Phenylketonuria (PKU), a prevalent genetic metabolic disorder, poses substantial diagnostic and treatment challenges globally. Current treatments primarily revolve around strict dietary management, necessitating lifelong commitment and frequent monitoring of phenylalanine (Phe) levels in the body. This study introduces an innovative diagnostic approach utilizing iron (III) chloride solution and highly porous polycaprolactone (PCL)-based solid biosensors for cost-effective, user-friendly detection of L-phenylalanine (L-Phe) in urine, which reflects systemic Phe levels. These biosensors operate through colorimetric changes, quantified using red, green, and blue (RGB), hue, saturation, and lightness (HSL), and cyan, magenta, yellow and black (CMYK) color models, to determine the concentrations of Phe in urine when incorporated with iron (III) chloride. Laboratory tests confirmed that the proposed iron chloride-based liquid and solid sensors are fast, sensitive, specific, and reliable depending on the Phe concentrations. This method promises to simplify home-based monitoring, providing a real-time, low-cost alternative to traditional blood tests, thereby potentially improving patient compliance and outcomes in managing PKU disease. The findings emphasize the potential use of the liquid and PCL-based biosensors in bridging gaps in access to essential diagnostic services for PKU patients. © This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply 2024.