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 change in the organic phase, is proposed. Depth profiling using X-ray photoelectron spectroscopy (XPS) revealed 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 elements. SEM analysis showed a slight increase in particle size after annealing. Additionally, Electron Spin Resonance (ESR) spectroscopy provided further validation, showing higher signal intensity in films treated with hydrochloric acid (HCl). 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.