Synthesis, characterization, and application of novel pseudostationary phases in micellar electrokinetic chromatography for separation of chiral and achiral molecules
Chirality (handedness) is a property of an object which is non-superimposable with its mirror image and is caused by the presence of one or more chirality elements (chirality axis, chirality plane, or chirality center).
Chirality of Molecules
Enantiomers (opposite portion), or optical isomers, are two chemically identical molecular species which differ from each other as non-superimposable mirror images. A mixture of equal amounts of the two enantiomers is said to be a racemic mixture.
Chiral molecules can show their different handedness in may ways, including the way they affect human beings.
Chirality and Human Body
The fact that the limonene or carvone enantiomers do not smell the same suggests that the receptor sites in our nose for these compounds are chiral and only correct enantiomer will fit its particular site.
Chirality and Health
“It was in the late 1950s that a German company called Grunenthal launched thalidomide, marketed as a cure for morning sickness, in 46 countries around the world.”
“It had never been tested on pregnant mammals, let alone women, and caused the deaths of up to 100,000 babies in the womb. Of the 10,000 born, nearly half died in the first year.”
“Those who survived were left with a range of disabilities, typically short limbs and flipper-like arms as well as deafness and blindness.” (http://www.dailymail.co.uk; 23 Apr 2008)
Development of polymeric and co-polymeric chiral and achiral surfactant micelles for separation of chiral/achiral pharmaceuticals and drug delivery systems
Surfactant-modified single (SWNT) and multi wall carbon nanotubes (MWNT) as pseudostationary phases in capillary electrophoresis
Separation of carbon nanotubes using capillary electrophoresis (CE) and high performance liquid chromatography (HPLC)
Separation science, especially chromatography, is an analytical chemistry discipline that is used to separate, isolate, and quantify compounds from complex mixtures. Chemists in pharmaceutical companies use the separation techniques to isolate a product from a reaction mixture, to isolate active components from natural products, to identify possible breakdown products of drugs, and to carry on chiral separation to isolate the active enantiomeric drug from inactive one. Environmental chemists use the separation techniques to quantify the amount of an analyte present in a sample and to assess the fate and transport of a compound in the environment.
My research interests are: 1) to synthesize, characterize, and utilize novel monomeric and polymeric chiral and achiral surfactants and apply them as pseudostationary phases for enantioseparation of chiral and achiral molecules using capillary electrophoresis (CE), a separation science technique; 2) to develop methods for real life experiments, e.g., quantification of chemicals (drugs and their metabolites) in body fluids and in environment (PAHs, PCBs, explosive residuals); 3) to investigate partitioning mechanisms between pseudo-stationary phases and analytes using linear salvation energy relationships (LSER) model; 4) to separate carbon nanotubes (CNTs), as a nanoscience project, using both CE and high performance liquid chromatography (HPLC); and 5) to use CNTs as pseudostationary phases in CE.
One of my major goals is to work with undergraduate students on small projects to make sure our students get involved in research and get enough experience for graduate school or for a better job. To accomplish this, I intend to use a variety of analytical tools such as CE, HPLC, gas chromatography (GC), fluorescence, ultraviolet, infrared spectrometry, densitometry, and surface tensiometry to make sure undergraduate students get familiar with a variety of analytical instruments. Which technique gets more attention will depend on the availability of the instrument we have in my research laboratory or in the department. There is a CE and density meter in my research laboratory. For other technique, I will use the department’s resources.
CE is a technique that separates compound mixtures on the basis of electrophoretic mobility differences. Recently, CE has become widely used in various fields. CE can offer advantages over other separation techniques such as HPLC and GC. CE is a powerful and practical tool because of its high resolution, ability to analyze impure samples, low reagent consumption, short analysis time and low running cost. Due to these strengths, CE has been used in the Genome Project as a powerful and practical separation tool.
By manipulating the separation media, separation systems can be devised for very specific purposes. There are several well known CE modes that can be utilized using the very same instrument. These modes are: capillary zone electrophoresis (CZE), capillary gel electrophoresis, capillary isotachophoresis, capillary isoelectric focusing, and micellar electrokinetic chromatography (MEKC). Due to their lack of electrical charge, neutral molecules cannot be separated by CZE. However, in MEKC, the separation media has been manipulated to allow for the separation of neutral as well as the charged compounds.
MEKC utilizes a micelle forming compound (called pseudo-stationary phase) to obtain separation of analytes. Virtually any micelle forming compound can be added to the buffer system to obtain separation. Sodium dodecyl sulfate (SDS, the soap found in shampoos), is a commonly used micelle forming additive. Analytes partition between the mobile buffer phase (usually aqueous or organic solvent modified) and the pseudo-stationary phase as they move through the separation capillary. Conventional surfactant (e.g., SDS) micelles are successfully used in separations of hydrophilic and slightly hydrophobic analytes. However, one should add organic modifiers to the buffer system for separation of highly hydrophobic analytes (such as polycyclic aromatic hydrocarbons, PAHs). Higher organic modifier concentrations tend to disrupt the conventional micelles and eventually affect the separation quality.
Polymeric surfactants (or molecular micelles) have gained popularity as potential pseudostationary phases for separations in MEKC in the recent years. A considerable interest in the use of polymeric surfactants arises because of their distinct advantages over conventional micelles. First, they have zero critical micelle concentration (cmc); thus, they may be used at concentrations well below the cmc of the unpolymerized surfactants. Second, molecular micelles are stable in the presence of a high content of organic solvents due to the covalent bond between surfactant monomers. Hence, organic additives do not disrupt the primary covalent structure of the micelle polymer. One should keep in mind that most biological samples typically comprise polar compounds that may also contain hydrophobic moieties. Thus, the use of organic solvents in combination with micelles is often required for the analysis of such compounds. In addition, the fixed micellar structure prevents dissociation of surfactant molecules during the electrospray process in mass spectrometry (MS). Third, due to their high molecular weight, molecular micelles can be conveniently used in MEKC-MS applications without background interference from surfactant monomers of low molecular weights. Fourth, lower surface activity and low volatility of molecular micelles provide a stable electrospray and hence less suppression of analyte signal in MEKC-MS.
Carbon nanotube (CNT), the forth allotrope of carbon, was discovered in 1991. The backbone of CNT is composed solely of carbon atoms, arranged in benzene rings forming graphene sheets, rolled up to give seamless cylinders with several micrometers in length and nanosized diameter. CNTs hold strong promise for nano- and biotechnological applications. There are two main types of CNTs, single wall (SWCT) and multi wall carbon nanotubes (MWCNTs). Depending on their diameter and chirality, CNTs can be metallic or semiconducting. The semiconducting CNTs offer possibilities to create semiconductor-semiconductor and semiconductor-metal junction which may be useful in electronic and sensor devices. CNTs have been proposed and used in a number of different applications, including field emission, energy storage, hydrogen storage, molecular electronics, atomic force microscopy, and many other areas such as drug delivery systems. Applications of CNTs in the field of biotechnology are raising great hopes. CNTs have been proposed as DNA and protein biosensors, ion channel blockers and as bioseparators. In addition, their use is becoming relevant in neuroscience research and tissue engineering. CNTs have also been used for detection of antibodies associated with human autoimmune diseases with high specificity.
A major drawback of CNTs is their complete insolubility in all types of solvents. In addition, CNTs tend to agglomerate in the form of close-packed arrays termed ‘‘nanoropes’’ owing to their similarity to conventional ropes. These arrays contain hundreds of nanotubes. Pure, monodisperse nanotubes will be essential for aforementioned applications. Dissolution and purification of CNTs are important steps to fully understand their properties and take a full advantage of their applications. Several methods have been introduced and proposed for purification of CNTs. For example, chemical oxidation removes amorphous carbon and catalyst particles, filtration and centrifugation are also used to purify CNTs. Size exclusion chromatography (SEC) and CE effectively purifies carbon nanotubes. Unfortunately, SEC, and CE do not provide direct, detailed information on particle size distributions. In addition, a pre-condition for efficient separation is that the CNT “ropes” are individualized. This can be achieved by suspending CNTs in aqueous surfactant solution under sonication, where CNT bundles (or ropes) split up and the individualized CNTs then being hindered by the surfactant to rebundle. The excess bundles can be removed by centrifugation. In this research project, we intended to develop a method that could be used for separate CNTs based on their chirality and length. As-prepared CNTs are composed of a variety of nanotubes with different diameter, length and chiralities. One has to sort them based on their chirality or length for certain applications. The separation of CNTs based on length and, especially, chirality remains a challenge for nanoscientists today.