Multi-wall carbon nanotubes bonding on silica-hydride surfaces for open-tubular capillary electrochromatography
Introduction
As the core of capillary electrochromatography (CEC) is column preparation, the open-tubular (OT) style is a comparatively straightforward one; it does not require the fabrication of any frits for packed formats or blending of monomeric reagents with suitable porogens in precise proportions for monoliths [1], [2], [3], [4]. However, OT-CEC suffers from a low phase ratio of available functional ligands attaching on the capillary wall. As such, etching the capillary wall surface and/or coating porous polymeric matrices are generally adopted methods to increase the loadability of phase materials [5], [6]. Additionally, the nanoparticles exhibiting high surface area would also create efficient phases for OT-CEC after their non-covalent or covalent bonding onto the columns. There are examples in the literature of ultizing this process with latex [7], human very-low-density lipoprotein [8], high-density-lipoprotein [9], mesoporous silica [10], and titanium dioxide [11]. One popular nanomaterial is carbon nanotubes (CNTs), which have unique properties including high electrical conductivity, mechanical strength, and chemical stability [12], [13]. In contrast to many studies and reviews concerning the use of CNTs as adsorbents [14], [15], LC stationary phases [16], GC stationary phases [17] and EKC pseudo-stationary phases [18], only four papers on CEC stationary phases were found [19], [20], [21], [22].
In general, non-covalent methods are simpler than covalent methods, but covalent methods provide a stronger and steadier incorporation of functional moieties onto the derived capillary wall surface. In two of the CEC papers on CNT-incorporated stationary phases, the negatively charged single-walled CNTs (SWNTs) were electrostatically adsorbed on the positively charged amine-based capillary by using either the simple acid-treated form [19] or blending in the monolithic monomers (vinylbenzyl chloride and ethylene dimethacrylate) [20]. Another paper also considered a non-covalent method where SWNTs conjugated with BSA proteins are physically encapsulated in the microchip electrophoresis channels through sol–gel condensation [21]. Another recent study seemed to use a covalent immobilization method [22]. However, the detailed chemistry dealing with the bonding between carboxylic multi-walled CNTs (MWNTs) and the glutaraldehyde-treated capillary following the silanization with the 3-aminopropyl triethoxysilane reagent was not unveiled.
We suggest the utilization of the double bonds within the CNTs structure as a reasonable approach to covalently attach the moieties onto the capillary wall. Carbon-atom pentagons at the curvature points in the CNTs are assumed to be involved in the π-bonds that break during the polymerization reaction [23], [24]. According to work done over the last decade, Pesek et al. have developed a novel method that replaces approximately 95% of the Si–OH groups on the bare capillary with Si–H groups and further attaches desired organic moieties with carbon double-bond functionality to the hydride surface [25], [26]. The reaction protocol, involving silanization and hydrosilation, leads to a stable Si–C bond between the capillary wall and the organic moiety. One of the primary advantages of this protocol is its versatility. Until recently, these hydride-based OT-CEC stationary phases included the attached moieties of diol, n-butylphenyl, cholesterol, n-C5, and n-C18 [27], [28], [5]. Furthermore, we incorporate ionizable carboxylate ligands in with the hydride phases to enhance the EOF drive in CEC separation [29], [30].
In this study, MWNTs treated by HNO3 and then HCl were attached to a hydride-based surface. The complete capillary was characterized by the measurements of SEM and ATR-IR. Furthermore, the effect of the EOF on the changes in pH and acetonitrile volume percentage in the running buffers were recorded and tracked for each intermediate capillary between the silanization and hydrosilation of different acid-treated MWNTs steps. A mixture of nucleosides and thymine were probed to prove that the CEC mechanism existing in the MWNT immobilized capillary. Furthermore, a modified capillary with higher loading content of MWNTs was tried to separate tetracycline samples.
Section snippets
Materials
Most chemicals used were of analytical or chromatographic grade. Purified water (18 MΩ cm) from a Milli-Q water purification system (Millipore, Bedford, MA, USA) was used to prepare samples and buffer solutions. All solvents and solutions for CEC analysis were filtered through a 0.45 μm cellulose ester membrane (Adventec MFS, Pleasanton, CA, USA).
SEM images
The MWNT materials used in this study were treated in 3 M HNO3, followed by a reflux process in 5 M HCl. After the acidic treatment and drying, the morphological appearance of the MWNT was as presented in the SEM image of Fig. 1(A). Although many nanotubes were clogged and formed in a mass, they will be dispersed in the dioxane medium. The SEM image in Fig. 1(B) demonstrates the completion of the MWNT spreading over the silica-hydride layer on the capillary wall surface. It is evident that
Conclusions
This study took advantage of the high reactivity of carbon-atom pentagons in the CNT structure. The bonded-phase CEC capillaries were successfully completed by hydrosilation of non-acid- or acid-treated MWNTs with the silica-hydride capillary. In addition to SEM and ATR-IR, these CEC capillaries were characterized by the EOF responses driven from the carboxylate groups on the MWNTs to the pH levels of buffers and to the addition of ACN modifier. The chromatographic contribution to the CEC
Acknowledgements
Support of this work by the National Science Council of Taiwan (NSC-98-2113-M-039-003-MY3) is gratefully acknowledged.
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