In this paper, the 3-n-propylpyridinium silsesquioxane chloride (SiPy⁺Cl⁻) polymer was used as a stabilizing agent for the synthesis of gold nanoparticles (AuNps). The formation of AuNPs-SiPy⁺Cl⁻ was confirmed by UV–vis spectroscopy from the plasmon band at 521 nm, which showed good distribution with size of about 5.0 nm, observed by transmission electron microscopy (TEM) and good stability (ζ = + 38.5 mV) obtained by zeta potential experiments. The surface of the glassy carbon electrode (GCE) was modified with gold nanoparticles (AuNPs-SiPy⁺Cl⁻) and subsequent formation of thiolactic acid (TLA) self-assembled monolayer (SAM). For the biosensor construction, horseradish peroxidase (HRP) was covalently immobilized on the surface modified with SAM. The steps of the formation of this biosensor was confirmed by electrochemical impedance spectroscopy (EIS) and field-effect scanning electron microscopy (SEM-FEG). The GCE/AuNps/TLA/HRP biosensor was applied for the detection of cathecol (CT). Under optimized experimental conditions, the biosensor showed an excellent electrocatalytic activity for CT in presence of 0.03 mmol L⁻¹ H₂O₂ in the range of 6.0 at 46.0 μmol L⁻¹, with a low detection limit (LOD = 0.852 μmol L⁻1) and good sensitivity (0.026 μA.mol L⁻¹). The modified electrode displayed good reproducibility for the determination of catechol and long-term stability of approximately 25 days. The formation of a TLA monolayer on the gold nanoparticles modifying GCE provided a suitable, simple, and low cost platform that could effectively be used to immobilize different enzymes.