**2.2 Biosensor applications for cardiovascular diseases**

Cardiovascular diseases (CVDs), which account for 17.9 million fatalities annually, are the leading cause of death worldwide. Research on cardiovascular disease relies mostly on biosensors that gauge cholesterol levels. Cholesterol oxidase (ChOx) and cholesterol esterase (ChEt) are frequently used in these biosensors as sensor components, allowing for the measurement of both free cholesterol and total cholesterol. An effective method for precisely determining cholesterol levels in the system is electrochemical transducers. MicroRNA-21 (miRNA-21), a biomarker linked to cardiovascular disorders, has been investigated as a potential target for a carbon nanodot-based biosensor. The carbon nanodots (CNDs) that were employed in the biosensor were created using a quick and eco-friendly process. The CNDs had advantageous optical and surface characteristics that made them co-reactant agents in the electrochemiluminescent process of the biosensor. The biosensor immobilized a complementary polynucleotide on a single-use gold electrode to achieve sensitive detection of miRNA-21. The approach was straightforward and did not need any difficult labeling steps, allowing for the immediate detection of miRNA-21 in serum specimens from cardiac patients [13]. Exosomal miRNAs are potential indicators for detecting cardiovascular disease. Using step polymerization catalytic hairpin assembly (SP-CHA), a fresh electrochemical biosensor has been created to find exosomal miR-181. This biosensor triggers the production of increased T-shaped concatemers on the electrode surface by miR-181. The detection limit is low (7.94 fM), while the linear detection
