This development represents an advancement in the field of portable microfluidic analysis systems, or lab-on-a-chip devices.

An advanced micro fluorescence detector (microFD) uses a hydrogenated amorphous silicon (a-Si:H) photodiode for high sensitivity biochemical analyses in micro-capillary electrophoresis (microCE) applications, representing an advancement in the field of portable microfluidic analysis systems, or lab-on-a-chip devices. The detector is highly sensitive to the emission wavelengths of commonly used fluorescent dyes, and in an integrated configuration using a Vertical Cavity Surface Emitting Laser (VCSEL) diode, proves efficacious for heavily multiplexed microfluidic biochemical analyses. This design can be fabricated and integrated at a low cost, making it ideal for the production of portable bioanalyzers that handle minute quantities of fluids (picoliters to nanoliters), as well as emerging large-scale integrated microfluidic technologies.

Fig. 1 depicts spectral response of an a-Si:H photodiode under a reverse bias voltage of 0 V with and without a SIO2/Ta2O5 multilayer optical interference filter.
The integrated design of this a-Si:H fluorescence detector is inherently scalable for heavily multiplexed microfluidic biochemical analysis. Laser light makes it highly sensitive to pigments such as fluorescein and green fluorescence proteins. The detector has demonstrated excellent quantum efficiency in the spectral range of ~520 to ~580 nm, expanding its limit of detection (LOD) compared to other detectors. Today’s ultrafine processing techniques for semiconductors simplify the production of its detection machine array. Inexpensive glass or plastic materials reduce costs and enable mass-production at lower temperatures than other designs. It is also reusable.

Sophisticated microfluidic analysis devices integrate various components for detecting fluorescence. These include laser diodes, photodiodes, electrophoresis, and emission filters. Highly sensitive fluorescence detection systems typically require large lasers, large optical systems, and large detectors. Some photodiode fabrication requires highprocess temperatures — in the range of ~1000 °C — rendering impractical the use of glass and plastic substrates.

Most practical labeling dyes used in biochemical assays, such as fluorescein, green fluorescence protein, ethidium bromide, and DNA intercalators, emit fluorescence in the visible light region, a range with narrow limits of detection (LOD) by conventional confocal microscope detection systems. Additionally, other detectors suffer from relatively high background photocurrents arising from specular laser light scattering during operation.

This detector is inherently scalable to micro-proportions, making it practicable for the next generation of handheld assay devices. The production process for its a-Si:H photodiode occurs at a relatively low temperature of ~200 °C.

This detector’s integrated design is characterized by expanded detection capabilities through the use of a half-ball lens in the detection system, which intensifies fluorescence detection by a factor of 3. The integrated components also eliminate much of the interference from the laser light used to excite the biochemical under examination, providing a dramatically higher quantum efficiency than other designs.

Applications include the fields of semiconductor quantum dots, which have been studied in transistors, solar cells, LEDs, and diode lasers. Quantum dot potential also includes the field of medical imaging. Other applications include point-of-care analysis and diagnosis in which diagnostic and analysis processes are mounted on a single chip. They will require miniaturized highly multiplexed microfluidic analyses, which are addressed by this invention.

The extension to a-Si:H-VCSEL-based excitation-detection arrays may play an important role in the development of large-scale integrated microfluidic devices.

Capitalizing on microfabrication technology, AIST scientists have used photolithographic patterning on a micrometer scale, making possible the manipulation of nano- and picoliter-sized samples of fluids through microcapillaries. These microfluidic capillary electrophoresis (CE) devices dramatically reduce biochemical analysis times.

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