This study reports the development of a highly sensitive electrochemical sensor for phosphate ion detection, utilizing silicon nanowires (SiNWs) as the transducing elements and a novel copper (II) phthalocyanine-acrylate polymer adduct (Cu (II) Pc-PAA) as the functional sensing layer. Silicon nanowires were fabricated via metal-assisted chemical etching (MACE) with etching durations of 15, 25, 35, 45, and 60 min. The SiNWs etched for 15 min exhibited the highest sensitivity, showing superior electrochemical performance. Functionalized SiNWs were systematically evaluated for phosphate ion (HPO₄²⁻) detection over a wide concentration range (10⁻¹⁰ to 10⁻⁶ M) using Mott–Schottky measurements. The surface morphology of the SiNWs was thoroughly characterized before and after Cu (II) Pc-PAA layer functionalization. The sensing material was analyzed using contact angle goniometry and scanning electron microscopy (SEM), confirming both its uniform distribution and effective immobilization. The sensor displayed a Nernstian behavior with a sensitivity of 28.25 mV/Decade and an exceptionally low limit of detection (LOD) of 1.5 nM. Furthermore, the capacitive sensor exhibited remarkable selectivity toward phosphate ions, even in the presence of potentially interfering anions such as Cl⁻, NO₃⁻, SO₄²⁻ and ClO₄⁻. These results confirm the sensor’s high sensitivity, selectivity, and fast response, underscoring its suitability for environmental phosphate ion monitoring.