# Ru(terpy)-Based Conducting Polymer in Electrochemical Biosensing of Epinephrine
## 1 Introduction  
Chemical sensors are a broad class of analytical devices in which a chemical recognition system, known as receptor, is combined with a physicochemical transducer to transform chemical information into an analytical signal. When the receptor is a biological recognition element, the recognition system is based on a biochemical mechanism, and these sensors are more properly called biosensors [1]. Usually, bioreceptors can be yeasts, enzymes, whole cells, bacteria, proteins, or antibodies that allow the sensor to be able to detect an analyte or a specific class of analytes with high selectivity. Various types of biosensors can be recognized according to the biological mechanism that determines their specificity or to the mode of signal transduction. In particular, the biocatalytic mechanisms promoted by the presence of enzymes as biological receptors are of considerable interest in scientific research. On the other hand, electrochemical methods often allow highly selective and sensitive determinations and low levels of interference. The most frequent drawbacks in the development of electrochemical biosensors are in the effectiveness of immobilization of the biological component on the electrode surface and its stability on replicated measurements. An efficient immobilization can be achieved by different approaches: physical adsorption, cross-linking, covalent bonding, or entrapment in gels or membranes. All these methods aim to obtain a stable anchoring of the enzyme, then the possibility to use the biosensor many times while retaining a constant quality in the analytical response in terms of sensitivity, linear dynamic range, limit of detection, and selectivity. In recent years, the use of conducting polymers (CPs) as immobilizing agents as well as transducerelements in biosensors has attracted much interest. Entrapping an enzyme into a CP film can be achieved during the electrochemical polymerization in a solution containing both the monomer and the enzyme, or through a suitable functionalization of the polymer film [2; 3; 4; 5]. Such an approach allows an effective immobilization of the enzyme, but it is sometimes not suitable because the conditions of polymerization can often cause a total or partial loss of the catalytic activity of the enzyme [6; 7]. In this context, the use of CPs-based electrochemical biosensors in the detection of neurotransmitters is becoming more relevant. Neurotransmitters are commonly investigated because of their central role in transferring signals across chemical synapses from one nerve cell to another or to non-neuronal cells. Changes in neurotransmitter concentrations induce a number of diseases, including Parkinson's, Alzheimer's, and some types of cancer, so a fast-responsive and sensitive analytical device is of particular interest to determine these chemical species. In this field, CPs have been used alone or combined with materials such as carbon nanotubes, pyrolytic graphite, and metal-containing nanoparticles as modifying agents for electrode surfaces. The most common CPs, such as polypyrrole, polyaniline, and polythiophene derivatives, have been considered [8; 9; 10]. Among neurotransmitters, epinephrine (EP, also known as adrenaline) is widely used in the treatment of heart diseases, allergic reactions, anaphylactic shock, and asthma. From the chemical point of view, epinephrine belongs to the catecholamines group along with norepinephrine and dopamine. Its detection is often based on the activity of the enzyme tyrosinase, which is a Cu-based metal-protein able to catalyze the oxidation of mono- and of \(o\)-diphenyl to the corresponding quinones and \(o\)-diquinones, respectively, in the presence of O\({}_{2}\)[11; 12]. The adequate anchoring of the enzyme on an electrode surface allows to determine the analyte using electrochemical techniques. In particular, differential pulse voltammetry (DPV) and chronoamperometry (CA) are often used to minimize the effect of interfering substances and, at the same time, to reach a high sensitivity [13].