A sensitive arecoline photoelectrochemical sensor based on graphitic carbon nitride nanosheets activated by carbon nanohorns

Abstract

For the first time, a sensitive and high efficient photoelectrochemical biosensor has been developed for the quantitative detection of arecoline based on graphitic carbon nitride nanosheets with the assistance of carbon nanohorns

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Photoelectrochemical (PEC) detection is a newly emerged yet dynamically developing analysis technique, which is based on the electron transfer between an electrode, semiconductor, and analyte after photoexcitation.¹ Coupling light-illumination with electrochemical detection, photoelectrochemical sensors have the advantages of not only optical methods, but also electrochemical sensors.² Therefore, this technique exhibits promise for analytical applications and has attracted substantial research interest. Recently, polymeric graphitic carbon nitride (g-C₃N₄) has exhibited excellent photocatalytic performance for the removal of organic pollutants under visible photoirradiation and photocatalytic hydrogen gas production from water splitting using solar energy.³ Hence, g-C₃N₄ is a useful and valuable material for photocatalytic-driven applications due to its high thermal and chemical stability, good semiconductivity, moderate band gap, and special optical features.⁴ However, the photo-to-current conversion efficiency of bare g-C₃N₄ is still limited because of the high recombination rate of photogenerated electron–hole pairs. So, as far as we know, there are still no reports utilizing g-C₃N₄ as a PEC element to fabricate a PEC sensor.

Carbon nanohorns (CNHs) are a new member of the carbon based materials family, which can be essentially be thought of as layers of graphite rolled up into horns.⁵ Nonetheless, they have some different characteristics to graphene and carbon nanotubes (CNTs) owing to their horn curvature and quantum confinement, for example, their large theoretical specific area, variable porosity, unique electronic properties and high electron mobility. Accordingly, many applications ranging from gas storage, nano-electronics to biomedicine and biosensing have been explored.⁶ Furthermore, compared to the syntheses of CNTs, which inevitably introduce some impurities and require tedious purification, CNHs are prepared from pure graphite rods at room temperature without the use of metal catalysts, which avoids any tedious purification and makes them more environmentally friendly. Accordingly, in contrast to metallic compounds or other carbon based materials, high purity metal-free CNHs can be utilized as a new photoactive hybrid material and are more conducive to the study of the mechanism of photosensitizers made of carbon nanomaterials. Herein, CNH-sensitized g-C₃N₄ nanosheets exhibit a higher photocatalytic activity and photoelectrochemical response under light irradiation due to the superior electric properties of CNHs.

Arecoline (AR), a major acreca alkaloid, has been reported to exert cytotoxicity and inhibit the growth of various cultured human cells including oral epithelial cells and fibroblasts.⁷ Up until now, various techniques, which require complex and expensive devices and sophisticated equipment have been used to detect AR.⁸ However, there are no reports concerning a PEC sensor for AR.

In this communication, we have proposed, for the first time, an ultrasensitive and high efficient photoelectrochemical biosensor for the quantitative determination of AR using a CNHs–g-C₃N₄ nanocomposite. The photoelectrochemical process is shown in Scheme 1. Photoirradiation of the g-C₃N₄ nanosheets leads to the transfer of electrons in g-C₃N₄ from the valence band (VB) to the conduction band (CB), thus yielding electron–hole pairs. The CNHs then effectively serve as an excellent electron-transport matrix to capture and transport electrons from the excited g-C₃N₄ nanosheets to GCE, with the simultaneous transfer of electrons from the AR solution to the holes located in the conduction band of g-C₃N₄, which results in a photocurrent.

Scheme 1 Schematic illustration of the PEC process for the oxidation of arecoline at the CNHs–g-C₃N₄ electrode.

Morphological characterization of CNHs was performed using TEM. As shown in Fig. 1A, a typical ‘dahlia-like’ morphology of CNHs was observed, where the nanohorns protrude from the aggregate surface and the CNHs are aggregated together forming spherical assemblies with diameters of about 40 nm, which is consistent with the size reported by Pagona.⁸ Fig. 1B shows the surface morphology of the synthesized g-C₃N₄, which was characterized using SEM. It appeared to be smooth, consisting not of particles, but of sheets, which means that the g-C₃N₄ nanosheets are comprised of graphitic planes that are stacked along the c-axis.

Fig. 1 (A) A TEM image of CNHs. (B) An SEM image of g-C₃N₄. (C) Cyclic voltammograms at the different electrodes, (a) GCE, (b) g-C₃N₄/GCE, (c) CNHs–g-C₃N₄/GCE obtained using 5 mM ferrocene in acetonitrile with 0.1 M tetrabutylammonium perchlorate. Scan rate: 100 mV s⁻¹. (D) Photocurrent densities of the (a) CNHs/GCE, (b) g-C₃N₄/GCE, and (c) CNHs–g-C₃N₄/GCE modified electrodes in 0.1 M phosphate buffer solution (PBS) with the light turned on and off. The applied potential was 0.6 V and the excitation wavelength was 390 nm.

The electrochemical response of the various modified electrodes was studied using ferrocene as redox probes by cyclic voltammetry (CV). Fig. 1C shows cyclic voltammograms using 5 mM ferrocene in acetonitrile at a scan rate of 100 mV s⁻¹. At the bare GCE (a), a classic pair of redox peaks was observed with a peak-to-peak separation of 38 mV. After the electrode was modified with g-C₃N₄ (b), the peak currents of the redox peaks decreased slightly, showing that the g-C₃N₄ nanosheets impeded the transfer of electrons. However, the peak currents of the redox peaks recovered CNHs–g-C₃N₄/GCE (c) and the corresponding electrochemical response was even bigger than at GCE, which was ascribed to the lower density of electronic states, the edge-plane-like sites and structural defects and the sp² bonding between the carbon atoms of the CNHs. This phenomenon showed that the CNHs in this nanocomposite effectively served as an excellent electron-transport matrix and accelerated the transfer of electrons from g-C₃N₄ to GCE.

To test the photochemical response of this nanocomposite, the modified GCEs were used as photoanodes and studied under the different light switching states. As shown in Fig. 1D, under optimal conditions, the CNHs/GCE (a) and g-C₃N₄/GCE (b) electrodes exhibited photocurrent densities of 0.0964 μA cm⁻² and 0.634 μA cm⁻² respectively, which indicated that the g-C₃N₄ nanosheets could function as an excellent photocatalyst. Moreover, we also observed that CNHs–g-C₃N₄/GCE (c) demonstrated a photocurrent density of 2.167 μA cm⁻², which is due to the energy levels of the CB of CNHs (+0.8 eV) and GCE being lower than that of the g-C₃N₄ nanosheets (−1.12 eV). Therefore, photoinduced electrons on the surface of the g-C₃N₄ nanosheets could directly inject into the CB of CNHs and then be transferred rapidly from CNHs to GCE (Fig. 2C), effectively avoiding electron–hole recombination and resulting in a much more sensitive response to the light being switched on and off. Furthermore, the photoresponse of g-C₃N₄-CNHs/GCE is superior to CeO₂ nanorods, graphene oxide modified TiO₂ nanotube arrays and In₂O₃ nanorods which have been reported in the literature by Lu, Song and Gan, respectively.⁹ Therefore, the favorable band energy, good distribution of g-C₃N₄ nanosheets and strong coupling of g-C₃N₄ and CNHs efficiently improved the photoelectric response of the CNHs–g-C₃N₄ nanocomposite. Moreover, the preparation of CNHs and g-C₃N₄ did not use metal catalysts, which means that there is no need to study the influence of metals on the mechanism of the new CNHs-g-C₃N₄ nanocomposite.

Fig. 2 Photocurrent density upon photoirradiation of (A) g-C₃N₄/GCE and (B) CNHs–g-C₃N₄/GCE 0.1 M PBS (at pH 9.0) in the absence (a and c) and presence (b and d) of 10 μM AR at 0.6 V. (C) Schematic illustration of the irradiation trapping mechanism and anodic photoelectrochemistry for the sensing of arecoline.

As shown in Fig. 2, 0.1 M PBS in the absence (a and c) and presence (b and d) of 10 μM AR was used as an electrolyte solution. The photocurrent density of the modified electrodes with the light switched on and off increased greatly in the presence of AR compared with that the modified electrodes in blank PBS. The explanation for this is that in the blank PBS solution, there are not enough effective electron donors for photo-generated holes to scavenge. Once there is an effective electron donor for scavenging of the holes, electron–hole recombination is inhibited and the photocurrent density can be enhanced. Therefore, in the present study, AR served as an electron donor and sacrificial reagent that can transfer electrons to the holes located in the excited state of the g-C₃N₄ nanosheets (Fig. 2C), effectively avoiding electron–hole recombination and leading to a sharp increase in the photocurrent. In addition, comparing curves a and c to b and d, it is easy to see that the response time at the CNHs–g-C₃N₄ modified electrode was shorter than the case at the g-C₃N₄/GCE, which shows that the PEC sensor has the advantage of a rapid response.

Fig. 3A shows the relationship between the photocurrent density and the variable concentration of AR. The photocurrent density increased with increasing concentration. As shown in Fig. 3B, the photocurrent density is proportional to the AR concentration logarithmically within the liner range from 1.0 × 10⁻¹⁰ M to 1.0 × 10⁻⁴ M (R² = 0.999). The linear response range was wider than those of others strategies for the quantitative determination of AR. In addition, the detection limit (LOD) was estimated to be 30 pM at a ratio of signal to noise of 3, which was much lower than that reported previously (Table S1, ref. S3, S4†).¹⁰

Fig. 3 (A) The effect of different concentrations (a–m) 0.0001, 0.0005, 0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 1, 5, 10, 50, and 100 μM of AR on the differential photocurrent density. (B) The corresponding calibration curve. (C) Selectivity of the proposed assay towards AR determined by comparing it to interfering substances at the 1 mM level: fructose, glucose, sucrose, amylum, KNO₃, KCl, Na₂SO₄ and trisodium citrate in 0.1 M PBS. (D) Time-based photocurrent density of the CNHs–g-C₃N₄/GCE electrode measured with the light switched on and off in 0.1 M PBS containing 10 μM AR. All of the photocurrent responses were recorded at a 0.6 V applied potential and 390 nm excitation wavelength.

In order to evaluate the selectivity of the prepared sensor, the influence of some potentially interfering substances was investigated by using 0.1 M PBS (pH 9.0) solutions containing 1 mM fructose, glucose, sucrose, amylum, KNO₃, KCl, Na₂SO₄, trisodium citrate, and 10 μM AR (shown in Fig. 3C). The results demonstrate that the interfering substance have negligible influence on the signal at 10 μM AR, suggesting that the high selectivity and excellent specificity of the present sensor for AR detection over other interfering substances. On the other hand, the reproducibility of the modified electrode is one of the important factors in deciding the applicability of the sensor. Fig. 3D shows the photocurrent density of the CNHs–g-C₃N₄ modified electrode under optimal conditions. Strong and stable photocurrent responses were observed with a relative standard deviation (RSD) of 0.46%, showing that this method had good reproducibility.

To further verify the analytical reliability and application potential of the proposed sensor, it was applied to determine the amount of AR in a betel nut sample. 2 μL of a pretreated sample solution was directly injected into 2998 μL 0.1 M PBS (pH 9.0), and 6.6 mg g⁻¹ AR was detected in the betel nut using this PEC method.

The content was consistent with the value of 7.5 mg g⁻¹, which was given in the report of Wang,¹¹ demonstrating the acceptable accuracy of the sensor. Afterwards, an amount of standard AR solution was added into the above solution, and the recovery of the added AR was found to be in the range of 102.2% to 111.5% (Table S2†) indicating that the prepared biosensor can be successfully used for monitoring AR in betel nut samples.

Herein, we found that g-C₃N₄ nanosheets could be used as an effective photoelectrochemical signal generating element. The presence of CNHs played a role in the sensitization of the photoelectric signal leading to favorable electrochemical mass transfer in the photoelectric sensing process, owing to the unique microstructure, and physical and chemical properties of CNHs. Meanwhile, the high purity of CNHs is convenient for the future study of the photoelectric behavior of the CNHs–g-C₃N₄ nanocomposite and research into the photoelectric catalysis mechanism. Therefore, on the basis of the PEC sensing principle and the outstanding photovoltaic properties of CNHs and g-C₃N₄, we proposed, for the first time, a PEC sensor which has the advantages of low cost, easy fabrication, rapid response, high sensitivity, a very broad linear range, a rather low detection limit, and excellent reproducibility. Moreover, it exhibited a high performance for the sensing of AR, in particular the LOD of this method is lower than existing methods being reduced by at least three orders of magnitude. Accordingly, the study of the CNHs–g-C₃N₄ nanocomposite broadens the scope of applications for PEC sensors.

This project was financially supported by the National Science Foundation of Fujian Province (2011J05020), the Education Department of Fujian Province (JA11062, JB13008, JA13068) and the National Nature Sciences Foundation of China (21205016, 21275031), and the Program for Changjiang Scholars and Innovative Research Team in University (no. IRT1116).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/c3ra46264a

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