Chiral recognition of tyrosine enantiomers on a novel bis-aminosaccharides composite modified glassy carbon electrode

Analytica Chimica Acta 1088 (2019) 35e44

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Chiral recognition of tyrosine enantiomers on a novel bis-aminosaccharides composite modified glassy carbon electrode Jiao Zou, Jin-Gang Yu* College of Chemistry and Chemical Engineering, Key Laboratory of Hunan Province for Water Environment and Agriculture Product Safety, Central South University, Changsha, Hunan, 410083, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Electrochemical chiral interface was constructed based on a bisaminosaccharides (CS-GalN) composite.
The CS-GalN modified GCE exhibited good anti-interference ability and stability.
Synergistic effect of CS and GalN contributes to the determination of L- and D-Tyr enantiomers.
It opens up the possibility of fabrication of novel chiral interfaces based on glycoconjugates.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 June 2019 Received in revised form 1 August 2019 Accepted 12 August 2019 Available online 13 August 2019

A polyaminosaccharide (chitosan, CS) and an aminosaccharide (D-galactosamine, GalN) were integrated together via hydrothermal assembly to obtain a bis-aminosaccharides composite (CS-GalN), and a novel and facile chiral sensing platform based on CS-GalN modified glassy carbon electrode (CS-GalN/GCE) was fabricated and used for electrochemical recognition of tyrosine (Tyr) enantiomers. CS-GalN composite was characterized by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and contact angle goniometry. It was observed that CS-GalN composite exhibited different binding ability for Tyr enantiomers. Under optimized experimental conditions, the oxidation peak current ratio of L-Tyr to D-Tyr (IL/ID) and the difference between their peak potentials (DEp ¼ ED-EL) were 1.70 and 28 mV at CS-GalN/GCE by square wave voltammetry (SWV). In addition, the peak currents increase linearly with the concentration of Tyr enantiomers in the concentration range 0.01e1.00 mM with detection limits of 0.65 mM and 0.86 mM for L-Tyr and D-Tyr (S/ N ¼ 3), respectively. CS-GalN/GCE also exhibited the ability to determine the percentage of D-Tyr in the racemic mixture. In addition, CS-GalN/GCE possessed remarkable sensitivity, great stability as well as fine reproducibility. It could be concluded that the chiral interface of CS-GalN/GCE can provide an ideal platform for electrochemical recognition and determination of Tyr enantiomers. © 2019 Elsevier B.V. All rights reserved.

Keywords: Chiral electrochemical sensor Chitosan D-galactosamine Tyrosine enantiomers Square wave voltammetry

1. Introduction

* Corresponding author. E-mail address: [email protected] (J.-G. Yu). https://doi.org/10.1016/j.aca.2019.08.018 0003-2670/© 2019 Elsevier B.V. All rights reserved.

Chirality is one of the significant features of nature and plays an important role in the molecular level of biology, medical science and biotechnology [1]. Amino acids (AAs), the most important and

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predominant chiral compounds in nature, could be used as important biomarkers for various metabolic diseases [2]. For example, tyrosine (Tyr) is an important precursor of various biosignal molecules, such as adrenaline, norepinephrine and dopamine, all of which can regulate mood. Tyr deficiency is associated with depression [3] and also a risk factor for predicting the development of diabetes [4]. Therefore, it is of great theoretical and practical significance to develop an effective and sensitive method for chiral recognition of AAs enantiomers. Among the reported methods for enantioselective recognition of AAs, recently there is increasing interest in electrochemical chiral recognition. In comparison with other strategies such as high performance liquid chromatography (HPLC) [5], fluorimetric [6], mass spectrometry [7], colorimetric techniques [8], capillary electrophoresis [9], molecular imprinting techniques [10] and Raman spectroscopy [11], electrochemical chiral sensing possessed many advantages such as lower cost, faster response, higher sensitivity, and simpler operation as well as easier regeneration [12e19]. The key requirement for electrochemical recognition is to construct a chiral platform that possesses different binding affinities for AAs enantiomers due to its enantioselective recognition sites [20]. To date, the most commonly used chiral recognition materials include bovine serum albumin (BSA) [21], human serum albumin (HSA) [22], AAs [23e26] and sodium alginate (SA) [27] which were composited with graphene oxide (GO) [28], reduced graphene oxide (rGO) [29] and graphene quantum dots (GQDs) [30], metal nanoparticles (AuNPs [15,31], PtNPs [32]), conductive polymers [33,34] and so on. In addition, electrochemical chiral recognition by cyclodextrins (CDs) (a-CD [35], b-CD [30] and g-CD [20]) and their derivatives [36,37], chitosan (CS) [38], soluble starch (SS) [39], potato starch (PS) [40] and so on has also drawn enormous attention owing to the excellent properties of these saccharides such as nontoxic, biocompatible, biodegradable [41] and relatively inexpensive [16]. For example, CS and Cu2þ-coordinated a-CD (Cu2-a-CD) modified GCE (Cu2-a-CD-CS/GCE) was prepared by an electrochemical deposition method, which could be used for electrochemical recognition of Tyr enantiomers [35]; Sodium carboxymethyl cellulose-chitosan (CMC-Na-CS) composite fabricated via amidation could be used for the electrochemical recognition of tryptophan (Trp) enantiomers [42]. Currently, little attention has been paid to the construction of novel electrochemical sensors based on the synergistic effects of various saccharides. Herein, a bis-aminosaccharides composite (CSGalN) based on two saccharides, CS and GalN, was hydrothermally assembled. CS-GalN modified glassy carbon electrode (CS-GalN/ GCE) was used as a novel electrochemical sensor for chiral recognition of Tyr enantiomers (Scheme 1A). The differences between the peak currents of D-Tyr and L-Tyr were observed at CS-GalN/GCE, making it an attractive sensor for the chiral recognition of Tyr enantiomers. Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and contact angle goniometry were used to characterize the CS-GalN composite. CS-GalN composite possessed a porous network structure, and the chiral recognition at CS-GalN/ GCE might be attributed to the directed and selective hydrogenbond interactions between Tyr enantiomers and CS-GalN composite (Scheme 1B). The proposed CS-GalN/GCE also exhibited good anti-interference and stability, making it an available platform for recognition and quantification of Tyr enantiomers. 2. Experiments 2.1. Reagents and chemicals D-tyrosine

(D-Tyr, purity of 98 wt%), L-Tyr (purity of 99 wt%) D-

Scheme 1. Schematic diagrams of the experimental setup: (A) The fabrication of CSGalN/GCE; (B) A proposed mechanism for the chiral electrochemical recognition of Tyr enantiomers at CS-GalN/GCE.

tryptophan (D-Trp, purity of 98 wt%), L-Trp (purity of 99 wt%), Dphenylalanine (D-Phe, purity of 98 wt%)), L-Phe (purity of 99 wt%), D-histidine (D-His, purity of 99 wt%), L-His (purity of 99 wt%) and Dgalactosamine hydrochloride (GalN, purity of 99 wt%) were obtained from Aladdin Chemistry Co., Ltd. (Shanghai, China). Chitosan (CS, B.R.; >90.0% deacetylation degree) was provided by Shanghai Zhanyun Chemical Co., Ltd. (Shanghai, China). Disodium hydrogen phosphate dodecahydrate (Na2HPO4$12H2O), sodium dihydrogen phosphate dihydrate (NaH2PO4$2H2O), concentrated nitric acid (HNO3), phosphoric acid (H3PO4), hydrochloric acid (HCl), acetic acid (HAc), boric acid (H3BO3), sodium hydroxide (NaOH), sodium acetate (NaAc) and tris(hydroxymethyl)aminomethane (Tris) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Potassium ferricyanide [K3Fe(CN)6] and potassium chloride (KCl) were bought from Tianjin Broadcom Chemical Co., Ltd. (Tianjin, China). Alumina polishing powder (0.05 mm) was obtained from Lab Testing Technology Co., Ltd (Shanghai, China). All other reagents were of analytical grade and used without further purification. Phosphate buffer solution (PB; 0.1 M, pH ¼ 7.0) was prepared with a certain amount of 0.1 M of NaH2PO4 and Na2HPO4. Electrochemical experiments were carried out in PB (0.1 M) solution. Stock solutions of D-Tyr and L-Tyr (1.0 mM) were prepared in PB (0.1 M, pH ¼ 7.0) solution. The working solutions of D-Tyr and L-

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Tyr were prepared by diluting the stock solution with PB (0.1 M) solution. 2.2. Apparatus The functional groups of CS and CS-GalN composite were analyzed by Fourier transform infrared (FT-IR) spectroscopy (PerkinElmer Co., Ltd., UK). The morphologies of CS and CS-GalN composite were observed on a field emission scanning electron microscopy (FE-SEM, TESCAN MIRA3 LMH/LMU, Czech). Energy dispersive X-ray spectroscopy (EDS) analysis was carried out on the same FE-SEM instrument to investigate the chemical composition of the samples. Contact angle measurements were carried out by a JC 2000D1 apparatus (Zhongchen Digital Equipment Co. Ltd., China). All electrochemical experiments were performed using a CHI660E Electrochemical Workstation (Chenhua Instrument Co., Ltd. Shanghai, China) with a classical three-electrode system composed of an Ag/AgCl reference electrode, a platinum wire counter electrode and a working electrode (bare GCE and modified GCE electrodes, diameter: 3.0 mm). All electrochemical experiments were carried out at room temperature (25  C) except for temperature control experiments. Error bars represent standard deviation for three independent measurements. 2.3. Preparation of bis-aminosaccharides (CS-GalN) composite 100.0 mg CS was dissolved in 100.0 mL of an aqueous solution of HAc (1.0%, v/v) to obtain a 1.0 mg mL1 CS solution, which was then stored in a refrigerator at 4  C before use. GalN was dispersed in double distilled water to form a homogeneous solution (1.0 mg mL1). 10.0 mL of GalN was added to 10.0 mL of above CS solution, and the mixture was sonicated for 5 min and then hydrothermal treated at 120  C for 1 h to obtain an assembled CSGalN composite. The mixture was naturally cooled to room temperature and stored in a conical flask for future use.

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and the characteristic peak of C]O (stretching vibration) is located at 1608.78 cm1; and an absorption peak of NeH (bending vibration) is located at 1503.47 cm1. CS-GalN (curve c) exhibits characteristic absorption peaks which are merged by CS and GalN. The shift of absorption band to 1516.05 cm1 can be observed, indicating there are newly generated intermolecular interactions between CS and GalN. It should be mentioned that a wider and stronger absorption peak for OeH and NeH at 3205.68 cm1 can be observed, which can be attributed to the hydrogen-bonded association of CS and GalN. Moreover, the distinctive stretching vibration of CeN bond at 1403.43 cm1 in CS-GalN is in accordance with that of CS, indicating the successful assembly of GalN with CS. Hydrogen bonding between the hydroxyl (-OH) groups and/or amino (-NH2) groups of CS and GalN is responsible for the integration. The morphologies of CS and CS-GalN films on GCE were investigated using FE-SEM. As can be seen from Fig. 1B, CS can form a very uniform and smooth film. Of particular interest is that CS-GalN film (Fig. 1C and D) exhibits a rough and porous network structure with irregular shapes and pore sizes, which is quite different from that of CS film. This observation suggested that CS-GalN composite has a porous structure, which is beneficial to the recognition of Tyr enantiomers due to the exposed polar/chiral networks. The surface composition of CS-GalN film was observed by EDS (Fig. S1), and the newly emerging peaks of Cl confirmed the successful assembly of GalN on CS. In order to evaluate the wettability of CS-GalN film, contact angle (CA) measurements of CS, GalN and CS-GalN were carried out on a water contact angle goniometer. As we can see in Fig. S2, the water CA of CS-GalN (48.56 ) was lower than that of CS (60.43 ) and GalN (54.69 ). Larger amounts of polar functional groups such as eOH and eNH2 were presented on the surface of CS-GalN compared with individual CS and GalN, resulting in an increased hydrophility. The results again confirmed the porous network structure of CS-GalN due to the hydrothermal assisted assembly, which was consistent with the FE-SEM data.

2.4. Fabrication of CS-GalN based chiral sensor 3.2. Electrochemical properties of CS-GalN/GCE Prior to modification, bare GCE was carefully polished with 0.05 mm alumina slurry and rinsed with double distilled water, followed by ultrasonic cleaning in double distilled water for 3 min and dried under nitrogen. A certain volume of CS-GalN solution was pipetted directly onto the freshly polished GCE, which was then airdried at room temperature to obtain CS-GalN/GCE. CS and GalN modified electrodes (CS/GCE and GalN/GCE) were prepared in the same manner, respectively. 3. Results and discussion 3.1. Characterization of CS-GalN composite The as-prepared CS-GalN composite was characterized by FT-IR spectroscopy, FE-SEM, EDS and contact angle goniometry. The FT-IR spectra of CS, GalN and CS-GalN are shown in Fig. 1A. As for CS (curve a), the broad peak at 3211.70 cm1 is ascribed to the stretching vibrations of OeH and NeH; the peak at 2870.95 cm1 belongs to the stretching vibration of CeH; the two characteristic peaks at 1638.11 and 1538.26 cm1 can be assigned to the stretching vibrations of amide I and II bands, respectively [42,43]; the stretching vibration of CeN is located at 1403.42 cm1; the asymmetrical stretching of CeOeC is located at 1147.48 cm1; and the peak located at 1086.75 cm1 can be attributed to the stretching vibration of CeO. GalN (curve b) shows multiple absorption peaks at 3188.38, 3245.55 and 3426.83 cm1 which can be attributed to the stretching vibrations of free and associating OeH, and NeH;

We investigated the electrochemical performance of CS-GalN/ GCE by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The electrocatalytic activity of bare GCE and modified electrodes in 10 mL of 1.0 mM [Fe(CN)6]3-/4- were evaluated by CV (Fig. 2A). A pair of well-defined and reversible redox peaks owing to the conversion between Fe(CN)64 and Fe(CN)3 6 can be observed for all tested electrodes. The peak-to-peak separation in potentials (DEp) of 56 mV was observed at CS-GalN/GCE, while the DEp at CS/GCE, GalN/GCE and bare GCE increased to 59 mV, 63 mV and 68 mV, respectively. In addition, the peak currents at CS/ GCE, CS-GalN/GCE, GalN/GCE and bare GCE decreased dramatically. The introduction of GalN onto GCE increased its electrochemical activity, suggesting GalN was effective in facilitating the electron transfer. The electrochemical activity and reversibility were enhanced a lot at CS/GCE, indicating that CS film on GCE also facilitated electron transfer at the electrode-solution interface. The strong electrostatic attractions between the protonated eNH2 on CS (or GalN) and [Fe(CN)6]3-/4- contributed a lot to the enhanced electrochemical activity [16]. In comparison with bare GCE, CS/GCE and GalN/GCE, CS-GalN/GCE exhibited the highest electrochemical activity and reversibility, indicating that the successfully assembly of GalN onto CS and the generated porous structure of CS-GalN composite greatly facilitated the electron transfer between the solution and the electrode owing to the synergistic effects of CS and GalN. A similar phenomenon also occurred in the EIS. As shown in Fig. 2B, bare GCE showed the largest charge-transfer resistance (Rct,

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Fig. 1. Characterization of the samples: (A) FT-IR spectra of the samples: (a) CS; (b) GalN and (c) CS-GalN composite; FE-SEM images of (B) CS and (C) (D) CS-GalN composite with two different resolutions.

219.9 U), which was higher than those at GalN/GCE (138.0 U), CS/ GCE (117.6 U) and CS-GalN/GCE (105.8 U). Obviously, CS-GalN/GCE possessed the lowest Rct, again confirming that the excellent electrocatalytic activity of CS and the synergistic effects of CS and GalN were beneficial for its excellent electrochemical performance. In order to understand the effects of effective surface area on the electrochemical behaviors of CS-GalN/GCE, CVs of bare GCE and CSGalN/GCE were performed in 1.0 mM K3Fe(CN)6 solution (containing 0.1 M KCl) at various scan rates (10e200 mV s1) (Fig. 2C and D). The results revealed that the absolute value of the peak currents and potentials of [Fe(CN)6]3-/4- redox couple at the surface of bare GCE and CS-GalN/GCE gradually increased with increase in scan rates. In addition, the redox peak currents of bare GCE (Fig. 2E) and CS-GalN/GCE (Fig. 2F) had a linear relation with the square root of the scan rates (n1/2), indicating that the electrode reaction at the electrode surface might be a diffusion-driven process [44,45]. The effective surface area (Aeff) of bare GCE and CS-GalN/GCE were estimated by the Randles-Sevcik equation: Ip ¼ (2.69  105) A D1/2 n3/2 n1/2 C (where Ip, A, D, n, n and C represent the redox peak current (A), the Aeff of electrode (cm2), the diffusion coefficient of K3Fe(CN)6 in 0.1 M KCl (6.67  106 cm2 s1), the number of electrons transferred (n ¼ 1), the scan rate (V s1) and the concentration of redox species (mol cm3), respectively) [46]. The reversible process was monitored in 1.0 mM K3Fe(CN)6 by CV. As shown in Fig. 2D and F, the slopes of the Ip- n1/2 [mA-(mV s1)1/2] curves of bare GCE and CS-GalN/GCE were 1.17 and 2.46, respectively. Aeff of bare GCE and CS-GalN/GCE were calculated to be 0.053 cm2 and

0.112 cm2, respectively. In comparison with that at bare GCE, the increased Aeff for signal transduction at CS-GalN/GCE could be attributed to the high surface area of CS-GalN composite [47]. To compare the efficiency of the electrodes, the current density was defined as J ¼ i/Aeff (where J is the current density (mA cm2), i is the current response (mA) and Aeff is the effective surface area of the electrode after surface modification (cm2)). The J of bare GCE and CS-GalN/GCE were calculated to be 43.46 mA cm2 and 102.00 mA cm2, respectively. The enhanced relative efficiency of CS-GalN/GCE can be attributed to the porous structure of CS-GalN composite [48]. The effects of scan rate on the peak potential were also investigated. As we can see in Fig. 2C and D, the separations of the anodic and cathodic peaks at bare GCE and CS-GalN/GCE increased with an increase in scan rates (10e200 mV s1), indicating that the redox process was quasi-reversible [47]. The redox peak potentials of bare GCE and CS-GalN/GCE were plotted against the logarithmic function of scan rates (log n), and linear variations can be observed (Fig. 2G and H). According to Laviron's theory [49], the anodic and cathodic slopes can be presented as 2.303RT/(1-a)nF and 2.303RT/anF, respectively [50]. Where, R is the ideal gas constant (8.314 J K1 mol1), T is the temperature (298 K), a is the electron transfer coefficient, n is the number of participating electron (n ¼ 1) and F is the Faraday constant (96485 C mol1). Based on the slopes of the two straight lines and the equation of ka/kb ¼ (a1)/a (where ka and kb are the slopes of the straight lines for Epa vs. log v and Epc vs. log v, respectively), the values of a at bare GCE and

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Fig. 2. Electrochemical behaviors of the electrodes: (A) CV of different electrodes in 1.0 mM of K3Fe(CN)6 solution (containing 0.1 M of KCl) at a scan rate of 50 mV s1; (B) Nyquist plots of different electrodes in 1.0 mM K3Fe(CN)6 solution (containing 0.1 M KCl) at open-circuit potential conditions (AC frequency range: 0.1 Hz to 100 kHz; AC amplitude: 5.0 mV), equivalent circuit (inset); CVs of bare GCE (C) and CS-GalN/GCE (D) in 1.0 mM K3Fe(CN)6 solution (containing 0.1 M KCl) redox peak currents at different scan rates (10, 20, 50, 100, 150 and 200 mV s1); Plots of bare GCE (E) and CS-GalN/GCE (F) redox peak currents as a function of scan rates (Ipa: oxidation peak current; Ipc: reduction peak current); The dependences of the redox peak potentials of bare GCE (G) and CS-GalN/GCE (H) on log v (Epa: oxidation peak potential; Epc: reduction peak potential).

CS-GalN/GCE were estimated to be 0.625 and 0.548, respectively. In addition, when the condition of nDEp < 200 mV is fulfilled, the electron transfer rate constant (ks) under a surface controlled process can be obtained using Laviron's equation: logks ¼ alog(1a) þ (1-a)loga - log(RT/nFn) - nFa (1-a)DEp/2.3RT (where DEp is the peak-to-peak potential separation, the scan rate (v) sets as

100 mV s1, other parameters are the same as abovementioned) [49]. A higher ks value for CS-GalN/GCE (1.086 s1) could be obtained in comparison with that (0.690 s1) for bare GCE, indicating that CS-GalN composite contributed to promote the electron transfer between solution and electrode [51].

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3.3. Enantioselective recognition of Tyr enantiomers

3.4. Optimization of experimental conditions

The electrochemical chiral recognition of Tyr enantiomers at different modified electrodes was investigated by SWV. And SWVs of D-Tyr and L-Tyr (1.0 mM, containing 0.1 M of PB, pH 7.0) at different electrodes were recorded in the potential range of 0.4e1.1 V (vs. Ag/AgCl) (Fig. 3). The SWV peak potential and peak current response of the Tyr enantiomers were almost the same at bare GCE (Fig. 3A), suggesting that bare GCE was incapable of recognizing Tyr enantiomers due to the absence of a chiral selector. As shown in Fig. 3BeD, higher peak current response ratio towards L-Tyr and D-Tyr enantiomers (IL/ID ¼ 1.70) and greater peak potential difference (DEp ¼ ED-EL ¼ 28 mV) at CS-GalN/GCE can be observed, which were much higher than those at CS/GCE (IL/ ID ¼ 1.04, DEp ¼ 4 mV) and GalN/GCE (IL/ID ¼ 1.13, DEp ¼ 4 mV). The results indicated that the porous network structure of CS-GalN, a novel bis-aminosaccharides composite which was assembled by CS and GalN, was sufficient for chiral discrimination of Tyr enantiomers. The higher hydrophilicity of CS-GalN based film was beneficial to the efficient and effective bonding between Tyr enantiomers and the composite [52]. Furthermore, CS-GalN on GCE preferentially bonded to L-Tyr rather than D-Tyr due to the different steric hindrance. The eOH/-NH2 groups of CS-GalN composite could bond to the eCOOH/-NH2 groups of L-Tyr, and the enrichment of L-Tyr on the surface of the chiral sensing platform due to the strong intermolecular forces enhanced the current response. In addition, a negative-shift in the peak potential of CS-GalN/GCE for L-Tyr could be observed. Therefore, the stereospecific interactions between Dor L-Tyr enantiomers and CS-GalN composite contributed to the chiral recognition (Scheme 1B). It should be mentioned that the value of IL/ID (1.70) in this study was much higher than that (IL/ ID ¼ 1.38) of previously reported works [39]. Therefore, the proposed CS-GalN/GCE are expected to meet potential applications in chiral electrochemical sensing.

To improve the electrochemical chiral recognition efficiency of Tyr enantiomers at the developed chiral sensor, supporting electrolytes, pH and temperature were investigated in detail. 3.4.1. Effects of types of supporting electrolyte and pH value Supporting electrolytes may play an important role in improving the conductivity of the solutions. The electrochemical chiral recognition of Tyr enantiomers (1.0 mM) at CS-GalN/GCE in five different buffer solutions (BR: 0.04 M, pH 7.0; Tris-HCl: 0.01 M, pH 7.0; PB: 0.1 M, pH 7.0; PBS: 0.01 M, pH 7.0; HAc-NaAc: 0.1 M, pH 7.0) was investigated. As shown in Fig. S3, it is obvious that the largest recognition efficiency (IL/ID ¼ 1.70, DEp ¼ 28 mV) can be observed in PB solution. Therefore, PB was selected as the supporting electrolyte in the following experiments for the chiral recognition of Tyr enantiomers. In addition, the electrochemical behaviors of Tyr enantiomers at CS-GalN/GCE can be greatly affected by pH value due to the protonation or deprotonation of Tyr enantiomers and the two aminosaccharides. As shown in Fig. S4A, IL/ID and DEp of Tyr enantiomers at CS-GalN/GCE gradually increased to the maximum values with an increase of pH from 5.0 to 7.0, while IL/ID and DEp values started to decrease when pH > 7. Bis-aminosaccharides CS-GalN composite was unstable under strong acid and alkaline conditions, which greatly affected its chiral recognition efficiency for Tyr enantiomers [53]. As we can see in Fig. S4B, the oxidation peak potentials of L-Tyr and D-Tyr at CS-GalN/GCE negatively shifted with an increase in pH value, indicating that the protons must be involved in the electrochemical oxidation process [54]. According to the regression equation of potential and pH, it can be seen that the slope value was close to the theoretical value (59 mV/pH), suggesting that the concerted equal proton-electron transfer in the electrochemical oxidation [55,56].

Fig. 3. SWV curve of 1.0 mM (a) L-Tyr and (b) D-Tyr at the electrodes: (A) bare GCE; (B) CS/GCE; (C) GalN/GCE; (D) CS-GalN/GCE.

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3.4.2. Effects of temperature Both intramolecular and intermolecular hydrogen bonding interactions are significantly affected by environmental temperature [57,58], therefore electrochemical chiral recognition might be also sensitive to environmental temperature. The recognition efficiency of CS-GalN/GCE toward Tyr enantiomers in temperature range from 5 to 40  C was evaluated (Fig. S5), and the highest recognition efficiency (IL/ID ¼ 1.70, DEp ¼ 28 mV) could be obtained at 25  C. Increasing or decreasing temperature would result decline in the efficiency of recognition. Normally, amino- and hydroxylcontaining molecules would maintain local molecular correlation and strongly limit their motions at low temperature, and the hydrogen-bonded Tyr-water networks were dominated by intermolecular coupling [59]. Therefore, the stereospecific hydrogen bonding interactions between CS-GalN and Tyr enantiomers were severely suppressed to obtain lower recognition efficiency (almost zero) of CS-GalN/GCE for Tyr enantiomers. As temperature increases, the interactions between Tyr and water would be decreased, leading to increased stereospecific hydrogen bonding interactions between CS-GalN and Tyr enantiomers, and an increase in the chiral recognition efficiency would be observed. It may be explained by the fact that CS-GalN could form stable stereospecific hydrogen bonding with L-Tyr at 25  C, and higher recognition efficiency can be obtained. On the other hand, the intermolecular hydrogen bonding formed between CS-GalN and LTyr would be broken at high temperatures of >25  C, and the chiral recognition efficiency of CS-GalN/GCE for Tyr decreased. As a result, an appropriate temperature of 25  C was used for chiral recognition of Tyr enantiomers.

3.5. Electrochemical chiral detection of Tyr enantiomers Under optimized conditions, a series of Tyr enantiomers with different concentrations were studied by SWV at CS-GalN/GCE in 0.1 M PB solution (pH 7.0) (Fig. 4 inset a, b). As shown in Fig. 4, peak currents increased linearly with increase in L-Tyr and D-Tyr concentrations over the range of 0.01e1.0 mM, and peak currents were plotted as a function of Tyr concentrations. Two fitted linear regressions for L-Tyr and D-Tyr enantiomers can be obtained, respectively: ip (mA) ¼ 15.22 CL-Tyr þ 0.32 (R2 ¼ 0.99127) and ip

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(mA) ¼ 11.43 CD-Tyr þ 0.24 (R2 ¼ 0.98968). The limits of detection at CS-GalN/GCE for L-Tyr and D-Tyr were determined to be 0.65 mM and 0.86 mM at a signal-to-noise ratio of 3 (S/N ¼ 3), respectively. The results showed that the proposed chiral sensor exhibited good detection ability for Tyr enantiomers. A comparison of CS-GalN/ GCE with previously reported sensors for chiral recognition of Tyr enantiomers is listed in Table 1. Obviously, CS-GalN/GCE has shown some advantages such as easy-to-manufacture, facile, rapid and low-cost, making it be a promising electrochemical chiral sensor. 3.6. Anti-interference, reproducibility and stability of CS-GalN/GCE Recognition selectivity and stability of the film electrode are critical parameters to evaluate a proposed sensor. The antiinterference ability of CS-GalN/GCE toward 0.1 mM of Tyr enantiomers in the presence of 10-fold higher concentrations of physiological substances and some mineral salts was investigated. It can be seen in Fig. S6 that the existence of ascorbic acid (AA), glucose (Glc), L-serine (L-Ser), L-lysine (L-Lys), L-cysteine (L-Cys), potassium chloride (KCl), sodium chloride (NaCl), calcium chloride (CaCl2), copper chloride (CuCl2), magnesium nitrate [Mg(NO3)2], aluminum nitrate [Al(NO3)3], zinc nitrate [Zn(NO3)2], cadmium nitrate [Cd(NO3)2] or lead nitrate [Pd(NO3)2] has no significant influence on the detection of Tyr enantiomers, and the decreases of peak currents were less than 5.68%, indicating that the proposed CSGalN/GCE possesses good selectivity and excellent antiinterference ability for detection of Tyr enantiomers. The reproducibility and stability of CS-GalN/GCE in 0.1 mM of Tyr enantiomers was tested by SWV. The stability of CS-GalN/GCE for 0.1 mM Tyr enantiomers was studied for six successive times, and relative standard deviation (RSD) of 2.88% for L-Tyr and 3.69% for D-Tyr can be obtained. The reproducibility was investigated by measuring six times using the same electrode in 0.1 mM Tyr enantiomers, and RSD of 3.99% and 4.14% for L-Tyr and D-Tyr can be obtained, respectively. Therefore, CS-GalN/GCE possessed high reproducibility and stability for the determination of Tyr enantiomers. 3.7. Chiral recognition of Tyr enantiomers in racemic solutions The ability of a chiral sensor to predict the ratio of L-to D-isomer in a racemic solution is critical to its practical applications [26]. The proposed CS-GalN/GCE was used to detect D-Tyr enantiomer (D-Tyr %) in different D-/L-Tyr mixtures at a total concentration of 1.0 mM by SWV. As we can see in Fig. 5A, only one broad oxidation peak can be found regardless of the relative amount of D-Tyr% in the mixture. Obviously, the peak currents (Ip) were linearly decreased and the oxidation peak potentials (Ep) were positively shifted with an increase in amount of D-Tyr% in the mixture (Fig. 5B). The proposed CS-GalN/GCE can provide a practical approach for qualitatively and quantitatively determination of D-Tyr% in the Tyr enantiomers mixture. 3.8. Chiral recognition of other amino acids

Fig. 4. The calibration plots of peak current versus Tyr concentration at the CS-GalN/ GCE. (Inset: SWVs of (a) L-Tyr and (b) D-Tyr with different concentrations, respectively.).

To further investigate its chiral recognition specificity, four amino acid enantiomers including Tyr, tryptophan (Trp), phenylalanine (Phe) and histidine (His) were detected at CS-GalN/GCE. As we can see in Fig. 6, the highest peak current ratio and largest peak potential difference can be obtained for Tyr enantiomers, indicating that the recognition efficiency of CS-GalN/GCE for Tyr was higher than those for other three amino acids. And the stereospecificity of CS-GalN/GCE for Tyr enantiomers maybe due to its chemical structure (Fig. S7). Obviously, Tyr and Phe belong to mono-aromatic ring amino acids, and the eNH2/phenolic hydroxyl groups of Tyr

42

J. Zou, J.-G. Yu / Analytica Chimica Acta 1088 (2019) 35e44

Table 1 A comparison of the developed sensors in the chiral recognition of Tyr enantiomers. Electrode

Method

Recognition difference (Ip, mA)

DEp (mV)

Linear range (mM)

Limit of Detection (mM)

Ref.

L-Cys-Au-Fe3O4/MGCE CyA/GCE [(SA)Zn(II)(L/D-Tyr)]/GCE b-CDs-GQDs/GCE

SWV DPV CV CV

IL/ID ¼ 1.85 e IL/ID ¼ 1.24 e

84 40 e e

1e125 e e 100e1000

[25] [26] [27] [30]

Electrodepositive AuNPs/GCE

CV

DIpa ¼ 7.9 DIpc ¼ 154.0

e

4e1000

0.012 (L-Tyr) 0.084 (D-Tyr) e e 6.07  103 (L-Tyr) 0.103 (D-Tyr) 1.3

[31]

Polypyrrole, Ni electrode MIP

CV

e

5000e45000

e

[34]

Cu2-a-CD-CS/GCE CS-GalN/GCE

DPV SWV

SL/SD ¼ 9.4 SD/SL ¼ 27.2 IL/ID ¼ 3.83 IL/ID ¼ 1.70

112 28

e 10e100

e 0.65 (L-Tyr) 0.86 (D-Tyr)

[35] This work

Notes: L-Cys: L-cysteine; MGCE: magnetic glassy carbon electrode; MIP: molecular imprint; SL/SD: selectivity ratio; CyA: cysteic acid.

Fig. 5. (A) SWV curves of D-Tyr% in the mixture of Tyr enantiomers (0, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100%); (B) Relationship between Ip (black) and Ep (blue) and D-Tyr% in the mixture solutions containing D- and L-Tyr enantiomers. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

4. Conclusion

Fig. 6. Recognition efficiency of CS-GalN/GCE for Trp, Tyr, His and Phe (1.0 mM). Error bars represent standard deviation for three independent measurements.

could form intermolecular hydrogen bonding with CS-GalN, while the absence of phenolic hydroxyl group on Phe would result in decreased intermolecular interactions and lower chiral recognition efficiency [35]. Furthermore, His possesses a heterocyclic ring, and the eNHe group of imidazole could form intermolecular hydrogen bonding with CS-GalN, and therefore the recognition efficiency of CS-GalN/GCE for His was higher than that for Phe. Although Trp is composed of a benzene ring and a five-membered pyrrole ring, while the intermolecular hydrogen bonding interactions between eNHe of pyrrole ring and eNH2/-OH of CS-GalN may be hindered by the benzene ring, and lowest recognition efficiency of CS-GalN/ GCE for Trp enantiomers would be found.

In conclusion, a novel chiral sensor based on hydrothermal assembled bis-aminosaccharides (CS-GalN) composite modified GCE was developed, and its ability for recognition and quantification of Tyr enantiomers was investigated by SWV. CS-GalN/GCE showed excellent recognition effects for Tyr enantiomers, and high peak current ratio between L- and D-Tyr (IL/ID ¼ 1.70) and large peak potential difference (DEp ¼ ED - EL ¼ 28 mV) can be observed. It is proposed that the selective formation of intermolecular hydrogen bonding between Tyr enantiomers and CS-GalN facilitated the efficient chiral recognition. Meanwhile, CS-GalN/GCE showed excellent stability and good reproducibility with antiinterference capability. The limits of detection of CS-GalN/GCE for L-Tyr and D-Tyr were 0.65 mM and 0.86 mM (S/N ¼ 3), respectively. More importantly, CS-GalN/GCE showed the ability to predict the percentage of D-Tyr in D-/L-Tyr mixtures of varying proportions. The proposed CS-GalN/GCE might find its practical application in the electrochemical chiral sensing of Tyr enantiomers. Notes The authors declare no competing financial interest. Declaration of interest statement The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

J. Zou, J.-G. Yu / Analytica Chimica Acta 1088 (2019) 35e44

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