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 turnover 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 .

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In this work, a flow system containing a micromachined lamella-type porous silicon reactor and a novel mid-IR fiber-optic flow cell were used for the enzymatic determination of sucrose in aqueous solution. The method relies on the enzymatic hydrolysis of sucrose to fructose and glucose catalyzed by β-fructosidase and on the acquisition of FT-IR spectra before and after complete reaction. β-Fructosidase was covalently bound to the porous silicon surface of the channels in the microreactor. The porous silicon was achieved by anodization of the silicon reactor in a HF/ethanol mixture. For the measurement of small amounts of aqueous solution, a miniaturized flow cell was developed which consisted of two AgCl(x)Br(1)(-)(x) fiber tips (diameter, 0.75 mm) coaxially mounted in a PTFE block at a distance of 23 μm. The flowing stream was directed through the gap of the two fiber tips which served to define the optical path length and to bring the focused mid-IR radiation to the place of measurement. Using this construction, a probed volume of ∼10 nL was obtained. The calibration curve was linear between 10 and 100 mmol/L sucrose. Furthermore, the potential of this method was demonstrated by the analysis of binary sucrose/glucose mixtures showing no interference from glucose and by the successful determination of sucrose in real samples.

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