Anal. Chem. 2002, 74, 3611-3615
Detection of Hg2+ Using Microcantilever Sensors Xiaohe Xu,† Thomas G. Thundat,*,‡ Gilbert M. Brown,§ and Hai-Feng Ji*,†
Chemistry Program Institute for Micromanufacturing, Louisiana Tech University, Ruston, Louisiana 71272, and Life Science Division, and Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Trace amounts of Hg2+ are detected by using a microcantilever coated with gold. The microcantilever undergoes bending due to accumulation of Hg2+ on the gold surface. It is found that a concentration of 10-11 M Hg2+ can be detected using this technology. Other cations, such as K+, Na+, Pb2+, Zn2+, Ni2+, Cd2+, Cu2+, and Ca2+ have little or no effect on the deflection of the cantilever. The selectivity of the Hg2+ sensor could be improved by coating the gold surface of microcantilever with a selfassembled monolayer of a long-chain thiol compound. Microfabricated cantilevers have recently attracted considerable interest in the development of a wide range of novel physical, chemical, and biological sensors.1-3 The use of microcantilevers in the development of miniaturized sensors offers advantages, including high sensitivity and the ability to work in air, liquid, and harsh environments. This sensor transduction mechanism is particularly attractive when it is recognized that the micromechanical components can be integrated with on-chip control electronic circuitry. The transduction mechanism of a microcantilever sensor is based upon changes in the deflection and resonance properties induced by environmental factors in the medium in which a microcantilever is immersed. It is known from the literature that the physical properties of the surface are influenced by molecular adsorption, including changes in surface forces (surface stress).4 If the two surfaces of a cantilever are different (bimaterial), molecular adsorption will result in a differential stress between the top and bottom surfaces of the cantilever. Molecular adsorption-induced surface stress on a microcantilever can be observed as changes in cantilever deflections. Using this concept, we have demonstrated the feasibility of chemical detection of a number of vapor-phase analytes, as well as highly sensitive cations and anions in solution.1-3 The radius of curvature of adsorption-induced surface stress of the microcantilever can be related using Stoney’s formula.4 The radius of curvature of the bending of the cantilever can be * Corresponding authors. E-mails:
[email protected] and
[email protected]. † Louisiana Tech University. ‡ Life Science Division, Oak Ridge National Laboratory. § Chemical Sciences Division, Oak Ridge National Laboratory. (1) Thundat, T.; Chen, G. Y.; Warmack, R. J.; Allison, D. P.; Wachter, E. A. Anal. Chem. 1996, 67, 519-521. (2) Chen, G. Y.; Thundat, T.; Wachter, E. A.; Warmack, R. J. J. Appl. Phys. 1995, 77, 3618-3622. (3) Thundat, T.; Warmack, R. J.; Chen., G. Y.; Allison, D. P. Appl. Phys. Lett. 1994, 64, 2894-2896. (4) (a) Preissing, F. J. J. Appl. Phys. 1989, 66, 4262. (b) Muller, P.; R. Kern, R. Surf. Sci. 1994, 301, 386 10.1021/ac0255781 CCC: $22.00 Published on Web 06/17/2002
© 2002 American Chemical Society
calculated using the physical parameters of the cantilever and can be written as
( )
1 1-v )6 δs R Et2
(1)
where R is the cantilever’s radius of curvature, E is Young’s modulus for the substrate, ν is Poisson’s ratio, t is the thickness of the cantilever, L is the length of the cantilever, and δs is the differential stress on the cantilever. An approximate relationship, derived for a rectangular cantilever, relates the observed deflection at the end of the cantilever and the change in surface stress using the physical properties of the cantilever.
∆z )
(
)
3(1 - v)L2 Et2
δs
(2)
The use of microcantilevers as force sensors in atomic force microscopy (AFM) has been perfected to the point of commercialization, and bending of a cantilever can be measured with subangstrom resolution using optical reflection, piezoresistive, capacitive, and piezoelectric detection methods.5-8 For chemical sensing, a specific chemical recognition agent (such as a chemical-selective self-assembled monolayer9,10) can be coated on one surface of a microcantilever to make it molecularly specific. For instance, a quaternary ammoniumterminated SAM11-modified cantilever was shown to be useful as a CrO42- sensor; a microcantilever modified with phosphateterminated SAMs12 was shown to be effective for the detection of alkaline earth metals,12 such as Ca2+. In this paper, we report Hg2+ sensors based on microcantilevers with or without chemical coating on the gold surface of silicon microcantilever for the detection of Hg2+. The development of such novel ultrasensitive sensors with high sensitivity and selectivity for in situ detection (5) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403-405. (6) J. Butt, H. J. J. Coll. Interface Sci. 1996, 180, 251-260. (7) Gimzewski, J. K.; Gerber, Ch.; Meyer, E.; Schlittler, R. R. Chem. Phys. Lett. 1994, 217, 589-594. (8) Raiteri, R.; Butt, H.-J. J. Phys. Chem. 1995, 99, 15728-15732. (9) Rubinstein, I.; Steinberg S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426-429. (10) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (11) Ji, H. F.; Thundat, T. G.; Dabestani, R.; Brown, G. M.; Britt, P. F.; Bonnesen, P. V. Anal. Chem. 2001, 73, 1567. (12) Ji, H. F.; Thundat, T. Biosens. Bioelectron. 2002, 17, 337-343.
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Figure 1. Schematic representation of the instrument showing the method for measuring cantilever deflection and the scheme for introducing solutions to the cell.
of Hg2+ has immediate relevance in environmental remediation and monitoring.13-16 EXPERIMENTAL SECTION 1. Reagents and Procedures. Hg(NO3)2, Cu2(NO3)2, NaNO3, KNO3, Zn(NO3)2, Cd(NO3)2, Pb(NO3)2, Ni(NO3)2, Ca(NO3)2, triethoxymethylsilane, and 1-dodecanethiol were used as received from Aldrich. High-purity deionized water was obtained with a Milli-Q water system (Millipore). We used commercially available silicon microcantilevers (Park Instrument, CA) in these experiments. The dimensions of the V-shaped microcantilevers were 200 µm length, 20 µm width, and 0.7 µm thickness. One side of the cantilever had a thin film of chromium (3 nm) followed by a 20-nm layer of gold deposited by e-beam evaporation. When preparing Hg(NO3)2 solutions, HNO3 was added to acidify the solution to pH ) 6.0 in order to prevent the formation of HgO particles. In our experiments, the pH of all the solutions we used was adjusted to 6.0 with HNO3. 2. Cantilever Preparation. The silicon cantilever tips (with Au on one side) were cleaned in piranha solution (7:3 H2SO4 (96%): H2O2 (30%)) for 1 min and rinsed with H2O (3 times) and EtOH (2 times) before measurement or SAM preparation. (Caution: Piranha solution reacts violently with many organic materials and should be handled with care.) The formation of 1-dodecanethiol SAM on the gold-coated cantilever was accomplished by immersing the cantilever into 10-3 M solution of 1-dodecancethiol in EtOH for 12 h, followed by rinsing with EtOH three times. A selfassembled film of silane on the silicon surface was formed by immersing the cantilever into a 10-2 M solution of triethoxymethylsilane in EtOH/H2O (95:5) overnight, followed by rinsing with EtOH three times. 3. Deflection Measurements. The deflection measurements were carried out with an AFM apparatus (Digital Instruments). A schematic diagram of the instrument used in this study is illustrated in Figure 1. The experiments were performed in a fluid cell that holds the V-shaped microcantilever in solutions maintained at pH ) 6.0 (i.e.,10-6M of HNO3 in water). The cantilever was equilibrated in the water until a stable baseline was obtained. Then solutions of different concentration of electrolytes (at pH)6.0) were injected into the fluid cell. The volume of the cell (13) Seitz, W. R. in Fiber Optic Chemical Sensors and Biosensors; Wolfbeis, O. S., Ed.; CRC Press: Boca Raton, 1991; Vol. 2, Chapter 9, pp 1-19. (14) Murkovic, I.; Wolfbeis, O. S. Sens. Actuators B38-39, 1997, 246. (15) Watson, C. M.; Dwyer, D. J.; Andle, J. C. Anal. Chem. 1999, 71, 3181. (16) Fakhari, A. R.; Ganjali, M. R.; Shamsipur M. Anal. Chem. 1997, 69, 3693
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Figure 2. Bending response as a function of time, t, for three independent silicon cantilevers coated with a 20-nm thickness of gold after injection of an solution at a concentration of 10-5 M at pH ) 6.0 (adjusted with HNO3).
was 0.2 cm3, ensuring a fast replacement of the solution. To eliminate thermomechanical motion of the silicon cantilever caused by temperature fluctuations, the fluid cell was mounted on thermoelectric coolers so that the temperature of the fluid cell could be controlled to 20 ( 0.2 °C. The bending of the cantilever was measured by monitoring the position of a laser beam reflected off the tip of the cantilever onto a four-quadrant photodiode. In our experiment, the laser beam was reflected off the gold surface (top surface in the experimental arrangement). We define “bending down” as bending toward the silicon side; “bending up” refers to bending toward the gold side. For each measurement, a new cantilever was used. The error induced by variation in the slightly different geometries of different cantilevers and the position of the focused laser spot at the end of the cantilever was found to be within (5 to 10%. RESULTS AND DISCUSSION Cantilever Defection Due to Hg2+ Accumulation. A gold -coated microcantilever was initially exposed to a solution containing HNO3 (pH ) 6.0), and the cantilever was equilibrated until a stable baseline was obtained (i.e., 0 nm deflection). When a 10-5 M solution of Hg2+ is injected into the fluid cell to replace the water, the cantilever bends down quickly during the first 5 min, and then the bending rate gradually slows until the cantilever deflection reaches its maximum in 1 h, as shown in Figure 2. Reproducibility was very good, as shown in Figure 2, by injection of a solution containing 10-5 M Hg2+ with another two freshly prepared cantilevers that were prepared under the same conditions. As shown in Figure 2, similar deflection amplitudes and bending rates ensure that the microcantilever prepared by the same procedure can be used for accurate Hg2+ analysis. Figure 3 shows the cantilever deflection at different concentrations of Hg2+ in the solution varied from a low concentration of 10-11 M to a high concentration of 10-5 M. It is observed that the cantilevers bend down (compressive stress); however, the rate
Figure 3. Deflection amplitude as a function of time, t, for a silicon cantilever coated with a 20-nm thickness of gold before and after exposure to different concentrations of Hg2+ solution at pH ) 6.0 adjusted with HNO3.
of bending is a function of the Hg2+ concentration. At higher Hg2+ concentration, the cantilever bends down faster with a greater bending amplitude. The increase in deflection at higher Hg2+ loading can be rationalized by a stress at high loading that is greater than at low loading. The surface stress caused by high loading of Hg2+ is significant. The high sensitivity is due in part to the extraordinarily large deflection response due to Hg2+ sorption. At a concentration of 10-5 M Hg2+, the bending response of the microcantilever is 0.5% of the length of the cantilever after 5 min. Figure 4 is a log-log plot of the deflection amplitude of a cantilever recorded at various times following injection (5, 10, 15, and 30 min, respectively) vs the concentrations of Hg2+. The plot shows that the deflection of a gold-coated microcantilever can be used to detect Hg2+ with a concentration as low as 10-11 M and that better sensitivity at a given concentration could be achieved at a sacrifice of time; i.e., a longer data-collecting time is preferred in this system to gain better sensitivity. Mechanism of Bending Caused by Accumulation of Hg2+ on the Cantilever Surface. In general, the bending of a cantilever can be attributed to absorbate-induced surface stress from the interaction of analytes with one surface of the microcantilever. One surface of the microcantilever used in these experiments was silicon (with surface oxidation); the other surface of the cantilever had a 20-nm thickness of gold. To answer the question of which surface of the microcantilever reacts with Hg2+, the surfaces of two closely matched cantilevers were modified using two different materials: one cantilever was modified by the formation of a monolayer of a long-chain thiol compound to inhibit the interaction of Hg2+ with gold; The other cantilever was modified by the formation of a self-assembled film of a silane compound on the SiOH surface17 to inhibit the contact of this surface with Hg2+ ions in solution. The deflection of these two cantilevers upon contacting with the Hg2+ solution was studied, as shown in Figure
Figure 4. Log-log plot of the deflection of a silicon cantilever coated a 20-nm thickness gold as a function of the concentration of Hg2+ at pH ) 6.0.
Figure 5. Bending response as a function of time, t, for three independent silicon cantilevers coated with (a) a gold surface, (b) a self-assembled monolayer of 1-dodecanethiol on the gold surface, and (c) a self-assembled film formed between triethoxymethylsilane and SiOH on the silicon surface after injection of a solution having 10-5 M Hg2+ at pH ) 6.0.
5. The cantilever modified by a film of triethoxymethylsilane on the silicon side shows a deflection amplitude and bending rate similar to those of an unmodified gold-coated cantilever. Conversely, the cantilever modified by a 1-dodecanethiol monolayer on the gold surface shows a much slower rate of bending than nonmodified cantilevers. These results suggest that the bending of an unmodified gold-coated cantilever upon exposure to Hg2+ is due to the interaction between gold and Hg2+, but not silicon (SiOH) with Hg2+. At this stage, it is not clear which reaction occurs between Hg2+ and the Au surface. The interaction between Hg2+ and the (17) (A) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (B) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136.
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Figure 6. Bending response of a gold-coated microcantilever to Hg2+ and different cations at the same concentration (1 × 10-5 M) in water at pH ) 6.0.
Au layer on the microcantilever appears to be irreversible. When water is injected into the fluid cell to replace the Hg2+ solution 15 min after the cantilever is exposed to 10-5 M of Hg2+, the defection of the cantilever remains at the same amplitude and does not return to its original position. Hg2+ is not a sufficiently strong oxidant (E0Hg2+/Hg ) 0.85 V) to spontaneously oxidize Au (E0Au+/Au ) 1.69 V), but it is known that Hg2+ in aqueous solution spontaneously amalgamates with Au. The underpotential deposition (UPD) of Hg2+ on Au has been studied in some detail,18,19 and it is known that anion-dependent deposition of surface species occurs at potentials that are positive of the Hg2+/Hg22+ couple at +0.52 V vs Ag/AgCl. The potential of zero charge of Au in 0.02 N Na2SO4 solution is +0.32 V vs SHE,20 and a possible explanation for the results observed is that the Hg2+ ions may be deposited on the surfaces as a result of the surface charge on the Au surface, the latter of which spontaneously forms as a result of having a conductor in the presence of an electrolyte.20 The formation of a Hg/Au amalgam on the gold surface of the cantilever causes the change of the surface stress and subsequent deflection of the cantilever. It should be noted that the area of the silicon chip covered with gold that is in contact with the solution (estimated to be 0.03 cm2) is far larger than the area of the actual microcantilever, and this large surface will effectively polarize the cantilever at a potential that is sufficiently reducing to promote the formation of a mercury-gold amalgam. Herrero and Abruna note that the latter process occurs at +0.50 V.19 These results are consistent with a model in which Hg2+ diffuses to the surface of a Au-coated microcantilever, initially reacting to form an adsorbed Hg(II) or Hg(I) species. At low Au(Hg) coverage (submonolayer coverage), it is anticipated that the stationary-state flux of Hg2+ to the surface of the cantilever will be linear in concentration, in analogy to the flux of an electroactive species to a microelectrode with the dimensions of the microcantilecver.21 (18) Herrero, E.; Buller, L. J.; Abruna, H. D. Chem. Rev. 2001, 101, 1897. (19) Herrero, E.; Abruna, H. D. Langmuir 1997, 13, 4446. (20) Bockris, J. O’M.; Reedy, A. K. N.; Gamboa-aldeco, M. Modern Electrochemistry. Fundamentals of Electrodes, 2nd ed; Kluwer Academic/Plenum Publishers: New York, 2000; Vol. 2A.
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Figure 7. Bending response of a silicon cantilever coated with a self-assembled monolayer of 1-dodecanethiol on the gold surface of a microcantilever as a function of the concentration of Hg2+ in water at pH ) 6.0.
Assuming the cantilever can be treated as a band microelectrode with a width of 20 µ (and a length 10 times greater), natural convection and diffusion will reach a stationary state after 102 sec and a species with a diffusion coefficient of 10-5 cm2 s-1 and at a concentration of 10-6 M will have a stationary state flux to the cantilever of ∼3 × 10-12 moles cm-2 s-1. Thus, lower concentrations are predicted to require longer reaction times to establish the same fractional monolayer coverage. Experiments indicated that the presence of a monolayer of 1-dodecanethiol on the surface of the gold functions to slow the rate of deposition of mercury on the surface. Although the Au surface is covered with a monolayer of 1-dodecanethiol, the cantilever gradually bends down with a rate slower than the unmodified cantilever upon adsorption of Hg2+. Self-assembled monolayers are generally regarded as perfect monolayers, and it was reported that 98% of the gold surface will be covered by 1-dodecanethiol 100 min after the cantilever is immersed into a 10-3 M solution of 1-dodecanethiol in ethanol.10 Our previous experiments also showed that the monolayer formation of 1-dodecanethiol on the surface of gold in 10-3 M of 1-dedecanethiol in EtOH is almost completed in 20 min.11 However, the results reported here suggest that some pinholes or defects in the selfassembled monolayer do exist so that Hg2+ can penetrate the monolayer to react with the gold. Selectivity of Microcantilever Hg2+ Sensors. To learn the selectivity of this gold-coated cantilever sensor to Hg2+ over other cations, the effect of other metal ions, such as Na+, K+, Pb2+, Zn2+, Cd2+, Cu2+, Ni2+, and Ca2+ on the deflections of a modified cantilever at the same concentration (1 × 10-5 M) have been studied. At this concentration, none of these cations causes as much deflection of the cantilever as Hg2+ does, as shown in Figure 6, suggesting that this gold-coated modified cantilever could be a selective Hg2+ sensor with both high sensitivity and selectivity. However, it is known that thiol compounds could form a monolayer on the surface of gold and subsequently cause a deflection (21) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; p168.
interference from naturally occurring thiol compounds will be mitigated, since the gold surface of the cantilever is already “fully” covered by 1-dodecanethiol monolayers. The bending of such a 1-dodecanethiol-modified cantilever upon injection of Hg2+ is shown in Figure 7, and the bending response vs the concentration of Hg2+ is shown in Figure 8. Good linearity in bending response vs the concentration is observed if sufficient time is allowed to establish stationary-state conditions. CONCLUSIONS Our studies have shown that a commercially available goldcoated cantilever can selectively respond to Hg2+ at concentrations as low as 10-11 M. Experiments are presently underway on sensors for other chemical and biological species based on microcantilevers modified with reagents for molecular recognition. Figure 8. Deflection amplitude of a silicon cantilever coated with a self-assembled monolayer of 1-dodecanethiol on the gold surface as a function of the concentration of Hg2+ in water at pH ) 6.0.
of the cantilever,22 potentially interfering with the detection of Hg2+. This might be a particular problem in the analysis of groundwater containing naturally occurring organic material. One solution is to use a cantilever that is already modified by a monolayer of a thiol compound on the gold surface, such as 1-dodecanethiol. Although the response time of this 1-dodecanethiol monolayer modified cantilever to Hg2+ is relatively slow compared with that of a bare gold-coated microcantilever, the (22) Berger, R.; Delamarche, E.; Lang, H. P.; Gerber, Ch.; Gimzewski, J. K.; Meyer, E.; Guntherodt, H.-J. Appl. Phys. A 1998, A66, S55-S59.
ACKNOWLEDGMENT H. F. Ji and X. Xu thank the Louisiana Board of Regents through the Board of Regents Support Fund under contract number LEQSF(2001-04)-RD-A-16 for financial support. T. Thundat and G. Brown were supported by the Environmental Management Science Program (EMSP), Office of Environmental Management, U.S. Department of Energy. Oak Ridge National Laboratory is operated and managed by UT-Battelle, LLC for the U.S. Department of Energy under contract number DE-AC05-00OR22725.
Received for review February 13, 2002. Accepted May 16, 2002. AC0255781
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