Miniaturized ultrathin slab gel electrophoresis with thermal lens microscope detection and its application to fast genetic diagnosis

Geometry-induced injection dispersion in single-cell protein electrophoresis

Arrays of microwells are widely used to isolate individual cells, facilitate high throughput cytometry assays, and ensure compatibility of those assays with whole-cell imaging. Microwell geometries have recently been utilized for handling and preparation of single-cell lysate, prior to single-cell protein electrophoresis. It is in the context of single-cell electrophoresis that we investigate the interplay of microwell geometry (circular, rectangular, triangular) and transport (diffusion, electromigration) on the subsequent performance of single-cell polyacrylamide gel electrophoresis (PAGE) for protein targets. We define and measure injector-induced dispersion during PAGE, and develop a numerical model of band broadening sources, experimentally validate the numerical model, and then identify operating conditions (characterized through the Peclet number, Pe) that lead to microwell-geometry induced losses in separation performance. With analysis of mammalian cells as a case study, we sought to understand at what Pe is the PAGE separation performance adversely sensitized to the microwell geometry. In developing design rules, we find that for the microwell geometries that are the most suitable for isolation of mammalian cells and moderate mass protein targets, the Pe is usually small enough (Pe < ∼20) to mitigate the effect of the microwell geometry on protein PAGE of single-cell lysate. In extreme cases where the largest mammalian cells are analyzed (Pe > ∼20), consideration of Pe suggests using a rectangular - and not the widely used circular - microwell geometry to maximize protein PAGE separation performance.

Thermal diffusivity imaging with the thermal lens microscope

A coaxial thermal lens microscope was used to generate images based on both the absorbance and thermal diffusivity of histological samples. A pump beam was modulated at frequencies ranging from 50 kHz to 5 MHz using an acousto-optic modulator. The pump and a CW probe beam were combined with a dichroic mirror, directed into an inverted microscope, and focused onto the specimen. The change in the transmitted probe beam's center intensity was detected with a photodiode. The photodiode's signal and a reference signal from the modulator were sent to a high-speed lock-in amplifier. The in-phase and quadrature signals were recorded as a sample was translated through the focused beams and used to generate images based on the amplitude and phase of the lock-in amplifier's signal. The amplitude is related to the absorbance and the phase is related to the thermal diffusivity of the sample. Thin sections of stained liver and bone tissues were imaged; the contrast and signal-to-noise ratio of the phase image was highest at frequencies from 0.1-1 MHz and dropped at higher frequencies. The spatial resolution was 2.5 μm for both amplitude and phase images, limited by the pump beam spot size.

Sensitive determination of concentration of nonfluorescent species in an extended-nano channel by differential interference contrast thermal lens microscope

A photothermal detector, named differential interference contrast thermal lens microscope (DIC-TLM), was used to determine the concentration of a nonfluorescent solution in a nanochannel. This method exploits a local change in the refractive index of a solution caused by light absorption. A solution was introduced into a 100-nm scale channel by a pressure-driven nanofluidic control system and its concentration was determined by DIC-TLM. The limit of detection (LOD) was 2.4 microM in a nanochannel that was 21 microm wide and 500 nm deep. The LOD was 3 orders of magnitude smaller than that of conventional method. Moreover, the detection volume was accurately determined to be merely 0.25 fL by using a nanochannel with an optical path length of 500 nm. Based on these results, the number of detected molecules was calculated to be 390. In addition, the concentration of a solution in a nanochannel that was 790 nm wide and 500 nm deep could be determined. Finally, the relationship between sensitivity and channel size was investigated and the sensitivity was found to decrease with decreasing nanochannel size, which indicates that the changes in the refractive indices of water and silica cancel each other out. The DIC-TLM realizes sensitive detection of nonfluorescent species in nanochannels without requiring any special fabrication techniques. Therefore, DIC-TLM is expected to be a highly useful analytical technique in nanofluidics.

Slab Optical Waveguide High-Acidity Sensor Based on an Absorbance Change of Protoporphyrin IX

A sensitive and fast-responsive evanescent wave absorption sensor has been constructed for pH measurements in highly acidic ranges. This sensor is based on a pH-dependent color change of protoporphyrin IX (PPIX). For the sensitive detection, a visible attenuated total reflection spectrometer with a slab optical waveguide (SOWG) was laboratory-made, and the guiding layer surface was modified with a PPIX-immobilized acrylamide-based thin membrane. The sensing membrane with a thickness of approximately 1 mum was directly fabricated on the SOWG glass surface by copolymerization of acrylamide, N,N'-methylene bisacrylamide, and PPIX in the narrow space confined by a cover plate. PPIX possesses two double bonds in its structure, and so it can be covalently incorporated into the membrane. The response characteristics of the PPIX-immobilized optode membrane were explored using aqueous solutions with different concentrations of HNO(3) or HCl. The optode membrane provided characteristic Soret band absorption spectra depending on the hydrogen ion concentration; the absorbance at 410 nm increased with increasing the concentrations in the range of 0.15-2 M, corresponding to the range of pH -0.3 to 0.8. The absorption signal reached 90% of its final value within 10 s, while the absorption signal was quite readily returned to background level simply by passing 2 mL of distilled water through a flow cell with a volume of 16.5 muL placed on the SOWG. Due to the rapid response and reversibility, this sensor could be operated in a flow-through mode as well as in a conventional static mode, where deionized water was conveniently used as a carrier and conditioning solution. In terms of the stability and precision, this sensor showed no significant change in response even after 100 assays and after being stored in a dry condition for over 6 months. Relative standard deviations for 10 replicate measurements were less than 1.8% in the linear range, and the detection limit calculated from 3 times of the standard deviation was 0.02 pH unit.

Ultrasensitive native fluorescence detection of proteins with miniaturized polyacrylamide gel electrophoresis by laser side-entry excitation

Direct detection of separated proteins inside polyacrylamide gels has many advantages compared to staining methods. Ultrasensitive native fluorescence detection of proteins with miniaturized 1-D and 2-D PAGE was achieved with laser side-entry excitation. The detection limit for R-phycoerythrin protein spots in 1-D SDS-PAGE with 532 nm excitation was as low as 15 fg, which corresponds to only 40,000 molecules. The average detection limit of six standard native proteins was 5 pg per band with 275 nm excitation. The dynamic range spanned more than three orders of magnitude. By using the same detection setup, approximately 150 protein spots from 30 ng of total Escherichia coli extraction were detected on a 0.8 cm x 1 cm gel in 2-D separation. The significant improvement in sensitivity for laser side-entry excitation comes from higher excitation power and lower background level compared with other excitation modes.

One- and Two-Dimensional Miniaturized Electrophoresis of Proteins with Native Fluorescence Detection

Miniaturized electrophoresis was successfully coupled with native fluorescence detection for direct analysis of proteins in one- and two-dimensional separations. The detection setup was based on direct observation of the UV-induced fluorescence of proteins using a CCD camera and a Hg (Xe) lamp for sample excitation. Protein mixtures were readily separated by size on a 1-cm segment of the one-dimensional gel in 8 min, and a detection limit of 0.04 ng per band was achieved. The dynamic range of the system was larger than 2 orders of magnitude. Miniaturized slab gel electrophoresis was performed on a special holder designed to couple isoelectric focusing with SDS-PAGE. Two-dimensional separation, including rehydration of IEF strip and fluorescence detection was completed in 2.5 h. Approximately 200 protein spots from Escherichia coli were detected on a 1 cm(2) area. A detection limit of 0.1 microg of total protein was achieved. The operation should be amenable to total automation.