Exploring single-cell dynamics using chemically-modified microelectrodes

Metabolite analyses of single cells

Single-cell analysis is a promising method for understanding not only cellular physiology but also biological mechanisms of multicellular organisms. Although neighboring cells in multicellular organisms originate from the same genomic information, different circumstances around cells or epigenetic differences have different influences on each cell, leading to differing expression of genes, and thus differing levels and dynamics of metabolites, in single cells. However, single-cell analysis is a tough challenge, even with recent technologies, because of the small size of single cells. Unlike genes, metabolites cannot be amplified, and therefore metabolite analysis is another issue. To analyze such a tiny quantity of metabolites in a single cell, various techniques have been tried and developed. Especially in mass spectrometry, marked improvements in both detection sensitivity and ionization techniques have opened up the challenge for the analysis of metabolites in single cells. In this review, we discuss the method for metabolite detection at the level of single cells and recent advancements in technology.

Amperometric micro-immunosensor for the detection of tumor biomarker

In this paper, a highly sensitive, reagentless, electrochemical strategy is reported for the detection of a cancer biomarker-Vascular Endothelial Growth Factor (VEGF). Disc shaped carbon fiber microelectrodes were used as the immunosensor platform. Ferrocene monocarboxylic acid labeled anti-VEGF was covalently immobilized on the microelectrode surface using a Jeffamine cross-linker. The formation of immunocomplexes leads to a decrease in the electrochemical signal of ferrocene monocarboxylic acid owing to increased spatial blocking of microelectrode surface. These signal changes enable quantitative detection of VEGF in solution. Voltammetric measurements were conducted to evaluate the interfacial immunoreactions and to quantitatively detect VEGF biomarker. The proposed immunosensing strategy allows a rapid and sensitive means of VEGF analysis with a limit of detection of about 38 pg/mL. This opens up the possibility of employing these electrodes for various single cell analysis and clinical applications. Further, experimental conditions such as concentration of the immobilized antibodies and incubation period were optimized. Following this, the stability and specificity of the immunosensors were also evaluated.

Metabolic monitoring of the electrically stimulated single heart cell within a microfluidic platform

A device based on five individually addressable microelectrodes, fully integrated within a microfluidic system, has been fabricated to enable the real-time measurement of ionic and metabolic fluxes from electrically active, beating single heart cells. The electrode array comprised one pair of pacing microelectrodes, used for field-stimulation of the cell, and three other microelectrodes, configured as an electrochemical lactate microbiosensor, that were used to measure the amounts of lactate produced by the heart cell. The device also allowed simultaneous in-situ microscopy, enabling optical measurements of cell contractility and fluorescence measurements of extracellular pH and cellular Ca2+. Initial experiments aimed to create a metabolic profile of the beating heart cell, and results show well defined excitation-contraction (EC) coupling at different rates. Ca2+ transients and extracellular pH measurements were obtained from continually paced single myocytes, both as a function of the rate of cell contraction. Finally, the relative amounts of intra- and extra-cellular lactate produced during field stimulation were determined, using cell electroporation where necessary.

Robotic sequential analysis of a library of metalloporphyrins as electrocatalysts for voltammetric nitric oxide sensors

A library of 83 metalloporphyrins with varying substitution pattern at the meso-position of the porphyrins and different central metal ions in the core region has been synthesized in small quantities using a parallel synthesis strategy. By means of a specially designed electrochemical robotic device integrating a 96-well microtitre plate and an easily movable assembly of working, counter and reference electrodes, the different porphyrins were automatically applied in sequence to an in-well electrochemical preparation and testing of NO sensors. Screening the entire compound collection suggested initial considerations concerning the influence of varied functionalities of the metalloporphyrins on their electrocatalytic properties for the oxidation of NO and helped to identify the quality of the investigated catalyst candidates. As compared to manually performed quality tests, the proposed strategy of automation has advantages in convenience, rapidity and especially reproducibility avoiding any inaccuracies introduced by manually performing all steps of the complex sensor formation and testing sequence.

Coupling of Solid-Phase Microextraction and Capillary Isoelectric Focusing with Laser-Induced Fluorescence Whole Column Imaging Detection for Protein Analysis

A coupling method of solid-phase microextraction (SPME) and capillary isoelectric focusing (CIEF) with laser-induced fluorescence (LIF) whole column imaging detection (WCID) was developed for the analysis of proteins. Unlike other liquid-phase separation methods and conventional CIEF, proteins are focused into stationary bands within a pH gradient in CIEF-WCID. Thus, CIEF-WCID is the most compatible liquid-phase separation method for coupling with SPME, which can effectively resolve the problems associated with the slow desorption kinetics of SPME in a liquid phase. By combining SPME and CIEF-WCID, the desorption time can be as long as necessary, allowing complete desorption without any band broadening and analyte carryover. By using this method, R-phycoerythrin in water can be extracted by SPME in 10 min, and subsequently analyzed by CIEF-LIF-WCID within 20 min, providing a limit of detection of 3.5 x 10(-12) M (S/N = 3). The feasibility of the SPME-CIEF-LIF-WCID method was demonstrated by extracting and analyzing extracellular phycoerythrins in cultured cyanobacteria samples. Extracellular phycoerythrins at the nanomolar level were extracted and analyzed in 30 min, while avoiding the interference of the cyanobacteria cells.

Transport, location, and quantal release monitoring of single cells on a microfluidic device

A novel microfluidic device has been developed for on-chip transport, location, and quantal release monitoring of single cells. The microfluidic device consists of a plate of PDMS containing channels for introducing cells and stimulants and a glass substrate into which a cell micro-chamber was etched. The two tightly reversibly sealed plates can be separated for respective cleaning, which significantly extends the lifetime of the microchip that is frequently clogged in cell analysis experiments. Using hydraulic pressure, single cells were transported and located on the microfluidic chip. After location of a single PC12 cell on the microfluidic chip, the cell was stimulated by nicotine that was also introduced through the micro-channels, and the quantum release of dopamine from the cell was amperometricly detected with our designed carbon fiber microelectrode. The results have demonstrated the convenience and efficiency of using the microfluidic chip for monitoring of quantal release from single cells and have offered a facile method for the analysis of single cells on microfluidic devices.

Carbon Fiber Nanoelectrodes Modified by Single-Walled Carbon Nanotubes

Microelectrode voltammetry has been considered to be a powerful technique for single biological cell analysis and brain research. In this paper, we have developed a simple method to get highly sensitive carbon fiber nanoelectrodes (CFNE) modified by single-walled carbon nanotubes (SWNTs) on the basis of our previous work. The electrochemical behavior of SWNTs/CFNE was characterized by potassium ferricyanide, dopamine (DA), epinephrine (E), and norepinephrine (NE) using cyclic voltammetry (CV). Compared with CFNE, SWNTs/CFNE has a much larger available internal surface area per external geometric area, which is supported by SEM images. The modified electrodes show very high sensitivity and favorable electrochemical behavior toward these neurotransmitters. The peak current increases linearly with the concentration of DA, E, and NE in the range of 1.0 x 10(-)(7)-1.0 x 10(-)(4), 3.0 x 10(-)(7)-1.0 x 10(-)(4), and 5.0 x 10(-)(7)-1.0 x 10(-)(4) M, respectively. The CV detection limit (S/N = 3) of DA, E, and NE is 7.7 x 10(-)(9), 3.8 x 10(-)(8), and 4.2 x 10(-)(8) M, respectively. The modified electrode exhibited almost the same electrochemical behavior after 15 days, indicating that SWNTs/CFNE is pretty stable and has good reproducibility.

Amperometric glucose sensor based on coimmobilization of glucose oxidase and Poly(p-phenylenediamine) at a platinum microdisk electrode

A miniaturized glucose biosensor in which glucose oxidase (GOD) and poly(p-phenylenediamine) (poly-PPD) were coimmobilized at the surface of a platinum microdisk electrode was developed and used successfully for amperometric determination of glucose. The performance of sensors prepared at different monomer concentrations and polymerization potentials with different media was investigated in detail. It was found that similarly to poly(o-phenylenediamine) (poly-OPD), (poly-PPD) noticeably eliminated the electrochemical interference of ascorbic acid, uric acid, and l-cysteine. The amperometric response of glucose with the biosensor under optimal conditions exhibited a linear relationship in the range of 5.0 x 10(-5) to 3.0 x 10(-3) M with correlation coefficient 0.9995. According to the Michaelis-Menten equation, the apparent Michaelis constant for glucose and the maximum steady-state current density of the poly-PPD/GOD-modified microelectrode were 3.94 mM and 607.5 microA cm(-2), respectively. The current density of the sensor responding to glucose in the linear range can reach 160 microA cm(-2) mM(-1), which is far greater than that obtained using poly-OPD and poly(phenol) film. In addition, the stability of the sensor was examined over a 2-month period.

Application of high-density optical microwell arrays in a live-cell biosensing system

In this paper, we use optical imaging fibers to fabricate a chemical and biochemical sensor that utilizes the ability of living cells to respond to biologically significant compounds. The sensor is created by randomly dispersing single NIH 3T3 mouse fibroblast cells into an optically addressable fiber-optic microwell array such that each microwell accommodates a single cell. The cells are encoded to identify their location within the array and to correlate changes or manipulations in the local environment to responses of specific cell types. The entire array can be simultaneously measured, yielding a rapid, repetitive, and high-density analysis method.

Channel electrophoresis for kinetic assays

A rectangular channel electrophoresis system and a cylindrical sampling capillary combination allows chemical changes in nanoliter-volume samples to be monitored as a function of time. The electrophoretic microseparation is carried out in a rectangular channel with a 7 -cm-long, 40-microm x 2.5-cm geometry and is coupled to a 50-microm-i.d. cylindrical sample introduction capillary. The channel width dimension is used as a time axis by moving the outlet of the sampling capillary across the entrance of the separation channel. Detection of the separated analyte bands is achieved with laser-induced fluorescence and spatially resolved detection based on a charge-coupled device. The system is characterized with a series of fluorescein thiocarbamyl amino acid derivatives; limits of detection are < 10(-8) M for amino acids and 10(-9)M (425 zmol) for fluorescein. The ability to achieve a time-based dynamic microseparation is demonstrated by monitoring fluorescent product formation during the enzyme-catalyzed hydrolysis of fluorescein di-beta-D-galactopyranoside (FDG), a commonly used fluorescent substrate for enzymological studies.