synthesis and application of dna-templated silver nanowires

Synthesis and application of DNA-templated silver nanowires
for ammonia gas sensing
Kai Zhao
a,b
, Qifei Chang
a
, Xing Chen
a
, Buchang Zhang
b
, Jinhuai Liu
a,
⁎
a
The Key Laboratory of Biomimetic Sensing and Advanced Robot Technology, Anhui Province, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, PR China
b
School of life science, Anhui University, Hefei 230039, PR China
abstractarticle info
Article history:
Received 1 April 2008
Received in revised form 8 September 2008
Accepted 19 September 2008
Available online 7 October 2008
Keywords:
DNA-templated silver nanowires
Gas sensing material
Selectivity
Ammonia
The DNA-templated silver nanowires have been synthesized by a simple chemical reduction method in
solution and used for gas detection of ammonia. Lactic acid as a stabilizer in reduction bath can effectively
decrease the nonspecific deposition of metallic silver by slowing down the reaction. In the present

experiments, the highly conductive DNA-templated silver nanowires consisting of grains improve the surface
area/volume ratio, which can increase the adsorption of ammonia gas molecules. The results of gas sensing
measurements indicate that these nanowires show a high selectivity to ammonia, a quick gas response
(~10 s) and a fast recovery (~7 s). Moreover, the possible gas sensing mechanism has been discussed in this
paper. Therefore, we found that the DNA-templated silver nanowires reported here might be a potential
candidate of gas sensing materials for ammonia gas detection.
Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
1. Introduction
Deoxyribonucleic acid (DNA) is an attractive bio-molecule as a
template of nanostructures [1–4] because of its narrow width (ca. 2nm)
and ability to self-recognize and self-assembly. Ever since Braun's group
synthesized a silver nanowire between gold electrodes [1], DNA has
been widely exploited as a template for the fabrication of a variety of
metallic and semiconducting nanowires, such as silver [1],gold[5],
palladium [6],cobalt[7],copper[8],CuS[9] and CdS [10]. The electrical
measurement has proved that the DNA-templated nanowires have high
conductivity as bulk metal, which can be potentially used in fabrication
of int erconnects [11],sensors[12], and integral device components [13].
As one of the noble metals which have high intrinsic conductivity
and resistance to oxidation under ambient experimental conditions
[5], silver was widely used in gas sensing detection to improve the gas
sensing performance [14,15]. During the last decades, DNA-templated
silver nanowires have been produced by different methods [1,16,17].
However, the application of these DNA-templated silver nanowires for
gas detection has rarely been reported.
Ammonia is one of the important industrial exhaust gases with high
toxicity [18,19]. With the increasing of the human awareness of
envir onmen tal problems in indu strial gases, the r eq uir emen t of detecting
ammonia has greatly been increased. Traditional semiconducting oxides
materials u sing for gas detection have some d ra wbacks, especially high

operative temperature and poor gas selecti vity etc [20,21].Oneofthe
important approaches o vercoming the disadvantag es is using nanoscale
materials as sensing elements due to their high surface area/volume
ratios, which is favorable to reduce working temperature and increase the
selectivity and sensitivity of the sensor [21]. Recentl y , many nanomaterials
have been developed as the sensing materials of ammonia gas sensor , such
as nanofibrous polyaniline [22], carbon nanotubes [23] and metal o xides
nanoparticles [24]. Because of using nanomaterials for gas sensing
materials, the working temperature of the gas sensors has been decreased,
meanwhile, t he selectivity and sensitivity hav e been improved compared
to the t radit ional gas sen sing materials. Therefore, the DNA-templat ed
silv er nan ow ir es wit h goo d condu cti vit y a nd na noscal e structu r e ma y be
potential for the application in gas detection.
In this paper, a simple chemical reduction method was used to
fabricate Ag nanowires with DNA as a template. Lactic acid as a stabi-
lizing agent in reduction bath effectively decreased the nonspecific
deposition of metallic silver by slowing down the reaction. Subsequently
these nanowires as gas sensing materials were employed for ammonia
gas detection. The gas sensing measurements were carried out by the
homemade gas detecting system at room temperature. The ultimate
objective of this study is to explore the possibility of application of the
DNA-templated silver nanowires in detecting ammonia gas.
2. Experimental
2.1. Preparation of DNA-templated silver nanowires
In experiments, Ag solution (2 mM) was prepared by dissolving
34 mg of AgNO
3
in 100 ml ddH
2
O. 50 μL λ-DNA solution (300 ng/μL,

Fermentas Inc.) and 250 μL 2 mM AgNO
3
solution were mixed and
Materials Science and Engineering C 29 (2009) 1191–1195
⁎ Corresponding author. Tel.: +86 551 5591142; fax: +86 551 5592420.
E-mail address: (J. Liu).
0928-4931/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.msec.2008.09.0 45
Contents lists available at ScienceDirect
Materials Science and Engineering C
journal homepage: www.elsevier.com/locate/msec
incubated for 1 h in the dark at room temperature. After that, 250 μL
the reduction bath, which contains 250 mg/L sodium citrate, 250 mg/
L 85% lactic acid, and 25 mg/L borane-dimethylamine (Aldrich), was
added to the mixture and incubated for some time.
2.2. Characterization of the DNA-templated silver nanowires
2.2.1. AFM
A5-μL drop of the prepared Ag nanowires solution was dropped onto
the surface of the fresh cleaved mica by micropipet. The samplewas then
imaged with a tapping-mode atomic force microscopy (AFM, Nanofirst-
3000, Shanghai Haizisi Optical-Electronics Co. Ltd., China) using a
Budget sensors Tap300 AFM tip with a force constant of 40 N/m. All AFM
images used in this paper were produced and analyzed by freely
available software: WSxM 4.0 (www.nanotec.es).
2.2.2. TEM
The prepared nanowires were dripped onto the copper grid, and
then observed by TEM. All TEM images were recorded using a Hitachi
H-800 transmission electron microscope (Japan) with a point-to-
point resolution of 0.45 nm operating at 200 kV.
2.2.3. UV–vis

After the reduction bath solution was added to the mixture of DNA
and AgNO
3
solution, the UV–vis absorbance spectra were immediately
recorded by a SHIMADZU-UV-2550 spectrophotometer (Japan). The
spectra were taken at 0, 10, 20, 30, 40, 50 and 60 min.
2.3. Detection of gas sensing properties
The as-prepared DNA-templated silver nanowires were deposi-
ted on the gold interdigital electrode to investigate the gas sensing
properties. The gas sensing experiments were carried out by the gas
sensing detection system, as shown in Fig. 1. The measurement of
the electrical signals was carried out by 6487 Picoammeter/Voltage
Source (Keithley, USA). These nanowires were exposed to various
concentrations of ammonia, hydrogen, ethanol, methanol and ace-
tone, respectively. The constant DC voltage mode was exploited
Fig. 1. Schematic diagram of thegas sensing detection system, (containing a gas chamber
with one inlet and one outlet, testing circuit and Picoammeter/Voltage Source). In
experiment, the sample was deposited on the surface of gold interdigital electrode and
detected in gas chamber. The current signal was measured by 6487 Picoammeter/
Voltage Source (Keithley, USA).
Fig. 2. UV–vis spectrum after addition of the reduction bath, 0, 10, 20, 30, 40, 50 and
60 min, respectively. The absorbance at 260 nm indicates DNA, and the absorbance at
415 nm indicates metallic Ag. UV–vis spectra showing the reductionprocess of DNA-Ag
+
complex. Following reduction, a peak formed at 415 nm, demonstrating the formation
of more and more metallic silver.
Fig. 3. Tapping mode AFM images of DNA-Ag nanowires with different reduction time,
(a) the reduction time =1 h, showing DNA-Ag cluster; (b) the reduction time=2 h,
showing more continuous DNA-Ag nanowires. In both images, the height scale is 25 nm,
the scan bar is 1500 nm.

119 2 K. Zhao et al. / Materials Science and Engineering C 29 (2009) 1191–119 5
during gas detecting. All experiments were carried out at room
temperature.
3. Results and discussions
3.1. Fabrication of the DNA-templated silver nanowires
The fabrication of DNA-templated silver nanowires was based on
electroless plating, a mature technology widely used in industry to make
metallic film [25]. In the work reported here, the same technology was
applied for the metallization of λ-DNA. But a complex reduction bath
solution was used to the fabrication. In experiment, λ-DNA solution
and AgNO
3
solution were mixed to incubate for several minutes. During
the incubation, the positively charged silver ions associated with the
negatively charged DNA structures (negatively phosphate groups) by
electrostatic attraction. Afterwards, the reduction bath was added to
reduce the Ag(I) ions to metallic Ag(0) on DNA molecules, resulting in
one-dimensional metallic Ag clusters. This reaction was proceed in the
dark for some time. Then, the clusters served as nucleation centres to
catalyse metallic Ag growth of continuous nanowires [26]. The dynamic
process of the reduction was monitored by UV–vis spectra. As shown in
Fig. 2, absorption peaks of DNA and Ag appear at 260 nm and 415 nm,
respectively [27]. Intensity of the absorption peak at ca. 415 nm is
increased with elongation of the reduction time, indicating that more
and more Ag(I) ions are reduced into metallic Ag.
The as-prepared nanowires were deposited on the fresh cleaved
mica and imaged with AFM. Fig. 3 shows the typical AFM morpholo-
gical structure of DNA-templated silver nanowires prepared at dif-
ferent reduction time (1 h and 2 h). Before treatment with Ag(I)
ions, the DNA molecule appeared uniform, and the diameter of the

naked DNA is 0.3–1 nm which is consistent with literature [28].After
treatment with Ag(I) ions and the reduction bath solution, the entire
DNA molecules became rough and nanoscale silver clusters grew
along the DNA chains, which we attribute to Ag(0) deposition. Fur-
thermore, the profile of the nanowires show that the diameters of
the nanowires obtained at 1 h and 2 h are 2–3 nm and 3–4 nm,
respectively. Combined with the UV–vis spectra, it is suggested that
increase of the diameters of DNA molecules through treatments could
be ascribed to dense growth of Ag on DNA. With extension of the
reduction time, not only do the diameters of the nanowires increased
but also the entire DNA chain was covered more uniformly and densely
by silver clusters. Therefore, it is believable to think that the diameter
of the DNA-Ag nanowires could be controlled by changing the re-
duction time.
In our experiments, we found that nonspecific metallic Ag de-
position rarely occurred, although there are some large aggregates on
local regions of DNA molecules, as shown in Fig. 4. We attributed this
phenomenon to the effect of lactic acid in the reduction bath solution,
which decreased the metallic Ag(0) formation in solution. Because
lactic acid is a common stabilizing agent in industry plating, slowing
down the reaction and to prevent unwanted cluster growth in solution
[29]. Our experiments also proved this hypothesis. We placed the as-
prepared sample solution for 5 h. We found there are rarely metallic
clusters precipitating out of the solution. Meanwhile, the diameters of
DNA silver nanowires could be up to 50 nm, indicating more densely
growth along DNA as shown in Fig. 4. In addition, it should be pointed
out that metallic DNA networks or loops could be easily formed in
solution, which is reflected from their nanowires structure (refer to
Figs. 3 and 4) and, importantly, the good agreement between the
present results and the previous report [30].

3.2. Gas sensing performance of the DNA-templated silver nanowires
3.2.1. Effect of NH
3
gas concentration at room temperature
Before detecting gas sensing properties, the pretreatment work
had to be carried out. The as-prepared DNA-Ag nanowires were
deposited on an interdigital gold electrode to measure the basic
electrical signals in air till the response baseline being stable. Then, the
Fig. 4. TEM images of DNA-Ag nanowires after 5 h reduction time. (a) shows a large
scale networks structure of DNA-templated silver nanowires, (b) shows enlargement of
the DNA-Ag nanowires.
Fig. 5. Conductivity variation of the DNA-templated nanowires exposed to different
concentrations of ammonia at room temperature. In experiments, 200 ppm ammonia
gas was been added in gas chamber at every 150 s. The response values were observed
to increase continuously with the gas concentrations being increased at room
temperature. However, the extent of increase in response was smaller.
Fig. 6. Variation of sensitivity exposed to different concentrations of ammonia. The
sensitivity was observed to increase continuously with the gas concentrations increasing
at room temperature.
119 3K. Zhao et al. / Materials Science and Engineering C 29 (2009) 1191–119 5
gas sensing measurements were started. All of experiments were
proceeded at ambient temperature. The variation of conductivity of
these hybrid nanowires with different ammonia concentrations at
room temperature is represented in Fig. 5. The electrode using DNA-
templated silver nanowires as gas sensing materials was exposed to
varying concentrations of ammonia gas. For the DNA-templated silver
nanowires samples, the response values were observed to increase
continuously with gas concentration at room temperature. How-
ever, the extent of increase in response was smaller, when an equal
amount of ammonia gas was added to gas chamber at every 150 s. A

little amount of gas molecules would be adsorbed and reacted to the
silver nanowires on the surface at lower gas concentrations, which
could interact more actively to give larger response. With gas con-
centrations being increased, a mass of gas molecules cumulated on the
surface of the nanowires, which could lead to saturation of adsorption
sites gradually.
The sensitivity to ammonia (S
NH
3
) was defined as:
S
NH
3
=
ΔI
I
0
=
I
g
− I
0
I
0
I
g
and I
0
represent the current values after exposure to ammonia
gas and air, respectively. Analogous to the observable tendency in Fig. 5,

Fig. 6 indicates the cumulative effect of the gas molecules, resulting in
saturation of the adsorption sites gradually on the nanowires.
3.2.2. Response and recovery of the DNA-templated silver nanowires
exposed to ammonia
The response and recovery of the DNA-templated silver nanowires
are represented in Fig. 7. The response is quick (~10 s) to 200 ppm of
NH
3
, whereas the recovery is considerably fast (~7 s). Its quick
response to ammonia and fast recovery to its initial chemical status
could be related to the structure of the materials surface and its high
volatility.
3.2.3. Selectivity to NH
3
against various gases
In further measurement, the DNA-templated silver nanowires
were exposed to ethanol, methanol, hydrogen and acetone of the
same concentration level at room temperature. Fig. 8 shows the
selectivity of the DNA-templated silver nanowires to ammonia against
other gases, in which no response to these above analytes could be
obtained. Therefore, the sensor has a remarkably good selectivity to
ammonia.
3.2.4. Mechanism of gas sensing properties
Generally, the gas sensing mechanism is explained in terms of
conductance change by adsorption of the gas molecules on the surface.
The electronic properties of the sensing materials are changed with the
adsorption of gas molecules. In the present case, the nanowires are
mainly composed of metallic silver clusters, which increase the surface
area/volume ratio and promote the adsorption of ammonia. As a result,
the quick gas response and high sensitivity can be observed.

As mentioned before, the electrode overlaid by the DNA-templated
silver nanowires is firstly pretreated in atmosphere condition before
the electrical experiments so as to obtain the stable current baseline.
During this process, an Ag
2
O layer would be formed on the surface of
silver nanoparticles or amongst metal particles, which is consistent
with the previous report [31]. A possible gas sensing mechanism can
be attributed to that “chemically responsive interparticle boundaries”
(CRIB) consisting of an Ag
2
O layer interposed between metal particles.
In the gas sensing experiment, the chemisorbed ammonia gas mole-
cules can modify the Ag
2
O barrier by either n- or p-doping this layer,
which could change the electrical conduction of the nanowires.
4. Conclusion
In this paper, we have demonstrated a simple method of fabricating
conductive silver nanowires through an efficient electroless deposition
in solution and have successfully deposited them on gold electrode
to study ammonia gas sensing properties at room temperature. The
results of gas sensing measurement indicate that the material pos-
sesses a high selectivity, quick gas response and fast recovery at room
temperature. Although further improvements are still needed to ma-
nufacture a good ammonia gas sensor, the report here indicates the
DNA-templated silver nanowires are a promising candidate of the
gas sensing materials for ammonia gas detection. In addition, the
fabrication could be easily adapted for making aligned DNA metallic
nanowires array for setuping more sensitive gas sensor.

Acknowledgments
This work was supported by the Natural Science Foundation of China
(NO. 60574095), the Knowledge Innovation Program of the Chinese
Academy of Science (kjcxz-sw-h12-02, 0723A11125) and the chief
foundation of Hefei Institutes of Physical Science, Chinese Academy of
Sciences (0721H11141).
References
[1] E. Braun, Y. Eichen, U. Sivan, G. Ben-Yoseph, Nature 391 (19) (1998) 775.
[2] J.J. Storhoff, C.A. Mirkin, Chem. Rev. 99 (1999) 1849.
[3] T.H. LaBean, H.Y. Li, Nano today 2 (2) (2007) 26.
[4] G.H. Clever, C. Kaul, T. Carell, Angew. Chem. Int. Ed. 46 (2007) 6226.
[5] A. Satti, D. Aherne, D. Fitzmaurice, Chem. Mater. 19 (7) (2007) 1543.
[6] K. Nguyen, M. Monteverde, A. Filoramo, L. Goux-Capes, Adv. Mater. (in press).
[7] Q. Gu, C.D. Cheng, D.T. Haynie, Nanotechnology 16 (8) (2005) 1358.
[8] C.F. Monson, A.T. Woolley, Nano lett. 3 (3) (2003) 359.
[9] W.U. Dittmer, F.C. Simmel, Appl. Phys. Lett. 85 (4) (2004) 633.
Fig. 8. Selectivity of the DNA-Ag nanowires to ammonia compared with other gases.
Fig. 7. Response and recovery of the DNA-templated silver nanowires upon exposure to
200 ppm NH
3.
Response time is about 10 s, and recovery time is about 7 s.
119 4 K. Zhao et al. / Materials Science and Engineering C 29 (2009) 1191–119 5
[10] L.Q. Dong, T. Hollis, B.A . Connolly, N.G. Wright, B.R. Horrocks, Adv. Mater.19 (2007)
1748.
[11] Y. Xia, P. Yang, Adv. Mater. 15 (5) (2003) 351.
[12] E.C. Walter, R.M. Penner, H. Liu, K.H. Ng, M.P. Zach, F. Favier, Surf. Interface Anal. 34
(2002) 409.
[13] T.E. Mallouk, N.I. Kovtyukhova, Chem. Eur. J. 8 (19) (2002) 4354.
[14] V.N. Singh, B.R. Mehta, R.K. Joshi, F.E. Kruis, S.M. Shivaprasad, Sens. Actuators,
B, chem. 125 (2007) 482.

[15] H.Q. Guo, S.Q. Tao, Sens. Actuators, B, chem. 123 (20 07) 578.
[16] J.T. Petty, J. Zheng, N.V. Hud, R.M. Dickson, J. Am. Chem. Soc. 126 (2004) 5207.
[17] L. Berti, A. Alessandrini, P. Facci, J. Am. Chem. Soc. 127 (2005) 11216.
[18] G.S. Devi, V.B. Subrahmanyam, S.C. Gadkari, S.K. Gupta, Anal. Chim. Acta 568
(2006) 41.
[19] D.R. Patil, L.A. Patil, P.P. Patil, Sens. Actuators, B, chem. 126 (2007) 368.
[20] J. Xu, Y. Shun, Q. Pan, J. Qin, Sens. Actuators, B, chem. 66 (2000) 161.
[21] G. Jiménez-Cadena, J. Riu, F.X. Rius, Analyst 132 (2007) 1083.
[22] L.H. Nguyen, T.V. Phi, P.Q. Phan, H.N. Vu, C. Nguyen-Duc, F. Fossard, Physica E 37
(2007) 54.
[23] M. Penza, F. Antolini, M.V. Antisari, Sens. Actuators, B, chem. 100 (2004) 47.
[24] H. Tang, M. Yan, H. Zhang, S. Li, X. Ma, M. Wang, D. Yang, Sens. Actuators, B, chem.
114 (2006) 910.
[25] Q. Gu, C. Cheng, R. Gonela, S. Suryanarayanan, S. Anaba thula, K. Dai, D.T. Haynie,
Nanotechnology 17 (2006) R14.
[26] J. Richter, Physica E 16 (2003) 157.
[27] L. Berti, A . Alessandrini, P. Facci, J. Am. Chem. Soc. 127 (2005) 11216.
[28] Z.X. Deng, C.D. Mao, Nano Lett. 3 (11) (2003) 1545.
[29] X. Chen, J. Yi, G. Qi, F. Liu, Electronic Materials and Packaging, 2000. (EMAP 2000).
International Symposium on, (2000) 12.
[30] J. Richter, R. Seidel, R. Kirsch, M. Mertig, W. Pompe, J. Plaschke, H.K. Schackert, Adv.
Mater. 12 (7) (2000) 507.
[31] B.J. Murray, E.C. Walter, R.M. Penner, Nano lett. 4 (4) (2004) 665.
119 5K. Zhao et al. / Materials Science and Engineering C 29 (2009) 1191–119 5