Graphite line on paper as an aqueous chemical sensor

This work investigates the ability of a graphite line drawn on a sheet of paper to be used as a sensor to detect different types of liquid chemicals. To the best of our knowledge, no work has been reported on identifying liquid chemicals based on their electrical resistance using graphite on paper. The sensor is capable of distinguishing between different types of liquids including acids, bases, solvents and water. A graphite pencil was used to sketch a 40 mm long by 6 mm wide line on a piece of white paper. The graphite on paper test strip was placed in a test fixture equipped with electrical contacts. The electrical contacts were attached to a data acquisition device (DAQ) along with LabVIEW software to measure and record the voltage outputs which were later used to calculate the electrical resistance. To make a consistent measurement in different samples, a test fixture was designed and fabricated using a 3D printing machine. The solvent tested in this experiment was acetone, the acid tested was nitric acid, and the base tested was tetramethylammonium hydroxide. Two droplets of each chemical were dropped onto the paper through a hole in the test fixture at room temperature. The voltage was recorded every millisecond for about 10 s after the liquid was dropped on the paper. The plots of electrical resistance values versus time showed a distinct curve for each type of liquid which can be used for liquid identification purposes.