Capacitive microsensors for biochemical sensing based on porous silicon technology

Porous EIS (electrolyte–insulator–semiconductor) structures of n-Si/SiO2/Si3N4 with a mean pore diameter of about 1 μm and a mean pore depth of about 2 μm have been realized for capacitive pH sensors. For the fabrication of the porous microsensors (down to “spot” sizes of 10 μm×10 μm), the n-doped silicon substrates have been photolithographically patterned by means of mask-matching technique using polyimide as a passivation material. The average pH sensitivity of the porous pH microsensor amounts about 56 mV/pH in the concentration range between pH 4 and pH 8. In order to prepare porous EIS biosensors the enzyme penicillinase has been adsorptively immobilized inside the porous structure. In the case of the porous biosensors an average penicillin sensitivity of about 90 mV/mM in the concentration range from 0.01 to 1 mM exists. Microreference electrodes, also prepared by the same porous silicon technology as for the pH- and biosensors, show a potential stability of more than 1 week in the long term.