Accuracy and resolution limits in quartz and silicon substrates with microelectrodes for electrochemical biosensors

Arrays of microelectrodes for electrochemical biosensing are commonly fabricated on standard silicon wafers. This choice positively profits from the optimized microelectronic technology, offering micrometric spatial resolution and the possibility to integrate electronic processing capabilities directly on the sensing chips. This paper analyzes the drawbacks on the accuracy and resolution in impedance detection arising when using such highly conductive substrates. To this aim, a direct experimental comparison is reported for identical 10 mu m disk electrodes fabricated on quartz (i.e. insulating) and on the silicon dioxide grown on a silicon wafer. The paper shows how the high conductivity of the substrate in contact with the solution promotes additional stray capacitances (in our experiment 280 times larger than the expected interfacial capacitance) that (i) heavily modify the measured impedance spectrum, thus affecting the biosensor accuracy, and (ii) increase the total current noise of the detector, thus affecting the resolution in the biosensor readout. Although the spectral distortion can be minimized through proper grounding of the substrate, the presence of the stray capacitance still degrades the resolution. We also show that even in the case of a microfluidic encapsulation system coupled to the chip, stray couplings through the substrate still affect the detection. However, as silicon appears irreplaceable for smart analytical micro-systems, the paper discusses the key aspects for an optimal design of the chips in terms of metal trace area, pad size, thickness of the insulating layers and bulk resistivity