Label-free DNA sensors using ultrasensitive diamond field-effect transistors in solution

Charge detection biosensors have recently become the focal point of biosensor research, especially field-effect-transistors (FETs) that combine compactness, low cost, high input, and low output impedances, to realize simple and stable in vivo diagnostic systems. However, critical evaluation of the possibility and limitations of charge detection of label-free DNA hybridization using silicon-based ion-sensitive FETs (ISFETs) has been introduced recently. The channel surface of these devices must be covered by relatively thick insulating layers ($\mathrm{Si}{\mathrm{O}}_2$, ${\mathrm{Si}}_3{\mathrm{N}}_4$, ${\mathrm{Al}}_2{\mathrm{O}}_3$, or ${\mathrm{Ta}}_2{\mathrm{O}}_5$) to protect against the invasion of ions from solution. These thick insulating layers are not suitable for charge detection of DNA and miniaturization, as the small capacitance of thick insulating layers restricts translation of the negative DNA charge from the electrolyte to the channel surface. To overcome these difficulties, thin-gate-insulator FET sensors should be developed. Here, we report diamond solution-gate FETs (SGFETs), where the DNA-immobilized channels are exposed directly to the electrolyte solution without gate insulator. These SGFETs operate stably within the large potential window of diamond $(>3.0\mathrm{V})$. Thus, the channel surface does not need to be covered by thick insulating layers, and DNA is immobilized directly through amine sites, which is a factor of 30 more sensitive than existing Si-ISFET DNA sensors. Diamond SGFETs can rapidly detect complementary, 3-mer mismatched $\left(10\mathrm{pM}\right)$ and has a potential for the detection of single-base mismatched oligonucleotide DNA, without biological degradation by cyclically repeated hybridization and denature.

Figure 1

(Color online) The static device characteristics of diamond SGFETs by hybridization and the mechanism of detecting signal depending on the Debye length. (a) Theorectially, dimond SGFETs show higher sensitivity to DNA charge than Si-ISFETs, because the channel surface is directly exposed to electrolyte solution without insulating layers. (b) The directly exposed channel surface is very sensitive to surface charge on diamond SGFETs. After hybridization with complementary target oligonucleotide on the functionalized channel surface, the surface negative charge increases and hole carrier density is enhanced due to the increased negative charge on the channel surface. (c) The sensitivity to detect DNA hybridization is stable about cyclic hybridization and denature on diamond SGFETs. (d) The Debye length compares to the concentration of NaCl in 1:1 electrolyte solutions.

Figure 2

(Color online) The sequential changes of gate potential and drain current in hybridization, and denature with complementary, 3-mer and 1-mer mismatched target DNA, respectively. (a) The sequential changes of drain current in hybridization and denature are stable and characteristic under various conditions. Drain current is characterized in 3-mer mismatched and complementary target oligonucleotide DNA by modifying the incubation temperature. (b) The shift of gate potential is different in complementary and 1-mer mismatched target oligonucleotide DNA at room temperature.

Figure 3

(Color online) The introduction of target DNA solution on the channel surface of diamond SGFETs with real-time detection.

Figure 4

(Color online) Real-time detection of complementary and noncomplementary target oligonucleotide DNA depending on the concentration and annealing time. (a) The shifted gate potential is enhanced by the concentration of complementary target oligonucleotide because the count density of hybridized oligonucleotide is increased. (b) Diamond SGFETs discriminate between complementary and noncomplementary oligonucleotide DNA in real-time detection, definitely. (c) The count of hybridized oligonucleotide DNA increases depending on the annealing time. The signal is a distinct difference between specific binding and non-specific binding in the target oligonucleotide DNA solution (10 nM). (d) The complementary target DNA hybridization is surely distinguishable compared 3-mer mismatched target DNA (100 nM).

Figure 5

(Color online) The device characteristics (${\mathrm{I}}_{\mathrm{DS}}\text{−}{\mathrm{V}}_{\mathrm{DS}}$, ${\mathrm{I}}_{\mathrm{DS}}\text{−}{\mathrm{V}}_{\mathrm{GS}}$) of diamond SGFETs on the H-terminated diamond surface in PBS solution. (a) ${\mathrm{I}}_{\mathrm{DS}}\text{−}{\mathrm{V}}_{\mathrm{DS}}$ (b) ${\mathrm{I}}_{\mathrm{DS}}\text{−}{\mathrm{V}}_{\mathrm{GS}}$.