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Sensors and Actuators B 168 (2012) 34–38 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Multi-wall carbon nanotube gas sensors modified with amino-group to detect low concentration of formaldehyde Haifen Xie a , Changhao Sheng a , Xin Chen a , Xingyan Wang a , Zhi Li a , Jia Zhou b,∗ a b Department of Physics, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, China ASIC and System State Key Lab, Department of Microelectronics, Fudan University, 220 Handan Road, Shanghai 200433, China a r t i c l e i n f o Article history: Received 30 September 2011 Received in revised form 29 December 2011 Accepted 31 December 2011 Available online 11 April 2012 Keywords: Multi-carbon nanotubes (MWCNTs) Gas sensor Formaldehyde Amino-group Modified electrodes a b s t r a c t Gas sensors with multi-wall carbon nanotubes (MWCNTs) modified with amino-groups on interdigitated electrodes (IDE) were fabricated to detect low concentration of formaldehyde at room temperature. Effects of content of amino groups on sensing responses against various interfering circumstances and low concentration of formaldehyde were investigated. The sensor behaved high relative resistance changes to formaldehyde and lower response to interfering gases such as acetone, carbon dioxide, ammonia, toluene and ethanol. When the concentration of formaldehyde was 20 ppb, the relative resistance changes of the sensor modified with 18.19% amino-group reached 1.73%. The sensor displayed high chemical selectivity, fast response and good reproducibility to low concentration of formaldehyde, which was attributed to the properties of MWNTs and the interaction between the surface of MWCNTs and amino-group. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Carbon nanotubes (CNTs) are one of the advanced functional materials today and has been researched extensively since its discovery in 1991 [1]. CNTs are widely used as sensitive material to detect low concentrations of gases such as nitrogen oxides [2,3], ammonia [4,5], hydrogen [6–9], carbon monoxide [10] and some organic gases [11,12]. Both CNT-based [13–16] and CNT-doped [17–19] gas sensors have been explored because of their specific properties [20,21] such as nanometer hollow geometry, high specific surface area, high electron mobility, surface modification, and functionalization. Electrical resistance change of CNTs induced by gas molecules adsorption on their surface via van der Waals force has found its applications to develop CNT-based gas sensors for various gases such as formaldehyde from indoor air [22]. Formaldehyde emitted from construction and decorative materials in our daily life, which is deleterious for our health with potentially carcinogenic [23], high rate of cancer, respiratory disease, and baby abnormalities [24,25]. The World Health Organization (WHO) has derived an air quality guideline of 80 ppb averaged over 30 min [26,27]. Therefore, detection of these indoor air pollutants is of particular concern because of their long-term effects on our health. Thus, gas sensors with fast response, high relative resistance changes and selectivity, and for ultra low concentration are of great wealth for exploration. So far, many methods have been investigated for detection of indoor formaldehyde, including spectrophotometry [28,29], potentiometric [30], difference-frequency generation [31], amperometric [32,33], electrochemical biosensors [34,35], optical methods [36,37], piezoelectric sensors [38] and filter color testing methods [39,40]. Although these methods provide safe detection of formaldehyde, they still have some limitations and challenges such as low relative resistance changes and selectivity, long-term instability and so on. In this paper MWCNTs modified with amino groups is proposed to enhance chemical reaction between amino groups of the MWCNTs and formaldehyde molecules. Functionalization of MWCNTs with amino groups is conducted and characterized. Sensors for detection of formaldehyde are fabricated by interdigitated electrodes with modification of functionalized MWCNTs. Effects of content of amino groups of the MWCNTs on sensing responses are investigated. Relative resistance changes, selectivity and repeatability of the sensors are demonstrated by various measurements. 2. Experimental ∗ Corresponding author. Tel.: +86 21 5566 4601; fax: +86 21 65643449. E-mail address: jia.zhou@fudan.edu.cn (J. Zhou). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.12.112 Microchips with interdigitated electrodes (IDE) were fabricated by micromachining technology. Specific sensors for detection of H. Xie et al. / Sensors and Actuators B 168 (2012) 34–38 35 formaldehyde were composed by functionalizing the electrodes and were characterized. 2.1. Reagents and functionalization of MWCNTs MWCNTs (diameter < 10 nm, length = 5–15 ␮m, purity > 95%) were obtained from Shenzhen Nanotech Port Co., Ltd., China. N,N-dimethylformamide (DMF), concentrated nitric acid (HNO3 ), concentrated sulfuric acid (H2 SO4 ), ethanediamine, thionyl chloride (SOCl2 ), and 1-propyl alcohol were products of Shanghai Lingfeng Chemical Reagent Co., Ltd. All chemicals were of analytical purity and used as received without further purification. Nafion (5 wt%) was purchased from DuPont. Purified water was used to prepare all solutions. Formaldehyde gas was obtained from Weichuang Gas Mixing Co., Ltd. (Shanghai, China) and N2 (99.99 vol%) was bought from Shanghai Wugang Gas Company. Dried carboxylated MWCNTs were obtained according to the reported literature [41]. The products were chlorinated by refluxing with SOCl2 at 80 ◦ C for 4 h and 12 h, respectively. After thoroughly evaporating remaining SOCl2 , amine functionalized MWCNTs (MWNT-NH2 ) were the reaction products of chlorinated MWCNTs and ethanediamine NH2 (CH2 )2 NH2 in DMF at 160 ◦ C for 4 h and 12 h, respectively. Followed by centrifugation at 8000 rpm of the suspension for 20 min, the insoluble part was got. Repeat this process several times. MWNT-NH2 powder was obtained from drying at 50 ◦ C in vacuum for 24 h. Two kinds of functionalized MWCNTs were obtained, i.e., 5.55% amino group-MWCNTs and 18.19% amino group-MWCNTs, representing the high N percentage with long processing time and low N percentage of MWCNTs with short processing time, respectively. Fig. 1. Schematic diagram of experimental setup. The flow rate was controlled at 0.2 L/min. Before starting measurement, sensors were heated in a drying oven at 90 ◦ C for 30 min, and then cooled at room temperature in a sealed glass chamber in nitrogen for about 10 min. For each test, the sensor was exposed to the target gas for 60 s, then to pure nitrogen for 100 s. 2.2. Sensor fabrication by modification of electrodes 30 mg MWCNTs/MWNT-NH2 was added into 3 mL of Nafion solution. After ultrasonicated for about 30 min, a uniform 10 mg/mL MWCNT/Nafion suspension was formed. A 10 ␮L of suspension was mounted on the top surface of the interdigitated electrode [42]. Cure the electrode on a hotplate at 90 ◦ C for 30 min. Three different gas sensors based on raw MWCNTs/Nafion film, MWNTs-N-1/Nafion film and MWNTs-N-2/Nafion film were obtained. We named them ECNT , ECNT-18%amino , ECNT-5%amino in sequence. Fig. 2. IR spectrum of MWCNTs modified with ethanediamine. The characteristic C O peak appears around 1650–1690 cm−1 , which is the characteristic absorption band of amide. 2.3. Characterization of sensors Characterization of the functionalized carbon nanotubes was performed with a Fourier transform infrared (Magna-IR 550, Nicolet, and USA) in the wave number ranging from 400 to 4000 cm−1 . Elemental analysis was done on Germany vario EL Ш. The morphology and microscopic structure of CNTs were characterized by scanning electron microscopy (SEM; JEOL JSM 6360, Japan). All resistance measurements were carried out by Agilent 34970A workstation (Agilent Instruments, USA). Schematic diagram of the experimental setup is shown in Fig. 1. A gas flow controller was connected to control the flow rate at about 0.2 L/min. Before starting measurement, sensors were heated in a drying oven at 90 ◦ C for 30 min, and then cooled at room temperature in a sealed glass chamber in nitrogen for about 10 min. For each test, the sensor was exposed to the target gas for 60 s, then to pure nitrogen for 100 s. The relative resistance changes of sensor is defined as (R − Ro )/Ro , where R and Ro are the resistances in target gas and in pure nitrogen, respectively. After characterization of the functionalized MWCNTs, relative resistance changes of sensors to formaldehyde gas were measured at concentrations of 20 ppb, 80 ppb, 120 ppb, 180 ppb and 200 ppb sequentially. Selectivity of sensors to formaldehyde was conducted at 200 ppb compared with methanol, ethanol, acetone, ammonia, and carbon dioxide. Repeatability of sensors was carried out in 50 ppb formaldehyde gas. Every detection was conducted three times for data acquisition and analysis. 3. Results and discussions 3.1. Characterization of surface functionalized MWCNTs Fig. 2 shows the infra-red (IR) spectrum of MWCNTs modified by ethanediamine. From Fig. 2, the characteristic C O peak appears around 1650–1690 cm−1 , which is the characteristic absorption band of amide. That indicates that amino-group is successfully grafted onto MWCNTs. The content of amino-group grafted onto MWCNTs was characterized by Vario ELШ through analysis of element N. Table 1 gives the mass fraction of N element on the surface of functionalized MWCNTs. From the table, the amino groups content of 5.55% amino group-MWCNTs and 18.19% amino group-MWCNTs were 18.19% and 5.55%, respectively. 36 H. Xie et al. / Sensors and Actuators B 168 (2012) 34–38 Table 1 Analysis results of content of amino-group in modified MWCNTs. Elemental content of nitrogen was measured by Vario ELШ. The content of amino-group in samples was calculated by the content of nitrogen dividing the molecular weight ratio of element N (28) over hexamethylene diamine (60). Sample Sample weight (mg) N% Average amino group % MWCNTs-N-1 1.388 1.392 1.036 1.031 8.38 8.59 2.54 2.64 18.19% MWCNTs-N-2 5.55% Morphology characterization of functionalized MWCNTs was also conducted by scanning electron microscopy (SEM). Fig. 3 demonstrates the SEM pictures of typical morphology (a) MWCNTs and (b) 18.19% amino group-MWCNTs. From Fig. 3, we can see that the MWCNTs are in the form of small bundles and single nanotubes, exhibiting a spaghetti-like porous reticular formation. Comparing with Fig. 3a and b, we found that no big change in morphology between raw MWCNTs and functionalized MWCNTs. 3.2. Gas sensor responses to the formaldehyde The relative resistance changes of three sensors to formaldehyde gas with different concentrations are shown in Fig. 4. The quick response time of all these three sensors is about 7–10 s. It is clear that sensor ECNT-18%amino has the highest relative resistance changes among the three sensors. Especially for detection of 20 ppb formaldehyde gas, relative resistance changes of sensors ECNT-18%amino , ECNT-5%amino and ECNT are 1.73%, 0.56% and 0.13%, Fig. 4. Typical relative resistance changes of sensor ECNT , ECNT-5%amino and ECNT-18%amino to formaldehyde gas with concentration ranging from 20 ppb to 200 ppb. respectively. The relative resistance change of ECNT-18%amino is about 2.4 times that of ECNT-5%amino and 13 times that of ECNT . Fig. 4 also illustrates the process of desorption of the three sensors. In the same desorption time of 100 s, the resistance of sensor ECNT-18%amino cannot return to its resistance base line. The results indicates that the sensor modified with higher percentage of N behaves more difficult for complete desorption of absorbed gas molecules at room temperature. The reason why these three sensors show different responses to formaldehyde gas is due to the different absorption and desorption ability of MWCNTs. For sensor ECNT , only MWCNTs were modified on electrodes. As we know, there only exists physical absorption due to large surface area of MWCNTs. The van der Waals attracting forces let formaldehyde molecules enter the MWCNTs, causing a change in the density of carrier electrons in the MWCNTs structures, leading to electrical resistance changes in sensor [43]. However, for sensor ECNT-18%amino and ECNT-5%amino with aminogroup modification, the responses of the resistance change are not only due to the physical absorption toward the formaldehyde gas molecules, but also to the chemical absorption. There is a reaction between amino groups of the MWCNTs and formaldehyde molecules as follows: HCHO + NH2 –R → H2 C N–R + H2 O Fig. 3. SEM images of (a) MWCNTs and (b) 18.19% amino group-MWCNTs. As each of the amino-group immobilized on MWCNTs is accessible to formaldehyde, both physical and chemical absorption decide the response time and the relative resistance changes of ECNT-18%amino and ECNT-5%amino . On one hand, the chemical reaction only takes about some microseconds [44]. That brings a relatively shorter response times of ECNT-18%amino and ECNT-5%amino than that of ECNT . The chemical reaction also increases the relative resistance changes of ECNT-18%amino and ECNT-5%amino and thus shows better performance to low concentration of than that of ECNT . On the other hand, chemical combination of formaldehyde and amino groups results in relatively longer desorption time of ECNT-18%amino and ECNT-5%amino than that of ECNT . The adsorption of sensor ECNT without modification of amino-groups only performs physical desorption. Purge of pure N2 can easily accomplish fully physical desorption but hardly give helps to chemical desorption. The residue chemical combination of formaldehyde and amino groups of ECNT-18%amino and ECNT-5%amino causes shifts of their resistance base lines. H. Xie et al. / Sensors and Actuators B 168 (2012) 34–38 Fig. 5. Linear relationship between relative resistance change of sensor ECNT , ECNT-5%amino and ECNT-18%amino and concentration of formaldehyde ranging from 20 ppb to 200 ppb. Data analyzed was from 3 measurements for each point. Fig. 5 demonstrates the linear relationship of the relative resistance changes verse concentrations. From the slopes of the lines, it is clear that the higher percentage of amino groups of MWCNTs, the stronger the binding forces between MWCNTs and formaldehyde molecules. However, the relative resistance changes of the sensors do not increase with the percentage of amino groups linearly. Because the resistance baseline of the sensor will increase quickly with increasing content of the amino groups modified on the MWCNTs, which will decrease the relative resistance changes greatly. To achieve high relative resistance changes, the percentage of amino groups modified on MWCNTs need to be optimized. In our experiments, in pure nitrogen, the resistance baselines of three sensors ECNT , ECNT-5%amino , and ECNT-18%amino are 0.2455 K, 0.917 K, 91.9 K, respectively. We found that with the content of amino-group more than 18.19%, the resistance baseline of the sensor was too large to be used as resistance sensors. 3.3. Selectivity and repeatability Fig. 6 shows the responses of sensor ECNT-18%amino to methanol, ethanol, acetone, ammonia, carbon dioxide, and formaldehyde gas with the same concentration of 200 ppb. From Fig. 6, sensor ECNT-18%amino demonstrates a good relative resistance change with 5.54% for formaldehyde, while relative resistance changes for 37 Fig. 7. Reproducibility of sensor ECNT-18%amino at 50 ppb formaldehyde. Relative resistance changes with time in continuous three test cycles indicates the reproducibility. methanol, ethanol, ammonia and carbon dioxide are 0.71%, 0.41%, 0.71%, 0.89%, respectively. When concentration of target gases dropped to 20 ppb, the relative resistance change of formaldehyde gas reached 1.73%, while for other five gases the response was very low. Even methanol and ethanol were difficult to be detected with our instrument. Fig. 7 shows responses of ECNT-18%amino to formaldehyde gas with the concentration of 50 ppb for three cycles. The reproducibility of the analysis data in response is almost the same. The relative resistance changes of the three cycles are 2.03%, 2.08%, 2.09%, respectively. 4. Conclusion In this paper, MWCNT films modified with different graft percentage of amino-groups have been studied as resistive formaldehyde sensors. Sensor responses were improved by successful functionalized MWCNT with amino-groups. The results show that the sensor electrode with MWCNT films modified with graft percentage of amino-groups around 18% provides excellent responses to low concentrations of formaldehyde. Its relative resistance change is 13 times that of pure MWCNT sensor when the formaldehyde is 20 ppb. It exhibits high selectivity to several other interference organic gases and inorganic gases. Both relative resistance change and selectivity strongly depend on the interaction between surface of MWCNT films and the modified amino-groups. Good repeatability of the sensor is also presented. This work offers fascinating opportunities for their applications as sensors to detect low concentration of formaldehyde. Future work will focus on study of the behavior of the sensor in the real combined gases at the room temperature and the interaction mechanism of gas molecules with functional groups in MWCNT sensors. Acknowledgement We are grateful for the support of this work from the ASIC and System State Key Lab, Department of Microelectronics, Fudan University. References Fig. 6. Selectivity measurements of sensor ECNT-18%amino . 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Biographies Haifen Xie is a professor of Physics Department in East China University of Science and Technology, received her PhD degree of Microelectronic Department at Fudan University in 2005. Her current fields of interests are microsensors, MEMS and biochemical sensor arrays. From September 2004 to January 2005 advanced visiting scholar at physics, applied physics and astronomy department of Rensselaer Polytechnic Institute. Changhao Sheng is a graduate student of Physics Department in East China University of Science and Technology. His research interest focuses on the development of micro sensors. Xin Chen is a graduate student of Physics Department in East China University of Science and Technology Her research interest focuses on nano materials fabrication and characterization. Xingyan Wang is a graduate student of Physics Department in East China University of Science and Technology. His research interest focuses on fabrication of sensors. Li Zhi received his BS in Chemistry Department of East China University of Science and Technology. His research interest focuses on nano materials fabrication and characterization, applications of nuclear technology and fine chemistry. Jia Zhou is a professor of ASIC and System State Key Lab, Department of Microelectronics, Fudan University. Prof. Jia Zhou received her PhD degree from Fudan University in 2004.1. Her research interests are in MEMS/NEMS based chemical, biochemical and biomedical sensors and their applications.