2.3. Sensor Configuration and Methodology

For the detection of L-Tyr, screen-printed carbon electrodes purchased from Metrohm-Dropsens (SPCE—model DRP- C110, diameter of 4 mm, surface of 12.56 mm²) were modified with PPy. For PPy deposition, three different solutions of monomer/doping agent (FeCN, SDS and NP) 0.1 M/0.1 M were used. An ultrasonic bath was used in order to homogenize the solutions by ultrasonication for 5 min. For SPCE modification, the CA method was used by applying a potential 0.8 V during 90 s at a constant temperature of 25 °C. After being manufactured, the sensors were rinsed with ultrapure water and kept in the dark at 4 °C until use. By modification of the SPCE surface, the following chemically modified sensors were obtained: PPy/FeCN-SPCE, PPy/SDS-SPCE, and PPy/NP-SPCE.

Then, the electrochemical behavior of PPy sensors PPy/FeCN-SPCE, PPy/SDS-SPCE, and PPy/NP-SPCE was analyzed through the CV technique in a solution of 0.1 M of KCl and then in a double solution containing 10⁻³ M L-Tyr and 0.1 M KCl. In the case of CV, the established parameters were: initial potential 0.0 V, positive vertex potential 0.5 V, negative vertex potential −1.0 V, and the scan rate was between 0.1 and 1.0 V × s⁻¹. For the CV studies with the polypyrrole based sensors immersed in KCl and KCl + Tyr solutions, the screen-printed reference electrode Ag/AgCl was used. The counter electrode was a screen-printed C electrode integrated on the commercial electrode DRP-110. Six cycles were recorded in the solution to be analyzed in order to stabilize the sensor signals. All the results reported in this study are after the stabilization stage. Prior to the electrochemical measurements, the electrolyte solution was deoxygenated with nitrogen.