A convenient approach to producing a sensitive MWCNT-based paper sensor

Abstract

MWCNT/octadecylamine hybrids with interesting structures were prepared via a simple ultrasonication and drip-drying method. The morphologies of the hybrids vary from lamellae, balls, to rose-like nanoflowers, depending on the weight ratio of the two components, which could be attributed to the introduction of nanotubes into the long-chained amine with self-organization characteristics. The paper chip sensor based on MWCNT/octadecylamine hybrid shows rapid response, high sensitivity and excellent repeatability to some volatile organic compounds, especially chloroform gas. The proposed sensing mechanism of the hybrid is via the swelling of octadecylamine upon the adsorption of organic gas, thus enlarging the nanotube distance in the MWCNT/octadecylamine hybrid and leading to the conductivity decrease of the paper chip.

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1. Introduction

Accompanying economic development, air pollution poses a serious problem throughout the world. One component of the pollutants, volatile organic compounds (VOCs), is known to affect people's health and plays an irreplaceable role in ozone production and secondary organic aerosol formation.^1–4 Numerous works have been done to detect different harmful gases, but compared with other gas-sensing fields, the development of VOC sensors still requires more convenient and lower-cost sensing methods.^5–7 The intrinsic electrical, thermal, optical and mechanical properties of carbon nanotubes (CNTs) afford them promising potentials in various research fields.⁸ Because of their small size and large specific surface areas, carbon nanomaterials have drawn considerable attention in the gas-sensing field.^9,10 CNT-based gas sensors, particularly the single-walled carbon nanotube (SWCNT)-based ones, are widely used to detect electron-donating/-withdrawing gas molecules, via the changes of surface electrons on the nanotubes and the conductivity of the SWCNT networks/arrays.^11–15 Multi-walled carbon nanotubes (MWCNTs) usually show worse performance in gas sensing compared with SWCNTs, owing to their inferior conductivity. Meanwhile, long recovery time, poor selectivity, and sensitivity to limited varieties of gases are the main weaknesses of the CNT-based gas sensors. Strong interaction between gas molecules and CNTs also means difficulty in desorbing the adsorbate, which results in a long recovery time. Subsequent thermal treatment or photoirradiation is often required for the recovery.¹⁶ Decorated CNT-based sensors have a large range of applications in detecting different varieties of gases such as CH₄, H₂, CO and so on. However, the use of noble metal modifiers (Ag, Pt, Rh, etc.) means high cost.¹⁷ On the other hand, the methods for incorporating CNTs into the sensors include chemical vapour deposition,¹⁸ spin-coating,¹⁹ printing²⁰ and dripping.²¹ Most of the methods require tedious steps or precise control, which seems unsuitable for the large-scale fabrication to meet the requirements of actual demand.

Recently, CNT-involved paper-based sensors have emerged as novel and promising gas sensors and drawn intensive attention owing to their advantages of flexibility, portability, low cost and easy fabrication.^22–26 Herein, we report a convenient approach to prepare a paper-chip gas sensor based on the hybrid of MWCNTs and a cheap organic molecule, i.e., octadecylamine (ODA). As a long-chain alkyl amine with good self-organization properties, ODA usually acts as template in the Langmuir–Blodgett technique^27–29 or as stabilizer and structure-directing reagent in the fabrication of 3-D architecture materials.^30,31 ODA has also been used in the preparation of solubilized SWCNTs^32,33 as well as the separation of SWCNTs because of its affinity with nanotubes.^34,35 We find that through a simple ultrasonic treatment of MWCNTs and ODA in chloroform and subsequent drip-drying process, CNT–ODA hybrid with various morphologies, including lamellae, balls, or rose-like nanoflowers, can be prepared depending on the initial weight ratio of the two components. The CNT–ODA hybrid with flower-like structure was utilized to fabricate the paper-based gas sensor by dripping the chloroform dispersion of CNT–ODA on the filter paper. The CNT–ODA-paper chip sensor shows excellent performance (fast recovery and good repeatability) in the detection of some VOCs, especially chloroform (CHCl₃) gas. The typical preparation process is outlined in Fig. 1.

Fig. 1 Schematic illustration for the preparation of paper-chip sensor based on MWCNT–ODA hybrid.

2. Experimental

Filter paper (thickness ∼ 150 μm, pore size ∼ 2.5 μm) was purchased from Whatman Co. MWCNTs (95% purity, length ∼ 20 μm, diameter 20–30 nm) were purchased from Shenzhen Nanotech Port Co. and purified in nitric-sulfuric acid with the aid of sonication for 12 h. ODA was purchased from Alfa Aesar and used as received.

Scanning electron microscopic (SEM) characterization was conducted on a JEOL JSM-7401F at an accelerating voltage of 5 kV. Transmission electron microscopic (TEM) images were captured on a JEOL JEM-2010 at an accelerating voltage of 200 kV through a Gatan model 780 CCD camera.

MWCNT–ODA hybrid was prepared through an ultrasonic method. Typically, 10 mg of purified MWCNTs and 200 mg of ODA were mixed in 2 mL of CHCl₃ and treated in an ultrasonic bath for 5 min, and then a black homogenous suspension was formed. For the SEM and TEM characterizations, the CHCl₃ suspension of the hybrid was dripped and dried on the silicon wafers and carbon-coated copper grids, respectively.

To fabricate the MWCNT–ODA-based paper sensor, 0.2 mL of the CHCl₃ suspension was dripped on a rectangular piece of filter paper (0.5 cm × 1.5 cm). After the paper was dried in air, a thin layer of gold (ca. 180 nm) was sputtered onto each end of the paper piece by using a JCP-200 magnetron sputter-coating machine. The paper chip between two gold electrodes was 0.5 cm × 0.8 cm. A statistical test system including a test chamber and a data acquisition system was utilized to measure the sensing performance of the chip device. The device was connected to an electrochemical workstation that acted as a direct current source (1.0 V) and a current reader. The sensor response was followed by the normalized change rate of conductivity, −ΔG/G₀, which was calculated by the normalized change of current, −ΔG/G₀% = [(I₀ − I)/I₀] × 100%, where G₀ and I₀ are the initial conductivity and current before exposure to the analyte, I is the current after exposure to the analyte, and ΔG is the change of conductivity after exposure to the analyte.

3. Results and discussion

The typical flower-like morphology of the MWCNT–ODA hybrid is shown in Fig. 2 (MWCNT : ODA weight ratio is 1 : 20). Obviously, in the low magnification, the hybrid appears in the shape of a sphere/clew with the diameter of 10 μm (Fig. 2a). In the high magnification, the flower-like structure composed of lamellae can be observed (Fig. 2b). Apparently, owing to the embedding of ODA matrix, there are no visible nanotubes in the SEM view. However, the TEM image of a petal edge reveals that MWCNTs are randomly distributed in ODA as the nanotube network (Fig. 2c).

Fig. 2 Typical SEM images of MWCNT–ODA hybrid (a) at low magnification, (b) at high magnification and (c) TEM image of MWCNT–ODA hybrid. MWCNT : ODA weight ratio = 1 : 20.

To investigate the influence of weight ratios of MWCNTs and ODA on the morphology of the hybrid, a series of experiments with different MWCNT : ODA weight ratios (1 : 1, 1 : 5, 1 : 20 and 1 : 50) were carried out. Fig. 3 demonstrates the morphology evolution process of the hybrid with the different ratios. When the ratio is 1 : 1, the dried solid exhibits aggregates of petals, with nanotubes distributed around them (Fig. 3a). For the ratio of 1 : 5, some flower-like balls composed of lamellae are formed, without obvious nanotubes in the view (Fig. 3b). When the ratio reaches 1 : 20, the hybrid yields flower-like structures (Fig. 3c), as mentioned above. For the ratio of 1 : 50, the majority of ODA causes the hybrid to lose the lamellae and flower-like structure (Fig. 3d). The morphology of pure ODA obtained from its CHCl₃ solution is shown in Fig. 3e. Due to its excellent film-forming property, pure ODA exhibits the leaf-like structure, with ∼100 μm lamella size, differing with the flower-like structure of MWCNT–ODA, which indicates the key role of nanotubes in the formation of the unique construction. In addition, it is found that the final structure of the hybrid is influenced by the concentration of MWCNT–ODA in the CHCl₃ solution. Fig. 3f shows the morphology of MWCNT–ODA hybrid from a dilute solution with the ratio of 1 : 20. Although the lamellar structure is formed, only aggregates of semi-finished flower-like structures can be seen, which is quite different from the closely packed structure shown in Fig. 3c.

Fig. 3 SEM images of MWCNT–ODA hybrids with different weight ratios: (a) 1 : 1; (b) 1 : 5; (c) 1 : 20; (d) 1 : 50; (e) 0 and (f) 1 : 20 (from dilute solution).

Due to its self-organization characteristic, ODA is apt to exist in the form of a film, as illustrated in Fig. 4a. Large ODA lamellae (∼100 μm) could be easily obtained through a dripping method (Fig. 3e). Meanwhile, MWCNT–ODA hybrids exhibit lamella/petal feature with the size of ∼10 μm. Obviously, the structure of the hybrids originates from ODA, and the introduction of nanotubes alters the organization fashion of ODA. In other words, the growth of ODA film is interrupted by the addition of nanotubes. In fact, the vigorous ultrasonication treatment of MWCNTs and ODA in CHCl₃ leads to the homogeneous MWCNT dispersion owing to the interaction with ODA, which acts as a surfactant. The interaction of SWCNTs and ODA has been proposed to be zwitterionic and via physisorption.³⁵ Further study indicates that the level of oxidation of SWCNTs plays an important role in the SWCNT–ODA interaction.³⁶ In our case, the carboxylic acid groups attached on the nanotubes should be responsible for the incorporation of nanotubes into the ODA assembly. As demonstrated in Fig. 4b, the MWCNTs anchor the ODA molecule through zwitterionic forces, and the latter attracts other ODA molecules through self-assembly. Subsequently, upon the evaporation of the solvent, ODA molecules are inclined to assemble along the nanotubes. That is, the nanotubes provide anchoring sites for ODA molecules, based on which ODA could self-organize into lamellae. As a result, the aggregation of lamellae yields the flower/ball-like structure. Due to the random distribution of nanotubes in ODA solution, a lamella could contain several nanotubes, which also accounts for the much-smaller size of the lamellae compared with the leaf from pure ODA.

Fig. 4 Schematic illustrations of (a) self-organization of ODA, (b) formation of MWCNT–ODA hybrid and (c) proposed mechanism of gas sensing.

The MWCNT–ODA hybrid was used to modify the filter paper to obtain a MWCNT–ODA paper sensor. Fig. 5a and b demonstrate the morphology of the filter paper before and after deposition of the MWCNT–ODA hybrid, respectively. Obviously, compared with the fibers of the original paper, the fibers of MWCNT–ODA-covered paper become much rougher due to the decoration of the hybrid. In principle, such roughness would favor the sensing ability of the sensors.

Fig. 5 SEM images of filter paper (a) without and (b) with coverage of MWCNT–ODA hybrid (1 : 20). (c) Responses of the paper chip to different concentrations of CHCl₃ from 100 ppm to 1000 ppm. (d) Repetitive response of the paper chip to CHCl₃ of 500 ppm. (e) Responses of the paper chip to different gases of 500 ppm.

The sensing capability of the MWCNT–ODA-paper chip was tested for detecting CHCl₃ gas. Fig. 5c shows the change rate of conductivity (−ΔG/G₀) with the injection of 1 mL of saturated CHCl₃ vapor into the chamber every 100 s ten times, corresponding to the increase of CHCl₃ concentration from 100 ppm to 1000 ppm. When the first dose of CHCl₃ is injected, the change of conductivity is 1.7%. The subsequent addition of the gas leads to a significant conductivity change, and each response is very quick, indicating the excellent sensing by the sensor. The stepwise growth of the change reached 51% at ten doses of CHCl₃ gas, 30 times higher than the initial change. Fig. 5d reveals the repetitive responses of the device to the CHCl₃ concentration of 500 ppm. It is found that the conductivity decreases immediately to a similar value for each cycle, implying the stability of the sensor and its recycling potential.

The responses of the paper chip to other VOCs are also measured, as shown in Fig. 5e. Obviously, for the same concentration of the gas, the device demonstrates quite different sensing capabilities. Besides chloroform, dichloromethane elicited an excellent response, while methanol, alcohol, acetone and diethyl ether elicited relatively low responses, and ethyl acetate only showed negligible response.

The detection responses of the paper chip to different VOCs are consistent with the solubility of ODA in the corresponding solvents, which might reveal the sensing mechanism of the MWCNT–ODA hybrid. As shown in Fig. 4c, when the gas molecules are adsorbed on the MWCNT–ODA hybrid, ODA is swelled to some extent, depending on the solubility of ODA in the solvent. The swelling of ODA enlarges the distances between the nanotubes, hence decreasing the conductivity of the nanotube network and leading to the current drop of the circuit. While the adsorbed gas volatilizes in air, the swelling of ODA diminishes, and the conductivity of the nanotube network is resumed.

4. Conclusions

In summary, MWCNT–ODA hybrids with interesting structures are prepared via a simple and efficient ultrasonication and drip-drying method. The unique morphology of the hybrid originates from the introduction of nanotubes into the long-chained amine with self-organization property. The MWCNT–ODA hybrid-covered paper chip sensor shows high sensitivity to chloroform gas with good repeatability. The sensing mechanism of the hybrid could be ascribed to the swelling effect of chloroform gas on the hybrid, which enlarges the distance of the nanotubes and results in the conductivity decrease of the chip. Our work should be helpful for the research of CNT/organic hybrids and their potentials in many fields.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21173266 and 21473250) and the Fundamental Research Funds for the Central Universities, the Research Funds of Renmin University of China (Grant No. 15XNLQ04 and 16XNLQ04).

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