Thin-Film Electrochemistry of Single Prussian Blue Nanoparticles

Oct 6, 2017 - Electrochemical behaviors of Prussian blue (PB) have been intensively studied for decades because it not only serves as a model electro-...
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Thin-film Electrochemistry of Single Prussian Blue Nanoparticles Revealed by Surface Plasmon Resonance Microscopy Dan Jiang, Linlin Sun, Tao Liu, and Wei Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03061 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 8, 2017

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Analytical Chemistry

Thin-Film Electrochemistry of Single Prussian Blue Nanoparticles Revealed by Surface Plasmon Resonance Microscopy Dan Jiang, Linlin Sun, Tao Liu, Wei Wang* State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China ABSTRACT: Electrochemical behaviors of Prussian blue (PB) have been intensively studied for decades because it not only serves as a model electro-active nanomaterial in fundamental electrochemistry, but also a promising metal-ion storage electrode material for developing rechargeable batteries. Traditional electrochemical studies are mostly based on bulk materials, leading to an averaged property of billions of PB nanoparticles. In the present work, we employed surface plasmon resonance microscopy (SPRM) to resolve the optical cyclic voltammograms of single PB nanoparticles during electrochemical cycling. It was found that the electrochemical behavior of single PB nanoparticles nicely followed a classical thin-film electrochemistry theory. While kinetic controlled electron transfer was observed at slower scan rates, intra-particle diffusion of K+ ions began to take effect when the scan rate was higher than 60 mV/s. We further found that the electrochemical activity among individual PB nanoparticles was very heterogeneous and such a phenomenon has not been previously observed in the bulk measurements. The present work not only demonstrates the thin-film electrochemical feature of single electro-active nanomaterials for the first time, it also validates the applicability of SPRM technique to investigate a variety of metal ion-storage battery materials, with implications in both fundamental nano-electrochemistry and electro-active materials for sensing and battery applications.

The demand for environment-friendly energy storage materials is dramatically increasing because of serious global environmental problems and the rapid development of electronic devices. Li-ions battery based on transition-metal oxide cathodes is the first commercialized battery and has been widely employed in portable electronics and electric vehicles,1-2 whereas Li resources in the earth are unable to meet the demand of large-scale applications in the near future due to the rarity of Li. Consequently, enormous efforts from the scientific community have been devoted to exploiting other alkali metals-based batteries in the past years, such as Kion batteries,3-5 owing to the abundance of K resources and the extremely low cost. Meantime, K+ ions often show higher mobility among alkaline ions at the electrode/electrolyte interface, and high-performance aqueous K-ion batteries have been successfully prepared, thereby affording greater safety and lower cost. Prussian blue (PB), as a pigment, has been used for about 300 years since it was synthesized by the paint maker Diesbach in Berlin for the first time, and it belongs to a part of transition metal hexacyanometallates consisting of mixed valence compounds.6 In the recent years, PB has attracted ever-growing attention as a potential alternative to transitionmetal oxide cathodes. The rigid and full open framework of PB with moderate interstitial space guarantees the reliable rechargeable insertion/extraction of alkali-ions. The simple preparation procedure, low cost, and non-toxicity of PB further render the large-scale application of PB possible.7-9 In practice, the PB materials consist of a large number of nanoparticles and the diffusion of alkali-ions such as Na+ and K+ among individual nanoparticles during cycling is not uniform. However, traditional electrochemical methods often

offer an average signal that washes out the intrinsic property of single nanoparticles. As a result, determining the electrochemical activity at the level of single PB nanoparticles is highly desired, which facilitates the rational design and optimization toward high-performance PB-based electrode materials. At present, in situ techniques such as transmission electron microscopy (TEM)10-15 and atomic force microscopy (AFM)1617 are often used to investigate the battery materials at the single nanoparticle level, whereas the low through-put, low time resolution, and high cost hinder their practical application severely. Moreover, since the diffusion of alkali-ions in PB nanoparticles during cycling have little relation with the change of the material volume, structure, and weight, the traditional methods are very difficult, if not impossible, to monitor the electrochemical change of single PB nanoparticles. Surface plasmon resonance microscopy (SPRM),18-20 an optical microscopy, is recently developed by us and others to image the refractive index (RI) change of single nanoparticles, which has the advantages of high sensitivity and high throughput. Moreover, visible light illumination with gentle power density employed in SPRM induced a neglectable disturbance to the reaction system compared with electron beam or scanning probes. In our recent work, we have successfully adopted SPRM to measure the optical voltammograms of single LiCoO2 nanoparticles, by virtue of the dependence of the RI of Li1-xCoO2 nanoparticle on its lithiation states (1-x).21 However, the applicability of SPRM to investigate other kinds of metal-ion storage electrode materials have not been demonstrated yet. In this work, we use SPRM to investigate the kinetics of K+ ions into and out of single PB nanoparticles during the

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electrochemical cycling. When K+ ions are intercalated into PB, the color of PB nanoparticles will alter from deep blue to colorless (from PB to Prussian White (PW)),22-25 leading to a significant change of the imaginary part of RI, and vice versa. This enables the dynamic process of K+ ions in single PB nanoparticles to be quantitatively monitored by SPRM as the optical contrast of single nanoparticles in SPRM images is sensitive to its RI. Cyclic voltammogram (CV) of single PB nanoparticles can be derived by analyzing time-lapse SPRM images during cycling, thereby providing an opportunity to resolve the electrochemical activity of single PB nanoparticles optically.26,27 EXPERIMENTAL SECTION Characterizations of PB nanoparticles. The PB nanoparticles and poly-dimethyl diallyl ammonium chloride (PDDA) were directly bought from Sigma. According to the manufacturer instructions, as-obtained PB nanoparticles have a chemical composition of Fe4[Fe(CN)6]3, which is known as the “insoluble” PB.22 X-ray diffraction (XRD) spectrum was recorded on Bruker AXS D8 ADVANCE using Cu Kα radiation at 40 kV and 40 mA in the 2θ degree range from 10° to 80°. The UV-Vis spectra were collected on Cary Series UVVis Spectrophotometer (Agilent Technologies). The transmission electron microscopy (TEM, JEM-2100, JEOL) was used to determine the morphology of single PB nanoparticles. The hydrodynamic size is measured by dynamic light scattering (DLS) performed on a Malvern Nano-ZS90 Zetasizer. Spectroelectrochemistry measurements. An indium tin oxide (ITO)-deposited conductive glass was cut into 1×2 cm2 size, and then the glass surface was washed by acetone and ethyl alcohol thoroughly. The ITO-deposited glass was immersed in 5% PDDA solution for 40 min to make the glass surface positively charged. After that, the glass surface was thoroughly rinsed with deionized water. Subsequently, the charged glass was immersed in the aqueous solution of PB for about 12 hours. Finally, the glass was exposed to the wind of N2 until being dried. As-prepared PB-modified ITO glass was connected with electrochemical workstation and placed in the sample zone of UV-Vis spectrometer to perform spectroelectrochemistry measurements. Optical System of SPRM. A total internal reflection illuminator, a fiber-coupled 680 nm superluminescence diode (Qphotonics LLC, QSDM-680-2, operating power 0.2 mW), a CCD camera (Pike F-032, Allied Vision Technology), and a high numerical-aperture oil immersion objective (60×, N.A. = 1.49) were integrated on an inverted optical microscope (Nikon, Ti-E), on which the SPRM experiment was performed. A traditional three-electrode potentiostat (ARFDE5, Pine Research Instrumentation) was employed for the electrochemical measurement. Signals from the potentiostat and CCD camera were synchronized by means of a data acquisition card (USB-6250, National Instruments) in order to link the electrode potential (and current) and the image frame number. Prior to experiment, PB nanoparticles were well dispersed in a suitable amount of deionized water (18.2 MΩ, Smart2Pure 3 UF, Thermo Fisher) through ultrasonic treatment. The gold-coated coverslip, serving as the working electrode, was placed on the sample stage of inverted microscope. A PDMS open chamber (flexiPERM micro12,

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SARSTEDT) was mounted on the coverslip to form an electrochemical cell. 300 uL 1M KNO3 electrolyte solution was subsequently added into the chamber, followed by additional 20 uL aqueous solution of PB nanoparticles that were injected and mixed. PB nanoparticles would gradually immobilize onto the PDDA-modified coverslip via electrostatic interactions. The immobilization process was monitored by SPRM simultaneously to ensure an appropriate immobilization density, which could be optimized by adjusting the concentration of PB nanoparticles and the waiting time. After 5 minutes, liquid was removed by pipette and thoroughly rinsed and dried. Then, the coverslip was baked for about 10 minutes in a baking oven under the condition of 50 °C in order to strengthen the interaction between nanoparticles and substrate. Finally, for further optical and electrochemical measurements, 1M KNO3 was injected into the chamber again, serving as electrolyte. RESULTS AND DISCUSSION PB nanoparticles used in the present work were bought from Sigma and subsequently characterized with XRD, DLS and TEM. According to the XRD pattern shown in Figure 1a, PB nanoparticles exhibit a face-centered cubic with Fm3m space group (JCPDS No.01-0239), which benefits to the insertion and extraction of K+ ions. Analyzing the XRD peak broadening effect with Scherrer equation revealed an averaged crystal diameter of 45 nm.28 Crystal size is much smaller than the hydrodynamic diameter of 150 nm determined by DLS (Figure 1b), suggesting a polycrystalline structure of PB nanoparticles. This result is consistent with the irregular morphology revealed by TEM image in the insert of Figure 1b.

Figure 1. (a) XRD patterns and (b) DLS and TEM (inset) of PB nanoparticles.

An electro-static adsorption method was adopted to immobilize PB nanoparticles onto the gold-coated coverslip using the procedure described in Experimental section. An extremely low surface density of PB nanoparticles was utilized to deposit 10-20 individual PB nanoparticles in a typical view field of 60×80 μm2. Figure 2a provides a representative SEM image of a region-of-interest (ROI) on the gold-coated coverslip, indicating the successful immobilization of PB nanoparticles. Corresponding SPRM image of the very same ROI is displayed in Figure 2b (see below for details on SPRM). The excellent correlation between SEM and SPRM images demonstrated the capability of SPRM to image individual PB nanoparticles. Figure 3a displays the schematic illustration of the principle used in this work to determine the optical CV of single PB nanoparticles with SPRM. A low-power red beam (wavelength = 680 nm, operating power = 0.2 mW) is collimated and

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Analytical Chemistry

directed into the gold-coated coverslip through the objective to generate surface plasmon polaritons at the gold-solution interface. Each individual nanoparticle immobilized on the gold film would exhibit a parabolic pattern on the SPRM images as long as its size is smaller than the optical diffraction limit (Figure 2b). This special pattern represents the point spreading function (PSF) of SPRM. A SPR intensity value can be extracted from the parabolic pattern in the SPRM image. Detailed descriptions on the optical setup, PSF, and image analysis methods can be found in our previous publications.21 Existing studies have shown that the SPR intensity value of a particular single nanoparticle is a function of its RI,21 volume18 and the vertical distance to the substrate.29

Figure 2. (a) SEM and (b) SPRM images of PB nanoparticles deposited in the same region-of-interest on the gold film.

In order to investigate the electrochemical activity of single PB nanoparticles, SPRM images of many PB nanoparticles in the same view-field were continuously recorded when a scanning potential was applied on the gold film-solution interface, from which the optical responses of each individual can be monitored and analyzed independently. In a typical experiment, a scan range from 0.3 to -0.3 V (vs Ag/AgCl) is imposed in an aqueous solution of 1 M KNO3 at a scan rate of 20 mV/s. According to the solid-state electrochemistry, the ion-transport process is accompanied by the electron transfer.30,31 During the reduction, the electrons are transferred through the interface between PB and electrode, while the K+ ions insert into PB with the growing PW layer. K+ ions are inserted into PB and the high-spin state FeIII atoms are reduced into low-spin state FeII atoms. Accordingly, PB is converted to PW and the color alters from deep blue to colorless. When K+ ions are extracted from PW, Fe atoms naturally recover from FeII to FeIII state and the deep blue color shows up again. The reversible transformation in color facilitates the optical readout of single nanoparticle electrochemistry by SPRM, since SPRM is very sensitive to the RI. It is well-known that the imaginary part of complex RI is associated with the absorption of light, i.e., the color. Figure 3a presents the experimental SPRM patterns of a single nanoparticle at PB (oxidized) and PW (reduced) states, respectively, and the corresponding linescan profiles are displayed in Figure 3b. Obviously, shape of the parabolic pattern is almost identical for nanoparticles with different RI. However, the SPRM intensity (image contrast) is sensitive to the RI of nanoparticle. This is expected because the shape reflects the PSF of the optical system, which is independent with the object smaller than the optical diffraction limit. Figure 3c shows the SPR intensity curve of a single PB nanoparticle that rapidly decreases with the intercalation of K+ ions into a single PB nanoparticle due to the decrease in optical absorption. Oppositely, when K+ ions are extracted out of PW during the reverse scan, SPR intensity completely recovers (Figure 3c), indicating the good electrochemical activity and reversibility of single PB nanoparticles. Figure 3d

(red curve) records an optical CV curve that is derived from the first order derivative of the SPR intensity curve in Figure 1c. Since SPRM intensity reflects the oxidation states of PB nanoparticle (charge quantity), the first order derivative naturally results in the electron transfer rate (current). Similar optical-to-electrochemical conversion principle has been adopted in previous studies involving different kinds of optical imaging techniques.21,32-35 The electrochemical CV curve from the bulk PB materials is simultaneously recorded as shown in Figure 3d (blue curve). The peak positions and the profile of the optical CV for the single PB particle agree well with the ensemble-averaged electrochemical CV, strongly demonstrating the reliability of our method. SPRM is able to visualize the electrochemical changes of tens of PB nanoparticles simultaneously (Figure 2b), showing that SPRM has the advantage of high throughput. Please note that the optical CV exhibited neglectable charging background compared with the electrochemical CV. This result is consistent with our previous studies on the electrochemical activity of LiCoO2 nanoparticles. It is because the charging background is subtracted by taking an adjacent ROI without PB nanoparticles as a reference. The peak potential separation of single PB nanoparticle is determined to be 52 mV (red curve). It is smaller than value (72 mV) in electrochemical CV from the bulk measurement, because single nanoparticle measurement eliminates the broadening effect associated with the heterogeneity among individuals. We conducted a spectroelectrochemistry experiment to prove that different optical extinction properties are responsible for the reduced SPR intensity of single PB nanoparticles (Figure 4). Positively charged PDDA molecules was introduced to facilitate the self-assembly of PB nanoparticles onto ITO electrode via electrostatic interaction. Three-electrode electrochemical cell is herein employed, consisting of an ITO substrate (working electrode), a AgCl/Ag (reference electrode), and a Pt wire (counter electrode). The UV-Vis spectra of the ITO substrate were collected when applying different surface potential as shown in Figure 4b. At the beginning, PB displays a deep blue color resulting from the intense absorption band around 690 nm. This band is due to the electronic transition associated with inter-valence charge-transfer.36,37 With the insertion of K+ ions, the UV-Vis spectrum is significantly declined due to the transformation of PB into PW that does not show any distinct bands in the visible range. Our SPRM apparatus adopts a red beam as the excitation light whose center wavelength is 680 nm with a bandwidth of 10 nm (pink region in Figure 4b). The decreased absorbance during electrochemical reduction alters the imaginary part of the RI of PB nanoparticles at this wavelength, resulting in reduced SPR intensity.38 We further notice that there are two parts in the spectra, one is from 0 V to -0.04 V, and the other is from -0.04 V to -0.9 V. The absorbance decreases significantly around -0.04 V. This result is expected because the formal potential of PB reduction is around -0.04 V (Figure 3d, blue curve). Note that existing studies using in situ XRD have shown that the volume expansion of PB during K+ insertion is only 4%.37 However, the SPRM intensity decreased by 36% during the conversion from PB to PW. We thus believe the SPR intensity change of a single PB nanoparticle here is dominated by the RI instead of its volume. Many techniques that are capable of studying ion insertion/extraction at single nanoparticle level often rely on the lattice expansion, such as TEM and AFM.11,17 As a result,

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Figure 3. (a) Schematic illustration of studying the optical CV of single PB nanoparticles with SPRM. Electrochemical reduction of bluecolored PB to form colorless PW alters the optical property of single nanoparticle, leading to decreased SPRM intensity. (b) The corresponding line-scan profiles of the SPRM patterns for PB (blue curve) and PW (red curve) indicate a decrease in the optical contrast after electrochemical reduction. The blue and red lines are marked in (a). (c) Continuously recorded SPRM intensity curve of a single PB nanoparticle shows a decrease during negative scan (reduction), which is almost completely recovered during the positive scan (oxidation). (d) Optical CV (red line) is obtained by performing a first-order derivative to the SPR intensity curve shown in (c). Both peak positions and peak widths are consistent with those in electrochemical CV of bulk PB materials (blue line).

it is more challenging for them to study positive electrode materials whose volume and morphological changes are often subtle, compared with negative electrode materials such as silicon and graphite. SPRM has particular superiority in this case as SPRM is sensitive to the electronic structure (optical spectrum) rather than atomic structure.

Figure 4. (a) Spectroelectrochemistry experiment confirms the evolution of color (a) and optical extinction spectra (b) of a PBdeposited ITO electrode during electrochemical reduction.

Next, we move on to explore the diffusion of K+ ions in single PB nanoparticles by altering the scan rate. Figure 5a presents the schematic illustration of K+ ions into the solid lattice of PB nanoparticle, and the process can be explained by the three-phase junction theory.40-42 In briefly, the exchange of

the electrons occurs at the electrode-particle interface when solid particles adhering to the electrode, while K+ ions diffuse in the crystal through the particle-solution interface. The junction where electrolyte solution, electrode, and particle meet is the most reactive zone. The extraction is the reverse process of the insertion. As a single PB nanoparticle contains limited K+ ions in its lattice, a thin-film electrochemistry theory is mostly suitable to understand the electrochemical behaviors of single nanoparticles. It is well known that the electrochemical process in electro-active film is either kineticcontrolled or diffusion-controlled depending on the thickness and the scan rate. Many existing studies on the PB film have shown that the electrochemical reaction is a kineticscontrolled process at slow scan rates but the process becomes diffusion-controlled when increasing the scan rate.43 Such trend is also validated in our work by collecting the electrochemical voltammograms on the gold film deposited by a quasi-monola yer of PB nanoparticles (Supporting Information Fig. S1). The semi-infinite diffusion model is applicable only when the scan rate is sufficiently high, causing a diffusion layer significantly smaller than the vertical height of nanoparticle. The SPR intensity curves of the same individual PB nanoparticle with different scan rates are displayed in Figure 5b. The SPR intensity drop (by ~250 IU)

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Figure 5. (a) Schematic of the insertion of K+ ions from the single PB film. (b) The SPR intensity curve of a single PB nanoparticle at different scan rate. (c) The first order derivative of SPR intensity of a single PB nanoparticle at different scan rate. (d) The peak current as a function of the scan rate (except 100 mV/s).

are almost identical under all scan rates in the range of 10-60 mV/s. Because SPR intensity quantitatively reflects the chemical composition (oxidation states), these results mean that the single PB nanoparticle was completely charged and discharged during electrochemical cycling in all cases. This is a clear sign of thin-film electrochemistry, because one of the major features associated with thin-film electrochemistry is that the electroactive species can be exhausted during electrochemical reactions. For single PB nanoparticle, the transferred charge is determined by the volume of the nanoparticle. When the scan rate is increased to 100 mV/s, the SPR intensity curve deviates from the others, suggesting that diffusion process begins to take effect. Given the K+ diffusion coefficient in PB nanoparticle of 5×10-16 m2/s 43 and a reactive time of 10 s (scan rate 60 mV/s and scan rage 600 mV), one can estimate the thickness of diffusion layer to be 70 nm. This value is just smaller than the vertical height of a single PB nanoparticle (100-150 nm), indicating that intra-particle diffusion began to take effect. Figure 5c shows the optical voltammograms at different scan rates, from which it can be seen that the peak currents linearly increase with the scan rate in the range of 10-60 mV/s. The peak current at 100 mV/s is obviously smaller than predicted by linear extrapolation. This further supports that the electron transfer rate of the single PB nanoparticle is kinetic-controlled at slow scan rates and becomes diffusion-controlled with scan rates higher than 60 mV/s. Furthermore, relatively higher transition scan rates were usually detected for nanoparticles with smaller size, because it took less time to completely charge or discharge a smaller nanoparticle (Supporting Information Fig. S2).

Another important feature associated with optical voltammograms is that the anodic peak current (ipa) is larger than the cathodic peak current (ipc), and different individual PB nanoparticles display quite diverse current ratio (ipa/ipc) values. In order to demonstrate the heterogeneity, optical voltammograms of 80 nanoparticles were investigated. Their current ratio values are shown in Figure 6b. Representative optical voltammograms of two extremes (NP1 and NP2) are shown in Figure 6a. Interestingly, the ratio is not constant and it ranges from 1.0 to 2.0, indicating the electrochemical activities among single PB nanoparticles are very heterogeneous. The reduction current is always larger than the oxidation current, indicating that the extraction process of K+ ions is kinetically preferred compared to the insertion process. Such trend is often observed in bulk measurement.25 The histogram of 80 ratio values is displayed in Figure 6c, from which it can be seen that the current ratio presents a Gaussianlike distribution and the expected value is around 1.4. This value is close to the ratio value (1.2) extracted from the electrochemical CV of ensemble nanomaterials (Figure 3d, blue curve). The structural basis of such heterogeneous activity among single PB nanoparticles is unclear at the present stage. Irregular polycrystalline structure and random nanoparticle-substrate contact are likely to play roles in such heterogeneity.44 Sophisticated synthesis of cubic-like PB nanoparticles with well-define single-crystalline structure is anticipated to build a better structure-activity relationship. It is believed that combining the capability of SPRM to resolve single nanoparticle electrochemistry and high-resolution electron microscope would point out a promising way to

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elucidate the structural basis of such heterogeneous activities in future.

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21327902), and the Natural Science Foundation of Jiangsu Province (BK20150013).

REFERENCES

Figure 6. (a) Optical voltammograms of two individual nanoparticles marked by NP1 and NP2 in (b), respectively. (b,c) The current ratio (ipa/ipc) values of 80 single PB nanoparticles are highly diverse in scattered point plot (b) and histogram plot (c). In conclusion, we used SPRM technique to measure the optical CV of single PB nanoparticles quantitatively during electrochemical cycling. The electrochemical-to-optical conversion is based on the sensitive dependence of RI on the oxidation states of single PB nanoparticles. The reduction of blue-colored PB to form colorless PW reduced the optical absorption at the wavelength used for SPRM imaging. By collecting the optical CV at different scan rates, it was found that electrochemical behaviors of single PB nanoparticles excellently followed thin-film electrochemistry theory. Electron transfer is kinetic-controlled at scan rates below 60 mV/s, and it begins to involve diffusion process at higher scan rate. Despite of the rapidly growing interests in studying single nanoparticle electrochemistry in recent years, it remains technically challenging to measure the cyclic voltammograms at single nanoparticle level. Therefore, we believe this work demonstrated both theoretical and technical advances by employing an advanced SPRM microscopy to resolve the optical cyclic voltammograms of important electroactive nanomaterials at single nanoparticle level. As PB represents a big family of hexacyanoferrate compounds which are able to incorporate different alkali cations,45 the present technique is anticipated to be generally applicable to explore the electrochemical and chemical activity of these PB-analogues nanomaterials at single nanoparticle level, and to benefit their broad applications in electrochemical sensing and batteries.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. CV of PB thin film and comparison between SPRM results of two different size nanoparticles (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] (W.W.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (NSFC, Grants 21522503, 21527807,

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Analytical Chemistry

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