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: jhliu@iim.ac.cn (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. 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