Photo-potentiometry: Sensing of sugars using a pH-probe coated with a film of intrinsically microporous polyamine containing graphitic carbon nitride photocatalyst

At proof-of-concept level, the photochemical transformation of glucose (or more generally of carbohydrates) can be detected analytically as a localised pH change. Using a conventional potentiometric pH-probe, a microporous coating is developed to explore carbohydrate sensing in the 200-800 μM concentration range based on localised pH changes induced by light. The photo-responsive film is based on fibrous cellulose (to aid permeability), photocatalytic graphitic carbon nitride (g-C₃N₄), and an intrinsically microporous polyamine host (PIM-EA-TB, as reaction environment and binder). The film-modified pH-probe is pre-conditioned in a pH 4 buffer (containing phthalate buffer). When immersed in an aqueous solution, switching on a blue LED (λ = 385 nm, approx. 60 mW cm^-2) causes a pH transient towards alkaline, which is correlated with the carbohydrate concentration (all three glucose, fructose, or sucrose give very similar signals). The LOD is typically 70 μmol dm^-3, with a linear range up to 800 μmol dm^-3. Non-linearity beyond 800 μmol dm^-3 is tentatively attributed to limited oxygen availability. The photo-electroanalytical mechanism is discussed in terms of competing proton generation and consumption in the photoactive film linked to oxygen depletion (causing alkaline drift) at the pH-probe surface.