Surface modification is a fundamental technique in the development of chemical sensors and biosensors. In this work, we established a strategy to incorporate boronic acids into the structure of polyethylenimine (PEI) through a multi-component reaction under microwave conditions. The presence of boronic acids on PEI was identified by NMR and IR spectroscopy. This borylated PEI polymer was coated on the glass substrate and its binding activity was tested by using 5-(diethylamino)-2-((methylimino)methyl)phenol (DAHMI) as a fluorescence tag. Moreover, the boronic acid-modified surface was applied to construct a liquid crystal (LC)-based sensor system for detecting glycated hemoglobin (HbA1C). The specific binding of HbA1C to boronic acid on glass surface lead to the reorientation of LCs, such that a dark-to-bright optical transition was observed under polarized light. Based on this mechanism, the limit of detection of the LC-based sensor for HbA1C was 100 μg/mL with good specificity. Our results demonstrated the applicability of the surface modification strategy of boronic acids on glass substrates for developing chemical sensors and biosensors.