detection by cyclic attraction (ON) and removal (OFF) of the nanowires to and from the channel exit. Conditions: separation medium, 35 mM NaOH; separation and injection potentials, +1000 V; injection time, 10 s; detection potential, +0.55 V (vs Ag/AgCl wire).
tips (inserted into the microchip) were filled with their respective solutions. The running buffer solutions were 35 and 10 mM NaOH solutions for glucose and amino acids, respectively. The injection was performed by applying the selected potential (+1000 V for sugars and +700 V for amino acids) for a given time (10 s for sugars and 5 s for amino acids) to the sample reservoir, with the detection reservoir grounded and other reservoirs floating. The separation was performed by applying the same potential to the running buffer reservoir with the detection reservoir grounded and other reservoirs floating. Electrochemical Detection. Amperometric detection was performed with a 621A electrochemical analyzer (CH Instruments, Austin, TX) using the “amperometric i-t curve” mode. Sample injections were performed after ∼1-2 min stabilization of the baseline. The electropherograms were recorded with a time resolution of 0.1 s and a detection potential of +0.55 V (vs Ag/AgCl wire). Nickel Nanowire Orientation and Modulation. The magnetic placement and alignment of the nanowires at the microchip channel outlet is displayed in the SI movie. Nickel nanowires were oriented in either “vertical” or “horizontal” positions by changing the orientation of a NdFeB/Ni-coated cube-shaped magnet (3/8 in. × 3/8 in. × 3/8 in., 12.4 kG), which was placed on the top surface of the microchip behind the channel outlet where the nanowires were positioned (see Figure 1). A 90° rotation of the magnet corresponded to a 90° change in the orientation of the nanowires (Figure 1, B vs C). Switching of the microchip detector between the active (ON) and passive (OFF) states was accomplished by removing the cube magnet from its switching position and moving it to the side of the detection reservoir. This removed the nanowires away from the centralized gold surface (around the channel exit) and placed them on a side location on the surrounding chip wall. Safety Considerations. The high-voltage power supply and related open electrical connections must be handled with extreme care to avoid electrical shock. The piranha solution should be handled with extreme care. 4722 Analytical Chemistry, Vol. 79, No. 12, June 15, 2007
Figure 3. Tuning the separation efficiency (postchannel band broadening) of microchips through controlled orientation of the nanowires. Response for a mixture of amino acids containing 500 µM arginine, histidine, and glycine, with the nanowires in the vertical (A) and horizontal (C) positions, as well as in the 45° orientation (B). Conditions: separation and injection potentials, +700 V; detection potential, +0.55 V; injection time, 5 s; separation medium, 10 mM NaOH. Also shown (top), optical images of the nickel nanowires oriented in the corresponding three positions at the end of the channel.
RESULTS AND DISCUSSION The magnetic attraction and removal of the nanowires to and from the channel exit (Figure 1) allow microfluidic devices to be activated periodically to perform measurements and reset them to the passive (OFF) state between such measurements. Such reversible magnetoswitchable control of the microchip detection is illustrated in Figure 2, which displays electropherograms for glucose obtained upon placement (ON) and removal (OFF) of the nanowires at the channel outlet. A well-defined glucose oxidation peak is observed in the presence of the nanowires at the channel outlet. In contrast, no signal is observed upon removing the nanowires. The process can be repeated multiple times by cyclic attraction and retraction of the nanowires to and from the channel exit, with highly reproducible peaks and no observable carryover between the ON and OFF detection states. A relative standard deviation of 4.7% was estimated for the height of the six glucose peaks (of the three ON/OFF cycles), reflecting the reproducible placement and removal of the nanowire detection elements. To the best of our knowledge, this represents the first example of a reversible positioning of a “temporary” microchip detector via external stimuli. In addition to reversible ON/OFF modulation of the microchip detection, the adaptive nanowires allow fine-tuning of the microchip separation through reversible modulation of the nanowire orientation at the channel outlet. Such magnetically controlled detector alignment at the exit of the microchannel is illustrated in the movie shown in the SI. Figure 3 demonstrates the influence of the nanowire detector orientation upon the separation performance in terms of peak width and symmetry. It displays electropherograms for a mixture of amino acids, with the nanowires placed in the vertical (A) and horizontal (C) orientations, as well as in the 45° alignment (B). Wide, partially overlapping peaks are observed using the horizontal orientation. In contrast, sharp wellresolved peaks are observed upon reorienting the nanowires to the vertical position. Intermediate separation efficiency is observed for the 45° nanowire orientation. The gradual change in the separation efficiency of the three nanowire alignments is reflected
Figure 4. Electropherogram for five repetitive injections of a mixture containing 500 µM arginine (a), histidine (b), and glycine (c) with the nickel nanowires detector in the ON (vertical) position. Conditions: separation and injection potentials, +700 V; detection potential, +0.55 V; injection time, 5 s; separation medium, 10 mM NaOH.
by the gradual increase of the plate number (N) from an average of 2560 for the horizontal configuration, to 4625 for the 45° alignment, and 17 470 for the vertical position (average half peak width of 29, 21, and 11 s, respectively; data for glucose, not shown). Such behavior is attributed to postchannel broadening of the analyte zone associated with the different geometries and alignments of the nanowire detector assembly. The optical images of the corresponding nanowire configurations (shown on top) indicate a “forestlike” structure in their vertical position (with nanowires aligned parallel to channel axis) and a “log pile” nanostructure in their horizontal position (nanowires aligned perpendicular to channel axis). While analytes exiting the channel are readily transported through the forestlike vertical detector configuration, they require additional diffusional paths to pass through the thick layers of horizontal detector nanostructure. Such dispersion in multiple directions and increased exposed electrode area broadens the analyte zone and results in inferior separation performance. The 45° orientation has a mixed forestlike/log pile nanostructure and, hence, leads to an intermediate behavior, with a mixed degree of dispersion. The new nanowires-based adaptive microchip detection is highly reproducible. A series of five repetitive measurements of a mixture of amino acids yielded reproducible electropherograms (Figure 4), with relative standard deviations (for the peak height) of 1.91, 2.40, and 1.78% for histidine, arginine, and glycine, respectively. Quantitative evaluation of the adaptive microchip detector is based on the correlation between the peak current and the analyte concentration. The concentration dependence was examined by recording the electropherograms for samples containing increasing levels of arginine in steps of 1.0 × 10-4 M (Figure 5, a-e). Defined peaks, proportional to the amino acid concentration, are observed. The resulting calibration plot (shown
Figure 5. Calibration data for arginine with the nickel nanowires detector in the ON (vertical) position. (a-e) Electropherograms for increasing levels of arginine in increments of 100 µM. Also shown (inset) is the resulting calibration plot. Conditions, as in Figure 4.
in the inset) is highly linear over the entire range, with a sensitivity of 142.7 nA/mM (correlation coefficient, 0.998). CONCLUSIONS In this study we have demonstrated the use of adaptive nanowires for controlling on-demand the separation and detection processes in microfluidic devices. Controlled reversible positioning and orientation of these nanowires at the microchannel outlet allows modulation of the detection and separation processes, respectively. Other detector materials (e.g., Au, Pt) can be used in connection to nickel-based dual segment nanowires. Additional microchip functions, such as pretreatment or mixing, could also be controlled on demand using various external stimuli. Current efforts in our laboratory aim at developing new stimuli-responsive materials for designing the next generation of smart chips and sensors, responsive to specific needs, opportunities, and unforeseen events. ACKNOWLEDGMENT This work was supported by grants from the EPA (STAR Program), NSF (Grant number CHE 0506529), and DOE (Grant DE-FG02-05ER63976). E.P. and R.L. acknowledge fellowships from the Brazilian and Thai governments (under the CAPES/CNPq and DPST programs, respectively). SUPPORTING INFORMATION AVAILABLE Related movie illustrating the magnetic positioning and orienting of the adaptive nanowire detector. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review March 19, 2007. Accepted April 6, 2007. AC0705519
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