Electric Charge Detection of Sparse Organic Acid Molecules Using an Organic Field-Effect Transistor (OFET)-Based Sensor

Detection and measurement of organic acids, such as small carboxylic acids, are useful for quality control of processed food and drinks and in the healthcare sector. Previously, chromatographic and electrophoresis methods requiring a bench-top device were used for their detection. In this article, we report an organic field-effect transistor (OFET)-based sensor capable of sensing electrical changes, which was applied to the detection of citric acid, chosen as a small organic acid model. When citric acid was injected into the OFET-sensor equipped with a nickel-nitrilotriacetic acid-immobilized gold-electrode (Ni-NTA-immobilized Au-electrode), the shifts of the OFET transfer characteristic curves were observed to depend on the citric acid concentration, in the range from 0 to 1000 mu M. On the other hand, there was no response after citric acid addition in the case of a quartz-crystal microbalance (QCM)-sensor, which is a mass-sensitive device, under similar low-salt conditions because the mass change on the sensor surface was below the detection limit of the QCM device owing to inter-anion repulsions. OFET-sensors have advantages in the detection of small organic molecules with an electro-charged group as well as the possibility to be incorporated into portable devices in the future.