Capacitive Current Induced by dsDNA for Biosensor Applications

DNA is usually composed of several nucleotide and form as a spring shape with good elasticity. It has been observed that DNA can be extended by external forces, such as magnetic or electric fields from literature. Through the changes of electric field, DNA can be deformed by electrical force. DNA can be compressed or stretched like a spring and store energy. When the applied forces disappear, the stored energy is released and DNA switch back to the original shape, leading to the capacitive current induced by the DNA motions. The capacitive current is recorded before the DNA fully relaxed. In this study, dsDNA was intercalated with different concentrations of Doxorubicin, causing different relation of DNA motion, and further leading to different capacitive current level. Doxorubicin is one of the commonly used drugs for cancer therapy, and it is only intercalate between G-C base pairs in dsDNA. In this study, it can be developed as a new type of biosensors and also facilitate the fundamental understanding of the dsDNA characteristics.

Figure 1(a) shows the schematic of device. The devices grew 3000 Å-thick SiNx on silicon wafer for dielectric layer, and plated 1000 Å-thick Au for electrodes. The sequence of the single strand DNA (ssDNA) and the Cy3 dye- attached complimentary DNA (Cy3-cDNA) are shown as 5'- thiol-TTT GCT TTT TCG TCG TTT GCT TTT CGT TTT- thiol- 3' and 5'-(Cy3)-AAA ACG AAA AGC AAA CGA CGA AAA AGC AAA-3', respectively. The Cy3-dsDNA solution was prepared by mixing and preparing 5 μM ssDNA, 5 μM Cy3-cDNA and 5 mM TCEP together in a 30 mM phosphate buffer solution (pH=8), followed by being heated to 90 ºC lasting for 2 minutes. The heated DNA solution was then gradually cooled down to room temperature to allow the hybridization of dsDNA. The hybridized dsDNA solution was then dropped on the gold substrate and waited for 36 hours in 25 ºC. Figure 1(b) shows the schematic of different pulse width. The bias applied for measuring the capacitive current was 0.5 V.

Figure 2 shows the capacitive current of dsDNA sample measured in different pulse widths. In short pulse (pulse width < 1 ms) width measurement, the capacitive current was recorded before the DNA fully relaxed. In long pulse width measurement (pulse width > 10 ms), the capacitive current was measured after dsDNA fully relaxed. Figure 3 shows the capacitive current measured when different concentration of Doxorubicin intercalating into dsDNA in 500 μs pulse width. As dsDNA molecule intercalated by higher concentration of Doxorubicin, the capacitive current was decreased with higher concentration of Doxorubicin.

In summary, different pulse widths affect the types of energy stored in dsDNA. And the stored energy is further manifested in different capacitive current. The relation between the stored energy and the capacitive current can be further confirmed by comparing with the sorter pulse widths measurement in the future.

This work was partially supported by National Science Council grant (101-2221-E-007-102-MY3) and by the research grant (101N7047E1) at National Tsing Hua University.