Graphitic carbon nitride/graphene oxide(g-C3N4/GO) nanocomposites covalently linked with ferrocene containing dendrimer for ultrasensitive detection of pesticide
Graphical abstract
A novel electrochemical biosensor for ultrasensitive detection of pesticide was constructed by utilizing graphitic carbon nitride (g-C3N4)/graphene oxide(GO) nanocomposite covalently linking with a ferrocene containing dendrimer(Fc-TED). The g-C3N4 with sufficient N atoms for providing lone pairs of electrons to an electron acceptor so as to enhance the adsorption towords organic molecules. The Fc-TED dendrimers with the native redox signaling center (Fe3+/Fe2+) can increase the electron transition of g-C3N4 from valence to conduction band. While GO can accelerate the electron transfer from g-C3N4 surface and Fc-TED to glassy carbon electrode (GCE), which would amplify the electrochemical signal of g-C3N4/GO/Fc-TED/GCE sensor and then improve the sensing performance.
Introduction
Electrochemical sensors are widely used in detecting environmental pollutants due to their convenient miniaturization, low cost as well as rapid quantification for analyte which make them suitable for routine inspection [1,2]. For the construction of an electrochemical sensor, generation and amplification of the electrochemical signal is crucial to improve the sensing performance of a sensor. At this point, various signal amplification strategies have been developed. Many researchers directly doped nanoparticles [[3], [4], [5]] or redox-active polymer [6] into the electrode material to enhance the sensitivity of the sensors. Among these nanoparticles, graphene oxide (GO) has been widely used in electrode material to improve sensitivity and response range of sensors due to its good conductivity and feasibility for surface functionalization [[7], [8], [9], [10]]. However, in the preparation and application of GO sensitized sensor, GO exhibits some chemical inertness towards analyte due to the fact that the surface plane of GO is composed by some strongly bonded sp2 hybridization of carbon atoms [11]. Thus, it is necessary to improve the adsorption ability of GO to analyte. Up to now, numerous strategies have been applied to further activate the GO to improve the adsorption ability, such as functionalizing and hybridizing GO with polymers [12], metal and metal oxides [[13], [14], [15], [16]], these methods have also been reported to some extent avoiding those defects [17,18].
Similar to the structure of GO, graphitic carbon nitride (g-C3N4) also possesses a lamellar structure with unique physicochemical properties, which is known for its applications in sensing [19,20] and redox reaction [[21], [22], [23]]. Especially, the sp2 hybridized N atoms in the tri-s-triazine structure of g-C3N4, in which the lone pair electrons remain, helps to improve the adsorption towards organic molecules [24,25]. And also, g-C3N4 nanosheets with a stacked two-dimensional (2D) structure and high surface area is appropriate for organic pollutants (pesticide residues) sensing application [26]. However, the poor conductivity and the large contact resistance of g-C3N4 make its unsatisfactory in electrocatalytic performance that limits its applications in the electrochemical field [27].
It was reported that [[28], [29], [30]] the uniqueness of the two-dimensional system of g-C3N4 and GO makes them enormous potential to promote their electrochemical performance by the construction of g-C3N4/GO composites. Accordingly, the g-C3N4/GO composites are expected to integrate the excellent electron conductivity of GO and the good adsorption property of g-C3N4, thereby showing better sensing performance in the fabrication of electrochemical sensor than single GO and g-C3N4 (e.g. sensitivity or response time).
Ferrocenyl-terminated dendrimer (Fc-TED) combines the particular electrochemical activity of ferrocene(Fc) with special topological structure of dendrimer that have sufficient adhesion on the electrode [31].The sensor fabricated by Fc-TED retains unique Fe3+/Fe2+ redox reaction and also overcomes the defect of poor adhesion of ferrocene on the surface of a sensor [32]. Fc-TED sensors have been successfully used in a number of sensors to monitor ATP2− [12], glucose [33], folic acid and boronicacid [34].Therefore Fc-TED is a good redox probe to directly sense analyte by monitoring the electrochemical signal in Fe3+/Fe2+ without any electronic medium [[35], [36], [37], [38]]. However, the sensitivities of these sensors have always been challenged due to the poor electron transfer activity of the polymer substrate inside ferrocene end-cap dendrimer. The working electrodes were usually modified by sensitizer during their preparation to achieve a high sensitivity.
This motivated us to combine the excellent electrochemical activity of Fc-TED with outstanding advantages of g-C3N4/GO composites by tethering ferrocene redox units onto the g-C3N4/GO composites for amplifying the electrochemical response of the sensor towards detection of organic pollutants. Grafting g-C3N4/GO composites onto the polymer is relatively easy because GO has abundant active groups such as hydroxyl group and carboxyl group [39,40]. On the other hand, using the nano-sized dendrimer to design electrochemical sensor will provide a compact matrix for the incorporation of g-C3N4/GO composites [41], which would improve the surface modification effect and the film forming performance those could further enhance the stability and sensitivity of the resultant sensor.
Metolcarb (3-methylphenyl-methylcarbamate) is a widely used broad-spectrum insecticide and herbicide in agricultural production [42]. It is also the key species for the detection of pesticide residues in vegetables, fruits and fermented food. Extensive studies have revealed that metolcarb can inhibit the activity of cholinesterase and damage the normal function of the nervous system [43]. Long-term exposure to metolcarb may cause serious health problems such as reproductive toxicity, mental derangement, fetal malformations and even cancer. Pesticide contaminants have recently caused much public concern [44], so effective and timely detection of metolcarb is a convenient way to prevent people from exposure to it.The commonly used methods for testing metolcarb include HPLC chromatography [45], fluorescence resonance energy transfer sensor [46], molecularly imprinted sensor [47], high performance liquid chromatography tandem mass spectrometry(HPLC-MS/MS) [48], gas chromatography-mass spectrometer (GS-MS) [49] and enzyme-linked immunosorbent assay [50,51]technology. Besides these, using nanoparticles modified GCE electrochemical sensors [52,53]to detect pesticide have attracted considerable interest, since they offered short diagnostic time and the possibility of miniaturization [[54], [55], [56]]. However, the existing electrochemical methods also had the disadvantages, such as complicated fabrication, high price and narrow detection range [57].
Therefore, it is necessary to develop novel sensitized modifying electrodes to further improve the detection range and the sensitivity as well as the convenience of preparation.
We have studied the synthesis, electrochemical behaviors of several ferrocene end-cap dendrimers [58,59] and their film electrodes to directly sense ATP2− and glucose [60]. Herein, we combined the excellent electrochemical characteristic of Fc-TED with outstanding advantages of g-C3N4/GO composites to prepare a novel sensitive sensor(g-C3N4/GO/Fc-TED/GCE) for pesticide detection. We found that compositing g-C3N4 into GO greatly enhances the sensing response of the sensor and covalently linking g-C3N4/GO with ferrocene containing dendrimer can be a potential candidate for the development of electrochemical sensors. This high sensitive response mechanism of the g-C3N4/GO/Fc-TED/GCE sensor toward metolcarb detection is also proposed.
Section snippets
Reagents and apparatus
Ferrocene-terminated dendrimer (Fc-TED) was synthesized according to the method in our laboratory [[58], [59], [60]]. GO sheets was prepared by a modified Hummers’ method [54]. Metolcarb were purchase from Lanboer Co., Ltd (Beijing, China). A 0.1 M phosphate buffer solution (PBS pH 7.0) was used as a supporting electrolyte. All other reagents were of analytical grade and all solutions were prepared with double distilled water. The electrochemical methods of cyclic voltammetry (CV), differential
Morphology of the synthesized samples and different modified electrodes
The morphologies of GO, g-C3N4 and g-C3N4/GO composite were characterized by SEM and TEM. The irregular lamellar nanostructure of GO sheets can be observed from Fig. 2a. The obtained g-C3N4 samples were illustrated by the micrometer-sized irregular flake structure as shown in Fig. 2b. From SEM and TEM images of g-C3N4/GO composites (Fig. 2c and g), one can see that g-C3N4 presents an irregular sheet structure with a size of about 0.5–1.0 μm. The TEM images of g-C3N4/GO composites also showed
Conclusions
A novel electrochemical sensor based on g-C3N4/GO nanocomposites covalently with ferrocene containing dendrimer for sensitive detection of metolcarb was introduced. The prepared g-C3N4/GO/Fc-TED/GCE sensor showed a wide linear range and low detection limit, superior to those previously reported electrochemical sensors. The ultra-high sensitivity and selectivity were verified by combining the unique redox active signaling center (Fe2+/Fe3+), the excellent electron transfer ability of GO and the
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 financially supported by Hebei Key Discipline Construction Project,China; Natural Science Foundation of Hebei Province, China (project no.B2014210014); the Key Research and Development Projects in Hebei Province and National Natural Science Foundation of China.
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