The sensors using liquid crystals (LCs) as the optical signal reporter, i.e., the LC-based sensors, have been considered as new sensing technology suitable for fast-screening and point-of-care applications due to their simplicity and visualized signals. In this review, we classified the LC-based sensors into five categories by the types of chemical reactions that involved in the sensing mechanisms. They are (1) DNA/aptamer hybridization, recognition, and cleavage, (2) immunobinding, (3) enzymatic reactions of proteases, (4) metal-ligand complexation, and (5) functional group transformation. In each mechanism, we discussed the types of target molecules and evaluate how the chemical reactions affects the performance of the sensing systems. Finally, we proposed the challenges and opportunities for current LC-based sensors studies. Overall, this review provides a whole scope of the research related to LC-based sensors, which is helpful for the researchers to find their space to contribute to this field from different viewpoints.
