Monitoring the chemical and structural changes in protein side chains and endpoints by infrared (IR) spectroscopy is important for studying the chemical reaction and physical adsorption process of proteins. However, the detection of side chains and endpoints in nanoscale proteins is still challenging due to its weak IR response. Here, by designing a double layered graphene plasmon sensor on MgF₂/Si substrate in the IR fingerprint region, we detect the vibrational modes in side chains and endpoints (1397 cm ¹ and 1458 cm ¹) of monolayer protein. The sensor could be applied on biochemistry to investigate the physical and chemical reaction of biomolecules.