- Email: [email protected]
New experimental studies on the phase diagram of the Al-Cu-Fe quasicrystal-forming system
Letter pubs.acs.org/ac
Cite This: Anal. Chem. 2019, 91, 6419−6423
Three-Dimensional CdS@Carbon Fiber Networks: Innovative Synthesis and Application as a General Platform for Photoelectrochemical Bioanalysis Yuan-Cheng Zhu,† Yi-Tong Xu,† Yi Xue,† Gao-Chao Fan,‡ Pan-Ke Zhang,*,† Wei-Wei Zhao,*,† Jing-Juan Xu,† and Hong-Yuan Chen*,†
Downloaded via 113.190.190.36 on October 21, 2019 at 13:29:24 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China ‡ Shandong Key Laboratory of Biochemical Analysis, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China S Supporting Information *
ABSTRACT: This Letter reports a novel synthetic methodology for the fabrication of three-dimensional (3D) nanostructured CdS@carbon fiber (CF) networks and the validation of its feasibility for applications as a general platform for photoelectrochemical (PEC) bioanalysis. Specifically, 3D architectures are currently attracting increasing attention in various fields due to their intriguing properties, while CdS has been most widely utilized for PEC bioanalysis applications because of its narrow band gap, proper conduction band, and stable photocurrent generation. Using CdS as a representative material, this work realized the innovative synthesis of 3D CdS@CF networks via a simple solvothermal process. Exemplified by the sandwich immunoassay of fatty-acid-binding protein (FABP), the asfabricated 3D CdS@CF networks exhibited superior properties, and the assay demonstrated good performance in terms of sensitivity and selectivity. This work features a novel fabrication of 3D CdS@CF networks that can serve as a general platform for PEC bioanalysis. The methodology reported here is expected to inspire new interest for the fabrication of other 3D nanostructured Cd-chalcogenide (S, Se, Te)@CF networks for wide applications in biomolecular detection and beyond. fiber (CF) networks and their application as a general platform for PEC immunoassay applications, which to our knowledge has not been reported.15
R
ecently, three-dimensional (3D) architectures have been of considerable attention in various fields because their unique structures allow the design and implementation of sophisticated and versatile platforms with intriguing properties.1−5 For example, 3D catalysts have been intensively developed by incorporating active species into 3D scaffolds such as graphene4 or nickel foam,6 which resulted in a significant enhancement of their catalytic activity because of the higher catalyst loading and better electrode contact.7−9 In the field of photoelectrochemical (PEC) bioanalysis,10−17 3D architectures also have unparalleled prospects due to their high specific surface areas for accommodating functional biomolecules while the numerous pores guarantee the accessibility of the electrolyte to the electrode surface.18−20 In addition, the excellent mechanical strength and outstanding electronic conductivity further favor the charge transfer and diffusion kinetics for solution-solubilized species. Despite their great potential, 3D architectures are rarely studied in PEC bioanalysis.18 Among various semiconductors, CdS has been most widely utilized for PEC bioanalysis because of its narrow band gap, proper conduction band, as well as efficient and stable photocurrent generation.21−26 Herein, this Letter reports the innovative synthesis of 3D nanostructured CdS@carbon © 2019 American Chemical Society
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RESULTS AND DISCUSSION Experimentally, as shown in Scheme 1, a typical 3D CdS@CF network can be easily prepared by directing the CF paper to a solvothermal process, and the applicability of as-fabricated 3D CdS@CF networks was exemplified by a sandwich immunoassay thereon with the fatty-acid-binding protein (FABP) as a model target. In the assay, with the aid of a biocatalytic precipitation (BCP) strategy,27,28 sandwich protein binding would surface-confine the enzyme label of horseradish peroxidase (HRP) to efficiently initiate BCP onto the 3D CdS@CF networks. (See the Supporting Information for the experimental section.) As demonstrated below, the CdS formed on CFs exhibited a highly porous netlike morphology that is especially advantageous for the deposition of insulating BCP species, which could effectively impede the interfacial Received: March 6, 2019 Accepted: April 29, 2019 Published: April 29, 2019 6419
DOI: 10.1021/acs.analchem.9b01186 Anal. Chem. 2019, 91, 6419−6423
Letter
Analytical Chemistry
Scheme 1. Schematic Illustration for the Synthesis of 3D Nanostructured CdS@CF Networks and Their Application for PEC Bioanalysis
Figure 1. (a, b) SEM images of pristine CFs and as-fabricated CdS@CF networks. Insets: the corresponding magnified images. (c) High-resolution SEM (HR-SEM) image of a CdS@CF network electrode. (d) TEM image of the as-fabricated composite. Inset: High-resolution TEM (HR-TEM) of a single CdS NW. (e) STEM image of CdS@CF and EDX elemental mapping images of C, Cd, and S. (f) Operational stability test of the CFs (black) and composite (red). The PEC tests were performed in 0.01 M PBS (pH 7.4) solution containing 0.1 M AA with 0.0 V applied voltage and 410 nm excitation light.
The morphological changes from the pristine CF paper to the 3D CdS@CF networks were tracked by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 1a and the inset, the unprocessed sample consists of randomly overlapped CFs with a very smooth surface. However, as shown in Figure 1b and the inset, after the processing, CdS grew densely on the CFs with a
mass and electron transfer and thus render the BCP-dependent suppression of the photocurrent signal. In such a protocol, the photocurrent response was correlated to the HRP-controlled BCP, which in turn depended on the amount of target FABP. As compared to current state-of-the-art PEC bioanalysis, this work features the innovative synthesis and use of 3D CdS@CF networks as a potential general PEC bioanalysis platform. 6420
DOI: 10.1021/acs.analchem.9b01186 Anal. Chem. 2019, 91, 6419−6423
Letter
Analytical Chemistry
Figure 2. (a) Photocurrent responses of the CdS@CF electrode (curve i), CdS@CF/Ab1, (curve ii), CdS@CF/Ab1/BSA (curve iii), CdS@CF/ Ab1/BSA/FABP (curve iv), CdS@CF/Ab1/BSA/FABP/Ab2 (curve v), and after the BCP reaction (curve vi). (b) EIS and (c) CVs of stepwise modified electrodes in 5.0 mM Fe(CN)63−/4− containing 0.1 M KCl. (d) Plot of the photocurrent variation vs different FABP concentrations. (e) Corresponding derived calibration curve. (f) Selectivity of the immunoassay to FABP with 10 ng mL−1 by comparison to the interfering proteins at the 100 ng mL−1 level: cTnT, p53, LpPLA2, PSA, CEA, IgG, and CK-MB. ΔI is the photocurrent decrement corresponding to the variable FABP concentrations. The PEC tests were performed in 0.01 M PBS (pH 7.4) solution containing 0.1 M AA with 0.0 V applied voltage and 410 nm excitation light.
indicating not only the good PEC property of the CdS but also the excellent contact between the formed CdS and the CFs as the current collector. With repeated on/off illumination cycles over 300 s, the current signal maintained reproducible responses revealing the high mechanical and photophysical stability of the electrode. Incidentally, as shown in Figure S4, linear sweep voltammograms (LSVs) of the 3D CdS@CF photoelectrode were also collected in the dark and upon illumination with a scan rate of 10 mV s−1. The as-fabricated 3D CdS@CF networks were then applied for the quantitative detection of FABP, a kind of biomarker of acute myocardial infarction (AMI). FABP mainly expresses by myocytes, which possesses important significance in the onset of AMI, and the normal concentration of FABP in healthy human plasma or serum is below 5 ng mL−1.31 Upon intermittent light irradiation by the chronoamperometric i−t technique, Figure 2a recorded the stepwise transient photocurrent responses during the immunoassay development corresponding to 100 ng mL−1 FABP. As shown, the CdS@ CF electrode exhibited an obvious PEC response (curve i), while the immunoassay development resulted in a gradual decrease of the signal (curves ii−v). Upon the introduction of BCP, the signal was further depressed obviously (curve vi). During this process, the interface properties of the electrode were also characterized by electrochemical impedance spectroscopy (EIS). As depicted in Figure 2b, both bare CF and CdS@CF exhibited a small semicircle in the EIS Nyquist plot. Subsequently, with the formation of the protein sandwich, the diameter of the resistance circle was increased, which revealed
netlike morphology. The high-resolution SEM image and the elemental mapping are shown in Figure 1c and Figure S1, respectively. Obviously, such a nanostructure with a large surface area and porosity could offer an excellent adsorption microenvironment for the BCP in a subsequent procedure. For better clarity, the sample was characterized by TEM as shown in Figure 1d and the inset. As shown, the netlike CdS consisted of crossing-linking CdS nanowires (NWs) that closely distributed on CFs with a nanoporous structure, and the single CdS NW was ca. 5 nm in diameter with a crystal lattice spacing of 0.336 nm that corresponds to the (111) facet of CdS.29,30 The existence of the porosity enables the composite to form a nanoporous 3D structure with abundant internal space and large surface area. Generally, for applications in various PEC directions, such a unique morphology can expose more active sites, accommodate more guest species, and also facilitate connection between the internal active material and electrolyte. Figure 1e shows the scanning transmission electron microscopy (STEM) image and corresponding energydispersive X-ray spectroscopy (EDX) elemental mapping, which proved the uniform distribution of Cd and S elements in the netlike CdS, and the results agreed well with the surface chemical composition survey by X-ray photoelectron spectroscopy (XPS), as shown in Figures S2 and S3. To study its PEC property, Figure 1f demonstrates the chronoamperometric i−t curves probed by photocurrent action spectra. As shown, the pristine CF paper had no response (black curve), whereas the as-fabricated 3D CdS@CF networks exhibited fast and strong photocurrent generation upon illumination (red curve), 6421
DOI: 10.1021/acs.analchem.9b01186 Anal. Chem. 2019, 91, 6419−6423
Analytical Chemistry
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the increasing charge-transfer resistance (Rct) at the electrode/ solution interface. Consistently, using Fe(CN)63−/4− as the electrochemical probe, the cyclic voltammetry (CV) was also conducted as presented in Figure 2c. As shown, it exhibited a pair of well-defined redox peaks with a small peak-to-peak separation (ΔEp) at the CdS@CF electrode. Along with the immunocomplexing and introduction of BCP on the electrode, ΔEp increased, and the redox peak current decreased, suggesting the diminished electron transfer rate in accordance with the tendency of EIS. These experimental phenomena were due to the fact that the nonconductive properties of the proteins and the insoluble precipitation layer could progressively obstruct the mass transport and electron transfer so that the hindrance and insulation effect elevated on the electrode surface. Because the extent of signal reduction depended upon the target concentration, a sensitive PEC FABP immunoassay can be achieved. Figure 2d shows the variation of the photocurrent signal corresponding to variable FABP concentration. Figures 2e demonstrates that the photocurrent increment linearly increased with the target concentrations from 10 pg mL−1 to 10 ng mL−1 and from 50 ng mL−1 to 1 μg mL−1, and the lowest detection limit was experimentally found as 10 pg mL−1, which was comparable to those obtained detection methods, as shown in Table S1. A relative standard deviation (RSD) of 5.7% was obtained by testing five electrodes at the concentration of 100 ng mL−1, suggesting the good reproducibility of the system. To verify the selectivity, as shown in Figure 2f, cardiac troponin T (cTnT), p53, lipoprotein phospholipase a2 (LpPLA2), prostate-specific antigen (PSA), carcinoembryonic antigen (CEA), immunoglobulin G (IgG), and creatine kinase isoenzymes (CK-MB) were investigated as interference. The concentrations of interfering agents were in 10-fold excess in comparison with the target FABP, and the photocurrent responses were very close to the blank test, demonstrating the favorable selectivity. These results proved the feasibility of the proposed PEC immunoassay and also confirmed the great potential of 3D CdS@CF network-based PEC bioanalysis.
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Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.9b01186. Experimental section, elemental mapping, XPS spectra, high-resolution XPS spectra, LSVs, and a comparison of different methods for FABP determination (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID
Wei-Wei Zhao: 0000-0002-8179-4775 Jing-Juan Xu: 0000-0001-9579-9318 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Science and Technology Ministry of China (Grant 2016YFA0201200), National Natural Science Foundation of China (Grants 21327902 and 21675080), and the Natural Science Foundation of Jiangsu Province (Grant BK20170073).
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REFERENCES
(1) Chaudhari, N. K.; Jin, H.; Kim, B.; Lee, K. Nanoscale 2017, 9, 12231−12247. (2) Ciornii, D.; Riedel, M.; Stieger, K. R.; Feifel, S. C.; Hejazi, M.; Lokstein, H.; Zouni, A.; Lisdat, F. J. Am. Chem. Soc. 2017, 139, 16478−16481. (3) Hou, Y.; Qiu, M.; Nam, G.; Kim, M. G.; Zhang, T.; Liu, K.; Zhuang, X.; Cho, J.; Yuan, C.; Feng, X. Nano Lett. 2017, 17, 4202− 4209. (4) Peurifoy, S. R.; Castro, E.; Liu, F.; Zhu, X. Y.; Ng, F.; Jockusch, S.; Steigerwald, M. L.; Echegoyen, L.; Nuckolls, C.; Sisto, T. J. J. Am. Chem. Soc. 2018, 140, 9341−9345. (5) Zhu, C.; Li, H.; Fu, S.; Du, D.; Lin, Y. Chem. Soc. Rev. 2016, 45, 517−531. (6) Tang, C.; Cheng, N.; Pu, Z.; Xing, W.; Sun, X. Angew. Chem., Int. Ed. 2015, 54, 9351−9355. (7) Ma, T. Y.; Ran, J.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Angew. Chem., Int. Ed. 2015, 54, 4646−4650. (8) Wang, J.; Zhong, H. X.; Wang, Z. L.; Meng, F. L.; Zhang, X. B. ACS Nano 2016, 10, 2342−2348. (9) Kong, D.; Wang, H.; Lu, Z.; Cui, Y. J. Am. Chem. Soc. 2014, 136, 4897−4900. (10) Luo, Z.; Zhang, L.; Zeng, R.; Su, L.; Tang, D. Anal. Chem. 2018, 90, 9568−9575. (11) Hao, N.; Zhang, Y.; Zhong, H.; Zhou, Z.; Hua, R.; Qian, J.; Liu, Q.; Li, H.; Wang, K. Anal. Chem. 2017, 89, 10133−10136. (12) Tu, W.; Wang, Z.; Dai, Z. TrAC, Trends Anal. Chem. 2018, 105, 470−483. (13) Kang, Z.; Yan, X.; Wang, Y.; Bai, Z.; Liu, Y.; Zhang, Z.; Lin, P.; Zhang, X.; Yuan, H.; Zhang, X.; Zhang, Y. Sci. Rep. 2015, 5, 7882. (14) Kang, Z.; Gu, Y.; Yan, X.; Bai, Z.; Liu, Y.; Liu, S.; Zhang, X.; Zhang, Z.; Zhang, X.; Zhang, Y. Biosens. Bioelectron. 2015, 64, 499− 504. (15) Zhao, W. W.; Xu, J. J.; Chen, H. Y. Anal. Chem. 2018, 90, 615− 627. (16) Metzger, T. S.; Chandaluri, C. G.; Tel-Vered, R.; Shenhar, R.; Willner, I. Adv. Funct. Mater. 2016, 26, 7148−7155.
CONCLUSIONS
In short, this work has developed an innovative method for the facile fabrication of 3D nanostructured CdS@CF networks, which was then characterized by various techniques including SEM, TEM, XPS, and electrochemical characterizations. In particular, the transient state photocurrent characterization by the chronoamperometric i−t tests demonstrated the favorable PEC performance of the 3D CdS@CF networks, indicating that it can be a competitive platform for the general development of PEC bioanalysis. Exemplified by FABP as a model target, such a potential was clearly demonstrated by a BCP-supported sandwich immunoassay event, and the asdeveloped assay possessed good performance in terms of sensitivity and selectivity. This work not only featured the novel synthesis of 3D CdS@CF networks for general PEC bioanalysis applications but also offered a new perspective for the preparation of other 3D semiconductor networks for innovative applications in this field. We further expect the implementation of these materials in the broad PEC field and beyond, e.g., photocatalytic H2 evolution, CO2 reduction, degradation of pollutants, and biohazard disinfection. 6422
DOI: 10.1021/acs.analchem.9b01186 Anal. Chem. 2019, 91, 6419−6423
Letter
Analytical Chemistry (17) Tang, J.; Zhang, Y.; Kong, B.; Wang, Y.; Da, P.; Li, J.; Elzatahry, A. A.; Zhao, D.; Gong, X.; Zheng, G. Nano Lett. 2014, 14, 2702− 2708. (18) Shi, X. M.; Wang, C. D.; Zhu, Y. C.; Zhao, W. W.; Yu, X. D.; Xu, J. J.; Chen, H. Y. Anal. Chem. 2018, 90, 9687−9690. (19) Song, H. S.; Kwon, O. S.; Kim, J. H.; Conde, J.; Artzi, N. Biosens. Bioelectron. 2017, 89, 187−200. (20) Balamurugan, J.; Thanh, T. D.; Karthikeyan, G.; Kim, N. H.; Lee, J. H. Biosens. Bioelectron. 2017, 89, 970−977. (21) Zhou, H.; Liu, J.; Zhang, S. TrAC, Trends Anal. Chem. 2015, 67, 56−73. (22) Zhao, W. W.; Wang, J.; Zhu, Y. C.; Xu, J. J.; Chen, H. Y. Anal. Chem. 2015, 87, 9520−9531. (23) Long, Y. T.; Kong, C.; Li, D. W.; Li, Y.; Chowdhury, S.; Tian, H. Small 2011, 7, 1624−1628. (24) Zhu, Y. C.; Xu, F.; Zhang, N.; Zhao, W. W.; Xu, J. J.; Chen, H. Y. Biosens. Bioelectron. 2017, 91, 293−298. (25) Li, P. P.; Cao, Y.; Mao, C. J.; Jin, B. K.; Zhu, J. J. Anal. Chem. 2019, 91, 1563−1570. (26) Wang, Y.; Zhang, L.; Kong, Q.; Ge, S.; Yu, J. Biosens. Bioelectron. 2018, 120, 64−70. (27) Zhao, W. W.; Ma, Z. Y.; Yu, P. P.; Dong, X. Y.; Xu, J. J.; Chen, H. Y. Anal. Chem. 2012, 84, 917−923. (28) Chu, Y.; Deng, A. P.; Wang, W.; Zhu, J. J. Anal. Chem. 2019, 91, 3619−3627. (29) Cheng, L.; Xiang, Q.; Liao, Y.; Zhang, H. Energy Environ. Sci. 2018, 11, 1362−1391. (30) Low, J.; Dai, B.; Tong, T.; Jiang, C.; Yu, J. Adv. Mater. 2019, 31, 1802981. (31) Ishii, J.; Ozaki, Y.; Lu, J. C.; Kitagawa, F.; Kuno, T.; Nakano, T.; Nakamura, Y.; Naruse, H.; Mori, Y.; Matsui, S.; Oshima, H.; Nomura, M.; Ezaki, K.; Hishida, H. Clin. Chem. 2005, 51, 1397−1404.
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DOI: 10.1021/acs.analchem.9b01186 Anal. Chem. 2019, 91, 6419−6423
Cite This: Anal. Chem. 2019, 91, 6419−6423
Three-Dimensional CdS@Carbon Fiber Networks: Innovative Synthesis and Application as a General Platform for Photoelectrochemical Bioanalysis Yuan-Cheng Zhu,† Yi-Tong Xu,† Yi Xue,† Gao-Chao Fan,‡ Pan-Ke Zhang,*,† Wei-Wei Zhao,*,† Jing-Juan Xu,† and Hong-Yuan Chen*,†
Downloaded via 113.190.190.36 on October 21, 2019 at 13:29:24 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China ‡ Shandong Key Laboratory of Biochemical Analysis, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China S Supporting Information *
ABSTRACT: This Letter reports a novel synthetic methodology for the fabrication of three-dimensional (3D) nanostructured CdS@carbon fiber (CF) networks and the validation of its feasibility for applications as a general platform for photoelectrochemical (PEC) bioanalysis. Specifically, 3D architectures are currently attracting increasing attention in various fields due to their intriguing properties, while CdS has been most widely utilized for PEC bioanalysis applications because of its narrow band gap, proper conduction band, and stable photocurrent generation. Using CdS as a representative material, this work realized the innovative synthesis of 3D CdS@CF networks via a simple solvothermal process. Exemplified by the sandwich immunoassay of fatty-acid-binding protein (FABP), the asfabricated 3D CdS@CF networks exhibited superior properties, and the assay demonstrated good performance in terms of sensitivity and selectivity. This work features a novel fabrication of 3D CdS@CF networks that can serve as a general platform for PEC bioanalysis. The methodology reported here is expected to inspire new interest for the fabrication of other 3D nanostructured Cd-chalcogenide (S, Se, Te)@CF networks for wide applications in biomolecular detection and beyond. fiber (CF) networks and their application as a general platform for PEC immunoassay applications, which to our knowledge has not been reported.15
R
ecently, three-dimensional (3D) architectures have been of considerable attention in various fields because their unique structures allow the design and implementation of sophisticated and versatile platforms with intriguing properties.1−5 For example, 3D catalysts have been intensively developed by incorporating active species into 3D scaffolds such as graphene4 or nickel foam,6 which resulted in a significant enhancement of their catalytic activity because of the higher catalyst loading and better electrode contact.7−9 In the field of photoelectrochemical (PEC) bioanalysis,10−17 3D architectures also have unparalleled prospects due to their high specific surface areas for accommodating functional biomolecules while the numerous pores guarantee the accessibility of the electrolyte to the electrode surface.18−20 In addition, the excellent mechanical strength and outstanding electronic conductivity further favor the charge transfer and diffusion kinetics for solution-solubilized species. Despite their great potential, 3D architectures are rarely studied in PEC bioanalysis.18 Among various semiconductors, CdS has been most widely utilized for PEC bioanalysis because of its narrow band gap, proper conduction band, as well as efficient and stable photocurrent generation.21−26 Herein, this Letter reports the innovative synthesis of 3D nanostructured CdS@carbon © 2019 American Chemical Society
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RESULTS AND DISCUSSION Experimentally, as shown in Scheme 1, a typical 3D CdS@CF network can be easily prepared by directing the CF paper to a solvothermal process, and the applicability of as-fabricated 3D CdS@CF networks was exemplified by a sandwich immunoassay thereon with the fatty-acid-binding protein (FABP) as a model target. In the assay, with the aid of a biocatalytic precipitation (BCP) strategy,27,28 sandwich protein binding would surface-confine the enzyme label of horseradish peroxidase (HRP) to efficiently initiate BCP onto the 3D CdS@CF networks. (See the Supporting Information for the experimental section.) As demonstrated below, the CdS formed on CFs exhibited a highly porous netlike morphology that is especially advantageous for the deposition of insulating BCP species, which could effectively impede the interfacial Received: March 6, 2019 Accepted: April 29, 2019 Published: April 29, 2019 6419
DOI: 10.1021/acs.analchem.9b01186 Anal. Chem. 2019, 91, 6419−6423
Letter
Analytical Chemistry
Scheme 1. Schematic Illustration for the Synthesis of 3D Nanostructured CdS@CF Networks and Their Application for PEC Bioanalysis
Figure 1. (a, b) SEM images of pristine CFs and as-fabricated CdS@CF networks. Insets: the corresponding magnified images. (c) High-resolution SEM (HR-SEM) image of a CdS@CF network electrode. (d) TEM image of the as-fabricated composite. Inset: High-resolution TEM (HR-TEM) of a single CdS NW. (e) STEM image of CdS@CF and EDX elemental mapping images of C, Cd, and S. (f) Operational stability test of the CFs (black) and composite (red). The PEC tests were performed in 0.01 M PBS (pH 7.4) solution containing 0.1 M AA with 0.0 V applied voltage and 410 nm excitation light.
The morphological changes from the pristine CF paper to the 3D CdS@CF networks were tracked by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 1a and the inset, the unprocessed sample consists of randomly overlapped CFs with a very smooth surface. However, as shown in Figure 1b and the inset, after the processing, CdS grew densely on the CFs with a
mass and electron transfer and thus render the BCP-dependent suppression of the photocurrent signal. In such a protocol, the photocurrent response was correlated to the HRP-controlled BCP, which in turn depended on the amount of target FABP. As compared to current state-of-the-art PEC bioanalysis, this work features the innovative synthesis and use of 3D CdS@CF networks as a potential general PEC bioanalysis platform. 6420
DOI: 10.1021/acs.analchem.9b01186 Anal. Chem. 2019, 91, 6419−6423
Letter
Analytical Chemistry
Figure 2. (a) Photocurrent responses of the CdS@CF electrode (curve i), CdS@CF/Ab1, (curve ii), CdS@CF/Ab1/BSA (curve iii), CdS@CF/ Ab1/BSA/FABP (curve iv), CdS@CF/Ab1/BSA/FABP/Ab2 (curve v), and after the BCP reaction (curve vi). (b) EIS and (c) CVs of stepwise modified electrodes in 5.0 mM Fe(CN)63−/4− containing 0.1 M KCl. (d) Plot of the photocurrent variation vs different FABP concentrations. (e) Corresponding derived calibration curve. (f) Selectivity of the immunoassay to FABP with 10 ng mL−1 by comparison to the interfering proteins at the 100 ng mL−1 level: cTnT, p53, LpPLA2, PSA, CEA, IgG, and CK-MB. ΔI is the photocurrent decrement corresponding to the variable FABP concentrations. The PEC tests were performed in 0.01 M PBS (pH 7.4) solution containing 0.1 M AA with 0.0 V applied voltage and 410 nm excitation light.
indicating not only the good PEC property of the CdS but also the excellent contact between the formed CdS and the CFs as the current collector. With repeated on/off illumination cycles over 300 s, the current signal maintained reproducible responses revealing the high mechanical and photophysical stability of the electrode. Incidentally, as shown in Figure S4, linear sweep voltammograms (LSVs) of the 3D CdS@CF photoelectrode were also collected in the dark and upon illumination with a scan rate of 10 mV s−1. The as-fabricated 3D CdS@CF networks were then applied for the quantitative detection of FABP, a kind of biomarker of acute myocardial infarction (AMI). FABP mainly expresses by myocytes, which possesses important significance in the onset of AMI, and the normal concentration of FABP in healthy human plasma or serum is below 5 ng mL−1.31 Upon intermittent light irradiation by the chronoamperometric i−t technique, Figure 2a recorded the stepwise transient photocurrent responses during the immunoassay development corresponding to 100 ng mL−1 FABP. As shown, the CdS@ CF electrode exhibited an obvious PEC response (curve i), while the immunoassay development resulted in a gradual decrease of the signal (curves ii−v). Upon the introduction of BCP, the signal was further depressed obviously (curve vi). During this process, the interface properties of the electrode were also characterized by electrochemical impedance spectroscopy (EIS). As depicted in Figure 2b, both bare CF and CdS@CF exhibited a small semicircle in the EIS Nyquist plot. Subsequently, with the formation of the protein sandwich, the diameter of the resistance circle was increased, which revealed
netlike morphology. The high-resolution SEM image and the elemental mapping are shown in Figure 1c and Figure S1, respectively. Obviously, such a nanostructure with a large surface area and porosity could offer an excellent adsorption microenvironment for the BCP in a subsequent procedure. For better clarity, the sample was characterized by TEM as shown in Figure 1d and the inset. As shown, the netlike CdS consisted of crossing-linking CdS nanowires (NWs) that closely distributed on CFs with a nanoporous structure, and the single CdS NW was ca. 5 nm in diameter with a crystal lattice spacing of 0.336 nm that corresponds to the (111) facet of CdS.29,30 The existence of the porosity enables the composite to form a nanoporous 3D structure with abundant internal space and large surface area. Generally, for applications in various PEC directions, such a unique morphology can expose more active sites, accommodate more guest species, and also facilitate connection between the internal active material and electrolyte. Figure 1e shows the scanning transmission electron microscopy (STEM) image and corresponding energydispersive X-ray spectroscopy (EDX) elemental mapping, which proved the uniform distribution of Cd and S elements in the netlike CdS, and the results agreed well with the surface chemical composition survey by X-ray photoelectron spectroscopy (XPS), as shown in Figures S2 and S3. To study its PEC property, Figure 1f demonstrates the chronoamperometric i−t curves probed by photocurrent action spectra. As shown, the pristine CF paper had no response (black curve), whereas the as-fabricated 3D CdS@CF networks exhibited fast and strong photocurrent generation upon illumination (red curve), 6421
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the increasing charge-transfer resistance (Rct) at the electrode/ solution interface. Consistently, using Fe(CN)63−/4− as the electrochemical probe, the cyclic voltammetry (CV) was also conducted as presented in Figure 2c. As shown, it exhibited a pair of well-defined redox peaks with a small peak-to-peak separation (ΔEp) at the CdS@CF electrode. Along with the immunocomplexing and introduction of BCP on the electrode, ΔEp increased, and the redox peak current decreased, suggesting the diminished electron transfer rate in accordance with the tendency of EIS. These experimental phenomena were due to the fact that the nonconductive properties of the proteins and the insoluble precipitation layer could progressively obstruct the mass transport and electron transfer so that the hindrance and insulation effect elevated on the electrode surface. Because the extent of signal reduction depended upon the target concentration, a sensitive PEC FABP immunoassay can be achieved. Figure 2d shows the variation of the photocurrent signal corresponding to variable FABP concentration. Figures 2e demonstrates that the photocurrent increment linearly increased with the target concentrations from 10 pg mL−1 to 10 ng mL−1 and from 50 ng mL−1 to 1 μg mL−1, and the lowest detection limit was experimentally found as 10 pg mL−1, which was comparable to those obtained detection methods, as shown in Table S1. A relative standard deviation (RSD) of 5.7% was obtained by testing five electrodes at the concentration of 100 ng mL−1, suggesting the good reproducibility of the system. To verify the selectivity, as shown in Figure 2f, cardiac troponin T (cTnT), p53, lipoprotein phospholipase a2 (LpPLA2), prostate-specific antigen (PSA), carcinoembryonic antigen (CEA), immunoglobulin G (IgG), and creatine kinase isoenzymes (CK-MB) were investigated as interference. The concentrations of interfering agents were in 10-fold excess in comparison with the target FABP, and the photocurrent responses were very close to the blank test, demonstrating the favorable selectivity. These results proved the feasibility of the proposed PEC immunoassay and also confirmed the great potential of 3D CdS@CF network-based PEC bioanalysis.
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Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.9b01186. Experimental section, elemental mapping, XPS spectra, high-resolution XPS spectra, LSVs, and a comparison of different methods for FABP determination (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID
Wei-Wei Zhao: 0000-0002-8179-4775 Jing-Juan Xu: 0000-0001-9579-9318 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Science and Technology Ministry of China (Grant 2016YFA0201200), National Natural Science Foundation of China (Grants 21327902 and 21675080), and the Natural Science Foundation of Jiangsu Province (Grant BK20170073).
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REFERENCES
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CONCLUSIONS
In short, this work has developed an innovative method for the facile fabrication of 3D nanostructured CdS@CF networks, which was then characterized by various techniques including SEM, TEM, XPS, and electrochemical characterizations. In particular, the transient state photocurrent characterization by the chronoamperometric i−t tests demonstrated the favorable PEC performance of the 3D CdS@CF networks, indicating that it can be a competitive platform for the general development of PEC bioanalysis. Exemplified by FABP as a model target, such a potential was clearly demonstrated by a BCP-supported sandwich immunoassay event, and the asdeveloped assay possessed good performance in terms of sensitivity and selectivity. This work not only featured the novel synthesis of 3D CdS@CF networks for general PEC bioanalysis applications but also offered a new perspective for the preparation of other 3D semiconductor networks for innovative applications in this field. We further expect the implementation of these materials in the broad PEC field and beyond, e.g., photocatalytic H2 evolution, CO2 reduction, degradation of pollutants, and biohazard disinfection. 6422
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Letter
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DOI: 10.1021/acs.analchem.9b01186 Anal. Chem. 2019, 91, 6419−6423