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Reversible strategy of water monitor aimed at amphiphilic pollutants Quan Zhang, Peng Meng, Yulong Wu, Rui-Ting Zheng, Xiaoling Wu, and Guo-An Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16186 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Materials & Interfaces

Reversible Strategy of Water Monitor Aimed at Amphiphilic Pollutants Quan Zhang, Peng Meng, Yulong Wu, Ruiting Zheng, Xiaoling Wu, Guoan Cheng*

College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China

Abstract For monitoring diverse pollutants in complicated water environment, design and development of various detection strategies are necessary. Here, we introduce a general strategy of water monitor aimed at amphiphilic pollutants using carbon nanotube-based film. The pollutants with amphiphilic characteristics can tune the wetting behavior between carbon nanotubes and water molecules, leading to the change in the interface resistance of the carbon nanotube-based film. The experimental results demonstrate that the change ratio of the film resistance is related to the concentration of pollutants in solution. This monitor strategy is general for detection of amphiphilic materials in mixed solution, such as surfactants and some organic solvent. The ability to achieve a sensitive and repeatable change in film resistance has potential applications in high-sensitivity, real-time, long-lasting and multiple water monitor.

Keyword Water monitor; amphiphilic property; wetting behavior; interface resistance; conductivity

Introduction Water monitor is an important and practical research focus due to enormous

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influence of water quality on public health and natural environment. Common chemical and biological pollutants, like heavy metal ion,1-3 acid group anion,4 organic reagent5, 6 and microbe,7 are difficult to distinguish due to solubility, dispersibility and their small size. Meanwhile, real-time and rapid monitoring of water quality is crucial to identify pollution level and design the subsequent treatment scheme. Up to now, diverse monitor approaches are designed aimed at specific pollutants. Electrochemical sensing technology is an in situ method to detect the heavy metal ion by redox reaction (either reduction or oxidation or both) at the electrodes (sensing material).1-3 This electrochemical technology is improved to monitor acid group anion in water.4 Spectroscopy analysis is an important detecting method for organic reagent.5 It is also suitable for microbe detection in water monitor. Biochemical oxygen demand (BOD) measurement is another method for biosensing in water monitor.3 In addition, zeta potential measurement and other monitor techniques are gradually developed based on the physical and chemical characteristics of various pollutants.6 Surfactants are common pollutants in industrial or domestic sewage. Many analysis technologies are used to detect surfactants by means of micelles in water. Time-resolved infrared double resonance spectroscopy can detect dioctyl sodium sulfosuccinate (AOT) of 6.2% mass fraction in water.8 In virtue of phenolphthalein as an indicator, ultraviolet–visible spectroscopy method can dynamically study the hydrolysis of 0.3mol/L AOT aqueous solution.9 Nuclear magnetic resonance spectroscopy is able to monitor the peak variation of 0.4mmol/L AOT/water/benzene mixed solution at high pressure (100~250 bar).10 This technology also can determine dodecyl sodium sulfate (SDS) of 4 mmol/L concentration in SDS/water/polymer solution.11 Small-angle X-ray scattering and electron paramagnetic resonance also are common detecting methods, which can determine SDS or hexadecyltrimethylammonium bromide (CTAB) of 5 mmol/L concentration in the solution.12 However, the researches about water monitor aimed at amphiphilic pollutants like surfactants are insufficient. Complicated analysis process and low detection sensitivity don’t satisfy the practical demands (e.g. real-time, rapid and ACS Paragon Plus Environment

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long-lasting monitoring). These problems interrupt a broader range of applications of the existing monitor methods. In this work, we demonstrate a mechanism of water monitor based on modulation of wetting behavior between CNTs and water caused by amphiphilic pollutants. The clear function relationship between the change ratio of the film resistance and the concentration of the solution confirms the feasibility of CNT-based film in water monitor for determining the content of the pollutants. In addition, the approach is general on diverse contaminations, including the surfactants and some organic solvents. The good reversibility makes it possible to continuously and repeatedly monitor quality of water.

Experimental method Materials: Vertical multi-walled CNT arrays are grown on (1-0-0) silicon wafer by microwave plasma-enhanced chemical vapor deposition (MW-PECVD) using 0.025mol/L ferric chloride ethanol solution, 5 sccm acetylene and 50 sccm hydrogen as the catalyst precursor, carbon source, and carried gas, respectively. The conductivity of deionized water is less than 6×10-8 S/cm. Hexadecyltrimethylammonium bromide (CTAB) (AR), dioctyl sodium sulfosuccinate (AOT) (CP), dodecyl sodium sulfate (SDS) (CP) and Potassium chloride (KCl) (AR) are purchased from Sinopharm Chemical Reagent Co., Ltd., China. Ethanol (AR) is purchased from Xilong Scientific Co., Ltd, China. Preparation of sensors: Au electrodes with a 100-nm thickness and 10-mm width are pre-deposited on the clean Teflon substrate by magnetron sputtering. The distance between Au electrodes is 10 mm. The well-ordered pristine CNT film is prepared by transferring the vertical MWCNT array to the organic substrate with Au electrodes.13, 14

Direction of rolling (namely axial direction of CNTs) is parallel to current. The

contact of the CNTs and the electrodes is coated by a layer of polydimethylsiloxane (PDMS) (as shown in Figure 1(a)). Characterization: The micro-morphology of the vertically multi-walled CNT



arrays and CNT film is determined by scanning electron microscopy (SEM) (Hitachi S-4800) and transmission electron microscopy (TEM). Contact angle is measured by the static sessile drop technique. Measurement of electric response: Hand-made equipment was used to monitor direct current change of CNT-based device. The measurement of time-resolved DC current is carried out by using Keithley 4200-SCS under a constant bias voltage of 1 V. Keithley 4200-SCS holds that the fluctuation of voltage and current is less than 50 µV and 10 nA, respectively. To avoid the fluctuation from operation, the CNT-based device is fixed on a support and the vessel is placed on a lifting platform. The CNT film is immersed into and pulled out of the liquid by raising and lowering the platform. In addition, the environmental temperature and humidity is 24±1 ℃ and 45±5 %, respectively.

Results and discussion Figure 1(a) is a SEM image of an as-grown CNT array. The thickness of the array is 316 µm and the mean diameter of the CNTs is 5 nm. The CNT film is prepared by transferred CNT array on Au electrodes deposited on a Teflon substrate (as shown in Figure 1(b)). To avoid the influence from the interface between the CNTs and the electrodes, a PDMS coating is deposited on the CNT/Au contacts. The inset of Figure 1(b) is a photograph of the device. The thickness and width of the Au electrodes are 100 nm and 10 mm, and the distance between the electrodes is 10 mm. The thickness of the CNT film is about 15 µm.


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Figure 1 (a) SEM image of an as-grown CNT array. The inset is a high-resolution TEM image of an individual CNT. (b) Schematic of a well-ordered CNT film on Au deposited on a Teflon substrate. The inset is a photograph of the device. (c,d) Schematic diagrams showing the microstructure of a CNT junction transforms from the (c) dry state to the (d) wet state. The black cylinders represent the CNTs, and the conductive paths are indicated by yellow lines.

Interface resistance, an important factor, influences the volume resistance of CNT-based assemblies. It’s dependent on the distance, overlapping area of adjacent CNTs and filling molecules in the intraspace of the film.15, 16 We have confirmed that liquid can directly regulate the resistance of the CNT film by infiltrating into their intraspace in the previous report.17 For the initial compacted CNT film, electrons can easily flow through the interfaces between the CNTs from low potential side to high potential side (as shown in Figure 1(c)). When the liquid molecules infiltrate into the intraspace of the film, the initial electron transport paths are destroyed, including the reduction of the efficient contact (like A or B to G in Figure 1(d)), the separation of the CNT junctions (like D, E, F, I and J in Figure 1(d)) and the filling of the liquid molecules in the intraspace. The structural damage of the junctions can directly



increase the interface resistance, and further decreases the volume conductivity of the film. The magnitude of the resistance change is related to the wetting property between the liquid and the CNTs.17-20 Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids or between a liquid and a solid. The surfactant aqueous solution of different concentration will cause the change in the wetting behavior between CNTs and water. Based on this mechanism, we propose that the CNT film can be used for water monitor aimed at the amphiphilic contaminant.

Ethanol is a common organic solvent, which is able to wet CNTs. Meanwhile, the surface tension of the ethanol aqueous solution decreases with the addition of ethanol.21 Figure 2(a) shows the change ratios of the film resistance (∆R/R0) exposed to the ethanol aqueous solutions with different mass fractions (WT). The change ratio of the resistance varies from –2.20% for pure water to 29.92% for pure ethanol, which shows the exponential relationship between the mass fraction of the solution and the change ratio of the resistance (as shown in Figure 2(a)). The R2 value is 0.998.

Figure 2 (a) Change ratio of the resistance of the CNT-based film responses with the


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mass fraction of ethanol aqueous solution. Contact angles of (b) water (H2O) droplet and (c) ethanol droplet on the well-ordered pristine CNT film.

Contact angles clearly reflect the variation of the wetting behavior between the solution and the film. The contact angle of pure water droplet on the CNT-based film is 106.5° (Figure 2(b)). It decreases with the addition of the ethanol. And the contact angle of pure ethanol droplet is only 31.4° (Figure 2(c)). Such distinct difference indicates the effect of ethanol molecules on regulating wetting property between water and CNTs. The fitting result shows that the electric response of the film is able to sensitively reflect the change in water quality caused by contamination of amphiphilic organic solvents (e.g. ethanol).



Figure 3 (a) Time-resolved ∆R/R0 plots of the film immersed in water, 10 mmol/L KCl aqueous solution and 10 mmol/L AOT aqueous solution. (b-e) Contact angles of (b) 1 mmol/L KCl solution droplet, (c) 10 mmol/L KCl solution droplet, (d) 1 mmol/L AOT solution droplet and (e) 10 mmol/L AOT droplet on CNT film. (f-i) Change ratios of the CNT film immersed in (f) KCl aqueous solution, (g) AOT aqueous solution, (h) SDS aqueous solution and (i) CTAB aqueous solution. Lines are the fitting results between the change ratio of the film resistance and the concentration of the solution.


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The mechanism of water monitor based on the variation of the wettability is general. We subsequently test the feasibility of the monitor strategy for diverse surfactants. Similar to the effect of ethanol in water, the surfactant also can effectively change the wetting behavior between CNTs and water. But the ionization of the surfactants in water will increase the conductivity of the solution, especially for high concentration. Figure 3(a) shows the time-resolved resistance plots when the film is immersed in different solution, including water, 10 mmol/L KCl solution and 10 mmol/L AOT solution. For water, there is a slight decline of the film resistance. When the film is immersed in 10 mmol/L KCl solution, the decrease degree of the resistance is distinctly larger than that of water. The measurements of contact angle indicate that the addition of KCL doesn’t obviously change the wetting property between CNTs and water (106.5° for water, 100.0° for 1 mmol/L KCl solution and 104.2° for 10 mmol/L KCl solution). Figure 3(f) shows the resistance of the film immersed in KCl aqueous solutions decreases from -2.16% for water to -12.06% for 1 mmol/L KCl solution. And there is a tiny change in resistance with the concentration of the KCl aqueous solution. We consider that this behavior is caused from the increase of the conductivity of the solution due to the addition of KCl. The ionization of KCl in water effectively increases the conductivity of the system, which further leads to the change in measured resistance when the film is immersed in the KCl solution instead of water. Different from 10 mmol/L KCL solution, 10 mmol/L AOT solution results in the significant increase of the measured resistance, although they have similar conductivity (as shown in Figure 3(a) and S2). The variation of the contact angle reflects that the addition of AOT markedly changes the wetting behavior between water and CNTs, and the wettability of aqueous solution is gradually improved by increasing concentration of AOT (from 42.2° for 1 mmol/L AOT solution to 25.1° for 10 mmol/L AOT solution). The measured change ratios of the resistance (∆R/R0) at immersion time of 60s are plotted in Figure 3(g) as a function of the concentration of the AOT solution (CB). The change law also applies to other surfactant (e.g. SDS and CTAB), as shown in Figure 3(h) and (i). It’s noted that the change ratios of the ACS Paragon Plus Environment


measured resistance deviates the fitting curve for CTAB solution with the concentration lower than 5 mmol/L. This deviation behavior maybe stem from the competitive relation between increase of the film resistance caused by molecule infiltration and decrease of the resistance of the solution caused by ionization of surfactants. Thus, the minimum detectable concentrations of different surfactants are about 1 mmol/L in our work.

Figure 4 (a) Change ratio of resistance when the CNT-based film is immersed in AOT aqueous solution (black square), AOT solution with KCl of 1 mmol/L concentration (red circle) and AOT solution with KCl of 10 mmol/L concentration (blue triangle). (b-e) Contact angles of (b) mixture droplet of 1mmol/L AOT and 1 mmol/L KCL, (c) mixture droplet of 10 mmol/L AOT and 1 mmol/L KCl, (d) mixture droplet of 1 mmol/L AOT and 10 mmol/L KCl and (e) mixture droplet of 10 mmol/L AOT and 10 mmol/L KCL on CNT film. Lines are the fitting results between the change ratio of the measured resistance and the concentration of the solution.

According to the contrast test of water and KCL solution, the competitive relationship between the conductivity and the wetting property of the solution will influence the change ratio of the film resistance. We further confirm the ability of this strategy to determine the amphiphilic pollutant in mixed system. Figure 4(a) shows the change ratio of the resistance when the film is immersed in AOT aqueous solution in presence of KCl. The addition of KCl really influences the change ratio of the ACS Paragon Plus Environment

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resistance. And the more KCl is added in solution, the more obvious variation of change in wet resistance is. The measurement of contact angles indicates that the addition of KCl has no effect on the wetting behavior between the CNTs and the solution (as shown in Figure 4(b)-(e)), but it increases the conductivity of the AOT solutions. Even so, there is a clear function relationship between the change ratio of the resistance and the concentration of the surfactant in the solution. However, the results indicate that the strategy of water monitor by means of the change in interface resistance is suitable to differentiate the surfactants and the electrolyte, although the conductivity of the solution influences the change ratio of the film resistance.

Figure 5 Repeating test of the film immersed (a) in the AOT aqueous solution with different concentration and (b) the ethanol aqueous solution with different mass fraction.

The cycling behavior of water monitor is shown in Figure 5. In Figure 5(a), the film is examined 5 cycles for the AOT aqueous solutions of different concentrations. It shows that the change of resistance is highly reversible. Meanwhile, there is an obvious variation of the change ratio for concentrations of the solution from 1 mmol/L to 10 mmol/L. The electric response of the film also owns a good reversibility for the ethanol aqueous solution (as shown in Figure 5(b)). The time-resolved plots show that the resistance has a sharp reduction when the film is exposed to the solutions. Then, the resistance slowly changes due to the further infiltration of the liquid. The change ACS Paragon Plus Environment


ratio of the resistance increases with the mass fraction of ethanol in the solution. The resistance can recover to the initial value after the evaporation of adsorbed solution and the calibration. The CNT-based film with excellent reversibility and stability is suitable for long-lasting and multiple water monitor in practice. The strategy of water monitor demonstrated with the diverse surfactant aqueous solution (AOT, SDS, CTAB) and the organic solvent aqueous solution (ethanol) is general. The monitored object can be more practical liquid environment for specific applications. The key is that pollutant or additive is able to effectively change the wetting behavior between the CNTs and the liquid medium. Although it is difficult to confirm the pollutants for the CNT-based film in this work, this way also can be used as an effective preliminary test in water monitor. Meanwhile, abundant modified methods for CNTs make it possible to identify the pollutants in the future.

Conclusion In summary, we have demonstrated an effective strategy to monitor water quality aimed at amphiphilic pollutants, by means of the change in the wetting property between water and CNTs. During the water pollution caused by amphiphilic materials, the interface tension between water and CNTs is distinctly changed. The infiltration of liquid molecules further leads to the measureable change in the resistance of the CNT-based film. The experiments of the surfactants (AOT, SDS, CTAB) and organic solvent (ethanol) aqueous solution verify that this strategy is feasible and general. This strategy also can be applied in other liquid system and some pollutants. The core in the mechanism is the modulation of wetting behavior between liquid and CNTs caused by pollutants. Reversible change in the resistance following the infiltration and evaporation of the solution as demonstrated in this work has potential application in long-lasting and multiple water monitor. This general strategy provides a new path for water monitor and further can be expanded further into other liquid system in practice.

Associated content Supporting information The Supporting Information is available free of charge on the ACS Publications ACS Paragon Plus Environment

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website. The photograph of the experimental setup is shown in the supporting information S1. The conductivities of the AOT and KCl solution are shown in the supporting information S2.

Author information Corresponding author Guoan Cheng

E-mail: [email protected]

ORCID Guoan Cheng: 0000-0002-0652-2534 Notes The authors declare no competing financial interests.

Acknowledgements This work is supported by the Beijing Science and Technology Major Project (No.Z171100002017008), the National Basic Research Program of China (No. 2010CB832905) and the National Nature Science Foundation of China (11575025). References: (1) Dey, S.; Santra, S.; Midya, A.; Guha, P. K.; Ray, S. K., Synthesis of CuxNi(1-x)O Coral-Like Nanostructures and their Application in the Design of a Reusable Toxic Heavy Metal Ion Sensor Based On an Adsorption-Mediated Electrochemical Technique. Environ.-Sci. Nano 2017, 4 (1), 191-202. (2) Guo, J.; Chai, Y.; Yuan, R.; Song, Z.; Zou, Z., Lead (II) Carbon Paste Electrode Based On Derivatized Multi-Walled Carbon Nanotubes: Application to Lead Content Determination in Environmental Samples. Sens. Actuator B-Chem 2011, 155 (2), 639-645. (3) Khani, H.; Rofouei, M. K.; Arab, P.; Gupta, V. K.; Vafaei, Z., Multi-Walled Carbon Nanotubes-Ionic Liquid-Carbon Paste Electrode as a Super Selectivity Sensor: Application to Potentiometric Monitoring of Mercury ion(II). J. Hazard. Mater. 2010, 183 (1-3), 402-409. (4) Xu, Z.; Zhou, W.; Dong, Q.; Li, Y.; Cai, D.; Lei, Y.; Bagtzoglou, A.; Li, B., Flat Flexible Thin Milli-Electrode Array for Real-Time in Situ Water Quality Monitoring in Distribution Systems. Environmental Science: Water Research & Technology 2017, 3 (5), 865-874. (5) Song, X.; Chen, J.; Shi, Y., Different Configurations of Carbon Nanotubes Reinforced Solid-Phase Microextraction Techniques and their Applications in the Environmental Analysis. TrAC Trends in Analytical Chemistry 2017, 86, 263-275. (6) Smith, S. C.; Ahmed, F.; Gutierrez, K. M.; Frigi Rodrigues, D., A Comparative Study of Lysozyme Adsorption with Graphene, Graphene Oxide, and Single-Walled Carbon Nanotubes:



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