Cellulose nanofibrils/carbon dots composite nanopapers for the smartphone-based colorimetric detection of hydrogen peroxide and glucose

[Display omitted] •Fe-doped carbon dots (FeCDs) with high peroxidase-like activity were synthesized.•FeCDs were immobilized on cellulose nanofibrils (CNF) to obtain a nanopaper.•H2O2 and glucose were colorimetrically detected by TMB as a chromogen.•The blue coloration of the composite nanopaper was monitored by a smartphone app.•CNF served as an excellent matrix and rendered the nanozyme reusable and stable. Fe-doped carbon dots (FeCDs) with high peroxidase (POD)-like activity were prepared by a rapid microwave-assisted method and immobilized on cellulose nanofibrils (CNF) to produce a composite nanopaper. The POD activity of the nanopaper was evaluated by the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) and applied to the colorimetric detections of hydrogen peroxide (H2O2) and glucose. CNF acted as a support for the nanozyme, thus imparting it with reusability. It also adsorbed the chromogen during the reaction to induce a color change, thus acting as a test strip for portable on-site detection. A smartphone was employed to monitor this color change which made the whole detection process simple and economical. Under optimal conditions, this method afforded linear ranges of 6–42 μM and 10–70 μM and detection limits of 0.93 and 1.73 μM for the H2O2 and glucose, respectively. The results of the quantification of glucose in human serum samples were comparable to those of the standard glucose oxidase (GOD)–POD method. Further, the nanopaper was potentially reused for ten cycles and presented over a month of shelf life.