Impregnation of Graphene Quantum Dots into a MetalâOrganic

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Impregnation of Graphene Quantum Dots into a Metal– Organic Framework to Render Increased Electrical Conductivity and Activity for Electrochemical Sensing Yu-Chuan Chen, Wei-Hung Chiang, Darwin Kurniawan, Pei-Chun Yeh, Ken-ichi Otake, and Chung-Wei Kung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11447 • Publication Date (Web): 18 Aug 2019 Downloaded from pubs.acs.org on August 18, 2019

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ACS Applied Materials & Interfaces

Impregnation of Graphene Quantum Dots into a Metal–Organic Framework to Render Increased Electrical Conductivity and Activity for Electrochemical Sensing Yu-Chuan Chen,a Wei-Hung Chiang,*b Darwin Kurniawan,b Pei-Chun Yeh,b Ken-ichi Otake c and Chung-Wei Kung*a Department of Chemical Engineering, National Cheng Kung University, 1 University Road, Tainan City, Taiwan, 70101. a

Department of Chemical Engineering, National Taiwan University of Science and Technology, 43, Keelung Rd., Sec.4, Da'an Dist., Taipei City, Taiwan, 10607. b

c Institute for integrated Cell-Materials Science (iCeMS), Kyoto University, Sakyo-ku Yoshida-honmachi, Kyoto, Japan, 606-8501. ABSTRACT: Graphene quantum dots (GQD) with an average size of 3.1 nm were incorporated into a mesoporous porphyrinic zirconium-based metal–organic framework (MOF) by direct impregnation to render the donor-acceptor charge transfer from GQD to porphyrinic linkers. The hybrid material still possesses around half porosity of the pristine MOF and shows a hundredfold higher electrical conductivity compared to the parent MOF. By utilizing the porphyrinic linkers as catalytically active units, the GQD-MOF material exhibits a better electrochemical sensing activity toward nitrite in aqueous solutions compared to both the pristine MOF and GQD. KEYWORDS: Conductive MOF; Donor-acceptor charge transfer; Electrocatalysis; Electrochemical sensor; Mesoporous; Nitrite oxidation; Zirconium-based MOF

INTRODUCTION Metal–organic frameworks (MOFs)1-3 have attracted great attention and have been utilized in a variety of applications4-11 for the past two decades due to their unique characteristics such as regular and interconnected porosity, ultrahigh specific surface area12, and intraframework chemical functionality13-14. Among various MOFs reported so far, zirconium-based MOFs (Zr-MOFs) and their hafnium-based analogs have been found to exhibit superior thermal and chemical stability.15-16 The crystallinity and porosity of numerous Zr-MOFs can keep intact in the presence of water, acidic solutions, steam, and even H2S vapour,15 which renders the use of Zr-MOFs in a range of applications operated in aqueous media,17 such as water splitting, water adsorption,18 and electrochemical sensors. Owing to their high porosity and exceptional water stability, thin films of Zr-MOFs should be attractive candidates as the catalysts for a range electrochemical applications in aqueous media. However, the electrically insulating nature of most MOFs strongly limits their practical use in electrochemical and electronic applications;19-20 the use of MOF-derived conductive carbons and inorganic compounds is comparatively more common for these applications.21-24 Although numerous approaches, e.g., introducing guest molecules to render donor-acceptor charge transfer in the frameworks,25-28 have been developed to design electrically conducting MOFs,29-37 examples of electrically conducting

Zr-MOFs are still very rare to date.25-26, 38-39 A more common strategy to render charge transport in Zr-MOFs is utilizing redox hopping under electrochemical conditions,40 but the slow redox hopping in Zr-MOFs usually restricts the electrochemical performance. For example, thin films of a porphyrinic Zr-MOF can be applied for electrochemical sensors to detect nitrite in aqueous solutions due to the electrocatalytic activity provided by the porphyrinic linkers, but the sensing activity is strongly limited by the sluggish charge transport presented in the framework.41 Graphene quantum dots (GQD) are nanometre pieces of single- or multi-layered graphenes that possess sizedependent bandgaps for light emission.42-43 A few recent studies have reported the incorporation of GQD or carbon dots in MOFs.44-49 However, all previous studies utilized encapsulation, i.e., growing MOF in the suspension of GQD or carbon dots, which usually causes the random and dispersed distribution of GQD or carbon dots in MOF crystals. Direct impregnation of GQD into MOFs may construct the continuous and more structurally regular alignment of GQD in the framework, but it is very challenging due to the small aperture size of most MOFs and the lack of interaction between GQD and MOFs to immobilize GQD in the framework. In this study, GQD with an average size of 3.1 nm were incorporated into a mesoporous porphyrinic Zr-MOF, PCN-222,50 by utilizing impregnation method (Figure 1). To the best of our knowledge, this is the first study reporting

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the direct impregnation of GQD into MOFs. As the GQD possess a more negative conduction band than the porphyrinic linkers presented in the MOF, donor-acceptor charge transfer can be observed from the electrondonating GQD to the electron-withdrawing linkers. As a result, the GQD-PCN-222 hybrid material shows an electrical conductivity increased by two orders of magnitude compared to the pristine MOF and exhibits a much better activity for electrochemical sensing toward nitrite compared to both the pristine GQD and MOF.

Figure 1. Incorporation of GQD into PCN-222 via direct impregnation.

EXPERIMENTAL SECTION Chemicals. Zirconium(IV) oxychloride octahydrate (Sigma-Aldrich, 98%), benzoic acid (Sigma-Aldrich, ≥99.5%), diethylformamide (DEF, TCI Chemicals, ≥99.0%), zinc chloride (Sigma-Aldrich, ≥98%), mesotetra(4-carboxyphenyl)porphine (TCPP, Strem Chemicals, 98%), dimethylformamide (DMF, ECHO Chemical Co, Ltd., Taiwan, ≥99.8%), hydrochloric acid (HCl, J. T. Baker, 36.5-38.0%), acetone (ECHO Chemical Co, Ltd., Taiwan, ≥99.0%), methanol (ECHO Chemical Co, Ltd., Taiwan, 99.9%), sodium hydroxide (Fluka, ≥98%), sodium nitrite (Alfa Aesar, 98%), starch (Sigma Aldrich, ≥98%), sodium hydroxide (Sigma Aldrich, ≥99%), and sodium chloride (VETECTM, 99%) were purchased and used as received. Deionized water was used as the water source throughout the work. Synthesis of GQD. GQD were synthesized in aqueous solutions using a direct current (dc) microplasma electrochemical reactor operated at ambient conditions. The experimental setup is similar to that shown in previous reports and details can be found elsewhere.51-52 Briefly, the reactor consisted of a platinum foil anode (1 cm2) immersed in an electrolyte and a microplasma cathode. The distances between the anode and cathode and cathode to the liquid surface are approximately 3 cm and 1.5 mm, respectively. A flow of 25 sccm argon was introduced in a hollow stainless steel capillary (i.d. = 180 m) to facilitate

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the formation of a microplasma. A dc power supply was used to ignite and maintain a stable plasma with a constant current of 9.6 mA. The process time was kept at 1 h. The starch (0.428 g), deionized water (15 mL), and 0.1 M aqueous solution of sodium hydroxide (2.5 mL) were mixed and heated at 70 oC for 10 min. Then, 6 mL of the mixed solution was used as the precursor for the microplasma synthesis. The electrolyte was transparent and colorless initially and gradually changed to yellow with increasing time for plasma treatment. After one hour of microplasma synthesis, the product was purified by mixing with 10 mL of ethanol for 10 min. In the end, the purified GQD suspension was obtained by vacuum filtration; the unreacted starch was removed by the filter membrane (polyvinylidene difluoride (PVDF) membrane filter. pore size: 0.2 μm, diameter: 47 mm, Pall cooperation). The purified GQD suspension was diluted to a concentration of 10 mg/mL by adding deionized water. The obtained GQD solution with a concentration of 10 mg/mL was used throughout the work. Dry GQD sample was prepared by the following procedure. First, 5 mL of the GQD solution was mixed with 5 mL of 0.1 M HCl in a scintillation vial, and the obtained solution was dried in a natural convection oven at 80 oC; solid with pale yellow color was obtained in the vial. Thereafter, 5 mL of methanol was added into the vial and the obtained mixture was sonicated for 20 min to partially re-disperse the GQD, and the mixture was filtrated by a 200-nm syringe filter to separate the solid salt from the solution. Methanol was then evaporated from the obtained solution at room temperature to get the yellow solid. The solid was then dried in a vacuum oven at 80 oC for 24 h, and the dry GQD sample was thus obtained. Synthesis of GQD-PCN-222. Free-base PCN-222 microcrystals were synthesized and activated following the reported procedure.53 For the synthesis of GQD-PCN-222 via direct impregnation, 7.53 mL of the GQD solution was first mixed with 9.72 mL of 0.1 M HCl aqueous solution in a 20 mL scintillation vial with a Teflon liner. Thereafter, 25 mg of activated PCN-222 was added into the obtained solution, and the suspension was kept for seven days at room temperature. The suspension was shaken periodically. Thereafter, the solid was separated from the suspension by using centrifugation at 13,000 rpm for 2 min, and was washed with 18 mL of 0.1 M HCl aqueous solution for three times by centrifugation.54 Between each time of centrifugation, the mixture was kept for 2 h, overnight, and 2 h, respectively, to ensure the complete removal of excess GQD from the solid. The solid was then washed with fresh acetone for three times by centrifugation, with the immersion for 2 h, overnight, and 2 h in acetone between each washing step to allow complete solvent exchange. Thereafter, the resulting solid was dried in a vacuum oven at 80 oC for overnight. A dark green solid was obtained and was designated as “GQD-PCN-222”. Preparation of pellets and thin films. The pellets of dry GQD, PCN-222, and GQD-PCN-222 were prepared according to the following procedure for measuring their electrical conductivity; similar approaches have been used to prepare the pellets of mesoporous Zr-MOFs


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previously.25-26 Around 15 mg of the solid sample was packed into a 7-mm die set and wetted with 40 μL of acetone. The wetted solid was pressed into a pellet by a mini pellet press (Specac) with a load of 0.5 ton. The obtained pellet was dried in a vacuum oven at 80 oC for overnight before the conductivity measurements. The thickness of the obtained pellet was measured by a digimatic caliper (Mitutoyo) after the conductivity measurements and was found to be in the range of 0.1-0.3 mm. For electrochemical measurements, the GQD, PCN-222, and GQD-PCN-222 were deposited on fluorine doped tin oxide (FTO) conducting glass substrates as thin films by drop casting.55 A FTO substrate (7 Ω/sq.) was washed by sonicating in soapy water, ethanol, and acetone for 5 min, sequentially. After drying the substrate with nitrogen flow, an insulating polyamide tape was used to obtain an exposed FTO area of 0.25 cm2 (0.5 cm × 0.5 cm). For preparing PCN-222 thin films, and GQD-PCN-222 thin films, 6 mg of the solid sample was dispersed in 0.5 mL of acetone by sonicating for 10 min, and 6 μL of the obtained suspension was drop-casted on the exposed FTO area. The drop-casting process was conducted five times successively to improve the film coverage; the thin film for electrochemical measurements was thus obtained. The GQD solution with a concentration of 10 mg/mL was directly used for drop-casting to prepare the GQD thin films. The drop-casting procedure for preparing GQD thin films is the same as that for PCN-222 thin films and GQDPCN-222 thin films except the following two changes: (1) the drop-casting process was conducted six times successively in order to get the same mass loading, and (2) drying in an oven at 80 oC was performed between each drop-casting process. Instrumentations. Scanning electron microscopic (SEM) images were collected by using SEM SU-8010 (Hitachi). UV-visible (UV-vis) spectra were collected by using a Halo RB-10 (Dynamica). Powder X-ray diffraction (PXRD) data was collected on beamline TPS09A at the Taiwan Photon Source (TPS) of the National Synchrotron Radiation Research Center. The wavelength of the X-ray source is 0.826569 Å. Nitrogen adsorption-desorption isotherms were measured by ASAP 2020 (Micromeritics). Thermogravimetric analysis (TGA) was performed on a TGA 4000 (Perkin Elmer) with a nitrogen flow at 19.8 mL/min and a ramping rate of 10 oC/min. Before temperature ramping, the sample was held at 120 oC for 2 h in order to remove all physically adsorbed water from the sample. Raman measurements were carried out at room temperature with a JASCO 5100 spectrometer (laser excitation wavelength: 32 nm). Thin films of materials were deposited on Si wafers by drop-casting and dried in ambient conditions for 24 hr. The laser power was maintained at 0.1 mW to avoid heat generation by the laser. Silicon wafer was used to calibrate Raman shifts using the 520 cm-1 peak. Photoluminescence (PL) spectroscopy measurements were performed at room temperature on the liquid as-produced GQD dispersion. Excitation and emission spectra were obtained by a commercial spectrometer (Horiba Jobin Yvon Nanolog-3

spectrofluorometer) equipped with an InGaAs NIR detector and a 20 nm bandpass for both emission and excitation. The PL spectra were scaled according to the measured excitation power. Transmission electron microscopy (TEM) was performed using a cold-field emission Cs-corrected TEM (JEOL ARM-200F, Japan) with an accelerating voltage of 200 kV. TEM samples were prepared from solution dry-casting of the colloidal solution on carbon-coated copper grids (300 mesh, Ted Pella Inc.). All electrochemical and current-voltage (I-V) measurements were performed on a CHI6273E electrochemical workstation (CH Instruments Inc.). For IV measurements, the dried pellet was removed from the vacuum oven and immediately sandwiched between two clean FTO substrates, and the I-V curve of the pellet was collected with a two-electrode setup. For electrochemical measurements, a three-electrode cell was used, with a platinum wire and a Ag/AgCl/NaCl (3 M) (BASi®) as the counter electrode and reference electrode, respectively. The FTO substrate modified with the thin film with an area of 0.25 cm2 was used as the working electrode. For nitrite sensing experiments, aqueous solutions containing 0.1 M NaCl were used as the electrolytes. Amperometric sensing experiments were conducted with a stirring speed of 150 rpm. Electrochemical impedance spectroscopy (EIS) was performed with a PGSTAT204 potentiostat/galvanostat (Autolab, Eco-Chemie, The Netherlands), equipped with a FRA2 module. The same three-electrode setup was used for EIS. Synchrotron X-ray diffraction measurements of GQDPCN-222 and PCN-222 were performed at 02B2 beamline of SPring-8 (λ = 0.7997 Å) with the step size of 0.006°. To determine the lattice parameter of GQD-PCN-222, the Pawley profile fitting was conducted using Topas3 software (Bruker AXS), which gave the parameter of P6/mmm, a = 42.191 Å, c = 16.997 Å with. The lattice parameter of PCN222 (P6/mmm, a = 41.968 Å, c = 17.143 Å) was used as the starting of the fitting. Using the obtained lattice parameter, the empty cage structure based on the singlecrystal analysis result of PCN-222 was constructed as the initial structure mode. Then, a different Fourier map was calculated based on the Rietveld analysis of the GQD-PCN222, which suggests extra electron density within the hexagonal channels of the structure. The Rietan-FP and VESTA softwares were used for the calculation.

RESULTS AND DISCUSSION TEM and high-resolution TEM images of the obtained GQD are shown in Figure 2(a) and Figure 2(b), respectively. It can be observed that the GQD sample is composed of spherical particles with an average size of 3.1 nm (Figure 2(c)), and the d-spacing between the graphene layers can be clearly observed in Figure 2(b). The photoluminescence of GQD was investigated by collecting the emission spectra of the sample with the excitation at various wavelengths. As shown in Figure 2(d), a strong emission centered at 420 nm can be observed when the GQD sample is excited at 310 nm. The band positions of GQD were then estimated by utilizing UV-vis spectra and electrochemical method, and


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the results were compared with those of the TCPP linkers presented in PCN-222 (see Figure S1, Table S1, and corresponding details in Supporting Information). As the GQD possess a wide band gap of 4.14 eV with a lowest unoccupied molecular orbital (LUMO) that is more negative than the LUMO of TCPP, the donor-acceptor charge transfer from GQD to TCPP should be feasible. Thus, we reasoned that by utilizing direct impregnation followed by successive washing, these GQD should be firmly immobilized in the hexagonal pore of PCN-222 by means of the donor-acceptor interaction between the GQD and the surrounding TCPP linkers.

Figure 2. (a) TEM image of GQD. (b) High-resolution TEM image of GQD, with the inset showing the d-spacing between graphene layers. (c) Size distribution of GQD estimated from TEM images. (d) Photoluminescence excitation-emissionintensity spectra of GQD.

The photo of PCN-222 and GQD-PCN-222 samples is shown in Figure S2. It can be observed that the color of the sample turns green after the incorporation of GQD, which implies that the TCPP linkers presented in GQD-PCN-222 are protonated, even after solvent exchange and drying.56 SEM and TEM images of PCN-222 and GQD-PCN-222 were then collected to investigate the change in microscopic morphology after the incorporation of GQD in PCN-222. As revealed in Figure S3(a) and Figure S3(b), both samples before and after the incorporation of GQD are composed of rod-like microcrystals with the width of around 1-2 μm; this morphology is consistent with that of PCN-222 reported previously.53 In addition, as revealed in the TEM images of GQD-PCN-222 shown in Figure S3(c) and Figure S3(d), no obvious quantum dots or nanoparticles can be observed on the external surface of the microcrystals in GQD-PCN-222. Figure 3(a) shows the PXRD patterns of PCN-222 and GQD-PCN-222, which suggest that the framework is still well crystalline after the incorporation of GQD. As

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revealed in its nitrogen adsorption-desorption isotherm in Figure 3(b), PCN-222 possesses mesoporous characteristics and a Brunauer–Emmett–Teller (BET) surface area of 2,420 m2/g, which agree with the features of free-base PCN-222 reported before.50 After the incorporation of GQD into PCN-222, the BET surface area decreases to 1,010 m2/g, but the resulting material is still mesoporous. However, the dry GQD sample was found to be nonporous with a BET surface area of around 10 m2/g. The density functional theory (DFT) pore size distributions of these materials were calculated from their isotherms, and the result is shown in Figure 3(c). The pore size distribution of PCN222 shows three peaks centered at 3.2 nm, 1.5 nm, and 1.2 nm, respectively, which agrees well with that of free-base PCN-222 reported before.50 The pore volumes in both mesoporous and microporous regions were found to decrease significantly after incorporating GQD into PCN222. In addition, it should be noted that a remarkable reduction in pore size in the mesoporous region from 3.2 nm to 2.7 nm can be observed after the incorporation of GQD into PCN-222, but such a reduction in pore size does not occur in the microporous region; this observation indicates that the quantum dots are presented in the mesoporous channels of the framework in GQD-PCN-222 rather than adsorbed on the external surface of MOF crystals. Raman spectroscopy was utilized to determine the presence of GQD in GQD-PCN-222, and the obtained spectra are shown in Figure 3(d). The pristine GQD sample shows two broad peaks centered at 1,345 and 1,595 cm-1 in its Raman spectrum, which correspond to the D band and G band of GQD, respectively.42 The spectrum of PCN-222 reveals multiple peaks at 1,238, 1,325, 1,358, 1,448, 1,492, and 1,553 cm-1; such characteristics are generally consistent with the reported Raman spectra of PCN-222.57-58 It should be noticed that besides the peaks from PCN-222 itself, the Raman spectrum of GQD-PCN-222 shows an obvious peak at 1,600 cm-1, which may be attributed to the G band of GQD presented in the sample; this observation indicates the presence of GQD within GQD-PCN-222.

Figure 3. (a) PXRD patterns of GQD-PCN-222 and PCN-222; the simulated pattern of PCN-222 is also shown. (b) Nitrogen adsorption-desorption isotherms (c) DFT pore size distributions, and (d) Raman spectra of GQD-PCN-222, PCN-


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222, and GQD. The calculated BET surface area is also shown in (b).

TGA was conducted in order to evaluate the loading of GQD in GQD-PCN-222. As revealed in Figure S4 and detailed discussion in Supporting Information, the mass loading of GQD within GQD-PCN-222 is around 5.3%. Synchrotron-based experiments were then performed to probe the spatial distribution of GQD within the framework structure of GQD-PCN-222. As shown in Figure S5, the obtained difference Fourier map shows that there is extra electron density appearing around the nodes and within the hexagonal channels after the installation of GQD, which implies that the guest species were incorporated in the entire framework homogeneously instead of being presented only on the surface or both ends of crystals.59 Emission spectra of GQD-PCN-222, PCN-222, and GQD measured with an excitation at 310 nm are shown in Figure 4. The GQD sample shows a strong and broad emission centered at 420 nm, agreeing with the data shown in Figure 2(d). PCN-222 reveals three emission peaks at around 410 nm, 500 nm, and 550 nm; such characteristics are consistent with the emission spectra of free-base PCN-222 and free-base porphyrinic linkers reported previously.60 For GQD-PCN-222, both the emission from GQD at 420 nm and the emission from PCN-222 at 410 nm are completely quenched and the emission from PCN-222 at 500 nm is partially quenched, which suggest that there is significant charge transfer from the electron-donating GQD to the electron-withdrawing framework. Electrical conductivity of each material was thereafter measured by utilizing the dry pellet of the sample (see Figure S6 and discussion in details in Supporting Information). In the presence of donor-acceptor charge transfer in the framework, the GQD-PCN-222 pellets achieve an average electrical conductivity of 9 × 10-11 S/cm, which is two orders of magnitude higher than the average conductivity of PCN222 pellets (6 × 10-13 S/cm) and close to that of the GQD pellets (3 × 10-10 S/cm).61

Given the improved electrical conductivity and the high porosity of GQD-PCN-222, we reasoned that GQD-PCN222 should exhibit a better electrocatalytic activity compared to the less conducting PCN-222 and nonporous GQD. As a demonstration, thin films of PCN-222, GQDPCN-222, and GQD deposited on FTO conducting substrates were utilized for electrochemical detection toward nitrite.41 Cyclic voltammetric (CV) curves of these thin films and bare FTO measured in the solutions with and without nitrite are shown in Figure 5(a) (see Figure S7 and Figure S8 for details). The GQD thin film shows an even smaller catalytic current for nitrite oxidation compared to the bare FTO, which implies that GQD is not catalytically active and acting as a blocking layer on the electrode surface. An obviously higher catalytic current with an onset potential of +0.68 V can be observed after adding nitrite for the PCN-222 thin film, which suggests that PCN-222 is electrocatalytically active; this activity is originated from the porphyrinic linkers presented in the framework.41 The GQD-PCN-222 thin film exhibits a much higher catalytic current and a more negative onset potential (+0.62 V) for nitrite oxidation compared to the PCN-222 thin film, which implies that the hybrid GQDPCN-222 material shows a better electrocatalytic activity toward nitrite compared to the pristine MOF. For comparison, thin films of the physically mixed GQD/PCN222 that possess a similar GQD-to-MOF ratio were also prepared by dispersing around 5 wt% of GQD and 95 wt% of PCN-222 in acetone followed by drop casting. As revealed in Figure 5(b) and Figure S7, the GQD/PCN-222 mixture behaves almost the same as the pristine PCN-222, which implies that the enhanced electrocatalytic activity of GQD-PCN-222 is attributed to the close interaction between GQD and TCPP in the entire framework and the improved electrical conductivity. Moreover, CV curves of both PCN-222 and GQD-PCN-222 show linear increase in current signal with increasing concentration of nitrite (Figure S7), which suggests that these materials can be utilized for electrochemical nitrite sensing. EIS was utilized to investigate the electrochemical redox activity of the catalytic TCPP linkers41 presented in PCN222 thin film and GQD-PCN-222 thin film (Figure S9(a)). As the electrolyte is common for both cases, the component for resistance associated with diffusion is not considered in the equivalent circuit; the equivalent circuit is solely used to obtain fitting results in order to gauge the charge transfer resistance (Rct) and series resistance (RS) of the modified electrodes. As estimated from Figure S9(b) and Figure S9(c), the GQD-PCN-222 thin film shows a RS of 63 Ω and a Rct of 30 Ω, which are both smaller than the RS (105 Ω) and Rct (190 Ω) of the PCN-222 thin film. This observation further suggests that the GQD-PCN-222 thin film exhibits a better conductivity and thus a higher redox activity of its TCPP linkers compared to the PCN-222 thin film.

Figure 4. Emission spectra of GQD-PCN-222, PCN-222, and GQD, measured with the excitation at 310 nm.


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from the incorporation of GQD can remarkably improve the electrochemical sensing activity of the Zr-MOF. The GQD-PCN-222 is still well crystalline after soaking in the aqueous solution containing 0.1 M NaCl and 2 mM NaNO2 for 24 h (Figure S12), which suggests that the MOF-based hybrid material is stable in the electrolyte used for nitrite detection; such a robust material stability in chloridebased aqueous electrolytes is similar to that of another ZrMOF reported before.64

Figure 6. Amperometric J-t plot recorded for the GQD-PCN222 thin film during successive droppings of the nitrite in a 0.1 M NaCl aqueous solution. Applied potential: +0.9 V. Inset shows the plot of current density vs. concentration of nitrite along with the calibration curve. Figure 5. (a) CV curves of the PCN-222 thin film, GQD-PCN222 thin film, GQD thin film, and the bare FTO substrate, and (b) CV curves of the GQD-PCN-222 thin film and the thin film of the physically mixed GQD and PCN-222, measured in 0.1 M NaCl aqueous solutions with or without adding nitrite.

Amperometric sensing experiments were performed with the GQD-PCN-222 thin films at an applied potential of +0.9 V.41 The current response was recorded during successive droppings of the nitrite solutions with various concentrations, and the obtained current density-time (J-t) plot is shown in Figure 6 (see Figure S10 for the lowconcentration region). The addition of nitrite was made every 100 s with various increments in the total concentration of nitrite in the solution, as indicated in Figure 6. A limit of detection (LOD; based on the signalto-noise ratio of 3) of 6.4 μM and a wide linear range of 4018,000 μM were achieved by utilizing the GQD-PCN-222 thin films; such sensing performances are much better than those of the pristine PCN-222 thin films, which show a much higher LOD of 50 μM and a similar linear range of 200-20,000 μM (see Figure S11). Compared to the electrochemical nitrite sensors based on another porphyrinic Zr-MOF (MOF-525) reported previously,41, 62 the GQD-PCN-222 thin film exhibits a slightly higher LOD but a much wider linear range extended by more than one order of magnitude.63 As the pristine GQD show negligible catalytic activity for nitrite sensing (Figure 5(a)), the findings suggest that the increased conductivity resulting

CONCLUSIONS In summary, graphene quantum dots (GQDs) with an average size of 3.1 nm have been successfully incorporated into a porphyrinic Zr-MOF, PCN-222, by utilizing direct impregnation. Both crystallinity and morphology of PCN222 can be preserved after incorporating GQD. The obtained GQD-PCN-222 shows a BET surface area of 1,010 m2/g, which is about 42% of the BET surface area of the pristine MOF but much higher than that of the nonporous GQD. Significant donor-acceptor charge transfer from the electron-donating GQD to the porphyrinic linkers presented in the framework can be observed, which results in the enhancement of electrical conductivity from 6 × 1013 S/cm (PCN-222) to 9 × 10-11 S/cm (GQD-PCN-222). The GQD-PCN-222 thin film exhibits a much better sensing activity toward nitrite compared to both the pristine GQD and PCN-222 thin films. An LOD of 6.4 μM and a linear range of 40-18,000 μM can be achieved by utilizing GQDPCN-222, which are much better than those of the pristine PCN-222 (50 μM and 200-20,000 μM). These findings suggest that GQD can be utilized as a new type of guests in the MOF hosts to render the donoracceptor charge transfer, improved electrical conductivity, and thus a better electrochemical performance. Since most previous studies reporting quantum dot-MOF composite materials utilized the obtained materials for optical sensing instead of electrochemical purposes, the findings


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here open up the opportunity for serving such hybrid materials for electrochemical applications. The obtained electrical conductivity of GQD-PCN-222 is still low, perhaps due to the low loading of GQD in the framework and the lack of fully continuous conducting pathways between those mesoporous channels on the a-b plane. However, as the size and band positions of GQD are highly tunable, Zr-MOFs with various pore structures and topologies may be utilized as the hosts for the adsorption of GQD with various sizes in future studies, and the loading of GQD and the resulting conductivity should be highly modulable. With the use of smaller GQD to increase their loading in MOFs and the selection of MOFs that allow to build more continuous conducting pathways, MOFs impregnated with GQD are expected to achieve higher conductivity, and such hybrid materials are desirable for a range of applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Additional experimental results including bandgap estimation, photo of the samples, SEM and TEM images, TGA results, synchrotron-based difference Fourier map, results obtained from conductivity measurements, CV and amperometric curves for nitrite sensing tests, EIS results, and evaluation of material stability

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (C.-W. K.) *Email: [email protected] (W.-H. C.)

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was sponsored by the Ministry of Science and Technology (MOST) of Taiwan, under projects MOST 1072218-E-006-054-MY3 and MOST 107-2628-E-011-002-MY3. We also thank the support from the Yushan Young Scholar Program, under Ministry of Education (MOE), Taiwan. This research was also supported in part by Higher Education Sprout Project, MOE, Taiwan to the Headquarters of University Advancement at National Cheng Kung University. We thank Mr. Chung-Kai Chang for PXRD measurements on beamline TPS09A at the Taiwan Photon Source (TPS) of the National Synchrotron Radiation Research Center.

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