Letter Cite This: Anal. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/ac
3D Semiconducting Polymer/Graphene Networks: Toward Sensitive Photocathodic Enzymatic Bioanalysis Xiao-Mei Shi,†,∥ Chao-De Wang,†,∥ Yuan-Cheng Zhu,†,∥ Wei-Wei Zhao,*,†,‡ Xiao-Dong Yu,*,† Jing-Juan Xu,† and Hong-Yuan Chen*,† †
State Key Laboratory of Analytical Chemistry for Life Science and Collaborative Innovation Center of Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, Jiangsu 210023, China ‡ Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States
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S Supporting Information *
ABSTRACT: This work reports the development of three-dimensional (3D) semiconducting polymer/graphene (SP/G) networks toward sensitive photocathodic enzymatic bioanalysis. Specifically, the porous 3D graphene was first synthesized via the hydrothermal and freeze-dry processes and then mixed with semiconducting polymer to obtain the designed hierarchical structure with unique porosity and large surface area. Afterward, the as-prepared hybrid was immobilized onto the indium tin oxide (ITO) for further characterizations. Exemplified by sarcosine oxidase (SOx) as a model biocatalyst, an innovative 3D SP/G-based photocathodic bioanalysis capable of sensitive and specific sarcosine detection was achieved. The suppression of cathodic photocurrent was observed in the as-developed photocathodic enzymatic biosystem due to the competition of oxygen consumption between the enzyme−biocatalyst process and O2dependent photocathodic electrode. This work not only presented a unique protocol for 3D SP/G-based photocathodic enzymatic bioanalysis but also provided a new horizon for the design, development, and utilization of numerous 3D platforms in the broad field of general photoelectrochemical (PEC) bioanalysis. enzymatic bioanalysis. However, such a possibility in the field of PEC bioanalysis has been little investigated. In this Letter, we have first designed, fabricated, and applied 3D SP/G networks for sensitive photocathodic bioanalysis, which to our knowledge has not been reported (see Supporting Information for Experimental Section). As demonstrated in Scheme 1, 3D SP/G networks were formed by the mixture of 3D graphene, the poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4benzo-{2,1′,3}-thiadazole)] (PFBT), and poly(styrene-comaleic anhydride) (PSMA). The specific preparation process was shown in Scheme S1. Thereafter, the synthesized 3D SP/ G networks were assembled onto the indium tin oxide (ITO) electrode. Afterward, the sarcosine oxidase (SOx) as a model biocatalyst was immobilized for further detection of sarcosine. The suppression of cathodic photocurrent was observed in the as-designed photocathodic enzymatic biosystem due to the competition of oxygen consumption between the enzyme− biocatalyst process and O2-dependent photocathodic electrode. According to a recent review on the PEC enzymatic biosensor,25 such a 3D SP/G-based photocathodic bioanalysis capable of sensitive and specific enzymatic detection has not been reported.
W
e report herein the three-dimensional (3D) semiconducting polymer/graphene (SP/G) networks for sensitive photocathodic enzymatic bioanalysis. Recently, with the use of p-type semiconductors, analysts noticed that the performance of photocathodic bioanalysis generally prevails over that of anodic mode due to its good anti-interference capability against reductive substances.1−4 Such a fascinating feature thus makes it very attractive for future photoelectrochemical (PEC) bioanalysis.5−10 On the other hand, 3D heterostructures integrating different properties of the individual components have exhibited great prospects in improving the performance in the biosensing field.11−13 Especially, 3D graphene-based architectures are of tremendous interest since such unique hierarchical structures possess multidimensional electronic networks, numerous open channels for the accommodation of guest biomolecules, enhanced accessibility of the electrolyte and thus favor diffusion kinetics for both electrons and solution-solubilized species.14−19 Their favorable characteristics and possible synergy have naturally ignited our strong interest to develop 3D graphene-based photocathodic enzymatic bioanalysis. Semiconducting (conjugated) polymers, a large family of versatile polymer materials with excellent photoresponsibility,20−24 may provide proper collaborative species for the development of functional 3D graphene-based architectures and thus ideal photocathodic © XXXX American Chemical Society
Received: June 22, 2018 Accepted: August 6, 2018 Published: August 6, 2018 A
DOI: 10.1021/acs.analchem.8b02816 Anal. Chem. XXXX, XXX, XXX−XXX
Letter
Analytical Chemistry
self-assembled polymers while the nanosheet turned into a thick one. Furthermore, Figure 1c inset shows that the 3D SP/ G solution is dark green, different from that of GO. Figure 1d shows the corresponding transmission electron microscopy (TEM) image of 3D SP/G hybrid, which clearly demonstrated the presence of semiconducting polymer on the 3D graphene. Incidentally, Figure 1d inset shows the TEM image of 3D graphene. Finally, the hybrid was modified onto the ITO electrode to obtain the 3D SP/G networks-based photocathode for further use, and the photograph of the asfabricated photocathode was shown in Figure S2. The successful preparation of 3D SP/G was further validated by X-ray photoelectron spectroscopy (XPS), ultraviolet−visible (UV−vis), Fourier transform infrared (FT-IR), and PEC techniques. As shown in Figure 2a, the XPS spectrum of GO
Scheme 1. Schematic Illustration of 3D SP/G NetworksBased Photocathodic Enzymatic Bioanalysis
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RESULTS AND DISCUSSION Characterization. Figure 1a shows the scanning electron microscopy (SEM) image of the pristine graphene oxide
Figure 1. (a) SEM image of the GO. Inset: the photograph of the homogeneous GO solution. (b) SEM image of the 3D graphene. Inset: the photograph of the 3D graphene resting on a dandelion. (c) SEM image of the 3D SP/G hybrid. Inset: the photograph of the homogeneous 3D SP/G solution. (d) TEM image of 3D SP/G hybrid. Inset: the TEM image of the 3D graphene.
Figure 2. (a) Full-scan XPS spectra of GO, 3D graphene, and 3D SP/ G. (b) UV−vis spectra of semiconducting polymer and 3D SP/G. (c) FT-IR of 3D graphene and 3D SP/G. (d) Photocurrent responses of 3D graphene, 3D SP/G modified electrode, and after the immobilization of the SOx. The PEC tests were performed in 0.1 M Tris-HCl solution (pH 7.0) at the bias voltage of 0 V.
(GO), which exhibited two-dimensional (2D) layered morphology with many wrinkles tiled on the substrate. Figure 1a inset demonstrates the homogeneous black solution of GO in the vial. To develop the 3D structure, as shown in Figure S1a, the GO was first subjected to a hydrothermal process for 12 h at 180 °C in a Telflon-sealed autoclave to produce the graphene hydrogel.26 The formation mechanism of graphene hydrogel was illustrated in Scheme S2 and discussed in detail in Figure S1. After freeze-drying of the graphene hydrogel, the structure and morphology of 3D graphene was revealed by SEM in Figure 1b. Significantly, the 3D graphene composed of multistacked nanosheets exhibited an extremely crossed 3D porous structure.26−31 Figure 1b inset displays that the 3D graphene could easily be placed on the finicky dandelion, demonstrating its very low density. Subsequently, the asobtained 3D graphene was integrated with the semiconducting polymer in the solution of tetrahydrofuran (THF), and the resulted 3D heterostructure was shown in Figure 1c. As revealed, after hybridization, the 3D graphene was coated with
exhibited a strong O 1s peak, while the O 1s peak of 3D graphene was relatively weak, reflecting the well reduction of the GO in the hydrothermal process.32,33 A similar comparison was also conducted by X-ray diffraction (XRD) with the results in Figure S3. Significantly, in addition to the O 1s and C 1s peaks, the N 1s and S 2p peaks appeared in the XPS spectrum of 3D SP/G, indicating the successful integration between semiconducting polymer and the 3D graphene. Figure 2b compares the UV−vis spectra of semiconducting polymer and 3D SP/G. Compared to the absorption peak of the semiconducting polymer, the UV−vis spectrum of the 3D SP/G exhibited the unique peak of 3D graphene at 253 nm, which further suggested the successful preparation of the 3D SP/G. As depicted in Figure 2c, the chemical compositions and groups of the 3D graphene and 3D SP/G were further characterized by FT-IR.33,34 In the pattern of 3D graphene, the characteristic peaks at 2350 and 1634 cm−1 were assigned to the stretching of CC, and the peaks at 1390 and 1108 cm−1 B
DOI: 10.1021/acs.analchem.8b02816 Anal. Chem. XXXX, XXX, XXX−XXX
Letter
Analytical Chemistry were ascribed to the C−C. As to that of 3D SP/G, the peaks at 2920 and 1850−1725 cm−1 of the CO and 1375−1061 cm−1 of the C−O were assigned to the anhydrides of PSMA. In addition, the peaks at 1627 and 1453 cm−1 were the CC of the benzene ring. Besides, the peak at 910 cm−1 was attributed to the C−H of unsaturated bond in 3D SP/G. These results further indicated the successful preparation of 3D SP/G. The PEC properties of the synthesized materials were characterized in Figure 2d. Upon light irradiation, the 3D graphene modified electrode did not exhibit photocurrent response. However, 3D SP/G modified electrode showed an obvious increase in photocurrent response, indicating the successful attachment of the semiconducting polymer onto the graphene substrate. Such an enhancement should be attributed to a synergy effect as illustrated in Scheme 1. As shown, upon light irradiation, electrons from the semiconducting polymer are photoexcited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).35 This allowed the electrons transferrance to the 3D graphene due to the suitable energy band matching. In addition, the unique structure of 3D graphene can promote the rapid charge separation and transfer.36 Then, the electrons are trapped in the dissolved oxygen, with the concurrent electrons supplemented from electrode to HOMO holes. After further immobilization of SOx, the photocurrent intensity was decreased due to the insulating feature of the protein biomolecules.37 PEC Bioanalysis. The developed SOx-3D SP/G photocathode was further applied for the proposed enzymatic detection of sarcosine. In such a protocol, the O2-dependent suppression of the photocurrent was observed because of the competition between the as-prepared O2-sensitive 3D SP/G electrode and the SOx-catalytic process toward O2 reduction.38 The concentration of sarcosine can thus be detected via the variation of the photocurrent. As shown in Figure 3a, with the increase of the sarcosine, the photocurrent decreased gradually, which was attributed to the competition of the dissolved oxygen by SOx. Figure 3b shows that the photocurrent response is linear to the logarithm of the sarcosine concentrations from 0.001 to 0.5 mM. After the calculation, the linear regression equation was I (nA) = 14.50 log (CSar/ mM) − 161.79 with the detection limit of 0.8 μM at a signalto-noise ratio of 3, and the corresponding correlation coefficient was 0.9985. As shown in Figure 3c, the system stability was verified by investigating the photocurrent responses to 0.1 mM target with the light switched on/off for 400 s. It can be seen that there was nearly no change in the photocurrent, demonstrating the good stability of the PEC enzymatic biosensor. Besides, the reproducibility of the PEC was evaluated by an interassay relative standard deviation (RSD) with five electrodes, and 5.6% of the RSD was obtained corresponding to the detection of 0.01 mM sarcosine. Figure 3d shows the selectivity of the PEC enzymatic biosensor. Compared with different kinds of interference species, such as inorganic salts, glucose, ascorbic acid, and amino acids, the biosensor showed obvious photocurrent responses to sarcosine, indicating its good selectivity and especially good antiinterference capability to reductive substances. These results proved the feasibility of using the 3D SP/G electrode toward novel photocathodic enzymatic bioanalysis, and exploration of a unique 3D platform with better performance is currently underway.
Figure 3. (a) Photocurrent responses toward increased sarcosine concentration and (b) corresponding logarithmic calibration curve. Error bars show the standard deviations of five replicate tests. (c) Photocurrent response to 0.1 mM sarcosine with the light switched on/off for 400 s. (d) Selectivity of the PEC enzymatic biosensor among different interference species (0.5 mM) in 0.1 M Tris-HCl solution (pH 7.0) at the bias voltage of 0 V. I0 and I were the photocurrents before and after the introduction of different substances.
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CONCLUSIONS In this work, an elegant 3D SP/G electrode was fabricated, characterized, and applied for innovative photocathodic bioanalysis. Due to unique interconnected porous structure of the 3D graphene, the as-developed SP/G electrode exhibited greatly enhanced cathodic photocurrent. For the enzymatic detection, the superiority of the system was exemplified by the SOx-catalytic conversion of sarcosine and good performance in terms of high sensitivity, stability, and selectivity was achieved. This work features the first use of 3D SP/G electrode for photocathodic enzymatic bioanalysis. We believe this work will open a new perspective for the future advancement of general conducting polymer and 3D platformbased PEC bioanalysis.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.8b02816. Experimental section; the synthesis of 3D SP/G networks; the formation mechanism of 3D graphene hydrogel; photographs of 3D graphene hydrogel and graphene; the photograph of the 3D SP/G networks modified photocathode; XRD patterns of GO and 3D graphene (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
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DOI: 10.1021/acs.analchem.8b02816 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry ORCID
(25) Zhao, W. W.; Xu, J. J.; Chen, H. Y. Biosens. Bioelectron. 2017, 92, 294−304. (26) Xu, Y. X.; Sheng, K. X.; Li, C.; Shi, G. Q. ACS Nano 2010, 4, 4324−4330. (27) Zhang, Q. Q.; Hao, M. L.; Xu, X.; Xiong, G. P.; Li, H.; Fisher, T. S. ACS Appl. Mater. Interfaces 2017, 9, 14232−14241. (28) Xu, X.; Li, H.; Zhang, Q. Q.; Hu, H.; Zhao, Z. B.; Li, J. H.; Li, J. Y.; Qiao, Y.; Gogotsi, Y. ACS Nano 2015, 9, 3969−3977. (29) Fu, G. T.; Yan, X. X.; Chen, Y. F.; Xu, L.; Sun, D. M.; Lee, J. M.; Tang, Y. W. Adv. Mater. 2018, 30, 1704609. (30) Xu, X.; Zhang, Q. Q.; Yu, Y. K.; Chen, W. L.; Hu, H.; Li, H. Adv. Mater. 2016, 28, 9223−9230. (31) Ni, Y.; Chen, L.; Teng, K. Y.; Shi, J.; Qian, X. M.; Xu, Z. W.; Tian, X.; Hu, C. S.; Ma, M. J. ACS Appl. Mater. Interfaces 2015, 7, 11583−11591. (32) Zhao, Q.; Zhu, X. Y.; Chen, B. L. Chem. Eng. J. 2018, 334, 1119−1127. (33) Li, M.; Huang, X. Y.; Wu, C.; Xu, H. P.; Jiang, P. K.; Tanaka, T. J. Mater. Chem. 2012, 22, 23477−23484. (34) Ma, C. L.; Peng, L.; Feng, Y. F.; Shen, J. X.; Xiao, Z. Q.; Cai, K. Y.; Yu, Y. H.; Min, Y.; Epstein, A. J. Synth. Met. 2016, 220, 227−235. (35) Wang, Q.; Ruan, Y. F.; Zhao, W. W.; Lin, P.; Xu, J. J.; Chen, H. Y. Anal. Chem. 2018, 90, 3759−3765. (36) Weng, B.; Xu, Y. J. ACS Appl. Mater. Interfaces 2015, 7, 27948− 27958. (37) Zhu, Y. C.; Zhang, N.; Ruan, Y. F.; Zhao, W. W.; Xu, J. J.; Chen, H. Y. Anal. Chem. 2016, 88, 5626−5630. (38) Dai, W. X.; Zhang, L.; Zhao, W. W.; Yu, X. D.; Xu, J. J.; Chen, H. Y. Anal. Chem. 2017, 89, 8070−8078.
Wei-Wei Zhao: 0000-0002-8179-4775 Jing-Juan Xu: 0000-0001-9579-9318 Author Contributions ∥
X.-M.S., C.-D.W., and Y.-C.Z. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support was from the Science and Technology Ministry of China (Grant No. 2016YFA0201200), National Natural Science Foundation of China (Grant Nos. 21327902 and 21675080), and the Natural Science Foundation of Jiangsu Province (Grant BK20170073).
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DOI: 10.1021/acs.analchem.8b02816 Anal. Chem. XXXX, XXX, XXX−XXX
