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PANnano films
Talanta 77 (2009) 1021–1026
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Highly sensitive electrochemical impedance spectroscopic detection of DNA hybridization based on Aunano –CNT/PANnano films Na Zhou, Tao Yang, Chen Jiang, Meng Du, Kui Jiao ∗ Key Laboratory of Eco-chemical Engineering (Ministry of Education), College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
a r t i c l e
i n f o
Article history: Received 12 March 2008 Received in revised form 30 July 2008 Accepted 31 July 2008 Available online 7 August 2008 Keywords: Electrochemical DNA biosensor Nanocomposite film Polyaniline nanofibers Electrochemical impedance spectroscopy PAT gene NOS gene
a b s t r a c t A polyaniline nanofibers (PANnano )/carbon paste electrode (CPE) was prepared via dopping PANnano in the carbon paste. The nanogold (Aunano ) and carbon nanotubes (CNT) composite nanoparticles were bound on the surface of the PANnano /CPE. The immobilization and hybridization of the DNA probe on the Aunano –CNT/PANnano films were investigated with differential pulse voltammetry (DPV) and cyclic voltammetry (CV) using methylene blue (MB) as indicator, and electrochemical impedance spectroscopy (EIS) using [Fe(CN)6 ]3−/4− as redox probe. The voltammetric peak currents of MB increased dramatically owing to the immobilization of the probe DNA on the Aunano –CNT/PANnano films, and then decreased obviously owing to the hybridization of the DNA probe with the complementary single-stranded DNA (cDNA). The electron transfer resistance (Ret ) of the electrode surface increased after the immobilization of the probe DNA on the Aunano –CNT/PANnano films and rose further after the hybridization of the probe DNA. The remarkable difference between the Ret value at the DNA-immobilized electrode and that at the hybridized electrode could be used for the label-free EIS detection of the target DNA. The loading of the DNA probe on Aunano –CNT/PANnano films was greatly enhanced and the sensitivity for the target DNA detection was markedly improved. The sequence-specific DNA of phosphinothricin acetyltransferase (PAT) gene and the polymerase chain reaction (PCR) amplification of nopaline synthase (NOS) gene from transgenically modified beans were determined with this label-free EIS DNA detection method. The dynamic range for detecting the PAT gene sequence was from 1.0 × 10−12 mol/L to 1.0 × 10−6 mol/L with a detection limit of 5.6 × 10−13 mol/L. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Last decades, astonishing achievements in the cultivation of the transgenic plants have been obtained all over the world [1,2]. However, their security has been brought into focus. The accurate, sensitive, and rapid detection of the transgenic plant products is of significance for understanding the security of the transgenic plants. Phosphinothricin acetyltransferase (PAT) gene and nopaline synthase (NOS) gene are two important screening detection transgenes of the transgenic plants. Detection of these two transgenes can be utilized to identify the transgenic plants. DNA electrochemical biosensors have been successfully applied for the transgene detection of the transgenic plants [3–5]. In order to improve the sensitivity, selectivity, and stability of the biosensor, various kinds of nanomaterial have been prepared and utilized recently for the biosensor fabrication [6–12]. One of these nano-
∗ Corresponding author. Tel.: +86 532 84855977; fax: +86 532 84023927. E-mail address: [email protected] (K. Jiao). 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.07.058
materials is polyaniline nanomaterial (PANnano ). PANnano has the properties of mechanical flexibilities, high surface area, chemical specificities, tunable conductivities and easy processing [13–16], which make this conducting polymer as a kind of the promising sensing material for ultrasensitive, trace-level biological and electrochemical nanosensors [17,18]. At present, combination of carbon paste with polyaniline is under intense investigation in order to get fast, sensitive and selective biosensors in different fields [19–21]. Ambrosi et al. [22] fabricated an ascorbic acid sensor via the drop-casting of dodecylbenzene sulphonic acid (DBSA)-doped polyaniline nanoparticles onto a screen-printed carbon paste electrode (CPE). The sensor was compared to a range of other conducting polymer-based ascorbate sensors and found to be comparable or superior in terms of analytical performance. More recently, hybrid nanomaterials, such as hybrid of gold nanoparticles and carbon nanotubes (CNTs), have also been applied for the construction of DNA biosensors [23]. The components of the hybrid have obvious synergistic effect on the performance of the sensor, such as high catalytic activity, enhanced conductivity, strengthened biosensing ability, improved sensitivity and selectivity.
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Fig. 1. SEM images of dendritic polyaniline nanofibers at different magnification.
In this paper, polyaniline nanofibers (PANnano ) were used as a kind of dopping material to prepare a modified carbon paste electrode, denoted as PANnano /CPE, which showed a good electric conductivity and served as an excellent affinity interface for the subsequent immobilization of the hybrid of carbon nanotubes and gold nanoparticles (Aunano –CNT). The Aunano –CNT/PANnano nanocomposite films could greatly enhance the loading of the DNA probe and hence markedly improved the sensitivity for the target DNA detection. The sequence-specific DNA of the PAT gene and the polymerase chain reaction (PCR) amplification of the NOS gene from a transgenic-modified bean sample were satisfactorily detected with the label-free electrochemical impedance spectroscopic (EIS) method. 2. Materials and methods
Limited Company. The base sequences of above DNAs and the preparation of their stock solutions (1.0 mol/L), and materials for the PCR amplification of NOS gene sample and the amplification procedure were as described in Ref. [9]. The DNA sample for PCR amplification was extracted from one kind of transgenic soybean according to the method of plant DNA mini prep kit (Shanghai Academy of Agricultural Sciences). All oligonucleotides stock solutions of 20-base oligomers (1.0 mol/L) were prepared using Tris–HCl solution (5.0 mmol/L Tris–HCl, 50.0 mmol/L NaCl, pH 7.0), and stored at 4 ◦ C. More diluted solutions were obtained via diluting aliquot of the stock solution with ultrapure water prior to use. The hybridization solution was diluted with 2 × SSC (pH 7.0), which was consisted of 0.30 mol/L NaCl and 0.030 mol/L sodium citrate tribasic dihydrate (C6 H5 Na3 O7 ·2H2 O).
2.1. Apparatus and reagents
2.2. Procedure
A CHI 660C electrochemical analyzer (Shanghai CH Instrument Company, China), which was in connection with a home-made carbon paste-modified working electrode (˚ = 4 mm), a Ag/AgCl reference electrode and a platinum wire auxiliary electrode, was used for the electrochemical measurement. The pH values of all solutions were measured by a model pHS-25 digital acidometer (Shanghai Leici Factory, China). Scanning electron microscopy (SEM) was carried out using a JSM-5900 machine (JEOL, Japan). The PCR amplification was performed by an Eppendorf Mastercycler Gradient PCR system (Germany). Aquapro ultrapure water system (Chongqing Yihe Company, China). Graphite powder and paraffine were purchased from Shanghai Colloid Laboratory and Shanghai Hua Ling Healing Appliance Factory, respectively. Polyaniline nanofibers were provided by College of Material Science and Engineering, Qingdao University of Science and Technology, and used without further purification [24]. The SEM image was shown as Fig. 1. HAuCl4 ·4H2 O (Sigma, St. Louis, MO, USA). Multi-wall carbon nanotubes (Shenzhen nanotech. Port Co., Ltd., China). K3 [Fe(CN)6 ] and K4 [Fe(CN)6 ] (Shanghai No. 1 Reagent Factory and Shanghai Heng Da Chemical Co. Ltd., respectively, China). Methylene blue (MB) (Shanghai Reagent Company, China). All the chemicals were of analytical grade and solutions were prepared with ultrapure water. The 20-base oligonucleotides probe (probe DNA), its complementary sequence DNA (cDNA, target DNA, namely a 20-base fragment of PAT gene sequence), single-base mismatched DNA, double-base mismatched DNA and noncomplementary sequence DNA (ncDNA) were synthetized by Beijing SBS Gene Technology
2.2.1. Synthesis of Aunano –CNT hybrid The Aunano –CNT hybrid was synthesized according to the literature [25]. Briefly, CNT was ultrasonically dispersed in citric acid aqueous solution. Then 50 mL of 4 g/L HAuCl4 solution was added dropwise to the as-prepared CNT suspension at 70 ◦ C under vigorous stirring, and kept for stirring for another 1 h. After that, the temperature of the hybrid suspension was controlled at 80 ◦ C for 8 h. 2.2.2. Preparation of polyaniline nanofibers-modified carbon paste electrode and Aunano –CNT hybrid films 3.0 g graphite powder, 1.0 g solid paraffine and 1.0 g polyaniline nanofibers were heated at 80 ◦ C for 2 h and mixed by hand to produce a homogenous carbon paste. It was tightly packed into a glass tube from one end by using a stainless steel rod and a copper wire was introduced into the other end for electrical contact. A fresh electrode surface was generated rapidly by extruding a small plug of the paste with the stainless steel rod and a smooth surface was obtained by smoothing the surface on a white paper. The prepared electrode was rinsed with absolute ethanol and ultrapure water for 5 min, respectively. 20 L of 1 mg/mL Aunano –CNT gelatin solution was dripped onto the fresh surface of the polyaniline nanofibers-modified carbon paste electrode and naturally dried in the air to form Aunano –CNT/PANnano /CPE. 2.2.3. Immobilization and hybridization of DNA The immobilization of the DNA probe on the electrode surface was carried out with following procedure: the
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Aunano –CNT/PANnano /CPE was immersed in 2.0 mL Tris–HCl buffer solution (pH 7.0) containing 1.0 mol/L probe DNA at +0.6 V for 500 s, followed by washing the electrode with 0.2% SDS solution for removing the unimmobilized ssDNA and then rinsing it with ultrapure water. The ssDNA/Aunano –CNT/PANnano /CPE electrode was immersed into hybridization solution of 1.0 mol/L target DNA with the working potential at +0.4 V for 600 s to complete the DNA hybridization. The electrode was washed with 0.2% SDS to remove the unhybridized DNA and this hybridization-modified electrode was denoted as dsDNA/Aunano –CNT/PANnano /CPE. The hybridization reaction of the probe DNA with the single-base mismatched DNA, double-base mismatched DNA and noncomplementary DNA was respectively conducted with the same procedure. 2.2.4. Electrochemical measurements Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used in this study. The following parameters were employed for CV and DPV, respectively—CV: scan rate 100 mV/s; DPV: pulse amplitude 50 mV, pulse width 60 ms, pulse period 0.2 s. The test solution was 1.5 × 10−5 mol/L MB in the B–R buffer solution of pH 6.0 including 25.0 mmol/L NaCl or the 1.0 mmol/L K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ] (1:1) solution containing 0.1 mol/L KCl. In order to obtain reliable response of MB at the working electrode, the background value was recorded after the working electrode was immersed into the B–R supporting electrolyte solution for 5 min to ensure equilibration. The electrode was then transferred into the MB solution and the signal was recorded after accumulation for 5 min. The response of MB was obtained by subtracting the background value from above recorded signal. The EIS measurement was also carried out with the CHI 660C electrochemical analyzer. Supporting electrolyte solution was 1.0 mmol/L K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ] (1:1) solution containing 0.1 mol/L KCl. The AC voltage amplitude was 5 mV and the voltage frequencies ranged from 10 kHz to 0.1 Hz. The applied potential was 172 mV. The reported result for every electrode in this paper was the mean value of three parallel measurements. 3. Results and discussion 3.1. Morphology of polyaniline nanofibers The morphologies of polyaniline nanofibers were characterized by SEM. Typical SEM images of dendritic polyaniline nanofibers were shown in Fig. 1. It was clear that these polyaniline nanofibers were interconnected to form dendritic or network structures, rather than isolated nanofibers or bundles. The diameters of the polyaniline branches ranged from 60 nm to 90 nm, and the lengths were several hundred nanometers. These polyaniline nanofibers took on exoteric porotic structure in evidence, which were propitious to enhance the sensitivity of chemical sensor.
Fig. 2. Cyclic voltammogram of 1.0 mol/L HCl at PANnano /CPE, scan rate: 10 mV/s.
than those of the curve ‘a’, indicating that the properties of the modified electrode had been significantly changed. After PANnano was doped in the bare CPE, PANnano had a strong electrocatalytic activity toward the redox of MB. The curve ‘c’ at the Aunano –CNT/PANnano /CPE had a couple of obviously enhanced redox peaks as compared with the curve ‘b’, which meant that Aunano –CNT film has been coated onto the PANnano /CPE successfully, and the Aunano –CNT/PANnano /CPE had a much larger electroactive surface. 3.3. Electrochemical characterization of DNA immobilization and hybridization at Aunano –CNT/PANnano /CPE 3.3.1. Differential pulse voltammetry Fig. 4 showed the hybridization detection of DNA by using DPV method. The curve ‘a’ was the DPV curve of MB at the bare CPE electrode. As can be seen, the electrode modified by PANnano could enhance MB signal (curve b). The DPV signal of MB at the Aunano –CNT/PANnano /CPE (curve c) was significantly enhanced as compared with that at the PANnano /CPE. Immobilization of the ssDNA on the Aunano –CNT/PANnano /CPE resulted in a further increase of the DPV signal of MB (curve d), which was attributed to the affinity of MB to the exposed guanine bases of ssDNA molecule [27,28]. After hybridization of the probe ssDNA with the target DNA, the DPV signal of MB decreased markedly (curve e) as expected. The reason was that the guanine bases were embedded in the double helix configuration after hybridization, which prevented MB from accessing the electrode [29]. These phenomena indicated that the Aunano –CNT/PANnano /CPE could be a fine platform for the immobilization and hybridization of DNA.
3.2. Cyclic voltammetry of MB at Aunano –CNT/PANnano /CPE Fig. 2 showed the cyclic voltammogram of 1.0 mol/L HCl solution at PANnano /CPE. The typical redox peaks of polyaniline in the acidic environment indicated that the PANnano doped in the CPE retained its good electroactivity, which was consistent with the previous report [26]. MB is a well-known hybridization indicator of DNA [27–29]. Fig. 3 showed the cyclic voltammograms of 1.5 × 10−5 mol/L MB at different kinds of modified electrodes. The curve ‘a’ at the bare CPE had a couple of small redox peaks in the potential range of 0.1 V to −0.7 V. The curve ‘b’ at the PANnano /CPE had a couple of welldefined redox peaks, the current peaks of which were much larger
Fig. 3. Cyclic voltammograms of 1.5 × 10−5 mol/L MB in B–R buffer solution (pH 6.0) at (a) CPE; (b) PANnano /CPE; (c) Aunano –CNT/PANnano /CPE, scan rate: 100 mV/s.
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backbone of the probe DNA prevented [Fe(CN)6 ]3−/4− from reaching the electrode surface, and led to a larger Ret value (curve c) than that at the Aunano –CNT/PANnano /CPE (curve b). When the probe DNA was hybridized with its complementary target DNA (cDNA) in solution, the Ret was further enhanced to a much larger value (curve d). After hybridization, the negative charges on the electrode surface increased remarkably and the surface membranes become thicker, which might raise the Ret value. Therefore, from the change of the Ret value, the immobilization and hybridization of DNA on this Aunano –CNT/PANnano /CPE platform could be understood clearly.
Fig. 4. Differential pulse voltammograms of 1.5 × 10−5 mol/L MB in B–R buffer solution at (a) CPE; (b) PANnano /CPE; (c) Aunano –CNT/PANnano /CPE; (d) ssDNA/Aunano –CNT/PANnano /CPE and (e) dsDNA/Aunano –CNT/PANnano /CPE. DPV parameters—amplitude: 50 mV, pulse period: 0.2 s, pulse width: 60 ms.
3.3.2. Electrochemical impedance spectroscopy Label-free electrochemical DNA biosensor may be characterized by the electrochemical impedance spectroscopic method [30–33]. In order to obtain more information from EIS results, the working electrode was modeled using a modified Randles equivalent circuit (inset of Fig. 5). Where, Rs is the electrolyte solution resistance, Ret the surface electron transfer resistance, Zw the Warburg impedance resulting from the diffusion of ions, and Cdl the double layer capacitance. In EIS, the semicircle diameter of the Nyquist diagram equals to the surface electron transfer resistance (Ret ) of the electrode. The immobilization and hybridization of DNA on the electrode surface can change the Ret value. Therefore, the properties of DNA immobilization and hybridization may be known by the EIS measurement. Nyquist diagrams of [Fe(CN)6 ]3−/4− at different modified electrodes were illustrated in Fig. 5. The curves a, b, c and d were the Nyquist diagrams of [Fe(CN)6 ]3−/4− at the PANnano /CPE, Aunano –CNT/PANnano /CPE, ssDNA/Aunano –CNT/PANnano /CPE and dsDNA/Aunano –CNT/PANnano /CPE, respectively. Compared with the PANnano /CPE, the Aunano –CNT/PANnano /CPE had a much larger electroactive surface and higher conductivity because of the Aunano –CNT film. After the probe DNA was immobilized at the Aunano –CNT/PANnano /CPE, the negatively charged phosphate
3.3.3. Optimization of conditions for DNA immobilization and hybridization 3.3.3.1. The selection of potential for immobilization of the probe DNA. The Nyquist diagram at the Aunano –CNT/PANnano /CPE in the [Fe(CN)6 ]3−/4− solution was recorded. Then, after the probe DNA was immobilized at the electrode at 0.4 V, the Nyquist diagram at the probe DNA-immobilized electrode was again recorded. The difference (Ret ) of the impedance values between before and after immobilization of the probe DNA was calculated. We performed the similar experiments as above except at 0.5 V, 0.6 V, 0.7 V and 0.8 V, respectively. The results indicated that Ret value increased with the positive shift of the potential from 0.4 V to 0.6 V. With further positive shift of the immobilization potential, the Ret did not increase anymore. An immobilization potential of 0.6 V was generally used in our experiments. 3.3.3.2. The selection of the immobilization time of the probe DNA. The probe DNA was immobilized at 0.6 V for from 100 s to 800 s, and the Nyquist diagrams at the electrode before and after every immobilization of the probe DNA were respectively recorded. The results showed that the Ret value rose with the increase of the immobilization time from 100 s to 500 s and reached a constant level beyond 500 s. 500 s was selected for the immobilization of the probe DNA. 3.3.3.3. The selection of potential for DNA hybridization. The probe DNA was hybridized with cDNA at different constant potentials from 0.2 V to 0.6 V, and the Nyquist diagrams at the electrode before and after hybridization of the probe DNA were respectively recorded. The Ret value calculated from the hybridization of the probe DNA indicated that the response of hybridization rose gradually with the positive shift of the hybridization potential from 0.2 V to 0.4 V and reached a constant level beyond 0.4 V. A hybridization potential of 0.4 V was generally used in our experiments. 3.3.3.4. The selection of the hybridization time of DNA. The probe DNA was hybridized with cDNA at 0.4 V for from 100 s to 800 s. The results showed that the Ret value rose with the increase of the hybridization time from 100 s to 600 s and reached a constant level beyond 600 s. 600 s was optimal for the hybridization. 3.4. Detection of sequence-specific DNA of PAT gene
Fig. 5. Nyquist diagrams recorded at (a) PANnano /CPE, (b) Aunano –CNT/PANnano /CPE, (c) ssDNA/Aunano –CNT/PANnano /CPE and (d) dsDNA/Aunano –CNT/PANnano /CPE. Supporting electrolyte solution is 1.0 mmol/L K3 [Fe(CN)6 ] and 1.0 mmol/L K4 [Fe(CN)6 ] containing 0.1 mol/L KCl. Inset: equivalent circuit used to model impedance data in the presence of redox couples.
The selectivity of DNA hybridization could be judged by the hybridization of the probe DNA with different DNA sequences. As shown in Fig. 6A, the curve ‘a’ was the Nyquist diagram of [Fe(CN)6 ]3−/4− at the probe DNA-modified electrode, which was the same as curve ‘c’ in Fig. 5. After hybridization of the probe DNA with the complementary DNA under the optimal experimental conditions, the Nyquist diagram of [Fe(CN)6 ]3−/4− was shown as the curve ‘b’. The Ret value rose obviously. When the noncomplementary sequence was used for the hybridization, the Ret value (curve c)
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Fig. 6. (A) Nyquist diagrams recorded at (a) ssDNA/Aunano –CNT/PANnano /CPE, (b) dsDNA/Aunano –CNT/PANnano /CPE (hybridized with cDNA), (c) the electrode hybridized with ncDNA, (d) the electrode hybridized with single-base mismatched DNA, (e) the electrode hybridized with double-base mismatched DNA. (B) Nyquist diagrams recorded at ssDNA/Aunano –CNT/PANnano /CPE (a) and after hybridization reaction with its complementary PAT gene sequence of different concentrations: (b) 1.0 × 10−12 mol/L, (c) 1.0 × 10−11 mol/L, (d) 1.0 × 10−10 mol/L, (e) 1.0 × 10−9 mol/L, (f) 1.0 × 10−8 mol/L and (g) 1.0 × 10−7 mol/L. Supporting electrolyte solution is 1.0 mmol/L K3 [Fe(CN)6 ] and 1.0 mmol/L K4 [Fe(CN)6 ] containing 0.1 mol/L KCl.
varied little as compared with the probe DNA-modified electrode. The single-base mismatched sequence (curve d) and the doublebase mismatched sequence (curve e) could also be recognized via comparing the change of the DNA Ret value of [Fe(CN)6 ]3−/4− . The results demonstrated that this DNA biosensor displayed a high selectivity for the hybridization detection. The difference between the Ret value (namely Ret ) of 1.0 mmol/L [Fe(CN)6 ]3−/4− solution containing 0.1 mol/L KCl at the probe DNA/Aunano –CNT/PANnano /CPE and that at the hybridizationmodified electrode (dsDNA/Aunano –CNT/PANnano /CPE) was used to be the measurement signal to determine the sequence-specific related to the PAT gene fragment. The concentration of the PAT gene fragment in the hybridization solution was changed from 1.0 × 10−7 mol/L to 1.0 × 10−12 mol/L, and the Nyquist diagrams of the above [Fe(CN)6 ]3−/4− solution at the DNA-modified electrode before and after hybridization were respectively recorded. The results were shown as in Fig. 6B. The Ret value versus the logarithm of the PAT gene fragment concentration presented a good linear correlation. The dynamic determination range for the PAT gene fragment was from 1.0 × 10−12 mol/L to 1.0 × 10−6 mol/L with the regression equation: Ret () = −961.5 lgC + 17064 and the correlation coefficient: = 0.9950. The detection limit was
5.6 × 10−13 mol/L using 3, where was the standard deviation of the blank solution with 11 parallel measurements. 3.5. Reproducibility and regeneration of DNA sensor The reproducibility of any biosensor is extremely important to practical applications. In our test, the probe DNA was hybridized with 1.0 mol/L complementary DNA for seven parallel measurements and relative standard deviation (R.S.D.) of 3.6% was estimated, showing the high reproducibility of the DNA electrochemical biosensor. The regeneration ability as for this impedance-based DNA hybridization sensor was also evaluated. The dsDNA on the hybridized electrode was hot denatured by immersing the electrode into boiling water for 8 min and then cooling it with the ice salt bath. The Nyquist diagrams of 1.0 mmol/L [Fe(CN)6 ]3−/4− solution at the regenerated probe DNA/Aunano –CNT/PANnano /CPE was recorded. The results indicated that the Ret values were almost the same values as that obtained in the first experiment. Successive experiments showed that the DNA biosensor could be reproduced for six times without losing its sensitivity, indicating the fine regeneration ability of the DNA electrochemical biosensor. 3.6. Detection of the PCR amplification of NOS gene sample
Fig. 7. Nyquist diagrams in 1.0 mmol/L K3 [Fe(CN)6 ] and 1.0 mmol/L K4 [Fe(CN)6 ] recorded at (a) Aunano –CNT/PANnano /CPE, (b) the NOS gene probe-modified electrode and (c) the electrode hybridized with the PCR amplified real NOS sample.
This label-free DNA electrochemical biosensor has been combined with the PCR technology for the detection of the NOS transgene in a kind of real transgenic soybean. The NOS gene template was extracted from this soybean sample with the DNA Mini preparation kit, which was purchased from Shanghai Academy of Agricultural Sciences. Then the template was amplified according to the PCR procedure. The purification of the PCR amplification was carried out as follows: added 5 L of agarose gel loading dye and loaded the above PCR reaction solution into a well of 1.0% low melting temperature agarose gel. Electrophosresed it for 30–60 min at 100–200 mA, and then excised the desired band visualized under UV light with a clean razor blade. Followed by purifying the DNA in a gel using the purification kit (TIANgel Maxi purification kit DP 210-02, Beijing, China). After the purification, the weight product of single molecule was obtained, and the A260 /A280 value of the PCR amplification sample was 1.82, indicating that it was pure enough.
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The purified PCR amplification of NOS gene sample was diluted with 5.0 mmol/L Tris–HCl buffer, and denatured by heating it in boiling water for 5 min and then cooled in an ice bath. After the immobilization of the NOS gene probe on the Aunano –CNT/PANnano /CPE, the electrode was immersed in 2 mL denatured PCR amplification for hybridization under the optimal conditions. The detection result was shown in Fig. 7. The curves a, b and c were the Nyquist diagrams of 1.0 mmol/L [Fe(CN)6 ]3−/4− , respectively, at the Aunano –CNT/PANnano /CPE, the NOS probe DNA/Aunano –CNT/PANnano /CPE and the hybridized electrode of the NOS probe with the PCR amplification of the NOS gene sample. Obviously, the Ret value of the curve ‘c’ was markedly larger than that of the curve ‘b’, whereas the latter was distinctly larger than that of the curve ‘a’. The impedance measurements confirmed that the ssDNA/Aunano –CNT/PANnano /CPE could successfully recognize and detect the PCR amplification of the NOS transgene in the real transgenic soybean sample. 4. Conclusion Nanogold (Aunano) and CNT hybrid was coated on the PANnano –modified CPE surface to form a Aunano –CNT/PANnano /CPE electrode. The Aunano –CNT and PANnano composite film is a very good platform for the immobilization and hybridization of DNA. The nanomaterials Aunano , CNT and PANnano in the composite film had remarkable synergistic effect on the recognition of DNA hybridization. The immobilization and hybridization of the probe DNA were characterized by DPV and EIS. The sequence-specific DNA of the PAT gene and polymerase chain reaction (PCR) amplification of the NOS gene from transgenically modified beans were detected by this electrochemical DNA biosensor with label-free EIS method. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 20635020, No. 20375020) and Doctoral Foundation of the Ministry of Education of China (No. 20060426001).
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Highly sensitive electrochemical impedance spectroscopic detection of DNA hybridization based on Aunano –CNT/PANnano films Na Zhou, Tao Yang, Chen Jiang, Meng Du, Kui Jiao ∗ Key Laboratory of Eco-chemical Engineering (Ministry of Education), College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
a r t i c l e
i n f o
Article history: Received 12 March 2008 Received in revised form 30 July 2008 Accepted 31 July 2008 Available online 7 August 2008 Keywords: Electrochemical DNA biosensor Nanocomposite film Polyaniline nanofibers Electrochemical impedance spectroscopy PAT gene NOS gene
a b s t r a c t A polyaniline nanofibers (PANnano )/carbon paste electrode (CPE) was prepared via dopping PANnano in the carbon paste. The nanogold (Aunano ) and carbon nanotubes (CNT) composite nanoparticles were bound on the surface of the PANnano /CPE. The immobilization and hybridization of the DNA probe on the Aunano –CNT/PANnano films were investigated with differential pulse voltammetry (DPV) and cyclic voltammetry (CV) using methylene blue (MB) as indicator, and electrochemical impedance spectroscopy (EIS) using [Fe(CN)6 ]3−/4− as redox probe. The voltammetric peak currents of MB increased dramatically owing to the immobilization of the probe DNA on the Aunano –CNT/PANnano films, and then decreased obviously owing to the hybridization of the DNA probe with the complementary single-stranded DNA (cDNA). The electron transfer resistance (Ret ) of the electrode surface increased after the immobilization of the probe DNA on the Aunano –CNT/PANnano films and rose further after the hybridization of the probe DNA. The remarkable difference between the Ret value at the DNA-immobilized electrode and that at the hybridized electrode could be used for the label-free EIS detection of the target DNA. The loading of the DNA probe on Aunano –CNT/PANnano films was greatly enhanced and the sensitivity for the target DNA detection was markedly improved. The sequence-specific DNA of phosphinothricin acetyltransferase (PAT) gene and the polymerase chain reaction (PCR) amplification of nopaline synthase (NOS) gene from transgenically modified beans were determined with this label-free EIS DNA detection method. The dynamic range for detecting the PAT gene sequence was from 1.0 × 10−12 mol/L to 1.0 × 10−6 mol/L with a detection limit of 5.6 × 10−13 mol/L. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Last decades, astonishing achievements in the cultivation of the transgenic plants have been obtained all over the world [1,2]. However, their security has been brought into focus. The accurate, sensitive, and rapid detection of the transgenic plant products is of significance for understanding the security of the transgenic plants. Phosphinothricin acetyltransferase (PAT) gene and nopaline synthase (NOS) gene are two important screening detection transgenes of the transgenic plants. Detection of these two transgenes can be utilized to identify the transgenic plants. DNA electrochemical biosensors have been successfully applied for the transgene detection of the transgenic plants [3–5]. In order to improve the sensitivity, selectivity, and stability of the biosensor, various kinds of nanomaterial have been prepared and utilized recently for the biosensor fabrication [6–12]. One of these nano-
∗ Corresponding author. Tel.: +86 532 84855977; fax: +86 532 84023927. E-mail address: [email protected] (K. Jiao). 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.07.058
materials is polyaniline nanomaterial (PANnano ). PANnano has the properties of mechanical flexibilities, high surface area, chemical specificities, tunable conductivities and easy processing [13–16], which make this conducting polymer as a kind of the promising sensing material for ultrasensitive, trace-level biological and electrochemical nanosensors [17,18]. At present, combination of carbon paste with polyaniline is under intense investigation in order to get fast, sensitive and selective biosensors in different fields [19–21]. Ambrosi et al. [22] fabricated an ascorbic acid sensor via the drop-casting of dodecylbenzene sulphonic acid (DBSA)-doped polyaniline nanoparticles onto a screen-printed carbon paste electrode (CPE). The sensor was compared to a range of other conducting polymer-based ascorbate sensors and found to be comparable or superior in terms of analytical performance. More recently, hybrid nanomaterials, such as hybrid of gold nanoparticles and carbon nanotubes (CNTs), have also been applied for the construction of DNA biosensors [23]. The components of the hybrid have obvious synergistic effect on the performance of the sensor, such as high catalytic activity, enhanced conductivity, strengthened biosensing ability, improved sensitivity and selectivity.
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Fig. 1. SEM images of dendritic polyaniline nanofibers at different magnification.
In this paper, polyaniline nanofibers (PANnano ) were used as a kind of dopping material to prepare a modified carbon paste electrode, denoted as PANnano /CPE, which showed a good electric conductivity and served as an excellent affinity interface for the subsequent immobilization of the hybrid of carbon nanotubes and gold nanoparticles (Aunano –CNT). The Aunano –CNT/PANnano nanocomposite films could greatly enhance the loading of the DNA probe and hence markedly improved the sensitivity for the target DNA detection. The sequence-specific DNA of the PAT gene and the polymerase chain reaction (PCR) amplification of the NOS gene from a transgenic-modified bean sample were satisfactorily detected with the label-free electrochemical impedance spectroscopic (EIS) method. 2. Materials and methods
Limited Company. The base sequences of above DNAs and the preparation of their stock solutions (1.0 mol/L), and materials for the PCR amplification of NOS gene sample and the amplification procedure were as described in Ref. [9]. The DNA sample for PCR amplification was extracted from one kind of transgenic soybean according to the method of plant DNA mini prep kit (Shanghai Academy of Agricultural Sciences). All oligonucleotides stock solutions of 20-base oligomers (1.0 mol/L) were prepared using Tris–HCl solution (5.0 mmol/L Tris–HCl, 50.0 mmol/L NaCl, pH 7.0), and stored at 4 ◦ C. More diluted solutions were obtained via diluting aliquot of the stock solution with ultrapure water prior to use. The hybridization solution was diluted with 2 × SSC (pH 7.0), which was consisted of 0.30 mol/L NaCl and 0.030 mol/L sodium citrate tribasic dihydrate (C6 H5 Na3 O7 ·2H2 O).
2.1. Apparatus and reagents
2.2. Procedure
A CHI 660C electrochemical analyzer (Shanghai CH Instrument Company, China), which was in connection with a home-made carbon paste-modified working electrode (˚ = 4 mm), a Ag/AgCl reference electrode and a platinum wire auxiliary electrode, was used for the electrochemical measurement. The pH values of all solutions were measured by a model pHS-25 digital acidometer (Shanghai Leici Factory, China). Scanning electron microscopy (SEM) was carried out using a JSM-5900 machine (JEOL, Japan). The PCR amplification was performed by an Eppendorf Mastercycler Gradient PCR system (Germany). Aquapro ultrapure water system (Chongqing Yihe Company, China). Graphite powder and paraffine were purchased from Shanghai Colloid Laboratory and Shanghai Hua Ling Healing Appliance Factory, respectively. Polyaniline nanofibers were provided by College of Material Science and Engineering, Qingdao University of Science and Technology, and used without further purification [24]. The SEM image was shown as Fig. 1. HAuCl4 ·4H2 O (Sigma, St. Louis, MO, USA). Multi-wall carbon nanotubes (Shenzhen nanotech. Port Co., Ltd., China). K3 [Fe(CN)6 ] and K4 [Fe(CN)6 ] (Shanghai No. 1 Reagent Factory and Shanghai Heng Da Chemical Co. Ltd., respectively, China). Methylene blue (MB) (Shanghai Reagent Company, China). All the chemicals were of analytical grade and solutions were prepared with ultrapure water. The 20-base oligonucleotides probe (probe DNA), its complementary sequence DNA (cDNA, target DNA, namely a 20-base fragment of PAT gene sequence), single-base mismatched DNA, double-base mismatched DNA and noncomplementary sequence DNA (ncDNA) were synthetized by Beijing SBS Gene Technology
2.2.1. Synthesis of Aunano –CNT hybrid The Aunano –CNT hybrid was synthesized according to the literature [25]. Briefly, CNT was ultrasonically dispersed in citric acid aqueous solution. Then 50 mL of 4 g/L HAuCl4 solution was added dropwise to the as-prepared CNT suspension at 70 ◦ C under vigorous stirring, and kept for stirring for another 1 h. After that, the temperature of the hybrid suspension was controlled at 80 ◦ C for 8 h. 2.2.2. Preparation of polyaniline nanofibers-modified carbon paste electrode and Aunano –CNT hybrid films 3.0 g graphite powder, 1.0 g solid paraffine and 1.0 g polyaniline nanofibers were heated at 80 ◦ C for 2 h and mixed by hand to produce a homogenous carbon paste. It was tightly packed into a glass tube from one end by using a stainless steel rod and a copper wire was introduced into the other end for electrical contact. A fresh electrode surface was generated rapidly by extruding a small plug of the paste with the stainless steel rod and a smooth surface was obtained by smoothing the surface on a white paper. The prepared electrode was rinsed with absolute ethanol and ultrapure water for 5 min, respectively. 20 L of 1 mg/mL Aunano –CNT gelatin solution was dripped onto the fresh surface of the polyaniline nanofibers-modified carbon paste electrode and naturally dried in the air to form Aunano –CNT/PANnano /CPE. 2.2.3. Immobilization and hybridization of DNA The immobilization of the DNA probe on the electrode surface was carried out with following procedure: the
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Aunano –CNT/PANnano /CPE was immersed in 2.0 mL Tris–HCl buffer solution (pH 7.0) containing 1.0 mol/L probe DNA at +0.6 V for 500 s, followed by washing the electrode with 0.2% SDS solution for removing the unimmobilized ssDNA and then rinsing it with ultrapure water. The ssDNA/Aunano –CNT/PANnano /CPE electrode was immersed into hybridization solution of 1.0 mol/L target DNA with the working potential at +0.4 V for 600 s to complete the DNA hybridization. The electrode was washed with 0.2% SDS to remove the unhybridized DNA and this hybridization-modified electrode was denoted as dsDNA/Aunano –CNT/PANnano /CPE. The hybridization reaction of the probe DNA with the single-base mismatched DNA, double-base mismatched DNA and noncomplementary DNA was respectively conducted with the same procedure. 2.2.4. Electrochemical measurements Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used in this study. The following parameters were employed for CV and DPV, respectively—CV: scan rate 100 mV/s; DPV: pulse amplitude 50 mV, pulse width 60 ms, pulse period 0.2 s. The test solution was 1.5 × 10−5 mol/L MB in the B–R buffer solution of pH 6.0 including 25.0 mmol/L NaCl or the 1.0 mmol/L K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ] (1:1) solution containing 0.1 mol/L KCl. In order to obtain reliable response of MB at the working electrode, the background value was recorded after the working electrode was immersed into the B–R supporting electrolyte solution for 5 min to ensure equilibration. The electrode was then transferred into the MB solution and the signal was recorded after accumulation for 5 min. The response of MB was obtained by subtracting the background value from above recorded signal. The EIS measurement was also carried out with the CHI 660C electrochemical analyzer. Supporting electrolyte solution was 1.0 mmol/L K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ] (1:1) solution containing 0.1 mol/L KCl. The AC voltage amplitude was 5 mV and the voltage frequencies ranged from 10 kHz to 0.1 Hz. The applied potential was 172 mV. The reported result for every electrode in this paper was the mean value of three parallel measurements. 3. Results and discussion 3.1. Morphology of polyaniline nanofibers The morphologies of polyaniline nanofibers were characterized by SEM. Typical SEM images of dendritic polyaniline nanofibers were shown in Fig. 1. It was clear that these polyaniline nanofibers were interconnected to form dendritic or network structures, rather than isolated nanofibers or bundles. The diameters of the polyaniline branches ranged from 60 nm to 90 nm, and the lengths were several hundred nanometers. These polyaniline nanofibers took on exoteric porotic structure in evidence, which were propitious to enhance the sensitivity of chemical sensor.
Fig. 2. Cyclic voltammogram of 1.0 mol/L HCl at PANnano /CPE, scan rate: 10 mV/s.
than those of the curve ‘a’, indicating that the properties of the modified electrode had been significantly changed. After PANnano was doped in the bare CPE, PANnano had a strong electrocatalytic activity toward the redox of MB. The curve ‘c’ at the Aunano –CNT/PANnano /CPE had a couple of obviously enhanced redox peaks as compared with the curve ‘b’, which meant that Aunano –CNT film has been coated onto the PANnano /CPE successfully, and the Aunano –CNT/PANnano /CPE had a much larger electroactive surface. 3.3. Electrochemical characterization of DNA immobilization and hybridization at Aunano –CNT/PANnano /CPE 3.3.1. Differential pulse voltammetry Fig. 4 showed the hybridization detection of DNA by using DPV method. The curve ‘a’ was the DPV curve of MB at the bare CPE electrode. As can be seen, the electrode modified by PANnano could enhance MB signal (curve b). The DPV signal of MB at the Aunano –CNT/PANnano /CPE (curve c) was significantly enhanced as compared with that at the PANnano /CPE. Immobilization of the ssDNA on the Aunano –CNT/PANnano /CPE resulted in a further increase of the DPV signal of MB (curve d), which was attributed to the affinity of MB to the exposed guanine bases of ssDNA molecule [27,28]. After hybridization of the probe ssDNA with the target DNA, the DPV signal of MB decreased markedly (curve e) as expected. The reason was that the guanine bases were embedded in the double helix configuration after hybridization, which prevented MB from accessing the electrode [29]. These phenomena indicated that the Aunano –CNT/PANnano /CPE could be a fine platform for the immobilization and hybridization of DNA.
3.2. Cyclic voltammetry of MB at Aunano –CNT/PANnano /CPE Fig. 2 showed the cyclic voltammogram of 1.0 mol/L HCl solution at PANnano /CPE. The typical redox peaks of polyaniline in the acidic environment indicated that the PANnano doped in the CPE retained its good electroactivity, which was consistent with the previous report [26]. MB is a well-known hybridization indicator of DNA [27–29]. Fig. 3 showed the cyclic voltammograms of 1.5 × 10−5 mol/L MB at different kinds of modified electrodes. The curve ‘a’ at the bare CPE had a couple of small redox peaks in the potential range of 0.1 V to −0.7 V. The curve ‘b’ at the PANnano /CPE had a couple of welldefined redox peaks, the current peaks of which were much larger
Fig. 3. Cyclic voltammograms of 1.5 × 10−5 mol/L MB in B–R buffer solution (pH 6.0) at (a) CPE; (b) PANnano /CPE; (c) Aunano –CNT/PANnano /CPE, scan rate: 100 mV/s.
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backbone of the probe DNA prevented [Fe(CN)6 ]3−/4− from reaching the electrode surface, and led to a larger Ret value (curve c) than that at the Aunano –CNT/PANnano /CPE (curve b). When the probe DNA was hybridized with its complementary target DNA (cDNA) in solution, the Ret was further enhanced to a much larger value (curve d). After hybridization, the negative charges on the electrode surface increased remarkably and the surface membranes become thicker, which might raise the Ret value. Therefore, from the change of the Ret value, the immobilization and hybridization of DNA on this Aunano –CNT/PANnano /CPE platform could be understood clearly.
Fig. 4. Differential pulse voltammograms of 1.5 × 10−5 mol/L MB in B–R buffer solution at (a) CPE; (b) PANnano /CPE; (c) Aunano –CNT/PANnano /CPE; (d) ssDNA/Aunano –CNT/PANnano /CPE and (e) dsDNA/Aunano –CNT/PANnano /CPE. DPV parameters—amplitude: 50 mV, pulse period: 0.2 s, pulse width: 60 ms.
3.3.2. Electrochemical impedance spectroscopy Label-free electrochemical DNA biosensor may be characterized by the electrochemical impedance spectroscopic method [30–33]. In order to obtain more information from EIS results, the working electrode was modeled using a modified Randles equivalent circuit (inset of Fig. 5). Where, Rs is the electrolyte solution resistance, Ret the surface electron transfer resistance, Zw the Warburg impedance resulting from the diffusion of ions, and Cdl the double layer capacitance. In EIS, the semicircle diameter of the Nyquist diagram equals to the surface electron transfer resistance (Ret ) of the electrode. The immobilization and hybridization of DNA on the electrode surface can change the Ret value. Therefore, the properties of DNA immobilization and hybridization may be known by the EIS measurement. Nyquist diagrams of [Fe(CN)6 ]3−/4− at different modified electrodes were illustrated in Fig. 5. The curves a, b, c and d were the Nyquist diagrams of [Fe(CN)6 ]3−/4− at the PANnano /CPE, Aunano –CNT/PANnano /CPE, ssDNA/Aunano –CNT/PANnano /CPE and dsDNA/Aunano –CNT/PANnano /CPE, respectively. Compared with the PANnano /CPE, the Aunano –CNT/PANnano /CPE had a much larger electroactive surface and higher conductivity because of the Aunano –CNT film. After the probe DNA was immobilized at the Aunano –CNT/PANnano /CPE, the negatively charged phosphate
3.3.3. Optimization of conditions for DNA immobilization and hybridization 3.3.3.1. The selection of potential for immobilization of the probe DNA. The Nyquist diagram at the Aunano –CNT/PANnano /CPE in the [Fe(CN)6 ]3−/4− solution was recorded. Then, after the probe DNA was immobilized at the electrode at 0.4 V, the Nyquist diagram at the probe DNA-immobilized electrode was again recorded. The difference (Ret ) of the impedance values between before and after immobilization of the probe DNA was calculated. We performed the similar experiments as above except at 0.5 V, 0.6 V, 0.7 V and 0.8 V, respectively. The results indicated that Ret value increased with the positive shift of the potential from 0.4 V to 0.6 V. With further positive shift of the immobilization potential, the Ret did not increase anymore. An immobilization potential of 0.6 V was generally used in our experiments. 3.3.3.2. The selection of the immobilization time of the probe DNA. The probe DNA was immobilized at 0.6 V for from 100 s to 800 s, and the Nyquist diagrams at the electrode before and after every immobilization of the probe DNA were respectively recorded. The results showed that the Ret value rose with the increase of the immobilization time from 100 s to 500 s and reached a constant level beyond 500 s. 500 s was selected for the immobilization of the probe DNA. 3.3.3.3. The selection of potential for DNA hybridization. The probe DNA was hybridized with cDNA at different constant potentials from 0.2 V to 0.6 V, and the Nyquist diagrams at the electrode before and after hybridization of the probe DNA were respectively recorded. The Ret value calculated from the hybridization of the probe DNA indicated that the response of hybridization rose gradually with the positive shift of the hybridization potential from 0.2 V to 0.4 V and reached a constant level beyond 0.4 V. A hybridization potential of 0.4 V was generally used in our experiments. 3.3.3.4. The selection of the hybridization time of DNA. The probe DNA was hybridized with cDNA at 0.4 V for from 100 s to 800 s. The results showed that the Ret value rose with the increase of the hybridization time from 100 s to 600 s and reached a constant level beyond 600 s. 600 s was optimal for the hybridization. 3.4. Detection of sequence-specific DNA of PAT gene
Fig. 5. Nyquist diagrams recorded at (a) PANnano /CPE, (b) Aunano –CNT/PANnano /CPE, (c) ssDNA/Aunano –CNT/PANnano /CPE and (d) dsDNA/Aunano –CNT/PANnano /CPE. Supporting electrolyte solution is 1.0 mmol/L K3 [Fe(CN)6 ] and 1.0 mmol/L K4 [Fe(CN)6 ] containing 0.1 mol/L KCl. Inset: equivalent circuit used to model impedance data in the presence of redox couples.
The selectivity of DNA hybridization could be judged by the hybridization of the probe DNA with different DNA sequences. As shown in Fig. 6A, the curve ‘a’ was the Nyquist diagram of [Fe(CN)6 ]3−/4− at the probe DNA-modified electrode, which was the same as curve ‘c’ in Fig. 5. After hybridization of the probe DNA with the complementary DNA under the optimal experimental conditions, the Nyquist diagram of [Fe(CN)6 ]3−/4− was shown as the curve ‘b’. The Ret value rose obviously. When the noncomplementary sequence was used for the hybridization, the Ret value (curve c)
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Fig. 6. (A) Nyquist diagrams recorded at (a) ssDNA/Aunano –CNT/PANnano /CPE, (b) dsDNA/Aunano –CNT/PANnano /CPE (hybridized with cDNA), (c) the electrode hybridized with ncDNA, (d) the electrode hybridized with single-base mismatched DNA, (e) the electrode hybridized with double-base mismatched DNA. (B) Nyquist diagrams recorded at ssDNA/Aunano –CNT/PANnano /CPE (a) and after hybridization reaction with its complementary PAT gene sequence of different concentrations: (b) 1.0 × 10−12 mol/L, (c) 1.0 × 10−11 mol/L, (d) 1.0 × 10−10 mol/L, (e) 1.0 × 10−9 mol/L, (f) 1.0 × 10−8 mol/L and (g) 1.0 × 10−7 mol/L. Supporting electrolyte solution is 1.0 mmol/L K3 [Fe(CN)6 ] and 1.0 mmol/L K4 [Fe(CN)6 ] containing 0.1 mol/L KCl.
varied little as compared with the probe DNA-modified electrode. The single-base mismatched sequence (curve d) and the doublebase mismatched sequence (curve e) could also be recognized via comparing the change of the DNA Ret value of [Fe(CN)6 ]3−/4− . The results demonstrated that this DNA biosensor displayed a high selectivity for the hybridization detection. The difference between the Ret value (namely Ret ) of 1.0 mmol/L [Fe(CN)6 ]3−/4− solution containing 0.1 mol/L KCl at the probe DNA/Aunano –CNT/PANnano /CPE and that at the hybridizationmodified electrode (dsDNA/Aunano –CNT/PANnano /CPE) was used to be the measurement signal to determine the sequence-specific related to the PAT gene fragment. The concentration of the PAT gene fragment in the hybridization solution was changed from 1.0 × 10−7 mol/L to 1.0 × 10−12 mol/L, and the Nyquist diagrams of the above [Fe(CN)6 ]3−/4− solution at the DNA-modified electrode before and after hybridization were respectively recorded. The results were shown as in Fig. 6B. The Ret value versus the logarithm of the PAT gene fragment concentration presented a good linear correlation. The dynamic determination range for the PAT gene fragment was from 1.0 × 10−12 mol/L to 1.0 × 10−6 mol/L with the regression equation: Ret () = −961.5 lgC + 17064 and the correlation coefficient: = 0.9950. The detection limit was
5.6 × 10−13 mol/L using 3, where was the standard deviation of the blank solution with 11 parallel measurements. 3.5. Reproducibility and regeneration of DNA sensor The reproducibility of any biosensor is extremely important to practical applications. In our test, the probe DNA was hybridized with 1.0 mol/L complementary DNA for seven parallel measurements and relative standard deviation (R.S.D.) of 3.6% was estimated, showing the high reproducibility of the DNA electrochemical biosensor. The regeneration ability as for this impedance-based DNA hybridization sensor was also evaluated. The dsDNA on the hybridized electrode was hot denatured by immersing the electrode into boiling water for 8 min and then cooling it with the ice salt bath. The Nyquist diagrams of 1.0 mmol/L [Fe(CN)6 ]3−/4− solution at the regenerated probe DNA/Aunano –CNT/PANnano /CPE was recorded. The results indicated that the Ret values were almost the same values as that obtained in the first experiment. Successive experiments showed that the DNA biosensor could be reproduced for six times without losing its sensitivity, indicating the fine regeneration ability of the DNA electrochemical biosensor. 3.6. Detection of the PCR amplification of NOS gene sample
Fig. 7. Nyquist diagrams in 1.0 mmol/L K3 [Fe(CN)6 ] and 1.0 mmol/L K4 [Fe(CN)6 ] recorded at (a) Aunano –CNT/PANnano /CPE, (b) the NOS gene probe-modified electrode and (c) the electrode hybridized with the PCR amplified real NOS sample.
This label-free DNA electrochemical biosensor has been combined with the PCR technology for the detection of the NOS transgene in a kind of real transgenic soybean. The NOS gene template was extracted from this soybean sample with the DNA Mini preparation kit, which was purchased from Shanghai Academy of Agricultural Sciences. Then the template was amplified according to the PCR procedure. The purification of the PCR amplification was carried out as follows: added 5 L of agarose gel loading dye and loaded the above PCR reaction solution into a well of 1.0% low melting temperature agarose gel. Electrophosresed it for 30–60 min at 100–200 mA, and then excised the desired band visualized under UV light with a clean razor blade. Followed by purifying the DNA in a gel using the purification kit (TIANgel Maxi purification kit DP 210-02, Beijing, China). After the purification, the weight product of single molecule was obtained, and the A260 /A280 value of the PCR amplification sample was 1.82, indicating that it was pure enough.
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The purified PCR amplification of NOS gene sample was diluted with 5.0 mmol/L Tris–HCl buffer, and denatured by heating it in boiling water for 5 min and then cooled in an ice bath. After the immobilization of the NOS gene probe on the Aunano –CNT/PANnano /CPE, the electrode was immersed in 2 mL denatured PCR amplification for hybridization under the optimal conditions. The detection result was shown in Fig. 7. The curves a, b and c were the Nyquist diagrams of 1.0 mmol/L [Fe(CN)6 ]3−/4− , respectively, at the Aunano –CNT/PANnano /CPE, the NOS probe DNA/Aunano –CNT/PANnano /CPE and the hybridized electrode of the NOS probe with the PCR amplification of the NOS gene sample. Obviously, the Ret value of the curve ‘c’ was markedly larger than that of the curve ‘b’, whereas the latter was distinctly larger than that of the curve ‘a’. The impedance measurements confirmed that the ssDNA/Aunano –CNT/PANnano /CPE could successfully recognize and detect the PCR amplification of the NOS transgene in the real transgenic soybean sample. 4. Conclusion Nanogold (Aunano) and CNT hybrid was coated on the PANnano –modified CPE surface to form a Aunano –CNT/PANnano /CPE electrode. The Aunano –CNT and PANnano composite film is a very good platform for the immobilization and hybridization of DNA. The nanomaterials Aunano , CNT and PANnano in the composite film had remarkable synergistic effect on the recognition of DNA hybridization. The immobilization and hybridization of the probe DNA were characterized by DPV and EIS. The sequence-specific DNA of the PAT gene and polymerase chain reaction (PCR) amplification of the NOS gene from transgenically modified beans were detected by this electrochemical DNA biosensor with label-free EIS method. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 20635020, No. 20375020) and Doctoral Foundation of the Ministry of Education of China (No. 20060426001).
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