Tracking the concentration of Al3+ in the aqueous system up to the nanomolar range using a modified biopolymer chitosan based fluorophore

Abstract

The paper reports on the development of a biopolymer chitosan based fluorophore for selective detection of Al³⁺ in aqueous streams. Fluorescence sensing based on the change in the photophysical properties of fluorophores is emerging as one of the favourite analytical techniques for environmental monitoring. We have developed a fluorophore by a simple and facile procedure involving two-step synthesis of (2-hydroxy napthalen-1-yl) azo chitosan (β-NAC) from the diazonium salt of the biopolymer chitosan and β-naphthol at low temperature. The resultant compound was characterized by UV-vis spectrophotometry, spectrofluorometry, FTIR, NMR, FESEM-EDX, PXRD and MALDI-TOF analysis. β-NAC shows characteristic absorption maxima at 327 nm in the UV region and fluorescence emission maxima at 427 nm with high intensity. The observed interaction of the β-NAC fluorescent compound was investigated as a fluorophore for several (alkali, alkaline and main group) metal ions and anions by monitoring the fluorescence response. The evident fluorescence quenching of β-NAC occurred in the presence of Al³⁺ metal ions up to the nanomolar range, confirming its high selectivity at low pH with a detection limit of 1.16 nM. The results of interference studies reinforce its selectivity in the presence of other metal ions and anions also. The distinct quenching parameters have been obtained by applying the Stern–Volmer plot at different temperatures and its analysis supported that the quenching mechanism is purely static in nature. The high K_SV values show strong ground state complexation between β-NAC and Al³⁺ ion. From thermodynamic studies, ΔG values were found to be negative which confirms that the complexation process is spontaneous.