Elsevier

Talanta

Volume 232, 1 September 2021, 122445
Talanta

A disposable dual-signal enantioselective electrochemical sensor based on stereogenic porous chiral carbon nanotubes hydrogel

https://doi.org/10.1016/j.talanta.2021.122445Get rights and content

Highlights

  • A strategy to fabricate chiral carbon nanotubes hydrogel-based electrochemical chiral sensor was presented.

  • The practicability of the sensor was validated by discriminating mandelic acid enantiomers.

  • Such chiral hydrogel has potential application in constructing simpler and more reliable chiral electrochemical sensors.

Abstract

As the highest form of molecular recognition, the chiral molecular recognition is the most difficult measurements. Herein, a disposable dual-signal enantioselective platform was fabricated based on stereoscopic porous chiral carbon nanotubes hydrogel modified screen printed electrode. This kind of chiral hydrogel was prepared by a simple heating method with l-cysteine and chiral single-walled carbon nanotubes of chirality (6,5), and its dispersion and morphology were characterized by several techniques including scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The stereogenic chiral interface was successfully employed to discriminate mandelic acid enantiomers via both oxidation peak intensity and peak potential value of cyclic voltammetry. The chiral recognition mechanism was discussed specifically, which resulted from the formation of an efficient three-dimensional chiral nanospace. The inherent chirality of chiral carbon nanotubes hydrogel, together with their orderly spatial arrangement, can significantly improve the efficiency of chiral recognition compared with traditional electrochemical chiral sensors. As a novel chiral sensing interface, such chiral carbon nanotubes hydrogel was simple to prepare, fast to operate, with good sensitivity and excellent stability for the construction of efficient and practical electrochemical chiral sensors.

Graphical abstract

We presented a new strategy to fabricate enantioselective electrochemical sensor based on stereogenic porous chiral carbon nanotubes hydrogel.

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Introduction

Because of the high selectivity of chiral molecular species in nature and life, chiral molecular recognition is one of the most fundamental and crucial properties of various natural systems [1]. The chiral sensor, due to its simple preparation, rapid detection, cheap apparatus and a possibility for real-time and on-line analysis, has drawn extensive attention in the field of chiral recognition, especially electrochemical sensors [2,3]. The key step for selectively sensing chiral molecules is to build an efficient chiral surface with recognition sites to enantiomers. However, one major drawback contributing to modest or labile chirality manifestations is that in most of the selectors considered, the chirality source is not intrinsic to the whole selector, but either the localized or the external. In this context, a breakthrough may come by exploiting the “inherent chirality” strategy, according to which the stereogenic element responsible for chirality coincides with the functional group responsible for the molecular material-specific property [4]. To our best knowledge, in chiral electrochemical sensing much attention has been given to the amplification effect of carbon nanotubes (CNTs) on chiral selectors [3,[5], [6], [7], [8], [9], [10]], whereas less research was conducted on the inherent chirality of CNTs [[11], [12], [13], [14], [15]]. Therefore, it is highly desirable to develop enantioselective electrochemical sensing based on such strategic chiral CNTs with good conductivity, chemical stability, biocompatibility and intrinsic chiroptical properties, which may have potential applications in constructing simpler and more reliable chiral electrochemical sensors.

As for the chiral recognition ability of chiral CNTs, Sholl et al. described atomistic computational modelling of chiral disubstituted cycloalkanes adsorbed in chiral single-walled carbon nanotubes (SWCNTs), and it was shown that chiral SWCNTs would not act as effective enantiospecific adsorbents [16]. Nevertheless, several reports also demonstrated the feasibility of using the inherent chirality of SWCNTs for chiral discrimination from a theoretical perspective. For instance, calculations performed by Girardet et al. demonstrated that enantiomers should in principle be selectively discriminated on chiral SWCNTs when being used in a resonator configuration [[17], [18], [19], [20], [21], [22], [23]]. Additionally, Dasgupta et al. reported the differential binding nature of pristine SWCNTs with tryptophan enantiomers, and it was shown that due to their topological complexity, topological chiral SWCNTs could discriminate geometrically chiral molecules [24]. However, the practical enantiorecognition of various chiral molecules via chiral CNTs still remains a necessary and challenging task.

In our previous studies, a chiral electrochemical sensor based on multi-walled carbon nanotubes/ionic liquids nanocomposite was first developed and applied to the enantiomeric recognition of propranolol. The mechanism for chiral sensing was proven to originate from the formation of an efficient chiral nanospace [11]. Then, an electrochemical sensor capable of providing antipodal signals was demonstrated for enantioselective recognition of 3,4-dihydroxyphenylalanine (DOPA) based on chiral SWCNTs. The presence of sulphuric acid and the application of square wave voltammetry (SWV) amplified the intrinsic chirality of chiral SWCNTs [12,13]. Last, a new strategy to fabricate vertically aligned chiral SWCNTs array-based electrochemical sensor was presented in combination with DOPA as a model molecule. The highly ordered standing of SWCNTs can further enhance the inherent chirality of chiral SWCNTs [14]. Meanwhile, Shi et al. presented a similar chiral electrochemical sensor which amplified the chiral selection on the electrode surface based on the left- or right-handed semiconducting SWCNT enantiomers with (6,5)-enriched chirality. It was shown how the structure of chiral SWCNTs influenced electrochemical chiral recognition [15].

The only reason why the design of chiral electrochemical sensors is a great challenge results from only one single unitary process of separation, which corresponds to one theoretical plate in chromatographic separations. Though the application of SWV with each tread superimposed by a symmetrical double pulse can extend the interaction time between chiral CNTs and enantiomers, and the highly ordered standing of SWCNTs on electrode surface seems to be encouraging in improving the efficiency of chiral recognition as well, the chiral recognition space of sensors is still very limited owing to two-dimensional and single-time chiral recognition. Using conductive hydrogels to fabricate stereogenic chiral interface is expected to realize multiple-time recognition of chiral molecules in a small space, which is similar to a multiple sequential separation unitary processes in high-performance separation methods with a large number of theoretical plates.

Herein, we prepared a hydrogel modified electrode (p-L-Cys-65CNTs/SPE, p stands for polymer) by using an uncomplicated heating method with simple and accessible l-cysteine (L-Cys), carboxylated chiral (6,5)-SWCNTs (named as 65CNTs) and disposable screen-printed electrode (SPE), which was further applied to discriminate electrochemical signals of mandelic acid (MA) enantiomers. As a kind of α-hydroxycarboxylic acid, MA is the structural unit of many natural products and drug molecules, and chiral MA plays a significant role in the pharmaceutical synthetic industry [25]. Compared with the published work regarding electrochemical sensors to enantio-selectively detect MA enantiomers [6,[25], [26], [27], [28], [29], [30], [31], [32]], the features of this new approach are simplicity, dual signal, sensitivity and disposability. Moreover, the use of disposable SPE can eliminate the problems of fouling and surface regeneration of the electrochemical sensor.

Section snippets

Apparatus

Electrochemical measurements were conducted with a CHI 660 E electrochemical workstation (Shanghai Chenhua Instruments, China). The three-electrode electrochemical cell contained a bare or modified SPE, a platinum foil (6 × 2 mm) as an auxiliary electrode and a saturated calomel reference electrode (SCE). Electrochemical impedance spectroscopy measurements were performed in the frequency range of 0.1 Hz–100 kHz at an amplitude of 5 mV. The electrolyte was 0.1 M KCl containing 2 mM K3[Fe(CN)6]/K4

Physicochemical characterization of chiral carbon nanotubes hydrogel

Scanning electron microscopy (SEM) was employed to characterize the morphologies of two polymeric materials, i.e. polymerized L-Cys (p-L-Cys) and conductive hydrogel (p-L-Cys-65CNTs) formed by L-Cys and 65CNTs. It can be observed from Fig. 1 that both polymeric materials have porous morphologies capable of providing permeable channels in the polymeric network. But the pore size of p-L-Cys is significantly larger than that of p-L-Cys-65CNTs, which might associate with whether thiol group (-SH)

Conclusions

Some CNTs with intrinsic chirality have the potential to open up new avenues for researchers seeking to develop a new chiral coordination environment for enantioselective applications. Such inherent chirality of chiral CNTs as a chiral selector can be enhanced through forming a so-called stereoscopic porous chiral hydrogel. Moreover, the use of disposable and low-cost SPE can radically eliminate the common problems of fouling and surface regeneration of the electrochemical device. In short,

Credit author statement

Yang Yang: Conceptualization, Methodology, Investigation, Data curation, Writing – original draft. Meixian Li: Supervision. Zhiwei Zhu: Methodology, Supervision, Writing- Reviewing and Editing

Declaration of competing interest

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.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21775007) and the National Key Research and Development Program of China (Grant No. 2016YFA0201300).

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