Synthesis, Characterization, and Multifunctional Applications of a Novel Nitro Substituted Caffeine Azo Dye: Antibacterial Activity, Textile Coloration, and Ph Sensing

A novel azo dye (K1) was synthesized via diazotization of p‑nitroaniline followed by coupling with caffeine at the C5 position of the imidazole ring. The structure was confirmed by FT‑IR, ¹H NMR, and ¹³C NMR; the absence of the H5 proton signal in the ¹H NMR spectrum confirmed the coupling site. The dye exhibited a pH‑dependent color change (yellow at pH 5.0–6.6 to orange‑red at pH 7.5–14.0), making it suitable as an acid–base indicator with an endpoint comparable to that of phenolphthalein. Antibacterial assays using the broth microdilution method revealed moderate activity against Staphylococcus aureus (Gram‑positive) with a minimum inhibitory concentration (MIC) of 62.5 µg mL⁻¹, but no significant activity against Escherichia coli (Gram‑negative) up to 250 μg mL⁻¹, highlighting the role of the Gram‑negative outer membrane as a permeability barrier. The dye showed excellent dyeing performance on wool fibers (washing fastness: grade 4; light fastness: grade 3–4) but poor affinity for cotton, consistent with hydrophobic interactions favoring proteinaceous fibers. This work demonstrates the potential of caffeine‑based azo dyes as low‑cost, multifunctional materials for analytical, textile, and biomedical applications.