## 1 Introduction  
Over the past decade, the interest of the scientific community in the electrografting of aromatic diazonium salts has increased notably [1, 2]. This interest is attributable to the stability and reproducibility of the covalent bond formed between the organic layer and the electrode surface [3, 4, 5, 6]. Specifically, electrochemical grafting of aryldiazonium salts represents a rapid, straightforward and versatile methodology for surface modification. The formation of a thin or thick film can be controlled by varying the electrografting conditions (cathodic potential, scan rate and number of scans), the concentration of the aryldiazonium salt, the use of aqueous or organic solvents, the temperature and the reaction time [7, 8, 9, 10, 11]. The use of aryldiazonium salts represents an elegant approach to modify a range of conductive substrates, including carbon [12] (glassy carbon [13], HOFG [14], graphene [15],diamond [16], carbon nanotubes [17]), metals (Au [18], Ni [19], Fe [20] and Pt [21]), semiconductors (Si [22] and BDD [23]) and other conductive polymeric surfaces (PEDOT [24] and ITO [25]). The grafted species can carry a variety of functional groups depending on the target applications, such as the detection of molecules for biomedical, environmental or food interests. Consequently, these organic layers are useful for the covalent immobilization of biological molecules (antibodies, DNA, enzymes, neurotransmitters and toxins) in the design of electrochemical sensors [1; 26; 27]. Tyrosinase (TYR) is a polyphenol oxidase enzyme with an active site consisting of two copper atoms, which are coordinated to six histidine residues, and it is stabilized by disulphide bonds [28]. From a clinical point of view, TYR enables catecholase activities, involved in catecholamine's physiological processes in the human body that can influence human health. Moreover, this metalloenzyme plays a role in melanogenesis, acting as a catalyst in the biosynthesis of melanin pigments found in human skin and hair. Briefly, in the presence of molecular oxygen as co-substrate, TYR initially hydroxlates monophenolic substrates to produce o-diphenyl, which are then oxidized to form o-quinones [29; 30]. For decades, as is widely described in the bibliography [31; 32; 33; 34; 35; 36], many authors have used its versatile catalytic properties in the field of electrochemical biosensors development, for either the point-of-care testing of catecholamines or portable monitoring devices for phenolic detection. It has also been revealed as a promising molecule for biotechnological applications in environmental remediation [28]. As stated above, one substrate of TYR is the catecholamine called dopamine (DA) which functions as a neurotransmitter in the nervous system, influencing a multitude of functions such as motor activity, learning, mood and attention span. DA has been extensively studied by electrochemical methods due to its electroactive 1,2-dihydroxyprophenyl groups and its terminal amino group which allows the formation of covalent bonds [37]. The interest shown by the scientific community towards this molecule is two-fold; firstly, in the analysis and monitoring in biological fluids by TYR-enzyme biosensors [38; 39] and, secondly, in its use as a modifier in the design of biosensors for the determination of other relevant molecules. Several methods for the immobilization of DA on surfaces can be found in the literature [40; 41; 42; 43]. In this context, the present study (based on a granted patent [44]) focused on the grafting of organic monolayers by the electrochemical reduction of 4-nitrobenzenediamine ions (4-NBD\({}^{+}\)) onto a screen-printed electrode (SPE). To confirm that the grafting was successful, voltammetric and spectroscopic techniques were used. Subsequently, two strategies were developed using this grafted platform. In the first one, DA was covalently immobilized on the aminophenyl-functionalized screen-printed carbon electrode (SPCE) with glutaraldehyde (GLU) as a cross-linking molecule.