Detection of phosphine with a Langmuir—Blodgett film of a N-octadecyl pyridinium, TCNQ

Abstract

We describe the behaviour of a Langmuir-Blodgett (LB) film of a mixed-valence charge-transfer salt regarding the detection of phosphine. This salt is the N-octadecyl pyridinium, tetracyanoquinodimethane ( NOPy +, TCNQ − 1.4, TCNQ°. Its structure and electrical properties have recently been studied. After deposition, the film shows a conductivity of 0.8 × 10 −3 ohm −1 cm −1. This conductivity is enhanced by doping with iodine to obtain a conductivity of 0.05 ohm −1 cm −1. This high conductivity value allows us to design a sensor based on a simple resistance measurement. On exposure to phosphine diluted in nitrogen, the resistivity of the film increases. Different parameters have been investigated: concentration of phosphine, nitrogen flow and temperature. The detection is limited by diffusion, but the signal after one minute is significant. Increasing the temperature results in a better response, which is optimal for a nitrogen flow of 15 l h −1 ( i.e., 0.3 m s −1 above the detector). The study demonstrates the ability of ( NOPy + TCNQ −) 1.4, TCNQ° to detect phosphine with a detection limit below 1 ppm.