A paper device for microRNA detection. The recognition of miR-21 is shown on the left and electrochemical response to the released DNAzymes is shown on the right, with CNTs-WE before and after adsorption of Fc-SDNA (upper right). Reprinted (adapted) with permission from X. Liu, X. Li, X. Gao, L. Ge, X. Sun and F. Li, A Universal Paper-Based Electrochemical Sensor for Zero-Background Assay of Diverse Biomarkers, ACS Appl. Mater. Interfaces, 2019, 11, 15381–15388. Copyright 2019 American Chemical Society.337