We reported an oligopeptide immobilization strategy for improving stability and sensitivity of liquid-crystal (LC) based protease assay. In this strategy, oligopeptides are immobilized by using N-terminal cysteine or free amine groups on lysine residues to react with surface aldehyde groups and form multiple anchoring points. Our results show that the number of anchoring points is an important factor for improving stability of the immobilized oligopeptides. Oligopeptides with multiple anchoring points are very stable on the surface. They can only be cleaved from the surface by using proteases such as trypsin or α-chymotrypsin. This phenomenon provides a principle for developing an assay for the detection of proteases. On the other hand, oligopeptides with a single anchoring point cannot form stable linkage on the surface, and they can be easily washed off during rinsing steps. By immobilizing oligopeptides with different surface densities, concentrations of trypsin and α-chymotrypsin can be estimated simply by counting the number of bright LC spots. The higher the concentrations of trypsin and α-chymotrypsin, the less LC spots can be observed. In addition, the limit of detection (LOD) of the assay can be lowered by allowing a very large oligopeptide fragment to desorb from the surface after the cleavage. This task can be accomplished by immobilizing an oligopeptide with all anchoring points located at both ends while the cleavage sites are located near the anchoring points. Based on the principle, optimized LODs for trypsin and α-chymotrypsin can reach 1 and 0.1 ng/mL, respectively.
