Citation

Sheng Feng, Pengcheng Yan, Li Xu, Jiexiang Xia, Huaming Li. Exploitation of a photoelectrochemical sensing platform for bisphenol A quantitative determination using Cu/graphitic carbon nitride nanocomposites[J]. Chin. Chem. Lett., 2018, 29(11): 1629-1632  

Sheng Feng^a, Pengcheng Yan^b, Li Xu^b, Jiexiang Xia^b, Huaming Li^b    

Bisphenol A is a significant chemical raw material, which is extensively applied in synthesizing epoxy resins, polyester resins, polycarbonate and flame retardants [1]. Bisphenol A can act as an endocrine disruptor, which can cause a variety of adverse effects including cancers, infertility, poor immune function, cardiovascular disease and so on [2]. Until now, several strategies are proposed for monitoring bisphenol A, such as liquid chromatography-mass spectrometry [3], gas chromatography-mass spectrometry [4], ultraviolet and fluorescence analysis [5], electrochemical detection [2]. The above-mentioned strategies are sensitive for monitoring bisphenol A, yet suffer several drawbacks, such as high equipment cost, time-consuming preparation, rigorous pretreatments, tedious operation and toxic solvents [6]. Electrochemical sensors are briefness, low equipment cost and miniaturization [7]. However, bisphenol A can be oxidized in the electrochemical detecting progress, which can cause the passivation and low sensitivity of the electrode [8]. Photoelectrochemical detection, a promising and satisfactory analytical technique, stimulates keen research interest because of its simple preparation, low cost, and miniaturization [9]. Photoelectrochemical detection couples advantages of conventional electrochemical and optical methods, because light sources are drawn into conventional electrochemical detection [10]. The photoactive electrode plays a fatal view in the photoelectrochemical detection system, which can generate photocurrent signal in optical excitation.

The graphitic carbon nitride (g-CN) has attracted a great deal of research interest in photoelectrochemical field due to the unique photoelectrochemical properties and high nitrogen content [11]. As an organic semiconductor, g-CN is seized of tri-s-triazine ring, which makes g-CN own well stability. In view of suitable band gap (2.7 eV), g-CN can exhibit photoresponsive ability in the visible light irradiation, resulting in excellent photoelectrochemical properties [12]. Additionally, g-CN as a Lewis base can adsorb metal ions by means of chelation, which can be applied in monitoring trace heavy metal ions in water environment [13]. Notwithstanding these properties are ostensibly advantageous, inadequate light absorption (λ< 460 nm) and fast-speed recombination of charge carriers have seriously confined the photoelectrochemical performance of g-CN. To conquer these deficiencies, modulation of different morphologies, doping g-CN with metal and combining other semiconductors are frequent strategies to enhance the photoelectrochemical performance [14]. Doping with metal such as Pt [15], Au [16], Ag [17], which can effectively improve the photoelectrochemical properties of g-CN by hindering the recombination of charge carriers. Nevertheless, noble metals are expensive, unbeneficial for extensive applications. Transition metal nanoparticles can be utilized as a candidate for noble metals in view of low cost and availability. As common transition metal, Cu nanoparticles can not only enhance the conductivity of g-CN to improve the separation efficiency of charge carriers, but also engender localized surface plasmon resonance (LSPR) to enhance the photoelectric translation efficiency and visible light absorption [18]. Recently, the Cu nanoparticles and carbon nitride have been combined to obtain high photocatalytic activities [19]. Inspired by this, Cu/g-CN nanocomposites can be designed to extend their applications in the field of photoelectrochemical detection.

Here, a simple and sensitive photoelectrochemical sensor for bisphenol A had been designed with Cu/g-CN composites (Supporting information). The Cu/g-CN composites were obtained via a facile solvothermal process in the presence of the copperbased ionic liquid. On account of the LSPR effect of Cu nanoparticles, the proposed Cu/g-CN composites can obtain the high light absorbance, rapid electron transport and low recombination of photogenerated charge carriers, resulting in excellent photoelectrochemical performance of the Cu/g-CN composites. The photoelectro-chemical sensor for bisphenol A was also fabricated by Cu/g-CN composites. The sensitivity and stability of photoelectrochemical monitoring platform was investigated detailed via a range of experiments.

Fig. 1A shows the XRD spectra of 5 wt% Cu/g-CN composites and the pure g-CN. It is used to investigate the microcrystal composition of materials. It can be seen that the diffraction peaks at 27.8° and 10.5° both exist in XRD spectra of 5 wt% Cu/g-CN composites and pure g-CN. The diffraction peak at 27.8° corresponds to the (002) crystal surface of g-CN, which is caused by the stacking of the conjugated aromatic system of g-CN [20]. The diffraction peak at the 10.5° of the pure g-CN appears a slightly shifting (from 13° to 10.5°) compared with that of bulk g-CN [21]. The three triazine unit can be assembled at the high temperature solvothermal process [22], which affects the layer arrangement and the interlayer ordering of the g-CN. Compared to the pure gCN, a better crystalline can be obtained through the XRD spectrum of the 5 wt% Cu/g-CN composites, which means that the structure of g-CN is assembled and arranged again at the high temperature solvent thermal condition in the preparation of 5 wt% Cu/g-CN composites. In the XRD pattern of the 5 wt% Cu/g-CN composites, the diffraction peaks at 43.5°, 50.4°, and 73.2° correspond to the standard card (JCPDS Card No. Cu single 04-0826), which indicates a small quantity of Cu modified in the surface of g-CN. The presence of g-CN and Cu planes can affirm the successful preparation of the Cu/g-CN composites. Fig. 1B shows FT-IR spectra of 5 wt% Cu/g-CN composites and pure g-CN. It can be seen that the typical characteristic peaks of Cu/g-CN composites basically match the characteristic peaks of the g-CN. There is no offset to appear. The characteristic peaks at 1380–1740 cm^-1 belong to C–N and C=N stretching vibration peak. The peak at about 800 cm^-1 corresponds to the typical breathing mode heptazine ring system [23]. From the characteristic peaks of Cu/g-CN composites, the introduction of Cu does not change the structure of the g-CN.

Figure 1

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Fig. 1. (A) XRD pattrns of the 5 wt% Cu/g-CN composites (a) and the g-CN (b), the inset shows the enlarged pattern of the 5 wt% Cu/g-CN composites. (B) FT-IR spectra of the 5 wt% Cu/g-CN composites and the g-CN. (C–F) XPS spectra of the 5 wt% Cu/gCN composites: (C) survey XPS spectrum, (D) C 1s, (E) N 1s and (F) Cu 2p.

XPS analysis is used to research the main composition and the valence of the material. Fig. 1C shows a survey spectrum of 5 wt% Cu/g-CN composites. It can be observed that the Cu/g-CN composites mainly consist of four elements: C, N, O and Cu. Fig. 1D shows the spectra of C 1s. Two strong characteristic peaks appear at 288.2 eV and 284.6 eV, belonging to sp2 hybrid carbon (N—C=N) and C—C [24], respectively. From the Fig. 1E, a strong characteristic peak at 399.1 eV corresponds to the sp2 hybrid of the nitrogen (C=N—C) [25]. Fig. 1F shows the spectra of Cu 2p. Two characteristic peaks at 932.6 eV and 952.6 eV are attributed to Cu (0) [26]. Moreover, no characteristic peaks of Cu(Ⅰ) or Cu(Ⅱ) can be observed. It is proved that the presence of Cu is Cu(0). Fig. S1 (Supporting information) shows the SEM images (a) and the surface element distribution mapping analysis (b–d) of the 5 wt% Cu/g-CN composites. It is easy to find a certain amounts of Cu in the Cu/g-CN composites from the distribution of mapping surface elements analysis (Fig. S1b–d). The specific distribution of Cu in the g-CN can also be observed in TEM image of Cu/g-CN composites (Fig. S1h). The results of the XPS, SEM and TEM indicate that the Cu/g-CN composites are prepared successfully.

The optical properties of 5 wt% Cu/g-CN composites and pure gCN are analyzed by DRS analysis, as shown in Fig. 2A. The absorption band edge of 5 wt% Cu/g-CN composites is obviously broadened compared with the pure g-CN, indicating that 5 wt% Cu/g-CN composites display an evidently increasing light harvesting ability in the visible-light region. The wide peak of 5 wt% Cu/g-CN composites at 600–750 nm corresponds to the LSPR effect of Cu nanoparticles [27]. From PL analysis (Fig. 2B), it can be seen that the luminescence intensity of Cu/g-CN composites is far lower than that of pure g-CN. It indicates that the introduction of Cu in Cu/gCN composites decreases the recombination efficiency of photoinduced electron-hole pairs, and then promotes the migration of carriers.

Figure 2

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Fig. 2. UV-vis diffuse reflectance spectra (A) and PL spectra (B) of the 5 wt% Cu/g-CN composites and g-CN. (C) Photocurrent response of (a) g-CN/ITO electrode and (b, c) 5 wt% Cu/g-CN/ITO electrode in PBS (0.1 mol/L, pH 7.0) in the absence (b) and presence (c) of 0.18 mmol/L bisphenol A at -0.2 V with light excitation. (D) Nyquist plots of g-CN/ITO and Cu/g-CN/ITO electrode.

The photoelectrochemical properties of Cu/g-CN composites have been investigated by i–t curves and EIS analysis. The i–t curves are obtained by constructing a standard three electrode system. The transient photocurrent experiments are performed in the phosphate buffer electrolyte solution (0.1 mol/L, pH 7.0) at -0.2 V, and the interval time is 20 s. In order to obtain the best photoelectrochemical performance of the Cu/g-CN composites, the photocurrent value of the Cu/g-CN composites with different proportions has been compared. As shown in Fig. 2C, it can be intuitively observed that the photocurrent of Cu/g-CN/ITO is higher than that of pure g-CN/ITO, indicating that the introduction of Cu can increase the photocurrent intensity. In addition, the 5 wt% Cu/g-CN/ITO has the largest photocurrent response compared with all Cu/g-CN/ITO electrodes with different proportions (Fig. S2 in Supporting information). The photocurrent response of 5 wt% Cu/g-CN/ITO is 7 times as much as that of the pure g-CN/ITO, indicating that the introduction of Cu can promote the separation efficiency of photoinduced electron-hole pairs, as a result of enhanced the photoelectrochemical properties of g-CN. The electron transfer capacity of the prepared materials is examined by Nyquist plots. Fig. 2D shows the Nyquist plots of the 5 wt% Cu/gCN/ITO electrodes and pure g-CN/ITO electrodes. It can be observed that the Nyquist curve of the 5 wt% Cu/g-CN/ITO and g-CN/ITO exhibit a similar semicircle shape. It is easy to find that the 5 wt% Cu/g-CN/ITO electrode has a smaller radius than that of pure g-CN/ ITO, indicating that the 5 wt% Cu/g-CN/ITO has small resistance and more rapid carrier transmission ability. Therefore, the 5 wt% Cu/gCN composites are an excellent photoelectrochemical material, which is helpful for constructing photoelectrochemical sensors. The mechanism of photocurrent enhancement is as follow (Fig. 3): When the Cu/g-CN composites are irradiated by light, the g-CN is excited and generates electron-hole pairs. The photogenerated electrons of g-CN are transferred from the valence band (VB) into the conduction band (CB) of g-CN, and the holes are remained on the VB of g-CN. The metallic Cu as an electron sink can promote the transfer of interfacial electron so as to prevent the fast charge-pair recombination [28]. On account of the LSPR effect of Cu nanoparticle, the electrons from the CB of g-CN can quickly move to the surface of Cu through the Schottky barrier [28]. It is beneficial for enhancing the separation efficiency of the photogenerated electron and hole of the g-CN, and promoting the carriers transfer. Finally, substantial electrons gathering on the surface of Cu nanoparticles and in the CB of g-CN can fast transfer to the ITO electrode to produce high photocurrent. Upon the injection of a certain amount of bisphenol A, the hole left on the VB of g-CN can be rapidly consumed to promote the ascension of photocurrent.

Figure 3

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Fig. 3. Schematic diagram of photocurrent generation mechanism of Cu/g-CN/ITO electrode.

Through the investigation about the photoelectrochemical performance of 5 wt% Cu/g-CN composites, it can be found that the introduction of Cu can greatly enhance the photocurrent intensity and rapid separation of photogenerated electron-hole pairs, which is favorable for fabricating the photoelectrochemical sensor. In order to investigate the application of Cu/g-CN material in photoelectrochemical sensor field, the construction of a three electrode system will be considered with 5 wt% Cu/g-CN/ITO as a photoelectrode. The applied potential on the PEC performance of 5 wt% Cu/g-CN/ITO is shown in Fig. S3 (Supporting information). The photocurrent is enhanced with the increasing applied potential and became nearly flat after the potential exceeded -0.2 V. Considering that low potential can minimize the interference from other coexisting species in the sample [29], the applied potential of -0.2 V is chosen as the PEC potential. Fig. 4A shows the photocurrent response of a photoelectrochemical sensor based on Cu/g-CN/ITO for bisphenol A detection. The photocurrent intensity increases by adding a certain concentration of bisphenol A. The concentration of bisphenol A can directly affect the photocurrent density of Cu/g-CN. The photocurrent value changes linearly with the concentration of bisphenol A in the range of 0.035mmol/L to 0.28 mmol/L, which can be seen from Fig. 4B. Its linear regression equation is I = 44.49C_{bisphenol A} + 4.66 (R² = 0.9929), where R² is a linear correlation coefficient. The linear correlation between the concentration of bisphenol A and photocurrent enhancement is relatively high. Moreover, a detection limit of bisphenol A is about 0.012 mmol/L. The results show that the Cu/g-CN composites have strong photoelectrochemical response to the bisphenol A, which can be used for sensitive detection of bisphenol A. In addition, this proposed photoelectrochemical sensor is compared to several analytical methods reported previously for the determination of bisphenol A (Table S1 in Supporting information) [8, 30-35]. This photoelectrochemical sensor at the basis of Cu/g-CN composite has the lowest limit detection among these methods for monitoring bisphenol A. As shown in Supporting information, the bisphenol A photoelectrochemical sensor also exhibits outstanding stability and acceptable anti-interference capacity.

Figure 4

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Fig. 4. (A) The effect of the bisphenol A concentration on the differential photocurrent responses with different concentration of bisphenol A from 0.035mmol/L to 0.28 mmol/L (a–g) for the Cu/g-CN/ITO electrode. (B) The linear relationship between the photocurrent and the bisphenol A concentration, the Cu/ g-CN/ITO electrode with different concentration of bisphenol A from 0.035mmol/L to 0.28 mmol/L. R2 represents the correlation coefficient.

In summary, a photoelectrochemical sensing method for sensitive bisphenol A determination applying Cu/g-CN composites are constructed and investigated. The Cu/g-CN composites were obtained via a facile solvothermal process in the presence of the copper-based ionic liquid. In view of LSPR of Cu nanoparticles, the induction of Cu nanoparticles greatly promoted light absorbance, accelerated electron transport and reduced recombination of photogenerated charge carriers, resulting in outstanding photoelectrochemical performance of the Cu/g-CN composites. Owing to the outstanding photoelectrochemical performance of the Cu/g-CN composites, a photoelectrochemical sensor based on Cu/g-CN composites were constructed for detecting bisphenol A. The detection limit of the bisphenol A photoelectrochemical sensor was below 0.012 mmol/L. The bisphenol A photoelectrochemical sensor exhibited an excellent stability and acceptable antiinterference capacity.

This work was financially supported by the National Natural Science Foundation of China (No. 41371446).

Supplementarymaterial related to this article canbefound, in the online version, at doi:https://doi.org/10.1016/j.cclet.2018.08.010.