Chiral PEDOT-Based Enantioselective Electrode Modification Material

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A chiral PEDOT-based enantioselective electrode modification material for chiral electrochemical sensing: Mechanism and model of chiral recognition Liqi Dong, Youshan Zhang, Xuemin Duan, Xiaofei Zhu, Hui Sun, and Jingkun Xu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b01095 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 16, 2017

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Analytical Chemistry

A chiral PEDOT-based enantioselective electrode modification material for chiral electrochemical sensing: Mechanism and model of chiral recognition Liqi Dong, Youshan Zhang, Xuemin Duan*, Xiaofei Zhu, Hui Sun and Jingkun Xu* School of Pharmacy, Jiangxi Science & Technology Normal University, Nanchang, 330013, China. *E-mail: [email protected], [email protected], Fax: +86-791-83823320, Tel.: +86-791-88537967

ABSTRACT: The development of electrochemical methods for enantioselective recognition is a focus of research in pharmaceuticals and biotechnology. In this study, a pair of water-soluble chiral 3,4-ethylenedioxythiophene (EDOT) derivatives—(R)-2’hydroxymethyl-3,4-ethylenedioxythiophene ((R)-EDTM) and (S)-2’-hydroxymethyl-3,4-ethylenedioxythiophene ((S)-EDTM)— were synthesized and electrodeposited on the surface of a glassy carbon electrode (GCE) via current-time (I-t) polymerization in an aqueous LiClO4 electrolyte. These chiral PEDOT polymers were used to fabricate chiral sensors and to investigate the enantioselective recognition of D-/L-3,4-dihydroxyphenylalanine, D-/L-tryptophan and (R)-/(S)-propranolol enantiomers, respectively. The results indicated that the (R)-PEDTM/GCE sensor showed a higher peak current response towards the levo or (S) forms of the tested enantiomers, while the opposite phenomenon occurred for (S)-PEDTM/GCE. The mechanism of the stereospecific interaction between these enantiomers and the chiral polymers was determined. Thereon, a model of the chiral recognition by the chiral conducting polymer electrodes and an electrochemical method was proposed. The chirality of the enantiomers was confirmed by two parameters: the chirality of the electrode and the peak current response. These findings pave the way for the application of chiral PEDOT as electrode modification materials in the electrochemical chiral recognition field.

The chirality of various stereoisomers is important in the production of pharmaceuticals, where typically only one enantiomer possesses the desired pharmaceutical activity and the other exhibits no therapeutic value and may even induce serious side effects. Furthermore, single-enantiomer pharmaceutical ingredients are preferred over racemates.1, 2 Thus, chiral resolution is highly significant to the pharmaceutical industry and for clinical purposes. The development of convenient methods for differentiating enantiomers has long been a challenge for chemists. Recently, an electrochemical approach for the discrimination of chiral drugs was introduced, offering some advantages over other techniques, such as low cost, simple equipment, and facile miniaturization. The key to the design and construction of an enantioselective electrochemical sensor is the construction of a chiral surface on the electrode that can identify minute differences between two enantiomers. There are two classes of chiral electrodes: (i) those in which the electrode itself is chiral, such as Cu (643), Au (321), Ag (643), Pt (531), and Pt (321)35 , and (ii) those in which an achiral electrode is modified with chiral molecules. The latter approach has the advantages of being easily constructed and having a wide range of possible modification materials. Various surface modifications that allow enantioselective electrochemical recognition have been reported, and four types of surface modification materials are commonly used: (a) protein-type molecules6-9, which recognize chiral molecules by antigen-antibody interactions or receptor-ligand binding; (b) metal complexes10-14, for which chiral recognition is mostly

based on the principle of chiral ligand exchange; (c) carbonbased nanocomposites, such as graphene-supported platinum nanoparticles15, hollow carbon microspheres16, and singlewalled carbon nanotubes17; and (d) chiral polymers18-24, which harness various interactions with analytes, such as oxidation and reduction, protonation and deprotonation, reactions with nucleophilic agents, ion-exchange, adsorption, and complexation. Research on using chiral polymers for enantioselective electrochemical recognition began in 1988 via the electropolymerization of a thiophene monomer that was covalently substituted with chiral substituents at the 3-position. Soon after, chiral discrimination by a chiral polypyrrole was demonstrated by Moutet et al.19 In recent years, Fu et al. have developed a simple and efficient electrochemical chiral sensor for the recognition of 3,4-dihydroxyphenylalanine (DOPA)25,26, tyrosine13, and lysine27 based on the electropolymerization of an amino acid. Most recently, Zhu et al. established an electrochemical chiral sensing method with high sensitivity and selectivity for monosaccharides based on a stimuli-responsive copolymer/graphene-hybrid-modified screen-printed carbon electrode28. Their tunable structure (via attachment of functional groups to the polymer backbone) and ability to form particles, membranes, and micro- and nano-dimensional fibres18 has led to the emergence of chiral polymers as the most promising surface modification material for chiral electrodes. However, most of the reported studies of these materials focus only on the discovery of new materials for chiral recognition, rather than understanding the stereospecificity mechanisms of the interactions between test enantiomers and chiral polymers.

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Therefore, increased understanding of this mechanism is crucial and may provide inspiration for the design of novel surface modification materials or lead to the development of innovative methods of chiral recognition. Poly(3,4-ethylenedioxythiophene) (PEDOT), one of the most stable and promising conducting polymers (CPs) available, has been employed for chemo/biosensors because of its high electrical conductivity, low bandgap, and outstanding environmental stability29-31. Therefore, chiral PEDOT is a promising electrode modification material for electrochemical chiral recognition. We previously reported the syntheses of various chiral substituted PEDOT species, such as chloromethyl-32, L-leucine-33, (R)-/(S)-2-phenylpropionic acid-34 and D/L-methionine-substituted PEDOT35, by grafting a chiral moiety to a side-chain functionalized PEDOT (Scheme 1). This common and effective synthesis strategy counterbalances the achirality of PEDOT. These polymers were fully characterized using electrochemical and chiral discrimination techniques and were found to exhibit enantioselective and chiroptical properties. Nevertheless, their chiral recognition mechanism is still not completely understood. One of the main drawbacks of PEDOT is the poor solubility of the 3,4ethylenedioxythiophene (EDOT) monomer in water (14.77 mmol L-1)30, which restricts the applicability of PEDOT. Thus, we designed water-soluble chiral PEDOT derivatives to enable wider use of PEDOT-based electrode materials and to unravel the mechanism of electrochemical chiral recognition.

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EXPERIMENTAL SECTION Reagents and Materials 3,4-Dimethoxythiophene (99%; Vita Chemical Reagent Co., Ltd.), p-toluenesulfonic acid monohydrate (p-TSA, 99%; Aladdin Industrial Inc.), (R)-/(S)-3-chloro-1,2-propanediol (e.e.: 98%; Daicel Chiral Technologies Co., Ltd.), D-/L-3,4dihydroxyphenylalanine (e.e.: 99%; Aladdin Industrial Inc.), D-/L-tryptophan (e.e.: 99%; Aladdin Industrial Inc.), D-/Lpropranolol hydrochloride (e.e.: 99%; Aladdin Industrial Inc.), sodium acetate anhydrous (99%; Aladdin Industrial Inc.), ammonia solution (NH3·H2O, 25-28%; Aladdin Industrial Inc.), sulfuric acid (H2SO4, 98%; Aladdin Industrial Inc.), disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O, 99%; Sinopharm Chemical Reagent Co. Ltd.), sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O, 99%; Sinopharm Chemical Reagent Co. Ltd.) and dimethyl sulfoxide (DMSO, 99%; Linfeng Chemical Reagent Co. Ltd.) were used directly without further purification. Lithium perchlorate (LiClO4, 99%; Energy Chemical) was dried under vacuum at 60 °C for 24 h before use. Toluene (analytical grade; Xilong Chemical) was purified using sulfuric acid and dried with anhydrous calcium chloride. Indium tin oxide (ITO)-coated glass was purchased from Zhuhai Kaivo Optoelectronic Technology Co. Ltd. Monomers Synthesis (R)-/(S)-(2,3-Dihydrothieno[3,4-b][1,4]dioxin-2-yl)methanol ((R)-/(S)-EDTM) was synthesized as described in Scheme 2. (R)-/(S)-2-(chloromethyl)-2,3-dihydrothieno[3,4-b][1,4]dioxine ((R)-/(S)-EDOT-MeCl) was prepared according to previously reported procedures32, 36. (R)-/(S)-(2,3-Dihydrothieno[3,4-b][1,4]dioxin-2-yl)methyl Acetate ((R)-/(S)-EDOT-MeOAc) (R)-EDOT-MeCl (4.490 g, 23.55 mmol) or (S)-EDOT-MeCl (5.066 g, 26.57 mmol), sodium acetate (2.898 g, 35.33 mmol), and DMSO (80 mL) were added to a three-necked flask (250 mL capacity) equipped with a nitrogen purge valve. The solution was stirred for 3 h at 120 °C. The reaction mixture was poured into water and extracted with CH2Cl2. After removing the CH2Cl2 under reduced pressure, the remaining crude product was purified by column chromatography (silica gel, petroleum ether/acetic ether, 10/1, v/v) to yield 3.704 g (73%) of a white solid for (R)-EDOT-MeOAc or 5.297 g (70%) of a white solid for (S)-EDOT-MeOAc.

Scheme 1. Chemical structures of the chiral PEDOT derivatives.

(R)-EDOT-MeOAc：[α]D25 = -35.400 (c = 1 g/100 mL, in CHCl3), 1H NMR (400 MHz, CDCl3, ppm) δ: 6.37-6.34 (m, 2H), 4.39-4.29 (m, 1H), 4.24-4.21 (m, 3H), 4.07-4.02 (m, 1H), 2.11 (s, 3H). The 1H NMR spectrum is shown in Figure S1.

We synthesized a pair of EDOT enantiomers bearing hydroxymethyl groups in their side chains (i.e. (R)-EDTM and (S)-EDTM). These water-soluble monomers could be electropolymerized in aqueous LiClO4 solutions. Films of the polymer surface modification materials were obtained on glassy carbon electrodes (GCE) and successfully distinguished D-/LDOPA, D-/L-tryptophan (Trp) and (R)-/(S)-propranolol (Pro). Finally, we proposed a mechanism of chiral recognition based on our experimental results and established a model of chiral recognition based on the chiral conducting polymer electrodes and an electrochemical method.

(S)-EDOT-MeOAc：[α]D25 = +33.200 (c = 1 g/100 mL, in CHCl3), 1H NMR (400 MHz, CDCl3, ppm) δ: 6.37-6.34 (m, 2H), 4.39-4.29 (m, 1H), 4.24-4.21 (m, 3H), 4.06-4.02 (m, 1H), 2.11 (s, 3H). (Figure S2) (R)-/(S)-(2,3-Dihydrothieno[3,4-b][1,4]dioxin-2-yl)methanol ((R)-/(S)-EDTM) (R)-/(S)-EDOT-MeOAc (3.702 g, 17.28 mmol) was added to a solution of NaOH (2.000 g, 50.0 mmol) in water (50 mL) in a round bottom f1ask (100 mL capacity) equipped with a reflux condenser. The mixture was refluxed for 1 h and then cooled to room temperature. Water (30 mL) was added, and the mixture was acidified then extracted with CH2Cl2. The


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solvent was removed under reduced pressure, and the remaining crude product was purified by column chromatography (silica gel, petroleum ether/acetic ether, 10/1, v/v) to give 2.940 g (99%) of a white solid for (R)-EDTM or 2.802 g of a white solid (94%) for (S)-EDTM. (R)-EDTM: [α]D25 = +31.400 (c = 1 g/100 mL, in CHCl3), 1 H NMR (400 MHz, CDCl3, ppm) δ: 6.35 (s, 2H), 4.26-4.22 (m, 2H), 4.12-4.07 (m, 1H), 3.91-3.84 (m, 2H), 2.05 (d, 1H). (Figure S3) (S)-EDTM: [α]D25 = -32.400 (c = 1 g/100 mL, in CHCl3), 1H NMR (400 MHz, CDCl3, ppm) δ: 6.35 (s, 2H), 4.28-4.22 (m, 2H), 4.12-4.07 (m, 1H), 3.91-3.81 (m, 2H), 2.06 (m, 1H). (Figure S4) Electrosynthesis and Electrochemical Tests Electrochemical polymerization was carried out using a solution of (R)-/(S)-EDTM (0.02 M) in aqueous LiClO4 (0.1 M). All solutions were deaerated under dry nitrogen flow for 15 min before polymerization and maintained under a slight overpressure during the electrochemical experiments to avoid the effects of oxygen. After polymerization, the polymer films were washed repeatedly with double-distilled deionized water to remove the electrolyte and monomer/oligomers. Electrochemical experiments were carried out in a onecompartment electrolytic cell controlled by a CHI 660E electrochemistry workstation (Shanghai Chenhua Instruments Co., China). The electrolytic cell contained two Pt wires as the working and counter electrodes and an Ag/AgCl electrode as the reference electrode. The electrolyte was purged with nitrogen before each experiment. Characterization Pt and stainless-steel sheets with surface areas of 4 and 6 cm2 were employed as the working and counter electrodes, respectively, to obtain sufficient polymer for characterization. Ag/AgCl electrodes directly immersed in the solutions served as the reference electrodes. For spectral analyses, the polymer films were dedoped by exposure to 25% ammonia for three days and then washed repeatedly with pure water. Finally, the polymer film was dried at 60 °C under vacuum for 24 h. 1 H NMR spectra were recorded on a Bruker AV 400 NMR spectrometer with chloroform-d as the solvent and tetramethylsilane (TMS) as an internal standard. Infrared spectra (FTIR) were recorded using a Bruker Vertex 70 Fourier spectrometer with samples pressed into KBr pellets. Scanning electron microscopy (SEM) measurements were taken using a JSM6701F cold field emission scanning electron microscope with the polymer deposited on ITO-coated glass. Circular dichroism (CD) spectroscopy (JASCO J-720) was used to characterize the polymer films. Thermogravimetric analysis (TGA) was performed using a Pyris Diamond TG/DTA thermal analyzer (Perkin-Elmer) under a nitrogen stream; the samples were heated from 25 to 1100 °C at a rate of 10 °C min-1. Chiral Sensor Apparatus Cyclic voltammograms (CVs) and differential pulse voltammetry (DPV) measurements were recorded with a CHI 660E electrochemistry workstation. Prior to modification, the working electrode bare GCE was carefully polished with 0.05 mm alumina slurry until a mirror-shine surface was obtained, followed by successive sonication in doubly-distilled deionized water and ethanol, and then drying in air. (R)-

PEDTM/GCE and (S)-PEDTM/GCE were obtained individually by one-step electropolymerization of 0.02 M monomers on GCE at a constant potential of 1.20 V vs. SCE in an aqueous LiClO4 electrolyte over a deposition time of 20 s. Then, the films were repeatedly washed with double-distilled deionized water to remove any electrolyte and monomers and dried in air. All measurements were conducted at room temperature (25 ± 2 °C).

RESULTS AND DISCUSSION Monomer Synthesis Most EDOT derivatives are poorly water-soluble and can only be electropolymerized in an organic solvent, forming films that are toxic to biologically active species. However, we found that the aqueous solubility of EDOT could be improved by appending a hydrophilic pendant side group, e.g. a hydroxymethylated group, onto the backbone of EDOT, resulting in a water-soluble EDOT deriverative37. A racemic mixture of hydroxymethylated-3,4-ethylenedioxylthiophene (EDTM) was first synthesized in 1992 by Blohm et al.38 Then, in 1998, Ng et al. described a method of preparing EDTM involving a complex series of six reaction steps, starting from thiodiglycolic acid39. However, these synthetic methods could not be reproduced in good yields. Lu et al. and Zhang et al. reported that the low yields of the previous EDTM syntheses could be overcome using nucleophilic substitution reactions based on a versatile intermediate EDOT derivative—EDOT-MeCl40, 41. EDOT-MeCl was produced based on the previously reported acid-catalyzed trans-etherification reaction between 3,4dimethoxythiophene and 3-chloro-1,2-propanediol enantiomers. The resulting EDOT-MeCl transformed to the acetic acid ester, and then pure EDTM was obtained after hydrolysis (Scheme 2). Chirality was introduced in EDTM using (R)-/(S)3-chloro-1,2-propanediol as a chiral source with a known configuration.

Scheme 2. The route used to synthesize (R)-/(S)-EDTM enan-

Figure 1. Cyclic voltammograms (CVs) of (R)-EDTM (a) and (S)-EDTM (b) that were electropolymerized in an aqueous LiClO4 solution (0.1 M) of the monomers (0.02 M) at a potential scan rate of 50 mV s-1. Inset: the corresponding anodic polarization curve of (R)-/ (S)-EDTM.



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tiomers Electrochemical Polymerization Even though their poor aqueous solubility means that CPs are typically electropolymerized in an organic solvent and at a high potential, water is an excellent, inexpensive, and “green” solvent compared with other organic solvents used in the electrochemistry of CPs. Successive CVs of (R)-EDTM (A) and (S)-EDTM (B) were recorded in an aqueous LiClO4 electrolyte at a potential scan rate of 50 mV s-1 (Figure 1). The onset oxidation potential (Eox) of (R)-EDTM (0.99 V, Figure 1A inset) and (S)-EDTM (0.98 V, Figure 1B inset) were essentially equal, and both were lower than that of other EDOT derivative analogues such as (R)-/( S)-EDOT-MeCl (1.26 and 1.27 V vs. Ag/AgCl)32, 36, (R)-/( S)-EDTM-PP (1.24 and 1.14 V vs. Ag/AgCl)34, EDOT-MeNH2 (1.25 V vs. Ag/AgCl)42, and C4EDOT-COOH (1.00 V vs. Ag/AgCl)43. These results indicated that the introduction of a hydroxyl group reduced the Eox of (R)-EDTM and (S)-EDTM. The lower value of Eox is beneficial for the preparation of high-quality PEDTM. As shown in Figure 1, the peak current intensities increased with successive scans, indicating that the reaction product was gradually deposited on the surface of the working electrode, increasing the thickness of the polymer films41, 44. Moreover, the shift in the potential of the wave current demonstrated that the electrical resistance of the polymer film increased, requiring a greater overpotential to overcome this resistance43, 44. Spectroscopic Characterization The FT-IR spectra of the monomers and their corresponding polymers were recorded to elucidate their structure and interpret the polymerization mechanism (Figure 2A and B). The results were similar to those for a previously reported racemic mixture of EDTM and PEDTM40. The strong peaks at approximately 3410–3440 cm-1 are typically attributed to –OH stretching vibrations. This peak was present in all the monomers and polymers, indicating that the chiral group is stable and is not destroyed during the electrochemical polymerization process. In the functional group region, the peak at 3111 cm-1 in the spectrum of (R)-EDTM and (S)-EDTM was attributed to the =C–H stretching vibration, which disappeared or weakened in the spectrum of (R)-PEDTM and (S)-PEDTM. This phenome-

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non is direct proof that the electrochemical polymerization of (R)-EDTM and (S)-EDTM occurred at the 2 and 5 positions of the thiophene ring. In the fingerprint region, the characteristic peaks at 930 cm-1 and 866 cm-1 for (R)-EDTM, and 911 cm-1 and 866 cm-1 for (S)-EDTM were assigned to out-of-plane deformation vibrations of C-H in the thiophene ring. However, these peaks disappeared in the spectrum of the corresponding polymers, further confirming that the polymerization of (R)PEDTM and (S)-PEDTM occurred at the 2 and 5 positions of thiophene. The peaks located at 1374 cm-1 and 1373 cm-1 of (R)- and (S)-EDTM were attributed to the C-O stretching vibration, which shifted to a lower wavenumber after electropolymerization (1322 and 1323 cm-1 for (R)- and (S)-PEDTM, respectively). Further details of the band assignments for monomers and polymers are given in Table S1. The as-prepared doped (R)-/(S)-PEDTM were dark blue in color. Their color changed to light purple when the films were dedoped over 3 days using 25% ammonia. However, the resulting dedoped PEDTM films were found to be incompletely soluble in DMSO or THF and exhibited poor solubility in other solvents such as acetonitrile and acetone. The UV-vis spectra of (R)- and (S)-PEDTM in DMSO are shown in Figure 2C and D. In the neutral state, the peaks at 592 nm for (R)PEDTM and 585 nm for (S)-PEDTM were attributed to π-π* transitions. In the oxidized state, the polymers both exhibited a broad peak at ~890 nm. Interestingly, the absorption band at a longer wavelength almost coincided with the new polaronic absorption band. The absorption band at 592 nm and 585 nm in the neutral state and the broad band between 750 to 1000 nm in the oxidized state are responsible for the light purple and dark blue colors in the dedoped and doped films, respectively. Circular dichroism (CD) spectroscopy is the most sensitive method for probing polymer chain conformations18, 45, 46. As shown in Figure 2E, (R)- and (S)-PEDTM exhibited a strong Cotton effect in the neutral state. The CD spectra of (R)PEDTM and (S)-PEDTM were mirror symmetric and otherwise essentially identical. This result suggests that the neutral polymers had a relatively high degree of freedom for internal main chain rotation. In the oxidized state, the CD spectra of the polymers showed that the intensity of the Cotton effect decreased, suggesting that the planarity of the main chain is

Figure 2 FT-IR spectra of (R)-PEDTM (A) and (S)-PEDTM (B): monomers (a) and polymers (b); UV-vis spectra of (R)-PEDTM (C) and (S)-PEDTM (D) in neutral and oxidized states; and CD spectra (E) of neutral and oxidized (R)-PEDTM and (S)-PEDTM. (UV-vis ACS Paragon Plus Environment and CD were measured in DMSO).

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enhanced by the intercalation of dopants (ClO4-) in the main chain24, 45-47. Both head-to-head (H-H) and head-to-tail (H-T) regioisomers of the polymers are possible, and we assumed that the presence of such structural variation would affect the optical properties of the polymer. However, control of the regioregularity of the polymers is difficult to achieve in electrochemical polymerization, which tends to produce polymers with random mixtures of H-H and H-T regioisomers. Surface Morphology The morphology of the surface of the film was studied using SEM, as shown in Figure S7A and B. Macroscopically, the asformed (R)- and (S)-PEDTM films were compact, spiral, and blue in color. Microscopically, the polymer films were nodular with irregular shapes and sizes. This observation suggests that these films had a large specific surface area, which is advantageous for use in chiral recognition experiments. Electrochemical Properties CVs were used to investigate the electroactivity and stability of (R)-PEDTM and (S)-PEDTM in monomer-free aqueous LiClO4 electrolyte solutions (Figure S8A and B). The results indicated that the redox processes were not diffusion-limited and that electroactive polymer films had been formed on, and strongly adhered to, the surface of the electrode32, 41, 48. Both (R)-PEDTM and (S)-PEDTM displayed very similar electrochemical behavior because the chirality of the polymers does not affect the redox behavior49. The redox stability of (R)-PEDTM and (S)-PEDTM was also investigated in monomer-free aqueous LiClO4 electrolyte solutions. As shown in Figure S8C and D, (R)-PEDTM and (S)-

lytes50-52. The relatively poorer stability of (R)-PEDTM and (S)-PEDTM was also ascribed to the films being slightly soluble in water, causing some oligomers to be dissolved from the electrode. Chiral Recognition DOPA, Trp, and Pro were used to verify the chiral selectivity of the enantiomers and chiral polymers. Based on previously reported methods17, 25, 53-57 and optimization of the experimental variables in Figure S9, H2SO4 (0.25 M) was used as the electrolyte for DOPA detection, and PBS (0.1 M, pH = 6.0) was used as the electrolyte for Trp and Pro detection. The data presented in Table S4 and Figure S10 show that the CV curves of (R)-/(S)-PEDTM/GCE are similar for each pair of enantiomers, with an insufficient difference between the peak currents (1–2 µA) for differentiation of the enantiomers. Thus, DPV analyses were carried out, resulting in evidence of chiral differentiation and providing a larger difference between the electrochemical responses of (R)-/(S)PEDTM/GCE (Figure 3). As shown in Figure 3, (R)PEDTM/GCE showed a higher peak current response for the L or (S) isomer; while (S)- PEDTM/GCE showed a higher peak current response towards the D or (R) isomer. The data presented in Table 1 and Figure 3 show a clear chiral discrepancy between enantiomers of DOPA, Trp, and Pro. These differences in the current changes demonstrate the potential for these materials to be applied in enantioselective recognition. In addition, we summarize the comparison of the proposed modified electrodes with other reported electrodes for the recognition of DOPA/Trp/Pro, as shown in Tables S5-S7. This demonstrated that (R)-/(S)-PEDTM/GCE have relatively moderate performances for the recognition of enantiomers.

Figure 3. DPV curves of (R)-PEDTM (A, C, E) and (S)-PEDTM (B, D, F) modified electrodes in (A, B) a H2SO4 (0.25 M) solution containing D-DOPA (0.3 mM, solid line) or L-DOPA (0.3 mM, dashed line); (C, D) in a PBS (0.1 M, pH = 6) solution containing D-Trp (0.3 mM, solid line) or L-Trp (0.3 mM, dashed line); (E, F) in a PBS (0.1 M, pH = 6) solution containing (R)-propanolol (0.5 mM, solid line) or (S)-propanolol (0.5 mM, dashed line). Increasing potential: 0.004 V; amplitude: 5 Hz; pulse width: 0.2 s; pulse period: 0.5 s

PEDTM had a relatively poorer stability in H2O-LiClO4. The electrochemical stability is highly dependent on the polymerization conditions, such as the solvent and supporting electro-

Mechanism and Model of Chiral Recognition



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Table 1. DPV curve parameters of (R)-PEDTM/GCE and (S)-PEDTM/GCE used for the chiral recognition of DOPA, Trp, and propranolol enantiomers. Polymers

(R)-PEDTM Potential of peaks (V)

(S)-PEDTM

Items and enantiomers

Peak currents of DPV (µA)

D-DOPA

155.6

L-DOPA

171.4

Current ratio

1.10

1.20

D-Trp

61.7

57.8

L-Trp

70.9

Current ratio

1.14

1.34

(R)-propanolol

74.5

128.6

(S)-propanolol

105.8

75.7

Current ratio

1.42

1.71

Chiral discrepancy

Peak currents of DPV (µA)

Potential of peaks (V)

Chiral discrepancy

~0.30

D>L

~0.80

D>L

~1.23

R>S

104.3 ~0.30

~0.85

L>D

L>D

~1.16

The chiral recognition mechanism is a central question in chiral recognition. Herein, we propose a mechanism for the chiral recognition of DOPA/Trp/Pro enantiomers based on our experimental results, as shown in Figure 4 and Figure 5. Furthermore, we provided a general guideline for confirm the chirality of an isomer through known of the chirality of electrode and the corresponding peak current response. We propose the analyte recognition consists of three steps: (i) The interaction between polymers and enantiomers. For DOPA, the amine and carboxyl groups are protonated on exposure to acid and change to their active form58, which can attach to the surface of the polymer-modified electrodes via Hbonds (Figure 4a-DOPA)59. According to literatures60, 61, which did not claim that Trp and Pro are protonated in the PBS (pH = 6.0) solution. Thus, we consider the interaction between polymers and Trp/Pro is as shown in Figure 4b-Trp and Figure 4b-Pro. There are two possible explanations of this chiral recognition process. The first is the “three-point interaction” model suggested by Dacankov’s theory62-64. Figure 5A-a shows that the L-enantiomers (i.e., L-DOPA, L-Trp, or (S)-Pro) can present three groups that match exactly with three sites of the D selector (i.e., the surface of (R)-PEDTM/GCE). However, after considering all possible rotations (Figure 5A-b, A-c), the Denantiomers (i.e., D-DOPA, D-Trp, or (R)-Pro) only have a maximum of two groups capable of interacting with two sites of the D selector. The binding constant of the chiral molecule in (a) (Figure 5A) will be higher than that of its mirror image. Thus, while the left-handed antipode (L-DOPA, L-Trp, and (S)Pro) interacts more strongly with the right-handed (R)PEDTM/GCE, the right-handed antipode is hindered, and vice versa (Figure 5B, a-c). The key feature of the three-point interaction model is that at least three simultaneous interactions are required, and they should occur with three different substituents attached to the stereogenic center. Nevertheless, other researchers have point-

87.1

43.3

S>R

ed out that the three-point interaction model is only a geometrical model, which is consistent with the geometric idea that two points define a line and three points define a plane60, 65. The second explanation is the “pseudo-two-point interaction” model66, 67, which suggests that three points of interaction are not required if the attractive interaction is strong enough to promote the formation of at least one attachment between the receptor and selector, or the repulsive interaction is so strong that the receptor and selector unable to connect. For example, hydrogen bonds are strong because the negative site can come

Figure 4. (a) A schematic illustration of the interaction between (R)-/(S)-EDTM and DOPA, Trp, or Pro enantiomers; (b) Mechanism of the redox reaction of DOPA, Trp and Pro.

very close to the hydrogen atom, which is depleted of any remaining repulsive electrons. Furthermore, for example, steric hindrance is repulsive and very strong at short ranges because of the intrinsic space required by each atom or group of


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Figure 5. a-c is the “three-point interaction” model of (R)-PEDTM/GCE(A) and (S)-PEDTM/GCE (B) (a is the case of enantiomer matched three sites of the chiral electrode; b and c are the case of enantiomer matched two sites of the chiral electrode). d and e are the “pseudo-two-point interaction” model of (R)-PEDTM/GCE(A) and (S)-PEDTM/GCE (B) (d is the case of enantiomer and chiral electrode occurred attractive interaction; e is the case of enantiomer and chiral electrode occurred repulsive interaction).

atoms. In this case, the amine and carboxyl groups of DOPA/Trp/Pro enantiomers attach to the surface of polymermodified electrodes via hydrogen bonds. Thus, (R)PEDTM/GCE (right handed) can readily connect with the lefthanded enantiomers (L-DOPA, L-Trp, and (S)-Pro) (Figure 5Ad). In contrast, the right-handed enantiomers (D-DOPA, D-Trp, and (R)-Pro) are hindered because of steric hindrance (Figure 5A-e). (S)- PEDTM/GCE has the opposite pattern with these enantiomers, as shown in (d) and (e) (Figure 5B). (ii) Redox reactions of enantiomers. The enantiomers displayed clear current responses in CV investigations, implying that they could be oxidized. The DPV results allowed the enantiomers to be distinguished by their voltammetric response. The mechanism for the redox reaction of the enantiomers (DOPA/Trp/Pro) is shown in Figure 4b in more detail. (iii) Signal export. A staircase waveform could effectively extend the interaction time between the electrode and enantiomers (DOPA/Trp/Pro) leading to an amplification of the signal that transformed the electrochemical spectrum. Thus, a model of chiral recognition by chiral PEDTM electrode materials was established. The chirality of the enantiomers could be confirmed by two parameters: the electrode chirality and the peak current response. If the (R) form of the electrode had a higher peak current value, the test enantiomer was the L or (S) form. Conversely, a higher peak current value for the (S) form of the electrode identified the D or (R) form of test enantiomer.

showed a higher peak current response towards L or (S) enantiomers, while the opposite phenomenon occurred for (S)PEDTM/GCE. Finally, we proposed that the mechanism of chiral recognition involved three steps: an interaction between the polymers and enantiomers, the redox reaction of the enantiomer, and a signal export. Based on this mechanism, a model of electrochemical chiral recognition by the chiral conducting polymer electrode was established. The chirality of these enantiomers could be confirmed by two parameters: the chirality of the electrode and the peak current response. This study confirms the suitability of chiral conducting polymers as electrode modification materials for electrochemical chiral recognition.

ASSOCIATED CONTENT Supporting Information 1

H NMR spectrum of (R)/(S)-EDOT-MeOAc and (R)/(S)-EDTM. Details of FT-IR spectra, thermogravimetric analytical experiments, surface morphology properties and electrochemical parameters for (R)/(S)-EDTM. Chiral recognition by CV curves and the parameters of (R)-PEDTM/GCE and (S)-PEDTM/GCE for the determination of DOPA, Trp, and Pro enantiomers. Optimization of experimental variables. Repeatability of (R)- or (S)PEDTM/GCE in chiral recognition. Comparison of the proposed electrode with other reported electrodes for the recognition of enantiomers. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION

CONCLUSIONS

Corresponding Author

In conclusion, we synthesized a pair of chiral EDOT derivatives that showed good solubility in water, i.e., (R)-EDTM and (S)-EDTM, and prepared the corresponding polymers by electrochemical polymerization. The polymers showed good redox activity and stability in monomer-free electrolytes. The CD spectra of (R)-PEDTM and (S)-PEDTM films in the neutral state were mirror symmetric. In addition, the obtained polymer films were investigated as surface modification materials for GCEs, and could successfully distinguish between D-/LDOPA, D-/L-Trp, and (R)-/(S)-Pro. The (R)-PEDTM/GCE

* E-mail: [email protected], [email protected]; Fax: +86-791-83823320; Tel: +86-791-88537967

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT



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The authors would like to acknowledge the financial support by the National Natural Science Foundation of China (51263010, 51272096), the Science and Technology Landing Plan of Universities in Jiangxi province (KJLD14069), and Jiangxi Science & Technology Normal University Program for Scientific Research Innovation Team (2015CXTD001).

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Chiral PEDOT-Based Enantioselective Electrode Modification Material

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