Fig. 4. (A) Fluorescence spectra of the sensing system with different concentrations of initiator I. (B) Calibration curves of (a) CHA–HCR, (b) HCR, and (c) CHA systems with different concentrations of their corresponding analytes. (C) Resulting linear correlationship curve of the CHA–HCR system. (D) Fluorescence intensity (at λ = 520 nm) of the CHA–HCR-based sensing system with different analytes for a fixed time interval of 5 h: (a) I, 50 nM, (b) I_a, 50 nM, (c) I_b, 50 nM, (d) I_c, 50 nM, and (e) no analyte. Here –ΔF represents the absolute fluorescence change of FAM. The system consisting of 400 nM H₁, 400 nM H₂, 400 nM H₃, 200 nM H₄, 200 nM H₅ and 200 nM H₆ was carried out in reaction buffer for 5 h. Error bars were derived from n = 5 experiments.