Anca Varlan

Design optimisation of planar electrolytic conductivity sensors

Planar conductivity sensors are the subject of increasing interest as basic transducers for biosensors. The high degree of control of the performance characteristics undoubtedly forms an important argument in favour of conductivity-based sensing. The paper provides an outline of the design rules to be followed if an optimal design of a planar conductivity cell is required. Based on a simplified model, it is shown that the required accuracy establishes a lower limit to the overall sensor dimensions. This lower limit is expressed as a minimum longitudinal path length necessary to obtain the desired accuracy. Given an available area, the optimum ratio of electrode-width over inter-electrode spacing for a basic two-electrode structure is shown to be close to unity. Furthermore, it is shown that the decomposition of the two electrodes into an interdigitated structure decreases the accuracy of the device if all other parameters are considered constant. If the sensing region has to be limited to within a thin sensitive layer, the splitting is proposed of one of the electrodes into a compound electrode. The optimum lay-out of this compound structure is calculated as a function of the layer thickness.

Nanoscaled interdigitated electrode arrays for biochemical sensors

Nanoscaled interdigitated electrode arrays were made with Deep U.V. lithography. Electrode widths and spacings from 500 nm down to 250 nm were achieved on large active areas (0.5 mm/spl times/1 mm). These electrodes allow for the detection of affinity binding of biomolecular structures (e.g. antigens, DNA) by impedimetric measurements. Such a sensor is developed, theoretically analyzed, experimentally characterized, and is demonstrated as an affinity biosensor.

Nondestructive Electrical Impedance Analysis in Fruit: Normal Ripening and Injuries Characterization

Electrical impedance was measured nondestructively in apples (Malus domestica) and tomatoes (Lycopersicon esculentum) over the frequency range 20 Hz-1 MHz. Raw data conform to a single time constant model. Fruit classification and data interpretation are based on the analysis of four derived parameters: low-frequency resistance R, high-frequency resistance RL, constant phase angle Φ, and characteristic frequency f0. The most sensitive parameters to ripening and injuries have proven to be low-frequency resistance and constant phase angle.

Covalent enzyme immobilization on paramagnetic polyacrolein beads

An optimized procedure of covalent glucose oxidase, urease, Bacillus subtilis alpha-amylase and Bacillus licheniformis alpha-amylase immobilization on paramagnetic, non-porous, polyacrolein beads is presented. The resulting insolubilized enzymes can be employed for extended periods of time without loss of activity. The conditions were optimized for maximizing the activity of the linked enzyme. Coated beads bearing up to 15 micrograms active enzyme/mg(beads) were obtained on reproducible basis. The paramagnetic feature of the particles facilitates the enzyme handling. In the magnetic field, the enzyme separation is fast and complete. Thus, the paramagnetic beads represent an excellent carrier for immobilized enzymes.

Capacitive sensor for the allatostatin direct immunoassay

Abstract The feasibility of allatostatin direct immunoassays based on capacitance measurements with TiO 2 modified electrodes is presented. The low conductance and good dielectric properties of the TiO 2 substrate insure good sensitivity. This was proven by a capacitance decrease of about 1 nF cm −2 for a 3000 times diluted insect serum sample. The simple construction and the miniaturisation possibility advocate this structure for direct immunoassay.

A new technique of enzyme entrapment for planar biosensors

Abstract We present a method of enzyme immobilization on planar sensors. The method combines covalent enzyme bonding on magnetic beads with physical entrapment on the sensor surface. This procedure is suitable for batch production of planar biosensors, with the facility of enzyme patterning on the wafer. The results for glucose oxidase (GOD) enzymatic layers are presented and commented.

Micromachined conductometric p(CO2) sensor

Abstract A novel micromachined conductimetric p(CO2) sensor is proposed. The p(CO2) measurement can be linearised with the theoretical formula: G th ( μS )= 0.055( μS cm −1 )+1.45( μS cm −1 ) p( CO 2 ) ( mmHg −1 ) cell const ( cm −1 ) The sensor design and evaluation is described. The linear behaviour was checked over the physiological ranges of p(CO2): 0–80 mmHg. The time response is in the range of 5 s. The dimensions allow its mounting in a French 5 catheter. Its advantage as compared with the traditional Severinghaus device is the simplicity of construction.

Characterisation of planar electrodes realised in planar microelectronic technology

THE STUDY of electrode-electrolyte interfaces began at the end of the 19th century (VARLEY, 1871); WARTBURG, 1899), and it revealed a complicated behaviour of the interracial impedance, also referred to as polarisation impedance. This impedance is frequency-dependent (SCHWAN 1963) and signal-dependent (SCHWAN, 1963; ONARAL and SCHWAN, 1982), and is also a function of the nature of the interface itself: electrode material (RAGHEB and GEDDES, 1990), surface topology DELEVIE, 1965; PAJKOSS! and NYIKOS, 1986), and the pH and i o n i c strength of the electrolyte (SCHEIDER, 1975). The interest in impedance measurement has resurfaced as planar microelectronic technology has become increasingly common in chemical sensor fabrication (LAMBRECHTS and SANSEN, 1992). Small structures exhibiting highly reproductible features promise inexpensive sensing devices. Moreover, the accurate design of conductometric devices, such as the urea sensor (JACOBS et al., 1993) or the haematocrit sensor (VARLAN et al., 1995), needs precise data about the electrode-electrolyte interface impedance. A correct design has to consider factors such as the following: