D4. Lab on a chipEdit
Micro total analytical systems (also called “chips”) are miniaturized microfluidic devices, fabricated from a variety of materials within which channels are constructed for the transport of samples and reagents. The small size minimizes the consumption of reagents, reduces manufacturing costs and increases the possibilities for automation. Miniaturization of detectors, however, leads to problems due to the reduced volume of liquid in the detector and to difficulties inherent in scaling down the size of the particular detector. One solution is to interface the chip with a macro-scale detector such as a photomultiplier tube; this is called the “off-chip” approach. This can be achieved, for example, by using optical fibres to carry light from the chip to the detector. An alternative solution – the “on-chip” approach - is to assemble a compact version of the detector and integrate this on the chip with the rest of the analytical system.
Chemiluminescence detection offers high sensitivity, low detection limits and instrumental simplicity but requires a relatively complex manifold on the microchip, the details depending on the chemiluminescence reaction system being used; for example, a Y-shaped channel junction works best when using peroxide-luminol chemiluminescence. Reagent is delivered by a micropump. The chip design must ensure that a high proportion of the emitted light enters the off-chip photomultiplier; this frequently involves coupling with an optical fibre. Such an arrangement typically achieves micromolar detection limits and has been used for a range of analytes including catechol, dopamine, amino-acids, cytochrome c and myoglobin as well as the determination of chip-separated chromium(III), cobalt(II) and copper(II). Horseradish peroxidise can be determined at sub-nanomolar levels. Micromolar concentrations of ATP (adenosine triphosphate) can be measured by means of luciferin-luciferase bioluminescence. The effect of antioxidants has been measured using a microfluidic system incorporating peroxy-oxalate chemiluminescence, by injecting the antioxidants into the hydrogen peroxide stream. The method is simple and rapid and excellent analytical performance is obtained in terms of sensitivity, dynamic range and precision. Electrochemiluminescence detection has been applied for microchip separations using electrodes installed during fabrication.
Photodiodes have been fabricated into chips at the bottoms of the microfluidic channels and have been used for on-chip chemiluminescence detection of DNA produced by the polymerase chain reaction and separated on the same chip by capillary electrophoresis. These devices have been used also to detect luminol chemiluminescence for the micromolar determination of hydrogen peroxide generated by the oxidation of glucose with glucose oxidase. Thin-film organic photodiodes can be fabricated by vacuum deposition and integrated into chips. Copper-phthalocyanine-fullerene small molecule diodes have high quantum efficiency and have been used to determine hydrogen peroxide by peroxy-oxalate chemiluminescence. Another example has been used for hydrogen peroxide determination by luminol chemiluminescence.
- Kuswandi B, Nuriman, Huskens J and Verboom W, Anal. Chim. Acta, 2007, 601, 141-155.