Porous silicon as the carrier matrix in microstructured enzyme reactors yielding high enzyme activities

Miniaturization and silicon integration of micro enzyme reactors for applications in micro total analysis systems (μTASs) require new methods to achieve structures with a large surface area onto which the enzyme can be coupled. This paper describes a method to accomplish a highly efficient silicon microstructured enzyme reactor utilizing porous silicon as the carrier matrix. The enzyme activity of microreactors with a porous layer was recorded and compared with a microreactor without the porous layer. The microreactors were fabricated as flow-through cells comprising 32 channels, 50 μm wide, spaced 50 μm apart and 250 μm deep micromachined in 〈110〉 oriented silicon, p type (20–70  cm), by anisotropic wet etching. The overall dimension of the microreactors was 13.1× 3.15 mm. To make the porous silicon layer, the reactor structures were anodized in a solution of hydrofluoric acid and ethanol. In order to evaluate the surface enlarging effect of different pore morphologies, the anodization was performed at three different current densities, 10, 50 and 100 mA cm−2. Glucose oxidase was immobilized onto the three porous microreactors and a non-porous reference reactor. The enzyme activity of the reactors was monitored following a colorimetric assay. To evaluate the glucose monitoring capabilities, the reactor anodized at 50 mA cm−2 was connected to an FIA system for glucose monitoring. The system displayed a linear response of glucose up to 15 mM using an injection volume of 0.5 μl. The result from the studies of glucose turn-over rate clearly demonstrates the potential of porous silicon as a surface enlarging matrix for micro enzyme reactors. An increase in enzyme activity by a factor of 100, compared to the non-porous reference, was achieved for the reactor anodized at 50 mA cm−2.