Ingenious Dual-Photoelectrode Internal-Driven Self-Powered Sensing

Jan 14, 2019 - To further heighten solar-energy utilization efficiency could be significantly meaningful for developing useful photoelectric devices. ...
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Letter Cite This: Anal. Chem. XXXX, XXX, XXX−XXX

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Ingenious Dual-Photoelectrode Internal-Driven Self-Powered Sensing Platform for the Power Generation and Simultaneous Microcystin Monitoring Based on the Membrane/Mediator-Free Photofuel Cell Xiaojiao Du,†,‡ Ding Jiang,† Qian Liu,† Nan Hao,† and Kun Wang*,†,§

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Key Laboratory of Modern Agriculture Equipment and Technology, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China ‡ Oakland International Associated Laboratory, School of Electrical and Optoelectronic Engineering, Changzhou Institute of Technology, Changzhou, Jiangsu 213032, P.R. China § Key Laboratory of Sensor Analysis of Tumor Marker, Ministry of Education, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, P.R. China S Supporting Information *

ABSTRACT: To further heighten solar-energy utilization efficiency could be significantly meaningful for developing useful photoelectric devices. Here, by integrating the nitrogendoped graphene-BiOBr (NG-BiOBr) nanocomposites as a photocathode with titanium dioxide (TiO2) nanoparticles as a photoanode synchronously, a dual-photoelectrode internally driven self-powered sensing platform was fabricated, which can work without an external energy input except for light illumination. In this design, the microcystin-LR (MC-LR) molecules function as the fuel and model analyte as well. Avoiding the use of the costly cathode, this is the first example of the integration of a dual photoresponsive electrode into a photofuel cell for self-powered sensing and paves a luciferous way for efficient multidimension energy conversion. Besides, in order to investigate the detailed sensing process of the self-powered system, the Nyquist curves of the interface are studied between the dual-photoelectrode before and after adding the target MCLR. The results demonstrated that the photoanode TiO2 contributed to the oxidation of MC-LR under photoirradiation rather than the photocathode. This work not only provides an appealing idea to construct the sensitive and easy-to-use assays of microcystins but also exhibits a successful prototype of a portable and on-site sensor.

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analyte without enzyme, representing another proof-of-concept advance in self-powered sensors.4,5 Wang et al. designed a dual-response self-powered sensor for solar-driven, real-time, and selective detection of H2O2.6 They have made remarkable progress in this emerging area. Nonetheless, they cannot avoid using the bioactive molecule hemin, dye Prussian Blue (PB), and noble metal Pt. This would involve high cost and intractable pollution. With a view to practical applications, two issues need to be addressed:9 (1) costly Pt cathode, hampering its wide application and (2) low light utilization efficiency with a single solar light responsive photoelectrode in the PFC system. As such, with the Pt cathode substitution, the introduction of a dual photoresponsive electrode may lower the cost and prominently boost the utilization of solar light. Taken in this

elf-powered sensors, a new concept, have came into the researchers’ consciousness because they can function without an external power supply and are beneficial to the miniaturization of detection devices.1,2 The concept was developed earlier in the biofuel cell (BFC) with the aid of biocatalysts (i.e., enzymes, organelles, and micro-organisms).3 However, BFC-typed self-powered sensors have the defects of complex operation, instability, and high-expense due to the intrinsic nature of these bioactive components. The aforementioned matters propel more researchers’ efforts on selfpowered sensors without biocatalysts. Very recently, photofuel cell (PFC) based self-powered sensors have come to the fore owing to several obvious advantages including fast and direct electrons transfer, simple operation, little temperature influence, and without cultivation due to no need of biocatalysts.1,2 The PFCs are typically compact two-electrode systems that convert both light and chemical energy into electricity.3 Only a few designs have been reported so far.4−8 For instance, Zhang’s group developed a visible-light driven PFC-type self-powered sensor with glucose as a example © XXXX American Chemical Society

Received: November 29, 2018 Accepted: January 14, 2019 Published: January 14, 2019 A

DOI: 10.1021/acs.analchem.8b05509 Anal. Chem. XXXX, XXX, XXX−XXX

Letter

Analytical Chemistry

Figure 1. TEM images of (A) TiO2 and (B) NG-BiOBr.

Scheme 1. Schematic Diagram of the Photoassisted Self-Powered MC-LR Sensor Consisting of the NG-BiOBr Photocathode and a TiO2-Based Photoanode

and further technology exploitation because they require a three-electrode system and a potentiostat to power the cell and regulate the sensor.1 In this regard, an idea is inspired whether a device can be designed that produces renewable energy and monitors of MCs pollution concurrently. Herein, we aim to transition a dual-photoelectrode driven self-powered sensor for refractory MCs monitoring based on dual nonenzymatic and membrane-free PFC. By incorporating the nitrogen-doped graphene-BiOBr (NG-BiOBr) photocathode with titanium dioxide (TiO2) nanoparticles as a photoanode, a solar-driven PFC was constructed for selfpowered MCs sensing, which produces renewable energy with simultaneous monitoring of MC pollution. In such a PFC, the prototype microcystin-LR (MC-LR) was oxidized on the TiO2 photoanode by solar light photocatalysis, while charge separation occurred in the NG-BiOBr photocathode under the solar light stimulation, resulting in effective conversion of chemical energy to electricity. This work possesses the unique advantages of no need for external power sources, simple instrumentation, low cost, and ease-for-miniaturization. The morphology of the TiO2 and NG-BiOBr nanocomposites was investigated by the TEM technique. Figure 1A revealed that the as-prepared TiO2 is in the form of the nanoparticle geometric shape via the hydrothermal treatment method. Furthermore, as depicted in Figure 1B, the BiOBr showed a nanoplate morphology and scattered on the wrinkle NG sheets. The light absorption properties of both electrode nanomaterials have been displayed in Figure S1.

sense, we envisage that both the anode and cathode can be irradiated concurrently in the newly designed PFC system. Besides, these PFC systems either adopted a free mediator, which is harmful to the environment, or need to use a membrane to make the anodic and cathodic compartments separate, causing system complexity.10 Meanwhile, the target analytes are rather limited in the reported PFC-based selfpowered sensors. Almost no efforts have been devoted to the application of such a system in microcystin (MC) analysis. As a result, these issues motivate us to develop novel PFCs with improvements of electrode materials enabling accelerated electron transfer without the need of these foreign redox mediators and expanding the scope of application. The presence of the detrimental cyanobacterial toxins in various aquatic environments stimulates the development of practical water quality monitoring technology with great urgency.11 Some of the most common cyanotoxins found in water environments are MCs, which have been regarded as a type of toxic cyclic heptapeptides and known to pose a serious threat to public health.11,12 Therefore, it is very necessary to implement on-site detection and make immediate response for the treatment. At present, many of the conventional methods for MCs detection are large-scale instrument dependent and laborious and thus unsuited for developing on-site assays, such as high-performance liquid chromatography and mass spectrometry-based assays.13−15 Although the recently developed research on electrochemical sensors in our group has circumvented some of these issues,16−18 electrochemical sensors are still bulky and unfavorable for field applications B

DOI: 10.1021/acs.analchem.8b05509 Anal. Chem. XXXX, XXX, XXX−XXX

Letter

Analytical Chemistry

the properties of the PFC in this work, the other cell was also established for a control test with TiO2 as the photoanode and Pt as the photocathode, with the results shown in Figure 3. Compared with the control group, the OCP of the newly built PFC (Figure 3A) was 0.57 V and even exceeds the classical Ptbased one (0.48 V). Moreover, from Figure 3B, the maximum output power (PMax) of the new one soars, indicating the superiority of PFC in this work. Meanwhile, to further evaluate the output performances of the proposed PFC, the dependence of power density (Figure S3A) is also investigated with different photocathodes. As is presented in Figure S3A, the PFC with NG-BiOBr/ITO as the photocathode (curve c) obviously exhibited more outstanding output performance on the part of OCP and PMax than the bare ITO (curve a) based one and BiOBr/ITO (curve b). This might be due to NG-BiOBr/ITO photocathode with a much faster charge transfer rate (Figure S3B) that can accelerate the photoelectron-transfer and thus facilitate the efficient electron−hole separation. In this regard, the inherent potential difference of the formed external circuit is increased and thus produced electricity is also enhanced. The polarization curves for both photoelectrodes have been provided to further reveal the feasibility of this photofuel cell as is presented in Figure S4. Obviously, the photocathode (NGBiOBr) catalyzes the reduction of O2 at a high potential while the photoanode (TiO2) oxidizes MC-LR at a low potential, suggesting the thermodynamics feasibility of the proposed PFC. The V−I and P−I curves of the PFC for MC-LR at different concentrations were recorded (Figure 4A,B). It is seen that the increase in power density was closely related to the increased concentration of MC-LR. The increase of the power output with the increase of MC-LR concentration powerfully suggests that the generated electrical energy is from the MC-LR fuel. Therefore, a light driven self-powered sensor for detecting MC-LR could be fabricated by tracking the increase in power density. The calibration curve was fitted between PMax and the logarithm of the MC-LR concentration over a linear range from 2 pM to 155 pM (Figure 4C) with a detection limit (3S/ N) of 0.67 pM, which is far lower than the safe values in drinking water (1 μg· L−1) and Tolerable Daily Intake (TDI) body weight per day (0.04 μg kg−1) issued by the World Health Organization (WHO). To investigate the sensing process of self-powered system, we studied the EIS data of the interface between the photoanode and the photocathode before and after adding

As shown in Scheme 1, the dual-photoelectrode PFC is established using two photocatalytic semiconductor electrode configuration including an n-type semiconductor TiO2 photoanode and p-type NG-BiOBr photocathode. As a matter of fact, when exposed to stimulated solar light, the two photoelectrodes can both produce electron−hole pairs. The photogenerated electrons of the TiO2 photoanode spontaneously migrate to the NG-BiOBr photocathode through an external circuit due to the inherent potential difference, and thus electricity is produced (details shown in Figure S2), which is critical for successfully combining the n-type photoanode with the p-type photocathode. In the process, holes of the photoanode and electrons of the photocathode can be released to oxidate MC-LR and the maximum power density increased with the concentration of MC-LR. Such oxidation of MC-LR effectively impedes the recombination of hole−electron pairs. As a result, the assembly of the photoanode and photocathode enables the simultaneous energy generation and MC-LR sensing. Figure 2 displays the OCP curves of the TiO2 photocathode without (curve a) and with (curve b) stimulated solar light

Figure 2. Dependence of open circuit potential (OCP) on the constructed photofuel cell in air-saturated PBS (pH = 5.0) without (curve a) and with (curve b) the simulated sunlight illumination.

illumination in 0.1 M PBS (pH 5). It is found that OCP of PFC shows a conspicuous increase when exposed to light irradiation. It forcefully demonstrates that the light on the two photoelectrodes substantially enhances the generation of electricity. In a classical PFC system, Pt was used as the cathode, while the TiO2 electrode functioned as a photoanode. To examine

Figure 3. (A) V−I and (B) P−I curves of PFC using TiO2/ITO as a photoanode with different electrodes: (a) Pt wire electrode and (b) NGBiOBr/ITO as the photocathode in air-saturated PBS (pH = 5.0). C

DOI: 10.1021/acs.analchem.8b05509 Anal. Chem. XXXX, XXX, XXX−XXX

Letter

Analytical Chemistry

Figure 4. Relationship between (A) V−I and (B) P−I responses and MC-LR concentration. (C) Typical calibration graph for MC-LR between Pmax and MC-LR concentration.

Figure 5. EIS curves of the (A) TiO2/ITO as photoanode and (B) NG-BiOBr/ITO as photocathode before (curve a) and after (curve b) adding MC-LR into the cell.

exchanger and has high permeability to cations,19−21 thus it can function as a natural barrier against the interaction of the negatively charged molecules. Besides, it is found that when the concentration of organic pollutants such as phenol derivatives and antibiotics is up to 20 nM, which is about 130-fold of MC-LR, there is a conscious PFC response observed, suggesting potential interference. Even so, as a tracelevel MC-LR detection method, it is still feasible. However, the selectivity of such a strategy is still limited, meanwhile related work to further enhance the selectivity is under way and will be written up. To characterize the long-term stability of the proposed PFC, the PMax results of the as-prepared PFC toward 155 pM MCLR was recorded across the period of 20 days (Figure S6). The result manifested that the PMax of the PFC sensing system still maintained 94.8% of the original response after about 3 weeks. It suggested that the constructed sensor was of considerable robustness in normal storage conditions and has great potential for manufacture, storage, and long-distance transport to remote regions and on-site determinations. In this study, natural pond water samples were collected and it was investigated whether the photoassisted self-powered sensors can be applied in practical applications. First, the water sample was left to rest for half an hour, then centrifuged at 2 000 rpm for 5 min on the centrifuge to remove the solid sediment from the water sample. Finally, the supernatant was taken out, and the MC-LR concentration value of each sample was measured using the proposed method. The experimental results are shown in Table S1. The values collected by the PFC

the target MC-LR. The results are shown in Figure 5. It can be seen from the careful observation of both of them that, after adding MC-LR into the system, the impedance of the photoanode interface increased significantly (Figure 5A), while the impedance of the photocathode basically remained unchanged (Figure 5B), indicating that the sensing process of the target in the system occurred at the photoanode instead of the photocathode interface. This may be due to the fact that TiO2 at the photoanode interface is a kind of excellent semiconductor, has a stronger oxidation capacity than photogenerated holes, and is thus more competitive in capturing target molecules compared with the photocathode NG-BiOBr. In this work, we aimed at proposing a novel detection configuration and demonstrated the possibility of environmental analysis. Therefore, several common environmental contaminants were selected as examples. As is shown in Figure S5, the interference studies were performed in MC-LR solution compared by adding the common photochemical interfering substances, such as dopamine (DA), uric acid (UA), and ascorbic acid (AA). The results show that these interfering agents induce almost no power density signal variation, suggesting outstanding specificity of the proposed self-powered assay. Such good selectivity may be that the negatively charged Nafion on the surface of the photoelectrode can selectively absorb neutral and positively charged molecules, but foreign species such as AA, paracetamol, UA, etc. are easily repelled. To our knowledge, Nafion is a perfluorinated sulfonated cation D

DOI: 10.1021/acs.analchem.8b05509 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry based self-powered sensor for the detection of MC-LR (2.11 ± 0.08 pM) were in agreement with liquid chromatography/mass spectrometry (LC/MS) data (2.17 ± 0.023 pM), suggesting the reliability of the self-powered sensor. The recovery percentage of the self-powered sensor measured by the standard addition method is in the range of ∼99.86− 100.14%, and the standard deviation is ∼3.44−5.23%. It can be seen from the table that MC-LR can still be detected in the sample without adding the MC-LR of standard concentration, which indicates that the selected water sample is from the water body that has been contaminated by MC-LR. The above results demonstrate that the proposed self-powered sensors hold great potential in environmental analysis. We have successfully constructed a PFC comprised of a TiO2-modified photoanode and a NG-BiOBr photocathode to develop a dual-photoelectrode internal driven self-powered sensor for detecting MCs. It avoided the use of a biocatalyst or Pt and boosted the efficiency of solar energy utilization. It was found that the maximum power output of the proposed selfpowered sensor shows a responsive signal with the concentration of MC-LR increasing without an external electric source. This work is a successful prototype of a portable and on-site assay in the field of environmental pollutants and would provide a powerful push for developing multiplex self-powered sensors.

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Laboratory of Sensor Analysis of Tumor Marker, Ministry of Education, Qingdao University of Science and Technology (Grant No. SATM201807).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.8b05509. The UV−vis absorption spectra; the energy-level and working mode diagram; P−I curves of PFC using TiO2/ ITO as photoanode with different electrodes: bare ITO, BiOBr/ITO, and NG-BiOBr/ITO as cathode; Nyquist plots of bare ITO, BiOBr/ITO, and NG-BiOBr/ITO; polarization curves of two photoelectrodes; selectivity and storage stability of the developed self-powered sensor; and determination of MC-LR contents in the pond water samples (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 511 88791800. Fax: +86 511 88791708. E-mail: [email protected]. ORCID

Kun Wang: 0000-0001-6764-8686 Author Contributions

The manuscript was written through the contributions of all the authors. All of the authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was supported by the National Natural Science Foundation of China (Grant Nos. 21375050, 21505055, 61601204, and 21675066), Provincial Natural Science Foundation of Jiangsu (Grant No. BK20160542), Research Foundation of Zhenjiang Science and Technology Bureau (Grant No. NY2016011), and the Foundation of Key E

DOI: 10.1021/acs.analchem.8b05509 Anal. Chem. XXXX, XXX, XXX−XXX