Enhanced separation of purine and pyrimidine bases using carboxylic multiwalled carbon nanotubes as additive in capillary zone electrophoresis

CE with multi-walled carbon nanotubes (MWCNTs). Part I. Functionalized and SDS coated MWCNTs as pseudo-stationary phases in nanoparticle EKC - Studies on retention energetics

In this study, multi-walled carbon nanotubes (MWCNTs) in either unmodified, hydroxylated (MWCNT-OH), carboxylated (MWCNT-COOH) or sulfonated (MWCNT-SO₃H) forms were incorporated into the running electrolytes in capillary electrophoresis (CE) to play the role of pseudo-stationary phases (PSPs) and perform nanoparticle electrokinetic capillary chromatography (NPEKC). MWCNT-COOH and MWCNT-SO₃H were derived from MWCNTs via their treatment with concentrated strong acids. These functionalized MWCNTs were characterized by Raman and FTIR spectroscopies to demonstrate their covalent functionalization. The study of MWCNT-SO₃H and MWCNT-OH as PSPs were introduced in this research report for the first time in NPEKC. The results obtained with functionalized MWCNTs were compared to those obtained using unmodified MWCNTs for better understanding the electrophoretic behavior of these functionalized MWCNTs. While only MWCNT-COOH allowed the separation of some nucleic acid bases and nucleosides, neutral solutes such as alkylbenzenes (ABs), phenyl alkyl alcohols (PAAs) and aniline derivatives in neutral forms (i.e., at basic pH) were not resolved in the presence of neither MWCNT-COOH nor MWCNT-SO₃H in the running electrolytes, indicating that these functionalized MWCNTs do not have enough surface charge density to function as effective PSPs in NPEKC. This necessitated the coating of the functionalized MWCNTs under investigation with sodium dodecyl sulfate (SDS) to bring about the separation of neutral solutes by NPEKC. The SDS coated MWCNTs whether unmodified or functionalized were characterized with two homologous series namely ABs and PAAs in order to evaluate their relative retention energetics under the same electrolyte composition. The results showed that the systems pairs SDS-MWCNT-COOH/SDS-MWCNTs and SDS-MWCNT-OH/SDS-MWCNTs were homoenergetics (i.e., same energetics) while the system pair SDS-MWCNT-SO₃H/SDS-MWCNTs was homeoenergetics (i.e., similar energetics). On the other hand, all the systems pairs SDS coated MWCNTs/SDS were homeoenergetics. Homoenergetics means that the solute retention has an identical physico-chemical basis and the differences observed in the magnitude of solute retention on the various PSPs are attributed to differences in the nonpolar phase ratios of the PSPs under otherwise the same electrolyte composition. Conversely, homeoenergetics signifies that the solute retention has a similar physico-chemical basis in the PSPs systems under investigation, which also differ in their phase ratios.

CE with multi-walled carbon nanotubes (MWCNTs). Part II. SDS coated functionalized MWCNTs as pseudo-stationary phases in nanoparticle EKC - Retention behaviors of small and large solutes

In this study, functionalized multi-walled carbon nanotubes (MWCNTs), namely hydroxylated MWCNTs (MWCNT-OH), carboxylated MWCNTs (MWCNT-COOH) and sulfonated MWCNTs (MWCNT-SO₃H) coated with sodium dodecyl sulfate (SDS) were demonstrated as effective pseudo-stationary phases (PSPs) in the separation of various species by the nanoparticle capillary electrokinetic chromatography (NPEKC) mode of capillary electrophoresis (CE). Due to the significant increase in their surface charge density in the presence of SDS, the three SDS coated MWCNTs yielded high performance separation for herbicides, barbiturates, dansyl-DL-amino acids (Dns-AAs), dipeptides and proteins by NPEKC. In addition, high resolution tryptic peptide maps of three standard proteins including myoglobin, cytochrome C and lysozyme were readily obtained. The three PSPs systems yielded high plate numbers that spanned a wide range of values depending on the type of species. The values of the observed selectivity factors (i.e., α values) were significantly different among the three PSPs for solutes that underwent strong interactions with the SDS coated functionalized MWCNTs while for negatively charged solutes (e.g., Dns-AAs) of the same charge sign as the PSPs the α values were about the same on the three different PSPs indicating weak association with the PSPs and signaling separation based chiefly on the differences in electro-migration arising from differences in charge-to-mass ratios.

Application of Carbon Nanotubes in Chiral and Achiral Separations of Pharmaceuticals, Biologics and Chemicals

Carbon nanotubes (CNTs) possess unique mechanical, physical, electrical and absorbability properties coupled with their nanometer dimensional scale that renders them extremely valuable for applications in many fields including nanotechnology and chromatographic separation. The aim of this review is to provide an updated overview about the applications of CNTs in chiral and achiral separations of pharmaceuticals, biologics and chemicals. Chiral single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) have been directly applied for the enantioseparation of pharmaceuticals and biologicals by using them as stationary or pseudostationary phases in chromatographic separation techniques such as high-performance liquid chromatography (HPLC), capillary electrophoresis (CE) and gas chromatography (GC). Achiral MWCNTs have been used for achiral separations as efficient sorbent objects in solid-phase extraction techniques of biochemicals and drugs. Achiral SWCNTs have been applied in achiral separation of biological samples. Achiral SWCNTs and MWCNTs have been also successfully used to separate achiral mixtures of pharmaceuticals and chemicals. Collectively, functionalized CNTs have been indirectly applied in separation science by enhancing the enantioseparation of different chiral selectors whereas non-functionalized CNTs have shown efficient capabilities for chiral separations by using techniques such as encapsulation or immobilization in polymer monolithic columns.

Analysis of five alkaloids using surfactant-coated multi-walled carbon nanotubes as the pseudostationary phase in nonaqueous capillary electrophoresis

In this paper, surfactant-coated multi-walled carbon nanotubes (SC-MWNTs) have been proposed as a novel pseudostationary phase (PSP) to enhance the separation of isoquinoline alkaloids in nonaqueous capillary electrophoresis (NACE). Several parameters affecting NACE separation were studied including the MWNT concentration, the electrolyte concentration, pH* and the separation voltage. In comparison to conventional NACE, the addition of an MWNT dispersion using surfactant solutions in the electrolyte produced an important enhancement in the resolution due to the π-π interactions between the analytes and the surface of the carbon nanotubes. Using SC-MWNTs (6μgmL(-1)) as a PSP in the background electrolyte (BGE) (i.e., 20mM sodium acetate in methanol-acetonitrile (80:20, v/v)) provided the complete separation of five alkaloids. Finally, the developed method has been successfully applied to the detection and quantification of the tested compounds of Rhizoma Coptidis.

Use of nanomaterials in capillary and microchip electrophoresis

This review gives an overview of different separation strategies with nanomaterials and their use in capillary electrophoresis (CE) and capillary electrochromatography, as well as in microchip electrophoresis, including metal and metal oxide nanoparticles, carbon nanotubes, fullerene and polymer nanoparticles, as well as silica nanoparticles. The paper highlights the new developments and innovative applications of nanoparticles as pseudostationary phases or immobilized on the capillary surface for CE separation. The separation and characterization of target nanoparticles with different sizes by CE are reviewed likewise.

Carbon nanotubes in capillary electrophoresis, capillary electrochromatography and microchip electrophoresis

Carbon nanotubes are among the plethora of novel nanostructures developed since the 1980s. Nanotubes have attracted considerable interest by the scientific community thanks to their extraordinary physical and chemical properties. Research areas have flourished in recent years and now include the nano-electronic, (bio)sensor and analytical field along with many others. This review covers applications of carbon nanotubes in capillary electrophoresis, capillary electrochromatography and microchip electrophoresis. First, carbon nanotubes and a range of electrophoretic techniques are briefly introduced and key references are mentioned. Next, a comprehensive survey of achievements in the field is presented and critically assessed. The merits and downsides of carbon nanotube addition to the various capillary electrophoretic modes are addressed. The different schemes for fabricating electrochromatographic stationary phases based on carbon nanotubes are discussed. Finally, some future perspectives are offered.

Computational algorithms for fast-building 3D carbon nanotube models with defects

Algorithms for generating defective carbon nanotubes have been developed and implemented in software. The algorithms were designed to create arrays of carbon atoms that form layers and interconnect. The parameters for construction were the following: Hamada indices that respond to topology (armchair, zigzag or chiral nanotubes) and diameter, the saturated or unsaturated nature of the nanotube, the length and, most importantly, the presence of defects that can be built individually or repetitively by rotating bonds, removing atoms, or adding additional carbon atoms. The output was written in a standard, exportable file format that contained atomic coordinates useful for further computational chemistry work.

Carbon nanotube based stationary phases for microchip chromatography

The objective of this article is to provide an overview and critical evaluation of the use of carbon nanotubes and related carbon-based nanomaterials for microchip chromatography. The unique properties of carbon nanotubes, such as a very high surface area and intriguing adsorptive behaviour, have already been demonstrated in more classical formats, for improved separation performance in gas and liquid chromatography, and for unique applications in solid phase extraction. Carbon nanotubes are now also entering the field of microfluidics, where there is a large potential to be able to provide integrated, tailor-made nanotube columns by means of catalytic growth of the nanotubes inside the fluidic channels. An evaluation of the different implementations of carbon nanotubes and related carbon-based nanomaterials for microfluidic chromatography devices is given in terms of separation performance and ease of fabrication.

NaDDBS as a dispersion agent for multiwalled carbon nanotubes in capillary EKC separation of nucleotides

A novel pseudostationary phase (PSP) of multiwalled carbon nanotubes (MWCNTs) dispersed with sodium dodecylbenzenesulfonate (NaDDBS) was used for the EKC separation of nucleotides. NaDDBS has a long hydrophobic chain and a benzylsulfonate group. It suspends more MWCNTs (about 100-fold) than SDS, and the π-π interaction between the benzene ring of NaDDBS and MWCNTs prolongs the slurry suspension time. Using NaDDBS as a surfactant can reduce the required amount of MWCNTs and decrease the baseline noise. To produce a stable suspension, the optimum ratio (w/w) of MWCNTs to NaDDBS was investigated with turbidimetry. In this context, several parameters affecting EKC separation were studied, including buffer pH, composition, concentration, and the organic modifier. Use of NaDDBS (8 mg/L)/MWCNTs (0.8 mg/L) as the PSP in a phosphate buffer (30 mM, pH 8) yielded complete resolution of seven geometric isomers of a nucleoside monophosphate. In stacking mode, with 10% MeOH in the sample plug, the mixture of nucleoside mono-, di-, and tri-phosphates was satisfactorily separated in phosphate buffer (50 mM, pH 9). The results indicate that nucleotides with bases containing more electron-withdrawing groups interact more strongly with MWCNTs. The system has been used to separate oligonucleotides, and to analyze nucleotides in a complex matrix sample.

Ionic liquids coated multi-walled carbon nanotubes as a novel pseudostationary phase in electrokinetic chromatography

A new CE system using ionic liquids coated multi-walled carbon nanotubes (ILs-MWNTs) as pseudostationary phase was developed for the simultaneous determination of four flavonoids, four phenolic acids and two saponins. Several parameters affecting the separation were studied, including the choice of ILs, ILs-MWNTs concentration, the respective use of ILs and MWNTs, buffer pH, SDS concentration and borate content. Results revealed that the addition of ILs-MWNTs in running electrolytes enhanced the separation of target compounds compared to conventional micelle because the surface of carbon nanotubes interacted favorably with the analytes. Under the optimum conditions, a baseline separation was achieved for these analytes within 11 min in a 41.5 cm of effective length fused-silica capillary. At a voltage of 28.0 kV, the separation was carried out in a 10mM borate buffer (pH 9.0) containing 100mM SDS, 6% propanol and 4 μg mL(-1) ILs-MWNTs. All calibration curves showed good linearity (r(2)>0.9990) within the test ranges. The intra- and inter-day precisions as determined from standard solutions were below 3.30% and 6.23%, respectively. The recoveries for ten compounds were found to range from 85.5 to 101.8%. The method was successfully applied for the determination of three types of compounds in Qishenyiqi dropping pills. Our experimental results indicated that the proposed method offered new opportunities for the analysis of complex samples.

A combination of single-drop microextraction and open tubular capillary electrochromatography with carbon nanotubes as stationary phase for the determination of low concentration of illicit drugs in horse urine

In this study we developed an interesting alternative to HPLC-mass spectrometry for the quantification of seven important drugs of abuse in racehorses. The procedure proposed in this work is a combination of single-drop microextraction (SDME) and an open tubular capillary electrochromatography (OT-CEC) using multi-wall carbon nanotubes (MWCTs) immobilized into a fused-silica capillary as a stationary phase. The SDME showed to be a powerful tool for extraction/preconcentration of the seven drugs analyzed in the study, showing an enrichment factor between 38- and 102-fold depending on the drug. We have investigated the electrophoretic features of MWCTs immobilized fused-silica capillary by covalent modification of the inner surface of the capillary. The results show a good run-to-run, day-to-day and capillary-to-capillary reproducibility of the method. Compared with the capillary zone electrophoresis (CZE), the coating of the capillary allowed the separation of the analytes with high resolution, with less band-broadening and without distortion of the baseline. The interactions between the analytes and the MWCTs resulted in an increased migration time and probably this was the reason of the front tailing effect. The results showed a good capillary efficiencies and an improved of the electrophoretic separation.

Recent applications of carbon-based nanomaterials in analytical chemistry: critical review

The objective of this review is to provide a broad overview of the advantages and limitations of carbon-based nanomaterials with respect to analytical chemistry. Aiming to illustrate the impact of nanomaterials on the development of novel analytical applications, developments reported in the 2005-2010 period have been included and divided into sample preparation, separation, and detection. Within each section, fullerenes, carbon nanotubes, graphene, and composite materials will be addressed specifically. Although only briefly discussed, included is a section highlighting nanomaterials with interesting catalytic properties that can be used in the design of future devices for analytical chemistry.

Determination of melatonin in wine and plant extracts by capillary electrochromatography with immobilized carboxylic multi-walled carbon nanotubes as stationary phase

The finding of melatonin, the often called "hormone of darkness" in plants opens an interesting perspective associated to the plethora of health benefits related to the moderate consumption of red wine. In this study, the implementation of a new method for the determination of melatonin in complex food matrices by CEC with immobilized carboxylic multi-walled carbon nanotubes as stationary phase is demonstrated. The results indicated high electrochromatographic resolution, good capillary efficiencies and improved sensitivity respect to those obtained with conventional capillaries. In addition, it was demonstrated highly reproducible results between runs, days and columns. The LOD for melatonin was 0.01 ng/mL. The method was successfully applied to the determination of melatonin in red and white wine, grape skin and plant extracts of Salvia officinalis L.

Novel application of carbon nanotubes for improving resolution in detecting human serum proteins with native polyacrylamide gel electrophoresis

This paper describes a novel application of carbon nanotubes for improving the resolution of a native PAGE in the detection of human serum proteins. Carbon nanotubes were functionalized and introduced into the gel of native PAGE system, and the electropherogram showed sharp, clear bands. Furthermore, the separation of some most important proteins was improved, and the established method could be applied for the detection of sera from patients with liver diseases.