Fabrication of Tris (bipyridine) ruthenium (II)-Functionalized Metal

Sep 12, 2018 - A simple strategy for one-step fabrication of tris(bipyridine)ruthenium(II) (Ru(bpy)32+)-functionalized metal–organic framework (Ru-M...
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Fabrication of Tris(bipyridine)ruthenium(II)-Functionalized Metal− Organic Framework Thin Films by Electrochemically Assisted SelfAssembly Technique for Electrochemiluminescent Immunoassay Xiaoli Qin,†,§ Xianhao Zhang,†,§ Minghan Wang,† Yifan Dong,† Junjie Liu,† Zhiwei Zhu,† Meixian Li,† Di Yang,‡ and Yuanhua Shao*,† †

Anal. Chem. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/12/18. For personal use only.

Beijing National Research Center for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ‡ Institute of Cardiovascular Disease, First Affiliated Hospital of Nanjing Medical University, Nanjing 210029, China S Supporting Information *

ABSTRACT: A simple strategy for one-step fabrication of tris(bipyridine)ruthenium(II) (Ru(bpy) 3 2+ )-functionalized metal−organic framework (Ru-MOF) thin films using a selfassembly approach assisted by an electrochemical way was introduced. In this protocol, the electrochemically driven cooperative reaction of Ru(bpy)32+ as an electrochemiluminescent (ECL) probe and a structure-directing agent, trimesic acid (H3btc) as a ligand, and Zn(NO3)2 as the Zn2+ source leads to an one-step and simultaneous synthesis and deposition of the MOF onto the electrode surface. Characterization of the Ru-MOF thin films was performed with scanning electron microscopy, Fourier transform infrared, and X-ray photoelectron spectroscopy. Scanning ion conductance microscopy was specially applied in situ to image the topography and thickness of the Ru-MOF thin films. The Ru-MOF thin films as a sensing platform show excellent ECL behavior because of plenty of Ru(bpy)32+ molecules encapsulated in the frameworks. On the basis of the Ru-MOF modified electrodes, an ultrasensitive label-free ECL immunosensing method for the human heart-type fatty-acid-binding protein has been developed with a wide linear response range (150 fg mL−1−150 ng mL−1) and a very low limit of detection (2.6 fg mL−1). The prepared immunosensor also displayed excellent stability and good specificity in the test of practical samples.

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Ruthenium(II) ploypyridyl (RuBpy) complexes are of specific interest in the synthesis of various MOFs due to their excellent luminescent properties.22−26 The RuBpy complexes usually, as the guest molecules, have been doped in the frameworks and/or encapsulated in the pores of MOFs. For example, Chi et al. synthesized Ru(bpy)32+-functionalized MOFs (Ru-MOFs) and developed a fast ECL sensing method for Hg2+ detection.22 The RuBpy complexes have also been reported as a templating agent for the synthesis of new MOFs, such as the synthesis of novel MOFs with specific photophysical properties formed from Zn(II) ions, and the 1,3,5tris(4-carboxyphenyl)benzene template by RuBpy was reported by Whittington el al.23 In this work, RuBpy complexfunctionalized MOFs are usually produced by a solvothermal approach, which needed a long reaction time, multisteps, expensive organic solvents, and high-temperature or highpressure conditions. Therefore, a simple, mild, and controllable

etal−organic frameworks (MOFs), as a novel type of porous crystalline materials, have attracted much attention due to their unique properties, such as their welldefined crystal structure, high volume of tunable micropores, large surface area, high agent loading, and versatile functionality.1−3 These porous and crystalline solids have been developed for applications in gas storage and separation,4 catalysis,5−7 and transporting and releasing drugs.8,9 Now another focus topic in this field is the syntheses and applications of functionalized MOFs for target sensing.10−13 For example, Yang et al. used the gold nanoparticales (AuNPs) and MOF composites as novel signal probes for electron transfer-mediated ultrasensitive electrochemical immunoassay to detect the C-reactive protein.14 Electrochemiluminescence (ECL) is a useful technique for biosensing because it has a high sensitivity and simple setup.15−20 An ECL biosensor was recently proposed for the determination of mucin 1 on MCF-7 cancer cells using an abundant N-(aminobutyl)-N-(ethylisoluminol) functionalized MOF as an ECL indicator, which was reported by Yuan et al.21 © XXXX American Chemical Society

Received: July 16, 2018 Accepted: August 31, 2018

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analytical performance demonstrated its promising potential application in clinical test.

method to design and synthesize RuBpy complex-functionalized MOFs is urgently needed and will be an important alternative for fabrication of MOF-based sensors. There have been numerous reports regarding the novel synthetic routes for MOFs, including electrochemical, mechanochemical, microwave, spray drying, and flow chemistry syntheses.27 Electrochemical synthesis of MOFs was developed by the BASF and has attracted great attention.28 Indeed, the electrochemical method can provide a continuous way under mild conditions within minutes or hours to produce the MOF materials and offers the ability to control MOF synthesis by adjustment of the applied voltage or passed current.29 More importantly, the electrochemical method has been successfully used for the deposition of homogeneous MOF thin films.30−33 Nematollahi et al. introduced a new, simple, and eco-benign method for the fabrication of mesoporous MOF thin film-modified electrodes based on the electrochemical synthesis and deposition.24 In this work, we introduced a new strategy for one-step fabrication of Ru(bpy)32+-functionalized MOF (Ru-MOF) thin films using the self-assembly approach assisted by the electrochemical way. In this protocol, the electrochemically driven cooperative reaction of Ru(bpy)32+ as an ECL probe and structure-directing agent, trimesic acid (H3btc) as a ligand, and Zn(NO3)2 as the Zn2+ source leads to an one-step and in situ synthesis and deposition of the MOF at a glassy carbon electrode (GCE) surface (Scheme 1). The Ru-MOF thin film-

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EXPERIMENTAL SECTION Chemicals and Materials. The following chemicals were employed as received, and all the reagents were analytical grade or better. Ru(bpy)3Cl2·6H2O and bovine serum albumin (BSA) were purchased from Sigma-Aldrich. Trimesic acid (H3btc) was obtained from Energy Chemical (Shanghai, China). Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and TEOA (≥98%) were obtained from Xilong Scientific Co., Ltd. (Guangdong, China). Absolute ethanol (C2H5OH), glutaraldehyde (GA), sodium chloride (NaCl), and potassium nitrate (KNO3) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The electrochemical reaction solution (0.1 M phosphatic buffer solution (PBS1), pH 7.0) and washing and blocking buffer solutions for immunoassay (0.01 M PBS2, pH 7.4) were prepared by the previously described procedure.34 Chitosan (CS) from crab shells (90% deacetylated) was from Sinopharm. CS solution (0.5 wt %) was prepared using 0.10 M acetate buffer solution (pH 5.4). Monoclonal antibody for h-FABP (anti-FABP), hFABP, and the plasma samples of patients were all provided by the First Affiliated Hospital of Nanjing Medical University. The ultrapure water (≥18 MΩ cm) was employed to prepare the aqueous solutions. Apparatus and Characterizations. Scanning electron microscopy (SEM) characterizations were carried out using a Merlin Compact field emission scanning electron microscope (ZEISS Co., Germany). Scanning ion conductance microscopy (SICM) experiments were performed on a ICnanoS2 scanning ion conductance microscope (Ionscope Ltd., UK). Nanopipettes with an 100 nm tip diameter were used as SICM probes and fabricated from borosilicate glass capillaries (0.58 mm inner diameter, 1 mm outer diameter, with filament, Sutter Instrument Co., USA) by a P-2000 laser-based puller (Sutter Instrument Co., USA). Fourier transform infrared (FT-IR) spectra were recorded from a KBr window by a Nicolet iS50 FT-IR spectrophotometer (Thermo Fisher Scientific Inc., USA) and Spectrum Spotlight 200 FT-IR microscopy (PerkinElmer, Inc., USA). X-ray photoelectron spectroscopy (XPS) measurements were carried out using an Axis Ultra spectrometer (Kratos Analytical Ltd., Japan). Electrochemical deposition experiments were carried out on a CHI 760E electrochemical workstation (Shanghai Chenhua Instruments, Co., China) with a typical three-electrode cell. After polishing and sonication, the GCE (diameter = 3 mm) served as the working electrode.35 A platinum sheet electrode and a KCl-saturated calomel electrode (SCE) were employed as the counter and reference electrodes, respectively. The ECL emission measurements were performed on a model MPI-EII electrochemiluminescent analyzer (Xi′An Remax Electronic Science & Technology Co. Ltd., China). A Pt wire electrode and an Ag/AgCl (3 M KCl) electrode were used as the respective counter and reference electrodes. The ECL signal was recorded in 0.1 M PBS1 containing none or TEOA with the immunoelectrode as the working electrode. The cyclic voltammogram was conducted from 0 to 1.35 V with the scan rate of 100 mV s−1. Electrochemical Deposition of the Ru-MOF-Modified Electrode and Fabrication of Immunoelectrodes. According to a previous report,31 0.266 g (0.9 mmol) of Zn(NO3)2·6H2O as a cation source and 0.0303 g (0.1 M) of

Scheme 1. Scheme for the Fabrication of the Ru(bpy)32+ Functional Metal−Organic Framework-Modified Electrode (Ru-MOFs/GCE) and the Construction of the Immunoassay System

modified electrode as a sensing platform shows excellent ECL behavior because massive Ru(bpy)32+ molecules encapsulated in the cavity of the frameworks can be electrooxidated. In this Ru-MOF, the triethanolamine (TEOA) as the co-reactants first diffuses through available channels and cavities within the MOF, then encounters the Ru(bpy)32+ molecules and undergoes ECL reactions, diffuses back through the channels/pores, and finally, exits the Ru-MOFs. Using this Ru-MOF-modified electrode, a label-free ECL immunosensor was developed for the analysis of human heart-type fatty-acidbinding protein (h-FABP), a cardiopathy biomarker, and the B

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Figure 1. SEM images of Ru-MOF on the GCE at the E = −1.3 V vs SCE and different electrolysis times: (A) t = 300 s, (B) t = 600 s, (C) t = 720 s, (D) t = 900 s, (E) t = 1200 s, and (F) t = 1500 s.

Figure 2. Film thickness−time curves (A) and SICM images (B−F) of Ru-MOF on the GCE at different electrolysis times: (B) t = 300 s, (C) t = 600 s, (D) t = 900 s, (E) t = 1200 s, and (F) t = 1500 s. Hopping scanning conditions: Ag/AgCl/120 mM NaCl/AgCl/Ag, bias voltage = 0.2 V, in the Petri dish.

As shown in Scheme 1, 3 μL of 0.5% CS solution was dropped onto the Ru-MOFs/GCE and dried in air. Then, 6 μL of 2.5% GA (in 10 mM PBS2, pH 7.4) was added to the modified electrode for 2 h, followed by being thoroughly rinsed with the ultrapure water. Then, 6 μL of anti-FABP (Ab) was coated on the GA-CS/Ru-MOFs/GCE surface and incubated at 4 °C overnight in the 100% moisture-saturated environment. After removing the excess Ab with PBS2, 6 μL of 3% BSA was dropped onto the electrode surface at 4 °C for 1 h to block the nonspecific binding sites, and also it was washed with PBS2 (BSA/Ab/GA-CS/Ru-MOFs/GCE). Then, the modified electrode was exposed to 6 μL of PBS2 containing different concentrations of antigen or serum samples at 37 °C for 1 h to form antigen/BSA/Ab/GA-CS/Ru-MOFs/GCE

KNO3 as a supporting electrolyte were dissolved in 3 mL of ultrapure water (solution A). Then, 0.105 g (0.5 mmol) of H3btc was dissolved in 3 mL of ethanol (solution B). Last, different volumes of the Ru(bpy)32+ solution (0.1 M) were added to the mixed solution of solution A and solution B under vigorous stirring. This preparation solution (pH 2.1) was aged under stirring for 3 h at room temperature before the electrodeposition process. As shown in Scheme 1, the pretreated GCE was immersed in the above prepared precursor solution, and the simultaneous synthesis and electrodeposition of the Ru-MOF thin films were completed by applying a potential (−1.3 V vs SCE) for a specific period. The modified GCE (Ru-MOFs/GCE) was finally rinsed with ultrapure water for further ECL experiments. C

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Figure 3. (A) SEM images of Ru-MOF on the GCE and (B) its corresponding EDX spectra. (C−E) Elemental maps for Zn-K and Ru-L, respectively. Electrolysis potential at the E = −1.3 V vs SCE and electrolysis time t = 1500 s.

Figure 4. (A) FT-IR spectra of Ru(bpy)32+, H3btc, and Ru-MOFs. XPS analysis for (B) the full region of XPS for Ru-MOFs, (C) the Zn 2p region, and (D) the Ru 3d region.

growth of the MOFs. The effects of encapsulation are evident from changes in ECL properties. Here, the electrogeneration of hydroxide ions in situ at the GCE surface by the applied potential is required.31 The water not only acts as a solvent, but also as an OH− source (NO3− reduction might also form hydroxide moieties) to manage ligand deprotonation and thin film formation at the electrode surface.32 Figures 1 and S1 (see the Supporting Information, SI) show SEM images of synthesized Ru-MOFs under controlled-potential conditions (E = −1.3 V vs SCE). All of these images show an almost perpendicularly oriented 3D thin film at the surface of the electrode. When the electrolysis time extended from 300 to 1500 s, electrogeneration of OH− was rather high, which indicates that a high nucleation rate, and the generated crystals had enough time to grow from spicules to a 3D polygon.

after being washed with PBS2. The prepared electrodes were finally washed with PBS2 for further ECL measurements.

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RESULTS AND DISCUSSION Characterization of the Ru-MOFs. As shown in Scheme 1, the self-assembly approach that was assisted electrochemically was employed for the preparation of a thin layer of Ru(bpy)32+‑functionalized MOFs on the electrode surface. The electrodeposition solution contained H3btc as a ligand, Zn(NO3)2 as a Zn2+ source, and Ru(bpy)32+ as a luminescent reagent and a structure-directing agent in the solution of KNO3. The electrostatic attraction of negatively charged ligands (btc3−) and positively charged cationic luminescent reagents (Ru(bpy)32+) leads to encapsulate of the Ru(bpy)32+ within the cavities of polyhedral MOFs and controls the D

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ECL Characterization of the Ru-MOFs and the Construction of Immunosensor. This method attempted to encapsulate the luminophores within the polyhedral MOFs, and the effects of encapsulation were evident from changes in ECL properties. As shown in Figure S9, when the deposition times increased, more Ru(bpy)32+ molecules could be confined within the MOFs, leading to the increase of the ECL signal, which indicated that the electrodeposited Ru-MOFs might have a better ECL property. It is clear that a longer deposition time (≥30 min) results in an undesired thicker MOF film, and finally, the ECL signals fall down. To obtain the high ECL signal, the effect of the concentration of Ru(bpy)32+ in the precursor solution was investigated by the ECL method and is shown in Figure S10. The ECL intensities increased gradually with an increase in the concentration of Ru(bpy)32+ from 0 to 5 mM. When the concentration of Ru(bpy)32+ is over 5 mM, the increased ECL signal begins to level off. Thus, 5 mM of Ru(bpy)32+ in the precursor solution was selected. Here, TEOA was chosen as a co-reactant due to it being much less toxic, more soluble,44 and easily diffusible through available channels and cavities within the MOFs in aqueous solution. The effect of its concentration was also investigated. The experimental results (Figure S11) show that the concentration of TEOA with 100 mM could provide the maximum ECL signal for the fabricated sensor; therefore, this value was used in the following study. The details of the construction of an ECL immunosensor are shown in Scheme 1 and characterized by ECL measurements after each step. As shown in Figure 5, the Ru-MOFs/GCE and

SEM imaging has a high spatial resolution; however, it can only perform 2D-imaging and cannot obtain 3D information on the electrode surface and the imagines in situ. Previous work has reported that the measurement of the thickness of the MOF-modified layer requires that the electrode was cut in the longitudinal direction.31 SICM is an in situ multifunctional nanopipette-based technique that has been widely used to image topography,36−38 such as living cells,39 Li-ion batteries,40 and conducting polymer electrodes.41 Glass pipettes with different tip diameters can be used as scanning probes according to different resolution requirements, and 3Dimaging from micrometer to nanometer can be realized under nondestructive conditions. To determine the surface topography, the film thickness of MOFs, and their relationships with the electrolysis time, various Ru-MOF-modified electrodes were characterized with SICM. Figures 2 and S2− S7 show 3D and 2D SICM images of Ru-MOF obtained on the GCE at different electrolysis times in situ. From these figures, we can see that the thickness of the MOF-modified layer formed under 300 s is the most uniform. With the reaction time being increased, the MOF film gradually grows into the multilayer structure, and the thickness is no longer uniform. Thicker polygon side sections can be seen in 3D images, which indicates the continuous deposition of the RuMOFs in the longitudinal direction. These results are consistent with those obtained by SEM. Meanwhile, the thickness of MOFs was measured by the obtained SICM images, and it increases from about 3 to 17 μm in proportion to the deposition time (Figure 3A). As a high-resolution 3Dimaging technique, SICM has unique advantages in the morphological characterization of modified layers on electrodes in situ, especially in thickness analysis. As shown in Figure 3, the elemental mapping distributions of some packed MOF polygons indicate that the MOFs contain the Zn and Ru elements. The Ru elements should come from Ru(bpy)32+, and the Ru(bpy)32+ was uniformly dispersed in the MOFs. It is worth noting, that the presence of Ru(bpy)32+ during the synthesis of MOFs can significantly influence the topography and charge of the resulting framework.23 Figure S8 also indicates a microporous MOF and a disorderly crystallization process and growth of the MOFs without Ru(bpy)32+.42 To study the bonding properties and functionality of the electrodeposition film at the surface of the electrode, FT-IR analysis of a scratched film was conducted. Figure 4A shows FT-IR spectra of Ru(bpy)32+, H3btc, and scratched Ru-MOFs. Compared with the spectra of the original H3btc and Ru(bpy)32+, two significant changes could be observed from the FT-IR spectra of Ru-MOFs. The disappearance of two unique peaks of the carboxylic acid (3086−2552 cm−1) and free COO− (1720 and 1404 cm−1) in the deposited film shows the contribution of the mentioned group in the formation of the MOFs.43 Then, the strong bands at 1627−1458 and 1437− 1373 cm−1 might be due to the skeletal vibration of bipyridine/ benzene and the symmetric and asymmetric vibration of carboxylate anions, respectively,31 which show the cooperation of the ECL probe in the electrocrystallization of the metal ion and ligand. XPS was also applied to characterize the elemental composition of Ru-MOFs. As anticipated, the characteristic peaks for Zn 2p, O 1s, N 1s, C 1s, and Ru 3d are presented in Figure 4(B−D). These proofs demonstrated that the Ru complex was successfully doped in the MOFs synthesized.

Figure 5. ECL intensity−potential curves were obtained on a RuMOFs/GCE (a), on a GA-CS/Ru-MOFs/GCE (b), on an antiFABP/GA-CS/Ru-MOFs/GCE (c), on a BSA/anti-FABP/GA-CS/ Ru-MOFs/GCE (d), and a FABP/BSA/anti-FABP/GA-CS/RuMOFs/GCE (e). All ECL signals were measured in 0.1 M PBS (pH 7.0) solution containing 0.10 M KCl and 100 mM TEOA. Concentration of h-FABP: 15 pg mL−1. Scan rate: 100 mV s −1. Scan potential: 0−1.35 V. PMT = 600 V.

GA-CS/Ru-MOFs/GCE showed high ECL signals (curves a and b). Then, the ECL signal decreased after the addition of anti-FABP and BSA, respectively (curves c and d). The decreasing of the ECL signal was due to the fact that the proteins could hinder the electron transfer. Finally, the obvious decreasing of the ECL signal was observed when the electrode was incubated with h-FABP (curve e). The reason is that the high specificity of anti-FABP toward FABP and the protein E

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Analytical Chemistry molecules were nonconductive. These results indicated that the proposed ECL immunosensor was successfully fabricated. ECL Detection of FABP. FABP is a cytosolic protein mainly expressed by myocytes, and it is the earliest available plasma marker of acute myocardial infarction.45−48 The proposed immunosensor was used for the analysis of the hFABP, and a good analytical performance was obtained. On the basis of Scheme 1, when the concentration of h-FABP in the test solution increased, more specific recognition sites of the binding of h-FABP to anti-FABP could be decreased in the molecular recognition progress, leading to the decrease of the ECL signal. The ECL signals decrease gradually with an increase in the concentration of h-FABP from 150 fg mL−1 to 150 ng mL−1 (Figure 6). A rather wider linear response

Figure 7. Selectivity of the immunoassay using our method for hFABP. h-FABP concentration is 150 pg mL−1, whereas those for other solutes are all 15 ng mL−1. Other conditions are the same as those described in Figure 6. (A) IgG, (B) BSA, (C) cTnI, (D) GOx, (E) DA, (F) L-cysteine, and (G) FABP.

though five replicative ECL measurements, the reproducibility of the immunosensor was evaluated, and the variation coefficient was 5.5 and 6.4%, which shows it has good repeatability. We also evaluated the immunosensor developed in human blood serum substrates by the standard addition method. As shown in Table S2, the acceptable recoveries and RSDs highlight the application potential of this proposed immunosensor in complex biological samples. Thus, these experimental results demonstrate that a new label-free method ECL approach has been developed to determine h-FABP. Then, the proposed immunosensor was incubated with 150 pg mL−1 h-FABP and cyclically scanned in 0.1 M PBS solution to test its stability. As shown in Figure S12, the 15 measurements of the sensor exhibited a stable ECL response with a relative standard deviation (RSD) of 1.2%, showing its satisfactory reversibility and reliability as a possible practical method.

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CONCLUSIONS

In this work, we have provided an eco-benign, simple, and onestep electrochemical method for the fabrication of Ru(bpy)32+functionalized MOF thin films. These Ru-MOF thin films could be immobilized onto the electrode surface in situ during the synthesis process. Also, these thin films were characterized by various surface and in situ techniques, especially when SICM was applied to topographical imaging and thickness of the Ru-MOF thin films. Because of massive Ru(bpy)32+ molecules encapsulated in the frameworks, the Ru-MOF thin films, as a sensing platform, show excellent ECL behavior. On the basis of the antibody−antigen interaction principle, the RuMOFs can achieve unique label-free ECL immunoassay with high sensitivity, stability, low LOD, and a wide linear range in h-FABP detection. In brief, this work is an alternative for typical ECL analytical strategies and opens new ways for the synthesis of luminescence-functionalized MOF materials for applications in ECL immunoassay. As for the numerous photocatalytic reaction schemes based on Ru(bpy)32+, the photophysical properties of these polyhedral Ru-MOFs and their applications are undertaken in our lab.

Figure 6. (A) ECL intensity−time curves of the immunosensor with different concentrations of FABP in 0.1 M PBS (pH 7.0) containing 0.10 M KCl and 100 mM TEOA. h-FABP concentrations: (a) 0, (b) 150 fg mL−1, (c) 1.5 pg mL−1, (d) 15 pg mL−1, (e) 150 pg mL−1, (f) 1.5 ng mL−1, (g) 15 ng mL−1, and (h) 150 ng mL−1. Curve a is the signal with BSA. (B) Relationship between the change of ECL intensity and the logarithm of the concentration of h-FABP. Scan rate: 100 mV s −1. Scan potential: 0−1.35 V. PMT = 800 V.

between the ECL intensity and the common logarithm of hFABP concentration could be obtained, and the detection limit was determined as 2.6 fg mL−1, which is quite sensitive (Table S1). In addition, the specificity and selectivity of this method were investigated. Six possible interfering substances, including IgG, BSA, human cardiac troponin I (cTnI), glucose oxidase (GOx), dopamine (DA), and L-cysteine, have been tested. Figure 7 shows that only FABP caused a significant ECL signal change, which indicated that the immunosensor has a high specificity. Using h-FABP at 15 pg mL−1 and 1.5 ng mL−1 F

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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.8b03186. Comparison of analytical performances of different methods, practical application of the proposed strategy, SEM imgaes, 2D SICM images, effects of Ru(bpy)32+, effects of electrodeposition, effect of the concentration of Ru(bpy)32+ and TEOA, and reversibility and reliability of the immunosensor (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: +86-10-62759394; Fax: +86-10-62751708 ORCID

Meixian Li: 0000-0001-8620-4191 Yuanhua Shao: 0000-0003-3922-6229 Author Contributions §

X.Q. and X.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial supports for this work from the National Key Research and Development Program of China (2016YFA0201300), National Natural Science Foundation of China (21335001 and 21575006), and China Postdoctoral Science Foundation (2016M600846) are gratefully acknowledged. The SICM instrument was funded by the School of Life Science and National Center for Protein Sciences at Peking University. We also thank Prof. Shiqiang Wang and Prof. Xuemei Hao for their useful discussion.

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DOI: 10.1021/acs.analchem.8b03186 Anal. Chem. XXXX, XXX, XXX−XXX