Fig. 4. Colorimetric signals (ΔPI) generated by different concentrations of gold nanoprobes ([Au]) modified with avidin when the paper substrate was modified with PAH (black dots) or PAH and PSS (red dots), and biotinylated BSA (full dots) or non-biotinylated BSA (open dots). Nanoprobes modified with avidin were obtained using carboxylate-coated (a), amine-coated (b) or PVP-coated (c) nanoparticles.