Porous Silicon -an enzyme coupling matrix for micromachined reactors

Abstract: The development of a miniaturised silicon wafer integrated enzyme reactor is described. The reactor was micromachined by anisotropic wet etching of (110) silicon. The enzyme glucose oxidase (GOx) was coupled to the reactor surface with standard methods of immobilising enzyme to silica. The glucose turn-over rate was monitored following a colourimetric assay. The advantage, in terms of high surface area per volume ratio, of utilising (110) silicon for microreactor fabrication compared to (100) silicon was demonstrated. Two reactors with different channel widths (50 and 20 µm) and channel densities (10 and 25 per millimetre) were compared, yielding a proportionally increased enzyme activity, for the reactor with the highest surface area (20 µm channels, 25 channels per millimetre). A novel method, utilising porous silicon as a coupling matrix, to increase the surface area for enzyme coupling was investigated. A porous silicon layer was fabricated on samples by anodic dissolution of silicon in hydrofluoric acid. Three different pore morphologies were generated by anodisation at (10, 50 and 100 mA cm^-2). Glucose oxidase was coupled to the samples and a non porous reference sample. The sample anodised at 10 mA cm^-2 displayed a 30-fold increased catalytic efficiency, compared to the non porous reference sample. The possibility of generating porous silicon on vertical channel type silicon reactors were also demonstrated. A porous silicon layer was generated on an anisotropically preetched vertical channel type microreactor. GOx was immobilised in the reactors and the glucose turn-over rate was monitored. A more than 100-fold increase in catalytic efficiency was recorded for a reactor anodised at 50 mA cm^-2, compared to a non porous reference reactor. The applicability of micro enzyme reactors was demonstrated by immobilising ß-fructose onto a porous channel type reactor. Sucrose monitoring was performed utilising Fourier transform infrared (FTIR) spectroscopy system with the micro enzyme reactor as a key component, the dissociation of sucrose into glucose and fructose. The miniaturised FTIR spectroscopy system was used for sucrose analysis of crude real samples from commercial soft drinks showing results in accordance with standardised sucrose test-kits. The influence of pore morphology was investigated to further improve the catalytic efficiency of enzyme activated porous silicon carrier matrices. Porous silicon was generated on substrates of p- and n-type silicon with different dopant concentrations. For each type of substrate, the porous layer was generated at three current densities (10, 50 and 100 mA cm²). The porous samples and a non porous reference sample was enzyme activated with GOx. A 350-fold increase in glucose turn-over rate, compared to the reference, was recorded for an n-type epilayer on n⁺-type substrate anodised at 100mAcm^-2. The influence of the porous silicon matrix depth was also investigated, for planar samples and vertical channel type reactors. In this investigation porous silicon layers were fabricated at two current densities and for three depths at each current density. A 170-fold increase in catalytic turn-over, compared to a non porous reference, was recorded for a reactor with an average porous depth of 10 µm.