Plastics are extensively used worldwide, with bisphenol A (BPA), produced in quantities exceeding 6 billion pounds annually, standing out as a commonly utilized plasticizer. BPA is known to leach into foods and beverages from containers and can also migrate from dental sealants and composite materials during everyday use. The widespread presence of BPA in daily life highlights an urgent need for rapid and sensitive detection methods to monitor and mitigate potential health risks from BPA exposure. In this study, we introduce a sustainable and eco-friendly approach to developing a simple, one-step surface-enhanced Raman-scattering (SERS)-based sensor substrate for BPA detection. This approach creates in situ reduction sites in a colloidal solution by leveraging chitosan’s properties as a cationic polymer, enabling precise control over silver ion reduction and optimizing particle size and distribution. Additionally, polyols rich in hydroxyl groups are incorporated to counteract chain scission in chitosan during the reduction process, whereby molecular chains are extended and the sensor is stabilized. This innovative process converts the colloidal solution into a film, yielding a highly efficient, one-step SERS-based sensor substrate for detecting BPA in various environments.