The technology used in this work is based

The technology used in this work is based on electrolyte–insulator–semiconductor (EIS) biosensors. EIS vary their potential because of a detection event at the insulator/electrolyte interface. For this structure, capacitance vs voltage (CV) measurements provide information on the interface potential [11]. The chemical changes that take place at this surface, such as the immobilization of biological molecules, or hybridization of a molecule of single strand DNA (ss-DNA, probe) with the complementary strand (target), will cause a potential variation, measured as a shift of the CV curve. The shift sign and amplitude will depend on the nature and coverage of material modification. The device capacitance is given by the series of three main capacitors: i) the semiconductor/insulator (CS/I) given by the “inorganic” part of device; ii) the biological sensing molecule (our probe) (Cprobe); and iii) the region spanning from the outer device surface and the bulk of the solution (Cint). Cprobe includes the Stern layer consisting of electrostatically bound water arecoline hydrobromide between the recognition and diffuse layers and, eventually, the recognized analyte [12]. The total capacitance Ctot, is then: The equation clearly states that the lowest capacitance dominates the total capacitance. Therefore, for the device design the tradeoff between the need to have a high insulating layer capacitance to lower its contribution (high oxide thickness), and the need to have a high sensitivity, hence a thin oxide, is important. The recognition event will change Cprobe value. The capacitance variation can be quantified by monitoring the voltage shift.
Materials and method Three different dielectrics were thermally grown, on p-type epitaxial Si wafers having a resistivity of the epi layer of ~2Ωcm, a thickness of 7μm, grown on CZ Si with a resistivity of ~2mΩcm to fabricate MOS-like structures. The dielectrics were:
Results and discussions The immobilization protocol effects on the device active surface were electrically characterized using the structure described in the previous section. Fig. 1 shows the comparison of the CV curves acquired for the sample Si/SiO2 after each immobilization step. Fig. 1 clearly indicates that the device is sensitive to a charge variation on its surface, as already observed in literature [14,15]. Indeed, after the silanization process, a net positive charge is anchored to the surface, as observed by the CV curve (blue dashed line), that exhibits a shift of ~−0.60V (left shift) with respect to the reference sample (SiO2, red dotted curve). Furthermore, the negative charge introduced by the ss-DNA used as a probe (anchoring phase, green solid line) produces a positive shift with respect to the reference of ~+0.20V. The full shift measured with respect to the step before anchoring is of ~0.8V. The Cmin/Cox values are roughly the same, ~0.32 for each curve. The flat band voltage (Vfb) values vary according to the surface transformation process of the sample, showing the measurement technique sensitivity to the potential variations on the surface. These results indicate that the CV curves shift can be valid system to detect DNA hybridization and can be used as transduction mechanism in biosensors. Finally, the BSA processing time does not produce detectable variations in the CV measurements. The measurements reported in Fig. 2 provide other interesting information. The BSA passivation of the surface, widely used in optical biosensors to reduce the artifacts, does not produce significant variation in the electrical measurements. BSA, under the experimental condition used (PBS pH7.4) does not improve the electrical device sensitivity between the anchoring and hybridization phases. Indeed, the measured shift between the curves of samples having only the probe anchored (blue dotted line) and after hybridization (light blue solid line) without BSA passivation of the surface is ΔV~0.60V, while between the curves with BSA (green dashed line and magenta dash dotted line for probe anchoring and hybridized samples, respectively) it is ΔV~0.40V.