Silver nanoparticles (AgNPs) exhibit strong potential for cancer biomarker detection due to their sharp and highly sensitive localized surface plasmon resonance (LSPR). However, their susceptibility to oxidation and agglomeration under harsh conditions, such as biological environments, limits their effectiveness in biosensing and therapy. To address this issue, we synthesized decahedral AgNPs and evaluated two stabilization approaches: a thin gold coating (2–3 nm, Au@Ag; shell@core) and a dual coating of gold and silica (SiO₂@Au@Ag) with varied thicknesses. The particles were characterized using UV–Vis spectroscopy, dynamic light scattering (DLS), transmission electron microscopy (TEM), and scanning electron microscopy with elemental analysis (SEM/EDX). With each additional layer, the UV–Vis spectrum exhibited a red shift, and particle size increased from 40 nm (Ag) to 45 nm (Au@Ag), and finally to 146 nm (SiO₂@Au@Ag). Stability was tested in biologically relevant buffers (PBS-T and HEPES), complete cancer cell media (DMEM and McCoy), and in strong oxidizing and basic agents (0.5 M H₂O₂ and NH₄OH). Uncoated AgNPs rapidly lost plasmon peak intensity in biological buffers, indicating oxidation, and showed agglomeration in cell media. In contrast, Au-coated AgNPs maintained strong LSPR and colloidal stability in most environments, though slight agglomeration appeared in McCoy and instability was observed in DMEM. The dual-coated SiO₂@Au@Ag nanoparticles exhibited superior stability across all tested buffers, media, and harsh environments, including high oxidant and alkaline conditions. These findings demonstrate that combining Au and SiO₂ layers markedly enhances AgNP stability under diverse conditions, ensuring safer and more reliable performance in cancer diagnostics and therapeutic applications. Current efforts focus on evaluating biological responses (e.g., MTT cytotoxicity) and investigating ligand-mediated targeting through surface plasmon resonance binding assays.