Gold nanoparticles (AuNPs) were homogeneously electrodeposited on nitrogen-doped reduced graphene oxide (N-rGO) to modify a glassy carbon electrode (GCE/N-rGO-Au) in order to improve the simultaneous detection of dopamine (DA), ascorbic acid (AA), and uric acid (UA). N-rGO was prepared by the hydrothermal treatment of graphene oxide (GO) and urea at 180 °C for 12 h. AuNPs were subsequently electrodeposited onto the surface of GCE/N-rGO using 1 mM HAuCl₄ solution. The morphology and chemical composition of the synthesized materials were characterized by field-emission scanning electron microscopy and X-ray photoelectron spectroscopy. The electrochemical performance of the modified electrodes was investigated through cyclic voltammetry and differential pulse voltammetry measurements. Compared to GCE/rGO-Au, GCE/N-rGO-Au exhibited better electrochemical performance towards the simultaneous detection of the three analytes due to the more homogeneous distribution of the metallic nanoparticles as a result of more efficient anchoring on the N-doped areas of the graphene structure. The GCE/N-rGO-Au-based sensor operated in a wide linear range of DA (3–100 µM), AA (550–1500 µM), and UA (20–1000 µM) concentrations with a detection limit of 2.4, 58, and 8.7 µM, respectively, and exhibited satisfactory peak potential separation values of 0.34 V (AA-DA), 0.20 V, (DA-UA) and 0.54 V (AA-UA). Remarkably, GCE/N-rGO-Au showed a very low detection limit of 385 nM towards DA, not being susceptible to interference, and maintained 90% of its initial electrochemical signal after one month, indicating an excellent long-term stability.

The reduced graphene oxide (rGO) and nitrogen-reduced graphene oxide (N-rGO) were electrochemically coated with gold nanoparticles and compared as the modifier of a glassy carbon electrode (GCE) for dopamine (DA) electrochemical sensors. The rGO and N-rGO were prepared by reduction of graphene oxide (GO) under hydrothermal conditions. Urea was applied as a nitrogen dopant. The modified GCEs were tested as working electrodes in the electrochemical DA detection in the presence of ascorbic and uric acids. Initially, the working parameters of sensors in a pure dopamine solution were determined. The GCE/N-rGO-Au electrode exhibited a lower limit of detection compared with the GCE/rGO-Au (385 vs. 700 nM). Both sensors had a wide linear range of 1-100 µM and high sensitivity of 0.78 µA/µM for GCE/N-rGO-Au and 1.78 µA/µM for GCE/rGO-Au. The presence of ascorbic and uric acids in the solution resulted in a decrease in the intensity of DA oxidation peak, maintaining acceptable limit of detection. This study showed that the surface modification of the graphene materials with gold nanoparticles allows to obtain satisfactory working parameters of DA sensors. The GCE/N-rGO-Au demonstrated the superior performance in DA sensing due to a homogeneous distribution of gold nanoparticles on the surface of the N-doped graphene material.