Potassium ion sensing using a self-assembled calix[4]crown monolayer by surface plasmon resonance

The precise detection of K+ ion is crucial because K+ ion plays a leading role in membrane transport. Current K+ ion detection methods suffered low resolution and detection limit. Calix[4]crown-5 derivatives are well known as K+ ionophores. We described here a K+ ion-sensing system using a self-assembled monolayer of calix[4]crown-5 derivative (calix[4]crown) modified gold chip based on surface plasmon resonance (SPR). The calix[4]crown sensing layer was characterized by atomic-force microscopy (AFM), SPR, Fourier transform infrared reflection absorption spectroscopy (FTIR-RAS) and cyclic voltammetry (CV). It was found calix[4]crown was assembled as a monolayer on Au surface. The SPR angle was found to be modulated by various concentrations of K+ ion due to the interaction between the calix[4]crown and K+ ion. This calix[4]crown monolayer showed a more sensitive and selective binding toward potassium ion over other alkali and alkaline earth metal ions. From the simple SPR spectroscopic analysis, we were able to monitor K+ ion concentration with a wide range of 1.0 × 10−12 to 1.0 × 10−2 M in an aqueous solution with a pH 6–8. These experimental results showed a useful method for the design of simple and precise potassium ion biosensors.