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

Abstract

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 29 Si CP-MAS NMR spectra of silylated kaolinite showed T 2 and T 3 signals corresponding to the linkage of APTES moieties. This was confirmed by the relative intensities of the signals in the quantitative 13 C 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.

Natural halloysite nanotubes as mesoporous carriers for the loading of ibuprofen

“…In the spectra of Hal/120-A, the signals at 43.7 (and 48.5), 21.3 (and 24.2 and 30.2), and 9.6 ppm are attributed to the carbon atoms at positions I, II, and III in the propyl groups (Fig. 5b), respectively; and the signal at 17.5 ppm is attributed to the carbon atom at position V in the ethoxyl groups [63][64][65][66]. Note that the 13 C MAS NMR spectra are more qualitative than quantitative, because the NMR peak areas for the three carbons in propyl group and for the two carbons in ethoxyl group are not accurately equivalent.…”

“…The chemical shift at À91.7 ppm is attributed to the Q 3 silicon of halloysite. The signals of T 2 and T 3 appear at approximately À56 ppm and À66 ppm (Table 2), respectively [63]. T 2 indicates that some APTES species preserve one ethoxyl (or hydroxyl) group that did not hydrolyze or condense.…”

“…One is that the chemical environments of C(I-V) atoms might be disturbed by the oligomerization of APTES, the other one is the intensities of 13 C NMR peaks and the concentrations of the surface groups are difficult to calibrate [66]. Herein the averaged values of the integrations of the carbons at different positions of propyl or ethoxyl group were used to roughly evaluate the percentage ratio of one ethoxyl carbon over one propyl carbon [63]. The result shows that the percentage ratio of one ethoxyl carbon over one propyl carbon is 0.059, which is much lower than the theoretical value (3.0) of the original APTES.…”

Silylation of clay mineral surfaces

“…The existence of crystal defects (e.g., isomorphous substitution, vacancy and broken bond) can obviously increase the reactivity of the siloxane surface (Carrado et al, 2001). The hydroxyl surfaces (e.g., Al-octahedral surface in kaolinite) are excellent sites for grafting since the surface hydroxyls can condensate with alkoxyl group and/or the hydroxyls in the hydrolyzed silane (Brandt et al, 2003;Tonlé et al, 2007Tonlé et al, , 2011Detellier, 1993, 1996). Due to the high ratio of terminal surface area to the basal surface area, hydroxyl groups located at broken edges play an important role in the silane grafting reaction.…”

“…Due to lack of swelling ability for non-swelling clay minerals such as kaolinite, pre-intercalation with small polar molecules is an indispensable step in the silylation of internal surfaces. This demands rigorous conditions for successful grafting such as high reaction temperature, inert atmosphere, and so on (Brandt et al, 2003;Tonlé et al, 2007Tonlé et al, , 2011.…”