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The fabrication of carbon nanotubes array-based electrochemical chiral sensor by electrosynthesis
Author’s Accepted Manuscript The fabrication of carbon nanotubes array-based electrochemical chiral sensor by electrosynthesis Hong Zhu, Fengxia Chang, Zhiwei Zhu
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To appear in: Talanta Received date: 25 November 2016 Revised date: 8 January 2017 Accepted date: 12 January 2017 Cite this article as: Hong Zhu, Fengxia Chang and Zhiwei Zhu, The fabrication of carbon nanotubes array-based electrochemical chiral sensor by electrosynthesis, Talanta, http://dx.doi.org/10.1016/j.talanta.2017.01.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The fabrication of carbon nanotubes array-based electrochemical chiral sensor by electrosynthesis Hong Zhua1, Fengxia Changb1, Zhiwei Zhub* a
Key Laboratory of Urban Agriculture (North) of Ministry of Agriculture, Beijing University of Agriculture, Beijing 102206, PR China b
Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China
Corresponding author. Tel.: +86 10 62757953; fax: +86 10 62751708. E-mail address: [email protected] (Z.
Zhu).
ABSTRACT How to align the single-walled carbon nanotubes (SWCNTs) onto the electrode vertically and to control their density and orientation are still a major challenge. Theoretically, properly selected chiral SWCNTs can discriminate enantiomers through their different dielectric response to the adsorption of chiral species, few reports can confirm this theoretic model. Herein, we presented a new strategy to fabricate SWCNTs array-based electrochemical chiral sensor. Carboxylated chiral SWCNTs were vertically attached to the oxidized glass carbon electrode with ethylenediamine as a linker by electrosynthesis. The electrode surface was characterized with atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). The practicability of the sensor was validated by chirally recognizing 3,4-dihydroxyphenylalanine as a model molecule. We found that both the highly ordered standing of SWCNTs and the application of square wave voltammetry (SWV) amplified the intrinsic chirality of chiral SWCNTs. Graphical abstract
1
These authors contribute equally to this work 1 / 12
A new strategy to fabricate SWCNTs array-based electrochemical chiral sensor was presented.
Keywords: Carbon nanotubes array; Chiral carbon nanotubes; Electrochemical chiral sensor; Electrosynthesis; 3,4-dihydroxyphenylalanine 1. Introduction The study of chiral recognition is of great importance in clinical and pharmaceutical applications since the steric effect of compounds really matters a lot in the organism metabolism [1,2]. For the chiral analysis of medicine, many methods have been applied, such as the routine high-performance liquid chromatography [3,4] and capillary electrophoresis [5], etc. The electrochemical method draws much attention recently due to the advantages of uncomplicated operation, low cost and high sensitivity [6-10]. However, the enantioselective electrochemical sensor is merely a burgeoning field because of the fewer plate number for the enantiomers on the electrode surface. So it’s imperative to increase the plate efficiency. With the development of nano materials and nanopore techniques, it was found that the smart design of chiral recognition space can amplify the difference between the electrochemical signals of enantiomers [11,12]. At present, the function of chiral electrochemical sensor mostly relies on the size effect of nano modification materials [12] or the chiral effect of the modified molecular on the electrode surface [1,10,13,14]. Since its discovery in 1991, carbon nanotube has been one of the frequently-used materials in the fabrication of the modified electrodes. Especially, the chiral carbon nanotube can be prepared by rolling up a graphite sheet into a seamless cylinder under the condition of the roll-up index being n≠m≠0 (n and m are two integers defining the characteristics of a tube according to the Hamada nomenclature), and its chiroptical activity owing to kink sites has been observed [15-16]. So the chiral carbon nanotubes possess both size effect and chiral space, and the micropore or nanopore 2 / 12
interface formed by its chiral surface can amplify the electrochemical signals effectively [1-3,17]. But most of the chiral carbon nanotubes were physically absorbed or adhered onto the electrode surface, and such a randomly orientation limited the formation of chiral space and then reduced the recognition effect of the chiral materials [2]. While the ordered carbon nanotubes array can further improve the electrochemical property and recognition space due to their high degree of order and high catalytic activity besides small diameter, high aspect ratio and superior conductivity [18-23]. Therefore, the vertically aligned single-walled carbon nanotubes (v-SWCNTs) array on the electrode surface is one of the research hotspots in the field of chiral recognition. Nevertheless, how to align the SWCNTs onto the electrode vertically and to control their density and orientation are still a major challenge. One of the two most commonly used methods for the fabrication of v-SWCNTs is chemical vapor deposition (CVD) [20,24-27] in which carbon nanotube grows vertically on the silicon substrates. Obviously, harsh growth conditions like high temperature were indispensable and the growth of carbon nanotubes is usually uncontrolled. The other method is connecting the carboxylated carbon nanotubes onto the substrate by covalent bond such as Au-S [28], amide bond [29], Si-O [30] and so on. Although this chemical method is frequently used in recent years, the procedure is rather complicated and long reaction time [22] or anhydrous oxygen-free environment is needed [31,32]. In this work, a new strategy was presented for the fabrication of v-SWCNTs. The v-SWCNTs array was successfully fabricated on glass carbon electrode (GCE) through a simple cyclic voltammetry (CV) technique, by which the carboxylated SWCNTs were attached orderly to the oxidized surface of GCE with ethylenediamine (EDA) as a double peptide bond linker and thus formed a monolayer SWCNTs array. And the utilization of chiral (6,5) SWCNTs make this array a chiral space which is expected to discriminate the enantiomers by electrochemical method. As a biological precursor of neurotransmitters in the brain, L-3,4-dihydroxyphenylalanine (L-DOPA), has been used in the treatment of Parkinson’s disease and plays a very important role in neurochemistry and clinic [2,10,11,33]. However, D-DOPA performs no active and even toxic property [10,34]. Therefore, it is of great necessity and significance to explore chiral recognition of DOPA enantiomers in biochemistry and pharmaceutics. Herein, as a model molecule, D- and L-DOPA were determined to validate the chiral recognition effect of the as-prepared chiral v-SWCNTs array modified electrode, and it shows effective chiral discrimination for DOPA enantiomers. 2. Materials and Methods 2.1. Apparatus Electrochemical measurements were conducted with a CHI 660C electrochemical workstation (Shanghai, China). The three-electrode system contains a GCE or modified GCE as working electrode, a platinum wire as auxiliary electrode and a saturated calomel reference electrode (SCE). All measurements were performed at room temperature (25 ± 2 oC). AFM results were achieved with Dimension Icon (Bruker AXS GmbH, Germany) using tapping mode in air atmosphere. XPS measurements were conducted with AXIS Ultra X-ray photoelectron spectrometer (Kratos, UK) using monochromatized Al KαX-ray as the excitation source and C1s (284.6 eV) as the reference. 3 / 12
2.2. Reagents Enantiomerically pure DOPA (99 %) and chiral (6,5) SWCNTs were purchased from Sigma Aldrich. The diameters of chiral (6,5) SWCNTs range in 0.7-0.9 nm. The carboxylated chiral (6,5) SWCNTs were prepared as follows. First, chiral (6,5) SWCNTs were placed in 2.6 M HNO3 and the solution was heated to reflux for 12 h. Then, after washed to neutral with ice water and dried, the SWCNTs were sonicated in strong acid solution containing H2SO4 and HNO3 (3:1 V/V) for 4 h while the temperature was maintained at 0 oC. Last, the mixture was filtered, followed by the carboxylated (6,5) SWCNTs being washed with water to neutral and dried at 90 oC. After these treatments, the chiral (6,5) SWCNTs were cut to be shorter and much carboxyl emerged on their ends. All other reagents were of analytical grade and used without further purification. All aqueous solution was prepared with triply distilled water. 2.3. Procedure First, prior to modification, the GCE (3 mm diameter) was polished with 0.1 μm and 0.05 μm alumina slurry, and washed ultrasonically in distilled water and ethanol for 1 minute, respectively. Second, the well-polished GCE was oxidized in the solution containing 2.5 % K2Cr2O7 and 10% HNO3 (wt %) using potentiostatic method at +1.5 V for 15 s. Third, the oxidized GCE was washed and immersed in 5 mM ethylenediamine (EDA) solution containing 0.1 M KCl, and CV was implemented for 10 to 60 cycles in the range of 0-1.9 V (vs. SCE). Then, the EDA-GCE was washed ultrasonically in triply distilled water and ethanol to remove the free EDA molecular adsorbed on the electrode surface. After that, the electrode was immersed in aqueous KCl solution containing 0.4 g/L carboxylated chiral (6,5) SWCNTs which had been previously sonicated for 1 min. Last, CV was implemented for 10 to 60 cycles in the range of 0-1.9 V (vs. SCE), thus a SWCNTs-EDA-GCE was prepared. 3. Results and discussion 3.1. Electrosynthesis strategy and characterization of the chiral interface The electrosynthesis strategy of the chiral v-SWCNTs was presented in Fig. 1. First, the carboxylated chiral (6,5) SWCNTs was gotten based on classical method. Then, the bare GCE was oxidized by potentiostatic method in K2Cr2O7 and HNO3 solution, through which the surface of GCE was of carboxylation. In the following step, CV was implemented on the carboxylated electrode with EDA-containing electrolyte solution. As electrochemical method has been widely applied to the amide formation [35-37]. Especially, CV method can be simply performed for the electrosynthesis [36]. There is research to show that anodic oxidation of carboxyl group can produce the radical intermediate, which then is attacked by the -NH2 group to form the amide [38]. To improve the efficiency of anodic oxidation, 1.9 V of high potential for CV was chosen. Thus EDA was connected to the electrode by a condensation reaction between carboxyl on GCE and one of the two amino of EDA. Last, the electrode was immersed in the carboxylated chiral (6,5) SWCNTs solution, followed with CV scan for 10-60 cycles. Now the chiral SWCNTs were linked to the other amino of EDA and thus a SWCNTs-EDA-GCE was prepared. Herein, the effect of CV scan cycle number on the fabrication was carefully investigated and the optimal number was 40 (as shown in Fig. S1). In addition, after 0, 10, 20 or 40 cycles of CV scan, the AFM was applied to 4 / 12
characterize the change of the electrode surface, and their respective images were shown in Fig. 2. As can be seen, the EDA-GCE shows no ordered interface (Fig. 2A). However, along with the electrochemical scans in chiral SWCNTs solution, the surface was gradually getting ordered (Fig. 2B,C,D) despite of few changes in height direction. And visually, the EDA-GCE and the SWCNTs-EDA-GCE present bright yellow and dark blue with gem luster, respectively. This is a direct evidence that SWCNTs are successfully attached to GCE with the help of EDA as a bridge. The comparison of XPS results of EDA-GCE and SWCNTS-EDA-GCE also demonstrates the SWCNT on the surface (Fig. 3). With the C-1s peak of external carbon as a benchmark (284.5 eV), the binding energy of the three C-1s peaks are 286.3, 287.5 and 289.5 eV, respectively. Among them, the first peak attributes to the carbon next to oxygen namely “C-O” and the others are respectively assigned to the carbons of amide and carboxyl [39-41]. Compared with the EDA-GCE, the carbons of amide and carboxyl become more abundant in the SWCNTS-EDA-GCE, which proves that amide or ester group generates on the electrode surface during the electrosynthesis process. Besides the fact that the peak of carboxylated SWCNTs multiply sharply, all the results illustrate that the carboxylated SWCNTs have been connected to the electrode surface via electrosynthesis. Unlike the chemical synthesis method mentioned above, this electrochemical method simply uses multiple CV scans to get a v-SWCNTs modified GCE without any other complicated treatment, and even more the morphology and contents of modification can be easily controlled by adjusting the cycle number of CV. With the utilization of chiral SWCNTs, the space formed on the electrode surface is orientated which is expected to favor the chiral recognition as demonstrated in many reports [2,42]. 3.2. Electrochemical response of DOPA on chiral (6,5) SWCNTs array-based electrode Basically, due to their intrinsical chirality, chiral SWCNTs should be able to show great potential in sensing the chirality of enantiomeric pairs [43]. Vardanega et al. have conducted careful calculations to demonstrate that chiral SWCNTs used in a resonator configuration can discriminate the enantiomers of chiral molecules through their different interaction energy and dielectric responses [44,45]. However, under a real environment, such differences are rather tiny to be negligible, and chiral SWCNTs alone are not able to efficiently recognize enantiomers. Hence chiral selector modified SWCNTs might have a promising application in resolution of chiral compounds [43]. Besides, the implementation of certain electrochemical techniques would also help with the enhancement of chiral recognition [2]. In our previous report, square wave voltammetry (SWV) was applied in the determination of D- and L-DOPA since SWV rather than CV can extend the interaction time between DOPA enantiomer and chiral SWCNTs, and thus making the recognition site reused, which can amplify the efficiency of chiral recognition [2]. In this study, the CV curves for either D- or L-DOPA are almost the same no matter which kind of GCE is used (Fig. 4A). However, the SWV peak potential of DOPA is around 0.56 V at the oxidized GCE bearing a great number of carboxyl and with this electrode the redox peak currents of D- and L-DOPA are consistent showing no chiral recognition (Fig. 4B). The peak current and background current at the EDA-GCE increase obviously because it may be the electrostatic attraction between the charged EDA and DOPA that facilitates the redox of DOPA at the electrode. The enhancement of peak currents further confirms 5 / 12
that EDA has been connected to the carboxyl on the electrode by covalent bond during the CV process. But no difference appears for the enantiomers of DOPA at the EDA-GCE either, since there’s no chiral recognition space yet. As for the SWCNTs-EDA-GCE, the peak currents of Land D-DOPA are larger than that at EDA-GCE, which should be owing to the good conductivity and electronic tunneling effect of SWCNTs. These results also prove that the amide bond between carboxylated SWCNTs and EDA can be achieved by such a simple electrosynthesis method. In contrast, the same SWV scans were directly performed in the carboxylated SWCNTs solution with the oxidized GCE. After that, the electrode did not show dark blue with gem luster, and more importantly, the peak currents of DOPA did not increase, again showing that EDA plays a crucial role as a linker between GCE and carboxylated SWCNTs. 3.3. Analytical application Despite that both the redox currents of the enantiomers increase obviously at the SWCNTs-EDA-GCE and that the current corresponding to D-DOPA is 1.9 times that of L-DOPA, the difference between the two enantiomers should be sufficient for the chiral determination of DOPA analytes. In our previous report regarding the chiral recognition of DOPA using chiral (6,5) SWCNTs modified GCE [2], the SWCNTs were adsorbed physically onto the GCE, and consequently either modification amount or distribution was uncontrollable. Also, larger amount of SWCNTs was needed since the modifications fall off easily. Nevertheless, with the proposed electrosynthesis method, a monolayer of SWCNTs is formed on the GCE with very small amount of SWCNTs. More importantly, the ordered arrangement of v-SWCNTs improves the formation efficiency of chiral space, and as a result, the peak current ratio of the enantiomers reaches up to 2. Additionally, the chiral (6,5) SWCNTs array-based electrode exhibits excellent stability because of the covalently link between the carboxylated chiral (6,5) SWCNTs and GCE, and specifically the chiral recognition of the modified electrode kept constant after a lapse of one month stored in refrigerator of 5 oC. In a word, the chiral (6,5) SWCNTs array-based electrode prepared by the electrosynthesis method possesses the advantages of high stability, high effectiveness, high sensitivity and good economy. For a better recognition effect, the measurement conditions were optimized. The surface of SWCNTs may adsorb DOPA by the π-π stacking interaction between SWCNT and the benzene ring in DOPA, and the intensity of adsorption is closely related to the steric hindrance. So the difference of adsorption can be used to promote the chiral recognition. Since scan rate has important influence on the adsorption time and potential increment determines the scan rate under certain conditions in SWV, potential increment was varied for optimization (Fig. S2). According to our results, the peak current ratio of D- and L-DOPA increases with the potential increment ranging from 2 to 25 mV. Because the molecular with favorable spatial orientation can be adsorbed in shorter time and shows higher current compared with its enantiomer, the difference is more obvious at high scan rate. However, when potential increment comes to 32 mV, the ratio of DOPA enantiomers decreases distinctly since some of the reactants has no enough time to spread to the diffusion layer of the electrode and thus the difference in the following adsorption cannot be taken advantage fully. Moreover, both the currents of D- and L-DOPA decrease with potential increment increasing, which adds proof to the illustration above. Considering adsorption time and 6 / 12
peak current ratio, 25 mV was chosen as the optimal. On the other hand, the concentration of DOPA has also important effect on the adsorption equilibrium. In fact, the chiral recognition originates from the discrepancy of adsorption amount scilicet the spatial interaction between the enantiomers and the modified electrode, which is presented as the difference of peak current. Therefore, with high concentration of DOPA, the adsorption gets to equilibrium quickly but the current difference caused by the conformation will decline. While the concentration is too low, the response of electrode will be insensitive because of small amount of molecular in the diffusion layer. As predicted, the experimental results (Fig. S3) show highest ratio of peak current when the concentration of DOPA is 12.5 μM, and it was chosen as the optimal. Similar to our previous report [2], the chiral SWCNTs array-based electrode shows almost no chiral recognition in the solution of phosphate buffer solution (pH 7.0) or low concentration of sulfuric acid (0.05 M), and the peak current ratio increases constantly with the sulfuric acid concentration. It means that the adsorbed bisulfate/sulfate on the chiral SWCNTs increases the surface roughness leading to more efficient chiral recognition, the results were shown in Fig.S4. Considering the influence of strong acid on the stability of amide bond, 0.25 M sulfuric acid was chosen as the background solution. Under the optimized conditions, the DOPA analytes with different enantiomeric fraction (EF value) of D-DOPA were detected with the SWCNTs-EDA-GCE (Fig. 5A). The peak currents show a good linear dependence on the EF value with the correlation coefficient of 0.995 (Fig. 5B). Thus it can be seen, the as-prepared v-SWCNT array-based GCE can distinguish DOPA enantiomers and show practical value and analytical application prospects. 4. Conclusions In summary, a novel, simple and facile electrosynthesis method for the fabrication of chiral SWCNTs array on the GCE was developed. With EDA as a linker, SWCNTs were controllably connected to the GCE surface via CV technique. The fabrication conditions were optimized thoroughly and the morphology change in the electrosynthesis process was analyzed. The as-prepared chiral (6,5) SWCNTs array-based electrode shows excellent recognition effect for the enantiomers of DOPA with the SWV technique. Overall, in the solution containing high concentration of H2SO4, the peak current ratio between D- and L-DOPA reached 2.55. This novel strategy enriches the fabrication of SWCNTs array and the preparation of chiral interface at electrodes. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC, No. 21475002; 21335001 and 21335001 and 21207077) and the Research Project of Beijing Educational Committee (No.KM201610020002). References [1] X. Chen, C. Sun, F. Jiao, J. Yu and X. Jiang, Current Anal. Chem., 2014, 10, 267-270. [2] L. Chen, F. Chang, L. Meng, M. Li and Z. Zhu, Analyst, 2014, 139, 2243-2248. 7 / 12
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Figure Legends: Fig. 1. Schematic representation of the sequential electrosynthesis of v-SWCNTs on GCE. Fig. 2. AFM characterization results of the changes in the electrosynthesis of SWCNTs-EDA-GCE. The number of CV scan: (A) 0, (B) 10, (C) 20, (D) 40. Fig. 3. XPS spectra of EDA-GCE (A,B) and SWCNTs-EDA-GCE (A,C). Fig. 4. (A) CVs of 50 μM D- or L-DOPA at GCE, EDA-GCE and SWCNTs-EDA-GCE. (B) SWVs of 50 μM D- or L-DOPA at GCE, EDA-GCE and SWCNTs-EDA-GCE. The background solution is 0.25 M H2SO4. SWV potential increments: 0.025 V; frequency: 15 Hz; quiet time: 2s. Fig. 5. SWVs of 12.5μM DOPA with different EF value of D-DOPA. The background solution is 0.25 M H2SO4. SWV potential increments: 0.025 V; frequency: 15 Hz; quiet time: 2s. Inset: The relationship curve of peak current and EF value of D-DOPA.
Highlights
A new strategy to fabricate SWCNTs array-based electrochemical chiral sensor was presented.
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Its practicability was validated by chirally recognizing 3,4-dihydroxyphenylalanine as a model molecule.
This method enriches the fabrication of SWCNTs array and the preparation of chiral interface at electrodes.
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S0039-9140(17)30135-2 http://dx.doi.org/10.1016/j.talanta.2017.01.036 TAL17218
To appear in: Talanta Received date: 25 November 2016 Revised date: 8 January 2017 Accepted date: 12 January 2017 Cite this article as: Hong Zhu, Fengxia Chang and Zhiwei Zhu, The fabrication of carbon nanotubes array-based electrochemical chiral sensor by electrosynthesis, Talanta, http://dx.doi.org/10.1016/j.talanta.2017.01.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The fabrication of carbon nanotubes array-based electrochemical chiral sensor by electrosynthesis Hong Zhua1, Fengxia Changb1, Zhiwei Zhub* a
Key Laboratory of Urban Agriculture (North) of Ministry of Agriculture, Beijing University of Agriculture, Beijing 102206, PR China b
Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China
Corresponding author. Tel.: +86 10 62757953; fax: +86 10 62751708. E-mail address: [email protected] (Z.
Zhu).
ABSTRACT How to align the single-walled carbon nanotubes (SWCNTs) onto the electrode vertically and to control their density and orientation are still a major challenge. Theoretically, properly selected chiral SWCNTs can discriminate enantiomers through their different dielectric response to the adsorption of chiral species, few reports can confirm this theoretic model. Herein, we presented a new strategy to fabricate SWCNTs array-based electrochemical chiral sensor. Carboxylated chiral SWCNTs were vertically attached to the oxidized glass carbon electrode with ethylenediamine as a linker by electrosynthesis. The electrode surface was characterized with atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). The practicability of the sensor was validated by chirally recognizing 3,4-dihydroxyphenylalanine as a model molecule. We found that both the highly ordered standing of SWCNTs and the application of square wave voltammetry (SWV) amplified the intrinsic chirality of chiral SWCNTs. Graphical abstract
1
These authors contribute equally to this work 1 / 12
A new strategy to fabricate SWCNTs array-based electrochemical chiral sensor was presented.
Keywords: Carbon nanotubes array; Chiral carbon nanotubes; Electrochemical chiral sensor; Electrosynthesis; 3,4-dihydroxyphenylalanine 1. Introduction The study of chiral recognition is of great importance in clinical and pharmaceutical applications since the steric effect of compounds really matters a lot in the organism metabolism [1,2]. For the chiral analysis of medicine, many methods have been applied, such as the routine high-performance liquid chromatography [3,4] and capillary electrophoresis [5], etc. The electrochemical method draws much attention recently due to the advantages of uncomplicated operation, low cost and high sensitivity [6-10]. However, the enantioselective electrochemical sensor is merely a burgeoning field because of the fewer plate number for the enantiomers on the electrode surface. So it’s imperative to increase the plate efficiency. With the development of nano materials and nanopore techniques, it was found that the smart design of chiral recognition space can amplify the difference between the electrochemical signals of enantiomers [11,12]. At present, the function of chiral electrochemical sensor mostly relies on the size effect of nano modification materials [12] or the chiral effect of the modified molecular on the electrode surface [1,10,13,14]. Since its discovery in 1991, carbon nanotube has been one of the frequently-used materials in the fabrication of the modified electrodes. Especially, the chiral carbon nanotube can be prepared by rolling up a graphite sheet into a seamless cylinder under the condition of the roll-up index being n≠m≠0 (n and m are two integers defining the characteristics of a tube according to the Hamada nomenclature), and its chiroptical activity owing to kink sites has been observed [15-16]. So the chiral carbon nanotubes possess both size effect and chiral space, and the micropore or nanopore 2 / 12
interface formed by its chiral surface can amplify the electrochemical signals effectively [1-3,17]. But most of the chiral carbon nanotubes were physically absorbed or adhered onto the electrode surface, and such a randomly orientation limited the formation of chiral space and then reduced the recognition effect of the chiral materials [2]. While the ordered carbon nanotubes array can further improve the electrochemical property and recognition space due to their high degree of order and high catalytic activity besides small diameter, high aspect ratio and superior conductivity [18-23]. Therefore, the vertically aligned single-walled carbon nanotubes (v-SWCNTs) array on the electrode surface is one of the research hotspots in the field of chiral recognition. Nevertheless, how to align the SWCNTs onto the electrode vertically and to control their density and orientation are still a major challenge. One of the two most commonly used methods for the fabrication of v-SWCNTs is chemical vapor deposition (CVD) [20,24-27] in which carbon nanotube grows vertically on the silicon substrates. Obviously, harsh growth conditions like high temperature were indispensable and the growth of carbon nanotubes is usually uncontrolled. The other method is connecting the carboxylated carbon nanotubes onto the substrate by covalent bond such as Au-S [28], amide bond [29], Si-O [30] and so on. Although this chemical method is frequently used in recent years, the procedure is rather complicated and long reaction time [22] or anhydrous oxygen-free environment is needed [31,32]. In this work, a new strategy was presented for the fabrication of v-SWCNTs. The v-SWCNTs array was successfully fabricated on glass carbon electrode (GCE) through a simple cyclic voltammetry (CV) technique, by which the carboxylated SWCNTs were attached orderly to the oxidized surface of GCE with ethylenediamine (EDA) as a double peptide bond linker and thus formed a monolayer SWCNTs array. And the utilization of chiral (6,5) SWCNTs make this array a chiral space which is expected to discriminate the enantiomers by electrochemical method. As a biological precursor of neurotransmitters in the brain, L-3,4-dihydroxyphenylalanine (L-DOPA), has been used in the treatment of Parkinson’s disease and plays a very important role in neurochemistry and clinic [2,10,11,33]. However, D-DOPA performs no active and even toxic property [10,34]. Therefore, it is of great necessity and significance to explore chiral recognition of DOPA enantiomers in biochemistry and pharmaceutics. Herein, as a model molecule, D- and L-DOPA were determined to validate the chiral recognition effect of the as-prepared chiral v-SWCNTs array modified electrode, and it shows effective chiral discrimination for DOPA enantiomers. 2. Materials and Methods 2.1. Apparatus Electrochemical measurements were conducted with a CHI 660C electrochemical workstation (Shanghai, China). The three-electrode system contains a GCE or modified GCE as working electrode, a platinum wire as auxiliary electrode and a saturated calomel reference electrode (SCE). All measurements were performed at room temperature (25 ± 2 oC). AFM results were achieved with Dimension Icon (Bruker AXS GmbH, Germany) using tapping mode in air atmosphere. XPS measurements were conducted with AXIS Ultra X-ray photoelectron spectrometer (Kratos, UK) using monochromatized Al KαX-ray as the excitation source and C1s (284.6 eV) as the reference. 3 / 12
2.2. Reagents Enantiomerically pure DOPA (99 %) and chiral (6,5) SWCNTs were purchased from Sigma Aldrich. The diameters of chiral (6,5) SWCNTs range in 0.7-0.9 nm. The carboxylated chiral (6,5) SWCNTs were prepared as follows. First, chiral (6,5) SWCNTs were placed in 2.6 M HNO3 and the solution was heated to reflux for 12 h. Then, after washed to neutral with ice water and dried, the SWCNTs were sonicated in strong acid solution containing H2SO4 and HNO3 (3:1 V/V) for 4 h while the temperature was maintained at 0 oC. Last, the mixture was filtered, followed by the carboxylated (6,5) SWCNTs being washed with water to neutral and dried at 90 oC. After these treatments, the chiral (6,5) SWCNTs were cut to be shorter and much carboxyl emerged on their ends. All other reagents were of analytical grade and used without further purification. All aqueous solution was prepared with triply distilled water. 2.3. Procedure First, prior to modification, the GCE (3 mm diameter) was polished with 0.1 μm and 0.05 μm alumina slurry, and washed ultrasonically in distilled water and ethanol for 1 minute, respectively. Second, the well-polished GCE was oxidized in the solution containing 2.5 % K2Cr2O7 and 10% HNO3 (wt %) using potentiostatic method at +1.5 V for 15 s. Third, the oxidized GCE was washed and immersed in 5 mM ethylenediamine (EDA) solution containing 0.1 M KCl, and CV was implemented for 10 to 60 cycles in the range of 0-1.9 V (vs. SCE). Then, the EDA-GCE was washed ultrasonically in triply distilled water and ethanol to remove the free EDA molecular adsorbed on the electrode surface. After that, the electrode was immersed in aqueous KCl solution containing 0.4 g/L carboxylated chiral (6,5) SWCNTs which had been previously sonicated for 1 min. Last, CV was implemented for 10 to 60 cycles in the range of 0-1.9 V (vs. SCE), thus a SWCNTs-EDA-GCE was prepared. 3. Results and discussion 3.1. Electrosynthesis strategy and characterization of the chiral interface The electrosynthesis strategy of the chiral v-SWCNTs was presented in Fig. 1. First, the carboxylated chiral (6,5) SWCNTs was gotten based on classical method. Then, the bare GCE was oxidized by potentiostatic method in K2Cr2O7 and HNO3 solution, through which the surface of GCE was of carboxylation. In the following step, CV was implemented on the carboxylated electrode with EDA-containing electrolyte solution. As electrochemical method has been widely applied to the amide formation [35-37]. Especially, CV method can be simply performed for the electrosynthesis [36]. There is research to show that anodic oxidation of carboxyl group can produce the radical intermediate, which then is attacked by the -NH2 group to form the amide [38]. To improve the efficiency of anodic oxidation, 1.9 V of high potential for CV was chosen. Thus EDA was connected to the electrode by a condensation reaction between carboxyl on GCE and one of the two amino of EDA. Last, the electrode was immersed in the carboxylated chiral (6,5) SWCNTs solution, followed with CV scan for 10-60 cycles. Now the chiral SWCNTs were linked to the other amino of EDA and thus a SWCNTs-EDA-GCE was prepared. Herein, the effect of CV scan cycle number on the fabrication was carefully investigated and the optimal number was 40 (as shown in Fig. S1). In addition, after 0, 10, 20 or 40 cycles of CV scan, the AFM was applied to 4 / 12
characterize the change of the electrode surface, and their respective images were shown in Fig. 2. As can be seen, the EDA-GCE shows no ordered interface (Fig. 2A). However, along with the electrochemical scans in chiral SWCNTs solution, the surface was gradually getting ordered (Fig. 2B,C,D) despite of few changes in height direction. And visually, the EDA-GCE and the SWCNTs-EDA-GCE present bright yellow and dark blue with gem luster, respectively. This is a direct evidence that SWCNTs are successfully attached to GCE with the help of EDA as a bridge. The comparison of XPS results of EDA-GCE and SWCNTS-EDA-GCE also demonstrates the SWCNT on the surface (Fig. 3). With the C-1s peak of external carbon as a benchmark (284.5 eV), the binding energy of the three C-1s peaks are 286.3, 287.5 and 289.5 eV, respectively. Among them, the first peak attributes to the carbon next to oxygen namely “C-O” and the others are respectively assigned to the carbons of amide and carboxyl [39-41]. Compared with the EDA-GCE, the carbons of amide and carboxyl become more abundant in the SWCNTS-EDA-GCE, which proves that amide or ester group generates on the electrode surface during the electrosynthesis process. Besides the fact that the peak of carboxylated SWCNTs multiply sharply, all the results illustrate that the carboxylated SWCNTs have been connected to the electrode surface via electrosynthesis. Unlike the chemical synthesis method mentioned above, this electrochemical method simply uses multiple CV scans to get a v-SWCNTs modified GCE without any other complicated treatment, and even more the morphology and contents of modification can be easily controlled by adjusting the cycle number of CV. With the utilization of chiral SWCNTs, the space formed on the electrode surface is orientated which is expected to favor the chiral recognition as demonstrated in many reports [2,42]. 3.2. Electrochemical response of DOPA on chiral (6,5) SWCNTs array-based electrode Basically, due to their intrinsical chirality, chiral SWCNTs should be able to show great potential in sensing the chirality of enantiomeric pairs [43]. Vardanega et al. have conducted careful calculations to demonstrate that chiral SWCNTs used in a resonator configuration can discriminate the enantiomers of chiral molecules through their different interaction energy and dielectric responses [44,45]. However, under a real environment, such differences are rather tiny to be negligible, and chiral SWCNTs alone are not able to efficiently recognize enantiomers. Hence chiral selector modified SWCNTs might have a promising application in resolution of chiral compounds [43]. Besides, the implementation of certain electrochemical techniques would also help with the enhancement of chiral recognition [2]. In our previous report, square wave voltammetry (SWV) was applied in the determination of D- and L-DOPA since SWV rather than CV can extend the interaction time between DOPA enantiomer and chiral SWCNTs, and thus making the recognition site reused, which can amplify the efficiency of chiral recognition [2]. In this study, the CV curves for either D- or L-DOPA are almost the same no matter which kind of GCE is used (Fig. 4A). However, the SWV peak potential of DOPA is around 0.56 V at the oxidized GCE bearing a great number of carboxyl and with this electrode the redox peak currents of D- and L-DOPA are consistent showing no chiral recognition (Fig. 4B). The peak current and background current at the EDA-GCE increase obviously because it may be the electrostatic attraction between the charged EDA and DOPA that facilitates the redox of DOPA at the electrode. The enhancement of peak currents further confirms 5 / 12
that EDA has been connected to the carboxyl on the electrode by covalent bond during the CV process. But no difference appears for the enantiomers of DOPA at the EDA-GCE either, since there’s no chiral recognition space yet. As for the SWCNTs-EDA-GCE, the peak currents of Land D-DOPA are larger than that at EDA-GCE, which should be owing to the good conductivity and electronic tunneling effect of SWCNTs. These results also prove that the amide bond between carboxylated SWCNTs and EDA can be achieved by such a simple electrosynthesis method. In contrast, the same SWV scans were directly performed in the carboxylated SWCNTs solution with the oxidized GCE. After that, the electrode did not show dark blue with gem luster, and more importantly, the peak currents of DOPA did not increase, again showing that EDA plays a crucial role as a linker between GCE and carboxylated SWCNTs. 3.3. Analytical application Despite that both the redox currents of the enantiomers increase obviously at the SWCNTs-EDA-GCE and that the current corresponding to D-DOPA is 1.9 times that of L-DOPA, the difference between the two enantiomers should be sufficient for the chiral determination of DOPA analytes. In our previous report regarding the chiral recognition of DOPA using chiral (6,5) SWCNTs modified GCE [2], the SWCNTs were adsorbed physically onto the GCE, and consequently either modification amount or distribution was uncontrollable. Also, larger amount of SWCNTs was needed since the modifications fall off easily. Nevertheless, with the proposed electrosynthesis method, a monolayer of SWCNTs is formed on the GCE with very small amount of SWCNTs. More importantly, the ordered arrangement of v-SWCNTs improves the formation efficiency of chiral space, and as a result, the peak current ratio of the enantiomers reaches up to 2. Additionally, the chiral (6,5) SWCNTs array-based electrode exhibits excellent stability because of the covalently link between the carboxylated chiral (6,5) SWCNTs and GCE, and specifically the chiral recognition of the modified electrode kept constant after a lapse of one month stored in refrigerator of 5 oC. In a word, the chiral (6,5) SWCNTs array-based electrode prepared by the electrosynthesis method possesses the advantages of high stability, high effectiveness, high sensitivity and good economy. For a better recognition effect, the measurement conditions were optimized. The surface of SWCNTs may adsorb DOPA by the π-π stacking interaction between SWCNT and the benzene ring in DOPA, and the intensity of adsorption is closely related to the steric hindrance. So the difference of adsorption can be used to promote the chiral recognition. Since scan rate has important influence on the adsorption time and potential increment determines the scan rate under certain conditions in SWV, potential increment was varied for optimization (Fig. S2). According to our results, the peak current ratio of D- and L-DOPA increases with the potential increment ranging from 2 to 25 mV. Because the molecular with favorable spatial orientation can be adsorbed in shorter time and shows higher current compared with its enantiomer, the difference is more obvious at high scan rate. However, when potential increment comes to 32 mV, the ratio of DOPA enantiomers decreases distinctly since some of the reactants has no enough time to spread to the diffusion layer of the electrode and thus the difference in the following adsorption cannot be taken advantage fully. Moreover, both the currents of D- and L-DOPA decrease with potential increment increasing, which adds proof to the illustration above. Considering adsorption time and 6 / 12
peak current ratio, 25 mV was chosen as the optimal. On the other hand, the concentration of DOPA has also important effect on the adsorption equilibrium. In fact, the chiral recognition originates from the discrepancy of adsorption amount scilicet the spatial interaction between the enantiomers and the modified electrode, which is presented as the difference of peak current. Therefore, with high concentration of DOPA, the adsorption gets to equilibrium quickly but the current difference caused by the conformation will decline. While the concentration is too low, the response of electrode will be insensitive because of small amount of molecular in the diffusion layer. As predicted, the experimental results (Fig. S3) show highest ratio of peak current when the concentration of DOPA is 12.5 μM, and it was chosen as the optimal. Similar to our previous report [2], the chiral SWCNTs array-based electrode shows almost no chiral recognition in the solution of phosphate buffer solution (pH 7.0) or low concentration of sulfuric acid (0.05 M), and the peak current ratio increases constantly with the sulfuric acid concentration. It means that the adsorbed bisulfate/sulfate on the chiral SWCNTs increases the surface roughness leading to more efficient chiral recognition, the results were shown in Fig.S4. Considering the influence of strong acid on the stability of amide bond, 0.25 M sulfuric acid was chosen as the background solution. Under the optimized conditions, the DOPA analytes with different enantiomeric fraction (EF value) of D-DOPA were detected with the SWCNTs-EDA-GCE (Fig. 5A). The peak currents show a good linear dependence on the EF value with the correlation coefficient of 0.995 (Fig. 5B). Thus it can be seen, the as-prepared v-SWCNT array-based GCE can distinguish DOPA enantiomers and show practical value and analytical application prospects. 4. Conclusions In summary, a novel, simple and facile electrosynthesis method for the fabrication of chiral SWCNTs array on the GCE was developed. With EDA as a linker, SWCNTs were controllably connected to the GCE surface via CV technique. The fabrication conditions were optimized thoroughly and the morphology change in the electrosynthesis process was analyzed. The as-prepared chiral (6,5) SWCNTs array-based electrode shows excellent recognition effect for the enantiomers of DOPA with the SWV technique. Overall, in the solution containing high concentration of H2SO4, the peak current ratio between D- and L-DOPA reached 2.55. This novel strategy enriches the fabrication of SWCNTs array and the preparation of chiral interface at electrodes. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC, No. 21475002; 21335001 and 21335001 and 21207077) and the Research Project of Beijing Educational Committee (No.KM201610020002). References [1] X. Chen, C. Sun, F. Jiao, J. Yu and X. Jiang, Current Anal. Chem., 2014, 10, 267-270. [2] L. Chen, F. Chang, L. Meng, M. Li and Z. Zhu, Analyst, 2014, 139, 2243-2248. 7 / 12
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Figure Legends: Fig. 1. Schematic representation of the sequential electrosynthesis of v-SWCNTs on GCE. Fig. 2. AFM characterization results of the changes in the electrosynthesis of SWCNTs-EDA-GCE. The number of CV scan: (A) 0, (B) 10, (C) 20, (D) 40. Fig. 3. XPS spectra of EDA-GCE (A,B) and SWCNTs-EDA-GCE (A,C). Fig. 4. (A) CVs of 50 μM D- or L-DOPA at GCE, EDA-GCE and SWCNTs-EDA-GCE. (B) SWVs of 50 μM D- or L-DOPA at GCE, EDA-GCE and SWCNTs-EDA-GCE. The background solution is 0.25 M H2SO4. SWV potential increments: 0.025 V; frequency: 15 Hz; quiet time: 2s. Fig. 5. SWVs of 12.5μM DOPA with different EF value of D-DOPA. The background solution is 0.25 M H2SO4. SWV potential increments: 0.025 V; frequency: 15 Hz; quiet time: 2s. Inset: The relationship curve of peak current and EF value of D-DOPA.
Highlights
A new strategy to fabricate SWCNTs array-based electrochemical chiral sensor was presented.
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Its practicability was validated by chirally recognizing 3,4-dihydroxyphenylalanine as a model molecule.
This method enriches the fabrication of SWCNTs array and the preparation of chiral interface at electrodes.
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