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Visible Light-Driven Membraneless Photocatalytic Fuel Cell towards Self-Powered Aptasensing of PCB77 Kai Yan, Yuhan Zhu, Weihao Ji, Fang Chen, and Jingdong Zhang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02302 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018
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Analytical Chemistry
Visible Light-Driven Membraneless Photocatalytic Fuel Cell towards Self-Powered Aptasensing of PCB77 Kai Yan, Yuhan Zhu, Weihao Ji, Fang Chen, and Jingdong Zhang* Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China ABSTRACT: Artificial photocatalytic systems have been extensively established to mimic the natural solar-energy conversion process for developing useful photoelectric devices. In this work, a membraneless, hydrogen peroxide (H2O2)-based photocatalytic fuel cell (PFC) operated under visible light was proposed and applied in self-powered sensing of 3,3',4,4'tetrachlorobiphenyl (PCB77). The PFC was comprised of a photoanode fabricated with gold nanoparticles-decorated graphitic C3N4 nanosheet and a cathode modified with hemin-graphene nanocomposites. The combination of both photocatalytic oxidation and electrochemical reduction of H2O2 processes led to electron transfer in the external circuit, which could generate a certain electric power output. Taking the advantage of the inhibited output performance of PFC by PCB77 which interacted with aptamer immobilize on the photoanode, a self-powered aptasensor driven by visible light was achieved, without applying external electric source. Thus, a highly sensitive and selective self-powered aptasensor for PCB77 was successfully demonstrated.
Self-powered sensors have emerged as a newly developed electrochemical detection technique for chemical and biological analysis. Different from conventional three-electrode electrochemical sensing systems using an external electric power supply, self-powered sensors generally consist of an anode and a cathode which can generate electrical output proportional to analyte concentration.1 Since such self-powered sensors have no demand of external electric power source during the entire detection process,2,3 they exhibit the advantages of simple configuration, flexible implementation, and low cost.4 Enzymatic fuel cells or triboelectric nanogenerators which could convert chemical or mechanical energy to electricity have been extensively explored for self-powered sensing.5,6 Photocatalysis has been one of the most promising approaches to the utilization of inexhaustible solar energy.7-9 In recent years, photocatalytic fuel cells (PFCs) that can generate electricity during the photocatalytic reactions have attracted considerable interest.10,11 Considering that PFCs could also generate fuel concentrationdependent output power, we proposed light-induced selfpowered sensors based on photocatalytic oxidation of glucose, which exhibited desirable accuracy and stability.12,13 Further, we introduced antibody mimetics, namely molecularly imprinted polymers (MIPs), as recognition elements to either photocathodic or photoanodic PFC to improve the selectivity of self-powered sensors.14,15 These reported self-powered sensors were constructed with two-chamber PFCs. Generally, a PFC consists of two electrodes that are placed in membrane-separated anodic and cathodic
chambers. Many organic species have been explored as the fuels of PFC to generate electricity under photoirradiation.16,17 Different from organic fuels, hydrogen peroxide (H2O2), a carbon-free energy carrier, has been considered as an outstanding candidate for developing “green” PFC since it can be facilely converted into oxygen or water to generate electric power.18 Moreover, considering that H2O2 can be utilized as both oxidant and reductant,19 single-compartment, membraneless cell based on H2O2 fuel has been proposed, which could simplify the cell configuration, develop miniaturization of fuel cell, and avoid the introduction of high cost proton-exchange membrane.20 A UV lightdriven membraneless, H2O2-based PFC constructed with TiO2 nanotubes photoanode and Prussian blue cathode has been designed and successfully operated under acidic condition with excellent long-term stability.21 However, membraneless PFC has never been explored for selfpowered sensing. On the other hand, aptamer is also a representative of antibody mimetics, which has the advantages of low cost and easy in vitro synthesis, less immunogenic response, high stability, and high specific binding ability to target.22,23 A variety of highly selective electrochemical and photoelectrochemical sensors have been developed with aptamers.24,25 Interestingly, aptamers have been incorporated in enzymatic fuel cells to construct selfpowered and intelligent logic aptasensors that could determine whether the two specific targets were both present in a sample.26,27
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Analytical Chemistry Herein, we described the first PFC-based self-powered aptasensor constructed with a membraneless cell. To avoid high background arising from the direct UV photolysis of analyte and H2O2,28 we designed a visible light-driven membraneless H2O2-based PFC. The PFC comprised of a photoanode prepared with gold nanoparticles-decorated graphitic C3N4 nanosheet (gC3N4-Au) and a cathode modified with hemin and graphene (HG) nanocomposites in a single-compartment cell. Under visible light irradiation, H2O2 was photoelectrocatalytically oxidized at the photoanode and electrocatalytically reduced at the cathode, inducing the generation of electric power in the external circuit. The proposed PFC exhibited a stable output performance in neutral media, which was further explored in selfpowered aptasensing. A structurally coplanar polychlorobiphenyl namely 3,3’,4,4’-tetrachlorobiphenyl (PCB77) which might cause cancer and other adverse health effects29,30 was chosen as the analyte, and thiol group-functionalized PCB77-binding aptamer (5’-SH(CH2)6-GGC GGG GCT ACG AAG TAG TGA TTT TTT CCG ATG GCC CGT G-3’) was employed as the recognition element immobilized on the photoanode. When PCB77 was present in solution, it could be captured by aptamer on the photoanode. The formation of PCB77aptamer complex on the photoanode would impact the electron transfer, leading to change of power output of PFC which could be directly utilized as the signal for selfpowered sensing of PCB77. Scheme 1 illustrates the working principle of the proposed PFC for self-powered aptasensing of PCB77. Scheme 1. Schematic illustration of the proposed H2O2-based PFC for self-powered aptasensing of PCB77. OCP and OCP’ represent the open circuit potentials of PFC in the absence and presence of PCB77, respectively.
nanoparticles with a diameter of ca. 15 nm are found to be deposited on the surface of g-C3N4 (Figure 1B). The thickness of the synthesized nanomaterials was determined by atomic force microscopy (AFM). As shown in Figure S1, the thickness of g-C3N4 nanosheets is ca. 23 nm and the deposition of Au nanoparticles does not influence the thickness of g-C3N4 nanosheets. Meanwhile, the energy dispersive spectroscopic (EDS) analysis of the g-C3N4-Au sample was carried out (Figure S2). The results confirm the presence and uniform distribution of Au, C, N and O elements in g-C3N4-Au. To analyze the crystalline phases of the prepared materials, X-ray diffraction (XRD) patterns of g-C3N4 and g-C3N4-Au composites were studied. As illustrated in Figure 1C, gC3N4 exhibits one diffraction peak at 27.4°, assigned to its (002) crystal plane.31 In the g-C3N4-Au sample, three additional peaks at 38.3°, 44.5° and 64.7° are clearly observed, which can be indexed to the (111), (200), and (220) planes of Au (JCPDS 04-0784), respectively.32 This result confirms that Au nanoparticles are successfully loaded on g-C3N4. On the other hand, the UV-vis absorption spectra of g-C3N4 and g-C3N4-Au were recorded to observe the effect of Au nanoparticles on the optical property of g-C3N4. As compared with pure g-C3N4, the g-C3N4-Au shows a significant enhancement of light absorption in the visible region (Figure 1D), which should be induced by well-known surface plasmon resonance of Au nanoparticles.33 Moreover, electrochemical impedance spectroscopic (EIS) analysis (Figure S3) reveals that incorporation of Au nanoparticles with g-C3N4 could facilitate the interfacial electron transfer on the electrode surface. Accordingly, the prepared g-C3N4-Au nanocomposites with improved optical and electrochemical properties are favorable to the fabrication of photoelectrode for visible light-driven PFC. A
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Before constructing the PFC, we synthesized g-C3N4-Au nanocomposites through a hydrothermal method as visible light-activated materials to fabricate the photoanode (see supporting information for details). The morphological structures of the prepared g-C3N4 and gC3N4-Au composites were characterized by scanning electron microscopy (SEM). As shown in Figure 1A, the exfoliated g-C3N4 displays the typical nanosheet-like structure. For g-C3N4-Au composite, many Au
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Figure 1. SEM images of (A) g-C3N4 and (B) g-C3N4-Au composites. Inset of (B) shows the SEM image of g-C3N4-Au at high magnification. (C) XRD patterns and (D) UV-Vis absorption spectra of (a) g-C3N4 and (b) g-C3N4-Au composite.
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In the designed membraneless PFC, H2O2 acted as the fuel, and thus the electrochemical behaviors of H2O2 were investigated to ascertain the reactivity of the fuel on both photoanode and cathode by cyclic voltammetry and photocurrent analysis. Figure 2A shows the cyclic voltammetric (CV) curves of H2O2 on g-C3N4-Au modified glassy carbon electrode (g-C3N4-Au/GCE) recorded in 0.1 M phosphate buffer solution (PBS, pH 7.4) with and without photoirradiation. As can be seen, after the addition of 10 mM H2O2 in electrolyte, the anodic oxidation current starting from 0.5 V (vs. SCE) enhances obviously, due to the electrochemical oxidation of H2O2 to O2 on g-C3N4Au/GCE. While the g-C3N4-Au/GCE is irradiated with visible light, the anodic current increases remarkably, accompanied by negative shift of the onset potential for the oxidation of H2O2. This result shows the high photoelectrocatalytic activity of g-C3N4-Au/GCE toward H2O2 oxidation. Furthermore, the photocurrent responses of different electrodes to H2O2 were recorded in 0.1 M PBS at a bias potential of 0 V (vs. SCE) (Figure 2B). In the dark, all the electrodes generate cathodic current, due to the reduction of H2O2. While the light is switched on, no photocurrent is observed on GCE; whereas both g-C3N4/GCE and g-C3N4-Au/GCE exhibit anodic photocurrent responses which are ascribed to the oxidation of H2O2 by photogenerated holes on g-C3N4 or g-C3N4-Au. Further observation indicates that the photocurrent on g-C3N4Au/GCE is almost three times higher than that on gC3N4/GCE, attributed to the enhanced light absorption property and better conductivity benefitting from the introduction of Au nanoparticles. Additionally, the influence of H2O2 concentration on the photocurrent response
On the other hand, the electrochemical behavior of H2O2 on HG/GCE cathode was investigated by cyclic voltammetry. As can be seen, a pair of well-defined redox peaks appear on the CV curve recorded in 0.1 M PBS (pH 7.4) on HG/GCE (curve a in Figure 2D), which are ascribed to single electron transfer process of iron centre in hemin shuttled by graphene.34 When H2O2 is present in the electrolyte, the cathodic current on HG/GCE drastically enhances, accompanied by positively shift of the onset potential of reduction (curve b in Figure 2D). By contrast, the reduction of H2O2 generates very weak cathodic current on bare GCE (curve c in Figure 2D). These results are consistent with our previous observation,12 indicating that HG nanocomposites can be employed as a highly efficient electrocatalyst toward H2O2 reduction. Accordingly, we selected HG/GCE as the cathode in the construction of membraneless PFC. To examine the thermodynamic feasibility of the proposed membraneless PFC, the polarization curves for photoanode and cathode were recorded in 0.1 M PBS (pH 7.4) containing 10 mM H2O2. As can be seen in Figure 3A, upon photoirradiation, the photoelectrocatalytic oxidation of H2O2 on g-C3N4-Au/GCE commences at -0.17 V (vs. SCE), which is obviously more negative than the onset potential (0.27 V (vs. SCE)) for the electrocatalytic reduction of H2O2 on HG/GCE (Figure 3B). This result indicates that the photoanode oxidizes H2O2 promptly at a low potential while the cathode catalyzes the reduction of H2O2 efficiently at a high potential, confirming the thermodynamic feasibility of the establishment of the H2O2-based PFC.35 Subsequently, a membraneless fuel cell was facilely assembled by associating the g-C3N4-Au/GCE photoanode and HG/GCE cathode in one-compartment cell containing 10 mM H2O2 as fuel. To investigate the output performance of the constructed PFC, we recorded the dependence of output voltage (V-I curves, Figure 3C) or power density (P-I curves, Figure 3D) on the cell current. Upon photoirradiation, the PFC constructed with g-C3N4-Au/GCE photoanode and HG/GCE cathode generates an open circuit potential (OCP) of ca. 0.45 V (curve a in Figure 3C) and a maximum output power density (PMax) value of 5.5 μW·cm-2 (curve a in Figure 3D). However, the PFC generates a low OCP (curve b in Figure 3C) and a negligible PMax (curve b in Figure 3D) in the dark, confirming the participation of photogenerated holes in the oxidation of H2O2. When g-C3N4-Au/GCE photoanode is replaced by gC3N4/GCE (curve c in Figure 3C and 3D) or HG/GCE cathode is replaced by bare GCE (curve d in Figure 3C and 3D), the OCP and PMax values of the PFCs upon photoirradiation obviously decline. Therefore, it can be concluded that the proposed H2O2-based PFC constructed with g-C3N4-Au/GCE photoanode and HG/GCE cathode exhibits a superior power output. Furthermore, to obtain
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Analytical Chemistry wards electrode surface to capture the photogenerated holes as revealed by EIS analysis (Figure S3). Thus, the different OCP responses upon the capture of analyte make it possible to be used for self-powered sensing of PCB77. To achieve a better sensing performance, the influences of the concentrations of aptamer and fuel on the OCP difference (ΔOCP) before and after the capture of PCB77 on photoanode were studied. According to the results displayed in Figure S6, 2.0 μM aptamer and 10 mM H2O2 exhibit the optimum performance for PCB77 sensing.
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Figure 3. Polarization curves of (A) g-C3N4-Au/GCE photoanode under photoirradiation and (B) HG/GCE cathode recorded in 0.1 M PBS (pH 7.4) containing 10 mM H2O2 at -1 scan rate of 1.0 mV·s . (C) V-I and (D) P-I curves of the PFCs fabricated with (a, b) g-C3N4-Au/GCE photoanode and HG/GCE cathode; (c) g-C3N4/GCE photoanode and HG/GCE cathode; (d) g-C3N4-Au/GCE photoanode and HG/GCE cathode. The dates were collected using 10 mM H2O2 as fuel. Curves a, c, and d were recorded under photoirradiation while curve b was recorded in the dark.
The developed PFC was utilized to construct the selfpowered sensor for detection of PCB77. To achieve the specific recognition of analyte, PCB77-binding aptamer was assembled on the g-C3N4-Au/GCE (ap/g-C3N4Au/GCE) photoanode through the formation of Au-S bond between Au nanoparticles and thiol groupfunctionalized aptamer. The OCP-time and P-I curves of PFCs constructed with g-C3N4-Au/GCE before and after assembly of aptamer were compared. The PFC constructed with ap/g-C3N4-Au/GCE exhibits an OCP of 0.426 V (curve b in Figure 4A) and a PMax of 3.8 μW·cm-2 (curve b in Figure 4B). Both values are lower than those obtained for the PFC constructed with g-C3N4-Au/GCE photoanode (curve a in Figure 4A and 4B), attributed to the blockage of aptamer to electron transfer. After ap/g-C3N4-Au/GCE is incubated with 200 ng·mL-1 PCB77, the OCP and PMax values decrease to 0.402 V and 2.9 μW·cm-2 (curve c in Figure 4A and 4B). This can be owing to the fact that the immobilization of aptamer and PCB77 on photoanode surface partly hinders the diffusion of electron donor to-
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the highest cell performance, the effects of the amounts of catalysts on the PMax response of PFC toward 10 mM H2O2 were investigated. As depicted in Figure S4, the photoanode prepared with 10 μL of 0.5 g·L-1 g-C3N4-Au and the cathode modified with 7 μL of 2.0 g·L-1 HG suspension are considered to be the optimum conditions for construction of PFC to exhibits a desirable output performance. Additionally, the stability of the optimized PFC was tested by exposing the photoanode under continuous photoirradiation. As shown in Figure S5, the PFC still maintains 95.9% of the initial OCP after being continuously operated for 60 min, suggesting a good stability.
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Figure 4. (A) OCP and (B) P-I curves of the PFC constructed with different photoanodes and HG/GCE cathode in presence of 10 mM H2O2 as fuel: (a) g-C3N4-Au/GCE, (b) ap/g-C3N4-Au/GCE, (c) ap/g-C3N4-Au/GCE incubated with -1 200 ng·mL PCB77. (C) OCP of the PFC constructed with ap/g-C3N4-Au/GCE incubated with different concentrations of PCB77. From a to h: 0, 10, 20, 50, 100, 200, 500, 1000 ng·mL 1 . (D) ΔOCP of the proposed self-powered sensor toward varied PCB77 concentrations. Inset represents the calibration curve for PCB77. Error bars were derived from the standard deviation of three measurements.
The ap/g-C3N4-Au/GCE-based PFC was explored for self-powered sensing of PCB77 under the optimum conditions. Figure 4C shows the OCP-time curves of the PFC for PCB77 at different concentrations. The OCP value of the sensor decreases linearly with logarithm of PCB77 concentration from 10 to 1000 ng·mL-1 (Figure 4D). The linear regression equation is expressed as ΔOCP (mV) = 14.0lg c (ng·mL-1) - 9.24 (correlation coefficient r = 0.996). The detection limit (S/N=3) is estimated to be 4.5 ng·mL-1, lower than that of many previously reported methods for PCB77 detection (Table S1). Moreover, the detection limit of the proposed self-powered sensor is close to that of an Ag nanocrown array-based surface enhanced Raman scattering (SERS) method.36 Although an immuno-PCR method37 and an aptamer-based fluorescence method38 exhibit obviously lower detection limit, they require tedious synthesis of PCB77 immunogens or time-consuming DNA hybridization procedure. To further assess the sensing performance of the proposed strategy, interference studies of the self-powered
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Analytical Chemistry sensor toward PCB77 over other species that might coexist in environmental samples as well as two structurally similar polychlorobiphenyls were carried out under the optimum conditions. As shown in Figure S7, all these species do not induce obvious deflection of detection signal, indicating the high selectivity of the proposed sensor owing to the specific affinity of aptamer with PCB77. As a matter of fact, our sensing procedure includes an incubation step and a measurement step (see supporting information for details). Before being put into the cell to conduct the measurement, the ap/g-C3N4-Au/GCE after incubation with analytical solution is rinsed with water to remove all loosely adsorbed species. Thus, some potential interfering metal ions such as Fe3+ and Co2+ that can easily react with H2O2 are actually removed before they enter the electrolytic solution of H2O2 and does not cause interference in the response even though they are present in the analytical solution. Moreover, the relative standard deviation of the ΔOCP values obtained on ten independently constructed sensors under the optimum conditions is calculated to be 5.8% (Figure S8), suggesting the good reproducibility of the self-powered sensor. Additionally, the feasibility of this method for PCB77 detection was evaluated in environmental water samples. The analytical results of spiked environmental water sample are summarized in Table S2. The good recovery (95.5% ~ 102.8%) suggests that this method shows great promise for PCB77 determination in real samples. In summary, we have successfully demonstrated a visible light-driven membraneless PFC using H2O2 as fuel. In such a PFC, H2O2 was oxidized on g-C3N4-Au photoanode by visible light photocatalysis and reduced on hemin-graphene cathode by electrocatalysis, resulting in effective conversion of chemical energy to electricity. The PFC could be operated at neutral condition and exhibited a good stability upon visible light illumination. After apatamer was coupled with the proposed PFC, a visible light-driven self-powered sensor for highly sensitive and selective detection of PCB77 was successfully developed. Our work provides a useful perspective for visible light-driven PFC and paves a new way to the development of aptasensors.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional experimental details, figures showing AFM patterns, EDS mapping results, EIS analysis, sensor optimization, selectivity and reproducibility tests, tables showing comparison of different sensors for PCB77 determination, and feasibility of the sensor in real samples.
AUTHOR INFORMATION Corresponding Author *Phone: +86-27-87543032. Fax: +86-27-87543632. E-mail:
[email protected].
Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes The authors declare no competing financial interest
ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 61571198). We also thank the Analytical and Testing Center of Huazhong University of Science and Technology for help in materials characterizations.
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