A pH optical and fluorescent sensor based on rhodamine modified on activated cellulose paper

A rhodamine-based colorimetric and fluorescent pH chemosensor (RhA) was designed and synthesized via a coupling reaction between rhodamine ethylenediamine and succinic anhydride. RhA showed excellent pH response in aqueous solutions. In addition, common cations (Na+, K+, Ag+, Mg2+, Ca2+, Pb2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Al3+, Cr3+, Fe3+, Au3+, Pt2+, and Ru2+) did not interfere with the pH response. As it has the potential to be used as a portable pH sensor, RhA was immobilized on activated cellulose paper using N,N'-dicyclohexylcarbodiimide (DCC) and N,N'-dimethylpyridin-4-amine (DMAP) as the coupling reagent to obtain a composite pH sensor (CP-RhA). CP-RhA was characterized by ATR-FTIR, UV-vis, and fluorescence spectroscopy, and by scanning electron microscopy (SEM). CP-RhA showed a rapid response in the pH range 1-8 through color and fluorescence changes. DFT calculations showed a blue-shifted spectrum in the protonated form compared to the neutral form. Moreover, the pH sensor paper could be reused by dipping in NaOH. Thus, our work demonstrates the potential of the rhodamine dye composite for visualizing pH changes in real systems.