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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Potential-Tunable Metal-Organic Frameworks: Electrosynthesis, Properties, and Applications for Sensing of Organic Molecules Liudi Ji, Junxing Hao, Kangbing Wu, and Nianjun Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10448 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019
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Potential-Tunable Metal-Organic Frameworks: Electrosynthesis, Properties,
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and Applications for Sensing of Organic Molecules
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Liudi Ji,†,§ Junxing Hao,† Kangbing Wu,*,† and Nianjun Yang*,‡
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†Key
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Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan
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430074, China.
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‡Institute
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§Hubei
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Science and Technology, Xianning 437100, China.
Laboratory for Material Chemistry of Energy Conversion and Storage, School of
of Materials Engineering, University of Siegen, Siegen 57076, Germany.
Collaboration Innovative Center for Nonpower Nuclear Technology, Hubei University of
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ABSTRACT: Properties of metal-organic frameworks (MOFs) are determined by metal centers,
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organic ligands, and applied synthesis methods. For electrosynthesis of MOFs, the applied
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potential is expected to play a key role in determining the morphology, thickness,
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electrochemical
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electrosynthesized at different cathodic potentials. They feature different morphology, thickness,
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and amounts of active copper centers, although they do show similar bonding properties,
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chemical compositions, phase purity, crystallinity, and surface electronic states of copper
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centers. Using NADH, the sensing application of these Cu-BTC films is explored, showing
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potential-dependent catalytic ability. Further monitoring of other six organic compounds (herein
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xanthine, hypoxanthine, diethylstilbestrol, estradiol, sunset yellow, and tartrazine) reveals the
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morphology and thickness of Cu-BTC films and the amount of copper centers inside these Cu-
properties
and
applications
of
MOFs.
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Cu-BTC
films
are
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BTC films determines the accumulation or sensing ability of Cu-BTC films. Highly sensitive
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detection of these molecules individually and simultaneously are achieved with
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Cu-BTC electrosynthesized at -1.30 V. Electrosynthesized Cu-BTC films are
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thus excellent electrode materials for sensitive sensing of various analytes.
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INTRODUCTION
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Metal-organic frameworks (MOFs) are extensively being employed as multifunctional materials
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for gas storage,1 adsorption/separation,2-5 drug release,6 energy storage,7 catalytic,8-10 magnetism
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/ferroelectric,12 and donor-acceptor13 applications. This is because MOFs generally feature large
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surfaces, high porosity, multiple functionalities, and variable host-guest interactions.14-16 In
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contrast to numerous and successful MOF applications in above fields, relatively few reports
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focused on the employment of MOFs for electrochemical sensing applications. Partially this is
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due to the lack of efficient methods to modify electroactive MOFs on conductive substrates,
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partially due to the poor stability of synthesized MOFs. Up to now, the widely utilized
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approaches for electrochemical sensing applications of MOFs are based on the fabrication of
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MOF/carbon paste electrodes17-19 and MOF composites modified glassy carbon electrodes
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(GCEs).20-22 The shortcomings of these MOF electrodes are of their complication, no suitability
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for automatic or on-site monitoring.
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Recently, electrosynthesis of MOFs on conductive substrates has been proved to be possible
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during the course of so-called oxidative or reductive processes.23-33 The properties of
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electrosynthesized MOFs are determined by the metal center(s) and the organic ligand(s) as well
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as the applied potential(s) or current density/densities. Most studies have focused on the
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utilization of zinc ions and organic ligands to synthesize MOFs as well as their applications.30-32
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Different carboxylic acids have been used as the organic ligands to electrosynthesize Cu-MOFs.
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Different from other metal-center (Zn, Co, Fe, etc.) based MOFs, tunable electrochemistry of
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electrosynthesized Cu-MOFs has been revealed.33 On other hand, relatively few reports deal with
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the clarification of potential effect on both properties and potential applications of
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electrosynthesized MOFs. For example, electrocatalytic active Cu-MOFs (owing to the presence
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of copper active centers)33 are expect to feature potential-dependent sensing performance
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towards different target molecules. However, such kind of reports is still missing in literature.
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Herein, reductive electrosynthesis of Cu-MOFs was first conducted at different negative
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potentials. The employment of these electrosynthesized MOFs for the sensitive and selective
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detection of several organic targets was then tested. As case studies, nitrogen-containing
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heterocyclic or hydroxyl of organic molecules were selected, including nicotine amide adenine
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dinucleotide (NADH), xanthine (XA), hypoxanthine (HXA), diethylstilbestrol (DES), estradiol
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(E2), sunset yellow (SY), and tartrazine (TT). The oxidation signals of these compounds are
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remarkably enhanced, dependent of the applied potentials for the electrosynthesis of Cu-MOFs.
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These Cu-MOFs are thus sensitive sensing materials for the detection of these organic
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compounds in different fields.
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EXPERIMENTAL
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Reagents and solutions. All chemicals were of analytical grades and used as received. NADH,
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XA, HXA, DES, E2, SY, and TT were purchased from Sigma (Shanghai, China). The stock
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solution of 0.1 M NADH was prepared using ultrapure water, 0.01 M XA and HXA using 0.1 M
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NaOH, 0.01 M DES and E2 using ethanol, 0.01 M SY and TT using ultrapure water. N,N-
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Dimethylformamide (DMF), Cu(NO3)2·3H2O, and H3BTC were purchased from Sinopharm
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Chemical Reagent Co., Ltd. (Shanghai, China). Et3NHCl was purchased from Aladdin Chemistry
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Co., Ltd. (Shanghai, China). High-purity distillated water was obtained daily from a Milli-Q
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water purification system at room temperature.
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Instrument. Electrochemical measurements were performed on a CHI 660E electrochemical
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workstation (Chenhua Instrument, China). The RDE experiments were carried out on the RRDE-
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3A rotating ring disk electrode apparatus (BAS Company, Japan). For these measurements, a
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conventional three-electrode system was used. The working electrode was a Cu-BTC film
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modified GCE or a GCE (diameter: 3 mm); the reference electrode was a SCE; and the counter
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electrode was a platinum wire/foil. FTIR spectra were recorded on an Equinox-55 Fourier
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transform infrared spectrometer (Bruker Company, Germany). For these spectra, KBr pellets
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were employed. XPS measurements were performed on an AXIS-UL TRA DLD-600W X-ray
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photoelectron spectrometer (SHIMADZU-Kratos Company, Japan). The binding energy of C1s
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emission peak at 284.8 eV was used as the reference for all XPS measurements. XRD patterns
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were measured using a X’Pert PRO diffractometer (Panalytical Company, Netherlands),
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operating with Cu kα1 radiation (0.154 nm) in the 2θ scan range from 5o to 40o. TEM images
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were measured using a Tecnai G2 F30 microscope (FEI Company, Netherlands). Brunauer-
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Emmett-Teller (BET) specific surface area was measured on a Micromeritics ASAP2420
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adsorption analyzer. Raman spectra were recorded using a LabRAM HR800 confocal Raman
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microscopy system using an Ar ion laser (514 nm) (Horiba JobinYvon, France).
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Electrosynthesis of Cu-BTC films. Different Cu-BTC films were electrosynthesized on GCEs
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by use of a potentiostatic mode. Prior to electrosynthesis of Cu-BTC films, the GCE was
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polished with 0.05-μm alumina slurry and then ultrasonically washed successively with ethanol
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and ultrapure water. The precursor solution for the electrosynthesis of these Cu-BTC films
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consisted of 10 mM Cu(NO3)2·3H2O, 15 mM H3BTC, and 10 mM Et3NHCl. All of them were
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dissolved in DMF. During the course of electrosynthesis, the precursor solution was vigorously
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stirred. The electrosynthesis duration was 5 min. Because proton reduction from a solution of
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Et3NHCl in DMF occurred at approximately -1.05 V.31 The cathodic potential applied was
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chosen as -1.1, -1.2, -1.3, and -1.4 V (vs. SCE). The Cu-BTC film coated GCEs after being
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washed with water were then dried at 50 oC in vacuum.
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RESULTS AND DISCUSSION
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The crystal structure of Cu-BTC is shown in Fig. 1, where its 3D unit cell owns paddle-wheel
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type metal corners, connected by trimesic acid linkers. It is thus composed of large central
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cavities that are surrounded by small pockets.34 This porous structure and functional centers are
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promising for accumulating analytes from solutions.
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Figure 1. The crystal structure of Cu-BTC in 3D unit cell (carbon atom: grey ball; hydrogen
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atom: white ball; oxygen atom: red ball; copper atom: orange ball).
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The effect of cathodic potentials on the properties of electrosynthesized Cu-BTC films was first
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revealed via synthesis and characterization of four Cu-BTC films grown at different potentials.
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For example, Fourier transform infrared (FTIR), X-ray diffraction (XRD), and X-ray
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photoelectron spectroscopy (XPS) have been applied to characterize the bonding properties,
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chemical compositions, crystallinity and phase purity as well as surface electronic states of
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copper centers of these Cu-BTC films. Similar features are found in their FTIR spectra (Fig. S1A)
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and XPS survey spectra (Fig. S1B). Their similar FTIR spectra confirm the coordination of
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carboxylate groups of organic ligands with copper centers.24,35 The detailed Cu2p XPS spectra
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(Fig. S1C) show the binding energies of 934.1 and 954.7 eV for Cu2p3/2 and Cu2p1/2,
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respectively. They reveal the divalent natures of copper atoms in these Cu-BTC films.36 To
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check the purity of synthesized Cu-MOF films, namely the co-existence of precursors including
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Cu(NO3)2·3H2O, Et3NHCl, H3BTC, their XRD patters were recorded. The main diffraction
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peaks of Cu-MOF films (Fig. S1D) match well with the simulation patterns of the Cu-BTC
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(CCDC no. 755080),37 except some deviations in the relative intensities. In addition, the
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characteristic diffraction peaks of precursors (Fig. S1E) are different from those for Cu-MOF
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films. Therefore, electrosynthesized Cu-MOF films are highly purified and there is no precursors
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existed inside Cu-MOF films.
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The morphology of these Cu-BTC films was then characterized using scanning electron
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microscopy (SEM). Their SEM images show that they are porous materials (Fig. 2A, B, C).
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When an electrosynthesized potential of -1.4 V was applied, cracks are clearly seen (Fig. 2D).
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These Cu-BTC films were further characterized using transmission electron microscopy (TEM),
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revealing that they contain layered nanosheets but featuring different sizes. When a potential of -
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1.1 V is applied, big pieces of nanosheets are seen (Fig. 2E). These nanosheets are somehow
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smaller (Fig. 2F) once a potential of -1.2 V is used. A further reduction of this potential to -1.3 V
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makes the sizes of these nanosheets even smaller, and some pores are noticed (Fig. 2G).
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However, when the applied potential is negative than -1.3 V, these nanosheets overlap
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completely (Fig. 2H) and the pores shown in Fig. 2G disappear. Their average thicknesses,
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estimated from their cross-sectional SEM images (insets in Fig. 2), are 3.86±0.1, 4.45±0.1,
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4.90±0.2, and 5.79±0.2 µm when the electrosynthesized potentials of -1.1, -1.2, -1.3, -1.4 V are
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applied, respectively. In other words, the thickness of these Cu-BTC films can be tuned when
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different electrosynthesized potential is applied. The formation of thicker films at more negative
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potentials probably results from faster nucleation rates.33 This is because a more negative
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potential accelerates the reduction of Et3NH+ in a faster speed, leading to an increase of the local
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pH value near the electrode surface. Consequently, the deprotonation rate of H3BTC is much
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enhanced and more BTC3- ions are available to coordinate with Cu(II) ions.33 Higher
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concentration of Cu(II) ions and/or longer electrosynthesized time is thus expected to obtained
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thicker Cu-BTC films. In short, four Cu-BTC films electrosynthesized at different cathodic
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potentials feature different thickness and morphology, although they do show similar bonding
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properties, chemical compositions, phase purity, crystallinity, and surface electronic states of Cu
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center.
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A
B
C
D
E
F
G
H
100nm
100nm
100 nm
100nm
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Figure 2. SEM (A, B, C, D) and TEM (E, F, G, H) images of Cu-BTC films electrosynthesized
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at -1.1 (A, E), -1.2 (B, F), -1.3 (C, G), and -1.4 V (D, H). The inset SEM images are tilted at 45
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degree and used for the estimation of the thickness of Cu-MOF films.
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The morphology and thickness of electrode materials is known to affect their electrochemical
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properties.33,38-40 Cyclic voltammograms of four Cu-BTC films deposited on GCEs in redox
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probes contained solutions were recorded (Fig. S2A). A pair of redox peak of Fe(CN)63-/4- is
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observed, while the magnitude of the anodic (cathodic) peak currents is different on these
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electrodes. The biggest peak current is obtained on the Cu-BTC film electrosynthesized at -1.3
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V. Moreover, the cathodic peak currents (Ipc) are proportional to the square roots of scan rates
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(Fig. S2B), suggesting diffusion-controlled electrode processes. Their electrochemical active
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areas, calculated using the Randles-Sevčik equation,41 are 0.05, 0.19, 0.21, 0.27, and 0.25 cm2
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for a GCE and for the Cu-BTC film coated GCEs fabricated at -1.1, -1.2, -1.3, and -1.4 V,
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respectively. The real surface areas of Cu-MOFs were calculated using BET measurements.
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Figure S2C shows one example BET result for Cu-MOFs electrosynthesized at -1.3 V. Their
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BET surface area is as high as 969.2 m2 g-1. The type-I hysteresis loops are seen, indicating their
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predominant microporous feature. Such BET surface areas are comparable with those for Cu-
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BTC prepared using sonoelectrochemical method (403.8 m2 g-1),42 sonication synthesis (792-
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1430 m2 g-1),43 interfacial synthesis (654.6 m2 g-1),44 and solvothermal method (1189.7 m2 g-1).45
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Additionally, the capacitive current (e.g., at 0.55 V) of a Cu-BTC film synthesized at -1.3 V is
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the biggest (Fig. S2D), confirming again its biggest area. This might be because its porous
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structure (Fig. 2C) benefits mass transport of redox probes and accelerates electron/charge
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transfer rates. The electron transfer resistances (Rct) of four Cu-BTC films deposited on GCEs
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were then estimated from the diameters of the semicircles in the related Nyquist plots (Fig. S2E).
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The value of Rct for a Cu-BTC film grown at -1.3 V is 424 Ω, smaller than that for a GCE (927
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Ω) and for the Cu-BTC films grown at -1.1 (816 Ω), -1.2 (611 Ω), and -1.4 (461 Ω) V. This
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smallest Rct confirms the fastest electron transfer ability of a Cu-BTC film grown at -1.3 V.
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Therefore, the cathodic potentials applied during the electrosynthesis of MOFs determine their
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morphologies, thickness and electrochemical properties.
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Furthermore, the voltammetric behavior of these Cu-BTC films deposited at different potential
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was investigated in 0.1 M KCl solution. As shown in Fig. 3, two pairs of redox waves are noticed
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for all four films. The used organic ligands have no redox activity in either water (due to their
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insolubility in water) or organic solvents. These waves thus result from the redox behavior of
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exposed copper active sites inside Cu-BTC films, probably in the reaction forms of Cu(II) + e- ↔
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Cu(I) for waves III/II and Cu(I) + e- ↔ Cu(0) for waves IV/I.33 Provided that these waves
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originate from the fast redox response of a monolayer or a few active Cu-BTC layers close to the
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electrode,33 the amount of copper ions inside four Cu-BTC films, calculated from the integrated
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charges of wave (III) in Fig. 3,33 is 5.91, 6.73, 9.82, and 8.13 µmol cm-2 for the Cu-BTC film
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electrosynthesized at -1.1, -1.2, -1.3, and -1.4 V, respectively.
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Figure 3. Faradaic behavior of the Cu-BTC film coated GCEs electrosynthesized at -1.1 (a), -1.2
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(b), -1.4 (c), and -1.3 (d) V in 0.1 M KCl at a scan rate of 100 mV s-1.
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Electrochemical applications of these Cu-BTC films were then tested. As a case study,
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electrochemistry of NADH was first studied on these films. Fig. 4A shows obtained cyclic
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voltammograms. An irreversible oxidation wave is observed at a potential of 0.60 V on GCE
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(Fig. 4A-a). This peak shifts negatively and the peak current enhances gradually (Fig. 4A-c to
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4A-f) when Cu-BTC modified GCEs are used, revealing that Cu-BTC has high catalytic ability
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towards the oxidation of NADH. Moreover, the highest oxidation current is achieved on the Cu-
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BTC film electrosynthesized at -1.3 V (Fig. 4A-f), indicating potential-dependent catalytic
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ability of Cu-MOFs. To find the catalytic activity center, GCEs were modified at -1.3 V only
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using the precursor of copper ions or H3BTC. In the former case, the decorated material is
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mainly CuO,46 as confirmed from the Raman and EDS spectra (Fig. S3). In the latter case, no
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film was formed on a GCE. Compared with a bare GCE, the oxidation peak potential on
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CuO/GCE has a negative shift, but no difference is found at peak current (Fig. 4A-b).
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Furthermore, it is known that copper ion is the catalytic activity centre.33 Therefore, copper ions
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inside these Cu-BTC films are catalytic centers. The big amount of cooper ions inside Cu-BTC
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film electrosynthesized at -1.3 V explains well the highest oxidation current of NADH on such a
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Cu-BTC film.
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Figure 4. (A) Cyclic voltammograms of 2.0 mM NADH in 0.1 M pH 7.0 phosphate buffer at a
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scan rate of 100 mV s−1 in pH 7.0 phosphate buffer on a GCE (a), CuO/GCE (b), Cu-BTC film
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electrosynthesized onto GCEs at -1.1 (c), -1.2 (d), -1.4 (e), -1.3 (f) V. (B-F) Hydrodynamic
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voltammograms of 2.0 mM NADH in 0.1 M pH 7.0 phosphate buffer at a GCE (B) and Cu-BTC
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film coated GCEs electrosynthesized at -1.1 (C), -1.2 (D), -1.3 (E), and -1.4 (F) V at different
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rotation rates: 100 (a), 400 (b), 900 (c), 1600 (d), and 2000 (e) rpm. The scan rate is 25 mV/s.
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Furthermore, the potential-dependent electrocatalytic oxidation of NADH on four Cu-BTCs was
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further elucidated by using rotating disk electrode (RDE). Figure 4B-F present the hydrodynamic
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voltammograms of different GCEs at different rotation rates. Based on the slopes of the
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Koutecky-Levich plots41 (Fig. S4), its direct electrooxidation kinetics, namely the apparent
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number of electrons exchanged for NADH oxidation, was estimated. It is about 2 on these
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electrodes, suggesting that two electrons are lost from the nitrogen-containing heterocycle in
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NADH. This is consistent with published results.47-49 The apparent heterogeneous reaction rate
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constant (k), calculated from the intercept of Koutecky-Levich plot, is 2.52×10-2 cm s-1 for the
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Cu-BTC film electrosynthesized at -1.3 V. It is bigger than that (1.02×10-2 cm s-1) for a GCE as
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well as that for other Cu-BTC films fabricated at -1.1 (1.38×10-2 cm s-1), -1.2 (1.71×10-2 cm s-1),
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and -1.4 (1.99×10-2 cm s-1) V. This result proves again that Cu-BTC films electrosynthesized at
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different potentials have varied electrocatalytic activity towards NADH oxidation. In other
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words, the catalytic activity is potential-dependent.
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Based on the electrocatalytic effects, we expect the electrosynthesized Cu-BTC films can be used
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in the electrochemical sensing determination of NADH. To realize sensitive monitoring of
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NADH, different pulse voltammograms (DPVs) were recorded using the Cu-BTC films
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electrosynthesized at different potentials (Fig. 5). In this way, the effect of background currents
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of these electrodes was minimalized. On a GCE, a small and broad oxidation wave is observed at
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0.37 V. When Cu-BTC films are electrosynthesized on GCEs, the oxidation peaks become
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sharper. The peak potentials shift negatively and currents increase dramatically. As seen in the
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inset of Fig. 5, the oxidation signal of NADH is enhanced on all Cu-BTC films and the Cu-BTC
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film electrosynthesized at -1.3 V shows the largest signal enhancement. The Cu-BTC film
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electrosynthesized at -1.3 V is expected to be a promising electrode material for electrochemical
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sensing applications.
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Figure 5. DPV curves of 100 µM NADH in pH 7.0 phosphate buffer on a GCE (a), Cu-BTC
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film electrosynthesized onto GCEs at -1.1 (b), -1.2 (c), -1.4 (d), -1.3 (e) V. The inset in (B) is the
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comparison of obtained oxidation peak currents (Ipa).
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To figure out the source of such signal enhancement towards NADH oxidation on these Cu-BTC
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films, the adsorption behavior of NADH was studies using double potential step
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chronocoulometry. The chronocoulometric curves were further redrawn using different functions
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(Fig. S5).41 For the forward (positive) step from 0.10 to 0.70 V, the function of Qf-t1/2 is applied
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and a straight line is obtained. Its intercept is the summation of the capacitive charge (Qdl) and
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the faradaic component from the oxidation of the adsorbed NADH (Qads,NADH). For the reverse
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step, the function of Q-f(t) is utilized and another straight line is obtained. Its intercept is the
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value of Qdl. The estimated Qads,NADH on a GCE is 0.44 µC, small than 1.04, 1.76, 2.41, and 2.10
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µC for Cu-BTC films electrosynthesized at -1.1, -1.2, -1.3, and -1.4 V, respectively. The
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increased Qads value reveals a higher accumulation ability of NADH on the surface of Cu-BTC
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films, due to varied thickness (namely porosities) of these Cu-BTC films, eventually leading to
256
the signal enhancement towards NADH oxidation. As shown in Fig. 2, the thicknesses of Cu-
257
MOF films electrosynthesized at different potentials are different, the surface adsorption capacity
258
and signal enhancement effect of these Cu-BTC films are thus altered. The thickness effect of
259
Cu-MOFs on the sensing performance of NADH was further clarified using Cu-BTC films
260
electrosynthesized at -1.3 V with different electrodeposition time. As shown in Fig. S6, the
261
oxidation peak current of NADH increases gradually with an enhancement of deposition time
262
from 1 min to 5 min, due to enhanced porosities of these films. In contrast, longer deposition
263
time leads to the reduction of the oxidation current, probably owing to decreased porosity on
264
these thick films as well as increased distance for electron transfer. In short, the thickness of the
265
Cu-MOF films is an important factor to affect the adsorption efficiency of target molecules and
266
eventually their obtained signal.
267
Since MOFs feature high porosity, multiple functionalities, and variable host-guest interactions,
268
Cu-BTC electrosynthesized at different potentials are expected to be utilized as excellent
269
interfaces to adsorb different kinds of targets from solutions and eventually to achieve their
270
highly sensitivity detection. To further support such a statement, DPVs of XA and HXA (A, B),
271
DES and E2 (C, D), SY and TT (E, F) are compared in Fig. 6. As expected, remarkable signal
272
enhancement is observed on the Cu-BTC films. Moreover, the obtained peak current is varied as
273
a function of the applied potential, showing similar tendency as what is achieved for NADH. In
274
addition, the accumulation efficiencies of Cu-BTC films towards these organic molecules were
275
also evaluated to verify our conjecture about the sensitization mechanism. The same approach
276
employed for the estimation of Qads,NADH was used. The calculated values of Qads for six organic
277
molecules from re-drawn chronocoulometric curves (Fig. S7-12) are summarized in Table 1.
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Clearly, the values of Qads on the Cu-BTC films are much enhanced in comparison to those on a
279
GCE. The biggest Qads for all six organic molecules is noticed on a Cu-BTC film
280
electrosynthesized at -1.3 V. Consequently, the efficient adsorption of organic molecules on a
281
Cu-BTC film is the main reason for highly sensitive detection of these organic molecules. Such
282
high adsorption efficiencies result from the porous structure of Cu-BTC films as well as the
283
interactions of functional groups inside Cu-BTC films with organic molecules. At least two types
284
of interactions exist: strong binding via the nitrogen or oxygen atoms in organic molecules with
285
copper atoms of the paddlewheel and hydrogen bonds with carboxylate groups of Cu-MOFs.50
286
At last, under the synergistic effect of copper ions, Cu-BTC films enhance the detection signals
287
of different targets. In summary, Cu-BTC films electrosynthesised under different cathodic
288
potentials show different morphology, thickness, electrochemical active area, electron transfer
289
ability and amount of catalytic center Cu2+, that make them have different kinetics for redox
290
reactions, accumulation ability and signal enhancement towards a series of small-sized organic
291
molecules. In view of this, Cu-BTC films can be used as sensitive electrode materials for
292
electrochemical sensing various targets.
293
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295 296
Figure 6. DPV curves of 1 µM XA and HXA (A), 300 nM DES and E2 (C), 50
297
nM SY and TT (E) in pH 7.0 phosphate buffer on a GCE (a), a Cu-BTC-1
298
electrosynthesized at -1.1 V (b), a Cu-BTC-2 electrosynthesized at -1.2 V (c), a
299
Cu-BTC-3 electrosynthesized at -1.3 V (d), and a Cu-BTC-4 electrosynthesized
300
at -1.4 V (e); (B, D, F) the comparison of oxidation peak currents (Ipa) on five
301
electrodes.
302 303
Table 1. Comparison of Qads for Different Organic Molecules on a GCE as well as on the Cu-
304
BTC Film Coated GCEs Fabricated at -1.10 (Cu-BTC-1/GCE), -1.20 (Cu-BTC-2/GCE), -1.30
305
(Cu-BTC-3/GCE), and -1.40 (Cu-BTC-4/GCE) V. Electrode
Qads (µC)
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The Journal of Physical Chemistry
GCE Cu-BTC-1/GCE Cu-BTC-2/GCE Cu-BTC-3/GCE Cu-BTC-4/GCE
XA 0.058 0.812 1.173 1.665 1.472
HXA 0.031 0.533 0.664 1.222 1.015
DES 0.050 0.164 0.304 0.438 0.392
E2 0.036 0.335 0.420 0.642 0.546
SY 0.035 0.233 0.354 0.427 0.390
TT 0.024 0.109 0.234 0.355 0.295
306 307
It is clear that the Cu-BTC film electrosynthesized at -1.3 V exhibits superior electrochemical
308
sensing activity. It is thus used for electrochemical sensing of these organic molecules. The DPV
309
curves recorded on a Cu-BTC/GCE electrosynthesized at -1.3 V for NADH, XA, HXA, DES,
310
E2, SY, and TT at their different concentrations are shown in Fig. S13. For each of them, the
311
related oxidation peak currents (Ipa) increase proportionally with their concentrations (C). No
312
mutual interference was noticed. The linear ranges, linear regression equations, and detection
313
limits (based on three signal-to-noise ratio) of these target molecules are summarized in Table 2.
314
Further comparison of the sensing performance of a Cu-BTC film electrosynthesized at -1.3 V
315
towards these compounds with that reported using other electrosynthesized materials are
316
summarized in Table S1-S4, leading one to conclude that such a film exhibits better sensing
317
performance (wider linear ranges and lower detection limits) towards these target molecules.
318
Therefore, it is a better platform for efficient accumulation, highly sensitive detection of these
319
organic molecules individually or simultaneously.
320 321
Table 2. Sensing Performance of Organic Molecules on a Cu-BTC Film Coated GCE
322
Electrosynthesized at -1.3 V. Targets NADH XA HXA DES
Linear range / nM 1000 ~ 100000 10 ~ 2000 10 ~ 5000 10 ~ 1000
Detection limit / nM Ipa = 0.0222 C, R = 0.998 140 Ipa = 0.00314 C, R = 0.998 1.9 Ipa = 0.00162 C, R = 0.998 2.2 Ipa = 0.000599 C, R = 0.998 2.6 Regression equation
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E2 SY TT
1.0~ 1000 1.0~ 100 1.0~ 100
Ipa = 0.00186 C, R = 0.999 Ipa = 0.0190 C, R = 0.998 Ipa = 0.00858 C, R = 0.999
Page 18 of 28
0.23 0.12 0.26
323 324
The reproducibility of sensing these target molecules was tested using several modified
325
electrodes synthesized under the identified conditions. First, the reproducibility of these Cu-
326
MOF film coated electrodes was checked. At each electrosynthesized potential, ten modified
327
electrodes were fabricated. The relative standard deviation (RSD) value of the cathodic peak
328
current of K3Fe(CN)6 obtained on ten electrodes was only 4.3, 5.4, 3.6, and 4.1% when an
329
electrosynthesized potential of -1.1, -1.2, -1.3, and -1.4 V was applied, respectively. Therefore,
330
these Cu-MOF film coated electrodes are highly reproducible. Then, taking the one
331
electrosynthesized at -1.3 V as a case study. The estimated relative standard deviation (RSD)
332
value of the oxidation peak current for individually determining 100 μM NADH, 1.0 μM XA and
333
HXA, 300 nM DES and E2, 50 nM SY and TT was only 5.4, 3.3, 4.7, 4.6, 3.8, 2.7, and 4.2%,
334
respectively. Therefore, sensing these molecules with these Cu-MOF film coated electrodes is
335
highly reproducible.
336 337
CONCLUSION
338
Cu-BTC films electrosynthesized at different cathodic potentials have similar bonding
339
properties, chemical compositions, phase purity, crystallinity, and surface electronic states of
340
copper centers. More interestingly, the reduction potentials tune the morphology, thickness and
341
electrochemical properties (more exactly different numbers of copper ions) of these Cu-BTC
342
films.
343
electrocatalytic activity, surface accumulation and sensing ability of Cu-BTC films, resulting in
344
signal enhancement for the detection of seven kinds of small-sized organic compounds: NADH,
In
other
words,
these
potential-tuned
properties
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345
XA, HXA, DES, E2, SY, and TT. Therefore, the properties and applications of
346
electrosynthesized Cu-BTC films are potential tunable, opening the door for the direct
347
construction of tunable sensors based on cathodic electrodeposited MOFs. Future activity can be
348
conducted on sensing the target molecules on MOFs with different thicknesses, metal centers
349
(e.g., Co, Fe, Ni, Zn, etc.), and ligands as well as on in situ observation of the
350
adsorption/interactions of the organic molecules with the functional groups of various MOFs. In
351
summary, potential-tunable Cu-BTC films are suitable, possible, and practical to be employed as
352
sensitive electrode materials for electrochemical sensing applications. The strategy of adopting
353
reductive processes to electrosynthesize MOFs will expand the MOF applications to other
354
electrochemical and related fields.
355 356
ASSOCIATED CONTENT
357
Supporting Information covers the experimental results about characterization and properties
358
of Cu-BTC films electrosynthesized at different potentials, Raman and EDS spectrums of CuO
359
nanoparticles,
360
chronocoulometric curves, electrochemical response of NADH on different electrodes, DPV
361
curves of linear and performance comparisons of electrochemical sensors.
Kouteckye-Levich
plots,
Re-drawn
plots
from
double
potential
step
362 363
AUTHOR INFORMATION
364
Corresponding Author
365
* E-mail:
[email protected] (K.W.); E-mail:
[email protected] (N.Y.)
366
Notes
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367
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The authors declare no competing financial interest.
368 369
ACKNOWLEDGEMENTS
370
This work was supported by the National Basic Research Program of China (973 Program, No.
371
2015CB352100), the National Natural Science Foundation of China (No. 21775050), the Hubei
372
Provincial Natural Science Fund for Distinguished Young Scholars (2016CFA039), and the
373
German Research Foundation (YA344/1-1). The Center of Analysis and Testing of Huazhong
374
University of Science and Technology was acknowledged for the help of TEM, XPS, XRD, and
375
Raman measurements.
376
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