Biology provides an array of materials and mechanisms to create sustainable high-performance materials. Here, we prepared a polysaccharide film and patterned it with redox-active catechol moieties using a biomimetic oxidative grafting method. In this proof-of-concept study, we show that the film allows results from an enzymatic biosensing reaction to be reported through cell phone imaging for the detection of glucose in food and beverage products (glucose is a marker for high-fructose corn syrup; HFCS). This method uses glucose dehydrogenase (GDH) to confer molecular recognition, while the NADH product transfers its electrons to catechol moieties that are patterned onto a polysaccharide film. The “accumulation” of electrons by the catechol pattern can be detected because the oxidized quinone state is darker in color than the reduced catechol state. Using a commercially available cell phone imaging app, we demonstrate that HFCS-containing products can be distinguished from analogous products containing either sucrose or artificial sweeteners. Thus, we envision that this method could allow buyers or consumers to detect glucose on-site or in-home. In addition, this work further illustrates the potential of catechol-based materials for performing redox-based functions.