Supramolecular cucurbit[7]uril@AuNPs self-assembled membranes for impedimetric sensing and selective removal of cationic dyes.

BACKGROUND: The development of industrial technology has led to rapid growth in the printing and dyeing industry, which subsequently generates a large volume of wastewater containing complex, difficult-to-biodegrade organic components such as dyes and auxiliaries. The release of this wastewater into the natural environment causes significant water deterioration and environmental pollution. Current treatment methods often involve a combination of physical, chemical, and biological processes to enhance removal efficiency. Cucurbit[7]uril (CB[7]) is a macrocyclic molecule capable of forming stable 1:1 host-guest complexes with small aromatic compounds commonly found in modern dye pollutants. Therefore, seeking an efficient and convenient method to treat complex printing and dyeing wastewater using CB-based materials is an indispensable part of current industrial development.

RESULTS: A self-assembled membrane system, CB[7]@AuNPs, was successfully fabricated by leveraging the affinity of CB[7] for gold, ensuring the CB[7] cavities remained accessible for adsorption. The construction involved immobilizing gold nanoparticles (AuNPs) onto a mercaptosilane-functionalized silicon substrate, followed by CB[7] attachment and spin coating with polyethersulfone (PES). Successful assembly was confirmed by X-ray photoelectron spectroscopy (XPS), which revealed characteristic gold and nitrogen peaks, and electrochemical impedance spectroscopy (EIS) measurements. Although the introduction of a membrane onto the gold electrode significantly increases the surface impedance relative to the bare electrode, the CB[7]@AuNPs self-assembled membrane showed a notably lower impedance compared to the pure PES membrane. This reduction in impedance is attributed to the charged CB[7]@AuNPs facilitating electron transfer. The fabricated membrane demonstrated remarkable selective adsorption capabilities for cationic dyes (Methylene blue, Malachite green, Crystal violet, and Rhodamine B) over anionic dyes (Methyl blue and Congo red). This selectivity was supported by significantly lower EIS values for cationic dyes. The membrane exhibits up to 65 % removal efficiency for cationic dyes, reaching equilibrium within 30 min, while anionic dyes show less than 10 % removal. Increasing cucurbit[7]uril (CB[7]) concentration from 1 to 5 mM enhances Methylene Blue adsorption, so 5 mM-modified membranes were used in final tests. Adsorption is most efficient under mildly acidic conditions (pH 2-6) and decreases in alkaline media. The membrane can be regenerated using ethanol at pH ∼10 and retains stable performance through multiple regeneration cycles.

SIGNIFICANCE: The developed CB[7]@AuNPs self-assembled membrane provides a convenient and efficient method for dye removal. Mechanistic studies confirmed that the adsorption is primarily driven by host-guest interactions between CB[7] and the cationic dye guests. Offering pronounced selectivity, excellent regenerability, and enhanced operational convenience compared to free macrocycles, this system presents a promising and sustainable platform for treating complex industrial printing and dyeing wastewater, aligning with green chemistry principles.