Article pubs.acs.org/est
Fabrication of a Novel and Simple Microcystin-LR Photoelectrochemical Sensor with High Sensitivity and Selectivity Kang Chen,† Meichuan Liu,†,‡ Guohua Zhao,*,†,‡ Huijie Shi,† Lifang Fan,† and Sichen Zhao§ †
Department of Chemistry, Tongji University, 1239 Siping Road, 200092 Shanghai, China Key Laboratory of Yangtze River Water Environment, Ministry of Education, Tongji University, Shanghai 200092, PR China
‡
S Supporting Information *
ABSTRACT: Microcystin-LR (MC-LR), an inert electrochemical species, is difficult to be detected by a simple and direct electrochemical method. In the present work, a novel photoelectrochemical sensor is developed on highly ordered and vertically aligned TiO2 nanotubes (TiO2 NTs) with convenient surface modification of molecularly imprinted polymer (MIP) (denoted as MIP@TiO2 NTs) for highly sensitive and selective determination of MC-LR in solutions. Molecularly imprinted polypyrrole (PPy) of MC-LR is chosen as the recognition element. The designed MIP@TiO2 NTs photoelectrochemical sensor presents excellent applicability in MC-LR determination, with linear range from 0.5 to 100 μg L−1 and limit of detection of 0.1 μg L−1. Moreover, the sensor exhibits outstanding selectivity while used in coexisting systems containing 2,4-dichorophenoxyacetic acid, atrazine, paraquat, or monosultap with high concentration, 100 times that of MC-LR. The sensor presents good photoelectric conversion efficiency and detection sensitivity, as well as broad linear detection range, mainly because of the high specific surface area and photoelectric activity of TiO2 NTs and the π bond delocalized electron system of PPy that promotes the separation of electron-holes. The prominent selectivity is from the MIP by forming multiple hydrogen bonds between PPy and MC-LR. Mechanisms for photoelectrochemical analysis and selective recognition are also discussed.
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INTRODUCTION Microcystin-LR (MC-LR), released by cyanbacteria during eutrophication, is kind of cyclic heptapeptides, which are strong inhibitors of protein phosphatases type 2A (PP2A) and type 1 (PP1) that play significant roles in dephosphorylation process of proteins.1,2 MC-LR is a soluble and stable chemical substance that may cause structural and functional disturbance of liver, a potential threat of cancer.2 Due to its ubiquity and high toxicity, MC-LR becomes an important element in water quality control and environmental monitoring. In 1998, the World Health Organization (WHO) put forward a maximum limit for MC-LR in drinking water, 1 μg L−1.3 Many analytical techniques have been used for MC-LR determination, such as high-performance liquid chromatography (HPLC), liquid chromatography/mass spectrometry (LC-MS), capillary electrophoresis, Raman spectroscopy, and protein phosphatase inhibition assays.2−5 Although these methods are currently accepted by most researchers, they suffer from bulky apparatus, high cost, and complicated prepreparation procedure. Electrochemical technique is an effective process with high sensitivity and fast response, which is simpler in instrument and easier to realize online detection compared to those apparatusapproaches.6,7 However, MC-LR is chemically inert, so it is difficult to be detected by direct electrochemical oxidation or reduction.2,3 Consequently, it is considered to develop a convenient and direct analytical method for photoelectrochem© 2012 American Chemical Society
ical (PEC) sensing of MC-LR by utilizing the photocatalytic oxidation of MC-LR on the surface of a highly efficient photocatalyst under a certain bias potential. The PEC sensor is considered to be a more sensitive technique attributed to different forms of energy for excitation (light) and detection (current). However, the active species such as hydroxyl radicals generated during photocatalysis in the PEC determination procedure are such powerful oxidants that most substances will be oxidized on the photoanode,8−11 which lowers the selectivity and makes it inadaptable for selective determination of MC-LR in multicomponent mixtures. How to realize the high selectivity of MC-LR detection by PEC sensor is thus a critical issue. Some special structure and function should be designed when constructing the PEC sensor to endow it with specific molecular recognition ability toward MC-LR. In this study, a novel photoanode based on TiO2 nanotubes (NTs) with secondary structure and MC-LR recognition sites is to be fabricated using surface molecular imprinting technology and a simple and convenient PEC determination method for MC-LR is proposed. In this process, vertically aligned TiO2 NTs are used as the substrate electrode for photocatalysis, molecularly imprinted polypyrrole (PPy) is chosen as the Received: Revised: Accepted: Published: 11955
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Scheme 1. Schematic Illustration for Fabrication of the MC-LR Sensor and Its EC (a,b) and PEC (c,d) Response in PBS Without (Dotted Line) or with (Solid Line) MC-LR
a new detection technique toward MC-LR in water environments.
recognition element, and MC-LR is employed as the template molecule. The construction is based on following two aspects. (1) Highly ordered and vertically oriented TiO2 NTs have outstanding photocatalytic ability, which will lead to more sensitive photo-oxidation current in analysis. In addition, their microstructure, high specific surface area and space utilization provide ideal conditions for the recognition element of PPy to grow in situ. (2) As so-called “simulation antibody”, molecularly imprinted polymers (MIPs) may specifically bind with target molecules like real antibody, which could probably improve the selectivity of electrode once employed in chemical sensors.12−14 Compared to the electrochemical immunosensor technique, which is adopted to obtain good selectivity toward target species by using biomolecules such as antibody-based immunoassay or enzyme-linked immunosorbent assay (ELISA),5,15−17 the avoidance of use any biomolecules obviously account for the higher stability and the lower cost, making the sensor adaptable in some extreme environments, such as high temperature and pressure, strong acid and alkali. PPy can easily grow in situ through electrochemical method and the formation of multiple hydrogen bonds between PPy and MC-LR molecules through H, O, and N atoms makes the electrode not only much more stable but also possible to improve the ability of molecule recognition and thereby enhance its selectivity.18−20 In addition, π bond delocalized electron system of PPy makes it capable of separating photogenerated electron holes efficiently and enhancing photoelectric conversion efficiency, which will enhance the PEC response of the sensor.20 Results show that this constructed photoelectrochemical MC-LR sensor not only presents excellent applicability in MC-LR determination with high sensitivity, low detection limitation, and broadened linear detection range, but also exhibits outstanding selectivity while used in coexisting of other pollutants such as 2,4dichorophenoxyacetic acid, atrazine, paraquat, monosultap. Mechanisms for PEC analysis and selective recognition are also discussed. It is promising to provide a new PEC sensor and
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EXPERIMENTAL SECTION Reagents and Apparatus. MC-LR (analytical grade) was purchased from Beijing Express Biotechnology Co. Ltd. 2,4dichorophenoxyacetic acid (2,4-D), atrazine, paraquat, monosultap, acetamiprid, profenofos, glyphosate, and aniline were all analytical reagents. Pure titanium foil (99.9% purity) was used for anodic oxidation. All the reagents in this experiment were of analytical grade and used without further purification. Doubledistilled water was used throughout the experiments. A 200W LA-410UV-3 lamp (Hayashi, Japan) was used as light source with wavelength of 250−700 nm. Scanning electron microscopy (SEM) (FE-SEM Hitachi-S4800, Japan) and ultraviolet diffuse reflectance spectroscopy (UV-DRS) (BWS002, BWtek) were used to examine the superficial structure and morphology. X-ray diffractometer (XRD) (D/max2550VB3+/PC, Rigaku) was used to detect the crystal phases with CuKα as radiation and was operated at 40 kV and 20 mA. All electrochemical experiments were carried out on CHI660c workstation (CH Instrument, USA). Fabrication of MC-LR PEC Sensor. The fabrication of MC-LR sensor is illustrated by Scheme 1. Polished Ti substrate was used for anodic oxidation to obtain highly ordered and vertically aligned TiO2 NTs.21,22 Before in situ polymerization of PPy, the electrode was first electrochemically pretreated at a constant potential of −1.5 V in neutral 2 M NH4Cl aqueous electrolytes for 3 s. After rinsed by distilled water and dried, the electrode was immersed in a solution containing 0.1 M LiClO4, 10 μM pyrrole and 1 μM MC-LR. Cyclic voltammetry (CV) was performed for electrochemical polymerization of pyrrole in the potential range from −0.2 to 1.0 V at scan rate of 50 mV/s for 15 sweep cycles. After that, the electrode was electrochemically treated at 1.3 V for 20 min in 0.2 M K2HPO4 solution to remove template molecules. Then the MC-LR PEC sensor 11956
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line) and with 20 μg/L MC-LR (solid line). No distinct difference is observed with MC-LR added, which reveals that MC-LR cannot be directly oxidized in a simple electrochemical way. The electrochemical (EC) response of the sensor for MCLR is almost the same as that for the blank PBS, illustrated as curves a, b in Scheme 1. Therefore, a photochemical method is considered to solve this problem. In order to evaluate the photochemical performance of the constructed MC-LR sensor, i-t technique was applied to measure the photocurrent on MIP@TiO2 NTs electrode, compared to that on TiO2 NTs and NIP@TiO2 NTs. Results show that, with the addition of MC-LR, the photocurrent is enhanced obviously compared to that of PBS, indicated by curves c, d in Scheme 1. The value of ΔI (=I − I0), where I and I0 are photocurrent in PBS before and after the addition of 20 μg/L MC-LR, respectively, is employed to describe the results. Figure 1B shows that under the bias of 0 V, the photocurrent responses increases on all the three electrodes with MC-LR added in the PBS solution, which means that photoelectrochemistry is a valid method to oxidize MC-LR directly. The values of ΔI on TiO2 NTs, NIP@TiO2 NTs and MIP@ TiO2 NTs electrode are 5.92 × 10−6, 3.27 × 10−5 and 9.06 × 10−5 A, respectively, which indicates that the photocurrent on MIP electrode is much higher than that on TiO2 NTs and NIP@TiO2 NTs. In other words, the prepared MC-LR sensor is much more sensitive in MC-LR sensing. The reasons for the superior sensitivity of MC-LR sensor are discussed as follows. On the one hand, excellent photocatalyst TiO2 NTs are chosen as the photoanode electrode. Figure 2
(MIP PPy@TiO2 NTs) with empty sites for MC-LR recognition was obtained. For comparison, nonimprinted polymer (NIP) on TiO2 NTs (NIP@TiO2 NTs) was constructed under the same conditions but without MC-LR in the solution while polymerizing. PEC Performance of the MC-LR Sensor. All PEC measurements were performed using a conventional threeelectrode cell system on CHI660c workstation. TiO2 NTs, NIP@TiO2 NTs or MIP@TiO2 NTs electrode was employed as the working electrode, while a saturated calomel electrode (SCE) and a platinum electrode served as the reference and counter electrode, respectively. I-t curve method was used for both sensitivity and selectivity experiments.
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RESULTS AND DISCUSSION
Improved PEC Performance of the MC-LR PEC Sensor. Scheme 1 illustrates the preparation process of this MC-LR PEC sensor. Highly ordered and vertically aligned TiO2 NTs were first fabricated on Ti substrate by in situ anodic oxidation. And then electrochemical polymerization of pyrrole was carried out with MC-LR as the template molecules to obtain the surface molecularly imprinted functionalization of the TiO2 NTs. Subsequently, the template MC-LR molecules were removed from the polypyrrole modified TiO2 NTs, leaving abundant recognition sites for MC-LR, and the MC-LR PEC sensor was obtained. Since MC-LR is a nonelectroactive species, it is difficult to be detected through direct electrochemical method. Figure 1A shows the CV curves of this MC-LR sensor in 0.1 M phosphorus buffer solution (PBS) (pH 7.0) without (dotted
Figure 2. SEM of TiO2 NTs (a) and MIP@TiO2 NTs (b), XRD patterns of TiO2 NTs, NIP@TiO2 NTs, and MIP PPy@TiO2 NTs (c).
presents SEM images of TiO2 NTs and MIP@TiO2 NTs and XRD patterns of TiO2 NTs, NIP@TiO2 NTs and MIP@TiO2 NTs. The microstructure of TiO2 NTs gives high specific surface area and excellent space utilization, providing PPy a wonderful substrate to grow in situ and significantly increasing the load amount of PPy. The modification of PPy neither completely covers the surface of TiO2 NTs nor destructs their tubular structure, so that photocatalytic properties of TiO2 NTs
Figure 1. (A) Current response without photo illumination on MCLR sensor before (dot) and after (solid) the injection of 20 μg/L MCLR in 0.1 M PBS (pH 7); (B) the difference (ΔI = I − I0) of photocurrent before and after the injection of 20 μg/L MC-LR on three different electrodes. 11957
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are well-retained. The XRD and Raman (S3, S4) patterns also show that the modification of PPy does not vary the crystalline structure of TiO2 NTs. These are the basis for the high sensitivity. With this ingenious design and PEC approach, MCLR is directly oxidized on the electrode. The employment of bias promotes the separation of electron−holes and enhances the photocurrent of inert MC-LR. On the other hand, PPy is chosen as imprinting material, since PPy can improve the photocatalytic property of TiO2 NTs (Figure 1B). Figure 3A shows the UV-DRS of TiO2 NTs
The photoelectric conversion efficiency is calculated as follows: η% = [(total power output‐electrical power input) /light power input] × 100 =jp (Erev − |Eapp|) × 100/I0
where jp is the photocurrent density (A/cm2), jpErev is the total power output, jp|Eapp| is the electrical power input, and I0 is the power density of the incident light (mW/cm2). Erev is the standard reversible potential (which is 1.23 V for the water splitting reaction at pH = 0), and Eapp is the absolute value of the applied potential, which is obtained as Eapp = Emeans−Eocp, where Emeas is the electrode potential of the working electrode at which jp is measured under illumination and is the applied potential at open circuit in the same solution and under the same illumination. Here the photoelectro conversion efficiency of TiO2 NTs is calculated to be 5.22%, while it rises to 16.7% on MIP@TiO2 NTs and NIP@TiO2 NTs electrodes. This can be attributed to the formation of p-π conjugated structure between N and C C in PPy, which results in a large delocalized π bond system and makes the difference between molecular orbital energy levels small to present conductor-like property, which can rapidly transfer electrons generated by TiO2 NTs, enhancing the conversion efficiency and sensitivity.18 The performance of this MC-LR PEC sensor is also evaluated by i-t curve to further investigate its sensitivity. As presented in Figure 4, the MC-LR sensor possesses a wide linear calibration range from 0.5 to 100 μg/L with detection limitation of 0.1 μg/L, the linear equation is I (mA) = 6.56 ×
Figure 3. (A) UV-vis DRS characterization of TiO2 NTs (a), NIP@ TiO2 NTs (b) and MIP@TiO2 NTs (c); (B) variation of photocurrent in 0.1 M Na2SO4 on TiO2 NTs (a), NIP@TiO2 NTs (b), and MIP PPy@TiO2 NTs (c) with UV irradiation and without UV irradiation (inset).
(a), NIP@TiO2 NTs (b) and MIP@TiO2 NTs (c). The absorption band edge of TiO2 NTs is 380 nm. Although no shift of the absorption band is observed on the NIP@TiO2 NTs and MIP@TiO2 NTs, the absorption in UV area is increased obviously. As is known, PPy is a conjugated polymer with good conductivity. For TiO2 NTs, photogenerated carriers will be easily recombined, lowering the photoelectric conversion efficiency, whereas for the PPy modified TiO2 NTs, PPy will promote the separation of electron holes, enhancing the conversion efficiency. Linear sweep voltammetry (LSV) was carried out to examine the photoelectro conversion efficiency of the three electrodes. As shown in Figure 3B, the dark current intensities are 2.68 × 10−5, 4.90 × 10−5 and 6.46 × 10−5 A/cm2, respectively, on TiO2 NTs, NIP@TiO2 NTs, and MIP PPy@ TiO2 NTs in 0.1 M Na2SO4 without UV irradiation. There is no obvious difference between the three electrodes. With the light illumination, the current increases considerably. The photocurrent density on MIP@TiO2 NTs and NIP@TiO2 NTs are 2.41 × 10−3 and 2.34 × 10−3 A/cm2, respectively, almost 6 times that of TiO2 NTs (4.06 × 10−4A/cm2). It is attributed to the π bond delocalized electron system of PPy, which makes it a good conductor, improving the photo electrochemical properties of TiO2 NTs.23,24
Figure 4. (A) Typical current−time curve of MC-LR with successive addition of MC-LR into 0.1 M PBS (pH 7) on MC-LR sensor; (B) the relationship between photocurrent and the concentration of MC-LR. 11958
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10−7C (μg/L) + 2.22 × 10−6 (mA) with a correlation coefficient of 0.997. Its sensitivity is comparable with that of the MC-LR immunosensor reported by Zhang et al.,5 and membrane-based ELISA assay reported by Lotierzo et al.17 Meanwhile, it is much more stable, cheaper and simpler owning to the direct oxidation of MC-LR through PEC way. With regard to the selective determination of environmental pollutants, electrochemical immunosensor is reported as an often-used technique, which can obtain good selectivity toward particular target species.17 As is well-known, the immunosensor is thought to tend to be less stable in the complicated environmental samples. Different to the electrochemical immunosensor, the obvious and excellent characteristics of the present work can obtain the high selective and sensitive determination of MC-LR without use any special biomoleculars. Superior Selectivity of the MC-LR PEC Sensor. To investigate the selectivity of the MC-LR sensor, 2,4-D, atrazine, paraquat, monosultap, acetamiprid, profenofos, glyphosate, and aniline were chosen as the interfering substances, and i-t technique was employed to detect the photocurrent for different systems on the MC-LR sensor. As shown in Figure 5, S0 is the photocurrent response for 20 μg/L MC-LR on the
MC-LR sensor, the current response of MC-LR is much higher than that of 2,4-D, atrazine, paraquat, monosultap, and acetamiprid, further demonstrating that the MC-LR sensor is excellent in selectivity. The super selectivity of the MC-LR sensor can be mainly contributed to the surface molecularly imprinted modification of the PEC sensor. MIPs develop fast in recent years based on immunology, which can specifically interact with target molecules like that of antigen with antibody. Therefore, the employment of MIPs is equivalent to introducing “artificial antibody” on photoanode, consequently realizes the selective determination of “antigen”. MIP mechanism can be schematically illustrated (S2). In this work, pyrrole is used as monomer and MC-LR as template molecule. This identification to MCLR on the MIP modified sensor is mainly based on the following two aspects: the shape identification and the hydrogen bond identification. The pyrrole monomer will coordinate with MC-LR target molecules and polymerize together, here the target molecule MC-LR is used as template. When the target molecules are removed, the leftover becomes a special polymer with voids in particular shape and size, which is a good complement to target molecules.27−30 Moreover, N, O, and H atoms of MC-LR can interact with H and N atoms of pyrrole to form multi hydrogen bonds. After the removal of MC-LR templates, the left MIP-PPy has special recognition sites and only MC-LR can interact with the MIP-PPy, since the hydrogen bond sites of MC-LR perfectly match the special recognition sites. That is, by employing MIP, molecular memory is introduced in PPy. This particular PPy shows strong ability on molecule recognition, and greatly improve the selectivity.27,31,32 This special characterization makes MIP modified sensor more superior than biosensor in selectivity, since biosensor usually rely on the inhibition of pollutant toward enzyme, while many species in environment may inhibit the same enzyme, lowering the selectivity. However, for the MCs variants, which are kind of cyclic heptapeptides, due to their highly similar molecular structure, size, and properties, the present MC-LR sensor might not be able to well distinguish different MCs variants in the way like what it does to MC-LR and other pollutants with totally different molecular structure and size. On the one hand, it can be understood from the recognition mechanism of the present PEC sensor. As stated above, the sensor indicated selectivity to MC-LR against pollutants such as 2,4-D, mainly due to shape identification and functional group identification. As we can see from S5 and S6, MC-LR and MC-YR have almost the same cyclic heptapeptides structure. Therefore, both the molecular shape identification and the functional group identification would be in the similar level for MC-LR and MC-YR on the constructed PEC sensor. So, in principle, it would be difficult for the sensor to present rather good selectivity toward MC-LR and other MCs, and it is hard for the sensor to selectively distinguish different MCs, such as MC-LR and MC-YR. On the other hand, after having extensively consulted literatures on the topic of “cross activity” of MC-LR sensor, it can be seen that some researchers have fabricated MC-LR immunosensors by using surface biological antibody modification method, which can be found as an often-used and interesting technique in MC-LR sensing.5,15,16 Some of meticulous work and detailed experimental results show that, even if using such biological antibody modified immunosensors, the selective distinguish for microcystins can hardly be realized because of their highly similar cyclic heptapeptides struc-
Figure 5. Relative photocurrent in different mixture systems on MCLR sensor for 20 μg/L MC-LR only (S0), 20 μg/L MC-LR + 2000 μg/L 2,4-D (S1), 20 μg/L MC-LR + 2000 μg/L atrazine (S2), 20 μg/ L MC-LR + 2000 μg/L paraquat (S3), 20 μg/L MC-LR + 2000 μg/L molosultap (S4), 20 μg/L MC-LR + 2000 μg/L acetamiprid (S5), 20 μg/L MC-LR + 2000 μg/L profenofos (S6), 20 μg/L MC-LR + 2000 μg/L glyphosate (S7), and 20 μg/L MC-LR + 2000 μg/L aniline (S8).
MC-LR sensor and is set as 100%, while S1, S2, S3, S4, S5, S6, S7, and S8 are two-component systems containing 20 μg/L MC-LR and 2000 μg/L 2,4-D, atrazine, paraquat, monosultap, acetamiprid, profenofos, glyphosate, or aniline, respectively. Results indicate that photocurrent deviations are all less than 3% even when the concentration of interfering substances is 100 times that of MC-LR, which adequately reveals that this MC-LR PEC sensor presents outstanding selectivity. Imprinting factor (S), represented by the ratio of the current of target substance to the current of interfering substance at the same concentration, is another important parameter in evaluating the selectivity.25,26 Therefore, the S value of the MC-LR sensor is used to further investigate the selectivity. In the experiments, photocurrents in PBS with different single components of 20 μg/L MC-LR, 2,4-D, atrazine, paraquat, monosultap and acetamiprid were recorded separately. The S values of 2,4-D, atrazine, paraquat, monosultap and acetamiprid are 1.56, 1.64, 1.67, 1.62, and 1.68, respectively. That is, on the 11959
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ture.17,33,34 Interesting, similar response activities have often been obtained for several different MCs on these immunosensors, and thus the sensors have been considered to present high cross activity toward MCs, which is thought to be significant and valuable for toxicity assessment in real water sample. For example, Lotierzo et al. have done a considerable cross activity test of their constructed membrane-based enzyme-linked immunosorbent (ELISA) assay and electrochemical immunosensor for microcystin-LR in water samples.17 Their results exhibited that cross reactivities were high with all cyclic peptides (MC-LR, MC-YR, MC-LF, MC-LW, NOD), with values ranging from over 100% to 50%, due to their similar common structural moieties in the cyclic peptides. Herranz et al. have also tested the cross reactivity of their constructed SPR biosensor for the detection of microcystins in drinking water, and results showed that cross reactivities were high for MC-LR, MC-RR, MC-YR with values ranging from over 100% to 88 ± 3%.34 Based on the above analysis, we think that by using surface artificial antibody (molecularly imprinted polymer) modified PEC sensor may not be apt to selectively distinguish MCs variations, such as MC-LR and MC-YR, whose size and the molecule nature are very similar. And hence the PEC sensor may also be anticipated to present good cross activities toward MC variants. The relating cross activities experiments of the sensor to MCs variants are being prepared and carried out. The related researches on the cross activities of the sensor to MCs variants, and a more in-depth mechanism of the system, together with the application in real sample monitoring would be further systematically studied in the future work. The MC-LR PEC sensor is fabricated with special and empty sites that could specifically interact with target MC-LR molecules. These particular sites may promote the adsorption of MC-LR on the surface of the sensor, not only improving the selectivity but also enhancing the sensitivity due to the enrichment of MC-LR. Therefore, chronocoulometry was applied to determine the absorption capacity Γ of the MCLR sensor toward MC-LR to investigate the selective property. This experiment was carried out in 0.1 M PBS and the charge amounts before and after injection of 20 μg/L MC-LR were recorded. For comparison, the absorption capacity of TiO2 NTs and NIP@TiO2 NTs was also measured. The value of Γ can be assessed according to the formula:
Figure 6. Stability test of MC-LR sensor in 0.1 M PBS (pH 7) containing 20 μg/L MC-LR.
stable within 1s. Same tests were conducted seven times and the relative standard deviation (RSD) was only 0.9%, which demonstrates the high stability of the MC-LR PEC sensor. There may be two reasons responsible for the excellent stability. (1) The cross-link structure of PPy is formed during the polymerization process, which makes PPy particularly stable and further stabilizes the PEC sensor. (2) The most critical defect of TiO2 NTs is the fast recombination of electron−hole which lowers the photocatalytic efficiency and makes the photocurrent unstable, restricting the application of TiO2 in electrochemical sensing, while in the fabricated MC-LR PEC sensor, the π bond system of PPy promotes the separation of photogenerated carriers, so that the stability is greatly improved.
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ASSOCIATED CONTENT
S Supporting Information *
Detail procedure of the preparation of TiO2 NTs and the in situ polymerization of pyrrole with template molecules, as well as the removal of target molecules MC-LR (S1), schematic illustration of surface molecularly imprinted polypyrrole mechanism and the specific identification for MC-LR (S2), the Raman patterns of TiO2 NTs, NIP@TiO2 NTs and MIP@ TiO2 NTs (S3, S4), structure of (A) Microcystin-LR (MC-LR) and (B) Microcystin-YR (MC-YR), comparisons of MC-LR and MC-YR. X and Y are variable L-amino acids. This material is available free of charge via the Internet at http://pubs.acs.org.
Q = nFA Γ
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where Q is the electric quantity obtained in experiment, n is the electron transfer number in redox process, F is the Faraday constant, A is the surface area of electrode and Γ is the absorption capacity (mol·cm−2). Results show that absorption capacities of TiO2 NTs and NIP@TiO2 NTs are 5.60 × 10−13 and 7.34 × 10−12 mol·cm−2, respectively. For the MC-LR senor, it is 1.22 × 10−11 mol·cm−2, which is 21.8 and 1.7 times that of TiO2 NTs and NIP@TiO2 NTs. It is attributed to the special recognition sites on MC-LR sensor, which is in favor of the absorption of MC-LR. The absorption results further reveal the excellent selectivity of the designed MC-LR PEC sensor. Excellent Stability of the MC-LR PEC Sensor. I-t curve method was used to check the stability of the MC-LR sensor. Photocurrent in 0.1 M PBS containing 20 μg/L MC-LR was continuously recorded. As shown in Figure 6, with light illuminating on the MC-LR PEC sensor, there is an immediate current rise on the electrode and the photocurrent becomes
AUTHOR INFORMATION
Corresponding Author
*Phone: (86)-21-65981180; fax: (86)-21-65982287; e-mail: g.
[email protected]. Present Address §
High School Affiliated to Fudan University, 383 Guoquan Road, 200433 Shanghai, China. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21077077, 21107081), 863 Program (2008AA06Z329) from the Ministry of Science and the Fundamental Research Funds for the Central Universities. We cordially thank Prof. Dr. Dongming Li for her careful and helpful revision for the English usage. 11960
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dx.doi.org/10.1021/es302327w | Environ. Sci. Technol. 2012, 46, 11955−11961
