Optimization of the complementary DNA (CPDNA) length (nucleotides, nt) and concentration. FAM-labeled DNA biosensor-GO was incubated for 1 h, then target ATP (blue, 500 μM), CPDNA (black), or a combination of CPDNA + target (red) was added. Relative Fluorescence (RF) was plotted after 1 h. RF = FT − FB, where FT is the fluorescence reading of a test mixture; FB is the fluorescence reading of the blank mixture. (A) CPDNA: 10 nM; (B) RF/RFc vs. CPDNA length, where RF is the red plot in (A) and RFc is the blank plot in (A); (C) CPDNA: 100 nM; (D) RF/RFc vs. CPDNA length, where RF is the red plot in (C) and RFc is the blank plot in (C); (E) CPDNA: 1000 nM; and (F) RF/RFc vs. CPDNA length, where RF is the red plot in (E) and RFc is the blank plot in (E). The data are an average of three independent experiments.