recently reported on the direct monitoring of adenosine, as a target molecule [34]. Upon the binding of adenosine to the cognate aptamer, changes in the electrical signal were monitored with an ISFET. Figure 2 shows the schematic diagram of an ISFET-based aptamer sensor for adenosine. Following a primary silanization of Al2O3 gate with 3-aminopropyltriethoxysilane, the surface was subsequently modified with glutaric dialdehyde. After the covalent immobilization of amine-functionalized aptamer on the gate surface, the hybridization of the nucleic acid to the aptamer was investigated by ISFET measurement. Consequently, the tested ISFET-based aptamer sensor exhibited the detection limit of approximately 5 × 10−5 M, and showed high specificity, as the aptamer-modified ISFET did not respond to other nucleotides, such as cytidine.
Besides the availability of ISFET-based DNA sensors, studies have raised the issue of the limitations of charge detection of DNA hybridization based on FET-type biosensors. In particular, the relatively thick insulator on top of the gate oxide can conspire to inhibit the effective detection of charges, because the thickness of the insulator may have a negative influence on the conversion of the charged DNA from the electrolyte to the sensing layer. To get around this, Song et al. [38,39] proposed a diamond solution-gate FETs (SGFETs), in which the surface is not covered by thick insulating layers, and DNA is immobilized directly onto amine-terminated sites, which is a critical factor in terms of sensitivity compared to existing DNA ISFET sensors. The diamond surface channel attached by DNA was exposed directly to the electrolyte lacking gate insulator, thereby leading to a large potential window for diamond up to >3.0 V. The tested device could rapidly detect 3-mer mismatched DNA, and potentially showed the possibility for the monitoring of single-base mismatched DNA, without sacrificing sensitivity. In addition, when DNA hybridization reaction takes place at the distance beyond the Debye length from the sensing layer, FET device fails to DNA detection. Thus, DNA hybridization measurement should meet the prerequisite where an electric field caused by redistributed charged molecules exists within the Debye length of the given solution.