Figure 1.

Schematic of paper-based detection of the bacterial enzyme ß-Gus, which is produced by more than 94% of all strains of E. coli [30]. An ultrathin coating of the cellulose fibers with functionalized chitosan affords the colorimetric sensing. The enzyme catalyzed reaction of the chromogenic substrate 5-bromo-4-chloro-3-indolyl-β-D-glucuronide (X-Gluc) affords in the presence of oxygen a deep blue indigo derivative, which is water-insoluble and can be quantified by adequate analysis of smartphone camera photographs.