Environ. Sci. Technol. 2010, 44, 4241–4246
A Simple, Stable and Picomole Level Lead Sensor Fabricated on DNA-based Carbon Hybridized TiO2 Nanotube Arrays MEICHUAN LIU, GUOHUA ZHAO,* YITING TANG, ZHIMIN YU, YANZHU LEI, MINGFANG LI, YANAN ZHANG, AND DONGMING LI Department of Chemistry, Tongji University, 1239 Siping Road, 200092 Shanghai, China
Received January 31, 2010. Revised manuscript received April 18, 2010. Accepted April 22, 2010.
An electrochemical lead sensor is developed on DNA-based vertically aligned conductive carbon hybridized TiO2 nanotube arrays (DNA/C-TiO2 NTs). The designed DNA/C-TiO2 NTs sensor is superior in determination of lead with high sensitivity, selectivity and repeatability, as well as wide pH adaptability, fast electro-accumulation capacity for lead and easy regeneration. Such remarkable characteristics for lead sensing are attributed to the immobilization of abundant target biomolecules, DNA, and the enhanced bioelectrochemical activity. The controllable carbon hybridization of the TiO2 NTs increases the conductivity of the electrode, while retaining the tubular structure, biocompatibility, and hydrophilicity. The results show that the lead sensor possesses a wide linear calibration ranging from 0.01 to 160 nM with the detection limitation at a picomole level (3.3 pM). The application of the present sensor is realized for determination of Pb2+ in real water samples.
Introduction Lead is a common environmental contaminant with high toxicity, which can result in various adverse physical and neurotoxic effects on human, particularly on children, in the brain, liver, kidneys, reproductive system, and central nervous system even with low-level exposure (1-3). Lead is also a persistent contaminant in the environment because of its nondegradability and accumulated tendency in the food chains (4). It is therefore of great importance to develop techniques for routine and effective monitoring of lead in the fields of environmental controlling, clinical toxicology, wastewater treatment, and industrial process. Current methods for lead detection, such as atomic absorption spectrometry, atomic emission spectroscopy, and inductively coupled plasma mass spectrometry, often require sophisticated equipment or sample treatment and are not suitable for on-site environmental applications (5-8). Simple and inexpensive methods for real-time sampling of Pb2+ are in great request. Compared to those spectroscopic competitors, electrochemical technique has been recognized as one of the most promising methods for ultra trace and on-site analysis of lead due to its favorable portability, suitability for automation, short analysis time, low power consumption, and inexpensive equipment (9-12). Screening of lead in * Corresponding author. 10.1021/es1003507
2010 American Chemical Society
Published on Web 05/04/2010
groundwater (13), seawater (14), drinking water (15), and other environmental water samples (16) by electrochemical analytical method has been reported. Among the electrochemical techniques, construction of new electrochemical biosensors with portable, reusable and environmentally friendly nature in virtue of the specific and high affinity between lead and biomolecules is an attractive strategy to improve the sensitivity and selectivity, and avoid the limitation of traditional electrodes such as the toxicity of mercury and the difficulty in handling, storage, and disposal (17). Especially the nucleic acid-based lead sensors have attracted much attention (18) because lead is known to have a great affinity for nucleic acids. Some nonelectrochemical nucleic acid-based lead sensors, such as leaddependent optical DNAzyme biosensors, have been developed by using DNA oligonucleotides strand with catalytic activities as the target recognition element and the cofactor with lead to produce fluorescent or colorimetric outputs (4, 18-20). The specific affinity of lead for nucleic acids has been proven by these optical methods. However, these methods suffer from some possible or inevitable drawbacks, including potential false signals from contaminating colorants, fluorophores, and quenchers, and frequently, a reliance on cumbersome optical equipment, which is not practical for on-site analysis (21). Electrochemical DNA-based biosensors may be a promising alternative technique to avoid the above-mentioned limitations. It is recently reported that the DNAzyme immobilized on Au electrodes can be used to construct an electrochemical lead biosensor (21, 22). Such sensors consist of the catalytic DNA strand and the hybridization of its complementary substrate strand. Moreover, a redox mediator, that is, methylene-blue (MB) or Ru(NH3)63+ (RuHeX), is bound to the anionic phosphate of DNA through electrostatic interactions and serves as the electrochemical signal transducer to lead. Parts-per-billion level (21) or 1 nM level (22) detected limitation of lead has been achieved on these sensors. Herein, the present work is focused on constructing a highly sensitive and selective DNA-based electrochemical lead sensor without any mediators to give direct electrochemical signal of Pb2+. A lead sensor with novel structured in favor of the electron transfer is developed in a simple fabrication route. For the first time, vertically aligned and conductive carbon hybridized TiO2 nanotube arrays (C-TiO2 NTs) are used to immobilize the target biomolecule, a traditional dsDNA. Abundant and swift immobilization of DNA is expected because the highly ordered vertically aligned TiO2 NTs with high surface area and precisely controlled morphology can provide much more multidimensional spaces, which are conceived to be excellent vessels for the biomolecules (23). The controllable carbon hybridization of the TiO2 NTs may decrease the impedance of the electrode surface, while retaining the tubular structure, biocompatibility, and hydrophilicity. The abundant immobilized dsDNA increases the amount of lead adsorbed, which will be quantitated by oxidizing lead from the sensor surface by differential pulse anodic stripping voltammetry. It is anticipated that direct use of the specific affinity of lead for traditional dsDNA improves the electrochemical determination of lead, so that special DNA oligonucleotides are not needed, to avoid the complicated, costly, and time-consuming biochemical modification of DNA. When used to probe lead, the designed DNA/C-TiO2 NTs lead sensor presents a broad linear range at a picomolar leveled detection limitation for lead. The potential relationship between the sensor VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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SCHEME 1. Schematic Illustration for DNA/C-TiO2 NTs Construction and its Pb Ion Monitoring
behavior and its microstructure is exploited. The lead sensor is further applied in real water samples.
Experimental Section Construction of DNA-Based C Hybridized TiO2 NTs (DNA/ C-TiO2 NTs) Lead Sensor. The construction of C-TiO2 NTs bioelectrochemical interface and further DNA-based lead sensor is illustrated by Scheme 1. Highly ordered and vertically aligned TiO2 NTs were primarily fabricated on Ti substrate (0.3 × 1.0 cm2) by anodic oxidation. Then, the TiO2 NTs were embedded with a saccharose solution (10 wt.%) containing ethoxyl aminopropyl trisiloxane (0.05 wt.%) and sulfuric acid (2 wt.%) under the vacuum of 6 × 10-2 Pa at room temperature. After dried at 120 °C for 5 h, carbonization at 500 °C under nitrogen atmosphere was performed to obtain the C hybridized TiO2 NTs (C-TiO2 NTs) for 4 h at heating and cooling rates of 5 °C/min. For comparison, C hybridized TiO2 film (C-TiO2 film) electrode was also prepared. The hybrid electrode was first electrochemically pretreated at a constant negative potential of -1.5 V in neutral 2 M NH4Cl aqueous electrolytes for 3 s. After rinsed by distilledwater and dried, the surface of the pretreated hybrid electrode was covered with dsDNA solution (60 µg mL-1) by dropping with a microsyringe, and was immediately dried under a constant flux of N2. This procedure was repeated three times. Then the electrode was immersed in acetate buffer solution (ABS) for given time and dried. The acquired DNA/C-TiO2 NTs was stored at 4 °C in a refrigerator when not used. For comparison, DNA/TiO2 NTs and DNA/C-TiO2 film electrodes were prepared by immobilizing DNA on the TiO2 NTs or C-TiO2 film in the same manner. The dsDNA sample used in the present work is DNA sodium salt from calf thymus, Type I, fibers, prepared from male and female calf thymus tissue, which was purchased from Sigma-Aldrich Co., U.S., and used without any pretreatment. It is a highly polymerized DNA containing double stranded DNA as the predominant form with molecular weight of 10-15 million Daltons, 41.9 mol % G-C and 58.1 mol % A-T. Probing Procedure for Pb2+ on DNA/C-TiO2 NTs. The trace Pb2+ was determined with two main steps including electrochemical accumulation and differential pulse anodic stripping voltammetry (DPASV). First, electrochemical accumulation of Pb2+ on the DNA/C-TiO2 NTs was carried out by immersing the electrode into the stirred Pb2+-containing solution at potentiostatic potential of -1.0 V for 180 s. Then, the electrode was rinsed thoroughly with deionized water and transferred to another electrolytic cell containing a leadfree fresh ABS (0.2 M, pH 7.0). DPASV was utilized to determine Pb2+ at pulse amplitude of 50 mV, pulse period of 0.2 s and quiet time of 2 s. Besides, the DNA/C-TiO2 NTs electrode can be easily regenerated in ABS containing 10 mM EDTA and 50 mM NaCl at +0.5 V.
Results and Discussion Enhanced Electrochemical Sensing Behavior for Pb2+ on the DNA/C-TiO2 NTs. The electrochemical features of the 4242
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DNA/C-TiO2 NTs are first evaluated by DPASV. In order to better understand the function of DNA/C-TiO2 NTs for Pb2+ analysis, the electrochemical DPASV responses on several electrodes are compared in Figure 1A. For a solution with 15 nM Pb2+, after electrochemical accumulation at -1.0 V for 180 s, a well-defined stripping peak appears at -0.66 V with a large stripping peak current density (curve a), which is attributed to the oxidation of metallic lead accumulated on the sensor from the sample solution. Under the same condition, no stripping peak for Pb2+ is observed on C-TiO2 NTs electrode (curve c) or DNA/C-TiO2 film electrode (curve d), on which no or much less DNA was loaded. An
FIGURE 1. (A) DPASV of different electrodes for 15 nM Pb2+. (B) DPASV for Pb2+ with various concentrations on DNA/C-TiO2 NTs. Inset: the calibration plot of stripping peak current density with Pb2+ concentration.
FIGURE 2. SEM patterns of as-grown TiO2 NTs (A: top view, B: cross sections) and C-TiO2 NTs (C: top view, D: cross sections). inconspicuous stripping peak appears at -0.66 V on DNA/ assigned to the in-plane vibrational mode of D-peak for TiO2 NTs electrode (curve b), but its striping peak current carbon defects and G-peak for graphite, which cannot be density for Pb2+ is less than 10% of that on DNA/C-TiO2 NTs. found for TiO2 NTs, confirming the hybridization of C on The above effects may be explained as follows. The DNA can TiO2 NTs. The Raman peaks at 144, 395, 514, and 640 cm-1 and the XRD diffraction peak at 2θ angles of 25.5° and 48.2° form a strong complex with Pb2+ and the C-TiO2 NTs improve the loading capacity of the DNA with a unique threeof TiO2 NTs and C-TiO2 NTs further demonstrate that TiO2 NTs mainly consist of anatase crystallites phase and the dimensional network structure, which has a larger surface hybridization of C does not change the crystal form of TiO2 area with more active sites for Pb2+ accumulation. Compared NTs. Therefore, the advantages of both TiO2 NTs and C are with TiO2 NTs, C-TiO2 NTs can improve the conductivity of the electrode. Thus the constructed DNA/C-TiO2 NTs can be well presented for dsDNA immobilization and its electroconsidered as an excellent lead sensor. The detailed reasons chemical sensor performance. for the enhanced electrochemical sensing toward lead can Second, the favorable wettability of the C-TiO2 NTs. It is known that the adsorption of biomolecules on a surface be further exploited from the preponderant tubular structure, depends strongly on the structure and topography, and in the better conductivity, the standout biocompatibility, and particular the surface wettability (24). The surface wettability hydrophilicity of the C-TiO2 NTs and the immobilization of abundant target biomolecule. of C-TiO2 NTs is studied by conventional sessile drop contact angle measurements. Results show that the contact angle First, the preponderant and tailed tubular structure of (CA) of water drop on C-TiO2 NTs is very small (ca. 10°), the C-TiO2 NTs. As illustrated in Scheme 1, the TiO2 NTs can be obtained on a Ti foil by an anodic oxidation process and reflecting its excellent hydrophilicity. Meanwhile, the DNA further be hybridized with C via a controllable technique drop almost completely spreads over the surface with CA of combining the surfactant-assisted vacuum impregnation and ca. 0°, so that the C-TiO2 NTs interface is ideal for DNA immobilization and can provide favorable microenvironment carbonization method. According to the SEM characterizafor the maintenance of its activity. It can further make the tion, the TiO2 NTs are self-organized, vertically aligned and compactly arranged with an average inner diameter of 80 ( immobilization of DNA onto TiO2 NTs more stable because of the hydrophilicity interaction, which would also benefit 15 nm, the thickness of the tube wall of ca. 15 nm, the tube the electrochemical ability. length of over 600 nm (Figure 2, A and B). The calculated Third, the much better conductivity of the C-TiO2 NTs. surface area of TiO2 NTs is thus estimated to increase at least 13 times that of Ti foil. With the hybridization, some of the Electrochemical impedance spectroscopy (EIS) can provide conductive C disperse over the nozzles of NTs, some cover the information on the conductivity change of electrode the nozzles or embed into the tubes or between the gaps, surface during the modification process. Therefore, EIS and some even grow into irregular form distributing over the measurements are performed in 0.1 M KCl solution containsurface of TiO2 NTs or enwrap the tubular mouth, which ing 10 mM [Fe(CN)6]3-/4- at the open circuit potential to makes the tube wall thicker, forming the flocculent porous assess the electrochemical properties of different electrodes. structure (Figure 2, C and D). The dispersion of the C is Results indicate that TiO2 NTs possess a very large electrochemical resistance, consistent with its semiconductor controlled and the nanotubular structure of TiO2 NTs maintains very well. Two distinct peaks at around 1311 and feature and the oxide barrier layer of tubular bottom 1619 cm-1 reflected by Raman spectra of C-TiO2 NTs can be originated in as-grown TiO2 NTs (25). Nevertheless, after VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. (A) Effects of the pH value on the stripping potential (a) and the stripping current density (b) for Pb2+ (50 nM) on DNA/C-TiO2 NTs. (B) Effects of the accumulation time at -1.0 V in Pb2+-containing solution (a) and the washing time at +0.5 V in EDTA solution (b) on the stripping peak current density for Pb2+ (15 nM) on DNA/C-TiO2 NTs, among which, the ratio of peak current density (y axis for b) was calculated by assuming the striping peak current density without washing is 100%. hybridization the C-TiO2 NTs present a very small and steady electrochemical resistance, indicating a much better conductivity compared with TiO2 NTs, which is significant in further electrochemical and biosensing applications. Uppermost, the large immobilization amount of DNA on C-TiO2 NTs. The EIS results indicate that, after the immobilization of DNA onto C-TiO2 NTs, the resistance of the electrode is obviously increased, suggesting the successful embedding of DNA. The surface coverage ratio (θ) of DNA on C-TiO2 NTs can be estimated by the relationship between 0 and Rct (26, 27), which are the resistances of the electrode Rct before and after DNA immobilization, respectively. For an optimized immobilization, the calculated θ of DNA on C-TiO2 NTs is 99.8%, indicating that an almost saturated immobilization is obtained. Moreover, the surface of the DNA/ C-TiO2 NTs sensor seems uniform. The large surface coverage and uniform surface of the sensor may be mainly ascribed to the vertically aligned structure, the high surface area and the three-dimensional nanotubular channel. The superior hydrophilicity and nanochannel effect are also helpful in protein immobilization. The greatly improvement on the DNA quantity provides much more binding sites for Pb and reasonably enhance the electrochemical anodic stripping response signal of Pb2+. The Superiority and Applicability of the Novel Lead Sensor. The performance of the DNA/C-TiO2 NTs as a lead sensor has been further investigated, indicating its domi4244
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FIGURE 4. (A) DPASV curves for 50 nM Pb2+ without (a), with Cd2+, Cu2+, Zn2+, Ni2+, Ca2+, Mg2+, Fe2+, Co2+, Ba2+, and Hg2+ (each at 50 nM) (b), or with Cd2+, Cu2+, Zn2+, Ni2+, Ca2+, Mg2+, Fe2+, Co2+, Ba2+, and Hg2+ (each at 50 µM) (c) on DNA/C-TiO2 NTs. (B) The stripping peak current density of Pb2+ (50 nM) for repetitive determinations on DNA/C-TiO2 NTs. nances of wide pH adaptability, fast electro-accumulation capacity for lead and easy regeneration, as well as the high sensitivity, selectivity and repeatability. Broad pH applicability is of great significance in environmental control and monitoring. The pH value of the solution may have obvious effects on the bioactivity of DNA, the interaction between heavy metal ions and DNA, and the anodic stripping peak potential and current. Thus the electrochemical response of Pb2+ on the DNA/C-TiO2 NTs is studied in the pH range from 3 to 10. As shown in Figure 3A, the potential (curve a) for the stripping peak of Pb2+ shifts slightly to a more positive direction as pH value decreases from 10 to 3. On the other hand, although the stripping current density (curve b) is much lower at pH of 3, 9, and 10, the sensitive level is almost the same in the pH range of 4-8, so that the effective detection of Pb2+ can be obtained in a wide pH range. A quick accumulation for lead is another favorable feature on the DNA/C-TiO2 NTs, which is of significance in the rapid analysis. Figure 3B shows the relationship between the stripping peak current density and the accumulation time (curve a), in which the anodic stripping peak current density for Pb2+ increases dramatically as the accumulation time increases up to 150 s, then increases gradually and finally changes little at 180 s due to the saturation of binding sites
TABLE 1. Comparison of the Results Obtained by the Present Work and the AAS for the Determination of Pb2+ in Real Water Sample concentration of Pb2+ no.
sample
present lead sensor
AAS
1 2 3 4 5
pure drinking water spiked drinking water 1a spiked drinking water 2b river water landfill leachatec
nondetectable (56 ( 2) pM (157 ( 8) pM (14.1 ( 0.5) nM (742 ( 31) nM
nondetectable (59 ( 7) pM (162 ( 19) pM (15.3 ( 1.2) nM (735 ( 63) nM
a 50 pM Pb2+ added to pure drinking water. landfill leachate was analyzed.
b
150 pM Pb2+ added to pure drinking water.
of DNA for Pb2+. It provides an important parameter for accumulation time (180 s) in the experiments. However, as to a C-TiO2 NTs electrode, under the same electrochemical accumulation conditions for 180 s, no stripping peak is observed (Figure 1A, curve c). The faster accumulation of Pb2+ on DNA/C-TiO2 NTs may be attributed to the larger loading amount of DNA on the sensor, which is in favor of binding Pb2+. It is noticeable that the sensor can be regenerated in a simple way. As the affinity of lead ions for EDTA is stronger than that for DNA (28), after each DPASV determination of Pb2+, the DNA/C-TiO2 NTs lead sensor is placed in ABS containing 10 mM EDTA to remove the remnants of Pb2+ at +0.5 V. Figure 3B shows that the remnant lead is rapidly removed from the sensor in the rinsing time of 90 s (curve, b). When the rinsing time increases to 150 s, Pb2+ ions cannot be detected on the sensor any longer. Figure 1B shows the DPASV curves of the present sensor for Pb2+ at various concentrations, the corresponding calibration curve being derived accordingly. The stripping peak current density (jpa) is proportional to the concentration of Pb2+ from 0.01 to 160 nM. The regression equation is jpa ) 0.3818 + 0.0367C with a correlation coefficient of 0.9994, where jpa is in mA cm-2 and C in nM. The sensitivity of the sensor to Pb2+ is 36.7 µA cm-2 nM-1, which is higher than that of the NHAP/ionophore/Nafion modified electrode with an open-circuit accumulation of 10 min (9). The detection limit is calculated to be as low as 3.3 pM. It should be noted that the DNA/C-TiO2 NTs has a wider linear dynamic range, lower detection limit, and higher sensitivity than most of other mercury-free electrodes for the determination of Pb2+ (12, 16, 29-31), and more sensitive than mercury-based electrodes (32, 33). The excellent sensitivity of the sensor benefits from the improvement on DNA/C-TiO2 NTs, since the quantity of loaded DNA is larger, the reactive-sites on the C-TiO2 NTs are increased, and the bioelectrochemical activity is enhanced. The specificity of the DNA/C-TiO2 NTs sensor is determined by challenging it with several divalent metal ions. In this work, metal ions including Cd2+, Cu2+, Zn2+, Ni2+, Ca2+, Mg2+, Fe2+, Co2+, Ba2+, and Hg2+ are chosen as potential interfering ions for investigating the selectivity of the sensor. The results are shown in Figure 4. In the potential range of -1.4 to 0 V, the stripping peak current signal for 50 nM Pb2+ changes little with each metal ion mentioned above at the same concentrations of 50 nM (curve b), compared with that of pure Pb2+ (curve a). When fixing the concentration of Pb2+ but increasing those of other ions, the signals can be observed for Cu2+, Cd2+, and Ni2+ at the concentration of 50 µM, but none for the rest (curve c). The peak for Pb2+ can be still observed except for a little widening in shape. All the results indicate that the response of Pb2+ on the constructed DNA/ C-TiO2 NTs is unaffected by the presence of other metals even at the concentration of 1000-fold. A good selectivity for Pb2+ is thus demonstrated for the present sensor, which is attributed to the selective transportation of Pb2+ from the
c
The 10-fold diluents of the
sample solution to the DNA via the stronger affinity of DNA for Pb. According to the literature, the complexation and interaction between Pb and DNA is stronger than that for the other metals such as Cd, Fe, Cu and Zn (32, 34, 35). Therefore, under the present experimental condition, where the electro-accumulation time is set as short as 3 min, the combination quantity of other metals onto DNA is in a very low degree. As a result, the coexisting concentration of other metals may be as large as 1000-fold without any obvious interference. Moreover, the repeated usage of the DNA/C-TiO2 NTs for lead probing under the same conditions is investigated. Figure 4B shows the stripping peak current response of lead for each DPASV. For the detection of 50 nM Pb2+, the relative standard deviation (R.S.D.) of 25 times is less than 5%, so that the lead sensor presents good repeatability, reproducibility and stability for Pb2+ detection. The stabilization of DNA is presumed mainly ascribed to the above-mentioned hydrophilicity interactions with C-TiO2 NTs owing to their favorable wettability confirmed by contact angle results. Hydrogen bonding interaction may also exist since there are abundance of oxygen-terminal functional groups on C-TiO2 NTs and plenty of amino groups on DNA. Furthermore, the potential confining effects provided by the unique 3-D nanotube channel structure of C-TiO2 NTs may also stabilize the DNA. In order to illustrate its accuracy in practical analysis, the environmental applicability of the DNA/C-TiO2 NTs is evaluated by determining Pb2+ in several real water samples, including drinking water, river water and landfill leachate water. Five parallel experiments were carried out for each measurement. All real water samples were filtered through a standard 0.45 µm filter, treated with UV digestion for release of the trace Pb2+ from the lead-organic complexes. Compared with atomic absorption spectrometry (AAS), satisfactory results are obtained and summarized in Table 1. It can be seen that the DNA/C-TiO2 NTs has a great potential for real sample analysis with good reliability, especially adaptable for the low-lead or free-lead water samples.
Acknowledgments This work was supported jointly by National Nature Science Foundation China (20877058), 863 Program (2008AA06Z329) from the Ministry of Science, Shanghai Nanometer Science Foundation (0852 nm01200) and Shanghai Educational Development Foundation (2007CG24).
Supporting Information Available Detailed experiment including the preparation of TiO2 NTs electrode and the physicochemical characterization of electrode materials (S1), the Raman spectra of C-TiO2 NTs and TiO2 NTs (S2), XRD patterns of C-TiO2 NTs and TiO2 NTs (S3), contact angles of water or DNA drops on C-TiO2 NTs surface (S4), the EIS for TiO2 NTs, C-TiO2 NTs and VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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DNA/C-TiO2 NTs (S5). This material is available free of charge via the Internet at http://pubs.acs.org.
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