Nanohybrid Kaolinite-Based Materials Obtained from the Interlayer Grafting of 3-Aminopropyltriethoxysilane and Their Potential Use as Electrochemical Sensors

The grafting of 3-aminopropyltriethoxysilane (APTES) onto the internal aluminol groups of two kaolinite minerals from Georgia and from Cameroon was achieved by utilizing their corresponding dimethyl sulfoxide (DMSO) intercalation compounds as intermediates. The modified clays were characterized by powder X-ray diffraction, thermal gravimetric analysis coupled with mass spectrometry, Fourier transform infrared, and solid-state MAS NMR. These techniques demonstrated the effectiveness of the interlamellar grafting process. An expansion of the interlayer distance from 11.2 to 15.9–16.4 Å on going from the precursors to the nanohybrid materials was observed. The study of the thermal behavior of the organoclays showed the release under heating of grafted APTES fragments, typical of the decomposition products of carbon, hydrogen, and nitrogen bearing organic matter. The hydroxyl stretching vibration zone (3700–3600 cm−1) of the starting clays and of the DMSO intercalated precursors was intensively perturbed in the final materials. New bands were observed, indicating strong interactions between APTES and the kaolinite interlayer surfaces. Solid-state 29Si CP-MAS NMR spectra of silylated kaolinite showed T2 and T3 signals corresponding to the linkage of APTES moieties. This was confirmed by the relative intensities of the signals in the quantitative 13C CP-MAS NMR spectra of the nanohybrid materials. Concomitant silylation and ethoxylation of the interlayer aluminol surface were obtained to give a mixed alkoxy–organosilyl kaolinite derivative. The silylation was roughly equally distributed between bidentate and tridentate fixations, with a minor amount of monodentate material. A remarkable feature of the new materials was their response to the [Ru(CN)6]4− electrochemical probe in acidic medium when the functionalized clays were coated on a platinum electrode. This is due to the protonated amine groups that act as anion exchange sites. These preliminary results open the way to further development of kaolinite-based electrochemical sensors.