Aqueous Redox Reaction of SDS-Encased Carbon Nanotubes with

Article pubs.acs.org/JPCC

Aqueous Redox Reaction of SDS-Encased Carbon Nanotubes with Mercuric Ions for Optical Sensing Azhar Kamel, Eric Gangluff, and Wei Zhao* Department of Chemistry, University of Arkansas at Little Rock, Little Rock, Arkansas 72204, United States S Supporting Information *

ABSTRACT: The redox reaction and the pseudo first-order reaction kinetics of single-walled carbon nanotubes (SWNTs) dispersed in solution of a surfactant sodium dodecyl sulfate (SDS) have been studied with Hg2+ ions. The reaction is monitored by using the unique near-infrared optical absorption properties of SWNTs. Various factors including SDS and SWNT concentrations, different pH levels, and buffer constituents have been systematically examined. It is found that the reactivity is sensitive to the concentrations of SDS and SWNTs, with an optimum at 1 wt % SDS and 0.1 mg/mL SWNTs. The reaction is pH sensitive as well, and the reactivity is higher at lower pHs. The suppressed SWNT spectral intensity by the redox reaction can be recovered by adjusting pH, enabling the reusability of SWNTs. SWNTs show high selectivity and sensitivity on Hg2+ over eleven other metal ions. The determined detection limit is 0.6 nM, lower than 10 nM, the maximum contaminant level for mercuric ions in drinking water. This work may provide insights for developing new SWNT-based optical sensors for mercuric ion detection.

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INTRODUCTION In recent years, single-walled carbon nanotubes (SWNTs) have attracted great attention for chemical and biological sensor applications.1−20 For example, SWNTs have been used to optically detect hydrogen peroxide and glucose5−9 based on their unique redox chemistry. More recently, SWNT-based field effect transistors have been used to sense mercuric ions.20 Solvated mercuric ions Hg2+ are considered as one of the stable forms of mercury.21 According to the United States Environmental Protection Agency, the maximum contaminant level for mercuric ions in drinking water are 2 ppb (10 nM).21 Numerous methods have been developed for mercuric ion determination as summarized in a recent excellent review article.21 As pointed out by Nolan and Lippard,21 because of continuing concern over mercury in the environment and its deleterious effects on human health, it is still of paramount importance to obtain new mercury detection methods that are cost-effective, facile, rapid, and applicable to the environmental and biological situations. For sensing mercuric ions, reversibility is an important considering factor for selecting a probe. Also, depending on the application, water solubility, pH sensitivity, cell permeability, and toxicity are other important factors for consideration.21 Our previous work has demonstrated that SWNT-based sensors can offer reversibility, water solubility, and pH sensitivity.2−9 Previous work done by Dai’s group22−24 and Weisman’s group13 has also shown that SWNTs are cell permeable and nontoxic. Thus, SWNTs may meet the requirements for developing new Hg2+ nanosensors for environmental and biological applications. Although SWNTs © 2012 American Chemical Society

have been studied for mercuric ion detection based on their electrical properties,20 there have been no systematic studies using SWNTs as an optical probe for mercuric ion detection. It becomes very interesting to see how the sensitive optical properties of SWNTs respond to the mercuric ions. In this work, the redox reaction of SWNTs with Hg2+ has been systematically studied by monitoring their near-infrared (NIR) spectral intensity changes under various reaction conditions, including SDS surfactant and carbon nanotube concentrations, different pH levels, and buffer constituents. It was found that the reactivity is dependent on various factors mentioned above. By varying the concentrations of SDS and SWNTs, an optimum of reactivity is determined when SDS concentration is ∼1 wt % and SWNT concentration is ∼0.1 mg/mL (optical density (OD) ≈ 0.3 at 1247 nm with 1 mm path length). The reaction is also pH-dependent; SWNTs can work at physiological pH near 7.0 and become more sensitive at lower pHs. SWNTs show high selectivity on mercuric ions over 11 other metal ions, with sensitivity ∼0.6 nM.

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EXPERIMENTAL SECTION Materials. Raw HiPco SWNTs (lot no. R0496, 2005) were purchased from Carbon Nanotechnologies, Inc. Their sizes ranged from 0.7 to 1.1 nm in diameter and a few hundred nanometers long. The reagents SDS (>99% pure), 2-(4morpholino) ethanesulfonic acid (MES, purity >99.5%), Received: April 27, 2012 Revised: June 28, 2012 Published: July 17, 2012 15591

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NIR spectrophotometer. The redox reaction under different SDS concentrations (0.1−2 wt %) was examined for maximizing SWNT response to mercuric ions. The SWNT concentration was fixed with OD ∼0.1 at 1247 nm, and Hg2+ concentration is 8.0 × 10−5 M. Other SWNT concentrations were also used, as summarized in Table S1 in the Supporting Information. Following that, the effects of SWNT concentration on the reactivity were also evaluated. The SWNT suspensions in 1 wt % SDS solutions with different SWNT concentrations were prepared by adding different aliquots of 1 wt % SDS solution to a SDS-SWNT stock suspension with a typical OD of ∼0.4 at 1247 nm. pH dependent measurements were conducted using an Orion model 420A pH meter with an Orion Thermo Scientific microprobe electrode. The study of pH reversibility of the NIR spectral intensity of SDS-SWNT suspensions was carried out in the presence of 8.0 × 10−5 M Hg2+. The pH was adjusted by titration with 0.1 M NaOH, which was added to the sample to gradually increase the pH from 5 to 10, while aliquots of 0.1 M HCl were added to decrease the pH from 10 to 5. For examining buffer effects, SDS-SWNT suspensions in four pH 7.0 buffers, TE buffer, Tris buffer, MES buffer, phosphate buffer, and a pH 7.0 SDS solution without containing buffer constituents were prepared. A peak shift of 1−4 nm was usually observed with reaction time or in different buffers, especially for the bands at longer wavelengths, which could be caused by the variance of the environment around SWNTs or by electron withdrawal in redox reactions. Because we focus on the relative magnitude of peak height change in this work, we do not take account of the peak shift.7 All measurements were repeated three times for reproducibility. In the selectivity study, various metal ions (solution pH was adjusted to 7.0) at calculated concentrations were introduced into pH 7.0 SDS-SWNT suspensions, so that the final ion concentration was 8.0 × 10−5 M. All measurements were conducted at room temperature. In addition to SDS surfactant, the effect of other surfactants such as SDBS and TCAB on the reaction was also examined. Our preliminary result showed that SWNTs encased with SDBS (1 wt %) or TCAB (1 wt %) were insensitive to mercuric ions under the same conditions used in this work (Figure S3 in the Supporting Information), suggesting the implication of the surfactants on the reactivity. The result was very similar to those observed previously in the reaction of SWNTs with hydrogen peroxide.5 Further results will be reported in a future paper. Detection Limit Measurement. Two different methods were used to calculate the detection limit, the standard curve method and the limit of detection (LOD) method, with the details similar to those described in ref 7. The standard deviation s of ΔA/A0 from 7 measurements of pH 7.0 SDSSWNT suspensions (1 wt % SDS and 0.1 mg/mL SWNTs) reacting with 6.6 × 10−6 M of Hg2+ was calculated for the three selected absorption bands. ΔA is defined as A0 − A30 where A0 is the absorbance of the SDS-SWNT suspension in the absence of Hg2+ and A30 is the absorbance of the SDS-SWNT suspension after 30 min reaction in the presence of Hg2+ ions. The typical A0 is 0.30 for 1247 nm, 0.26 for 1317 nm, and 0.25 for 1375 nm, respectively. The determination of the detection limit dl was based on the equation dl = 3s/m, where m is the slope of the linear standard curve.7,28 The LOD method is defined as the concentration at which the magnitude of the spectral intensity change is three times larger than the instrumental noise (∼0.0003).7

tris(hydroxymethyl)-aminomethane (Tris, >99.9%), and (ethylenedinitrilo)-tetraacetic acid disodium salt (EDTA) were purchased from Sigma-Aldrich. Sodium dodecylbenzenesulfonate (SDBS, 88% pure) and tetraoctylammonium bromide (TCAB, 99.0% pure) were from Acros Organic. Mercury bichloride was from J. T. Baker Chemical. Hydrochloric acid and sodium hydroxide were from Fisher Scientific. The pH 7.0 buffer solutions were Tris buffer (10 mM), TE buffer (10 mM Tris +1 mM EDTA), MES buffer (20 mM), and phosphate buffer (50 mM). For the ion selectivity study, stannous chloride (98.2%), manganous sulfate (99.4%), cadmium nitrate (>98%), ferric chloride FeCl3·6H2O (97.0%), lead chloride (99.0%), and calcium carbonate (99.8%) were from Fisher Scientific. Anhydrous magnesium sulfate (99%) was from Spectrum Chemical. Tetra hydrate iron(II) chloride (99.9%) was from Acros Organic. Cobalt nitrate (99.0%) was from Aldrich. Zinc acetate was from Matheson Coleman and Bell, and methyl mercury chloride CH3Hg+ (99.9%) was purchased from Fluka. Preparation of SDS Encased HiPco SWNT Suspensions. In a typical experiment, ∼2.5 mg of pristine HiPco SWNTs were weighed on a microgram-scaled balance and mixed with 5 mL of 1 wt % SDS solution. The pH of the SDS solution was adjusted to pH 7.0 by adding 0.1 M NaOH and 0.1 M HCl. The mixture was then ultrasonicated in an ice− water bath (Branson Model 1510R-MT, 42 kHz, output power 70 W) to disperse the nanotubes.5 After sonication, the mixture was subjected to centrifuge at 14 000 rpm (VWR Galaxy 16 Microcentrifuge) for 16 h. The resulting supernatant was fetched as a stock solution for the next measurements. To optimize the dispersion of SWNTs in SDS solution, various periods of sonication time were investigated. It was found that 15 min sonication resulted in the best dispersion of SWNTs as shown in the Figure S1 of the Supporting Information. With the optimized sonciation time, the effects of SDS concentration (0.05−2 wt %) on the SWNT dispersion were also examined. The results indicated that, except in 0.05 wt % SDS, SWNTs in the solutions of 0.1−2 wt % SDS were dispersed, with the best dispersion outcome in 1 wt % (Figure S2 in the Supporting Information), in agreement with previous work.25,26 Therefore, if not specifically stated, for most of the SWNT-Hg2+ reaction studies in this work, the SDS concentration was fixed at 1 wt %. In a typical experiment, the SWNT stock solution was diluted with the SDS solution. The exact dilution volume was determined by measuring the absorbance of SWNTs at 1247 nm in a 1 mm path length quartz cell so that, after dilution, the absorbance (or OD) was ∼0.3 (∼0.1 mg/mL SWNTs5,7,27). Three-hundred microliters of SDS-SWNT samples was used for the experiments. The UV−vis-NIR absorption spectrum was measured on a Varian Cary 5000 UV−vis-NIR spectrophotometer with the SDS solution as the background. Three distinctive SWNT absorption peaks at 1247, 1317, and 1375 nm were chosen for further analysis. Because a SWNT band at 646 nm was insensitive to the environment changes,7 it was selected to normalize all absorption spectra for correcting dilution effects caused by the addition of Hg2+ or HCl and NaOH solutions. Solutions for Mercuric Ions-Related Experiments. A series of SDS-SWNT suspensions were prepared with different concentrations of Hg2+ ranging from 4.0 × 10−6 to 8.0 × 10−4 M. A 10 μL Hg2+ solution of a calculated concentration was added to a 300 μL SDS-SWNT suspension. The sample was immediately mixed in a 1 mm path length quartz cell, and timedependent absorption spectra were measured on the UV−vis15592

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surface of SWNTs forming a structureless, adsorbed layer.26 The spherical micelles of SDS at 0.5 wt % are reduced by ∼20%, while, at 1 and 2 wt %, they are almost unperturbed.26 As shown in Figure 2a, the spectral intensity decays at 1247 nm for SDS-SWNT suspensions reacting with 8.0 × 10−5 M Hg2+. It can be seen that the fastest decay occurs around 0.5 wt % SDS, the next is 1 wt % SDS. The decay curves at various SDS concentrations can be fit exponentially over reaction time. Similar results were observed for the bands at 1317 and 1375 nm. Throughout this work, it was observed that the timedependent spectral intensity decayed exponentially with reaction time, suggesting a pseudo first-order kinetic mechanism for the reaction of SWNTs with Hg2+, a phenomenon similar to the reaction of SWNTs with hydrogen peroxide7 and other oxidants.15 Figure 2b presents the spectral change by plotting (A0 − A30)/A0 against SDS concentration. It clearly showed that the reactivity reached a maximum at about 0.5−1 wt % SDS. At lower SDS concentrations, the surface of SWNTs functionalized with SDS molecules may not be optimized for Hg2+ ions to get access easily, with possible SWNT bundle formation. However, the higher SDS concentration >1 wt % has a significantly negative impact on the reactivity. The results did not change, even when SWNT concentration was increased to OD ≈ 0.6 at higher SDS concentrations (sample set #3 in Table S1 in the Supporting Information). The same results in Figure 2b obtained under the variance of SWNT concentrations listed in Table S1, Supporting Information, suggest that SDS concentration plays a more important role in the observed reactivity. Possible explanations include that a higher SDS concentration leads to the excess SDS micelles, which may attract Hg2+ through the electrostatic interaction between Hg2+ and the −SO3− functional group, or results in a higher SDS coating density on the surface of SWNT sidewalls with more Na+ ions surrounding the SWNT surface.32−36 The Na+ ions repel Hg2+. Both situations prevent Hg2+ ions from reaching the SWNT surface, resulting in lower reactivity. The detailed mechanisms may warrant further investigation. A similar reactivity reduction was observed in Tris and TE buffer solutions as presented in the later section. SWNT Concentration Effects. The SWNT concentration effects were also examined in the pH 7.0 SDS-SWNT suspensions with the addition of 8.0 × 10−5 M Hg2+. For the reaction, the initial reaction rate −(ΔA/Δt) 0 = k[SWNT]0[Hg2+]0. With [Hg2+]0 fixed at 8.0 × 10−5 M, it is expected that the initial reaction rate should linearly increase with initial SWNT concentrations [SWNT]0. We measured the decay curves of SDS-SWNT suspensions at various SWNT

RESULTS AND DISCUSSION Figure 1 shows a set of time-dependent absorption spectra of a SDS-SWNT suspension in a pH 7.0 solution reacting with 8.0

Figure 1. Absorption spectra of a SWNT suspension in 1 wt % SDS at pH 7.0 as a function of reaction time after adding 8.0 × 10−5 M of Hg2+. The arrows mark the three bands used in the work.

× 10−5 M Hg2+. The first interband transitions of semiconducting SWNTs (S11) occur from 900 to 1450 nm in which the spectral intensity from 1040 to 1450 nm are the most sensitive to Hg2+. The second interband transitions of semiconducting SWNTs (S22) locate at the region 6.2 > 7.2 > 8.1 > 9.2. In addition, at the selected pH level, the rate constants follow 1375 nm >1317 nm >1247 nm. These behaviors can be well explained based on eq 1,7,15 first given by Doorn’s group:15 ⎛ ⎛ 1 ⎞⎞ ln(k′) ∝ −ΔG ≈ nF ⎜EA − ⎜E F + E bg ⎟⎟ ⎝ ⎝ 2 ⎠⎠

(3)

(1)

where n is the number of electrons, F is the Faraday constant, EA is the redox potential of the electron acceptor, EF is the Fermi level of a given nanotube, and Ebg is the nanotube band gap energy. The equation suggests two ways in which rate constants k′ could be affected. The first one is the change in the nanotube diameter, which will cause a change in EF and Ebg. 0 This change leads to a smaller ESWNT + /SWNT as the diameter 15 increases. The second one is an increase in EA (E0Hg2+/Hg) as pH decreases from basic to acidic. Both situations will lead to a

Figure 6. (a) Absorption spectra of an SDS-encased SWNT solution containing 8.0 × 10−5 M Hg2+ at various pHs. (b) Ratio A30/A0* at 1247 nm from the Hg2+ reacting samples responds hysteretically to the pH changes. The A0* is the absorbance of 1247 nm band at pH 6.0. The red solid square represents the ratio for SWNT stock solution at pH 7.0 without reacting with Hg2+. 15595

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Figure 7. (a) Decay curves at 1247 nm for SDS-SWNT suspensions in different buffer solutions after the addition of 8.0 × 10−5 M Hg2+. The pH of the solutions is 7.0. (b) Pseudo first-order rate constants k′ at 1247 nm for the SDS-SWNT suspensions in different buffers after adding 8.0 × 10−5 M Hg2+.

increases in following order: H2O > phosphate (PHP) > MES > TE ≈ Tris. The rate constant k′ shown in Figure 7b follows H2O > phosphate (PHP) > MES. The buffer effects have been observed previously in the reaction of SWNTs with H2O2.7 In that work, the reaction is more efficient in Tris buffer, which was explained based on the stronger interaction of Tris with coating material DNA, causing loosened SWNT coating surface, available for more H2O2 molecules to gain access. In this work, however, SWNTs were insensitive to mercuric ion in Tris and TE buffers. One explanation is that, in Tris buffer, the SDS molecules may cover the SWNT surface more densely, as observed by Doorn’s group,34 thus making less SWNT surface area available for Hg2+ to react. In addition, it is known that mercuric ions can form coordination compounds with donors containing nitrogen.21 In Tris buffer, the Tris molecules (10 mM) may bind to Hg2+ ions by forming a coordination compound. In TE buffer, in addition to Tris molecules, the EDTA molecules (1 mM) may also chelate with Hg2+ ions. Both cases may result in no free mercuric ions available for reaction. In MES, although MES molecules contain nitrogen, the containing six membered ring structure where nitrogen and oxygen atoms stay on, may cause stereohindrance, making MES molecules less efficient to bind to Hg2+ than Tris molecules do. Thus, the reaction may still happen in MES buffer but at a slower rate as shown in Figure 7. Similarly, the slower reaction rate in the phosphate buffer than that in 1 wt % SDS solution might also suggest that phosphate binds to Hg2+ to some extent. To further elucidate the Tris effect on the reactivity, the reaction under different Tris concentrations was performed. The result is shown in Figure 8. Clearly, even at a low concentration [Tris] = 1 × 10−6 M, Tris molecules start to impede the reaction of SWNTs with Hg2+. Selectivity. The selectivity of SWNTs over 12 metal ions was examined by selecting a set of metal ions, including Hg2+, Zn2+, Cd2+, and methyl mercury ions. They were added into pH 7.0 SDS-SWNT suspensions so their final concentration is 8.0 × 10−5 M. Figure 9 shows the spectral intensity changes caused by these ions. Their time-dependent absorption spectra are shown in Figure S9 in the Supporting Information. In Figure 9, a significant change is observed for Hg2+, which is far more pronounced than other ions. The SWNTs respond to methyl mercury ion as well but with much lower sensitivity. A spectral change is also observed for Fe3+ (E0Fe3+/Fe2+ = 0.771 V), which is 0 able to oxidize some types of SWNTs that possess ESWNT + /SWNT < 0.771 V. Negligible changes in the absorption spectra were observed for metal ions Mg2+, Ca2+, Mn2+, Co2+, Zn2+, Cd2+, and Pb2+. These ions have negative standard reduction potentials, not being able to react with SWNTs.

Figure 8. Decay curves at 1247 nm for SDS-SWNT suspensions at pH 7.0 in the presence of different Tris concentrations after the addition of 8.0 × 10−5 M Hg2+.

Figure 9. (A0 − A50)/A0, the normalized absorbance changes at 1247 nm for SDS-SWNT suspensions at pH 7.0 after 50 min reaction with various ions at a concentration of 8.0 × 10−5 M, plotted as a function of different ion species.

However, instead of causing spectral intensity suppression, metal ions Fe2+ (E0Fe3+/Fe2+ = 0.771 V) and Sn2+ (E0Sn4+/Sn2+ = 0.139 V) induce a small spectral intensity increase, as shown by the negative bars in Figure 9. Sn2+ ions cause an apparent change in the SWNT absorption spectra where a doublet at ∼1247 nm appears (Figure S9, Supporting Information). One possible explanation for the observed increase in spectral intensity could be due to the interaction of Fe2+ and Sn2+ with some residual byproducts produced during the sonication process.3 The residual byproducts include H2O2, which may subsequently react with SWNTs by forming charge-transfer complexes.3,5 These residual complexes could be reduced by Fe2+ and Sn2+, leading to the recovery of the spectral intensity of SWNTs.3 In addition, the electrostatic interactions between metal ions and surfactant coating materials may also be a factor for consideration. Barron’s group has observed that the transition metal ions may have profound effects on SWNT 15596

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Figure 10. Standard curves of log[(A0 −A30)/A0], plotted against log[Hg2+] (log CHg2+) for SDS-SWNT suspensions at pH 7.0 at (a) 1246 nm, (b) 1317 nm, and (c) 1375 nm after 30 min reaction.

Figure 11. Pseudo first-order rate constant k′, plotted against [Hg2+] (CHg2+) for the band at (a) 1246 nm, (b) 1317 nm, and (c) 1375 nm of pH 7.0 SDS-SWNT suspensions.

fluorescence quenching due to the electrostatic interactions.37,38 More detailed studies are needed in the future in order to elucidate the detailed mechanisms. Sensitivity. For determination of the sensitivity to Hg2+, the reaction at various Hg2+ concentrations (4.0 × 10−6 to 8.0 × 10−4 M) was studied under optimized reaction conditions. The standard curves of the three absorption peaks were established by plotting log(ΔA/A0) (ΔA = A0 − A30) as a function of log[Hg2+] (log CHg2+), which shows a linear relationship (Figure 10). In addition, the rate constants k′ were obtained by fitting the decay curves at various Hg2+ concentrations. As shown in Figure 11, k′ is linearly proportional to the concentration of Hg2+ for these three bands, a characteristic of pseudo first-order kinetics with respect to [Hg2+].7 The Hg2+ concentration used for the rate constants is between 8.0 × 10−5 and 8.0 × 10−4 M, about 10 to 100 times larger than the valence electron concentration as discussed in the SWNT Concentration Effects section. This satisfies the condition that the Hg2+ concentration should be much larger than the SWNT concentration, as discussed previously in the reaction of SWNTs with H2O2.7 Thus, the observation of the linear relationship is warranted. In order to calculate the detection limit, the slope m and the intercept b were obtained from the linear fit in Figure 10. The standard deviation s was determined on the basis of 7 measurements of ΔA/A0 from SDS-SWNT suspensions reacting with 6.6 × 10−6 M of Hg2+. A modified equation (eq 4) was used to calculate the detection limit log[Hg 2 +] =

log 3s − b m

Table 1. Slope m, Intercept b, and Standard Deviation s for the Three Bands constants

1247 nm (10,5)

1317 nm (9,7)

1375 nm (10,6)

m b s

0.412 0.445 2.4 × 10−3

0.346 0.581 6.9 × 10−3

0.330 0.585 5.3 × 10−3

Table 2. Detection Limit of Hg2+ Determined from Two Methods in pH 7.0 SDS-SWNT Suspensions detection limit (M) standard curve method LOD method

1247 nm (10,5)

1317 nm (9,7)

1375 nm (10,6)

4.9 × 10−7 5.3 × 10−8

2.8 × 10−7 1.7 × 10−9

5.9 × 10−8 6.4 × 10−10

most sensitive one, which has the detection limit of 59 nM based on the standard curve method and 0.6 nM based on the LOD method. The detection limit on the basis of standard curve method can be further improved by reducing the standard deviation s. The LOD method offers the lowest detection limit, below 10 nM, the maximum contaminant level for mercuric ions in drinking water, which suggests the promise using SWNT-based sensors for mercuric ion detection.

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CONCLUSIONS The redox reaction and the pseudo first-order reaction kinetics of SDS-encased SWNTs with Hg2+ were systematically studied under different conditions, including different SDS and SWNT concentrations and different pH levels and buffer constituents. The reactivity was sensitive to the concentrations of SDS and SWNTs, and it was determined that the optimal condition for the redox reaction at pH 7.0 is 1 wt % SDS and 0.1 mg/mL SWNT (OD = 0.3 at 1 mm path length). The relationship between the reactivity and the concentrations of both SDS and SWNTs suggests that a balance is necessary between these two

(4)

where b is the y-intercept of the standard curve, m is the slope, and s is the standard deviation. Table 1 summarizes the values of m, b, and s needed for the detection limit calculations. The detection limits of the three bands based on the standard curve method and the LOD methods are summarized in Table 2. The 1375 nm band is the 15597

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(13) Cherukuri, P.; Bachilo, S. M.; Litovsky, S. H.; Weisman, R. B. J. Am. Chem. Soc. 2004, 126, 15638−15639. (14) Dukovic, G.; White, B. E.; Zhou, Z. Y.; Wang, F.; Jockusch, S.; Steigerwald, M. L.; Heinz, T. F.; Friesner, R. A.; Turro, N. J.; Brus, L. E. J. Am. Chem. Soc. 2004, 126, 15269−15276. (15) O’Connell, M. J.; Eibergen, E. E.; Doorn, S. K. Nat. Mater. 2005, 4, 412−418. (16) Zheng, M.; Diner, B. A. J. Am. Chem. Soc. 2004, 126, 15490− 15494. (17) Barone, P. W.; Baik, S.; Heller, D. A.; Strano, M. S. Nat. Mater. 2005, 4, 86−92. (18) Star, A.; Gabriel, J. P.; Bradley, K.; Gruner, G. Nano Lett. 2003, 3, 459−463. (19) Strano, M. S.; Huffman, C. B.; Moore, V. C.; O’Connell, M. J.; Haroz, E. H.; Hubbard, J.; Miller, M.; Rialon, K.; Kittrell, C.; Ramesh, S.; et al. J. Phys. Chem. B 2003, 107, 6979−6985. (20) Kim, T. H.; Lee, J.; Hong, S. J. Phys. Chem. Lett. 2009, 113, 19393−19396. (21) Nolan, E. M.; Lippard, S. J. Chem. Rev. 2008, 108, 3443−3480. (22) Kam, N. W. S.; Jessop, T. C.; Wender, P. A.; Dai, H. J. J. Am. Chem. Soc. 2004, 126, 6850−6851. (23) Liu, Z.; Davis, C.; Cai, W.; He, L.; Chen, X.; Dai, H. J. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 1410−1415. (24) Schipper, M. L.; Nakayama-Ratchford, N.; Davis, C. R.; Kam, N. W. S.; Chu, P.; Liu, Z.; Sun, X.; Dai, H. J.; Gambhir, S. S. Nat. Nanotechnol. 2008, 3, 216−221. (25) Poulin, P.; Vigolo, B.; Launois, P. Carbon 2002, 40, 1741−1749. (26) Yurekli, K.; Mitchell, C. A.; Krishnamoorti, R. J. Am. Chem. Soc. 2004, 126, 9902−9903. (27) Zhao, W. J. Phys. Chem. Lett. 2011, 2, 482−487. (28) Harris, D. C. Exploring Chemical Analysis, 3rd ed.; W. H. Freeman: New York, 2005. (29) Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Smalley, R. E.; Weisman, R. B. Science 2002, 298 (5602), 2361−2366. (30) Weisman, R. B.; Bachilo, S. M. Nano Lett. 2003, 3, 1235−1238. (31) Tu, X. M.; Manohar, S.; Jagota, A.; Zheng, M. Nature 2009, 460 (7252), 250−253. (32) Duque, J. G.; Densmore, C. G.; Doorn, S. K. J. Am. Chem. Soc. 2010, 132, 16165−16175. (33) Niyogi, S.; Densmore, C. G.; Doorn, S. K. J. Am. Chem. Soc. 2009, 131, 1144−1153. (34) Duque, J. G.; Gupta, G.; Cognet, L.; Lounis, B.; Doorn, S. K.; Dattelbaum, A. W. J. Phys. Chem. C 2011, 115, 15417−15153. (35) Tummala, N. R.; Striolo, A. ACS Nano 2009, 3, 595−602. (36) Xu, Z.; Yang, X.; Yang, Z. Nano Lett. 2010, 10, 985−991. (37) Brege, J. J.; Gallaway, C.; Barron, A. R. J. Phys. Chem. C 2009, 113, 4270−4276. (38) Brege, J. J.; Barron, A. R. Main Group Chem. 2011, 10, 89−104. (39) Crochet, J. J.; Duque, J. G.; Werner, J. H.; Doorn, S. K. Nat. Nanotechnol. 2012, 7, 126−132.

variables in order to reach an optimal surface coating structure of SDS on SWNTs and maximize the accessibility for Hg2+ to the surface of SWNTs for reaction. It also suggests that the oxidants used in the redox reactions such as Hg2+ in this work and H2O2 in previous work5−9,39 could serve as probes to detect the microenvironment around SWNTs, thus providing a new tool to study SWNT properties in solution. Furthermore, this work has revealed that the reaction is pH sensitive and the reactivity is higher at lower pHs. The suppressed spectral intensity can be recovered by raising pH to basic, then bringing the pH back to pH 6.0−7.0. This feature enables the reusability of SWNTs. SWNTs show high selectivity on Hg2+ over 11 other metal ions (Mg2+, Ca2+, Mn2+, Zn2+, Fe2+, Fe3+, Co2+, Cd2+, Sn2+, Pb2+, and CH3Hg+). The determined detection limit based on the LOD method is 0.6 nM, lower than the maximum contaminant level for mercuric ions (10 nM). This study may give an insight into the development of a new, SWNT-based NIR optical Hg2+ sensor for environmental, chemical, and biological applications.

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ASSOCIATED CONTENT

S Supporting Information *

SDS effects, different surfactants, SWNT concentration effects, and absorption spectra of SWNTs reacting with 12 ions. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank one reviewer for bringing refs 32−34, 37, and 38 to our attention. REFERENCES

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dx.doi.org/10.1021/jp304063x | J. Phys. Chem. C 2012, 116, 15591−15598