Strategy for Polychlorinated Biphenyl Detection Based on Specific

Oct 15, 2012 - Meet the Winners of the 2018 ACS Infectious Diseases Young ... Top Sessions for Librarians & Information Professionals at the 256th ACS...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/ac

Strategy for Polychlorinated Biphenyl Detection Based on Specific Inhibition of Charge Transport Using a Nanogapped Gold Particle Film Yang Yu,†,‡ Xing Chen,† Yan Wei,§ Jin-Huai Liu,† and Xing-Jiu Huang*,†,‡ †

Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, PR China ‡ Department of Materials Science & Engineering, University of Science and Technology of China, Hefei 230026, PR China § Department of Chemistry, Wannan Medical College, Wuhu 241002, PR China S Supporting Information *

ABSTRACT: A new strategy to detection of polychlorinated biphenyls (PCBs) using an electrical nanogap device is presented. This strategy is based on specific inhibition of charge transport when PCBs are captured by the cavities of β-cyclodextrin (β-CD) that are modified onto gold nanogapped electrodes' surfaces. The binding of PCBs to the cavities of β-CD leads to readily measurable conductivity decreases associated with the formation of guest−host complexes. PCB-29, PCB-77, PCB-101, PCB-153, and PCB-187 were chosen for the experiments. Six persistent organic pollutants, substituted benzenes with different sizes1,2-dichlorobenzene, 1,4-dichlorobenzene, pnitrophenol, 2,4-dichlorophenol, 1,3,5-trichlorobenzene, 1,2,4,5-tetrachlorobenzenewere chosen to investigate the selectivity properties of the nanogap devices. A modified “thermionic emission” electron tunneling model is used to account for the mechanisms for “inhibition of charge transport”. We believe that this specific inhibition of charge transport in a electrical nanogap device provides a promising approach to detect those pollutants having chemical inertness and insulating properties in the environment.

P

polyaniline nanowires in the nanogaps of a pair of interdigitated microelectrodes.16 An aggregate of gold nanoparticles also can be used as a conductive tag to bridge the nanogap electrodes for electrical detection of oligonucleotides.17 Similarly, polysaccharide templated silver nanowires in nanogaps can be used for ultrasensitive electrical detection of nucleic acids.18 Of particular interest is the nanogapped gold particle film based on interdigitated gold electrodes. This technique provides an opportunity to realize label-free DNA electronic detection. The basic principle relies on modification using a thiolated probe on the gold nanoparticles, which functions as both the linker between the gold nanoparticles and the spacer for producing the tunneling barrier, the energy of which changes upon hybridization occurring between the particles.19−23 However, interest is often concentrated on the conductivity increasing when the target molecules bridge or connect (not break) the gaps. Here, we report a strategy for electrical detection of PCBs that is based on specific inhibition of charge transport in a nanogap device. HS-β-Cyclodextrin (β-CD) was introduced

olychlorinated biphenyls (PCBs), the major class of the persistent organic pollutants existing in the environment, cause several adverse toxic effects in humans, such as genotoxicity, immunosuppression, tumor promotion, and oxidative stress.1 Current approaches for detection of PCBs are either GC (gas chromatography)/MS (mass spectrometry)2,3 and LC (liquid chromatography)/MS,4 a phosphorimetry method,5,6 and chromatographic analysis,7 or optical methods that are based on porous ZnO, Ag dendritic nanostructures, Ag-capped Au nanopillar arrays, and the membrane of the fluorophore phenyl isothiocyanate-immobilized porous anodic aluminum oxide.8−11 However, the effective detection of PCBs still remains challenging owing to their chemical inertness and insulating and hydrophobic properties. There have been several studies discussing nanogap devices for electrical sensing due to the most intriguing feature of the devices (that is, directly transducing events of the molecules' specific binding into useful electrical signals, such as resistance/ impedance, capacitance/dielectric, or field effect outstanding works).12−15 Typically, a nanogapped microelectrode-based biosensor array is fabricated for ultrasensitive electrical detection of microRNAs by depositing conducting polymer nanowires, © 2012 American Chemical Society

Received: July 23, 2012 Accepted: October 15, 2012 Published: October 15, 2012 9818

dx.doi.org/10.1021/ac302078d | Anal. Chem. 2012, 84, 9818−9824

Analytical Chemistry

Article

Figure 1. Schematic of an electrical nanogap device modified with CDs for PCBs detection. (a) Electrical nanogap device is constructed by a nanogapped gold particle film on an interdigitated microelectrode with a 2.5-μm gap. (b) Conceptual illustration of specific inhibition of charge transport. Functionalized point “traps” made through the modification of HS-β-CDs between the gaps in an electrical nanogap device. Cavities of HS-β-CDs act as specific grabbers to PCBs. Charge transport can be inhibited by electrical barriers (i.e., PCB molecules) in the transport channels.

Before depositing the gold nanoparticles (GNPs) on the substrate between the microelectrodes to reducing the gap sizes, the microelectrodes were washed thoroughly and modified with amino groups. The microelectrode was first immersed in acetone for 2 h and then in piranha solution (H2SO4/H2O2 = 3:1, v/v) for 10 min. After washing by ultrapure fresh water, 1 mM 3-aminopropyl-trimethoxysilane (APTMS) ethanol solution was utilized to treat the substrate for 2 h, followed by rinsing in ultrapure fresh water baths, and the microelectrode was placed in a 120 °C oven for 30 min to complete the Si−O bond formation.23,24 GNPs were synthesized following Frens’ method.25,26 In brief, 100 mL of 1.64% chlorauric acid (HAuCl4) aqueous solution was brought to a rolling boil with stirring. The rapid addition of 8 mL of 0.82% trisodium citrate to the solution resulted in an immediate color change from pale yellow, to black, to purple, then to wine-red. The boiling was continued for 30 min, and then the solution was allowed to cool to room temperature with rapid stirring. The colloidal solution of GNPs was finally separated by centrifuge and washed twice using ultrapure fresh water. The concentrated wine red-solution was stored at 4 °C until use. A microelectrode with a −NH2 group obverse was immersed in the GNPs colloidal solution for 8 h at room temperature to get the gold nanogapped electrode.17 Decoration of Cyclodextrin, and Host−Guest Inclusion. A saturated aqueous solution of HS-β-cyclodextrin (HSβ-CD) was employed to form “traps” between gaps. The nanogap devices were immersed in the HS-β-CD solution at room temperature for 24 h,27 and then β-CDs were decorated onto the nanogap surface via the Au−S bond. Subsequently, the nanogap devices were washed using ultrapure fresh water and then dried under a stream of N2. The modified nanogap device was then quietly put into an anhydrous ethanol solution

onto the gold nanogapped electrodes' surface for capturing PCBs. We find that, just as an inhibitor breaks the electron transport, the binding of PCBs to the cavities of β-CD in an electrode gap leads to readily measurable conductivity decreases associated with the formation of guest−host complexes. We believe that this specific inhibition of charge transport in the electrical nanogap device provides a promising approach to detect those pollutants having chemical inertness and insulating properties.

■

EXPERIMENTAL SECTION Chemicals. All the chemicals used in this study were of analytical grade and were used as purchased without further purification. 3-Aminopropyl-trimethoxysilane (APTMS) was bought from Sigma-Aldrich. Five different PCBsnamely, 2,2′,4,5,5′-pentachlorobiphenyl (PCB-101), 2,2′,3,4′,5,5′,6-heptachlorobiphenyl (PCB-187), 2,2′,4,4′,5,5′-hexachlorobiphenyl (PCB-153), 3,3′,4,4′-tetrachlorobiphenyl (PCB-77), and 2,4,5trichlorobiphenyl (PCB-29)were purchased from J&K Chemical Ltd., Shanghai. Gold(III) chloride tetrahydrate (HAuCl4·4H2O, 99.9%); trisodium citrate; and other reagents, including 1,2-dichlorobenzene (1,2-DCB), 1,4-dichlorobenzene (1,4-DCB), p-nitrophenol (p-NP), 2,4-dichlorophenol (2,4DCP), 1,3,5-trichlorobenzene (1,3,5-TCB), 1,2,4,5-tetrachlorobenzene (1,2,4,5-TeCB), were all received from Sinopharm Chemical Reagent Co., Ltd., China. Ultrapure fresh water was obtained from a Millipore water purification system (Milli-Q, specific resistivity >18 MΩ cm, S.A., Molsheim, France) and used in all runs. Gold Nanogapped Electrode Fabrication. The interdigitated gold microelectrode (2 × 2 array, gold 60 nm, titanium 10 nm) with 2.5-μm gaps was fabricated using electron beam lithography on a Si wafer with a 1-μm coating of SiO2. 9819

dx.doi.org/10.1021/ac302078d | Anal. Chem. 2012, 84, 9818−9824

Analytical Chemistry

Article

Figure 2. Tapping mode AFM characteristics of a nanogapped gold particle film. (a−b) Height image of nanogapped gold film before and after decoration with HS-β-CDs, respectively. Scale: 0.5 μm × 0.5 μm. (c−d) Phase image of nanogapped gold film before and after decoration with HSβ-CDs, respectively. Scale: 1.0 μm × 1.0 μm.

containing the target molecules to be detected (such as PCBs and substituted benzenes) for 1 h. The nanogap device was finally washed using anhydrous ethanol and ultrapure fresh water and completely dried under a stream of N2. Materials Characterization. The GNPs were characterized by UV−vis (ultraviolet visible absorption spectra, Solidspec-3700 spectrophotometer), SEM (Quanta 200 FEG fieldemission scanning electron microscope), and HRTEM (JEOL JEM-2010, 100 kV). Surface morphology and microstructure were characterized by AFM (Veeco Autoprobe CP atomic force microscopy). The entrapment of HS-β-CD and polychlorinated biphenyl was verified by XPS (Philips X’Pert diffractometer with Cu Kα radiation λ = 1.5418 Å). Molecular dimensions of the eleven chemical molecules were obtained with the help of molecular structure simulation using the software ChemBio3D Ultra 11.0, ChemBioOffice 2008, CambridgeSoft. Electrical Measurements. The decoration of the CDs and the inclusion of target molecules were detected by measuring the change in the electrical characteristics of the gold nanogapped electrode using a semiconductor parameter analyzer (HP4156C). The electrical characteristics of the gold

nanogapped electrode were measured after each designed step; in other words, by measuring the data after formation of the gold nanogap, decoration of CDs, and inclusion of target molecules (including different concentrations), respectively. The currents were measured while the bias voltage was swept from −1.5 to 1.5 V with a scan rate of 40 mV s−1. Current− time curves were also measured under a steady voltage of 1.5 V for 250 s. All measurements were performed on the probe station at room temperature. The voltages were applied through probing tips that were directly in contact with each probing pad in the device and fixed by vacuum. The currents were measured through the probing tips while the corresponding voltage was applied. The I−V data-fitting was performed using the software MATLAB R2007b, MathWorks.

■

RESULTS AND DISCUSSION Figure 1 shows the detection principle of polychlorinated biphenyls (PCBs) based on specific inhibition of charge transport. The strategy uses the specific binding of PCBs to the CD molecules in a nanogap electrode. Figure 1a illustrates the fabrication process of a nanogapped gold particle film. The 9820

dx.doi.org/10.1021/ac302078d | Anal. Chem. 2012, 84, 9818−9824

Analytical Chemistry

Article

characteristic sizes of the nanogaps can be modulated by changing the concentration of the GNP colloid and the assembling time. Cyclodextrins have the special molecular structure of a hydrophobic internal cavity and a hydrophilic external surface. This determines that CDs can be used as wellknown molecular hosts for recognition of small hydrophobic molecules in aqueous media, such as persistent organic pollutants in environment.1,27,28 As depicted in Figure 1b, the HS-β-CDs were modified on the surface of GNP as traps. When the PCBs are trapped by the hydrophobic internal cavities of CDs between the nanogaps, they act as electrical barriers because of the insulating property; the charge transport can be inhibited in the transport channel. Thus, by monitoring the decrease in current, it is possible to detect the specific binding of PCBs to CDs. The characterization of GNPs is shown in Supporting InformationFigure S1. As seen from the TEM image (Figure S1a), the as-synthesized GNPs are well-dispersed, and their average diameter is ∼15 nm. The SEM image of a random area between the microelectrodes demonstrates that most GNPs distribute well on the substrate, and the average distance between two adjacent GNPs is about 34 nm (Figure S1b). The UV−vis absorption spectra of GNPs proves the maximum surface plasmon resonance absorption peak appears at 522 nm (Supporting Information Figure S2), which confirms the GNPs' diameter to be ∼15 nm on average.29 The surface properties of the nanogapped gold film before and after decoration of HS-β-CDs were characterized using an atomic force microscope (AFM, Figure 2). From the height images (Figure 2a−b) after decorating CDs, the surface has a greater root-mean-square (RMS) roughness (RMS = 4.830 nm) compared with the bare nanogapped gold film (RMS = 3.397 nm). Meanwhile, we found that the size of the gold nanoparticles clearly increases and the displayed morphologies of the particles are not as spherical as that of GNPs before CD modification, indicating that the small CD molecules have been connected to the GNP surface randomly and have combined closed adjacent particles together when decorated with CD. More importantly, we observed that each nanoparticle is linked with its adjacent particle by means of a washer-like structure (examples are shown by the white arrows). It could be that the gaps between the nanoparticles are partially filled as a result of the CDs decoration. These results could be further verified by the corresponding phase images (Figure 2c−d). The phase image provides a clearer observation of fine features. As seen, the RMS roughness of the surface after decorating with β-CDs is 19.372 nm, which is much larger than that before decoration (RMS = 13.396 nm). The increase in the surface roughness is believed to be the result of the CD-decorated gold surface. Especially, the gaps between nanoparticles can be clearly seen (Figure 2d). To verify the concept of specific inhibition of charge transport for PCB detection in a nanogap device, the electric conductivity changes of the nanogap devices after CDs were immobilized and PCB-101 was trapped in the nanogap are recorded (Figure 3). As seen from Figure 3a, the results show an increase in the current when the air gap is filled with CD molecules to form “traps” under the positive voltage bias, followed by a decrease when the PCB-101 is captured by the cavities of the CDs. Upon increasing the concentration of PCB101, the current decreases further. Quantitatively, at a bias voltage of 1.5 V, the current runs up from 37 to 740 nA after filling the nanogaps with β-CDs. When the nanogap devices

Figure 3. Electrical characteristics of a nanogap device. (a) I−V curve measurements were performed by applying a bias voltage from −1.5 to 1.5 V at a scan rate of 40 mV s−1. (b) I−t curve measurements were performed at a fixed bias voltage of 1.5 V for 250 s. The inset is a plot of ΔI as a function of the logarithmic value of the PCB-101 concentrations.

were dipped into 1, 2, and 4 nM PCB-101 anhydrous ethanol solution, the tunneling currents decreased sharply to 395, 296, and 226 nA, respectively (Figure 3b). These results demonstrate that charge transport can be inhibited by PCB molecules in the transport channel. Meanwhile, a linear relation between ΔI and the logarithmic value of PCB-101 concentrations was seen (inset in Figure 3b). The linear regression equation is ΔI (nA) = 379.5 + 252.8 log cPCB (nM) with a correlation coefficient of 0.999. According to the linear equation, one could detect the PCB-101 concentration quantitatively. ΔI is the current difference before and after captured PCB-101. The reproducibility of experimental results is checked by performing 10 repetitive preconcentration−measurement−regeneration cycles under the same conditions. The error bars indicate good reproducibility of the present nanogap sensing system. X-ray photoelectron spectroscopy (XPS) measurements were carried out to verify the host−guest behavior of CD-PCB (Supporting Information Figure S3). The β-CD-decorated GNPs film was treated with 1 nM PCB-101 in ethanol for 1 h and then thoroughly washed with anhydrous ethanol and ultrapure fresh water to remove the physically absorbed PCB-101. The Cl 2p XPS spectra exhibit one peak at 199.2 eV, corresponding to the Cl 2p spin−orbit 9821

dx.doi.org/10.1021/ac302078d | Anal. Chem. 2012, 84, 9818−9824

Analytical Chemistry

Article

Table 1. Fitting Parameters of the plots of ln(I) vs V for I−V Measurements at Positive Bias Voltages by Fitting As ln(I) = a′Vn + b

peak of PCB-101. All the Cl 2p spin−orbit peaks join together, forming a slightly weak peak because of the exact low concentration of PCB-101 host in the CDs cavities. No Cl 2p peak could be observed at the same region of CD-decorated GNPs film without PCB-101 ethanol solution treatment. It is considered that the PCB-101 can favorably host inside the CDs. Two factors contribute to the conductivity change of nanogap device during the measurements: (1) the relative distances between the two adjacent gold nanoparticles, d; and (2) the relative dielectric constant after filling in the nanogap, εi.30 A thermionic emission electron tunneling model is used to account for these two mechanisms for “inhibition of charge transport”.31−33 I = AT 2ea

V − qΦ / kT

where

a=

q 2

q 4πεiε0d

a′ b adjusted R2

where

b = ln AT 2 − qΦ/kT

a′ =

q 2

CD

PCB-101 1 nM

PCB-101 2 nM

PCB-101 4 nM

6.45 −3.00 0.954

9.16 −3.11 0.990

7.61 −2.03 0.988

8.71 −3.41 0.987

7.33 −2.28 0.990

How well does eq 2 model the measured conductivity of our CD-modified gold nanogap? Focusing first on the conductivity data at the bare nanogaps, eq 2 predicts a tunneling current that is too low for the nanogaps when a width of 34 nm (see Supporting Information Figure S1b) is studied. After decorating β-CDs in the nanogaps, d′ obviously decreases, resulting in an increased a′. Meanwhile, the hydroxyl groups on the β-CDs can provide the charges for enhancing the conductivity, and they result in εi′ decreasing while β-CDs fill the nanogap to replace part of the air. So the tunneling current highly rises. When the PCB-101 molecules enter into the cavities of βCDs instead of air, three contributing factors to the conductivity decreasing should be considered. First, εi′ increases gradually as the concentration increases as a result of the ultrahigh dielectric properties of PCB-101. Second, d′ does not change or changes only a little, which can be ignored during the PCB-101 hosting, because the PCBs are entrapped into the cavities of the β-CDs. Finally, entrapment of the PCB molecules into the CD cavities will engender a further decrease in the charge mobility by altering the scattering potentials of the CD molecules.27 In addition, according to eq 2, b has a slight change due to the tiny change in the thermal emission barrier height Φ (approximately several electronvolts) compared with the main effect parameter, a′. We also used another four PCB molecules (PCB-187, PCB153, PCB-77, and PCB-29) to examine how the concept of specific inhibition of charge transport can be used for PCBs detection (Supporting Information Figure S4). Linear relations between ΔI and the logarithmic values of PCB-187, PCB-153, PCB-77, and PCB-29 concentrations were obtained (Supporting Information Figure S5), the linear regression equations and correlation coefficients can be seen in the figure. We see clear quantitative increases and decreases in the electrical signals of the nanogap associated with immobilization of CDs and PCBs. Good reproducibility could also be seen by checking the error bars in the calibration plots. The results confirm that PCB molecules can be captured by the cavities of CDs and inhibit the charge transport. For different devices, the current magnitude varies slightly because of variation in device area, but the rectifying behavior is always observed. Devices with nanogap lengths equal to sub-30 nm were studied to confirm the gap length that is efficient for modification of CDs and PCBs because the height of the CDs layer is about 0.7 nm and it seems that the spacing between the gaps can be enough for capturing PCBs. Unfortunately, the tunneling current is too high in this stage. On the other hand, although the conductivity change after CD and PCB-101 molecule modification could be observed, the signals were quite unstable, and no significant changes could be seen. This may be explained using the “steric hindrance” of the CD and PCB molecules. We cannot explain this effect within the context of following eqs 1 and 2.

(1)

In this equation, A is the effective Richardson constant multiplied by the current injection area, q is the electron charge, Φ is the thermal emission barrier height, V is the bias, k is Boltzmann’s constant, T is the temperature, εi is the relative dielectric constant after filling, ε0 is the vacuum dielectric constant, and d is the thickness of the molecular dielectric film (equal to the size of the nanogap), respectively. Given that each nanogap unit can generate a tunneling current under a certain potential, the model should be modified as below: the linear relationship between ln(I) ∼ V1/2 is amended as ln(I) ∼ Vn numerically, considering that there are plenty of nanogap units in our nanogap device. The experimental results at positive bias voltages are fitted by ln(I ) = a′V n + b

bare electrode

q 4πεi′ε0d′ (2)

The results are shown in Figure 4. From eq 2, it can be found that the parameter a′ is related to εi′ and d′. Thus, a′ shows the exact properties in the nanogaps between gold nanoparticles. The fitting results of a′, b, and n are listed in Table 1. According to the fitting results, n is defined as 0.16 ± 0.01 by counting, and all the adjusted R2 data are higher than 0.950.

Figure 4. Plots of ln(I) vs V. Hollow doted lines represent I−V measurements at positive bias voltages; solid lines are corresponding fitting data. 9822

dx.doi.org/10.1021/ac302078d | Anal. Chem. 2012, 84, 9818−9824

Analytical Chemistry

Article

dielectric constant of the gap area, εi, is the dominant factor in detecting the decrease in the tunneling current. Such a device having “traps” between the gaps addresses the fundamental problem in that it is difficult to force a “solution” containing the target species to be detected into the small size of the nanogap.12 In addition, it is expected that such a CD-modified nanogap device will have the capability to detect a number of organic compounds in solution, which could be captured by the hydrophobic inner cavity of CDs. Thus, we have designed not only a convenient PCBs sensor in this study but also a methodology for more extensive application of the nanogap sensor in detection of other persistent organic pollutants having chemical inertness, insulating, and hydrophobicity properties.

Six persistent organic pollutants, substituted benzenes with different sizes1,2-DCB, 1,4-DCB, p-NP, 2,4-DCP, 1,3,5TCB, and 1,2,4,5-TeCBwere chosen to investigate the selectivity properties of the nanogap devices (see Supporting Information Figure S6 for their molecular structures). The I−V curves were recorded, as shown in Supporting Information Figure S7. Almost no obvious current decrease happens for 1,2DCB and 1,4-DCB treatment at a certain bias voltage. 1,2,4,5TeCB holds the biggest current decrease ratio at 14.69%. As demonstrated in Figure 5, the current decrease ratios are all

■

ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: 86-551-559-1167. Fax: 86-551-559-2420. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21105073, 61102013, 21073197, and 90923033) and the National Basic Research Program of China (No. 2011CB933700). Y.W. thanks the State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences (KF2011-18), and X.-J.H. acknowledges the One Hundred Person Project of the Chinese Academy of Sciences, China, for financial support.

Figure 5. Selectivity studies of an electrical nanogap device modified with CDs. Five different PCBs together with six benzene compounds were employed as the guests.

higher than 60% for 4 nM PCBs and much higher than that for 10 nM substituted benzenes. The main reason might be that the substituted benzenes (1,2-DCB, 1,4-DCB, p-NP, 1,2-DCP, 1,3,5-TCB, 1,2,4,5-TeCB) are smaller than PCBs (Supporting Information Table S1). It is clear that all the PCBs' molecular volumes are around 200 Å3, but substituted benzenes are smaller than 150 Å3 (1,2,4,5-TeCB, 156.2 Å3). In contrast to the cavity of β-CD (0.7−0.8 nm), substituted benzenes are too small to be entrapped. Thus, these small molecules can be easily removed from hosting in β-CDs by exhaustive washing, or they cannot offer an effective hindrance to block the tunneling current paths. Hence, considering the microcalorimetric titration study between CD and other guests,27 the equilibrium constant (K) values for the complexation of CDs with different objective molecules and the molecular volumes are two main factors correlated with the selectivity due to the concept of molecular recognition. Finally, it is worthwhile to point out that the electrodes can be refreshed by transferring them after determination to an ethanol-saturated solution of sodium acetate (Supporting Information Figure S8).

■

REFERENCES

(1) Wei, Y.; Kong, L. T.; Yang, R.; Wang, L.; Liu, J. H.; Huang, X. J. Chem. Commun. 2011, 47, 5340. (2) Ozcan, S.; Tor, A.; Aydin, M. E. Anal. Chim. Acta 2009, 647, 182. (3) Lambropoulou, D. A.; Konstantinou, I. K.; Albanis, T. A. J. Chromatogr. A 2006, 1124, 97. (4) Focant, J. F.; Cochran, J. W.; Dimandja, J. M. D.; DePauw, E.; Sjodin, A.; Turner, W. E.; Patterson, D. G. Analyst 2004, 129, 331. (5) Belfatmi, R.; Lamotte, M.; Ait-Lyazidi, S.; de Violet, P. F. Chemosphere 2005, 61, 761. (6) Arruda, A. F.; Goicoechea, H. C.; Santos, M.; Campiglia, A. D.; Olivieri, A. C. Environ. Sci. Technol. 2003, 37, 1385. (7) Schurig, V.; Reich, S. Chirality 1998, 10, 316. (8) Huang, Z. L.; Meng, G. W.; Huang, Q.; Yang, Y. J.; Zhu, C. H.; Tang, C. L. Adv. Mater. 2010, 22, 4136. (9) Li, M. T.; Meng, G. W.; Huang, Q.; Yin, Z. J.; Wu, M. Z.; Zhang, Z.; Kong, M. G. Langmuir 2010, 26, 13703. (10) Yang, Y. J.; Meng, G. W. J. Appl. Phys. 2010, 107. (11) Wang, M. L.; Meng, G. W.; Huang, Q.; Li, M. T.; Li, Z. B.; Tang, C. L. Analyst 2011, 136, 278. (12) Chen, X.; Guo, Z.; Yang, G. M.; Li, J.; Li, M. Q.; Liu, J. H.; Huang, X. J. Mater. Today 2010, 13, 28. (13) Lim, D.-K.; Jeon, K.-S.; Hwang, J.-H.; Kim, H.; Kwon, S.; Suh, Y. D.; Nam, J.-M. Nat. Nanotech. 2011, 6, 452. (14) Park, S. J.; Taton, T. A.; Mirkin, C. A. Science 2002, 295, 1503. (15) Venkatesan, B. M.; Bashir, R. Nat. Nanotech. 2011, 6, 615. (16) Fan, Y.; Chen, X. T.; Trigg, A. D.; Tung, C. H.; Kong, J. M.; Gao, Z. Q. J. Am. Chem. Soc. 2007, 129, 5437.

■

CONCLUSION We have demonstrated a strategy of specific inhibition of charge transport to detection of polychlorinated biphenyls coupled with a 34-nm gold nanogap. The selectivity and sensitivity of this strategy is due to the specific binding of PCBs to the cavities of β-CD. The lower detection limit reaches at least 1 nM for PCBs. In addition to the gap size, d, the relative 9823

dx.doi.org/10.1021/ac302078d | Anal. Chem. 2012, 84, 9818−9824

Analytical Chemistry

Article

(17) Fang, C.; Fan, Y.; Kong, J. M.; Gao, Z. Q.; Balasubramanian, N. Anal. Chem. 2008, 80, 9387. (18) Kong, J. M.; Ferhan, A. R.; Chen, X. T.; Zhang, L.; Balasubramanian, N. Anal. Chem. 2008, 80, 7213. (19) Shiigi, H.; Tokonami, S.; Yakabe, H.; Nagaoka, T. J. Am. Chem. Soc. 2005, 127, 3280. (20) Tokonami, S.; Shiigi, H.; Nagaoka, T. Anal. Chem. 2008, 80, 9071. (21) Tokonami, S.; Shiigi, H.; Nagaoka, T. Electroanalysis 2008, 20, 355. (22) Tokonami, S.; Shiigi, H.; Nagaoka, T. J. Electrochem. Soc. 2008, 155, J105. (23) Wang, C. J.; Huang, J. R.; Wang, J.; Gu, C. P.; Wang, J. H.; Zhang, B. C.; Liu, J. H. Colloids Surf. B 2009, 69, 99. (24) Sato, T.; Ahmed, H.; Brown, D.; Johnson, B. F. G. J. Appl. Phys. 1997, 82, 696. (25) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306. (26) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 55. (27) Kong, L. T.; Wang, J.; Meng, F. L.; Chen, X.; Jin, Z.; Li, M. Q.; Liu, J. H.; Huang, X. J. J. Mater. Chem. 2011, 21, 11109. (28) Wei, Y.; Kong, L. T.; Yang, R.; Wang, L.; Liu, J. H.; Huang, X. J. Langmuir 2011, 27, 10295. (29) Haiss, W.; Thanh, N. T. K.; Aveyard, J.; Fernig, D. G. Anal. Chem. 2007, 79, 4215. (30) Im, H. S.; Huang, X. J.; Gu, B.; Choi, Y. K. Nat. Nanotechnol. 2007, 2, 430. (31) Wang, W.; Lee, T.; Reed, M. A. Phys. Rev. B 2003, 68, 035416. (32) Zhou, C.; Deshpande, M. R.; Reed, M. A.; Jones Ii, L.; Tour, J. M. Appl. Phys. Lett. 1997, 71, 611. (33) Ibañez, F. J.; Zamborini, F. P. Small 2012, 8, 174.

9824

dx.doi.org/10.1021/ac302078d | Anal. Chem. 2012, 84, 9818−9824