Letter Cite This: Anal. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/ac
Enzyme-Initiated Quinone-Chitosan Conjugation Chemistry: Toward A General in Situ Strategy for High-Throughput Photoelectrochemical Enzymatic Bioanalysis Guang-Li Wang,*,† Fang Yuan,† Tiantian Gu,† Yuming Dong,† Qian Wang,‡ and Wei-Wei Zhao*,‡,§ †
Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China ‡ 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 210023, China § Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: Herein we report a general and novel strategy for high-throughput photoelectrochemical (PEC) enzymatic bioanalysis on the basis of enzymeinitiated quinone-chitosan conjugation chemistry (QCCC). Specifically, the strategy was illustrated by using a model quinones-generating oxidase of tyrosinase (Tyr) to catalytically produce 1,2-bezoquinone or its derivative, which can easily and selectively be conjugated onto the surface of the chitosan deposited PbS/NiO/ FTO photocathode via the QCCC. Upon illumination, the covalently attached quinones could act as electron acceptors of PbS quantum dots (QDs), improving the photocurrent generation and thus allowing the elegant probing of Tyr activity. Enzyme cascades, such as alkaline phosphatase (ALP)/Tyr and β-galactosidase (Gal)/Tyr, were further introduced into the system for the successful probing of the corresponding targets. This work features not only the first use of QCCC in PEC bioanalysis but also the separation of enzymatic reaction from the photoelectrode as well as the direct signal recording in a split-type protocol, which enables quite convenient and high-throughput detection as compared to previous formats. More importantly, by using numerous other oxidoreductases that involve quinones as reactants/products, this protocol could serve as a common basis for the development of a new class of QCCC-based PEC enzymatic bioanalysis and further extended for general enzyme-labeled PEC bioanalysis of versatile targets.
H
present, achieving advanced PEC enzymatic bioanalysis remains a challenge. QCCC has of longstanding interest due to its unique capability of covalently conjugating quinone residues to the nucleophilic groups of the aminopolysaccharide chitosan, which is a naturally occurring versatile biopolymer with high permeability, biocompatibility, and nontoxicity.18 The mechanism of the QCCC is incompletely understood because quinones can undergo a complex set of reactions.19 Previous studies have demonstrated that Tyr-generated quinones are conjugated to chitosan’s amines through Schiff base and/or Michael-type adduct linkages.20 For example, selective binding of Tyr-generated reactive quinone derivatives of phenols onto the nucleophilic amino functionalities of chitosan have been reported and applied for environmental remediation for the phenol and bisphenol A (BPA) removal21,22 as well as to create functional polymers.23 In addition, the application of QCCC in
erein we report the use of tyrosinase (Tyr)-initiated quinone-chitosan conjugation chemistry (QCCC) to in situ generate a covalently attached electron acceptor on the photocathode, based on which a general and high-throughput split-type photoelectrochemical (PEC) enzymatic bioanalysis can be realized. PEC bioanalysis is currently being extensively investigated due to its attractive properties in terms of low-cost and high sensitivity.1−6 Especially, PEC enzymatic bioanalysis has become a hot subject of new research interests because of its vital importance in the analysis of diverse targets in disease diagnosis and biological and biomedical research.7 For example, various enzymes, e.g., horseradish peroxidase (HRP),8 glucose oxidase (GOx),9−11 alkaline phosphatase (ALP),12,13 βgalactosidase (Gal),14 and acetylcholine esterase (AChE)15−17 have been integrated with different photoelectrodes and signaling strategies for the corresponding bioanalysis purposes. Despite these desirable progress, the research in this area is still constrained by, e.g., complicated and vulnerable biomolecules immobilization steps, tedious and time-consuming incubation and rinsing process associated with the electrodes, as well as limited sensing protocols and efficient signaling strategies. At © XXXX American Chemical Society
Received: November 8, 2017 Accepted: January 15, 2018
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DOI: 10.1021/acs.analchem.7b04625 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry
Scheme 1. Illustration of the Targeted (a) Enzymatic Reactions or (a′) Enzyme Cascades in the Microwell Plate, (b) the QCCC between Chitosan and Benzoquinones to Yield Schiff Base and Michael-Type Adduct, and (c) Signal Generation Mechanism of the Split Photocathodic Probing of Enzyme Activity
Obviously, the separation of enzyme immobilization (and thus the enzymatic reaction) from the photoelectrode and the direct signal recording of the photocathode in this split-type methodology enables quite convenient and high-throughput detection, as compared to existing PEC enzymatic bioanalysis. More importantly, since there are numerous other oxidoreductases that involving quinones as reactants/products, including HRP,28 laccase,29 coenzyme Q,30 etc., this approach could be easily extended for general enzyme-labeled PEC bioanalysis of versatile targets. According to our recent review on PEC enzymatic bioanalysis,7 such a QCCC-based split-type photocathodic enzymatic bioanalysis has not been reported. Experimentally, PbS QDs sensitized NiO31 is adopted as the photocathode because the PbS QDs are attractive in terms of their small band gap, promisingly high photo to current conversion efficiency,32 and especially their long exciton lifetime33 to facilitate their interaction with redox partners under illumination. The details for the synthesis and characterization of the PbS/NiO/FTO electrode are included with Figures S1−S4 in the Supporting Information. As shown in Figure S5A,B, we also verified that the PbS QDs had proper positioning of band energy that can match with that of NiO for constructing an ideal photocathode. Specifically, the photoexcitation of PbS QDs will lead to separated electrons and holes on their conduction band (CB) and valence band (VB), respectively. Because their VB potential (at +0.90 V vs the normal hydrogen electrode (NHE)) is more positive than that of NiO (0.54 V vs NHE),34 the holes on the VB of PbS QDs will spontaneously transport to that of NiO and then be captured with the electrons transferring from the FTO electrode. Meanwhile, the electrons in the CB of PbS QDs
paper bioassay for the colorimetric detection of phenolic compounds was reported.24 While chitosan has been known to have good adhesion for different substrates and are used widely as a coating material in analytical chemistry, it sparks our interest to utilize such a chemical binding between the chitosan and enzymatically generated quinone in the field of PEC enzymatic bioanalysis. If possible, harnessing this kinetics could allow the innovative operation of PEC enzymatic bioanalysis. Given the wide usage of quinonic compounds that oxidized from phenols by polyphenol oxidases (PPOs) in current bioanalysis,25−27 the success of this hypothesis could further pave the way to a different and general PEC bioanalytical protocol as well as the implementation of new PEC bioanalysis devices. Unfortunately, such a possibility has not been verified. As shown in Scheme 1, this Letter presents the use of QCCC for in situ enzymatically generation of quinonic compounds as covalently attached electron acceptors on the photocathode, on the basis of which a high-throughput, split-type cathodic PEC enzymatic bioanalysis was accomplished. Using Tyr as a model quinones-generating oxidase (Scheme 1a), 1,2-benzoquinone or its derivative of dihydroxyphenylalanine (DOPA) quinone, produced by Tyr-stimulated hydroxylation of phenol or tyrosine in a 96-well microplate, could easily conjugate onto the surface of the chitosan deposited PbS/NiO/FTO photocathode via the QCCC (Scheme 1b). Upon light illumination, the covalently attached quinones acting as electron acceptors effectively improved the photocurrent of the photocathode (Scheme 1c), allowing elegant probing of Tyr activity. We further introduced enzyme cascades (Scheme 1a′), such as ALP/Tyr or Gal/Tyr, and the proposed system was easily extended for selective probing of the corresponding targets. B
DOI: 10.1021/acs.analchem.7b04625 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry
Figure 1. (A) Photocurrent responses of the PbS/NiO/FTO electrode in deaerated Tris-HCl (pH 7.0) solution without (a) and with (b) 1.0 × 10−5 mol/L 1,4-benzoquinone. (B) Photocurrent responses of the PbS/NiO/FTO electrode in deaerated Tris-HCl (pH 7.0) solution before (a) and after the addition of (b) 1.0 × 10−4 mol/L catechol, (c) 5 U/mL Tyr, or (d) the product by 1.0 × 10−4 mol/L catechol and 5 U/mL Tyr. (C) Photocurrent responses of the chitosan deposited PbS/NiO/FTO electrode before (a) and after (b) covalently attached with 1,2-bezoquinone produced from 1.0 × 10−4 mol/L catechol and 5 U/mL Tyr.
Figure 2. (A) Photocurrent responses of the chitosan deposited PbS/NiO/FTO electrode in the presence of different concentrations of Tyr (0, 0.1, 0.5, 1.0, 5.0, 50, 100, 500, 1000 U/L) employing 1.0 × 10−4 mol/L phenol as the substrate. (B) Calibration curve relative to the relative photocurrent change (ΔI) and Tyr concentrations on a logarithmic scale. (C) Selectivity investigations of the assay for Tyr. The concentrations of phenol, BSA, Lys, His, and Arg were 1.0 × 10−4 mol/L; the concentrations of Tyr, pepsin, trypsin, HRP, GOx, and ALP were 5 U/mL.
chitosan modified PbS/NiO/FTO electrode. All these experiments demonstrated that the produced quinones were effective electron acceptors33,35 to accept the electrons of PbS QDs to promote the cathodic photocurrent generation. As shown in Figure S5C, the electrochemical characterizations further revealed that the 1,2-benzoquinone possessed a reduction potential of −0.087 V (vs Ag/AgCl), which was in accordance with the previous literature.36 By comparison, this value was more positive than the CB level of the PbS QDs, implying that there was a potential gradient that drove the photo induced electron transfer (PET) from the excited electrons of the PbS QDs to 1,2-benzoquinone. Next, the two substrates, i.e., phenol and tyrosine, were applied to follow the activity of Tyr by the photocurrent variation. Specifically, as shown in Figure 2A,B, in the case of phenol, the photocurrent increment (ΔI) was found to be linear with the logarithmical concentration of Tyr ranging from 0.1 to 1000 U/L with the detection limit of 0.03 U/L (S/N = 3). When using tyrosine as the substrate of Tyr, as shown in Figure S8, the photocurrent increment (ΔI) was linear with the logarithmical concentration of Tyr ranging from 0.3 to 5000 U/ L with a detection limit of 0.1 U/L (S/N = 3). This sensing platform, as compared in Table S1, exhibited a competitive detection limit that was lower than other assays acquired by fluorometry,37−40 electrochemistry,41 and photoelectrochemistry.42 Compared to the reported PEC method42 for the detection of Tyr, it is a new sensing strategy which is more facile without complex functionalization process for the sensing interface. Its higher sensitivity may be resulted from two reasons: on one hand, this split-type method avoids the adverse effects of the enzyme immobilization for crippling the
will migrate toward their surface and react with an electron acceptor (if there is any) in the electrolyte, producing cathodic photocurrent (Scheme 1c). To reveal the feasibility, as shown in Figure 1A, the effect of 1,4-benzoquinone upon the PbS/ NiO/FTO electrode was studied in a preliminary experiment, the result of which turned out its presence in the electrolyte could enhance the cathodic photocurrent of the electrode obviously. Subsequently, as shown in Figure 1B, the effect of catechol upon the photocurrent in the absence and presence of Tyr was investigated, demonstrating only the 1,2-benzoquinone produced by the Tyr biocatalyzed oxidation of catechol could cause the increase of cathodic photocurrent. On the basis of these results, we employed the QCCC to realize the covalent attachment the enzymatically generated quinones on the chitosan deposited FTO/NiO/PbS electrode for photocurrent inspections. As shown in Figure S6, the successful deposition of chitosan on the PbS/NiO/FTO electrode and the subsequent conjugation of quinone compounds on the chitosan modified electrode were exemplified by 1,2-benzoquinone and confirmed by cyclic voltammograms and absorption spectra. As expected, after the 1,2-benzoquinone was covalently conjugated on the FTO/NiO/PbS electrode through the connector of chitosan, an increment in the photocurrent was also observed (Figure 1C). For the PEC response of the chitosan deposited FTO/ NiO/PbS electrode to 1,2-benzoquinone, an optimum concentration of chitosan deposited at 0.5 wt % and an optimum applied potential of 0 V vs saturated Ag/AgCl were found (Figure S7). In addition to 1,2-benzoquinone, as shown in Figure S8, its derivative of dihydroxyphenylalanine quinone, produced by biocatalyzed oxidation of tyrosine through Tyr, was also found to enhance the photocurrent generation of the C
DOI: 10.1021/acs.analchem.7b04625 Anal. Chem. XXXX, XXX, XXX−XXX
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Figure 3. Photocurrent responses of the chitosan deposited PbS/NiO/FTO electrode for different concentrations of (A) ALP (from a to h), 0, 0.005, 0.01, 0.02, 0.2, 2.0, 20, 50 U/L and inset, calibration curve relative to the photocurrent change (ΔI) and ALP concentrations on a logarithmic scale. (B) Selectivity of the protocol for ALP (20 U/L) analysis. The concentrations of HRP, GOx, and Gal are 20 U/L, the concentrations of Na+, Ca2+, Zn2+, Cl−, Glu, and Lac are 1.0 × 10−4 mol/L. (C) Photocurrent responses of the chitosan deposited PbS/NiO/FTO electrode for different concentrations of Gal (from a to j), 0, 7.0 × 10−4, 1.0 × 10−3, 5.0 × 10−3, 0.01, 0.05, 0.1, 0.5, 1.0, 5.0 U/L; insert, calibration curve relative to the photocurrent change (ΔI) and Gal concentrations on a logarithmic scale. (D) Selectivity investigations for Gal. The concentrations of K+, Ca2+, Na+, Zn2+, His, Thr, and Arg are 1.0 × 10−4 mol/L, the concentrations of Gal, HRP, GOx, and ALP are 5.0 U/L.
sensitivity. In the conventional protocols, the fixed enzymes on the photoelectrodes may be damaged by light irradiation (especially when using the UV light as the excitation source) or by the oxidative/destructive species (photogenerated holes, hydroxyl radicals, superoxide anion radicals, and so on)43−45 produced by the photoelectrochemically active materials. Also, the immobilized enzymes will not only absorb/scatter light but also sterically hinder the interaction between the PEC transducers and the signal reporting species. On the other hand, different from the conventional PEC assays that rely on the diffusion of the electron acceptor from the bulk electrolyte solution to the electrode surface to induce photocurrent change, the quinones as efficient electron acceptors are in close proximity of PbS QDs to promote the photocurrent generation. As shown in Figure 2C, by monitoring the responses from comparatively high concentration of potential interfering substances, such as bovine serum albumin (BSA), pepsin, trypsin, Cu2+, HRP, GOx, ALP, lysine (Lys), histidine (His), arginine (Arg), we can see that this sensing platform had good selectivity for probing Tyr. The potential of the protocol for real sample detection was then evaluated by analyzing the recoveries of Tyr in 20-fold diluted serum samples. As demonstrated in Table S2, the recoveries obtained by standard addition approach varied from 95.3% to 104.5%, and the relative standard deviation for repeated measurements (RSD, n = 5) was from 3.1% to 4.7%, indicating the feasibility of the proposed method for real sample analysis. To demonstrate the general applicability of the proposed method, the probing of Tyr activity was then extended to
follow other Tyr-involved enzymatic cascades. This was exemplified by the assay of ALP and Gal using the bienzyme cascades consisting of ALP or Gal and Tyr. Results also confirmed that the approach enabled the sensitive detection of ALP with good selectivity. As shown in Figure 3A,B, ALP can be detected selectively within a linear detection ranging from 5.0 × 10−3 to 50 U/L with the detection limit as low as 1.6 × 10−3 U/L, while Gal could be detected selectively with a detection limit as low as 2.0 × 10−4 U/L in the linear range from 7.0 × 10−4 to 5.0 U/L with the results shown in Figure 3C,D. No remarkable responses were observed in the presence of other potential interfering agents other than ALP or Gal. As compared in Tables S3 and S4, the present assays were more sensitive than the previously reported ALP46−51 or Gal52−54 assays.
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CONCLUSIONS In conclusion, this study demonstrates a new enzyme-enabled QCCC-based format for the novel PEC enzymatic bioanalysis that is facile, easy-to-use, reagent/time-saving, sensitive, and high throughput. The methodology has a split-type detection format without the necessity to immobilize enzymes onto the electrode, and the mechanism is exemplified by the combined use of enzymatic oxidation of the phenolic compounds by Tyr and the subsequent specific binding of the reaction product to chitosan on a PbS/NiO/FTO electrode. The enhanced photocurrent can be achieved by the PET from the PbS QDs to the surface bioconjugated products, making possible the sensitive probing of Tyr activity (with the detection limit of D
DOI: 10.1021/acs.analchem.7b04625 Anal. Chem. XXXX, XXX, XXX−XXX
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(4) Li, R. Y.; Zhang, Y.; Tu, W. W.; Dai, Z. H. ACS Appl. Mater. Interfaces 2017, 9, 22289−22297. (5) Li, R. Y.; Yan, R.; Bao, J. C.; Tu, W. W.; Dai, Z. H. Chem. Commun. 2016, 52, 11799−11802. (6) Liu, S. S.; Cao, H. J.; Wang, Z. Y.; Tu, W. W.; Dai, Z. H. Chem. Commun. 2015, 51, 14259−14262. (7) Zhao, W. W.; Xu, J. J.; Chen, H. Y. Biosens. Bioelectron. 2017, 92, 294−304. (8) Chen, D.; Zhang, H.; Li, X.; Li, J. H. Anal. Chem. 2010, 82, 2253−2261. (9) Tanne, J.; Schäfer, D.; Khalid, W.; Parak, W. J.; Lisdat, F. Anal. Chem. 2011, 83, 7778−7785. (10) Golub, E.; Niazov, A.; Freeman, R.; Zatsepin, M.; Willner, I. J. Phys. Chem. C 2012, 116, 13827−13834. (11) Yao, W. J.; Le Goff, A.; Spinelli, N.; Holzinger, M.; Diao, G. W.; Shan, D.; Defrancq, E.; Cosnier, S. Biosens. Bioelectron. 2013, 42, 556− 562. (12) Zhao, W. W.; Ma, Z. Y.; Xu, J. J.; Chen, H. Y. Anal. Chem. 2013, 85, 8503−8506. (13) Zhao, W. W.; Ma, Z. Y.; Yan, D. Y.; Xu, J. J.; Chen, H. Y. Anal. Chem. 2012, 84, 10518−10521. (14) Zhao, W. W.; Chen, R.; Dai, P. P.; Li, X. X.; Xu, J. J.; Chen, H. Y. Anal. Chem. 2014, 86, 11513−11516. (15) Pardo-Yissar, V.; Katz, E.; Wasserman, J.; Willner, I. J. Am. Chem. Soc. 2003, 125, 622−623. (16) Zhu, W.; An, Y. R.; Luo, X. M.; Wang, F.; Zheng, J. H.; Tang, L. L.; Wang, Q. J.; Zhang, Z. H.; Zhang, W.; Jin, L. T. Chem. Commun. 2009, 45, 2682−2684. (17) Zhao, W. W.; Shan, S.; Ma, Z. Y.; Wan, L. N.; Xu, J. J.; Chen, H. Y. Anal. Chem. 2013, 85, 11686−11690. (18) Cruz, J.; Kawasaki, M.; Gorski, W. Anal. Chem. 2000, 72, 680− 686. (19) Chen, T. H.; Small, D. A.; Wu, L. Q.; Rubloff, G. W.; VazquezDuhalt, R. G. R.; Bentley, W. E.; Payne, G. F. Langmuir 2003, 19, 9382−9386. (20) Kumar, G.; Smith, P.; Payne, G. F. Biotechnol. Bioeng. 1999, 63, 154−165. (21) Ispas, C. R.; Ravalli, M. T.; Steere, A.; Andreescu, S. Water Res. 2010, 44, 1961−1969. (22) Sun, W. Q.; Payne, G. F. Biotechnol. Bioeng. 1996, 51, 79−86. (23) Wu, L. Q.; Embree, H. D.; Balgley, B. M.; Smith, J. P.; Payne, G. F. Environ. Sci. Technol. 2002, 36, 3446−3454. (24) Alkasir, R. S. J.; Ornatska, M.; Andreescu, S. Anal. Chem. 2012, 84, 9729−9737. (25) Cosnier, S.; Mousty, C.; Cui, X.; Yang, X.; Dong, S. Anal. Chem. 2006, 78, 4985−4989. (26) Akanda, M. R.; Ju, H. Anal. Chem. 2016, 88, 9856−9861. (27) Liu, Z.; Liu, B.; Kong, J.; Deng, J. Anal. Chem. 2000, 72, 4707− 4712. (28) Yuan, J. P.; Guo, W. W.; Wang, E. K. Anal. Chem. 2008, 80, 1141−1145. (29) Xu, F. Biochemistry 1996, 35, 7608−7614. (30) Qin, L. X.; Ma, W.; Li, D. W.; Li, Y.; Chen, X. Y.; Kraatz, H.; James, T.; Long, Y. T. Chem. - Eur. J. 2011, 17, 5262−5271. (31) Dai, W. X.; Zhang, L.; Zhao, W. W.; Yu, X. D.; Xu, J. J.; Chen, H. Y. Anal. Chem. 2017, 89, 8070−8078. (32) Yang, Y.; Rodríguez-Córdoba, W.; Lian, T. Q. Nano Lett. 2012, 12, 4235−4241. (33) Knowles, K. E.; Malicki, M.; Weiss, E. J. Am. Chem. Soc. 2012, 134, 12470−12473. (34) Odobel, F.; Le Pleux, L.; Pellegrin, Y.; Blart, E. Acc. Chem. Res. 2010, 43, 1063−1071. (35) Gill, R.; Freeman, R.; Xu, J. P.; Willner, I.; Winograd, S.; Shweky, I.; Banin, U. J. Am. Chem. Soc. 2006, 128, 15376−15377. (36) Vicentini, F. C.; Garcia, L. L.; Figueiredo-Filho, L. C.; Janegitz, B. C.; Fatibello-Filho, O. Enzyme Microb. Technol. 2016, 84, 17−23. (37) Yang, X. M.; Luo, Y. W.; Zhuo, Y.; Feng, Y. J.; Zhu, S. S. Anal. Chim. Acta 2014, 840, 87−92.
0.03 U/L). Enzyme cascades, i.e., ALP/Tyr coupled hydrolysis and oxidation of P-Tyr as well as Gal/Tyr hydrolysis of P-GP and subsequent oxidation of phenol, are then also synergized with the system for the successful detection of corresponding analytes. Since that, numerous other oxidoreductases also involve with the production/consumption of quinones, we believe this work will lead to a new class of QCCC-based PEC enzymatic bioanalysis and further be extended for general enzyme-labeled PEC bioanalysis of versatile targets. In addition, this method relies on the binding of the enzymatically generated product to the immobilized chitosan, if for other specific PEC purposes, the desired photocurrent might be tuned by simply adjusting the amount of chitosan (as matrix for the formation of quinone-imine).
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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.7b04625. Experimental section, the synthesis and characterization of PbS QDs and the PbS/NiO/FTO electrode, the energy levels of PbS QDs and the reduction potential of 1,2-benzoquinone, the confirmation of chitosan deposition on the PbS/NiO/FTO electrode, the optimized detection conditions, the performance of the Tyr detection with tyrosine as the substrate, the comparison of this detection with other protocols, and its potential for real sample detection (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected];
[email protected]. ORCID
Guang-Li Wang: 0000-0002-6662-839X Yuming Dong: 0000-0002-2999-1325 Wei-Wei Zhao: 0000-0002-8179-4775 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant Nos. 21575052, 21676123, and 21675080), the Natural Science Funds of Jiangsu Province (Grant BK20170073), the MOE & SAFEA for the 111 Project (Grant B13025), the Opening Foundation of the Public Health Centre at Jiangnan University (Grant JUPH201507), and the Fundamental Research Funds for the Central Universities (Grant JUSRP51314B) is appreciated. This work was also supported by a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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