Fluorescent Self-Supporting Composite Film Formed from Chitosan and the Neutral Poly(3-hexylthiophene-<i>co</i>-1,4-phenylene) Polymer with Enhanced Dispersion Properties for a Small Molecule

A composite film of chitosan (Ch) with a neutral conjugated polymer, poly(3-hexylthiophene-<i>co</i>-1,4-phenylene) (PTPh), was developed to combine the adsorption capacity of Ch with the fluorescence sensitivity of PTPh. Characterization of the films using thermogravimetric analysis, microscopy, and infrared, absorption, and fluorescence spectroscopies revealed that the dispersity of the target small molecule, 4-aminoazobenzene (4-AAB), was improved in the composite film compared to the pristine Ch film as evidenced in microscopy studies. In the presence of 4-AAB, the Ch/PTPh film exhibited fluorescence quenching at low 4-AAB concentrations and changes in emission spectra at higher concentrations. Photoisomerization studies suggested that the improved dispersity of 4-AAB in the composite film is due to an increase in the free volume provided by PTPh, with faster cis-to-trans isomerization observed when PTPh was present. Proof-of-concept adsorption experiments showed that the composite film adsorbed 4-AAB from an aqueous solution, leading to a change in the emission properties of the film. This qualitative characterization uncovered a dual role for the conjugated polymer in the composite film: the addition of the polymer changed the morphology and robustness of the film, and the polymer also provides the fluorophore to sense adsorbed molecules over a wide range of 4-AAB concentrations. These results show that the strategy of incorporating water-insoluble polymers at low concentrations into a versatile biopolymer leads to enhanced functionalities of a composite material.