Fig.1 Absorbance spectrum of GO(A) and absorption and fluorescence emission spectra of P1(B)|||λem=405 nm.

Fig.2 AFM(A, B) and TEM(C, D) images of GO(A, C) and P1(B, D)|||Inserts of (A) and (B) show the height of line in AFM images; insert of (D) shows the HRTEM image.

Fig.3 Image of native PAGE(A) and gel filtration chromatography of P1 and the complex hybridizedwith complementary DNA(B)||| (A) Lane 1: P1, lane 2: P1+cT1, lane 3: P1+cT2, lane 4: P1+cT1+cT2.

Fig.4 Quenching efficiency of GO for P1(A) and QDs(B)

Fig.5 Fluorescent intensity of P1 at different concentrations of GO(A), fluorescent intensity of P1-GO complex with complementary DNA(cT1-18, cT2-18)(B) and fluorescence response of the “OR” logic gate(C)|||λem=405 nm.

Fig.6 Fluorescence of P1 with different concentrations of cT1(A) and cT2(B)|||Inserts: corresponding concentration calibration curve for target DNA detection with the excitation wavelength of 405 nm. Concentrations of complementary DNA is 0, 20, 30, 40, 60, 80, 100, 300, 500, 800, 1000, 1500, 2000 nmol/L, respectively.

Fig.7 Concentration calibration curve for target DNA detection with the excitation wavelength of 405 nm when the other target reaches saturation||| (A) cT1; (B) cT2. Concentration of the saturated DNA is 2 μmol/L; concentrations of complementary DNA is 60, 80, 100, 250, 500, 750, 1000 nmol/L, respectively.

Fig.8 Specificity of the P1-GO probe

Fig.9 Schematic illustration of the cyclic P1-GO biosensor

Fig.10 Switching progress of the P1-GO probe

Fig.11 Theoretical variation range of the P1 on GO when hybridized with one complementary DNA

Fig.12 AFM plots of each step products shown in Fig.9(A—G) and the relative height of every plot(A'—G')|||(A, A') P1-GO; (B, B') P1-GO-cT; (C, C') P1-GO-cT-T'; (D, D') P1-GO-cT1; (E, E') P1-GO-cT2; (F, F') P1-GO-cT-T1'; (G, G') P1-GO-cT-T2'.