Luminescence detection of cysteine based on Ag-mediated conformational change of terbium ion-promoted G-quadruplex

In this work, we developed a simple and sensitive method for the detection of cysteine (Cys) by employing terbium ion (Tb³⁺)-promoted G-qudraplex (G4/Tb) as a luminescent probe, which is based on Ag⁺-mediated conformational change of G4/Tb. Due to Ag⁺ is able to compete with Tb³⁺ to bind guanine at G4, the presence of Ag⁺ can lead to the formation of G4/Tb–Ag⁺ complex and disrupt the structure of G4/Tb. Meanwhile, the binding of Ag⁺ with G4/Tb will also cause the alteration of the excited state of G4 and more efficient energy transfer from G4 to Tb³⁺, enhancing the luminescence of G4/Tb. However, upon the addition of Cys, Ag⁺ will be released from G4/Tb–Ag⁺ complex because of the high affinity of Cys to Ag⁺. This results in the re-formation of the conformation of G4/Tb and the decrease of the luminescence of G4/Tb. So, Ag⁺-enhanced luminescence of G4/Tb is associated with its conformational transformation. As a luminescent probe for Cys, G4/Tb not only shows excellent selectivity and sensitivity with a detection limit of 20 nM, but also possesses the features of simple preparation, easy reproducibility, and eliminating the interferences from background fluorescence. We envision that the presented strategy might provide new insight into the biosensing applications of lanthanide complex.