Fig. 1.

Photographs of the paper immunosensor and key analytical steps for detecting IL-6; (A) The biosensors consists of a piece of paper containing antibody-decorated nanoparticles, and a paper strip with three capture sites (CS); (B) After adding a drop of sample in each capture site and drying it, the paper is folded; (C) After removing the reservoir, a colored spot appears whose pixel intensity is the sum of the colorimetric signal generated in each capture site; (D) After washing away excess reagents the colorimetric signal is measured with a previously developed smartphone app within seconds; (E) Unfolding the paper strip in (C) reveals that nanoparticles are transferred vertically to the 3 binding sites; (F) Schematic representation of biomolecular interactions; IL-6 is adsorbed onto the paper substrate and subsequently detected by antibody-decorated gold nanoparticles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)