A porphyrin-chitosan Schiff base polymer for sequential ratiometric detection of Fe3+ and ascorbic acid via UV-Vis spectroscopy.

A water-soluble porphyrin-chitosan (PC) conjugate was facilely synthesized through a one-step Schiff base reaction, establishing a novel dual-functional sensing platform for sequential detection. The PC probe itself functions as a highly selective ratiometric colorimetric sensor for Fe3+, exhibiting a distinct concentration-dependent shift in its Soret band (A439/A408) with a linear range of 0∼100 μmol/L and a low detection limit of 3.5 μmol/L. Crucially, the in-situ formed PC-Fe3+ complex is ingeniously repurposed as a sensitive "signal-recovery" sensor for ascorbic acid (AA), primarily monitored through a highly convenient UV-Vis spectrophotometric method. This ratiometric UV-Vis approach for AA detection demonstrated a linear range and achieved an impressive detection limit of 5.3 μmol/L, outperforming the fluorescence mode in both sensitivity and practicality. The sensor exhibits excellent selectivity, stability across a broad pH range, and rapid response. Its real applicability was successfully validated by the accurate determination of AA in commercial fruit juices with satisfactory recovery rates. This work highlights the unique advantages of covalent grafting and ratiometric optical sensing, presenting a simple yet effective strategy for sequential analysis with significant potential in food quality control and environmental monitoring.