Introduction

The central nervous, renal, hormonal, and cardiovascular systems depend on dopamine (4-(2-aminoethyl)benzene-1,2-diol, DA), one of the most important neurotransmitters [1,2]. It also has a significant role in regulating cognitive functions, including behaviour, stress, and focus. DA is present as an organic cation in body fluids and brain tissues. Low dopamine levels are common in patients with Parkinson's disease due to the death of dopamine-containing neurons in the midbrain. Abnormal DA concentrations are also associated with schizophrenia, Huntington's disease, attention deficit hyperactivity disorder, and restless legs syndrome [3-5]. Therefore, a straightforward, sensitive, and selective method for detecting DA is needed to track DA levels in the human body.

Tyrosine, an essential amino acid involved in many bodily functions, is sometimes referred to as 4-hydroxyphenylalanine (Tyr). It is crucial for regulating protein synthesis. Tyr helps maintain the body's nitrogen balance. Tyr is present in foods, drugs, and dietary supplements [6,7]. Since phenylalanine cannot produce tyrosine without phenylalaninase, the production of phenylpyruvic acid, the secondary metabolic product, is greatly increased. Because of its effects on neurotransmitters, Tyr is used to treat conditions such as growth hormone stimulation, appetite suppression, and mood enhancement. Furthermore, studies show that abnormal tyrosine concentrations are directly associated with a number of human illnesses. For example, high Tyr levels may cause Parkinson's disease and enhanced sister chromatid exchange. The development of inborn disorders, including hawkininuria, tyrosinemia I, II, and III, and alkaptonuria, also depends on Tyr, a dopamine precursor. Furthermore, the development of type-2 diabetes, liver cancer, and obesity is significantly influenced by the concentration level of Tyr [8-10].

In the pharmaceutical industry, enzymatic processes can convert Tyr into DA [11] and it is important that Tyr and DA are measured at the same time in biological fluids. The methods reported for determining DA and Tyr include chemiluminescence [12,13], spectrophotometry [14,15], gas chromatography [16,17], and high-performance liquid chromatography [18,19]. These methods sometimes require multi-step sample preparation, despite their reputation for excellent accuracy. They should be handled appropriately because they also have certain disadvantages, including higher cost and greater time commitment. When it comes to DA and Tyr estimations, electrochemical methods were superior despite the wide range of other analytical techniques available [20,21].

Electrochemical methods have attracted significant interest for their inherent advantages, including rapid response times, high detection capabilities, user-friendly operation, non-toxicity, and the potential to create smaller devices. The techniques reduce analytical time and costs by handling samples without complex preparatory work and enabling analysis without it [22-25]. The adsorption of oxidized compounds leads to bare electrode fouling, reducing electrode performance and stability across multiple testing cycles. The direct electrochemical oxidation of DA and Tyr at bare electrodes requires substantial overpotential. The detection limit increases due to this factor, while the background current rises. The research results demonstrate that electroanalytical methods achieve improved performance with chemically modified electrode systems, which researchers use to create electrochemical sensors [26-30].

Electrochemical sensor development enables researchers to create devices that enable fast electron transfer while maintaining high selectivity through advances in nanomaterials. The scientific community has renewed its interest in nanomaterials due to their numerous applications in nanosensor development and other fields. Their special characteristics include a combination of large surface area and catalytic, mechanical and electrical properties [31-34].

All equipment needed for electrochemical analysis is portable, enabling screen-printed electrode (SPE)-based electrochemical sensors to function as in situ screening devices. The special features of screen-printed electrodes, which include compact size, low detection limit, fast reaction time and high repeatability, have made the technology popular for analytical use [35-38].

Metal-organic frameworks (MOFs) are extremely porous materials composed of metal cations linked by organic linkers. The extensive porosity and large internal surface area of MOFs, together with their tuneable pore structure and distinct active sites, enable their application in gas storage operations and separation processes, catalytic functions and the new field of electrochemical sensor detection [39-41]. The low electrical conductivity of the material restricts its development and application in electrochemical sensing technologies. The use of MOFs as direct electrode materials remains challenging.

Researchers have developed multiple techniques to achieve outstanding electrical conductivity in MOFs. The formation of bimetallic MOFs by partially replacing the primary metal with a secondary metal is an effective and straightforward method to enhance MOF conductivity. The electrical conductivity and charge-conduction properties of bimetallic MOFs can be controlled by adjusting the metal ratio in the material [42-45].

The researchers developed Co/Ni-MOF electrode materials through their basic method. The researchers used field-emission scanning electron microscopy (FE-SEM) to examine the material's surface structure. The researchers created a Co/Ni-MOF-modified electrode (Co/Ni-MOF/SPCE) by drop-casting a Co/Ni-MOF suspension onto the SPCE surface. The researchers used various electrochemical methods to evaluate the sensor's performance and optimized the detection conditions for DA. The produced electrode demonstrated excellent linearity over the range of 0.01 to 660.0 μmol L-1. The Co/Ni-MOF/SPCE sensor determined DA levels in the presence of Tyr, yielding two distinct voltammetric peaks for DA and Tyr.

The Co/Ni-MOF/SPCE sensor demonstrates effective performance for detecting both DA and Tyr in authentic sample analysis.