ELECTRICAL CHARACTERIZATION OF

SEQUENCE-SPECIFIC LABEL-FREE DNA BY USING

POLYSILICON WIRE

You-Lin Wu, Po-Yen Hsu

Department of Electrical Engineering, National Chi Nan University, Puli, Nantou, Taiwan

Chung-Ping Hsu

Division of Chest Surgery, Department of Surgery, Taichung Veterans General Hospital, Taichung, Taiwan

Wen- Cheng Liu

Graduate Institute of Biomedicine and Biomedical, National Chi Nan University, Puli, Nantou, Taiwan

Keywords: DNA bases detection, Polysilicon wire sensor, Label-free.

Abstract: In this paper we report the electrical measurement of sequence-specific and label-free deoxyribonucleic acid

(DNA) by using poly-silicon (poly-Si) wire. Four single-strained (ss) DNA bases, adenine (A), thymine (T),

cytosine (C) and guanines (G), as well as double-stranded (ds) DNA sequences, ds(A-T) and ds(C-G), with

different lengths and concentrations were dropped onto the poly-Si wire surface, and currents flowing

through the poly-Si wire channel were determined. It is found that the amount change of the channel current

ΔI for ssDNAs with fixed length and fixed concentration is that T > C > G > A. For dsDNA sequence, we

observe that ds(A-T) has higher ΔI than does ds(C-G). We also prove that single base change in ssDNA is

feasible by using the poly-Si wire sensor.

1 INTRODUCTION

Determination of DNA bases has long been an

intensive research topic in biotechnology and

medical diagnostics since the genetic information

carried by an organism is inscribed in DNA. On the

other hand, measurements of DNA conductivity,

hybridization and melting using electronic means

have also drawn much attention in recent years due

to the possible applications in molecular electronics.

Typically, the detection of DNA uses techniques

such as radiochemical, enzymatic and fluorescent

(Lander, 1999) (Wang, 2000) based on detecting

different labels or reagents added in the molecules

under test. These techniques, however, are proven to

be time-consuming, expensive and complicated to

implement. Therefore, label-free DNA detection

methods such as cyclovoltametry and

chronopotentiometry have also been developed

(Mastrangelo, 1999) (de-los-Snatos-Alvarez et al,

2004). However, all these methods are wet type in

which the sensor has to be immersed in the solution

under test, and the selectivity as well as the detection

limit would not be accurate enough owing to the

influences of thermal drift of the electrolyte solution.

Thanks to the rapid progress in semiconductor

processing and nanofabrication techniques, many

semiconductor/nano electronic devices and nano-

scale measurement tools have been developed either

to measure the electrical properties of DNA or for

DNA detection. For example, Storm and his co-

workers used nanogap junctions to measure the

electrical resistance of hybridized and denatured

DNA molecules (Storm et al, 2001). Xu et al. (2007)

measured the conductance of DNA by using

scanning probe microscopy. On the other hand, field

effect devices such as electrolyte-insulator-silicon

(Fritz et al, 2002) and silicon nanowire (SiNW)

(Hahm and Lieber, 2001) have also been proposed

for the detection of hybridyzation and mutation of

170

Wu Y., Hsu P., Hsu C. and Liu W. (2010).

ELECTRICAL CHARACTERIZATION OF SEQUENCE-SPECIFIC LABEL-FREE DNA BY USING POLYSILICON WIRE.

In Proceedings of the Third International Conference on Biomedical Electronics and Devices, pages 170-173

DOI: 10.5220/0002712601700173

 SciTePress

DNAs. Instead of using SiNW, in this work we used

poly-Si wire for sequence-specific ssDNAs as well

as ds DNAs, A, T, C, G, A-T and C-G, detection

from the consideration of simplicity, economy and

easy-fabrication. In the present work, current

flowing through the poly-Si wire channel rather than

time-dependent conductance was determined. We

found that the amount of current change before and

after dropping the DNA solution under test on the

poly-Si wire surface is different for each ssDNA and

dsDNA. This result indicates that each of ssDNAs

and dsDNAs has its own characteristic amount of

current changes, and the poly-Si wire sensor can be

used as a promising DNA detection device.

2 EXPERIMENTS

In this work, p-type (100) Si wafer was used as the

substrate. After standard RCA cleaning, a 12 nm-

thick thermal oxide was grown at 900℃. Following

that, a phosphorous-doped polysilicon layer with a

thickness of 80 nm was deposited at 620℃ by

vertical furnace and having a sheet resistance of 40-

50 Ω/□. An e-beam writer was then used to define

the pattern of the poly-Si wire. After development,

the poly-Si wire was obtained by reactive-ion-

etching. The line width and length of the poly-Si

wire is about 200nm and 2um, respectively. Figure 1

and Figure 2 depicts respectively the schematic

diagram and the SEM image of the poly-Si wire

sensor used in this work. To increase the detection

sensitivity, an enzyme layer (γ-APTES) is deposited

onto the poly-Si wire surface and then cured at

120℃ for 5 min on a hot plate, which can enhance

the adhesion of DNA molecular with the poly-Si

wire surface as well as increase the sensitivity of the

sensor. The same sensor has been reported being

successfully used for cancer cells detection (Wu et al,

2008). In this work, the sensor was used for DNA

detection.

Sequence-specific DNA reagents (Invitrogen,

U.S.A.), ssA, ssT, ssC, ssG, ds(A-T) and ds(C-G),

with different concentrations (50, 100 nM) and

lengths (10, 20 mer) were prepared and dissolved in

a 0.165 M phosphate buffer solution. The DNA

solution under test was then dropped onto the poly-

Si wire sensor surface by using a micropipette. A

voltage was applied between the source and the

drain of the poly-Si wire without side gate bias, and

the current flowing through the poly-Si wire was

measured by using the semiconductor parameter

analyzer HP 4156B. All the experiments were

carried out at room temperature.

Figure 1: Schematic diagram of the poly-Si wire biosensor.

Figure 2: SEM picture of the Poly-Si wire, the line width

and length are about 200nm and 2um, respectively.

3 RESULTS AND DISCUSSION

Figure 3 and Fig. 4 show respectively the I-V

characteristics of 50 nM and 100 nM ssA, ssT, ssC

and ssG, with different lengths under forward bias.

As observed, the current I

flowing through the

poly-Si wire increases with increasing source-drain

voltage V

. It is interesting to note that the current

flowing through the poly-Si wire channel after the

solution under test was dropped onto the poly-Si

wire surface is higher than that before it was

dropped for all the sequence-specific ssDNAs except

for ssC. This result indicates that the polarity of the

surface charge of ssC is opposite to that of other ss

DNAs. Figure 5 compares the absolute value of the

amount of current changes (which is defined as ΔI

=⎥ I

(after dropping) – I

(before dropping)︳) of

ssA, ssT, ssC and ssG DNA with different

concentrations and lengths measured at V

= 5V.

We find that ΔI increases with increasing DNA

length as well as concentration as expected because

the longer length and the higher concentration of

ssDNA, the more charges within the poly-Si wire

channel will be induced. We also observe that, for

the same length and concentration of ssDNA, the

amount of current changes ΔI is that T > C> G > A.

It is reported that the Fermi level of DNA bases is

that T < C < G < A (Zwolak and Di Ventra, 2008).

Therefore, we believe that the surface charge of the

ELECTRICAL CHARACTERIZATION OF SEQUENCE-SPECIFIC LABEL-FREE DNA BY USING POLYSILICON

WIRE

171

poly-Si wire is modified by the electron transfer

between the wire surface and the DNA bases

attached.

Figure 3: I-V characteristics of the poly-Si wire after

dropping different 50 nM sequence-specific ssDNA

solution with a length of 10 mer and 20 mer.

Figure 4: I-V characteristics of the poly-Si wire after

dropping different 100 nM sequence-specific ssDNA

solution with a length of 10 mer and 20 mer.

Figure 5: Current difference comparison of ssA, ssT, ssC

and ssG with different concentrations and lengths

measured at V

= 5V.

Figures 6 and 7 show respectively the I-V

characteristics of 50 nM and 100 nM sequence-

specific dsDNAs, ds(A-T) and ds(C-G) with

different lengths under forward bias. As observed,

ds(A-T) has higher I

than ds(C-G) does. Since the

charge polarity of C is opposite to that of A, T and G,

it is believed that the net surface charge is reduced

when ds(C-G) is dropped onto the poly-Si wire

surface and would reduce the amount of induced

charge in the poly-Si wire channel, hence less

current would flow. Figure 8 compares the ΔI for the

all the dsDNA s and ssDNAs. It is reported that the

ds(C-G) and ds(A-T) might be treated respectively

as p-type and n-type semiconductor (Kim et al,

2006), which also explains the ΔI difference in Fig.

8. The detection of single base change in ssDNA

was also conducted in this work. The result (not

shown) indicates that the poly-Si wire sensor can be

used for the detection of single base change in

ssDNA.

Figure 6: I-V characteristics of the poly-Si wire after

dropping different 50 nM sequence-specific dsDNA

solution with a length of 10 mer and 20 mer.

Figure 7: I-V characteristics of the poly-Si wire after

dropping different 100 nM sequence-specific dsDNA

solution with a length of 10 mer and 20 mer.

BIODEVICES 2010 - International Conference on Biomedical Electronics and Devices

172

Figure 8: Current difference comparison of ssA, ssT, ssC,

ssG, ds(A-T) and ds(C-G) with different concentrations

and lengths measured at V

= 5V.

4 CONCLUSIONS

In this paper we report the detection of ssDNAs and

dsDNAs with specific sequence by using the poly-Si

wire sensor. For ssDNA, we find that each ss DNA

base has its own characteristic ΔI, and the amount

changes of the current flowing through the poly-Si

wire is that T > C > G > A. For dsDNAs, we also

observed that ds(A-T) has higher ΔI than ds(C-G).

Our experimental result confirms that the surface

charge state is modified after the DNA solution is

dropped, which is believed to be related to the Fermi

levels of the DNA bases. We also prove that the

poly-Si wire sensor can be used for the detection of

single base change in ssDNA. In conclusion, the

poly-Si wire sensor can be used as a promising DNA

detection device.

ACKNOWLEDGEMENTS

This work is supported by the Taichung Veterans

General Hospital-National Chi Nan University Joint

Research Program no.TCVGH-NCNU987910.

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