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DOI: 10.1039/C7CE00416H (Paper) CrystEngComm, 2017, 19, 4194-4200

Electrochemical sensing and catalysis using Cu3(BTC)2 coating electrodes from Cu(OH)2 films†

Kenji Okada a, Shota Sawai b, Ken Ikigaki b, Yasuaki Tokudome b, Paolo Falcaro c and Masahide Takahashi *b
aDepartment of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
bDepartment of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan. E-mail: masa@photomater.com
cGraz University of Technology, Institute of Physical and Theoretical Chemistry, Stremayrgasse 9/Z2, 8010 Graz, Austria

Received 28th February 2017 , Accepted 10th April 2017

First published on 10th April 2017

Abstract

Metal–organic framework (MOF) coatings were prepared on gold electrodes through the conversion from Cu(OH)2 nanobelts to Cu3(BTC)2 MOFs (BTC = 1,3,5-benzenetricarboxylic acid). In the conversion from Cu(OH)2 to Cu3(BTC)2, the porosity and homogeneity of the coatings are tunable by controlling the EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratio of the MOF precursor solution. The kinetics and mechanism of the conversion from metal hydroxide nanobelts to MOFs were investigated by using the quartz crystal microbalance (QCM) technique. The Cu3(BTC)2 coatings were finally tested for the size-selective electrochemical sensing and non-enzymatic electrocatalysis of glucose.

Introduction

Metal organic frameworks (MOFs), also known as porous coordination polymers (PCPs), have attracted interest due to their high accessible surface area, uniform and tunable pore sizes, and diverse functionalities.1 Among different envisaged practical uses of MOFs, electrochemical application is a burgeoning field.2 However, the employment of MOFs in such a field, which includes photovoltaics, electrochromics, sensing and electrocatalysis, requires homogeneous MOF films or coatings with a physical interface between the MOFs and an electrode.3 Fabrication of MOF coatings has been achieved by different methods4 including the Langmuir–Blodgett (LB) method,5 a direct MOF growth method on self-assembled monolayer (SAM)-modified electrodes,6 gas-phase deposition,7 the use of MOF colloidal solutions or powders,8 electrochemical preparation methods9 and direct conversion from metals.10 However, MOF synthesis was also achieved via the conversion of metal hydroxides. The conversion can be performed at room temperature using a water–ethanol solution.11 Various types of MOFs including Cu3(BTC)2, Cu2(BDC)2, Cu2(BDC)2(DABCO) and Fe-BTC can be synthesized from metal hydroxides.11 Furthermore, the conversion reaction of metal hydroxides as MOF precursors also allows the structuralization of MOFs as free-standing MOF membranes, 2D patterns and 3D complex architectures with control over their composition.11b,12,13 Recently, we reveal a novel property of these systems as MOF films perfectly oriented along the direction both normal and parallel to the substrate (out-of-plane and in-plane) can be fabricated on a metal hydroxide film via hetero-epitaxial growth.14

In the present study, we report on the fabrication of Cu3(BTC)2 coatings on a gold electrode through the conversion of Cu(OH)2 nanobelts to Cu3(BTC)2 MOF crystals. As various types of supports can be used.14 This versatile technique can be integrated on different electrodes used in electrochemical applications. The characteristics of the coating, such as film homogeneity, porosity and composition, are tunable by controlling the water-to-ethanol ratio of the MOF precursor solution. The diameter of Cu3(BTC)2 MOF particles increased with the fraction of water. The conversion was investigated by monitoring the weight change using a quartz crystal microbalance (QCM). Functional properties such as size-selective sensing and non-enzymatic electrocatalysis of glucose using the converted Cu3(BTC)2 coating on a gold electrode were tested.

Results and discussion

Preparation of a Cu3(BTC)2 coating on gold electrode

Fig. 1 is a schematic illustration showing the concept of the present study. At first, Cu(OH)2 nanobelt films were deposited on gold electrodes (Fig. 1a and b). The Cu3(BTC)2 MOF coating was obtained by exposing the Cu(OH)2 nanobelt films, for 10 min at room temperature, to an aqueous ethanolic mixture containing H3BTC linkers (Fig. 1c). The composition of the solvents was changed to obtain Cu3(BTC)2 coatings with different features. The obtained Cu3(BTC)2 coating on a gold electrode was subsequently tested for sensing and electrocatalysis (Fig. 1d). A schematic representation of the set-up for the sensing and electrocatalysis is shown in Fig. 1e, where Pt, Ag/Ag+ and the Cu3(BTC)2 coating on a gold electrode were used as the counter electrode (CE), the reference electrode (RE) and the working electrode (WE), respectively.
image file: c7ce00416h-f1.tif
Fig. 1 Schematic illustration showing the concept of the present study. (a)–(c) The method to fabricate the Cu3(BTC)2 coating on a gold electrode. (a) Deposition of Cu(OH)2 nanobelts on the gold electrode where the yellow part indicates the gold coating and the light blue part indicates the quartz substrates. (b) Cu(OH)2 nanobelts on the gold electrode, where the plate-like emerald green objects indicate Cu(OH)2 nanobelts. (c) MOF coating on the gold electrode, where the blue objects indicate Cu3(BTC)2 crystals. (d) and (e) Schematic representation of the set-up for sensing and electrocatalysis.

Fig. 2 shows the scanning electron microscopy (SEM) images and X-ray diffraction (XRD) patterns of Cu3(BTC)2 coatings prepared by soaking Cu(OH)2 nanobelt films into an H3BTC-containing solution of water (H2O) and ethanol (EtOH) mixture with different volume ratios. There was no morphological change from the initial Cu(OH)2 nanobelt film for H3BTC solution using pure ethanol (Fig. 2a). The XRD pattern is the same as that of the initial Cu(OH)2 nanobelt film, indicating that the Cu(OH)2 nanobelts did not react with H3BTC in pure EtOH (Fig. 2e). Octahedral crystals on Cu(OH)2 nanobelts were found in the sample prepared in the solvent with EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O = 5[thin space (1/6-em)]:[thin space (1/6-em)]2. The crystalline octahedral morphology is well-known as typical Cu3(BTC)2.15 The characteristic peaks of Cu3(BTC)2 MOFs were detected in the XRD patterns (Fig. 2e).16 The larger the amount of H2O, the bigger the MOF crystal formed (Fig. 2c and d). In the sample prepared with EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O = 2[thin space (1/6-em)]:[thin space (1/6-em)]5, small MOF particles with several hundred nm in diameter were also detected along with MOF particles larger than 3 μm in diameter, suggesting a process dominated by Ostwald ripening.11b Intense peaks from Cu3(BTC)2 were detected in the samples prepared at a lower EtOH/H2O volume ratio, indicating that an increased amount of H2O induced a high conversion from Cu(OH)2 into Cu3(BTC)2 (Fig. 2e). The obtained Cu3(BTC)2 crystals were preferentially oriented along the direction normal to the substrate because the peaks at 11.62° (222) and 17.62° (333) showed a higher intensity compared to the other peaks. The a–c plane of Cu(OH)2 corresponding to the surface of the nanobelts induced the oriented growth of Cu3(BTC)2 crystals along the out-of-plane direction, as reported in our previous study.14 This process allowed the fabrication of homogenous coatings for all samples (Fig. S1†). SEM and XRD investigations revealed that an increased H2O ratio led to faster conversion into MOFs with preferential orientation and the formation of larger MOF particles.


image file: c7ce00416h-f2.tif
Fig. 2 SEM images (a–d) of the Cu3(BTC)2 coatings prepared at different EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratios: (a) 1[thin space (1/6-em)]:[thin space (1/6-em)]0, (b) 5[thin space (1/6-em)]:[thin space (1/6-em)]2, (c) 1[thin space (1/6-em)]:[thin space (1/6-em)]1, (d) 2[thin space (1/6-em)]:[thin space (1/6-em)]5. (e) XRD patterns of the Cu(OH)2 nanobelt film, the powder Cu3(BTC)2 MOF prepared by solvothermal synthesis and the MOF coatings prepared at different EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratios.

The porosity of the obtained Cu3(BTC)2 coatings was investigated using N2 adsorption measurement (Fig. 3a). The weight ratio of components (Cu(OH)2 and Cu3(BTC)2) in the coatings was investigated by thermogravimetric–differential thermal analysis (TG-DTA) (Fig. 3b). The sample prepared with 1[thin space (1/6-em)]:[thin space (1/6-em)]0 (EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratio) consisted of unreacted Cu(OH)2 nanobelts, as confirmed by SEM and XRD, leading to a low specific surface area of 93.8 m2 g−1, thus confirming the absence of MOFs. From literature, Cu3(BTC)2 MOFs are reported to have a specific surface area in the range from 1300 to 1800 m2 g−1.11a Although the increase of H2O resulted in an increase of the weight ratio of Cu3(BTC)2 in the coatings from ∼30 wt% to ∼80 wt% (Fig. 3b), there was a little increase in the specific surface area (from ∼750 m2 g−1 to ∼900 m2 g−1). The specific surface area of the Cu3(BTC)2 component in the coatings was calculated from the specific surface area and the weight obtained from BET and TG-DTA investigations (Fig. 3a and b), respectively, following the same procedure previously reported11b (Fig. S2†). The specific surface area of Cu3(BTC)2 in the coatings was ca. 1500 m2 g−1 which is in agreement with those of reported Cu3(BTC)2 MOFs.11a Various types of coatings were prepared by changing the EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratio.


image file: c7ce00416h-f3.tif
Fig. 3 BET specific surface area (a) and weight ratio of components (Cu(OH)2 and Cu3(BTC)2) (b) resulting from different conversion solutions with varying EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O ratios.

The EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratio affected the conversion rate and crystal size of Cu3(BTC)2, as shown above. Quartz crystal microbalance (QCM) measurement was employed in order to investigate the conversion kinetics and mechanism from Cu(OH)2 into Cu3(BTC)2 under different conditions with varying EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O ratios. QCMs are highly sensitive mass sensors that measure quantitative mass changes by monitoring vibration frequency of a piezoelectrically driven quartz crystal. This technique has been used for the in situ investigation of the reaction kinetics and mechanism of step-by-step MOF growth on a SAM-modified QCM.17 Herein, QCM measurements were employed for the first time to investigate the kinetics and mechanism of the MOF synthesis via the conversion from metal hydroxides. First, Cu(OH)2 nanobelt films were deposited on a gold-coated QCM (Fig. 4a). Then, the coating was soaked in EtOH–H2O solution at different EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratios, which does not contain an H3BTC ligand. In situ investigation was carried out during the conversion from Cu(OH)2 to Cu3(BTC)2 by adding a prescribed amount of H3BTC solution with the corresponding EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratio. In this conversion process, weight loss confirmed by a positive frequency shift indicates the dissolution of Cu(OH)2, while weight gain confirmed by a negative frequency shift indicates the formation of Cu3(BTC)2, because the molar mass of Cu3(BTC)2 with respect to Cu(OH)2 per each Cu atom is 7-fold. Fig. 4b shows the results of in situ QCM measurements for the reaction of the Cu(OH)2 film in the H3BTC solution at different EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratios. At 180 s, a small droplet of H3BTC solution was added to the EtOH–H2O solution. Only weight gain was observed in the solution with 5[thin space (1/6-em)]:[thin space (1/6-em)]2 and 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratios. These results indicate that the Cu2+ ions dissolved from Cu(OH)2 quickly react with the abundant H3BTC in solution to form Cu3(BTC)2 crystals on the Cu(OH)2 nanobelts. The conversion rate in the 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratio was higher than that in 5[thin space (1/6-em)]:[thin space (1/6-em)]2 because the plot of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 showed higher weight gain than that of 5[thin space (1/6-em)]:[thin space (1/6-em)]2. Only the case of 2[thin space (1/6-em)]:[thin space (1/6-em)]5 showed two stages: weight loss at the first stage (180 to 450 s) and then weight gain (after 450 s), which implies the rapid dissolution of Cu(OH)2 and simultaneous nucleation and crystal growth of Cu3(BTC)2 MOFs. Indeed, the dissolution rate of Cu(OH)2 was decreased at higher EtOH/H2O ratios when benzoic acid was used instead of H3BTC in order to investigate only the effect of dissolution of Cu(OH)2 in the reaction solution (Fig. 4c). Larger MOF crystals were reportedly synthesized in a higher-polarity solvent for the conversion from copper hydroxysulfates to Cu3(BTC)2.13 A high polarity solvent promotes the deprotonation of organic ligands and increases the acidity of the reaction mixture, resulting in more rapid crystal growth of MOFs and larger Cu3(BTC)2 particles.18 In the present study, a lower EtOH/H2O volume ratio led to more deprotonation of the organic ligands and higher acidity of the conversion solution because the polarity (dielectric constant) of water (80.1) is higher than that of ethanol (24.5), which resulted in the formation of larger crystals and a high conversion ratio. Indeed, the conversion solution showed higher acidity at a lower EtOH/H2O volume ratio (Fig. S3†). The weight loss after 600 s in the 2[thin space (1/6-em)]:[thin space (1/6-em)]5 sample can be explained by the dissolution of Cu3(BTC)2 MOFs in highly acidic solution.19 Based on the in situ QCM measurements and results from SEM and XRD, the conversion mechanism is proposed in Fig. 4d–f. In the EtOH-rich solution, the limited deprotonation of the organic linkers and the corresponding low acidity of the conversion solution result in slower dissolution of Cu2+ from Cu(OH)2 and more rapid reaction with abundant H3BTC linkers to form smaller Cu3(BTC)2 particles where the composition of Cu3(BTC)2/Cu(OH)2 (wt%/wt%) is low due to a low conversion rate. On the other hand, in the H2O-rich solution, the promoted deprotonation of the organic linkers and the high acidity of the conversion solution result in the rapid dissolution of Cu2+ from Cu(OH)2 and reaction with abundant H3BTC linkers to form a Cu3(BTC)2 nucleus. Then, Cu2+ dissolved in the solution is used for the crystal growth of the nucleus. A longer reaction time of over 600 s leads to dissolution of Cu3(BTC)2 MOFs. Under these conditions, a high Cu3(BTC)2/Cu(OH)2 (wt%/wt%) ratio is related to a high conversion rate.


image file: c7ce00416h-f4.tif
Fig. 4 QCM investigation and the conversion mechanism from Cu(OH)2 to Cu3(BTC)2 MOFs. (a) Schematic illustration of QCM investigation where the Cu(OH)2 coatings deposited on the gold-coated QCM are reacted with H3BTC linkers in the conversion solution. The QCM frequency as a function of time recorded in situ during the reaction with (b) H3BTC and (c) benzoic acid in the conversion solution with different EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratios. At 180 s, a small amount of the corresponding solution containing carboxylic acids was added. The bare gold-coated QCM was used as the reference. (d)–(f) Schematics showing the mechanisms in H3BTC containing solution with different EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratios and the corresponding size of Cu3(BTC)2 crystals and Cu3(BTC)2/Cu(OH)2 ratio (wt%/wt%); where the blue octahedral objects indicate Cu3(BTC)2 crystals.

Size-selective sensing of a Cu3(BTC)2 coating on a gold electrode in solution

MOFs have been investigated for different sensors including solvatochromism/vapochromism, luminescence-based, and electromechanical sensors.20 Among them, electrochemical sensors are expected to be a highly sensitive detection system. For instance, MOFs deposited on interdigital electrodes and a MOF pellet pressed between gold disk electrodes can be used for humidity sensing at water concentrations below 10% and at low temperature.21 QCM-based sensing devices have also been investigated so far. A MOF-coated QCM has been demonstrated to detect water vapor and small molecules such as methanol, hexane and pyridine.9b,22 Most MOF sensors are based on the chemical selectivity of the target molecules with specific interactions with the organic linkers or metal ions of MOFs. There are few reports on size-selective MOF sensors which exclude molecules larger than the pore sizes of the MOFs.23 Only gas and vapor phase analytes have been detected by size-selective MOF sensors.24 To the best of our knowledge, there are no reports on a size-selective MOF sensor in solution by electrical and electrochemical methods. This is because such a sensor requires the formation of homogenous and intracrystalline and intercrystalline defect-free MOF coatings on electrodes. Otherwise, analytes larger than the size of MOF pores can easily pass through the defects and pinholes and then react with electrodes in the solution, losing the size-selectivity. The approach reported here allows the fabrication of homogeneous Cu3(BTC)2 coatings which can be used for size-selective sensors in solution. To find the best MOF coatings for the size-selective sensing, redox active tetraoctylammonium tetrafluoroborate ([CH3(CH2)7]4N(BF4), ∼23 Å (ref. 25)), which is larger than Cu3(BTC)2 pores (3.5–13.3 Å (ref. 26)), was used as an electrolyte in the electrochemical system. Electrochemical impedance spectroscopy revealed that the Cu3(BTC)2 coating electrodes prepared at different EtOH/H2O volume ratios showed different charge-transfer resistances, Rct (Fig. S4†). The order of Rct was 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ≫ 2[thin space (1/6-em)]:[thin space (1/6-em)]5 > 5[thin space (1/6-em)]:[thin space (1/6-em)]2 ≫ 1[thin space (1/6-em)]:[thin space (1/6-em)]0 (EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratio). The coatings showing low Rct indicate that [CH3(CH2)7]4N(BF4) reacted on the electrodes through the defects, while the coating prepared at 1[thin space (1/6-em)]:[thin space (1/6-em)]1 could protect the percolation of larger molecules, [CH3(CH2)7]4N(BF4), than the pore size of MOFs. The low Rct can be explained by the presence of larger spaces between the unconverted Cu(OH)2 nanobelts and intercrystalline defects between the large and small Cu3(BTC)2 crystals in the coating prepared at 5[thin space (1/6-em)]:[thin space (1/6-em)]2 and 2[thin space (1/6-em)]:[thin space (1/6-em)]5, respectively, as observed from the SEM investigation (Fig. 2). Although the Cu(OH)2 nanobelts existed in the coating prepared at 1[thin space (1/6-em)]:[thin space (1/6-em)]1, the packed Cu3(BTC)2 crystals can refuse the invasion of [CH3(CH2)7]4N(BF4) into the coating. Thus, the coating prepared at 1[thin space (1/6-em)]:[thin space (1/6-em)]1 was tested for the size-selective MOF sensing by electrochemistry (Fig. 5). Redox active iron compounds with different molecular sizes were selected as target analytes; ferrocene (FC): 5.7 Å, iron(III) acetylacetonate (Fe(acac)3): 9.3 Å, and iron(II) phthalocyanine (FePc): 15 Å.25 The reduction and oxidation of Fe(II/III) can be observed by cyclic voltammetry when the iron compounds reach the electrode through the MOF pores (3.5–13.3 Å). Fig. 5(b)–(d) show the cyclic voltammograms of the Cu(OH)2 nanobelt film and the Cu3(BTC)2 coating on gold electrodes in acetonitrile containing 1 M FC, Fe(acac)3 and FePc, respectively. The reduction and oxidation of Fe(II/III) for all iron compounds were observed in the Cu(OH)2 nanobelt film because the large interspaces from a few hundred nm up to 1 μm between the Cu(OH)2 nanobelts allowed the percolation of the compounds. Meanwhile, the Cu3(BTC)2 coatings on electrodes showed size-selective responses; the reduction and oxidation were only observed for FC and Fe(acac)3 which are smaller than the Cu3(BTC)2 pores. These results indicate that the Cu3(BTC)2 coatings can be used as sensors for size-selectively detecting electroactive analytes in the solution.
image file: c7ce00416h-f5.tif
Fig. 5 Size-selective sensing by the Cu3(BTC)2 coating on a gold electrode. (a) Schematic representation of the set-up for the sensing. Cyclic voltammograms of the Cu(OH)2 nanobelt and the Cu3(BTC)2 coatings for 1 M FC (b), Fe(acac)3 (c) and FePc (d) in acetonitrile.

Electrocatalytic oxidation of glucose using Cu3(BTC)2 coatings

Cu-based materials such as metallic copper, Cu(OH)2, CuO, and Cu3(BTC)2 MOFs have been considered as promising materials for non-enzymatic glucose electrocatalysis and sensing.27 The electrocatalytic oxidation of glucose using the Cu3(BTC)2 coatings was investigated. Fig. 6(a) shows the cyclic voltammetry responses of the Cu(OH)2 nanobelt film and the Cu3(BTC)2 coating prepared in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 water–ethanol solution of H3BTC. The cyclic voltammetry investigation was carried out in 0.1 M NaOH solution containing 100 μM glucose at a scan rate of 50 mV s−1. Fig. 6(b) shows the chronoamperometry responses at +0.60 V after successive injections of different amounts of glucose into 0.1 M NaOH (aq) under stirring of the Cu3(BTC)2 coatings. The MOF coating electrode exhibited an increase in current response after successive injections of glucose even over 1500 μM concentration. The sensitivity was calculated from the plot of electrocatalytic current of glucose versus its concentrations (Fig. S5†). The calculated sensitivity was around 273 μA mM−1 cm−2 which is higher than that of nanostructured metallic copper.28 However, the degradation of the MOF coatings in the NaOH electrolytic solution was confirmed by the XRD and FTIR investigations (Fig. S6†). The MOF coating was almost eliminated after immersing in the electrolyte. An improvement in the stability of the MOF coating has yet to be achieved.
image file: c7ce00416h-f6.tif
Fig. 6 (a) Cyclic voltammograms of the Cu(OH)2 nanobelts and the Cu3(BTC)2 coatings in 0.1 M NaOH solution containing 100 μM glucose at a scan rate of 50 mV s−1. (b) Chronoamperometry responses of the Cu3(BTC)2 coating after successive injections of different amounts of glucose into 0.1 M NaOH (aq) at +0.60 V under stirring.

Experimental

Synthesis of Cu(OH)2 nanobelt films on a gold electrode

Cu(OH)2 nanobelt films were prepared on a gold electrode by a spin-coating method. The copper hydroxide nanobelts were synthesized by the following procedure. At first, 3 mL of 0.15 M NH4OH solution was added dropwise to 10 mL of 0.04 M CuSO4·5H2O aqueous solution. After stirring for around 5 min, 0.6 ml of 12 M NaOH aqueous solution was added dropwise under stirring and then stirred for 1 hour at room temperature. The solution was then held for 30 min at 40 °C without stirring. By centrifuging the resultant mixture, colloidal milky blue powders were separated and then washed with water and ethanol. The resultant copper hydroxide nanobelts (0.3 g) were dispersed in ethanol (10 mL). The ethanolic colloidal solution was spin-coated on gold electrodes, thus producing the Cu(OH)2 nanobelt film.

Conversion of the Cu(OH)2 nanobelt film into the Cu3(BTC)2 coating

The conversion from Cu(OH)2 nanobelts to Cu3(BTC)2 MOFs was performed at room temperature by immersing the obtained Cu(OH)2 nanobelt film on the gold electrode into 10 mL of an ethanol (EtOH)–water (H2O) mixture containing 7 mg H3BTC where the EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O (volume to volume) ratio was varied as 1[thin space (1/6-em)]:[thin space (1/6-em)]0, 2[thin space (1/6-em)]:[thin space (1/6-em)]5, 1[thin space (1/6-em)]:[thin space (1/6-em)]1 and 5[thin space (1/6-em)]:[thin space (1/6-em)]2. After 10 min, the product was removed from the solution and washed with water and ethanol, and then dried under air. The samples for N2 sorption measurements were prepared from Cu(OH)2 nanobelt powder. 0.15 g of Cu(OH)2 nanobelts were reacted in the same solution as in the Cu3(BTC)2 coating. After reaction, the resulting powders were washed with water and ethanol by centrifugation, and then dried under air.

Characterization

The morphology of the coatings was observed by field emission SEM (S-4800, Hitachi, Japan). Crystal phase identification was obtained via XRD measurements; CuKα radiation (λ = 0.154 nm), Rigaku diffractometer (SmartLab; Rigaku, Japan). BET surface area was determined by N2 sorption measurements (BELSORP-mini II; Bel Japan Inc., Japan). Activation of the samples was carried out at 150 °C in a vacuum for 12 hours. Thermogravimetric–differential thermal analysis (TG-DTA; Thermo Plus Evo, Rigaku, Japan) was carried out with a temperature profile of 10 °C min−1 while continuously supplying air at a rate of 300 mL min−1. Estimation of the Cu(OH)2 and Cu3(BTC)2 weights was calculated according to the method in our previous report.11b Based on these values and the surface area obtained by BET, the specific surface area of the Cu3(BTC)2 component in the coatings was calculated. The weight change in the conversion of the Cu(OH)2 spin-coated on the gold electrode to Cu3(BTC)2 was determined by the in situ QCM method. Electrochemical experiments were performed using a potentiostat: HZ-7000 (Hokuto Denko Co., Ltd.) with a conventional three-electrode system. The Cu3(BTC)2 coating on a gold electrode obtained using a water to ethanol ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 was used as the working electrode. A Pt wire and a Ag/AgCl electrode acted as the counter and reference electrodes, respectively.

Conclusions

We report the fabrication of Cu3(BTC)2 coatings on gold electrodes through the conversion of Cu(OH)2 nanobelts to Cu3(BTC)2 MOFs. The film homogeneity, porosity and composition of the coating can be tuned by controlling the EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratio of the conversion solution. The conversion was investigated by in situ QCM measurement. The lower EtOH/H2O volume ratio promoted the deprotonation of H3BTC in the conversion solution, leading to larger crystals, a high conversion ratio due to the rapid dissolution of Cu2+ from Cu(OH)2 and a quicker reaction with abundant H3BTC. The MOF coating prepared at the 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O volume ratio showed the best size-selective sensing properties for redox active iron compounds in solution due to the formation of a homogenous and defect-free MOF coating on the electrode. Also, the MOF coating was tested for the electrocatalytic oxidation of glucose. This approach for the fabrication of MOF coatings with tunable composition, porosity and homogeneity is expected to be compatible with MOF-based electrical and electrochemical applications.

Acknowledgements

This work was partly supported by a Grant-in-Aid for Scientific Research (B) (26288108), a Grant-in-Aid for Young Scientists (B) (30750301), a Grant-in-Aid for Scientific Research on Innovative Area (26630322) from the Ministry of Education, Culture Sports, Science and Technology of Japan, and the Strategic Young Researcher Overseas Visits Program for Accelerating Brain Circulation from the Japan Society of Promotion of Science.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ce00416h
This journal is © The Royal Society of Chemistry 2017
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