Development of an ‘OFF-ON-OFF’ colorimetric and fluorometric pH sensor for the study of physiological pH and its bioimaging application

Abstract

2-hydroxyl-1-naphthaldehyde based probe 1,1′((1E,1′E)((4hydroxy1,3phenylene) bis(azanylylidene)bis(methanylylidene)bis(naphthalen-2-ol) (NAP) has been synthesized and thoroughly characterized by IR, 13C & 1H NMR, fluorescence, UV-Vis, and mass spectral studies. The detection of pH is a necessary feature of the intracellular atmosphere and probably affects all cellular systems. NAP acts as a highly sensitive pH sensor in ethanol/HEPES (7:3, v/v) medium. Upon increasing the pH of NAP solution from acidic-neutral-basic medium, a distinct color change occurs from colorless-yellow-brown followed by a subsequent bathochromic shift in UV-Vis spectra and fluorescence response from ‘OFF-ON-OFF’. Photo-induced electron transfer (PET) mechanism has been proposed for the fluorescence change on the basis of theoretical studies on protonated, neutral and deprotonated NAP. Bioimaging investigations display a pH dependent fluorescence behavior of NAP in gut tissue of drosophila 3rd instar larvae. Additionally, NAP paper strips have been successfully developed for solid state pH sensing based on colorimetric and fluorescence responses. © 2021 Elsevier B.V.