Multiple sampling in single-cell enzyme assays using capillary electrophoresis with laser-induced fluorescence detection

Electrochemical coupled-enzyme assays at carbon nanotubes

The recently developed internally calibrated electrochemical continuous enzyme assay (ICECEA) has proved to work well for single-enzyme systems. In the present work, its relevance to more challenging coupled-enzyme measurements was investigated by using a model enzyme pair comprising aspartate transaminase (AST) and malic dehydrogenase. The ICECEA was performed at an electrode modified with carbon nanotubes (CNTs), which were dispersed in a polysaccharide chitosan that acted as an adhesive. The 7 min assay required a 100 μL sample and relied on an AST-free calibration. It had a limit of detection equal to 5.0 pM AST (0.10 U L(-1)) with no need for the incubation period. Its linear range extended up to 3500 pM (70 U L(-1)). Perhaps the most promising was the fact that the assay and its calibration could be performed in the same solution even though the composition of the assay solution for the coupled-enzyme assays is typically more complex than that for the single-enzyme assays. This and the fast electrode kinetics of the signal transducing reaction of nicotinamide adenine dinucleotide at CNTs accounted for the low limit of detection. The unique shape of the ICECEA amperogram allowed for the selective determination of AST in the complex matrix of serum samples containing redox active potentially interfering species. Given these advantages, the prospects for the ICECEA in the development of other coupled-enzyme assays were also discussed.

Determination of ascorbic acid in individual rat hepatocyte cells based on capillary electrophoresis with electrochemiluminescence detection

A novel method to detect ascorbic acid (AA) in individual rat hepatocyte cells was developed by combining CE with electrochemiluminescence (ECL) based on tris(2,2'-bipyridine) ruthenium(II) (Ru(bpy)(3)2+). A single cell, followed by 0.1% SDS as cell lysis solution, was injected into the inlet of the separation capillary by electromigration. After optimizing the analytical conditions, the RSDs of migration time and peak height were 0.38% and 2.6% for 1.0x10(-5) M AA (n=10), respectively. The linear range of AA was from 1.0x10(-8) to 5.0x10(-5) M with a correlation coefficient of 0.9979 and the LOD (S/N=3) was estimated to be 1.0x10(-8) M. This method has been successfully applied to determine AA in single rat hepatocytes and the amount of AA in seven rat hepatocytes ranged from 16 to 62 fmol. The above results demonstrated that CE coupled with ECL is convenient, sensitive, and will become an attractive alternative method for single-cell analysis.

Fast-lysis cell traps for chemical cytometry

Electrically addressable cell traps were integrated with capillary electrophoresis for the analysis of the contents of single adherent cells. Electrodes composed of indium tin oxide were patterned on a glass surface followed by formation of topographical cell traps using 1002F photoresist. Single cells trapped in the holes could be lysed in less than 66 ms by applying a brief electric field (10 ms) across the electrode beneath the cell and the ground electrode placed in the aqueous media above the cell traps. The gas formed during cell lysis remained localized within the cavity formed by the 1002F photoresist. The retention of the gas in the cell trap enabled the cell traps to be coupled to an overlying capillary without blockage of the capillary. Single cells cultured in the traps were loaded with fluorescein and Oregon Green and then electrically lysed. By simultaneous application of an electric field to the capillary, the cell's contents were loaded into the capillary and electrophoretically separated. Orgeon Green and fluorescein from a single cell were fully resolved in less than two minutes. The use of a single patterned electrode beneath the 1002F cell trap yielded a simple easily fabricated design that was robust when immersed in aqueous solutions. Moreover, the design can easily be scaled up to create arrays of adherent cells for serial analyses using a single capillary or for parallel analysis by mating to an array of capillaries. Enhancing the rate of analysis of single adherent cells would enable a greater understanding of cellular physiology.

Chemical analysis of single cells

Chemical analysis of single cells requires methods for quickly and quantitatively detecting a diverse array of analytes from extremely small volumes (femtoliters to nanoliters) with very high sensitivity and selectivity. Microelectrophoretic separations, using both traditional capillary electrophoresis and emerging microfluidic methods, are well suited for handling the unique size of single cells and limited numbers of intracellular molecules. Numerous analytes, ranging from small molecules such as amino acids and neurotransmitters to large proteins and subcellular organelles, have been quantified in single cells using microelectrophoretic separation techniques. Microseparation techniques, coupled to varying detection schemes including absorbance and fluorescence detection, electrochemical detection, and mass spectrometry, have allowed researchers to examine a number of processes inside single cells. This review also touches on a promising direction in single cell cytometry: the development of microfluidics for integrated cellular manipulation, chemical processing, and separation of cellular contents.

Coaxial flow system for chemical cytometry

Over the past decade, chemical cytometry performed by capillary electrophoresis (CE) has become increasingly valuable as a bioanalytical tool to quantify analytes from single cells. However, extensive use of CE-based chemical cytometry has been hindered by the relatively low throughput for the analysis of single adherent cells. In order to overcome the low throughput of CE-based analysis of adherent cells and increase its utility in evaluating cellular attributes, new higher throughput methods are needed. Integration of a coaxial buffer exchange system with CE-based chemical cytometry increased the rate of serial analyses of cells. In the designed system, fluid flow through a tube coaxial to the separation capillary was used to supply electrophoretic buffer to the capillary. This sheath or coaxial fluid was turned off between analysis of cells and on during cell sampling and electrophoresis. Thus, living cells were not exposed to the nonphysiologic electrophoretic buffer prior to lysis. Key parameters of the system such as the relative capillary-sheath positions, buffer flow velocities, and the cell chamber design were optimized. To demonstrate the utility of the system, rat basophilic leukemic cells loaded with Oregon green and fluorescein were serially lysed and loaded into a capillary. Separation of the contents of 20 cells at a rate of 0.5 cells/min was demonstrated.