<p>Industrial and technological advancements have led to a significant increase in the contamination of water by heavy metal ions. As a result, the detection of these hazardous ions has become a crucial focus in various physical and chemical methods. However, these conventional systems often face challenges such as high costs, complex equipment, intricate chemical processes, and time-consuming procedures, despite offering relatively low detection limits and high sensitivity. In this study, we propose an innovative photonic crystal fiber device (PCFD) to detect Cu²⁺ and Mg²⁺ ions in drinking water. The PCFD features a simple design with a circular hole treated as a core gap, and the full-vector finite element method (FEM) is applied using COMSOL Multiphysics software. The proposed PCFD achieves an ultra-high relative sensitivity (RSE) of 98.43% at 2 THz. Additionally, the device exhibits minimal loss, demonstrating its effectiveness. The tunable design and straightforward structure of the PCFD allow for easy fabrication using existing manufacturing techniques. As such, this proposed PCFD has the potential to play a pivotal role in chemical and biological testing across a wide range of applications.</p>