A simple and cost-effective electrochemical synthesis of carbon-based nanomaterials for electrochemical biosensor is of great challenge these days. Our study describes a single-step electrochemical deposition strategy to prepare a nanocomposite of electrochemically reduced graphene oxide (ErGO), multi-walled carbon nanotubes (MWCNTs), and polypyrrole (PPy) in an aqueous solution of pH 7.0 for dopamine (DA) detection. The ErGO/MWCNTs/PPy nanocomposites show enhanced electrochemical performance due to the strong π–π* stacking interactions among ErGO, MWCNTs, and PPy. The efficient interaction of the nanocomposites is confirmed by evaluating its physical and electrochemical characteristics using field-emission scanning electron microscopy, Raman spectroscopy, electrochemical impedance spectroscopy, cyclic voltammetry, and amperometry. The deposited nanocomposites are highly stable on the substrates and possess high surface areas, which is vital to improve the sensitivity and selectivity for DA detection. The controlled deposition of the ErGO/MWCNTs/PPy nanocomposites can provide enhanced electrochemical detection of DA. The sensor demonstrates a short time response within 2 s and is a highly sensitive approach for DA detection with a dynamic linear range of 25–1000 nM (R2 = 0.999). The detection limit is estimated to be 2.3 nM, and the sensor sensitivity is calculated to be 8.96 μA μM−1 cm−2, with no distinct responses observed for other biological molecules.

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A simple one-step electrochemical deposition/activation of graphitic carbon nitride (g-C3N4) is highly desired for sensor configurations and remains a great challenge. Herein, we attempt an electrochemical route to exfoliate the g-C3N4 nanosheets in an aqueous solution of pH 7.0 for constructing a sensor, which is highly sensitive for the detection of serotonin (5-HT). The significance of our design is to exfoliate the g-C3N4 nanosheets, a strong electrocatalyst for 5-HT detection. Investigations regarding the effect of neutral pH (pH 7.0) on the bulk g-C3N4 and g-C3N4 nanosheets, physical characterization, and electrochemical studies were extensively carried out. We demonstrate that the g-C3N4 nanosheets have a significant electrocatalytic effect for the 5-HT detection in a dynamic linear range from 500 pM to 1000 nM (R2 = 0.999). The limit of detection and sensitivity of the designed 5-HT sensor was calculated to be 150 pM and 1.03 µA µM−1 cm−2, respectively. The proposed sensor has great advantages such as high sensitivity, good selectivity, reproducibility, and stability. The constructed g-C3N4 nanosheets-based sensor platform opens new feasibilities for the determination of 5-HT even at the picomolar/nanomolar concentration range.

Bacterial infections in fish farms increase mass mortality and rapid detection of infection can help prevent its widespread. Lactate is an important biomarker for early diagnosis of bacterial infections in farmed olive flounder (Paralichthys olivaceus). To determine the lactate levels, we designed a disposable amperometric biosensor based on Prussian blue nanozyme and lactate oxidase (LOX) entrapped in copolymer-reduced graphene oxide (P-rGO) on screen-printed carbon electrodes. Because LOX is inherently unstable, P-rGO nanosheets were utilized as a base matrix to immobilize it. After optimization in terms of enzyme loading, operating potential, and pH, the biosensor displayed maximum current responses within 5 s at the applied potential of –0.1 V vs. internal Ag/AgCl. The biosensor had Langmuir-type response in the lactate concentration range from 10 µM to 1.6 mM, a dynamic linear response range of 10–100 µM, a sensitivity of 15.9 µA mM−1 cm−2, and a lower detection limit of 3.1 µM (S/N = 3). Additionally, the biosensor featured high reproducibility, good selectivity, and stability till four weeks. Its practical applicability was tested in olive flounder infected by Streptococcus parauberis against the uninfected control. The results were satisfactory compared to those of a standard colorimetric assay kit, validating our method.

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