sensors Article Simple and Cost-Effective Electrochemical Method for Norepinephrine Determination Based on Carbon Dots and Tyrosinase Sylwia Baluta 1 , Anna Lesiak 1,2 and Joanna Cabaj 1, * 1 Faculty of Chemistry, Wroclaw University of Science and Technology, Wybrze˙ ze Wyspia ´ nskiego 27, 50-370 Wroclaw, Poland; sylwia.baluta@pwr.edu.pl (S.B.); anna.lesiak@pwr.edu.pl (A.L.) 2 Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wybrze ˙ ze Wyspia ´ nskiego 27, 50-370 Wroclaw, Poland * Correspondence: joanna.cabaj@pwr.edu.pl; Tel.: +48-71-320-4641 Received: 6 July 2020; Accepted: 10 August 2020; Published: 14 August 2020   Abstract: Although neurotransmitters are present in human serum at the nM level, any dysfunction of the catecholamines concentration may lead to numerous serious health problems. Due to this fact, rapid and sensitive catecholamines detection is extremely important in modern medicine. However, there is no device that would measure the concentration of these compounds in body fluids. The main goal of the present study is to design a simple as possible, cost-effective new biosensor-based system for the detection of neurotransmitters, using nontoxic reagents. The miniature Au-E biosensor was designed and constructed through the immobilization of tyrosinase on an electroactive layer of cysteamine and carbon nanoparticles covering the gold electrode. This sensing arrangement utilized the catalytic oxidation of norepinephrine (NE) to NE quinone, measured with voltammetric techniques: cyclic voltammetry and differential pulse voltammetry. The prepared bio-system exhibited good parameters: a broad linear range (1–200 μM), limit of detection equal to 196 nM, limit of quantification equal to 312 nM, and high selectivity and sensitivity. It is noteworthy that described method was successfully applied for NE determination in real samples. Keywords: biosensor; carbon dots; norepinephrine; tyrosinase; voltammetry 1. Introduction One of the primary goals of worldwide scientific endeavors is to improve quality of life. Achieving this is directly related to the rapid analysis of common disorders, quality control in the food industry and environment monitoring. Constant, fast and sensitive in situ monitoring is a priority in diagnostic control, and most of all, in medical diagnostics. The devices that meet these requirements are biosensors. According to Cammann, biosensors are analytical devices allowing for conversion of a biological signal to a measurable signal, like for instance amperometric response in the case of electrochemical sensors [1]. In electrochemical biosensors, differential pulse voltammetry (DPV) is frequently used. DPV applies a linear sweep voltammetry with a series of regular voltage pulses superimposed on the linear potential sweep [2]. Due to this, the current can be estimated instantly before every change of potential. In consequence, the influence of the charging current is minimized, reaching a better sensitivity [3]. DPV is frequently used in voltammetry-based techniques, not only due to its good sensitivity, but also because of resolving power. Norepinehrine (NE) is a monoamine neurotransmitter engaged in a broad range of physiological actions. The main function of NE is to help the organism adapt to internal and external environmental Sensors 2020, 20, 4567; doi:10.3390/s20164567 www.mdpi.com/journal/sensors