Laterally selective adsorption of pH sensing coatings based on neutral red by means of the electric field directed layer-by-layer self assembly method

In this work, electric field directed layer-by-layer assembly is proposed for fabricating patterned sensing coatings. The Neutral Red (NR) colorimetric pH sensitive indicator and the polymers poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) which had been already proposed in the past by several groups to incorporate diverse indicators are used here in two different structures with the aim of obtaining selective deposition. The first structure studied combines NR and PAA while the second structure combines both NR and PAH with PAA. The results obtained in this work revealed that the LbL material adsorption over the electrodes was enhanced or even inhibited when applying a specific direct current voltage between them under some other particular fabrication parameters. Particularly, by adjusting to 7 the pH of the solutions used for the fabrication of the films and applying 1.4 V between the electrodes, laterally selective deposition was obtained with both of the pH sensing structures proposed here. This strategy allows the fabrication of pH-sensitive patterned films.