Introduction

Metal ions, together with organic ligands, form metal-organic frameworks (MOFs), a special family of three-dimensional crystalline materials [1]. The properties of coordination compounds depend on what kind of metal ions are used, as well as which bonding ligands are used [1]. Often, metal ions are linked through aromatic carboxylate-type ligands such as 1,4-benzene-dicarboxylate (BDC) [2], benzene-1,3,5-tricarboxylate (BTC) [3], while nitrogen heterocyclic-based ligands are also common, like pyrrole-, imidazole-, pyrazole-, triazole-, thiophene-, and furan-carboxylates [4].

MOFs have attracted considerable attention for their exceptional properties: crystallinity, high surface area, low density, excellent thermal stability, and tuneable chemical functionality [5]. Applications for MOFs are many and include gas storage and separation [6], sensors [7], drug release [8], lithium-ion batteries [9], and more.

Some conductive metal-based MOFs maintain a high density of active sites in electrochemical sensing, enabling high rates of electron transport across the electrolyte [8, 9]. To improve the sensitivity of detecting specific biological species in various fluids, it is desirable to design unique MOF-based sensors with a large surface area and high conductivity [10]. Notably, Mg-based MOFs have been utilized in several applications lately, including sensors [10].

The ionic radius and elastic coordinative habits of magnesium ions are said to be almost the same as those of other transition metal ions, like zinc and cobalt ions. Magnesium is also a promising metallic component in MOF synthesis, mainly because it is readily available, has low toxicity, and has a smaller environmental footprint [11]. The stronger links arising from the low electronegativity of Mg ions and the lone pairs on carboxyl groups are often described as strong binding interactions between the Mg ions and the carboxyl moieties. Through those interactions, 3D Mg-based MOFs can form, directly boosting gas adsorption, catalytic activity, sensor response, and energy-related applications [11].

Doctors prescribe methotrexate (MTT) as an anticancer medication. Chemically known as 2,4-diamino-N--10-methyl folic acid, MTT is used to treat a variety of carcinomas, including acute lymphoblastic leukaemia, head and neck cancer, gastric cancer, breast cancer, choriocarcinoma, and more [12]. MTT is absorbed into the body approximately one to two hours after oral intake.

MTT's therapeutic value is limited because it is highly toxic and inhibits the growth of healthy cells [13]. Therefore, to prevent the adverse consequences associated with MTT, there is a need for rapid, sensitive methods for its early, or at least timely, detection. Also, because MTT is not safe for long, it is crucial to monitor its therapeutic levels in the human body [13]. Many analytical options, such as enzyme multiplication immunoassay [14], radioimmunoassay [15], enzyme inhibition and protein binding [16], fluorimeter-based tests, microbiological methods [17], and high-performance liquid chromatography (HPLC) [18], have been introduced to track MTT. HPLC is the most widely used and reliable method for measuring MTT in blood samples [18]. These techniques are limited by their high cost and complexity. In addition to these traditional techniques, electrochemical techniques have been developed for MTT analysis [19]. Electroanalytical techniques are inexpensive, straightforward, sensitive, selective, quick to react, and simple to use for on-the-spot tests. Highly active electrocatalytic materials capable of selectively measuring MTT in both in vitro and real-sample settings are needed for these techniques [20].

Leucovorin calcium, another name for calcium folinate (CFT), is a folic acid derivative. This molecule is a naturally occurring substance found in living cells, where it is essential to several metabolic processes [21]. In the pharmaceutical industry, CFT is a medication used as an antidote to MTT and as a vitamin against anaemia [22]. For many years, it has been used in clinical settings as a rescue agent after MTT treatment [23]. CFT can be used to reduce MTT-induced toxicity without reducing its anticancer efficacy [24]. For the simultaneous measurement of MTT and CFT in biological samples, quick and straightforward analytical techniques are therefore used [25].

Since these methods are expected to be accurate and sensitive, and to detect and quantify different materials with their varying attributes in real-life samples, analytical chemists often face the challenge of developing approaches that enable rapid in situ analyses [25]. And for in situ, point-of-care setups aimed at quality control, environmental monitoring, and healthcare monitoring, people tend to rely less on current commercial lab tests. That’s mostly because they’re described as pretty complicated and also expensive [25]. Over the years, sensors built on screen-printed electrodes, or SPEs, have been among the key topics in electrochemical studies, mainly for fast, focused, portable, sensitive, inexpensive, and reliable assessments, plus they are claimed to open up new use cases [25]. The idea that screen printing can be much less expensive than traditional manufacturing routes was an important reason behind these big breakthroughs [26]. Screen printing has been suggested for the mass production of repeatable, affordable, dependable, single-use sensors for on-site monitoring in the microelectronics sector over the past 30 years [26]. On the one hand, SPEs enable the combination of functionalized compounds to produce many carbon electrodes in repeatable, inexpensive, and disposable formats. Well, SPEs get used in a bunch of areas of electrochemistry, especially for converting energy into usable forms and storing it, for detecting chemical or biological molecules, and even in microelectronics. On top of that, flexible electronics, a rapidly growing field in which you print electrical devices directly onto flexible plastic substrates such as polycarbonate, polyamide, and polyether ether ketone, often rely on these electrodes. When it comes to the most practical carbon types to deposit on them, people commonly choose graphite, activated carbon, or carbon black [27].

SPEs offer benefits over older electrode manufacturing methods because electrode thickness, surface characteristics, and overall composition can be adjusted relatively easily, and catalysts can be incorporated into the printing ink in a straightforward way. Plus, they allow more experimental statistical confirmation of the results, even if you end up having duplicate electrodes. The main downside is that they’re basically limited to flatbeds [28]. SPEs enable a range of tests using only small amounts of materials and reagents, without the need for electrode storage or pretreatment. In industries such as agriculture, pharmacy, medicine, the food sector, and the environment, these electrodes are frequently used for analysis [29,30].

To the best of our knowledge, most of the earlier published electrochemical studies relied on screen-printed or other electrode formats, and the electrodes were modified to determine CFT or MTT individually. So, in this paper, we report on how to craft a new screen-printed electrode (SPE) based on an Fe and Mg linked to a 1,4-benzene dicarboxylate ligand metal organic frameworks (FeMg-BDC MOF/SPE) and then evaluate its performance for the electrocatalytic detection of CFT in water-based solutions. We also evaluate the analytical performance of the modified electrode for CFT quantification in the presence of MTT.