Marion Thust

Porous silicon as a substrate material for potentiometric biosensors

For the first time porous silicon has been investigated for the purpose of application as a substrate material for potentiometric biosensors operating in aqueous solutions. Porous silicon was prepared from differently doped silicon substrates by a standard anodic etching process. After oxidation, penicillinase, an enzyme sensitive to penicillin, was bound to the porous structure by physical adsorption. To characterize the electrochemical properties of the so build up penicillin biosensor, capacitance - voltage (C - V) measurements were performed on these field-effect structures.

Capacitive microsensors for biochemical sensing based on porous silicon technology

Abstract 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.

Miniaturization of potentiometric sensors using porous silicon microtechnology

A new capacitive field-effect microsensor based on a porous EIS (electrolyte-insulator-semiconductor) structure is presented. The porous silicon sensor was prepared using standard techniques of semiconductor processing. A well-defined macroporous layer was formed on silicon by electrochemical etching and a SiO2Si3N4 sandwich was deposited as insulating and pH-sensitive layer. The porous sensor exhibits a high, near-Nernstian pH sensitivity of about 54 mV per decade in the concentration range from pH 4 to pH 8, similar to a planar non-porous EIS structure with the same layer sequence. The enlargement of the active sensor area (surface) due to the porous structure increases the measured capacitance and thus allows a scaling down of the sensor. The preparation of biosensors based on the same structure is demonstrated by immobilization of the enzyme penicillinase as biosensitive component.

Enzyme immobilisation on planar and porous silicon substrates for biosensor applications

Abstract Two methods for the immobilisation of enzymes on silicon-based so-called electrolyte–insulator–semiconductor (EIS) structures are suggested. These EIS structures are used as a basis for potentiometric biosensors. In the first method, heterobifunctional cross-linker molecules are employed to covalently bind enzymes to these capacitive layer structures which possess a planar surface that contains amine groups. Porous EIS sensors which, in comparison to planar sensors, exhibit an enlarged surface area, are used in the second method. For the first time, pH-sensitive Si3N4 was deposited on the walls and bottoms of the SiO2-covered pores. Here, a large amount of enzyme molecules can adsorptively be bound inside the porous structure. Penicillinase is used as a model enzyme. Capacitance–Voltage and Constant Capacitance measurements are performed in order to examine the respective penicillin sensor responses and thus to validate both immobilisation methods. Whereas the sensitivity of the sensors prepared by both methods is nearly identical for low penicillin concentrations up to around 0.25 mM, a difference of the calibration curves in the higher concentration range indicates a larger amount of immobilised enzyme in the case of the porous structures.

Cross-sensitivity of a capacitive penicillin sensor combined with a diffusion barrier

Abstract The sensitivity of a capacitive penicillin sensor based on a pH-sensitive electrolyte–insulator–semiconductor (EIS) sensor with respect to four different kinds of penicillin is investigated. A total of 10 μl of a solution containing penicillin G, ampicillin, amoxicillin or cloxacillin, respectively, are dried on a plastic disc as a diffusion barrier which is then brought in contact with the sensor. In order to correct the sensor signals for a possible pH interference, all measurements are performed with a penicillin sensor as well as with a pH sensor. The resulting calibration curves show no sensitivity for cloxacillin and nearly equal sensitivities for the other three kinds of penicillin. Lower detection limits of about 0.05 μg are obtained for penicillin G, ampicillin and amoxicillin.

Biochemical sensors with structured and porous silicon capacitors

Abstract Chemical sensors with structured and porous semiconductor/insulator capacitors have been developed. The sensors have been prepared using standard techniques of semiconductor processing, i.e. by means of an anisotropic and anodic etching process, respectively. Both sensor types exhibit a high, near-Nernstian pH sensitivity of about 54 mV per pH decade. Due to the enlargement of the active sensor area, the measured capacitance values increase up to a factor of 30. These structured and porous electrolyte–insulator–semiconductor (EIS) sensor substrates are suitable as transducer material for biosensors which has been demonstrated by depositing the enzyme penicillinase through physical adsorption onto the respective pH-sensitive EIS capacitors.

Novel electrochemical sensors with structured and porous semiconductor/insulator capacitors

Abstract Novel electrochemical sensors with structured and porous semiconductor/insulator capacitors, which allow an efficient miniaturization, have been developed. These sensors have been prepared using standard techniques of semiconductor processing. Both sensor types exhibit a high, near-Nernstian pH sensitivity of about 54 mV per decade. Due to the enlargement of the active sensor area, the measured capacitance values increase up to a factor of 30.

Feldeffekt-Enzymsensor zur Detektion von Pestiziden (Field-effect Enzyme Sensor for the Detection of Pesticides)

Abstract Hochtoxische Organophosphatverbindungen werden vielfach als Pestizide, Insektizide und Nervengase eingesetzt, sodass es eines Detektors zum Nachweis dieser Neurotoxine dringend bedarf. Der hier vorgestellte Feldeffekt-Enzymsensor arbeitet schnell, selektiv, reversibel, ist einfach herzustellen und hat eine hohe Langzeitstabilität.

Investigations on porous silicon layers with regard to chemical microsensor applications

A new concept for silicon microsensors based on porous EIS (Electrolyte-Insulator- Semiconductor) structures is presented. The porous sensor was prepared by anodic etching of n-doped silicon and subsequent deposition of a dielectric layer of SiO2. Experimental conditions were investigated to realize a well-defined macroporous formation of the porous silicon. To compare the chemical sensor properties with similar built-up planar Si/SiO2 structures, C/V (Capacitance-Voltage) measurements have been performed. The porous EIS structures have been characterized by SEM (Scanning Electron Microscopy), XPS (X-ray Photoelectron Spectroscopy) and cyclic voltammetry. This solid-state technology allows the preparation of transducer materials for pH microsensors.