J. Phys. Chem. C 2007, 111, 17227-17231
17227
Redox Reaction of DNA-Encased HiPco Carbon Nanotubes with Hydrogen Peroxide: A Near Infrared Optical Sensitivity and Kinetics Study Xiaomin Tu,† Pehr E. Pehrsson,‡ and Wei Zhao*,† Department of Chemistry, UniVersity of Arkansas, 2801 South UniVersity AVenue, Little Rock, Arkansas 72204, and Chemistry DiVision, NaVal Research Laboratory, Washington, D.C. 20375 ReceiVed: July 27, 2007; In Final Form: September 4, 2007
The near-infrared optical properties of single-walled carbon nanotubes (SWNTs) have attracted particular attention for nanobiosensors based on their redox chemistry. In this work, we studied the redox reaction of single-stranded DNA (ssDNA)-encased HiPco nanotubes with hydrogen peroxide. The absorption intensity of the near-infrared interband transitions of semiconducting nanotubes decays exponentially with reaction time. The rate constant increases linearly with the H2O2 concentration, consistent with pseudo first-order kinetics. The spectral changes are reversible by tuning the pH, and the sensitivity is enhanced at lower pH in the pH range of 6-8. The reaction rate depends on the buffer, which follows the order MES > Tris > phosphate > TE. The detection limit for H2O2 is determined by three different methods based on the concentration-dependent rate constant, spectral intensity change, and signal-to-noise ratio. Our current work provides new insights into the solution chemistry of the ssDNA-SWNT hybrids for optical biosensing.
Introduction Single-walled carbon nanotubes (SWNTs) have drawn much attention because of the chemical and electrical properties derived from their unique one-dimensional nature.1-12 One of the most promising research areas is the surface modification of SWNTs using various coating materials, in particular, biological molecules such as DNA,1 for applications including biological sensors9,10-13 and separation of carbon nanotubes.1,14 However, SWNTs’ poor solubility in aqueous and nonaqueous environments and uneven distribution in suspension present major challenges1,2 It is reported that single-stranded DNA (ssDNA) and double-stranded DNA effectively disperse SWNTs into aqueous solution by forming stable complexes.1,4 The ssDNA encapsulates the SWNTs, thus preventing them from getting close enough to each other that van der Waals forces cause them to form bundles.1 Earlier computational modeling and experimental studies by Zheng et al. have found that oligonucleotides with repeat units of the thymine (T) and guanine (G) bases (poly d(GT)) have a much higher dispersion efficiency than those with adenine (A) and cytosine (C),1 probably because of the differences in the π-π interactions between the bases and the sidewall of carbon nanotubes. Hence, poly d(GT) oligonucleotides are extensively used, as they are here, to disperse the nanotubes in aqueous solution.1,14 In our previous study, we found that sodium dodecyl sulfate (SDS)-encased carbon nanotubes are optically sensitive to hydrogen peroxide (H2O2), which is a product of the glucose oxidase-catalyzed glucose reaction.7 H2O2 is a major reactive oxygen species in living organisms and its overproduction is implicated in the development of many severe diseases such as cancers and angiogeneses.15 Recently, H2O2 has been found to * Corresponding author. E-mail:
[email protected], phone: (501) 5698823. † University of Arkansas. ‡ Naval Research Laboratory.
play an important role in cell signaling.16 Therefore, the development of a sensitive and specific probe for H2O2 is physiologically and pathologically important. Being the product of many enzyme catalyzed reactions, H2O2 can act as an indicator to monitor the quantity of molecules such as glucose.17 Electrochemical detection methods for H2O2 offer the prospect of very low detection limits. However, they also have disadvantages in selectivity and device size.18 The exquisite near infrared optical properties of SWNTs are extremely sensitive to the environment, including the coating material. Therefore, it is essential to investigate the reaction of H2O2 and SWNTs functionalized with different surfactants. Combined with the information from SDS-encapsulated SWNTs from our previous studies, the interaction of H2O2 with ssDNA-SWNT hybrids may be useful for optical H2O2 sensing and could provide a rich solution redox chemistry for the development of an ultrasensitive miniature H2O2 detection device. In this work, the reaction kinetics between ssDNA-SWNTs and H2O2 were studied under different H2O2 concentrations, pH levels, and buffer constituents. The spectral intensity of the first interband transitions of semiconducting nanotubes decreases after addition of H2O2 according to pseudo first-order kinetics. The sensitivity of ssDNA-SWNT hybrids to H2O2 is pH-dependent as well. The spectral changes are reversible by tuning pH in the range of pH 6.0-8.0 and are more dramatic at lower pH. Experimental Section Materials. Raw Hipco SWNTs (lot no. 79, December 2, 2001) were purchased from Carbon Nanotechnologies, Inc. Their diameters ranged from 0.8 to 1.2 nm, and they had average lengths of a few hundred nanometers. The oligonucleotide (poly d(GT)20) was purchased from Integrated DNA Technologies, Inc. Reagents including 2-(4-morpholino)ethanesulfonic acid (MES, purity >99.5%), tris(hydroxymethyl)-aminomethane (Tris, >99.9%), (ethylenedinitrilo)-tetraacetic acid disodium salt
10.1021/jp075966c CCC: $37.00 © 2007 American Chemical Society Published on Web 10/30/2007
17228 J. Phys. Chem. C, Vol. 111, No. 46, 2007
Tu et al.
(EDTA), and 30 wt % hydrogen peroxide (H2O2, ACS) were purchased from Sigma-Aldrich. Phosphoric acid, sulfuric acid, hydrochloric acid, and sodium hydroxide were from Fisher Scientific. Acid and base solutions of 0.1 and 1.0 M were prepared in distilled water immediately before use. The buffer solutions were TE buffer (10 mM Tris +1 mM EDTA, pH 8.0), Tris buffer (10 mM, pH 7.0), MES buffer (20 mM, pH 7.0), and phosphate buffer (50 mM, pH 6.0, 7.0, and 8.0). MES, phosphate, and TE are commonly used biological buffers, and they have relatively good buffer capacity at pH 7 or 8. Tris buffer is chosen to elucidate the effect of EDTA in the reaction. ssDNA-Encased HiPco SWNT Suspensions. In a typical experiment, 1.0 mg of pristine HiPco SWNTs and 1.0 mg of ssDNA were weighed on a microgram-scaled balance and mixed in a 1.5 mL Eppendorf Flex-Tube with 1.0 mL of a buffer such as MES. The sample was ultrasonicated in an ice-water bath for 2 h (Sonics model VCX 130 PB, 20 kHz, output power ∼8 W) to disperse the nanotubes. The sample was then centrifuged at 16 000g (VWR Galaxy 16 Microcentrifuge) for 18 h. The supernatant was extracted and diluted with the buffer so that the absorbance at 1270 nm was 0.1 (∼0.03 mg SWNT/mL) for the experiments. Optical and pH Measurements. H2O2 solution (1.5 wt %) in calculated volumes was added to 0.3 mL ssDNA-SWNT suspensions. The final concentration of H2O2 in the samples ranged from 1 to 200 ppm. The samples were then immediately transferred into 1 mm path length quartz cells for optical measurements. Time-dependent absorption spectra were measured on a Varian Cary 5000 UV-vis-NIR spectrophotometer. Buffer solutions without HiPco nanotubes were used as references for background subtraction. The pH was measured using an Orion model 420 pH meter with a Fisher AccupHast Microprobe electrode. Reversibility measurements were performed by pH titration with 0.1 M HCl and 0.1 M NaOH followed by optical absorption measurements. Each experiment was conducted three times to ensure reproducibility. The detection limit determinations were repeated at least seven times with ssDNA-SWNT samples in pH 7.0 MES buffer after addition of 1 ppm of H2O2. The detection limit dl was determined using the following equation,19
dl )
3s m
(1)
where s is the standard deviation of the measurements, and m is the slope of the linear calibration curve.19 We measured the rate constant k′ as well as the relative magnitude of spectral peak height change, ∆A, which is defined as:
∆A ) (A0 - At)30min)S11/AS22(650 nm)
(2)
The A0 and At)30min are the absorbance (peak height) of the selected first interband transition S11 band at reaction time 0 and 30 min. The As22(650 nm) is the absorbance of the second interband transition S22 band at 650 nm, which is insensitive to the reaction. After subtraction of the reference signals, the baseline is determined at the region between 2000 and 2500 nm, where there is no absorbance of ssDNA-SWNTs. Since the study is only concerned with the relative magnitude of peak height change, we did not take account of negligible peak shift or change in peak shape. For example, the peaks in S11 bands may shift ∼(3 nm in the experiments. This is partly caused by the variance of the environment in which the ssDNA-SWNTs hybrids are prepared, for example, the buffer solution. All experiments were conducted at room temperature.
Figure 1. Absorption spectra of ssDNA-SWNT in pH 7.0 MES buffer as a function of time after adding 50 ppm H2O2: (a) 0 min, (b) 5 min, (c) 15 min, (d) 35 min, (e) 65 min, (f) 125 min, (g) 185 min, (h) 16 h.
Results and Discussion Figure 1 shows the time-dependent absorption spectra of an ssDNA-SWNT suspension in pH 7.0 MES buffer after mixing with 50 ppm H2O2. There are three distinctive absorption bands at roughly 1130, 1190, and 1270 nm. These belong predominantly to the first interband transitions (S11) of (8, 4), (12, 1), and (10, 5) semiconducting nanotubes, respectively.3-7 In comparison, the S11 bands of the SDS-SWNTs have a different tube composition.7 Other weaker absorption bands above 900 nm are the S11 bands of other semiconducting SWNTs, while those below 900 nm but greater than 600 nm are the second interband transitions (S22) of the semiconducting SWNTs. Larger diameter nanotubes have S11 bands at longer wavelengths, i.e., they have smaller interband transition energies.3-7 The measured spectral intensity was corrected for the dilution caused by the addition of H2O2 by normalizing to the S22 band at 650 nm, which is insensitive to pH and H2O2. As shown in Figure 1, the spectral intensity of the S11 bands decreases over time as the SWNTs react with H2O2, while most of the S22 bands are insensitive to H2O2. The decrease in absorption reflects a diminished transition strength.3-5,7,8,20 This results when H2O2 forms a charge-transfer complex with the sidewall of SWNT, which removes electrons from the valence band and increases the band gap.3-5,7,8,20,21 The result is consistent with that of SDSencased SWNTs.7 Our previous work showed that the reaction of SDS-SWNT suspensions with H2O2 is sensitive to pH.7 Similar results were observed in the ssDNA-SWNT hybrids. Figure 2 shows the spectral intensity changes of the 1270 nm S11 band, representing mainly (10, 5) ssDNA-SWNTs, in phosphate buffers at different pH levels after addition of 50 ppm of H2O2. The intensity decays exponentially with the reaction time. As summarized in Table 1, the rate constants k′ for the three S11 bands at 1130, 1190, and 1270 nm, all increase at lower pH. The reversibility of the changes in the near-infrared spectral intensity of the ssDNA-SWNT hybrids was also examined by titration. A suspension prepared in pH 6.0 phosphate buffer was reacted with 100 ppm H2O2. After 1 h, aliquots of 0.1 M NaOH were added to the sample to gradually increase the pH from 6 to 8. Aliquots of 0.1 M HCl were then added to decrease the pH from 8 to 6. The time-dependent spectra at different pH values were measured after the pH was adjusted. The spectral
DNA-Encased HiPco Carbon Nanotubes
J. Phys. Chem. C, Vol. 111, No. 46, 2007 17229
Figure 2. The absorption intensity of ssDNA-SWNT in phosphate buffers of pH 6.0, 7.0, and 8.0, changes as a function of time after addition of 50 ppm H2O2. The data are the normalized ratio of As11(1270 nm)/As22(650 nm). The solid lines show the exponential fits.
Figure 4. The normalized ratio of As11(1270 nm)/As22(650 nm) vs time after addition of 50 ppm H2O2 to ssDNA-SWNT in different pH 7.0 buffers: (a) TE buffer, (b) phosphate buffer, (c) Tris buffer, and (d) MES buffer. The solid lines are the exponential fits.
TABLE 2: Pseudo First-Order Rate Constants k′ of the First Interband Transitions of SWNTs in Different Buffers at pH 7.0 (50 ppm H2O2) rate constant k′ (×10-4 s-1)
Figure 3. pH titration of ssDNA-SWNT in phosphate buffer with 100 ppm H2O2. Restoration and suppression of the near-infrared spectral peak intensity is plotted as the ratio of As11 (1270 nm)/As22 (650 nm).
TABLE 1: Pseudo First-Order Rate Constants k′ of the First Interband Transitions of SWNTs in Phosphate Buffers at Different pH Values (50 ppm H2O2) rate constant k′ (×10-4 s-1)
pH 6.0 pH 7.0 pH 8.0
1130 nm (9, 4)
1190 nm (12, 1)
1270 nm (10, 5)
3.7 ( 0.1 3.3 ( 0.2 2.8 ( 0.8
6.8 ( 0.2 3.7 ( 0.4 3.3 ( 0.2
7.7 ( 0.7 4.0 ( 0.2 3.2 ( 0.2
intensity reached a constant value about 40 min after a pH adjustment. As shown in Figure 3, the pH-induced spectral intensity changes were completely recoverable, just like those in the SDS-encased SWNTs.7 However, in the DNA-encased SWNT system, the reversible behavior occurs in a much narrower pH range from 6 to 8, while in the SDS-encased SWNT system, a broader pH range from 6 to 11 is observed.7 The narrower pH response range between 6 and 8 may be an advantage for biological applications. The reversibility of the reaction between H2O2 and ssDNA-SWNTs was also observed in MES, TE, and Tris buffer solutions. The reaction mechanism may resemble that of the HiPco nanotubes reacted with oxygen, as proposed by Brus et al. who suggested that O2 reacts with the aromatic ring to form a 1,4-endoperoxide.21 Under acidic conditions, protonation of the endoperoxide forms a carbocation that is stabilized via resonance structures. The addition of base reverses the reaction and inhibits carbocation formation.21 Buffers affect the reaction rates and mechanisms of biological molecule, such as enzymes and DNA.22 In the past, many biological experiments failed because of the interactions of
TE phosphate Tris MES
1130 nm (9, 4)
1190 nm (12, 1)
1270 nm (10, 5)
2.6 ( 0.9 3.3 ( 0.2 9.2 ( 1.2 17.0 ( 0.5
2.1 ( 0.3 3.7 ( 0.4 16.0 ( 3.0 31.5 ( 2.4
2.1 ( 0.1 4.0 ( 0.2 15.2 ( 0.9 25.6 ( 2.1
buffer molecules with DNA and competitive inhibition of enzymes.22 Therefore, the buffer must be optimized for the reaction conditions. We examined four buffers that are commonly used in biological research: TE, MES, Tris, and phosphate buffer. The reaction rate was the slowest in TE and the fastest in MES. Figure 4 shows the spectral intensity changes of the 1270 nm S11 band in different buffers at pH 7.0 after addition of 50 ppm of H2O2. The spectral intensity decays exponentially. The rate constant k′ is obtained after the exponential curve fitting and is summarized in Table 2 for the three S11 bands. The reaction rate order is MES > Tris > phosphate > TE. These results suggest possible electrostatic interactions between amine-containing buffer molecules such as Tris or MES and the phosphate backbone of the DNA.23,24 Molecular simulations suggest that ssDNA wraps helically around SWNTs, thus preventing exposure of the hydrophobic interface to the aqueous medium.25,26 If so, then buffer molecules interacting with the phosphate backbone of the DNA molecules may pull them away from the SWNT surface. Such a distortion would enhance the access of H2O2 molecules to reaction sites on the SWNTs, thus enhancing the reaction rate. On the other hand, the EDTA in TE buffer might reduce the reactivity of H2O2 because it complexes with metal ion impurities which catalyze H2O2-related redox reactions. However, controlled experiments indicate that the reaction in TE buffer was slower than that in either 10 mM Tris or EDTA solution alone. Therefore, EDTA alone does not reduce the reactivity of H2O2 with ssDNA-SWNTs but may work cooperatively with Tris in the buffer to slow the H2O2 reaction. Similar phenomena of buffer dependence were also observed in SDS-SWNTs. The
17230 J. Phys. Chem. C, Vol. 111, No. 46, 2007
Tu et al.
Figure 5. Pseudo first-order rate constant (k′) plot against [H2O2] for the S11 band (1270 nm) of (10, 5) ssDNA-SWNT in pH 7.0 MES buffer.
detailed effects of buffers on the reaction kinetics are under investigation and will be discussed in a future report. Doorn et al.5,27 reported that the natural log of the rate constant k′ (the pseudo first-order rate constant) for a redox reaction of SWNTs is approximately linear with the nanotube band gap energy (Ebg) according to:
1 ln(k′) ∝ -∆G ≈ nF EA - EF + Ebg 2
(
(
))
(3)
where n is the number of electrons, F is the Faraday constant, EA is the redox potential of the organic acceptor, and EF is the Fermi level of a given nanotube.5,27 The equation suggests an inverse relationship between rate constant and band gap energy. Consequently, SWNTs with small band gaps are expected to have larger rate constants. However, Table 2 shows that except for the result in phosphate buffer, the highest rate constant in MES and Tris buffer occurs for the band at 1190 nm, (predominantly due to (12,1) SWNTs), while in TE buffer, it occurs for the band at 1130 nm. The result might suggest that the buffer may affect the reaction of nanotubes in a selective way dependent on the chirality. It may also be possible that the spectral overlap among the three dominant S11 bands in the nearinfrared absorption spectra limits precise rate constant determination as compared to the SWNT near-infrared fluorescence spectra, which have much better resolved emission bands.5 To further address this issue, chromatographically purified ssDNASWNTs enriched with single type nanotubes will be used to elucidate the correlation of sensitivity and tube diameter in a future study. The time-dependence of the absorption spectra of ssDNASWNT in pH 7.0 MES buffer was measured after addition of 1 ppm H2O2 to determine the detection limit. We used three different methods to obtain the detection limit, the rate constant, the relative magnitude of spectral height change, and the signalto-noise ratio. The rate constants were determined at different H2O2 concentrations using time-dependent absorption spectra of ssDNA-SWNT in pH 7.0 MES buffer. As shown in Figure 5, the rate constant k′ (the pseudo first-order rate constant) is linearly proportional to the concentration of H2O2, from which the slope (m) is obtained. The standard deviation of the rate constants from seven measurements of SWNTs reacting with 1 ppm of H2O2 was calculated, and the detection limit was determined using eq 1. For example, the standard deviation (s) of k′ for the 1270 nm band was 1.12 × 10-4 s-1. The slope (m) was 3.8 × 10-5 ppm-1 s-1 as determined using the standard
Figure 6. The normalized spectral intensity change ((A0 - At)30min)s11/AS22(650 nm)) of the S11 peak at 1270 nm from (10, 5) ssDNA-SWNT as a function of H2O2 concentration in pH 7.0 MES buffer.
TABLE 3: H2O2 Detection Limit Determined from Various Methods in pH 7.0 MES Buffer on the Base of the First Interband Transitions of SWNTs detection limit (ppm)
rate constant k′ method ∆A method LOD method
1130 nm (9, 4)
1190 nm (12, 1)
1270 nm (10, 5)
44.1 0.97 0.59
8.6 0.94 0.37
8.8 0.86 0.28
curve, so the detection limit was 8.8 ppm for the 1270 nm band. The calibration curve for the relative magnitude of the spectral peak height change was established by plotting the magnitude of the spectral peak height change of the S11 bands 30 min after reacting with 1 ppm H2O2 against the spectral intensity of the S22 band at 650 nm, ∆A ) (A0 - At)30min)S11/AS22(650nm). As shown in Figure 6, the magnitude of the spectral change of the 1270 nm band increases linearly with the log of H2O2 concentration (C), and the slope m is also acquired from the linear fit.19 The standard deviation of the magnitude of the spectral changes ∆A is obtained for the S11 band at 1270 nm. Using a modified version of eq 1,
(3s - a)/m ) log[H2O2]
(4)
where a is the y-intercept of the calibration curve, the detection limit was determined to be 0.86 ppm for the 1270 nm band. We also determined the limit of detection (LOD), defined as the concentration at which the magnitude of the spectral intensity change was three times larger than the instrumental noise (δ, ∼0.0003). The LOD was determined to be 0.28 ppm for the 1270 nm band. As shown in Table 3, the LOD is better than that in SDS-encased SWNTs where the LOD was 0.4 ppm for the 1322 nm band and 1 ppm for the 1245 nm band, respectively.7 The detection limits of three S11 bands from all three methods are summarized in Table 3. It appears that the LOD method affords the lowest detection limit as expected. The method involving rate constants provides the highest detection limit, and the ∆A method was intermediate. It is noted that all measurements are done at room temperature. Higher temperatures significantly increase the reaction rate and so might yield improved sensitivity. Purifying the SWNTs may also reduce the errors and deviations. In Figure 5, k′ is linearly proportional to the concentration of H2O2, which suggests that the reaction follows pseudo firstorder kinetics with respect to the SWNTs. Figures 2 and 3 show that the decay of the spectral intensity is well fit by a first-
DNA-Encased HiPco Carbon Nanotubes
J. Phys. Chem. C, Vol. 111, No. 46, 2007 17231
order decay curve. For each 100 carbon atoms, there are approximately 0.2-0.4 valence electrons.5,20 The H2O2 concentration in our experiments is therefore approximately 102000 times greater than the concentration of available valence electrons. Detailed titrations of the ssDNA-SWNTs samples after completing the reaction with H2O2 by using a KMnO4 solution reveal no detectable changes in the concentration of H2O2 after the reaction, which confirms that H2O2 is the excess reactant in the reaction. For the reaction of ssDNA-SWNTs and H2O2,
rate ) k[ssDNA - SWNT][H2O2]
(5)
where [ssDNA-SWNT] is the concentration of ssDNA-SWNT hybrids, [H2O2] is the concentration of H2O2, and k is the second rate constant.28 Since [H2O2] is in large excess, the rate equation can be expressed as,28
rate ) -
d[ssDNA - SWNT] ) k′[ssDNA - SWNT] (6) dt
where k′ ) k[H2O2]. k′ is the pseudo-first-order rate constant.28 As shown in Figure 5, k′ is linearly proportional to [H2O2], characteristic of the pseudo first-order reaction of ssDNASWNTs with H2O2. Conclusions In this work, we have measured the near-infrared spectral sensitivity of ssDNA-SWNT to H2O2, which suggests their potential in detecting target analytes that produce H2O2 by enzymatic reactions. The detection limit for H2O2 is determined for the three bands at 1130, 1190, and 1270 nm and for the most sensitive band at 1270 nm, it is found to be 8.8, 0.86, and 0.28 ppm by three different methods. In addition, we have determined the pH dependence of and the buffer effects on the optical sensitivity of ssDNA-SWNT hybrids reacted with H2O2. The relative reaction rates are MES > Tris > phosphate > TE. The solution chemistry of ssDNA-SWNT involves interactions between the ssDNA, carbon nanotubes, H2O2, and the solution environments. Our current study has built the foundation to further investigate the solution redox chemistry of ssDNAassisted purified SWNTs by ion-exchange chromatography.1 More importantly, the current finding enables us to explore the potential in carbon nanotube-based sensor applications17,29-32 in the biologically transparent near-infrared window for in ViVo imaging hydrogen peroxide as a contrast agent33 and for in ViVo glucose sensing that may offer minimally invasive and continuously monitoring technology for diabetes diagnosis.17,31,32 Acknowledgment. We thank Dr. Yang Xu and Dr. Chulho Song for their assistance in this work. W.Z. acknowledges the
financial support from ARO under Award No. DAAD 190210140, ONR, and DTRA. References and Notes (1) Zheng, M.; Jagota, A.; Semke, e. D.; Diner, B. A.; Melean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Mater. 2003, 2, 338342. (2) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95-98. (3) Zhao, W.; Song, C.; Pehrsson, P. E. J. Am. Chem. Soc. 2002, 124, 12418-12419. (4) Kelley, K.; Pehrsson, P. E.; Ericson, L. M.; Zhao, W. J. Nanosci. Nanotechnol. 2005, 5, 1029-1032. (5) O’Connell, M. J.; Eibergen, E. E.; Doorn, S. K. Nat. Mater. 2005, 4, 412-18. (6) Weisman, R. B.; Bachilo, S. M. Nano Lett. 2003, 3, 1235-1238. (7) Song, C.; Pehrsson, P. E.; Zhao, W. J. Phys. Chem. B 2005, 109, 21634-21639. (8) Itkis, M. E.; Niyogi, S.; Meng, M. E.; Hamon, M, A.; Hu, H.; Haddon, R. C. Nano Lett. 2002, 2, 155-159. (9) Shim, M.; Kam, N. W. S.; Chen, R. J.; Li, Y.; Dai, H. Nano Lett. 2002, 2, 285-288. (10) Hu, C.; Zhang, Y.; Bao, G.; Zhang, Y.; Liu, M.; Wang, Z. L. J. Phys. Chem. B 2005, 109, 20072-20076. (11) He, P.; Bayachou, M. Langmuir 2005, 21, 6086-6092. (12) Staii, C.; Johnson, A. T., Jr.; Chen, M.; Gelperin, A. Nano Lett. 2005, 5, 1774-1778. (13) Star, A.; Gabriel, J. C.; Bradley, K.; Gruner, G. Nano Lett. 2003, 3, 459-463. (14) Zheng, M.; Jagota, A.; Strano, M. S.; Santos, A. P.; Barone, P.; Chou, S. G.; Diner, B. A.; Dresselhaus, M. S.; Mclean, R. S.; Onoa, G. B.; Samsonidze, G. G.; Semke, E. D.; Usrey, M.; Walls, D. J. Science 2003, 302, 1545-1548. (15) Chang, M. C. Y.; Pralle, A.; Isacoff, E. Y.; Chang, C. J. J. Am. Chem. Soc. 2004, 126, 15392-15393. (16) Rhee, S. G. Science 2006, 312, 1882-1883. (17) Song, C.; Pehrsson, P. E.; Zhao, W. J. Mater. Res. 2006, 21, 28172823. (18) Bakker, E. Anal. Chem. 2004, 76, 3285-3298. (19) Harrris, D. C. Exploring Chemical Analysis, 3rd ed.; W. H. Freeman: New York, NY, 2005; pp 96-98. (20) Zheng, M.; Diner, B. A. J. Am. Chem. Soc. 2004, 126, 1549015494. (21) Dukovic, G.; White, B. E.; Zhou, Z.; Wang, F.; Jochusch, S.; Steigerwald, M. L.; Heinz, T. F.; Friesner, R. A.; Turro, N. J.; Brus, L. E. J. Am. Chem. Soc. 2004, 126, 15269-15276. (22) Good, N. E.; Winget, G. D.; Winter, W.; Connolly, T. N. Biochemistry 1966, 5, 467-477. (23) Stellwagen, N. C.; Bossi, A.; Gelfi, C.; Righetti, P. G. Anal. Biochem. 2000, 287, 167-175. (24) Wenner, J. R.; Bloomfield, V. Anal. Biochem. 1999, 268, 201212. (25) Lustig, S. R.; Jagota, A.; Khripin, C.; Zheng, M. J. Phys. Chem. B 2005, 109, 2559-2566. (26) Gao, H. J; Kong, Y. Annu. ReV. Mater. Res. 2004, 34, 123-50. (27) The sign of the band gap energy Ebg in eq 3 should be positive based on the Figure 2’s illustration of ref 5. (28) Atkins, P. Physical Chemistry, 6th ed.; W. H. Freeman: New York, NY, 1997; pp 765-766. (29) Xu, Y.; Pehrsson, P. E.; Chen, L.; Zhang, R.; Zhao, W. J. Phys. Chem. C 2007, 111, 8638-8643. (30) Satishkumar, B. C.; Brown, L. O.; Gao, Y.; Wang, C.-C.; Wang, H.-L.; Doorn, S. K. Nature Nanotechnol. 2007, 2, 560-564. (31) Barone, P. W.; Parker, R. S.; Strano, M. S. Anal. Chem. 2005, 77, 7556-7562. (32) Barone1, P. W.; Baik, S.; Heller, D. A.; Strano, M. S. Nat. Mater. 2005, 4, 86-92. (33) Lee, D.; Khaja, S.; Velasquez-Castano, J. C.; Dasari, M.; Sun, C.; Petros, J.; Taylor, W. R.; Murthy, N. Nature Mater. 2007, 6, 765-769.
