**5.1 25OHD**

25-hydroxyvitamin D (25OHD) plays a critical role in assessing vitamin D status and is widely used as a biomarker for vitamin D deficiency [89]. Sensing 25OHD levels accurately is crucial for diagnosing deficiencies and guiding appropriate treatment strategies. Various sensing methods have been developed to measure 25OHD, including immunoassays, liquid chromatography-mass spectrometry (LC–MS), and electrochemical biosensors [90]. Electrochemical biosensors offer several advantages, such as rapid response, high sensitivity, and simplicity of operation [91]. These biosensors utilize specific recognition elements, such as antibodies or aptamers, to selectively capture

25OHD and convert the binding event into an electrochemical signal. The development of 25OHD biosensors holds great promise for point-of-care testing, enabling real-time and noninvasive monitoring of vitamin D levels [92]. Further advancements in the design and optimization of 25OHD biosensors are expected to contribute to improved diagnosis, management, and prevention of vitamin D-related disorders.
