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Journal of Solid State Electrochemistry

, Volume 22, Issue 8, pp 2405–2412 | Cite as

Electrochemical recognition for tryptophan enantiomers based on 3, 4, 9, 10-perylenetetracarboxylic acid–chitosan composite film

  • Zunli Mo
  • Xiaohui Niu
  • Huhu Gao
  • Zhenliang Li
  • Shujuan Meng
  • Ruibin Guo
Original Paper
  • 88 Downloads

Abstract

A novel and simple chiral sensing platform had been successfully fabricated by means of amidation reaction between 3, 4, 9, 10-perylenetetracarboxylic acid (PTCA) and chitosan (CS) to form 3, 4, 9, 10-perylenetetracarboxylic acid–chitosan (PTCA–CS) composite film. Since CS has chiral center and PTCA has excellent electrical conductivity, the PTCA–CS composite modified glassy carbon electrode (PTCA–CS/GCE) could be treated as an effective electrochemical chiral sensor and applied for chiral discrimination of tryptophan (Trp) enantiomers theoretically. PTCA–CS composite was characterized by Fourier transform infrared (FTIR) spectroscopy and cyclic voltammetry (CV). When the prepared chiral sensing interface interacted with tryptophan isomers, a higher selectivity was received from D-Trp by differential pulse voltammetry (DPV). It indicated that the PTCA–CS/GCE can be treated as an electrochemical chiral sensor for the discrimination of Trp enantiomers. Further study demonstrated that the peak currents were linearly increased with the increasing percentage of L-Trp of Trp racemic mixture. Furthermore, the enantioselective interaction of the PTCA–CS/GCE was systematically studied by other experimental factors, such as the incubation time and acidity.

Keywords

Electrochemical chiral sensor Tryptophan enantiomers Enantioselectivity recognition 3, 4, 9, 10-perylenetetracarboxylic acid–chitosan composite 

Introduction

Chirality is the ubiquitous phenomenon of nature, playing a vital role in living organisms. Macroscopic chirality is caused by the chiral small molecules that constitute them. L-amino acids are the foundational component of protein synthesis in humans and animals. However, the monosaccharides that make up the polysaccharides are in the D-configuration. Several studies have suggested that individual enantiomer of a drug often has different pharmacological activity and biological activity. One enantiomer is therapeutically effective, while the other may not be effective, and even cause dangerous side effects [1]. At present, numerous analytical techniques have been applied for chiral discrimination of optically active chiral compounds, such as capillary electrophoresis [2, 3], fluorescence spectroscopy [4], high performance liquid chromatography [5, 6, 7], quartz crystal microbalance [8], and electrochemical measurements [9, 10, 11, 12, 13, 14]. A great deal of attention has been focused on electrochemical techniques for study of chiral discrimination due to their advantages of simple operation, low equipment requirements, low-cost, easy repeatability, environmentally friendly, and real-time operation [15, 16, 17, 18, 19].

3, 4, 9, 10-Perylenetetracarboxylic acid (PTCA), as an excellent aromatic organic dye, has drawn increasing attention in the fields of electrochemistry and field-effect transistors due to its brilliant chemical stability as well as good solubility [20, 21]. Moreover, PTCA has redox activity for its electronic property. Its electrical conductivity ranges from 10−1 to 10−2 S cm−1 [22]. In addition, PTCA-based materials have been carried over into the development of electrochemical sensors for its simple membrane-forming performance [23, 24]. In the regular methods, PTCA was just used for semiconductor template to develop PTCA-based materials for electrochemical sensors. Apparently, it has great potential and prospects for construction of electrochemical sensors.

Tryptophan is one of the essential amino acids (L-Trp) related to the component of proteins, which plays a significant role in maintaining nitrogen balance in humans and animals and can also act as an anti-depressant [25]. However, D-Trp has no apparent effect on physiological and pharmacological activity. Consequently, it is of great importance to identify optically active compounds in the field of pharmaceutical industry and biological sciences. The basic step in developing an electrochemical chiral biosensor is to fabricate a chiral interface with recognition sites for chiral recognition. Chitosan (CS), a natural polysaccharide, possesses outstanding chiral selectivity owing to the existence of large amounts of chiral sites [26]. Benifiting from its excellent water permeability, easy film-forming ability, good adhesion, and as nontoxicity, chitosan is commonly used for constructing electrochemical sensors and biosensors [27, 28, 29]. However, the construction of electrochemical chiral sensor based on chitosan-containing suffered some restrictions owing to dielectric and insulating properties of CS. But an effective method for electrochemical identifying tryptophan (Trp) was proposed by group of Yong Kong, offering an effective evidence for electrochemical chiral recognition of Trp enantiomers with protonated CS from the viewpoint of electrochemistry [30].

In this article, we prepared a novel electrochemical chiral sensor based on PTCA–CS composite film electrodeposited on a glassy carbon electrode (PTCA–CS/GCE) to design a simple method for enantiorecognition of Trp enantiomers. The electrodeposited PTCA–CS chiral interface combined the corresponding advantages of each element, such as the stereoselectivity of CS and the excellent electrical conductivity of PTCA. Therefore, the chiral interface was used for electrochemical discrimination of Trp enantiomers by differential pulse voltammetry (DPV). PTCA–CS/GCE demonstrated completely different selectivity to L-Trp and D-Trp, indicating a larger current response to D-Trp.

Experimental

Materials and reagents

N-(3-Dimethylaminopropyl)-N-ethylcarbodiimidehy drochloride (EDC), N-hydroxy succinimide (NHS), L-Trp, and D-Trp were purchased from Aladdin Chemistry Co., Ltd. (China). 3, 4, 9, 10-Perylenetetracarboxylic dianhydride (C24H8O6, PTCDA) was obtained from BaiYin LiangYou Chemical reagents Co., Ltd. Chitosan (CS), K4Fe(CN)6, and K3Fe(CN)6 were from Tianjin FuYu Fine Chemicals Co., Ltd. 0.1 M phosphate buffer solution (PBS) at different pH values was prepared with 0.1 M KH2PO4 and 0.1 M K2HPO4 containing 0.1 M KCl as supporting electrolyte. 5.0 mM [Fe(CN)6]4−/3− solution was prepared by K4Fe(CN)6 and K3Fe(CN)6. The supporting electrolyte was 0.1 M KCl. Ultra-pure water was applied throughout all the experiments.

Apparatus

Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were carried out on a CHI660 electrochemical workstation (Chen Hua Instruments Co., Shanghai, China) with a conventional three electrode system containing a saturated calomel electrode (SCE) as the reference electrode, a platinum wire as the counter electrode, and a modified glassy carbon electrode(GCE) as the working electrode. Fourier transform infrared (FT-IR) spectra of the samples were obtained from an EQUINOX-55 FTIR spectrometer.

Synthesis of PTCA-based composite

The whole procedure involved in preparing the PTCA–CS composite was displayed schematically as shown below: in short, PTCA was obtained from hydrolyzing PTCDA in KOH aqueous solution (2%) [31]. Subsequently, 40 mg of EDC and 60 mg of NHS were added into the PTCA solution respectively to activate carboxyl sufficiently on PTCA accompanied by continuous stirring for 2 h. One hundred milligram CS powder was added in 50 mL of 0.1 M acetic acid solution with continuous stirring for 2 h, and the as-prepared CS solutions were added dropwise into the activated PTCA within 20 min. The amidation reaction between PTCA and CS was stirred continuously for 4 h at room temperature. The synthesized product (expressed as PTCA–CS) was filtered and then washed thoroughly with 0.1 M acetic acid solution and distilled water to remove unreacted PTCA and CS, finally stored at 5 °C.

Fabrication procedure of the electrochemical biosensors

Prior to modification, the bare glassy carbon electrodes (GCEs) were polished respectively with 1.0, 0.3, and 0.05 mm alumina slurry and then cleaned by sonication in alcohol and double distilled water sequentially for 5 min each. Then, the bare GCE was placed into the PTCA–CS solution (2 mg mL−1) with a constant potential of + 0.5 V for 150 s. By contrast, the CS/GCE (60 mg of CS was dissolved in 30 mL of 0.1 M acetate for protonation) and PTCA/GCE were also fabricated by electrodeposition for 150 s at a constant potential of − 0.5 and + 0.5 V, respectively. Thus, the PTCA–CS/GCE, CS/GCE, and PTCA/GCE were fabricated for the subsequent experiments. Scheme 1 depicted the preparation process of the chiral composites and the stepwise construction of the working electrode.
Scheme 1

Schematic diagram of the construction of chiral sensor and the electrochemical responses to Trp enantiomers

Electrochemical chiral recognition of tryptophan isomers

Electrochemical chiral recognition of Trp isomers by PTCA–CS/GCE was conducted by differential pulse voltammetry. The as-prepared PTCA–CS chiral interface was immersed into 25 mL 5 mM L-Trp or D-Trp solution (pH 6.0) for 5 min, respectively. Then, the differential pulse voltammograms of these systems were compared (PTCA-L-Trp vs PTCA-D-Trp, CS-L-Trp vs CS-D-Trp; PTCA–CS-L-Trp vs PTCA–CS-D-Trp). Every differential pulse voltammetry was repeated five times to calculate for the error bars. To study the pH-sensitive properties of the PTCA–CS chiral sensor, the enantiorecognition between PTCA–CS and Trp isomers was performed at t pH range from 4.5 to 7.5.

Results and discussion

Characterization of PTCA–CS

Figure 1 displayed the FT-IR spectra of PTCDA, CS, and PTCA–CS. The assignments of the major chemical bands of PTCDA could be shown as follows: the peaks at 1583.92 and 3110.65 cm−1 were ascribed to stretching vibration of the aromatic C=C and –CH in PTCDA, respectively. The peak at 1690.01 cm−1 in the spectrum of PTCDA was assigned to vibration of C=O, and 1294.32 cm−1 was the vibration of C–O–C. For the FT-IR spectrum of pure CS, the characteristic peaks of –NH2 appeared at 3441.49 cm−1. PTCA−CS displayed similar bands compared with CS and PTCDA. It was observed that the characteristic peak of 1090.21 cm−1 (C–N) and 3409.01 cm−1(N–H) in the spectra of PTCA−CS indicates the successful formation of amide bond (CO–NH) between PTCA and CS.
Fig. 1

FT-IR spectra of PTCDA, CS, and PTCA–CS

Electrochemical properties of different modified electrodes

The electrochemical behaviors of the different working electrodes were investigated by cyclic voltammetry (CV) at a scan rate of 100 mV s−1. Herein, the CVs of bare GCE, PTCA/GCE, CS/GCE, and PTCA–CS/GCE were studied in 5.0 mM [Fe (CN)6]4−/3− solution containing 0.1 M KCl (Fig. 2). There was a pair of well-defined redox peak appearing at the bare GCE (curve a), which was ascribed to redox reaction between Fe(CN)64− and Fe(CN)63−. After PTCA modification, the peak currents decreased evidently owing to the repulsion between [Fe(CN)6]4−/3− and the oxygen-containing functional groups on PTCA (curve b). When PTCA–CS/GCE immersed in 5.0 mM [Fe (CN)6]4−/3− solution, a higher peak current was observed (curve c), which was attributed to the affinity between negatively charged [Fe(CN)6]3−/4− and positively charged CS. However, the peak current at the PTCA–CS/GCE still descended a little compared with bare GCE, providing an indirect evidence that PTCA attach to the CS successfully. After protonated CS was modified on a glassy carbon electrode, the maximum peak current was observed (curve d), which can be attributed to the strong attraction between [Fe(CN)6]3−/4− and positively charged CS.
Fig. 2

The CVs of different electrodes in 5.0 mM [Fe (CN)6]4−/3− containing 0.1 M KCl: (a) bare GCE, (b) PTCA/GCE, (c) PTCA–CS/GCE, (d) CS/GCE

The PTCA–CS/GCE was repeatedly cycled in 5.0 mM [Fe (CN)6]4−/3− solution containing 0.1 M KCl. After scanning for 40 cycles continuously, the chiral biosensor had excellent reproducibility and stability with the relative standard deviation (RSD) of 3.1% (n = 13).

The interaction between PTCA–CS and the Trp enantiomers

The electrochemical chiral recognition of L- or D-Trp bound to PTCA–CS/GCE was investigated by DPV in 5 mM L-Trp and D-Trp (pH = 6) for 5 min. To compare the recognition efficiency, several other working electrodes including bare GCE, PTCA/GCE, and CS/GCE were applied for the control experiments. It was observed that the peak currents of L- and D-Trp almost overlapped on bare GCE (Fig. 3a). Although the peak currents increased remarkably at the PTCA/GCE for the excellent electron transport ability of PTCA, the ratio of the peak currents (IL/ID) was too small (1.1) to discriminate the Trp enantiomers effectively (Fig. 3b), which was ascribed to the lack of chiral microenvironment on PTCA. When CS modified electrode (Fig. 3c), the differences in the peak currents ratio increased to a certain extent (IL/ID = 1.2). There can be no doubt that the rising recognition efficiency was related to the existence of chiral microenvironment derived from CS for stereoselective recognition Trp enantiomers, since the CS can produce different steric hindrance when combined with L-Trp (or D-Trp). However, the recognition results were still not satisfactory. The peak current ratio (IL/ID = 2.6) was maximum at the PTCA–CS/GCE (Fig. 3d). To our interest, it was observed that PTCA–CS showed completely different selectivity for optically active Trp. That is, PTCA–CS exhibited higher affinity for D-Trp than L-Trp, resulting in larger peak current of L-Trp because the insulation of Trp inhibited the transmission of electrons. There was no doubt that PTCA and CS played synergistic effect in improving the recognition efficiency. PTCA can expand electrochemical signal and CS can provide chiral sites.
Fig. 3

Differential pulse voltammograms of L-Trp and D-Trp at bare GCE (a), PTCA/GCE (b), CS/GCE (c), and PTCA–CS/GCE (d). Scan rate, 50 mV s−1; scan range, − 0.2 to 0.6 V

The general mechanisms for chiral recognition were ligand exchange and three-point interaction. When the metal ions such as Cu2+, Fe2+, and Zn2+ were absent, chiral recognition mechanism was three-point interaction which is caused by the intermolecular forces including π-π interactions, steric effects, inclusion formation, hydrogen-bond interactions, absorption interactions, or Van der Waals’ force between the chiral selectors and analytes [32]. It was observed that CS had different interaction for L-Trp and D-Trp, which was probably ascribed to stronger three-point interaction between PTCA–CS chiral composites and D-Trp through hydrogen-bond interactions and π-π interactions. However, when L-Trp approached to PTCA–CS, L-Trp may exhibit larger steric hindrance compared with D-Trp and cannot effectively establish three-point interaction model. There can be no doubt that the higher recognition efficiency was related to the existence of chiral microenvironment derived from PTCA–CS, since step of chiral recognition can form diastereoisomeric complexes between PTCA–CS, thus resulted in the different Gibbs free energy between the PTCA–CS-two diastereoisomeric enantiomer complexes [33]. A mechanism of chiral recognition was displayed in Fig. 4.
Fig. 4

The possible chiral recognition mechanism

Influence of incubation time and pH on enantiorecognition

Finally, the influences of incubation time and pH-sensitive for the chiral discrimination were studied. As shown in Fig. 5a, recognition efficiency (IL/ID) of L-Trp and D-Trp increased significantly ranging from 1 to 5 min, then the peak currents ratio (IL/ID) decreased with prolongation of the incubation time of 5 to 6 min. And a response almost invariably appeared after 6 min, indicating the active sites at the modified electrode surface were saturated. The recognition efficiency reached the maximum response signal when the interaction time was 5 min. Consequently, the optimal interaction time was 5 min.
Fig. 5

Influence of incubation time (a) and pH (b) on the enantiorecognition efficiency of the PTCA-modified GCE toward Trp isomers. Errors bars represent the standard deviation for four independent measurements

As displayed in Fig. 5b, the electrochemical chiral recognition of Trp enantiomers was dependent on the pH of the solution. The electrochemical properties of Trp enantiomers on the PTCA–CS chiral sensing platform at pH values of 4.5, 5.0, 5.5, 6.0, 6.5, 7, and 7.5 were investigated. It is obviously that peak current ratio increased with increasing pH (4.5–6.0). However, the recognition efficiency had not risen but deteriorated with the increase of pH. The low efficiency at low pH values was most likely due to the decomposition of natural polysaccharide including CS in strong acidic medium, leading to the instability of the PTCA–CS recognition systems on GCE [34]. It was reported that the isoelectric point of tryptophan is 5.89 [35]. Therefore, Trp isomers were negatively charged when pH exceeded 6. The electrostatic repelling action between negatively charged L-Trp or D-Trp and a large quantity of oxygen groups on PTCA would reject the combination of Trp enantiomers with the PTCA–CS composite, resulting in reduced recognition efficiency with the PTCA–CS/GCE. Therefore, it was obviously that any interaction might undoubtedly affect enantioselectivity. It can be observed that the maximum peak current ratio appeared at pH = 6. Accordingly, pH = 6 was applied in experiments.

Current response to different concentrations of Trp enantiomers

To get further understanding about the relationship between concentration of Trp enantiomers and current response, DPV measurements were conducted over a series of concentration of L-Trp and D-Trp solution (pH = 6). Figure 6 has shown the peak current change (∆I=I0-I) after the chiral interface (the peak current expressed as I0) was incubated in Trp enantiomers solution for 5 min (the peak current expressed as I). The linear regression for D-Trp and L-Trp was expressed as ∆I = (9.8 ± 0.2) cD-Trp + (4.20 ± 0.16) (R2 = 0.997) (line b) and ∆I = (4.60 ± 0.16) cL-Trp + (2.7 ± 0.4) (R2 = 0.993) (line a), respectively. As the results showed, D-Trp had larger current response than L-Trp at the chiral interface and provided further evidence that the prepared chiral interface could be applied for chiral recognition.
Fig. 6

Peak current change of the chiral interface toward (a) L-Trp and (b) D-Trp with different concentrations (0.01, 0.10, 0.50, 1.0, 2.0, 3.0, 4.0, 5.0 mM) (pH = 6)

Application of chiral biosensor

The PTCA–CS/GCE was also used to detect the current responses of Trp enantiomer in different fixed ratio racemic solution (L-Trp %). Different racemic solutions were prepared with the percentage composition of L-Trp range from 0 to 100% with the total concentration maintaining at 1.0 mM (line b) and 5.0 mM (line a). It was obviously that the peak currents (Ip) were uniformly increased with the increasing amount of L-Trp. Figure 7 displayed linear relationships between the percentage of L-Trp and Ip at different concentrations. The linear regression was expressed as Ipa = (0.400 ± 0.008) L-Trp% + (25.0 ± 0.5) (R2 = 0.998) (line a) and Ipb = (0.250 ± 0.006) L-Trp% + (14.0 ± 0.4) (R2 = 0.997) (line b). To the best of our knowledge, Table 1 gave comparison of the proposed PTCA–CS/GCE with other techniques applied in the electrochemical enantiorecognition of Trp isomers. A new method was proposed for chiral recognition of Trp enantiomers with higher selectivity than most of previous studies [30, 36, 37, 38].
Fig. 7

Current response of enantiomeric composition L-Trp % at different concentrations: (a) 5.0 mM; (b) 1.0 mM

Table 1

Comparison of different modified electrodes for enantiorecognition of Trp isomers

Electrode substrate

pH

Recognition mechanism

I L /I D

Ref.

l-Cys/Au

5.4

Ligand exchange

1.4

[36]

HSA/MB–MWNT/GCE

7.4

Three-point interaction

2.4

[37]

GQDs–CS/GCE

7.0

Three-point interaction

2.1

[30]

β-CD-PtNPs/GNs/GCE

7.0

Three-point interaction

1.3

[38]

PTCA–CS/GCE

6.0

Three-point interaction

2.6

This work

Conclusion

In summary, a simple and feasible electrochemical chiral sensor with high sensitivity for recognition of L- and D-Trp was designed in this study. The recognition efficiency at the PTCA–CS/GCE was higher than those at PTCA/GCE and CS/GCE (or bare GCE) owing to the synergistic effects between PTCA and CS. This work may open up a new application of PTCA for the fabrication of the feasible and convenient electrochemical chiral biosensors for the electrochemical chiral recognition of other optically active compounds.

Notes

Funding information

This work was supported by the National Natural Science Foundation of China (51262027), the financial support of the Natural Science Foundation of Gansu Province (1104GKCA019 and 1010RJZA023), the Science and Technology Tackle Key Problem Item of Gansu Province (2GS064-A52-036-08) and the fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201011).

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical EngineeringNorthwest Normal UniversityLanzhouPeople’s Republic of China

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