Synthesis, characterizations and colorimetric pH sensors of freestanding Janus conjugated Covalent Organic Framework (COF) films

Ultrathin, freestanding and optically active conjugated COF films based on 2,4,6-trihydroxybenzene-1,3,5-trialdehyde (TFP) and 4,4',4"-triaminotriphenylamine (TPA) have been synthesized and studied. A new mechanism of interfacial polymerization-interfacial film formation, based on the direct observation of the aqueous droplet volume changes in the organic phase, is proposed. Depth profiling using X-ray photoelectron spectroscopy (XPS) revealed the changes in the oxygen element composition after annealing, with a corresponding increase in hydrophilic groups exposed near the film's surface. Capacitance tests demonstrated a slight decrease in capacitance after annealing, attributed to a lower percentage of oxygen element. SEM analysis revealed a slight increase in particle size after annealing. Additionally, the COF films treated with hydrochloric acid (HCl) exhibited stronger Electron Spin Resonance (ESR) signals. It was also discovered that the as-prepared conjugated COF films undergo pH-dependent color changes, rendering them suitable for optical sensors. A colorimetric optical pH sensor based on the conjugated COF films was fabricated and evaluated.