Using a nanocomposite consist of Boron-doped reduced graphene oxide and electropolymerized β-cyclodextrin for Flunitrazepam electrochemical sensor

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Highlights

  • A sensor is based on a modified GCE with a nanocomposite of B-RGO and Eβ-CD.

  • The sensor shows sensitive and selective behavior for Flunitrazepam.

  • Two linear ranges of 2 nM to 0.5 μM and 0.5 μM to 20 μM were achieved by the sensor.

  • Limit of detection (LOD) was estimated to be 0.6 nM.

  • The sensitivity value was calculated to be 8.4 μA µM cm−2.

  • A clever composition of this materials makes active sites for detection target.

Abstract

Flunitrazepam, or the same “Rohypnol” amongst other names, is a very powerful benzodiazepine that is prescribed to cure intense insomnia. Electropolymerized β-cyclodextrin (Eβ-CD) and Boron-doped reduced graphene oxide (B-RGO) were used to prepare a composite material which was used for modifying glassy carbon electrode (GCE). The resulting modified GCE was then used for the determination of traces of Flunitrazepam. Using various analytical techniques the surface of the modified GCE was investigation. The modified GCE was used in differential pulse voltammetry (DPV) and cyclic voltammetry (CV) analyses. Experimental factors affecting the results were evaluated and optimized. Two linears range for Flunitrazepam determination were from 2.0 nmol L−1 to 0.5 μmol L−1 and 0.5 μmol L−1 to 20.0 μmol L−1. The detection limit (DL) and sensitivity were calculated to be of 0.6 nmol L−1 and 8.4 μA µmol L−1 cm−2, respectively for the modified electrode. The modified electrode by nanocomposite consist of B-RGO and Eβ-CD demonstrated a number of advantages: a simple preparation route, high selectivity and sensitivity, a low detection limit, excellent reproducibility and high surface-to-volume ratio. The sensor was applied for the analysis of Flunitrazepam concentration in human blood serum samples.

Introduction

Benzodiazepines such as Flunitrazepam (Scheme 1) have been receiving more attention in relation to their illicit use in assaults and robberies. Flunitrazepam is an anxiolytic and hypnotic drug know better under the name “Rohypnol” which is generally administered as a brief term treatment for sleeping disorders like insomnia [1]. Flunitrazepam is used illegally as a “date rape drug” by spiking alcoholic drinks above the recommended pharmacological dose of 0.5–1.0 mg in adults to produce a prolonged and extreme intoxication [2]. The sedative effect of the drug is increased by alcohol consumption which creates psychomotor impairment and causes the victim to suffer from a “blackout”, a type of a short term amnesia that forestalls the victim from recalling much, if any, of the attack. The low dosage and high biotransformation makes its analysis very problematic drugs since it is so rapidly cleared out from the body [2].

Flunitrazepam can be analysed via traditional analytical methodologies such as Fluorescence spectroscopy [3], Desorption Electrospray Ionization Mass-Spectrometry [4] and Gas Chromatography-Mass Spectroscopy (GC–MS) [5], [6], [7].

Despite their accuracy and reliability, most of the methods have some practical disadvantages such as high cost, time consummation, complexity and lack of sufficient sensitivity. Thus, although a significant improvement has been made, a portable and reliable method for the sensitive and selective measurement of Flunitrazepam still faces challenges. Currently, Flunitrazepam detection strategies are trended toward development a few techniques based on electrochemical methods. Electrochemical sensors have recently been used for this purpose [8], [9], [10] because of their ease of operation, the simplicity of the equipment required, the reasonable costs, short analysis times and high sensitivity [11], [12]. However, there is still a need for sensitive and selective analysis tools. Various nanomaterials have been used in analytical tools and techniques such as capacitors, electronic devices and sensors [13], [14]. An example is graphene, a 2D carbon-based nanostructure with high conductivity and catalytic activity, which has been used in the development of various sensing devices [11], [12], [13], [14].

The graphene utilized in such research is produced through the reduction of graphene oxide (GO), which might introduce defections like hydroxyl and carboxyl into the structure of graphene, depressing the electrons transferring property, increasing the defects of graphene and leading to the decrease of electrochemical effects [15]. To solve this problem, two common methods are utilized: first is to construct graphene to a 3D structure aerogel due to improved electron transport properties and second is to dope graphene with heteroatoms such as N, B, S and P attributing to the enhancement of conductivity and electron transport property [16]. Boron atom than other heteroatoms has been more applied. Boron atom can create extra p-electrons to the π-bonding system, giving rise to the increase of carrier's density and then raise the electron transportation speed in graphene. Therefore, the recombination between the photo-excited electrons and holes would be reduced, and the electrochemical and photocatalytic performance is enhanced. Moreover, boron substitutional doping in graphene lattice can conserve sp2 bonding structure, leading to the decrease of defection caused by sp3 bonding and Boron-doped reduced graphene oxide owns property of p-type semiconductor [16].

Overall, as compared with none-doped graphene, Boron-doped reduced graphene oxide is more suitable for composite system and excellent electrochemical property of Boron-doped reduced graphene oxide allow it to serve as a good support to enhance the efficiency and rate of by nanoparticles and nanocomposites [15], [16].

β-cyclodextrin (β-CD) (Scheme 2) is a natural cyclic oligosaccharide with a toroidal configuration having an inner hydrophobic cavity and an outer hydrophilic shell, which consists of seven glucose units [14]. The structure of β-CD enables it to incorporate molecules of appropriate size into its cavity and form stable host–guest inclusion complexes [14] through non-covalent van der Waals, hydrophobic and electrostatic interactions [14], among which hydrogen bonding is a key element in stabilizing the complexes. Naturally a stronger hydrogen bonding species than the guest molecule can form stronger intermolecular interactions with the cavity of the host species. Different analytes may differ not only in the nature and location of their hydrogen bonding groups, but also in the number of hydrogen donors and the resulting host–guest interaction energies, which allows for the selective determination of specifically suitable target species. Consequently, β-CD has been widely evaluated as a modifier for electrodes for the selective detection of electroactive molecules [14].

Electrochemical routes offer advantages like moderate cost, simplicity and high speed, and therefore the possibility of use in miniaturized applications. To enable use of this technique, various materials (e.g. conducting polymers [14], metal nanoparticles [13], selective membranes [17], and carbon nanomaterials [18]) have been evaluated as modifiers.

Recently, efforts have been made to synthesize cyclodextrin/graphene hybrid materials, which simultaneously offer the excellent electrochemical properties of graphene and the high supramolecular sensing potentials of cyclodextrin [19]. An instance is the work of Wang et al. [20], who prepared a β-CD-graphene composite via covalent bonding. These methods led to the formation of cyclodextrin/graphene hybrids; however, the focus of these studies has been on the covalent bonding of the components, which limits the extent of cyclodextrin immobilization and consequently impairs the material’s supramolecular recognition ability. Also, given that monomeric β-CD is not a conductive species, introducing β-CD onto the graphene surface decreases its electrical conductivity [21]. Hence a great deal of effort has been made to develop new methods for preparing cyclodextrin/graphene hybrids. For instance, recent studies indicate that β-CD polymers not only offer good conductivity, but also provide a large number of β-CD units to interact with the guest molecules [14].

It seems that a combination of the B-RGO and Eβ-CD may be a good choice to form a promising nanocomposite with high supramolecular recognition capabilities as well as enhancing conductivity. The objective of the present work is to develop a sensor by Eβ-CD on the B-RGO modified GCE in measuring of the Flunitrazepam (Scheme 3). To the simplest of the authors’ knowledge, no report has been introduced yet on electrochemical detection of the Flunitrazepam supported the Eβ-CD/B-RGO nanocomposite. The properties of the developed Eβ-CD/B-RGO/GCE were studied using electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM), and it had been also used for cyclic voltammetric (CV) and differential pulse voltammetric (DPV) determination of Flunitrazepam. Applicability of the GCE modified with the Eβ-CD/B-RGO was checked by sensing in human serum with acceptable results.

Section snippets

Materials and instruments

β-cyclodextrin and Flunitrazepam from Sigma-Aldrich (http://www.sigmaaldrich.com), Graphite powders and H3BO3 from Merck (http://www.merck.com) were used as received. The electrolyte was a phosphate buffer solution (PBS) prepared using Na2HPO4 and NaH2PO4 from Sinopharm Chemical Reagent Company (http://en.reagent.com.cn/). Deionized water (DIW) was used throughout the experiments. All reagents were with none treatments to use. The Flunitrazepam solutions were prepared by diluting a 1.0 × 10−3 M

Characteristics of the synthesized B-RGO

To confirm the synthesized B-RGO and GO nanosheet, FT-IR technique was recorded (Fig. 1A). Consistent with Fig. 1A, the hydroxyl peak is sharp and observed at around 3220 cm−1 due to incorporation of boron atoms and consequently the reduction of GO. Also, there are three peaks observed additionally at 1460 cm−1, 1197 and 794 cm−1, respectively. These peaks might be assigned for the stretching vibration of B–O, B–C and C–O bonds [22]. From the spectral analyses so, we will suggest that the B–O

Conclusions

A GCE was modified using the efficient nanocomposite based on the Eβ-CD/B-RGO and used for analyzing Flunitrazepam through an electrochemical method. The electrode had a wide response linearity range, low limit of detection, and high response stability and reproducibility and efficient applicability in measuring of the Flunitrazepam. Utilizing B-RGO is suitable for composite system and excellent electrochemical property that allow it to serve as a good support to enhance the efficiency and rate

Novelty statement

A comparison of the results with other proposed electrode and methods display that it produces better results. A clever combination of Utilizing the B-RGO and Eβ-CD can be a better result in the detection limit and linear ranges of Flunitrazepam concentration, simplicity of the fabrication procedure without using toxic reagents and the very good electrical conductivity. Also, the sensor was also successfully used in the analysis of flunitrazepam concentrations in blood serum samples.

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.

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