Electrochemical Detection of Sarcosine and Supercapacitor Based on a New Ni–Metal Organic Framework Electrode Material

: A new Ni metal organic framework based on 2,2 ′ -Biphenyldicarboxylic, 4,4 ′ - bipyridine as linker is prepared by hydrothermal reaction and directly used as an electrode material for supercapacitor and the detection of sarcosine. [Ni 3 (BIPY) 3 (BPDA) 2 (HCOO) 2 (H 2 O) 2 ] n ( Ni-1 ; BIPY = 4,4 ′ -bipyridine; BPDA = 2,2 ′ -Biphenyldicarboxylate) displays the specific capacitance of the Ni-1 are 667 F/gat 1 A/g and retention is 82% of initial capacitance at 1 A/g. The excellent electrochemical property is ascribed to the intrinsic nature of Ni-1 . Furthermore, the sarcosine sensing performance of the Ni-1 electrode is evaluated in 0.1 M of NaOH solution and the electrode showed a wider range of linear response 1 × 10 − 4 M to 1 × 10 − 3 M. Thus, the results show that the Ni-1 is a potential candidate for not only sensing of sarcosine but also supercapacitor application.


Introduction
Potential threatening energy crisis and environmental pollution force scientists and engineers to develop clean and very efficient energy storage technologies to overcome these issues. Supercapacitors are among such novel energy solutions because they demonstrate high-power density, long-termstability, and fast charging and discharging when needed [1][2][3][4][5][6]. Pseudocapacitance is stored by the rapid Faraday redox reactions occurring on the active material surfaces. Pseudocapacitive materials have higher specific capacitances than carbon materials (EDLCs). For the purposes, MOFs have been demonstrated to be useful for electrochemical energy storage and are considered as the most promising electrode material candidatefor SCs due to their high surface area, tunable pore size, controllable pore structure, and special structures with potentialpseudo-capacitive redox centers [7][8][9][10].
Panprostate cancer (PCa), the most common malignant tumor diagnosed in males, if diagnosed early, could be successfully treated. Early diagnosis of PCA also significantly reduced the mortality rate [8]. Sarcosine, is considered as a differential metabolite, since sarcosine is increased during PCa progression to metastasis sarcosine and can be detected non-invasively in urine. Recently, many papers have reported sarcosine sensing [11][12][13][14][15]. Sarcosine detection techniques include high-performance liquid chromatography-mass spectrometry (HPLC-MS) [16,17] and gas chromatography. However, these methods are very specialized, expensive, and time-consuming. Therefore, new, very sensitive, specific, fast, and inexpensive alternative methods are still desired.
Herein, a new Ni metal organic framework was synthesized and directly used as an electrode material for super capacitor and electrochemical sensing for sarcosine. The [Ni3(BIPY)3(BPDA)2(HCOO)2(H2O)2]n (Ni-1; BIPY = 4,4′-bipyridine; BPDA = 2,2′-Biphenyldicarboxylate, CCDC number 1935262) exhibited a high specific capacitance, and long-term cycling stability. The Ni-1 exhibited good sensitivity and a wider range of linear response 1 × 10 −4 M to 1 × 10 −3 M for the determination of sarcosine. This is the first time that a new metal organic framework is served as supercapacitor electrode and sensor.

Synthesis of Ni-1
Firstly, 2,2′-Biphenyldicarboxylic Acid and 4,4′-bipyridine were dissolved in DMA to form 1 mol/L DMA solution, and nickel nitrate hexahydrate was dissolved in water to form 1 mol/L aqueous solution, The above solution 200 μL:300 μL:300 μL and 8 mL H2O was added to a 25 mL Teflon-lined stainless autoclave, sealed and heated at 160 °C for 72 h, then cooled to room temperature. The green block crystals of Ni-1 were obtained (yield: 75% based on Ni(II))

Single-Crystal Structure Determination
Crystal of Ni-1 was collected from the mother liquor. Single-crystal data of Ni-1 were collected on a Rigaku Oxford CCD diffractometer equipped with graphite-monochromatic Mo-Kα radiation (λ = 0.71073 Å) at 293 K. The structure was solved by direct methods, and refined by full-matrix least-square method with the OLEX-2 program package. The crystallographic data and refinements and the selected bond lengths and angles for Ni-1 are listed in Tables 1 and 2.

Fabrication of Working Electrode
80% of Ni-1, 10% of acetylene black and 10% of PVDF were mixed in N-methyl pyrrolidone (NMP) until a homogeneous slurry was obtained, which was then applied to Ni foam and dried at 60 °C in the air. Electrochemical sensing tests were performed using the glassy carbon electrode (GCE, model CHI104, 3 mm in diameter). Prior to the tests, it was polished with 0.05 μm alumina, followed by ultrasonicated in 50% HNO3, absolute ethanol and distilled water mixture for 30 at each step. Two milligrams of MOF was dispersed in 1 mL of DMF ultrasonically. Five microliters of this suspension was dropped onto the GCE surface to obtain a Ni-1 based working electrode (NHCPs-GCE).

Electrochemical Measurements
All the electrochemical measurements were conducted on an electrochemical workstation (CHI760E). In a three-electrode system, the Ni-1 was used as the working electrode, with Pt plate as the counter electrode, Hg/HgO as the reference electrode, and 6 M KOH as the electrolyte, respectively.

Structure Description
The Compound Ni-1 shows two-dimensional structure and belongs to C2 space group. The asymmetric unit contains one and a half nickel (II) centers, one bridging BPDC 2− group, one and a half BIPY, one formic acid group, and one water molecule. As shown in Figure 1, the Ni 2+ are six-coordinated by four O atoms and two N atoms. In coordination mode of Ni(1), N1, and N2 come from two bipyridine molecules, O1 is from biphenyl dicarboxylic acid molecules, O6 and O6d are from two formic acid molecules, and O5 is from water molecules. Formic acid comes from the decomposition of solvent molecules. In coordination mode of Ni(2), N3 and N4a are from two bipyridine molecules, O2, O3, O2c, and O3c come from two BPDC 2− group (Figure 1). As a bridging ligand, two N atoms in pyridine participate in the coordination, and only three oxygen atoms of BPDC 2− group are involved in the coordination. It is interesting that in this structure, the conformation of the bipyridine molecule in which N1 is located is quite different from that of N3 ( Figure 2). The bipyridine molecule in which N3 is located has a symmetry plane, and the two planes of N3 and N4 form a certain angle of 38.60(9)°, which is close to the normal bipyridine molecular structure, while the bipyridine molecule of N1is relatively deformed (without plane of symmetry), N1 and N2 the two planes rotate to the same plane through the δ bond between C17 and C18 (the angle is 14.89°). Although this conformation is not a stable conformation of the bipyridine molecule, the structure δ-bond rotation precisely enhances the relationship between pyridine rings π-π accumulation. In this two-dimensional structure, the bipyridine molecule where N1 resides forms a straight chain with Ni1 via a coordination bond. Similarly, the bipyridine molecule in which N2 resides forms another straight chain with Ni2 via a coordination bond. The Ni(1) chain and the other Ni1 chains are connected by O6 atom in two coordinated formic acid molecules. After connecting with Ni1 chain, Ni1 is coordinated with Ni2 chain via O atom of Biphenyldicarboxylic Acid Molecule. Finally, a two-dimensional layered structure is formed by O-coordination and π-π stacking ( Figure 3)

Purity and Thermal Stability
In order to explore the properties of Ni-1, the purity and thermal stability of the material were investigated. Figure 4a shows the PXRD patterns of the Ni-1, The PXRD patterns of Ni-1showed the diffraction peaks corresponding to the simulated Ni-1 pattern. FT-IR spectrum of Ni-1 showed bands at 1610 and 1493 cm −1 , which correspond to bending and stretching of C = O (Figure 4b), while peaks at 1305 and 1542 cm −1 were ascribed to the pyridine bending. The Ni-1 thermal stability was analyzed by the thermogravimetric analysis (TGA) performed in the 50-800 °C range under N2 (see Figure 4c). The first weight-loss stage occurred at 100-350 °C due to the loss of weakly bonded water and DMA solvent molecule. The second weight-loss step occurred at 350-400 °C due to the decomposition of the organic ligand in the MOF.

Investigation of Electrochemical Properties
CV curves of the Ni-1 supercapacitor electrode were recorded at 10-100 mV/s scan rates at 0-0.6 V showed faradaic redox peaks (see Figure 5a), which is indicative of the Faradaic behavior. As the scan rate was increased, no changes in the redox peaks were detected, which indicates excellent rate capability of the Ni-1 based electrode. The GCD test showed that the specific capacitances of the Ni-1 based electrodes were equal to 667, 334, 300, 267, and 240 F/g at 1, 2, 5, 8, and 10 A/g current densities, respectively (see Figure 5b). EIS tests showed straight curves in the low-frequency region (see Figure 5c), which indicates very low diffusion resistance and fast ion diffusion of electrolyte in the Ni-1based electrode. Such excellent electrochemical properties are very beneficial for excellent capacitive performance. Analysis of the EIS curve in the high-frequency region revealed low resistance (Rs), judging by the small intercept of the arc with the real axis. Thus, the Ni-1based electrode possessed low intrinsic and ionic resistances as well as excellent contact resistance with the current collector. The retention rate is up to 82% after 5000 cycles at a current density of 1 A/g, which indicates good cycling performance and stability. The capacitance in comparison with those reported ones is depicted in Table 3.

Selectivity and Stability of the Sensor
The selectivity of the Ni-1/GCE-based electrode was tested under the presence of 1.0 mM species (Streptomycin, Erythromycin, Norfloxacin, Tetracycline, Tryptophan) with 0.1 M KOH as an electrolyte. The presence of the compounds did not affect the performance of our electrode (see Figure 6b), which indicates excellent anti-interference properties of our Ni-1/GCE.

Conclusions
We developed a novel Ni 2+ -containing metal-organic framework (Ni-MOF) material, which was then used as an active material for sarcosine sensing and for a high-performing supercapacitor (with excellent capacitance 667 F/g at 1 A/g and long-term cycling stability 82% after 5000 cycles). In fact, the sensor was very sensitive to sarcosine (with a low detection limit), and its response was linear relative to the increasing sarcosine contractions. Sarcosine was even detected when other interfering substances (such as Streptomycin, Erythromycin, Norfloxacin, Tetracycline, Tryptophan) were present. Our results strongly indicate that the Ni-MOFs are aspiring materials for next-generation supercapacitors and sensors.