ISFET-based detection of cDNA perfect or partial matching kinetics

Microelectronic devices can be fruitfully used for DNA hybridization detection: in this work we show that a Si-based n-ISFET (Fig.1) can detect surface potential changes resulting from the surface adsorption of charged molecules in aqueous environment. To promote cDNA adhesion the gate dielectric was coated with poly-L-lysine at 0.2%. When a probe cDNA was bound to the poly-L-lysine layer the ISFET drain current ID decreased, and this effect was further enhanced when a complementary target DNA was later bound to the corresponding probe (fig.2). We evaluated the DNA quantity retained on the gate area as a function of different probe concentrations by measuring the ID shift (fig.3): noticeably all binding sites were already saturated with the lowest probe density. We used our ISFETs to evaluate the different hybridisation kinetics of perfect matching or partial matching targets. We performed a preliminary temperature calibration of our devices in order to account for electrical effects related to the operating temperatures used in the hybridisation experiments (Fig.4). The normalized shift of the drain current (ƒ´ID/ID) is reported in Fig.5 for two different targets. Parasitic effects due to temperature have been properly subtracted. For the perfect match target (whole experiment at 22¢XC) the drain current abruptly and quickly fells, remaining almost constant at about 20% of its initial value. Conversely, for partially matching target the current fells slowly at 22C, as expected from a-specific chemical bonds between the target and the probe DNA. Once the temperature rose to 70¢XC most of (but not all) a-specific bonds are broken. The different kinetics shown in Fig. 5 demonstrate that ISFET is able to discriminate between specific and a-specific hybridisation of DNA targets to DNA probes fixed on their surfaces with good electrical and temporal experimental sensitivity for a thorough investigation of the occurring phenomena