Synthesis and Characterization of ZIF 67 Manganese Bimetal for Electrochemical Sensor Application

The field of sensor applications has witnessed substantial growth in diverse domains, including agriculture, food, oil, environment, and medicine. Among the varied methodologies employed, electrochemical methods stand out. The judicious selection of appropriate materials in this context significantly augments sensor efficiency. Zeolitic Imidazolate Framework 67 (ZIF 67), a highly porous material with an expansive surface area, offers facile synthesis, yet is impeded by limited conductivity. The introduction of additional metal derivatives has been reported to enhance its conductivity, with manganese, a transition metal, identified for its potential conductivity improvement and its influence on particle size. Thus, this paper aims to comprehensively explore the properties of manganese-modified ZIF 67, spanning structural, morphological, and electrochemical properties. Research findings indicate that manganese doping enhances crystallinity, as evident from X-ray diffraction analysis, while also impacting particle size (from x¯ 514.4 nm to 944 nm), as assessed through SEM measurements. Furthermore, electrochemical performance reveals heightened peak current during redox processes reaching ±64 µA, indicative of improved conductivity from the ZIF 67 (±13 µA) and the bare (±43 µA). In light of these outcomes, this material emerges as a promising candidate for electrochemical sensor development.


Introduction
In various fields, sensor technologies have enhanced human life.Sensors detect changes and collect signals for appropriate responses.They function with light, temperature, motion, and pressure.Modern sensors have revolutionized everyday life, healthcare, fitness, and manufacturing.Electrochemical sensor is the method chosen in the development of the sensors that are based on the redox reaction of the target analyte that occurs on the modified electrode material due to cheap and easy modification [1].Choosing the right electrode is crucial for effective analysis.Recent advancements are closely associated with sensors and biosensors.This approach is extensively applied in agriculture, food, oil, environment, and medicine [2].
Metal-Organic Frameworks (MOFs) are porous material that consists of metal ions and organic linkers (ligands) interacting with each other, forming a crystalline polymer structure of MOFs [3].This material is regarded as a promising electrode material candidate for modification due to its high surface area, porous crystal structure, elevated porosity, redox metal active site, and modifiability flexibility.The high surface area facilitates material modification such as conjugation, immobilization, chemical modification, and similar processes [4].Meanwhile, the material's pores aid in facilitating electron transfer during ongoing redox reactions, enhancing its physicochemical characteristics [5].The adaptability of MOFs in their synthesis process presents the potential for modification as an alternative material for electrochemical sensor applications [6].
ZIF 67 represents a type of MOF material with cobalt as the central atom and 2-methylimidazole as the ligand.This material exhibits relatively safe stability and ease of synthesis.While MOFs excel in terms of high surface area and abundant porosity, ZIF 67 encounters limitations in its conductivity.To address this, numerous studies have reported combining two metals within the ZIF 67 framework to enhance the material's conductivity while maintaining the MOFs' structural integrity for subsequent application processes.Cobalt is often paired with other transition metals such as nickel, resulting in a bimetal ZIF 67.Nazir et al. (2021) combined cobalt with nickel, cobalt with manganese, and cobalt with iron, as materials for metal detection [7].It is interesting to note that manganese has been suggested as a potential material that assists in the transfer of electrons in electrochemical mechanisms.Due to their cost-effective and broad voltage window [8].According to Xu et al. (2019), manganese-MOF was developed and indicated good electrical conductivity and reversibility in supercapacitor applications [9].
Cobalt and manganese were discovered together by Kazemi et.al (2018), resulting in manganese cobalt-MOF.The performance of its cyclic voltammogram demonstrates that manganese cobalt-MOF exhibits superior electrical conductivity compared to manganese-MOF itself.In addition, the manganese cobalt-MOF exhibits a sizable surface area and a well-balanced pore size distribution [10].Another study reported that the addition of manganese to silver causes a face-centered cubic crystal form, as shown by X-ray diffraction examination.With higher manganese concentrations, crystal size increases.Studies on particle size and surface area have been conducted on manganese because of its complicated crystal structure [11].Although several combinations of transition metals have been thoroughly investigated, manganese is still rarely used.ZIF 67 manganese has not yet been combined for use in electrochemical sensor applications.
Research concerning ZIF 67 continues to evolve, both by incorporating additional metal materials to enhance material conformation and electrical characteristics, as well as by physically modifying the material itself.This study presents the structural and electrochemical properties resulting from modifications of ZIF 67 doped with manganese as an additional transition ion.This provides a perspective for further research in the realm of electrochemical-based sensor applications.

Synthesis of ZIF 67, ZIF manganese and ZIF 67 Manganese
The ZIF 67 material was synthesized using the coprecipitation method by mixing cobalt nitrate hexahydrate as the metal ion (Sigma Aldrich) and 2-methylimidazole (2-Meim) (Sigma Aldrich) as the ligand.Each component was dissolved in a 30 mL methanol solution.Each solution was homogenized for one hour, and then the metal ion solution was combined with the 2-Meim solution.The mixture was incubated for 24 hours to allow material precipitation.After incubation, the solution was washed repeatedly until a neutral pH was achieved and then centrifuged to obtain the precipitate.The resulting precipitate was subsequently dried at 60-70°C.For the synthesis of ZIF manganese and ZIF 67 manganese, the same method was employed, with the addition of manganese to the metal ion solution followed by the precipitation and washing processes.

Morphology and electrochemical characterization
For the characterization of crystallinity and morphology, X-ray Diffractometer Bruker D8 Advanced and Scanning Electron Microscopy (SEM) using the Quanta 650 by Thermo Fisher Scientific were employed.Electrochemical characterization was carried out through cyclic voltammetry measurements by depositing the material onto a screen-printed carbon electrode consisting of counter, working, and reference electrodes.A 10 µL drop of the material was applied to the working electrode and incubated overnight.Subsequently, cyclic voltammetry measurements were conducted at a scan rate of 20 mV/s using the electrolyte K3Fe(CN)6 (5 mM) and KCl (0.1 M) in a 0.01 M PBS (pH = 7.4) solution.The setup was connected to a potentiostat (PalmSens) and a computer device.
However, peak intensities are compared, and it can be seen that manganese incorporation slightly improves ZIF 67's crystallinity.This may indicate an improved ordering of the crystal lattice, as noted by Sahin and Kaya (2016) [13].Importantly, ZIF 67 manganese remains pure, as evidenced by the absence of supplementary peaks originating from other phases like the peak of the ZIF manganese.The pink box inset in figure 1 shows that the ZIF manganese has 4 strong peaks that are not found in the modified ZIF 67_manganese.Additionally, the absence of any discernible 2θ shifts in the peak positions strongly supports the conclusion that manganese ions have effectively substituted some of the cobalt ions in the lattice.Notably, these XRD peak positions align satisfactorily with the established XRD pattern from earlier investigations, as documented by Qin et al. (2017) [14].

Figure 1. XRD pattern of ZIF manganese, ZIF 67, and ZIF 67 manganese
The morphology of ZIF 67 manganese is compared with the ZIF 67 that was inspected via SEM (See figure 2), which reveals a rhombic dodecahedral shape in agreement with prior literature [15].Upon closer examination, it becomes evident that the introduction of manganese has induced alterations in the particle size of ZIF 67 (See in figure 2 (a) and (b)).Notably, the average particle size of pure ZIF 67 measures approximately 514.4 nm, while the ZIF 67 manganese results in an increased average particle size of about 944 nm.It is relevant to note that these dimensions fall within the particle size range characteristic of ZIF 67, which spans from 228 nm to 5.2 μm, as previously documented by Qian et al. (2012) [16].It is reasonable to relate these fascinating phenomena to a protracted nucleation phase, which leads to the production of bigger particle sizes.This increase in size can be attributed to the growth process using excess precursor materials.This hypothesis finds support in the investigations conducted by Lan et    By performing a cyclic voltammetric scan in the potential range of -0.5 V -1 V, the electrochemical properties of ZIF 67 and ZIF 67 manganese were evaluated.Figure 3 illustrates the corresponding voltammogram, effectively showcasing the contrast performance between the two materials.Specifically, the modified ZIF 67 manganese/SPCE exhibits a superior electrochemical response with a higher peak when compared to the unmodified ZIF 67 in scan rate 20 mV/s, as evidenced by its distinct oxidation-reduction outcomes between material and K3Fe(CN)6 (See in figure 3 (a)).Figure 3 (b) demonstrates that the cyclic voltammetric peak increases with the increasing scan rate.Drawing insights from the cyclic voltammetric scan outcomes, a resounding conclusion emerges: ZIF 67 manganese possesses elevated electrochemical properties characterized by heightened conductivity.The modified ZIF 67 manganese reached a high peak of approximately 64 µA compared to the ZIF 67 (13 µA) and the bare (43 µA).Moreover, ZIF 67 manganese possesses higher conductivity compared to another ZIF 67 bimetallic.ZIF 67/Zn has a peak current of approximately 43 µA [19], while ZIF 67/Ce@NC has a peak current of 37 µA [20].
The occurrence of this reversible oxidation-reduction pair can be attributed to the redox reaction of Mn 2+ , which is supported by the intercalated metal ions in the manganese in the ZIF 67 manganese modification.Hence, this phenomenon could increase the current peak that contributes to the increase of material conductivity material.This augmented conductivity serves as a pivotal attribute in the context of sensor applications, wherein the material's capacity to effectively contribute or accept electrons during redox reactions or electron transfers significantly impacts sensitivity.Consequently, the ZIF 67 manganese material emerges as a promising candidate for utilization in electrochemical sensors.

Conclusion
This study presents an investigation into the properties of ZIF 67 and its modified ZIF 67 manganese.The analysis contains structural, morphological, and electrochemical aspects, revealing ZIF 67 manganese's high crystallinity, unique dodecahedral morphology, and enhanced electrochemical behavior compared to ZIF 67.The successful manganese incorporation is confirmed through distinct Xray diffraction patterns that have no 2θ shifting in the peak positions and have spectrum displays matched to the previous crystallographic planes.Scanning electron microscopy showcases altered particle size and shape.The modified ZIF 67 manganese shows rhombic dodecahedral and increasing particle size from the ZIF 67 ± 514.4 nm to ZIF 67 manganese about 944 nm.Moreover, the modification of manganese to ZIF 67 could increase the conductivity of the material through the electrochemical performance by the height of the redox peak at approximately 64 µA compared to unmodified ZIF 67 which reaches the height of the peak at 13 µA.The electrochemical analysis demonstrates that ZIF 67 manganese are potential for superior oxidation-reduction responses and heightened conductivity, positioning it as a promising candidate for sensitive electrochemical sensor applications.This study contributes valuable insights into the multifaceted applications of ZIF 67 manganese in material science and sensor technology.

Acknowledgment
This work was supported by Institut Teknologi Bandung and the Ministry of Education, Culture, Research, and Technology Indonesia.