**2. Materials used in electrochemical biosensors**

Vitamin D electrochemical biosensors utilize various materials for their construction, with glassy carbon, carbon dots, and carbon paste being some of the most commonly used materials [2]. Each material offers unique advantages and disadvantages in terms of their sensitivity, stability, and selectivity, making them suitable for different applications in the detection of vitamin D.

### **2.1 Glassy carbon electrode (GCE)**

GCE is a type of carbon electrode that has a smooth, glassy surface. It is commonly used in electrochemical analysis due to its stability, high conductivity, and low background current.

### **2.2 Carbon dots**

Carbon dots are tiny carbon-based nanoparticles that have a size range of 1–10 nm. They have unique optical and electronic properties, making them useful in various applications, including biosensors [3, 4].

## **2.3 Carbon paste**

Carbon paste is a composite material made up of graphite powder and a binder such as paraffin oil or mineral oil. It is a cheap and versatile electrode material that can be easily modified for use in various biosensing applications [5, 6].
