Highly Selective Environmental Nanosensors Based on Anomalous

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2009, 113, 19393–19396 Published on Web 10/20/2009

Highly Selective Environmental Nanosensors Based on Anomalous Response of Carbon Nanotube Conductance to Mercury Ions Tae Hyun Kim, Joohyung Lee, and Seunghun Hong* Department of Physics and Astronomy, Seoul National UniVersity, Seoul 151-747, Korea ReceiVed: September 15, 2009; ReVised Manuscript ReceiVed: October 11, 2009

We have developed a selective, sensitive, and fast single-walled carbon nanotube (swCNT) field effect transistor (FET) sensor for Hg2+ ion detection. This sensor is based on the anomalous response of swCNT conductance to the exposure of Hg2+, which provides the selectivity toward Hg2+ over various other metal ions through a strong redox reaction between swCNTs and Hg2+. Our sensor system exhibited a detection limit of 10 nM for Hg2+ in water, which is comparable with the maximum allowable limit of mercury ions in drinking water set by most government environmental protection agencies. It also has a wide measurable detection range from 10 nM to 1 mM and a sensitive quantifying range with a steep slope for Hg2+ detection. The rapid detection of mercury is crucial because of its harmful effect on human health and natural environments. In particular, solvated mercuric ions (Hg2+) are one of the stable inorganic forms of mercury, causing various human ailments such as prenatal brain damage, serious cognitive motion disorders, and Minamata disease.1 Various techniques have been demonstrated for the detection of Hg2+.2-7 However, those techniques often suffer from bulky system size, complicated instrumentation, and limitations in sensitivity and selectivity. Thus, there is a high demand to develop new approaches for Hg2+ detection with high sensitivity and selectivity and great simplicity to protect our health and environment. Carbon nanotubes (CNTs) hold salient advantages as sensing elements because their one-dimensional quantum confinement properties should make charge transport extremely sensitive to scattering from adsorbates.8,9 Especially, the semiconducting single-walled CNTs (swCNTs) are very sensitive to their chemical environment due to the high sensitivity of their band gap energies to the local dielectric or redox environment, which can be exploited for chemical sensing.9-11 However, the detection limit of previous swCNT-based sensors to hazardous metal ions has been somewhat inferior to conventional methods and cannot meet the regulations set by most government environmental protection agencies.17 Herein, we report the extremely strong response of swCNT conductance to the exposure of Hg2+ caused by the strong redox reaction between swCNTs and Hg2+ ions, which is quite unique compared to that with other metal ions. Furthermore, we successfully demonstrated highly selective and sensitive Hg2+ sensors based on this response. Our sensors exhibited spontaneous and selective detection of Hg2+ ions with the detection limit comparable with the maximum allowable limit of mercury ions in drinking water set by most government regulations. This work provides new insight into the response of swCNTs to metal ions * To whom correspondence should be addressed. Telephone: +82-2880-1343. Fax: +82-2-884-3002. E-mail: [email protected].

10.1021/jp908902k CCC: $40.75

Figure 1. Optical absorption spectra of an aqueous swCNT solution dispersed in SDS after the addition of (a) Hg2+, (b) Ca2+, and (c) Pb2+ solution (0, 20, 40, 60, 80, 100, 120, 140, 160, 180, 200 µM). (d) FESEM image of Hg metal particles decorating swCNTs on SiO2 after the drop of the Hg2+ ion solution.

and enables the rapid detection of hazardous mercury ions for environmental safety. As a control experiment, we introduced various metal ions (all of them as perchlorate salts) such as alkali and alkaline earth and several heavy metal ions including Hg2+, Cu2+, Pb2+, Zn2+, and Cd2+ into aqueous swCNT solutions dispersed by the surfactant sodium dodecyl sulfate (SDS) and measured the effect on the absorption spectra of the swCNT dispersions via UV/vis/NIR spectrophotometry. Figure 1a-c shows the absorbance of swCNT dispersions after the addition of Hg2+, Ca2+, and Pb2+ at various concentrations from 20 to 200 µM. The absorption spectra of a swCNT solution with the addition of Hg2+ exhibited a significant decrease in absorption bands with a wavelength > ∼1000 nm, which is a typical indication of redox titration of an aqueous swCNT solution with progressive oxidation of swCNTs caused by an oxidant (Figure 1a).10 In contrast, only minor changes in  2009 American Chemical Society

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Figure 2. (a) Fabrication of swCNT-based sensors for the selective detection of Hg2+ ions. (b) Optical (left) and atomic force microscopy topographic (right) images of a swCNT-based mercury ion sensor array.

the spectra were observed when Ca2+, Pb2+ (Figure 1b, c), and other metal ions (Figure S1 in Supporting Information) were added to swCNT solutions. These results can be explained by the standard potential difference (∆E 0 ) E 0metal ion - E 0swCNT) between swCNTs (E 0swCNT ) 0.5-0.8 V versus NHE)9,10,12 and metal ions (E 0metal ion).13 E 0swCNT varies depending on the band gap of the swCNT.10 Since E 0 of Hg2+ (0.8535 V versus NHE)13 is higher than that of swCNTs, the above redox reaction between the

Letters swCNT and Hg2+ is thermodynamically favorable (∆G ) -nFE < 0). On the contrary, in the case of other metal ions, the redox reactions with a swCNT are unfavorable due to the negative or lower E 0’s (e.g., Ca2+ ) -2.924 V, Pb2+ ) -0.126 V vs NHE)13 of the metal ions than that of swCNTs. To confirm the reduction of Hg2+ by swCNTs, a 1 mM Hg2+ solution was dropped onto the swCNTs on a SiO2 substrate. One minute later, the sample was rinsed with deionized water, dried in air, and characterized by a field emission scanning electron microscope (FE-SEM). Figure 1d shows the typical SEM image of Hg metal particles coating the swCNTs on SiO2 after the drop of Hg2+ solution. Hg metal particles were spontaneously formed on the swCNT sidewalls due to the direct redox reaction between swCNTs and Hg2+ ions. It is also consistent with previous reports about the redox reaction of metal ions on individual swCNTs.12 However, the treatment with other metal ions exhibited no changes on swCNTs when imaged by the SEM (data not shown). For the creation of Hg2+ sensors with great simplicity and sensitivity, we have examined the effects of metal ions on swCNT-based field effect transistors (FETs) (Figure 2a). swCNT-FETs were fabricated as reported previously.14,15 Briefly, an octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) with nonpolar terminal groups was patterned on a SiO2 surface using photolithography. When the patterned substrate was placed in the solution of swCNTs (usually 0.1 mg mL-1 in o-dichlorobenzene), a single layer of swCNTs was selectively adsorbed onto bare SiO2 regions. Afterward, electrodes (30 nm Au on 10 nm Pd) were fabricated. Finally, the electrodes were selectively passivated with an AZ5214 photoresist layer using conventional photolithography. The passivation on metal electrodes was crucial to measure only the interactions between swCNTs and metal ions because exposed electrodes in solution may cause unexpected electrochemical reactions and leakage currents. It should be noted that the self-limiting adsorption behavior of swCNTs allows us to prepare uniform-density swCNT networks, which enhances the reproducibility of our FET device compared with previous devices comprised of a

Figure 3. Response of swCNT-based sensors to various metal ions. (a) A real-time current measurement obtained from the swCNT-FET after the introduction of Hg2+ at various concentrations. Arrows indicate the points of Hg2+ injections. (b) Conductance change of the swCNT-FET by the introduction of various concentration Hg2+ ion solutions. The red line in the inset represents the fitting curve for the estimation of the equilibrium constant (K). (c) Response to various metal ions with concentrations from 1 nM to 1 mM. (d) Plausible mechanisms for Hg2+ (upper) and Pb2+ detection (lower).

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single swCNT.15 Since we utilized only conventional microfabrication facilities, our sensors can be easily mass-produced for practical applications such as healthcare and environmental monitoring (Figure 2b). We carried out extensive control experiments to measure the effect of various metal ionic species on the conductance of swCNT-FETs (Figure 3). First, a 9 µL droplet of water was placed on a swCNT-FET, and the source-drain current was monitored after the addition of the metal ion solution. A 100 mV bias voltage was maintained at all times during the electrical measurement. Figure 3a shows the source-drain current of our swCNT-FET sensor after the introduction of Hg2+ at various concentrations from 1 pM to 1 mM. The addition of Hg2+ from 1 pM to 1 nM showed no significant effect on the source-drain current, while drastic current increase with fast response was observed by the addition of Hg2+ from 10 nM, which is the maximum allowable level of Hg2+ ions in drinking water set by EPA regulation.16 It is also comparable to the detection limits of other state-of-the-art assays based on fluorescent dyes and nanoparticles (∼10 nM).3,6 The high sensitivity of our sensors can be attributed to the strong redox reaction directly between swCNTs and Hg ions without any intermediate layers. The sensitivity of swCNT-based sensors usually depends on the transconductance of the swCNT-FETs. Thus, the detection limit of our sensors is expected to be improved by using FET devices with larger transconductances such as those based on only semiconducting CNTs or aligned CNT networks.18 It also should be noted that the redox reaction between swCNTs and Hg2+ is irreversible; therefore, the sensor is not reusable. The calibration curve (Figure 3b) for Hg2+ estimations with swCNT-FETs exhibited a wide dynamic range (10 nM-1 mM) over which there were measurable responses and a sensitive linear range with a steep slope of ∼0.22/decade. For the linear range

∆G ) 1.7 + 0.22 log[Hg2+] G0

(1)

The Hg2+-swCNT equilibrium constant (K) was estimated by fitting the data with the Langmuir isotherm equation given by

∆G )

∆Gmax[Hg2+] 1/K + [Hg2+]

(2)

(inset Figure 3b),17 where Gmax represents the maximum conductance change due to the interaction between Hg2+ and swCNTs. K is found to be 1.4 × 106 M-1, which can be translated to E 0 by the following equation13

∆E 0 ) E 0Hg2+ - E 0swCNT )

0.05916 log K n

(at 25 °C)

(3) From the eq 3, E 0swCNT is estimated to be ∼0.67 V, which is consistent with previously reported values.9,10,12 To investigate the selectivity, we compared the sensitivity (∆G/G0 by percentage) of our swCNT sensors to Hg2+ and other metal ionic species at a concentration from 1 nM to 1 mM. Figure 3c indicates that only the addition of Hg2+ caused a conductance increase starting from the 10 nM concentration, revealing the exceptional selectivity of this sensor. The mea-

surement of FET gating effects also confirmed the selective conductance enhancement only by the addition of Hg2+ ions in the mixed solution of Ca2+, Na+, and Hg2+ (Figure S2 in Supporting Information). It also should be noted that the swCNT-FET-based sensing measurements showed strong responses to metal ions with much lower concentrations than those in the case of optical measurements (Figure 1a, c). Presumably, since our FET devices are based on pristine swCNTs, the swCNT-FET-based sensors can be more sensitive to surrounding metal ions than the SDS-coated swCNTs used for the optical measurements. As shown in absorption spectra and FE-SEM data (Figure 1), the response of our swCNT-based sensors can be explained by the Hg2+ reduction and swCNT oxidation (upper in Figure 3d).12 When swCNTs give electrons to Hg2+, the consequential hole injection causes a conductance increase in swCNT junctions due to the p-type characteristics of swCNTs. The reduced conductance of the sensor observed in the presence of Pb2+ at a rather high concentration (10 µM-1 mM) was most likely due to the chemical gating effect by the adsorption of Pb2+ on swCNTs, which induces additional negative charges in the swCNTs, thus reducing the conductance of p-type swCNTs (lower in Figure 3d).19 In summary, we report quite a unique effect of mercury ions on the swCNT conductance which can be explained by the selective and spontaneous redox properties between swCNTs and Hg2+. Furthermore, we successfully demonstrated highly sensitive and selective swCNT-based nanosensors for the detection of mercury ions based on this effect. This sensor system exhibited a detection limit of 10 nM for Hg2+, which is the limit of Hg2+ ions in drinking water by most government environmental protection agencies.16 It also has a wide measurable detection range from 10 nM to 1 mM and a sensitive quantifying range with a steep slope for Hg2+ detection. Considering the high sensitivity and selectivity of our sensor, it can be a simple but extremely powerful tool for the immediate applications in the environmental protection area. Experimental Methods UV/Vis/NIR Titration Experiment. The purified swCNT product was purchased from Carbon Nanotechnologies, Inc. (CNI, average length of ∼1 µm and diameter of 1.5-2 nm). The purchased nanotube product (50 mg) was dispersed and stirred in 100 mL of deionized water with 1 wt % SDS. The dispersion was treated by weak sonication (4 W, 20 min) and then ultrasonication (540 W, 15 min). After sonication, the sample was ultracentrifuged at 200 kg for 4 h. An individual swCNT-rich solution was then obtained in the upper supernatant. The absorption spectra were recorded on a UV/vis/NIR spectrophotometer (Varian Cary-5000). Fabrication of swCNT-FETs. To prepare swCNT suspensions for FET device fabrication, purified swCNTs (CNI) without SDS were dispersed in 1,2-dichlorobenzene by applying ultrasonic vibration for 20 min. The typical concentration for swCNT suspensions was 0.1 mg mL-1 or less. For swCNT assembly, OTS SAM was first patterned by a photolithography technique on a substrate comprising a degenerately doped Si wafer covered with a 300 nm thick thermal oxide layer. The patterned surface was placed in the suspension, usually for 10 s, and rinsed thoroughly with 1,2-dichlorobenzene. The contact electrodes were fabricated using conventional photolithography followed by Au/Pd (30 nm/10 nm) evaporation and the lift-off method. The gap distance between the source and drain electrode was 4 µm. The metal electrodes, except the active nanotube

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channels, were then covered with an AZ5214 postive photoresist to avoid current passage through the liquid. Acknowledgment. This work was supported by the KOSEF Grant (No. 2009-0079103) and the TND program. S.H. acknowledges the support from the Ecotechnopia 21 project and the Basic Research Promotion Fund (KRF-2008-314-C00118). Supporting Information Available: Figure S1. Results of UV/vis/NIR titration experiment for various metal ions. Figure S2. Liquid gated conductance measurement (Ids versus Vlg) in mixed metal ion solution. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Clarkson, T. W.; Magos, L.; Myers, G. J. N. England J. Med. 2003, 349, 1731–1737. (2) Hollenstein, M.; Hipolito, C.; Lam, C.; Dietrich, D.; Perrin, D. M. Angew. Chem., Int. Ed. 2008, 47, 4346–4350. (3) Lee, J. S.; Mirkin, C. A. Anal. Chem. 2008, 80, 6805–6808. (4) Liu, X. F.; Tang, Y. L.; Wang, L. H.; Zhang, J.; Song, S. P.; Fan, C.; Wang, S. AdV. Mater. 2007, 19, 1471–1474. (5) Xue, X. J.; Wang, F.; Liu, X. G. J. Am. Chem. Soc. 2008, 130, 3244–3245.

Letters (6) Yang, Y. K.; Yook, K. J.; Tae, J. J. Am. Chem. Soc. 2005, 127, 16760–16761. (7) Kim, H. J.; Park, D. S.; Hyun, M. H.; Shim, Y. B. Electroanalysis 1998, 10, 303–306. (8) Tans, S. J.; Devoret, M. H.; Groeneveld, R. J. A.; Dekker, C. Nature 1998, 394, 761–764. (9) Barone, P. W.; Baik, S.; Heller, D. A.; Strano, M. S. Nat. Mater. 2005, 4, 86–92. (10) Ming, Z.; Diner, B. A. J. Am. Chem. Soc. 2004, 126, 15490–15494. (11) Xu, Y.; Pehrsson, P. E.; Chen, L. W.; Zhao, W. J. Am. Chem. Soc. 2008, 130, 10054–10055. (12) Choi, H. C.; Shim, M.; Bangsaruntip, S.; Dai, H. J. J. Am. Chem. Soc. 2002, 124, 9058–9059. (13) Patnaik, P.; Dean, J. A. Dean’s analytical chemistry handbook. 2nd ed.; McGraw-Hill: New York, 2004; section 13. (14) Rao, S. G.; Huang, L.; Setyawan, W.; Hong, S. H. Nature 2003, 425, 36–37. (15) Lee, M.; Im, J.; Lee, B. Y.; Myung, S.; Kang, J.; Huang, L.; Kwon, Y. K.; Hong, S. Nat. Nanotechnol. 2006, 1, 66–71. (16) Mercury Update: Impact on Fish AdVisories; EPA-823-F-01-011; U.S. EPA, 2001. (17) Forzani, E. S.; Li, X. L.; Zhang, P. M.; Tao, N. J.; Zhang, R.; Amlani, I.; Tsui, R.; Nagahara, L. A. Small 2006, 2, 1283–1291. (18) Lee, M.; Noah, M.; Park, J.; Seong, M.-J.; Kwon, Y.-K.; Hong, S. Small 2009, 5, 1642–1648. (19) Heller, I.; Janssens, A. M.; Mannik, J.; Minot, E. D.; Lemay, S. G.; Dekker, C. Nano Lett. 2008, 8, 591–595.

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