While basic concepts of paper-based microfluidic analytical devices (μPADs) have appeared considerably, elemental technology development is worth working from different specialized points of view. Herein, we show that 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofiber (TOCN) can be reasonably used as a module of μPADs; we adopted a biochemical reaction of indigo formation by oxidation of indoxyl acetate (IDA) via its hydrolysis with acetylcholinesterase (AChE) as an example. First, we demonstrate that TOCN stably stores unstable AChE and IDA when dried and acts as a biochemical reaction field when wet. The thixotropic nature of TOCN aqueous dispersion gave inkjet printability, which will facilitate the μPADs production. The exchange of substances in the wet TOCN network took more time than in solution, but this can be read as control of reaction kinetics. Finally, we constructed semiquantitative μPADs for an organic phosphorus pesticide. We illustrate that by mounting TOCN, various functions can be incorporated on one sheet of paper to lead to the extension of design flexibility and universal use for μPADs.
Keywords: biosensors; cellulose nanofiber; enzymes; paper-based microfluidic analytical devices (μPADs); stabilization.