An integrated relay/nitrate reductase field-effect transistor for the sensing of nitrate (NO3-)

An integrated enzyme-functionalized field-effect transistor (ENFET) device for the sensing of nitrate ions is described. An aminosiloxane-functionaliz ed gate interface is modified with N-methyl-N'-(carboxyalkyl)-4,4'-bipyridi nium relay units. The complex formed between nitrate reductase and the bipy ridinium units on the gate surface is crosslinked with glutaric dialdehyde to yield a stable relay-enzyme layer on the gate interface. In the presence of sodium dithionite as electron donor, the biocatalyzed reduction of nitr ate to nitrite ion is stimulated. The ratio between the oxidized and reduce d states of the bipyridinium sites regulates the gate potential, and is con trolled by the concentration of NO3- ions in the system. The effect of the chain length tethering the N-methyl-N'-(carboxyalkyl)-4,4'-bipyridinium uni ts to the gate surface on the biocatalyzed reduction of NO3- ions, and on t he NO3- FET sensor performance is discussed. The devices that include the b ipyridinium units tethered to the gate interface with methylene chain lengt h, -(CH2)(n), where n greater than or equal to 7, reveal a detection limit of 7 x 10(-5) M for nitrate and a sensitivity of 52 +/- 2 mV dec(-1). The r esponse time of the device is as low as 50 s, and the operational time of t he system is ca. 85 s. We estimate the surface coverage of nitrate reductas e on the gate surface to be ca. 1.2 x 10(-12) mol cm(-2).