Tetraphenylethylene-Modified Halloysite Nanotubes Enabling Cu(Ⅱ)-Sensitive Nanocomposite

The increasing risk of heavy metals causes many concerns as one of the nonnegligible problems in modern environmental conservation. As one of the most common heavy metals, copper (Ⅱ) has a dominant effect in such a pollution. The accumulation of copper (Ⅱ) in water can eventually give rise to the overexpression of copper in humans, showing to be directly associated with Alzheimer’s, Parkinson’s, Menkes, and Wilson’s disease. As a result, the residue of copper (Ⅱ) in wastewater becomes a ubiquitous environmental challenge. The development of smart materials that are capable of detecting and removing copper (Ⅱ) simultaneously can pave an effective approach to solving this puzzling issue. Halloysite nanotubes (HNTs) are natural hollow aluminosilicate clays with a chemical composition similar to kaolin (i.e., Al₂Si₂O₅(OH)₄·nH₂O). HNTs display a superior water dispersibility due to the large cavity volume and surface charge repulsion. The attractive properties including low-cost, environmental friendliness, biocompatibility, good stability, and ease of surface modification also make HNTs promising composite materials in the fields of environmental protection, catalysts, bio-medical materials, and sensors.. In this study, tetraphenylethene (TPE) moiety was immobilized on aminated HNTs with oxalyl chloride for the preparation of TPE-functionalized HNTs (HNTs-TPE-NH₂).

TPE-2NH₂ (184 mg, 0.5 mmol), HNTs-COCl, mL and and 10 N,N-Dimethylformamide (DMF) were mixed in a flask. The system was stirred at room temperature to make nanoparticles well-dispersed and then reacted at 60 ℃ for 8 h. After cooling to ambient temperature, the suspension was centrifugated and the obtained residue was washed by DMF, tetrahydrofuran (THF), and ethanol.

The coumarin unit was bonded to the HNTs-TPE-NH₂ via a condensation reaction by treating 7-hydroxy-2-oxo-2H-chromene-5-carbaldehyde with HNTs-TPE-NH₂ to afford the Schiff base-containing product (HNTs-TPE-Cou). HNTs-TPE-NH₂ (1.0 g) and compound M1 (50 mg, 0.25 mmol) were added to a flask containing absolute ethanol (15 mL), and vacuum-filled with argon gas was repeated for three times. The mixture was refluxed and stirred at 70 ℃ for 12 h. After cooling to ambient temperature, the suspension was centrifugated and the obtained residue was washed sequentially by THF and ethanol. After vacuum-drying, the product HNTs-TPE-Cou was obtained as a faint yellow solid. The obtained HNTs-TPE-Cou was then deposited on the surface of polyurethane foam (PUF) to afford the composite PUF@HNTs-TPE-Cou based on the surface charge of the nanotubes. The obtained product HNTs-TPE-Cou was characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, solid-state nuclear magnetic resonance spectroscopy, thermogravimetric analysis, transmission electron microscopy, respectively.

HNTs-TPE-Cou aqueous solution has a fluorescence emission peak at a wavelength of 560 nm. The chelation between Cu²⁺ and HNTs-TPE-Cou can destroy the aggregation state of TPE units and give rise to a “turn off” effect on the fluorescence intensity of HNTs-TPE-Cou aqueous solution. he maximum fluorescence intensity of HNTs-TPE-Cou appears as the Cu²⁺ ion concentration is 0 μmol/L. The fluorescence intensity of HNTs-TPE-Cou decreases gradually with the increase of Cu²⁺ ion concentration, showing a typical concentration-dependent manner. F₀ and F represent the fluorescent intensities at 560 nm without and with different concentrations of Cu²⁺ ion, respectively. The F₀/F of HNTs-TPE-Cou at 560 nm has a good linear correlation to the concentration of Cu²⁺ ion when the Cu²⁺ ion concentration in the solution to be measured is between 0.2 μmol/L and 0.6 μmol/L. Other metal ions, including Ni²⁺, Mg²⁺, Ca²⁺, Co²⁺, Mn²⁺, Al³⁺, Ba²⁺, Na⁺, K⁺, Zn²⁺, and Cd²⁺, do not exhibit the quenching phenomenon with the addition to HNTs-TPE-Cou aqueous solution.

The yielded PUF@HNTs-TPE-Cou composites are cut into special shapes to fill the glass columns. A Gel-like separation column is prepared, which can be used to retain Cu²⁺ ions selectively. Moreover, the fluorescence intensity can serve as a nake-eyes identified indicator, which is useful to display the degree of usage. When the fluorescence intensity changes to non-emission, it means the end of useful life, and new columns are needed. The column (20 cm × 2 cm) is used to purify the Cu²⁺-containing water ([Cu²⁺] = 1×10^–4 mol/L). The concentration of Cu²⁺ effluent water is detected. The results indicate the concentration of Cu²⁺ is lower than LOD calculated in the first 100 mL eluting water, having a high purification efficiency.

In this study, HNTs were used as scaffolds and surface modified with tetraphenylethylene and coumarin moieties. The obtained product HNTs-TPE-Cou could be used to detect and separate of Cu²⁺ ions in water solution. HNTs-TPE-Cou displayed an intense fluorescence emission peak at a wavelength of 560 nm in water under the excitation of 365 nm light, while a repaid “turn off” effect on fluorescence appeared, showing a chelation behavior after the addition of Cu²⁺ ions. The as-prepared HNTs-TPE-Cou had high specificity and precision in detecting and removing Cu²⁺ without the recourse to energy consumption. Furthermore, HNTs-TPE-Cou was developed as composite materials by a facile deposition method, which could be further used as gel-like separation columns. The established method could pave a path on the simultaneously detecting and removing metal ions in aqueous solution.