Anal. Chem. 2001, 73, 5625-5628
Gold Nanoparticle-Enhanced Microchip Capillary Electrophoresis Martin Pumera,† Joseph Wang,*,† Eli Grushka,*,‡ and Ronen Polsky†
Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003, and Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel
We describe here the use of gold nanoparticles in conjunction with chip-based capillary electrophoresis to improve the selectivities between solutes and to increase the efficiency of the separation. We coated the microchannel wall of a microfluidic device with a layer of poly(diallyldimethylammonium chloride) (PDADMAC) and then collected on it citrate-stabilized gold nanoparticles. The resolutions and the plate numbers of the solutes were doubled in the presence of the gold nanoparticles. Such selectivity improvements reflect changes in the observed mobility accrued from interactions of solutes with the particle surface. The electrochemical detection and the quantitation of the solutes were not effected by the PDADMAC and the gold nanoparticles.
particles in conventional CE systems.7-12 For example, Huber and co-workers7,8 as well as Rodriguez and Colon,9,10 used polymerbased nanoparticles to coat fused-silica capillaries for use in CE. Fujimoto and Muranaka11 used commercially available silica gel nanoparticles as a run buffer additive in CE. Neiman et al.12 recently reported on the use of colloidal gold nanoparticles dispersed in the run buffer for capillary electrophoretic separations. To the best of our knowledge, nanoparticles have not been used in microchip CE systems. In the following sections we extend the use of gold-based nanoparticles to chip-based capillary electrophoresis devices and demonstrate that the presence of such nanoparticles in the microchannels acts as a selectivity modifier by changing both the apparent electrophoretic mobilities of the solutes and the electroosmotic mobility.
The development of microscale (chip-based) analytical devices integrating all the steps of an assay (including sample preparation, separation, and detection) on a small footprint of a microchip device has witnessed an explosive growth in recent years.1-3 Such miniaturized devices represent the ability to shrink conventional “benchtop” analytical systems with major advantages of speed, performance, integration, portability, sample size, solvent/reagent consumption, multiplexing, automation, and cost. Particular attention has been given to the development of high-performance capillary electrophoresis (CE) on planar structures.4 Although offering remarkable separation efficiencies, with close to 1 000 000 theoretical plates,5 there are also major needs for manipulating the selectivity of CE microchips. Such tailoring of the selectivity can be accomplished through control of the microchannel surface chemistry or by the presence of additives in the run buffer. The addition of SDS to the running buffer for forming micelles and separating neutral compounds is just one example.6 This note reports on the use of gold nanoparticles for manipulating the selectivity in CE microchips. Several groups, including ours, have recently demonstrated the use of nano-
EXPERIMENTAL SECTION Reagents. The run buffer was prepared by mixing 0.02 M sodium acetate solution (Sigma) and 0.02 M acetic acid (Fisher Scientific) at a 70:30 volume ratio. The resulting pH of the buffer was 5.0. Citrate-stabilized colloidal gold particles (10-nm diameter; 0.01% concentration as HAuCl4; CAS no. 7440-57-5) was purchased from Sigma and served as a stock solution. Poly(diallyldimethylammonium chloride) (PDADMAC) was used to modify the inner surface of the capillary. For that purpose, the PDADMAC solution (20 wt % in water; d ) 1.040) (Aldrich) having molecular weight of ∼100 000-200 000 was used. Aminophenols were obtained from Aldrich. Stock solutions of aminophenols were prepared in the run buffer at concentrations of 1 × 10-3 M. Sodium hydroxide was obtained from Sigma. The gold atomic absorption standard solution (1000 mg/L Au(III)/0.1 M HCl) was received from Aldrich. All chemicals were used without any further purification. Apparatus. Details of the integrated CE glass chip/detection microsystem were described previously.13 The thick-film carbon electrodes were printed with a semiautomatic printer (model TF 100, MPM, Franklin, MA) using the Acheson ink (Electrodag 440B; Acheson Colloids, Ontario, CA).13 A Plexiglas holder was fabricated for holding the separation chip (Micralyne, model
* Corresponding authors. E-mails:
[email protected]; Eli_Grushka@ huji.ac.il. † New Mexico State University. ‡ The Hebrew University of Jerusalem. (1) Figeys, D.; Pinto, D. Anal. Chem. 2000, 71, 330A. (2) Kutter, J. P. Trends Anal. Chem. 2000, 19, 352. (3) Jakeway, S. C.; de Mello, A. J.; Russell, E. L. Fresenius’ J. Anal. Chem. 2000, 366, 525. (4) Dolnik, V.; Liu, S.; Jovanovich, S. Electrophoresis 2000, 21, 41. (5) Culbertson, C. T.; Jacobson, S. C.; Ramsey, J. M. Anal. Chem. 2000, 72, 5814. (6) Moore, A. W.; Jacobson, S. C.; Ramsey, J. M. Anal. Chem. 1995, 67, 4184. 10.1021/ac015589e CCC: $20.00 Published on Web 10/20/2001
© 2001 American Chemical Society
(7) Kleindienst, G.; Huber, C. G.; Gjerde, D. T.; Yengoyan, L.; Bonn, G. K. Electrophoresis 1998, 19, 262. (8) Huber, C. G.; Premstaller, A.; Kleindienst, G. J. Chromatogr. A. 1999, 849, 175. (9) Rodriguez, S. A.; Colon, L. A. Anal. Chim. Acta 1999, 397, 207. (10) Rodriguez, S. A.; Colon, L. A. Chem. Mater. 1999, 11, 754. (11) Fujimoto, C.; Muranaka, Y. J. High Resolut. Chromatogr. 1997, 20, 400. (12) Neiman, B.; Grushka, E.; Lev, O. Anal. Chem., in press. (13) Wang, J.; Tian, B.; Sahlin, E. Anal. Chem. 1999, 71, 5436.
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Figure 1. Top view of the capillary electrophoresis microdevice: (a) the glass chip, (b) separation channel, (c) run buffer reservoir, (d) sample reservoir, (e) unused reservoir, (f) platinum cathode for separation, (g) screen-printed electrode detector, (h) Ag/AgCl wire reference electrode, (i) platinum counter electrode, and (j) detection reservoir.
MC-BF4-SC, Edmonton, Alberta, Canada) and housing the detector and reservoirs. The microchip, shown in Figure 1, consisted of two crossed channels with a half-circle cross section and three reservoirs, including a four-way injection cross. The separation channel was 77 mm long, 20 µm in depth, and 50 µm in width. The gold-modified screen-printed working electrode14 was placed opposite the channel outlet at 60-µm distance (controlled by a plastic screw and a thin-layer spacer). Preparation of Gold Nanoparticle-Coated Capillaries. Prior to its coating, the chip was sequentially rinsed with 1 M NaOH, deionized water, and the 0.02 M acetate buffer (pH 5.0), each for 10 min. A run buffer solution containing 0.2% (wt.) PDADMAC was pumped through all of the channels of the microchip using a syringe for 30 min. After the polymer adsorption, the channels were flushed for 10 min with the acetate buffer containing 0.02% (wt) PDADMAC. Subsequently, an acetate buffer solution containing the citrate-stabilized gold nanoparticles (at 1000-fold dilution of the gold-nanoparticles stock solution) and 0.02% (wt) PDADMAC was pumped through the channels for 30 min.12 Electrophoresis Procedure. The channels of the glass chip were treated before use by rinsing with 0.1 M NaOH and deionized water for 20 and 5 min, respectively. The channels of the PDADMAC-modified chip were treated before use by rinsing with deionized water for 20 min. The electrophoresis buffer was an acetate buffer (20 mM, pH 5.0). The run buffer and unused reservoirs (Figure 1c,e) were filled with the electrophoresis run buffer solution, and the sample reservoir (Figure 1d) was filled with a mixture of p-aminophenol, o-aminophenol and m-aminophenol (in the run buffer). After initial loading of the sample in the injection channel, sample was injected by applying a +1500 V potential between the sample reservoir (d) and the grounded detection reservoir (j). This drove the sample “plug” into the separation channel through the intersection. By switching the high-voltage contacts, the separation potential was subsequently applied to the run buffer reservoir (c) with the detection reservoir (j) grounded and all other reservoirs floating for the separation of the analytes. Safety Considerations. The high-voltage power supply should be handled with extreme care to avoid electrical shock. Aminophenols are toxic and skin and eye contact and accidental inhalation or ingestion should be avoided. Amperometric Detection. The electropherograms were recorded with a time resolution of 0.1 s while the detection (14) Wang, J.; Chatrathi, M. P.; Tian, B. Anal. Chim. Acta 2000, 416, 9.
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Figure 2. Electrophoregrams showing the separations of 1 mM p-aminophenol (a), o-aminophenol (b), and m-aminophenol (c) in a bare glass capillary (A) without treatment and (B) after PDADMACgold coating. Conditions: acetate buffer (20 mM, pH 5.0) as an electrophoresis buffer; sample injection at +1500 V for 3 s; separation voltage, +2000 V; detection at +0.8 V using a gold-coated screenprinted carbon electrode with 60-µm channel-electrode spacing.
potential (usually +0.8 V vs Ag/AgCl wire) was applied. Sample injections were performed after stabilization of the baseline. All experiments were performed at room temperature. RESULTS AND DISCUSSION The addition of gold nanoparticles to the run buffer in CE adds another separation vector to the orthogonal electrophoretic vector. The coexistence of these two vectors results in selectivity changes that can be quite beneficial. Figure 2 displays two electropherograms of aminophenols, one obtained with an untreated capillary (trace A) and the second with a gold- and PDADMAC-modified capillary (trace B). The dramatic increase in the selectivity and, therefore, in the resolution are clearly evident. Under the conditions of the experiment (acetate buffer, pH 5, applied voltage of +2000 V), in the untreated channel, the resolution between p- and o-aminophenol is ∼1, and o- and m-aminophenols are only partially resolved. With the PDADMAC and gold nanoparticles, the resolutions between the solutes are much greater than 1. However, the higher resolution is achieved at the cost of the analysis time; ∼2.5 min, as compared to 1.5 min with the untreated channel. Figure 2 indicates also that the presence of PDADMAC and gold nanoparticles does not have a deleterious effect upon the electrochemical signal. The peaks observed in the PDADMAC/goldmodified microchip device are actually higher than those observed on the unmodified microchip. The exact reason for these sensitivity changes is not fully understood at this stage. No apparent tailing of the peaks was observed upon comparing the response for the individual compounds at the two microchips (not shown). The reason for the use of PDADMAC is to allow the adsorption of the nanoparticles onto the channel walls. In the presence of PDADMAC but without the gold nanoparticles, the electroosmotic flow is anodic, and no analytes reached the detector (immersed in the cathodic reservoir; not shown). The addition of gold nanoparticles reverses the electroosmotic flow. The combination of electroosmotic mobility and the apparent mobilities of the solutes results in the improved resolution observed in Figure 2B. Such changes in the mobilities in the presence of the gold nanoparticles reflect the interactions of solutes with the surface
Figure 3. Influence of separation voltage on the resolution of p-aminophenol and o-aminophenol (A); on the total resolution of p-aminophenol, o-aminophenol and m-aminophenol (B); on the number of theoretical plates per meter for o-aminophenol (C) before (a) and after (b) the PDADMAC-gold pretreatment. Other conditions as in Figure 2.
of these additives; these effects were described in detail in our previous work.12 The voltage applied to the separation channels affects the performance of the nanoparticle-enhanced CE microchip. Figure 3 shows the influence of the voltage on the resolution between two of the solutes (panel A), on the overall total resolution (panel B), and on the separation efficiency (panel C) in the absence (a) and presence (b) of PDADMAC/gold nanoparticles. The total resolution, SR, in panel B is defined as
SR ) exp[
∑ln(Rs )] i
where Rsi is the resolution between each pair of neighboring solutes.15 As can be seen, the performance of the channel treated with PDADMAC and gold nanoparticles is superior to that of the untreated channel. The resolution and the plate count in the treated channel are double the values obtained with the untreated capillary. Note also that the performance of the electrophoretic system varies slightly with the applied potential up to +1500 V and decreases thereafter. Although one would expect an increased number of plates at higher voltages, other factors, such as endcolumn broadening and incomplete isolation of the detection system and Joule heating, apparently dominate at voltages higher than +1500 V. Figure 4 depicts hydrodynamic voltammograms (HDV) at the gold-coated detector for the oxidation of p-aminophenol (a), o-aminophenol (b) and m-aminophenol (c). The curves were recorded pointwise by making 100 mV changes in the applied potential over the 0 to +1.0 V range using the separation voltage +1500 V. All three compounds display similar profiles, with the waves starting at +0.2 V (a,b) or +0.5 V (c) and leveling off at +0.8 (a) or +0.9 (b,c). The half-wave potentials are +0.28 V (a), +0.34 V (b), and +0.67 V (c). Subsequent amperometric detection work employed the detection potential of +0.8 V, which offered the most favorable signal-to-noise characteristics. A dramatic increase in the baseline current, its slope, and the corresponding noise was observed at higher potentials. Higher operation poten(15) Schoenmakers, P. J. Optimization of Chromatographic Selectivity; Elsevier: 1986.
Figure 4. Hydrodynamic voltammograms for 100 µM p-aminophenol (a), o-aminophenol (b), and m-aminophenol (c). Separation performed at +1500 V. Other conditions as in Figure 2.
tials would be required in connection with higher separation voltages that may shift the voltammetric profile to the anodic direction.13 The nanoparticle-mediated CE microchip displays a welldefined concentration dependence. Electrophoregrams for sample mixtures containing increasing levels of p-aminophenol and o-aminophenol (in 50 µM steps) are shown in Figure 5 (A-F). Defined peaks, proportional to the analyte concentration, are observed for both compounds. The resulting calibration plots are linear, with sensitivities of 0.050 and 0.031 nA/µM for p-aminophenol and o-aminophenol, respectively (correlation coefficients, 0.9993 and 0.9980). Apparently, the linearity is not compromised by the presence of PDADMAC and gold nanoparticles in the run buffer. A series of eight repetitive injections of a mixture containing 100 µM p-aminophenol and o-aminophenol resulted in reproducible peak currents with relative standard deviations of 1.96 and 1.81%, respectively. Good reproducibility was obtained also in connection with different coatings and chips (not shown). In addition, the channel coatings were found to be quite stable. When the PDADMAC-gold-nanoparticle-modified microchip was stored filled with water, its lifetime was more than 1 month. When the procedure was repeated, a useful coating resulted 85% of the time. The day-to-day reproducibility of migration times was also good Analytical Chemistry, Vol. 73, No. 22, November 15, 2001
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gold nanoparticles in the running buffer. Such selectivity improvements reflect changes in the observed mobility as a result of interactions of solutes with the particle surface. Although the concept of nanoparticle-enhanced CE microchips has been illustrated in connection with electrochemical detection of aminophenols, it could be readily extended to other classes of analytes and detection modes. Other additives (separation vectors) that may manipulate the selectivity of CE microchips are currently under investigation.
Figure 5. Electrophoregrams of mixtures containing (A) 50, (B) 100, (C) 150, (D) 200, (E) 250, and (F) 300 µM p-aminophenol (a) and o-aminophenol (b). Separation performed at +1500 V. Other conditions as in Figure 2.
(RSD < 3%). In conclusion, we have demonstrated that the selectivity of CE microchips can be enhanced by the presence of
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ACKNOWLEDGMENT This research was supported by grants from the United States-Israel Binational Science Foundation (BSF), Jerusalem, Israel (no. 1999199) and from the Office of Naval Research (Award no. N00014-01-1-0213).
Received for review August 8, 2001. Accepted September 19, 2001. AC015589E
