Tailor-Made Chitosan-Azovanillin Photochromic Schiff Bases for Antimicrobial Applications

Functional biomaterials with responsive properties are in high demand due to their potential application as antimicrobial agents. In this study, we have developed a photochromic chitosan-Azo-Schiff base using condensation reactions of chitosan with three derivatives of azobenzene. The biopolymers thus produced were characterized by Fourier-transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, scanning electron microscopy, and X-ray diffraction. The results showed that the materials had good thermal stability, an amorphous nature, and belt-like morphology. The optical properties of the Schiff bases as a function of the azobenzene moieties present in chitosan were examined by absorption spectroscopy. The photoresponsive behavior of the Schiff bases was studied under UV illumination. Of the three Schiff bases, ChS-Azo-P showed π-π* and n-π* features with no reversible changes, while peak broadening with absorbance enhancement was observed in ChS-Azo-O (87% at λ_abs = 404 nm) and ChS-azo-S (93.2% at λ_abs = 364 nm). The antimicrobial activities of the Schiff bases were evaluated against human pathogenic fungi, viz.,Candida albicans ATCC10231 (MIC₉₀ 4-64 μg/mL), Candida glabrata NCYC388 (MIC₉₀ 16-128 μg/mL), Cryptococcus neoformans ATCC34664 (MIC₉₀ 4-16 μg/mL) as well as Gram-positive bacteria, Staphylococcus aureus ATCC29213 (MIC₉₀ 0.5-4 μg/mL) and Gram-negative bacteria, Escherichia coli ATCC10799 (MIC₉₀ 1-8 μg/mL), Klebsiella pneumoniae ATCC700603 (MIC₉₀ 2-16 μg/mL), and Acinetobacter baumannii ATCC19606 (MIC₉₀ 2-8 μg/mL). Chitosan-Azo-Schiff bases inhibited the growth of the human pathogens (fungal and bacterial) at concentrations lower than those of clinically used antifungal and antibacterial drugs. Interestingly, none of the ChS-Azo-Schiff bases showed significant hemolytic activity (<5%) and were found to be hemocompatible. The finding opens an avenue for developing biodegradable polymers comprising azo-dye-based photoresponsive materials for biointerface applications.