Elsevier

Electrochimica Acta

Volume 241, 1 July 2017, Pages 386-394
Electrochimica Acta

Enantiorecognition of Tyrosine Based on a Novel Magnetic Electrochemical Chiral Sensor

https://doi.org/10.1016/j.electacta.2017.04.155Get rights and content

Highlights

  • A novel magnetic electrochemical chiral sensor was fabricated and characterized.

  • The enantiorecognition efficiency was improved by magnetic field-induced assembly.

  • The sensing interface exhibited higher affinity for L-Tyr than D-Tyr.

  • The sensor showed wide linear range, low detect limit and high stability.

  • The enantiomeric composition can be determined within the Tyr mixtures.

Abstract

Chiral selector-functionalized magnetic nanoparticles were achieved by self-assembly anchoring L-cysteine (L-Cys) on the surface of the Au/Fe3O4 nanocomposites, which was synthesized via a facile in-situ reduction process. The as-prepared L-Cys-Au/Fe3O4 magnetic nanoparticles (MNPs) were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, ultraviolet-visible spectroscopy, Fourier-transform infrared spectroscopy and electrochemical methods The typical MNPs with uniform globular morphology not only can provide the enhanced pore accessibility and excellent electrocatalytic active site for the guest species, but also effectively improve the load capacity of chiral selector of L-Cys. Based on magnetic field-induced assembly, a novel magnetic electrochemical chiral sensor (MECS) was first constructed by MNPs for the discrimination and determination of tyrosine (Tyr) enantiomers. Under the optimized experimental condition, the oxidation peak current ratio of L-Tyr to D-Tyr (IL/ID) and the difference between the peak potential of the two enantiomers (ΔEp = EDEL) was observed to be 1.85 and 84 mV according to square wave voltammetry (SWV). With the synergic effect of the magnetic Fe3O4 NPs and the chiral electrocatalytic activity of L-Cys functionalized AuNPs, a linear relationship between the peak current and the concentration of the two enantiomers was obtained over the range of 1–125 μM, with the limit of detection of 0.021 μM and 0.084 μM for L- and D-Tyr, respectively. The preferential chiral recognition for L-Tyr was further verified by circular dichroism spectroscopy. Furthermore, the sensitive MECS has been successfully used for the practical determination of L- or D-enantiomer in the Tyr mixture solutions by continuously increasing the concentration of one isomer, showing the consistent and reproducible results.

Introduction

Chirality is a universal phenomenon in nature, which plays a prominent role in the molecular/biomolecular level and has tremendous impact on the fields of biology, medicine, pharmaceutical sciences and biotechnology [1]. Many biologically active substances (amino acids, enzymes, proteins, carbohydrates, DNA, ect.) and more than 50% of the 500 top selling drugs are chiral [2]. So far has been recognized, the two enantiomers of a chiral molecule possess identical physical and chemical properties, except for the rotation effect upon a plane of polarized light. However, they may make quite a difference in terms of pharmacological activities, biological interactions and metabolic behaviors in biological systems [3]. Usually, one of the enantiomers may be effective in the disease treatment and exhibit the desirable traits in the chiral recognition process, while the other may be ineffective or even cause serious side effects and toxicity [4], [5]. Accordingly, considering the enantioselective bioactivity of chiral molecules, a strong demand has arisen to develop efficient analytical methods which could discrimination and determination the individual enantiomers in complex samples, especially in the fields of pharmaceutical science, food control and clinical analysis [6].

Various techniques to discriminate enantiomers, such as gas chromatography (GC) [7], high-performance liquid chromatography (HPLC) [8], capillary electrophoresis (CE) [9], colorimetric analysis [10], [11], fluorescence spectroscopy [12], have been performed by using either indirect or direct strategies [1]. However, most of these techniques mentioned above require expensive instrumentation, complicated sample pretreatment or time-consuming process. Nowadays, electrochemical sensor has been received considerable attention as a promising approach for chiral recognition with the advantages of high sensitivity, fast response and relative simplicity [2], [13]. The more preferable direct recognition was achieved by the formation of the diastereomeric complexes between a specific chiral selector and the analyte (a pair of enantiomers) via the combinations of different molecular recognition modes including hydrogen bonds, ionic interactions, ion-dipole or dipole-dipole interactions, π–π interactions and van der Waals interactions [14]. The most common chiral selectors used are cyclodextrins (CDs) and their derivatives [15], bovine serum albumin [16], cellulose nanocrystal (CNCs) [17], crown ethers [18], calf thymus DNA [19], and so on. The key step for fabricating an electrochemical chiral sensor is to construct a chiral sensing surface possessing recognition sites with different affinities towards certain enantiomers. Despite several efforts have been devoted to the electrochemical recognition of enantiomers, novel chiral electrode materials with unique properties as transducer platform for the electrochemical sensing still remain a considerable challenge.

The distinguished magnetic and electrochemical properties of Fe3O4 nanoparticles (Fe3O4 NPs) make them a desirable candidate for the design of electrochemical sensors [20]. However, unfunctionalized Fe3O4 NPs are easily oxidized in air and tend to irreversibly aggregate, which might in turn hinder the electron transfer ability [21]. On the other hand, metallic nanoparticles, especially, gold nanoparticles (AuNPs) exhibit several desirable electrochemical features for the fabrication of sensor, such as good electrocatalytic activity, excellent biocompatibility, easy surface modification and other related properties [22]. Therefore, incorporation of AuNPs with Fe3O4 NPs could effectively enhance the electrical conductivity, catalytical activity and chemical stability of the nanocomposites. Meanwhile, the introduced Au nanoparticles also could facilitate the binding affinity of thiolated compounds through Au-S interaction for further functionalization on the surfaces [23].

Amino acids, the building blocks for the proteins, are very important chiral bioactive substances [24]. They have found widespread usage in the synthesis of drugs, agrochemicals, food additives and fragrances [25]. Among the aromatic amino acids, L-tyrosine (L-Tyr) is an important nonessential amino acid which serves as a precursor of several neurotransmitters, including dopa, dopamine, epinephrine, catechol and thyroxin [26]. Depletion in tyrosine level could lead to Parkinson’s disease, hypochondrium, albinism, alkaptonuria and other psychological diseases, while excess of tyrosine in vitro culture medium could cause the increase of sister chromatid exchange [27]. As a non-protein amino acid, D-Tyr was used as a probe to study the conformation and dynamics of protein molecules [44]. Since tyrosine is an electro-active chiral compound, it is a feasible, promising alternative for chiral analysis via electrochemical techniques.

In our previous studies, magnetic field directed self-assembly (MDSA) has been confirmed to be an efficient, convenient and specific approach to construct the magnetic molecularly imprinted electrochemical sensors (MMIES) for the determination of different biomarkers with the controllable and orderly oriented nanostructures [28], [29], [30], [31], [32], [33]. Here, we use thiol-containing natural amino acid, L-cysteine (soluble in aqueous solution with a wide pH range) not only as a chiral selector but also as a stabilizing ligand to synthesis Au/Fe3O4 MNPs via a facile reduction process. The as-prepared hybrids were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), UV, FT-IR, circular dichroism (CD) spectroscopy and electrochemical methods. Then, based on magnetic field-induced assembly, a magnetic electrochemical chiral sensor (MECS) was constructed and employed for the fast, sensitive determination of tyrosine enantiomers. Besides, the practical enantiorecognition of L- or D-enantiomer in the Tyr mixture solutions was also evaluated by square wave voltammetry.

Section snippets

Reagents and chemicals

D/L-tyrosine, D/L-cysteine (Cys), D/L-tryptophan (Trp), D/L-phenylalanine (Phe), uric acid and dopamine (DA) were purchased from Sigma-Aldrich Corp (USA). Chloroauric acid tetrahydrate (AuCl3·HCl·4H2O) was obtained from Sinopharm Chemical Reagent (Shanghai, China). Dopa, epinephrine, nor-epinephrine, (3-aminopropyl) triethoxysilane (APTES) and ascorbic acid (AA) were purchased from Aladdin reagent (Shanghai, China). Phosphate buffer (PB) solutions were prepared with 0.1 M NaH2PO4, 0.1 M Na2HPO4

Characterization of L-Cys-Au/Fe3O4 MNPs

Scanning electron microscopy (SEM) was used to characterize the surface morphologies and nanostructures of Fe3O4–NH2, Au/Fe3O4 and L-Cys-Au/Fe3O4 on the modified MGCE. As shown in Fig. 1A, the surface of Fe3O4–NH2/MGCE exhibits a porous, reticular structure. After in-situ incorporation of AuNPs (Fig. 1B), many observable light dots spread uniformly and evenly over the entire surface, indicating the formation of Au/Fe3O4 NPs, and the diameter of the gold nanoparticles was about 20 nm. Fig. 1C

Conclusions

To summarize, a novel magnetic electrochemical chiral sensor with the assistance of magnetism was developed for the discrimination and quantitative determination of Tyr enantiomers. In the fabricated MECS, the L-Cys-Au/Fe3O4 MNPs could be well-oriented based on magnetic field-induced assembly and assembled orderly on the surface of MGCE, leading to the formation of uniform magnetic porous nanointerface. Ascribing to the synergistic effect of magnetic Fe3O4 NPs and gold nanoparticles as well as

Acknowledgments

This work was supported by Natural Science Foundation of Jiangsu Province (No. BK20141433), National Natural Science Foundation of China (No. 81572081).

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