Introduction

Epinephrine (EP), also known as noradrenaline, is an important chemical mediator in the central nervous systems of mammals. EP occurs in nervous tissues and biological fluids, mainly as an organic cation [1,2]. From a chemical standpoint, EP belongs to the catecholamine family. This hormone is produced only in the adrenal glands and is derived from the amino acids phenylalanine and tyrosine. EP is a highly influential factor in biological functions and neurochemical activities within the human body. It increases heart rate, causes vasoconstriction, and increases the blood supply to muscle tissue. It can be employed to deal with different types of medical emergencies, including a major allergic shock (anaphylaxis), the stoppage of heart activity (cardiac arrest) or a slight external haemorrhage [3,4]. Repeated injections of EP may cause serious adverse reactions such as haemorrhaging in the brain due to the sudden increase in arterial pressure and disturbances in heart rhythm. However, there are more serious adverse effects associated with repeated injections of EP, such as intracranial bleeding owing to the sudden increase in arterial pressure and disturbances in heart rhythm [5]. Thus, the determination of EP levels is particularly significant in studying neural functions and other biological processes.

Acetaminophen (AC), N-acetyl-para-aminophenol, or paracetamol, is a well-established drug used extensively around the world. This common medication works as an antipyretic for the alleviation of fever and also as an analgesic agent for pain reduction, therefore being used worldwide as a potent medication to relieve mild to moderate pains related to headaches, arthritis and post-operations [6,7]. Moreover, it has no carcinogenic potential and, when a patient is intolerant to aspirin, it is a good alternative, since at recommended doses its safety record is well established. Acetaminophen is predominantly metabolized through the liver, so an overdose can result in the production of highly toxic intermediates to the point where, without proper treatment, serious liver damage can occur [8,9].

In addition, the use of AC is associated with increased brain serotonin (5-HT) levels. The elevation in serotonin levels can then lead to inhibition of the liver enzyme tryptophan-2,3-dioxygenase (TDO). It is also well established that 5-HT can markedly influence epinephrine secretion in the brain. Conversely, the mechanism of action of AC primarily involves reducing the synthesis of cyclooxygenase-derived compounds in the central nervous system. The activating influence of AC on cyclooxygenase function in viable cells can be counteracted by the presence of EP [10,11]. Therefore, developing a straightforward, economical, and highly responsive method to detect both EP and AC at the same time would be highly advantageous.

Various analytical techniques, including flow injection analysis [12,13], chromatographic methods [14,15], and ultraviolet-visible (UV-Vis) spectrophotometry [16,17], have been utilized for the concurrent detection of EP and AC. Nonetheless, the widespread implementation of those strategies has been hindered by their operational complexity, prolonged periods and significant price.

Because EP and AC have electroactivity, they can be quantified with electrochemical methods [18,19]. Beyond that, electrochemical sensors have been of great interest for future development. Electrochemical sensors do provide fast readouts, simple design, are cheap and easy to carry, making them a very exciting technology. In addition, these sensors can be miniaturized and made convenient to allow real-time detection of target analytes by non-specialist persons [20,21].

In recent times, electrochemical sensors derived from screen-printed carbon electrodes (SPCEs) have received great interest in research. This is due to the incredible benefits SPCEs have over traditional sensors, such as miniature, field-deployable size, portability, ease of production and cost. The SPCE surface is small and can therefore have a high active surface, making it very useful for analysing samples with low pH levels. In this way, screen-printed carbon electrodes are highly advantageous for their ease of use, reproducibility and reliability as well as excellent analytical behaviour demonstrating high sensitivity, greater selectivity and reproducible performance [22,23].

Yet, it is still a challenge to measure the EP and AC simultaneously. The oxidation peak of these two compounds is not well separated in most electrodes, posing a challenge to accurate measurement. Over the last few years, significant advancements in nanotechnology and novel manufacturing processes of SPCEs have substantially enhanced their electrochemical performance [24].

Nano-structured materials offer an extremely high surface-to-volume ratio; this property makes them great candidates for sensing objects. As a consequence, different nanoarchitectures have demonstrated better performance in applications such as sensor technology and optoelectronic devices. The properties of nanomaterials can be fine-tuned for a specific application by adjusting their dimensions or geometric shape [25,26]. Electroanalytical properties are determined with great significance by the surface characteristics of the material, since they directly affect key factors such as selectivity and sensitivity, stability or response speed. In general, nanomaterial-modified electrodes are widely used in analytical applications due to their higher selectivity and sensitivity [27,28].

Over the years, metal oxide nanoparticles (NPs) have found significant use in various scientific fields. This is largely based on their precautions for adjusting its physicochemical characteristics with exact control of particle size, structural morphology and crystalline face [29–31]. In particular, nickel oxide (NiO) nanostructures have continued to attract scientific attention during the past two decades. These factors can be attributed to high theoretical capacity, low cost, non-toxicity, high stability and abundance. Of particular note, NiO's remarkably high surface area makes it one of the best-known materials with catalytic activity. Additionally, NiO nanostructures are p-type semiconductors that have been useful in various physicochemical applications due to their wide band gap (3.6 to 4.0 eV). Which are pandas for solar cells 32P21, fuel cell electrodes 33P22, adsorbent materials 34P23, magnetic agents and gas & electrochemical sensors [32-34].

Herein, we fabricated NiO nanostructures via a simple and economical hydrothermal method. These nanostructures were subsequently used to fabricate a screen-printed carbon electrode, yielding a NiO/SPCE sensor. The NiO/SPCE sensor was then used as an electrocatalytic platform for the simultaneous determination of EP and AC. The NiO/SPCE sensor exhibited an impressive wide linear detection range from 0.01 to 400.0 μmol L-1 for EP with extraordinarily low limit of detection (LOD) of 0.005 μmol L-1 through differential pulse voltammetry (DPV). In addition, the NiO/SPCE sensor was successfully applied to detect EP and AC. The sensor is also successful at identifying EP and AC within relevant sample matrices.