Singlet Oxygen Production and Tunable Optical Properties of Deacetylated Chitin-Porphyrin Crosslinked Films

The increasing need for biocompatible materials as supports to immobilize photosensitizer molecules for photodynamic therapy (PDT), led us to investigate the use of chitin as a support for 4,4′,4″,4-(porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid) (mTCPP) for singlet oxygen production. Chitin was first extracted from shrimp shells using the ionic liquid 1-ethyl-3-methyl-imidazolium acetate ([C₂mim][OAc]), coagulated as a floc into water, and then deacetylated to varying degrees of deacetylation using 4 M NaOH. The deacetylated chitin (DA-chitin) was dissolved in [C₂mim][OAc] and mTCPP was covalently attached by reaction between the amino groups of DA-chitin and the carboxyl groups of mTCPP using N-(3-(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) as activators. The resulting composite polymers were cast as a film and coagulated with water to remove IL and excess reagents, resulting in homogeneous DA-chitin/mTCPP films. Attempts to prepare films by coagulation from a solution containing chitin and mTCPP to physically entrap the porphyrin, resulted in aggregation of mTCPP in the film. The DA-chitin/mTCPP films had strong optical absorbance and their absorbance intensity could be tuned by changing the mTCPP content and degrees of deacetylation of DA-chitin in a predictive manner. In addition, metal ions (Cu²⁺, Zn²⁺, Gd³⁺, and Fe³⁺) could be easily chelated into the DA-chitin/mTCPP films through mixing metal salt solutions with the films and heating. After chelating metal ions, optical properties, such as absorption region and intensities, of the films changed, suggesting chelating metal ions could tune their optical properties. Moreover, the DA-chitin/mTCPP films could generate singlet oxygen under light irradiation and, hence, might serve as a photosensitizer in PDT. The methodology used in this study is also applicable for developing other functional biomaterial devices.