Inkjet-Printed Graphene Sensors for the Bedside Detection of Tear Film pH

To determine whether an inexpensive, graphene thin-film electronic pH sensor could be used to measure tear film pH.

The pH-sensitive electrolyte-gated graphene field-effect transistors (EG-GFETs) were fabricated by patterning graphene ink and ultraviolet-cured dielectric onto 125 µm-thick polyimide substrate using a nanomaterials inkjet printer. A flow-cell was used to exchange test solutions and record current flow through the EG-GFET. Laboratory reference pH test solutions were used to calibrate the sensor. Contrived tears with lipids were pH buffered using HCL (1 M) or NAOH (1 M) to produce tear solutions ranging in pH from 2.0 to 9.5. A laboratory-reference pH meter was used to verify the pH of each solution. Dirac curves that demonstrate pH-dependent changes in current flow through the EG-GFET were measured for each test solution, using dual sourcemeters.

Graphene EG-GFET devices were highly sensitive to changes in artificial tear-film pH. The Dirac voltage was defined as the gate voltage at which minimum source drain current was measured. The relationship between Dirac voltage and tear film pH was highly linear with a slope of 17.2 mV per pH unit over the range of solutions tested, from pH 2.0 to pH 9.5 (r2 = 0.977).

Graphene field-effect transistors accurately measure tear film pH and may be useful in the emergency management of ocular adnexal exposure to acids or bases.

Thin-film graphene sensors are low cost and can rapidly map tear-film pH at multiple sites on the ocular surface and within the conjunctival fornices, avoiding subjective, colorimetric test-paper methods.