An ultrasensitive cyanine dye-based fluorescent probe for the determination of iron(III) and oxalate.

An ultrasensitive fluorescent probe based on the pentamethine indocyanine dye (Ind2) was developed for the quantitative determination of trace iron(III) and oxalate in aqueous solution. The analytical principle relies on fluorescence quenching of Ind2 by Fe3+ through radical oxidation under acidic conditions (pH 1.9 ± 0.05), whereas oxalate determination relies on a reduction of the effective oxidizing activity of Fe3+ upon complex formation. HRMS analysis confirmed the formation of a dye radical cation, followed by cleavage of the polymethine chain. Comparative studies revealed that pentamethine dyes (Ind2) are more efficient reagents for Fe3+ detection than trimethine (Ind1) and heptamethine (Ind3) cyanine dyes. The probe exhibits excellent linearity in the ranges of 0.1-1.1 μM (R2 = 0.9968) for iron(III) and 0.1-1.8 μM (R2 = 0.9987) for oxalate, with identical limits of detection of 0.05 μM for both ions. These values are several orders of magnitude lower than the WHO guideline concentration for iron in drinking water (5.4 μM) and typical oxalate levels in black tea infusion (≈430-9200 μM). The probe shows satisfactory selectivity, with interference associated with anions forming competing Fe3+ complexes (F- and H2PO4-). The developed probe operates in the red spectral region (≈655 nm), exhibits high sensitivity and functions in an aqueous medium (ethanol <1% v/v). The probe was successfully applied to the determination of Fe3+ in sorrel and soluble oxalate species in tea infusion, demonstrating strong potential for monitoring of iron and oxalate in leafy vegetables and for food quality control.