synthesis and application of dna-templated silver nanowires

Synthesis and application of DNA-templated silver nanowiresfor ammonia gas sensing Kai Zhaoa,b, Qifei Changa, Xing Chena, Buchang Zhangb, Jinhuai Liua,⁎ a The Key Laboratory of Biomimeti

Synthesis and application of DNA-templated silver nanowires

for ammonia gas sensing

Kai Zhaoa,b, Qifei Changa, Xing Chena, Buchang Zhangb, Jinhuai Liua,⁎

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

a b s t r a c t

a r t i c l e i n f o

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 2 nm)

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 interconnects[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

environmental problems in industrial gases, the requirement of detecting

ammonia has greatly been increased Traditional semiconducting oxides

materials using for gas detection have some drawbacks, especially high

operative temperature and poor gas selectivity etc[20,21] One of the

important approaches overcoming the disadvantages 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] Recently, many nanomaterials have been developed as the sensing materials of ammonia gas sensor, such

as nanofibrous polyaniline[22], carbon nanotubes[23]and metal oxides nanoparticles [24] Because of using nanomaterials for gas sensing materials, the working temperature of the gas sensors has been decreased, meanwhile, the selectivity and sensitivity have been improved compared

to the traditional gas sensing materials Therefore, the DNA-templated silver nanowires with good conductivity and nanoscale structure may 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 AgNO3in 100 ml ddH2O 50μL λ-DNA solution (300 ng/μL, Fermentas Inc.) and 250μL 2 mM AgNO3solution were mixed and

⁎ Corresponding author Tel.: +86 551 5591142; fax: +86 551 5592420.

E-mail address: jhliu@iim.ac.cn (J Liu).

0928-4931/$ – see front matter Crown Copyright © 2008 Published by Elsevier B.V All rights reserved.

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Materials Science and Engineering C

j o u r n a l h o m e p a g e : w w w e l s ev i e r c o m / l o c a t e / m s e c

incubated for 1 h in the dark at room temperature After that, 250μL

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

A 5-μL drop of the prepared Ag nanowires solutionwas dropped onto

the surface of the fresh cleaved mica by micropipet The sample was then

imaged with a tapping-mode atomic force microscopy (AFM, Nano

first-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 AgNO3solution, 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 inFig 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 the gas 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 reduction process of DNA-Ag +

complex Following reduction, a peak formed at 415 nm, demonstrating the formation

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,

1192 K Zhao et al / Materials Science and Engineering C 29 (2009) 1191–1195

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

metallicfilm[25] In the work reported here, the same technology was

applied for the metallization ofλ-DNA But a complex reduction bath

solution was used to the fabrication In experiment,λ-DNA solution

and AgNO3solution 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 3shows 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 inFig 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 inFig 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 NH3gas 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

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

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 inFig 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 (SNH 3) was defined as:

SNH

I0 =

Ig− I0

I0

Igand I0represent the current values after exposure to ammonia

gas and air, respectively Analogous to the observable tendency inFig 5,

Fig 6indicates 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 inFig 7 The response is quick (~ 10 s) to 200 ppm of

NH3, 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 NH3against 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 isfirstly pretreated in atmosphere condition before the electrical experiments so as to obtain the stable current baseline During this process, an Ag2O 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 Ag2O layer interposed between metal particles

In the gas sensing experiment, the chemisorbed ammonia gas mole-cules can modify the Ag2O 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)

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