Synthesis of Highly Dispersed CuPd@UiO-66-NH₂ for Nonenzymatic Hydrazine Sensing

ZrCl₄, CuCl₂, PdCl₂ and 2-aminoterephthalic acid (H₂N-BDC), were purchased from Aladdin Chemical Co., Ltd. (Shanghai, China). Acetic acid (HAc), NaBH₄, DMF, n-hexane and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. ultrapure water was used in the experiment.

The product was characterized by a transmission electron microscope (TEM, Talos F200X). The surface element composition of the synthesized sample was characterized by energy dispersive X-ray spectrometer (EDX). The structure and morphology of the samples were marked by X-ray diffraction spectroscopy (XRD, Bruker D8-ADVANCE). Electrochemical measurements were carried out in a conventional three-electrode electroanalysis system controlled by CHI660D electrochemical workstation (Shanghai CH Instrument Co. Ltd., China). A conventional three-electrode cell was used, including a glassy carbon electrode (GCE, geometric area = 0.07 cm²) as the working electrode, a saturated calomel electrode as the reference electrode and platinum foil as the counter electrode. All potentials given in this work were referred to the saturated calomel electrode.

Synthesized UiO-66-NH₂ according to the method reported in literature. ²⁶ (0.107 g, 0.45 mmol)ZrCl₄ and CH₃COOH (750 μl) were added in DMF (17 ml) solution. The mixture was stirred for 10 min, then, added (0.083 g, 0.45 mmol) NH₂-BDC and DMF solution into the mixture solution and stirred continued for 10 min. And then transferred into a 50 ml Teflon-lined autoclave and heated at 120 °C, 24 h. After that, the orange-red solid product was separated from the solution by centrifugation and washed 3 times with DMF and MeOH.

100 mg of activated NH₂-UiO-66(Zr) was suspended in 25 ml of hydrophobic n-hexane. In order to obtain CuPd@UiO-66-NH₂ nanocomposite with 5%wt metal nanoparticles, the resulting suspension was sonicated for 0.5 h and stirred for 1 h, and then the CuCl₂ (0.53 mg, 12.5 μl) and PdCl₂ (0.42 mg, 12.5 μl) (Cu:Pd = 1:1) solution were added dropwise to the above suspension with vigorous stirring.After stirring for 1.5 h, NaBH₄ solution was added and the suspension was stirred for another 1.5 h to ensure a complete reduction of the metal particles inside the pore channels of NH₂-UiO-66(Zr). Centrifugation to obtain a brown-black solid product, which was washed with ethanol and dried under 60 °C.

The glassy carbon electrode (GCE) is prepared by a simple method. Before use, first polish GCE with 0.3 μm alumina powder to remove dirt and scratches, and rinse with distilled water to obtain a mirror-like surface, and then sequentially ultrasonically treat it in distilled water. Then, wipe the GCE dry for later use. The prepared nanocomposite material (1 mg) was ultrasonically dispersed in a chitosan (1 ml, 0.5%) solution for 30 min. Then drop the obtained suspension (6.5 μl) on the GCE indicator and dry it naturally in the air. The prepared electrode is expressed as CuPd@UiO-66-NH₂/GCE. And then a UiO-66-NH₂/GCE was prepared by the same method as the former.

Figure 1A shows the powder XRD spectrum of NH₂-UiO-66(Zr) framework. As shown, The UiO-66-NH₂ crystal shows sharp diffraction peaks in the figure, indicating that its crystallinity is excellent, and the positions of all its characteristic diffraction peaks (curve a) are consistent with the peak positions reported in the literature. ²⁷ After composite metal nanoparticles, the peak position of CuPd@UiO-66-NH₂ is roughly the same as UiO-66-NH₂, but the peak intensity is significantly reduced. ²⁵ From the research of Farzaneh ²⁸ et al., we can know that one possible reason for the decrease of the peak intensity of the XRD diffraction peak is that when depositing CuPd nanoparticles, the hydrophilic Pd²⁺ and Cu²⁺ enter the porous channel of UiO-66-NH₂ under the action of hydrophobic and capillary. However, since metal nanoparticles enter the pore channel, their particle size is larger than that of the cavity channel, which may damage the adjacent cavities and cause small regular changes in the pore structure frame. ²⁹ Another reason may be that high voltage makes the shape of UIO-66-NH₂ irregular when we are doing the mapping. Certain interactions between the amino groups on the MOF and the Pd particles will affect the effect to a certain extent. ³⁰ Curve a shows the successful synthesis of UiO-66-NH₂, an ultra-broad peak emerged near 41.8° for the CuPd@UiO-66-NH₂ (curve b), which located between those of pure Pd (111) crystal plane and Cu (111) crystal plane, suggesting the formation of CuPd alloy rather than the simple mixture of monometallic Cu and Pd. The FTIR spectrum (Fig. 1B) obtained by the KBr tablet method shows that the symmetric and asymmetric N-H tensile vibration peaks of the amino group in UiO-66-NH₂ are located at 3500 cm^-1 and 3426 cm^-1, respectively, and the bending vibration of N-H is 1573 cm^-1,and 1346 cm^-1 is the shear stretching vibration of the C-N structure in arylamine. ³¹ Figure 2d is the chemical composition diagram of CuPd@UiO-66-NH₂ nanocomposite characterized by EDS. It can be seen that Cu and Pd metal nanoparticles successfully entered the pore structure of UiO-66-NH₂, further indicating that the obtained sample was CuPd@UiO-66-NH₂ nanocomposite.

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Figure 2a is the SEM image of NH₂-UiO-66(Zr), which shows the successful synthesis of MOF with a particle size of about 90 nm. From Fig. 2b, it can be seen that there are only very few metal nanoparticles on the outer surface of CuPd@NH₂-UiO-66 (Zr), and these metal nanoparticles do not enter the MOF structure and gather near the outer surface. Figure 2c is an enlarged image of (b), and its structure shows a little irregularity, which may be caused by the entry of bimetallic nanoparticles into the pore size. By the high-angle circular dark-field scanning TEM and energy-dispersive X-ray spectroscopy (EDX) mapping image [Fig. 2d] further proves the formation of CuPd nanoclusters in the UiO-66-NH₂ (Zr) cavity. It can be seen that the CuPd alloy nanoparticles successfully enter the UiO-66-NH₂ channel, and the CuPd alloy nanoparticles are extremely small in size. Generally, due to the high surface energy of nanoclusters, they aggregate during the preparation process, so it is difficult to obtain such small size nanoclusters, however, according to Iglesia ³² and Luo ³³ research team's report, ethylenediamine was used to introduce amino groups in MIL-101-Cr. The amino group stabilizes Pb²⁺ through coordination and enhances the adsorption capacity of MOF to Pb²⁺.The study by Wang ³⁴ et al. also pointed out that the amino group in UiO-66-NH₂ and Ag⁺ enter the MOF channel under the coordination effect to stabilize metal ions. Through the research of the Ning ³⁵ research group, we can also know that there is a certain interaction between the amino group and the palladium nanoparticles, which all indicate that the amino group can stabilize the metal precursor to a certain extent. The pore channel can act as a nanoreactor, limiting the growth of these nanoparticles and promoting the preparation of CuPd nanoclusters in the MOF cavity. This shows that the double-solvent method and different reduction processes can growth of small-sized nanoparticles in the MOF cavity.

The electrochemical characteristics of various modified electrodes were studied. Figure 3A shows that in the presence of (a, b, c) 1.0 mM N₂H₄ and the absence of (a, b, c) 1.0 mM N₂H₄ in 0.1 M PBS (pH = 7.4) bare/GCE, UiO-66-NH₂/GCE and CuPd@UiO-66-NH₂/GCE Cyclic Voltammetry (CV), at this time the rate is 100 mVs⁻¹. Figure 3A shows that in the presence of (a, b, c) 1.0 mM N₂H₄ and the absence of (a', b', c') 1.0 mM N₂H₄ in 0.1 M PBS (pH = 7.4) solution, the Cyclic Voltammetry (CV) of bare/GCE, UiO-66-NH₂/GCE and CuPd@UiO-66-NH₂/GCE, at this time with a rate of 100 mVs⁻¹. It can be easily seen that the bare GCE (a'), UiO-66-NH₂/GCE (b') and CuPd@UiO-66-NH₂/GCE (c') in the absence of N₂H₄ have almost no electrochemical response. When added 1.0 mM N₂H₄ in solution, no obvious oxidation peak appeared at bare/GCE (a), which indicated that the oxidation kinetics of N₂H₄ on the bare/GCE surface was very slowly. The sluggish reaction increased oxidation over potential. UiO-66-NH₂/GCE (b) shows a lower current response than bare/GCE due to the poor conductivity of the metal organic framework. And we can obvious seen that CuPd@UiO-66-NH₂/GCE (c) showed a higher oxidation peak response at 0.6 V, the intensity is about 45 μA, indicating that CuPd@UiO-66-NH₂ has excellent electrocatalytic activity for N₂H₄. The reaction process of N₂H₄ on the electrode surface may be as follows: ³⁶

$$\begin{eqnarray*}&&{{\rm{N}}}_{{\rm{2}}}{{\rm{H}}}_{{\rm{4}}}+{{\rm{H}}}_{{\rm{2}}}{\rm{O}}\to {{\rm{N}}}_{{\rm{2}}}{{\rm{H}}}_{{\rm{3}}}+{{\rm{H}}}_{{\rm{3}}}{{\rm{O}}}^{+}+{{\rm{e}}}^{-}\end{eqnarray*}$$

$$\begin{eqnarray*}&&{{\rm{N}}}_{{\rm{2}}}{{\rm{H}}}_{{\rm{3}}}+{{\rm{3H}}}_{{\rm{2}}}{\rm{O}}\to {{\rm{N}}}_{{\rm{2}}}+{{\rm{3H}}}_{{\rm{3}}}{{\rm{O}}}^{+}+{{\rm{3e}}}^{-}\end{eqnarray*}$$

And which should be attributed to the synergistic effect of Cu and Pd. ³⁷ Figure 3B is the electrochemical sensing experiment of adding Pd@UiO-66-NH₂/GCE and Cu@UiO-66-NH2/GCE to 0.1 M PBS (pH = 7.4) solution containing 1.0 mM N₂H₄. It is found that copper and palladium nanometer under the coexistence of particles, CuPd@UiO-66-NH₂/GCE has higher catalytic activity at lower potential, indicating that there is a synergistic effect between Cu and Pd.

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Figure 3C studied the electrocatalytic activity of CuPd@UiO-66-NH₂/GCE on N₂H₄. After adding N₂H₄ to 0.10 M PBS (Ph = 7.4), it can be seen, with increasing concentration of N₂H₄, a significant increase in the oxidation current, the oxidation peak potential increased correspondingly. Figure 4A studied the effect of scanning rate on CuPd@UiO-66-NH₂/GCE electrocatalytic oxidation of 1.0 mM N₂H₄. As the scan rate increases in the range of 10∼200 mVs⁻¹, the peak current also increases with a certain rule, and the peak potential moves in the positive direction. In addition, when plotting the relationship between the peak current and the square root of the scan rate, good linearity was obtained, which indicates that the oxidation of N₂H₄ is a diffusion-controlled process [Fig. 4B].

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The sensing characteristics of CuPd@UiO-66-NH₂/GCE were further studied by current-time electrochemical technology. Figure 5A shows the ampere response of CuPd@UiO-66-NH₂/GCE when a 0.1 M PBS (pH = 7.4) solution containing 0.01 mM N₂H₄ is continuously stirred under a gradient potential. As shown in Fig. 5A, the current response increases as the operating voltage increases, the highest current response occurs at 0.65 V. However, when the working voltage is 0.65 V, the interfering substance is activated and generates a lot of noise. Therefore, in order to ensure excellent current response, it is beneficial to choose 0.6 V with low noise and less interference as the working potential. Figure 5B records the ampere response of CuPd@ UiO-66-NH₂/GCE when N₂H₄ is continuously added to continuously stirred 0.1 M PBS (pH = 7.4) under the working potential of 0.6 V, which shows that after adding N₂H₄ the response increases rapidly. When N₂H₄ diffuses from the solution surface to the electrode/solution interface, the current response increases rapidly, and N₂H₄ will rapidly oxidized. However, when a high concentration of N₂H₄ is added, the response time is longer and the current response is slower. This may be the electrode surface cannot provide enough space for the high concentration of N2H4 reaction. Figure 5C shows the linear correlation between the current response and the N₂H₄ concentration, thereby obtaining a wide linear range from 0.25 μM to 1.38875 mM. The linear regression equation can be expressed as Ip (μA) = 0.544 + 27.206 C (mM). When using the formula LOD = 3S_B/N to calculate, the lowest detection limit obtained is 0.08 μM (where S_B represents the standard deviation of the blank solution and b represents the slope of the calibration curve), when the slope of the standard curve is 27.206, and the GCE surface area is 0.07cm², the sensitivity of the sensor is estimated to be 386.7 μA·mM^-1 ·cm^-2. Table I is the this sensor compared with other electrocatalytic N₂H₄ sensors,CuPd@UiO-66-NH₂/GCE shows excellent N₂H₄ detection performance. Owing to the three-dimensional porous structure of UiO-66-NH₂ provides a large number of N₂H₄ reactive sites for CuPd alloy and the synergy of copper and palladium, which is beneficial to enhance the current response. The detection limit of this sensor is low and the sensitivity is high.

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As shown in Fig. 6A, the influence of some common interfering substances was studied. The addition of N₂H₄ leads to an enhanced current response, which indicates that CuPd@UiO-66-NH₂/GCE has electrocatalytic activity for N₂H₄ oxidation. However, the current response does not change much after the addition of 10 times the concentration of inorganic interferences, and the current response is extremely low when the 2 times the concentration of organic interferences AA and UA are added, which means that these interfering substances have almost no effect on the detection of N₂H₄. Figure 6B shows that the initial current response of CuPd@UiO-66-NH₂/GCE at 4000 s basically maintains a straight line, indicating acceptable stability, and Fig. 6C shows that five CuPd@UiO-66-NH₂/GCE prepared to detect 0.01 mM N₂H₄, and the relative standard deviation (RSD) obtained was 4.0%.

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