Potential-Tunable Metal-Organic Frameworks: Electrosynthesis

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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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The Journal of Physical Chemistry

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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.

ACS Paragon Plus Environment

Herein,

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

248

(Fig. S5).41 For the forward (positive) step from 0.10 to 0.70 V, the function of Qf-t1/2 is applied

249

and a straight line is obtained. Its intercept is the summation of the capacitive charge (Qdl) and

250

the faradaic component from the oxidation of the adsorbed NADH (Qads,NADH). For the reverse

251

step, the function of Q-f(t) is utilized and another straight line is obtained. Its intercept is the

252

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

253

µC for Cu-BTC films electrosynthesized at -1.1, -1.2, -1.3, and -1.4 V, respectively. The

254

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

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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

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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

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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

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highly sensitivity detection. To further support such a statement, DPVs of XA and HXA (A, B),

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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.


14

Page 15 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


278

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


15


Page 16 of 28

294

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)


16

Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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


17


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


further

regulate

the

18

Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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


19


367

Page 20 of 28

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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