**2.10 Gold (Au)**

Gold is a noble metal that has been extensively used in biosensors due to its excellent electrical conductivity and biocompatibility [19, 20]. It is often used as a support material for enzyme immobilization in electrochemical biosensors [21].

The use of various materials and functionalization strategies have been explored to improve the performance of electrochemical biosensors for vitamin D detection. For instance, the use of antibodies or vitamin D-binding proteins in the biosensors can enhance the selectivity and sensitivity of the detection [22].

However, there are still some challenges that need to be addressed in the development of electrochemical biosensors for vitamin D detection, such as the need for improved reproducibility, stability, and specificity [23, 24]. Additionally, the detection of various forms of vitamin D, such as 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, also needs to be explored. In summary, electrochemical biosensors have shown great potential in the detection of vitamin D, and further research is needed to overcome the existing challenges and explore their practical applications.
