Fig. 4. Tuning the response of the bioluminescent molecular beacon towards targets with varying affinities by stabilizing the stem (i.e. increasing the number of base pairs). The assays were performed using 10 pM ODN–NanoLuc, 20 pM stem-loop and 1 pM–5 μM target oligonucleotides in TE/Mg/NaCl buffer supplemented with 1 mg mL^–1 BSA (10 mM Tris–HCl, 1 mM EDTA, 12.5 mM MgCl₂, 150 mM NaCl, 1 mg mL^–1 BSA, pH 8.0, 1 : 5000 substrate). Errorbars represent the standard error of the mean of duplicate measurements. The solid lines are global fits of eqn (S1) (ESI†) to the experimental data per stem loop length.