Small Schiff base molecules derived from salicylaldehyde as colorimetric and fluorescent neutral-to-basic pH sensors

Abstract

The development of pH sensors is very important to distinguish different pHs in several applications. In this study, we examined the change of the side chain moiety on the photophysical properties of eight pH-sensitive small Schiff bases (SBs) that can act as both, colorimetric and fluorometric probes. For that, we synthesized all SBs by condensation between salicylaldehyde with a primary amine (methyl-, ethyl-, isopropyl, propyl-, butyl-, pentyl-, cyclohexyl-, and benzyl-amine). The complete molecular structure was confirmed by 1H NMR, 13C NMR, FTIR, UV–Vis, and by GC–FID purity over 90% was confirmed for all synthesized SBs. Theoretical calculations were also performed to give support to our findings. All synthesized samples were tested as a colorimetric and fluorometric pH sensor in the range of pH 3.0–12.0. All SBs exhibited similar behavior, despite the side chain lengths. The SBs are quite long-lived in water solution, allowing their usage as pH sensor. While between pH 3.0–7.0 they are colorless due to the presence of the protonated species, HSB, above pH 7.0 they turn yellow due to the formation of SB−, indicating that the chromogenic property depends on the pH. Additionally, these SBs also exhibit fluorogenic characteristic as they show fluorescence off at acidic pH 3.0–7.0 and fluorescence on (508 nm) above pH 8.0, exhibiting a linear response between pH 7.0–9.0. The pKa value was calculated using the Henderson–Hasselbalch equation for all SBs, obtaining values from 8.20 to 8.52. All SBs act as pH-sensitive colorimetric and fluorometric probes in neutral-to-basic solutions and this process is reversible.