An Ultrathin Phthalocyanine Conjugated Polymer Nanosheets-based

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An Ultrathin Phthalocyanine Conjugated Polymer Nanosheets-based Electrochemical Platform for Accurately Detecting H2O2 in Real Time Wenping Liu, Houhe Pan, Chenxi Liu, chaorui su, Wenbo Liu, Kang Wang, and Jianzhuang Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22686 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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ACS Applied Materials & Interfaces

An Ultrathin Phthalocyanine Conjugated Polymer Nanosheets-based Electrochemical Platform for Accurately Detecting H2O2 in Real Time

Wenping Liu, Houhe Pan, Chenxi Liu, Chaorui Su, Wenbo Liu, Kang Wang,* and Jianzhuang Jiang*

Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, Department of Chemistry, University of Science and Technology Beijing, Beijing 100083, China

Keywords: phthalocyanine, nanosheets, multilayer films, electrochemical sensors, H2O2, real-time monitoring

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Abstract: As a vital biological mediator and a widely used industrial oxidant, the accurate detection of hydrogen peroxide (H2O2) is of significance for both academic purpose and practical applications. Herein, we report a novel approach for the development of high performance electrochemical H2O2 sensor constructed by iron phthalocyanine (FePc)-based diyne-linked conjugated polymeric nanosheets (NSs), FePc-CP NSs. The FePc-CP NSs were delaminated from the bulk material via a defect and disorder induced synthetic strategy. By the quasi-Langmuir-Shӓfer method, the prepared FePc-CP NSs were self-assembled into multilayer films with controllable thickness on electrodes. Owing to the highly exposed active centers on the surfaces, the FePc-CP NSs film modified electrode exhibited excellent H2O2 determination performance with a wide linear detection range (0.1-1000 μM), a short response time (the response current approached the maximum value within 0.1 s), a low limit of detection (0.017 μM), and excellent sensitivity (97 μA cm-2 mM−1), comparable to the best results reported so far for electrochemical H2O2 sensors. In addition, the fabricated electrochemical H2O2 sensor also displayed satisfactory stability, reproducibility, and selectivity. Furthermore, the obtained FePc-CP NSs films sensor can be applied in the real-time monitoring of H2O2 in commercial orange juice and beer as well as secreted from A549 live cells, revealing its application potential towards the accurate detection of H2O2 in real-sample analysis.


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Introduction

Hydrogen peroxide (H2O2) is a valuable oxidant which is widely used in a variety of industries, including wastewater treatment, paper and pulp, textile, medical instruments, food manufacturing, and chemical industries.1-3 Furthermore, H2O2 is also one of side products of various biological processes, such as DNA strand breaks, apoptosis, endothelial tissue permeability, and vascular remodeling.4,5 However, H2O2 is cytotoxic to cells and tissues, which could lead to various kinds of illness, such as Parkinson's, Alzheimer's, cancer, and diabetes.6 As a result, monitoring the concentration level of H2O2 is of significance for both academic research and practical application. In the past decades, a variety of analytical strategies have been developed for the H2O2 determination, including titrimetry,7 spectrometry,8 fluorometry,9 colorimetry,10 and electrochemistry.11 Among which, the last method has attracted more attentions than others owing to its advantages of easy operating, cost-effective instrumentation, high sensitivity and selectivity, and rapid response. Catalysts on the electrode are the key materials for the electrochemical H2O2 sensors. Heme proteins, such as horseradish peroxidase (HRP),12 catalase (CAT),13 hemoglobin (HB),14 and myoglobin (MB)15, were initially used as the catalytic active materials towards electrochemical H2O2 sensing, owing to their high sensitivity and selectivity. However, these enzymes suffer from instability, low reproducibility, and slow rate of electron transfer, which severely restricted their practical application.16,17 It has been found that heme, an iron porphyrin (FePor) derivative, is the catalytic active center of these enzymes. As a result, developing biomimetic catalytic materials containing heme-like structures have been considered as a promising way to improve the catalytic activity and stability.18,19 Meanwhile, since the discovery of graphene in 2004, a variety of artificially synthesized

two-dimensional

(2D)

nanosheets

(NSs),


such

as

transition

metal


dichalcogenides,20

graphitic

carbon

nitride,21-23

layered

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double

hydroxides,24,25

graphdiyne,26-29 and 2D metal-organic framework (MOF) NSs,30-32 have been widely explored. Owing to the ultrathin nature, 2D NSs have a number of unique physicochemical properties, such as remarkable electron transport capability, large surface area, and high densities of accessible active sites on their surfaces. Combination of these advantages of 2D NSs and the high catalytic activity of heme-like structures may provide an ideal system as the electrode catalytic materials for electrochemical sensors with improved property. This is confirmed by the recent work of Zhang and co-workers.33 They synthesized a batch of 2D MOF NSs using an iron porphyrin ligand via the surfactant-assisted synthetic method. The electrodes modified by the as-synthesized MOF NSs films exhibited high H2O2 sensor performance with a detection limit of 0.15 μM, superior to the sensors constructed by natural heme protein. However, the H2O2 determination properties of these 2D MOF NSs are still limited in terms of its relatively higher limit of detection.33 In order to further enhance the sensor performance, introducing catalytic centers with enhanced activity onto the surfaces of 2D NSs seems to be necessary. Phthalocyanines (Pcs) are a well-known class of artificially synthesized porphyrin analogue.34,35 In particular, by a comparative research on the oxygen reduction reaction (ORR) catalytic activities of the mixed Pc-Por-based 2D conjugated microporous polymers (CMPs), FePcZnPor-CMP and ZnPcFePor-CMP, it has been revealed that the FePc is more favorable to boost the fracture of O-O bonds in the O2 and peroxide compared to FePor.36,37 This suggests the higher catalytic activity of FePc towards H2O2 reduction. Moreover, our group recently developed a defect and disorder induced synthetic strategy of Pc conjugated polymers 2D NSs.38 The prepared Pc-based 2D NSs afford highly exposed heme-like active centers on the surfaces owing to their ultrathin structure, which provide a unique platform to develop high efficiency catalysts. Herein, we report the design and synthesis of a new


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FePc-based diyne-linked (−C≡C−C≡C−) conjugated polymers 2D NSs, FePc-CP NSs, for the first time. Importantly, the as-prepared NSs were readily dispersed in various organic solvents, which enabled the FePc-CP NSs assembling into a thin film with controllable thickness on solid substrates by using a conventional quasi-Langmuir-Shӓfer (QLS) method.39-42 The obtained 2D FePc-CP NSs film exhibited high H2O2 determination performance with a wide linear detection range of 0.1-1000 μM and a low detection limit of 0.017 μM. These values are comparable to the best results reported so far for electrochemical H2O2 sensors. Moreover, the fabricated 2D FePc-CP NSs sensor was able to be applied in real-time monitoring of H2O2 in commercial samples as well as secreted by live cells.

Figure 1 (a) Schematic diagram for the synthesis of the FePc-CP NSs. (b) TEM photo, (Inset: SAED pattern of the FePc-Pc NSs.) (c) AFM image, and (d) the height profiles along the



marked white line of FePc-CP NSs. (e) Typical HRTEM image of FePc-CP NSs. (f) HAADF-STEM image as well as EDS mapping of the FePc-CP NSs. (g) UV–vis absorption spectra of FePc-CP NSs suspension in ethanol.

Results and Discussion

The synthesis of FePc-CP NSs is illustrated in Figure 1a. The bulk FePc-CP material was firstly synthesized via the Yamamoto homo coupling of Fe[Pc(ethynyl)4] {[Pc(ethynyl)4] = dianion of 2(3),9(10),16(17),23(24)-tetra(ethynyl)phthalocyanine} according to a reported procedure.22,23 By heating Fe[Pc(ethynyl)4] in a mixed solvent of anhydrous THF:Et3N (1:2) under the catalyst of Pd(PPh3)2Cl2 at 70°C for 48 h, the bulk FePc-CP was obtained in the yield of 98%. The formation of the diyne (−C≡C−C≡C−) linkages in the bulk FePc-CP was confirmed by the Fourier transform infrared (FT-IR) analysis. As can be seen in Figures S1 in Supporting Information, the two peaks at ca. 3289 and 2108 cm−1 in the FT-IR spectrum of Fe[Pc(ethynyl)4] assigned to C−H stretching and C≡C vibration, respectively, disappeared in the IR spectrum of the bulk FePc-CP, demonstrating the coupling of Fe[Pc(ethynyl)4] into the crosslinked polymer.36-38 Moreover, the FT-IR spectrum of bulk FePc-CP also exhibits the typical vibration absorptions of the Pc macrocycle, such as the C=N aza group stretching vibrations (1584-1596 cm-1), isoindole ring stretching vibrations (1070-1420 cm-1), and wagging and torsion vibrations of C-H groups (672-753 cm-1) (Figure S1a in Supporting Information),39-42 unambiguously revealing its Pc composition and structure. Raman spectrum of bulk FePc-CP was also recorded, which exhibits the typical vibration of conjugated diyne links (−C≡C−C≡C−) at 2204.9 cm−1 (Figure S1b in Supporting Information). These results provide further demonstration for the successful coupling of Fe[Pc(ethynyl)4] into the crosslinked polymer. The structural information of the bulk


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FePc-CP was further identified by solid-state UV-vis diffuse reflectance, X-ray photoelectron (XPS), and powder X-ray diffraction (PXRD) spectroscopy as well as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The solid-state UV-vis diffuse reflectance spectrum (DRS) of the bulk FePc-CP shows the Pc Soret band at ca. 418 nm and Q-band at ca. 714 nm, observably red-shifted compared to Fe[Pc(ethynyl)4] probably owing to the lager π-conjugated system of FePc-CP network than Fe[Pc(ethynyl)4] (Figure S2 in Supporting Information). XPS analysis indicates that the bulk FePc-CP is composed of Fe, N, C, and O, (Figure S3 in Supporting Information). The Fe 2p spectrum exhibits the typical peaks for Fe 2p3/2 and Fe 2p1/2 at 709.8 eV peak 723.4 eV, respectively, which are the characteristic peaks of Fe3+ cations.36-38 The high resolution N 1s spectrum of the bulk FePc-CP can be deconvoluted into two peaks for metal coordinated pyrrole nitrogen at 398.8 and aza nitrogen at 398.2 eV (Figure S3 in Supporting Information).36-38 PXRD pattern shows no obvious evident peaks, suggesting the amorphous nature of the bulk FePc-CP (Figure S4 in Supporting Information). SEM and TEM images indicate the sheet-like morphology of the bulk FePc-CP (Figure S5 in Supporting Information). It has been demonstrated that individual layers of the 2D phthalocyanine CPs fabricated by tetra-β-substituted phthalocyanines possess intrinsic disordered network structure.38 This could diminish the degree of overlap between adjacent individual layers and weaken the interlayer π-π interaction in the CP, enable to easily exfoliate the bulk 2D phthalocyanine CP materials into ultrathin nanosheets.38 This is also true for the present FePc-CP, owing to the tetra-β-substituted phthalocyanine nature of Fe[Pc(ethynyl)4]. As a result, the bulk FePc-CP was also exfoliated into ultrathin NSs, FePc-CP NSs, in a yield of ca. 60% by liquid sonication for 8 h in ethanol (Figure 1a). As can be seen in Figure 1b, the TEM image of FePc-CP NSs exhibits a graphene-like 2D structure with a lateral size of hundreds of nanometers and transparent appearance, confirming that the bulk FePc-CP



material has been successful exfoliated into ultrathin NSs. The atomic force microscopy (AFM) image and corresponding height profiles reveal that the FePc-CP NSs possess a uniform thickness of ca. 4.0 nm, corresponding to ca. 10 layers (Figures 1c and 1d). As expected, no obvious lattice fringes are observed in the high-resolution TEM (HTEM) image (Figure 1e), due to the amorphous nature of the phthalocyanine conjugated polymer monolayers.23 This is further confirmed by the broad halo rings gave by the selected area electron diffraction (SAED) pattern of the FePc-CP NSs (Figure 1b). Energy dispersive spectroscopy (EDS) results reveal that the chemical composition of the obtained FePc-CP NSs is Fe, N, C, and O (Figure S6 in Supporting Information), in line with the composition of bulk FePc-CP material. Figure 1f exhibits a typical high-angle annular dark-field scanning TEM (HAADF-STEM) image of the FePc-CP NSs as well as the corresponding elemental mapping images in the selected area. It can be seen that C, N, and Fe element distributions are homogenous throughout the whole FePc-CP NSs. Moreover, the FT-IR, and XPS spectra as well as the UV-vis DRS of FePc-CP NSs keep almost unchanged from those of the bulk FePc-CP, indicating the same structures of the bulk FePc-CP material and FePc-CP NSs. (Figures S1-S3 in Supporting Information). For the reference purpose, CoPc-CP NSs and NiPc-CP NSs were also synthesized by the same preparation procedure of FePc-CP NSs, which exhibit similar shape and thickness to the FePc-CP NSs (see Figures S7-S9 in Supporting Information).


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Figure 2 (a) Schematic representation of the preparation of the FePc-CP NSs films. (b) UV-vis absorption spectra of the as-prepared ITO/(FePc-CP NSs)n (n = 1-6) films as well as the bare ITO. (c) Plot of the maximum absorbance of ITO/(FePc-CP NSs)n (n = 1-6) films at 801 nm vs n. Inset shows the photo of the as-prepared ITO/(FePc-CP NSs)n (n = 1-6) films as well as the bare ITO.

Remarkably, the FePc-CP NSs are readily dispersed in organic solvent, due to that the improved interactions with the solvent molecules reduce the aggregation (Figure S10 in Supporting Information). In particular, the absorption of FePc-CP NSs in ethanol possesses a nearly linear correlation with its concentration in the range of 0.04-0.20 mg mL−1, further revealing the excellent dispersibility of the exfoliated FePc-CP NSs (Figures 1g and S11 in Supporting Information). Furthermore, the formation of the stable dispersions enabled the FePc-CP NSs to be assembled into a thin film with controllable thickness on solid substrates



by using liquid-deposition methods, which has great significance for electrochemical sensing.43,44 Herein, the layer-by-layer assembly of FePc-CP NSs into a scalable thin film was performed via conventional air-water quasi-Langmuir-Shӓfer (QLS) method (Figure 2a).33,39-42 Water was selected as the liquid phase owing to the hydrophobicity of FePc-CP NSs, while CH2Cl2 was selected as the spreading solvent owing to its low boiling points. Briefly, the colloidal dispersion of FePc-CP NSs in CH2Cl2 (0.2 mg mL1) was slowly dropped onto the water surface in a Petri dish. Along with the CH2Cl2 evaporation, the FePc-CP NSs gradually diffused at the water surface to generate a thin film. Then the film can be easily transferred to a solid substrate including Si wafer, indium tin oxide (ITO) plate, or GC electrode, by horizontal lifting. Finally, the residual water on the substrate, was removed by a stream of N2. The above process is defined as one deposition cycle. The process was repeated to obtain required number of deposition cycles, denoted as the substrate/(FePc-CP NSs)n (n = number of deposition cycles). We recorded the UV-vis absorption spectra of the as-prepared ITO/(FePc-CP NSs)n (n = 1-6) films. As can be seen in Figures 2b and 2c, the absorbance is gradually enhanced along with the increase of n, revealing the formation of multilayer 2D FePc-CP NSs films. Moreover, AFM images also indicate the thicknesses of 2D FePc-CP NSs film increases with n, in line with the absorbance spectra (Figure S12 in Supporting Information). In addition, SEM images show that the surface morphology of FePc-CP NSs films on Si wafer was relatively flat. (Figure S13 in Supporting Information).


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Figure 3 (a) CVs of GC/(FePc-CP NSs)n (n = 1-6) electrodes as well as the bare GC electrode in 0.1 M PBS (pH = 7.4) with 0.5 mM H2O2 at a scan rate of 50 mV s−1. (b) Amperometric response of the GC/(FePc-CP NSs)4 with successive addition of H2O2 at an applied potential of -0.15 V vs Ag/AgCl and (c) the corresponding calibration curve of the current response to the H2O2 concentration. Inset: The calibration curve for the H2O2 concentration range of 0.1-2.0 μM. (d) Amperometric responses of GC/(FePc-CP NSs)4 with successive additions of O2 and 0.5 mM different analytes, including H2O2, caffeic acid, ascorbic acid (AA), bisphenol A, uric acid (UA), and glucose (Glu), and NaCl into 0.1 M PBS (pH = 7.4) under different applied potential.

It has been found that the FePc is favorable to boost the fracture of O-O bonds in the



O2 and peroxide. This, incorporation with the highly exposed active sites, suggests 2D FePc-CP NSs films exhibit improved electrochemical activity to detect H2O2. In order to prove this point, cycle voltammetry (CV) curves were first measured on the GC/(FePc-CP NSs)n (n = 1-6) electrodes prepared by the aforementioned procedure in 0.1 M phosphate buffer solution (PBS) (pH = 7.4) containing 0.5 mM H2O2. As can be seen in Figure 3a, compared to bare GC electrode, the current for CV curves of all the GC/(FePc-CP NSs)n electrodes gets significantly increased in the range of 0.1 to -0.6 V vs Ag/AgCl with a cathodic peak observed at -0.15 to -0.2 V vs Ag/AgCl, due to the H2O2 reduction. These results clearly reveal the effective catalytic activity of the FePc-CP NSs films toward H2O2 reduction. Noted, the current of the peaks for the GC/(FePc-CP NSs)n increases with increasing the number of deposition cycles (n) in the range from 1 to 4, Table S1 (Supporting Information), possibly owing to more active sites are accessible. However, the catalytic activity starts to reduce with n further increasing to 6, Table S1 (Supporting Information), which may derive from the decreased electron transfer from GC electrode to the FePc-CP NSs film surfaces along with the increase in the film thickness. This is confirmed by the electrochemical impedance spectroscopy (EIS) of GC/(FePc-CP NSs)n (n = 1-6), which reveals the electron transfer capability of GC/(FePc-CP NSs)n (n = 1-6) gets reduced along with increasing the number of n (see Figure S14 in Supporting Information). As a result, the GC/(FePc-CP NSs)4 electrode exhibited the best catalytic behavior and was then used for further quantitative H2O2 determination by the chronoamperometry method. Figure 3b exhibits the amperometric response curve for the GC/(FePc-CP NSs)4 electrode upon the successive addition of H2O2 into 0.1 M PBS (pH = 7.4) at an applied potential of -0.15 V vs Ag/AgCl. As can be seen, along with adding H2O2, the response current rapidly approached the maximum value within 0.1 s. Moreover, the catalytic current has a significant linear correlation to the concentration of H2O2 with a sensitivity of 97 μA cm−2 mM−1 in the range


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of 0.1-2.0 μM and 2.4 μA cm−2 mM−1 in the range of 2.0-1000 μM (Figure 3c), respectively, revealing a wide detection range. In particular, the limit of detection (LOD) was calculated to be 0.017 μM,45 more than one order of magnitude lower compared to the FePor-based electrochemical sensors and representing one of the best results reported thus far for H2O2 sensors, Tables S2 and S3 (Supporting Information). In addition, the CoPc-CP NSs and NiPc-CP NSs modified electrodes, GC/(CoPc-CP NSs)4 and GC/(NiPc-CP NSs)4, fabricated by the same preparation procedure of GC/(FePc-CP NSs)4, were also used to detect H2O2. As can be seen in Figures 3 and S15 as well as Table S2 (Supporting Information), GC/(FePc-CP NSs)4 electrode exhibited significantly higher detection performance compared to GC/(CoPc-CP NSs)4 and GC/(NiPc-CP NSs)4 electrodes, clearly indicating the key role of FePc-CP NSs for detecting H2O2 in such high detection performance. These results demonstrate that the FePc-CP NSs thin film electrodes assembled by QLS method possess great application potential in the electrochemical analysis. Besides the catalytic activity, the stability, reproducibility, and selectivity of H2O2 sensors are also crucial factor for their practical applications. In present work, the stability of the GC/(FePc-CP NSs)4 electrode was firstly evaluated by recording the current response to 0.1 mM H2O2 in 0.1 M PBS (pH = 7.4) over a period of 2000 s. As shown in Figure S16 (Supporting Information), the current could retain ca. 80.2% of the starting value after 2000 s test, revealing the high stability of the GC/(FePc-CP NSs)4 electrode. This is further supported by the almost same IR spectra and similar morphology of FePc-CP NSs before and after the steady-state current-time test (see Figure S17 in Supporting Information). In addition, the amperometric response by adding 0.5 mM H2O2 into 0.1 M PBS (pH = 7.4) was also recorded every one week by CV to evaluate the long-term storage stability of the GC/(FePc-CP NSs)4 electrode. As can be seen in Figure S18 in Supporting Information, the peak current at -0.15 to -0.2 V vs Ag/AgCl maintained 95.5% of the original value, further



confirming the excellent stability of the GC/(FePc-CP NSs)4 electrode. For assessing the reproducibility of the GC/(FePc-CP NSs)4 electrode, response current was recorded ten times for one electrode in 0.1 M PBS (pH = 7.4) by adding 0.5 mM H2O2 (Figure S19 in Supporting Information). The relative standard deviation (RSD) is calculated to be as small as 3.44%. Meanwhile, amperometric curves were carried out on ten GC/(FePc-CP NSs)4 electrodes in the aforementioned test condition, which showed a RSD of 5.01% (Figure S20 in Supporting Information). These results indicate the good reproducibility of the GC/(FePc-CP NSs)4 electrode for monitoring H2O2. In order to evaluate the selectivity of the GC/(FePc-CP NSs)4 electrodes for H2O2 detection, potential interfering substances including O2 and 0.5 mM caffeic acid, ascorbic acid (AA), bisphenol A, uric acid (UA), glucose (Glu), and NaCl were added into 0.1 M PBS (pH = 7.4) during the steady-state current-time test. As revealed in Figure 3d, compared to H2O2, these substances only caused a slight current increase, revealing the high catalytic selectivity of the GC/(FePc-CP NSs)4 electrode toward H2O2. Inspired by the high performance of the GC/(FePc-CP NSs)4 electrode for H2O2 senses, this electrode was further used in practical food monitoring and clinical diagnosis. By using the GC/(FePc-CP NSs)4 electrode as the electrochemical platform, we detected the H2O2 concentration in commercial orange juice and beer (obtained from a local market) by using the chronoamperometry method at a potential of -0.15 V vs Ag/AgCl. For the study, the real samples were diluted 10 times by 0.1 M PBS (pH = 7.4), and then spiked with standard concentrations of H2O2. As shown in Figure 4a and Table S4 in Supporting Information, the detected H2O2 contents in both orange juice and beer were in line with the theoretical value with the recoveries ranging from 95.8 to 107 %. Moreover, conceptual verification application for clinical diagnosis, the GC/(FePc-CP NSs)4 electrode was also used for the real-time monitoring of H2O2 secreted from living A549 cells (human lung carcinom cells). It


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is well known that H2O2 can be released by the A549 cells in the presence of N-formyl-methionyl-leucyl-phenylalanine (fMLP). As can be seen in Figure 4b, a prominent increased current was observed at the GC/(FePc-CP NSs)4 electrode after adding 100 μM fMLP into the electrolyte with living A549 cells, and then the current reached a stable value after 150 seconds. In comparison, no response signal was found upon the addition of fMLP into the electrolyte without living cells. In addition, the cell bio-compatibility of FePc-CP NSs film towards living cells have also been investigated by adding the FePc-CP NSs into the nutrient solution of living A549 cells. As can be seen in Figure S21 in Supporting Information, the cells were still alive after adding the FePc-CP NSs for 24 h, revealing the good cell bio-compatibility of FePc-CP NSs. These results demonstrate the good practicability of the developed H2O2 sensor based on the FePc-CP NSs.

Figure 4 (a) Determination of H2O2 in commercial orange juice and beer. (b) Amperometric responses of the GC/(FePc-CP NSs)4 electrode in 0.1 M PBS (pH = 7.4) by the addition of fMLP with (black curve) and without (red curve) A549 cells at an applied potential of -0.15 V vs Ag/AgCl. Inset: Microscopy image of A549 cells.

Experimental Section



Synthesis of bulk FePc-CP, CoPc-CP, and NiPc-CP. For the synthesis of bulk FePc-CP, Fe[Pc(ethynyl)4] (160.0 mg, 0.24 mmol) and Pd(PPh3)2Cl2 (5.6 mg, 30 μmol) were added into Et3N/THF (40 ml/20 ml). Then the mixture was stirred at 70°C for 2 days under N2. After cooled to room temperature, the blue product was collected by filtration, washed with THF, toluene, and methanol, and dried under reduced pressure at 80°C for 12 h to generate bulk FePc-CP (156 mg, yield 98%). For the synthesis of bulk CoPc-CP and bulk NiPc-CP, Co[Pc(ethynyl)4] and Ni [Pc(ethynyl)4] were used to replace Fe[Pc(ethynyl)4], respectively, while all other steps were the same to the synthesis of bulk FePc-CP. The yields of CoPc-CP and NiPc-CP are 98 and 95%, respectively.

Preparation of FePc-CP NSs, CoPc-CP NSs, and NiPc-CP NSs. In a typical experiment, bulk FePc-CP (40 mg) was added into ethanol (100 mL). The mixture was sonicated for 8 h at room temperature, and then centrifuged with a speed of 2000 rpm for 10 min to remove the bulk FePc-CP, generating the colloidal dispersion of FePc-CP NSs. CoPc-CP NSs and NiPc-CP NSs were obtained by the same preparation method of FePc-CP NSs with bulk FePc-CP replaced by bulk CoPc-CP and bulk NiPc-CP, respectively. The yield of FePc-CP NSs, CoPc-CP NSs, and NiPc-CP NSs is 60, 60, and 51%, respectively, which is calculated by measuring the weight of collected residual bulk MPc-CP after sonication and compared to the original weight of bulk MPc-CP.

Preparation of 2D FePc-CP nanosheet films on solid substrates. First, Si wafers, the ITO glass, and GC electrodes were sonicated for 15 min in ethanol and deionized water, respectively. The 2D FePc-CP NSs films were deposited onto the aforementioned cleaned substrates through conventional air–water quasi-Langmuir–Shӓfer (QLS) method. Typically,


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the colloidal dispersion of FePc-CP NSs in CH2Cl2 (0.2 mg mL1) was slowly dropped onto the surface of water in a Petri dish. 2D FePc-CP NSs spontaneously diffused at the surface of water to generate a thin film. The film was then transferred onto a solid substrate, including Si wafer, ITO, or GC electrode, by horizontal lifting. Finally, the residual water on the substrate was removed by a stream of N2. The process was repeated to obtain required number of deposition cycles, denoted as the substrate/(FePc-CP NSs)n (n = number of deposition cycles). The GC/(CoPc-CP NSs)4 and GC/(NiPc-CP NSs)4 were fabricated by the same preparation method of substrate/(FePc-CP NSs)4 with the FePc-CP NSs replaced by the CoPc-CP NSs and NiPc-CP NSs, respectively

Electrochemical measurement. The conventional three-electrode system, using the prepared GC/(MPc-CP NSs)n as working electrode, a Pt wire as auxiliary electrode, and a Ag/AgCl electrode as reference electrode, was employed for the electrochemical test. CVs of the GC/(MPc-CP NSs)n electrodes were measured in 0.1 M PBS (pH = 7.4) with 0.5 mM H2O2 in the voltage range of -0.6 to 0.6 V vs Ag/AgCl at a scan rate of 50 mV s−1. The amperometric response curves were recorded in the stirring 0.1 M PBS (pH = 7.4) with successive addition of H2O2 at -0.15 V vs Ag/AgCl. The electrolyte solutions were purged with N2 for half an hour to remove O2 before being used in electrochemical measurements. All the potentials shown in the text are the data with respect to the Ag/AgCl reference electrode.

Conclusion

In summary, FePc-based diyne-linked conjugated polymers 2D NSs with a uniform thickness of 4.0 nm have been designed and synthesized for the first time. Significantly, an efficient H2O2 electrochemical sensor has been fabricated based on the multilayer films of



the FePc-CP NSs prepared by QLS method. Owing to the highly exposed active centers on the surfaces, the sensor based on the FePc-CP NSs films exhibited excellent H2O2 determination performance with a detection limit as low as 0.017 μM, which is comparable to the best results reported so far for electrochemical H2O2 sensing. Moreover, the fabricated FePc-CP NSs sensor was able to be applied in the real-time monitoring of H2O2 in real-sample analysis. The present result is surely helpful for developing high-efficiency electrochemical H2O2 sensors with practical application value.

Associated Content Supporting Information AFM, TEM, and EDS images. IR, UV XRD, and XPS spectra. Experimental details (PDF). This material is available free of charge via the Internet at https://pubs.acs.org.

Author Information Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Acknowledgement. Financial support from the Natural Science Foundation of China (Nos. 21631003, 21671017, and 21871024), the Fundamental Research Funds for the Central Universities (No. FRF-BD-17-016A), and University of Science and Technology Beijing is gratefully acknowledged.

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Table of Contents Graphic


An Ultrathin Phthalocyanine Conjugated Polymer Nanosheets-based

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