Chitosan polymers randomly labeled with pyrene: synthesis, photophysics, and selective quenching by metal cations and anions

The synthesis together with a comprehensive electronic spectral and photophysical study of two chitosan polymers randomly labeled with different degrees of pyrene were here undertaken. The polymers, denoted as ChiNPy-A and ChiNPy-B, have different degrees of substitution with pyrene (%DSPy), with values of 0.5 and 4.5, respectively. The resulting polymers consist in four distinct units, including the deacetylated and acetylated units and the grafted imine and amine pyrene units, obtained by a Schiff base formation reaction and subsequent reduction, respectively. Time-resolved fluorescence measurements in aqueous solution revealed triple exponential decay fits that characterizes the fluorescence properties of the amine- and imine-pyrene-linked units, as well as self-quenching of pyrene units. The ChiNPy polymer showed fluorescence quenching of its pyrene amine units, with lead (II) and mercury (II) exhibiting the highest degree of fluorescence quenching. From Stern-Volmer type plots, two distinct populations of fluorophores were observed. These findings emphasize the potential of chitosan pyrene-labeled polymers as sensitive fluorescent probes, particularly for detecting metal cations.